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DEPARTMENT OF THE INTERIOR
MONOGRAPHS
OF THE
UNITED STATES GHOLOGICAL SURVEY
VOLUME XLVII
WASHINGTON
GOVERNMENT PRINTING OFFICE
1904
UNITED STATES GEOLOGICAL SURVEY
CHARLES D. WALCOTT, Drrecror
TREATISE ON METAMORPHISM
BY
CHARLES RICHARD VAN HISE
WASHINGTON
GOVERNMENT PRINTING OFFICE
1904
CONTENTS.
EPPER AO Me LRANSMITT Aye a2 ane s/2c occas Se Sek cise ean eee eee ee
CHAPTER Dt ——ENTRODUGDION aise se saya /o ns 5a a a ey os en
Generales funevotya ite rail O10 Sees eee
Classification orametaniorphismysns soe ee] eee ene
Geological factors affecting the alterations of rocks --..-..--...----
Com positionet = pe ara ae seen = sae See ee ae one eee
Structuressandate x tires tess ees eye ee ee ey
IROTOSIby A eee rein Reames ete Seb abe koe reas ree ane ee
Waterandicascousicontentasen ss e ees eee eee eee
IDA Vil at ono coaddacsnuse=eqqnadaaamaneneSsensoswesouckouLose
CuHaprer JJ.—THE Forces or METAMORPHISM ._.---..-.-.------------
Permanent strain with closing of openings and welding
Wratenaction=eceeere er asec ts on ct Ree eer ee eee
Generalltetatenre mba ote eyes eet eee ae aries le a athe
3)
41
a es
Sites for) at a IG ING) tS)
6 CONTENTS.
CHAPTER TI —Tan) AGrents or MEnAMORPHISM=s-- 22-25 se oie alee =o eaenie = erin ele el loam
General¥statementierere: aeereeee sence ee ee aCe eee CE eee eee Eee eee eae ee eae
Partin Gaseouskso lutions es soe ea seo satya ete ee ie yen tee pete ae en bette era
Section 1. Chemical and physical principles controlling the action of gases...-.-----
(Gases presen erase retas oe era ater tee sete a eee eee etm see ae ee
MUMS TORT 5 kb Spo dst cabo n sasase sano codaccdodasesessoacausoseedarcas
Mbeitemperatuneee setae ees see eee eee eee ee eee aeee ea aot ek eR ABE ar HE
Sectioni2snGeolocicalaworkioi cases sas = eens ser Pe see ee eee ee see eee ne aes
Part leeAqueousisolutionsiamdlsolids) 0-552). e state) lee ya ore eee gestae relat
Generalliconsidenation sis sspears toa e Pea varar a ee
Section 1. Chemical and physical principles controlling the action of ground water. -
Principles of solutions applicable to ground water -........------.-------------
Solution of gases in groumd waters: 22. 5222-22-_.22222. 2225202. (AE Nats
Gases present -<. - = == 2 2222 22 enn ne =
WDoVe) (VRE oe ee os SceocodpecdcoscnssosapecbocsosoSsouebeSoEeeocac
ON aysy MECN NAAR a ceo se sco uoososocooosod sobs ooSodaboescoecosccsecs
Solidsiimisolutionys S432 suse ase ten cinoe eee tee eer eevee tae apees
Solutionwofsolidsumyeroundawatersesee sees asso ee eo eee eee eee eee
Compound shpresen tries ar eee eee eee eee eee eater
Relations of solutioniandlypressurewessse sees wees e Ee eee eee aeeee
Soewel OF SOMIMOM S56 Sos eae sooo soccosossccoesaseacecaschbonsasces
Quantity of material which may be held in solution
Relations of solution to absorption and liberation of heat.....------
IDMRNENON So oso oacocecenasoonGeacesalodaoduanusonboseooobecpauuSoooeadneS
Principles of chemical reactions applicable to ground waters. ..-..---.----------
Generalastateml em tian ane ee ere ree eee eee ee eee eee
Definitionsye ee eee een ee CEES ee LEE eee eer ee menaer crise
IDNERO CAMO earcdc sacndoso esos dsdoo spac osasesacdodnocsacnesscesccauS
JEGWOMROMIENS co sasces ess se oeeae on Soscen san cesedsosddsus5us Seancdeusous
Reactions: Sees tee ne en a sae okstoe Se ae ee CE Ee eee Cee eae
ID oqHUbH Opa, = Sa ob one eas Soe noosoScesecoosHoscnscesbeTonesaseedS
Homogeneous and heterogeneous systems. ..-.----.------------------- .
Natureiandispeediotreactionss.--- ee her eses sae eeee ee eee ee eee
Mhercompoun d skesss esse see ele elses eee eee eee
Strenothiotitheysolmtioms ese eis sae retele tere n alata eae te eereeeteteratetatr
Miggamaiel emo. «oso bd eagassuns sb ocoonocnsueboussoosscucbenbsdesd
Straimiawithoubanupture soe eee erent seal slays es
Stiralmiavalt nie up GUC ee ee sears rete rare terete
Readjustmentiofparticlesssse a sesee eee seer eae
lmdinect#heatietiec tase ee serene eee tie tee ees errerataera
Nature of the chemical reactions. .-...-.---------- ye Saletan ie eee
62
65
DD aD
Se COm COROT
NINN NON os
InDwbw Ww wv
nnn nm 0 Mm mM WI I AI
Ore FEN | OO O
87
oOo Oo O
or St or Ot
CONTENTS.
Cuaprer II].—THEe AGents or Meramorpaism—Continued.
Part II. Aqueous solutions and solids—Continued.
Section 1. Chemical and physical principles, ete. —Continuec
Principles of chemical reactions applicable to ground waters—Continued.
Nature and speed of reactions—Continued.
Mechanical action—Continued.
Nature of the chemical reactions—Continued.
= Smaller rock volume as the result of solution and deposition
without change in chemical composition. -_.-...------------
Recrystallization and condensation without change of min-
Recrystallization and condensation with change of minerals.
Smaller volume as the result of solution and redeposition with
changennichemicalicompositions=seeeeeeeesceeeeeeeeeeree ee
Crystallization and condensation of amorphous compounds.
Recrystallization and condensation of crystallized com-
OUT A Sif sea Ss esis Mere ee a ee re fa ee
Generaliistatements's ase -..o se aep eee eae eee eee eae
Relations of chemical action, mechanical action, and heat_.........-----
Precipitation assets sue cee jasss sc bys nent eee errr RO nee nate
BreCiIpiLAtlOnyDyACh an ee rOl PLeSSUNE tae eye eee ee ee eee
Precipitation by change of temperature..-......--.-------------------
Precipitation by reactions between aqueous solutions --.-..------------
Precipitation by reactions between aqueous solutions and gases.---.----
Frecipitation by reactions between solutions and solids ...-...---------
Section)2:) Circulationjand work of ground water: =sss2--46 sess seeeeece. sees ese
Wmmvyersalpresence-olawateninerOck see s= === ae =e eee eee eee ee eases eee
IPOTCTS PACE LOL LOCKS Es sa ate Assia aos Sache eee See eee ee Bee eae Eee eecix eee
Cinculationvotyonoundawaltensseenee renee seee eee ere Aer eee ene er eee ce eeee
(Oyorerantoyeqe) rial sole tacfa as ee i at RE Ta SAR Ayes cate Le co Pas aR
Form and continuity of openings
Size of openings
Soyoonceyollleray Opes. 5 Gos beascgcansacSucomeccesoaseodeeses
Canilllanycopenin esses sae eee eee Pee ae eae ae ean ee eae
Gravity and heat
Mechanical action
Vegetation
General statements
8 CONTENTS.
Cuaprer III.—THr Acrents or MrramorpHism—Continued.
Part II. Aqueous solutions and solids—Continued.
Section 2. Circulation and work of ground water—Continued.
Circulation of ground water—Continued.
The factor opposing water circulation
General istatementsaassas8ee ee eee ee eee er eee eee eee eer epee
Geologicaliworkiolenoundwate mee eeere eee ee eee ee eee eee eee
Cuaprer [V.—THE ZoNES AND Betts or MmeraMORPHISM......-.-._....-------------------
Gemeral¥consid erationsy: Sesser t re ee atop tise 5 ate eam eaters and a te ne ov aL ea seh
Zoneno felka tama Ovp Lis rans ses yest ee arse esta Sag Ce aoa ag em ec aE
Beltrobaweath erin ays se S015) age wee ces ois ie MSE are ei reth ay psig ee ae Dee pera
Beltioficem entation! ose ety es pete aay ee oe Oe Lt ab arta alm Rr a eels paoe eaa nee PI ae a
Belts of weathering and cementation contrasted ...-....-.-.------.-.-----+--------
Zone olanamorphismy.\-2se. 226 scis\2 ses ease sos ase eee ERO RU Eee eee eee eee
Relations of zones of katamorphism and anamorphism...........-.-....-.------.------
Generalkconsid erationsise ec ee eee Soe be Meee Se ee ee aE Ee te a eae
Relations of zones of katamorphism and anamorphism to zones of fracture and flowage --
Uippenslimitvotzonexrotilowace mess see eee aeee tere eae ee eee eee eee eee
(CHAPTER Ve —— MINERALS epee Ses eet eae Re Se etary ep ee OU
Section 1. Chemical and mineral composition of the known crust of the earth.........-.
Section*aGeneralimatunerofialteratioms sms apy serra ee pear e are e ee
Alteration without change in chemical composition..........--.--------------
Molecularsrearrangement ass ie ote sate ate orotate 2 Sp ney
Simple recrystallization-.....-- SP eerie PMN Bees Sar OL EONS pa
Alteration with change in chemical composition...-.......--.--.--------+---------
Alteration without addition or subtraction of material .......-..-...--.--------
Alteration with addition or subtraction of material........--...----...--..-..---
Generalistatementss2 Js hee sate os eee eee eee eS eee eee ees teense
Sections 3egRock-m alka omni eral] Sessa esate a
Manneriof treatmem tien sas sess oe pe eee a een ae eee cere
Generalistatementsie nes eens ee ee eer ae dete ei Lies eee
Nativeelementsis- 2s Se Sos Soe eas ot 25S nats eel Ge nan sce eaten epee Seay
Graphite ys se ese re aya lee Sar aN is /feeoc cutee 2] om uefa a alae om ns en
Oceurrence 5285 ec es ASS Shee sees Oe ate eee a eee
WA Cerationsseye sys aces aro ec al eee eae ee
Theisulphidess-2 Wsssdascce cesses teas olsis eis See ee eee eee eee Eee eee
Pycrhotibempycite wan Cem ancaslic== eee ee eee ee ee ee eee
Occlrren cele jsse 25s sees cies Sate ee Se ee See neeneee
Alterations) sacs de Bens Sete siaiacreinte Snare rea See oe eater tere
Pe HO rides ese ses SI ara teeta ae SHS Re ar PN ac een cS
TOIT be eye ase Ue sen: PS Ie Ab rat pea ye pare oe AEAL pct SNe py tre BE
Page.
155
154
156
159
159
160
163
164
166
H
for)
~I
170
iw) i fe er es
(=) 2weee
bo
(=)
KOR RO) ROE ROE URS) ST Sy
CONTENTS.
CHaprer V.—MiInERALS—Continued.
Section 3. Rock-making minerals—Continued.
The oxides—Continued.
Quartz—Continued.
WIOGIIR CRONE cocacescoa5 oboenescoqsescaseso00se6
GUAGE) < oosocesocdo oesasaseseessaasanssesraseessasesosaesn=
OWGUIERANES 4 coop usaboconosusecucooosSessopeaonuSe
Modif catlons sees e eee eee ee eee eee eee ee eee ee
Owwalll cooostoscoessooscosaden scone nacocosecasauascgcosusasSsces
OWHHIEREANCO sos seb bbe sobs osbondasedsododssdoccdcces
Modifications
Chert, chalcedony, ete
Hematite group
Corundum, hematite, and ilmenite -..--.----.--------------
Corundum
Occurrence
JNUGRATIONS o oaoscunsconsoucScoacqoocesoSssosesoese
Hematite
Occurrence
Alterations
Ilmenite
Occurrence
Alterations
Spinel group
Spinel, magnetite, and chromite
Spinel
OWCUIBRINGO 26 oad Sscscoadodcdoouccsstossosceosesseo
/MWigmmimt@me{. So eonsSossboseoasseccoscocoonassass
Magnetite
Occurrence
Chromite
Occunnence eee eee ee eee eee
Alterations
Rutile group
Rutile, octahedrite, and brookite
OWaviERaNGS 5 cog asussbocceocqosesaasoosceSseedsese
JNVIGMMOMG | sGo55 coos ecosdoscoucdeseSsecsecesssoos5
INWEENOME so cdaGosaceooedooasebeesaessoucouoosdse
Limonite
Occurrence
Alterations
INWereatiionng Gocosoacdccunssseussteooccestoeacogcsadosossseceoda
Page.
232
232
232
232
232
233
10 CONTENTS.
CuHarter V.—MINERALS—Continued.
Section 3. Rock-making minerals—Continued.
The oxides—Continued.
Brucite group
Brucite and gibbsite
IB UCU Ee ese eect Vt SU UL Tepe peat hoc aang eT eA wea Ug aaah say
efor heats) Kees ieas Geese ee se eee eR es A ear ae ee RU es ek A
WAN LOT ALIOM Sie eoe eeers ea No ah ep ese rs leap = aa RNR ee Syn Me Us
Gilbbsites ess nace Sete sag cs eee eee RS See aera is otra Sete
MGCULKEM CE See pee Ses eee aS ET ote eae Eder cr
PAN CCTATTONS mize hs eae eee cite ete erage a pet ne een re I Co OO
MPG CAT bONAtESE Sas cess 2S RUINS Cia ate a Cae ora) ale Re Ep
Callentesero up ei eal a= este sieve aie age ra oy Seley eae sree ee eee pret eS Ne ADR payee re ‘
Calcite, dolomite, ankerite, parankerite, magnesite, and siderite....2.._.__.
Call Ciba as seers SOS ee STMT Nl 2 es (Ue pepe Ur ea UC iP
Ankerite and parankerite-__..-_--.--- ere a aie ais Wipe ee sre NE
@@eumner cay ae eh EN Se aa HNN SD gt et ranean ere gure ed OL
AMT nat] OMS} eraye les Pees oo le ots ue ata tee et ha UR TES Sk to
Mla ome siitie ss = ae Sits 2 eee Oe Me ek TP oye anata On ap mie ee ab
Occurrence ys see set eee eae ee eee ee er
Ailite rast Orig Be Sat Se eee NS A cay ey ee a a ey ie SE a
SLL CTT CS Was eee SY er EE oe or eee a ee nV ay ay
Allterations eee re Se Ure tc UES EA UW) sila ini sli ea puri iy i ail enya See
NTAPOMITSSLOUP Se Oe ash aren ae AN eM ASE Oe DLL ei aes a Lit esters Eee
ATAGOMIUGC ee phe e See ae ices le aay ope la te tere Se fe are ey ec ee a
EeuaTereS ra Cee ee ea oh ee El TI Sra es at rn RR
ANE atOM Siete eile Ste RAY ASE Ce EL im os ee tere Ota ese ey RR pene RE
Evidence that devitrification takes place._.------.........-...-.--
P Scalevoisdevatriti cation cae ee Sia Bae oh ee pale See ae Stl
Ratevoide vitriti cates rescore cyte pays ae oy ae eee yan ee hoya
Minerals producedis sors esses senna Se oe ee heme ns eee
Eleatiandsvolume relations seeseen ey eaeee see eee eee eee
Meldspar group. s)riceitacierseisce ase eieisice Slee oe erie es eeeee ete lte = Se ease
Monoclinic ioxpseudomonoclinicle=seeene seas eeee eee eee ee eee eee eee teen
Orthoclase, microcline, and anorthoclase -.-...--.---.----.---------------
Orthoclaseyandimicroclines sm sssen eee eae eee eee Cee eee
Page.
bo
ey
Cia
[ey [Ke) TRS)
oo oo oO
or OV
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iw)
CONTENTS.
CuaptER V.—Mrnerats—Continued.
Section 3. Rock-making minerals—Continued.
The silicates—Continued.
Feldspar group—Continued.
Monoclinic or pseudomonoclinice—Continued.
Orthoclase, microcline, and anorthoclase—Continued.
Orthoclase and microcline—Continued.
OCGUTTETC OM Sas ee ee eee Ne ee Fee ad ora
Alterations
eT @ Isr TS apse et a a arene A DSR RR, et Sg RCT Peg ENC
Albite, oligoclase, andesine, labradorite, bytownite, and anorthite _..__.__-
@cetrnen Ce Hee see mre eo ep eae ete ieee Sees SB) ani snl SEL gia ees Seva ce
IA te ratti ON Sye sets eters asi sie ala ee ee ES ies ee Ae hla
TSU CILEY ONO WD Rees sie stare tetas ae ine ie eee oy oe eee ee ere eee Bea eae
JEPEROEUCSS pe eff SEIS AS ON ah sere ea a An oe ede ee tear pad enc
JEAPDRODS SHC) CAMO DUO or ene er eee See Ae ee naa Se ses oer pes Le
Oxthoch om chp ymOxen CS eres eee eet ate eae ee ee eee a
Momo climicwpymOXeNeS Seater Sete eae eine ae eee eee
Diopside, sahlite, hedenbergite, augite, acmite, spodumene, wollastonite,
AO g Dec LONI eS tse ere se se mre ate as eee Be eee OP cine ere a
Mecurnence 3 sar: Seiya eee eS eR eat ee ee ees See ae
Alterations of the diopside-augite series.--................--------
Alterations of pyroxenes other than the diopside-augite series. _____
Am phiboleveno wpe cccse can ose see eee eee eee oat eee eee oe enone
OninorN@ENole Amaoall NOES ooo oo ncascucceesconcooeSee) Haas sabdonoadetaas
AMI AO HANI GiNGl: CECHAIS. oo soso cesconcaseaoesaasaosseesoocseenae
Occurrence: Clarke, F. W., The relative abundance of the chemical elements: Bull. U.S. Geol. Survey No.
78, 1891, p. 34.
31
32 A TREATISE ON METAMORPHISM.
reasonably full and satisfactory knowledge, not only of the known principles
of geology, but of the observed phenomena in the parts of the world which
had been studied. While by many years of work a geologist may still be
able to learn the important facts concerning various provinces, it is no
longer possible for one man to have anything like complete information as
to the local geology of many parts of the world. Not only is this so, but
no one geologist can know all the important discovered facts concerning a
particular branch of geology. Moreover, in recent years the accumulation
of facts has gone on much faster than the development of geological theory.
Nowhere is this more true than in the branch of geology known as
petrology, and in petrology it is perhaps more true of the phenomena of the
alterations of rocks than of any other. Scarcely a paper on petrology
appears that does not contain some account of the alterations of minerals or
of rocks, but in most cases there is no serious attempt to arrange the observed
phenomena in order under recognized principles. Indeed, there is no
general set of recognized principles under which the phenomena can be
reduced to order.
Some years ago, finding myself lost in the vast accumulation of data,
T began to formulate principles applicable to the alterations of rocks. The
result of this work is the present treatise, which is an attempt to reduce the
phenomena of metamorphism to order under the principles of physies and
chemistry, or, more simply, under the laws of energy. It is but a part of
the larger task of reducing to order under the same laws the entire subject
of physical geology.
As a result of the development of the science of petrology, especially
microscopical petrology, it has been ascertained that changes are continu-
ally occurring within the rocks constituting the outer part of the earth.
This statement is equally applicable to the most porous rocks at the surface
of the earth and to the densest rocks as deep below the surface as observa-
tion gives exact knowledge. All changes, by whatever forces, agents, and
processes caused, and in whatever classes of rocks occurring, whether
solidified magmas, chemical precipitates, organic deposits, or mechanical
deposits, may be called metamorphism.
Metamorphism, as here used, means any change in the constitution of
any kind of rock.
It will be shown that at any given time and place, under any given
ee
ROCKS ADAPTED TO ENVIRONMENT. 33
set of conditions, minerals tend to form which remain permanent under
those conditions. his tendency is more potent with minerals crystallizing
from magmas than with minerals which constitute the sedimentary rocks
or with the secondary minerals which form by metamorphism. ‘The reason
for this is that adjustment to existing conditions is so much more readily
accomplished in fluids than in solids; but the tendency to form minerals
which are permanent under the existing conditions controls in the solid
rocks, although there is a great amount of lag in the process of modifica-
tion. If adjustment be reached in a given case and if the conditions
remain the same, the minerals formed do not again alter, but may remain
the same through eons. This is illustrated by the meteorites, the minerals
of which may persist without change during the evolutions of stellar sys-
tems. However, when an important change of conditions occurs, as when
a meteor gives up its separate existence in the interstellar spaces and joins
a planet, as the earth, readjustment begins at once.
Although the changes of conditions upon the earth are not so great as
the change when a meteor falls to the earth, the range of conditions upon
the earth is large and varied. The conditions may be those of ordinary
pressure and temperature at or near the surface of the earth, or they may
be those of very high pressure and temperature, such as exist well below
the surface of the earth. A rock mass may alternately be subject to each
of these sets of conditions and to various intermediate conditions. Changes
of physical conditions result from surficial transfer of material by epigene
agents—bringing rocks to the surface here, burying them there—from
igneous intrusions, from orogenic movement, and from other causes. The
changes upon the earth are therefore profound, although usually slow.
During the changes the rocks are always modified in the direction of
adjustment to the new conditions. Such modification of rocks has led to
the idea of adaptation to their environment. As conditions change, species
of plants and animals are so rapidly modified that at first sight adaptation
seems almost perfect. Indeed, so sensitive are plants and animals to their
environment that since the theory of evolution gained ascendancy the fact
of approximate adaptation is taken for granted. The variety and complexity
of the structures, colors, etc., of life forms resulting from adaptation to
environment is a constant source of wonder. Almost daily some remarkable
structure or form is described, and its existence explained by showing how
MON XLVII—04——3
34 A TREATISE ON METAMORPHISM.
it is advantageous to the animal under the conditions in which it lives.
Since adaptation is an assumed law, in those cases where there seems to be
lack of adaptation, as where some peculiar structure is present which
apparently is not of advantage to an animal or plant, it is believed that the
facts are not fully known or that the structure was once useful and is a
survival. However, the very idea of survival shows that to a certain degree
the development of plants and animals lags behind their changing environ-
ment. Upon a priori grounds it would be certain that this is the case; and
the existence of rudimentary organs, such as the muscles for moving the
human ear, which at one time may have had a use, is positive evidence of
the lag of organic species during their adaptation to changing environment.
Likewise it is believed that minerals constantly tend to change to
forms that are relatively stable under existent conditions. This, however,
is accomplished by granulation or recrystallization or some analogous
process, and is adaptation only in the sense that the old particles break
up into smaller particles or develop into new mineral particles which
conform to the existent conditions. Some minerals are stable under a
considerable variety of conditions, and therefore are less sensitive to
change than are others. For instance, quartz develops directly from an
igneous rock, and it also forms as a deposit from water. It persists under
both quiescent and dynamic conditions. Other minerals require rather
definite conditions for their existence. Such are leucite and olivine, which
abundantly form as original minerals in igneous rocks of certain composi-
tion, but which readily change under new conditions to other minerals.
However, no mineral persists without reference to its environment, and so
it may be said that there is a tendency in all mineral substances to form
minerals adjusted to the conditions under which they exist. Rocks are
composed of aggregates of different minerals. Therefore rocks, like
minerals, have a tendency toward adjustment to their environment.
Even if the chemical composition of a small mass, say a cubie milli-
meter, remains exactly the same, the mineral constituents of the mass
may greatly change. At the end of the change the original minerals may
not be in the same proportions as before and minerals which did not
originally exist in the rock may have formed. But the adjustment of rocks
is not confined to redistribution of the elements present in a small space.
There may be a change in the average chemical composition of rocks.
ADAPTATION TO ENVIRONMENT SLOW. 30
Material may be intruded, or may be brought in by water solutions, or
may be abstracted by water solutions. By any one of these processes or
by any combination of them a considerable change in the chemical
composition of a rock may take place.
While it may be safely asserted that all rocks, under all conditions,
at all times, are being adapted to their environment, the change in a rock
goes on so slowly that its lag behind the change in the environment may
be measured by millions of years. Often the lag is so great that the con-
ditions again change before the process of adjustment has made much
advancement, and, therefore, before one set of changes is near completion
another set is begun. Indeed, a later change in conditions may be a
reversal of an earlier change, and, therefore, in the process of adaptation,
work done in an early stage may be reversed at a later stage. But even in
such a case it is clear that the principle of adaptation applies, just as in the
case of many plants and animals, although there may be, in fact, little more
than a tendency toward adaptation to existent conditions.
Because the adjustment of rock to environment is so slow, in order that
it may be approximately complete it is necessary that a rock remain under
substantially the same conditions for a very long time. This has happened
in some regions in which important mechanical movements have not
occurred for a period or an era and the rocks of which have remained
buried-to a moderate depth for most of the time. Such were the conditions
of the ancient volcanics of certain parts of the Lake Superior region. These
have escaped important mechanical movement since the beginning of Pale-
ozoic time. They were buried under Paleozoic sediments to a moderate
depth. Denudation since the begmning of Cretaceous time brought them
to the surface. Finally glacial erosion removed a skin of weathered
material and exposed the volcanic rocks, approximately adapted to their
past environment, that of the belt of cementation under quiescent condi-
tions. (See Chapter VII, pp. 594 et seq.) So far as they have reached the
surface they are subject to a new set of conditions; and a new cycle of
change, begun at the end of the Glacial epoch, but not far advanced, is in
progress.
In considering metamorphism, the fundamental hypothesis of geology
will be applied as in other branches of the subject. That is to say,
the Huttonian principle, that the present is the key to the past, is
36 A TREATISE ON METAMORPHISM.
assumed. Where certain phenomena are now produced by certain com-
binations of forces and agents, and by these only, and similar phenom-
ena are found in the rocks long since formed, it is assumed that the like
phenomena, present and past, are due to essentially the same combina-
tions of forces and agents. For instance, if alterations of a certain
kind are now being produced by a complex set of. geological factors, and
by these only, where similar alterations are found in ancient rocks it is
assumed that they are due to practically the same combination of the
forces and agents of alteration.
But the above statement does not imply that the changes are now
taking place with the same speed as that with which they occurred in the
past, as might have been held by Lyell; nor is it assumed that the various
forces and agents have the same relative values. Indeed, it is believed to
be highly probable that there have been changes in the rate of alteration
of rocks and in the nature and effectiveness of the factors producing the
alterations.
While the Huttonian principle is of service in the study of metamor-
phism, the alterations of rocks take place so slowly that it does not have
nearly the value that it has in the study of the work of the epigene agents,
such as air and water and ice; nor the value it has in the study of such
hypogene agents as volcanoes and earthquakes.
Many of the rock alterations are now taking place under conditions
which can not be directly observed, but must be inferred from the records
of the change. This statement applies to all changes below a mile in
depth, and it is very largely applicable to all but the mere outer film of the
rocks, for most excavations and cuttings are not deeper than a few score or
a few hundred feet and the deepest shafts are but little over a mile. For-
tunately it frequently happens that in a rock formation now at the surface
the results of various stages of change under deep-seated conditions are
preserved, so that the character of the alterations and the nature of the
forces and agents which have produced them may be inferred from a close
study of the different stages of alteration.
Tn such cases, instead of observing the forces and agents accomplishing
certain results, and of reasoning that similar results produced in the past
are due to these forces and agents, we observe the results at various stages
of development and infer from them the nature of the forces and agents
producing them; we then infer that similar forces and agents are at work
METHOD OF REASONING. a7
beyond the zone of observation, accomplishing at the present time similar
results. This is a complete reversal of the Huttonian method. Hence, in
treating of metamorphism we must argue both from the present to the past
and from the past to the present. By studying the action of the forces and
agents now at work in the zone of observation and the stages of alteration
preserved in the rocks brought into the zone of observation we are able to
push the boundaries of the known for a certain distance into the domain of
the unknown, and infer with considerable certainty the nature of the
changes which have taken place in the far-distant past and of those which
are now taking place but which we can not directly observe.
It will be generally agreed that the majority of the altered rocks,
including a large portion of the schists and gneisses, have been metamor-
phosed from aqueous and igneous rocks like those now being produced.
This is in accordance with the Huttonian principle. But some may hold
that the most ancient of the schists and gneisses, those of the so-called
Basement Complex, had a different origin. For instance, it has been held
by some that these ancient rocks are direct precipitates in a primeval
ocean. On later pages it will be seen that the most ancient schists and
gneisses are in all respects like those produced from more recent rocks by
the processes of alteration, and therefore that the probable, but not certain,
inference is that they were produced from rocks not fundamentally unlike
those now being formed by processes of change not radically different from
those now at work. But in ascertaining the forces and agents and their
method of work in both the ancient and the modern rocks, we must for the
most part follow the reversal of the Huttonian principle—i. e., argue from
past results as to the nature and method of work of present forces and
agents.
Whatever the origin of rocks—whether solidifications from magmas,
chemical precipitates, organic deposits, or mechanical deposits—as already
noted, they may be altered so as to modify their structures, so as to change
their mineral composition, and so as to change their chemical composition.
In place of the original characteristic structures of the igneous rocks, such
as flowage structure and massive structure, and in place of the original
structures of the sedimentary rocks, such as bedding, there may be pro-
duced secondary structures, such as cleavage, fissility, joints, slatiness,
schistosity, and gneissosity. In place of the original textures of igneous
rocks, such as granolitic, porphyritic, ophitic, and poikilitic, and in place
38 A TREATISE ON METAMORPHISM.
of the original textures of sedimentary rocks, such as granular and oolitic,
there may be produced textures characteristic of the metamorphic rocks,
such as cataclastic, parallel orientation, ete. The alteration may result in
the change to minerals all of which may wholly differ from any of the
original minerals, or it may take place by recrystallization without change
in mineral character, as in the case of the formation of marble from lime-
stone. Chemical change may result in the addition of constituents, as in
the case of oxidation and hydration of compounds already existing, or in
the deposition of additional material in the interstices, or in the abstraction
of material. Any given mineral may gain additional elements, or a greater
proportion of some of the elements; it may lose a part or all of some of its
elements, or it may be wholly replaced by another mineral. While the
chemical composition of the rock may be greatly affected by such changes,
in other cases the alterations may result merely in a redistribution of the
elements without affecting the average composition of the rock, as in the
case of marmorization, some cases of devitrification, various cases of
metasomatism, ete.
After one set of changes has taken place, or while they are in progvess,
a change of physical conditions may come about in consequence of which
a different set of changes may be set up. Thus rocks may be partly moedi-
fied under mass-static conditions and subsequently modified under mass-
mechanical conditions. They may be modified near the surface of the
earth, and as a result of burial be later modified at much greater depth;
or they may be modified at great depth, and as a result of erosion be
brought near the surface and there be again modified. Therefore one set
of changes may be superimposed upon another. In many cases it is cer-
tain that rocks have gone through several very complex sets of modifica-
tions. For instance, a rock may be modified under conditions at the
surface, afterwards be buried under other strata and thus pass into a deep
zone, where it may be modified in a different manner, and still later, as a
result of denudation, be brought to the surface and in the passage undergo
successive alterations in intermediate belts, and when it reaches the surface
once more be altered by the same forces and agents as at first. Substan-
tially this history has been gone through by the jaspilites of the Lake
Superior region. (See pp. 831-833.) Many other rocks have had an
equally intricate but very different history.
AGENCIES CONCERNED IN ROCK CHANGES ag
CLASSIFICATION OF METAMORPHISM.
The forces of metamorphism are chemical energy, gravity, and heat
and light. ‘The agents of metamorphism are gases, liquids, and organic
compounds.
A critical examination of the published classifications of metamorphism
shows that the kinds of metamorphism recognized are based upon the idea
that one force or agent or process is dominant in the production of a
particular kind of rock. But in all of the various kinds of metamorphism
ordinarily recognized in classifications, such as thermo-metamorphism,
hydro-metamorphism, chemical metamorphism, static metamorphism, pres-
sure metamorphism, dynamo-metamorphism, regional metamorphism, and
contact metamorphism, all of the forces above mentioned are required, and
also the chief agent, water. There is no metamorphism of a rock without
the presence of heat, and hence all metamorphism is partly thermo-
metamorphism; there is no metamorphism without the presence of water
solutions, and hence all metamorphism is partly hydro-metamorphism; there
is nd metamorphism in which chemical action does not enter, and hence all
metamorphism is partly chemical metamorphism; there is ho metamorphism
without motion, and hence, in an exact sense, all metamorphism is dynamic.
In the alterations of rocks the forces of metamorphism in each case produce
atomic, molecular, and mechanical changes.“ When it is realized that in
all the varieties of metamorphism mentioned chemical action, heat, and
dynamic action enter as important factors, and that water is present and
active wherever metamorphism occurs, it becomes self-evident that the
classifications ordinarily given are not satisfactory. Moreover, the classifi-
cations involve different factors not belonging to the same category, some
being physical, some chemical, some geological, some referring to an agent,
others to a cause. For instance, thermo-metamorphism refers to heat;
hydro-metamorphism refers to the presence of water; chemical metamor-
phism refers to the action of chemical forces; static metamorphism and
pressure metamorphism refer to quiescent conditions; dynamo-metamorphism
refers to conditions of motion; regional metamorphism refers to the extent
“Tf this be true, it is clear that a classification of metamorphism into paramorphism, metatrophy,
and metataxis, restricting these terms to atomic, molecular, and mechanical changes, respectively,
as proposed by A. Irving, is wholly impracticable. Irving, A., Metamorphism of rocks, London,
1889, pp. 4-5.
40) A TREATISE ON METAMORPHISM.
of the alterations; and contact metamorphism refers to the contiguity of
an igneous rock.
As a matter of fact, all of these different kinds of metamorphism are
related in the most intricate manner, and certain metamorphic results
which have been attributed to one of these forces, agents, or processes
could equally well be attributed to another. For instance, in many cases
metamorphism known as thermo-metamorphism might just as well be
called hydro-metamorphism, or regional metamorphism be called dynamic
metamorphism, or contact metamorphism be called thermo-metamorphism
or chemical metamorphism.
It follows from the above that a satisfactory classification of meta-
morphism based upon chemical forces alone, or physical forces alone, or
individual processes, is quite out of the question. It appears to me that
the only workable classification of metamorphism is geological. (See
pp. 43-44.)
GEOLOGICAL FACTORS AFFECTING THE ALTERATIONS OF ROCKS.
The more important geological factors affecting the alterations of rocks
are: Composition; structures and textures; porosity; water and gaseous
content; climatic and geographic conditions; time; environment; degree
of movement; depth. Many physical factors enter into each of these
geological factors.
At present only general statements will be made with reference to
these factors, but on later pages the effect of each of them will more
clearly appear.
Composition], so far as rocks are composed of minerals which are
permanent under the existing conditions, or are composed of minerals
which may exist under a wide variety of conditions, this is favorable to
stability.
Structures and textures—T so far as there are coarse structures and textures,
this is favorable to permanency, for it will be seen that fine material is
more readily altered than coarse material.
Porosity.— Porosity has a very important influence upon the rapidity of
change. In proportion as rocks are porous the agents of alteration, gases
and water, may enter and rapidly circulate. In proportion as they are
dense, the amount of water present is small and the circulation is slow.
GEOLOGICAL FACTORS AFFECTING ROCK ALTERATION. 41
Hence porosity is favorable to rapid change; density is favorable to
stability.
Water and gaseous content.—[n proportion as rocks contain water and gas
they are readily altered. In proportion as water and gas are absent they
are stable.
Climatic and geographic conditions.—'[he speed of alteration of rocks is affected
by their geographical position. The alteration of surface rocks is more
rapid in tropical than in arctic regions; it is more rapid in humid than in
arid regions; it is more rapid on steep than on gentle slopes; it is more
rapid along coasts than in the interior. In short, the nature of the altera-
tions of the upper belt of rocks varies with every varying factor of climate
and geography.
Time—T'ime is a factor of the very highest importance in metamor-
phism. ‘Time can not be included among the forces or the agents of meta-
morphism, but the amount of metamorphism is a function of the time.
Where a given set of forces and agents is at work under a given set of
conditions, increase of time increases the metamorphism, but not in a direct
ratio, for im proportion as adjustment to environment is approached the
alterations decrease in speed. The importance of time in geology can not
be too strongly emphasized, for a comparatively weak force or agent
working through a great length of time may accomplish an almost incred-
ible amount of work. We are accustomed to judge of the efficiency of a
force or agent by observations in the chemical or physical laboratory, but
the time through which an experiment may be continued in the laboratory
is an almest infinitely small fraction of the time through which the forces
and agents have been at work in nature. To illustrate, in the chemical
laboratory the amount of crystallized silica which can be dissolved in water
and transported to another place within the time during which an ordinary
experiment is carried on is so small as to be unmeasurable, and yet it is
certain that in nature water has dissolved and transported to other places
enormous quantities of silica. (See Chapter VII, pp. 622-623.) This illus-
tration enforces the fact that the geologist has very much more time at his
command than has the chemist or the physicist. If the geologist ignores
this fact, and reasons in reference to the potency of forces and agents in
metamorphism as a chemist or physicist would in the laboratory in refer-
ence to the same forces and agents, he is certain to fall into very serious
42 A TREATISE ON METAMORPHISM.
error. The importance of the time factor has been recognized by most
geologists with respect to erosion and many of the other geological
processes, but it is of even greater importance in metamorphism. Most of
the metamorphic processes are very slow indeed, but the amount of time
available in a single geological period is great, and the metamorphic results
are often stupendous.
In general it may be said that in proportion as rocks are old they are
likely to have been greatly altered; in proportion as they are young they
are likely to have been little altered. While time is a most important
factor in the amount of alteration, time alone, without the other necessary
conditions for change, is not sufficient to insure important metamorphic
results. Further, when the other conditions are very favorable to change,
extensive alteration may take place in°a comparatively short time, consid-
ering this factor from a geological point of view. It follows, because of
variations in other factors than time, that in some regions very ancient
rocks may be little modified and in other regions comparatively young
rocks may be greatly modified.
Environment—I) many cases environment may be important. If the
rocks surrounding a given rock be porous, this condition readily permits the
entrance of the agents of alteration—water and gases—and therefore much
more profound change may occur than if the rock were surrounded by
comparatively impervious material. This is illustrated by the diabase dikes
of the Penokee series of Michigan, which where surrounded by the broken
rocks of the iron-bearing formation are completely altered, but which
where surrounded by the impervious black slates are comparatively
unaltered. A further very important factor in environment is the
presence of intruded igneous rocks. Igneous rocks, by conduction, may
directly heat the adjacent rocks; but of even greater importance is the
fact that igneous rocks may furnish solutions to the adjacent rocks or heat
the solutions which percolate through them. These illustrations show that
the alteration of a rock may be greatly affected by the surrounding rocks.
Degree of movement—()ne of the most important of the factors affecting
alterations is movement; indeed, the factor of movement is so important
that it has frequently been made a basis for a classification of metamorphism.
Changes of rocks take place with comparative slowness under conditions
of quiescence and take place with comparative rapidity under conditions of
DEPTH THE MOST IMPORTANT GEOLOGICAL FACTOR. 43
movement. Furthermore, the alterations which occur under dynamic con-
ditions are far more profound than those which take place under static
conditions. For instance, very ancient sedimentary rocks which have been
undisturbed by orogenic movements may be in almost the original condition
in which they were deposited. On the other hand, rocks of comparatively
recent age which have been in mountain-making areas and been deeply
buried may be profoundly modified. Little metamorphosed rocks of great
age are illustrated by the St. Peter sandstone of Wisconsin and the uncon-
solidated Cambrian sands of Russia. Profoundly metamorphosed rocks
of comparatively recent age are illustrated by the Eocene and Neocene
rocks of the Coast Range of California and the Eocene of the Alps.
Depth.—Rocks at or near the surface of the earth are ordinarily under
conditions of slight pressure and low temperature. Rocks at some depth
below the surface are under conditions of considerable pressure and
temperature. It will be shown that the alterations of a given rock under
these varying conditions are very different. Therefore depth is a matter of
great consequence in the consideration of metamorphism. Indeed, depth
is believed to be the most important of the influences which determine the
character of the alterations of rocks. Therefore the geological factor
which in this treatise will serve as the primary basis for a classiffcation of
metamorphism is the dominant factor of depth. On this basis metamor-
phism will be classified into (1) alterations in the zone of katamorphism
and (2) alterations in the zone of anamorphism. The zone of katamor-
phism is subdivided into (a) the belt of weathering and (b) the belt of
cementation. The zone of katamorphism may be defined as the zone in
which the alterations of rocks result in the production of simple com-
pounds from more complex ones. The zone of anamorphism may be
defined as the zone in which the alterations of rocks result in the pro-
duction of complex compounds from more simple ones. The belt of
weathering is the belt which extends from the surface to the level
of ground water. The belt of cementation is the belt which extends
from ground-water level to the zone of anamorphism.
It is to be noted not only that this classification is geological, but that
the factor is one which is universally applicable. Geological factors of
different kinds, such as movement, contact action, ete., are not introduced.
It is therefore clear that the proposed classification follows one law of all
44 A TREATISE ON METAMORPHISM.
good classifications, viz, that a factor or factors of the same class shall be
used throughout as a primary basis. While the primary classification of
metamorphism will be based upon depth, it is recognized that there are no
sharp dividing lines between the zones and belts. In metamorphism, as in
every other branch of geology and of science, there is complete gradation
between the phenomena of the various classes.
However, it has been seen that depth is not the only geological factor
of consequence in metamorphism. It is recognized that various other
geological factors enter into the alteration of a given rock. Moreover, these
various factors overlap. In the discussion of the zones of metamorphism.
the geological factors of subordinate importance will be given proper
consideration.
Before considering the general alterations in the zones of katamor-
phism and anamorphism, and the alterations of the mdividual minerals
and rocks in these zones, it is necessary to consider the forces and the
agents of metamorphism from chemical and physical points of view.
It should therefore be recalled that the forces of metamorphism are
chemical energy, gravity, and heat and light, and that the agents of
metamorphism are gases, liquids, and organic compounds. The rocks
are the materials upon which these forces and agents work. ‘The forces
of metamorphism are considered in Chapter I, the agents of metamor-
phism in Chapter TIT, and the work of these forces and agents upon the
rocks in the later chapters.
Glo We i Ike
THE FORCES OF METAMORPHISM.
As already seen, the important forces of metamorphism are chemical
energy, gravity, and heat and light.
CHEMICAL ENERGY.
When different compounds are brought together molecular interchange
may occur between them. As a result the compositions of the compounds
are mutually changed. Such interchange is chemical action. Chemical
action usually involves expenditure of chemical energy, which is one of the
main original sources of energy; but it will be seen that other forms of
energy may be transformed into chemical energy, and chemical action in
this way be promoted.
Chemical action may take place between gas and gas, gas and liquid, gas
and solid, liquid and liquid, liquid and solid, and solid and solid. Chemical
action, or molecular interchange, involves movement between the atoms and
molecules. Chemical action therefore never takes place without dynamic
action. So far as we know chemical action never takes place without the
presence of heat. Under the conditions obtaining in the crust of the earth
chemical action is usually promoted by heat and by mechanical action, As
chemical action always produces a heat effect, positive or negative, such
action may result in the liberation or in the absorption of heat. The heat
effect may hasten or retard further chemical action. In so far as chemical
action results in the liberation of heat, it usually hastens further chemical
action, and therefore promotes metamorphism; in so far as chemical action
results in the absorption of heat, it usually retards further chemical action,
and therefore stays metamorphism. It is shown (pp. 170-186) that both
classes of reactions take place on a very extensive scale.
In consequence of chemical action material may be added to or sub-
tracted from a given mineral. A mineral may alter into two or more other
minerals with the simultaneous addition or subtraction of material. Two
or more minerals may unite to produce a single mineral. Either of these
45
46 A TREATISE ON METAMORPHISM.
changes may take place without addition of material, or added material
may be derived from some other particle or particles near or remote.
Material subtracted from any given mineral particle may be added to
another mineral particle at a greater or less distance. Illustrating the above
are the alterations of feldspar into muscovite and quartz, and of olivine
into serpentine, magnesite, magnetite, and quartz. Chemical action is in
most cases accomplished through solutions. Therefore its detailed discus-
sion is considered in connection with the agents of metamorphism, gaseous
solutions, and aqueous solutions. (See Chapter ITZ.)
GRAVITY.
Gravity is now the great dominating force of the universe. Indeed,
it is a main original source of energy. Certainly it is the source of energy
which has largely controlled the development of the solar system, including
the sun and all the planets and satellites. The transformations of gravity
into chemical energy, heat, light, and other forms of energy are important
factors in the development of the solar system, including the earth. More-
over, gravity still remains as the great dominating force which controls
earth movements,” both vertical and horizontal, and also the circulation of
the water, both overground and underground. By earth movements are
meant all movements of the solids or rocks of the earth not in solution. In
this broad sense the movement of glaciers is an earth movement.
The direct work of gravity in metamorphism may be considered under
two headings—mechanical action and water action.
MECHANICAL ACTION.
Rocks may be stressed within the elastic limit, or the stress may
extend beyond the resisting power of the material. In either case the
rocks are strained. Strain may occur with or without chemical action.
Strain is always accompanied by some transfer of energy into heat.
When the rocks are strained the molecules are moved with reference to
one another. If the strain be within the elastic limit and chemical change
does not take place, the molecules are only slightly farther apart or closer
together, and when the stress is removed they may return to their original
«Van Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11,
1898, pp. 512-514.
MECHANICAL ACTION. AT
positions, or nearly so. If under the stress chemical interchange also takes
place between the molecules, when the stress is removed the body may still
return to nearly its original form. But if the strain extends beyond the
elastic limit the form of the body is notably changed, as when a piece of
wrought iron or steel is drawn out or when a piece of cast iron is crushed.
Mechanical action may therefore be considered as molecular or mass.
By molecular mechanical action is meant differential movements of the
molecules. By mass mechanical action is meant differential movements of
large masses of the rocks. Molecular movement also frequently involves
differential movements of the atoms. Metamorphism by molecular move-
ment has generally been called static metamorphism. But molecular
mechanical action is always accompanied in some degree by mass
mechanical action, though this process may be subordinate. The term
‘dynamic metamorphism” has usually been restricted to alterations in con-
nection with mass deformation. But mass mechanical action is always
accompanied by molecular mechanical action as an important and essential
concomitant, although this iivariable relation has not always been recog-
nized. Further, as mass movement becomes important molecular moye-
ment, instead of becoming less important, is likely to be of even greater
consequence. There is therefore gradation between molecular mechanical
action and mass dynamic action.
MOLECULAR MECHANICAL ACTION.
Molecular mechanical action involves various degrees of movements.
Presumably the lesser movements are the cases of change in crystalline
form and of strain within the elastic limit. In the change of a substance
from one crystalline form to another—as, for instance, of aragonite to cal-
cite—the movement of the molecules may not involve more than a rear-
rangement of those which are adjacent. In the case of substances strained
within the elastic limit, the molecules are simply pressed slightly closer
together or pulled slightly farther apart, and yet these very slight adjust-
ments may have a profound effect upon the physical properties of the
materials. For instance, amorphous glass when strained but slightly and
well within its elastic limit becomes an anisotropic substance. Leucite
crystallizes in the isometric system at high temperatures. As the mineral
cools it passes at once into an anisotropic form. The transformation from
48 A TREATISE ON METAMORPHISM.
one to the other may be seen by alternately heating and cooling this
mineral under the microscope. In the foregoing cases, while we can not
doubt that movement occurs, the readjustment is molecular, and it is there-
fore beyond the power of the microscope to determine its character.
It might at first be supposed that such shght movements as are involved
in strains within the elastic limit are unimportant, but it is to be remembered
that strains of this kind not only affect every mineral particle, but displace
the individual molecules with reference to one another, so that the strained
masses are affected throughout. While, therefore, it requires polarized
light to detect the strained condition in minerals, it is certain that the effect
is pervasive. It will be seen (pp. 95-98) that such state of strain is of
fundamental importance in the matter of solution and deposition through
the agency of solutions.
In a second class of movements there is molecular interchange between
substances by which the compounds are modified in composition. Such
interchanges involve chemical action. The motions which occur during
chemical changes in solids are commonly for such short distances that the
naked eye does not discover the relations of the original and secondary
minerals. Such movements are microscopic. Chemical interchange may
be mainly accomplished by chemical forces and the movement be an
incident of this process. On the other hand, mechanical action may be the
inciting cause which leads to chemical action. And, finally, the purely
chemical and mechanical forces may interact, each promoting the other.
The more important chemical reactions resulting from mechanical action
are accomplished through the agency of solutions, and hence are treated
in Chapter III. But Prof. Walther Spring* has shown that chemical
changes may be induced by mechanical action alone, without the presence
of solutions. For instance, when barium carbonate and solid sodium
sulphate were mixed in equal molecular proportions and subjected to a
pressure of 6,000 atmospheres a change took place by which 80 per cent
of the barium carbonate and sodium sulphate were changed to barium
sulphate and sodium carbonate, respectively; and conversely, when barium
sulphate and sodium carbonate were mixed together in equal molecular
proportions and subjected to a like pressure about 20 per cent was changed
«Professor Spring on the physics and chemistry of solids, review by C. F. Tolman, jr.: Jour.
Geol., vol. 6, 1898, p. 323.
MASS MECHANICAL ACTION. 49
to barium carbonate and sodium sulphate.“ In all such changes the
fundamental principle controlling is that reactions shall ‘take place which
result i in smaller volumes. Spring * found that in the case of dry reactions
imilneadl by mechanical action time is a very important factor, the reactions
taking place much more slowly than when compounds are moist and water
is an intermediate agent.
MASS MECHANICAL ACTION.
Mass mechanical action (a) may permanently strain the rocks without
openings, (b) may strain the rocks with rupture and openings, and (¢) may
close the openings in rocks and produce welding.
Permanent strain without openings. —]n order that permanent strain beyond
the elastic limit without openings may take place in the rocks it is nec-
essary that deformation shall occur while the rocks are under a sufficient
‘pressure in all directions to hold the molecules so close together that the
molecular attraction is effective. This will be true only where the pressure
is greater in all directions than the crushing strength of the rocks. It is
well illustrated by Adams and Nicolson’s experiment on the deformation
of marble while under pressure in all directions... The molecules were
held close to one another, and the deformed marble retained considerable
strength.
Later we shall see that the process of readjustment may be mechanical
or chemical or partly each. When the process is mechanical the mineral
granulated—that is, finely fractured. When the
particles are usually g
process is chemical the particles are recrystallized. Also the process of
readjustment may be accomplished by any combination of granulation and
recrystallization (See pp. 737-748.) Under natural conditions, in order
that the pressure in all directions shall be greater than the crushing strength
of a rock, it is necessary that it be in the zone of flowage for that rock.
Permanent strain with openings.— When the rocks are strained beyond the elastic
limit and the pressure is not greater in all directions than the crushing
strength of the rocks, rupture and openings are produced. The ruptures
may be regular or irregular. The regular ruptures may be of great extent
«Nernst, W., Theoretical chemistry, transiated by C. 8S. Palmer, Macmillan & Co., London, 1895,
p. 390.
o Spring, op. cit., p. 322.
¢ Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble:
Philos. Trans. Royal Soc. London, ser. a, vol. 195, 1901, pp. 363-401.
MON XLV1II—04——4
50 A TREATISE ON METAMORPHISM.
and wide apart, as in the case of faults; or of moderate extent and width,
as in the case of joints and bedding partings; or close together, as in the
case of fissility. The irregular ruptures may be continuous and the open-
ings wide, as in the case of the coarse breccias; or discontinuous and the
openings small; or so minute as to affect the individual particles, and thus
grade into deformations without openings or granulation.
Under natural conditions, in order that the pressure shall not exceed
the crushing strength of a rock in all directions, it is necessary that it shall
be in the zone of fracture for that rock.
Permanent strain with closing of openings and welding.— Mechanical action may close
openings in rocks and weld the separated parts. In this case there is a
diminution of volume due to bringing the particles closer to one another.
In order that welding shall take place there must be sufficient pressure i
all directions to bring the particles so close together that the molecular
attractions are effective, or the pressure in all directions must be greater
than the crushing strength of the rock.
Of course, the pressure required to satisfy the above conditions tor
welding depends very greatly upon the character of the material. Moder-
ate pressure may be sufficient to weld material composed of small and
weak particles. For instance, moderate pressure of clay may bring many
of the minute particles of kaolin so close to one another as to place them
within the limits of effective molecular attraction. When the clay is dried
the mass becomes harder. This hardening is doubtless due in part to the
precipitation of the dissolved material contained by the water and the conse-
quent cementation of the particles, as explained on pages 617-621. In pro-
portion as the particles are coarse, strong,
few points of contact, the pressure necessary to produce welding increases.
and large, and have relatively
To produce deformation with welding of the separated large particles of
the strong minerals considerable pressure is necessary.
WATER ACTION.
The movement of water under the force of gravity is of the utmost
importance in metamorphism. It is, indeed, the great agent of transporta-
tion of material both overground and underground, and is the dominating
agent through which metamorphism is accomplished. Its work is fully
considered in Chapter III, on ‘The agents of metamorphism.”
FORCES OF METAMORPHISM. 51
HEAT AND LIGHT.
Heat and light are forms of energy of the first importance. It has
already been noted that their ultimate source is largely gravity. Heat
is always present as a factor in metamorphism, for nowhere upon the surface
of the earth nor within the earth is the temperature absolute zero. Other
things being equal, the higher the temperature the more rapidly do alterations
of rocks take place. Light also affects all parts of the earth at the surface.
In metamorphism heat and light should be considered from two points of
view—(1) sources of heat and light, and (2) effect of heat and light upon
the alterations of rocks.
SOURCES OF HEAT AND LIGHT.
Heat and light agential in the alteration of rocks are derived (a) from the
sun, (b) from deep within the earth by conduction or by convection through
water or magma, (c) from mechanical action, and (d) from chemical action.
The heat from all these sources is important; light, however, is derived
chiefly from the sun, that from the other three sources being of little
consequence.
THE SUN AS A SOURCE OF HEAT AND LIGHT.
The heat and light of the sun are forces of the first order of magnitude
in the alterations of rocks. The effect of these forces needs to be considered
in four cycles—the cycle of the solar system, that of the seasons, that of
the cyclone, and that of the day.
The solar-system cycle is the most important. This cycle involves
two factors—the absolute temperature and change in temperature.
As to the absolute temperature, were it not for the heat and light of
the sun it is certain that the temperature of the surface of the earth would
not greatly exceed that of the interstellar spaces. Probably it would be
—200° C., or even lower. At the present time the temperature of the
surface of the earth averages 10° C. (283° C. absolute) or more. Therefore
the temperature of all the upper zone of the earth is 200° C., or more,
greater than it would be without the heat from the sun. Were it not for
this heat the water in the outer zone of the earth would be congealed, and
the atomic and molecular energy would be greatly diminished. As a
comparatively slight increase of temperature over that prevalent at the
surface of the earth increases greatly the speed of alteration of rocks, it is
52 A TREATISE ON METAMORPHISM.
to be presumed that under such low temperatures changes in rocks would
be so slow as to be negligible.
How deep below the surface of the earth the heat of the sun produces
an effect can not be accurately determined. It is highly probable that it
has an important effect to a depth of thousands of meters, probably beyond
the limits of the zone of observation. If the sun were not furnishing heat
to the earth, and the increment of increase in temperature were the same
as at present (1° C. for 80 meters), and the temperature at the surface were
200° C. lower it would be necessary to penetrate to a depth of 6,090 meters
to reach a temperature as high as that at the surface under the present
conditions. Below 6,000 meters the temperature would increase approxi-
mately as it does now from the surface downward. However, it is not to
be supposed that the effects of metamorphism would be the same as those
in the outer 6,000 meters at the present time, for the conditions of pressure
would be very different. (See Chapter I, p. 43, and Chapter IV, pp. 159-
160.) The assumption that the increment of temperature would remain
the same were not the sun giving heat to the earth is only approximately
true; but when it is remembered that 6,000 meters is an exceedingly small
fraction of the earth’s radius, it seems probable that the increment of
increase of heat with depth in the outer part of the crust of the earth
would not be greatly different, even if the sun had long ceased to be a
source of heat; but if it were not for the heat of the sun, the temperature
of that part of the lithosphere directly under observation would be so low
that all chemical changes would be very slow, if indeed they were not
inappreciable.
The absolute temperature at the surface is also dependent upon
latitude. The average temperature at the warmest tropical regions is about
300° ©. absolute, or, stated in the ordinary scale, 27° C.; the average
temperature of the coldest polar region where observations have been made
(latitude 81° 44’) is 252.9° absolute, or, in the ordinary scale, —20.1° C.*
At intermediate latitudes there are all gradations between these extremes.
At any place the temperature may be presumed to increase with depth from
these surface temperatures at the rate of 1° C. per 30 meters.
It would be fruitless to attempt a discussion of the changes of the
temperature of the outer part of the earth due to the solar cycle. So far
«Hann, Julius, Handbuch der Klimatologie, J. Engelhorn, Stuttgart, 1883, p. 733.
CHANGES IN TEMPERATURE. 53
as rock alterations now taking place are concerned, the sun may be regarded
as furnishing to the earth a uniform amount of heat.
The seasonal changes of temperature are very important, at the surface
ranging from 30° C. or less to as much as 80° C. However, the depth to
which the seasonal change produces an effect is not great, probably about
15 meters.
The cyclonic changes of temperature may be very great, ranging from
a fey degrees to about 70°, but the depth to which these changes extend is
slight, probably less than 3 meters.
The diurnal changes in temperature are scarcely less than the cyclonic,
ranging from 0 to 50° C. or more; but the depth to which the diurnal
changes extend is insignificant, probably but a fraction of a meter.
From the foregoing it is plain that the heat and light derived from the
sun are of very great direct importance in the chemical and mechanical
changes that rocks undergo. It will also be seen that the various changes
of temperature as well as the absolute temperatures are of great consequence.
Moreover, the heat and light of the sun exert a very important indirect
influence upon metamorphism by reason of their being the sole source
of the energy which produces plants and animals, and these agents will be
seen to have a far-reaching effect upon the alterations of rocks. The effects
of the heat and light derived from the sun are fully considered in Chapters
VI, VU, and VIII, on “The belt of weathering,” ‘The belt of cementation,”
and “The zone of anamorphism.”
HEAT DERIVED FROM WITHIN THE EARTH BY CONDUCTION OR CONVECTION THROUGH
WATER OR MAGMA.
The amount of heat derived by the crust of the earth from the interior
depends upon the conductivity of the various rocks and upon the convee-
tional movements of magma and water.
The heat conductivity of the majority of rocks is between 0.4 and 0.6,
silver having a conductivity of 100. It is apparent that the conductivity
of rocks is very low as compared with that of the metals, but it can not be
doubted that there is a steady but slow flow of heat by conduction from the
interior of the earth to the zone of observation.
The amount of heat derived by the crust of the earth from intrusions
of igneous rocks is very great. So far as this heat passes into the adjacent
54 A TREATISE ON METAMORPHISM.
rocks by conduction, the coefficients are the same as in the transfer of heat
from the interior of the earth. The transfer of the heat of magma to adjacent
rocks is probably largely accomplished by convection. The magmas fre-
quently furnish heated solutions. Ordinary circulating waters approach or
come in contact with the igneous rocks; they thus become heated. The
heated waters move through the rocks controlled by the laws of under-
ground circulating waters (see pp. 146-153), and give up a part of their heat
to the surrounding rocks. The important metamorphosing effects of the
great igneous masses through water convection may extend several miles.
Contact metamorphism is sometimes restricted to the very marked effects
due to high temperature immediately adjacent to the igneous rock. How-
ever, the alterations thus produced by high temperature as the result of
direct conduction are probably small, compared with the widespread effects
resulting from the dispersal of heat and material by means of underground
waters.
MECHANICAL ACTION AS A SOURCE OF HEAT.
It is a well-known principle that when work is done involving strain
of solids within the elastic limit, or subdivision of solids, or differential
movement between solids in contact, the energy is partly transformed to
heat. Hence strain within the elastic limit, subdivision of the rocks, and
differential movement between rock masses and particles and within the
particles raise the temperature of the rocks, and this greatly increases the
speed and extent of the chemical reactions. Heat developed by mechanical
action is therefore an important factor in the metamorphism of rocks.
Indeed, the resultant metamorphic products are very different under con-
ditions of movement and under conditions of quiescence; but heat is only
one of the factors entering into the differences. (See pp. 685-707.)
CHEMICAL ACTION AS A SOURCE OF HEAT.
Chemical action always produces a positive or negative heat effect,
and thus promotes or retards metamorphism.
EFFECTS OF HEAT AND LIGHT ON ALTERATIONS OF ROCKS.
The relations between metamorphism and heat and light may be gener-
ally stated as follows: The kinetic energy of the molecules of substances,
whether in the form of gas, liquid, or solid, is increased by heat and light.
EFFECTS OF HEAT AND LIGHT. YS)
The speed of metamorphism is therefore largely dependent upon the amount
of heat and light present, especially the former.
In rock alteration heat and light produce direct effects and indirect
effects.
DIRECT EFFECTS OF HEAT AND LIGHT.
The more important direct effects may be either mechanical or
chemical.
Mechanical effects —he mechanical effects are desiccation, baking, and
fusion. At the surface of the earth the heat of the sun frequently results
in evaporating the moisture and desiccating the rocks; an attendant result
is induration. This process is especially important in the clay sediments,
and occurs to the greatest extent in the hot and arid regions, although
desiccation is not unimportant in the colder regions. The details of the
process especially concern the belt of weathering and are treated in the
chapter on that subject. (See Chapter VI, pp. 541-550.) Where igneous
rocks as a consequence of volcanism are brought into contact with other
rocks the latter may be baked for a longer or shorter distance from the
igneous rocks. The process of baking as here used is restricted to modifi-
cations similar to those which take place in the baking of bricks; that is, to
effects which are mainly due directly to the heat. This process is restricted
to the belt above the level of underground water—the belt of weathering—
and is therefore treated in detail in the chapter on that subject. (See
Chapter VI, pp. 488-494.) Below the belt of weathering the rocks are
saturated with water and the heat effects are mainly produced through that
agent. Even in the belt of weathering the baking effect is not wholly due
to heat, but is partly accomplished through the agency of the contained
water, precisely as is the transformation of clay to brick by burning; for all
rocks under natural conditions contain gas and water, and usually consid-
erable quantities. During the baking process the original molecules are
brought nearer together, but there are also important chemical changes.
Where the masses of the igneous rocks are very great, and especially
where adjacent rocks are included in masses of igneous rocks, the rocks
may be softened by the heat or even absorbed by the magma. Where
the rocks are softened they are likely to be very greatly changed, perhaps
recrystallized. Where they are absorbed by the magma they are lost as
original rocks and become a part of the magma by which they are absorbed.
56 A TREATISE ON METAMORPHISM.
When the modified magma crystallizes it takes the form of an ordinary
igneous rock, and may show no evidence of the fact that previously
solidified rocks have contributed material.
Chemical effects. —In proportion as the temperature is high chemical reactions
are likely to take place between solids. This is illustrated by the case-
hardening of iron. When soft iron is placed in contact with pulverized
charcoal and the temperature is raised to a red heat, but not to the point
of fusion, some of the carbon unites with the iron, transforming the outer
part of it into steel. Thus it is casehardened. Just how the union takes
place between the iron and the carbon is uncertain. It is supposed to be due to
the direct union of the solids, but we can not be quite sure that the result
is not accomplished through the agency of a gas. The carbon may be partly
oxidized, and thus be transformed to the gas carbon monoxide. ‘This may
penetrate the iron, which may reduce the carbon monoxide to carbon again.
The reduced carbon may at the instant of reduction unite with the iron,
forming the carbide, or steel. While it is certain that high temperature is
favorable to the mutual chemical reactions of solids, when the temperature
becomes so high as to transform the solids to liquids the chemical reactions
are those of liquids rather than those of solids.
INDIRECT EFFECTS OF HEAT AND LIGHT.
The indirect effects of heat and light are accomplished through the
agents of metamorphism—gases, water, and organic forms. The move-
ments of the atmosphere and hydrosphere are the conjoint effect of heat
and light and gravity. It has already been noted that the movements of
these bodies are the agents which do the main work of epigene transfer
of material. Not only do gas and water act as agents of transfer, but they
act as agents for chemical changes. It has already been seen that chem-
ical action may be a direct result of heat. However this may be, it is
certain that by far the more important, indeed the dominant, effects which
heat and light have upon chemical reactions are accomplished through the
agency of gases and water and organic forms. Of the forces heat and
light, the former is the important one in the reactions accomplished through
the agency of gases and water solutions; but light is very important in the
production of organic agencies. The indirect effects of heat and light and
all other conjoint forces are considered in connection with the agents of
alteration in Chapter III.
EFFECTS OF HEAT AND LIGHT. 57
GENERAL STATEMENTS.
From the foregoing it is apparent that the effects of chemical energy,
gravity, and heat and light are not independent of one another; on the
contrary, they are most intricately interlocked. To a considerable degree
any one of the forms of energy may be transformed into the others. Con-
sequently the action of one almost always produces an effect upon the
action of the others. Moreover, one almost never. acts without the action
of the others. Frequently all of the forces of metamorphism are important
simultaneous factors in the results; again, one or two of the forces may be
prominent, or even dominant, the others playing a subordinate part. But,
in every transformation of metamorphism, if all the energy factors of the
entire system affected be taken into account, some of the energy is changed
into the lowest form of energy, heat, and at least a portion of this heat is
dissipated.*
a Daniell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York,
1895, p. 51.
Cite 18 i IMI,
THE AGENTS OF METAMORPHISM.
GENERAL STATEMENT.
The agents through which the alterations of rocks take place are
gaseous and liquid solutions and organisms. Solutions are the special
subject of this chapter. Organisms are influential only in the belt ot
weathering, and their action is therefore considered in connection with that
belt. (See Chapter VL.)
The circulation and work of solutions involve a consideration of the
circulation and work of the gases of the earth, of which the atmosphere is
the dominant portion, and a consideration of the circulation and work of the
water of the earth, of which the ocean is the dominant portion. While the
circulation and work of the atmosphere and of overground water may from
a purely theoretical point of view be considered as a part of a treatise on
metamorphism, the work of these epigene agents is the subject of that
division of geology which has been named physiography, and as the work
of the atmosphere and overground water is so fully dealt with in connection
with that subject, this branch of metamorphism will not be discussed here
at all. But the circulation and work of underground gas and water solu-
tions are of fundamental importance in metamorphism and must be some-
what fully considered.
Gas and water below the surface in the openings of the rocks will be
called ground gas and ground water, to discriminate them from gas and
water above the lithosphere.
Solutions “are homogeneous mixtures which can not be separated into
a
their constituent parts by mechanical means.”" ‘The properties of solutions
vary continuously and regularly with the concentration.” Under the
aOstwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., London,
189i), pp. L-
>Cameron, F. K., Application of the theory of solutions to the study of soils: Rept. No. 64, Field
Operations of the Division of Soils, 1899, U. 8. Dept. of Agric., 1900, pp. 142-143.
58
CHARACTER OF THE SOLUTIONS. og
definition, solutions may be made by mingling gases and gases, gases and
liquids, gases and solids, liquids and liquids, liquids and solids, and solids
and solids. he solutions resulting from these various combinations may
be gases, liquids, or solids, or partly two or all. Gaseous solutions may be
formed by the mingling of gases and gases, of gases and liquids, and
of gases and solids.“ Liquid solutions may be formed by the mingling of
gases and gases, of gases and liquids, of liquids and liquids, of solids and
liquids, and of gases, liquids, and solids. Solid solutions may be formed
by the mingling of gases and solids, of liquids and solids, of solids and
solids, and of gases, liquids, and solids. But however complex the origin
and however numerous the components, the compounds with which the
geologist has to deal are gases, liquids, and solids. The two common
combinations which he has to consider are gaseous solutions and solids,
and liquid solutions and solids. The liquid solutions are universally
aqueous. The solids are the rocks. The combinations gaseous solutions
and solids, and aqueous solutions and solids will be treated under Parts I and
II of this chapter.
PART I. GASEOUS SOLUTIONS.
Since the geological work of gases and vapors can not be practically
discriminated, the term gas is here used to cover both gases and vapors.
The gases which are important in rock alteration are oxygen (O,),
sulphur (S, to 8,), water gas (H,O), ammonia (NH;), carbon dioxide (CO.),
sulphurous oxide (SO,), boric acid (H;BO,), hydrochloric acid (HCI), and
hydrofluoric acid (HF).
Never is one of these chemical compounds at work alone upon the
rocks; at the place of action there are always solutions of several gases.
Mineralizers in rocks, according to the original definition, are substances
which act in the gaseous condition;’ but it will be seen (pp. 490-494)
that the term has been practically restricted to peculiar gases under special
circumstances. Notwithstanding the definition of the term, the action of
water solutions containing certain compounds, which if alone would be
gaseous, has been spoken of as due to mineralizers. The term miner-
alizers, if it is to serve any useful purpose, should be definitely restricted
«Daniell, Alfred, A text-book of the principles of physics. 3d ed., Macmillan Co., New York,
1895, p. 330.
> Expression ‘‘ Agents minéralisateurs” first used by Elie de Beaumont and defined by H. Ste:-
Claire Deville: Comptes rendus des Séances de l’ Académie des Sciences, vol. 52, 1861, pp. 920, 1264.
60 A TREATISE ON METAMORPHISM.
to the action of some particular compound or compounds, or else to some
form of compound, such as gases.
Gaseous solutions require consideration from two points of view—the
chemical and physical principles controlling the action of gases and the
geological work of gases.
SECTION 1. CHEMICAL AND PHYSICAL PRINCIPLES CONTROLLING THE ACTION
OF GASES.
The chemical and physical principles controlling the work of gases
may be considered under (1) the gases present, (2) the pressure, and (3)
the temperature.
Gases present.—The law of greatest importance controlling the chemical
action of gaseous solutions is: The properties of a homogeneous mixture or
solution of various gases are the sum of the properties of the constituents of
the mixture. To illustrate, when carbon dioxide (CO,) and oxygen (O,) are
mixed the properties and activities of each are the same as if the same
quantity of each were free from the other and occupied the same space.
Therefore, in the belt of weathering, where gases are active, the carbon
dioxide and oxygen are both doing their work, the one that of carbonation
(see pp. 473-480), the other that of oxidation (see pp. 461-473), as if the
other were not present. It is clear, therefore, that the properties of gaseous
mixtures are additive.
Slight deviations from this law have been noted under certain condi-
tions, but these mainly concern the exact physics of the gaseous solutions
rather than their geological work, and hence are not here considered. In
applying the law, however, we must be sure that the gases do not unite
chemically and produce a new compound. To illustrate, while the law is
certainly applicable to the case mentioned, that of a mixture of carbon
dioxide and oxygen, it is not certain that this is the case when water gas
(H,O) and carbon dioxide cr sulphurous oxide (SO,) are mixed, for these
compounds may unite with water gas, producing carbonic acid (H,CO,)
and sulphurous acid (1,503) gases. Certainly the law will not apply to a
mixture of the gases ammonia (NH) and water, for these gases will largely
unite and produce ammonium hydroxide (NH,OH), which may exist in the
form of gas. In case a gas be formed by the union of two or more gases,
PRESSURE AND TEMPERATURE OF THE GASES. 61
the law controlling the action of gases is applicable to the new compound,
and to the other gases with which it is mingled but does not unite
chemically.
As to the relative importance of the gases, it might at first be thought
that the strong acids, such as hydrochloric and hydrofluoric, are of greater
consequence than the much less active compounds, carbon dioxide and
oxygen; but it should be remembered that carbon dioxide and oxygen are
everywhere at work upon the surface of the earth, whereas the presence
of the strongly active compounds in more than minute quantities is excep-
tional. It therefore follows that the action of the universally present
weaker agents, such as carbon dioxide and oxygen, is of immeasurably
greater geological importance than the action of the stronger but much less
abundant gases.
The pressure—Increase of pressure increases the chemical activity of a gas.
This law follows from the fact that the number of molecules which act upon
a given space is directly as the pressure. The varying atmospheric pressure
may be taken as illustrating this principle. When the pressure increases,
say, by .05, this means that 1.05 times as many molecules of gas are actively
at work upon a given area as before.
One of the best illustrations of the increased activity of gases in accom-
plishing chemical work when under pressure is that of carbon dioxide. As
shown in another place (pp. 175-176), carbon dioxide is capable of decom-
posing many silicates at ordinary temperatures; but Struve and Mueller*
have shown that when carbon dioxide is under pressure its effect in decom-
posing silicates is very much greater than under ordinary conditions. This
is in accordance with the law of mass action. In proportion as the pressure
increases the number of active molecules increases, and therefore the
geological work increases in proportion.
The temperature— he activity of gases increases with increase of temper-
ature. In proportion as the temperature is high, the kinetic molar energy
of the molecules of gases is great. The absolute temperature of a perfect
gas is believed to be a direct measure of its kinetic molar energy. By
molar kinetic energy is meant the energy of translation of the molecule of
a gas, and not the vibratory or rotary motions of the molecules themselves.
«Mueller, Richard, Untersuchungen tiber die Einwirkung des kohlensiurehaltigen Wassers auf
einige Mineralien und Gesteine: Tschermaks mineral. Mittheil., vol. 7, 1877, p. 47.
62 A TREATISE ON METAMORPHISM.
The kinetic energy of a moving body is the product of one-half of its mass
into the square of its velocity. When a gas is very dense its molecules
are closely crowded, and on account of the molecular attraction there is
an appreciable decrease in the theoretical pressure, which is a measure of
the kinetic molar energy. Since the kinetic energy of the gaseous molec-
ular projectiles increases as the squares of the velocities, this may explain
why a slight increase of temperature often greatly increases the chemical
reactions of the gases in contact with the solids of the earth’s crust, for
the likelihood of a chemical union depends, among other things, upon the
energy with which the particles of a gas come in contact with the minerals
of the rocks.
SECTION 2. GEOLOGICAL WORK OF GASES.
The observable geological work of gases is mainly above the level of
ground water, or in the belt of weathering. In the belt of cementation,
below the level of underground water, the rocks are saturated with water
solutions. Gaseous substances, if present, would be in solution in water,
and their action would therefore fall under water solutions, treated on later
pages.
In the belt of weathering oxygen and carbon dioxide are immeasura-
bly the most important of the mineralizers, because they are present in the
interstices of the rocks in this belt throughout the land areas. However,
in volcanic districts any or all of the geologically important gases may be
present and have a very marked metamorphosing effect upon the rocks.
But of these gases that of water is of vastly the greatest consequence.
The consideration in detail of the effects of these various mineralizers and
of their action in conjunction with other agents properly falls in Chapter VI
on “The belt of weathering.”
In the deep-seated zone of anamorphism water itself is mainly above
its critical temperature (see pp. 659-661), and is therefore in the form of a
gas. On account of the great pressure the gases are dense. Under these
conditions most or all of the substances held in solution would also be in
the form of gases. The active substances would be solutions of gases in
gases. One would expect that the action of water gas holding in solution
other gases under such conditions of pressure and temperature would be
different from the action of highly heated water, in that its viscosity would
ACTION OF THE GASES. 63
be very small. It would therefore have a greater penetrating power than
water, and would be more highly energetic in its action. Under these
conditions the minutest spaces would be somewhat readily traversed. The
rocks of the deep zone in which action of this kind has taken place can
reach the surface only by passing through the zone in which water is in the
liquid form. Therefore the effects which were produced by the mineralizers
in the deepest zone will have been modified by the action of water solu-
tions during the long time the rocks were in the belt of cementation. The
details of the effect of water gases in the zone of rock flowage will be con-
sidered in Chapter VIII, on ‘‘The zone of anamorphism.”
All of the gases may act in either of two ways: (1) By their presence
they may influence crystallization or recrystallization without entering into
combination. (2) They may enter into the combinations forming oxides,
hydroxides, carbonates, sulphates, etc. The first of these actions is spoken of
as that of crystallizers, and the second as that of mineralizers. In the meta-
morphic rocks it is ordinarily difficult to prove the past action of gases, not
in water solutions. Occasionally the materials of volcanic cones have been
rendered porous and the rocks altered in consequence of the action of
gaseous exhalations. In such cases the gases usually have united to some
extent with the materials through which they have passed, and in this way
furnish evidence of their past action.
PART IL. AQUEOUS SOLUTIONS AND SOLIDS.
GENERAL CONSIDERATIONS.
The one liquid through which the greater part of the alterations of
rocks occur is water solution. Indeed, this is so profoundly true that the
water of the earth has been compared with the blood of an organism. And it
is certainly true that the transformations of tissues by the blood are scarcely
more far-reaching than those of the lithosphere by the agency of water. It
has been determined by laboratory experiments that pure water at ordinary
temperatures is capable of dissolving all compounds to some extent. Cor-
responding with this fact, analyses of ground waters show that they contain
in solution all of the elements which occur in nature. The solutions may
yary from very dilute to rather strong. So far as the gases are dissolved in
water, their action is to be treated under water solutions, not under gases.
64 A TREATISE ON METAMORPHISM.
In the belt of weathering, above the free surface of ground water,
gaseous solutions and liquid solutions work together. In this belt the rocks
are not ordinarily saturated with water, but on the average contain a con-
siderable amount of water held by adhesion between the liquid and the solid
mineral particles. It is believed that in this belt the gases act upon the
rocks chiefly through water solutions. As evidence of this is the small
amount of decomposition of the disintegrated rocks in arid regions. (See
pp. 496-498.) It therefore appears that the dominant agents of alterations
in the belt of weathering are aqueous solutions.
In the belt of cementation below the free surface of ground water the
rocks are practically saturated, and in this belt aqueous solutions are the
chief agents of alterations.
Water solutions are also a chief agent in the transportation of material
from one place to another.
At this point it is necessary to understand that the places of interaction
of aqueous solutions and solids are the contacts between the two. It will
be seen later that, on account of the molecular attraction between water and
rock, a thin film of water adheres to the solid particles with which it is in
contact. This film is not in active circulation, yet it is the part of the
agent, water, which is immediately concerned in the transfer of mineral
material from the rocks to the solutions and from the solutions to the rocks.
The contact film may take material of the rock into solution. From
this film the materials taken into solution migrate to other parts of the
solution. Probably the migration from the contact film to the free water is
largely by diffusion (see pp. 82-83); but, once beyond the contact film, the
migration is largely accomplished by convectional movements. Material
may be supplied to the contact film by migration of material from the free
parts of the solution. From the contact film material may be deposited in
the rocks.
In this connection it is interesting to note that in the portions of the
solutions near the contact with solids ‘there is often a concentration of
the dissolved material. This phenomenon has been called adsorption.”“
The phenomena of adsorption seem to show with great clearness, not only
that the contact film is the active agent in transfer between the free solu-
«Cameron, Frank K., Application of the theory of solutions to the study of soils: Report No. 64,
Field Operations of Division of Soils, 1899, U. 8. Dept. of Agric., 1900, p. 142.
ACTION OF AQUEOUS SOLUTIONS. 65
tions and the solids, but that in this film the migration of the dissolved
material is to some extent stayed by the molecular attraction of the
crystals.
Aqueous solutions as a geological agent require consideration from two
points of view—the chemical and physical principles controlling the action
of ground water, and the circulation and geological work of ground water.
These are treated in the following sections I and IT, respectively:
SECTION 1. CHEMICAL AND PHYSICAL PRINCIPLES CONTROLLING THE ACTION
OF GROUND WATER.
The work of ground water, like any other work, requires the expendi-
ture of energy. The energy by which the water accomplishes its work is
derived from chemical action, heat, and mechanical ; +tion.
In order to comprehend the processes of alteration of rocks it will be
necessary to summarize the important conclusions of physical chemistry as
to solutions and chemical reactions. The principles here contained are
mainly taken from the works of Ostwald and Nernst.
Chemical action will be considered under the headings, ‘Principles of
solutions applicable to ground waters,” and ‘“ Principles of chemical reactions
applicable to ground waters.”
PRINCIPLES OF SOLUTIONS APPLICABLE TO GROUND WATERS.
While the consideration of the principles of solution logically falls
under general chemical action, and, perhaps, ought to be treated as a special
case under the general treatment of chemical reactions, it seems advisable,
because the subject of solutions is somewhat simple as compared with the
interactions of complex chemical compounds, to take up this subject first,
after which the general laws controlling chemical reactions will be given.
The water of rocks, whether at ordinary temperatures and pressures or
at higher temperatures and pressures, may take any of the substances
with which it comes in contact into solution; it may deposit substances
from solution; it may combine with substances forming hydroxides, as in the
case of many of the zeolites and limonite; it may part with its hydrogen in
exchange for bases, thus at the same time changing the composition of the
rock and taking the bases replaced into solution. This is illustrated by the
alteration of enstatite to tale. (See Chapter V, p. 268.) There may be
MON XLvVII--~04——5
66 A TREATISE ON METAMORPHISM.
reactions as a result of different substances being taken into solution at
different times; there may be reactions as a result of different solutions
coming together, and thus mingling; there may be reactions between
substances in solution and the solid material with which the water is in
contact; there may be reactions as a result of changing temperature and
pressure. All these changes are in the nature of chemical action. There-
fore by chemical action through solutions is meant the taking of material
into solution, the deposition of material from solution, the interchange
between materials in solutions, the interchange between materials in solu-
tions and adjacent solids, and, finally, the interchange of the adjacent solid
particles, for such an interchange is usually accomplished through the
medium of a separating film of water. In this case the apparently simple
reaction between solids is really accomplished by transfers through sepa-
rating solutions. In all these interchanges the materials pass through a
stage of solution.
Salts are combinations of the metals and the acid radicals. Thus
Na,SO, is a combination of Na, and SO, and KCIO, of K and ClO.
Faraday called these constituents ions. This term will be used as defined
by Faraday without any implication that a compound in solution separates
into its constituent ions or is dissociated.
According to many chemists® salts in various solutions are at least
partly separated into their ions. Such supposed separation has been called
electrolytic dissociation. If electrolytic dissociation takes place to a consid-
erable extent, the properties of the compounds are practically the sum of
the properties of their separated ions. In its power of dissociation of
dissolved salts water is held to exceed all other solvents. Water itself is
held to be slightly dissociated, or the H,O separates into the ions OH and
H. According to the theory of dissociation the presence of free ions in
water solutions is therefore universal. By the advocates of the theory it
is held that it is by the interaction of these free ions that chemical
interchanges are accomplished. But dissociation is held to be very imper-
fect in strong solutions, relatively far advanced in dilute solutions, and in
very dilute solutions nearly or quite complete. As the greater portion of
@Nernst, W., Theoretical chemistry, trans. by C. 8. Palmer, Macmillan & Co., London, 1895,
p- 307. Ostwald, W., Outlines of general chemistry, trans. by James Walker, Macmillan & Co., Lon-
don, 1895, pp. 266-290.
FORM OF SILICA IN SOLUTIONS. 67
underground solutions are very dilute, at least where somewhat free circu-
lation is the rule, if the theory of dissociation be true we may suppose
that the salts held in solution are largely separated into their ions. While
the theory of dissociation and the explanation of chemical reactions by
interchange of free ions (see pp. 84-85) have a strong foothold in theoretical
chemistry, they have never gained universal support; and recently the
theory has been strongly attacked by Kahlenberg, who not only holds
that the theory is unnecessary to explain chemical reaction, but brings
together many facts which appear to controvert it." He has shown, more-
over, that instantaneous chemical changes take place in solutions that are
the best of insulators.’
Until recently it has not been known how the most important of the
geological compounds, the silicates, behave when dissolved. However,
Kahlenberg and Lincoln’ have shown that when dilute solutions of sili-
cates are made the silica exists in such solutions in the form of colloidal
silicie acid. To illustrate: If a sufficiently dilute solution of sodium
silicate be made, but much more concentrated than ordinarily occurs in
underground waters, the: compound breaks up into NaOH and colloidal
silicic acid. From this fact it would not be supposed that the silicic acid is
a chemically active compound, and it is not active near the surface of the
earth at ordinary temperatures and pressures; but on subsequent pages it
will be seen that at considerable depth, where the pressure and temperature
are much above the normal, silicic acid is a most active compound.
Before the ionic theory of solutions gained recognition it was cus-
tomary in the published analyses of underground waters to suppose that
the bases and acids of the dissolved materials are united in a definite way.
For instance, chlorine was ordinarily considered as united with the potas-
sium, sodium, or calcium. The sulphuric: oxide radical SO, was supposed
to be united with the oxides of potassium, magnesium, calcium, and sodium.
The carbon dioxide radical CO; was supposed to be united with the oxides
of iron, magnesium, sodium, and calcium. The aluminum and silica were
«Kahlenberg, L., The theory of electrolytic dissociation as viewed in the light of facts recently
ascertained: Bull. Univ. of Wisconsin No. 47, 1901, pp. 299-351; also Jour. Phys. Chem., vol. 5, 1901,
pp. 339-392.
> Kahlenberg, L., Instantaneous chemical reactions and the theory of electrolytic dissociation:
Jour. Phys. Chem., vol. 6, 1902, p. 1.
¢ Kahlenberg, L., and Lincoln, A. T., Solutions of silicates of the alkalies: Jour. Phys. Chem.,
vol. 2, 1898, pp. 88-90.
68 A TREATISE ON METAMORPHISM.
usually regarded as oxides, although in some cases the aluminum was
treated as united with the chlorine.* However, results of recent analyses
have ordinarily been given on the basis of ions.”
In a solution, under the law of mass action, each of the bases is to be
considered as divided between all acids, and under the theory of disso-
ciation there are also present in the solutions the free ions of both the
bases and the acids. For example, suppose a strong underground water
solution to contain three bases and three acid radicals; as, for instance, the
bases sodium, calcium, and magnesium, and the radicals of carbonic,
sulphuric, and hydrochloric acid; then the following nine compounds are
present, Na;CO;, Na,SO,, NaCl, CaCO,, CaSO,, CaCl, MgCO;, MgSO,,
MeCl,, and also the six free ions, Na, Ca, Mg, CO;, SO,, and Cl, making
altogether fifteen separate combinations of the elements. However, under
the theory of dissociation, if the solutions be so weak that the substances in
solution are wholly ionized the nine compounds first mentioned will not be
present. If the dissociation theory be rejected, under ‘the law of mass
action in all cases all of the nine compounds will be present, but not the
free ions.
Under the principles of solutions it is necessary to consider the cases
of (1) the solution of gases in ground waters, (2) the solution of solids in
ground waters, and (3) diffusion.
SOLUTION OF GASES IN GROUND WATERS.
The quantity of gases which can be dissolved in underground water
depends upon the gases present, the pressure, the temperature, and the
solids in solution.
Gases present —A]] the natural gases may be dissolved in water or may
unite with water. In the latter case the resultant compounds are dissolved.
Tn both cases solutions are formed.
Since below the level of the free surface of underground water it is
clear that the gases enter into solution either by absorption or by combina-
tion, it follows that the more far-reaching effects of these substances in
metamorphism are not as gases, but as aqueous solutions. The gases are
«Peale, A. C., Lists and analyses of the mineral springs of the United States: Bull. U. 8S. Geol.
Survey No. 32, 1886, pp. 48, 115, 133.
> Clarke, F. W., and Hillebrand, W. F., Analyses of rocks and analytical methods, U.S. Geol.
Survey, 1880-1896: Bull. U. 8S. Geol. Survey No. 148, 1897. Clarke, F. W., Analyses of rocks,
laboratory of the U. 8. Geol. Survey, 1880-1899: Bull. U. 8. Geol. Survey No. 168, 1900.
GASES IN SOLUTIONS. 69
therefore important factors in the action of ground waters, but they are of
course only a small portion of the substances which ground waters carry.
The more important of these gases which pass into ground waters are:
Oxygen (O,), carbon dioxide (CO,), hydrosulphuric acid (HS), sulphur-
ous oxide (SO,), hydrochloric acid (HCI), hydrofluoric acid (HF), boric
acid (H,BO,), and ammonia (NH;). Sulphur and boric acid as gases occur
mainly in connection with voleanic action. If the above-mentioned or
other gases unite with the water the laws below given as to solubility do
not hold; thus carbon dioxide unites with water, forming carbonic acid
(CO,+ H,O=H,CO,); sulphurous oxide unites with water, producing sul-
phurous acid (SO,+ H,O=H.SO,); ammonia unites with water, producing
ammonium hydrate (NH;+H,0=NH,OH). In some of these cases, for
instance, that of ammonia and sulphurous oxide, the water may unite with
many times its volume of the gas, with increase of volume; thus water at
0° C. and atmospheric pressure absorbs 1,050 volumes of ammonia as a
result of the union of the two. What portion of CO, contained in ground
water remains as CO, in solution, and what part unites with water, forming
carbonic acid, is uncertain, but it is definitely known that much of the CO,
contained in the ground water is in the form of the so-called bicarbonates—
for instance, such salts as Na,CO,+H.,CO, or 2NaHCo,
united with the water.
When new compounds are formed by the union of the gases with the
and therefore is
liquids, the substances held in solution are the new compounds. When
these new compounds are gases the laws below given concerning the solu-
tion of gases in liquids apply only to the new compound, not to the original
gas. Where the compound is a solid—as, for instance, a bicarbonate—the
laws for the solution of gases in water do not apply, but such compounds
are held under the laws controlling the solution of solids in liquids. (See
pp. 72-82.)
In some cases in nature a part of a gas may unite with a substance in
solution and make a new compound and a part may unite with water and
be dissolved in this form. If both the compounds be gases the laws for the
solution of gases in liquids hold. If the new compound formed be a solid
salt the laws for the solution of solids in liquids apply to it, and the laws
for the solution of gases in liquids apply to the uncombined gas. This case
is illustrated by carbon dioxide, already mentioned.
70 A TREATISE ON METAMORPHISM.
The pressure— “The quantity of a gas dissolved by a specified quantity of
« This statement is true
a liquid is proportional to the pressure of the gas.”
of each gas without reference to whether a gas be alone or mixed with other
gases. Thus the solubility of each of a number of mixed gases is controlled
by the pressure exerted by that gas, not by the total pressure exerted by
the mixture. It is therefore clear that under natural conditions the press-
ure of that part of any gas which is in the atmosphere and the pressure of
that part which is held in solution in the water immediately adjacent are
the same when the two are in equilibrium, and the water is therefore just
saturated.
So far as ground waters are concerned, there are two cases; first, the
waters of the belt of weathering, or those to the level of ground water;
and second, those below the level of ground water, or the belt of satura-
tion. In the belt of weathering the pressure is atmospheric. Changes of
pressure are barometric. In so far as the atmospheric pressure varies—and
this is by fractions up to one-fifteenth—the solubility of the natural gases
in the water of the belt of weathering also varies directly as the pressure of
each of the gases varies, without reference to the pressure and solubility of
the other gases.
In the belt of saturation, just at the level of ground water, the amount
of gases held in solution is proportional to atmospheric pressure; but
at greater depths higher degrees of concentration of gases are possible,
although it might at first be thought that the atmospheric pressure or vapor
pressure at the free surface of the water would determine the concentration
of the solution. The pressure which really is determinative as to the
amount of gas which may be held in solution is that of a column of water
extending to the free surface, plus the atmospheric pressure. Since,
however, water is so much heavier than the atmosphere, at considerable
depths below the level of ground water the atmospheric pressure may be
neglected; and the pressure, and therefore the solubility of underground
gases in water, is almost directly proportional to the depth below the level
of ground water. For instance, at a depth of only 100 meters below the
level of ground water the pressure of the atmosphere is only one-tenth
that of the water pressure; at a depth of 1,000 meters it is only one
«Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., New York,
1891p 298
RELATIONS OF PRESSURE AND SOLUTION. 71
one-hundredth, and at still gi ater depths the fraction of pressure due to
the atmosphere is insignificant.
But in order that saturation for any gas corresponding to the pressure at
any given depth shall occur, it is necessary that a sufficient amount of gas
shall there exist. Gases may be produced below the level of ground water
by the chemical reactions, as by the liberation of carbon dioxide in the
process of silication. Later it will be seen (see Chapter VIII, pp. 677-679)
that this is one of the fundamental processes of the lower physical-chemical
zone. It follows from the above that at depth the amount of carbon
dioxide or other gas in solution per unit of water may be many score times
ereater than near the surface. The pressure of carbon dioxide at the
surface is only about 0.0006 of an atmosphere. ‘The water pressure at a
depth of 1,000 meters is almost 100 atmospheres; therefore the amount of
free carbon dioxide which may be held in solution, if pressure were the
only factor concerned, might be 166666 times as great as that held in
solution in the belt of weathering.
But it must be remembered that, as shown below, the increase of
temperature due to increase of depth somewhat reduces this multiple.
It should be remembered also that carbon dioxide combines with
water, producing carbonic acid, and the amount of this compound which
may be held in solution at the surface of ground water is not dependent
upon the pressure of the atmospheric carbon dioxide. But it is evident
that deep ground waters, where the pressure is great, may hold a vastly
greater quantity of carbon dioxide than can be held in solution near the
level of ground water.
As already pointed out, the law which obtains in reference to geological
work is that the activity of the carbon dioxide increases in direct ratio with
its quantity.
The theoretical conclusion that the action of carbon dioxide would be
increased by pressure, and consequent greater quantity, has been experi-
mentally verified by Mueller® and Struve, who found that strong pressure
increased the action of carbon dioxide in the decomposition of the silicates
more than did increase of time.
@Mueller, Richard, Untersuchungen tiber die Einwirkung des kohlensiiurehaltigen Wassers auf
einige Mineralien und Gesteine: Tschermaks mineral. Mittheil., vol. 7, 1877, p. 47-
12 A TREATISE ON METAMORPHISM.
The temperature-—Increase of temperature generally results in decrease of
solubility of a gas.° Increase in temperature with depth, or because of
voleanism, lessens the solubility of gases in ground water, and to this
extent works against the effect of increased pressure.
Solids in solution— There is still another factor which enters to a slight
extent into the solubility of gases. Water holding solids in solution, in
most cases, absorbs less of a gas at a given pressure than does pure water.’
However, the solutions near the surface are ordinarily so dilute that this
law is probably not important, but at depth it may be of some consequence
in working against the effect of increased pressure.
SOLUTION OF SOLIDS IN GROUND WATER.
Where a solid is placed in a liquid some or all of it dissolves, and thus
forms a homogeneous mixture composed of the two, or a solution.
It has been found that if a liquid be placed in a vessel having two com-
partments separated by a membrane through which the solvent but not the
dissolved substance may pass, when a soluble compound—for instance,
sugar—is dissolved in the liquid in one of the compartments, pressure
against the membrane is produced. This pressure has been called osmotic
pressure, to distinguish it from ordinary gas pressure, known as vapor
pressure. According to van’t Hoff, the osmotic pressure ‘is independent
of the nature of the solvent, and in general obeys the laws of gases.” That
is to say, ‘‘the osmotic pressure is proportional to the concentration; the
osmotic pressure. is proportional to the absolute temperature; the same
osmotic pressure can be obtained by equimolecular quantities of the most
various substances in the same solvent; the osmotic pressure is exactly the
same as the gas pressure which would be observed if the solvent were
removed and the dissolved substance were left filling the same space in the
Ne
gaseous state at the same temperature.”° ‘These somewhat sweeping state-
ments need various modifications. For instance, where the solutions are
very concentrated the molecules in solution are believed to be so close to
«Ostwald, W., Outlines of general chemistry, translated by James Walker, Macmillan & Co.,
London, 2d ed., 1895, p. 121.
> Ostwald, op. cit., p. 121.
¢ Nernst, W., Theoretical chemistry, translated by C. 8. Palmer, Macmillan & Co., London, 1895,
pp. 134-137. Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co.,
New York, 1891, pp. 112-117.
CONDITIONS OF SOLIDS IN SOLUTION. Ue}
one another that molecuiar attraction produces an effect, and in this case
the osmotic pressure does not vary directly as the concentration. But,
Cameron says, in so far as the molecules in solution are sufficiently sepa-
rated so that they may act as a gas, “the volume, pressure, and temper-
ature relations are dependent only upon the number of molecules involved.” “
Since all of these relations are the same as the laws controlling the
behavior of gases, it is held by many physical chemists that when a solid
passes into solution it is transformed to a gas. Under this explanation the
osmotic pressure is a gaseous pressure. ‘‘The kinetic energy of the
molecules of the dissolved substance is equal to that of the gas at the same
temperature; and, moreover, as the kinetic energies of the molecules of the
dissolved subtance and of the solvent must agree, because these molecules
are in immediate contact, it follows also that the kinetic energy of the
molecules of the liquid must, on the whole, be the same as that of gaseous
molecules at the same temperature.””
If the above theory be correct, it follows that the solution of solids in
liquids is similar to that of gases in liquids; for in both cases the compound
when dissolved is in the form of a gas; and the geological work of under-
ground water, whether the solutions be produced by a mingling of gases
and water, solids and water, or the three combined, could be considered as
aunit. (See pp. 63-64.)
In ecase.a salt dissolved in water be an electrolyte, under the dissocia-
tion theory it is separated into ions to some extent. If this be so, the
number of dissolved particles is represented by the number of ions plus the
number of undissociated molecules. Therefore in very dilute solutions,
where the dissociation is held to be complete, the number of dissolved
particles and consequently the osmotic pressure is doubled in the case of
a salt of a monad acid with a monad base. Thus the law of equal gaseous
pressure for equal number of molecules is believed by many to still hold
good. For instance, if NaCl dissociates into the ions Na and Cl, or KOH
into the ions K and OH, thus giving twice as many molecules as in the
case of a compound which does not ionize, under the law the csmotic
pressure is twice as great as that of the compound which does not dissociate.
«Cameron, F. K., Application of theory of solutions to the study of soils: Report No. 64, Field
Operations of Division of Soils, 1899, U. 8. Dept. of Agric., 1900, p. 144.
bQstwald, op. cit., p. 148.
74 A TREATISE ON METAMORPHISM.
The conclusions of van’t Hoff, Ostwald, and others in reference to
osmotic pressure being due to gaseous pressure of the dissolved substances
have never been accepted by Mendeléeff, and have recently been strongly
opposed by Kahlenberg. Certainly there are many discrepancies between
the observations made as to the amount of osmotic pressure and the amount
which the pressure should be under the gas law. But, so far as the
observations of geology show, I see nothing that controverts or confirms
van't Hoft’s theory. In studying the work of underground solutions I
have been unable to discover any criteria which will separate the work of
gases in water solutions from the work of solids in water solutions. So
far as geology is concerned, solutions of gases in water and solutions of
solids in water can’ not be discriminated. It has been held by some that
the presence of fluorite and other minerals is evidence of gaseous action,
but, as yet, I have not been able to find valid evidence offered by any
author for this conjecture. It may be that gases dissolved in water and
solids dissolved in water are held in solution in consequence of chemical
affinity, as held by Mendeléett, or they may be in solution as gases, as
held by van’t Hoff, but in either case the manner of action of the two is
the same, and therefore there is no warrant for attributing the development
of fluorite, tourmaline, etc., to the presence of ‘‘mineralizers” in the sense
that these compounds are the products of the action of gases as opposed to
water solutions.
When a soluble solid is placed in a liquid solvent it at once begins to
dissolve. The temperature and pressure remaining constant, if an excess
of the solid be present after a sufficient time there is no further decrease
in the amount of the solid present, nor is there any increase. When this
state is reached the solution is saturated.
When a solid is in a saturated solution, and therefore constant in
amount, even if temperature and pressure remain constant it does not follow
that no interchange takes place between the dissolved and solid salt. The
kinetic theory of solutions leads to the conclusion that many molecules are
released from the solid to the solution, and pass from the solution into the
solid, but these amounts balance. This is well illustrated by sugar solu-
tions. If finely pulverized sugar be placed in the bottom of a saturated
sugar solution and sugar-covered threads be suspended in the solution,
Oe
5°
sticks of rock candy will be formed. The crystals of the candy grow at
GROWTH OF LARGE CRYSTALS. 7a
the expense of the sugar below, which is being constantly taken into solu-
tion and deposited as crystals about the string; and, therefore, although the
solution is continuously saturated, there is continuous solution and deposi-
tion. Even if no sugar-coated strings were placed in the sugar, after a
time it would be found to be coarser grained or to have recrystallized.
Thus the constant interchange between a saturated solution and that of an
adjacent solid is certain.
The change occurs under the law by which large crystals grow at the
expense of small ones. In order that crystals shall grow in a solvent, it is
necessary that the solutions shall be saturated or supersaturated at the
immediate place of crystal growth. Since underground there is always a
superabundance of many materials as compared with the amount of water,
we may suppose that at a moderate depth below the surface, and especially
in the smaller spaces, where movement is very slow (see pp. 138-146), the
solutions are often saturated. It is well known that the growth of larger
crystals at the expense of smaller ones, under conditions of saturation and
superabundance of material, goes on more rapidly in proportion as the
temperature is high and the pressure is great. The principle is taken
advantage of in the chemical laboratory in the production, before filtration, of
a coarse precipitate by boiling or other means. During the process the finer
particles of the precipitate are dissolved and the coarser ones are enlarged
at their cost. The growth of the large crystals at the expense of the small
ones is due to the fact that the smaller crystals are somewhat more soluble
than the larger. The explanation of this change, as given by Ostwald,”
lies in the ‘surface tension which exists on the boundary surfaces between
solids and liquids, as on those between liquids and gases—the so-called
free surfaces of liquids. This tension acts so that the surfaces in question
are reduced in size, with the consequent enlargement of individual crystals
(the total amount of precipitate remaining practically unaltered), i. e., with
the coarsening of the grains.” During the change, for a given volume of
solid the lessening of the total surface of the crystals, and consequently the
lessening of the surface tension, results from the fact that the surfaces are
small in proportion as the individuals are large. For a given volume of a
substance the surfaces of the crystals are inversely as their diameters. (See
@ Ostwald, W., The scientific foundations of analytical chemistry, translated by George McGowan,
Macmillan & Co., London, 1895, p. 22.
76 A TREATISE ON METAMORPHISM.
p- 98). The increase in the size of the crystals, lessening the surface
tension, may be considered as a transfer of potential into kinetic energy.
This passes into heat and is dispersed under the apparently general law of
the dissipation of energy. Why the tendency to the transformation of all
forms of energy into heat and the dissipation of heat should be a law of
nature it is not my purpose here to discuss. But such the law seems to
be, and in its application we carry the causal sequence as far as we are
now able.
The growth of large individuals at the expense of small ones in ground
water is of the most profound significance in the metamorphism of rocks.
It is illustrated by the secondary enlargement of minerals and by the por-
phyritie. crystals which frequently develop in schists and gneisses, such as
the porphyritic crystals of feldspar, hornblende, ‘garnet, staurolite, ete.
(See pp. 643-644, 699-700.)
The above principle in reference to the growth of large crystals at the
expense of small ones is very clearly applicable to the growth of segregations
of minerals of a certain kind as compared with smaller segregations. If,
for instance, at one place there be a mineral aggregate, this, so far as the
surface tension and the free surface of liquids are concerned, acts as a unit
and tends to draw to itself the material of smaller aggregates or of individual
mineral particles. For aggregates which do not have crystal boundaries
the form which would be assumed under ideal conditions is spherical.
This principle of the growth of large aggregates at the expense of small
ones is illustrated by chert nodules. (See pp. 816-818.)
The quantity of a solid which can be dissolved in aqueous solutions
depends upon the compounds present, the pressure, and the temperature.
When the limit of solubility is reached the solution is said to be saturated.
COMPOUNDS PRESENT.
Theoretically all compounds are soluble to some extent in water. This
statement applies to all natural compounds; that is, the minerals of nature
are elements, oxides, or salts which are soluble in water. No substance is
wholly insoluble in the ground solutions, even at the ordinary temperatures
and pressures. This statement is illustrated by the solution of quartz and
the more refractory silicates at the surface. Under surface conditions
«Hayes, C. W., Solution of silica under atmospheric conditions: Bull. Geol. Soc. America, vol. 8,
1897, pp. 214-217.
MUTUAL INFLUENCE OF COMPOUNDS. Ct
quartz grains are sometimes etched by meteoric waters, and the decompo-
sition and partial solution of the refractory silicates is universal. Under
conditions of deep-water circulation solution of quartz and the refractory
silicates may be accomplished with relative rapidity. This is illustrated
by the Calumet and Hecla conglomerate, many of the pebbles of which
have been partly or even completely dissolved and the space once occupied
by them taken by copper."
Since underground solutions always contain a number of compounds,
and often many, the influence of one compound upon the solubility of
another is of consequence in various ways. For instance, when several
compounds are present, a unit quantity of water will not dissolve as much
of a given salt as it would if it were alone. But if a number of units of
water are each saturated with a single salt, and the solutions are mingled
without chemical reaction, the mixture is capable of taking additional quan-
tities of the salts into solution. In other words, a unit of solution simul-
taneously saturated with* each of several compounds contains a greater
total of solids than a unit of solution saturated with fewer of these com-
pounds, but less of any individual salt than it would were it saturated with
that salt alone.’
In the ground solutions the different compounds frequently react upon
one another, and therefore important modifications in the above statement
are necessary, as is explained under “Precipitation,” pp. 113-123. :
RELATIONS OF SOLUTION AND PRESSURE.
In general, the volume of the solvent plus that of the dissolved
compound is greater than that of the solution. Fora given quantity of
the solid the contraction is greater the more of the solvent is used.“ In
some cases, however, the volume of the dissolved compound and solvent is
less than that of the solution, or expansion results from dissolving the selid.
Ammonium chloride in water is an illustration of this case. From the fore-
going relations we obtain a rule as to the relations of pressure to solubility.”
In the common ease in which the volume of the solution is less than that of
«Pumpelly, R., The paragenesis and derivation of copper and its associates on Lake Superior: Am.
Jour. Sci., 3d ser., vol. 2, 1871, p. 34.
>Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., New York,
1891, pp. 83, 84.
¢ Ostwald, W., op. cit., p. 82.
dNernst, W., Theoretical chemistry, translated by C. 8. Palmer, Macmillan & Co., London,
1895, p. 567.
78 A TREATISE ON METAMORPHISM.
solvent and solid, pressure increases solubility; for in that case solution
tends to bring the molecules nearer together and works in conjunction with
the pressure. A mixture of water and ice furnishes an excellent illustration
of this principle. At any moment the volume of the water is less than
that of the equivalent water and ice. Hence pressure promotes solution and
prevents freezing, or in other words, crystallization.- In the reverse case,
that in which the volume of the solution is greater than that of solvent and
solid, pressure decreases the solubility, the reason being the reverse of that
of the previous case.
The above law may be illustrated by fig. 1. A given amount of salt,
say 10 cc. in volume, may be supposed to be placed in 90 cc. of water, and
the salt be of such a nature as to saturate the water at that
temperature and pressure. Before solution begins the
space occupied is 100 ce. After solution this space may
be greater or less than 100 ce., say 105 ce. or 95 ce.; that
is, the water surface instead of being at aa will be at ce or
vb. Ifit be at bb, where the volume is less, and the pres-
sure be increased, an additional amount of salt may be
added and taken into solution. If it be at cc, and the
pressure be increased, a part of the salt already in solution
will be precipitated from the solution.
It is well known that the solubility of calcium car-
Fic. 1—Change of vol bonate and of some other carbonates is increased by pres-
ume resulting from so-
lution, andrelationsof sure.” It is a fair inference from Barus’s work that the
solution and pressure.
solubility of the silicates is also increased by pressure.
Barus’ found that when soft glass is dissolved in water at temperatures
above 210° C., the volume is 20 to 30 per cent less than the two sepa-
rately. This glass is one which contains alkalies, alkaline earths, and
lead, and therefore is somewhat similar in composition to many natural
silicates. The carbonates and the silicates are the dominant compounds
in underground solutions. The solubility of many other salts, besides
the carbonates and silicates, occurring underground is increased by pres-
sure. Therefore, in the majority of the complex underground solutions
aQLindgren, W., Gold-quartz veins of Nevada City and Grass Valley, California: Seventeenth
Ann. Rept. U. 8. Geol. Survey, pt. 2, 1896, pp. 176-178.
> Barus, C., Hot water and soft glass in their thermodynamic relations: Am. Jour. Sci., 4th
ser., vol. 9, 1900, p. 173.
RELATION OF SOLUTION AND TEMPERATURE. (
the totals of the salts in solution are in general increased by pressure,
and the volumes of the solution are less than those of the salts and solvents
separately.
RELATIONS OF SOLUTION AND TEMPERATURE,
The relations of solution and temperature have three phases; first, the
speed of solution; second, the quantity of material which may be held in
solution; third, the relations of solution to absorption and liberation of heat.
Speed of solution — The speed of solution is commonly increased greatly by
rise of temperature.” A slight increase in temperature may increase the rate
of solution out of all proportion to the absolute change in temperature. At
temperatures above 100° C., and especially above 185° C., the activity of
water may increase to an amazing degree. The rapid solution of glass, by
Barus,’ at temperatures about 185° C. illustrates this. At any temperature
solution will continue until the point of saturation is reached, but this state
will be attained at high temperatures in but a small fraction of the time
required at low temperatures. [or instance, to saturate an underground
solution with the refractory silicates or sulphides at ordinary temperatures
might require months, or even years, while to saturate them at temperatures
above 185° C. might require only an equal number of minutes, or at most,
hours. The capacity of water for action at high temperatures combined
with pressure, considered above, is adequate to explain the complete
recrystallization of great volumes of rock. (See pp. 749-751.)
Quantity of material which may be held in solution The effect of temperature upon
the quantity of material which may be held in solution does not admit of a
simple general statement.’ For most substances moderate increase of
temperature gives greater capacity for solution; but for many substances
there exists a temperature at which there is the maximum capacity for
solution, and the amount of material which may be held in solution at
higher and lower temperatures is less than this maximum. ‘The quanti-
tative relations of solution and temperature at ordinary pressure between
«Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
p. 568.
> Barus, C., Hot water and soft glass in their thermodynamic relations: Am. Jour. Sci., 4th ser.,
vol. 6, 1898, p. 270, and vol. 9, 1900, pp. 167-168.
¢Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., New
York, 1891, pp. 55-77.
80 A TREATISE ON METAMORPHISM.
0° CG. and 100° C. are shown by fig. 2, taken from Ostwald.* For various
substances the maximum capacity for solution lies between 60° and 140°
C., and for many substances it is probably below 200° C. It therefore
follows, in respect to underground solutions, that a general statement can
not be made as to how change of temperature may affect solubility.
However, it is highly probable that up to temperatures of 100° C., and
therefore under normal conditions to depths of 3,300 meters, increase of
temperature increases the average capacity of underground water to hold
material in solution; and it is probable that the average capacity of
ground water increases to temperatures considerably above 100° C., and
therefore to depths greater than 3,300 meters. But when water passes
downward to the deeper parts of the zone of fracture the increase in temper-
ature may lessen the average capacity for holding material in solution,
provided the joint effect of pressure be
barred. But it has been seen that increas-
ing pressure with increasing depth pro-
motes solubility. It is almost certain that
Solubility.
high temperature and pressure combined
greatly increase the capacity of water for
0 40 a0 Bo 40° 50° G9? 70° a0? vo? 100° SOlution. This is proved by the experi-
Temperature.
ments of Barus upon the solubility of
Fic. 2.—Quantitative relations between solution and
MS a a a glass. He has shown that at temperatures
above 185° C. and below 200° C it is possible “to impregnate glass
with water to such an extent as to make it fusible below 200° C. The
solution oceurs with contraction of bulk relatively to the ingredients and
increasing compressibility.” . . . ‘If these solutions are sufficiently
concentrated they coagulate at ordinary temperature and the congealed
aqueous glass is not different in general appearance from common glass.
The melting point of the coagulated aqueous silicate frequently les below
200° C., probably above 150° C., depending on the glass.” And he con-
cludes that ‘‘Glass as a colloid is miscible in all proportions with water.””
Since glass is one of the important silicate rocks which occur in nature,
these statements are directly applicable to one set of rocks. They may
«Ostwald, W., Grundlinien der anorganischen Chemie, Engelmann, Leipzig, 1900, p. 222.
> Barus, C., Remarks on colloidal glass: Am. Jour. Sci., 4th ser., vol. 6, 1898, p. 270. See also Am.
Jour. Sci., 4th ser., vol. 9, 1900, pp. 161-175.
GRADATION BETWEEN LIQUIDS AND SOLIDS. 81
not be applicable to the same extent to crystallized silicate rocks, but it
seems to me highly probable that they apply in large measure to many.
In so far as Barus’s final conclusion is applicable, there may result all grada-
tions, from solutions in which the water is the dominant constituent to
those in which it is the subordinate constituent. This principle of the
increased quantity of material which may be held in solution as a result of
combined high pressure and temperature is believed to possess very great
significance in alterations in the zone of anamorphism, and to be of impor-
tance in alterations in the belt of cementation. (See pp. 602-603, 659-661.)
Relations of solution to absorption and liberation of heat——Ag al ready explained, when
material passes into solution the molecules are separated and acquire
kinetic energy, and are believed by many to change from the solid to the
gaseous form. This process absorbs heat. On the other hand, where the
volume of the solution is less than the volume of the solvent and salt sepa-
rately, the molecules of the solvent and salt combined are brought closer
together and heat is therefore liberated. In the reverse case, where the
volume of the solution is greater than that of the solvent and salt separately,
the molecules are pushed farther apart, and heat is absorbed. If the com-
pounds in solution separate into ions this process is believed to be usually
attended by liberation of heat.” Whether there is a rise or fall of tempera-
ture of the solution will depend upon the relative values of these factors.
In the common case where there is decrease in the volume as a result of
solution, the heat thus liberated by change in volume plus the supposed
heat of ionization are together preponderant, and there is, therefore, libera-
tion of heat and a rise in temperature. However, in the case where there
is increase in the volume as a result of solution, the heat thus absorbed and
the heat absorbed in changing the salt from the state of a solid to that of a
gas is greater than that supposed to be liberated by dissociation. The first
two factors are dominant, and there is usually a marked absorption of heat
and, consequently, a fall in the temperature of the solution. This is illus-
trated by the solution of ammonium chloride in water. The volume is
considerably decreased and the fall in temperature is very decided.
aNernst, W., Theoretical chemistry, translated by C. 8. Palmer, Macmillan & Co., London, 1895,
p- 562.
MON XLVII—04——6
82 A TREATISE ON METAMORPHISM.
DIFFUSION.
It has been seen that the molecules of gases and of solids when dis-
solved in water are distributed through the solution. When the material
dissolved is not evenly distributed the molecules are more abundant here
and less abundant there. If the theory be true that the dissolved solids are
gaseous the molecules would exert a greater pressure where more closely
packed. Under these conditions molecules where more closely packed
move toward places where they are less closely packed. This move-
ment is regarded by many as the explanation of osmotic pressure.
Kahlenberg, however, does not accept this explanation, but regards osmotic
pressure as due to the “mutual attraction between solvent and dissolved
substance.”* Without reference to either theory the more important
conclusions in reference to diffusion may be summarized.
The force which drives the dissolved substances from place to place,
and the velocity with which a dissolved substance wanders in a solvent, is *
proportional to the degree of concentration.” Therefore, ‘the quantity of
a salt which diffuses through a given area is proportional to the difference
between the concentrations of two areas infinitely near one another.”* In
other words, diffusion is proportional to the difference in strength. The
quantity diffused is proportional to the square root of the time of diffusion,
and the distance over which a determinate concentration extends is also
proportional to the square root of the time of diffusion.” Several salts in a
solution diffuse almost independently of one another, each at its own specific
rate.’ At 20°, according to Ostwald, there is twice as much diffusion as at
0°, and at 40° twice as much as at 20°/ When a solution is in equilibrium
the concentration of the solution varies inversely as the temperature. It
follows that when the temperature of the solution varies, equilibrium is
obtained not by equal distribution of the solutes, but by unequal distri-
bution. Ifthe temperature be the same throughout a solution with equal
«Kahlenberg, Louis, The theory of electrolytic dissociation as viewed in the light of facts recently
ascertained: Bull. Univ. of Wisconsin No. 47, 1901, p. 349.
> Nernst, W., Theoretical chemistry, translated by C. 8. Palmer, Macmillan & Co., London, 1895,
pp. 143-144.
¢ Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., New York,
1891, p. 120.
@ Solutions, cit., p. 135.
€ Solutions, cit., p. 139.
F Solutions, cit., p. 13€.
SLOWNESS OF DIFFUSION. 83
distribution of the dissolved compounds, a deviation from uniformity in the
temperature of the solution will disturb the equilibrium and result in
unequal distribution of the dissolved substances.“
The values of the coefficient of diffusion (D) of certain substances in
water solutions at various temperatures are given by the following table
from Nernst:’ (The table gives the number of grams of the dissolved
substance which will pass in one day through a section of 1 sq. em. when
the difference in concentration of the cross section 1 cm. apart amounts to
1 gram in a cubic centimeter.)
Rates of diffusion of certain substances in water solutions at various temperatures.
Temps. -D:
Ey rock loniciaci dieses eels haere ors ee ca ate DR bac ne yaa ee 0.0 1.4
ADD) ep eS sy aN Sa Hse ety pa De pee agere eat By 11.0 1.84
MINGTHtTSL GY ACU CL eye pe a dT aE Tee Nt 2 Lipa ead lie 2 ALS A ie 9.0 1.75
Sulphuriciacidies 3254255 ase ee eae sae e aan Ne ees Caen 7.5 1.04
PAYCO LL CVA CTCL peat este eee ee oe oe Mia Nea oN Sask cee PIA PS AN NS 14.0 .81
Rotassiumayluydmosxd dl Ceasers epee aie ate arete eee nea pe ual 13.5 1.66
Scocivamalpliny, chro cl esas une oss a ee EE ett eee eee at 8.0 1.96
Amn oniumy by dro xid Giese see ease a eee ee eee tes ee 4.5 1.06
‘Soclaumaye ll ort dl eyes acta aes esta esa east eats ea lee Nyce AME aa 6009 75:
Armano pit OCnlloysls) oosecossosdosacuasodooneeoaseebosaoedeses RO neal Sil
Rotassiumiychilorid ee sets ee mere see er cen tee eta 9.0 66
JBEahoben Colloratelay GAeamo usw amet Ns Le etna Ue Ll lett 8.0 .65
Potassiumablmitratey sees ceo Saye Ue ale eel Se eae is retiars Deal yur eaOiaien 92
SOG irritate eS eye ety ma dea ar ch at ices AN ated tae LSEOR90)
Silyveraiitrate yey uy say vay Soh Cl ate EN ioe Suara neces Ohi dees ate 7.5.90
Mead ymitrate is sess esis pee ee ee eT a eee ie a eas Mayan WO 50)
OSE ys Br a aN hae ene a ee aE TOM ol
Chioralvhydratejesse seen eee eens Tee iia ate As Rs OSE seven i 9.0. .55
BMT earn Va Ge peep pe as ae teats era a Eni ae ea eGR TE 10.0 .38
This table shows that diffusion is extremely slow. The slowness
with which diffusion occurs is due, according to Nernst, to “the resistant
friction experienced by the dissolved substance in its movement through
the solvent.”’ This friction is very great, because the molecules themselves
are exceedingly small.
Later it will be seen that the process of diffusion is of very considerable
importance in the migration of compounds in ground water. (See pp:
636-639.) This is illustrated by the very important process of solution
and deposition or recrystallization. .
4 Ostwald, Solutions, cit., pp. 150-151.
bNernst, W., Theoretical chemistry, translated by C. 8S. Palmer, Macmillan & Co., London, 1895,
p. 144.
¢ Nernst, op. cit., p. 145.
84 A TREATISE ON METAMORPHISM.
PRINCIPLES OF CHEMICAL REACTIONS APPLICABLE TO GROUND WATERS.
GENERAL STATEMENT.
DEFINITIONS.
Before taking up chemical reactions it is advisable to give a number
of elementary definitions.
‘Compounds whose aqueous solutions contain the hydrogen ion (H)
are termed acids, and those which contain the hydroxyl ion (OH) bases.” “
To illustrate, HCl is an acid; NaOH is a base. When the hydrogen ion
united with one or more nonmetallic elements is mingled in solution with
the hydroxyl ion united with a metal a double reaction occurs, resulting
in the union of the hydrogen ions with the hydroxyl ions, forming water,
and the union of the nonmetallic parts of the compound with the metallic
parts. This latter union forms a salt. For example—
HCl+NaOH=NaCl + H,0.
H,CO,+-2NaOH =Na,CO,-++2H,0.
The acids and salts which contain only a single nonmetallic element are
called binary compounds. The acids and salts which contain two non-
metallic elements are called ternary compounds. For example, HCl is a
binary acid; NaCl is a binary salt; H,CO, is a ternary acid; Na,CO, is
a ternary salt. Compounds having the composition of acids, bases, and
salts may be separated from solution as solids, and of course all of these
solids may pass into solution.
Some salts also contain a certain amount of acid, and such salts are
called acid salts. For instance—
Na,00,+H,CO,=2NaHCO,.
The latter compound is acid sodium carbonate. On the other hand, some salts
contain some additional base, and such salts are called basic. For example,
Fe,(SO,); may be united with Fe,(OH),, producing mFe.(SO,);.nFe.(OH)<.
This compound is basic ferric sulphate.
DISSOCIATION,
In explaining chemical reactions the theory of dissociation as advocated
by Arrhenius, Ostwald, Nernst, and others is followed for the most part.
«Ostwald, W., The scientific foundations of analytical chemistry, translated by George McGowan,
Maemillan & Co., London, 1895, p. 117.
THEORY OF DISSOCIATION. 85
This theory is firmly placed in the text-books. No opinion is expressed
by me as to its correctness. Indeed, I have no right to any opinion on the
subject. As already pointed out, this theory has been vigorously opposed
by Kahlenberg, but as yet that author has offered no constructive theory
to take its place. I therefore follow the theory of the standard text-books
so far as necessary to show how it would apply to the work of ground solu-
tions if it prove to be true; but so far as practicable I make the statements
in such form that they will be correct even if the theory of free ions and
reactions between such ions is finally abandoned.
Under the theory of dissociation the superiority of water as a solvent
for chemical interchanges is regarded as largely due to the fact that the
dissolved substances are separated into their ions to a greater degree than
in any other solvent. To the fact of active reactions in water, whatever
their cause, are very largely due the profound changes which occur in rocks
through the medium of water solutions.
Under the theory of dissociation water solutions, acids, bases, and salts
separate into their ions. For instance, HCl separates into the free ions H
and Cl; NaOHd separates into the free ions Na and OH; and NaCl into the
free ions Na and Cl. However, in solutions the dibasic acids are supposed
to separate into free ions somewhat differently from what might be expected.
For instance, it might be expected that H,CO; would separate into the free
ions H, and CO,, but it is supposed to separate thus:
H,CO, = H+ HCO,.
Other dibasic acids are thought to dissociate in a similar manner. However,
if the dibasic acid be very strong the compound ion may again break up.
Thus, H,SO, is thought to first break up into the free ions H and HSO,
and the latter to break up into the free ions H and SO,, so that these would
be the ions present in the water. But in the case of the weak acid, carbonic,
it is thought that the last change does not take place, and that the free ions
remain H and HCO,.
Ostwald regards the absence of the second stage of dissociation as the
explanation of the peculiar characteristics of carbonic acid.” Since carbonic
acid is, next to silica, the most important rock-making acid, the manner in
which it breaks up is of great consequence in metamorphism.
“Ostwald, W., Grundlinien der anorganischen Chemie, Engelmann, Leipzig, 1900, pp. 276-278,
397-398.
86 A TREATISE ON METAMORPHISM.
HYDROLYSIS. a
Under the theory of dissociation, not only do acids, bases, and salts
separate into ions in water solutions, but the water itself is believed to
dissociate to a very small extent, according to the equation HJO=H-+OH8,
thus simultaneously forming free hydrogen and hydroxyl. If this be true
the hydrogen ions and the hydroxyl ions cvexist and water solutions to a
small extent contain free acids and free bases at the same time. The
excellence of water as an agent for reactions between the substances it
holds in solution is held to be partly due to hydrolysis.
When strong bases and acids are in solution the amount of their dis-
sociation is believed to be so much greater than that of water that the
dissociation of the latter is of little consequence. But if a very strong base
be united with a weak acid the solution will give au alkaline reaction, and
this is regarded as showmg the presence of free hydroxyl ions or of
hydrolysis. For instance, if the strong base, sodium, be united with the
weak acid, carbonic, and a water solution be made, it is held that hydrolysis
will take place to some extent, thus:
Na,CO,++-H,O=NaHCO,+NaOH.
It is supposed that NaHCO, breaks up into the ions Na and HCO,, and
the NaOH into the ions Na and OH. Therefore, in a solution of Na,CO,
in water the coexistent ions are thought to be H, HCO,, Na, and OH.
Since the base, NaOH, is stronger than the acid, HCO,, the separation into
the ions is thought to be the explanation of the alkaline reaction.
Cameron has shown that sodium silicate in solution gives an alkaline
reaction, and his explanation is that this compound is hydrolized in a man-
ner precisely similar to that of sodium carbonate.’ Not only do solutions
of sodium silicate give alkaline reactions, but Clarke has shown’ that many
natural mineral silicates, when treated with pure water, show an alkaline
reaction. The following gave permanent alkaline reactions: Phlogopite,
oligoclase, albite, cancrinite, sodalite, analcite, natrolite, pectolite, apophyl-
lite, egirite. The following gave more or less distinct colorations to the
phenolphthalein indicator, but in time faded: Muscovite, lepidolite, ortho-
a Ostwald, W., Grundlinien der anorganischen Chemie, Engelmann, Leipzig, 1900, pp. 254-257.
>Cameron, F. K., Application of the theory of solutions to the study of soils: Rept. No. 64, Field
Operations of the Division of Soils, 1899, U.S. Dept. of Agric., 1900, p. 169.
- ¢Clarke, F. W., Alkaline reaction of some natural silicates: Jour. Am. Chem. Soce., vol. 20, 1898,
pp. 739-742. ;
HYDROLYSIS OF COMPOUNDS. 87
clase, leucite, nephelite, spodumene, scapolite, laumontite, stilbite, chabazite,
heulandite, thomsonite.” If the theory of dissociation be true, this shows that
the silicates are hydrolized, thus:
R,SiO, +4HOH=2R(OH),+H,Si0,.
However, according to Kahlenberg and Lincoln, the H,SiO, does not dis-
sociate into the radicals H and SiO,, but forms colloidal silicic acid.2 Thus
this compound is inert and the reaction reverses only to a small extent and
under favorable conditions, and the hydrate of the alkali metal gives an
alkaline reaction.
In a similar manner hydrolysis is held to occur in water solutions of the
strong base sodium with the weak acid hydrosulphuric, thus:
Na,S+H,O=NaHS+Na0OH.
Cameron further states that hydrolysis is to be expected in the case of the
aluminates and ferrates.° When a strong acid is united with a weak base
the solution gives an acid reaction, and this is also explained by dissociation,
the free acid supposed to result from hydrolysis being stronger than the
weak base.@
Since the three most abundant acids of nature are silicic, carbonic, and
hydrosulphuric, all weak, hydrolysis, if true, is a reaction of fundamental
importance in metamorphism.
REACTIONS.
When, after a number of chemical substances are brought together,
and especially when they are united by a solvent, interactions between
them may occur which after a time appear to cease. When the conditions
have become such that there is no increase or decrease in the amount of
any one of the chemical compounds, the system is in a condition of
chemical equilibrium.’ When two substances in solution, A and B, react
upon each other so as to produce two other substances, C and D, if
solutions of C and D are mixed they in turn will react upon each other to
«Clarke, cit., pp. 740-741.
> Kahlenberg, L., and Lincoln, A. T., Solutions of silicates of the alkalies: Jour. Phys. Chem., vol.
2, 1898, pp. 77-90.
¢Cameron, cit., p. 169.
@ Ostwald, Grundlinien, cit., pp. 276-278, 397.
éNernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
pp. 355-356.
88 A TREATISE ON METAMORPHISM.
produce more or less of the substances A and B.* That is, the reaction is
reversible to a greater or less degree. To illustrate, if two solutions, one
of them containing MgSO, and the other Na,CO,, come together, the ions
are Mg, Na, SO, and CO;. Nernst, W., Theoretical chemistry, translated by C. 8. Palmer, Macmillan & Co., London, 1895,
pp. 358-360.
¢ Nernst, cit., p. 357.
@ Nernst, cit., p. 391.
HETEROGENEOUS SYSTEMS. 91
in which each phase is present in the system.”* To illustrate, if an excess
of salt be in a solution, so that it is saturated, and an additional amount
of salt be added, this does not in the least change the quantity of salt
held in a given volume of the solution. Therefore the equilibrium in a
saturated solution is independent of the amount of undissolved salt in the
solution. It follows that in a heterogeneous system ‘the condition of
equilibrium is independent of the relative mass of each of the phases.”’ A
simple case of heterogeneous equilibrium is that between ice and liquid
water, or between liquid water and water vapor. ‘For a definite external
pressure there corresponds a definite temperature at which the two systems
can exist beside each other; thus ice and water are coexistent at atmos-
pheric pressure at 0° C.; and liquid water and water vapor, at atmospheric
pressure and at 100° C. If we change the external pressure, at a tempera-
ture which is kept constant, or if we change the temperature, at an external
pressure which is kept constant, then the reaction advances”’ to equilibrium
in one direction or the other. ‘The process is ended as soon as the
expansive force of the evaporating or dissolving substance is held in equi-
librium by the gas pressure of the vaporized molecules or by the osmotic
pressure of the dissolved molecules, respectively.”°
NATURE AND SPEED OF REACTIONS.
The fundamental principle of chemical dynamics is that chemical action
4 This is the law of mass action.
is proportional to the active mass.
The speed of a chemical reaction which occurs under any given con-
ditions depends upon the compounds, the strength of the solutions, the
mechanical action, and the beat. Hence each of these features requires con-
sideration.
THE COMPOUNDS.
The reactions depend upon the compounds present, or, in other words,
upon the nature of the ions composing them; for the conditions under which
two ions, A and B, unite may be different from those under which one of
these ions will unite with a third, as A with C, or different from those under
which two other ions, C and D, unite. In order that ions shall unite in
solution they must meet or come within the limits of molecular attraction of
4 Nernst, cit., pp. 391-392.
> Nernst, cit., p. 393.
¢ Nernst, cit., p. 403.
@Ostwald, W., Outlines of general chemistry, translated by James Walker, Macmillan & Co., 2d ed.,
London, 1895, p. 292.
92 A TREATISE ON METAMORPHISM.
one another under certain definite conditions which are peculiar to each sub-
stance. Therefore not every time such a meeting occurs are compounds
formed. The ratio between meeting and union in the case of any two com-
pounds is a constant, which can be compared with the constant of any
other two compounds, each pair of which has its constant. This is merely
another statement of the old law that different substances have different
affinities for one another, and it is well known that the chemical affinities
are developed only when the molecules are in immediate contact with one
another.
The ions which are present in ground waters in any given case
largely depend upon the character of the adjacent rocks. In a lime-
stone region, for instance, the water may quickly take into solution all
the calcium and magnesium it can hold, considering the acids present.
Under such circumstances the acid ions will be mainly balanced by the cal-
cium and magnesium. The other substances, such as sodium and _potas-
sium, perhaps in more readily soluble forms than the calcium and magnesium,
will be largely kept from going into solution, or if in solution will be partly
thrown down, because these substances are obliged to compete for the acid
radicals with the vastly greater number of calcium and magnesium molecules.
Is it not possible that the agricultural advantage of having calcium and mag-
nesium abundantly in the soil is largely, or at least partly, due to the fact
that the presence of these soluble substances in abundance prevents the
solution and washing out of the elements potassium and sodium which the
plants need?
Ostwald divides the bases into strong, moderately strong, and weak."
The alkalies and alkaline earths, with the exception of magnesium, are
strong bases; magnesium is a moderately strong base; iron and aluminum
are weak bases—ot the two aluminum is the weaker. It follows that,
other things being equal, in underground solutions the alkalies and alkaline
earths, with the exception of magnesium, largely take possession of the acids.
To a less extent this is true of magnesium, and to a still smaller degree of
iron and aluminum. Thus we have the partial explanation of the relative
solubilities of the bases in the belt of weathering. In this belt the alkalies
are dissolved to the greatest extent; next in order comes calcium, then
magnesium, and finally iron and aluminum. (See p, 518.)
«Ostwald, W., The scientific foundations of analytica! chemistry, translated by George McGowan,
Macmillan & Co., London, 1895, pp. 55-86.
WEAKNESS OF ACIDS COMPENSATED BY QUANTITY. 93
Ostwald divides the acids into strong, moderately strong, weak, and
very weak. The acids H,SO,, HCl, and HNO,, are strong acids. The
acids H,SO, and H,PO, are moderately strong. The acids H,S, H,BO,,
and H,CO, are weak acids. The acids of silica are very weak.”
The strong acids H,SO,, HCl, HNO,, when present in ground solu-
tions, as they sometimes are, of course take possession of the bases in
proportion to their quantity. However, in the crust of the earth strong
acids are not abundant on the average, although under exceptional con-
ditions, as in voleanic districts, they may be rather plentiful. Also the
moderately strong acids H,SO, and H,PO, are not abundant, although
phosphoric acid is rather widespread. Of the weak acids H.S and H,BO,
are not plentiful.. The two ereat acids of nature are carbonic and silicic
acids, and the major contest in the rocks, so far as the acids are concerned,
is between the weak carbonic acid and the very weak silicic acid. These
two acids are everywhere very abundant in the rocks. While, therefore,
the moderately strong and the strong acids play a relatively important
part in proportion to their quantity, one weak and one very weak acid,
because of their dominant quantity, under the law of mass action play
the greatest part in rock alterations; and in the contest the very weak
acid, silicic, holds its own against the weak acid, carbonic, partly because
its far greater abundance compensates for its relative weakness. The fact
of the formation of carbonates and the simultaneous decomposition of
the silicates under surface conditions the world over is well known.
(See pp. 163, 473-486.) The partial explanation of the phenomena is the
relative abundance of carbonic acid under the conditions. in the zone of
katamorphism. As shown in another place (see p. 479), the reaction is also
one which liberates heat, and this is a favorable factor in the process.
In the zone of anamorphism, where the pressure is great, the reaction
of the upper zone is reversed. (See pp. 173-178, 677-679.) The replace-
ment of carbonic by silicic acid results in decrease in volume (see p. 177).
Therefore, under the great pressures of the zone of anamorphism,
the relative volumes of the original and secondary compounds is a most
important, probably dominant, factor in the process. But also it is
probable that at the high temperatures and pressures which obtain in the
lower zone silicic acid gains strength as compared with carbonic acid.
a Foundations, cit., p. 55.
94 A TREATISE ON METAMORPHISM.
It may under these conditions be a stronger acid than at the surface,
and if this were the case the reactions would be partly explained.
Bearing in this direction is the experiment of Bischof, who has shown
that at 100° C. silicic acid, when present in abundance, may partially
replace carbonic acid of carbonates."
Ostwald’s explanation of the varying strength of the bases and acids is
based on the varying amount of supposed dissociation.
The velocity of a reaction is proportional to the masses of the active
components, and according to Ostwald these are the free ions. Therefore
the speed depends upon the number of free ions which are acting. But
the number of free ions which are present is dependent upon the degree
of dissociation, and in this matter different compounds vary greatly.
Therefore the degree of electrolytic dissociation of the va ‘ous bases and
acids determines their respective strengths and is “the measure of the
reaction capacities of all substances.” ”
From this it follows that an acid or base which is strongly dissociated
is stronger than, or, in other words, is able to largely replace, an acid or
base which is but slightly dissociated; for the number of free ions of the
stronger compound far exceeds that of the weaker. It therefore becomes
important, from Ostwald’s point of view, to know the comparative strength,
or the relative amounts of dissociation, of the abundant bases and acids
which occur in the rocks. According to Ostwald the strong bases and
strong acids may be largely dissociated; the moderately strong bases and
acids under ordinary conditions are dissociated to a much less extent; the
weak acids, carbonic, hydrosulphuric, and boric, are usually not dissociated to
the extent of 1 per cent; silicic acid under ordinary conditions is scarcely
dissociated at all.
STRENGTH OF THE SOLUTIONS.
Saturated and strong solutions are more active than weaker solutions;
for the amount of the active compound increases with the concentration,
but not in a simple ratio. Weak solutions are relatively more active than
strong solutions, and by those who believe in dissociation this is attributed
to their nearer approach to complete dissociation; but the greater relative
activity of weak solutions never compensates fully for the greater dilution.
“ Bischof, Gustay, Elements of chemical and physical geology, translated by Paul and Drummond,
Harrison & Sons, London, 1854, vol. 1, p. 6.
> Nernst, W., Theoretical chemistry, translated by C. 8. Palmer, Macmillan & Co., London, 1895,
p. 440.
EFFECT OF QUANTITY OF ELEMENTS. 95
Although, as just seen, strong bases and acids have a great advantage
over weak bases and acids, the quantity of an element present is a very
important factor in the final result of the action of the solutions on the solids.
If a certain element is abundant in the ground solutions, it may to a large
extent replace another element in the solids, an element of the solid going
into solution at the same time. This may take place to a large extent even
if the element in solution is weaker than the one it replaces in the solid.
For instance, the relatively weak base, magnesium, when abundant in
solutions, is known to replace the stronger base, calcium, on a large scale
in calcium carbonate, thus changing limestone to dolomite. In this reaction,
while the abundance of magnesium is a very important factor, a number of
others enter; and therefore its detailed consideration is given under the
process of rock dolomitization. (See pp. 802-808.)
MECHANICAL ACTION.
It has already been seen that no changes in rocks take place without
movements of material, small or great, for long or short distances. Even
in the case of a mineral passing from one form to an allotropic form, there
is movement of the molecules. In short, wherever there is rearrangement
of the elements there must be movements.
Mechanical action alone is one of the processes of metamorphism of
the utmost importance. (Sée pp. 46-50.) However, the effect of mechan-
ical action in the promotion of chemical action is even more important
than mechanical action alone.
Mechanical action influences chemical action in two general ways—the
speed is promoted, and the nature of the reaction is modified.
SPEED OF CHEMICAL ACTION.
The speed of chemical action is promoted directly by the deformation,
and indirectly by the heat liberated.
DIRECT DEFORMATION EFFECT.
As already shown (pp. 49-50), mechanical action produces deformation
in three different ways—by producing strain without rupture, strain with
rupture, and readjustment of the particles
Strain without rupture— When material is strained without rupture, even if
the amount of deformation be slight, a great change in the molecular con-
stitution may be involved. This is well shown by a common experiment
96 A TREATISE ON METAMORPHISM.
on glass. If a piece of glass, free from stress, be placed under the micro-
scope with crossed nicols, the light is cut off because the glass is isotropic.
If, however, the glass be slightly flexed, well within the elastic limit, it
immediately becomes anisotropic, and brilliant colors flash out. So far as
light is concerned—and this is one of the best agents for giving an insight
into the molecular constitution of bodies—the strained glass behaves wholly
different from unstrained glass. Evidently when glass is alternately strained
and freed from strain it undergoes a profound change in molecular consti-
tution. The greatness of the molecular change in material when strained
within the elastic limit is dwelt upon to show that such changes might
greatly affect chemical action; and it will be seen below that the facts
correspond to this expectation.
Barus has shown“ in the case of metals strained to the point of rupture
that a considerable per cent of the energy expended in straining them is
potentialized; in ‘ glass-hard” steel 50 per cent, in brass 40 per cent, in
copper 25 per cent. A larger percentage of the energy was potentialized
in the earlier stages of strain than in the later stages. By stating that
energy is potentialized is meant that the mechanical equivalent in heat of
the work done on the metals was only partially developed; the remainder
of the energy is stored up in the strained metals. Now, considering a brittle
substance which is analogous in physical characters to rocks, Prince Rupert
drops, the explosion of a drop when a point is broken shows that a large
amount of energy is potentialized, or that the glass is in a high state of
strain. The experiments of Barus and the condition of the Rupert drop
show that in strained materials energy is probably potentialized. If this
be true, must it not be the case that the atoms and molecules of a strained
body are in a more than ordinarily favorable condition for chemical action?
Bodies in which energy is potentialized are believed to be in an
exceptionally favorable condition for chemical action. For instance, if a
strained metal, in which on that account more than the usual amount of
energy is stored, be dissolved in an acid, less than the usual amount of
chemical energy is expended, for the resultant salts in the solution have the
same energy of combination in each case. But in the strained metal work
has been done, the equivalent of which has not escaped as heat during
strain, and is therefore stored energy. Therefore this energy is available
@ Barus, C., The mechanism of solid viscosity: Bull. U. 8. Geol. Survey No. 94, 1892, pp. 107-108.
STRAINED MINERALS EASILY DISSOLVED. ST
to assist the chemical reaction. That it is utilized is shown by the fact
that the heat of combination of the resultant chemical compound must
be the same whatever the condition of the metal. Hence less chemical
energy is required for the solution of a strained metal, and the reaction is
promoted by the state of the strain.
The validity of this reasoning is dependent upon the principle of the
conservation of energy. Asa result of my studies in the phenomena of
recrystallization,” I became convinced that strained minerals are more
readily acted upon by underground solutions than unstrained minerals.
(See pp. 690-692.) Barus’s experiments already cited suggested the above
explanation. I then predicted that experiments would show that strained
metals are more readily acted upon chemically than unstrained ones, and
asked that this prediction be tested experimentally. This Mr. Harmbuechen
has done in reference to iron, with the following results:
The application of stress to metals causes an increase in chemical activity, this
increase being especially marked after the elastic limit has been reached.
It is possible to get a curve showing the relation of electro-motive force to
strain which is similar to that of stress to strain.
There is a definite relation between the electrical potential of iron toward an
electrolyte and the amount of energy stored up in the metal through the application
of stress.’
Thus complete experimental confirmation of this prediction is made so
far as iron is concerned; and it can hardly be doubted that this illustrates
the general principle above given.
Applying the above principles to strain and chemical action, it may be
said that im so far as minerals are strained either within or beyond the
elastic limit, this potentializes energy and puts such minerals into a
condition more favorable for chemical reactions than unstrained minerals.
All rocks, except at the very surface of the earth, are under stress, and
therefore strained to some extent at all times. It is true that the amount
of stress may not be great within a few meters of the surface; but with
increase of depth the average amount of stress becomes more important.
In most cases of ordinary horizontal rocks near the surface it is customary
«Compare Van Hise, C. R., Metamorphism of rocks and rock flowage: Bull. Geol. Soc. America,
vol. 9, 1898, p. 300.
> Hambuechen, Carl, An experimental study of the corrosion of iron under different conditions:
Bull. Univ. of Wisconsin No. 42 (Engr. ser., vol. 2, No. 8), 1900, p. 255.
MON XLVII—04——7
98 A TREATISE ON METAMORPHISM.
to regard them as practically free from stress and strain. However, not
infrequently rapid deformation by uplift of an arch or by fracture when a
few meters of load is removed, as at the Chicago drainage canal and at
the combined lock of Appleton, shows that such rocks are under very con-
siderable stress, and therefore must be strained.
Not only are rocks generally under stress, but because of the com-
plexity and variability of rock compositions, structures, and textures,
wherever rocks are under stress the amount of stress and therefore of strain
continually varies with changing direction and changing position. Variable
amount of strain is therefore a universal law. In so far as any mineral
particle is strained to a greater degree than an adjacent mineral particle of
the same kind similarly strained, the particle under greater strain is more
rapidly altered by chemical action. In so far as any portion of a mineral
particle is strained to a greater degree than another portion of the same
particle similarly strained, the part under greater strain is more rapidly
altered by chemical action. Finally, for the same mineral particle or some
part of the same the strain varies continually during deformation.
From the foregoing it follows that the almost universal state of strain,
and the not less universal variability in the amount of strain, are of the
most profound significance in metamorphism. (See Chapters VI, VI, VII.)
Strain with rupture— Where deformation produces rupture, another feature
enters, also favorable to chemical action. Rupture is favorable to chemical
action since thereby the surface exposed to the underground waters is
inversely as the average diameter of the mineral particles. Granulation
very greatly increases the surface of action.
Readjustment of particles. — The readjustment of the rock particles with refer-
ence to one another can hardly fail to give better opportunities for the
chemical action of the ground waters; for during the adjustment the
water will necessarily be moying and will come in contact with a succession
of mineral particles, and thus promote chemical interchange. Hence I
conclude that mechanical action is favorable to metamorphism by chemical
action, whether the deformation be strain without rupture, with rupture, or
merely readjustment of the rock particles, or, finally, any combination of
these.
«Cramer, Frank, On the rock fracture at the Combined Locks mill, Appleton, Wis.: Am. Jour.
Sci., 3d ser., vol. 41, 1891, pp. 4382-434.
HEAT PRODUCED BY MECHANICAL ACTION. 39
INDIRECT HEAT EFFECT.
It is a well-known law that mechanical action develops an equivalent
amount of heat, except for the part of the energy which is potentialized,
It has already been seen that heat is ordinarily favorable to chemical
action. Therefore mechanical action promotes chemical action, because it
develops heat and raises the temperature. Indeed, the heat developed by
mechanical action is frequently one of the most important favorable con-
ditions for metamorphism. It will be shown (Chapter VIII, p. 740) that
where mechanical action is strong the complete recrystallization of rocks
may occur much nearer the surface than under quiescent conditions. This
result is largely attributed to the rise in temperature due to deformation,
which results in vastly greater efficiency of the water as an agent of
chemical action.
It therefore becomes of the utmost importance to consider to what
extent the temperature is raised in the rocks by mechanical action.
The heat, as already intimated, is produced by the transformation of
work into heat as a result of straiming the rock particles within the elastic
limit, by rupturing them, and by their frictional movements over one
another. Mallet” has held that the heat thus developed may be sufficient
to liquefy rocks by aqueo-igneous fusion. He thus accounts for the crys-
tallized cores of many mountain ranges. He even holds that the material
fused by mechanical action may intrude the adjacent solid rocks. LeConte
follows Mallet in this belief. It may be theoretically possible that rock
material can be ground so fine as to develop sufficient heat to fuse it.
However, as explained (Chapter VIII, pp. 728-732), we have no evidence in
the field that this has occurred. It is shown (Chapter VIII, pp. 690-696),
that when the temperature of water-saturated rocks rises a certain amount,
readjustment occurs, not by mechanical subdivision and grinding of the
particles over one another, but by recrystallization. The process is thus
chemical, not mechanical, and the expenditure of energy and the conse-
quent development of heat are tar less than by the former process. How-
ever, it is probable that, as a result of the interior kneading of rocks, the
temperature may be materially increased, perhaps several hundred degrees
beyond the normal temperature which obtains as a result of the depth of
«Mallet, Robert, Volcanic energy; an attempt to develop its true and cosmical relations: Philos.
Trans. Royal Soe. London, vol. 163, 1873, pp. 147-227.
i100 A TREATISE ON METAMORPHISM.
burial. And it is certain that the temperature can be very materially
increased, and therefore that the chemical activity is enormously increased.
NATURE OF THE CHEMICAL REACTIONS.
Pressure influences chemical reactions under the following law: If a
chemical system be compressed at a constant temperature, there follows a
displacement of the equilibrium in that direction, which is associated with
a diminution of volume. This law in relation to pressure and chemical
activity may be stated in a more general form, as follows: ‘‘Those chemical
forces are strengthened by compression which condition a diminution of
volume; and those chemical forces are weakened by compression which
condition an increase in volume.”“ In other words, so far as pressure
influences chemical reactions, changes go on in directions which produce
smaller volumes. Therefore pressure at all times and places is influencing
chemical reactions in the direction of the production of more condensed
svstems. It has been seen (Chapter II, pp. 48-49) that pressure alone,
without the presence of solutions, may produce reactions under this law.
However, in nature, the vast majority of reactions under the law are
accomplished through the agency of water. The importance of water in
this connection is well illustrated by Spring’s experiments upon the con-
solidation of clay when dry and wet. By pressure upon moist clay
confined in a cylinder he was able to consolidate the clay into a body as
compact as a piece of shale—indeed, so compact that it was difficult to
scratch it with the finger nail. But using the same pressure upon dry clay
he produced a substance so little consolidated that it was easily scratched
with the finger nail. In the case of the moist clay, he attributed the consoli-
dation to the escape of the plastic material about the piston, and to the
precipitation of material from solution at the moment of escape.’ Spring’s
explanation therefore does not introduce chemical readjustment of the com-
pounds. However, it will be seen that pressure does promote chemical
interchange, producing compounds which are, on the average, denser than
the original ones. This, as will be shown on the following pages, is
believed to be a dominant process for a great many chemical reactions
p- 567.
>Tolman, C. F., jr., Professor Spring on the physics and chemistry of solids: Jour. Geol., vol. 6,
1898, p. 323.
CHANGE OF VOLUME BY RECRYSTALLIZATION. 101
resulting from pressure as the chief motive force; and it may be that
chemical interchange is one of the processes which explain the consolidation
of the clay in Spring’s experiment.
In the rocks a smaller volume may result in either of two ways:
Material may be taken into solution and deposited in a more compact
form without change in chemical composition, or with change in chemical
composition.
SMALLER ROCK VOLUME AS THE RESULT OF SOLUTION AND DEPOSITION WITHOUT CHANGE IN CHEMICAL COMPOSITION.
It has already been explained (pp. 77-78) that pressure promotes solu-
tion in case the volume of the solution is less than that of the solvent and
solid, and that pressure promotes precipitation in case the volume of the
solution is greater than that of the solvent and solid. Thus the solubility
of a salt increases with pressure, provided the dissolving is associated with
a contraction of the volume of the solution plus the salt; and, conversely,
the solubility decreases if the separation of the salt (from the solution) is
associated with a diminution of the volume of the system.“ In ground
solutions the general law is that the volume of the solution is less than that
of the substances dissolved and the water. It follows from this law that
pressure in rocks, the interstices of which are filled with water, promotes
recrystallization and condensation.
The production of a smaller rock volume without change in chemical
composition may occur where the recrystallization and condensation take
place without change of minerals, and where the reerystallization and
consolidation take place with change of minerals.
Recrystallization and condensation without change of minerals— As an illustration of the
principle, we may consider a stratum of unconsolidated crystallized calcium
carbonate over which is a layer of water saturated with calcium carbonate.
Inasmuch as the calcium carbonate is porous, the water in the rock is free
to move and is under the pressure of the hydrostatic column above it.
The particles of CaCO, are under this pressure, and also that of the solid
above All the water in the crevices and pores small enough to hold water
by eapillarity is under both the pressure of the water and in part that of
the rock. This water is saturated under this pressure, and it can hold more
p. 567.
102 A TREATISE ON METAMORPHISM.
is constantly carried on between the free and the capillary water, and as the
capillary water becomes free it is supersaturated and deposits some of its
load in the interstices of the rock. But gravity ever pulls the material
downward, and although this process is not rapid, it is continuous, and in
course of time the particles are cemented. A solidified and recrystallized
limestone is produced. Evidently the greater the pressure the more rapid
and complete is this change.
Another example of ‘solidification without change in mineral composi-
tion is the change of snow or separate ice crystals where mingled with
water to solid ice, as at the head of glaciers. Ice has its melting point iow-
ered by pressure. Where the granules are under more than the average
pressure some of them melt. The water flows out into the free spaces and
is again frozen. Or, as expressed above, under more pressure more of the
ice is dissolved in the water than under less pressure. When the pressure
is relieved in the more open spaces the ice is reprecipitated.* As the
process goes on the particles are finally cemented. This process, like that
of the recrystallization of limestone, is continuous, and finally the separated
snow granules are transformed to continuous ice.
Recrystallization and condensation with change of minerals. —Recrystallization and con-
densation with change of minerals but without change in chemical composi-
tion may take place by precisely the same processes as already given. The
resultant minerals, where the inducing cause is pressure, are more compact
than the original minerals. Illustrating this principle, pressure induces
the transformation of amorphous calcium carbonate to calcite. Similarly,
pressure may induce the transformation of many other amorphous substances
to crystalline forms. Pressure also induces minerals to change to forms
having higher specific gravities. Thus pressure tends to transform tridymite,
sp. gr. 2.28-2.33, to quartz, sp. gr. 2.653-2.660; and marcasite, sp.
4.85-4.90, to pyrite, sp. gr. 4.95-5.10. (See pp. 220-221, 215.)
We may also safely argue that, where the pressure is great, minerals
or
gy.
are not likely to crystallize in forms having low specific gravities. ‘Thus
under great pressure it is to be expected that silica will crystallize as
quartz and not as tridymite. Doubtless this principle explains why
quartz is always found in the plutonic rocks, and why tridymite often is
aLe Chatelier, in Theoretical chemistry, by W. Nernst, p. 654. Zeitschr. phys. Chemie, vol. 9,
1892, p. 335.
VOLUME DECREASED BY RECRYSTALLIZATION. 103
found in the voleanic rocks. The plutonic rocks crystallize under condi-
tions of great pressure, while the volcanic rocks crystallize under conditions
of moderate or slight pressure. It would be interesting to know the
relations of quartz and tridymite in the matter of depth in the lavas, and
therefore in reference to pressure at the time of crystallization.
SMALLER VOLUME AS THE RESULT OF SOLUTION AND REDEPOSITION WITH CHANGE IN CHEMICAL COMPOSITION.
Pressure inducing chemical reactions involving changes in cheiical
composition may produce crystallization and condensation of amorphous
compounds and recrystallization and condensation of crystallized com-
pounds.
Crystallization and condensation of amorphous compounds.— ]}) general the amorphous
compounds occupy more volume than their complex crystalline equivalents.
“Therefore, since the crystallized state is generally that which takes the
smallest volume, pressure aids crystallization.”“ According to Delesse, in
passing from the crystalline to the glassy state, granite decreases in density
9 to 11 per cent, syenite 8 to 9 per cent, diorite 6 to 8 per cent, dolerite 5
to 7 per cent, and trachyte 3 to 5 percent.’ Thus glass occupies from 3 to
11 per cent more volume than the equivalent crystallized rocks. It there-
fore follows that pressure is one of the potent forces which result in the
devitrification of glass. In general it may be said that rocks near the
surface, whether original magmas, sediments, or schists and gneisses partly
altered in the belt of weathering, very frequently contain amorphous prod-
ucts; whereas rocks which have been altered while deeply buried rarely
contain any considerable quantity of amorphous material. It is believed
that the explanation of the difference is largely due to difference in. pres-
sure. At depth where pressure is forceful the amorphous products which
occupy more space than their crystallized equivalents either have not -
formed or if formed at the surface and deeply buried have become ecrystal-
lized, the pressure being one of the important forces in the process.
Reerystallization and condensation of crystallized compounds. — Pressure may induce chem-
ical action upon crystallized compounds, producing recrystallized products
of a different kind and with more compact molecules, and therefore of
greater specific gravity. In some cases the recrystallization has occurred
«Tolman, C. F., jr., Professor Spring on the physics and chemistry of solids: Jour. Geol., vol. 6,
1898, p. 320.
>See Dana, J. D., Manual of geology, American Book Co., 4th ed., 1895, p. 265.
104 A TREATISE ON METAMORPHISM.
at least twice. After one set of compounds was produced recrystallization
again occurred, producing heavier compounds. The first change may be
illustrated by the rearrangement of minerals which constitute mud so as to
produce mica, quartz, and feldspar, and the second stage may be illustrated
by the development from the latter rock of the still heavier minerals, garnet,
staurolite, ete. (See p. 685.) However, the process of recrystallization
in nature works in connection with more rapid solution of minerals where
strained (see pp. 95-98) and with other forces. Its full consideration is
therefore deferred to Chapter VIII (pp. 686-698).
GENERAL STATEMENTS.
Where pressure is unimportant, as near the surface of the earth, the
chemical reactions are ordinarily controlled by other factors than pressure;
but as the pressure increases, due to depth below the surface or other causes,
it becomes a more and more important factor in the reactions which occur.
But it is shown in Chapter VI, on ‘‘ Weathering,” that pressure may be an
important factor in chemical reactions comparatively near the surface. This
is illustrated by granitic rocks in the District of Columbia, described by
Merrill,” which when brought to the surface underwent rapid disintegration,
hydration, and expansion. The pressure of a few feet of rock was appa-
rently sufficient to prevent the completion of these reactions, and thus it is
clear that the adjustment between chemical reaction and pressure may be
very delicate.
However, as explained in Chapter VI, chemical reactions near the sur-
face do extensively take place with expansion of volume, and therefore in
spite of some pressure. But it is also shown (Chapter VIIT) that the
pressure becomes a more and more potent factor in controlling the reac-
tions; and, finally, that there exists a lower zone of anamorphism in which
this is the dominant force. In this zone the chemical changes so take place
as to lessen the volume of the compounds, and therefore to produce heavy
minerals. Moreover, the reactions which occur in this lower zone are
frequently just the reverse of those which take place in the upper zone,
where pressure is unable to control, or the reactions in the two zones
reverse each other.
“ Merrill, G. P., Disintegration of the granitic rocks of the District of Columbia: Bull. Geol. Soc.
America, vol. 6, 1895, pp. 322-332. ‘2
THE HEAT OF SOLUTION. 105
ina
Heat is a very important factor in chemical action. In the heat factor
two points are involved: first, the general effect of heat; and, second, the
effect of change in temperature in consequence of the reactions.
As to the first of these, in the lithosphere the higher the temperature in
general the more rapid the alteration. To this law there may be excep-
tions, but none are positively known to me.
As to the second point, the chemical effect due to the change in tem-
perature in consequence of a reaction is much more complicated.
In considering whether heat be liberated or absorbed as a result of a
chemical reaction it is necessary to take into account the heat changes in
solution, the heat changes in precipitation, the heat changes in mixing
solutions, and the heat effects of chemical reactions.
‘By the heat of solution is meant the quantity of heat produced by
the solution of 1 gram molecule of a substance in a large quantity of the
solvent.”” It has already been seen that in general the volume of the
solvent and salt is greater than that of the solution, and that in this case
there is usually liberation of heat and consequently rise in temperature;
but in exceptional cases the volume of the salt and solvent is less than that
of the solution, and im this case there is generally absorption of heat, and
consequent fall in temperature. The total effect as to the liberation or
absorption of heat depends upon whether the total of the factors, change
in volume, change of the solid to its dispersed form in the solution, and
the heat factor of dissociation, provided this occurs, is plus or minus.
Decrease of volume tends to liberate heat; increase of volume tends to
absorb heat. The change from the solid to the dispersed state of solution.
absorbs heat. The supposed dissociation of a substance into its ions is
regarded as attended with either a liberation or an absorption of heat,
though liberation is held to occur more frequently.’
In precipitation the heat effect is just the opposite from that of solution
and is equivalent to the heat effect of the solution of an equal amount of
the like salt. ‘In general, in comparing substances which are chemically
analogous and soluble with difficulty, the heat of precipitation (=the
negative value of the heat of solution) is the greater the more insoluble
«Nernst, W., Theoretical chemistry, translated by C. 8. Palmer, Macmillan & Co., London,
1895, p. 503.
DNernst, cit., p. 562.
106 A TREATISE ON METAMORPHISM.
the substance is.”” If this law be applicable to quartz and to silicates it is
of great importance in metamorphism, because these are the substances
most largely dissolved and deposited by the ground water, with the pos-
sible exception of the carbonates.
As to the heat relations when two solutions are mixed, Ostwald states
that in mixing solutions heat is produced by the work between the hetero-
geneous molecules, and heat is used in separating and spreading out the
homogeneous molecules. The sum of these may be positive or negative,
but in most cases the former is the case, and hence the two liquids usually
become warmer when they are mixed.’ Upon the same point Nernst says:
“No heat phenomena result from the mixture of salt solutions [provided
that no precipitate (and no volatile compound) is produced ].”*
When chemical reactions occur there is a certain amount of heat of
formation of the compounds. “By the ‘heat of formation’ of a chemical
compound is meant the quantity of heat which is given off in the formation
nd
of the compound from its respective ingredients. “The ‘heat-toning’ of a
reaction is equal to the sum of the resulting heats of formation minus the
”@ In whatever
sum of the heats of formation of the vanished molecules.
way a chemical result is accomplished, and however many the stages of
process of the change, ‘the energy differences (and therefore the heat dif-
ferences) between two identical conditions of the system must be the same,
independently of the way by which the system is transferred from one
condition to the other.”
This last is an important law so far as the work of ground waters is
concerned, for in most cases we know only the opening and closing stages
of the processes of alteration, and can ascertain whether the heat effect of
a reaction is plus or minus only by comparing the heat required for the
production of the original minerals with that required for the production of
the secondary minerals, and their gaseous and fluid by-products.
While the above gives the conclusions as to the heat effect of indi-
vidual reactions, the reaction which is likely but not certain to obtain at
moderate pressure and temperature is covered by the rule of Berthelot.
He says, ‘Every chemical change gives rise to the production of those
«Nernst, W., Theoretical chemistry, translated by C. 8. Palmer, Macmillan & Co., London, 1895,
p- 504.
>bOstwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., New York,
1891, p. 308.
¢Nernst, cit., p. 508. 4 Nernst, cit., p. 505." eNernst, cit., p. 496.
CHEMICAL REACTION AND HEAT. 107
substances which occasion the greatest development of heat.”* And there-
fore, ‘other things being equal, there is the more chance that a substance
can be formed, the greater its heat of condensation.”’ While these are the
usual rules, they are not broad enough to cover the reactions of meta-
morphism under all pressures and temperatures. A more general statement
of the law as to the relations of heat and chemical reactions is that of van’t
Hoff: ‘On the whole, the preponderating chemical reactions at lower tem-
peratures are the combinings (associations) which take place with a devel-
opment of heat, while the reactions preponderating at higher temperatures
are the cleavings (dissociations) which take place with the absorption of
heat.”*° The meaning of this law may be illustrated by the following
reactions: At ordinary temperatures CO combines with O, producing CO,,
with great liberation of heat; at very high temperatures CO, dissociates
into CO and O, with very great absorption of heat. This illustration
makes it clear, as stated by Nernst, that to cover all cases van’t Hofft’s
law must replace that of Berthelot, above given. Still more general laws
as to the relations of heat and chemical reactions are the following: “If
we heat a chemical system, at constant volume, then there occurs a
displacement of the state of equilibrium, and in that direction towards
which the reaction advances with absorption of heat.”” “Those chemical
forces which condition a development of heat will always be weakened by
an increase of temperature; and, conversely, those which condition an
absorption of heat will be strengthened by such an increase in tempera-
ture; and it is this fact which, primarily, gives the preceding proposition
its universal validity.”
“Tf we heat the system, therefore, the reaction
which takes place will be accompanied by absorption of heat; if we cool
the system, the corresponding reaction will develop heat.”*
Now that the general laws covering the mutual influences of heat and
chemical action have been given, we may consider in more detail their
meaning. The speed of the reaction is commonly increased much more
rapidly than the increase in absolute temperature. Thus, the speed of
reaction of two similar solutions, one of which is at higher temperature
«Nernst, cit., p. 581, quoting Berthelot.
> Nernst, cit., pp. 585-586.
¢ Nernst, cit., p. 583.
4 Nernst, cit., p. 566.
e Ostwald, W., Outlines of general chemistry, translated by James Walker, 2d ed., Macmillan &
Co., London, 1895, p. 312.
108 A TREATISE ON METAMORPHISM.
than the other, may be far greater in the solution at high temperature than
would be calculated from the relative absolute temperatures of the solutions.
Indeed, the velocity of a chemical reaction commonly increases
enormously with moderate increase of temperature. The partial explana-
tion of the phenomena lies in the fact that in most cases the reactions
themselves, as already seen, develop heat, which immediately reacts
to increase the kinetic energy of the remaining molecules, and this again
increases the kinetic energy of the molecules, and so on, there being
continual action and reaction between the chemical activity and the rising
temperature.
Another illustration of the very important way in which increase of
temperature increases chemical action is the increased activity of substances
which at low temperatures are relatively inert. While at ordinary tempera-
tures carbon dioxide replaces silica in silicates, at temperatures of 100° C.
silica, if present in abundance, may replace carbon dioxide in carbonates.“
While this is explained in part by the increase of activity of silicic acid
with increase of temperature, it doubtless in part is explained by the law
of mass action and the increased volatility of carbon dioxide at higher
temperatures.
If the dissociation theory be true, a third factor which may have some
effect in producing speed of reaction with increase of temperature is the
increase in the amount of hydrolysis with increase of temperature. This is
illustrated by ferric chloride, which at low temperatures is regarded as but
little hydrolized, but at high temperatures is believed to be hydrolized to a
perceptible extent according to the equation:
Fe,Cl;+6H,0=Fe,(OH),+6HCI.
The presence of the ferric hydroxide is shown by the color of the solu-
tion.’ In a similar manner, the carbonates and silicates are believed to be
hydrolized to a much greater extent at high temperatures than at low
temperatures. This is illustrated by calcium carbonate, which in solution
at high temperatures gives a strong alkaline reaction of calcium hydroxide,
and this is regarded as evidence of strong hydrolysis. It is possible that
hydrolysis is an important factor in the reactions which take place in the
different zones of metamorphism.
“Bischof, Gustay, Elements of chemical and physical geology, translated by Paul and Drum-
mond, Harrison & Sons, London, vol. 1, 1854, p. 6.
Ostwald, W., Grundlinien der anorganischen Chemie, Engelmann, Leipzig, 1900, p. 583.
CHEMICAL CHANGES ACCELERATED BY HEAT. 109
While in general, speed of chemical change is promoted by rise of
temperature, as indicated by the second part of van’t Hoff’s law, there is a
limit to the increase of speed due to action and reaction between chemical
change and heat, for when the temperature becomes too high a reverse
tendency is set up, since the compounds formed by the chemical reactions
frequently can not exist at very high temperatures. In such cases the rate
of reaction may cease to increase with increase of temperature, and, indeed,
the reactions which obtain at lower temperatures may be reversed.
Just as a slight increase of temperature may enormously increase the
speed of chemical reactions, so a slight decrease of temperature may very
greatly lessen the speed of reactions. Therefore, if the reaction be one
which itself absorbs heat, and thus lowers the temperature, the slight
decrease in the kinetic energy of the molecules may greatly retard the
speed of the reaction.
At the very moderate temperatures which generally prevail within the
outer part of the crust of the earth the heat resulting from the chemical
changes does not become so great as to stay the reactions. Therefore, it
may be said that the chemical reactions which take place with liberation of
heat promote metamorphism, and those which take place with absorption
of heat retard metamorphism. The great importance of these two tenden
cies, as applied to rocks, will be shown on subsequent pages in connection
with the discussion of the zones of katamorphism and anamorphism.
On subsequent pages it will be seen that in the zone of katamorphism
the first part of van’t Hoff’s law or the rule of Berthelot generally prevails
in the alterations of rocks for a considerable distance from the surface.
That is to say, on the whole the preponderating chemical reactions are
those which take place with the liberation of heat. Moreover, as a
consequence of increase of heat with depth, at a very moderate depth
the temperature is rather high. Also, igneous rocks give high temperatures
to the surrounding rocks and solutions. Asa result of any of these causes,
water may reach the moderate temperature of 100° to 200° C., and such
temperatures increase the activity of water in an amazing degree. (See
pp- 79-81.) Thus we see that in the zone of katamorphism the heat of
chemical action, and that derived from the interior of the earth through
conduction and convection by means of magma and water, all work
together to increase the speed of chemical action, and therefore to hasten
metamorphism.
110 A TREATISE ON METAMORPHISM.
However, it will also be seen that in the zone of anamorphism, with
pressure as a dominant factor, reactions very generally occur with the
absorption of heat under the second part of van’t Hoff’s law. Thus, in this
zone the heat effect of the chemical reactions is to stay metamorphism.
But while the reactions which occur at depth are very generally those
which absorb heat, it must be remembered that in the zone of anamorphism
the amount of heat available, due to increase of heat with depth and to the
difficulty with which the heat escapes from intrusive rocks, is very great.
Therefore, notwithstanding the fact that the chemical reactions themselves
absorb heat, the temperature is much higher than in the upper zone.
Consequently one would expect that the chemical activity would be
greater in the zone of anamorphism than in the upper zone of katamorphism;
and with these expectations the facts correspond. (See pp. 660-661, 690—
692, 749-751.)
RELATIONS OF CHEMICAL ACTION, MECHANICAL ACTION, AND HEAT.
All transformations of material upon the earth, provided all the energy
factors be taken into account, involve the expenditure of energy and the
dissipation of part of it as heat. If this were not true it would be possible
to manufacture an engine by means of which an equal or greater amount
of energy is available for work than is expended in driving the engine, and
perpetual motion would be possible. In metamorphism of rocks, in order
that the above general statement as to the expenditure of energy shall be
true, it is necessary to take into account the chemical force, mechanical
force, and heat which promote the transformations. In those cases where a
transformation of material does not at first sight appear to demand the
expenditure and dissipation of energy, this is due to the fact that some of
the energy factors are overlooked.
It has been noted that chemical actions are reversible, and it will be
seen subsequently that chemical reactions which take place on a large scale
in the zone of katamorphism are reversed in the zone of anamorphism.
When a chemical reaction takes place, and later that reaction is reversed
and the cycle is repeated, exterior energy must have been expended and
dissipated. To illustrate, let us consider the reversible reaction
Fe,0,+3H,02Fe,0;.3H,0.
The reaction may advance from left to right by the expenditure of
chemical energy alone, and as a result of the process heat is liberated.
CHEMICAL ACTION, MECHANICAL ACTION, AND HEAT. 111
However, to reverse the reaction or to advance it from right to left
requires the expenditure of a greater amount of external energy than the
chemical energy expended in the first reaction. In the reaction given
the available external energy may be from one of two sources—heat. or
mechanical action. The ferric hydrate may be broken into ferric oxide
and water by heating. Also, if the pressure be very great and water have
a chance to escape the same transformation may take place by the
expenditure of mechanical energy. Doubtless in nature in many cases
both of these forces unite in the process, but whether the dehydration takes
place as a result of the expenditure of heat energy alone or mechanical
energy alone, or the two combined, a greater amount of energy must be
expended than the chemical energy expended in the hydration of the iron.
Hence, when hydration takes place in the zone of katamorphism energy is
expended. This is potential chemical energy. When dehydration takes
place in the zone of anamorphism, reversing the first process, energy is also
expended. ‘This is either potential mechanical energy or the energy of
heat, or the two together. I say potential mechanical energy, for I have
held in another place that all earth movements, provided all the factors are
taken into account, result in bringing the material moved nearer the center
of the earth, and therefore the energy expended is the potential gravitative
energy of position.”
The reasoning applied to the case of hydration and dehydration of
ferric oxide is applicable to every other reversible reaction in metamor-
phism; hence, when we take all the energy factors into account, at the end
of the process energy has been expended. Furthermore, a part of this
energy during the process has been transformed to heat and dissipated; for
in all transformations of energy there is an inevitable tendency for some
of the energy to run down into the lowest form, heat, a portion of which
is lost.’
In conclusion, therefore, in the zone of katamorphism, while chemical
reactions frequently take place which liberate heat and expand the volume,
and in the zone of anamorphism chemical reactions take place which absorb
heat and condense the volume, in both zones alike when all of the energy
aVan Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11,
1898, pp. 487, 488, 512-514.
> Daniell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York,
1895, p. 51.
112 A TREATISE ON METAMORPHISM.
factors are taken into account the reactions take place in such a way as to
demand the expenditure of energy and the loss of a part of it.
Where chemical force, mechanical force, and high temperature work
together, with an abundance of water, as an agent of metamorphism, the
speed of rock metamorphism is very great as compared with the slow
alterations which occur at the surface of the earth. For instance, Barus
finds that water at temperatures above 185° C. and under high pressure is
capable of very rapidly uniting with glass, forming a new compound, which
at these temperatures is liquid, and which he calls water glass. In a retort
he combined 210 grams of glass and 50 grams of water in twelve hours
at a temperature of 210° C. into water glass, which was liquid at that
temperature, but became a clear solid at ordinary temperatures.”
Not only amorphous compounds but crystalline minerals also are acted
upon rapidly at such temperatures and pressures. At 180° C., with
pressure sufficient to keep the water in the liquid form, Lemberg? has
completely dissolved zeolites in pure water. Under similar conditions
it has also been shown that pure water acts rapidly upon powdered
anhydrous silicates. For instance, Forchhammer showed that water under
these conditions dissolves potassium silicate from powdered orthoclase.
Within the zone of rock flowage temperatures and pressures higher
than those with which these experiments have been made are available, and
it is therefore to be supposed that in the zone of anamorphism there is rapid
transformation of the minerals to forms which are relatively stable under
the conditions obtaining at any given time and place. So far as substances
have not a compact state of aggregation energy is potentialized. Pressure
being a very potent factor, the transformations would of course be into
condensed systems, or into minerals having high specific gravity and
probably complex molecular structure. It is evident that in the forces of
chemical action, mechanical action, and heat, and the agent, water, we have
adequate causes for the crystallization of amorphous compounds, for the
recrystallization of strained minerals, and for the recrystallization of highly
« Barus, C., The compressibility of liquids: Bull. U. 8. Geol. Survey No. 92, 1892, pp. 78-84.
Hot water and soft glass in their thermo-dynamic relations: Am. Jour. Sci., 4th ser., vol. 9, 1900,
pp. 164-65.
» Doelter, C., Allgemeine chemische Mineralogie, Wilhelm Engelmann, Leipzig, 1890, p. 189.
¢ Forchhammer, G., Ueber die Zusammensetzung der Porcellanerde und ihre Entstehung aus
dem Feldspath: Poggendorff, Annalen, vol. 35, 1835, p. 354.
NATURE OF PRECIPITATION. BS
potentialized minerals to lower potentialized forms. These changes are
illustrated by the passage of glass to a crystalline form, by the passage of
minerals from a strained to an unstrained condition, and by the passage
of minerals of low specific gravity to minerals of higher specific gravity.
PRECIPITATION.
From solutions, by changing conditions, solids may separate. This
process is called precipitation. Since precipitation from ground water
solutions is of the utmost importance in metamorphism, it is necessary to
consider fully the conditions under which precipitation takes place. It has
already been seen that in solutions the ingredient which is present in excess
is called the solvent and the ingredients which are subordinate are the sub-
stances dissolved. When from solutions the substance in excess, or the
solvent, separates, this is called a freezing of the solution. When in the
solution the substances dissolved first separate, this is called crystallizing
out of the materials dissolved. ‘‘The processes of freezing and of crystal-
lizing out are both to be considered from the same point of view; and when
we are not dealing with dilute solutions where one ingredient is present in
large excess, but with a mixture, where both ingredients are present in
about the same proportions, then we would be in actual doubt whether the
separation should be regarded as a freezing (of the solvent) or a crystal-
lizing out (of the substance dissolved), or perchance of both processes.””
The necessary condition for precipitation is supersaturation; for if a
solution be not saturated it will take more material into solution; but if a
solution be sufficiently supersaturated some of the material must be thrown
down or be precipitated. If solids are present similar to the compounds
in solution, considerable supersaturation does not occur. This is very
frequently the case with ground solutions. Under such circumstances
the salts in solution separate out upon the minerals already present, or the
minerals grow. At any given pressure and temperature, provided the
changes occur slowly, equilibrium is nearly retained by this continuous
adjustment. This relation between minerals already present and solutions
is one of the most important factors which control the growth of minerals
which are present. If, for instance, in a complex solution containing
«Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York,
1895, p. 414.
" MON XLVvII—04——8
114 A TREATISE ON METAMORPHISM.
various ions there are also various crystalline minerals, the moment that
the solution becomes supersaturated with reference to several ions which
may unite to produce one of the solids present, this union will take place,
the material will be precipitated upon the minerals of that kind, and thus
they will grow.
This process of mineral growth applies alike to minerals in magmas and
to minerals in sedimentary rocks. If, for instance, m a magma plagioclase
and pyroxene individuals once begin to form, they may grow to large size
and produce a gabbro. In a sedimentary rock in which quartz and feld-
spar particles are present and the solutions are of a kind which furnish
constituents for their growth, these particles are likely to be enlarged.
Supersaturation, and consequently precipitation, may result in various
ways, of which the following are the more important: (1) Precipitation
by change of pressure, (2) precipitation by change of temperature, (38)
precipitation by reactions between aqueous solutions, (4) precipitation.
by reactions between aqueous solutions and gases, and (5) precipitation by
reactions between solutions and solids.
PRECIPITATION BY CHANGE OF PRESSURE.
Change of pressure may result in supersaturation, and therefore in
precipitation. Where the volume of the solution is less than that of the
solvent and substance dissolved, decrease of pressure is favorable to
precipitation. Where the volume of the solution is greater than that of
the solvent and substance dissolved, increase of pressure is favorable to
precipitation. The volume relations are opposite in the cases of the
crystallization of minerals from solutions of ground water and the crystal-
lization of minerals from magmas. In the case of substances dissolved in
ground solutions the volumes of the solutions are commonly less than those
of the solvent and the substances dissolved; therefore decrease of pressure
is favorable to precipitation. But in the case of crystallization, from
magmas the volume of the solution is greater than that of the crystallized
minerals; therefore pressure is favorable to crystallization.
In another connection it is suggested that under certain conditions water
and magma are miscible in all proportions. (See Chapter VIII, p. 723.)
In other words, there is every gradation from water containing compounds
in solution to magmas containing subordinate amounts of water. If this
LAWS OF PRECIPITATION. ills
be so, ideally there must be a neutral point in which the volume of the
material is the same whether as a solution or as a solid. In this case
pressure would have no effect upon precipitation. However, the precipita-
tion of any part of the material from a solution modifies the character of
the remainder of the solution, and it is not to be supposed that a case is
likely to oceur in which crystallization of material takes place without there
being any pressure effect.
Where circulating waters are descending the pressure is increasing,
and where ascending the pressure is decreasing. Therefore, in the case of
ordinary ground-water solutions the direction of water circulation which is
favorable to precipitation is ascension.
Pere ee BY CHANGE OF TEMPERATURE.
Change of temperature may result in supersaturation, and therefore in
precipitation. In general, in ground solutions increase in temperature
increases solubility. (See pp. 79-81.) Therefore decrease in temperature
is favorable to supersaturation and precipitation. While this statement is
true for most substances at temperatures below 100° C., and is correct for
many substances at temperatures considerably higher than this, at very
high temperatures the conditions are reversed for some substances. (See
p- 79.) In the common case, that of precipitation with decrease of
temperature, the freezing point of the solution is lower than that of the
solvent. Apparently the amount of lowering is proportional to the
molecular weights, and is stated by Raoult as follows: “One molecule of
any compound when dissolved in 100 molecules of a liquid lowers the
solidification point of the liquid by an amount which is nearly constant,
viz, 0.62°;” or, the molecular depression, when the solvent is to the
solute as 1:100, is 0.62°.2 In dilute solutions of salts in water, the
molecular depression may be larger than this, in which case the substance
is regarded by many as dissociated.“ It was by the application of the
principle of molecular depression that Kahlenberg and Lincoln were able
to reach the conclusion already given (p. 87), that silica goes into
solution as colloidal silicic acid: When the silicates are dissolved and
@Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., New
York, 1891, p. 199 et seq.
b Ostwald, Solutions, cit., p. 208.
¢ Ostwald, Solutions, cit., p. 214.
116 A TREATISE ON METAMORPHISM.
decomposed by hydrolysis into colloidal silicic acid and metallic hydroxides,
the latter (or, according to the dissociation theory, their ions) caused the
molecular depressiou, which was unaffected by the colloidal silicic acid.
In precipitation from complex mixtures the substances do not solidify
at the same time. The compounds crystallize in such order, and the
separated solid is of such a character, that “the freezing point of the
remaining liquid is lowered.” * After one compound has separated another
follows, which again lowers the freezing point, and finally a liquid is left
with the lowest freezing point, and this liquid is the last compound to
crystallize.
Change in temperature is the rule for underground circulating waters.
The waters which are passing to lower levels are, on the average, becoming
warmer. Waters which are rising to higher levels are, on the average,
becoming colder. Also there are changes of temperature, both positive
and negative, due to varying local conditions; for instance, the presence of
intruded igneous rocks. Ascending waters are, on the whole, precipitating
material, because they are losing heat. The increase in the capacity to
hold material in solution with rising temperature, and the simply enormous
increase in this capacity as the temperature becomes very high, have
already been pointed out. (See pp. 79-81.) During the upward journey
of the water the temperature continuously falls, and if the journey be long
the total loss of heat is great, and the amount of precipitation is correspond-
ingly large. Since the upward course of the water is likely to be in the
larger openings (see p. 583), such as the spaces of porous sandstones,
faults, joints, ete., we have the partial explanation of the filling of these
openings in the belt of cementation. However, this general statement
needs various modifications, dependent upon many variable factors. (See
pp. 629-640.)
PRECIPITATION BY REACTIONS BETWEEN AQUEOUS SOLUTIONS.
It has already been seen that when solutions containing various salts
are mixed the resultant solution will contain all the salts which can be
made by the various combinations of their positive and negative ions.
(See p. 68.) The first law of precipitation may be stated thus: When any
combination of the various ions in a solution can form to a sufficient extent
a Nernst, W., Theoretical chemistry, translated by C. 8. Palmer, Macmillan Co., New York,
1895, p. 111.
LAWS OF PRECIPITATION. LIke
\
to be insoluble in the liquid present, such compound will be produced and
precipitated. To illustrate, if a solution of BaCl, be added to a solution
of Na,SO, the ion Ba can unite with the ion SO, and produce the insoluble
compound BaSQO,, which will be precipitated.
The above is a statement of the empirical facts. The explanation of
these facts under the theory of dissociation is given by Ostwald as follows:
In any given case there is a constant relation between the amount of a
compound which can be held in solution and the number of free ions of
that compound. Upon this statement are based the laws of precipitation
from solutions. Says Ostwald:
In solutions a state of equilibrium subsists between the ions of the electrolyte
and the nondissociated portion. To take the simplest possible case, if we have a
binary electrolyte C, which can break up into ions A and B’, and if a, b, and e
represent the concentrations of these three constituents in a given solution, then the
following simple formula holds good: ab=ke.
Now, the two kinds of ions are produced in equivalent quantities, in the above
case, hencea=b. If, further, the total amount of the electrolyte =1, and a repre-
1
° 6 o a —a . 5
sents the ionized portion, then a=b=— and c= ,v being the volume of the solu-
Vv Vv
tion in which unit quantity (a molecular weight in grammes) of the electrolyte is
al ae which
expresses the state of ionisation of an electrolyte at the dilution v.“
In the saturated aqueous solution of an electrolyte we have a complex equilib-
rium. On the one hand the solid is in equilibrium with the nonionised portion of
itself which is in solution, while on the other hand this nonionised portion is in
equilibrium with the dissociated part—i. e., with the ions of the same substance.
The first equilibrium comes under the law of proportional concentration, or, since
we are dealing here with a substance of unalterable concentration on the one hand,
the concentration of the nonionised portion in the solution must have a perfectly
definite value. For the second equilibrium we have in the simplest case—i. e., when
the-ions of the compound are monovalent—ab=ke, a and b representing the concen-
trations of the ions and ¢ the cuncentration of the nonionised portion.
Now, since ¢ is constant at a given temperature, as we have already seen, ke,
and therefore ab, must be constant also. Equilibrium is thus established between a
precipitate and the liquid above it when the product of the concentrations of the two
ions, into which the precipitate falls, has a definite value. This product may be
termed the solubility product for the sake of brevity.
contained.. By carrying out the substitution we get the formula
«Ostwald, W., Foundations of analytical chemistry, translated by George McGowan, Macmillan &
Co., London, 1895, p. 59.
118 A TREATISE ON METAMORPHISM.
If the electrolyte consists of polyvalent ions in the proportion mA:nB, the
solubility product takes the form: a™b"= constant.@
From the foregoing follows Ostwald’s statement of the first law of pre-
cipitation, already given: ‘‘Whenever in any liquid the solubility product
of a solid is exceeded, the liquid is supersaturated with respect to that solid,”
and therefore precipitation of the salt follows. Of the various salts which
may be precipitated from a solution, that one will be precipitated first
whose solubility product exceeds its constant of solubility.
Ostwald illustrates this by the cases already cited: If a solution of
BaCl, be added to Na.SO,, BaSO, will be precipitated. According to
Ostwald’s view, this happens because the solubility product of the ions in
BaSO, is very small.
The second law of precipitation follows from the faet that ‘the solu-
bility of one salt is depressed in the presence of another having a common
he
ion. This is equivalent to saying that ‘the solubility of each molecular
species in a mixture is always smaller than for the particular species when
alone.”?
Hence, when to a solution containing certain ions a solution is
added which has an ion in common with one of those already in the solu-
tion, supersaturation and precipitation are promoted. An example of this
is the addition of HCl to a solution of BaCl,. The chlorine ion is common,
and if the solution is near saturation before the HCl is added, BaCl, will be
precipitated. Again, if one adds a saturated solution of NaClO, to a satu- ,
rated solution of KC1O,, an abundant precipitate of the latter salt will form.
The above law is a general statement which includes the rule that ‘‘ The
addition to a solution of a liquid which is able to form a homogeneous whole
with the solution causes precipitation of more or less of the substance in
solution if that substance is insoluble in the liquid which is added.”*’ This
rule is illustrated by the same examples. It follows from this that ‘in
order to precipitate a substance completely from its solution, an addition of
an excess of the precipitant is an advantage.”?
The converse of the second law of precipitation is: The solubility of a
salt increases on the addition of a second salt containing no ion in common. .
aOstwald, W., Foundations of analytical chemistry, translated by George McGowan, Macmillan
& Co., London, 1895, p. 76.
> Ostwald, Foundations, cit., pp. 76-77.
¢Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York,
1895, p. 446. :
4 Nernst, cit., p. 453.
€ Ostwald, Solutions, p. 90.
J Nernst, cit., p. 449.
LAWS OF PRECIPITATION. 119
To illustrate: ‘If one adds some KNO, to AeBrO, a number of molecules
of AgNO, and also of KBrO, will be formed. This will result in a diminution
of the number of the molecules of AgBrO,, which must be replaced from
the solid salt,” or the solubility will be increased."
Jameron gives two illustrations of this converse which are of great
importance in ground solutions :
Gypsum, which is essentially the salt calcium sulphate containing some water,
is sparingly soluble in water. But the addition of an electrolyte with no common
ion, such as sodium chloride, will considerably increase the solubility of the gypsum.
Some experiments made in this laboratory have shown that in moderately strong
brines containing only sodium chloride gypsum can be regarded as a soluble salt.
The reason for this is readily seen when the substances which are formed are con-
sidered, both the calcium chloride and the sodium sulphate being very soluble salts.
The transportation of large quantities of lime by the drainage and ground waters in
arid regions where these salts are found is readily explicable from this point of view.
Calcium carbonate, so abundant and so important in nature, is dissolved ina
precisely similar way; but the ionization of carbonates being relatively small, the
effect is not so striking and relatively much less lime is transported in the solution.
Treadwell and Reuter? have recently published investigations on this point and find
the solubility of calcium carbonate in sodium chloride solutions does not become
markedly large until considerable concentrations of the latter salt are reached. The
effect of carbon dioxide in forming the more soluble bicarbonate of lime undoubtedly
is an important element in this connection, but as the ionization is but little affected
by its presence its influence must be small in the presence of such a salt as sodium
chloride.°
PRECIPITATION BY REACTIONS BETWEEN AQUEOUS SOLUTIONS AND GASES.
Another case of precipitation occurring in nature follows as a result
of mixing solutions, one of which is a gas which acts upon the compounds
in the aqueous solution, producing ions of a different kind from those before
present, and in some cases forming compounds, the solubility of which is
so small that precipitation results. Perhaps the most important case of
this kind is the mixing of oxygen with a solution containing salts of iron
protoxide. As a result of this the iron is changed from ferrous to ferrie
form, and the latter is precipitated as a sesquioxide or hydrosesquioxide of
iron. In the latter case hydration occurs siinultaneously with oxidation.
« Nernst, cit., p. 450.
> Treadwell, F. P., and Reuter, M., Ueber die Loslichkeit der Bikarbonate des Calciums und
Magnesiums: Zeitschr. fiir anorgan. Chemie, vol. 17, 1898, p. 170.
¢Cameron, F. K., Application of the theory of solutions to the study of soils: Rept. No. 64, Field
Operations of Division of Soils, 1899, U. S. Dept. of Agric., 1900, pp. 150-151.
120 A TREATISE ON METAMORPHISM.
PRECIPITATION BY REACTIONS BETWEEN SOLUTIONS AND SOLIDS.
“Tf one pours a solution of KBr over solid AgCl, ... the bromine
existing in the solution will be largely replaced by chlorine, because as
AgBbr is much less soluble than AgCl an equivalent quantity of AgCl will
be changed into AgBr. This is also established by experiment. If one
knows the solubilities of AgCl and AgBr, then for a given concentration of
KBr we may state the point of equilibrium which the system strives to
reach.”* Hence we conclude that if a salt, A, is treated with a saturated
solution of another salt, B, a greater or less part of the salt B may separate
out, the salt A being taken into solution at the same time. In this case
‘the active mass of the solid substance is a constant.” The meaning of
this is that if any of a solid salt is present after the reaction has ceased
there was sufficient to produce equilibrium between the salt and the
solution.
An excellent case illustrating precipitation from solution in nature by
the action of a solid, one of the most fundamental importance, is the partial
dolomitization of the calcium carbonate of shells and corals by the sea
waters, which contain both calcium and magnesium salts. In this case,
under the law of chemical equilibrium, there is constant action and reaction
between the magnesium salts in solution and the solid CaCO, The
magnesium and calcium partially interchange, the calcium going into
solution by uniting with the ions before combined with the magnesium, and
the magnesium simultaneously uniting with the CO; ion before united with
the calcium and thus being thrown down as MgCO;. Thus the calcite is
partially dolomitized.
This case of dolomitization well illustrates the principle that simul-
taneously with the precipitation of one element or mineral another element
or mineral may be dissolved, one being conditioned upon the other. There
are very numerous complicated cases of this kind which need investigation.
(See pp. 203-206.)
The solids present exert an important influence in precipitation
independently of the passage of elements of the solids into the solutions.
That is to say, if there be solids present, even if none of the elements. of
any of such compounds pass into solution, these solids may influence the
«Nernst, W., Theoretical chemistry, translated by C. 8. Palmer, Macmillan Co., New York, 1895,
p. 452. ;
> Nernst, cit., p. 450.
LAWS OF PRECIPITATION. IAT
nature of the precipitation. This statement is applicable both to com-
pounds present in solutions before precipitation begins and to compounds
formed by precipitation itself. Once any precipitate begins to form, par-
ticles of that precipitate are present and influence further precipitation,
precisely as do other solids which were present before the precipitation
began. The proof of the influence of the solids present is furnished by
the very well-known tendency to the enlargement of mineral particles
already existing in preference to the formation of new individuals.
The growth of mineral particles already present is probably con-
nected with the phenomenon of adsorption, described on pages 64-65. It is
there noted that the contact film of solutions with solids contains more than
an average amount of material in solution. It may be suggested that this
is due to the molecular attraction of the crystal for the molecules in solu-
tion, just as the adherent film of the liquid itself is due to the molecular
attraction between the solids and liquids. As the particles in solution move
about they continually impinge against the solids in the solutions. These
particles thus come within the limits of the molecular attraction of the solids
and are to a certain extent held, and hence the concentration. It would
follow that the adherent films of liquid are likely to become supersaturated
in advance of the remainder of the solutions. Under these circumstances the
moment supersaturation is reached with reference to the compounds forming
a given particle, these materials will be deposited upon the particle, and
will grow. Precipitation immediately follows supersaturation of the con-
centrated film because of the orienting and selecting power of the mineral
particle already existing. It is probable in the case of a given mineral that
for compounds other than those which can unite to produce the mineral
supersaturation can take place to some extent, and that from this slightly
supersaturated adherent film this material may escape into the free solution.
However, when such solutions become supersaturated in the presence of a
mineral which could use them they would be thrown down. By this process
is explained the selective power by which each mineral particle is able to
take from solution material like itself and add it to itself; and also the fact
that particles once formed abstract materials like themselves from solutions
in preference to the formation of new particles.* The presence of any
«For explanation of adsorption see Ostwald, W., Grundlinien der anorganischen Chemie,
Engelmann, Leipzig, 1900, pp. 387-389.
122 A TREATISE ON METAMORPHISM.
mineral species will prevent considerable supersaturation of the solution, so
far as the compounds of that species are concerned. The result is that if
there be materials in a solution which can unite to produce mineral species
which are present they will do so. In this way the minerals control or
guide to a considerable extent the character of the solids which are deposited,
since when a certain mineral is absent, before that mineral can begin to be
precipitated supersaturation must occur with reference to the- chemical
combination which composes it.
Therefore the mineral species which are present in a solution have
an advantage over other kinds of minerals which are absent. To a less
degree, minerals which are abundantly present have an advantage over
those which are sparse. To illustrate, if quartz be present and the solu-
tions contain ions of silica, it will be apt to abstract the silica from
the solutions the moment supersaturation occurs. In the same way, if
feldspar be present and there are ions of sodium, calcium, aluminum, and
silica in proper proportions, these are likely to be grouped together to
produce feldspar. Moreover, it appears to be the case that the feldspar
may so nearly control that a closely analogous feldspar is produced, and
twinning and other phenomena. characteristic of the original grains be
continued in the secondary growth. The same statements apply to
hornblende, tourmaline, calcite, and, in fact, to all minerals in which a
secondary growth has been noted. Of course, in a rock in which there
are present a large number of mineral particles, the particular mineral
which is formed will depend upon the various ions in the solution, their
relative proportions, and the relative insolubility of the salts. For instance,
tourmaline can not form unless the boric acid ions are present; horn-
blende can not be produced unless there are in the solution all the bases
demanded by that mineral in sufficient abundance. Thus the particular
mineral which forms depends upon a complicated adjustment of the mineral
particles present, the ions present in the solution, their relative proportion,
and the solubility of the mineral particles.
In the above chemical principle lies a partial explanation of the strange
fact that minerals are so firmly cemented by material like the dominant
original mineral. In quartzose sandstone the chief cement is silica; in feld-
spathic sandstone the chief cement is likely to be feldspar; in strongly horn-
blendice rocks one of the chief cements is hornblende, and so on. Another
:GROUND WATER UNIVERSAL. 123
important factor in the process is the extension of the rock masses from
the places of solution to the places of deposition. For instance, in any
rock which extends from the belt of weathering to the belt of cementation
the water at the places of solution (especially the belt of weathering)
would obtain material adapted to the enlargement of the minerals of the
same rock at the place of deposition (especially the belt, of cementation).
Consequent upon the two factors above given, rocks in many instances
are cemented by minerals like those present before cementation began.
SECTION 2. CIRCULATION AND WORK OF GROUND WATER.
UNIVERSAL PRESENCE OF WATER IN ROCKS.
It has already been explained at the opening of this chapter that water
is the great dominating agent through which the greatest transformations
are accomplished. Free water is present to some extent in all rocks within
the zone of observation. That it is abundant in porous rocks is well
known. Water has also the power to slowly penetrate the apparently solid
rocks. Between the mineral particles there is space sufficient for water to
make its way, and a small amount of water is found in the most massive
and relatively impervious rocks.
Besides the free water in rocks, there is always present water in a
combined form. ‘The combined water varies from’a small fraction of 1 per
cent to several per cent. Commonly the combined water does not. fall
below 0.50 per cent, and seldom is higher than 8 per cent. It therefore
appears that all rocks contain water, both in the free and in the combined
form. ‘The amounts of each of these are very variable. Bischof many
years ago noted the penetration of basalt by water. The permeation of
apparently solid rocks by water is well illustrated by the readiness with
which agate, chalcedony, and such materials are affected by a staining
solution. When agates are boiled in colored solutions, the liquid makes its
way through the minute subcapillary spaces so small that the niicroscope
can not detect them, and the bands are differently tinted, the amount
of deposited coloring’ material depending upon the relative sizes of the
minute openings.
@ Bischof, Gustav, Chemical and physical geology, translated by Paul and Drummond, Harrison
& Sons, London, vol. 1, 1854, p. 10.
124 A TREATISE ON METAMORPHISM.
The water in rocks may completely or partly fill the openings. Where
the openings of a rock are completely filled, the rock is saturated. Unless
all the openings in a rock are subcapillary it will remain saturated only
so long as it is surrounded or partly surrounded by the saturating liquid.
If withdrawn from the saturating liquid, all the water may be drawn
off by ordinary physical means except that adhering to the walls of the
openings. ‘This residual amount of water is called the water of imbibition.
The difference between the water of saturation and that of imbibition,
which, as will be seen, is the water which may flow somewhat readily, may
be called the water of hygrometricity. In the rocks having subeapillary
openings (see pp. 143-146) the attraction extends from wall to wall, and
therefore the entire film of water in the spaces adheres to the rock particles,
or is water of imbibition. In the rocks having subeapillary pores only, the
water of imbibition and saturation is the same.
The next question which arises is as to the source of the ground water.
On pages 661-668 reasons are given for the belief that the circulation in the
zone of anamorphism, which corresponds to the zone of rock flowage, is
very slow indeed. In this deep-seated zone decarbonation, dehydration,
and to some extent deoxidation of the rocks take place. It is shown (see
pp- 764-766) that with these exceptions, excluding igneous rocks, the compo-
sition of the rocks metamorphosed in the zone of anamorphism closely
corresponds with their original composition, contrasting greatly in this
respect with the rocks metamorphosed in the zone of fracture. From these
and other facts it is certain that the circulation of water in the zone of
anamorphism is very slow. However, it is probable that a large portion of
the carbon dioxide and water liberated slowly makes its way into the zone
of fracture. It is also explained that some water may join the zone
of fracture through the agency of igneous rocks which enter this zone.
But the amount of these supplies of water at any one time is small—indeed,
insignificant compared with the amount required to keep up the active
circulation which we know exists in the zone of fracture. Since, then, it can
not be shown that any considerable fraction of the water of circulation of
the zone of fracture is derived from the zone of rock flowage, we can only
suppose that this water is derived from precipitation. The subterranean
water is therefore predominantly of meteoric origin.
PORE SPACE IN ROCKS. 125
PORE SPACE OF ROCKS.
The pore space of rocks varies from a small fraction of 1 per cent to 50
per cent or more. The pore space in compact, strong, igneous rocks is
exceedingly small. For instance, in fresh, strong granites the percentage
of water absorbed by the dry rock varies from 0.08 to 0.20 per cent, which
corresponds to a pore space of 0.20 to 0.50 per cent. The more compact
limestones also contain very little pore space. Some of them absorb as
small an amount as 0.20 per cent by weight of water, which corresponds to
a pore space of about 0.55 per cent.
Ordinary compact limestones used for building material, when satu-
rated, contain from 1 to 5 per cent of water by weight, and this corre-
sponds to a pore space of about 2.5 to 12.5 per cent. The more porous
limestones are capable of absorbing 10 per cent or more of water by weight.
Sandstones are ordinarily very porous, holding from about 2 or 3 to 15
per cent of water by weight. This corresponds to a pore space of from
about 5 to 28 per cent Capacity to hold about 10 per cent by weight, and
therefore a pore space of about 20 per cent, is very common in sandstones.
The extreme of porosity for sandstones yet reported is the Dunnville sand-
stone of Wisconsin, which, according to Buckley, contains a fraction more
than 28 per cent of air space when dry,*and therefore when saturated is
capable of having 28 per cent of its volume occupied by water. According
to Merrill,’ chalk may contain as much as 20 per cent by weight of water.
Supposing the specific gravity of the chalk to be 2.8, this corresponds to a
pore space of about 41 per cent. However, in coherent rocks, pore spaces
of more than 25 per cent are rather uncommon.
In unconsolidated rocks where cementation has not taken place at all,
and in products of the belt of weathering, the pore space may be even
greater than the above amounts. If grains of sand are spherical, of uniform
size, and “are arranged in the most compact manner possible, each grain
will touch the surrounding grains at twelve points.”° In this case the pore
space will be 25.95 per cent.” If the particles be spherical, of uniform
aBuckley, E. R., Building and ornamental stones of Wisconsin: Bull. Wisconsin Geol. and Nat.
Hist. Survey, No. 4, 1898, p. 225.
6 Merrill, G. P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, p. 198.
¢Slichter, C. 8., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1899, p. 306.
@Slichter, cit., p. 310. Becker, G. F., Geology of the quicksilver deposits of the Pacific coast:
Mon. U.S. Geol. Survey, vol. 13, 1888, p. 399.
126 A TREATISE ON METAMORPHISM.
size, and arranged ‘‘so that the lines joining their centers form cubes,” ® this
will be the most open possible arrangement. In this case the pore space
will be 47.64 per cent.’
King has made a number of experimental determinations of the pore
space of unconsolidated sands, of broken rocks, and soils, the material being
packed as closely as he was able to pack it. Where quartz sand com-
prising materials varying greatly in coarseness was used, a pore space as
low as 25.43 per cent was obtained.“ But ‘‘ well-rounded grains of nearly
uniform diameter tend to give a pore space which lies between 32 and 40
per cent. * * * For simple sands with angular grains the pore space is
much larger than it is for the rounded sands of the same size of grains, and
in the case of the crushed glass, whose grains are more angular than those
of the crushed limestone, which have a tendency to be cuboidal in form,
the pore space is the largest of all.”°
Seelheim found that clays when allowed to settle in water have a pore
space of 50 to 79 per cent, and that there is no sensible reduction of this
space under a pressure of 30 meters of water’
In clay loams and clays pore spaces as high as 48 to 52 per cent were
obtained by King.’ He suggests that the high pore space of clays may
possibly be partly explained by the angularity of the grains, it being well
known that the very fine mechanical sediments are largely composed of
angular particles.”
It is evident from these experimental results of King’s that the grains
of sands and soil are not packed by nature in the most compact manner
possible; otherwise the pore spaces would run lower, rather than higher,
than Slichter’s minimum pore space (25.95 per cent); for the natural grains
@ Slichter, cit., p. 308.
b Slichter, cit., p. 309.
¢ Tn order to get the closest packing, the material was added ‘‘in small lots at a time and gently
tamped with a broad, flat-faced pestle until the vessel was filled. ... The vessel, after being filled
by tamping, was ‘struck off’ with a piece of plate glass, then held firmly while with light blows the
walls of the tubes were struck gently, but repeatedly, as long as any reduction in volume could be
produced.’’—King, F. H., Principles and conditions of the movements of ground water: Nineteenth
Ann. Rept. U. S. Geol. Survey, pt. 2, 1899, p. 208.
d King, cit., p. 211.
e King, cit., p. 215.
f Seelheim, Zeitschr. fiir anal. Chemie, vol. 19, p. 387; cited in King, F. H., Principles and con-
ditions of the movements of ground water: Nineteenth Ann. Rept. U.S. Geol. Survey, pt. 2, 1899, p. 78.
g King, cit., pp. 213-215.
h King, cit., pp. 217-218.
PORE SPACE IN ROCKS. 2m
of soil and sand are not spherical in shape, or of uniform size. In so far as
the grains vary from regular forms and uniform magnitude the pore space
would be less than calculated; but in so far as the method of packing is
not the most compact possible the pore space would be greater than caleu-
lated. Thus these two factors neutralize each other to a considerable
degree, and we are obliged to turn to experiment to ascertain approxi-
mately the facts. It is probable that King’s experimental results* on sands
composed of well-rounded grains of nearly uniform diameters, where the
pore space was between 32 and 40 per cent, represent approximately the
original pore space in the coarser assorted mechanical sediments. The
more porous sandstones, where the pore space, as ascertained by Buckley,”
varies from 18 to 28 per cent, have a crushing strength varying from 172
to 413 kilograms per square centimeter; indeed, are strong enough to serve
for building stones. It is clear that a considerable amount of cementing
material has been added, and that the pore space measured is much less
than the original space in the sands before cementation. Hence it appears,
both from experimental work by King and by deductions from actual
measurements of the space in partially cemented sandstones, that the
original pore space in clean, well-assorted sands probably varies from one-
fifth to as much as two-fifths, with a probable average of about one-third.
It is much more difficult to give a statement as to the average pore
space of the lavas. Some of these rocks are rather dense and had orig-
inally a very small amount of pore space; others are exceedingly vesicular
and originally had pore spaces amounting to 50 to 75 per cent, or even
more. It is rather probable that where a succession of thin-bedded basic
lavas are piled up one on the other, as in the Keweenawan of the Lake
Superior region, the pore space averages as much as in ordinary sandstones;
but from this maximum the average runs down as the lava flows become
thicker and as they become more acid. Therefore the average pore space
of the vesicular lavas is probably not more than one-third to one-half as
great as in the mechanical sediments. °
It is even more difficult to make an estimate of the amount of pore
space due to fracttires in the rocks, such as faults, joints, fissility, the open-
@ King, cit., pp. 147-157.
> Buckley, E. R., Building and ornamental stones of Wisconsin: Bull. Wisconsin Geol. and Nat.
Hist. Sury. No. 4, 1898, pp. 393-395, 402-403.
128 A TREATISE ON METAMORPHISM.
ings of autoclastic rocks, etc. In the case of some breccias the pore space
is certainly as large as in the mechanical sediments, and such breccias in
some places are present in considerable volume. From this maximum
amount the pore space of course varies to a fraction of 1 per cent.
I am therefore wholly unable to give any general averages of the
amount of pore space, taking the world as a whole. But Shaler has esti-
mated that the amount of igneous and vein material of certain regions of
the New England coast is from 3 to 5 per cent of the superficial area."
Since the volumes are as the cubes of the dimensions, if the amount of vein
material were the same in other directions this would involve a filled pore
space of from 0.52 to 1.12 per cent.
From the foregoing it is plain that, while it is easy to ascertain the
amount of pore space in a given rock, it is very difficult indeed to make any
estimate of the average amount of pore space in the zones of katamorphism
and anamorphism. It is shown on pages 187-191 that these zones corre-
spond, respectively, to the zones of fracture and flowage. It is certain that
the pore space in the zone of fracture is far greater than in the zone of flow-
age. It is also equally certain that the pore space in the belt of weather-
ing is vastly greater than in the belt of cementation. When these various
zones and belts are discussed it will be shown that both the unconsolidated
materials and the coherent rocks of the belt of weathering are exceedingly
open and have a very large pore space. It will further be seen that in
passing downward from the belt of weathering to the belt of cementation
there is a sudden diminution in the amount of pore space available, the
rocks becoming almost at once far less open. Doubtless on the average
the amount of pore space in the belt of cementation steadily diminishes
from the upper to the lower part; and in the zone of anamorphism the pore
space is almost certainly but a fraction of 1 per cent.
It is to be remembered that below the comparatively thin belt of
weathering, the rocks, with unimportant exceptions, are saturated. Dana
estimates the average amount of water contained in the rocks as 2.67 per
cent of their weight.’? Supposing that the specific gravity of the crust is 2.7,
this would mean a pore space of 6.89 per cent of the volume of the rocks;
or, if the rocks were saturated, about 69 liters of water in every cubic
«Shaler, N. 8., The crenitic hypothesis and mountain building: Science, vol. 11, 1888, p. 281.
> Dana, J. D., Manual of geology, American Book Co., 4th ed., 1895, pp. 205, 311.
CHARACTER OF OPENINGS IN ROCKS. 129
meter. Supposing the pore space for the upper part of the zone of fracture
to be one-fifth of that suggested by Dana and to diminish to zero at the
lower part of that zone, this would give an average pore space for that
zone of 0.69 per cent. Supposing that the zone of fracture extends to a
depth of 10,000 meters and that the pore space is saturated, the amount of
contained water, if concentrated to the exclusion of rock, would make a
sheet 69 meters thick, extending throughout the continental areas. This
calculation is of course made upon an hypothetical basis (see pp. 569-571),
but it shows that the underground water is truly a great subterranean sheet.
This subterranean sheet may be compared to the blood of an organism,
and the comparison has force to the degree that it is the chief medium
through which the transformations of the rocks are accomplished.
CIRCULATION OF GROUND WATER.
Subterranean water must be considered from two points of view—
its circulation and its work.
The actual ground-water circulation depends upon the openings in the
rocks, the forces producing water circulation, and the forces opposed to
circulation.
OPENINGS IN ROCKS.
The rate and amount of flowage of water is largely dependent upon
the openings in rocks. The openings in rocks in reference to flowage
need to be considered from the following points of view: The form and
continuity of the openings, the size of the openings, and the percentage of
openings, or pore space.
FORM AND CONTINUITY OF OPENINGS.
For a given cross section, in proportion as an opening approaches a
circular form—that is, as it approaches a minimum of wall area per unit of
volume—the flow increases, because the friction between the moving water
and the film of fixed water upon the walls is less per unit volume. In
proportion as the openings are continuous in rocks the flow increases.
The openings in rocks include (1) those which are of great length and
breadth as compared with their width, and thus are essentially flat parallel-
opipeds; (2) those in which the dimensions of the cross sections of the
openings are approximately the same, and therefore resemble tubes of
various kinds; and (8) irregular openings.
9
MON XLVII—O4
130 A TREATISE ON METAMORPHISM.
(1) The openings which have great length and breadth as compared
with their width are those of bedding partings, of faults, of joints, and of
fissility. It is recognized that many of the fractures are exceedingly
complex. They are, indeed, in many instances a series of parallel or
intersecting fractures, forming a zone of' brecciation. However, for such a
a zone, as a whole, the statement still holds that the openings have great
length and depth as compared with their width.
Bedding partings are parallel to the layers. Since ground waters
very frequently follow formations, the bedding partings become important
factors in the promotion of flowage parallel to the formation This is
especially true of the contacts of formations of different character. These
contacts are places of maximum differential movements, of consequent com-
plex fracturing, and therefore of important openings and large circulation.
In position the fault, joint, and fissile openings ordinarily have an
important vertical element, or at least traverse the beds. Frequently they
are nearly vertical, or traverse layers or formations at right angles. In
consequence of this they are very important factors in the vertical move-
ments of ground water.
As to continuity, bedding partings are likely to be the most continuous;
faults come next in continuity, joints next, and fissile openings are those
that are least continuous. .
Bedding partings are likely to be continuous for long distances, and
because of this and their size (considered on pp. 137-138), they are fre-
quently important factors in the flowage of ground water."
Faults may have very great continuity. Thrust faults of 15 kilometers
and more along the dip are known; and along the strike faults may extend
for even hundreds of kilometers, although ordinarily their extent is much
less. From their great persistence and from the fact that they are likely to
cut across formations, thus frequently severing and displacing impervious
strata and consequently connecting porous strata separated by impervious
strata with one another, faults are of very great consequence in the flowage
of ground water.
Joints are less extensive than faults, but they may extend across an
entire formation, or even across two or more contiguous formations. The
«King, F. H., Principles and conditions of the movements of ground water: Nineteenth Ann.
Rept. U. 8. Geol. Survey, pt. 2, 1899, p. 126.
NATURE OF OPENINGS. ie)
extent of joints along the strike may be many kilometers. While joints
are less extensive than faults, they are far more numerous. Probably their
number, as compared with faults, more than compensates for their lack of
extent. Joints are therefore of very great importance in the flowage of
ground water. On the average they may be of even greater importance
than faults. Joints, like faults, may connect separated porous strata, but
very frequently the joints do not pass through the relatively plastic
separating impervious strata, and therefore in this respect are of less
consequence than faults.
Fissility openings usually have less extent than bedding partings,
faults, or joints; and the openings are small. While they doubtless have an
important influence in water flowage, they are not of such consequence as
bedding partings, faults, or joints.
(2) Openings in which the dimensions of the cross sections are
approximately the same are those of the mechanical deposits, including
conglomerates, sandstones, soils, tufts, ete.
The openings of mechanical sediments have a strong tendency to a
definite form, and are continuous. ‘The forms of these openings have been
fully discussed by Slichter.*. The openings alternately narrow and widen.
At the wider parts their sections are roughly polygonal, the polygons
having more than three sides, and these curved. At their narrowest places
the cross sections of the openings approximate triangles, and where the
grains are of equal size the triangles are equilateral. The form of the
tubes at their minimum is due to the contact of three grains in a plane,
the space between which is nearly triangular. (Fig. 3.)
Professor Slichter has further shown that there are various possible
natural systems of packing of particles. In nature one system of packing
may hold for a certain distance, and then be replaced by another system. |
Within any system of packing all the openings are connected with one
another by straight or curved tubes, triangular at their minimum cross
section, and no opening is shut off from any other opening. Slichter has
shown that in the various natural systems of packing of the particles there
is at least one direction in which the tubes are straight; in other words,
there is one direction in which a straight wire may be thrust without coming
@Slichter, C. 8., Theoretical investigation of the motion of ground water: Nineteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1899, pp. 305-323.
132 A TREATISE ON METAMORPHISM.
in contact with any grain. (Fig. 4.) In any other than the one direction,
where the grains are naturally arranged, the tubes are ordinarily interrupted,
In any case the continuity of the tubes in straight lines persists so far as the
arrangement of grains is by one system of piling. Slichter has shown that
the openings in the directions in which the tubes are not straight may be
neglected so far as the flowage of water is concerned; he therefore con-
cludes the quantity of flowage to be dependent upon the continuous straight
or nearly straight tubes. These of course vary in size, but the water
may be reckoned as passing through continuous tubes of the minimum size,
made by the cross section between three grains arranged in a plane at right
angles to the direction of the tubes. Of course it is understood that any
Fic. 3.—Triangular cross sections of pore space. After Slichter,
one system of arrangement does not extend indefinitely, and that where
one system of packing changes into another there are, ordinarily, bends in
the tubes.
Slichter further shows that the amount of space in mechanical sediments
before cementation takes place is largely dependent upon the system of
packing. It is also dependent upon the regularity of the grains and their
variation in size. The more nearly spherical the grains and the more
nearly uniform the size, the greater is the pore space.
Ordinarily the continuous tubes of mechanical sediments are limited
by the boundaries of a stratum or formation. However, a porous formation
may extend for hundreds of kilometers and have a thickness of hundreds
IMPORTANCE OF OPENINGS IN SANDSTONES. 133
of meters. This is well illustrated by the strata bearing artesian water,
many of which certainly transmit great quantities of water for hundreds of
kilometers. An excellent illustration of porous strata of this class is the
Dakota sandstone. This sandstone yields great quantities of water along
the James River Valley, and the nearest feeding area, so far as known, is
in the Black Hills, 400 kilometers distant. The volume of water which issues
Fic. 4.—Spheres packed in the most compact manner possible,. showing continuous open-
ings in one direction. After Slichter.
from sandstone strata in artesian basins shows how important is the class of
openings under consideration.
Since the continuous openings of sediments are commonly limited to
a formation, it is plain that such openings are very favorable to the flowage
of water along a formation, but are less potent in the transference of water
from one stratum or formation to another.
(3) Irregular openings are those of the vesicular lavas and of the
134 A TREATISE ON METAMORPHISM.
irregular fractures of rocks. In rocks where the openings are exceedingly
irregular in form the flowage of water is limited by the continuous openings,
however small they may be.
Irregular openings may be of any form. In the lavas they are fre-
quently spherical or ovoid. In the compact rocks they are confined to the
very minute, exceedingly irregular interspaces between the mineral par-
ticles, which apparently are in perfect contact. As already seen, in the very
vesicular lavas the pore space may vary from a small per cent to a very
large’ amount, even to 75 per cent or more. The openings are more likely
to be continuous where the pore space is large than where it is small. But
even where the pore space is very large the openings of lavas are not nearly
so continuous nor the minima of the tubes so large as in sands. In the
igneous rocks and in the rocks metamorphosed under deep-seated conditions
the openings are minute; they are controlled by the form of the grains.
They are, therefore, very irregular and discontinuous.
SIZE OF OPENINGS.
The size of the openings is very important in the circulation of ground
water. The size of openings must be discriminated from the amount of
pore space. The amount of pore space may be the same in two cases, but
in one the openings may be very few and large, and in the other very
numerous and small. The flowage in the two cases, other conditions being
equal, is very different. For a given mass of water the internal friction,
both within the moving water and between the moving and fixed water
increases very greatly as the openings decrease ini size. It is, therefore,
necessary to consider the various classes of openings in reference to size.
Upon the basis of size openings in rocks may be divided into (a) open-
ings larger than those of capillary size, or supercapillary openings; (b)
capillary openings, and (c) openings smaller than those of capillary size, or
subcapillary openings.
For water, openings larger than capillary openings, according to
Daniell,” may be circular tubes which exceed 0.508 mm. in diameter, or may
be sheet openings, such as bedding partings, faults, joints, ete., the widths of
which exceed one-half of this, or 0.254 mm. To movement of water in such
«Daniell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York,
1895, pp. 315-317.
SIZE OF OPENINGS IN ROCKS. 135
openings the ordimary laws of hydrostatics apply. Capillary openings for
water solutions include those which, if circular tubes, are smaller than
0.508 mm. in diameter, and those which, if sheet spaces, are narrower than
0.254 mm., and which in either case are larger than the openings in which
the molecular attractions of the solid material extend across the space.
Such openings in the case of circular tubes are those smaller than 0.0002 mm.
in diameter, or, if sheet passages, are below 0.0001 mm. in width. Capil-
lary openings, therefore, include circular tubes from 0.508 to 0.0002 mm.
in diameter, and sheet passages from 0.254 to 0.0001 mm. in width. Capil-
lary openings of other forms have a range limited between 0.508 and
0.0001 mm., but no one form has so wide a range as this. ‘To movement
of water in openings such as these the laws of capillary flow apply. By
subeapillary openings are meant those in which the attraction of the solid
molecules extends from wall to wall. These include all tubes smaller than
0.0002 mm. in diameter, and sheet. openings smaller than 0.0001 mm. in
width. For intermediate forms the subeapillary openings have as their
maximum limit a range from 0.0002 to 0.0001 mm.
It is not supposed that supercapillary openings, capillary openings,
ana subeapillary openings are sharply separated from one another. They
grade into one another, and the laws below given which control the flowage
in one class of openings are gradually modified until they pass into the
laws which control the flowage in another class of openings. For instance,
water in circular tubes slightly larger than 0.508 mm. in diameter would
to some extent obey the laws of flowage of capillary openings, and water
in tubes slightly less than 0.508 mm. in diameter would to some extent
obey the laws of supercapillary flow. In short, flowage in openings near
the dividing line between two classes obeys laws intermediate between
those controlling flowage in the typical cases of each class.
The areas of openings of variable size and similar form vary as the
squares of their respective diameters. The circumferences of openings of
variable size and similar form vary as their respective diameters. It follows,
for a given volume of water, that the larger the openings in which it is
contained the less is the surface of contact. For instance, if for an opening
of any form, of given diameter, the surface of contact for 1 em. of length
be 1 sq. em., if the cross diameter be doubled, the length remaining the
same, the volume of the water is four times as great, but the surface of
136 A TREATISE ON METAMORPHISM.
contact is only twice as great. If the diameters be decreased to one-third,
the volume of the water is decreased to one-ninth, but the surface of contact
to one-third only.
As a consequence of the relation between size of openings and area of
contact, it follows that in small openings a given volume of water is capable
of performing much more work upon the rocks than in openings of larger
size, for the surfaces of contact are the places where chemical interaction
between the water and rock takes place. How important is the factor of
small size in the amount of work which may be accomplished by ground
water can be adequately comprehended only when the surface of action for
a given volume of water for small openings is calculated. To illustrate, if
the openings are circular tubes of a size at the border line between those of
supercapillary and capillary size—that is, tubes 0.508 mm. in diameter—1
cu. cm. of water would have a surface contact with the rocks of about 78.74
sq cm. If the openings be sheet openings at the boundary between super-
capillary and capillary—that is, 0.254 mm. in width—1 cu. em. of water
would have a surface contact of about 78.74 sq. cm. If the openings be
circular tubes at the border line between those of capillary and subeapillary
openings—that is, 0.0002 mm. in diameter—1 cu. em. of water would have
a surface contact of about 200,000 sq. cm. Tf the openings be sheet open-
ings at the border line between those of capillary and subcapillary size—
that is, have a width of 0.0001 mm.—1 cu. em. of water would have a
surface contact of 200,000 sq. cm. ‘Therefore 1 cu. cm., or 1 gram of water,
has a surface contact varying from 0.007874 to 20 square meters in circular
capillary tubes; and in sheet passages has a surface contact varying from
0.007874 to 20 square meters. It has been calculated by Whitney that
“the grains in a cubic foot of soil have, on the average, no less than 50,000
square feet of surface area.”* The magnitude of these numbers shows how
important a factor in the work of a given volume of ground water is the
size of the openings in which the water is contained.
It follows from the above relations that the area of contact, and
therefore the friction between moving water and the fixed film of water
adherent to the walls, is inversely as the size of the openings. As will be
«Whitney, Milton, The physical principles of soils in their relations to moisture and crop distri-
bution: Bull. Weather Bureau No. 4, U. 8. Dept. of Agric., 1892, p. 14.
FLOWAGE IN SUPERCAPILLARY OPENINGS. IBY -
seen, this is a matter of controlling consequence in flowage in small and
especially in very small openings.
Supercapillary openings.—'he flowage of water through supercapillary tubes
is controlled by the ordinary laws of hydrokinetics. Ignoring friction,
the flowage of water is as the square root of the pressure or head. If
V=velocity, H=head, and G=force of gravity, then V per second
=/2GH. For instance, the velocity resulting from a pressure of 1 atmos-
phere or a head of 1033.3 em. would be the square root of 2 981 x
1033.3 = 1423.8 cm. per second.”
This formula is only approximately correct, for the internal friction in
supercapillary tubes is dependent upon the viscosity of the solutions, upon
the regularity of the openings, upon their length and size, and upon the
velocity of flowage. If the openings be not straight, or vary in size, or
both, eddies form, which increase the internal friction and decrease the
speed of movement. The friction between the moving liquid and that fixed
to the walls increases with increase of length, with decrease of size, with
roughness of surface, and with increase in velocity. If the available area
be great and the movement consequently very slow, the resistance per unit
of length due to friction becomes so small as to be almost inappreciable.
But even if the openings be large and continuous the formula gives some-
what too high results. If the flow be rapid in long, rough, irregular
underground passages, the resistance is so great as to make the formula
above given inapplicable.
Supercapillary openings include the greater number of bedding part-
ings, fault openings, joint openings, some of the openings of fissility, and
the openings in the coarser mechanical sediments, such as coarse sandstones
and conglomerates. The distance from an angle to the opposite side of the
roughly triangular tubes (fig. 3, p. 132) in sandstones composed of spherical
grains of equal size, which average 3 mm. in diameter, somewhat exceeds
0.508 mm.’ The average diameter of the pores in the system of closest
packing is 43: per cent greater than the mmimum section of the triangular
pores.’ It therefore follows that a sediment composed of grains just large
a@Daniell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York, 1895,
p. 303.
bSlichter, C. S., Theoretical investigation of the motion of ground water: Nineteenth Ann.
Rept. U. 8. Geol. Survey, pt. 2, 1899, p. 316.
¢Slichter, cit., p. 317.
138 A TREATISE ON METAMORPHISM.
enough to make the pores capillary at the smallest section have super-
capillary pores in other parts of the section. Hence, it may be said that
sandstones and conglomerates the grains of which exceed 3 mm. in diameter
have tubes which are greater than those of capillary size. But the grains
in the great majority of sandstones average less than 3 mm. in diameter,
and hence the pore openings in sandstones are for the most part capillary,
and are considered under the next heading.
It is through openings exceeding those of capillary size—that is, cir-
cular tubes larger than 0.508 mm. in diameter and sheet openings greater
than 0.254 mm. in diameter—that the rapid circulation of underground
water is accomplished. For instance, the openings through which springs
of large size issue mainly exceed those of capillary dimensions.
Capillary openings —Capillary openings include the great majority of the
openings of sands and sandstones, many of the openings of fine conglom-
erates, many of the openings of porous lavas, and many of the openings
produced by fracture. As already noted, the superior limit of size of
grains of sands and sandstones composed of grains of uniform size, the
smallest openings of which are capillary, is 3mm. in diameter. The inferior
limit of size are grains, the diameters of which are six times the maximum
diameter of subcapillary tubes, or 0.0012 mm. The majority of the par-
ticles of most clays, shales, and slates are much smaller than this, and
therefore the openings of these rocks are subcapillary. Hence capillary
openings in mechanical sediments range from very fine sands to very
coarse sands. Many of the openings of fissility are capillary; but the
majority of bedding partings, fault openings, and joint openings are partly
supercapillary, although often the walls of such fractures are so close
together as to make even these openings capillary in part.
In capillary openings the resistance to flow increases very rapidly as a
tube diminishes in size. This is due to the fact, already explained, that
the area of contact between the moving liquid and that fixed to the wall
increases inversely as the size of the openings. Indeed, the friction between
the moving and the fixed liquid becomes the dominant factor in the resist-
ance to flowage in capillary tubes. As openings decrease in size, at the
diameter at which this factor controls for a given liquid the openings
become of capillary size for that liquid.
FLOWAGE IN CAPILLARY OPENINGS. Isis)
According to Poiseuille, the general formula for the flow through a
tube of circular section is
Tap
S= Sul b)
in which fis the discharge in cubic centimeters per second, a is the radius
of the tube, / its length, p is the difference in pressure at its ends in dynes
per square centimeter, and w is the coefficient of viscosity of the liquid.”
According to Slichter, ‘‘if A is the area of cross section, this formula may
be written
ABD
Ie rT
and the mean velocity of the fluid in the tube is given by
In a triangular tube the flow per second is represented by the formula
Me OAS
I~ 30/3 fl
and the velocity by the formula
v= (o.02ssTyer
“The mean velocity for a circular tube of equivalent area of cross section
»> Slichter finds the volume and
was found to be about 38 per cent more.
velocity of flow in an elliptical cylinder to vary but slightly from that of
a circular tube. ‘‘Even an eccentricity of 0.866 will change the flow by
but 10 per cent, and an eccentricity of one-half will reduce the flow by
about one-half of 1 per cent. ‘Thus it is clear that a slight change in the
shape of the cross section of a tube will change but slightly the flow
through it. Analogy warrants us in extending this truth to tubes having
other than elliptical sections. For example, we may conclude that the flow
through a tube whose section is an oblique triangle is given approximately
by the formula for a tube whose section is an equilateral triangle of the
same area, even though the shape of the section of the given tube differs
db
slightly, or even materially, from that of an equilateral triangle.”” Further-
@Slichter, C. S., Theoretical investigation of the motion of ground water: Nineteenth Ann. Rept.
U. S. Geol. Survey, pt. 2, 1899, p. 317.
bSlichter, cit., p. 319.
140 A TREATISE ON METAMORPHISM.
more, in capillary tubes “the velocity of flow through a tube of variable
section will be less than the velocity of flow through a tube having a
uniform section equal to the mean section of the first tube, because of
the viscosity or internal friction of the expanding or contracting stream.”
Daniell expresses a part of the laws of capillary flow in words, instead
of in a formula, as follows: ‘The flow in capillary tubes is proportional not
to the square, but to the fourth power of the radius; the velocity is propor-
tional not to the square root of the pressure, but to the pressure itself.
The resistance in capillary tubes varies directly as the velocity; in wide
tubes approximately as the square of the velocity. This seems discrepant;
but it is due to the formation of eddies in the wider tubes; in a capillary
tube the flow is steady.””
From the foregoing it follows that the flow in a tube with a radius
one-fifth millimeter in diameter is sixteen times as great as in a tube
one-tenth millimeter in diameter. Furthermore, in a tube of any definite
length, if the pressure be doubled the flow is doubled; if trebled the flow is
trebled, ete. However, experimental work by King upon the flowage of
water through capillary openings of sandstones and sands gave results
showing that under the conditions in which he performed his experiments
the flowage increased faster than the pressure. The pressure in the experi-
ments varied from a small fraction of an atmosphere to somewhat more than
an atmosphere. The departure from Poiseuille’s law varied from less than
1 per cent to more than 50 per cent.’ In the experiments the departures
seemed to be greater, on the average, when very low pressures were used
than when moderate pressures were used. The very variable results may
be partly explained by the conditions under which the experiments were
performed, but it is entirely possible that the departures are partly to be
explained by the relative importance of internal friction due to viscosity
when the rates of movements are slow. (See pp. 141-143.)
Also, according to Poiseuille’s law, the flowage is inversely as the
viscosity. When it is remembered that the viscosity of water decreases
rapidly with increase of temperature, it is seen that this is a very important
“$lichter, C. 8., Theoretical investigation of the motion of ground water: Nineteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1899, p. 320.
>Daniell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York,
1895, p. 316.
cKing, F. H., Principles and conditions of movements of ground water: Nineteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1899, pp. 135-157.
FLOW AGE IN CAPILLARY OPENINGS. 141
factor. The relative viscosity of water at various temperatures below
100° C. is as follows:*
Relative viscosity of water at different temperatures.
OP aieidod ESS OA SAAR RAE SUAS Se ae ren ema OO RD SAUNT aki tog 100. 00
1S 3 Gb55 pea OUSaC SOS Batis SAP Sas Meee Roe ee a eee na 63. 60
GU! Sete cGadben cde an HORSES SES Seen ae See aerate men RANE 44.90
GO a8 AS Sea Soe4 SOAS EOS He See Eee oe ee ae Ae EDI He Se eR ER 33. 89
GUY hotieH Uae be ete Sarasa se Meera eh on miiesm ieee een MSE e Umea staat 26. 94
lO ieee Se ney teint Safle ye a Sih tee eae ie ea lh Uae Ae Su 21.75
0 Sark ciates eee ee eta a arate a epee Siac sas ewe Sweetie Seeman ole 18.16
From this table it appears that the viscosity of water at 45° C. is about
one-third its viscosity at 0° C.; at 90° C., less than one-fifth as great as
at 0° C. It therefore follows that temperature is a factor of the greatest
importance in the flowage of water through capillary openings in the litho-
sphere. It is shown (pp. 138, 145-146) that the openings in the lithosphere
are largely those of capillary or subcapillary size; hence the importance of
the temperature element.
Another factor entering into the flowage of ground water is the
influence of the meniscus where the openings are not fully occupied by
water. Wolff’ has shown that if water be introduced into an empty
capillary tube, the meniscus in advance of the column is an important
retarding influence, and consequently that the movemént is slower than
under circumstances where there is no meniscus. This influence is likely
to be important in many cases in the belt of weathering, where partial
filling is the rule, but is probably of little consequence in the belt of
cementation below the level of ground water, where saturation is the rule.
In conclusion, it should be fully understood that the laws of capillary
flow, as developed by Poiseuille and others, involve rather rapid movement
through the capillary openings. It has already been stated that viscosity
of the solutions and friction between the moving and the fixed water are
the determinative factors in reference to capillary flow. It is highly
probable that where the movements are very slow the friction is minute or
inappreciable and that the consequent departures from Poiseuille’s laws are
very great. Apparently in the exceedingly slow movements of many of
Landolt and Bornstein Tabellen, 1894, p. 288; supplemented by experimental data furnished by
Mr. C. F. Bowen.
b Wolff, H. C., The unsteady motion of viscous liquids: Trans. Wisconsin Acad. Sci., Arts, and
142 A TREATISE ON METAMORPHISM.
the larger masses of ground water the viscosity of water and the friction
becomes almost zero per unit area. Evidence of this is furnished by the
fact that artesian water flowing through rocks for hundreds of kilometers,
the openings of which are capillary, may have nearly the full pressure due
to head. For instance, the artesian water adjacent to Lake Michigan at
Chicago at the early wells, before they became so numerous as to interfere
when allowed to flow, had a head of 30 meters above the surface, and the
feeding area is only about 80 meters above Chicago;" yet the water has
traveled underground from 150 to 250 kilometers. The resistance causing
the loss of head of 50 meters is to be distributed through this distance;
therefore the friction per meter must have approached an infinitesimal
amount. The same thing is again finely illustrated by the artesian wells
of the James River Valley of South Dakota. The water of these wells
must have traveled at least from the eastern border of the Black Hills, 400
kilometers. The elevation at the source is 1,500 meters and at the James
River 500 meters. The consequent loss of head of considerably less than
1,000 meters is due to resistance through the entire distance, and again must
D
be almost immeasurably small per meter.’ In all such instances the average
movement is exceedingly slow, for it will be shown that to accomplish the
first of the above journeys more than a century was perhaps required, and
for the second possibly centuries were necessary. (See pp. 585-586.)
But the moment the speed of movement becomes appreciable the resist-
ance promptly runs tip. This is shown by the very slow fall of a slanting
water table in sands as the result of lateral flowage. The best illustration of
this of which I know is that kindly furnished me by J. B. Lippincott, city
engineer, of Los Angeles, Cal: The Los Angeles River is mainly fed by
ground waters derived from granitic and other sands which are of moderate
coarseness, but the openings of which are capillary. The water table
about
16.1 kilometers—from a little more than 180 meters to a little more than
rises from the headwaters of the river to a point north of Fernando
330 meters, or 9.3 meters per kilometer. My. Lippincott says that from
1896 to 1900, inclusive, five years, there was practically no rainfall, and
aLeyerett, Frank, The water resources of Illinois: Seventeenth Ann. Rept. U. 8. Geol. Survey,
pt. 2, 1896, pp. 805-806, 811.
bDarton, N. H., Artesian waters of the Dakotas: Seventeenth Ann. Rept. U. 8. Geol. Survey,
pt. 2, 1896, pp. 665-670, pl. Ixx.
FLOW AGE IN SUBCAPILLARY OPENINGS. 145
therefore no addition to the ground waters During that time the water
table fell in the granitic sand, on an average, at the rate of 0.38 meter per
kilometer per annum. This fall of water during these years in the granitic
sands alone, Mr. Lippincott says, is sufficient to account for the entire dis-
charge of the Los Angeles River. A head of 9.4 meters per kilometer in
large channels where friction is small would result in the outpouring of the
great quantity of water held in the gravels into the Los Angeles River in a
very short time. But the openings in the sands are capillary, and the resis-
tance due to friction and to viscosity is such that thé water was very slowly
delivered to the river under a head of 9.4 meters per kilometer, the average
fall being, as explained, 0.38 meters per kilometer per annum.
Movement as slow as this must be rapid as compared with the exceed-
ingly slow movement of the ground water in the artesian basins referred
to. It follows from these illustrations that the ordinary rates of movement
in the belt of cementation are very much slower than were the move-
ments under the conditions in which Poiseuille, King, and others carried
on their experiments. It is plain that the laws derived from experiments
as given by Poiseuille and King in reference to capillary flow are only
very partially applicable to movements of ground water; indeed, their
application is probably limited to the somewhat rapid movements of the
water in the capillary tubes above the level of ground water in the belt of
weathering where gravity has its full effectiveness, and adjacent to large
openings, either natural or artificial.
Subcapillary openings—By subcapillary openings, as already explained, are
meant openings smaller than capillary openings. In subeapillary openings
the attraction of the solid molecules extends from wall to wall, and there-
fore in these openings the water is wholly that of the films attached to the
walls by molecular attraction. There is no free water, in the sense that
the molecules are free to move among themselves, resisted only by the vis-
cosity of the fluid. The ratio of the resistance to movement of water thus
attached as films to solids is almost infinitely great as compared with that
of free molecules. Water thus attached is as if glued to the walls.
Quincke has determined that the attractive influence of glass upon a
fluid extends through a silver film 0.00005 mm. thick; or, stated in another
way, he finds that the distance through which molecular attraction acts is
144 A TREATISE ON METAMORPHISM.
in general 0.00005 mm.* Plateau made the distance through which mole-
cular attraction acts ;;4;> mm.,’ which amount is slightly greater than
Quincke’s determination. Since each wall holds a film of water, sheet pas-
sages below 0.0001 mm. in diameter are subcapillary. ‘The maximum size
for the subcapillary circular openings is twice as great, or 0.0002 mm. in
diameter.
The laws of flowage of water through tubes of such small size have
not been investigated, so far as I am aware. However, upon theoretical
grounds one would expect that the flow would be exceedingly, indeed
indefinitely, slow even as compared with flow in capillary tubes. This
anticipation is fully justified by the observed facts of geology. It is well
known that natural oil and gas may be held in anticlinal arches and domes
for long periods of time, even when under great pressure. It is certain in
these cases that the escape of oil, or even gas, through the subcapillary
openings of the shales is slower than the manufacture of these products in
nature’s laboratory. he facts as to the retention of oil and gas under
shale roofs render it highly probable that flow in subcapillary openings
is so slow as to be inappreciable during the time through which an experi-
ment is ordinarily continued; but the flow in subeapillary openings during
geological periods is probably of great consequence. (See pp. 892-904.)
It may be anticipated that the slow movement of water in subeapillary
openings is greatly influenced by change of temperature. At high temper-
atures the viscosity of water is an important element in flow, and _ this
rapidly decreases with increasing temperature. That water gas does not
obey the law of flow of liquids in subeapillary tubes is shown by the
experiment of Daubrée,’ in which the vapor of water at a temperature of
160° C., and consequently at a pressure of 6 atmospheres, passed through
a layer of apparently solid rock 2 cm. in thickness, and gave a pressure on
the other side of 1.9 atmospheres. This experiment shows beyond all
question that water gas under high pressure and temperature does not
adhere to the walls strongly, and has such a small viscosity that it slowly
but surely passes through subeapillary openings. However, ground water
at all temperatures below the critical temperature under ordinary conditions
“Quinecke, M., Ueber die Entfernung in welcher die Molecularkrafte der Capillaritat noch wirk-
sam sind: Poggendorff, Annalen, vol. 138, p. 402.
> Plateau, J., Statique des liquides, vol. 1, 1873, p. 210.
¢Daubrée, A., Géologie expérimentale, Paris, 1879, vol. 1, pp. 236-238.
FLOWAGE IN SUBCAPILLARY OPENINGS. 145
is held by the pressure in the form of a liquid. But at temperatures
higher than 865° C., or the critical temperature of water, whatever the
pressure, the water is in the form of water gas. In this case it may be
supposed to have a much greater penetrating power than in the form of
liquid, since it can not be considered as adhering to the walls of the
openings.
Even if subeapillary openings be very small and the flow very slow,
it does not follow that the water within these minute openings is not an
agent through which important geological work is accomplished. The
water in such spaces is capable of taking into solution the substances with
which it is in contact, of depositing material from solution, of reacting upon
the substances by hydration; in short, is capable of performing all the
transformations which freely moving water is able to accomplish. Indeed,
it has already been seen that all transfers of material between water and
rock must take place through the fixed films of water. (See p. 64.)
The transfer of material in subcapillary openings is confined to short
distances because there is no free circulating water. ‘The interchanges of
material are probably slow, except between adjacent or nearly adjacent
given volume
to)
mineral particles; therefore it seems highly probable that a
of water in the subcapillary openings is far more effective in transforming
rocks than an equivalent volume in larger openings. The same reasoning
applies here as in the case of the capillary openings as compared with
supercapillary openings. The surface of action per unit volume in the
subeapillary tubes is vastly greater than in larger openings. As shown
on pages 686-698, the above conclusion as to the efficacy of water in
subcapillary openings is fully justified by the facts. It is there seen
that the minute amount of water contained in the subcapillary openings is
the medium through which the complete transformation of rocks to schists
and eneisses has been accomplished. I therefore conclude that, while it
is probable that the actual flow of water and transfer of material in
subcapillary openings is comparatively slow, it is certain that most
profound alterations of rocks take place through this water as the agent of
transformation.
Subcapillary openings include the openings of mechanical sediments
the particles of which, if spherical and of uniform size, are not greater than
00012 mm. in diameter. As a matter of fact, many of the openings in
MON XLVII—04—10
146 A TREATISE ON METAMORPHISM.
which a portion of the particles are larger than this have subeapillary
openings, since the larger openings are occupied by grains as small as or
smaller than the above dimensions. The great majority of the clays,
shales, and slates are largely composed of particles smaller than 0.0012
mm. in diameter and their openings are subcapillary. Minute openings
between the grains of the igneous rocks and of the rocks metamorphosed
to schists and gneisses are also usually subcapillary. Where practically
all of the openings are subcapillary, whether they be the openings of
sedimentary, igneous, or metamorphic rocks, such rocks constitute practi-
cally impervious strata; for the contained water is in fixed films held by
molecular attraction, and the circulation, as already explained, is so slow as
to be negligible during short time intervals.
PERCENTAGE OF OPENINGS, OR PORE SPACE.
The percentage of openings in the rocks, or the pore space, is a func-
tion of the number and the size of the openings. In so far as the openings
in rocks are large and numerous, there is a large pore space. It has
already been seen (pp. 124-129) that the absolute amount of openings in
rocks, as shown by observation, varies from a small fraction of 1 per cent to
over 50 per cent. ‘The larger the pore space the more favorable the condi-
tions for circulation, but since the variation in pore space is so great it is
evident that the flowage of water dependent upon porosity is very variable.
Water passes readily through rocks which contain much pore space; water
does not flow to an appreciable extent through rocks which have a small
fraction of 1 per cent of pore space. Other factors being the same, and the
pore space of the same character, the flowage is in direct ratio to the amount of
pore space.
FORCES PRODUCING WATER CIRCULATION.
The forces producing circulation of ground water are gravity, heat,
mechanical action, molecular attraction, and vegetation. The dominant
force, upon which the movement of ground water mainly depends, is gravi-
tative stress.
Gravity— Gravity ever tends to pull the water downward. And this
never-ceasing force at work throughout the zone of water circulation, on
the average continuously carries the circulating water to lower levels. This
condition of affairs is analogous to the work of gravity in earth movements."
@ Van Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11., 1898,
pp. 465-516.
GRAVITY PROMOTES UNDERGROUND CIRCULATION. 147
But in earth movements and water circulation alike, all the elements of the
movement must be taken into account. The downward movement of a
greater mass of earth or water may result in the upward movement of a
lesser mass. The upward movements of water dependent upon downward
movements of other water are of relatively greater importance in the water
circulation than are the upward movements of rocks consequent upon
downward movements of larger masses of material in earth movements.
Indeed, it will be seen that commonly the circulation of a system of
ground water in the belt of cementation involves both downward-moving
and upward-moving masses. In such systems of ground-water circulation
gravity is effective in the movement in proportion to the head. Head is
due to the fact that the water entering the ground at a certain level, after a
short or long underground journey, issues at a lower level.
Also where there is a difference in the density of the two columns due
to difference in the amount of material held in solution, gravity promotes
circulation independently of head, the column holding more salts being
pulled down and the lighter column driven upward. Probably the amount
of material in solution is usually not so great as to make this an important
factor in the process, but in salt regions it may be important. The density of
the water of the sea as compared with fresh water is 1.02765 to 1.02795,” and
the density of a saturated solution of sodium chloride at 4° C., as experi-
mentally determined by Mr. 8. H. Ball, is 1.2063. Of course in actual cases
such differences as these are not found, for both columns are sure to have
salts in solution; but where springs empty under the sea the first case is
approached. In such instances, the increased density of the sea water
opposes the head of the lighter stream of relatively pure water.
Heat—Change in temperature may result in the expansion and contrac-
tion of water, and such changes in volume necessarily involve some move-
ment. The volume of water varies as the temperature. Taking the
volume of water at 4° C. as 1, its volume at 50° C. is 1.0120, at 75° C. is
1.0258, and at 100° C. is 1.0432.2 Therefore the increase in the
temperature of underground water may increase its volume and lessen its
density as much as 4 per cent without exceeding its boiling point at atmos-
pheric pressure, and a difference in the density of two columns by 1 per
@ Bischof, Gustav, Elements of chemical and physical geology, translated by Paul and Drummond,
Harrison & Sons, London, vol. 1, 1854, p. 97.
> Austin, L. W., and Thwing, C. B., Exercises in physical measurements, Allyn & Bacon, Boston,
1895, p. 151.
148 A TREATISE ON METAMORPHISM.
cent or more is probably not uncommon. Decrease in temperature may
correspondingly increase the density of water.
Gravity and heat_ While change of temperature necessarily involves some
movement, its chief effect in water circulation is as a force subordinate to
gravity. In so far as water in a connected descending and ascending
system is warmer at its point of issuance than it was when it joined the sea
of underground water, this gives gravity an effect in circulation in the
same direction as head. This is consequent upon the fact, noted above,
that the density of water varies inversely with the temperature.
It is therefore evident that in columns of water of equal length the
stress of gravity is greater upon the column having the lower temperature.
That the diffence in gravitative stress due to difference in temperature may
be sufficient to produce rapid circulation in pipes that are supercapillary is
shown by the use of the principle in the hot-water system of heating
buildings. Underground, as in the hot-water system of heating, heat is the
energy which causes the water to expand, and gives a difference in density.
When heat has produced a difference in density of the two columns,
gravity is the foree which inaugurates and maintains the circulation.
It is believed that underground circulation may be promoted to an
important extent by difference in temperature of the descending and
ascending columns of water, resulting from heat abstracted from the rocks
due wholly to their normal increment of temperature with depth. Later it
will be shown that the downward-moving water is ordinarily dispersed in
many small openings and moves relatively slowly; therefore it may be
supposed at any given place to have approximately the temperature of
the rocks. The upward movement of water, on the contrary, is shown to
be usually in the larger openings and relatively rapid; therefore at any
given place its temperature is probably higher than is normal for the rocks
at that depth. The result is a difference in temperature between the des-
cending and ascending columns, the ascending column being the warmer.
In regions where volcanism, or. mechanical action, or both, have
recently occurred, the difference in density resulting from difference in
temperature between the descending and ascending columns is likely to be
a much more important influence in the circulation of the ground waters
than in regions where the difference in temperature is due to the normal
heat of the rocks. Such a region is the Yellowstone Park.
HEAT INFLUENCES IN UNDERGROUND CIRCULATION. 149
In some countries the issuing waters throughout great regions are very
clearly at a higher temperature than the entering waters, and in such
regions the difference in temperature must be a very important factor in
the underground circulation. In such cases the difference in temperature
between descending and ascending waters generally results from a combina-
tion of the normal increase of temperature due to depth, from regional
volcanism, and from the rocks having a higher temperature than normal
because of recent orogenic movements. An excellent illustration of such
regions is the Cordilleran region of western United States. (See pp. 591-
592.)
5 As already noted, the expansion of water with increase of temperature
is considerable, amounting to over 4 per cent between 0° and 100° C.;
that is, a given mass of water occupies a volume 4 per cent greater at the
latter than at the former temperature. In other words, if there be an
average difference of 100° C. between the ascending and descending
columns, 100 meters of the downward-moving water balances 104 meters of
the upward-moving water. If we suppose the descending and ascending
columns to be connected, of equal height, and having an average difference
in temperature of 100° C., this would be equivalent to a head of 4 meters per
100 meters for the entire height of the column. Probably the difference in
temperature between two columns is not often so great as 100° C., but if it
be sufficient to give a difference in density of 1 per cent, and the ascending
and descending columns be the same length, this is ample to give a stress
sufficient to overcome friction and viscosity, and give a decided movement
to ground water. As an illustration of the principle may be mentioned the
water power of the sea mills of Cephalonia, which, according to the Crosbys,
is wholly due to difference in temperature between the descending and
ascending waters.“
Mechanical action—A third force influencing ground-water circulation is
mechanical action. Earth movements may close or partly close the
openings in the rocks, and in this process squeeze out the water, as in the
production of the schists and gneisses from the sedimentary rocks. If the
deformation of the rocks be referred to their ultimate cause, gravity, even
the circulation of the water resulting from deformation is indirectly due to
the stress of gravity.
a@Crosby, W. F., and Crosby, W. O., The sea mills of Cephalonia: Tech. Quar., vol. 9, 1896,
pp. 6-23.
150 A TREATISE ON METAMORPHISM.
Molecular attraction—'he fourth force affecting the movement of ground
water is molecular attraction. This attractive force works between the
particles of water themselves (cohesion) and between the particles of water
and rock (adhesion).
As a result of molecular attraction water may rise against gravity in
capillary or hair-like openings, thus saturating the rocks at higher altitudes
than it would were it not for this cause; it may creep along the walls of
the openings of rocks without extending from wall to wall, and therefore
without saturating the rocks.
The rise of water when it fills capillary openings raises the free surface
of water above the normal level. ‘This rise of the free surface is explained
by the attraction between the water and the walls, and the attraction of the
molecules of water for one another. The strong attraction between the
surfaces of mineral grains and water has already been alluded to. Asa
result of this, water tends to rise along a wall or tube. This is dependent
upon the fact that there is greater attraction between the molecules of rock
and water (adhesion) than between the molecules of water themselves
(cohesion). However, the molecular attraction between the particles of
water is very great. The strength of the surface tension of a film of pure
water is dependent upon cohesion, and is 81.96173 dynes per square centi-
meter." When a molecule is surrounded on all sides by free water the
attractions in the various directions equalize one another, and so particles
are comparatively free to move. However, at the surface the upward com-
ponent of the attraction is zero; hence there is effective tangential and
downward attraction. The rise of the water along the walls is due to
adhesion. As a result of this attraction a film of water is drawn along the
walls. Because of the attraction of cohesion the film of adherent water
draws up the next row of molecules away from the walls; these molecules
in turn exert an attractive force on the next adjacent molecules, and so on.
The attractive force of the surface film of water for the water below draws
up the molecules constituting it; this in turn acts upon the film below, and
soon. The total effect of the molecular attraction between the walls and
«Naniell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York,
1895, pp. 271-279. Ostwald, W., Outlines of general chemistry, translated by James Walker, 3d ed.,
Maemillan Co., New York, 1895, pp. 107-111. Barker, Geo. F., Physics, Holt & Co., New York, 1892,
pp. 200-211.
MOLECULAR ATTRACTION AND GROUND CIRCULATION. 151
the water and the molecular attraction between the particles of water is to
produce an elevation above the normal surface of the water, the upper
surface of which is of a shape as though it were an elastic membrane
adhering to the walls and being stretched by the weight of the water above
the ordinary level below.
The height to which water rises above this natural level is indirectly
as the diameter of the capillary openings. In circular glass tubes 1 mm. in
diameter, at 20° C., pure water rises 3.32 em.“ Between plates 1 mm. apart
it rises half of this amount. Since the height is inversely as the diameters
of the openings, in circular tubes 0.01 mm. in diameter, the height in tubes
would be 3.32 m. and in sheet openings 1.66 m.
In circular tubes 0.001 mm. in diameter the height in tubes would be
33.2 m., and in sheet openings 16.6 m.; and in circular tubes 0.0002 mm. in
diameter—that is, openings of a size intermediate between subcapillary and
capillary—the water would rise to a height of 166 m, and in sheet open-
ings 83 m. Since many rocks have openings as small as or even smaller
than this, capillary attraction may be very important in the position of the
ground-water level. (See pp 411-412.) If the openings are inclined the
lengths of the openings thus filled are correspondingly great.
The height to which the water rises is independent of the character of
the walls, provided the walls are wetted,’ and hence the above numbers are
applicable to rocks. However, the height to which the water rises dimin-
ishes as the temperature increases; hence, the above numbers should be
modified somewhat as the top of the sea of ground water has a temperature
below or above a temperature of 20° C. Ordinarily this modification is of
minor importance.
Above the level to which the water may be raised as a continuous sheet
in the capillary openings, the water may still creep along the walls of the
openings without filling them. The obstinacy with which a film of water
holds to the rock surface has already been explained. This water is that of
imbibition (p. 124). In proportion as the water of imbibition varies in
amount the water under molecular attraction creeps from areas of greater
humidity to areas of less humidity. To the rise of the free surface due to
capillarity there is a definite limit; there is no limit to the creep of water
along the walls. It is presumable, however, that such movement is rela-
@ Barker, cit., pp. 209-210. » Barker, cit., p. 210.
152 A TREATISE ON METAMORPHISM.
tively slow, and that the amount of water which is thus transferred for
a given surface is small. But in the soils very large surfaces are available
for creep, and therefore this process is a very important one, especially
in connection with plant growth. The process is one which especially
pertains to the belt of weathering and is therefore later considered. (See
pp. 412-423.)
The rise of the free surface of ground water above thenormal level
saturating the rocks, the creep of water along the walls without saturation,
and the flowage of water through small tubes where there is no free sur-
face are generally described under the term capillarity. However, it is
evident that under the term thus used are included three very different
things. The principles involved in the flow of water through capillary
tubes are very different from those which control the free surface of ground
water in capillary tubes, and these laws again are different from those
which control the creep of water along the walls of openings.
Vegetation The roots of plants absorb ground water and transport it to
the surface. The absorption of water by plant roots causes a relative
deficiency of water. This deficiency is remedied by the movement of
water from other places toward the roots by the forces already considered.
But the influence of roots upon the flow of ground water mainly concerns
the belt of weathering. The subject is therefore later considered. (See
pp. 417, 422-423.)
General statements—[n conclusion, it may be said that the immediate cause
of movements of ground water are five—gravity, heat, mechanical action,
molecular attraction, and vegetation.
So far as the forces are concerned, the vertical component of the move-
ments of ground water is of far the greatest importance.
But whatever the cause of the flow of ground water, the direction of
movement is from places of greater pressure to places of less pressure. A
current going in any direction is evidence of an excess of pressure in the
rear of the current. Thus water which enters by seepage or through capil-
lary tubes into a larger opening, such as a fissure, must be under greater
pressure than the column of water into which it makes its way. Whether
the motive force in the movement of the water be difference in gravitative
stress or temperature, or any other cause, the excess of pressure resulting
in movement is behind the current.
VISCOSITY RETARDS UNDERGROUND CIRCULATION. 153
THE FACTOR OPPOSING WATER CIRCULATION.
The factor opposing water circulation is internal friction of the water.
The internal friction is dependent upon the viscosity of the solutions. The
elements entering into viscosity are the concentration of the solutions and
the temperature. The more concentrated the solutions the greater the
viscosity; but as the underground solutions of water are commonly not
strong, this is ordinarily not an important element. The viscosity of water
decreases very rapidly with increase of temperature. The relative viscosity
of pure water at 0° C., 45° C., and 90° C. is respectively 100.00, 33.89, and
18.16. (See p. 141.) From these ratios it is apparent that the viscosity of
water at 45° C.is about one-third of that at 0° C., and at 90° C. only about
one-fifth of that at 0° C.
It is therefore clear that the higher the temperature the less the
viscosity and the less the internal friction. Internal friction due to viscosity
results from the variable speeds of different parts of moving water columns
and from the friction between the moving and fixed portions. ‘The greater
the variations in speed of the moving parts the greater the internal friction
due to this cause.
Water usually wets the surface of the rocks. In other words, there is
molecular attraction between the water solutions and the minerals com-
posing the rocks. This attraction is so strong that a thin film of water is
firmly held by the walls of the openings—so firmly that it may be consid-
ered as fixed; at least the only interchange which occurs between it and
the passing water currents is that of diffusion, not that of flow. Daniell
says the friction between the layer of adherent water and the rock is
infinite as compared with the friction within the liquid. That the friction
is between the moving liquid and the fixed film of liquid is shown by the
fact that for any liquid the composition of the walls has no effect upon the
flowage.? This being the case, it is clear that in the flowage of water
through tubes there is no friction of the water against the rock walls. The
adherent films of water are the walls of the moving columns, and the internal
friction between the water and the walls is that between the fixed films and
moving water.
aDaniell, Alfred, A nebo of Aine aaa: oe physics, 3d ed., Macmillan Co., Neowavor
1895, p. 306.
>Daniell, cit., p. 316.
154 A TREATISE ON METAMORPHISM.
The greater the speed of the moving water the greater the internal
friction, because of the differential movements both in the moving water
and between the moving water and the films fixed to the walls. Where
the rate of movement is sufficiently slow the internal friction due to viscosity
drops to an almost inappreciable factor. Therefore where the movement
is very slow, even if the passages be long and small, the pressure due to
head may diminish very slowly. Indeed, nearly the full pressure may be
maintained for long distances—many or even hundreds of kilometers..
This principle is of the utmost importance in the flowage of ground water,
and its applications are later developed. (See pp. 585-588.)
GENERAL STATEMENTS.
In general it may be said that in proportion as the driving forces,
gravity, mechanical action, ete., are great, circulation is likely to be rapid.
In proportion as the opposing force, internal friction, is great, circulation is
likely to be slow. In proportion as the openings approach the circular
form, circulation is likely to be rapid. In proportion as the openings are
continuous, the circulation is likely to be rapid. In proportion as the pore
space is great, circulation is likely to be rapid.
However, of all these various factors dependent upon the character of
the openings, that of size is probably the most important; for rocks which
do or do not readily transmit water may have the same proportion of pore
space. For instance, if the grains be supposed to be spherical, of the same
size, and arranged in the most compact fashion possible, the unoccupied
space is 0.26 of the entire space, without reference to the size of the grains.
Thus the relative proportion of the openings in a great bowlder conglom-
erate and a fine-grained clay may be the same. But the capacity for the
transmission of water by the former will be indefinitely greater than by the
latter. As illustrating this, an experiment showed that a quartz sand, the
water of saturation of which was the same as that of a certain chalk, trans-
mitted water under a certain pressure six hundred times as fast as the chalk.“
In the compact soils, the particles of which are exceedingly small (see
pp. 188-146), the openings between the particles are of capillary or sub-
“Prestwich, Joseph, Geology, chemical, physical, and stratigraphical, Clarendon Press, Oxford,
vol. 1, 1886, p. 159.
PORE SPACE AND UNDERGROUND CIRCULATION. 155
capillary size. In the case of the fine soils and clays the pores may be almost
wholly subeapillary, or the water is that of imbibition. In this fact we
have the explanation of the retention of soil moisture in fine clays. The
moisture is glued to the grains. There is practically no circulation, and
the water is removed only by high temperature or high pressure, or the
two combined. It follows from the foregoing that, under given conditions
with a given pore space, the coarse conglomerates furnish a much larger
flow than fine conglomerates, the fine conglomerates a larger flow than the
sandstones, and these a vastly greater flow than the soils, clays, and shales.
Bedding, fault, joint, and fissility openings may be so close together
that the pore space is very large. Ordinarily fault openings are wider
spaced but larger than the joint openings, and joint openings are wider
spaced and larger than the openings of fissility. It can not be said which
kind of opening gives, on the average, the larger pore space. Since,
however, large openings are favorable to rapid flow, for a given pore space
the fault openings are likely to give a greater flow than joint openings, and
jomt openings a greater flow than those of fissility. This follows from the
ereater size of the fewer openings. To this is to be added the element of
greater continuity of the larger openings, as explained on pages 130-
131. Therefore, with a given pore space the flow may be vastly greater
in the case of faults than in the case of joints, and much greater in the
case of joints than in the case of fissility.
In this connection it may be said that the capacity of a rock for
imbibition gives a very good idea as to its power of transmission. The
water of imbibition, it may be recalled (see p. 124), is the amount which
adheres to the walls of the openings. It is evident that in rocks containing
the same percentage of water when saturated the power. of transmission
varies inversely as their capacity for imbibition. If the openings of a rock
be very small, but numerous, there is in a cubic centimeter a large surface
to which the water can adhere. If the openings be subcapillary, the water
of imbibition and saturation are the same and the powers of transmission
practically nil. ' If the spaces be capillary, the water of imbibition is much
less and the power of transmission greatly increased. If the spaces be
supereapillary, the water of imbibition is slight in amount and the power
of transmission very great.
156 A TREATISE ON METAMORPHISM.
As already noted, there are two zones ‘of metamorphism, that of kata-
morphism and that of anamorphism, and the former consists of a belt of
weathering and a belt of cementation.
The major part of the water entering the ground must finally reach the
surface. A small part may be combined with the rocks in the underground
course of the water. A small part may possibly penetrate deep within the
zone of anamorphism, but it is safe to say that at least 99 per cent of the
water entering the ground reappears at the surface in some manner. A
very large part of the water penetrating the soil is drawn to the surface
after having taken a longer or shorter journey in the belt of weathering. A
lesser part of the water joins the sea of ground water and takes a journey
of greater or less distance in the belt of cementation before it reaches the
surface. This journey may be merely from the top of a small hill to its
base, or it may be hundreds of kilometers. An exceedingly small fraction
of the water doubtless penetrates the zone of anamorphism, although, as
explained (pp. 665-668), the general movement is from rather than to this
zone. The underground journeys of water, whether the exceedingly
short ones within the belt of weathering or the longer journeys in the belt
of cementation or the zone of anamorphism, may be resolved into two
components, one parallel to the surface of the earth and one at right
angles to this surface. he first may be called the horizontal component,
the second the vertical component. On the average, the horizontal com-
ponent of the journey is many times longer than the vertical component.
GEOLOGICAL WORK OF GROUND WATER.
From the foregoing it follows that the geological work of ground
water is favored by smallness of openings, by length of time, by pressure,
and by high temperature. Water enters the rocks mainly through the
smaller openings. A very large surface of the rock material is exposed to
water action. In so far as the water passes from the smaller openings to
the larger openings its geological work is lessened. The geological work
may be considered as directly proportional to the time. The smaller the
openings the greater the resistance, and therefore the greater the time for a
given journey. That the resistance runs up very rapidly as the openings
become small, and especially as they become capillary or subeapillary, has
GEOLOGICAL WORK OF GROUND WATER. IBY ¢
already been shown. Since the horizontal journey is, on the average, long
as compared with the vertical journey, the element of time is of much
greater importance in the horizontal component of the journey than in the
vertical component. The capacity for geological work is increased by
pressure and by temperature. ‘These forces, under ordinary conditions, are
a function of depth, and these factors in the work mainly concern the ver-
tical component of movement. During the downward journey the pressure
and temperature steadily increase, and the amount of material in solution
increases. During the upward journey the pressure and temperature
diminish and the tendency for material to pass from solution or to be
precipitated increases, and the amount held in solution diminishes.
Pressure and temperature are ever working together according to
definite laws. Both increase in efficiency with depth, and they greatly
promote the activity of deep ground waters. However, of all the vary-
ing factors, varying temperature is the one which is of imcomparably
the greatest importance. High temperature ordinarily results from depth
of penetration; but it has been pointed out that it may result from various
other causes, of which chemical action, mechanical action, and the pres-
ence of intrusive igneous rocks are the more important. The capacity
which water has for taking and holding in solution various relatively
insoluble compounds, and the velocity of chemical reactions, increase
enormously with increase of temperature. Not only is high temperature
favorable to geological work, because of the chemical activity of the water,
but, as already pointed out, high temperature greatly decreases its viscosity,
and this, as already explained, is favorable to depth of penetration and
flow through minute openings. Since the temperature changes of ground
water are commonly dependent upon depth, the vertical component of the
movement of underground water is ordinarily far more important than the
longer horizontal component.
The underground journey of water may occupy hundreds of years.
(See pp. 585-586.) The surface of contact in very small openings is
very great. Under these conditions of slow movement and small openings
there is sufficient time nearly to establish complete equilibrium between
the solutions and the solids with which they are in contact; but it has been
seen (pp. 34-35) that rarely or never is the adjustment of a rock to its
158 A TREATISE ON METAMORPHISM.
environment complete. In so far as the adjustment is not complete, changes
are going on, and the conditions are everywhere those of chemical dynamics,
although the chemical action may be so slow that if the operations were
conducted in a laboratory it might be concluded that the conditions were
those of chemical statics. This, however, but illustrates the importance of
time in geological operations.
Thus far the treatment of the circulation and work of ground water
has been general. There are many other factors concerned in the circula-
tion which have not yet been considered, but these are factors special to
the different belts and zones. They wiil therefore be treated in Chapters
VI, VU, and VIII, on the belt of weathering, the belt of cementation, and
the zone of anamorphism, respectively.
Glelale eI IY,
THE ZONES AND BELTS OF METAMORPHISM.
GENERAL CONSIDERATIONS.
The various geological factors which bear upon metamorphism have
been briefly discussed in the introductory chapter. It is there held that
the geological factor of dominating importance is depth. Upon the basis
of depth it is stated that the known crust of the earth is divisible into upper
and lower zones of metamorphism; the first is called the zone of katamor-
phism, and the second the zone of anamorphism. It is further stated that
the zone of katamorphism is divisible into two belts, an upper belt of
weathering and a lower belt of cementation.
While in the introductory chapter these general statements were made,
there was no attempt to show that they are correct. It is one of the pur-
poses of this and the following chapters to furnish evidence of the utility
of this classification, and to show that very different metamorphic effects
follow from the work of the same forces and agents in the different belts and
zones. In the present chapter a brief general statement will be made as to
the characteristic reactions of the different zones and belts. This statement
is primarily from physical and chemical points of view. ‘The next chapter
will treat of the alterations of minerals with reference to the different zones
and belts. In succeeding chapters the alterations of the rocks in the belt
of weathering, the belt of cementation, and the zone of anamorphism will
be taken up in detail. The treatment will be primarily from the geological
point of view, but with reference to physical and chemical principles.
Finally, the alterations of the individual rocks will be considered. This
and the following chapters might be regarded as a consideration of the
metamorphism of the crust of the earth from the point of view of the
physical-chemical principles developed in Chapters I] and IIT.
159
160 A TREATISE ON METAMORPHISM.
It has just been stated that the nature of the metamorphism varies
ereatly with depth. The physical reasons for this are that, as depth
increases, temperature and pressure increase. It has been seen in Chapters
II and III that where the pressure is moderate chemical reactions are likely
to be such that heat is liberated, and this is a fact whether the reactions
decrease or increase the volume. It has also been seen that where the
pressure is great this is likely to be the controlling factor, and that under
such circumstances reactions take place which lessen the volume of the
materials. Whether the reactions take place with liberation of heat or
with absorption of heat is a subordinate matter; but very commonly the
reactions are of a kind that absorb heat.
When the law of chemical affinity controls, and the reactions take
place with liberation of heat irrespective of the volume change, the reac-
tions may be said to be chemical-physical reactions. Where pressure is a
dominant factor and reactions take place with diminution of volume
irrespective of the heat change, the reactions may be said to be physical-
chemical. It is because variations in the geological factor of depth result
in these contrasting reactions that the lithosphere is divisible into a zone of
katamorphism and a zone of anamorphism.
ZONE OF KATAMORPHISM.
From the surface of the earth to a very considerable depth below the
surface (for strong rocks possibly 10,000 or 12,000 meters under quiescent
geological conditions) the rocks as originally formed may contain many
openings, as, for instance, those of sandstones, vesicular lavas, ete. Even if
uot originally porous deformation may fracture the rocks and thus produce
many openings. Where the rocks contain openings chemical reactions may
take place, increasing the volume of the material without rupturing the
rocks and without raising them to a higher position. In the outer litho-
sphere the pressures and temperatures are moderate. Under such circum-
stances the reactions which take place are controlled mainly by the laws
of chemical affinity, not by the influence of pressure. At low temperatures
the fundamental chemical law is that, on the whole, the preponderating
chemical reactions are those which take place with the liberation of heat in
accordance with the first part of van’t Hoff’s law. Therefore in this zone the
occurrence of a reaction in the alteration of a rock is favorable to further
REACTIONS IN ZONE OF KATAMORPHISM. 161
alteration; for the heat developed by the first reaction is retained by the
adjacent material, at least for a time, and this promotes further reaction, ete.
But this tendency, as has been seen, may be reversed if the temperature
becomes too high. (See p. 79.)
Since the law of chemical reactions with the liberation of heat is the
dominant factor in this upper zone, alterations may take place which
work with or against pressure. In the first case both the chemical reaction
and the compression in volume result in the liberation of heat. In the
second case the heat liberated is that developed by the chemical reaction
minus that absorbed as a result of the work done in expanding the volume.
As a matter of fact, near the surface of the earth the very important
reactions from the point of view of the nonmetallic elements, aside from
solution, are those of oxidation, hydration, and carbonation. Oxidation
and hydration commonly involve the addition of material, although the
former frequently occurs by substitution of oxygen for sulphur, and
therefore by desulphidation. Carbonation frequently involves the addition
of material, but more commonly occurs by the substitution of CO, for SiO,
and the decomposition of silicates. Often the freed silica, or a part of it,
remains in situ. All of these reactions are well known to liberate heat,
Commonly they decrease rather than increase the specific gravity of the
minerals. Since they usually involve addition of material, it is clear that
where all the residual material, or a large part of it, remains in situ the
volume of the rocks is considerably increased. However, it will be seen
that solution is also a very important reaction in parts of the zone of
katamorphism, and where this takes place to a sufficiently great extent
the volume of material may be decreased.
The main part of the oxygen and much of the carbon dioxide for
oxidation and carbonation is directly or indirectly derived from the atmos-
phere. The water is chiefly that of the ground circulation. It is there-
fore clear that in the upper zone oxygen and carbon dioxide are being
steadily abstracted from the atmosphere and fixed in the rocks, and ground
water is steadily becoming fixed by hydration. The amount of oxygen
and carbon dioxide thus fixed is great. If it were not for replenishment,
it is little short of certain that the carbon dioxide of the atmosphere
would have long since become exhausted. But probably the amount of
water fixed by hydration is even greater than that of the gases, oxygen
MON XLVU—04.——11
162 A TREATISE ON METAMORPHISM.
and carbon dioxide. Analyses of rocks in the upper zone of metamorphism
show that the amount of combined water runs as high as 4.42 per cent in
shales (see p. 744), and it probably averages as high as 1.64° per cent.
When it is remembered that the zone of katamorphism extends to a depth
of thousands of meters, it is apparent that the amount of water which is
thus fixed in the rocks by the process of hydration is enormous. However,
it will be seen that the process of hydration, like that of carbonation, is
reversed in the zone of anamorphism.
By the statement that oxidation, carbonation, and hydration are the
very important characteristic reactions of the zone of katamorphism it is
not meant to imply that the reverse reactions do not take place to some
extent. In fact, deoxidation, decarbonation, and dehydration all occur;
but oxidation, carbonation, and hydration are greatly preponderant, and
indeed dominant over the reverse reactions.
Summarizing so far as the energy factors are concerned, the changes in
volume commonly absorb heat, the chemical reactions dominantly liberate
heat and only exceptionally absorb heat. The heat liberated by the chem-
ical reactions is certainly very much greater than the sum of that absorbed
by the volume changes and that absorbed by the exceptional chemical
reactions. Therefore, so far as the rocks of the zone of katamorphism are
concerned, the total of the volume and chemical changes results in the
liberation of heat and the dissipation of energy.
The minerals formed in the zone of katamorphism are comparatively
few in number, with low specific gravities and probably for the most part
comparatively simple molecules; hence the propriety of calling this zone
the zone of katamorphism, or katamorphic, zone. This use of the term
katamorphism is parallel to the use of the term katabolism in biology to
designate those chemical changes within a living body which result in the
production of simple compounds from more complex ones. ‘The zone of
katamorphism may therefore be defined as the zone in which alterations of
rocks result in the production of simple compounds from more complex ones.
The zone of katamorphism is divisible into two belts, (1) an upper
belt of weathering, and (2) a lower belt of cementation. The belts are
a@This is the average taken from analyses of shales, sandstones, limestones, and volcanic and
crystalline rocks, given by F. W. Clarke in Bulls. U. 8. Geol. Survey No. 78, pp. 36-37, and No. 168,
pp. 16-17.
e
REACTIONS IN BELT OF WEATHERING. 163
delimited by the level of ground water. The separation of the belt of
weathering from the belt of cementation is therefore based upon the posi-
tion of an agent of metamorphism. It has been seen that the zone of kata-
morphism is separated from the zone of anamorphism by a reversal of the
physical-chemical factors. As one would suppose, the latter distinction is
of much more fundamental importance than the former.
BELT OF WEATHERING.
By some it has been proposed to call the belt of weathering that of
demorphism; and to call the alterations of all rocks below this belt
metamorphism. The fact that the alterations in the belt of weathering are
very different from the belts below has been well known for many years.
But it has not been generally recognized that the belts of weathering and
cementation are delimited by the level of ground water. This is doubtless
due to the fluctuations of that level and to a considerable transition band
between the two belts (see pp. 423-429, 560-561); but in many places the
change in the character of the alterations in passing from the belt of
weathering to the belt of cementation is very sudden, and at such places
is very clearly connected with the level of ground water.
The belt of weathering is therefore defined to extend from the surface
to the level of ground water. In this belt all of the very important reac-
tions characteristic of the zone of katamorphism—viz, oxidation, carbona-
tion, hydration, and solution—are at their maximum activity; but on the
whole, of these three reactions the most characteristic, but not the dom-
inant one, is that of the carbonation of the silicates. This reaction takes
place on a vast scale, producing carbonates from the silicates, and at the
same time setting free silica or colloidal silicic acid. Hydration is the most
extensive simple reaction in the belt of weathering. Oxidation is also very
important. As will be seen, this reaction is very general in this belt,
because not being saturated with water the oxygen of the atmosphere very
rapidly makes its way through the porous rocks and continually supplies
oxygen to replace that element used in the process of oxidation. The total
effect of these chemical reactions is decomposition, While hydration and
oxidation are usual for this belt, under special conditions these reactions may
be reversed. In places of luxuriant vegetation and very high humidity
deoxidation may take place. In regions of great heat and temporary or
®
164 A TREATISE ON METAMORPHISM.
permanent aridity dehydration may locally occur. As already noted, as a
result of oxidation, carbonation, and hydration, the volume of the rocks
would be greatly increased if all the compounds formed remained in situ;
but the complex process of solution is dominant. Many of the compounds
formed are dissolved in large quantities and transferred by the overground
water circulation to the sea, or by the underground water circulation to the
belt of cementation below. Consequently the volume of the rocks contin-
uously decreases in the belt of weathering; and finally the resultant material
may occupy but a small fraction of the original volume.
In the belt of weathering, in addition to the characteristic chemical
reactions, mechanical disintegration is the rule. Thus the complex results
of weathering may be classified into disintegration, decomposition, and
solution. As a final result of the various mechanical and chemical changes,
rocks soften and degenerate. As coherent solids they are destroyed. ‘The
processes of the belt of weathering are therefore destructive. The minerals
which remain are usually few and simple, and ordinarily are not well
crystallized. In the destructive processes all of the agents of meta-
morphism, both inorganic and organic, are actively at work. The details
of these processes are fully developed in Chapter VI, on ‘*'The belt of
weathering.”
BELT OF CEMENTATION.
The belt of cementation extends from the bottom of the belt of
weathering to the bottom of the zone of katamorphism. On the average
this belt is therefore much thicker than the belt of weathering. All of the
very important reactions characteristic of the zone to which the belt
take place. Water
is everywhere abundantly present in the belt, and hence hydration is the
most important of the three reactions. ‘The minerals produced by meta-
belongs—viz, oxidation, carbonation, and hydration
somatic change from the original minerals and those deposited from the
solutions are likely to be strongly hydrated. The processes of carbonation
and oxidation in the belt of cementation are largely limited by the amount
of carbon dioxide and oxygen there contained.
Tt will be seen (pp. 608-610) that carbon dioxide is derived from several
sources and that carbonation is usual throughout the belt, but that the
oxygeu is limited to that derived from above, and consequently that oxidation
REACTIONS IN BELT OF CEMENTATION. 165
/
is usual in only a very limited part of the belt. Not only are the processes
of carbonation and oxidation subordinate to hydration, but the process of
oxidation not infrequently is stopped or reversed in all but the upper part
of the belt of cementation. This anomaly is due to the fact that many of
the rocks contaim organic materials or sulphides or both which have a
strong affinity for oxygen. When the oxygen is exhausted from the water
derived from the belt of weathering the reducing compounds may act
directly as reducing agents or may produce reducing solutions. The
demands of these reducing agents for oxygen may abstract this material
from highly oxidized compounds, such as ferric oxide, basic ferric sulphate,
ete. Deoxidation in the belt of cementation is most commonly the result of
the burial of the higher oxide of iron and sulphates with a considerable
amount of organic material in the presence of abundant water. Under
these circumstances the ferric compounds may be reduced to ferrous
compounds and the sulphates to sulphides.
But it is to be noted that the reduction of these compounds involves
simultaneous oxidation of the organic compounds, the resultant products
being CO, and water. The carbon dioxide may escape from the belt or
enter into other combinations. For instance, as explained fully in another
place, the ferrous compounds largely unite with the carbon dioxide, pro-
ducing carbonates. Similar reactions may take place with reference to
other less abundant metals, as, for instance, manganese, and some metals
may even be reduced to the metallic condition, for instance, copper, silver,
and gold. These reducing reactions in the belt of cementation, except in
the case of iron, are of small consequence from a geological point of view,
but they have a most important bearing upon the deposition of ores. (See
Chapter XII.) It thus appears that oxidation and deoxidation are both
rather important in the belt of cementation.
The changes in the belt of cementation ordinarily produce crystalline
minerals. Minerals which were partly altered by processes in the belt of
weathering may be regenerated. This applies only to those minerals which
are adapted to the belt of cementation. ‘The average specific gravity of the
rocks is usually lessened.
It has been noted that the most characteristic reaction of the belt
of weathering is solution. In contrast with this the most characteristic
reaction of the belt of cementation is deposition in the openings of the
166 A TREATISE ON METAMORPHISM.
rocks. ‘The material deposited is derived from the belt of weathering or
from the alterations within the belt of cementation itself. Much of the
material dissolved in the belt of weathering is continuously transferred to
the belt of cementation by the downward movement of water. The total
amount of material which is thus derived from the belt of weathering is not
limited to the thin belt which exists at any given time; for, as a result of
denudation, the belt of weathering is constantly migrating downward and
encroaching upon the upper part of the belt of cementation; and thus
there is never a lack of material for solution in the belt of weathering which
may be dissolved and transferred to the belt of cementation. Within
the belt of cementation itself the reactions of oxidation, carbonation, and
hydration all increase the volume, provided all the compounds formed, or
a large part of them, remain as solids. The material added to the belt of
cementation from the belt of weathering, and the reactions within the belt
of cementation, furnish an abundant supply of material for deposition in
the openings of the rocks, whether these openings be those originally
present or produced by orogenic forces. And, as a matter of fact, in the
belt of cementation the openings are continuously filled by mineral
matter and finally closed; but this does not show that solution may not
preponderate over deposition in this belt if the effect upon the original rocks
and the openings both be considered. (See pp. 612-617.) The mechanical
result of the various processes is to indurate the rocks. The processes of
the belt of cementation are constructive. The belt of cementation, from a
geological point of view, is fully considered in Chapter VII.
BELTS OF WEATHERING AND CEMENTATION CONTRASTED.
The alterations in the belts of weathering and cementation, while not
so fundamentally different as those in the zones of katamorphism and
anamorphism, contrast strongly. In the belt of weathering, of the great
reactions characteristic of the zone of katamorphism—oxidation, carbonation,
and hydration—all are important, but carbonation is most characteristic. In
the belt of cementation, of these reactions hydration is most important. In
the belt of weathering, solution greatly dominates over deposition. In the
belt of cementation solution and deposition are more nearly balanced, but
because of reactions which increase the volume of the rocks the openings are
REACTIONS OF ZONE OF ANAMORPHISM. 167
filled. In the belt of weathering, the material continuously decreases in
volume due to solution; in the belt of cementation it continually increases in
volume due to deposition of material through reactions involving expansion
of volume. These changes of volume due to addition or subtraction of
material commonly involve decrease in specific gravity. In the belt of
weathering the mechanical results are disintegration and softening; in the
belt of cementation, cementation and induration. The belt of weathering
is therefore especially characterized by solution, decrease of volume, and
softening, resulting in physical degeneration. The belt of cementation is
especially characterized by deposition, increase of volume, and induration,
resulting in physical coherence.
ZONE OF ANAMORPHISM.
At a variable depth below the surface of the earth the pressure is so
ereat that it can not be supposed that considerable openings permanently
exist. The depth at which this condition of affairs is reached depends
largely upon the character of the rocks. For the strong rocks, as already
noted (p. 160), this depth, under quiescent geological conditions, may be as
ereat as 10,000 or 12,000 meters. If openings be originally present in
the rocks of the zone of anamorphism, as, for instance, sandstones, vesicular
lavas, ete., or be due to fracture while the rocks are not deeply buried,
when such rocks become sufficiently deeply buried to be in the zone of
anamorphism, it is certain that rock flowage will take place and the
openings will be closed, except possibly those of subeapillary size and
other minute openings in which water, carbon dioxide, or other liquids and
gases are occluded. In the zone of anamorphism there is great pressure in
all directions, and mechanical energy becomes the dominant factor which
controls the reactions. Changes consequently take place which diminish
the volume of the rocks. This volume change increases the specific grav-
ity, and contrasts with the volume changes of the zone of katamorphism.
The fundamental chemical law of energy in reference to heat is subordinate.
Reactions take place with the liberation or absorption of heat, depending
upon what is demanded by the pressure. Commonly, the preponderant
chemical reactions are those which take place with absorption of heat.
The depth at which pressure becomes dominant is variable, depending
168 A TREATISE ON METAMORPHISM.
upon the character of the rock and upon whether the conditions are mass-
static or mass-mechanical.
It has been seen that at the moderate temperatures of the zone of kata-
morphism the preponderant chemical reactions are those which take place
with the liberation of heat. As the depth below the surface increases, the
temperature ever becomes higher; and consequently the temperature may
become so high that the tendency for chemical reactions to take place
with the liberation of heat is less'dominant, and at sufficiently great depths
the heat may be so great that this tendency ceases, or is even reversed.
Or, using the words of van’t Hoff, at high temperatures the preponderating
chemical reactions, or associations, which take place at lower temperatures
with the development of heat are replaced by preponderating chemical
reactions, or dissociations, which take place with the absorption of heat.”
However, at moderate depths in the zone of anamorphism under ordinary
conditions the temperatures are not very high. For instance, at a depth of
9,000 meters the temperature is probably in the neighborhood of 300° C.
Therefore, so far as the temperature is concerned, for that part of the crust
of the earth within observation the preponderant chemical reactions would
probably take place under the first part of van’t Hoff’s law, rather than under
the second part, if it were not for the pressure. But the pressure is the
dominant factor which controls the reactions. The rocks in this zone are
under so great pressure in all directions that this fact demands chemical
reactions which produce diminished volumes irrespective of whether heat is
liberated or absorbed by them.
The very important reactions in the zone of anamorphism are silica-
tion, or union of silicic acids with bases producing silicates, and dehydra-
tion. Deoxidation is subordinate. The process of silication commonly
takes place upon carbonates, and consequently involves decarbonation and
the liberation of the carbon dioxide, which may escape and thus the volume
be decreased. To what extent the pressure is the controlling factor in the
production of this reaction is difficult to say. Probably it is the dominant
cause, but it is possible that at the temperatures which prevail in this zone
silicic acid may be relatively more active than at the lower temperatures of
the zone of katamorphism, where carbonic is the stronger acid.
aNernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
p- 583.
REACTIONS OF ZONE OF ANAMORPHISM. 169
As illustrations of the process of silication may be mentioned the
formation of wollastonite from pure limestone, of tremolite from dolomitic
limestone, of actinolite from ankerite, and of griinerite from siderite. (See
pp. 239, 241, 243, 244.) In the impure limestones under deep-seated con-
ditions, where numerous bases are present, various complicated silicates
form, such as other pyroxenes and amphiboles, tourmaline, chondrodite, ete.
The process of dehydration involves the liberation of water. This
reaction, it is safe to say, is one which is controlled by pressure. The
combined water is actually squeezed out of the hydrated mineral particles,
transforming them to less hydrous and to anhydrous forms in a manner
similar to that in which free water is pressed from a sponge.
Whether or not pressure in the zone of anamorphism is sufficient to
deoxidize compounds is uncertain. Certainly it can not be asserted that
the pressure is sufficient to squeeze out a part of the oxygen of hematite,
thus transforming it to magnetite. So far as deoxidation oceurs, probably
the oxygen abstracted from the rocks usually unites with the elements of
organic compounds, thus producing carbon dioxide and water. Thus the
chief products liberated by silication, dehydration, and deoxidation are car-
hon dioxide and water. ‘These join the interstitial water in the subeapillary
spaces and probably slowly escape into the zone of katamorphism above.
(See pp. 665-667.) This results in loss of material, and since the specific
gravity of the minerals is increased on the average, the volume of the
rocks is decreased.
Besides the above processes, condensation may also be accomplished
by recrystallization, although this process generally takes place in connec-
tion with them. The process of recrystallization produces a rearrangement
of the elements in such a way as to form compounds of higher specific
gravity. This is well illustrated by the devitrification of glass.
The minerals produced in the zone of anamorphism are numerous,
definite, stable, crystalline, of high specific gravities, and probably have
complex molecules. The rocks formed are compact and strong. The
lower zone may therefore properly be called the zone of anamorphism, or
anamorphic zone. This use of the term anamorphism is parallel to the use
of the term anabolism in biology to designate those chemical changes in a
living body which result in the production of complex compounds from more
170 A TREATISE ON METAMORPHISM.
simple ones. The zone of anamorphism may be defined as the zone in
which alterations of rocks result in the production of complex compounds
from more simple ones.
Summarizing the energy factors in the zone of anamorphism, so far
as the volume change is concerned, the result is to liberate heat; so far
as the chemical reactions are concerned, heat may be liberated or absorbed,
but the latter reaction is more common. In the latter case the heat
absorbed is almost certainly much greater than that liberated by decrease of
volume. If it were not that a considerable number of the chemical reactions
liberate heat, it would be certain that heat is absorbed in the zone of
anamorphism But the heat liberated by some chemical reactions must
be added to that liberated by decrease of volume. Whether this sum is as
ereat as the heat absorbed by the preponderating chemical reactions is
somewhat uncertain; but it is thought to be rather probable, for the com-
pounds immediately concerned in the reactions, that the total effect is to
absorb heat and store energy. However, in order to accomplish this, energy
must be derived from an outside source, and when all the factors which in
any way affect the reactions are taken into account, including the movement
of the superincumbent material, heat is dissipated and energy lost. (See
p. 182.)
RELATIONS OF ZONES OF KATAMORPHISM AND ANAMORPHISM.
We shall now consider the zones of metamorphism together with
reference to the energy factors. So far as the chemical reactions are
concerned, it has been seen that they may take place with lberation or
absorption of heat. So far as heat is liberated energy is dissipated. So
far as heat is absorbed energy is stored. The change in volume may also
result in the dissipation or storage of energy. Where increase of volume
is preponderant energy may be stored (1) by increasing the volume of the
rocks affected by the reaction or (2) by elevating the overlying rocks in
order that the space shall be available for the expenditure. In a given
case the energy may be stored by (1) or (2) or a combination of them.
Where decrease of volume is prepouderant energy is dissipated (1) by the
decrease of volume of the rock affected by the reaction or (2) by subsid-
ence of the overlying material, or by both. Below the extreme outer film
of the earth the factor of elevation or subsidence of the overlying rocks is
of vastly greater importance than the volume change, and the relative
RELATIONS OF ZONES. ileal
importance of this factor steadily increases with depth. This is more
broadly true in the case of increase of volume than in, that of decrease
of volume; for in the latter case in the zone of katamorphism the strength
of the rocks near the surface may prevent subsidence, and the decrease of
volume simply produce porosity. A common illustration of that is vesicu-
lar dolomite. However, in the zone of anamorphism, when the reactions
result in decrease of volume, subsidence occurs and energy is dissipated.
The importance of the necessity of lifting the overlying material in order
to find more room in the case of increase of volume is well illustrated by
the frequent rapid hydration or slacking, with great expansion and rapid
disintegration, which follows when a partly hydrated rock, buried but a
few feet, is brought to the surface." Apparently when buried the tendency
for hydration and necessary expansion with liberation of heat was not
sufficient to lift the superjacent material. When the necessity of elevat-
ing the superjacent material was removed by transfer to the surface the
process of hydration and expansion went on to completion with great
rapidity.
I conclude from the foregoing that in so far as energy is concerned
there are four cases. Chemical reaction may (1) release energy and result
in the liberation of heat, or (2). may consume energy and result in the
absorption of heat. The change of volume may be (3) by decrease of
volume, and result in the release of energy and the liberation of heat, or
(4) by increase of volume, and result in the consumption of energy and
in the absorption of heat. (1) and (3) will be called plus, and when they
are combined the heat developed is equal to their sum; (2) and (4) will
be called minus, and when they are combined the heat absorbed is equal
to their sum. When (1) and (4) or (2) and (8) are combined heat may
be liberated or absorbed, and consequently energy dissipated or stored,
depending upon the relative values of the opposing factors.
It has been noted that the three important reactions in the zone of
katamorphism are oxidation, carbonation, and hydration; and in the zone
of anamorphism are deoxidation, silication, and dehydration.
Since all of the abundant metallic elements except iron are completely
oxidized as they occur in the original rocks, the important inorganic com-
pounds which are oxidized in the zone of katamorphism are mainly those
“Merrill, G. P., Disintegration of the granitic rocks of the District of Columbia: Bull. Geol. Soe.
America, vol. 6, 1895, p. 332.
172 A TREATISE ON METAMORPHISM.
of iron. Iron occurs extensively in the ferrous form, in magnetite, in car-
bonates, and in silicates. To a considerable extent it occurs as a sulphide.
To a small extent it occurs as metallic iron. In all of these forms it is
capable of oxidation. The main result of the oxidation of these com-
pounds, so far as the iron is concerned, is to change the monoxide to
ferric oxide. But where it is present as a sulphide it may be changed to a
sulphate, and then be thrown down as a basic ferric sulphate. Ferric oxide,
hydrous or anhydrous, is an important constituent in the sedimentary
rocks, and its presence is, without doubt, largely due to oxidation in
the zone of katamorphism. To a far less extent other metals, such as
copper, lead, zinc, ete, occur in the native form, in partially oxidized
forms, or as sulphides. All these substances may be oxidized. These
substances have little importance in general geology, but are of great
importance in the production of ores. All of the reactions of oxidation
take place with great liberation of heat and with increase of volume. In
the zone of anamorphism partial or complete deoxidation of the highly
oxidized compounds may occur. The ferric iron may be reduced to the
ferrous form. The sulphates of iron and the other metals may be reduced
to sulphides. In most cases the reducing agent is organic matter. The
reduction of the metals by organic compounds results in the oxidation of
the carbon and hydrogen, thus producing carbon dioxide and water. The
carbon dioxide and water largely escape. Where reducing agents are not
present the highly oxidized materials produced in the zone of katamorphism
commonly remain in this condition even if the material passes into the
zone of anamorphism. Deoxidation can not, therefore, be.said to be char-
acteristic of the zone of anamorphism to the degree that oxidation is
characteristic of the zone of katamorphism. The reducing reactions all
take place with great absorption of heat, so far as the metals are concerned,
and with decrease of volume. However, si1ice heat is liberated by the
oxidation of the carbon and hydrogen, it is probable that the sum total
of the heat reaction in deoxidation in the zone of anamorphism is to
liberate heat.
In the matter of oxidation and deoxidation, the zone of katamorphism
presents a case in which the chemical law of the liberation of heat controls,
without reference to change in volume, while in the zone of anamorphism
the pressure tending to produce decrease of volume and chemical reactions
with the liberation of heat probably work together.
CHEMICAL RELATIONS OF SILICON AND CARBON. IN}
Another set of reactions, of the most fundamental importance and
widespread character, which occur in an opposite sense in the two zones of
metamorphism are the mutual replacements of carbon dioxide and silicon
dioxide. It has already been noted that near the surface, or in the zone
of katamorphism, carbonic replaces silicic acid. Deep below the surface,
or in the zone of anamorphism, silicic replaces carbonic acid. Under the
conditions near the surface, where the pressure is small and the tempera-
ture is low, carbonic is the stronger acid; and under the conditions deep
below the surface, where the pressure is great and the temperature is
high, silicic is the stronger acid. The importance of the mutual replace-
ment of these compounds under different conditions makes it advisable to
summarize the chemical analogies of silicon and carbon. Silicon is the
characteristic element of inorganic compounds; carbon is the characteristic
element of organic compounds. How closely analogous are these two
elements is shown by the following comparative table:
Chemical relations of silicon and carbon.
SILICON. CARBON.
SiO, silica, anhydride, solid.........------------- CO, carbon dioxide, gas.
Sik lasiluconvlyyduid eyoaseery sees s elas aivatelsisrele eer CH, methane, gas.
Si@lj silicon chloride; liquids] 22222-2025 - 520-2222. CCl, carbon tetrachloride, liquid.
IB OU Statib Hepa cre ie eee tere ae etae olor sya) Boils at 76°.
SiHCl, silicon chloroform, liquid.-.....---------- CHCl, chloroform, liquid.
Roly ety vO | coacdgceneeosdsoppeESoEcsences Boils at 60°.
Si(C,H;), silicon ethyl; liquid ..-.----.---.------- C(C,H;), tetraethylmethane, liquid.
Boil stata 0 Crepes tees ere sia Seeie sain m= Boils at 120°.
Si(OC,H;), ethyl orthosilicate, liquid. ..-.-.------ C(OC,H;), ethyl orthocarbonate, liquid.
IBollstatpl OO sme ree ec eets oe csiciaccei== Boils at 158°. (See Mendeléeff, Vol. IJ,
Chap. XVIII, pp. 99-100.)
E91 O)orthostlicicacida ate e j= eel ee = » H,CO, orthoearbonic acid.
OH OH
si /OH 70H
\OH NOH
OH OH
SiO,(C,H;), ethyl orthosilicate.
(MgFe),SiO, olivine | Exists only in certain artificial organic compounds,
CaAl, (SiO,), anorthite ‘Natural orthosilicates. as ethyl orthocarbonate, CO,(C,H;),.
R/’,R//’, (SiO,)3 garnet, ete.|
HES Ospmetacilicictach dyeemeeriseeiee nine easel H,CO, carbonic acid.
OH OH
O=siC o=0%
OH OH
Exists in salts and in solution. Forms normal
(neutral) and acid salts (‘‘bicarbonates”’ ).
od
174 A TREATISE ON METAMORPHISM.
Chemical relations of silicon and carbon—Continued.
SILICON. CARBON.
KAI(SiO,), leucite (normal salts)
O—Na
AlZOSSi= 0=0%¢
OX . Natural metasilicates. O—Na
1
K—07 0-06. Ca
Be,Al, (SiO,), beryl, K
ete. (acid salts)
OH
o=cK
ONa
or
OH
Za
O=CY
Oa
YO
OH
H,Si,O; disilicic or dimetasilicic acid..... anata H,C,0; dicarbonic or pyrocarbonic acid.
yOu /OH
O=SiC 0=Cy
vi ww
O=SiC O=C\
OH OH
Si:O ::2:7 for basic salts. Known only in salts, as C,03(NaO),, produced by
Si:O::2:5 for acid salts. heating the acid salt.
Not known in free state.
LiAl (Si,O;), petalite.
H,Si,O, diorthosilicic acid... . 2... cece cccceccces The corresponding carbon acid does not exist.
(OH)
icant
0
siZ
\(OH)s
H,Mg;S8i,0, serpentine (normal salt).
H,CaSi,O,+H,0 okenite (acid salt).
H,Si,O, polysilicic acid or trisilicic acid......-.--- The corresponding carbon acid does not exist.
(May be considered as metasilicic acid plus
disilicic acid. )
/OH
Si=O
O
siZo
O
Si= (OH);
(neutral salts of trisilicie acid)
KAISi,0, orthoclase.
NaAISi,O, albite.
VOLUMES OF SILICON AND CARBON COMPOUNDS. 175
Another close analogy which exists between the carbonates and the
silicates is the fact that many salts of both give alkaline reactions, or under
the theory of dissociation are hydrolized as explained (pp. 86-87), and
that alkalinity creases with the temperature.
The specific volumes of the silicates and carbonates also have very
close relations. In general the specific volumes (the molecular weights
divided by the specific gravities) of the silicon compound are slightly the
ereater. The comparative specific volumes of a number of the correlative
silicon and carbon compounds are as follows:“
Specific volumes of silicon and carbon compounds.
SUCH a ei eet eed eer a TTD Fae COT ps ye Alc an a ae 94
SSIET@ Ipaaater RR Meat) JU BOEHNER C Iga WAS seers ay ee eae aoe 81
SHI(COCOLISI-) 7 es a i ee ee DOIEKC (OCH eoe cee aan sne ain clea uni 186
Casi OM ERR TAnee con aN SOU IE CAC Oe Hecate en aan A eee: 37
The specific volumes of SiO, and CO, are wholly different, but this is
explained by the fact that one is a solid and the other a gas.
Since the specific volumes of the carbon compounds are less than those
of the silicon compounds, if there be a simple substitution of carbon for
silicon the volume is decreased; if silicon for carbon, the volume is
increased. However, as a matter of fact, the changes in the rocks are
never so simple as this. The volume changes in carbonation with desilica-
tion, and in silication with decarbonation im the rocks largely depend upon
whether the reacting and resultant compounds are gaseous, liquid, or solid,
and whether the products remain as solids or are dissolved and transported
elsewhere.
In the zone of katamorphism carbon dioxide replaces silicon dioxide
ordinarily with liberation of heat
The fact of the carbonation of the silicates is well known. So far as I
know, the importance of this process was first realized by Bischof. He
attributes the general decomposition of the rocks near the surface mainly
to the action of carbonic acid, thus producing the carbonates which are
found in spring water. He shows by experiment that ‘the silicates of
alkalies, alkaline earths, protoxides of iron and manganese are decomposed
@Mendeléeff, D., The principles of chemistry, translated by Geo. Kamensky, Longmans, Green
& Co., London, 1897, vol. 2, pp. 99-100.
176 A TREATISE ON METAMORPHISM.
by carbonic acid at ordinary temperatures.”* But he says that, since
carbonic acid does not combine with alumina or peroxide of iron, the
silicates of these compounds are not decomposed by carbonic acid.” How-
ever, we now know that the process of carbonation takes place with all the
natural silicates. It will be shown in Chapter VII that this process of
carbonation goes on throughout the entire zone of katamorphism, but it is
in the upper of the two belts of the zone of katamorphism, that of weath-
ering, in which the process of carbonation goes on with greatest rapidity
and is especially characteristic. Simultaneously with the substitution of
the carbon dioxide for the silica much of the silica separates as colloidal
silicic acid, is taken into solution, and is carried downward to the belt of
cementation by the percolating waters. In this belt the silica is deposited
on an enormous scale. The carbon dioxide is furnished in solution, being
mainly derived directly or indirectly from the atmosphere. When carbon
dioxide replaces silicon dioxide the volume would be decreased, provided
all of the silicic acid were abstracted in solution. But it is probable that
the larger portion of the silica set free in the zone of katamorphism by
carbonation is deposited in the belt of cementation, and therefore the
volume of the zone of katamorphism as a whole, so far as this reaction is
concerned, is increased. The deposition of silica in the belt of cementation
is probably accompanied by a considerable absorption of heat, under the
law that the negative value of the heat of solution is greater the more
insoluble the substance.
Carbonation in the zone of katamorphism may take place without
replacing silica, as in the case of the union of carbon dioxide with iron
oxide in magnetite, thus producing iron carbonate. In this case the
liberation of heat and the increase in volume are both great.
In the zone of anamorphism, and especially under mass-mechanical
conditions, silica replaces carbon dioxide in the carbonates on the most
extensive scale. So far as I am aware, Bischof was the first to realize that
under proper conditions the process of carbonation of the silicates could be
reversed. He shows by experiment that carbonates of calcium, magnesium,
and iron are decomposed by silica at a boiling temperature, and cor-
@ Bischof, Gustav, Elements of chemical and physical geology, translated by Paul and Drummond,
Harrison & Sons, London, vol. 1, 1854, p. 2.
b Bischof, cit., vol. 1, pp. 4-5.
VOLUMES OF SILICON AND CARBON COMPOUNDS. 177
rectly infers that when any of these carbonates occur with quartz at a
sufficient depth within the earth, where a temperature of 100° C. is reached,
this reaction may take place. He calculates that this depth will be 2,440
meters. He correctly infers that the presence of abundant carbon dioxide
in deep-seated waters is probably due to this process of silication.* We
now understand that under conditions of moderate pressure and temperature
not only are the carbonates which Bischof mentioned decomposed, but other
carbonates may be altered in a similar manner. However, it is noteworthy
that the carbonates which Bischof mentioned are those of predominant
importance.
The substitution of silicon for carbon would result in increase of
volume provided silica were derived from the solutions and the carbon
dioxide passed into the solutions. But in the process of silication in the
belt of anamorphism little material is available from outside sources.
Therefore the most of the silica which replaces carbon dioxide in carbonates
must be considered as a solid. It is probable that a large part of the freed
carbon dioxide slowly escapes; for at temperatures prevailing in the zone
of anamorphism the carbon dioxide is above its critical temperature, and
therefore a gas, and probably slowly makes its way through the subeapil-
lary spaces to the zone of katamorphism (see p. 667.) Hence the volume
comparison must be made between the carbonate and replacing silica
combined and the resultant silicate. On this basis there is a marked
diminution of volume. One of the simplest illustrations of the formation
of the silicates with condensation of volume is the development of wollasto-
nite from calcium carbonate and quartz. In this change the volume of the
solid remainder is decreased 31.48 per cent. However, this calculated
decrease is somewhat too great; for it will be seen (p. 667) that some
of the carbon dioxide does not escape, but is retained in the rocks in the
form of numerous inclusions.
It appears from the foregoing that in the replacement of silicon dioxide
by carbon dioxide in the zone of katamorphism, the chemical law of reac-
tions with liberation of heat dominates over that of pressure; and that in
the substitution of silicon dioxide for carbon dioxide in the zone of
anamorphism the physical law that pressure demands decrease of volume:
dominates over the chemical law of reactions with liberation of heat.
aBischof, cit., vol. 1, pp. 237-241.
MON XLVII—04 12
178 A TREATISE ON METAMORPHISM.
The third important case in which the reactions occur in the opposite
sense in the zones of katamorphism and anamorphism are hydration and
dehydration.
Hydration is a characteristic reaction of the zone of katamorphism,
only less important than that of carbonation; moreover, hydration occurs
on a great scale both in the belt of weathering and in that of cementation.
That hydration occurs extensively deep in the belt of cementation is
evidenced by the hydrated minerals which develop in the cavities of the
rather deeply buried rocks, such as the amygdules of amygdaloids.
Hydration represents, in the words of the first part of van’t Hoff’s law, ‘an
association which takes place with great liberation of heat.” This process
also results in very considerable increase of volume, provided all or nearly
all of the products formed remain in situ.
Dehydration is a characteristic reaction of the zone of anamorphism,
only less important than that of silication. When the hydrated minerals
formed in the belt of katamorphism pass into the zone of anamorphism by
deep burial they are dehydrated. The pressure, or the high temperature,
or the two combined, unite to drive off a large part of the water. Dehy-
dration, in the words of the second part of van’t Hoff’s law, represents ‘‘a
dissociation which takes place with great absorption of heat” and it takes
place with decrease of volume.
Therefore, so far as hydration and dehydration are concerned, in the
upper zone the first part of van’t Hoff’s law, that of chemical reactions with
the liberation of heat obtains, but in the lower zone the law of diminution
of volume controls, regardless of the heat effect. The first part of this
statement is sufficiently evident; the second possibly needs further expla-
nation. To drive off the combined water of rocks at ordinary pressure
usually requires a temperature above 110° C. This temperature under
mass-static conditions would not be found untila depth of 3,300 meters had
been reached. It is certain that at depths much less than this, and at
temperatures lower than this, dehydration takes place on an important scale;
for it will be shown (p. 744) that in the transformation of mudstones to
shales there is a loss of about one-half of the combined water. I conclude
that under many circumstances the increase in temperature is not suffi-
cient to reverse the reaction of hydration, and therefore the reversal must
VOLUME EFFECTS OF HYDRATION AND DEHYDRATION. 179
be due to the pressure. However, in the lower part of the zone of ana-
morphism the temperature is frequently higher than 110° C., and under
such circumstances both the pressure and the temperature may work together
to produce dehydration.
The statement that the volume is decreased by dehydration is only
true provided the separated water, or a large part of it, escapes; for the
volume of the hydrated solid is less than that of the residual solid plus the
separated water; therefore, if the water could not escape, pressure would
tend to preserve the combination. Hence, the fact that the reaction does
take place in the zone of anamorphism shows that there is sufficient
pressure not only to separate the combined water from the rocks, making
it free water, but to squeeze the free water from the rocks as one can
squeeze the water from a sponge. The effective pressure doing the work
is equal to the pressure of the adjacent rocks less the weight of an equal
column of water extending to the surface. Thus, under mass-static condi-
tions, if the rocks have a specific gravity of 2.7, the effective weight in
producing dehydration and driving out the free water at a depth of 3,300
meters is that of a column of material of this height with specific gravity
of 1.7. Under mass-mechanical conditions, where the pressure as a result
of thrust may be much greater than that due to weight, the effective
pressure tending to separate the combined water is much greater. Conse-
quently, under such conditions dehydration may occur at much less depth
than under mass-static conditions. (See pp. 766-768.)
One or two minerals may be mentioned which illustrate the processes
of hydration and dehydration in the two physical-chemical zones. Near
the surface and to a considerable depth, under mass-statie conditions,
limonite and other hydrated oxides of iron develop. Deeper down, and
especially in connection with mass-mechanical action, limonite is dehy-
drated, and hematite is produced. As another illustration may be men-
tioned the somewhat similar compounds, chlorite and biotite. Near the
surface and under quiescent geological conditions chlorite forms. Deep
below the surface, and especially under mass-mechanical conditions, biotite
ordinarily develops. This is nowhere better illustrated than in the Michi-
gamme formation in the Marquette district of the Lake Superior region,
where these two minerals directly replace each other under the law just
180 A TREATISE ON METAMORPHISM.
stated.“ In the zone of katamorphism the complex hydrous silicates, such
as the kaolins, serpentines, and zeolites form. In the zone of anamorphism
these minerals are largely dehydrated, and such minerals as muscovite,
andalusite, garnet, staurolite, ete., are produced.
The physical-chemical principles cited (pp. 45-123) give reasons for
the existence of the above reverse sets of reactions in the two zones. We
can now give chemical or physical causes why oxidation, carbonation, and
hydration take place in the zone of katamorphism, and deoxidation,
silication, and dehydration in the zone of anamorphism, and so on for other
reactions.
While each of these sets of processes is particularly characteristic of
one zone, it is not meant to imply that each reaction may not occur in
both zones. But in the zone of katamorphism, oxidation, carbonation,
and hydration greatly predominate over the reverse processes. On the
other hand, in the zone of anamorphism, deoxidation, silication, and dehy-
dration predominate over the reverse processes.
If all of these sets of processes reversed as preponderant reactions at
the same depth, it would be possible to sharply separate the zones of
katamorphism and anamorphism. If, for instance, for a given region above
a depth of 10,000 meters the sum totals of the oxidation, carbonation, and
hydration were greater than the sum totals of reverse processes, the zone
of katamorphism would be sharply separated from the zone of anamor-
phism at this depth. But this is not the case. The reversal of each pair
of processes occurs at different depths; and, further, the reversal for a
given pair of processes is at different depths under different conditions.
One of the most important of these is as to whether the conditions are
mass-static or mass-mechanical.
Of the three sets of reversing reactions, oxidation and deoxidation,
carbonation and silication, hydration and dehydration, the first reverses
with the. least depth and pressure, the second requires the greatest depth
and pressure, and the last a mean depth and pressure. It has already been
noted that oxidation very frequently is replaced by deoxidation in the
lower part of the zone of katamorphism. It is certain that the process of
hydration is very greatly stayed, if it does not altogether cease, and may
«Van Hise, C. R., and Bayley, W.S., The Marquette iron-bearing district of Michigan: Mon. U.S,
Geol. Survey, vol. 28, 1897, pp. 444-459.
CONTRASTING REACTIONS OF DIFFERENT ZONES. 181
even be reversed in the lower part of the zone of katamorphism. It is
therefore apparent that the two zones are not sharply delimited. In
general, however, it may be said that the outer zone to a depth in which
oxidation, carbonation, and hydration preponderate is that of katamor-
phism, and that the deeper-lying zone, in which the reverse of these
processes preponderate, is that of anamorphism. But carbonation and its
opposite, desilication, are the most fundamental reactions of the zone of
katamorphism. Silication and decarbonation are the most fundamental
reactions of the zone of anamorphism. By these reactions more than by
any others, these zones are delimited. The three sets of reversing reac-
tions, oxidation and deoxidation, carbonation and silication, hydration and
dehydration, constitute three cycles in metamorphism. The second of
these cycles was recognized many years ago by Bischof (see pp. 176-177),
and was called the carbono-silicic cycle.
From the foregoing statement it is clear that the work of the zones of
katamorphism and anamorphism are opposed to each other. What the one
is doing the other is undoing. At the present time it is therefore possible
that in the case of any one of the pairs of opposed reactions, consider-
ing both the zones, either one of them preponderates, or that they are
approximately balanced. For instance, the amount of water being fixed
in the zone of katamorphism may be greater or less than the amount of
water being freed by dehydration in the zone of anamorphism, or the
two may be nearly balanced. The same statement may be made in refer-
ence to the other reversing reactions. Upon the preponderance of these
opposing sets of reactions in the opposite zones depends the answer to
the question whether, on the whole, oxygen, carbon dioxide, and water
from the atmosphere and hydrosphere are being fixed or freed by meta-
morphism. This question is considered in Chapter XI.
While the zones of katamorphism and anamorphism are separated
from each other by contrasting reactions, all reactions do not reverse in the
two physical-chemical zones. The first part of van’t Hoff’s law of heat
and the law of pressure may work together—that is, in both zones reactions
may oceur which, simultaneously with the liberation of heat by chemical
action, also result in liberation of heat by condensation. In so far as
there are cases of this kind it is to be presumed that such reactions are
common to both zones. As an instance in which heat is probably evolved
182 A TREATISE ON METAMORPHISM.
both by the chemical reactions and by the volume change in both zones may
be mentioned the devitrification of glass. (See Chapter V, pp. 251-252.)
The chemical reaction is presumably under the first part of van’t Hoff’s
law, and the, volume is decreased. Another instance of chemical reaction
with the liberation of heat and condensation of volume is the replacement
of calcium by magnesium in limestone, thus transforming the rock into
dolomite.“
It is thought to be certain that the total of all the changes taking place
in the whole of the mass of rocks concerned in any given modification of
the lithosphere results in the dissipation of energy, and it is believed that
such is the fact for each of the physical-chemical zones separately. In the
zone of katamorphism the chemical reactions result in liberation of heat;
the average volume reaction results in absorption of heat. It is, however,
thought certain that the residual is in favor of the former. In the zone
of anamorphism the average of the chemical reactions results in absorption
of heat; the average of the volume reactions results in the liberation of
heat. It has already been seen (pp. 170-171) that the amount of energy
required for the volume change rapidly increases with depth, and in the
lower zone it is thought that the heat liberated from the volume changes is
ereater than the heat absorbed by the chemical reactions, and therefore
‘that the residual is in favor of the liberation of heat.
Hence, it is concluded that the changes which take place in each of
the zones are under the general law of the running down of energy into
the form of heat which is dissipated, and this accords with the apparent
order of the universe. :
A corollary to the foregoing pages is the conclusion that in the upper
zone, where pressure is relatively unimportant, on the average, alterations
result in the expansion of the volume of the rocks; and that in the
deeper-seated zone, where pressure is important or dominant, on the average
the alterations result in the contraction of the volume of the rocks. It
follows as a further conclusion from this that the tendency of the alterations
« The verification from authorities of the heat of the chemical reactions and the volume relations
for the majority of the changes above mentioned have been very kindly made for me by Mr. A. T.
Lincoln. Mr. Lincoln either has found the results used in the works of Thomsen, Ostwald, Mendeléeff,
or other standard authorities, or from the data there found has been able to calculate results which
answer the specific questions I gave to him.
SPECIFIC GRAVITIES OF MINERALS IN THE ZONES. 185
in the first zone 1s, on the average, to produce. minerals of lower specific
gravity than the original minerals, while in the deeper-seated zone the
tendency, on the average, is to produce minerals of higher specific gravity.
Illustrations of the first rule are the minerals produced by the disinte-
gration and decomposition of rocks near the surface, out of which the sedi-
mentary rocks are built. Some of these are kaolinite (sp. er. 2.6—2.63),
quartz (sp. gr. 2.65), calcite (sp. gr. 2.72), chlorite (sp. gr. 2.60-2.96),
serpentine (sp. gr. 2.5-2.65), tale (sp. gr. 2.7-2.8), zeolite (sp. er. 2—2.4),
limonite (sp. gr. 3.5-3.96), ete. All of these minerals and most of the other
abundant undecomposed minerals, such as feldspar (sp. gr. 2.55—2.75),
which make up great masses of sedimentary rocks, have comparatively
low specific gravities.
The second rule is illustrated by the change from low to high specific
gravity of the minerals where the sedimentary rocks are metamorphosed.
As just seen, the minerals which compose the unaltered sedimentary rocks
are originally those of low specific gravity. Some of the abundant result-
ant minerals in the equivalent metamorphosed rocks have considerably
higher specific gravities, as, for instance, muscovite (sp. gr. 2.76-3), biotite
(sp. gr. 2.7-3.1), pyroxene (sp. gr. 3.2-3.6), and amphibole (sp. gr. 2.9-3.4),
and the still heavier minerals, garnet (sp. gr. 3.15—-4.3), staurolite (sp. gr.
3.65-3.75), chloritoid (sp. gr. 3.52-3.57), hematite (sp. gr. 4.9-5.3), and
magnetite (sp. gr. 5.168—-5.180). Less common heavy minerals are andalusite
(sp. gr. 3.16—3.2), fibrolite (sp. gr. 3.23-3.24), and chondrodite (sp. gr. 3.118-
3.24). With the above are the lighter minerals, quartz and feldspar; but
even these are quite as heavy as the average of the original minerals.
It is noticeable in the altered rocks that in proportion as deep-seated
metamorphism is advanced the heavier of the above minerals appear. In
the early stages of the metamorphism of shales, mica develops plentifully,
and the rocks become slates. Where the metamorphism is more intense
the heavier minerals, garnet and staurolite, appear, the material of the pre-
viously developed micas being absorbed at the places occupied by the
garnet and staurolite.
The garnet-, staurolite-, chloritoid-, andalusite-, and tourmaline- bearing
schists and gneisses of the Penokee and Marquette districts of Michigan
and Wisconsin and the Black Hills of South Dakota, produced by the
184 A TREATISE ON METAMORPHISM.
alteration of clastic rocks, are perfect illustrations of the above changes.
In these rocks the acid feldspars (sp. gr. 2.55-2.67) have extensively
altered into quartz (sp. gr. 2.65) and mica (sp. gr. 2.76-3.01), and therefore
have passed into minerals denser on the average than those from which
they were derived. Also the heavier minerals, garnet, etc., have developed
on an extensive scale in the more metamorphosed varieties.
When all the minerals formed are taken into account the average
is as given. But it is not supposed that there are not exceptions to each of
the rules that in the upper physical-chemical zone lighter minerals form
and in the lower zone heavier minerals develop. Indeed exceptions are
known to both. An illustration of such exceptions in the upper zone is the
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201
ROCK-MAKING MINERALS.
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202 A TREATISE ON METAMORPHISM.
SECTION 2.—GENERAL NATURE OF ALTERATIONS.
Minerals may be altered (1) without chemical change and (2) with
change of chemical composition.
ALTERATION WITHOUT CHANGE IN CHEMICAL COMPOSITION.
The alterations which occur without changes in chemical composition
3 I
are (a) molecular rearrangement and (b) simple recrystallization.
MOLECULAR REARRANGEMENT.
Molecular rearrangement alone means passage from one crystalline
form to another erystalline form. Such change of form may result from
changed physical conditions, as, for instance, change in temperature or
pressure or movement. As an example of molecular rearrangement due
to change of temperature may be mentioned leucite, which crystallizes
from a hot magma in the regular system, but which changes upon cooling
to ordinary temperatures to a complex twinned anisometric form. An
example of a change due to pressure is furnished by orthoclase, which is
said for this reason to alter to microcline.* Molecular readjustments such
as above are simply changes of form, and are therefore called paramorphism.
SIMPLE RECRYSTALLIZATION.
Simple reerystallization usually but probably not always occurs through
the medium of a certain amount of water, which is able to take material into
solution and deposit it from solution. Changing pressure and comparatively
high temperatures are favorable conditions for such recrystallization. Per-
haps the most common example of recrystallization without chemical
change is that of the transformation of amorphous or finely crystalline
calcium carbonate to crystalline or more coarsely crystalline calcium
carbonate, such as occurs in limestones and marble. This process has
been called marmorosis. Another instance of recrystallization without
change in chemical composition which takes place, on an extensive scale, is
alteration of flinty or finely crystalline quartz to coarsely crystalline
quartz. ; .
ALTERATION WITH CHANGE IN CHEMICAL COMPOSITION.
Alterations with chemical change may take place (1) without the
addition or subtraction of material or (2) with the addition or subtraction
“Dana, J. D., A system of mineralogy, Descriptive mineralogy by E. 8. Dana, Wiley & Sons, New
York, 6th ed., 1892, p. 318.
ALTERATIONS OF MINERALS. 203
of material. For either of these changes the presence of water is required
in most instances, the alterations taking place through solution and redepo-
sition, although it is not impossible that solids may act upon one another
to an important extent without the help of water.
ALTERATION WITHOUT ADDITION OR SUBTRACTION OF MATERIAL.
In the changes which oceur under this case the material moves only
short distances. Such changes may be (a) a crystallization of an amor-
phous substance or (b) interior alteration of minerai particles.
An instance of the crystallization of an amorphous substance is
furnished by the devitrification of glass. In this alteration the uniform
homogeneous solid glass changes into a heterogeneous crystalline solid, the
different mineral particles of which have differmg compositions. This
involves segregation of the different elements in various proportions into
the different minerals. It is therefore clear that the materials have moved
very short distances.
Interior alteration of mineral particles is effected by the change of
one mineral into two or more minerals. his is illustrated by the change
of pyrope into enstatite, spinel, and quartz; the change of pyrope into
hypersthene, spinel, and quartz; the change of spodumene into eucryptite
and albite; the change of almandite into hypersthene, spinel, and quartz;
and the change of titanite into perovskite and quartz.
ALTERATION WITH ADDITION OR SUBTRACTION OF MATERIAL.
The changes which take place with the addition or subtraction of
material may vary from those which involve the slightest addition or
subtraction to complete substitution. The added material may come from
afar or from the adjacent mineral particles. The subtracted material may
enter into an adjacent mineral particle or may be transported great
distances before entering into a new mineral. Reactions between adjacent
minerals may produce new minerals. ‘Two or more minerals may unite to
produce a single mineral. For example, olivine and quartz may pass into
anthophyllite; nephelite and halite into sodalite; albite and halite into
marialite. Or two or more minerals may unite to produce two or more
new minerals. For example, rutile and magnetite may pass into ilmenite
and hematite; diopside and magnetite into tremolite and calcite; sahlite,
siderite, and magnesite into actinolite and calcite; augite, siderite, and
204 A TREATISE ON METAMORPHISM.
magnesite into hornblende and calcite; or the reverse of this, hornblende
and calcite into augite, siderite, and magnesite.
The changes here belonging are by far the most numerous and impor-
tant of the various classes; indeed, are vastly more important than all of
the other classes together. By far the greater number of reactions written
out on the succeeding pages for the alterations of the various minerals fall
under this heading.
The more important of these alterations, considered from the point of
view of the nonmetallic elements, may be classified into:
(1) Oxidation. (2) Deoxidation.
(8) Hydration. (4) Dehydration.
(5) Carbonation. (6) Decarbonation.
(7) Silication. (8) Desilication.
(9) Silicification. (10) Desilicification.
Less important reactions are:
(11) Sulphidation. (12) Desulphidation.
(13) Sulphation. (14) Desulphation.
(15) Titanation. (16) Detitanation.
(17) Phosphation. (18) Dephosphation.
(19) Chloridation. (20) Dechloridation.
(21) Fluoridation. (22) Defluoridation.
(23) Boration. (24) Deboration.
A number of these reactions are of small consequence so far as the
alterations of rocks are concerned; but all are important with reference to
the development of minerals, and especially in reference to economic
products. This phase of the subject in reference to the metallic products
is treated in Chapter XII.
(1) Oxidation is the addition of oxygen. Frequently the added oxygen
is substituted for another element, often sulphur.
(2) Deoxidation is the subtraction of oxygen. Often the subtracted
oxygen is replaced by another element—for instance, sulphur.
(3) Hydration is the addition of water, producing hydroxides.
(4) Dehydration is the subtraction of water from hydroxides. When
carried to completion, anhydrous compounds are formed.
(5) Carbonation is the union of carbonic acid and base, or the substi- —
tution of carbonic acid for another combined acid, in either case producing
carbonates. The oxide with which carbonic acid most frequently unites is
CLASSIFICATION OF ALTERATIONS. 205
iron oxide. Carbonic acid may replace any of the other ternary rock-
forming acids, including silicic, titanic, and phosphoric, and thus become
united with any of the important bases. The carbonation of the silicates
is of fundamental importance. The carbonation of the titanates and phos-
phates is unimportant.
(6) Decarbonation is the separation of carbonic acid from a base with-
out the addition of other compounds, or with the substitution of another
acid for the carbonic. The most frequent substituted acid is silicic.
(7) Silication is the union of silicic acid and base, or the substitution
of silicic acid for a combined acid, in either case producing silicates. The
only important oxide with which silicic acid unites as a rock-forming con-
stituent is iron oxide. Silicie acid may replace carbonic, titanic, or phos-
phoric acid, thus becoming united with any of the bases with which it can
combine. The silication of the carbonates is of fundamental importance.
The silication of the titanates and phosphates is unimportant.
(8) Desilication is the separation of silicic acid and bases without the
addition of other compounds, or with the substitution of another acid for
the silicic acid. The most frequent acid substituted is carbonic.
(9) Silicification involves the addition of silica without union with
bases. The added silica may or may not replace other compounds.
(10) Desilicification involves the subtraction of free silica. The sub-
tracted silica may or may not be replaced by other compounds.
(11) Sulphidation is the union of sulphur with a metal forming
sulphides. Added sulphur may be substituted for another element, usually
oxygen.
(12) Desulphidation involves the subtraction of sulphur. Generally
the subtracted sulphur is replaced by another element, usually oxygen.
(13) Sulphation is the union of sulphuric acid with base or the
substitution of sulphuric acid for another combined acid, in either case
producing sulphates.
(14) Desulphation is the separation of sulphuric acid and base, or the
substitution of another acid for the sulphuric.
(15) Titanation is the union of titanic acid with base, or the substitution
of titanic acid for another combined acid, in either case producing titanates.
(16) Detitanation is the separation of titanic acid and base, or the
substitution of another acid for the titanic.
206 A TREATISE ON METAMORPHISM.
(17) Phosphation is the union of phosphoric acid with base, or the
substitution of phosphoric acid for another combined acid, in either case
producing phosphates.
(18) Dephosphation is the separation of phosphoric acid and base, or
the substitution of another acid for the phosphoric acid.
(19) Chloridation is the addition of chlorine, forming chlorides.
(20) Dechloridation is the subtraction of chlorine, destroying chlorides.
(21) Fluoridation is the addition of fluorime, forming fluorides.
(22) Defluoridation is the subtraction of fluorine, destroying fluorides.
(23) Boration is the union of boric acid with base, or the substitution
of boric acid for another combined acid, in either case producing borates.
(24) Deboration is the separation of boric acid and base, or the
substitution of another acid for the boric.
GENERAL STATEMENTS.
The foregoing processes are seen to be in pairs, in each case one of a
pair being the reverse of the other. That is, deoxidation is the reverse of
oxidation, dehydration is the reverse of hydration, etc. Moreover, one of
the processes of a pair is in several of the cases frequently the complement
of that of another pair. To illustrate, the processes of the following pairs
are often complementary of each other, viz, oxidation and desulphidation,
sulphidation and deoxidation, carbonation and desilication, silication and
decarbonation. By complement is meant that one takes place simultaneously
with the other, and that the two may really be one chemical reaction. In
such a case the change may be considered from either of two points of view.
To illustrate, the process of carbonation may be also a process of desilication,
and the process of silication may be also a process of decarbonation.
In general the process is named on the basis of the substance added rather
than that subtracted, for such substance is the active agent which drives
off the other and takes it place. It has been shown (pp. 168, 170-181)
that for several reactions one of a pair is particularly characteristic for one
of the zones of metamorphism. To illustrate, oxidation and its complement
desulphidation, carbonation and its complement desilication, and hydration
are particularly characteristic of the zone of katamorphism; sulphida-
tion and its complement deoxidation, silication and its complement decar-
bonation, and dehydration are particularly characteristic of the zone of
anamorphism.
MINERALS. 207
SECTION 3.—ROCK-MAKING MINERALS.
MANNER OF TREATMENT.
GENERAL STATEMENTS.
In this treatise only the principal rock-forming minerals will be consid-
ered. The point of view is not that of mineralogy but that of metamorphism.
So far as it seems advisable without too much repetition, I shall consider
each mineral in reference to the following:
(1) Its composition, crystallization, specific gravity, and source, so far
as it is a rock-making mineral; but its occurrence in veins will not be
considered.
(2) The minerals into which it may pass, giving their crystallizations,
specific gravities, and compositions.
(3) The chemistry and physics of the processes of change, including
the volume relations.
(4) The natural conditions under which the changes occur, and the
causes of the changes.
With many minerals this outline can be carried out nearly to com-
pletion. With others the present state of knowledge is such that it can
be only very incompletely done. Consequently there is great variation
in the satisfactoriness of the discussion of the different minerals. When
the treatment of each of the minerals from these various points of view
can be carried out we shall have an interlocking system by which each
mineral is considered in its most important metamorphic connections.
To a certain extent the plan involves repetitions, but in each case the
important facts which concern an individual mineral are brought together.
The method of treatment proposed seems advisable, for many minerals
are both primary and secondary, and only by considering each mineral
from both poiuts of view is it possible to understand the causes of the
changes as well as the changes themselves. Ordinarily the latter only are
considered. When one of the sources of a mineral is the alteration of
another the exact reactions concerned in the change are not given under
the former, but may be found by referrmg to the latter mineral which is
mentioned as its source. Ordinarily, however, qualitative statements are
made. To illustrate, a source of limonite is siderite. The reactions involved
in this change are to be found under siderite, not under limonite; but under
208 A TREATISE ON METAMORPHISM.
the latter mineral the statement is made that the change generally involves
liberation of heat and decrease of volume. But when a mineral is derived
by precipitation from a solution, or results by the combination of several
minerals, it is necessary to consider the chemistry and physics of the change
in connection with the sources, for otherwise this important part of the
history of metamorphism of minerals would be omitted.
In discussing the sources of a mineral when it is derived from other
minerals the natural conditions of the alterations are not given, but may be
found by referring to the minerals from which the one under discussion
is derived. But where a mineral is derived by the interaction or union of
several other minerals the natural conditions are discussed under the source
of the mineral, for otherwise this part of the subject would be omitted.
As this treatise was originally planned it was designed to include the
heat and volume changes with the chemical reactions. But with the
present state of knowledge of the heat relations in chemical transformations
the first has been found impracticable. While very few quantitative
results can be given, in many cases it is possible to make a qualitative
expression of the heat reaction. To illustrate, the heat of combination of
calcium is far greater than that of iron in all analogous compounds in
which determinations have been made; but such determinations have not
been made with reference to the silicates. Where calcium is replaced by
iron in the alteration of the silicates it is inferred that a considerable amount
of heat is absorbed, though the exact amount can not be specified. Vice
versa, where iron is replaced by calcium, a considerable amount of heat is
liberated. Of course, in each reaction the other chemical combinations
which occur simultaneously should be considered, for they constitute a part
of the chain, and in obtaining a correct end result their effects are vital. If,
for instance, a salt of iron and a salt of calcium interchange acids, no general
statement can be made as to the heat reaction. Therefore, if at the same
time the iron replaces the calcium the calcium unites with an acid which
was before in combination with the iron, the inference above given as to
absorption of heat can not be made.
For the calculation of the volume changes, the equations of the chem-
ical reactions written out by me and the specific gravities of the minerals,
taken from the standard Mineralogies, were turned over to Mr. A. T. Lincoln,
who made the numerical computations. Subsequently Mr. R. M. Chapman
CALCULATION OF VOLUME RELATIONS. 209
repeated the work in order to verify it. The following well-known principle
was employed:
The volume of the original compound is to the volume of the compound
produced directly as their molecular weights and indirectly as their specific
gravities.
Under this general principle are two cases:
Case 1. Where one solid compound alters into another solid compound.
This case is illustrated by the well-known changes of limestone to dolomite.
In this change we have 2CaCO, replaced by MgCa(CO,),. The molecular
weight of 2CaCO,; is 198.62. The molecular weight of MgCa(CoO,), is
182.96. ‘The specific gravity of calcite may be taken as 2.7135; of dolomite,
as 2.85. The compound proportion is therefore as follows:
wi. 198. 62 : 182. 96
Vi Ver 9185: 2. 7135
or the volume of the dolomite is 87.70 per cent of that of the calcite;
or, therefore, there is a decrease in volume of 12.30 per cent.
Case 2. This has three phases: (a) where two or more solid compounds
unite to produce a single solid compound; (b) where a single solid
compound breaks up, producing two or more compounds, and (c)
where two or more solid compounds unite to produce two or more solid
compounds. In this case the method of calculation is slightly different
from case 1. The molecular weights of each of the compounds represented
in the equations are divided by the specific gravities of the respective
compounds. This gives their relative volumes. In phase (a) the volume
of the resultant single compound is divided by the sum of the volumes of
the producing compounds, and this gives the percentage of change. In
phase (b) the sum of the volumes of the resultant compounds is divided by
the volume of the original compound. In phase (c) the sum of the volumes
of the resultant compounds is divided by the sum of the volumes of the
original compounds. These different phases are so similar in method that
it is necessary only to illustrate one of them. The first phase is illustrated
by the formation of wollastonite by the union of calcite and quartz, the
reaction being:
CaC0,-++Si0, =CaSi0,+-CO,.
The molecular weights of the three solid compounds are, respectively,
MON XLyim—04— 14
210 A TREATISE ON METAMORPHISM.
99.31, 59.94, and 115.58. Their specific gravities are 2.7135, 2.6535, and
2.85, respectively. The volume of the wollastonite is, therefore:
That is, the decrease in volume in this case of silication of calcite is 31.5
per cent.
In order to expedite the laborious numerical calculations of the volume
relations for the very numerous alterations, Mr. Lincoln completed the table
on pp. 195-201 by adding the molecular weights, the logarithms of the
molecular weights, the logarithms of the specific gravities, the molecular
volumes, and the logarithms of the molecular votumes of each of the min-
erals. These determinations have been carefully verified by Mr. Chapman,
and may be used to check the volume changes given in the succeeding
pages, and also to make additional volume calculations.
In calculating the volume relations, unless otherwise specified, the
compounds on both sides of the equations are regarded as solid except those
which by themselves independent of the solvents are liquids or gases, such
as H,O and CO,. All such compounds are supposed to be added in the
solutions or to be taken away by the solutions, and therefore are not taken
into account in the volume calculations. In general these liquid and
gaseous compounds do undoubtedly escape in large measure, although in
some ‘cases they are confined as inclusions within the minerals formed.
(See p. 678.)
Where +k is added to the equation, this signifies that heat is liberated;
where —k appears, this means that heat is absorbed by the reaction.
No claim is made that the equations which are written in the following
pages exactly represent the changes that take place in the alterations of
the various minerals into other minerals — Indeed, the probability is that
not half exactly represent the facts; for the great majority of the reactions
are more complicated than written, and in many cases substances in the
solutions or as solids not taken into account are concerned. Since these
are the facts, the question may be asked why the equations are written.
The answer is, first, that at some time the attempt must be made to give
a first approximation to quantitative exactness in the alteration of minerals.
The equations found on the following pages represent such an attempt.
Before the appearance of this treatise scarcely more than a score of mineral
CALCULATION OF VOLUME RELATIONS. 211
alterations have been expressed by chemical equations, and in fewer still
have the volume relations been calculated. Second, the imperfect
equations herein contained will be sure to lead to closer imvestigations of
the nature of the alterations, and to improved equations representing them.
Thus the progress of science will be promoted by the set of equations here
given, even if the great majority of them are defective. Third, it is
believed that when a more nearly correct set of equations is written it will
be found that the large majority of the equations herein contained substan-
tially represent the facts, ‘and consequently that the volume changes are in
most cases roughly approximate. Many of them may be changed by a
few per cent one way or the other; but the sign of few will be changed,
and this is the fundamental point in reference to the zones in which the
alterations occur. |
The weakest point in the accuracy of the volume reactions is not tound
in the chemical equations, but in the inexactness of the specific gravities of
the minerals as given in the text-books. For most minerals there is a con-
siderable range of specific gravity given; and with the exception of one or
two minerals, such as calcite and quartz, it is impossible to ascertain the
exact specific gravity of the pure minerals. In the table the mean between
the two best determined extremes is given as the best approximation
available of the specific gravities of the pure minerals. For most minerals
these extremes are taken from Dana’s System of Mineralogy.
The facts as to the occurrences and alterations of the various minerals
given in the following pages are largely taken from the standard text-
books of mineralogy and petrology, and especially from Dana’s great
System of Mineralogy. The information available is especially imperfect
as to the manner in which the complex minerals, and particularly the
complex silicates, break up into simpler compounds in the belt of weather-
ing. As explained fully in the following chapter, this is a general process.
For the better known of these changes equations are written, but no
attempt is made to express by equations the manner in which many of the
minerals decompose and degenerate, because so little exact formation is
available upon which to base such equations.
As already stated, only those minerals will be considered which are
important rock-making constituents. It is impracticable at the present time,
to consider the physical-chemistry of the rarer minerals.
212 A TREATISE ON METAMORPHISM.
Following the ordinary classification, the abundant rock-making
constituents may be considered under the headings: Native Elements,
Sulphides, Fluorides, Oxides, Carbonates, Silicates, Titanates, Phosphates,
and Sulphates.
NATIVE ELEMENTS.
GRAPHITE.
Graphite:
Crystallized carbon (C).
Rhombohedral.
Sp. gr. 2.09-2.23; av. 2.16.
Occurrence. —_(Graphite occurs as a very widely disseminated constituent
in the extremely metamorphosed sedimentary rocks, which in their original
condition contained carbonaceous material. It is especially prevalent in
scales in the marbles, schists, and gneisses. In some instances the original
beds were so heavily carbonaceous as to give considerable layers a large
percentage of which is graphite. Such layers are illustrated by the
graphitic shales of Worcester, Mass.* Graphite occurs to some extent with
the very hard anthracite coals, a part of the carbon having passed over to
the graphitic condition. Such graphitic coals occur in the Rhode Island
coal field.” The reaction producing graphite as a metamorphic mineral
requires great pressure and takes place with decrease in volume. This
mineral in the sedimentary rocks is therefore a product of the zone of
anamorphism.
Graphite is said to occur as an original constituent in some basaltic
rocks. During the alterations of carbonaceous rocks the hydrocarbon com-
pounds, as gases, oils, and bitumen, wander widely in the solutions. In
some cases such compounds are deposited in the openings of original rocks
Later these compounds may be altered to graphite, and yet the carbon not
be an original constituent of the magma from which the rocks crystallized.
aiterations—A]terations of graphite are not recorded, but it is by no
means certain that this mineral is not very slowly oxidized under favorable
conditions in the belt of weathering.
THE SULPHIDES.
The sulphides which are important as rock-making minerals are
pyrrhotite, pyrite, and mareasite. Many other sulphides are important in
“Perry, J. H., Note ona fossil coal plant found at the graphite deposit in mica-schists at Worcester,
Mass.: Am. Jour. Sci., 3d ser., vol. 29, 1885, pp. 157-158.
bShaler, N. S., Woodworth, J. B., and Foerste, A. F., Geology of the Narragansett Basin: Mon.
U.S. Geol. Survey, vol. 33, 1899, p. 82.
OCCURRENCE OF PYRRHOTITE, PYRITE, AND MARCASITE. 213
the genesis of ore deposits. These, however, will be considered only in the
chapter on that subject.
PYRRHOTITE, PYRITE, AND MARCASITE.
Pyrrhotite:
Fe,S, to Fe,;S,.; chiefly Fe,,S,..
Hexagonal.
Sp. gr. 4.58+4.64.
Pyrite:
Fe§,.
Isometric.
Sp. gr. 4.95-5.10.
Marcasite:
Fe§,.
Orthorhombic.
Sp. gr. 4.85-4.90.
Occurrence —Pyrrhotite, pyrite, and marcasite are very widespread acces-
sory minerals, occurring in rocks of all ages and all kinds. So far as known,
these minerals aru not abundant original pyrogenic constituents, although
they frequently are fcund along the contact between intrusive and other
rocks, occurring in both the intrusive and the intruded rocks. Pyrrhotite
is an original mineral in meteorites. These minerals extensively form in
rocks in volcanic districts through the action of solutions of hydrogen
sulphide and other sulphide solutions upon iron salts. As secondary minerals
in the sedimentary rocks, and to a less extent in the igneous rocks, the
sulphides are extensively formed through the reducing action of organic
compounds upon the sulphites and sulphates, especially the latter, and par-
ticularly iron sulphate. Such reduction is characteristic of the belt of
cementation and the zone of anamorphism; but in the latter zone pyrrhotite
or pyrite, rather than marcasite, probably forms.
The reducing agent of the sulphites and sulphates may be either a
solid organic compound or one of its gaseous products of decomposition,
such as carbon monoxide (CO) and carburetted hydrogen (CH,). If the
reducing agent be taken as CO, the reaction for pyrite and marcasite
may be:
2FeSO,+7CO=FeS,+ FeCO,+6C0,+k4
and for pyrrhotite:
12F eSO,-+-45C0=Fe,,S,.-+ FeCO,+4500,-+k.
If the reducing agent were taken as carbon, similar results would be
obtained, except that the amount of CO, would be less. This action, while
aSee page 210.
214 A TREATISE ON METAMORPHISM.
ordinarily called a reduction, is reduction so far as the iron sulphate is
concerned, but is oxidation so far as the carbon compound is concerned,
and hence the explanation of the liberation of heat.
Pyrite, mareasite, and pyrrhotite are also doubtless produced by the
action of soluble sulphides upon the iron oxides or iron salts. In the
change from crystallized Fe,O, (hematite) to FeS, Gn the form of pyrite),
the volume increases 56.14 per cent.
Alterations — The first alteration to be considered is that of marcasite into
pyrite. In this alteration there is recrystallization, an increase of symmetry,
a decrease of 2.98 per cent in volume, but no change in chemical compo-
sition. The heat effect is undetermined, but probably heat is liberated.
The mineral pyrrhotite by recrystallization passes into pyrite. This
change may occur in voleanie districts by the action of hydrogen sulphide
upon the pyrrhotite, the reaction perhaps being:
Fe,,S+-10H,S=11F eS, + 10H).
In this change the volume is increased 21.13 per cent.
The minerals pyrite and marecasite may by oxidation pass directly
into (1) hydrated sesquioxide of iron, of which, ordinarily, limonite (not
crystallized; sp. gr. 3.80) is the most common kind; (2) magnetite (isomet-
ric; sp. gr. 5.174); (3) ferrous sulphate, which may be removed in solution,
or (4) may be decomposed by further oxidation, either at the place of
formation or elsewhere, after a longer or shorter time, into hydrated sesqui-
oxide of iron, ordinarily limonite. The reactions for marcasite and pyrite
may be as follows, assuming in each case that the sulphur, or a part of it,
is also oxidized:
(1) 4FeS,-+220+3H,0=2Fe,0,.3H,0+880,+k.
(2) 3FeS,+160=Fe,0,+680,-+k, or
3F eS, +4H,0+40=Fe,0,-+4H,S+2S0,+k.
(3) FeS,+60=FeS0,+80,-+k, or
FeS,+30+H,0=FeS80,+H,S+k.
(4) 4FeS0,+20+7H,O=2Fe,0,.3H,0+4H,S0,+k.
As shown in Chapter XI, on ‘‘Ore deposits,” pyrite and mareasite also
alter to hematite without oxidation by the reaction of an alkaline carbonate.
The alteration of common pyrrhotite into magnetite and limonite may
be written as follows:
(5) 3¥Fe,S)+1160=11Fe,0,+3680,-+k, or
3Fe,,8,.+36H,0+80=11 Fe,0,+36H,S-+k.
(6) 4Fe,,8,.+33H,0-+1620=11(2Fe,0,.3H,0)+4880,-+k. »
ALTERATIONS OF PYRRHOTITE, PYRITE, AND MARCASITE. 215
If in the production of the limonite the pyrrhotite passes through the
stage of ferrous sulphate the reaction producing the sulphate may be:
(7) FeyS,,+460=11FeS80,+80, +k, or
Fe,,8,.+H,0-+430=11FeS0,+-H,S-+k.
The change from the ferrous sulphate to the limonite is the same as in
the case of pyrite and mareasite. Where water is present the SO, produced
in the above reactions would unite with water and form H,SOz,, or if further
oxidized H,SO,.
As the end results of alteration are usually limonite or magnetite, the
volume relations for these two compounds will be given. In the change
of pyrite to limonite the volume is increased 2.93 per cent; to magnetite,
is decreased 37.48 per cent. In the change from marcasite to limonite the
volume is decreased 0.14 per cent; to magnetite, is decreased 39.34 per
per cent. In the change of pyrrhotite to magnetite the volume is decreased
24.27 per cent; to limonite, is increased 24.68 per cent.
When pyrite and marcasite pass into limonite there is a change from
a crystalline to an amorphous form. In the alteration of pyrite to magne-
tite the system does not change. In the alterations of pyrrhotite and mar-
casite to pyrite there are changes from lower degrees of symmetry to the
highest degree of symmetry, that of the isometric system. The change
from marcasite to pyrite occurs especially in the zone of anamorphism,
subject to the principle there obtaining that the changes take place with
decrease in volume. The change of marcasite to pyrite is an excellent
illustration of the principle that where the pressure is great minerals tend
to pass into other minerals having a higher degree of symmetry and a higher
specific gravity (see pp. 360-365). The abundance of marcasite as an
autogenic constituent in rocks not deeply buried, its absence in the rocks
which have been in the lower zone, and the presence of pyrite in these
rocks, are thus all explained. Where the pressure is small near the surface
marcasite with lower symmetry and lower specific gravity than pyrite may
abundantly form. At depth where the pressure is great pyrite of higher
specific gravity and higher symmetry forms. If rocks near the surface in
which marcasite has formed are buried to a great depth by superimposed
strata the marcasite previously formed changes to pyrite.
Similar statements can not be made concerning pyrrhotite and pyrite,
for these minerals have unlike compositions. Doubtless where the necessary
216 A TREATISE ON METAMORPHISM.
chemical reactions can take place there is a tendency in the lower zone for
pyrrhotite to alter to pyrite.
The natural conditions for the transformation of pyrite, marcasite, and
pyrrhotite to limonite are those of abundance of oxygen and moisture.
These conditions are found in the zone of katamorphism, and especially in
the belt of weathering. In this belt the process goes on with such rapidity
that pyrite, marcasite, and pyrrhotite have generally been completely
oxidized where the rocks have been long exposed to the reactions of the
belt. The reactions are oxidation and hydration. They take place with
great liberation of heat and, for pyrite and pyrrhotite, with some expansion
of volume, and these changes may therefore be taken as typical illustrations
of alterations of the belt of weathering.
The conditions for the formation of magnetite from pyrite, marcasite,
and pyrrhotite are the presence of some oxygen, but not a sufficient amount
to fully oxidize the iron, and considerable pressure. Where iron carbonate
is present, which also alters to magnetite, oxygen is not necessary. | This
reaction is of great consequence. (See p. 244.) The alterations of the
sulphides to magnetite involve a decrease of volume of 24 to 39 per cent
and liberation of heat. Corresponding with this fact, the changes take
place in the belt of cementation or in the zone of anamorphism.
THE FLUORIDES.
Among the fluorides the only important rock-making mineral is fluorite.
FLUORITE.
Fluorite:
CaF,.
Isometric.
Sp. gr. 3.01-3.25.
Occurrence.—T]"]uorite occurs as an accessory constituent, especially in
granitic and syenitic rocks. It is also found in other eruptive rocks, and
in metamorphic rocks, such as the schists and marbles. It therefore has a
somewhat widespread occurrence, but is of very subordinate importance.
Alterations.—By the action of alkaline waters fluorite alters into calcite
(rhombohedral; sp. gr. 2.7135). Supposing the alkaline compound to be
sodium carbonate, the reaction is:
CaF,-+ Na,CO,=CaC0,+2NaF +k.
The increase in volume of the calcite as compared with the fluorite is 47.66
per cent.
OCCURRENCE OF QUARTZ. Palit
THE OXIDES.
The more important oxides occurring as rock-building constituents are
those of silicon, iron, and titanium. The oxides of silicon are quartz,
tridymite, and opal. The important oxides of iron are hematite, magnetite,
and limonite. The important oxides of titanium are rutile, octahedrite,
and brookite. One oxide of iron and titanium, or else a ferrous titanate,
has a widespread occurrence; this is ilmenite.
QUARTZ.
Quartz:
SiO,.
Rhombohedral.
Sp. gr. 2.653-2.654.
Occurrence.—()uartz is second in abundance only to the minerals of the
teldspar group. According to Clarke,* quartz comprises 12 per cent of
the lithosphere. It is very abundant as an original pyrogenic constituent
of the igneous rocks, as an allogenic constituent of the clastic rocks, and
as an autogenic mineral in all classes of metamorphosed rocks. The
material for secondary quartz may be derived from the alterations of many
minerals in situ, or from the decomposition of minerals at some distance.
The most widespread of all the alterations which furnish silica to the
solutions is that of the decomposition of the silicates by carbonic acid in
the belt of weathering, with the simultaneous production of carbonates and
quartz, or a solution of colloidal silicic acid from which opal, chert, or
quartz may later separate. Such quartz may be extensively deposited from
the solutions in the porous rocks of the belt of cementation. It there fills
the minute spaces between the individual grains of sedimentary rocks.
It occupies spaces in porous tufts or in vesicular igneous rocks. It fills
openings between laminze, and joint, fault, and breccia openings. The
quantity of quartz thus deposited is far greater than that of any other
mineral, and not improbably greater than that of all other minerals com-
bined. By this process the rocks are cemented. (See pp. 617-621.) Not
only may the openings be occupied by quartz, but at the time of the
deposition of the quartz other minerals may dissolve and their places be
taken by the quartz. This process of deposition of silica as quartz is called
silicification. (See p. 205.)
«Clarke, F. W., Analyses of rocks from the laboratory of the United States Geological Survey,
1880-1899: Buil. U. S. Geol. Survey No. 168, 1900, p. 16.
218 A TREATISE ON METAMORPHISM.
As a metamorphic mineral, quartz is derived from actinolite, anorthite,
anorthoclase, anthophyllite, augite, biotite, bronzite, chalcedony, cumming-
tonite, diopside, enstatite, epidote, garnet, grossularite, hornblende, hypers-
thene, microcline, olivine, opal, orthoclase, plagioclase, prehnite, pyrope,
sahlite, scapolites, serpentine, tridymite, and zoisite.
Modifications —The most frequent and important modification of quartz is
by recrystallization. Crystallized quartz is dissolved under conditions of
weathering, as are all other minerals. This process is, however, exceed-
ingly slow. As a result of solution the quartz crystals may be corroded.
a @
Such corrosion has been described by Hayes.“ In the belt of cementation,
and especially adjacent to trunk channels of circulation, quartz may be ex-
tensively dissolved from veins and from the wall rocks. (See pp. 848-849.)
Granulation and recrystallization of quartz occur on a most extensive
scale in all quartzose rocks which are subjected to mass-mechanical action
or other favorable conditions in the zone of anamorphism. These changes
involve no heat and volume reactions so far as the quartz itself is concerned,
except that as the original minerals may be strained, or the new grains are
imperfectly adjusted, the change may involve a slight expansion. But
such expansion is followed by an equal contraction when the material is
recrystallized into quartz free from strain. In the recrystallization many
small individuals may be merged into one large individual. In some
instances of recrystallization, where large grains are produced from smaller
ones, the large individuals may average more than a million times as great
as the small individuals from which they are derived. (See p. 695.) In
the production of a comparatively few large individuals from a multi-
tude of small individuals there is probably a release of energy. (See
p. 771.) During recrystallization the material taken into solution may
be deposited practically in situ or may travel far and be extensively
deposited elsewhere. Often quartz deposited in situ, or nearly so, can not
be discriminated from quartz deposited from solutions coming from distant
sources, as above described.
A second modification of quartz only less important than that of
recrystallization is silication by the union with bases united with other
acids, thus forming silicates. Of such acids carbonic is by far of
«Hayes, C. W., Solution of silica under atmospheric conditions: Bull. Geol. Soc. America, vol.
8, 1897, pp. 213-220.
MODIFICATIONS OF QUARTZ. MAUS)
the greatest consequence. Some of the more common minerals in which
silication occurs on an extensive scale are calcite, dolomite, ankerite, and
siderite, thus producing wollastonite, diopside, tremolite, sahlite, actinolite,
and griinerite. The silica may unite with the bases of various carbonates
producing various complex silicates, such as chondrodite, augite, horn-
blende, garnet, ete. At the same time the material of previous silicates
may be absorbed. The heat and volume reactions in many of these
changes may be found under the carbonates mentioned.
In this process of silication of carbonates it is not often possible to
identify the remnants of the quartz mdividuals which furnished the silica
for the reactions. But apparently the quartz particles which furnished the
silica for the process of silication may be identified im some instances.
This is best seen for such fibrous minerals as serpentine, talc, and actinolite,
the needles or fibers of which appear to grow into the quartz, in some
instances deeply. In such cases it seems clear that the silica of the quartz
furnished at least a part of the silica for the silicate, the bases being
furnished by the solutions.
One of the best instances of the extensive union of quartz with bases,
producing serpentine pseudomorphous after quartz, is that described by
Becker.* He describes the exteriors of original clastic grains of quartz to
be “entirely occupied by felted fibers of serpentine, and long, slender
microlites pierce the quartz grain toward its center, like pins in a cushion.”
This is but one illustration of a very widespread replacement of quartz
by serpentine in the Coast Ranges. The growth of actinolite into quartz
is illustrated in the Tyler slate of the Penokee district of Wisconsin.”
In instances where the quartz furnishes the silica for the penetrating
silicates the migration of the silica is microscopical, and it might be sup-
posed that the reactions occur without the solution of the silica of the
quartz; but it seems probable, even in such cases as these, that there is
solution of the silica before combination with the bases. In such reactions
it is presumed that the bases which unite with the silica were before united
with some other acid, and it is only when the previous combination is known
that the heat and volume relations of the reactions can be ascertained.
«Becker, G. F., Geology of the quicksilver deposits of the Pacific slope: Mon. U: S. Geol.
Survey, vo]. 13, 1888, pp. 120-127.
> Becker, cit., p. 124.
¢Trying, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin:
Mon. U.S. Geol. Survey, vol. 19, 1892, pp. 210-215.
220 A TREATISE ON METAMORPHISM.
In a third class of changes quartz may be wholly replaced by other
minerals, as by magnetite and hematite. Very frequently the deposition of
the new minerals seems to be conditioned upon the solution of the quartz.
The replacement of quartz by iron oxide is illustrated in the Lake Superior
region in both the iron-bearing and the slate formations.“
The most favorable conditions for the solution of silica, especially of that
formed by the decomposition of the silicates by carbonation, are furnished
by the belt of weathering. The most favorable conditions for the deposi-
tion of silica as quartz are those of the belt of cementation. The solution of
silica in the belt of weathering of the zone of katamorphism and its deposi-
tion in the belt of cementation of this zone is perhaps the best illustration
of the principle explained on pages 634-636, that material dissolved in
the belt of weathering may be extensively deposited in the belt of
cementation. Recrystallization of quartz mainly takes place in the zone
of anamorphism, although it undoubtedly occurs to some extent in the
zone of katamorphism, and especially in the belt of cementation. The
process of silication takes place almost invariably with decrease in volume,
provided all the compounds concerned are solids. Where the carbonates
are silicated the decrease in volume ranges from 20 to 40 per cent. Silica-
tion occurs upon a great scale in the zone of anamorphism—is, indeed, one
of the most distinctive chemical reactions of that zone.
TRIDY MITE.
Tridymite:
SiO,
Hexagonal, or pseudo-hexagonal.
Sp. gr. 2.28-2.33.
Occurrence —T'ridymite usually occurs as an autogenic mineral in cavities
in lavas, such as rhyolite, andesite, trachyte, ete.
Modifications —T'ridymite is dissolved more readily than quartz. The
material of tridymite may go through any of the changes which silica of
quartz may pass through, with the difference that its arene would
result in the production of quartz (rhombohedral; sp. gr. 2.652—2.6: 54)
rather than the original mineral, tridymite. The changes of tridymite into
other minerals than quartz need not be discussed in detail, since the reac-
tions are the same as with nee tZ, excep! that the volume decrease is greater
«Van Hise, ©. R. antl Bay ley, w. 8., The Marquette iron-bearing Gighitas of MiChioe law U.S.
Geol. Survey, vol. 28, 1897, pp. 370, 400-405.
OPAL. 221
in the changes of tridymite than with quartz. In the change of tridymite
to quartz there is a diminution of volume, amounting to 14.24 per cent, and
there is also probably liberation of heat. Energy is therefore potentialized
in tridymite as compared with quartz. The change is one which is particu-
larly likely to occur in the zone of anamorphism, where pressure is the
dominant factor. In the fact that quartz is a denser mineral than
tridymite we probably have a reason not only for the passage of tridymite
into quartz in the lower zone, but for the absence of tridymite as an
original pyrogenic constituent in the plutonic igneous rocks which crystal-
lized originally in this zone. Under its conditions the denser mineral,
quartz, formed.
OPAL.
Opal:
SiO,.nH,O (H,O 2 to 13 per cent; but mostly 3 to 9 per cent.)
Amorphous.
Sp. gr. 2.1-2.2.
Occurrence —Opal, like most other hydrous minerals, is a product of the
zone of katamorphism. Opal is a direct deposit from hot springs. In the
sedimentary rocks it is abundantly formed from the siliceous skeletons of
certain animals and plants, such as radiolaria, sponges, and diatoms. Opal
is plentifully deposited in cavities in rocks by subterranean waters. Its
most common places of occurrence are the limestones, where it is largely
of organic origin, and the porous igneous rocks, especially as amygdules of
the amyegdaloids, where it is a chemical precipitate.
In general, as a metamorphic product opal may be derived from the
same minerals as quartz.
Modifications— The most frequent change of opal is to quartz (rhombohe-
dral; sp. gr. 2.652-2.654). Frequent intermediate products are chalcedony
and chert, which appear to be partly crystalline substances. (See p. 222.)
In the passage of opal into quartz, the changes are three: dehydration,
reduction of volume, and recrystallization. Supposing the composition of
the opal is SiO,1H.0, which would be about 6 per cent of water, the
decrease of volume would be 22.81 per cent. The change from opal to
quartz above given is commonly accomplished by solution and redeposition
or recrystallization. When the material is taken into solution this silica may
be deposited near by or transported elsewhere. It may unite with free
bases, producing silicates; it may displace other acids combined with bases,
222 j A TREATISE ON METAMORPHISM.
as, for instance, carbonic acid, thus also producing silicates. The heat and
volume relations of these reactions are discussed under ‘“ Quartz.”
The reactions of dehydration, crystallization, and lessening of volume,
as seen on pages 167-170, are particularly characteristic of the zone of
anamorphism, and it is in this zone that the change from opal to quartz
probably most extensively occurs. As evidence of this is the frequent
occurrence of opal in the zone of katamorphism, and the general absence
of opal in the rocks which have been metamorphosed in the lower zone.
CHERT, CHALCEDONY, ETC.
Standing between opal and quartz are numerous varieties of partly
crystallized or very finely crystallized silica, of which chert and chalced-
ony may be taken as the more important kinds. With these substances
are frequently small but variable amounts of opal containing combined
water. The specific gravities of chert and chalcedony are intermediate
between those of opal and quartz, i. e., between 2.15 and 2.65. Their most
frequent occurrence is as veins, nodules, belts, and members in carbonate
formations. Ordinarily they are derived from organic forms, such as
radiolaria, diatoms, and sponges, which lived under conditions similar to
those under which the limestone-building animals lived. (See p. 817.)
Chert and chalcedony are derived from opal. The material here
included varies from that which is close to opal, having only a few minute
crystallized spots, through material which shows more and more evidence of
crystallization, to material which contains comparatively little amorphous
silica, and thence into fully crystallized silica or quartz. The transition
varieties may have the peculiar spotty appearance in polarized light char-
acteristic of ordinary chert or the peculiar radial fibrous polarization of
chalcedony or any combination of the two.
The alterations of chert and chalcedony are into quartz, or by combi-
nation with bases producing silicates, the same as opal. The chemistry and
physics of the change are the same as for opal except that the decrease in
volume is less, and therefore they need not be repeated.
OCCURRENCE OF CORUNDUM. 223
HEMATITE GROUP.
CORUNDUM, HEMATITE, AND ILMENITE.
Corundum:
Al,O,,
Rhombohedral.
Sp. gr. 3. 95-4.10.
Hematite :
Fe,03.-
Rhombohedral.
Sp. gr. 5.20-5.25.
Ilmenite :
FeTiO,; varies to mFeTiO, .nFe,Os.
Rhombohedral.
Sp. gr. 4.50-5.02.
CCRUNDUM.
Occurrence —In Canada at one locality corundum occurs as an original
constituent of a syenite.“ Also, corundum as an accessory mineral has been
noted in granite, andesite, and other rocks. Corundum is, therefore, an
original pyrogenic constituent of igneous rocks. Corundum occurs along
the contact of intrusive basic rocks rich in alumina, especially those con-
taining more than 30 per cent, such as peridotites and pyroxenites. The
intruded rocks may be either igneous rocks or gneisses and schists. But
where corundum occurs in ves along contacts it is in many cases an
aqueo-igneous product (see pp. 720-728) or an aqueous deposit. Corundum
is a widespread accessory constituent in various micaceous, chloritic, and
hornblendic schists and gneisses, and in marble. Corundum, as a meta-
morphic mineral, is associated with chlorite and corundophilite. It is often
associated with other heavy metamorphic minerals, such as andalusite,
sillimanite, cyanite, spinel, rutile, etc. As a metamorphic mineral it is
derived from andalusite, cyanite, diaspore, gibbsite, sillimanite, staurolite,
and topaz.
Atterations.—Corundum alters into diaspore (orthorhombic; sp. gr. 3.40),
gibbsite (monoclinic; sp. gr. 2.35), spinel Gsometric; sp. gr. 3.8), sillimanite
(orthorhombic; sp. gr. 3.235), cyanite (triclinic; sp. gr. 3.615), muscovite
(damourite), (monoclinic; sp. gr. 2.88), margarite (monoclinic; sp. gr. 3.035),
and zoisite (orthorhombic; sp. gr. 3.31). The reactions for the formation of
diaspore and gibbsite are simple reactions of hydration. The reactions for
«Miller, W. G., Economic geology of eastern Ontario; corundum and other. minerals: Seventh
Rept. Ontario Bureau of Mines, 1897, Toronto, 1898, p. 213.
224 A TREATISE ON METAMORPHISM.
the production of the other minerals require the addition of various other
constituents—in the case of spinel, magnesia; in the case of sillimanite and
cyanite, silica; in the case of the complicated silicates, muscovite, margarite,
and zoisite, various bases and a large amount of silica. Therefore in these
cases it is clear that the common statement that corundum alters to the
minerals muscovite, margarite, and zoisite can have only the meaning that
the relations are such that corundum furnishes the alumina for the resultant
compound, and that the additional compounds are derived from another
source. It will be assumed in the alterations that the magnesia, lime,
and potash are derived from the solid carbonates and that the silica is
added as quartz. The equations for the reactions are as follows:
Al,O,--H,0=2[ Al0. (OH)]-+k.
Al,0, +3H,0=2Al(OH),+k.
Al,O,-+-MgCO,=MgAl,0,+00,-+k.
Al,0,+8i0,=A1,Si0,-+k.
3A1,0,+6Si0,+K,CO,+2H,0=2H,K Al,Sis0,,+CO,+-k.
2A1,0,+28i0,+CaCO,+H,0=H,CaAl,Si,0,,+CO,-+k.
3A1,0,+68i0,+4CaCO,+H,0O=H,Ca,Al,Si,0.,+4C0,-++k.
ES St ER SCS IES
NOD OP WW HE
RSS NN
The increase in volume as compared with corundum is, for diaspore
(equation 1), 39.25 per cent; for gibbsite (equation 2), 161.83 per cent.
The volume of the corundum and the magnesite in passing to the spinel
(equation 3) is decreased 29.17 per cent. The volume of the corundum
and quartz in passing into sillimanite (equation 4) is mereased 4.38 per
cent; into cyanite (equation 4) is decreased 6.59 per cent. If the volume
of the corundum be compared with that of the muscovite (equation 5),
with that of the margarite (equation 6), and with that of the zoisite
(equation 7), there will be great volume increases. If, on the other
hand, all the products which unite with the corundtm in each case, with
the exception of the water, be counted as solid, there would be small
increase in the volume for muscovite, a considerable decrease for zoisite,
and a small decrease for margarite. On the first hypothesis the increase in
the volume in the production of muscovite is 264.25 per cent; in margarite,
159.02 per cent; in zoisite, 261.34 per cent. On the second hypothesis the
increase in volume in the production of muscovite is 1.62 per cent; to form
margarite the decrease is 1.22 per cent; to form zoisite the decrease is 23.58
per cent.
It is reasonably certain that the passage of corundum to diaspore and
eibbsite is a reaction characteristic of the zone of katamorphism, and
OCCURRENCE OF HEMATITE. 225
especially the belt of weathering. It is almost equally certain that the
passage of corundum into spinel, sillimanite, and cyanite is characteristic
of the zone of anamorphism.
The case, however, is not clear in reference to the muscovite, margarite,
and zoisite. The equations as written are those of silicifiation and slight
hydration. If these equations be correct, they should occur in the lower
part of the belt of cementation or in the zone of anamorphism. It is
tolerably certain that margarite, zoisite, and muscovite form in the lower
part of the belt of cementation; but the zone in which muscovite charac-
teristically develops is that of anamorphism. It is not at all impossible
that the potassium carbonate, and perhaps the calcium carbonate, or even
the silica, are added in solution for the margarite and zoisite. In this case
there would be a considerable volume increase. Whether the same may
be assumed for the muscovite is uncertain. Very likely the materials
added to the corundum are in some cases carried in by the solutions, in
others are derived from adjacent minerals, and in still others partly from
both. Where the lime and potash are derived from minerals adjacent, they
may come from other compounds than carbonates, and the silica may have
been previously united with other bases. So far as this is so, in considering
the variations in volume the minerals from which the elements added to the
corundum to produce the muscovite, margarite, and zoisite were derived
must be taken into account. It is clearly impracticable in the present state
of knowledge to give definite statements as to the volume changes for these
minerals.
HEMATITE.
Occurrence— Hematite is a pyrogenic constituent in igneous rocks and is
an abundant metamorphic mineral. Its most abundant source in the
metamorphic rocks is by the dehydration of limonite, a reaction occurring
with the absorption of heat and reduction of volume. A second important
source of hematite is from iron carbonate by loss of carbon dioxide and by
oxidation, a reaction occurring with the liberation of heat and reduction of
volume. Hematite may also be produced by the oxidation of magnetite, a
reaction resulting in liberation of heat and expansion of volume. Fre-
quently after this change the hematite has the isometric form of the original
magnetite and is called martite. A fourth source of hematite is by the
oxidation of the ferrous iron of silicates at the time of their decomposition.
MON XLVII—O+ 15
226 A TREATISE ON METAMORPHISM.
A fifth source is by oxidation of ferrous iron solutions, which may result in
the precipitation of hematite. The first reaction occurs most extensively
in the zone of anamorphism; the other four occur in the zone of katamor-
phism, and to these positions the heat and volume reactions correspond.
Finally, as shown in Chapter XII, on ‘Ore deposits,” hematite may be
formed from pyrite by the action of alkaline carbonate solutions.
In summary, hematite is derived from actinolite, ankerite, anthophyl-
lite, biotite, bronzite, garnet, greenalite, griinerite, hornblende, hypers-
thene, ilmenite, limonite, magnetite, olivine, parankerite, pyrite, serpentine,
and siderite.
Alteration. The most frequent alteration of hematite is into limonite
(amorphous; sp. gr. 3.6—-4). The reaction is as follows:
2Fe,0,-+3H,0=2Fe,05.3H,0-+k.
In the change the volume is increased 60.72 per cent. A second altera-
tion of hematite is into magnetite (isometric; sp. gr. 5.168-5.18). This
may be accomplished by any of the reducing agents furnished by organic
compounds. Supposing the reducing agent to be the partially oxidized
carbon compound CO, the reaction is:
3Fe,0;CO=2Fe,0,+C0,+k.
While a reduction of the oxide of iron occurs a simultaneous oxidation of
the organic compound occurs, and the end result is the liberation of heat.
In the change the volume is decreased 2.38 per cent. A third alteration
of hematite is to pyrite (isometric; sp. gr. 5.025) or marcasite (orthorhom-
bic; sp. gr. 4.875). In the best-known instances siderite (rhombohedral,
sp. gr. 38.855) or some other iron-bearing carbonate is simultaneously
produced. The reaction may be:
Fe,0,-+-2H,8+C0,=FeS,-+-FeC0,+2H,0-+k.
In the change to pyrite and siderite the volume is increased 76.12 per cent,
and to mareasite and siderite 78.73 per cent.
The alterations of hematite to limonite occur in the zone of katamor-
phism, and especially in the belt of weathering. Corresponding with this
position the reaction is with liberation of heat and expansion of volume.
The alteration of hematite into magnetite occurs in the belt of cementation
and the zone of anamorphism. This agrees with the fact that the reaction
ALTERATION OF ILMENITE. 227
liberates heat and diminishes the volume. The alteration of hematite to
pyrite and marcasite is best known where organic compounds are present
to reduce sulphuric acid to hydrosulphurie acid and to furnish carbonic acid
to form the carbonates. The reaction is especially characteristic of the belt
of cementation, and to this position the expansion of volume and _ the
liberation of heat correspond.
ILMENITE.
Occurrence.— []menite is an abundant pyrogenic constituent of the igneous
rocks. It is found both as an allogenic and as an autogenic constituent im
metamorphic rocks. As an autogenic constituent the compounds which
unite to produce it have not been worked out. As a metamorphic mineral
ilmenite is derived from perovskite and rutile.
Alteration. —I]menite alters to titanite (monoclinic; sp. gr. 3.48), to rutile
(tetragonal; sp. gr. 4.18-4.25), and to octahedrite, or anatase (tetragonal;
sp. gr. 3.82-3.95). With these minerals magnetite (isometric; sp. gr. 5.174)
or hematite (rhombohedral; sp. gr. 5.225) or limonite (amorphous; sp.
er. 3.80) is simultaneously produced. One of the most frequent reactions
in the production of titanite is probably along the following lines:
3FeTi0,+3CaCO,+3S8i0,+O0=3CaTiSiO, + Fe,0,+3C0,+k.
The decrease in volume of the ilmenite, calcite, and quartz in passing into
titanite and magnetite, supposing the CO, to escape, is 22.35 per cent; but
the increase in volume of the titanite as compared with the ilmenite alone
is 76.35 per cent. The alteration of ilmenite to rutile and octahedrite, with
combined magnetite, is as follows:
3FeTi0, +O=3Ti0, + Fe,0,+k.
In case hematite is produced instead of magnetite the reaction is:
2FeTiO,+0=2TiO,+Fe,0O,+k.
In case limonite is produced, one and one-half molecules of water are added
to both sides of the equation.
The increase in volume of the ilmenite in passing into rutile and mag-
netite is 6.02 per cent; into octahedrite and magnetite, 11.07 per cent. In
case hematite or limonite be produced, the increase in volume is corre-
spondingly greater.
It is certain that titanite forms from ilmenite in the lower zone. In this
zone, as explained on pp. 764-765, the CaCO, and SiO, can not be supposed
228 A TREATISE ON METAMORPHISM.
to have been brought in from the outside, and therefore the change takes
place with decrease in volume. It is also certain that titanite forms exten-
sively in connection with chlorite, which commonly develops in the belt of
cementation. In this case the calcium carbonate and silica may be intro-
duced in solution from an outside source, under which circumstances the
volume is increased.
The alterations of ilmenite to rutile and octahedrite, or any combina-
tion of them, certainly occur in the zone of katamorphism, and to this
position the heat and volume reactions correspond. However, I have not
found sufficient information on the subject to assert that these reactions do
not also occur in the zone of anamorphism.
SPINEL GROUP.
SPINEL, MAGNETITE, AND CHROMITE.
Spinel:
MgAl,O,.
(Hereynite, FeA1,0,. )
(Pleonaste, [MgFe] [AlFe].0,. )
(Picotite, [MgFe] [AlCr],0,. )
Isometric.
Sp. gr. 3.5-4.1.
Magnetite:
Fe,0,.
Isometric.
Sp. gr. 5.168-5.180.
Chromite:
FeCr,O,.
Isometric.
Sp. gr. 4.32-4.57.
SPINEL.
Occurrence —Spinel occurs as an original constituent in the igneous rocks,
but is much more abundantly present as a secondary constituent in the
metamorphic rocks, especially those which are rich in magnesium. In
many cases it is secondary to olivine and other minerals rich in magnesium.
The more important minerals from which spinel is derived are almandite,
biotite, chlorite, corundum, diaspore, garnet, gibbsite, olivine, and pyrope.
Alterations.
According to Dana, spinel has been observed as altering to
tale (orthorhombic or monoclinic; sp. gr. 2.75), serpentine (monoclinic;
sp. gr. 2.575), and mica (monoclinic; sp. gr. 2.88-2.90). However, the
character of the alterations and the conditions under which they occur are
so little known that I shall not attempt to treat them from the physical-
chemical point of view.
SPINEL GROUP. 229
MAGNETITE.
Occurrence.
Magnetite is a very abundant pyrogenic constituent in
igneous rocks. It is abundantly deposited from solutions, and especially
from solutions bearing iron carbonate, according to the reaction:
3FeCO,-O=Fe,0,+3C0,+k.
Magnetite also extensively forms from siderite in situ. These changes
liberate heat and decrease the volume. A third source of magnetite is by
incomplete oxidation of pyrite and marcasite, reactions occurring with
liberation of heat and diminution of volume. Fourth, frequently siderite
and iron sulphide together pass into magnetite with decrease in volume.
(See pp. 244, 845.) A fifth way in which magnetite may be produced is
by the reduction of hematite by organic compounds, a reaction occurring
with the liberation of heat, because of the simultaneous oxidation of the
organic compounds, and with diminution of volume. A sixth way in which
magnetite is produced is by the incomplete oxidation of ferrous iron of
silicates; for instance, olivine and garnet.
In summary, magnetite is derived from actinolite, ankerite, arfvedsonite,
augite, biotite, bronzite, diopside, garnet, greenalite, griinerite, hematite,
hornblende, hypersthene, ilmenite, marcasite, and pyrite.
Alterations —Magonetite alters into hematite (rhombohedral; sp. gr. 5.225), -
limonite (amorphous; sp. gr. 3.80), and siderite (rhombohedral; sp. gr. 3.83-
3.88). The reactions are as follows:
(1) 2Fe,0,+0=3Fe,0,+k.
(2) 4Fe,0,+20+9H,0=3(2Fe,0;.3H,O) +k.
(3) Fe,0,+CO+2CO0,=3FeCO,+k.
In the change the increase in volume is, for (1), 2.44 per cent; for (2),
64.63 per cent, and for (3), 101.30 per cent. The increase in volume in the.
change from magnetite to siderite—over 100 per cent—is the greatest
volume change in which only two minerals are concerned which the calcu-
lations of Mr. Lincoln have given, with the exception of the alteration of
corundum into gibbsite. (See p. 224.) All of the above changes are well
known to occur in the zone of katamorphism, and corresponding with this
position they all take place with the liberation of heat, expansion of volume,
and decrease in symmetry.
CHROMITE.
Occurrence—Chromite occurs in the igneous rocks, especially those rich
in magnesium. It also occurs in the metamorphic rocks, often in connec-
230 A TREATISE ON METAMORPHISM.
tion with serpentine. In these positions it is very frequently a secondary
product of olivine. The reactions occurring in its production are given
fo) to}
under that mineral.
The alteration of chromite into other minerals has not been
Alterations.
noted.
RUTILE GROUP.
RUTILE, OCTAHEDRITE, AND BROOKITE.
Rutile:
TiO,.
Tetragonal.
Sp. gr. 4,184.25.
Octahedrite:
TOs
Tetragonal.
Sp. gr. 3.82-3.95.
Brookite:
TiO).
Orthorhombic.
Sp. gr. 3.87-4.082.
Occurrence—Rutile is a pyrogenic constituent in igneous rocks, and has a
widespread occurrence in the clastic and metamorphic rocks, both as an
allogenic and as an autogenic constituent, in the latter case generally being
derived from ilmenite. Rutile is also derived from brookite, ilmenite, octa-
hedrite, and titanite.
Octahedrite in the metamorphic rocks is a secondary alteration of
other titanium-bearing minerals, especially of titanite. It is also derived
from ilmenite.
Brookite occurs sparingly, both in altered igneous rocks and in sedi-
mentary rocks. In some cases it is secondary to titanite.
alterations — Both octahedrite and brookite alter to rutile (tetragonal; sp.
er. 4.18-4.25). In the case of octahedrite the decrease in volume is 7.83 per
cent; in the case of brookite the decrease is 5.69 per cent. The heat change
is undetermined, but probably the alterations occur with the liberation of
heat. If this be the case the alterations involve recrystallization, diminu-
tion of volume, and liberation of heat. In the case of octahedrite the sym-
metry remains the same; in the case of brookite the symmetry is increased.
It may be inferred that such changes as these occur in both zones,
being in all respects analogous to the changes which take place in dolomiti-
zation. (See pp. 238-240.) However, the geological occurrences of these
alterations have not been given with such definiteness as to enable one to
make definite statements as to the actual facts.
——————
ALTERATIONS OF RUTILE. yolk
In this connection the experiments of Hautefeuille* are very interesting.
He produced rutile, brookite, and octahedrite from the same compounds,
but at different temperatures, rutile forming when red heat was used,
brookite when the temperature was between that required for the volatiliza-
tion of cadmium and zine, and octahedrite when the temperature was a little
below that for the volatilization of cadmium. Rutile, the mineral with the
highest specific gravity, forms at the highest temperature, and high tempera-
ture is especially characteristic of the zone of anamorphism.
Rutile may alter into ilmenite (rhombohedral; sp. gr. 4.75) and into
titanite (monoclinic; sp. gr. 3.48). In the change to ilmenite the reactions
may be:
(1) TiO,+Fe,0,=FeTi0,+Fe,0;+k, or
(2) TiO,+-FeCO,=FeTi0,+C0,+k.
In (1) the decrease in volume of the ilmenite and hematite as compared
with the rutile and magnetite is 1.88 per cent. In (2) the decrease in
volume is 34.77 per cent, provided the iron carbonate is present as solid
siderite and the CO, escapes.
In the change to titanite the most probable reaction is:
(3) TiO, +-CaCO,+Si0,=CaTiSi0, +CO,+k.
The decrease in volume is 28.17 per cent, provided the compounds which
unite with the rutile are solids and the liberated CO, escapes.
Those who have described the changes of rutile to ilmenite and titanite
have not indicated whether or not they occur as deep-seated alterations. — It
may, however, be anticipated that such is the case, for they are changes
whieh involve liberation of heat and condensation of volume, and therefore
the kind which normally oceur in the zone of anamorphism. :
DIASPORE GROUP.
DIASPORE AND LIMONITE.
Diaspore:
AIO(OH).
Orthorhombie.
Sp. gr. 3.3-3.5.
Limonite:
2¥Fe,0, .3H,0.
Amorphous.
Sp. gr. 3.6-4.00.
@Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. 8S. Dana; Wiley & Sons,
New York, 6th ed., 1892, p. 239.
232 A TREATISE ON METAMORPHISM.
DIASPORE.
Occurrence—])iaspore is especially found in the serpentine- or chlorite-
bearing schists and gneisses and in dolomites. In these rocks it is frequently
associated with corundum. Diaspore has been recorded as a constituent
of granite, nepheline-syenite, and basaltic rocks. As noted on other pages,
for the alterations of various minerals in the zone of katamorphism, espe-
cially the belt of weathering, it may be produced as one of the alteration
products of the following minerals: Biotite, corundum, gibbsite, haiiynite,
muscovite, nephelite, noselite, phlogopite, scapolites, sodalite.
Alterations—No alterations of diaspore are recorded. However, it is
probable that where diaspore is deposited in sedimentary rocks and is deeply
buried, so as to undergo alteration in the zone of anamorphism, it passes
into corundum (rhombohedral, sp. gr. 4.025); or, like corundum, unites
with other bases to produce such minerals as spinel (isometric, sp. gr. 3.8);
sillimanite (orthorhombic, sp. gr. 3.235), and cyanite (triclinic, sp. gr. 3.615);
muscovite (monoclinic, sp. gr. 2.88); margarite (monoclinic, sp. gr. 3.035);
and zoisite (orthorhombic, sp. gr. 331). So far as the hydrous minerals
muscovite, margarite, and zoisite are formed, the water may have been
derived from the diaspore, which contains more water, and thus their forma-
tion be really a process of dehydration and silication. For these supposed
reactions the equations may be:
2A10(OH)=Al,0,-+H,O—k.
1)
2) 2A10(0H)-+MgC0,=MgAl,0,+C0,+H,0—k.
3) 2A10(0H)+S8i0,=Al,SiO;+H,O—k.
4)
5)
6A10 (OH) +68i0, +-K,CO,=2H,KAl,Si,0,.+CO,+-H,O—k.
4A410(OH) +28i0,-+CaCO,=H,CaAl,Si,0,,+CO0,+H,0—k.
6) 6A10(OH)+68i0,-+4CaCO,;=H,Ca,Al,Si,0,+4C0,+2H,0—k.
SS OSA ES
*
Supposing that all the compounds in the first member of the equation are
solids, and that the liberated CO, and H,O escape, the decrease in volume
for corundum (1) would be 28.18 per cent; for spinel (2), 40.39 per cent;
for sillimanite (3), 13.52 per cent; for cyanite (3), 22.61 per cent; for
muscovite (4), 54.21 per cent; for margarite (5), 14.08 per cent; for zoisite
(6), 29.44 per cent. All take place with absorption of heat. Therefore
all of these reactions are characteristic of the zone of anamorphism.
LIMONITE.
Occurrence—Timonite as a mineral is produced either as an original
chemical precipitate or by the alteration of other minerals. It is not,
OCCURRENCE AND ALTERATIONS OF LIMONITE. 233
so far as known, an original pyrogenic constituent of the igneous rocks.
The most important source of bodies of limonite is precipitation from
iron-bearing solutions, especially iron carbonate. For iron carbonate the
reaction is—
4FeCO,+20+3H,0=2Fe,0,.3H,0-+4C0,+k.
A second important source for limonite is the oxidation and hydration of
solid iron carbonate in rocks, especially siderite, ankerite, parankerite, and
iron-bearing limestone or dolomite. The source next in importance is the
oxidation and hydration of pyrite, marcasite, and other sulphides. A fourth
important source of limonite is the oxidation and hydration of the ferrous
iron of silicates. A fifth source is the hydration of hematite. A. sixth
but unimportant source is the oxidation and hydration of magnetite. All the
reactions involve oxidation or hydration, or both, and therefore take place
with the liberation of heat. In the production of limonite from iron
carbonate there is an important contraction of volume; in the other cases
the volume of the limonite is greater than that of the compounds from
which it is derived. All the above reactions producing limonite occur in
the zone of katamorphism, and the controlling factor is the first part of
van’t Hoff’s law, that of chemical reactions with the liberation of heat.
Limonite does not develop in the zone of anamorphism.
In summary, limonite is derived from the following minerals: Actino-
lite, ankerite, anthophyllite, arfvedsonite, biotite, bronzite, epidote, garnet,
greenalite, griinerite, hematite, hornblende, hypersthene, ilmenite, mag-
netite, marcasite, olivine, parankerite, pyrite, pyrrhotite, serpentine, and
siderite.
Alterations—The important alterations of limonite are into hematite
(rhombohedral; sp. gr. 5.20-5.25) and siderite (chombohedral; sp. gr.
3.83-3.88). Hematite produced from limonite may be earthy or crystalline.
The reaction is—
2Fe,0;.3H,O=2Fe,0,+3H,0—k.
The decrease of volume is 37.78 per cent. The change is therefore one
of dehydration, reduction of volume, and crystallization. The transforma-
tion takes place on a great scale in the zone of anamorphism, that in which
pressure controls whether heat is absorbed or liberated.
The second important change of limonite is into iron carbonate.
Where this change occurs organic compounds are commonly present and
234 A TREATISE ON METAMORPHISM.
decomposing to serve as reducing agents and to furnish abundant CO, to
unite with the iron. The reduction may be by the passage of CO into
CO,, of C into CO,, or of C into CO, as follows:
(1) 2Fe,0,.3H,0-+2C0-+2C0,=4FeC0;+3H,0-+k.
(2) 2Fe,0,.3H,0-+C+3C0,=4FeC0,+3H,0-+k.
(3) 2Fe,0,.3H,0+20-+-4C0,=4FeCO,+3H,0-+2C0+k.
So far as the iron is concerned, its reduction and dehydration absorb heat,
but the oxidation of the C or CO and the union of the CO, and FeO both
liberate heat, the amount of which is greater than that absorbed, so that in
each of these reactions heat is liberated. In all of the reactions the volume
is increased 22.27 per cent.
The reduction of the iron of limonite so as to produce protoxide for
the formation of iron carbonate may of course be accomplished by carbu-
reted hydrogen, especially methane (CH,), rather than by the compounds
suggested; but the carbureted hydrogen compounds are so numerous and
the resultant compounds so uncertain that no attempt will be made to
formulate equations for possible changes with these substances as reducing
agents.
The change of limonite to siderite is one which occurs extensively in
rocks bearing organic compounds in the zone of katamorphism. The
formation of the abundant siderites which are used as iron ores of Carbon-
iferous and later age are believed for the most part to be thus derived from
limonite in the upper zone. The reactions correspond perfectly to this
position, being those which occur with liberation of heat and very consid-
erable expansion of volume. The siderite thus formed may later be
decomposed into various other compounds, or even reproduce limonite, but
the consideration of such changes belongs under ‘‘Siderite.”
BRUCITE GROUP.
BRUCITE AND GIBBSITE.
Brucite:
Mg(OH),.
Rhombohedral.
Sp. gr..2.38-2.40.
Gibbsite (hydrargillite) :
Al(OH )s.
Monoclinic.
Sp. gr. 2.28-2.42.
BRUCITE AND GIBBSITE. 235
BRUCITE.
Oceurrence—Brucite is one of the minerals which is produced in the
upper physical-chemical zone, especially in the belt of weathering. Brucite
is produced by the alterations of minerals rich in magnesia, being recorded
as secondary to chondrodite, clinohumite, humite, and serpentine. It is
especially prevalent in serpentinous rocks and veins. Doubtless in many
instances it forms simultaneously with the serpentine and perhaps other
minerals, rather than secondary to them.
alterations—T'he one alteration of brucite noted is that of carbonation,
into hydromagnesite (monoclinic; sp. gr. 2.145-2.180). The reaction
representing the change is—
4Mg(OH),+3C0,=Mg,(CO,) ;.2Mg(OH).3H,0+k.
The increase in volume is 73.08 per cent. The alteration is therefore
one of simple carbonation, and takes place in the zone of katamorphism,
especially in the belt of weathering, with expansion of volume and
liberation of heat.
GIBBSITE.
Occurrence.—Gibbsite occurs as an accessory constituent in many of
the schists and gneisses, especially those which have been subjected to the
forces of the upper physical-chemical zone, and particularly in the belt of
weathering. As noted on subsequent pages, it may be a result of the
alteration of many minerals, the more important of which are as follows:
Anorthoclase, andalusite, biotite, cancrinite, corundum, cyanite, epidote,
haiiynite, microcline, muscovite, nephelite, noselite, orthoclase, phlogopite,
plagioclases, pyrope, the scapolites, sillimanite, sodalite, topaz, tourmaline,
and zoisite. By reference to the discussion of these minerals and the
minerals which simultaneously form, the conditions of its formation may
be ascertained.
Alterations —No alterations of gibbsite are recorded in the standard text-
books, but where sedimentary rocks containing gibbsite are so deeply
buried as to pass into the zone of anamorphism it may become partly
dehydrated, producing diaspore (orthorhombic; sp. gr. 3.40), or wholly
dehydrated, producing corundum (rhombohedral; sp. gr. 4.025); or the
aluminum may unite with other compounds, producing the same minerals
that are produced by corundum or diaspore. It is believed that these
236 A TREATISE ON METAMORPHISM.
alterations from diaspore and gibbsite have taken place on an extensive
scale, even if they have not been recorded. There is no doubt about the
formation of gibbsite abundantly in the zone of katamorphism, especially
in the belt of weathering. To my mind there is as little doubt that the
widespread corundum of the schists, gneisses, and marbles is derived in
large measure from gibbsite. I am confident that the hydrous aluminum
oxides furnish the bases for much of the spinel (isometric; sp. gr. 3.80),
sillimanite (orthorhombic; sp. gr. 3.235), and cyanite (triclinic; sp. er-
3.615) which occur in these rocks. And it is little short of a certainty that
gibbsite furnishes alumina for the silicates, muscovite (monoclinic; sp. gr.
2.88), margarite (monoclinic; sp. gr. 3.035), and zoisite (orthorhombic;
sp. gr. 3.31). As with diaspore, the reactions producing all the above-
mentioned silicates are those of dehydration and silicifiation. The following
equations may be written for the above supposed reactions:
(1) Al(OH),=Al10(OH)-+H,0O—k.
(2) 2A1(OH),;=Al,0;+3H,0—k.
(8) 2A1(OH);+Mg00,=MegAl,0,+C0,+3H,0—k.
(4) 2A1(OH);+Si0,=Al,Si0,+3H,0—k.
(5) 6A1(OH),+6Si0,--K,CO,=2H,IAl,Si,0,.+-CO,-+-7H,O—k.
(6) 4A1(OH),+2S8i0,+CaCO,=H,CaA1,Si,0,,.+CO,+5H,0—k.
(7) 6A1(OH),+6Si0,+4CaCO,=H,Ca,Al,Si,0,,+4C0, +8H,O—k.
Regarding all the minerals as solid, the decrease of volume for diaspore (1)
is 46.82 per cent; for corundum (2), 61.81 per cent; for spinel (3), 60.12
per cent; for sillimanite (4), 43.68 per cent; for cyanite (4), 49.61 per cent;
for muscovite (5), 64.99 per cent; for margarite (6), 38.92 per cent; for
zoisite (7), 43.06 per cent. The decreases of volume are greater for the
corresponding minerals than for diaspore because of the greater amount of
water in the gibbsite. In all the reactions heat is absorbed. The reactions
are therefore typical of the zone of anamorphism.
THE CARBONATES.
The important carbonates which occur as rock-making constituents
are the calcite group, including calcite, dolomite, ankerite and parankerite,
magnesite, and siderite, and the aragonite group, of which aragonite is the
only important rock-making member.
OCCURRENCE OF CALCITE. 23
CALCITE GROUP.
CALCITE, DOLOMITE, ANKERITE, PARANKERITE, MAGNESITE, AND SIDERITE.
Calcite:
CaCO.
Rhombohedral.
Sp. gr. 2.713-2.714.
Dolomite:
CaMgC,0,.
Rhombohedral.
Sp. gr. 2.8-2.9.
Ankerite:
CaFeC,0,-CaMgO,0,;(CaMgC,0,:CaFeC,0,: :3:1 to 2:1).
Rhombohedral.
Sp. gr. 2.95-3.1.
Paranker ite:
CaFeC,0,.2CaMgC,0,;(CaMgC,0,:CaFeC,0,: :2:1 to 10:1).
Rhombohedral.
Sp. gr. 2.95-3.1.
Magnesite:
MgCOs.
Rhombohedral.
Sp. gr. 3.00-3.12.
Siderite:
FeCo,.
Rhombohedral.
Sp. gr. 3.83-3.88.
CALCITE.
Occurrence —The chief sources of calcite are (1) organic precipitates, (2)
chemical precipitates, (3) by alteration of aragonite, and (4) by carbonation
of silicates.
The chief direct source of calcite is organic. Corals and innumerable
other kinds of shell animals, especially in the sea, abstract calcium carbonate
from the water and build it into their external or internal structures. Calcite
as a chemical precipitate may be deposited from the waters of the sea,
especially in inclosed lagoons; by the waters of inland lakes, especially
those having no outlet; by springs and streams, especially hot springs and
desert streams; and by underground waters in the cpenings of rocks, such
as the interstices between grains, the cavities of porous igneous rocks,
especially amygdules, and in cave, fault, joint, and fissility openings. The
deposited calcite may replace a considerable number of other minerals. As
a deposit in the openings of rocks calcite is second in abundance only to
quartz. Calcite is an alteration product of a large number of minerals, of
which the following are the more common: Actinolite, ankerite, antho-
238 A TREATISE ON METAMORPHISM.
phyllite, aragonite, augite, diopside, dolomite, epidote, fluorite, garnet,
grossularite, gypsum, haiiynite, hornblende, noselite, parankerite, pyrope,
sahlite, scapolites, tremolite, and zoisite.
While the abundant direct sources of calcite are (1), (2), and (3)
above, the indirect and ultimate source which has probably furnished the
great quantity of calcium carbonate is the carbonation of the silicates. (See
pp: 473-480.) This process occurs on a great scale in the zone of kata-
morphism, especially in the belt of weathering. It is a reaction which
takes place with liberation of heat and increase of volume in case the
replaced silica separates as quartz in situ. Many of the individual carbo-
nation reactions of the silicates, as, for instance, wollastonite, diopside, etc.,
are given under that class of minerals.
Alterations —J‘he first of the alterations of calcite is recrystallization. Cal-
cite is the most mobile of the abundant rock-making minerals. It responds
readily to changes of physical conditions, and is very susceptible to weak
chemical agents. A slight stress may produce in it twinning structure. A
state of unequal strain favors its solubility. Where the pressure increases,
solution increases; where pressure is lessened, deposition takes place.
Increase of temperature greatly increases its solubility, and vice versa.
The increase of carbon dioxide in water greatly increases its solubility,
and vice versa. Thus it happens that in rocks where the calcite is almost
constantly subjected to changing pressure, temperature, and varying
amounts of carbon dioxide it is constantly being taken into solution
and, after a greater or less journey, being deposited from solution or
carried to the sea to be ultimately precipitated by organic agents. The
recrystallization of great masses of calcite, the solution of calcite in the
belt of weathering and its partial deposition in the belt of cementation, the
formation of caves, cave deposits, ete., are considered later.
The second important change of calcite is partial replacement of cal-
cium by magnesium, often producing dolomite (rhombohedral; sp. gr.
fo)
2.8-2.9). The generalized reaction is:
(1) 2CaCO,+Mg=CaMg(CO,),+Ca+k.
Supposing the calcium to be present as a carbonate, and supposing the
added magnesium to be a chloride—and this is believed to be a very
common case—the reaction would be:
(2) 2CaCO;+MgCl,=CaMgC,0,+CaCl,-+k.
ALTERATIONS OF CALCITE. 239
Or supposing that the magnesium salt is a carbonate, and that this is depos-
ited and an equivalent amount of calcium carbonate is taken into solution,
the reaction would be:
(3) 2CaCO,+Mg00,=CaMg0,0,+CaC0,+k.
Either of these changes is accompanied by the decrease in volume of 12.30
per cent if the original calcite be compared with the produced dolomite.
There might be no diminution in volume, or even an increase in volume,
in case less than the molecular weight of calcium salt equivalent to the
introduced magnesium was dissolved. For instance, in an extreme case
the reaction might be:
(4) CaCO,+MgC0,=CaMg(CO,),+k,
the MgCO, being added through solutions, and no caleium carbonate
dissolved. In this case the expansion in volume over the original calcite
would be very great—75.41 per cent. However, the normal case in
dolomitization, as noted below, appears to be the molecular replacement
represented by the specific equations (2) and (3). The compounds
concerned in these reactions are so important that the heat relations have
been determined as above given; so it can be asserted positively, from
chemical studies, that heat is liberated by them.
The calcium of calcium carbonate may be replaced by other metals
besides magnesium, or calcite may be replaced by an oxide. The most
important of the elements which enter into such combinations, and the
only one which need be mentioned, is iron. At many localities, partly
or wholly occupying the place once held by calcite, iron carbonate is
found. For any definite proportion of iron replacing the calcium, equations
may be written paralleling those for the replacement of calcium by
magnesium.
The third important alteration of calcite is to wollastonite (monoclinic ;
sp. gr. 2.8-2.9). This alteration is, indeed, the chief source of wollastonite.
The equation is:
(5) CaCO,+S8i0,=CaSi0O,+CO,—k.
In the change the volume is decreased 31.48 per cent, provided the silica
used is a solid and the carbon dioxide escapes. In case the silicic acid be
brought in solution from an outside source, the volume of the solid is
increased 10.81 per cent. Between these extremes there are theoretically
240 A TREATISE ON METAMORPHISM.
all gradations, but, as noted below, an approach to the former extreme
probably is the common case.
Recrystallization of calcite and dolomitization take place on the most
extensive scale at all depths and under both mass-static and mass-dynamic
conditions; they are therefore alterations which are common to both
physical-chemical zones. By dolomitization it is believed that great masses
of calcite have been transformed to dolomite. The evidence of this trans-
formation and the detailed facts in connection with the change are given
under dolomite. (See pp. 798-808.) The fact that dolomite forms in both
zones would be sufficient evidence that the reactions producing this com-
pound liberate heat, even if this had not been experimentally determined
to be the fact. It has been pointed out before (pp. 181-182) that the
formation of dolomite is a typical illustration of an alteration in which both
the volume and the chemical changes liberate heat, and which therefore
may occur in all zones and belts of the lithosphere.
The change from calcite to wollastonite occurs chiefly or wholly in
the very deep-seated rocks, especially in the zone of anamorphism. In this
zone, as noted (pp. 764-766), it can not be assumed that material is added
in considerable quantity from an outside source by circulating water; hence
in this zone silica for the change is believed to have been a solid. The
reaction is therefore one taking place with the absorption of heat and
condensation of volume. The silication of calcite to wollastonite in the
zone of anamorphism may be taken as a typical example of the heat and
volume change of silication of carbonates in that zone.
DOLOMITE.
Occurrence —The chief souree of dolomite is believed to be the dolomiti-
zation of calcite (see pp. 238-239), but dolomite is also a direct chemical
precipitate. Dolomite also forms in subordinate amount by the alteration
of ankerite. The ultimate source of the magnesium carbonate for the
dolomitization of the calcite is the magnesium liberated by the carbona-
tion of the silicates in the zone of katamorphism, especially in the belt
of weathering. The reactions for the decomposition of some of the
simple silicates, such as diopside and tremolite, are given under those
minerals. The magnesium for the dolomitization need not be directly
derived from a silicate, but may be from the solutions of the sea or from
ALTERATIONS OF DOLOMITE. 241
a previously formed magnesium limestone or dolomite which is in the
belt of weathering. Dolomite produced by the carbonation of the silicates
or by solution of dolomitic formations is an important chemical precipitate
in caves and small crevices in the rocks, the same as calcite.
In summary, dolomite is chiefly derived as a secondary mineral from
ankerite, calcite, and parankerite.
Alterations—An important alteration of dolomite is to diopside (mono-
clinic; sp. gr. 3.2-3.38). This alteration is a typical example of silication.
(See p. 205.) The most probable reaction is:
(1) MgCaC,0,+28i0, =MgCaSi,0,+2C00, —k.
The decrease in volume is 40.11 per cent, provided all of the silica entering
into the combination was a solid. In case all of the silica were introduced
through water solutions there would be an increase in volume of 2.03 per
cent. More important alterations of dolomite are into tremolite (mono-
clinic; sp. gr. 2.9-3.1) and calcite (@hombohedral; sp. gr. 2.713-2.714),
or into tremolite and wollastonite (monoclinic; sp. gr. 2.8-2.9). In the first
case the reaction is:
(2) 3CaMgC,0,+4Si0,=Mg,Ca8i,0,.+2CaCO,+4C0,—k.
The decrease in volume, provided the silica is present as a solid, the
calcite remains as a solid, and the carbon dioxide escapes, is 25.20 per cent.
However, the excess of calcium carbonate may simultaneously change to
wollastonite. In this case the reaction would be:
(3) 3CaMgC,0,+6Si0,=Mg,CaSi,O,,+2CaSi0, +6CO,—k.
The decrease in volume as compared with the dolomite and quartz of the
tremolite and wollastonite is 33.09 per cent. In both of the changes, if a
portion of the silica be supposed to be introduced from an outside source
the decrease in volume would be lessened, and if all of it were thus sup-
posed to be introduced there would be an increase in volume from the
solid dolomite of 9.89 per cent in the case of tremolite and calcite, and 14
per cent in the case of tremolite and wollastonite.
The space once occupied by dolomite, like that occupied by calcite,
may be taken by other carbonates or by various oxides. The most impor-
tant of these are carbonate of iron and oxide of iron. The carbonate may
be a replacement, or possibly a substitution, of the iron of some other iron
MON XLV1II—04——16
242 A TREATISE ON METAMORPHISM.
salt for that of the calcium and magnesium. The oxide of iron is an illus-
tration of a pure replacement, not of an alteration.
The formation of diopside, tremolite, and wollastonite is known to
occur in deep-seated rocks, and especially in connection with mass-
mechanical action where the rocks are deformed by flowage. As
repeatedly noted, in the zone of anamorphism the circulation of water is
reduced to a minimum; and it can not be supposed that important addi-
tions are made from the outside, and therefore the silica must be supposed
to have been previously present in the rocks. Indeed, we know that silica
usually accompanies deposits of calcite and dolomite; hence I conclude that
the reactions take place with substantially the decrease in volume above
assigned to the changes. In the reactions heat is absorbed. The changes
are therefore again typical illustrations of silication in the lower physical-
chemical zone.
ANKERITE AND PARANKERITE.
Occurrence —A]] the compounds from normal ankerite and parankerite to
the extremes of composition given above (p. 237) are included under the
general term ferro-dolomite, which I have elsewhere used as covering all
the ferriferous compounds standing between dolomite on the one side and
siderite on the other. (See p. 823.)
The sources of ankerite and parankerite are the same as siderite,
with the difference that at the time of the formation of the iron carbonate,
calcium and magnesium carbonate are present, or formed, and unite with it.
Alterations — The more common alterations of ankerite and parankerite
are to limonite (amorphous; sp. gr. 3.80), hematite (rhombohedral; sp.
er. 5.225), and magnetite (isometric; sp. gr. 5.174), the calcium and
magnesium carbonates either separating or simultaneously undergoing the
alterations given under ‘‘Calcite” and “Dolomite.” Equations may easily
be written for any definite compound by which the iron carbonate passes
into the minerals mentioned in the same way that siderite does and the
calcium-magnesium carbonates separate. The volume changes are in the
same direction, and the physical conditions under which ankerite and par-
ankerite alter to limonite, hematite, and magnetite are the same as those
for the alteration of siderite to the like compounds. Therefore the equa-
tions and summary of physical conditions will not be here repeated.
Other important alterations of the ferro-dolomites are to sahlite (mono-
ALTERATIONS OF ANKERITE AND PARANKERITE. 243
clinic; sp. gr. 3.25-3.4) and to actinolite (monoclinic; sp. gr. 3.00-3.20).
Supposing that the magnesium and iron are present in equal quantity in
the sahlite, the reaction in the case of normal ankerite is:
(1) CaFeC,0,.CaMeC,0,+48i0,=MgFeCa,Si,0,,-+-400,—k.
Supposing the silica to be present as a solid, the decrease in volume is
37.27 per cent. In the formation of actinolite from normal ankerite, on
the supposition that the iron and magnesium are present in equal quantity
in the actinolite, the reaction is:
(2) 3CaFeC,0,.CaMgC,0,+8S8i0,=Mg,Fe,Ca,Si,0,,+4CaCO,+8CO,—k.
The decrease in volume, supposing the silica to be present as a solid and
the CaCO; as a solid, is 22.62 per cent. Of course, if the ferro-dolomite
were one in which the calcium carbonate is not so plentiful, being replaced
in equal molecular parts by magnesium and iron, it would not be necessary
for any calcium carbonate to form as a result of the reaction. For instance,
if the ferro-dolomite were CaFe;C,O,..CaMg,C,O,, the reaction would be as
follows:
(3) CaFe,C,0,).CaMg,0,0,,-+8Si0, =Mg,Fe,Ca,Si,0,,+8CO,—k.
Using the specific gravity of normal ankerite, the decrease of volume of
the actinolite as compared with the ankerite and quartz is 32.72.
Sahlite and actinolite are both known to form abundantly in the zone
of anamorphism. Sahlite is found in the marbles of eastern United States.
Actinolite is very abundant in the iron-bearmg formations of the Lake
Superior region. The development of these silicates may be taken as
typical illustrations of the reaction of silication in the lower physical-
chemical zone, with condensation of volume and absorption of heat.
MAGNESITE.
Occurrence—Magnesite may be a product of the alteration of any of the
heavily magnesian rocks. It is especially prevalent in the olivinitic
rocks and the chloritic, serpentinous, and talcose schists and gneisses,
being a product which is produced by the alteration of original minerals
simultaneously with the formation of chlorite, serpentine, and tale. It
is also found in dolomite. The more important minerals from which it is
recorded as forming are common garnet, olivine, pyrope, and serpentine.
Alterations.— No alterations are recorded for magnesite. There is, how-
ever, no doubt that this compound does break up in the zone of anamor-
244 A TREATISE ON METAMORPHISM.
ism, the carbon. dioxide being liberated and the magnesia being furnished
for the formation of various dense magnesian minerals, such as enstatite,
tremolite, olivine, pyrope, ete. These changes would involve a diminution
of volume and an absorption of heat.
SIDERITE.
Occurrence—The chief source of siderite is believed to be the reduction,
dehydration, and carbonation of limonite. (See pp. 233-234.) This change
is one occurring with the liberation of heat if the reaction upon the organic
compound be taken into account, and increase of volume. A subordinate
amount of siderite is also derived from magnetite. This change takes
place with liberation of heat and increase of volume. Siderite also forms
from ankerite and parankerite, arfvedsonite, garnet, hematite, hornblende,
hydrous ferrous silicate, limonite, magnetite, and olivine, and replaces
calcite and dolomite.
Alterations.— The important alterations of siderite are into limonite
(amorphous; sp. gr. 3.6-4.0), hematite (hexagonal-rhombohedral; sp. gr.
5.225), magnetite (isometric; sp. gr. 5.168—-5.18), and griinerite (monoclinic;
sp. gr. 3.713). The reactions are as follows:
(1) 4FeCO,+20-+3H,0=2Fe,0;.3H,0-+400,+k.
The decrease in volume is 18.22 per cent.
(2) 2FeCO,+0=Fe,0,-+2C0,+k.
The decrease in volume is 49.11 per cent.
(3) 3FeCO,+0=Fe,0,+3C0,+k.
Very often iron sulphide, as pyrite (isometric; sp. gr. 5.025) or
mareasite (orthorhombic; sp. gr. 4.875), unites with the siderite to form
magnetite. This reaction is probably of great consequence in forming the
heavy beds.of magnetite. (See p. 845.) It may be written:
(4) 2FeCO,+FeS,+2H,0=Fe,0,+2H,8+2C0,—k.
The decrease in volume for the siderite alone to the magnetite, equation
(3), is 50.32 per cent; for siderite and pyrite, 46.67 per cent; for siderite
and marcasite, 47.135 per cent.
(5) - FeCO,+Si0,+-nH,0=FeSi0, +CO,+-nH,0—k.
The decrease in volume, regarding the silica as a solid, is 32.53 per cent.
The alteration to limonite occurs in the zone of katamorphism, especially
OCCURRENCE AND ALTERATIONS OF ARAGONITE. 245
in the belt of weathering. The alteration to hematite occurs as a somewhat
deeper seated change, usually in the belt of cementation of the zone of
katamorphism. The alteration to magnetite is especially characteristic of
the zone of anamorphism, but it can not be asserted not to take place in
the belt of cementation. The alteration to griinerite occurs under deep-
seated conditions, and is in its heat and volume relations a characteristic
reaction of the lower zone. Magnetite and griinerite often form simul-
taneously. (See p. 284) The series of changes from siderite are very
interesting, in that the volume changes are all diminutions, and therefore,
so far as this factor is concerned, might take place in either zone. The first.
three reactions (equations 1, 2, and 3) liberate heat, and hence these reac-
tions in their physical-chemical relations are similar to those of dolomite,
discussed on pages 182, 240, and may take place in both zones. But the
reaction of equation (4) probably absorbs heat, and that of (5) certainly does.
Magnetite having the origin represented by equation (4) is probably, and
griinerite is certainly, confined to the zone of anamorphism, where pressure
is a controlling factor.
ARAGONITE GROUP.
The only important rock-making member of this group is aragonite.
ARAGONITE.
Aragonite:
CaCO,.
Orthorhombic.
Sp. gr. 2.93-2.95.
Occurrence—A chief source of aragonite is as an organic precipitate.
It occurs intimately associated with calcite in numerous marine shells:
While abundant, it is very subordinate to calcite as an organic deposit. A
second abundant source of aragonite is as a chemical precipitate, frequently
in association with beds of iron carbonate and gypsum. It also occurs as
a chemical precipitate from ground-water solutions, in openings in rocks,
especially at places where the temperature of the solutions is from 30° to
100° C. or more. Aragonite is not mentioned as an alteration product
of other minerals.
aiterations— "he chief change of aragonite is to calcite (rnombohedral;
sp. gr. 2.713-2.714). This is a change involving recrystallization, increase
of symmetry, and lowering of specific gravity. The increase in volume is
8.35 per cent. The heat effect of the change has not been found; but it
246 A TREATISE ON METAMORPHISM.
seems probable that heat is liberated, for the transformation of aragonite to
calcite occurs in both the physical-chemical zones, and I know of no excep-
tion to the principle that such reactions take place under the first part of
van’t Hoft’s law (see pp. 107, 181).
The change from aragonite to calcite is so complete in rocks of mod-
erate age that the presence of aragonite in the metamorphosed rocks is
almost unknown. The alteration of aragonite to calcite in both zones is of
considerable interest, as it presents a somewhat exceptional case. As
explained on pages 182-186, the common rule of change in the zone of
anamorphism is increase in specific gravity and increase of symmetry,
provided the volume change demanded will allow this. However, the
change of specific gravity in this case is a decrease rather than an increase,
and hence aragonite conforms only to the second of these rules—the first,
and usually the controlling rule, for the zone of anamorphism being
violated. These facts suggest the conclusion that in this instance sym-
metry is a more important factor than density—a very exceptional thing.
If this be so, the conclusion would follow that the symmetrical arrange-
ment of the molecules in calcite are those which best resist the changing
conditions of mass-static and mass-mechanical action in the lower zone.
The suggestion occurs to one that, if rocks were very deeply buried, so as
to be extraordinarily deep in the lithosphere, pressure might control the
form, and calcite alter to aragonite. This, however, is a speculation which
has no verification.
THE SILICATES.
The silicates are the most important of rock-making constituents.
They include natural glass and many mineral groups. The groups of
rock-making silicates are as follows: Feldspar, leucite, pyroxene, amphi-
bole, nephelite, sodalite, garnet, chrysolite, scapolite, zircon, aluminum-
silicate, epidote, humite, zeolite, mica, clintonite, chlorite, serpentine-tale,
and kaolin. Besides the members of the above groups are a number of
important rock-making silicates not so included.
GLASS.
Glass, while not a definite silicate or ordinarily included among the
specific minerals, is an important rock-making constituent, and therefore must
be treated in connection with the silicates in a treatise on metamorphism.
DEVITRIFICATION OF GLASS. 247
Occurrence.— Natural glass is an abundant constituent of the effusive rocks.
It is especially prevalent in the more acid ones, but is not confined to them,
being not infrequently abundant in the intermediate rocks, such as basalts.
A lava or tuff may be almost wholly composed of glass, or glass may con-
stitute but a small part of the background. There are thus all gradations
between completely crystallime rocks and glassy rocks. Of the more recent
effusive rocks glass not infrequently composes a large part of the flows.
An instance is Obsidian Cliff, in the Yellowstone National Park. But in
proportion as lavas are old, glass is less and less likely to be found, and in
the more ancient lavas is ordinarily absent. The explanation of this
absence is devitrification after solidification.
Evidence that devitrification takes place—That certain rocks now wholly composed
of minerals were once glasses is shown by the preservation in perfection of
the flow structures and very delicate trichitic, perlitic, spherulitic, and other
textures characteristic of glass.
Scale of devitrification.—It is also certain that the process of devitrification
has taken place in nature on a great scale. As evidence of this may be
cited the well-known American instances of devitrified glass in the original
Huronian district, described by Williams,” the aporhyolite of South Moun-
tain, Pennsylvania, described by Williams and Bascom,’ the metarhyolites
of the Fox River Valley of Wisconsin, described by Weidman,’ and the
devitrified glasses of the Crystal Falls district of Michigan, described by
Clements.” In the papers of these authors many other instances of
devitrification are cited, including European instances.
Not only does devitrification of natural glass take place, but under
proper conditions artifical glass devitrifies in a similar manner. Well-
known cases of the devitrification of artificial glass under conditions of
weathering are those of the buried ancient glasses of Nineveh and of Rome.
«Williams, G. H., Notes on the microscopical characters of rocks from the Sudbury mining
district, Canada: Ann. Rept. Geol. and Nat. Hist. Survey of Canada, vol. 5, Pt. F, Appendix 1, 1890-
1891, pp. 74-82.
> Williams, G. H., The volcanic rocks of South Mountain, in Pennsylvania and Maryland: Am.
Jour. Sci., 3d ser., vol. 44, 1892, pp. 486-490.
Bascom, Miss Florence, The ancient volcanic rocks of South Mountain, Pennsylvania: Bull. U. 8.
Geol. Survey No. 136, 1896, pp. 42-61.
¢ Weidman, Samuel, A contribution to the geology of the pre-Cambrian igneous rocks of the Fox
River Valley, Wisconsin: Bull. Wisconsin Geol. and Nat. Hist. Survey No. 3, 1898, pp. 4-31.
@Clements, J. Morgan, and Smyth, H. L., The Crystal Falls iron-bearing district of Michigan:
Mon. U.S. Geol. Surv., vol. 36, 1899, pp. 87, 101-103, 138.
248 ~ A TREATISE ON METAMORPHISM.
In glass found in the lake at Walton Hall, near Wakefield, Bingley” found
that the alkalies had been wholly removed by decay. Another case of
devitrification largely due to original state of strain is the glass of certain
old buildings, such as cathedrals. A well-known instance is that of St.
Andrew’s Chapter House.”
Rate of devitrification—'The rate of devitrification of glass depends, among
other things, upon (1) composition, (2) strain or lack of strain, (3) pressure,
(4) mass-mechanical action, (5) temperature, (6) moisture.
In any given case of devitrification several and sometimes all of these
factors enter, and hence it is impossible to discriminate the effect of each.
Very often devitrification has been described as hydro-metamorphism, but
by this no more can be meant than that water is usually an important
factor in the process.
(1) The rate of devitrification of glass increases with its basicity. This
follows from the ready solubility of basic glasses. It has also been deter-
mined that glasses rich in soda devitrify faster than those rich in potash.
This corresponds with the fact emphasized in another place (see p. 516)
that minerals rich in soda are more readily decomposed than those rich
in potash.
(2) It is shown in another place that a state of strain in minerals
promotes alteration. (See pp. 95-98.) The same is true of glass. It is
definitely known that unannealed glass, which therefore cooled irregularly
and is in a state of strain, independently of pressure or movement may
partly devitrify in a few years. For instance, drawn-glass tubing, such as
is used in the chemical laboratory, if kept for a few years may devitrify so
as to become useless. Another well-known case of devitrification probably
due to strain is the glass of certain cathedral windows. As large masses
of glass cool under natural conditions, they must often be almost at the
extreme of the unannealed condition, and therefore in a high state of
strain. So far as glass is in this condition, even without reference to any
extraneous pressure or movement, there is a marked tendency toward
devitrification. The stage of the process due to this cause is dependent
upon the amount of strain and the time.
«Bingley, C. W., On the peculiar action of mud and water on glass, as more especially illustrated”
by some specimens of glass found in the lake at Walton Hall, near Wakefield: Rept. Twenty-eighth
Meeting British Assoc. Ady. Sci., London, 1859, pp. 45-46.
> Brewster, Sir David, On the decomposition of glass: Rept. Tenth Meeting British Assoc. Adv.
Sci., London, 1841, pp. 5-7.
DEVITRIFIGATION OF GLASS. 249
(3) Pressure produces a state of unequal strain, and hence is favorable
to devitrification.
(4) Mass-mechanical action not only produces a state of unequal
strain in minerals, but fractures the material, and this gives a large surface
of action for the solutions. It is therefore clear that mass-mechanical action
is very favorable to devitrification.
(5) Experiments in the laboratory show that if glass be raised to a
temperature short of fusion the tendency to devitrification is greatly
promoted. It is therefore certain that conditions of dry heat after solidifi-
cation are favorable to devitrification. As glass occurs in considerable
bodies in a state of nature, it must for a long time, perhaps hundreds of
thousands of years, have a high temperature due to the residual heat of
the magma, and only very gradually assumes the normal temperature
corresponding with its depth of burial. It is rather probable that micro-
lites and crystallites, which so frequently occur in glass, largely form
during this process of cooling after solidification. :
(6) While devitrification of glass may occur without the presence of
abundant water, it is probably rare indeed that in nature the process occurs
without the presence of some moisture, and in general moisture is a very
important factor favorable to devitrification.
It is therefore clear that each of the above factors may give a condi-
tion favorable to devitrification, but in general actual devitrification is
due to a combination of two or more of them.
Devitrification in the two zones—In the zone of katamorphism under ordinary
conditions it is probable that the devitrification occurs somewhat slowly.
But in areas of regional volcanism, and often in those of local volcanism,
the lava flows follow one another in such rapid succession that beds are
piled up so deep that the water is held at a high temperature. By complex
intrusion the entire mass of a cooled glass may again be raised to a high
temperature. Orogenie movement if severe may produce a high tempera-
ture. Under any of these circumstances the conditions are furnished for
the complete and rapid devitrification of the glass.
The nature of the devitrification is certainly different in the belt of
weathering and the belt of cementation, although available descriptions do
not furnish data for accurate statements as to the differences. But it is
certain that in the belt of weathering the several changes are along the
250 A TREATISE ON METAMORPHISM.
lines given on pages 506-527 for that belt, finally resulting in the oblitera-
tion of textures and structures and producing an incoherent rock.
In the belt of cementation ordinarily the alterations do not result in
the obliteration of the original textures and structures of the glasses. This
is sufficiently evident where the alterations occur under mass-static condi-
tions, and even where mass-mechanical conditions prevail. The glass is
simply fractured, as explained on pages 601-602, and the individual blocks
are altered by metasomatism under mass-static conditions.
So far as we know, glasses originally form only in the zone of
katamorphism, and mainly at or near the surface. Therefore a glass can
get into the zone of anamorphism only by being buried under succeeding
lava flows or tuffs or under sedimentary rocks. Hence, before glass
reaches the lower zone, it must have been subjected for a long time to
devitrification in the belt of cementation, and the question arises whether
or not a glass would not be completely devitrified before it becomes
sufficiently deeply buried to reach the zone of anamorphism. However, if
glass ever does reach the lower zone, it is certain that its devitrification will
take place rapidly under either mass-static or mass-mechanical conditions.
The rocks in this lower zone are everywhere at temperatures exceeding
100° C; they contain water; hence, even under conditions of absolute
quiescence, it is certain that glass could not long exist. The crystallization
would be eyen more rapid under mass-mechanical conditions.
In so far as the glass had devitrified in the zone of katamorphism, and
had produced minerals characteristic of that zone, in the lower zone these
minerals would be recrystallized and minerals formed characteristic of the
latter zone. If mass-static conditions prevail this recrystallization may take
place without obliterating previous textures and structures. However, if
recrystallization takes place under conditions of mashing, the original
textures and structures are lost, and minerals are produced of such kinds
and proportions as correspond with the composition of the glass. More-
over, when the glass passes into the zone of anamorphism, textures and
structures may be formed characteristic of the slates, schists, and gneisses.
When such alteration is complete it is often impracticable to determine
whether the rock was originally glass or not. There can be little doubt
that many of the finer-grained schists are derived from rocks which were
DEVITRIFICATION OF GLASS. yD)
originally partly or wholly glassy. For instance, the Berlin gneiss of
central Wisconsin is in chemical composition the same as that of various
associated aporhyolites. The aporhyolites show that they were orginally
glasses by retaining the characteristic textures of glass. The Berlin gneiss
which was altered under conditions of mashing in the deep-seated zone is
entirely devoid of any structure which can be attributed to glass, and one
can not be certain that it did originally have a glassy base, although this
seems probable.
Minerals produced.—Ihe minerals which are produced by the alterations of
glass are very numerous. It has already been noted that glasses form
from the most acid magmas, and also from those which are intermediate or
basic in character. Furthermore, it has been seen that glass is devitrified
in both the upper and the lower physical-chemical zones, and in the upper
zone both in the belt of weathering and in that of cementation. In each of
these zones and belts minerals form from the glass which are characteristic
of them. It is plain from the foregoing that every mineral which may be
a metamorphic product of an igneous rock of any kind may result from the
devitrification of glasses of different kinds under the different conditions
which obtain in the zones and belts of alteration.
Heat and volume relations—"T'he devitrification of glass is a process which
probably results in the liberation of heat. This is certainly true for the
zone of katamorphism, where oxidation, hydration, and carbonation take
place. As to the volume relations of the change, the devitrification itself by
means of which the substance passes from an amorphous to a crystalline
condition would decrease the volume, provided there were no additions of
other compounds. But where devitrification is accompanied by oxidation,
carbonation, and hydration there are considerable additions of material.
Therefore, in the belt of cementation there can be little doubt that
expansion of volume is the rule where glasses are devitrified; but in the
belt of weathering, where solution is prominent, doubtless there is diminution
in volume with glass as with other compounds. In the zone of anamorphism
devitrification takes place with decrease of volume, the reactions being
controlled by pressure. Whether heat be liberated or absorbed in the zone
of anamorphism doubtless depends in large measure upon how far the
reactions of the zone of katamorphism have taken place during the time
252 A TREATISE ON METAMORPHISM.
the glass was passing through that zone. If these had gone far, the undoing
of the oxidation, hydration, and carbonation would probably absorb heat.
But if the glass reached the zone of anamorphism in an anhydrous condition,
the crystallization, producing a decrease in volume, would liberate heat.
Thus no general statement can be made as to the heat reaction in the zone
of anamorphism.
FELDSPAR GROUP.
The minerals of the feldspar group are the most abundant of the
silicates. According to Clarke,” the feldspars comprise 60 per cent of the
minerals of the lithosphere. The feldspars include minerals of two classes
of symmetry, monoclinic or pseudomonoclinic, and triclinic. Those of the
first class comprise orthoclase, microcline, and anorthoclase; those of the
second class include albite, oligoclase, andesine, labradorite, bytownite,
and anorthite.
In chemical composition the feldspars vary from orthosilicates, through
metasilicates, to polysilicates. The readiness of decomposition is indirectly
proportional to the acidity, the orthosilicates being the most easily decom-
posed, and the polysilicates being the most difficult to decompose.
The more frequent alterations of the monoclinic feldspars and of the
polysilicate plagioclase feldspars are to mica, especially muscovite, and to
hydrated silicate of aluminum, especially kaolin. In this alteration there
is simultaneous liberation of silica, which may separate as quartz. Very
frequently also gibbsite is formed at the same time. Where the mica biotite
is produced it is necessary that iron and magnesium shall be added. The
most common alterations of the orthosilicate plagioclase feldspars are to
zeolites, epidote, and zoisite, frequently with the simultaneous formation of
another plagioclase and chlorite. Where epidote is produced it is necessary
that iron be added from some other source; where chlorite is produced it is
necessary that magnesium and iron be added from some other source.
All the important minerals produced by the alterations of the feldspars,
with the exception of quartz and plagioclase, are hydrated, though in
varying degrees; hence, in general, water is added during the alteration of
the feldspars. From the intermediate plagioclases there may be produced
any of the foregoing minerals.
“Clarke, F. W., Analyses of rocks, laboratory of the U. 8. Geol. Survey, 1880-1899: Bull. U. S.
Geol. Survey No. 168, 1900; p. 16.
ORTHOCLASE AND MICROCLINE. 253
MONOCLINIC OR PSEUDOMONOCLINIC.
ORTHOCLASE, MICROCLINE, AND ANORTHOCLASE.
Orthoclase :
KAIS1,03.
Monoclinic.
Sp. gr. 2.57.
Microcline:
KAISi,O,.
Triclinic.
Sp. gr. 2.54-2.57.
Anorthoclase:
mNaAl,Si,0,.nKAl1,Si,0,. (Na-silicate:K-silicate:: 2:1 or 3:1, usually.)
Pseudomonoclinice or triclinic.
Sp. gr. 2.57-2.60.
ORTHOCLASE AND MICROCLINE.
Occurrence —Orthoclase and microcline have a very widespread ocecur-
rence as chief pyrogenic constituents. The minerals also are allogenic
constituents of the clastic rocks. They further have a very widespread
occurrence in the metamorphic rocks, being chief constituents both as
allogenic and as autogenic constituents of the schists and gneisses of both
aqueous and igneous origin. In the development of the feldspars as
autogenic constituents it is usually necessary that two or more minerals
unite, except in the case of the derivation of the acid feldspars from the
more basic ones or from leucite. As a metamorphic mineral orthoclase is
derived from analcite, heulandite, leucite, laumontite, and stilbite. Micro-
cline is recorded as derived from spodumene.
Alterations—Qne of the most important alterations of orthoclase and
microcline is to kaolinite (monoclinic; sp. gr. 2.60-2.63). The most prob-
able reaction, for reasons given below, is believed to be:
(1) 2KAISi,O,+2H,0+C0,=H,Al,8i,0,+48i0,+K,CO,+k.
The decrease in volume, supposing the freed silica to separate as quartz,
and K,CO, dissolved, is 12.57 per cent. If all of the freed silica be dis-
solved, the decrease in volume would be 5444 per cent. In calculating
these volume changes and those which follow, the specific gravity of
orthoclase is used.
While the ordinary alteration of the potash feldspars to the kaolin
group is to kaolinite as indicated, the alteration may be to other minerals
of this group; for instance, to pyrophyllite (monoclinic (?); sp. gr. 2.8-2.9),
halloysite (massive; sp. gr. 2.1), newtonite @hombohedral; sp. gr. 2.37),
254 A TREATISE ON METAMORPHISM.
cimolite (amorphous; sp. gr. 2.24), allophane (amorphous; sp. gr. 1.87), and
perhaps others. he chief differences are in the amounts of water added
and the amount of silica which separates. Pyrophyllite (H,A1,(SiO;),)
differs from kaolinite in that less silica is removed and less basic water is
added; it therefore might be considered as an intermediate stage in the
alteration. Halloysite (H,A1Si,O, Aq.) differs from kaolinite only in having
water of hydration. Newtonite (H,A1,Si,0,,Aq.) differs from kaolinite in
containing twice as much basic water as that mineral, and in being hydrated.
Cimolite (H,Al,(SiO;).3H,0) differs from kaolinite in containing more
silica, more basic water, and water of hydration. Allophane (A1,8i0;.5H,O)
differs from kaolinite in containing less silica and much water. It would
be easily possible to formulate equations along the line of that given for
kaolinite for each of these minerals and to calculate the volume relations.
However, this hardly seems necessary since these minerals as secondary
products to orthoclase and microcline appear to be very subordinate in
amount.
Another alteration of orthoclase and microcline of some little impor-
tance is into gibbsite (monoclinic; sp. gr. 2.3-2.4). The reaction is:
(2) 2KAISi,0,+3H,0+C0,=2Al(OH),+6Si0,+K,CO,-+k.
The decrease in volume of the gibbsite and quartz as compared with the
orthoclase is 6.61 per cent.
Another of the very important alterations of orthoclase and microcline
is to muscovite (monoclinic; sp. gr. 2.76-3.0) and quartz (rhombohedral;
sp. gr. 2.653-2.654). The reaction is:
(3) 3KAISi,O,-H,0-+CO,=—KH,Al,Si,0,,+68i0,-+K,CO,+k.
Provided the silica separates as quartz and the potassium unites with
carbonic acid and the potassium carbonate be removed in solution, the
decrease in volume is 15.58 per cent.
While this reaction may take place under exceptional conditions, it is
believed, as explained below, that where muscovite forms from orthoclase
one of the rich aluminous minerals often unites with the orthoclase to
produce the mica. Supposing the aluminous mineral to be gibbsite, the
reaction is:
(4) KAISi,O,+2A1(OH),=KH,Al,Si,0,.+2H,0—k.
The decrease in volume of the muscovite as compared with the orthoclase
and gibbsite is 20.81 per cent.
ALTERATIONS OF ORTHOCLASE AND MICROCLINE. 255
The alteration may be to hydro-muscovite or damourite (monoclinic;
sp. gr. 2.76-3.00). This is believed by most mineralogists to differ from
muscovite only in containing more water, but Dana states that a greater
content of water in damourite than that contained by ordinary muscovite
is not necessary.
From orthoclase and microcline, with the addition of magnesium and
iron compounds, biotite (monoclinic; sp. gr. 2.7-3.1) may be formed. If
the hydrogen and potassium be supposed to be present in equal proportions
and the same supposition be made with reference to magnesium and iron,
and the latter elements are supposed to be present as carbonates, the
reaction may be as follows:
(5) 4KAISi,0,+-2MgC0,+2FeCO,+H,0=
2HKMegFeAl,$i,0,)+58i0,+-K,8i0,+4C0,+k.
The decrease in volume of the feldspar, magnesium carbonate, and iron
carbonate in passing into the biotite and quartz is 22.64 per cent. But, as
with muscovite, the more frequent reaction probably involves gibbsite,
thus:
(6) KAISi,O, +MgCO, +FeC0,+Al(OH),=HKMgFeAl,Si,0,,+-H,0-+2C0,+k.
This greatly simplifies the cquation. The decrease in volume of the bio-
tite as compared with the compounds from which it is derived is 22.33
per cent.
Orthoclase and microcline are said also to alter to epidote (monoclinic;
sp. gr. 3.25-3.50); but if this be so calcium and iron must be introduced.
The forms in which these compounds are present during the alteration are
doubtless variable. If they be assumed to be present as calcium carbonate
and iron sesquioxide, the reaction might be as follows:
(7) 4KAISi,0,+Fe,0,+-4CaCO,+H,0=2HCa, Al, FeSi,0,5+68i0,+2K,CO,+200,-+k.
Supposing the Al is to the Fe as 2 is to 1, the decrease in volume of the
epidote and quartz as compared with the feldspar, calcite, and iron oxide
together is 33.73 per cent. However. it is so uncertain as to the forms of
the accessory compounds, both before and after reaction, that it is impos-
sible to make a definite statement as to the volume relations.
The alteration of orthoclase and microcline to minerals of the kaolin
group and to gibbsite occurs in the zone of katamorphism. The process
takes place on the most extensive scale in the belt of weathering, especially
256 A TREATISE ON METAMORPHISM.
in the soil horizon. Wherever the feldspathic rocks are exposed to atmos-
pheric agencies this change steadily goes on, though not so rapidly as with
the orthosilicate feldspars. (See p. 519.) But wherever the potash feld-
spars have been very long exposed to the weathering agencies they have
been partly or wholly decomposed, and in some places to a depth of several
hundred feet. The change is one of the most important of all those which
affect rocks. It is partly because the alterations take place near the
surface, where carbon dioxide is abundant, that it is believed that the freed
alkali largely unites with carbon dioxide, as given in the reaction. The
silica freed in the belt of weathering is in part undoubtedly taken into
solution as colloidal silicic acid and carried downward to the belt of
cementation. Indeed, the silica for the process of silicification in this belt,
which, as explained on page 480, is derived from the decomposition of the
silicates, probably in good part comes from the alteration of the feldspars.
Under the same conditions in which a part of the feldspar breaks up into
kaolinite another part of the feldspar may produce gibbsite, quartz, and
potassium carbonate. The potassium carbonate liberated at the time of
the formation of the kaolinite and gibbsite is largely dissolved and _ trans-
ported elsewhere, although the soluble potassium compounds are often held
in the soil to a considerable extent. (See pp. 498, 541-543.)
The alteration of orthoclase and microcline to minerals of the kaolin
group and to gibbsite is not, however, confined to the belt of weathering.
It takes place on an important scale in the belt of cementation, though not
on a scale comparable to that in the belt above. So far as known, kaolin-
ization is not a reaction which occurs in the zone of anamorphism; at least,
if it does there take place it is a very subordinate phenomenon. As seen
above, the reaction is one taking place with liberation of heat and fre-
quently with decrease of volume, since much and perhaps the most of the
freed silica is taken away in solution. The heat reaction controls, and
hence the change is under the rules of the upper physical-chemical zone.
The alteration of orthoclase and microcline to mica occurs in rocks
which have been somewhat deeply buried, and the change has been noted
in connection with both mass-static and mass-mechanical action. Under
either of these conditions the alteration may be nearly or quite complete.
But it has taken place on the most extensive scale in connection with mass-
mechanical action, where the secondary structures, such as cleavage, are
OCCURRENCE OF ANORTHOCLASE. 257
produced, and therefore in the belt of rock flowage. In the formation of
muscovite and quartz from feldspar by equation (3), as the specific gravity
of the separated quartz is somewhat greater than that of the original
feldspar, and that of the muscovite is considerably greater than that of the
feldspar, the condensation in volume above calculated is accounted for,
although the alteration is one involving hydration and possibly carbonation.
In the zone of anamorphism the wateradded is doubtless largely derived from
other minerals, as this is a belt of dehydration, and destruction of previous
minerals containing hydroxides. This passage from one mineral to another
would involve no increase in the total volume, the controlling consideration.
The most doubtful point concerning equaticn (3) is the carbonation of the
potassium. It might be supposed that the potassium unites with a part of
the freed silica and with other elements to form potassium minerals. But it
is not easy to suggest such minerals, as leucite is not recorded as a meta-
morphic mineral. The more probable solution of the problem is that
potassium and a portion of the silica unite with the alumina of the gibbsite
or some other minerals and produce one molecule of mica from one of
orthoclase, as suggested in equation (4). This suggestion is rendered
especially plausible for the slates, schists, and gneisses derived from sedi-
ments, for such rocks usually contain residual orthoclase and also aluminum
hydroxide. (See pp. 232, 235, 898-900.) The reaction of equation (4) pro-
duces great decrease in volume, is one of dehydration, and thus absorbs heat;
it is therefore a perfect example of the rules of the zone of anamorphism.
The same remarks are applicable to equations (5) and (6), respectively, for
the production of biotite, as to (3) and (4) for the formation of muscovite,
with the addition that the development of biotite involves silication and
decarbonation, and therefore still better than muscovite illustrates the
reactions of the zone of anamorphism.
The physical-chemical principles for the alteration of orthoclase and
microcline to epidote are the same as for the alterations of the more basic
feldspars to epidote. As the process occurs much more extensively in con-
nection with the latter minerals, it is discussed under the basic plagioclases.
(See pp. 263-264.)
ANORTHOCLASE.
Occurrence —T'his mineral is subordinate in quantity to orthoclase and
microcline. It occurs in both deep-seated and effusive igneous rocks; in
MON XLVII—04——17
258 A TREATISE ON METAMORPHISM.
the latter, chiefly in the andesitie lavas. As an allogenic mineral it also
is found in the sedimentary rocks. Whether it occurs as an autogenic
mineral in the metamorphic rocks has not been determined.
Alterations. —-Both orthoclase and microcline contain some sodium. When
the sodium becomes important the mineral is anorthoclase. It naturally
follows from this fact that the alterations of anorthoclase are in all respects
like those of orthoclase and microcline, with the exception that the freed
alkalies are in good part sodium. The reactions are analogous to those
already given for orthoclase, but with the muscovite or biotite the soda-mica
paragonite (monoclinic; sp. gr. 284) is formed. Supposing the sodium
silicate is to the potassium silicate as 2 to 1, the more important reactions
may be written as follows:
uy
2NaAlSi,0,. K AISi,0,+6Al (OH) ,=KH, Al,Si,0,+2NaH, Al,Si,0,,+6H,0—k.
2NaAlSi,O,. KAISi,0,+-MgCO,-+-FeCO,+5Al (OH),=
HK MeFeA1,8i,0,,+2NaH, Al,Sis0,,+-5H,0+2C0,—k.
(1) 2
(2) 2(2NaAlSi,0,.K AlSi,0,) +9H,0+3C0,=6Al(OH),+188i0,+K,CO,+21
(3)
(4)
Supposing the sodium silicate is to the potassium silicate as 3 to 1, we have:
(5) 2(3NaAISi,05.K A1Si,0,) +2Fe,0,+8CaCO,+2H,0=
4HCa, Al, FeSi,Oy3 +12Si0,-+ K,CO, +3Na,CO,+4C0, +k.
The equations corresponding to (3) and (5) under orthoclase and
microcline are not written, since their occurrence is very doubtful. The
decrease in volume of the kaolinite and quartz as compared with the
anorthoclase, equation (1), is 9.56 per cent, or of the kaolinite alone is 52.19
per cent. The decrease in volume of the gibbsite and quartz as compared
with the anorthoclase, equation (2), is 3.30 per cent, or of the gibbsite alone
is 68.02 percent. The decrease in volume of the muscovite and paragonite,
as compared with the anorthoclase and gibbsite, equation (3), is 20.04 per
cent. The decrease in volume of the biotite and paragonite, as compared
with the anorthoclase and gibbsite, equation (4), is 10.91 per cent. The
decrease in volume of the epidote and quartz, as compared with the anortho-
clase, hematite, and calcite, equation (5), is 28.30 per cent. Equations cor-
responding with the above and the volume relations can be easily worked
out along analogous lines for other ratios of the sodium-bearing and potas-
sium-bearing silicates, but the general results would be the same, so this is
hardly worth the while.
The geological positions and physical conditions under which the
5
THE PLAGIOCLASE FELDSPARS. 259
changes take place are identical with the corresponding changes of ortho-
clase and microcline—i. e., alterations represented by equations (1) and (2)
take place in the zone of katamorphism, and especially the belt of weathering.
Alterations (3) and (4) occur in the zone of anamorphism, and that of
equation (5) is known for the belt of cementation. One general point is
clear from the above, that in the anorthoclase rocks we have a source for
paragonite in the paragonite-schists and paragonite-gneisses.
TRICLINIC.
The plagioclase feldspars are a group of triclinie feldspars which range
from sodium-aluminum silicate to caleium-aluminum silicate. The former
is a polysilicate and the latter an orthosilicate, hence there is great variation
both as to composition and as to acidity. The names, compositions, and
specific gravities of the species, as given by Tschermak and Dana, are as
follows:
ALBITE, OLIGOCLASE, ANDESINE, LABRADORITE, BYTOWNITE, AND ANORTHITE.
Albite:
NaA1$i,0s.
Triclinic.
Sp. gr. 2.62-2.65.
Oligoclase:
Ab to AbzAn,.
Triclinic.
Sp. gr. 2.65-2.67.
Andesine:
Ab;An, to Ab, An).
Triclinic.
Sp. gr. 2.68-2.69.
Labradorite:
Ab,An, to Ab, Ang.
Triclinic.
Sp. gr. 2.70-2.72.
Bytownite:
Ab, An; to An.
Triclinic.
Sp. gr. 2.72-2.74.
Anorthite:
CaAl,Si,0s.
Triclinic.
Sp. gr. 2.74-2.76.
Oceurrence—The plagioclases are probably the most important rock-
making constituents, being approached in abundance only by the orthoclase
feldspars and by quartz. The plagioclases are present as pyrogenic con-
stituents in the great majority of igneous rocks. They also occur very
260 A TREATISE ON METAMORPHISM.
abundantly as allogenic constituents in the sedimentary rocks. In such
rocks the more siliceous plagioclases are more plentiful than the less siliceous
plagioclases, because of the more ready decomposition of the latter. The
plagioclases develop abundantly as autogenic constituents in the metamor-
phic rocks of both sedimentary and igneous origin. The plagioclase albite
is recorded as being derived from analcite, heulandite, laumontite, plagio-
clases (with orthoclase), sodalite, spodumene, and stilbite. The plagioclase
anorthite is not recorded as being derived from other minerals. But it is
seen in the zone of katamorphism that anorthite passes into various zeolites
by simple hydration. It can hardly be doubted that when such zeolites
pass into the zone of anamorphism by dehydration they are sources of anor-
thite. Doubtless also anorthite is produced in different ways from the com-
binations of various minerals, just as it passes into different combinations of
minerals. The intermediate feldspars, which are intermolecular mixtures of
albite and anorthite, may be derived from any of the minerals from which
albite and anorthite are formed.
Alterations —In treating the alterations of the plagioclases the only prac-
ticable plan is to calculate equations and volume reactions separately for
albite and for anorthite. For any of the intermediate feldspars the corre-
sponding equations may be written by multiplying the albite and anorthite
equations by the number of molecules of these compounds, respectively,
and adding the products. However, the alterations of the plagioclases are
so complicated that I have not been able to make the treatment more than
very partial.
The species belonging to the more siliceous half of the plagioclase feld-
spars—i. e., albite, oligoclase, and andesine—trequently undergo alterations
similar to those of the monoclinic feldspars, producing kaolin (monoclinic;
sp. er. 2.615), gibbsite (monoclinic; sp. gr. 2.35), and quartz (chombohe-
dral; sp. gr. 2.6535). These alterations may be considered as coming
from the albite molecule.
But the more common alterations of the plagioclases are into the zeo-
lites, epidote (monoclinic; sp.gr.3.38), quartz, the scapolites, and paragonite
(monoclinic; sp. gr. 2.84), and the less siliceous feldspars into more siliceous
plagioclase feldspars. The plagioclases are also recorded as altering into
prehnite (orthorhombic; sp. gr. 2.875) and albite. By pyrochemical
methods plagioclase and sodium carbonate at 220° C. produce the zeolite
analcite (isometric; sp. gr. 2.255), and this process is more rapid in propor-
ALTERATIONS OF PLAGIOCLASE FELDSPARS. 261
tion as the feldspars are less siliceous. The alteration of a given feldspar
may be into two or more of the above minerals. Doubtless often orthoclase
and plagioclase together pass into other minerals. One such reaction has
been considered by Becke, and is given below.
The less siliceous plagioclases, labradorite, bytownite, and anorthite,
alter rarely to kaolin alone, but this mineral may separate simultaneously
with zoisite (orthorhombic; sp. gr. 3.31) or epidote.
The alteration of albite to kaolin and quartz, to gibbsite and quartz,
and of albite and gibbsite to paragonite, respectively, may be written as
follows:
(1) 2NaAlsi,0,+2H,0+C0,=H,Al,Si,0)+4S8i0,-+Na,CO,+k.
(2) 2NaAlSi,O,+3H,0-+CO,=2[Al(OH),]+6Si0,--Na,CO,+k.
NaAlSi,0, +2A1(OH),;=NaH,Al,Si,0,.+2H,0 +k
The decrease in volume in equation (1) of the kaolin and quartz is 4.89
per cent; in equation (2) the increase for the gibbsite and quartz is 1.58 per
cent; in equation (3) the decrease for the paragonite, as compared with the
albite and gibbsite, is 18.85 per cent.
Analcite Gsometric; sp. gr. 2.22-2.29) may be derived from albite
according to the following reaction:
(4) 2NaAlSi,0;+2H,0=Na,Al,Si,0,).2H,0-+28i0,+k.
The increase in volume is 20.82 per cent, supposing the silica separates as
a solid.
Natrolite (orthorhombic; sp. gr. 2.20-2.25) may also be derived from
albite according to the reaction:
(5) 2NaAlSi,O,+2H,0=H,Na, Al,Si,0;,+3Si0,+k.
The increase in volume is 19.95 per cent, supposing the silica separates as
a solid.
From anorthite a number of zeolites are derived. Clarke is one of the
latest authors who has discussed the relations of the zeolites to the feldspars,
and the chemical alterations given are obtained mainly from his paper.”
The equations for the more common varieties may be written as follows:
i
Thomsonite (orthorhombic; sp. gr. 2.3-2.4) is derived from anorthite
according to the following reaction:
(6) 3CaAl,Si,0,+7H,0=Ca,A1,Si,0,,.7H,0+k.
The increase in volume is 34.65 per cent.
«Clarke, F. W., The constitution of the silicates: Bull. U. 8S. Geol. Survey No. 125, 1
RD
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262 A TREATISE ON METAMORPHISM.
Gismondite (monoclinic; sp. gr. 2.265) is derived from anorthite
according to the following reaction:
(7) 3CaAl,$i,0,+12H,0=Ca,Al,Si,0.,.12H,0-+k.
The increase in volume is 52.76 per cent.
For laumontite (monoclinic; sp. gr. 2.305) the change does not appear
to have been determined with reasonable certainty. It may be supposed
to be derived from anorthite by the simultaneous union of freed calcium
and aluminum with other compounds, the calcium perhaps passing into the
carbonate and the aluminum into the hydrate. On this hypothesis the
reaction is:
(8) 2CaAl,$i,0,+-7H,0+CO,=H,CaAl,Si,0,,.2H,O+CaCO,+2[Al (OH),]+k.
The increase in volume is 33.65 per cent, supposing the calcium carbonate
to be dissolved and the aluminum hydroxide to remain as gibbsite. How-
ever, Clarke regards laumontite as derived from equal quantities of anorthite
and the hypothetical compound trisilicic anorthite“ (CazA1,(Si305)6)-
The zeolite phillipsite (monoclinic; sp. gr. 2.20) may be regarded as
derived from albite, anorthite, and leucite, as follows:
(9) 6CaA1,$i,0,+4NaAlSi,0,+6K AlSi,O,+48H,0+2C0,=
3(K,Ca,Al,Si,,0,5.14H,0) +2Na,CO,+4Al (OH), +k.
The leucite is added as a source of the potassium. The increase in volume
of the three compounds in passing into phillipsite is 31.98 per cent.
Heulandite (epistilbite) (monoclinic; sp. gr. 2.20) and stilbite (mono-
clinic; sp. gr. 2.1495) are regarded by Clarke as derived from the
hypothetical compound trisilicie anorthite. Chabazite hombohedral; sp.
gr. 2.12) is regarded by him as derived from this compound and from
normal anorthite. All four, however, may be equally well considered
as derived from intermediate plagioclases with carbonation of the excess
of calcium and hydration of the excess of aluminum. On these hypotheses
the reactions for the four may be written as follows:
(10) 4NaAlsi,O,+3CaAl,$i,0,+21H,0-+2CO,=
3(H,CaAl,Si,0,,.3H,O) +2Na,CO, + 4[ Al (OH),]+k.
(11) 4NaAlSi,O,+3CaAl,Si,0,+24H,0+2C0,=
Ca, Al,(Sij0,)q-18H,O+2Na,CO,+4A1l(OH),+k.
(12) 6NaAISi,0,+6CaAl,Si,0,+3C0,+45H,0=
2[Ca,A],(Si0,)s(SisOs)g-18H,0] +3Na,CO,+6[ Al(OH ),]+6Si0,+k.
«Clarke, F. W., The constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, p. 42.
ALTERATIONS OF PLAGIOCLASE FELDSPARS. 263
Supposing the sodium carbonate is dissolved and the other compounds
are solids, the increase in volume for (10) is 37.14 per cent, for (11) is
43.50 per cent, and for (12) is 46.76 per cent.
Scolecite (monoclinic; sp. gr. 2.16-2.40) may be derived from anorthite,
according to the following reaction:
(13) 3CaAl,Si,0, +9H,0-+C0,=2CaAl,$i,0,).3H,0+2Al(OH),+Ca00,+k.
The increase in volume is 35.23 per cent, provided the gibbsite separates
as a solid and the CaCO, is dissolved.
Mesolite (monoclinic or triclinic; sp. gr. 2.29), according to Clarke,“
is an isomorphous mixture of equal quantities of natrolite and scolecite;
therefore the reaction for this compound may be expressed by the following:
(14) 4NaAlSi,O,+3CaAl,Si,0,+13H,0+C0,=
2(H.Na,CaAl,Si,0,,.H,0) -+6Si0,+2A1(OH),+CaCO,+k.
The expansion in volume is 24.96 per cent, provided the silica and gibbsite
separate as solids and the CaCO, is carried away in solution. If all products
are solid the increase in the volume is 30.19 per cent.
Turning now from the zeolites to other minerals, the plagioclases are
recorded as altering into prehnite (orthorhombic; sp. gr. 2.875). Since
albite is recorded as simultaneously separating, the most probable reaction
is by hydration of the anorthite molecule.
(15) 4CaAl,Si,O,+-SH,O=2H,Ca, Al,Si,0,,+4Al(OH),+28i0, +k.
Supposing the compounds formed to be solids, the increase in volume is
14.85 per cent.
Another important alteration of the plagioclases is into zoisite
(orthorhombic; sp. gr. 3.25-3.37) or epidote (monoclinic; sp. gr. 3.25-3.5),
with the simultaneous formation of kaolinite or gibbsite. The reaction for
anorthite in the case of zoisite is probably:
(16) 4CaAl,Si,0,+3H,0=H,Ca,Al,Si,0,+H,AlSi,0)+k,
or
(17) 4CaAl,Si,0,+4H,0=H,Ca,Al,Si,0o,+2Al(OH) +28i0,+k.
The decrease in volume of the solids in (16) is 7.77 per cent, and in (17)
is 4.58 per cent.
In the formation of epidote the reactions are of a similar kind, but
Fe,O, replaces some Al,O, of the feldspar molecule. Supposing the
«Clarke, cit., Bull. 125, pp. 35-36.
264 A TREATISE ON METAMORPHISM.
aluminum is to the iron as 2 to 1, that the iron is derived from hematite, and
that excess of aluminum separates as gibbsite, the reaction for anorthite is:
(18) 4CaAl,S$i,0,+Fe,0,+6H,O=H,Ca,Al,Fe,Si,O.,+H,Al,Si,0.+2Al(OH),-+k,
or
(19) 4CaAl,Si,0,+7H,0-+ Fe,0,=H,Ca,Al,Fe,Si,0,+4A1(OH),+2Si0,+k.
The increase of the volume of the epidote, kaolin, and gibbsite as compared
with the anorthite and hematite, equation (18), is 3.6 per cent. The
increase in volume of the epidote, gibbsite, and quartz as compared with
the anorthite and hematite, equation (19), is 6.57 per cent.
The scapolites include marialite (tetragonal; sp. gr. 2.566), meionite
(tetragonal; sp. gr. 2.70-2.74), and various isomorphous mixtures. (See
pp. 811-312.) The alterations of the plagioclases into these two minerals
are given. From these equations those for any definite isomorphous mix-
ture of marialite and meionite can easily be formulated. According to
Clarke,” albite changes into marialite, and anorthite into meionite. The
reactions may be written as follows:
(20) 3NaAlSi,O,+NaCl=Na,Al,8i,0,,Cl+k.
Supposing the NaCl to be in solution, the increase in volume of the solid
compound is 10.29 per cent. If the NaCl be supposed to be a solid, the
increase in volume of the solid compound is 1.84 per cent. The change
from anorthite to meionite is represented by the following reaction:
(21) 3CaAl,Si,0,+CaC0,=Ca,Al,Si,0,;+ CO,+k.
If the calcium carbonate be supposed to be present as a solid, the decrease
in volume is 3.78 per cent; if it be supposed to be added in solution, the
increase in volume is 7.87 per cent.
Becke records the alteration of orthoclase and plagioclase into albite
(triclinic; sp. gr. 2.6385), zoisite (orthorhombic; sp. gr. 3.31), muscovite
(monoclinic; sp. gr. 2.88), and quartz (rhombohedral; sp. gr. 2.6535).
His equation for this reaction is as follows: ?
(22) x(NaAISi,O,) +4 (CaAl,Si,0,) +K AISi,0,+2H,0=
x(NaAlSi,O,) +2(HCa, Al,Si,0,5) +H,K Al,Si,0,)+28i0,.
It is impracticable at the present state of knowledge to write reactions
representing the changes of the less siliceous plagioclases into more sili-
ceous plagioclases and other minerals, because the exact compositions of
the original and resultant minerals are not known. The alterations of the
“Clarke, F. W., The constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, p. 29.
> Becke, F., Ueber Beziehungen zwischen Dynamometamorphose und Molecularvolumen: Neues
Jahrbuch fiir Mineralogie, etc., vol. 2, 1896, p. 182.
RELATIONS OF ALTERATIONS OF FELDSPARS. 265
plagioclases to kaolinite, equation (1), and to gibbsite, equation (2), are
by reactions of carbonation and hydration; the alterations to the zeolites,
equations (4) to (14), inclusive, are by reactions of hydration, and (8)
to (14), inclusive, also involve carbonation. The alteration to prehnite,
equation (15), is a reaction of hydration and desilication. The alterations
to zoisite and epidote, equations (16), (17), (18), and (19), are by reactions
of hydration. All but (1), (3), (16), and (17) take place with nerease of
volume ranging from 1.58 to 52.76 per cent, provided all the compounds
formed remain in situ. All take place with liberation of heat. The altera-
tions in all particulars are characteristic of the zone of katamorphism.
The development of kaolin is probably more characteristic of the belt
of weathering than of the belt of cementation. The development of the
zeolites and cpidotes is known to occur on an extensive scale in the belt of
cementation, both within the bodies of other minerals and within the open-
ings in rocks. Amyegdules and veins of these minerals, with quartz, are of
great importance in cementing rocks. The material for this work is doubt-
less in large part, though not altogether, derived from feldspathic minerals.
' For the intermediate scapolites, equations (20) and (21), the alterations
take place with a slight increase in volume for marialite and a slight
decrease for meionite, provided all the compounds which enter into them
are solids. The reactions are those of silication and decarbonation to some
extent, and this involves absorption of heat. The geological occurrences
correspond with the physical-chemical facts. The most common of the
scapolites which occurs as a secondary product in the altered rocks is
wernerite, an isomorphous mixture of meionite and marialite molecules.
As stated by Dana, wernerite ‘“‘occurs in metamorphic rocks, and most
abundantly in granular limestone near its junction with the associated
granitic or allied rock.”* Wernerite is associated with such minerals as
pyroxene, amphibole, and garnet, which occur as deep-seated alterations.
The formation of wernerite from feldspar is probably, therefore, a deep-
seated change which occurs in the zone of anamorphism.
The alteration of orthoclase and plagioclase together to albite, zoisite,
muscovite, and quartz, equation (22), is a reaction of hydration and desili-
cation. It involves an increase in volume. One would therefore . expect
the reaction to take place in the zone of katamorphism.
a@Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. 8. Dana; Wiley «& Sons,
New York, 6th ed., 1892, p. 470.
266 A TREATISE ON METAMORPHISM.
LEUCITE GROUP.
Leucite is the only rock-making mineral belonging to this group.
LEUCITE.
Leucite:
KAISi, 05.
Isometric.
Sp. gr. 2.45-2.5.
Occurrence—Ieucite is « common constituent of volcanic rocks, especially
the more recent ones. The fact that it is not abundant in the older volcanic
rocks is probably due to its ready alteration. It may have been present
originally.
Leucite is not known as a constituent of the schists and gneisses
derived from the sediments. Leucite has been produced by pyro-chemical
methods from analcite and potassium chloride. This is a reversal of the
reaction in the case of change of leucite to analcite. .
Alterations —Leucite frequently alters to analcite (isometric; sp. gr. 2.22—
2.29); to a mixture of orthoclase (monoclinic; sp. gr. 2.53-2.6) and kaolinite
(monoclinic; sp. gr. 2.6—2.63); to a mixture of orthoclase and muscovite
(monoclinic; sp. gr. 2.76-3.0); and to a mixture of orthoclase and nephe-
lite (hexagonal; sp. gr. 2.55—-2.65).
The change of leucite to analcite requires a substitution of sodium for
potassium; hence sodium carbonate or some other sodium compound must
be supposed to be present. Supposing sodium carbonate to be the com-
pound, the reaction is:
(1) 2KAISi,0,4+-Na,CO,+2H,0=Na,A1,Si,O,).2H,0-+K,CO,+k.
Tenoring the carbonates, the increase in volume is 10.74 per cent.
The passage of leucite mto orthoclase and kaolin is as follows, sup-
posing the freed potassium to unite with carbon dioxide:
(2) 4KAISi,0,+CO,-+2H,O=2K AlSi,0,+-H,Al,Si,0)+K,CO,+k.
Supposing the potassium carbonate to be taken into solution, the decrease
in volume of the orthoclase as compared with the leucite is 38.57 per cent,
and the decrease of the orthoclase and kaolinite together as compared with
the leucite is 10.58 per cent.
In a similar way the passage of leucite into orthoclase and muscovite
is as follows:
(3) 6KAISi,0,+CO,+H,0=3K Alsi,0,+KH,Al,$i,0,.+-K,CO,+k.
THE ORTHORHOMBIC PYROXENES. 267
As before, supposing the potassium carbonate to be taken into solution, the
decrease in volume of the orthoclase and muscovite as compared with the
leucite is 12.43 per cent.
In the passage of leucite into orthoclase and nephelite it is necessary
to suppose that a part of the potassium of the leucite is replaced by sodium.
Supposing the nephelite formed to be a pure soda-nephelite, the reaction
would be:
(4) 4KAISi,O,+Na,CO,=2K AlSi,0,+2NaAlSi0,+K,CO,+k.
Ignoring the carbonates, the decrease in volume of the orthoclase and
nephelite as compared with the leucite is 7.59 per cent.
The reactions above given, except the last, are those of hydration,
and the second and third are those of carbonation also. They are,
therefore, reactions which are to be expected in the zone of katamorphism.
The change of leucite into orthoclase and nephelite gives decrease in
volume, with neither hydration nor dehydration, carbonation nor silication.
It is, therefore, to be expected that the change is one which takes place in
the zone of anamorphism.
The artificial transformation of leucite into analcite by treatment
with soda solutions, and the reverse alteration of analcite into leucite
a
by treatment with potassium solutions, as shown by Lemberg,® is an
excellent illustration of the law of mass action and proves the importance
of this principle under natural conditions.
PYROXENE GROUP.
ORTHORHOMBIC PYROXENES.
ENSTATITE, BRONZITE, AND HYPERSTHENE.
Enstatite:
MgSi0,.
Orthorhombie.
Sp. gr. 3.1-3.3.
Bronzite:
(MgFe)SiO; where Mg: Fe :: 8:1, 6:1, and 3:1.
Orthorhombie.
Sp. gr. 3.2-3.3.
Hypersthene:
(MgFe)SiO, where Mg: Fe :: 3:1, nearly to 1:1.
Orthorhombic.
Sp. gr. 3.4-3.5.
«Lemberg, J., Ueber Silicatumwandlungen: Zeitschr. Deutsch. geol. Gesell., vol. 28, 1876, pp.
536-545.
268 A TREATISE ON METAMORPHISM.
Occurrence—The rhombic pyroxenes are common pyrogenic constituents
of igneous rocks rich in magnesium. They are common in the normal
diabases, gabbros, and basalts, and are abundant in the norites, peridotites,
ete. They also occur in the intermediate, basic, and ultrabasic volcanic
rocks, including both lavas and tuffs. A very common associate of the
rhombic pyroxenes is olivine. The rhombic pyroxenes also occur in the
schists and gneisses, especially those derived from eruptives. In such
rocks they are frequently associated with the monoclinic pyroxenes. They
further occur as vein materials and are found in meteorites.
As metamorphic minerals, enstatite is derived from pyrope and
hypersthene from almandite, biotite, and common garnet.
Alterations— The most frequent alteration of the rhombic pyroxenes is
to tale (orthorhombic or monoclinic; sp. gr. 2.7-2.8). The less frequent
alterations are to serpentine (monoclinic; sp. gr. 2.50-2.65), bastite (ortho-
rhombic; sp. gr. 2.50-2.75), actinolite (monoclinic; sp. gr. 3-3.2), and
anthophyllite (orthorhombic; sp. gr. 3.1-3.2).
For the sake of simplicity it is assumed that where pure tale or
serpentine is produced these materials are derived from enstatite; and that
where bastite, actinolite, and anthophyllite are produced these minerals are
derived from bronzite or hypersthene. Of course, serpentine or tale may
be produced from bronzite or hypersthene, the iron separating as oxide or
carbonate. One such possible alteration is written. However, the ordinary
alterations of the ferriferous pyroxenes are to bastite, which is iron-bearing.
The change of enstatite to tale is as follows:
=
(1) 4MeSi0,+CO,+H,0O=H,Me,Si,0,,.+MgC0,+k.
Supposing the magnesium carbonate to be dissolved, the increase in
volume is 9.93 per cent. If a ferriferous pyroxene be supposed to alter
to tale, iron oxide must separate. Supposing this to be in the form of
magnetite (isometric; sp. gr. 5.174), and supposing that the magnesium
is to the iron as 3:1, or that the mineral is intermediate between bronzite
and hypersthene, the reaction may be written:
(2) 3Mg,FeSi,O,.+3H,0+0=3H,Meg.S8i,0,.+Fe,0,-\-k.
Similar equations may be written by which, instead of magnetite,
hematite (rhombohedral; sp. gr. 5.225) or limonite (amorphous; sp. gr.
3.80) is produced, in which case the expansion of volume would be
ALTERATIONS OF ORTHORHOMBIC PYROXENES. 269
greater. The calculated increase in volume of the tale and magnetite, as
compared with the pyroxene, is 14.68 per cent, provided the average
specific gravity of bronzite be used, and 21.73 per cent provided the
average specific gravity of hypersthene be used. Probably the real
increase in volume is the average of the above, or about 18.20 per cent.
In the case of a hypersthene in which the iron is to the magnesium as
1:1 the alteration to tale may be as follows, provided the iron separate as
magnetite :
(3) 8MgFeSi,0,+H,0-+O=H,Mg,Si,0,+Fe,0,+28i0,+k.
The increase of volume of the tale, magnetite, and quartz as compared
with the hypersthene is 12.84 per cent.
Serpentine is produced from enstatite by the following reaction:
(4) 3MgSiO, +2H,0O=H,Mg,Si,0,+Si0,+k.
In case the SiO, is dissolved, the increase in volume is 14.25 per cent;
if it separates as quartz (rhombohedral; sp. er. 2.6535) the increase in
volume is 38.36 per cent.
If a rhombic pyroxene be taken in which the magnesium is to the iron
as 3:1—i. e., stands on the border line between hypersthene and bronzite—
serpentine might be produced by the following reaction, with the simul-
taneous separation of hematite and quartz:
(5) 2Mg,FeSi,O,.4H,0+0=2H,Mg,Si,0,+Fe,0,-+48i0,+k.
Using the specific gravity of hypersthene, in case only serpentine and
hematite separate as solids, the decrease in volume is 221 per cent, and if
the silica separates as quartz the increase of volume is 33.94 per cent.
Supposing the calcium is to the iron as 1:1 and the excess of iron
separates as magnetite, the reaction is:
(6) 3MgFeSi,0,+2H,0-+O—H,Mg,$i,0,-+Fe,0,+-48i0,-+k.
The increase in volume of the serpentine, magnetite, and quartz as
compared with the hypersthene is 20.24 per cent.
Other reactions may be written which represent the alterations of
bronzites and hypersthenes, in which the proportions of magnesium and
iron are different. Also reactions may be written in which the oxide of
iron forms as magnetite or limonite. Where magnetite forms, the increase
in volume would be less than for hematite, and where limonite forms the
increase in volume would be considerably greater.
270 A TREATISE ON METAMORPHISM.
If in the formation of bastite, a pyroxene be taken which stands
i. e., in which the magne-
intermediate between bronzite and hypersthene
sium and iron are as 3:1—and if the same proportions of these constituents
be supposed to hold in the bastite, the reaction is as follows:
(7) 3Mg,FeSi,O,,+8H,0=H,,Mg,Fe,Si,0,,+-4810, +k.
Using the specific gravity of hypersthene, if the silica be dissolved the
increase of volume is 22.77 per cent (if the specific gravity of bronzite be
employed, 15.65 per cent); if the silica separates as quartz, 46.87 per cent.
Similar reactions may be written which represent the formation of bastites
which are richer and poorer in iron, in which cases the volume changes are
slightly different.
The passage of ferriferous rhombic pyroxene into anthophyllite may
be one of pure paramorphism, since in anthophyllite the proportions of
magnesium to iron have ranges paralleled by bronzite and hypersthene.
Therefore, the only necessary change is a molecular one, a mineral being
produced of lower symmetry and lower specific gravity as a result of the
alteration. If the specifie gravity of hypersthene be used, the calculated
increase in volume due to the lower specific gravity of the resultant
mineral is 8.70 per cent.
In the formation of actinolite from a rhombic pyroxene, it is necessary
that lime and silica be added. Supposing the magnesium is to the iron as
3:1 in both the rhombie pyroxene and actinolite, the equation is as follows:
(8) 3Mg,FeSi,O,,+4CaCO,+4Si0, =MgyFe,Ca,Si,,0,,+4C0,-+k.
The decrease in volume of the actinolite as compared with pyroxene,
calcite, and quartz is 7.40 per cent if the specific gravity of hypersthene
be used, and if that of bronzite is 10.77 per cent. Similar equations may
be written in which the proportions of magnesium and iron are different.
The changes of the rhombic pyroxenes to tale involve reactions of
carbonation and hydration, or of hydration and oxidation, or of all three
together. The changes of the rhombic pyroxenes to serpentine and bastite
involve hydration alone, or hydration and oxidation. All take place with
increase of volume and liberation of heat.
Corresponding with these facts, as a matter of observation the devel-
opment of serpentine, bastite, and tale from the rhombic pyroxenes takes
place in the zone of katamorphism. The development of tale is especially
characteristic of the belt of weathering, and serpentine and bastite of the
MONOCLINIC PYROXENES. Zs Il
belt of cementation, although it can not be asserted that the formation of
any of these minerals is confined to either belt.
The paramorphic change of rhombic pyroxene into anthophyllite
being one involving lessening of specific gravity and decrease of symmetry,
one would expect the change to take place in the upper physical-chemical
zone, but I have been unable to ascertain from the literature the facts in
this case.
The formation of actinolite from a rhombic pyroxene requires the
assistance of calcite and silica. This reaction is one of silication and
decarbonation. It occurs with diminution of volume and absorption of
heat. As a matter of observation, corresponding with these facts it is
well known that the change is a deep-seated one.
MONOCLINIC PYROXENES.
DIOPSIDE, SAHLITE, HEDENBERGITE, AUGITE, ACMITE, SPODUMENE, WOLLASTONITE, AND PECTOLITE.
Diopside:
CaMgSi,0,.
Monoclinic.
Sp. gr. 3.2-3.38.
Sahlite:
Ca(MgFe) Si,O,.
Monoclinic.
Sp. gr. 3.25-3.4.
Hedenbergite:
CaF eSi,0,.
Monoclinic.
Sp. gr. 3.5-3.58.
Augite:
Ca(MgFe)Si,O, with (MgFe) (AlFe),SiO,.
Monoclinic.
Sp. gr. 3.3-3.5.
Acmite:
NaFeSi,0,.
Monoclinic.
Sp. gr. 3.50-3.55.
Spodumene:
LiAISi,0,.
Monoclinic.
Sp. gr. 3.13-3.20.
Wollastonite:
CaSi03.
Monoclinic.
Sp. gr. 2.8-2.9.
Pectolite:
HNaCa,Si,0,.
Monoclinic.
Sp. gr. 2.68-2.78.
272 A TREATISE ON METAMORPHISM.
The minerals diopside, sahlite, and augite constitute the so-called
diopside-augite series.
Occurrence—The pyroxene group is one of the most widespread and
important. One or another variety of pyroxene may occur in almost any
rock; but the pyroxenes are much more abundant in the intermediate and
basic than in the acidic rocks. Pyroxene is found in the plutonic and
voleanie rocks, as an original constituent of the clastic rocks, and as an
original and secondary constituent of the metamorphosed rocks, both of
igneous and of aqueous origin. ‘The minerals of the pyroxene group occur
extensively in veins.
Diopside occurs in marbles, especially magnesian marbles. Indeed,
this is the common form of pyroxene which develops as a secondary
constituent during the metamorphism of the magnesian limestones. It also
occurs in veins. As a metamorphic mineral diopside is derived from
dolomite.
Sahlite occurs in ferriferous magnesian marbles. Like diopside, it is
also found in veins. Unlike diopside, it is a common product in many
hornblendie schists and gneisses, such rocks probably having been in their
original condition calcareous, magnesian, and ferriterous. Sahlite is derived
from ankerite and parankerite.
Hedenbergite occurs as a rather common constituent of some nepheline
syenites and other basic syenites.
Augite is a common form of pyroxene in the eruptive rocks, both
plutonic and voleanice. It occurs in many mechanical sediments. It also
is found in metamorphic rocks of both igneous and sedimentary origin,
though in the sedimentary metamorphosed rocks it is less common than
diopside and sahlite. But augite develops to a considerable extent in the
sedimentary rocks which are intermediate between the chemical and
mechanical roeks—that is, those which contain abundant calcium carbonate
and also are rich in aluminum. Augite is recorded as a metamorphic
mineral derived from hornblende.
Wollastonite occurs especially in the metamorphosed calcareous and
sedimentary rocks, it being a secondary product produced by metamorphism.
It is found abundantly in marbles, and in schists and gneisses which were
originally calcareous, especially the calcareous feldspathic schists. It also
develops in calcareous inclusions in eruptive rocks, and is found as a contact
ALTERATIONS OF DIOPSIDE-AUGITE SERIES. 273
product of igneous and calcareous rocks. The schists and gneisses contain-
ing wollastonite are often garnetiferous and epidotic.
The very frequent development of the above pyroxenes in the sedi-
mentary rocks which are calcareous, rather than amphiboles, is due to the
fact that the pyroxenesare richer in calcium than are the amphiboles. Where
sedimentary rocks contain magnesium abundantly with the calcium, the
amphiboles are likely to form rather than the pyroxenes.
Aemite occurs mainly in the eruptive rocks, and especially in those
which are rich in alkalies. According to Rosenbusch, it occurs especially
in granites and syenites rich in sodium, in the eleeolite-syenites, phonolites,
aud leucitophyres. As a metamorphic mineral acmite is derived from
arfvedsonite.
Spodumene sometimes occurs as an accessory constituent in the granites,
schists, and gneisses, and in some cases as considerable masses.
Pectolite, while not an abundant mineral, is present as a secondary
constituent in many basic eruptive rocks, both plutonic and voleanic. It is,
however, especially prevalent in the volcanic rocks, since these are more
porous, and pectolite is especially likely to occur in cavities or seams.
Occasionally pectolite is found in the metamorphic rocks as a product of
apophyllite.
Alterations of the diopside-augite series. —'he most common alteration of the non-
aluminous diopside and sahlite is into tale (orthorhombic or monoclinic;
sp. gr. 2.7-2.8). They also often alter into serpentine (monoclinic; sp. gr.
2.5-2.65). These changes are accompanied by the formation of calcium
carbonate, and frequently by the separation of a part of this carbonate as
calcite (Whombohedral; sp. gr. 2.7135).
The aluminous pyroxenes, augite, and diallage, under the conditions of
the zone of katamorphism, change into chlorite (monoclinic; sp. gr. 2.08—
2.16), with which are usually associated epidote (monoclinic; sp. gr. 3.25—
3.5), this mineral often being embedded in the chlorite and calcite. Under
conditions of weathering, any of the minerals of the diopside-augite series
may be partly or entirely replaced by quartz (rhombohedral; Sp. er.
2.6535), chalcedony (cryptocrystalline; sp. gr. 2.6-2.64), or calcite. Such
replacements are particularly common in the case of the porous andesites
and trachytes, and also in tuffs. Not infrequently this replacement of
the pyroxene occurs without the feldspar being greatly affected.
MON XLvII—04——18
274 A TREATISE ON METAMORPHISM.
However, perhaps the most frequent and characteristic of the altera-
tions of the diopside-augite series is uralitization or change to amphibole
(monoclinic; sp. gr. 2.9-3.4). This process is particularly characteristic of
the ancient igneous rocks, and especially those which are under compara-
tively deep-seated conditions, although the alteration is by no means con-
fined to deep-seated rocks. It occurs on a great scale under the conditions
of the transformation of the igneous rocks into schists and gneisses. During
the process of uralitization epidote also very frequently forms. Not intre-
quently also magnetite (isometric; sp. gr. 5.16—5.18) and calcite separate.
In some cases the change is accompanied by the development of a feldspar,
such as albite (triclinic; sp. gr. 2.62-2.65). The kind of amphibole which
forms depends upon the variety of the pyroxene. From diopside, tremolite
(monoclinic; sp. gr. 2 9-3.1) is the ordinary product; from sahlite, actino-
lite (monoclinie; sp. gr. 8.0-3.2) is normally to be expected; from diallage
and omphacite (according to Zirkel,* varieties of augite), smaragdite (a
variety of hornblende) is ordinarily produced; and from ordinary augite,
hornblende (monoclinic; sp. gr. 3.05-3.47) is usually developed. Finally,
it not infrequently occurs that augite changes directly into biotite (mono-
clinie; sp. gr. 2.90).
The change of diopside to tale may be written as follows:
(1) 3CaMgSi,O,+3C0, +H,O=H,Mg,$i,0,+3CaC0,+2S8i0,+k.
The increase in volume, supposing all compounds remain as solids, is 48.74
per cent. If only the tale remains, the decrease is 30.13 per cent. Sup-
posing the diopside were one in which a part of the calcium and magnesium
were replaceable by iron, so that the calcium and magnesium and iron are
present in equal proportion, thus approaching sahlite in composition, and
supposing the iron to pass into magnetite, the reaction is—
(2) 3CaMgFeSi,0, +3CO,+-H,0-+0=H,Mg,Si,0,,+3CaCO,+Fe,0,+58i0,+k.
The increase in volume of the talc, calcite, magnetite, and quartz, as com-
pared with the diopside, is 27.88 per cent.
The change of diopside to serpentine may be represented by the
following equation:
(3) 3CaMegSi,0,+3C0,+2H,0=H,Mg,Si,0,+4S8i0,+3CaCO,+k.
«Naumann, ©. F., and Zirkel, F., Elemente der Mineralogie, Leipzig, 1898, p. 696.
ALTERATIONS OF DIOPSIDE-AUGITE SERIES. 2795
Supposing all the compounds separated as solids, the increase in volume is
56.32 per cent. If only the serpentine and quartz remain as solids, the
increase in volume is 0.44 per cent.
The change from sahlite to ferriferous bastite, provided that in both
compounds the magnesium is to the iron as 3:1, is—
(4) 3Ca,Mg,FeSi,O,,+12CO, +8H,O=H,,MgFe,Si,Ozg-+ 16810, + 12CaC0,-+k.
If all the compounds separate as solids, the increase in volume is 56.41 per
cent; if the bastite and quartz remain as solids, 1.93 per cent. Supposing
the calcium, magnesium, and iron were in equal proportions in the sahlite,
and that in the bastite the magnesium were to the iron as 3:1, the equation
may be written—
(5) 9CaMgFeSi,0,-+9CO,+8H,0+20=H,,Mg,Fe,Si,0z,+2Fe,0,+9CaC0,+198i0,--k.
The increase in volume of the serpentine, magnetite, calcite, and quartz,
as compared with the sahlite, is 37.50 per cent.
It is, of course, not impossible that serpentine shall develop as one of
the products from augite. In this case it doubtless forms from the sahlite
molecule of the augite compound, the sesquioxide compounds passing into
some other mineral. It hardly seems advisable to attempt to write equa-
tions representing such an alteration.
In writing equations for the alterations of the aluminous pyroxenes into
chlorite and epidote it is necessary that certain assumptions shall be made
in reference to the relative proportions of the various elements. Moreover,
if equations are written which produce chlorite alone, a large amount of the
sesquioxide bases must be left over. If an equation be written for the forma-
tion of epidote, a large amount of magnesium is unaccounted for. Since it
is very common for the minerals chlorite and epidote to form simultaneously,
an equation is written on this supposition. In order to give definiteness to
the compound, it is supposed that there are twice as many molecules of the
diopside part of the augite molecule as of the other part. Furthermore, it
is supposed for the diopside molecule that the magnesium is to the iron as
2:1; and for the other molecule that the aluminum is to the iron sesquioxide
as 3:1. An epidote is taken in which the aluminum is to the iron as 2:1.
A chlorite between clinochlore and prochlorite is taken, as such a chlorite
is at about the middle of the series. As may be seen by reference to the
analyses of augites and epidotes, the proportions taken represent about their
276 A TREATISE ON METAMORPHISM.
average compositions. With all these hypotheses, and supposing the extra
silica to separate as quartz, the magnesia to separate as magnesium carbonate,
and the iron as sesquioxide of iron, the equation may be written as follows:
(6) 6[2(Ca;Mg,FeSi,0,.). Mg,Fe,Al)Fe,Si,0,5] -+-12CO,+39H,0+120
18(HCa, Al, FeSi,O,3) -+3(HopMgy, Al,Si,0,;) +33Si0,+12Fe,0,+12MgC0,-+k.
If all the compounds remain as solids the increase in volume is 15.43 per
cent. If the magnesium carbonate be dissolved the increase in volume is
8.58 per cent.
It is evident that many other equations could be written if other sup-
positions be made as to the relative proportions of the magnesium to the
iron and the aluminum to the iron in the respective compounds, and if other
chlorite than the particular one chosen be produced. For the complex
silicates, present knowledge is not sufficient to determine whether or not
particular equations written accurately represent the alterations which take
place, although closer study in the future may possibly determine this.
But there is little doubt that substantially the change represented by equa-
tion (6) has occurred in many instances, whether it can be verified in an
individual case or not, as doubtless have also a multitude of alterations
which might be represented by other possible equations. The difficulty is
to ascertain in a given instance which of the equations represents a given
alteration. It is hoped that the quantitative statement of the problem given
by equation (6) and following equations will lead to closer study of the
compounds which enter into new compounds and the compounds which are
produced, and thus to more exact knowledge of the various alterations of
augite.
According to the above reactions, as would be expected from the nature
of the compounds, the alteration of diopside and sahlite more frequently
produces tale, serpentine, and bastite, while the alteration of augite more
frequently produces chlorite and epidote.
As already noted, perhaps the most characteristic of the alterations of
the pyroxenes is to the amphiboles. This alteration involves the substitu-
tion of magnesium, or magnesium and iron, for calcium. It is supposed
that the iron and magnesium are added in the form of carbonate, and that
the liberated calcium separates in the form of carbonate. Parallel equa-
tions can, however, readily be written on the basis of any other magnesium
compound being added and similar iron and calcium compounds being
ALTERATIONS OF DIOPSIDE-AUGITE SERIES. 210
produced. It is assumed, further, that the alteration of a pyroxene results
in the production of the most closely allied amphibole. Of course this is
not always the fact, but it is believed to be usual. Following this assump-
tion, the alteration of diopside is to tremolite, of sahlite is to actinolite, of
augite is to hornblende.
The change from diopside to tremolite may be written as follows:
(7) 2CaMgSi,0,+MgCO,=CaMg,Si,O,.+CaC0,+k.
Regarding the magnesium carbonates as added in solution and the calcium
carbonate as subtracted in solution, the increase in volume is 5.68 per cent.
If the magnesium carbonate be considered as present as magnesite, and the
calcium carbonate be considered as present as calcite, the increase in volume
is 10.55 per cent.
The change from sahlite to actinolite, supposing the magnesium and
iron to be present in the sahlite in equal proportions, is as follows:
(8) 2Ca,MgFeSi,O,,+FeCO,+ MzCO,=Ca,Mg,Fe,S$i,0.,+2CaCO,+k.
Supposing the sahlite and actinolite only to be solids, the increase in vol-
ume is 7.28 per cent. I all the compounds are regarded as solids on both
sides of the equation, the increase in volume is 10.81 per cent.
Supposing that in the augite compound there are two of the sahlite
molecules to one of the sesquioxide molecule, and supposing that the mag-
nesium and iron are in equal proportions in both the augite and hornblende,
the general alteration may be written as follows:
(9) 2[Ca,MgFeSi,0,,. (MgFe) (AlFe), Si0,]-+FeCO, ~MgC0O,=
Ca,Mg,Fe,Si,0,,.(MgFe), (AlFe),Si,0,,-+2CaCO,+k.
But before the volume relations can be calculated it is necessary to
assume definite proportions between Mg and Fe, and Al and Fe, in the
second members of the augite and hornblende molecules. If the magne-
sium be taken to the iron as 2:1 and the aluminum to the iron as 2:1,
an average case, and only the augite and hornblende be considered as
solids, the increase in volume is 4.30 per cent. If all the compounds in
both equations are solids, the increase in volume is 6.14 per cent.
An inspection of the above equations giving the alterations of the
diopside-augite series to amphibole shows that the chemical change in the
alteration of diopside and sahlite to tremolite and actinolite is relatively
greater than in the alteration of augite to hornblende. Moreover, if it be
278 A TREATISE ON METAMORPHISM.
supposed that the last half of the augite and hornblende molecules are
present in greater proportion than given in the equations, the chemical
change would be of still less relative importance. This is of interest because
the alteration of augite to hornblende is a far more common phenomenon
than the alterations of diopside and sahlite to tremolite and actinolite The
equations, also give reasons for the very frequent occurrence of calcite with
uralite. The nature of the alterations is such that calcium carbonate must
be produced, and very naturally a portion of this substance frequently
separates as calcite.
In the change of augite to biotite it is necessary that potassium be
derived from some source. Supposing it to be furnished in the form of
potassium carbonate, as a result of the decomposition of some of the potas-
sium-bearing silicates, the simplest form of reaction may be written as
follows:
(10) 2[Ca(MgFe)Si,0,. (MgFe) ( AlFe),8i0,] +K,CO,+H,0+C0,=
2HK (MeFe),(AlFe),Sis0,.-+2CaCO,+k.
Supposing the MgO: FeO:: 2:1, and the Al,O,: Fe,O,::3:1—these ratios
being chosen because they represent about an average of the analyses—
and multiplying the above equation by 6, we have:
(11) 2[CasMg,Fe,Si,,0s5. Mg, Fe, AlgF'e,Si,035 ] +-6K,CO;+ 6H,O+6C0,=
2(H,K,Meg,Fe,Al)FesSi,,0;.) +12CaCO,-+-k.
Disregarding all other compounds, the increase in volume of the biotite as
compared with the augite is 17.26 per cent.
The alteration of diopside and sahlite to tale, serpentine, and bastite,
equations (1), (3), and (4), all involve increase in volume and liberation of
heat; also they are alterations involving carbonation and hydration. Equa-
tions (2) and (5) involve carbonation, hydration, and oxidation. In all
except equation (1), even if all of the separated quartz and calcite is
dissolved, there is still an increase in volume. They therefore stand as
alterations that are typical of all the principles of metamorphism in the
zone of katamorphism.
The changes of the pyroxenes, especially augite, to chlorite and epidote,
equation (6), involve hydration, carbonation, and oxidation. The change
occurs with increase in volume and liberation of heat, even if the resultant
oxide of iron and magnesium carbonate be ignored. If these separate as
solids, the increase in volume is considerable. The alteration is, therefore,
ALTERATIONS OF DIOPSIDE-AUGITE SERIES. US)
like the change to talc, serpentine, and bastite, one characteristic of the
upper physical-chemical zone. The change of pyroxene to the fibrous
amphibole known as uralite occurs in the belt of cementation on an exten-
sive scale, and to this position the volume change corresponds.
But the passage of pyroxene into definite amphibole individuals is one
of the most common alterations in the zone of anamorphism, and especially
under conditions of mashing. The rule for this zone is for alterations to
oceur which result in minerals of higher specific gravity. The alteration
of pyroxene to amphibole seems to be an exception to this rule; for the
specific gravity of the pyroxenes ranges between 3.2 and 3.6, while that of
the amphiboles varies from 2.9 to 3.4. .
Mainly in consequence of this decrease in specific gravity the increase
in volume, as already seen, of all compounds entering into the reactions in
the change from diopside to tremolite, equation (7), is 10.55 per cent; of
sahlite to actinolite, equation (8), 10.81 per cent; of augite to hornblende,
equation (9), 6.14 per cent, supposing that the necessary chemical constitu-
ents added to the pyroxene are solid carbonates and the other compounds
produced are solid carbonates.
Unlike the previous alterations, these changes do not involve oxidation,
hydration, or carbonation; nor, on the other hand, do they involve deoxi-
dation, dehydration, or silication. They are substitution reactions, by which
magnesium, or iron, or both take the place of calcium. They are, there-
fore, analogous to the dolomitization or ferritization of the limestones; but
the volume change is in an opposite sense from those alterations.
But another factor may enter into the problem, the effect of which is
hard to estimate. The exchange of the magnesium and iron for calcium is
supposed to take place with the separation of a carbonate. If such carbon-
ate were simultaneously silicated, the entire volume change for all the fac-
tors concerned would be decrease. It is necessary to consider the volume
relations of all the resultant minerals rather than those of the pyroxene and
amphibole alone, and hence it may be that in the change of pyroxene to
amphibole in the lower physical-chemical zone, if one could ascertain the
entire effect of this alteration in connection with other alterations, the
volume would not be expanded but contracted, and thus there be no real
exception to the law that the reactions here take place with condensation
of volume.
280 A TREATISE ON METAMORPHISM.
But, even if this be true, it is freely admitted that the case is not fully
covered, for it is very uncommon indeed for the chief resultant mineral of
an alteration in the zone of anamorphism to have a lower specific gravity
than the minerals from which it is derived with comparatively small chem-
ical change. Apparently, for some reason the amphiboles are more stable
under conditions of moderately deep-seated metamorphism than the pyrox-
enes. This view is confirmed by the fact that, while the majority of the
schists and gneisses are amphibolitie rather than pyroxenitic, in some of the
gneisses and schists which have been altered under very deep-seated condi-
tions the pyroxenes are present instead of the amphiboles. The significance
of this fact is probably that an unusually high pressure is required in order
to produce the mineral of the highest specific gravity in the case of the
pyroxene-amphibole group.
The change from augite to biotite, equations (10) and (11), is one which
takes place in the zone of anamorphism especially under conditions of
mashing. In this change the volume of the biotite produced is greater
than that of the pyroxene; in the case of the equation (11) 17.26 per cent.
However, this case is similar to that of hornblende. Potassium salt must
be added from some other mineral and a calcium salt is produced. In
order to get the real volume relation of the reaction it would be necessary
to know the source of the potassium and the place to which the calcium
goes; and as present information does not enable us to determine this, no
definite statement can be made as to the total effect of all the changes
involved in the alteration of augite to biotite.
Alterations of pyroxenes other than the diopside-augite series —NoO equations are written
for the alterations of wollastonite, hedenbergite, acmite, and pectolite,
because the character of the alterations of these compounds has not been
described in the standard authorities, although there is no doubt that these
minerals, like all others, do undergo various alterations. All these minerals
form under deep-seated ccnditions; and it is to be expected that under the
conditions of the zone of katamorphism, especially in the belt of weathering,
they would be decomposed; but, if so, the minerals into which they change
are unknown.
Alterations of spodumene are recorded. According to Dana, the first
stage in the alteration of spodumene is to beta-spodumene (crystallization
not determined; sp. gr. 2.644-2.649), in which one-half of the hthium is
ORTHORHOMBIC AMPHIBOLES. 281
replaced by sodium. The second stage in the process of alteration is the
beta-spodumene passing into eucryptite (hexagonal; sp. gr. 2.667) and albite
(triclinic; sp. gr. 2.62—2.65), or into muscovite (monoclinic; sp. gr. 2.76-3)
and albite, the uniform mixture of which has been known as cymatolite
(sp. gr. 2.69-2.70); or spodumene may pass into muscovite and microcline
(triclinic; sp. gr. 2.54-2.57). The reactions representing the above changes
may be expressed in the manner shown by the equations given below.
‘The change from spodumene to beta-spodumene may be written:
(12) 4LiAISi,O,+-Na,CO,=2LiNaAl,Si,0,,+Li,CO,+k.
The increase in volume is 24.72 per cent. Where the beta-spodumene
breaks up into eucryptite and albite the reaction is:
(13) LiNaAl,Si,O,,=LiAISiO,+NaAlsi,O,+k.
The increase in volume is 0.05 per cent. Where the beta-spodumene
passes into muscovite and albite the reaction is:
(14) 6LiNaAl,Si,O,,+K,CO,+2H,0+2C0,=2H,K Al,Si,0,,+6NaA$i,0,+3Li,CO,-+k.
The decrease in volume is 0.76 per cent. In case the spodumene changes
into muscovite and microcline the reaction is:
(15) 12LiA1Si,0,4+-4K,CO,+2C0, +2H,0=2H, K Al,8i,0,) +6K AlSi,0,+6Li,CO,-++k.
The increase in volume is 31.74 per cent.
One would expect reactions (12) and (15) to take place in the zone of
katamorphism, but I know of no observations on this point, nor as to the
conditions under which reactions (18) and (14) occur.
AMPHIBOLE GROUP.
ORTHORHOMBIC AMPHIBOLES.
ANTHOPHYLILITE AND GEDRITE.
Anthophyllite:
(MgFe) SiO, Mg: Fe::4:1, 3:1, ete.
Orthorhombie.
Sp. gr. 3.1-3.2.
Gedrite:
(MgFe).Si,O,. MgAl,Si0,.
Orthorhombic.
Sp. gr. 3.1-3.2.
Oceurrence—Anthophyllite and gedrite occur in the schists and gneisses,
both those derived from sedimentary and those derived from igneous rocks.
282 A TREATISE ON METAMORPHISM.
They are frequently associated with hornblende and mica. Anthophyllite
occupies the same position in the rhombic amphiboles that bronzite does in
the rhombic pyroxenes, and gedrite the same position as hypersthene. As
already described (p. 270), the bronzites and hypersthenes alter into
anthophyllite. It is to be expected that gedrite in a similar manner forms
from hypersthene, but this particular alteration is not mentioned in the
standard books of reference. Also, as described (p. 310), anthophyllite
forms as a secondary product from olivine.
Alterations —Anthophyllite by hydration passes into talc’ (orthorhombic
or monoclinic; sp. gr. 2.75) or bastite (orthorhombic; sp. gr. 2.6). Also,
Lacroix states* that rarely it alters into calcite (rhombohedral; sp. gr.
2.7135). Supposing the magnesium is to the iron as 3:1, and that the
freed iron separates as hematite (rhombohedral; sp. gr. 5.225), the alteration
to tale may be written as follows:
(1) 2Mg,FeSi,O,,+2H,0-+0=2H,Mg,$i,0,)+Fe,0, +k.
The increase in volume of the tale and hematite, as compared with the
anthophyllite, is 11.41 per cent. If the iron oxide be supposed to be
hydrated into limonite (not crystallized; sp. gr. 3.80), the imcrease in
volume would be still greater. If bastite be produced, and it be supposed
that the magnesium is to the iron as 3:1, the same as in the anthophyllite,
the equation may be written:
(2) 3Mg,FeSi,O,)+8H,O=H,,MgyFe,Si,Oz5+48i0,-+k.
The increase in volume of the bastite and quartz (rhombohedral; sp. gr.
2.6535) as compared with the anthophyllite is 34.09 per cent. If the silica
be supposed to be dissolved the increase in volume is 12.09 per cent.
The particular alterations which gedrite undergoes are not described
in the standard text-books; therefore no attempt is made to write equations
for changes of this mineral.
The alteration of anthophyllite to tale and iron oxide involves hydra-
tion and oxidation. The alteration of anthophyllite to bastite involves
hydration and desilication. Both sets of reactions are, therefore, char- .
acteristic of the zone of katamorphism; and it is in this zone, especially in
the belt of weathering, that the changes occur.
«Lacroix, A., Minéralogie de la France, Paris, 1893-95, vol. 1, p. 637.
? ? i=) ? ? BF , }
OCCURRENCE OF MONOCLINIC AMPHIBOLES. 283
MONOCLINIC AMPHIBOLES.
The monoclinic amphiboles include the following rock-making minerals:
TREMOLITE, ACTINOLITE, CUMMINGTONITE, GRUNERITE, HORNBLENDE, GLAUCOPHANR. RIEBECKITE, AND
ARFVEDSONITE.
Tremolite:
CaMg;Si,O,,
Monoclinic.
Sp. gr. 2.9-3.1.
Actinolite:
Ca(MgFe),Si,O,>.
Monoclinic.
Sp. gr. 3-3.2.
Cummingtonite:
(MgFe)SiO,.
Monoclinic.
~ Sp. gr. 3.1-3.32.
Grimerite:
FeSi0,.
Monoclinic.
Sp. gr. 3.713.
Hornblende:
Chiefly Ca(MgFe),Si,O,, with (MgFe),(AlFe),Si,O,., and Na,Al,Si,O,).
Monoclinic.
Sp. gr. 3.05-3.47.
Glaucophane:
NaA1Si,O05. (FeMg)Si03.
Monoclinic.
Sp. gr. 3.103-3.113.
Riebeckite:
Na, Fe,Si,O,,. FeSiO,.
Monoclinic.
Sp. gr. 3.3.
Arfvedsonite:
(Na,CaFe ),Si,O,..(CaMg),(AlFe),Si,Oj5.
Monoclinic.
Sp. gr. 3.44-3.45.
Occurrence—The monoclinic amphibole group of minerals is one of the
most important of the rock-making minerals. Like the pyroxenes, one
form or another of amphibole may occur in almost any kind of rock,
running from the most basic to the most acid, including both plutonic and
voleanic rocks, the unmodified sedimentary rocks, and metamorphosed,
igneous, and sedimentary rocks. The amphiboles develop extensively as
secondary minerals, and especially is this true for the variety of amphibole
known as uralite, which, as seen on pp. 274-275, 276-278, is derived from
corresponding pyroxenes.
Tremolite and actinolite are very abundant in the schists metamor-
phosed from carbonate rocks, especially those rich in magnesium and iron.
284 A TREATISE ON METAMORPHISM.
They also occur in the metamorphosed calcareous fragmental sediments.
Where iron is not abundant, as in the marbles, tremolite is the mineral
which ordinarily develops. Where ferrous iron is plentiful actinolite
normally forms. Where iron is the chief or only carbonate, griinerite
ordinarily develops. Tremolite and actinolite also occur as alteration
products in igneous rocks, being noted in diabases, gabbros, and more basic
rocks. The secondary products frequently take the form of asbestos and
jade. They are frequently associated with tale and serpentine in steatite-
schists or serpentine-schists. Often, also, tremolite and actinolite are asso-
ciated with pyroxene, epidote, and chlorite. ‘These amphiboles also occur
in veins. Summarizing, as metamorphic minerals, tremolite is derived from
diopside, dolomite, and olivine; actinolite from ankerite, bronzite, hyper-
sthene, olivine, parankerite, and sahlite.
Cummingtonite, the monoclinic amphibole corresponding in composi-
tion with the orthorhombic amphibole anthophyllite, occurs in various
schists of metamorphic origin. It is not known as an original constituent
of the igneous rocks.
Griinerite occurs most extensively in connection with magnetite and
quartz, or with quartz alone, thus constituting griinerite-magnetite-quartz-
schists, or griinerite-quartz-schists. The griinerite in such cases often
develops as a secondary product from the alteration of siderite, as explained
on page 245. Greenalite, probably having the formula FeSiO;.nH,O, occurs
extensively, as in the Biwabik formation of the Mesabi series of Minnesota.”
If such material were so deeply buried as to be altered under the conditions
of the zone of anamorphism, dehydration would take place and griinerite
would be formed. The mineral also occurs in the garnetiferous micaceous
schists; but in some of these rocks the griinerite itself develops from the
siderite, as in the case of the pure griinerite-quartz-schists and griinerite-
magnetite-quartz-schists.
Hornblende is the most abundant of the amphiboles, and has a very
widespread occurrence, being found as a principal constituent in various
igneous rocks, including plutonic and volcanic rocks, and among’ the latter
both in lavas and in tuffs. It also is a constituent of some of the sedi-
mentary rocks. It is a chief constituent of many of the metamorphosed
“Leith, C. K., The Mesabi iron-bearing district of Minnesota: Mon. U. 8. Geol. Survey, vol. 43,
1903, pp. 101-115.
ALTERATION OF MONOCLINIC AMPHIBOLES. 285
rocks, especially those of igneous origin. Not infrequently it is also an
abundant constituent in the metamorphosed sedimentary rocks. The schists
in which the hornblende is the chief constituent, whether of aqueous or
igneous origin, are generally known as amphibolites. In many other schists
and gneisses which are chloritic and micaceous it is an importaut constitu-
ent. As a metamorphic mineral hornblende has been noted as derived from
almandite, augite, melanite, and pyrope.
Glaucophane occurs abundantly in certain of the amphibole-schists,
especially those which are derived from the débris of basic rocks which were
originally rich in sodium. Naturally, such rocks have a somewhat limited
occurrence; but where they do occur abundantly, as in the Coast Ranges
of California, glaucophane is also very abundant—in fact, is the chief con-
stituent of some of the schists, so that they may properly be called
elaucophane-schists.
Riebeckite occurs in some eruptive rocks which are rich in sodium and
iron, and also in metamorphosed rocks of both sedimentary and igneous
origin. Like glaucophane, it may locally occur abundantly, but is not a
widespread mineral.
Arfvedsonite, a soda-amphibole, very naturally occurs in the soda-
bearing igneous rocks, especially in elzeolite-syenites and nepheline-syenites.
Alterations —The minerals of the monoclinic amphibole group, of such a
wide variety of composition and extensive occurrence, have naturally a
large number of alteration products. The more common of these are tale,
serpentine, bastite, chlorite, epidote, and biotite. These are frequently
accompanied by more or less magnetite, hematite, and limonite. In some
cases the amphiboles alter into the zeolites, pinite, and chabazite.
Taking up the individual minerals, tremolite is most frequently trans-
formed into tale (orthorhombic or monoclinic; sp. gr. 2.75), which may be
accompanied by calcite (Whombohedral; sp. gr. 2.7135). Actinolite com-
monly alters into tale or serpentine (monoclinic; sp. gr. 2.575), often
with the simultaneous formation of calcite, quartz (rhombohedral; sp. gr.
2.6535), and iron oxide. Cummingtonite commonly alters to bastite (ortho-
rhombic; sp. gr. 2.6). The standard text-books do not describe the altera-
tions of griinerite, although it is believed to alter to the iron oxides.
Hornblende under weathering conditions ordinarily changes to chlorite
(monoclinic; sp. gr. 2.71-2.725), which is often accompanied by epidote
286 A TREATISE ON METAMORPHISM.
(monoclinic; sp. gr. 3.38), calcite, quartz, iron oxides, and siderite (rhombo-
hedral; sp. gr. 3.855). Under deep-seated conditions biotite (monoclinic;
sp. gr. 2.90) is frequently a product of the alteration of hornblende, and with
the biotite epidote may simultaneously form. Rarely serpentine is also pro-
duced. While these are the usual alterations of hornblende, in some cases,
under conditions of high temperature, hornblende alters into augite (mono-
clinic; sp. gr. 3.4), with the simultaneous separation of magnetite (isometric;
sp. gr. 5.174). Such alteration of the hornblende has been noted, according
to Lacroix, both in lavas and in bombs." In the lava the change is attributed
to the action of the magma, being analogous to resorption; but in the bombs
it is attributed to heat alone. The alterations of glaucophane and riebeckite
are not described in the standard text-books. Arfvedsonite under certain
conditions changes into an acmite (monoclinic; sp. gr. 3.525) free from
calcium.’ With the acmite occur limonite (amorphous; sp. gr. 3.80), mag-
netite, and sometimes lepidomelane (monoclinic; sp. gr. 3.0-3.2). The
change of hornblende to augite and the change of arfvedsonite to acmite
are the reverse of the process of uralitization.
The alteration of tremolite to tale may be written as follows:
(1) CaMg,$i,0,,+H,0+C0O,=H,Mg,Si,0,.+CaC0,+k.
The increase in volume of the tale and calcite as compared with the
tremolite is 25.61 per cent. The decrease in volume of the tale alone as
compared with the tremolite is 0.83 per cent.
The alteration of actinolite to tale, supposing the excess of iron to sepa-
rate as hematite (rhombohedral; sp. gr. 5.225), and supposing that the
Mg: Fe::2:1, is as follows:
(2) 6CaMg,FeSi,0,,+4H,0+6C0, +30=4H,Mg,$i,0,, + 6CaCO,-+3Fe,0;+88i0,+k.
Supposing all the compounds to remain as solids the increase in volume
is 20.33 per cent. If magnetite be formed instead of hematite the increase
in volume is somewhat less; if limonite be formed, considerably more. If
it be supposed that all the compounds except the tale are dissolved the
decrease in volume is 36.51 per cent.
The alteration of actinolite to the variety of serpentine known as
bastite may be written as follows:
(3) Ca(MgFe),Si,0,, +2H,0+C0,=H,(MgFe),Si,0,+CaC0,+28i0, +k.
@ Lacroix, A., Minéralogie de la France, Paris, 1893-95, vol. 1, pp. 668-669.
» Brogger, W. C., Die Mineralien der Syenitpegmatitgange der Stidnorwegischen, Augit- und
Nephelin-Syenite: Zeitschr. fur Kryst. und Min., vol. 16, 1890, pp. 406-407.
ALTERATIONS OF HORNBLENDE. 287
Supposing that the Mg: Fe::2:1, the equation is—
(4) CaMg,FeSi,0,, +2H,0+CO,=H,Mg,FeSi,0, +CaCO,--28i0,+k.
The increase in volume, supposing all the compounds to separate as solids,
is 38.67 per cent. If only the bastite remains as a solid, the decrease in
volume is 18.06 per cent.
The alteration of cummingtonite into bastite is as follows:
(5) 3(MgFe)Si0,+2H,0=H,(Mg¥Fe),Si,0,+Si0,+k.
Supposing that the Me: Fe::3:1, the equation is—
(6) 3Mg,FeSi,0,,+8H,O=H,,Mg,Fe,Si,0,,+4S8i0,+k.
The increase in volume of the bastite and quartz as compared with the
cummingtonite is 36.76 per cent; of the bastite alone, 14.2 per cent.
In writing equations for the alteration of hornblende into chlorite and
accompanying minerals the soda-bearing part of the molecule will be
omitted, since the amount of soda present in ordinary hornblende is small.
Supposing that there are eight actinolite molecules in the hornblende to
two of the sesquioxide molecules, that the MgO: FeO::2:1, and the
Al,O,: Fe,O,::2:1, that the chlorite produced is on the border line between
prochlorite and clinochlore, and that the Al: Fe in the epidote as 2:1, the
reaction may be written as follows:
(7) 8CaMg,FeSi,O,).2Mg,Fe,Al,Fe,Si,0,,+21H,0+16C0,=
2( Hy )Mg,,Al,Si,0,;) +2HCa, Al, FeSijO,,+4CaCO,+12FeCO,+248i0,+3Fe,0,+k.
Provided all the compounds separate as solids the increase in volume is
25.39 per cent.
It is noticeable that the equation for the alteration of the hornblende
to chlorite as a chief resultant product demands that epidote, calcite, sid-
erite, quartz, and hematite be produced; and corresponding with this,
Lacroix noted all of these minerals as accompaniments of the chloritic
alteration with the exception of hematite.* Of course all or a larger part
of the iron may pass into the form of iron oxide—magnetite, hematite, or
limonite—in which case some oxygen would need to be added to the equa-
tion; the amount of CO, required will be less; and the oxide of iron will
replace the iron carbonate partly or wholly. As these modifications can
easily be made in the equation, it hardly seems necessary to write out
formulee for them.
«Lacroix, cit., pp. 667-668.
288 A TREATISE ON METAMORPHISM.
The change of hornblende to biotite requires the addition of potassium.
The potassium can be derived from some other mineral. It is perhaps
most frequently derived from orthoclase, although it is undoubtedly in
many cases derived from leucite. Supposing it is derived from orthoclase,
and therefore is in the form of potassium silicate, K,SiO,, and that the
actinolite molecule in the hornblende is to the sesquioxide molecule as 2: 8,
the change may be represented as follows:
(8) 2Ca(MgFe) ,Si,0,).3(MgFe),(AlFe),Si,0,,+-3K,Si0, +-Si0,-++3H,0-+2C0,
6HK (MgFe),(AlFe),Sis0,,+-2CaCO,+k.
In order to ascertain the volume relations it is necessary to make assump-
tions with reference to the proportions of the Mg and Fe, and of the Al and
Fe. Supposing the magnesia is to the iron protoxide as 2:1, and the
alumina is to the iron sesquioxide as 2:1, the equation is:—
(9) 2(CaMg,FeSi,0,,). Mg, Fe, Al,Fe,Si,Oa5+3KSi0;+Si0,+3H,0+2C0,=
H,K,Mg,Fe,AlgFe,Si,,072+2CaCO,4-k.
3
The increase in volume of the biotite and calcite as compared with the
hornblende and quartz is 4113 per cent. It has been noted, however,
that the alteration of hornblende to biotite is often accompanied by the
separation of epidote; and this is natural, since there is residual calcium in
the hornblende not needed by the biotite, which could pass into the
epidote. Supposing this residual calcium to pass into the epidote, the
reaction may be written as follows:
(10) 8Ca(MegFe),Si,O,».18(MgFe),(AlFe),Si,O,)+15K,Si0;+19Si0,+17H,0O=
30HK(MgFe),(AlFe),Sis0,.+4HCa,( AlFe) ,Si,0,,-+k.
In order to calculate the volume relations it may be supposed that the
MgO: FeO as 2:1, and the Al,O,: Fe.O, as 2:1, the equation being—
(11) 8CaMg,FeSi,O,).6Mg,Fe, Al, Fe,Si,Ox,+15K,Si0,+198i0,-+17H,0=
10H, K,Mg,Fe,Al, Fe, Sig Ogg +H ,CagAlpFe,Si,.05.+k.
The increase in volume of the biotite and epidote as contrasted with the
hornblende and quartz is 30.05 per cent. The increase in volume for
equations (9) and (11) would be much less if the K,SiO, were taken into
account.
Where serpentine also occurs as an alteration product accompanying
the chlorite, biotite, epidote, and other products, this mineral is doubtless
derived from the actinolite part of the molecule, and an equation may be
readily written which represents the simultaneous formation of the bastitic
ALTERATIONS OF HORNBLENDE. 289
form of serpentine by supposing that the number of actinolite molecules is
ereater than given in the above equations and that such excess of these
molecules passes into bastite, according to equation (8).
While the more common alterations of hornblende are to chlorite,
biotite, epidote, and accompanying minerals, as above explained, the change
of hornblende into augite, just the reverse of that of augite into hornblende
described on pages 274-278, does take place, and probably on a great scale
at sufficient depth. The equation for one case may therefore be written:
(12) Ca,Mg;Fe,Si,0,,. (MgFe),(AlFe),Si,0,,+2CaCO,=
2[Ca,MgFeSi,O,).(MgFe) (AlFe) SiO,]-+FeCO,+MgC0,+k.
Ifthe Mg: Fe::2:1, and the Al: Fe::2:1, the equation is—
(13) Ca,Mg,Fe,Si,Q.,.Mg,Fe, Al, Fe,Si,0,,+-2CaC0,=
2[Ca,MgFeSi,O,,. Mg, FeAl, Fe,8i,0,,]+FeCO,+MgC0,+k.
The decrease in volume of the augite as compared with the amphibole is
4.13 per cent.
It is not supposed that the above equations for the alteration of horn-
blende necessarily represent the actual facts of specifie cases. Doubtless in
most instances materials from minerals aside from those given enter into
the alterations, and the actual changes are more complex than represented.
However, the equations very clearly show why it is that the production of
chlorite from hornblende demands also the production of other minerals
which Lacroix says so generally accompany chlorite. Also, they show why
epidote so frequently accompanies biotite secondary to hornblende. The
equations may be considered as average cases, which approximate to the
alterations that actually occur in many instances. The volume relations
calculated from the equations also are probably averages, for the proportions
of the elements taken in the equations given are chosen from a considera-
tion of analyses of the various minerals. The equations at least make a
quantitative estimate of the relations of the original and secondary minerals,
and therefore will lead to closer observations as to the minerals which
resuit from the alteration of hornblende, and their relative proportions.
The change of arfvedsonite into acmite is so uncertain in its character
that no attempt is made to write out the equations. In order to satisfactorily
write equations for this alteration it is necessary to know the composi-
tion of the particular arfvedsonite which changes into the particular acmite,
and what other minerals aside from the acmite are produced in the change.
MON XLVu—04——_19
290 A TREATISE ON METAMORPHISM.
The alteration of tremolite to tale, equation (1), is that of hydration
and carbonation. The alteration of actinolite to talc, equation (2), is that
of hydration, carbonation, desilication, and oxidation. The alteration of
actinolite to bastite, equations (3) and (4), is that of hydration, carbonation,
and desilication. The alteration of cummingtonite to bastite, equations (5)
and (6), is that of hydration and desilication. All these changes take place
with the liberation of heat and with expansion of volume, provided the
compounds which form mainly separate as solids. Whether or not there is
an actual increase in the volume as a result of the changes depends, of
course, upon the amounts of the secondary material which is dissolved. It
is therefore clear that all of these changes are those which are typical of the
zone of katamorphism, and especially the belt of weathering. Moreover,
some of the changes, like that of actinolite to tale and the accompanying
compounds, illustrate all the processes normal to this position; 1. e., hydration,
carbonation, oxidation, and desilication. The fact that calcite is so
frequently found associated with the tales and serpentines secondary to
tremolite, cummingtonite, and actinolite, is rendered perfectly clear by the
equations; for there is always a residuum of calcium which evidently, under
the conditions of the upper physical-chemical zone, unites with the carbon
dioxide and produces calcium carbonate, which frequently separates as the
mineral calcite in large part, but which doubtless is frequently largely or
altogether carried away in solution.
The alteration of hornblende into chlorite and accompanying minerals
is one of liberation of heat and expansion of volume. It is an alteration
also of carbonation, and of oxidation in case some of the ferrous iron be
changed to sesquioxide. It is therefore to be expected in the upper
physical-chemical zone, and as a matter of fact it occurs there. The change
from hornblende to biotite is a much deeper seated alteration. It involves
hydration, silication, and possibly carbonation, and thus includes an
unusual combination of reactions. Corresponding with these facts the
change of hornblende to biotite is one which takes place under rather
deep-seated conditions, particularly in connection with profound mechan-
ical action. The physics of the interchanges between hornblende and
augite are elsewhere discussed (see pp. 279-280); but it may be said that
the change of the first to the second involves decrease of volume, and,
corresponding with this fact, is known to take place under very deep-
seated conditions of metamorphism.
OCCURRENCE AND ALTERATION OF IOLITE. 291
IOLITE (CORDIERITE).
Iolite (cordierite) -
H, (MgFe),A1,8i,403,.
Orthorhombie.
Sp. gr. 2.60-2.66.
Occurrence—Tolite occurs in a great variety of schists and gneisses. In
some cases it is so abundant as to make the rock a cordierite-gneiss. It is
associated with the very heavy metamorphic minerals, such as tourmaline,
andalusite, sillimanite, garnet, etc. Iolite occurs, likewise, in ejected frag-
ments of volcanoes and as a contact mineral in connection with dikes; also
rarely as an original mineral in igneous rocks.
Alterations — "he most common alteration is simple hydration. Further
changes may remove some of the ferrous iron or introduce alkalies, or
both, forming pinite (massive; sp. gr. 2.775). Simultaneously with this an
isotropic substance is said to be formed. Jolite sometimes passes into a
chlorite similar to tale.
By the hydration of iolite, according to Clarke, chlorophyllite (crystal-
lization not given; sp. gr. 2.77) is formed.* Supposing the Mg and Fe to
be in the same proportions both in the iolite and in the chlorovhyllite, the
reaction is simple:
(1) H,(MgFe),A1,Si,,0,,+3H,0O=H,(MgFe),Al,Sij)Oy) +k.
If it be supposed that the Me: Fe::3:1 in both compounds, the equation is—
(2) H,Mg,FeAl,Si,,03,4-3H,O=H,Mg,FeAl,Si,Oy+k.
The decrease in volume is 0.86 per cent.
The reaction bemg hydration, one would expect it to involve increase
of volume, but the chlorophyllite produced is enough heavier to compensate
for this. One would expect the reaction to take place in the zone of kata-
morphism, but observations on this point are not known to me.
The character of the product which forms simultaneously with pinite
being unknown, and the character of the chlorite which forms as a sec-
ondary product not being ascertained, it seems hardly worth while to
attempt to write equations for these alterations, for they would be largely
conjectural.
a4Clarke, F. W., The Constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, p. 83.
292 A TREATISE ON METAMORPHISM.
NEPHELITE GROUP.
NEPHELITE AND CANCRINITE,
The nephelite group includes—
Nephelite:
NaAlsiO,.
Hexagonal.
Sp. gr. 2.55-2.65.
Cancrinite :
H,Na,Ca (NaCO,)A]SigOso-
Hexagonal.
Sp. gr. 2.42-2.50.
NEPHELITE.
Occurrence—Nephelite is a sodium-aluminum silicate. Commonly the
sodium is in part replaced by potassium. Nephelite occurs in both ancient
and modern igneous rocks, both surface and deep seated. It is abund-
ant in the syenite-schists and syenite-gneisses of certain localities, but
is not known in the metamorphosed secondary rocks. This is doubtless
due to its ready alteration. Nephelite has been produced artificially at
220° C. by a reaction between kaolinite and an alkaline carbonate. As a
secondary product nephelite forms from leucite, but this alteration is not
an important source of the mineral. Nephelite is also probably derived
from sodalite.
Alterations— The most frequently observed alteration of nephelite is to
the zeolites, and especially to hydronephelite (hexagonal; sp. gr. 2.263),
natrolite (orthorhombic; sp. gr. 2.20-2.25), thomsonite (orthorhombic; sp.
er. 2.3-2.4), and analcite (isometric; sp. gr. 2.22-2.29). Simultaneously
with the formation of some of the zeolites diaspore (orthorhombic; sp. gr.
3.3-3.5), or gibbsite (monoclinic; sp. gr. 2.35), or kaolinite (monoclinic; sp.
er. 2.615), or some combination of these, is frequently formed.
The reaction for hydronephelite is—
(1) 6NaAlSiO,+7H,0+CO,=2(HNa,Al,Si,0,.3H,0) +Na,CO,+k.
The increase in volume is 23.49 per cent.
The alteration next in importance is to natrolite and gibbsite, or to
natrolite and diaspore. The reaction in the former case is:
(2) 6NaAlSiO,+7H,0+CO,=2Na,Al,H,Si,0,)+2A] (OH),+Na,CO,+k.
ALTERATIONS OF NEPHELITE. 293
Supposing the sodium carbonate to be carried off in solution, the increase
in volume would be 24.46 per cent. If two molecules less of water were
added, instead of two molecules of gibbsite, two molecules of diaspore
would be formed, according to the reaction:
(3) 6NaAlSi0,+5H,0+CO, =2Na, Al,H,Si,0,,+2A410(OH)-+Na,CO,+k.
In this case the increase in volume would be only 15 per cent.
In the production of thomsonite, calcium must replace the sodium. It
will be assumed that this calcium is derived from calcium carbonate. The
reaction will then be—
(4) 6NaAlSi0,4-7H,0+3CaCO,=Ca,Al,Si,0.4-7H,O+8Na,CO,+k.
Supposing the calcium carbonate to have been brought in solution and the
sodium carbonate carried away in solution, the increase in volume is 24.60
per cent.
The less common alteration of nephelite to the zeolite analcite, with
the simultaneous production of diaspore or gibbsite, is expressed by the
following reactions:
(5) 4NaAlSi0,+3H,0+C0,=Na,Al,Si,O,.2H,O-+2[A10(OH)]+Na,CO,+k
or
(6) 4NaAISiO,+5H,0-+CO,=Na, Al,Si,O,).2H,O+2A1(OH),+Na,CO,+k.
In the first case diaspore is simultaneously formed, and in the second case
gibbsite. Supposing the sodium carbonate to be carried away in solution
the increase in volume is 5.49 per cent if diaspore be formed, and 19.68
per cent if gibbsite be formed.
Alterations of nephelite to muscovite (monoclinic; sp. gr. 2.88), to hydro-
muscovite (pinite) (massive; sp. gr. 2.775), and to kaolinite Qnonoclinic; sp.
er. 2.6—2.63) have also been noticed. Where this alteration takes place
the nephelite is probably a potassium-bearing one. Assuming that the
amount of potassium is one-third of the sodium, the reaction may be written:
(7) 2KNa,Al,Si,0,,+4H,0-+3CO,=2KH, Al,Si,0,.-+H,Al,Si,0)+3Na,CO,+k.
The decrease in volume of the muscovite and kaolin as compared with the
nephelite is 16.50, provided the sodium carbonate is carried away in solu-
tion. The decrease is 13 per cent if the products are pinite and kaolinite.
The volume of the muscovite alone is 38.46 per cent less than that of the
nephelite.
294 A TREATISE ON METAMORPHISM.
Another alteration of nephelite of some importance is to sodalite
(isometric; sp. gr. 2.14-2.30)— d
(8) 8NaAlSiO,+NaCl=NaCl.3NaAlsiO,+k.
Supposing the NaCl to be added in solution, the increase in volume is
33.14 percent. If the sodium chloride be present as solid halite Gsometric;
sp. gr. 2.1-2.6), the increase in volume would be 15.64 per cent.
While the change is not recorded, it is believed to be highly probable
that nephelite during mass deformation under deep-seated conditions may
change into feldspar, probably albite (triclinic; sp. gr. 2.62—-2.65). This
reaction would require the addition of silica, as follows:
(9) NaAlSiO,+2Si0, =NaAlSi,O,+k.
Supposing the silica to have been present as quartz (rhombohedral; sp. gr.
2.653-2.654), the decrease in volume would be 0.41 per cent.
The formation of the zeolites, and simultaneously the minerals gibbsite
or diaspore, equations (1) to (6), are all alterations of hydration, carbona-
tion, and expansion of volume, except that of thomsonite, equation (4),
which does not involve carbonation. It is therefore to be expected that
these are reactions which take place in the zone of katamorphism, and such
is the fact. As a result of the alteration of the nephelites to the zeolites in
this zone, a part of the sodium separates and probably goes into solution as
sodium carbonate, and thus we have one of the sources of this compound
which so frequently occurs in underground waters, especially in volcanic
regions. ‘The formation of muscovite and kaolinite from “nephelite is a
reaction involving hydration and carbonation and decrease of volume, and
therefore is characteristic of the zone of katamorphism. The formation of
sodalite from nephelite is one which might take place in either physical-
chemical zone, only in the upper zone the sodium chloride would probably
be added in solution, while in the lower zone it would probably be derived
from solid halite.
CANCRINITE.
Occurrence,
Cancrinite is known only in the nepheline syenites.
Alterations —By Dana it is mentioned as altering to natrolite (orthorhombic;
sp. gr., 2.225). The reaction, supposing the excess of alumina passes into
gibbsite (monoclinic; sp. gr., 2.35), may be as follows:
H,Na,Ca(NaCO,),Al,Sis0,,+-6H,0=
3 (Na, Al,Si,0,9-2H,O) +2A1(OH),-+ CaCO,-+Na,CO,+k.
OCCURRENCE AND ALTERATIONS OF SODALITE. 295
The imerease in volume of the natrolite, gibbsite, and calcite (rhombo-
hedral; sp. gr., 2.71385) as compared with cancrinite is 8.64 per cent.
The reaction is that of hydration and breaking up of a complex com-
pound into several simpler compounds requiring greater volume, and is
therefore typical of the zone of katamorphism.
SODALITE GROUP.
SODALITE, HAUYNITE, AND NOSELITE.
The sodalite group includes—
Sodalite:
NaCl.3NaAlSiO,.
Isometric.
Sp. er., 2.14-2.30.
Haivynite:
Na,Ca(NaSO,. Al) Al,Si,0,5.
Isometric.
Sp. gr., 2:4-2.5.
Noselite:
Na,(NaSO,. Al) Al, Si,O,..
Isometric.
Sp. gr., 2.25-2.4.
SODALITE,
Occurrence.—Sodalite is sodium aluminum silicate with some chloride.
Sodalite occurs as an original constituent in the igneous rocks, both surface
and deep seated. It is not known in the secondary rocks or their metamor-
phosed equivalents. In fact, the occurrence of sodalite is almost identical
with that of nephelite, which mineral is one of its sources.
Alteration—The alteration products of sodalite are also identical with
those of nephelite, except that nephelite passes into sodalite, and the
reverse reaction is not recorded, although, as noted below, it is believed to
occur. The alterations of sodalite into minerals similar to those into
which nephelite alters is natural, as sodalite is made up of the nephelite
molecule with the addition of sodium chloride.
Sodalite alters to the same zeolites as does nephelite, viz, to hydro-
nephelite (hexagonal; sp. gr., 2.263), natrolite (orthorhombic; sp. gr., 2.2—
2.25), thomsonite (orthorhombic; sp. gr., 2.3-2.4), and analcite (isometric;
sp. gr., 2.22-2.29). Simultaneously with the formation of some of the
296 A TREATISE ON METAMORPHISM.
zeolites, diaspore (orthorhombic; sp. gr., 3.3-3.5) or gibbsite (monoclinic;
sp. gr., 2.3-2.4) is frequently formed.
In the production of hydronephelite the reaction is—
(1) 2(NaCl.3NaAlSiO,) +4H,0+C0,=2HNa,Al,Si,0,..3H,0+2NaCl+Na,CO,+k.
Supposing that the sodium chloride and sodium carbonate are dissolved,
the decrease in volume is 7.25 per cent.
In the alteration of sodalite to natrolite, gibbsite or diaspore is also
produced. The reaction, provided gibbsite be produced, is—
(2) 2(NaCl.3NaAlSiO,)-+7H,O-+CO,=2Na,Al,H,Sis0,.+2Al(OH),+2NaCl+Na,C0,+k.
If two molecules less of water were added, in place of the gibbsite two
molecules of diaspore would be produced, according to the reaction:
(3) 2(NaCl.3NaAlSiO,) +5H,0+CO, =2Na, Al,H,Si;0,.+2A10(OH) +2NaCl+Na,CO,+k.
Supposing the sodium chloride and sodium carbonate to be dissolved,
the decrease in volume in the first case would be 6.52 per cent, and in the
second case 13.62 per cent.
In the production of thomsonite the reaction is—
(4) 2(NaCl.3NaAlsiO,)-+7H,O-+3CaCO,=Ca,A],Si,0.,.7H,0+2NaCl+4-3Na,CO,+k.
Supposing the calcium carbonate to have been in solution and the sodium
chloride and sodium carbonate to be taken into solution, the decrease in
volume is 6.41 per cent.
The alteration of sodalite to analcite and to diaspore may be written
as follows:
(5) 4(NaCL3NaAlsiO,) +9H,0+3C0,=
3(Na,Al,Si,O,)-2H,O) +6A10(OH)+4NaCl+3(Na,CO,) +k.
Tf six additional molecules of water were added, as in the case of the
reaction written for natrolite, gibbsite instead of diaspore would be formed.
The reaction is—
(6) 4(NaCl.3NaAlSiO,)+15H,0+300,=
3(Na,Al,$i,O,).2H,O) +6Al(OH)s+4NaCl+3Na,C0,+k.
Supposing the sodium chloride and sodium carbonate to be taken into
solution, the decrease in volume is 20.77 per cent in the case of diaspore
and 10.11 per cent in the case of gibbsite.
OCCURRENCE OF HAUYNITE AND NOSELITE. AAS)
The reaction for the alteration of sodalite to muscovite (monoclinic;
sp. gr., 2.76-3) and kaolinite (monoclinic; sp. gr., 2.6-2.63), supposing
potassium to replace one-fourth of the sodium of the silicate, would be—
(7) 2(4NaCl.K,NayAlj) Si,,0,,) +12H,0-+9C0,=
6KH, Al,Si,0;.-+3H,A1,Si,0)-8NaCl-+9NajCO,-Lk.
Provided the sodium chloride and sodium carbonate are dissolved, the
decrease in volume is 37.07 per cent.
As in the case of nephelite, it is suspected that sodalite may pass into
albite or other feldspar. However, as this change is conjectural, no reaction
will be written.
The various reactions above given are analogous, both from a physical-
chemical point of view and from a geological point of view, with the corre-
sponding reactions in the case of nephelite. Hence it need only be said
that the changes written are those occurring in the zone of katamorphism,
in which rock fracture occurs and ground solutions are active. These
ground solutions by the changes become bearers of sodium chloride and
sodium carbonate.
The relations between the alterations of nephelite and sodalite illustrate
very well the law of mass action. In the laboratory, if nephelite be exposed
to the “slow action of fused sodium chloride with the addition of vaporized
NaCl” it is changed into sodalite.” On the contrary, however, in nature,
where water is abundant and the amount of sodium chloride is small, the
reverse reaction takes place, and sodium chloride is abstracted. Probably
at the same time the nephelite molecule is altered as above indicated.
Thus, while observation does not as yet record nephelite as an alteration
product of sodalite, it is believed to be highly probable that this mineral is
really formed as a stage in the process of alteration of sodalite.
HAUYNITE AND NOSELITE.
Occurrence—Haiiynite is sodium-calcium-aluminum silicate with some
sulphate. Noselite is sodium-aluminum silicate with some sulphate.
«Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. 8. Dana, Wiley & Sons, New
York, 6th ed., 1892, p. 430. See also Rosenbusch, Mikroskopische Physiographie, Stuttgart, 1885, p. 284.
298 A TREATISE ON METAMORPHISM.
Haiiynite and noselite are common in certain igneous rocks, especially
those which contain nephelite and leucite. Neither of these minerals is
known in the schists and gneisses derived from the sedimentary rocks.
Alterations — The minerals alter to zeolites, especially to natrolite (ortho-
rhombic; sp. gr. 2.20-2.25), stilbite (monoclinic; sp. gr. 2.094—2.205), and
chabazite (rhombohedral; sp. gr. 2.08-2.16). Simultaneously with certain
of these alterations calcite (rhombohedral; sp. gr. 2.713-2.714) also forms.
Noselite passes into natrolite according to the following reaction :
(1) 2Na,(NaS0,..A1)Al,8i,0,,+CO,-+7H,0=
2(H,Na,Al,Si,0,,)+-2A1(OH),+2Na,S0,+Na,CO,+k.
It appears that the change requires the formation of gibbsite (mono-
clinic; sp. gr. 2.3-2.4) or diaspore (orthorhombic; sp. gr. 3.3-3.5), although
these minerals are not recorded as forming contemporaneously with the
natrolite. Supposing the gibbsite to separate as a solid, and the sodium
sulphate and sodium carbonate to be taken into solution, the decrease in
volume is 16.44 per cent.
The parallel reaction for the passage of haiiynite into natrolite and
gibbsite is as follows:
(2) 2Na,Ca(NaS0,. Al) Al,8i,0,.+2CO0,+8H,0=
2(H,Na,Al,Si,0,)-+2A1(OH) 5+2CaCO,-+2NaHSO,+k.
Supposing the gibbsite and calcite to remain as solids with the natrolite, but
the sodium acid sulphate to pass into solution, the mecrease in volume is 4.99
per cent.
As stilbite is a calcium-bearing silicate, it may be assumed that this
forms from haiiynite rather than noselite. The reaction is as follows:
(3) 6Na,Ca(NaS0,. Al) Al,Si,0,.+36H,0-+6C0,=
Ca, A1,(Sis0,),-18H,O-+12A1 (OH )3+3CaCO,+6Na,SO,+3Na,CO,+-k.
It appears that the reaction for the formation of stilbite thus requires the
formation of calcite, and also of gibbsite or diaspore. The equation is
written for the former mineral, but could readily be changed to the latter.
Supposing the calcium carbonate and the gibbsite, as well as the stilbite, to
be solids, and the other compounds to be taken into solution, the increase
in volume is 0.460 per cent.
MINERALS OF GARNET GROUP. 299
The reaction for the formation of chabazite from haiiynite is—
(4) 4Na,Ca(NaSO,. Al) Al,Si,0,,+24H,0-+6C0,=
Ca;Al,(SiO,)3(SisO,)3-18H,O+4A1 (OH),+CaS0,+ Al, (SO,);+-6Na,CO,--k.
This reaction again requires the formation of gibbsite or diaspore. Sup-
posing the compound to be gibbsite, and it and the chabazite to remain as
solids, and the other compounds to be taken into solution, the decrease in
volume is 7.46 per cent.
The alterations of haiiynite and noselite to the zeolites, calcite, and
gibbsite or diaspore are all reactions of hydration and carbonation and
liberation of heat. If the readily soluble compounds are dissolved, as is
probable, the volume is decreased in most instances. The reactions are
therefore characteristic of the zone of katamorphism.
GARNET GROUP.
GROSSULARITE, PYROPE, ALMANDITE, SPESSARTITE, MELANITE, AND UVAROVITE.
The garnet group includes the following rock-making species:
Grossularite:
Ca; A1,Si,0,).
Isometric.
Sp. gr. 3.55-3.66.
Pyrope:
Mg;A1,Si,0,).
Isometric.
Sp. gr. 3.70-3.75.
Almandite:
Fe, A1,Si,O,.
Isometric.
Sp. gr. 3.9-4.2.
Spessartite:
Mn, Al,Si,O,9.
Isometric.
Sp. gr. 4.00-4.30.
Melanite:
Ca,Fe.Si,O,>.
Isometric.
Sp. gr. 3.80-3.90.
Uvarovite:
Ca,Cr,8i;0),.
Isometric.
Sp. gr. 3.41-3.52.
d00 A TREATISE ON METAMORPHISM.
Oceurrence—Some form of garnet is a very common mineral in a great
variety of the schists and gneisses, including those which are derived from
sediments and from all forms of igneous rocks, plutonic and volcanic, both
lavas and tuffs. Ordinarily the garnet is a subordinate constituent in these
rocks, although in some cases it becomes one of the chief constituents. The
mineral has its most widespread occurrence in the metamorphosed rocks
which have altered under the influence of mechanical action, or with the
assistance of igneous injections, or both. Not infrequently where garnet is
particularly abundant combined contact and mechanical action have assisted
in furnishing the conditions favorable to its formation. In many instances
the garnet develops after the mechanical action has ceased, showing that it
was not the movements themselves but the other favorable conditions result-
ing therefrom which produced the garnets. It appears, therefore, that the
conditions favorable for the extensive development of the mineral are heat,
moisture, and high pressure. The mineral garnet is the most important of
a group of heavy metamorphic minerals which form under the conditions
mentioned. Other minerals which form under similar conditions and are
frequently associated with garnet are wollastonite, cordierite, vesuvianite,
scapolite, chondrodite, staurolite, andalusite, sillimanite, eyanite, tourmaline,
zircon, etc. These minerals are all anhydrous, or nearly so, and mostly
of a high specific gravity, many of them having a high symmetry. All of
them are formed by the union of silica with bases, and are therefore
produced by processes of silication. In many instances this simultaneously
involves decarbonation, and this change, as already explained, p. 177,
absorbs heat and lessens the volume of the compounds. They are
therefore minerals which form normally in the zone of anamorphism.
Garnet thus produced can not in general be said to have been derived
from any single mineral. It is usually the result of the rearrangement of
material of two or more adjacent minerals. Dana notes” that when garnet
is fused, and the material recrystallizes, the resultant minerals are usually
pyroxene, melilite, monticellite, scapolite, anorthite, nephelite, ete.
This doubtless gives an indication as to some of the minerals which
are rearranged under the conditions above described for the development of
garnet, which are very different from those of dry fusion. Also it is
«Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. 8. Dana, Wiley & Sons, New
York, 6th ed., 1892, p. 447.
MINERALS OF GARNET GROUP. 301
certain that various hydrous minerals furnish material for the formation of
garnet, and also the limestones and dolomites. As already noted, garnet
has a great variation in composition, and in a given case one of the pure
species mentioned, or a combination of the molecules of two or more of
them, will be formed which can be derived from the elements available.
For instance, from an impure limestone, calcium-aluminum garnet, grossu-
larite, is likely to form. In the magnesian rocks, magnesium-aluminum
garnet, pyrope, is likely. to be produced. In the impure aluminous
carbonates of calcium, magnesium, and iron, some combination of two or
more of the species grossularite, pyrope, almandite, and melanite is likely
to be produced.
Garnet may be an original constituent of some of the igneous rocks.
If this be so, this source of garnet is comparatively insignificant, as it is
very rare indeed that garnet is found in an unaltered igneous rock. In
some of the little altered igneous rocks it is found in lithophyse, but the
garnets in this position are apparently the latest products of crystallization,
the conditions of their formation being analogous to those producing garnets
under the ordinary conditions of rock metamorphism.
Considering the garnets individually, the following statements can be
made as to their occurrence:
Grossularite is especially common in the marbles, where it is frequently
associated with vesuvianite, wollastonite, diopside, ete. It also occurs in
the calcareous schists and gneisses, especially in the calcareous siliceous
rocks, such as calcareous quartzites and calcareous novaculites. Grossularite
also is associated with common garnet in other schists and gneisses. It is
recorded as being derived from melilite and gehlenite.
Pyrope, the magnesium garnet, as would be expected, is especially
prevalent in peridotites and their derivatives, such as serpentine and tale,
since these rocks are rich in magnesium. It also occurs in some basalts.
Almandite, one of the most widespread of the pure garnets, occurs in
eranites, schists, gneisses, and granulites, and thus is present in both
feldspathic and feldspar-free schists. Almandite is also known in certain
andesites. It rarely has crystalline forms.
Spessartite occurs in large and small grains in contact rocks, in
porphyritic crystals of large size in quartzites, and is abundant in certain
whetstone-schists. With topaz, it is known in lithophysee in rhyolite.
302 A TREATISE ON METAMORPHISM.
Melanite is common in basic eruptive rocks rich in alkali. It occurs
especially with nephelite and leucite in phonolites, leucitophyres, nephe-
linites, and tephrites. In connection with contact metamorphism it occurs
with wollastonite and fassaite. It is also found in many serpentines.
Uvarovite is at home in the serpentines, particularly those which con-
tain chromite. It is also found in the marbles.
Common garnet, ordinarily a molecular mixture of two or more of the
species grossularite, pyrope, almandite, and melanite, is of course more
abundant than the pure species. It occurs in such rocks as amphibolites
and eclogites, in the metamorphosed diabases and gabbros, in the pyroxenic
rocks and their derivatives, and in the schists and gneisses both of igneous
and of sedimentary origin.
Alterations—'|he minerals into which garnets alter are very numerous,
chlorite (monoclinic, sp. gr. 2.71-2.725), tale (orthorhombic or monoclinic,
sp. gr. 2.75), and serpentine (monoclinic; sp. gr. 2.575), however, being the
more common products. Only the secondary products which occur on
an important scale in the rocks will be discussed, mere mineralogical
occurrences and pseudomorphs being ignored.
Alterations of grossularite are not described in the standard text-books;
but it is known that meionite (tetragonal; sp. gr. 2.72) and zoisite (saus-
surite) (massive; sp. gr. 3.-3.04) are sometimes secondary products of garnet,
and it is natural to suppose that these minerals are derived either from
erossularite or from the grossularite molecule of common garnet, since
erossularite contains the elements in about the right proportions to produce
meionite and zoisite.
Tale and serpentine are minerals which are secondary to garnet, and
from their chemical composition ought to be derived from the pyrope mole-
cule, either from the pure garnet or from the pyrope molecule in com-
bination with other garnet molecules. Pyrope is known to alter into
chlorite. As chlorite is regarded as a molecular mixture of serpentine and
amesite (crystallization not determined; sp. gr. 2.71), equations are written
for its alterations into amesite and into average chlorites. Pyrope further
alters into enstatite (orthorhombic; sp. gr. 3.2) and spinel (isometric; sp. gr.
3.8), these minerals frequently forming kelyphite rims about the garnet.
Almandite is recorded as altering into chlorite and into hypersthene
(orthorhombic; sp. gr. 8.45) and spinel, which minerals form kelyphite rims
ALTERATIONS OF MINERALS OF GARNET GROUP. 303
about the garnets. It seems probable that in such cases with the alman-
dite there is also present the pyrope molecule, and the reactions for the
formation of chlorite, spinel, and hypersthene, after almandite, as written
include the pyrope molecule. In the case of the Spurr mine chlorite,
secondary to garnet, the species has been determined to be aphrosiderite®
(massive; sp. gr. 2.90).
Alterations of spessartite, melanite, and uvarovite, as pure species, are
not described in the standard text-books
Common garnet most frequently alters into chlorite. Often also it
changes into epidote (monoclinic; sp. gr. 3.88) or into hornblende (mono-
clinic; sp. gr. 3.26). ‘The mixture of almandite and pyrope altering into
aphrosiderite, and into hypersthene and spinel, may be considered as alter-
ations of common garnet. Where epidote is produced it is probable that
the molecules from which it is derived are a mixture of grossularite and
melanite. Where hornblende is produced it is probable that the molecules
are a mixture of pyrope. almandite, and melanite. In the alterations of
the common garnets any of the iron oxides, magnetite (isometric; sp. gr.
5.174), hematite (chombohedral; sp. gr. 5.225), or limonite (amorphous;
sp. gr. 3.80), may be produced.
The change from grossularite to meionite may be written as follows:
(1) 3Ca,A1,Si,0,,+5CO,=Ca,Al,Si,0,,+5CaCO,+38i0,-+k.
The increase in volume of the meionite, caleite (rhombohedral; sp. gr.
2.7135), and quartz (rhombohedral; sp. gr. 2.6535) as compared with the
erossularite is 54.62 per cent.
The change of grossularite to zoisite may be written as follows:
(2) 3Ca,A1,Si,0,.+5CO,+ H,0=2HCa, Al,Si;0,;+5CaCO,+3S8i0,-+k.
The increase in volume of the zoisite, calcite, and quartz as compared
with the grossularite is 40.49 per cent.
The alteration of pyrope to tale may be written in two ways, depend-
ing upon whether the excess of magnesium over that required for the for-
«Pumpelly, Raphael, On pseudomorphs of chlorite after garnet: Am. Jour. Sci., 3d ser., vol. 10,
1875, pp. 14. Penfield, S. L., and Sperry, F. L., Pseudomorphs of garnet from Lake Superior and
Salida, Colo.: Am. Jour. Sci., 3d ser., vol. 32, 1886, pp. 307-311.
304 A TREATISE ON METAMORPHISM.
mation of tale is regarded as passing into magnesite (rhombohedral; sp.
er. 3.06) or into spinel. The first reaction is—
(3) 4Me,Al,Si,0,.+15H,0+3C0, =3H,Mg,Si,0,,+3MgC0,+8Al(OH),+k.
The increase in volume of the tale, magnesite (rhombohedral; sp. gr. 3.06),
and gibbsite (monoclinic; sp. gr. 2.35) as compared with the pyrope is 75.91
per cent. The second reaction is—
(4) 4Mg,A1,Si,0,.-+6H,O=3H,Mg,$i,0,,+3MgAl,0,+2Al(OH);+k.
The increase in volume of the tale, spinel, and gibbsite as compared with
the pyrope is 36.84 per cent.
The change of pyrope into serpentine is—
(5) Mg,A1,Si,0,.+-5H,0=H,Mg,Si,0,+2Al(OH),+Si0,+k.
The increase in volume of the serpentine, gibbsite, and quartz as compared
with the pyrope is 81.61 per cent.
If amesite (hexagonal plates; sp. gr. 2.71) is produced from pyrope
the equation is—
(6) Mg,Al,Si,0,.-+-2H,0+CO,=H,Mg, Al,Si0,+MgCO,+28i0,-+k.
The increase in volume of the amesite, magnesite, and quartz as compared
with the pyrope is 62.26 per cent.
In the alteration of pyrope to chlorite, supposing an intermediate
chlorite be taken, the reaction is—
(7) 3Mg,A1,Si,0,.+8H,0=H,,Mg,A1,Si;05,+-48i0.--k.
The increase in volume of the chlorite and quartz as compared with the
pyrope is 56.02 per cent. Reactions could be written which represent
other varieties of chlorite.
The change of pyrope to enstatite and spinel is—
(8) Mg,Al,Si,0,.=2Mg Si0,4-MgAl,0,4Si0,+k.
The increase in volume of the enstatite, spinel, and quartz as compared
with the pyrope is 13.51 per cent.
The alteration of almandite and pyrope to chlorite (aphrosiderite),
ALTERATIONS OF MINERALS OF GARNET GROUP. 305
supposing the Fe:Mg:: 2:1, about the proportion shown by analysis in the
case of the Lake Superior, chlorite at the Spurr mine,” is—
(9) 4Fe,A1,Si,0,).2Mg,A1,Si,0,,.+15H,O=3H,,Fe,Mg, Al,Si,O,+6Si0,-+k.
The increase in volume of the aphrosiderite and quartz as compared with
the garnet is 50.98 per cent.
The alteration of almandite and pyrope to hypersthene and spinel,
supposing the Mg: Fe:: 1:1 in the hypersthene, is as follows:
(10) Fe,A1,Si,0,).2Mg,A1,Si,O,.=3MgFeSi,0,+3MgAl,0,+38i0,-+k.
The increase in volume of the hypersthene, spinel, and quartz, as compared
with the garnet, is 12.66 per cent. If a hypersthene be produced which is
less rich in iron, the amount of pyrope molecule in the original garnet
must be increased.
The alteration of grossularite and melanite to epidote, supposing an
average epidote be produced, in which the Al: Fe:: 2:1 is probably
ge e} P ) } »)
(11) 2Ca,A1,Si,0,).CagFe,Si,0,.-+-5C0,-+H,0 =2HCa, Al, FeSi,0,3-+-5CaCO,-+3S8i0,-+k.
The increase in volume of the epidote, calcite (rhombohedral, sp. gr.
2.7135), and quartz, as compared with the garnet, is 40.88 per cent.
Similar equations can be written in which the pyrope molecule takes the
place of the grossularite molecule in large part. In this case magnesite,
instead of calcite, would be produced. Other reactions could be written
for the formation of epidote, in which the original molecule is a combination
of grossularite, pyrope, and melanite. The simplest case is as follows:
(12) Ga, A1,Si,0,>.Mg,A1,$i,0,,.Ca,Fe,8i,0,.+H,0+5C0,=
2HCa, Al, FeSigO,,+-2CaC0,+3MgC0,+3S8i0,+k.
In this case the increase in volume of the epidote, calcite, magnesite, and
quartz, as compared with the garnet, is 39.53 per cent.
The reaction for the passage of pyrope, almandite, and melanite into
hornblende may be written in many ways, depending upon the composition
of the particular hornblende produced. Taking the case of an average
hornblende, in which there are five of the actinolite molecules to two of the
a@Penfield, S. L., and Sperry, F. L., On pseudomorphs of garnet from Lake Superior and
Salida, Colo.: Am. Jour. Sci., 3d ser., vol. 32, 1886, pp. 307-311.
MON XLVII—04——20
306 A TREATISE ON METAMORPHISM.
aluminous molecules, in which the MgO: FeO:: 2:1, and Al,O,: Fe,O,:: 3: 1,
the reaction is as follows:
ry
(13) 3[2Mg,A1,SijQ,o.FesAl,Si,0,5.CagFe,Si,0,)] +4C0,=
5CaMg,FeSi,O,»:2[(Mg,Fe, ) (AlgFes) SigOg5]-+4CaCO,+48i0,+k.
The increase in volume of the hornblende, calcite, and quartz, as compared
with the garnet, is 24.55 per cent.
The alterations of the ferriferous garnets frequently produce iron
carbonate or iron oxides. No reactions are written to illustrate these
changes; nor would it be easy to express these alterations by reactions
without knowing what becomes of the remainder of the garnet material.
Of course, the alterations which are written above, instead of taking
place separately, may occur simultaneously. Thus the garnet may be a
complex one, which contains molecules of several of the simple garnets,
and there would be simultaneously produced a considerable number of
secondary minerals. Thus, chlorite and hornblende, chlorite and epidote,
or epidote and hornblende, might be simultaneously produced. For defi-
nite cases such as these, reactions might be written by combining the reac-
tions for the production of the individual minerals.
An examination of the equations as written shows that in almost all
cases, Simultaneously with the production of the minerals which are recorded
as secondary to garnet, quartz also appears, and in some cases calcium
carbonate also must separate, which may be deposited in the form of calcite.
Less frequently siderite and iron oxide form. It is well known that with
the minerals chlorite, epidote, hornblende, etc., secondary to garnet, quartz,
and calcite are often found, and that with serpentine, tale, spinel, hyper-
sthene, and enstatite, quartz is often found. However, the quartz and calcite
are usually not regarded as derived from the garnet and called minerals
secondary to them. But the equations clearly show that these minerals
should be regarded as secondary to garnet, just as certainly as epidote,
chlorite, ete. The almost universal presence of quartz with the minerals
mentioned, and the frequent presence of calcite, are thus completely
explained. The equations also seem to demand in the alteration to serpen-
tine and tale that gibbsite or diaspore shall be produced. However, some
of the alumina may unite with silica and water and form kaolin. The
equations suggest that a search be made for gibbsite, diaspore, and kaolin
ALTERATIONS OF MINERALS OF GARNET GROUP. 307
where the serpentines and tales are secondary to garnet. Of course, in
many cases the silica, calcium carbonate, and possibly the excess of
aluminum hydrate, may be dissolved and transported elsewhere, and thus
their absence would be no proof that the compounds were not really pro-
duced by the alteration of the garnet.
The alterations of the various kinds of garnet into different combina-
tions of the following minerals, serpentine, tale, chlorite, epidote, and zoisite,
magnesite, and gibbsite (equations 2, 3, 4, 5, 6, 7, 9, 11, 12), are all
alterations of hydration, and the majority of them of carbonation and
desilication. These reactions are notable in the amount of increase in
volume, ranging from 36 to 80 per cent. - This increase in volume is a
natural consequence of the high specific gravity of the garnet. The altera-
tions of grossularite to meionite, calcite, and quartz (equation 1), and of
pyrope, almandite, and melanite to hornblende, calcite, and quartz (equa-
tion 13), are alterations of carbonation and desilication. There can be no
better illustrations of reactions characteristic of the zone of katamorphism.
It will be seen (pp. 683-685) that the development of garnet is a process
of the zone of anamorphism where the pressure is great and the tempera-
ture probably high. Naturally the extensive destruction of garnet is a
process of the upper physical-chemical zone.
The alterations of pyrope to enstatite, spinel, and quartz (equation 8),
and of almandite and pyrope together to hypersthene and spinel (equation
10), are common reactions. They do not involve hydration. They do,
however, involve desilication. The increase in volume for these changes
is comparatively small, 12 or 13 per cent. One would expect that these
reactions would take place either in the lower part of the belt of cementa-
tion or possibly in the upper part of the zone of anamorphism.
308 A TREATISE ON METAMORPHISM.
CHRYSOLITE GROUP.
FORSTERITE, OLIVINE, AND FAYALITE.
The chrysolite group includes—
Forsterite:
Mg,SiO,.
Orthorhombic.
Sp. gr. 3.21-3.33.
Olivine:
(MgFe),SiO, where Mg: Fe::16:1, 12:1, to 2:1, in the last case the mineral being
known as hyalosiderite. (Sp. gr. 3.566.)
Orthorhombic.
Sp. gr. 3.2-3.6 according to Hintze, but ordinarily being, according to Dana, 3.27-3.37.
Fayalite:
Fe,Si0,. '
Orthorhombic.
Sp. gr. 4.1.
Occurrence —T'schermak considers olivine as an isomorphous mixture of
fayalite and forsterite. The occurrence of the three minerals is the same,
except that fayalite and forsterite are not nearly so widely known as the
intermediate common mineral, olivine. Olivine is an abundant constituent
in intermediate and basic igneous rocks, both plutonic and voleanié, in lavas
and tuffs alike. In rare cases in the volcanic rocks fayalite occurs, as, for
instance, in nodules in voleanic rocks and in lithophysze of the rhyolites of
the Yellowstone Park. Forsterite also very rarely occurs in connection
with voleanic rocks. Olivine is also an accessory constituent in the very
basic schists and gneisses, such as the amphibolites, pyroxenites, eclogites,
ete. Finally, it not infrequently occurs in marbles. In rocks of this class
forsterite also rarely occurs. It therefore appears that the chrysolite group
of minerals occurs most abundantly as original constituents, but are also
rather widely found as secondary developments in the metamorphosed rocks,
including both the carbonates and the basic schists.
Alterations—The alterations of fayalite and forsterite are exceptional;
theretore the chief alterations which are considered are those which pertain
to olivine.
The most common alteration of olivine is to serpentine (monoclinic;
sp. gr. 2.50-2.65). This is a change from an anhydrous orthosilicate to a
hydrous orthosilicate. Doubtless this explains why serpentine rather than
ALTERATIONS OF OLIVINE. 309
tale develops so generally from the olivines, because tale is a metasilicate,
Ordinarily accompanying the serpentine one or more of the following
minerals may be found: Tremolite (monoclinic; sp. gr. 3.0), actinolite
(monoclinic; sp. gr. 3.10), tale (orthorhombic or monoclinic; sp. gr. 2.75),
hydrotalcite (hexagonal; sp. gr. 2.04—2.09), magnesite (rhombohedral, sp. gr.
3.06), breunnerite (rhombohedral; sp. gr. 3-3.2), siderite (rhombohedral;
sp. gr. 3.83-3.88), quartz (rhombohedral; sp. gr. 2.6535), opal (amorphous;
sp. gr. 2.15), magnetite Gsometric; sp. gr. 5.174), chromite (isometric; sp. gr.
4.445), hematite (hombohedral; sp. gr. 5.225), and limonite (amorphous;
sp. gr. 8.80). One of the most frequent combinations of minerals with
serpentine is magnesite, quartz or opal, and magnetite. Frequently the
magnetite may partially or completely replace the hematite or limonite.
The formation of the serpentine is frequently accompanied by tremolite or
actinolite with iron oxide. It is much less frequently accompanied by tale.
In some instances the olivine has passed directly into magnesium carbonate
and hematite or limonite, but the former commonly being largely removed
in solution.
Other alterations of olivme are into anthophyllite (orthorhombic; sp.
gr. 3.15) into actinolite, hematite, and spinel (isometric; sp. gr. 3.8), but
these are by no means comparable in importance to the change to serpentine.
Beginning with the simplest alteration to serpentine, if an olivine be
taken in which the magnesium is to the iron as 3:1, and magnetite being
the only mineral which accompanies the serpentine, the reaction may be
written as follows:
(1) 3Me,FeSi,0,+6H,0--O=3H,Me,Si,0,+-Fe,0,+k.
The increase in volume of the serpentine and magnetite as compared with
the olivine is 29.96 per cent.
Supposing the magnesium is to the iron as 1:1 and the iron passes into
magnetite, the reaction is—
(2) 3MgFeSi0,+2H,0+0=H,Mg,Si,0,--Fe,0,+Si0,+k.
The increase in volume of the serpentine, magnetite, and quartz as compared
with the olivine is 15.19 per cent.
If it be supposed that a third of the magnesium passes into magnesite,
and that silica also separates, the reaction may be written as follows:
(3) 3Mg,FeSi,O,+-3C0, +4H,0+0=2H,Mg,Si,0, +Fe,0,+3MeC0,+28i0,-+k.
310 A TREATISE ON METAMORPHISM.
The increase in volume of the serpentine, magnetite, magnesite, and quartz
as compared with the olivine is 387.13 per cent. Supposing the Mg and Fe
are present in equal proportions, the equation stands—
(4) 3Mg,Fe,Si,0,+4H,0+2 0=2H,Me,Si,0.+2Fe,0,+28i0,-+k.
In this case, the olivine of which nearly corresponds to that of many rocks,
the increase in volume is 12.43 per cent.
It would be easy to write other equations for different proportions of
magnesium and iron in the olivine, but this seems unnecessary. Also it
would be easy to write reactions by which other forms of iron compounds
than magnetite are produced, such as siderite, hematite, and limonite. If
this be done, and the volume reaction calculated, it will be found that the
increase in volume is still greater than when magnetite forms.
Olivine is described by Becke as passing into anthophyllite (where
Mg: Fe::4:1, 3:1, ete., orthorhombic; sp. gr. 3.1-3.2). If the proportion
of the magnesium to the iron be taken as 3:1 in both the olivine and the
anthophyllite, the reaction may be written as follows:
5) Meg,FeSi,O,+2Si0,=Mg,FeSi,O,.— k.
S3 2 2 £3 40»
The decrease in volume of the anthophyllite as compared with the original
olivine and quartz is 1.48 per cent.
Various authors have also described the alteration of olivine into
actinolite. Supposing that the magnesium is to the iron as 3:1 in both the
olivine and the actinolite, and supposing the calcium to be derived from
carbonate and the silica from quartz, the reaction is as follows:
(6) 3Mg,FeSi,O,+4CaCO,+ 10Si0O, = Mg, Fe,Ca,8i,,0,,+4C0,—k.
The decrease in volume of the actinolite as compared with the olivine,
calcite, and quartz, is 13.34 per cent.
In some instances the alteration into actinolite is described as taking
place in connection with feldspar asa reaction rim. In this case the caleuim
may be supposed to be derived from anorthite, as calcium silicate. The
aluminum may be supposed to pass into common spinel and hereynite
(isometric; sp. gr. 3.93), which are well known to be alteration products of
olivine. The reaction may be:
(7) 4Mg,FeSi,O,+4CaAl,Si,0,=Mg,Fe,Ca,Si,,0y.+3MgAl,0,+ FeAl,0,—k.
SCAPOLITE GROUP. lel
The volume decrease of the actinolite and spinels as compared with the
olivine and feldspar is 7.18 per cent.
The reactions in the alterations of olivine into tremolite are parallel
with those for actinolite, with the exception that no iron is present, and the
mineral therefore probably forms from forsterite. The reaction may be
written:
(8) 3Mg,Si0,+2CaC0,+58i0, =2Me,CaSi,O,.+2C0,—k.
The decrease in volume of the tremolite as compared with the forsterite,
calcite, and silica is 12.29 per cent.
The alteration of olivine to serpentine and the accompanying minerals
is the common one. It takes place in the zone of katamorphism on a great
scale, both in the belt of weathering and in the belt of cementation. Corre-
sponding with the position in the upper physical-chemical zone, the reactions
occur with hydration, oxidation, expansion of volume, and liberation of heat.
The developments of anthophyllite, actinolite, and tremolite from
olivine and actinolite, and of spinel from olivine and feldspar, are all deep-
seated reactions of the zone of anamorphism. Corresponding to this position
the change to anthophyllite, equation (5), is a reaction of silication; the
changes to actinolite and to tremolite, equations (6) and (8), silication
and decarbonation; and the change of olivine and anorthite to actinolite
and spinel, equation (7), rearrangement of the silicates into denser silicates;
and all take place with diminution of volume and absorption of heat.
SCAPOLITE GROUP.
MEIONITE, WERNERITE, and MARIALITE.
The scapolite group includes:
Meionite :
Ca,Al,Si,O,5-
Tetragonal.
Sp. gr. 2.70-2.74.
Wernerite:
mNa,Al,Si,O,,Cl.nCa,Al,Si,O0.;.
Tetragonal.
Sp. gr. 2.66-2.73.
Marialite:
Na,A1,Si,O,,Cl.
Tetragonal.
Sp. gr. 2.566,
Sule A TREATISE ON METAMORPHISM.
As is well known, the scapolite group is analogous to the plagioclase
group, both consisting of sodium-aluminum-silicate molecules and calcium-
aluminum-silicate molecules in various proportions. Wernerite is a
combination of the marialite and meionite molecules in various ratios.
Generally the ratios vary between 2:1 to 1:3.
Occurrence— Dana summarizes the occurrence of the scapolites as follows:
(1) in volcanic rocks, as in ejected masses on Mte. Somma (meionite);
(2) in crystalline limestone, often as the direct result of ,contact meta-
morphism; (3) crystalline schists, augite-gneiss, ete; (4) as an alteration
product of a plagioclase feldspar, sometimes on an extensive scale, as with
amphibole.” “
Alterations —Dana states that the scapolites are readily alterable. The
more common products of alteration are kaolin (monoclinic; sp. gr. 2.6—2.63),
tale (orthorhombic or monoclinic; sp. gr. 2.7-2.8), muscovite (hydromusco-
vite, pinite) (monoclinic; sp. gr. 2.76-3.0), and epidote (the Al and Fe
varying from 6:1 to 3:2; monoclinic; sp. gr. 3.25-3.50). It is also recorded
that the scapolites alter into biotite (monoclinic; sp. gr. 2.7-3.1). Accom-
panying various of these alteration products quartz (rhombohedral; sp. gr
2.653-2.654) separates. Also, it is probable that in connection with some
of them, gibbsite (monoclinic; sp. gr 2.3-2.4) or diaspore (orthorhombic;
sp. gr. 8.3-3.5) forms, and very likely also calcite (rhombohedral; sp. gr.
2.713-2.714).
In writing out equations for the alterations to the above minerals, one
is handicapped by lack of knowledge as to whether the marialite or the
meionite, or a combination of the two, produces a given mineral. In this
state of affairs the particular molecule is chosen which is most analogous to
the compound produced. It seems probable that kaolin and tale together
are produced from marialite, according to the reaction :
(1) 2Na,A1,Si,0,,C1+9MeC0,+9H,0=
3H, Al,Si,0,-+3H,Mg,Si,0,,+3Na,C0,+2NaCl+6C0,+k.
The increase of volume of the kaolin and tale, as compared with the
marialite, is 7.69 per cent.
It may be that kaolin and calcite are also produced from meionite, as
follows:
(2) Ca,Al,$i,0,,+6H,0+400,=3H,Al,Si,0,+4CaC0,-+k.
«Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. 8. Dana, Wiley & Sons, New
York, 6th ed., 1892, p. 467.
ALTERATIONS OF MEIONITE AND MARIALITE. Sik
Supposing all of the CaCO, to remain as calcite, the increase of volume is
35.40 per cent.
The passage of the scapolites into muscovite may be-written as follows:
For marialite:
\
(8) 2Na,Al,Sij0,,C1+K,CO,+2H,0+200,=2KH,Al,Si,0,.+128i0,+2NaCl+3Na,CO,+k.
In this reaction, as in the case of the passage of the acid feldspars into
muscovite, a large amount of the silica separates. The decrease in volume
of the muscovite and quartz as compared with the marialite is 16.74 per
cent, but if the soluble sodium salts be also taken into account the volume
is increased.
For meionite the reaction may be—
(4) Ca,Al,Si,0.5+K,CO,+3C0, +2H,0=2KH,Al,Si,0,.+4CaCO,+k.
The increase in volume of the muscovite and calcite as compared with the
meionite is 29.42 per cent.
As the composition of epidote is very analogous to meionite, and as it
is a calcium-bearing compound, it is thought likely, where epidote is second-
ary to a scapolite, that it is derived from a meionite molecule. Therefore,
supposing that the epidote is one in which the aluminum is to the iron as
2:1, and supposing that the iron is derived from ferric oxide (Ie,O;), the
reaction may be written as follows:
(5) (Ca,Al,Si,0,;+-Fe,0,+4H,0=2HCa,Al,FeSi,0,;+2A1(OH),+k.
Supposing the hematite (hexagonal-rhombohedral; sp. gr. 5.225) to have
been present as a solid, and the gibbsite to remain as a solid, the decrease
in volume is 1.62 per cent. It is thought likely that iron for the reaction
is often derived from iron carbonate in solution, combined with simulta-
neous oxidation. In this case the reaction would be—
(6) Ca,Al,Si,0,;+2FeCO,+4H,0-+0=2HCa, Al, FeSi,0;,+2A1(OH),+2CO,+k.
The increase in volume of the epidote and gibbsite as compared with the
meionite is 7.55 per cent.
The passage of marialite into kaolinite and tale involves hydration,
expansion of volume, and liberation of heat. The change of meionite to
kaolinite involves hydration, carbonation, increase in volume, and libera-
tion of heat. The change of the scapolites to muscovite and accompany-
ing compounds are reactions of hydration, carbonation, increase of volume,
d14 A TREATISE ON METAMORPHISM.
and liberation of heat. The change of meionite to epidote is a reaction of
hydration and possibly of oxidation.
Corresponding with these facts the alterations to kaolin and tale are
known to take place in the zone of katamorphism, and the same is probably
true of the alterations to muscovite and epidote, although the latter reac-
tions may be more characteristic of the belt of cementation than of the belt
of weatherin g.
MELILITE.
Melilite:
(CaMgNa,),(AlFe),Si;O,,. (Groth.)
Tetragonal.
Sp. gr. 2.9-3.10.
Occurrence—Melilite has a widespread distribution in the leucite and
nephelite rocks. Aside from leucite and nephelite the most characteristic
associates are augite and perovskite. Some of the rocks in which melilite
occurs are leucitophyre, nepheline-syenite, and basalt.
aiterations—"The alterations of this mineral are not recorded, although
from its composition there can be no doubt that in the upper physical-
chemical zone it decomposes into less complicated silicates.
GEHLENITE.
Gehlenite:
CazA1,Si,04o.
Tetragonal.
Sp. gr. 2.9-3.07.
Occurrence.— The only occurrence of gehlenite recorded in rocks is as a
contact product in limestone.
According to Dana it alters to tale (orthorhombic or mono-
Alterations.
clinic; sp. gr. 2.75), to fassaite (monoclinic; sp. gr. 2.965-3.291), and to
erossularite (isometric; sp. gr. 3.605).
The change to grossularite involves the addition of SiO,, thus:
Ca, A1,8i,0,)+S8i0,=Ca,Al,$i,0,,+k.
6 re PRAY 2 3 1
The decrease in volume of the grossularite as compared with the gehlenite
is 4.42 per cent. If the SiO, be added as a solid, the decrease in volume
is 18.56. :
As gehlenite is so rare, and the manner of the alteration into tale and
fassaite is not clear, no attempt is made to write equations for the changes.
VESUVIANITE AND ZIRCON. aay 5)
VESUVIANITE.
Vesuvianite:
HR’,Al,Si,0., (Clarke).
Tetragonal.
Sp. gr. 3.35-3.45.
Clarke states that the R, in the typical mineral is replaced by calcium
and magnesium in the proportion of 5:1, giving HCa;MgAl,Si,O,,.
Occurrence.— Vesuvianite occurs in ancient ejections of Vesuvius. It is
most abundant in marbles. It is also found in various gneisses and schists,
especially those which are calcareous. It often forms in connection with
contact action. It is frequently associated with such other metamorphic
minerals as garnet, and also the micas and chlorites.
Alteration —F' rom the literature it is impracticable to ascertain which
particular garnet, mica, or chlorite forms from a certain vesuvianite, and
the accompanying minerals which must simultaneously form are unknown;
it therefore does not seem advisable to attempt to write equations represent-
ing the alterations, since they must be so largely speculative.
ZIRCON GROUP.
The only important rock-making mineral of the zircon group is zircon.
Zircon:
ZrSiO,.
Tetragonal.
Sp. gr. 4.66-4.70.
Occurrence—Zircon is especially common in marble. It also occurs both
in massive igneous rocks, such as syenite and granite, and in the schists and
oneisses.
Alterations—According to Clarke the only alteration described is that of
hydration, producing hydrous zircon (malacon) (tetragonal; sp. gr. 3.905),
the reaction being:
3ZrSi0, + H,C =H,Zr,Si,0,3.
The increase in volume in the change is 24.05 per cent.
316 A TREATISE ON METAMORPHISM.
ALUMINUM-SILICATE GROUP.
TOPAZ, ANDALUSITE, SILLIMANITE, AND CYANITE.
The aluminum-silicate group includes—
Topaz:
AI,F,SiO, or Al,(F,OH),Si0,.
Orthorhombic.
Sp. gr. 3.4-3.6.
Andalusite:
ALSi03.
Orthorhombie.
Sp. gr. 3.16-3.20.
Sillimanite:
ALSiO,.
Orthorhombic.
Sp. gr. 3.28-5.24.
Cyanite (disthene):
AL,Si0,.
Triclinic.
Sp. gr. 3.56-3.67.
Occurrence—T'opaz is a much less common mineral than andalusite,
sillimanite, and cyanite. Like them, it occurs in the schists and oneisses
of sedimentary origin, especially those in which other fluorine minerals are
found, such as tourmaline and beryl. Unlike andalusite, sillimanite, and
cyanite, it is sometimes found in cavities in fresh volcanic rocks, as, for
instance, rhyolite.
Andalusite is a frequent constituent of the metamorphosed sedimentary
rocks, especially of the argillaceous kinds. It often occurs in crystals,
including many other minerals in the partly metamorphosed sedimentary
rocks; but is also found in large, well-formed crystals in the schists.
Frequently in the metamorphosed sedimentary rocks its development has
been promoted by the contact effect of igneous rocks, especially the
granitic rocks. Its most characteristic associates are sillimanite and cyanite.
With the former it frequently has parallel intergrowths. Also it is fre-
quently associated with garnet and staurolite, and not infrequently with
tourmaline. Andalusite is rare, if indeed not altogether absent in the
metamorphosed igneous rocks.
Sillimanite is a common mineral in the strongly metamory hosed sedi-
metary rocks, such as schists and gneisses, where it frequently replaces
: ALUMINUM-SILICATE GROUP. d17
andalusite to a large extent. Like andalusite, its development may be
promoted by the presence of intrusive rocks, especially granites. In such
cases sillimanite frequently develops nearer the intrusive masses than does
the andalusite, the sillimanite therefore being the mineral which forms under
conditions of more advanced metamorphism. It is frequently associated
with garnet and with spinel and staurolite, sometimes with iolite (cordierite).
Sillimanite is derived from andalusite, biotite, corundum, cyanite, diaspore,
and gibbsite. :
The occurrence and associates of cyanite are similar to those of silli-
manite; but a very frequent additional associate is corundum, and where
formed by the assistance of igneous rocks the cyanite is likely, on the
average, to be closer to the intrusive than the sillimanite, although of
course they ordinarily overlap. As a metamorphic mineral, cyanite is
derived from andalusite, corundum, diaspore, and gibbsite.
Tremolite, actinolite, and diopside are frequent associates of andalusite,
sillimanite, and cyanite, especially of the last two.
The special homes of the aluminum-silicate eroup of minerals are
the metamorphosed argillaceous sedimentary rocks. As is well known,
kaolin is one of the chief constituents of such rocks, and doubtless it is
from this mineral in large part, under deep-seated conditions, that the
aluminum-silicate minerals are formed. If it be supposed that these heavy
minerals develop from kaolin, the process would be one of dehydration
and separation of silica. This silica may separate either as quartz or may
unite with other compounds, such as calcium and magnesium or other
bases, to form silicates. The breaking up of the kaolin may be repre-
sented by the following equation:
(1) H,A1$i,0,=Al,Si0,+2H,0+Si0,--k.
Supposing the mineral produced were andalusite, the volume of the anda-
lusite and quartz is 25.40 per cent less than that of the kaolin. If it be
supposed that calcium carbonate is present at the same time, and that the
freed silica unites with it, the equation may be written:
(2) H,A1,Si,0,+CaCO,=Al,Si0,+CaSi0,+2H,0+C0,+k.
In this case the volume of the andalusite and wollastonite is 32.32 per cent
less than that of the kaolin and calcite. If the heavier mineral silli-
manite or cyanite be produced the decrease in volume is even greater.
d18 A TREATISE ON METAMORPHISM.
While for the sake of simplicity wollastonite is supposed to form, the
more frequent association of the aluminum-silicate group is with tremolite,
actinolite, and diopside. For the first and last of these minerals the freed
silica unites with the calcium and magnesium together, and for the second
with the calcium, magnesium, and iron. The equations representing the
changes are analogous to (2), and the volume changes are in the same
direction.
alterations.— [he standard stated alterations of the aluminum-silicate
group are to tale (steatite) (massive; sp. gr. 2.75) and to muscovite
(damourite) (monoclinic; sp. gr. 2.88). It is recorded also that topaz and
andalusite alter to kaolin (monoclinic; sp. gr. 2.615). Occasionally also
andalusite may alter into the heavier mineral cyanite (triclinic; sp. gr.
3.56-3.67).
The alterations of the minerals into tale require an entire change of
base; that is, from aluminum silicates to magnesium silicates. The reac-
tions being those of the zone of katamorphism, the most probable source
of the magnesium is doubtless the carbonate, which may be derived from
the decomposition of magnesium rocks such as the pyroxenites, olivinites,
ete. The process, however, requires the separation of aluminum either as
corundum (rhombohedral; sp. gr. 4.025), corundophilite (monoclinic; sp.
gr. 2.90), diaspore (orthorhombic; sp. gr. 3.40), gibbsite (monoclinic; sp.
gr. 2.35), or some other form. Since the reaction takes place in the upper
physical-chemical zone, gibbsite will be regarded as the product formed.
The change of the aluminum-silicate minerals to muscovite requires the
addition of potassium. This is doubtless derived from the liberation of
potassium during the decomposition of the potash feldspars, and will there-
fore be regarded as added as a carbonate. The change from andalusite to
cyanite is simply a molecular one, the result bemg a mineral of great
specific gravity. It has already been seen that andalusite is a product of
less intense metamorphism, and that more intense metamorphism produces
sillimanite and cyanite. The change of andalusite to these heavier minerals
is therefore one which requires deep-seated conditions, and is characteristic
of the zone of katamorphism.
The equations representing the change of andalusite, sillimanite, and
cyanite to tale with gibbsite may be written as follows:
(1) 4Al,3i0,+3MgCO,+13H,0=H,Mg,Si,0,.+8Al(OH),+300,-+k.
ALUMINUM-SILICATE GROUP. alls)
The decrease in volume of the tale as compared with the andalusite is 32.37
per cent; as compared with the sillimanite, 31.20 per cent; as compared
with the cyanite, 23.12 per cent. But if the gibbsite be included as a solid
the increases in volume are 97.67 per cent, 101.09 per cent, and 124.71 per
cent, respectively.
The change of the three minerals to kaolin may be written as follows:
(2) 2A1,Si0,+5H,0=H,Al,Si,0,+2Al(OH),+k.
The change in volume of the kaolin as compared with the andalusite is a
decrease of 3.15 per cent; as compared with. the sillimanite, a decrease of
1.47 per cent; and as compared with the cyanite, an increase of 10.11 per
cent. But if the gibbsite be a solid, the increases in volume are 61.87
per cent, 64.67 per cent, and 84.02 per cent, respectively.
The alterations of the same minerals to muscovite (damourite) may be
written as follows:
(3) 6A1,Si0,+K,CO,+11H,0=2H,K Al,Si,0,,+6Al(OH),+CO,-+k.
The decrease in volume of the muscovite as compared with the andalusite
is 9.55 per cent; as compared with the sillimanite, 7.98 per cent; the
increase as compared with the cyanite is 2.83 per cent. But if the gibbsite
be regarded as a solid, the increases in volume are 55.47 per cent, 58.16
per cent, and 76.74 per cent, respectively.
The alterations of the aluminum-silicate minerals to tale, kaolin, or
muscovite, with the accompanying gibbsite, are all reactions of hydration.
They involve great increase of volume, from 55 to 125 per cent. To
produce the original heavy aluminum-silicate minerals in the zone of
anamorphism undoubtedly required great condensation of volume. When
the reactions are reversed in the zone of katamorphism, there is a corre-
spondingly great expansion of volume. The change of the heavy aluminum-
silicate minerals to the much lighter hydrous minerals gives one of the best
illustrative cases of typical reactions of the zone of katamorphism.
The change of andalusite to cyanite, as already explained, being a
molecular one, involves a volume relation inversely as the specific gravity,
and therefore by the change the volume is decreased 12.03 per cent. . The
change of andalusite to cyanite is a reaction of the zone of anamorphism.
320 A TREATISE ON METAMORPHISM.
EPIDOTE GROUP.
ZOISITE, EPIDOTE, PIEDMONTITE, AND ALLANITE.
The epidote group includes the following minerals:
Loisite :
Ja,( AIOH) Al, (SiO,)s.
Orthorhombice.
Sp. gr. 3.25-3.37,
Epidote :
Ca,(AIOH)(AlFe),(SiO,), where Al: Fe as 6:1 to 3:2.
Monoclinic.
Sp. gr. 3.25-3.50.
Piedmontite:
Ca, (AIOH) (MnA1),(SiO,) 5.
Monoclinic.
Sp. gr. 3.404.
Allanite (orthite):
Ca, (AIOH) (AlCeFe),(Si0,)5.
Monoclinic.
Sp. gr. 3.5-4.2.
Occurrence. —Zoisite is not known as an original pyrogenic constituent of
igneous rocks. It is found in the schists and gneisses, especially those
containing the amphiboles. Thus it is very common in the amphibolites,
glaucophane-schists, eclogites, ete. Zoisite frequently occurs with albite
as one of the constituents of the so-called saussurites, which develop as
eabbros. Zoisite also
an alteration of the basic feldspars, especially in g
occurs in the altered granites and other acid igneous rocks, although it is,
on the whole, less abundant than in the more basic rocks, but in some
localities it is plentiful even in the acid rocks. Zoisite is a very frequent
constituent in grits, graywackes, and other sediments of similar composition.
In such rocks the minerals were partly altered to zoisite during the forma-
tion of the sedimentary rocks, and this zoisite is to be classed with the
allogenic constituents of the mechanical sediments. Zoisite further develops
in the altered sedimentary rocks as a frequent and sometimes abundant
product of metamorphism. From the foregoing statement of occurrence it
is plain that zoisite develops in the zone of katamorphism, and especially
in the belt of cementation. As shown under the discussion of the other
minerals, it is seen that zoisite may be derived from the following minerals:
Corundum, diaspore, gibbsite, grossularite, and the plagioclases.
OCCURRENCE OF EPIDOTE. 321
Epidote, like zoisite, is rarely if ever a pyrogenic constituent in igneous
rocks. It is, however, a secondary constituent in all varieties of metamor-
phosed igneous rocks, whether plutonic or voleanic, whether lavas or tufts.
It is an allogenic constituent of the sedimentary rocks, and it extensively
develops in the sedimentary rocks as a secondary product. It is particularly
likely to form in rocks rich in calcium and iron, whether igneous or sedi-
mentary; and thus is especially abundant in those metamorphosed igneous
rocks which contain ferriferous varieties of pyroxene and amphibole, and
in metamorphic sedimentary rocks which contain a considerable amount
of calcium, as, for instance, caleareous schists and gneisses and marble.
In the metamorphosed rocks epidote occurs alike in those which have a
strongly developed schistose or gneissose structure and in those which have
merely undergone metasomatic change. It is found as one of the important
filling constituents of amygdaloids. It frequently develops at the contact
of two rocks, especially an igneous rock with other rocks, either igneous or
sedimentary. A list of different rock species which contain epidote includes
almost every variety of massive, schistose, semischistose, and little altered
igneous and sedimentary rocks. Epidote is, in fact, one of the most
important secondary constituents of all the silicates. It is an almost
constant accompaniment of the chlorites. Wherever the calcium-iron-
magnesium-silicate rocks break up, the magnesium passing into chlorite, a
part of the calcium and iron is likely to pass into epidote. The equations
for these alterations may be found under the minerals from which epidote
is derived. Where epidote becomes so abundant as to be a chief constituent
it may give a name to a rock; for instance, epidosite. From the foregoing
statements it is apparent that epidote develops abundantly under mass-
static and under mass-mechanical conditions. It forms with ease and on a
great scale in the belt of cementation of the zone of katamorphism, and it
is probable that it develops to some extent in the zone of anamorphism.
Whether it forms at all in the belt of weathering can not be stated.
Epidote is derived from the following minerals: Anorthoclase, augite,
biotite, garnet, hornblende, melanite, microcline, orthoclase, the plagio-
clases, and the scapolites.
Piedmontite, or manganese-epidote, is apt to replace epidote in those
schists and gneisses in which manganese happens to be an important con-
stituent. Thus it is rather common in certain manganese-bearing schists of
21
MON XLVII—O4
B22 A TREATISE ON METAMORPHISM.
Japan, in the manganese-chlorite-sericite-gneisses of eastern United States,
and at other localities. In some cases piedmontite occurs as nuclei sur-
rounded by ordinary epidote. Piedmontite is occasionally so abundant as
to be one of the chief constituents ot rocks.
Allanite occurs as an original subordinate constituent of a ereat number
of eruptive rocks, such as granite, rhyolite, diorite, tonalite, andesite, dacite,
and syenite. In short, it is a common accessory in the acid and intermediate
eruptives, but is not so characteristic of the basic eruptive rocks. It also
occurs in the metamorphic rocks, such as the schists and gneisses, especially
those which are calcareous, and it may oceur also in the marbles.
Alterations.— Definite alterations of zoisite, epidote, and piedmontite are
not recorded. But it is certain in the belt of weathering that zoisite and
epidote break up into calcite @hombohedral; sp. gr. 2.7135), quartz (rhom-
bohedral; sp. gr. 2.6535), iron oxides, kaolin (monoclinic; sp. gr. 2.615),
and perhaps gibbsite (monoclinic; sp. gr. 2.35); and piedmontite and
allanite alter into other minerals in a similar fashion.
It has already been seen that in the alteration of mica, pyroxene,
amphibole, and other minerals chlorite and zoisite are frequent simultaneous
produets which together use up all the material of the original minerals.
It has also been noted that the chlorite and epidote are abundantly
developed together in the sedimentary rocks. If the conditions so change
that these sedimentary rocks or other rocks in which epidote and zoisite
have formed in the zone of katamorphism become so deeply buried as to
pass into the zone of anamorphism, it is highly probable that the consti-
tuents which form epidote and zoisite and those which form chlorite reunite
to produce minerals that are on the average denser, such as mica, amphibole,
pyroxene, etc., out of which they are originally developed. This is believed
to be probable from the fact that in the most profoundly metamorphosed
sedimentary rocks, those which are true schists and gneisses, little or no
epidote and chlorite is contained, unless they have again been subjected to the
conditions of the upper physical-chemical zone. Such schists and gneisses,
having been derived from and traced into ordinary sediments, in all prob-
ability did originally contain both chlorite and epidote, which have doubt-
less united to reproduce heavy minerals similar to those from which epidote
and chlorite formed originally.
ALTERATIONS OF ZOISITE AND EPIDOTE. By:
It is not easy to approach accuracy in writing equations for the altera-
tions of the epidotes in the belt of weathering. In the equations given
below it is supposed that the calcium passes into carbonate, that the
Al (OH) goes into gibbsite, that the remainder of the aluminum goes into
kaolin, and that the excess of silica separates as quartz. In the epidote the
Al is supposed to be to the Fe as 2:1, and the iron is supposed to pass into
limonite (amorphous; sp. gr. 3.8). Upon these suppositions the alterations
stand—
For zoisite—
(1) Ca,(AlOH) Al,(SiO,),-+2CO,+3H,0=2CaCO,+ Al(OH), +-H,Al,Si,0,-+Si0,--k
and for epidote—
(2) Cag(AlOH),A1,Fe,Sig0,,+6CO,+83H,0-=
6CaCO,+3Al(OH) ,+2H,Al,8i,0.-+Fe,O;.1$H,0-+58i0,-+k.
The increase in volume of all the compounds formed as compared with
the zoisite is 66.22 per cent, and as compared with the epidote is 69.08 per
cent.
Of course there are many other ways in which the equations could be
written. All of the aluminum might pass into gibbsite or diaspore and
more quartz form. The iron may pass into hematite in whole or in part,
etc. While all this is true, it is believed that the above equations represent
correctly the fundamental fact that by hydration and carbonation zoisite
and epidote in the belt of weathering pass into simpler compounds.
Similar reactions could be written for the alterations of piedmontite
and allanite, but considering the comparative rarity of these compounds
this will not be done.
AXINITE.
Awinite:
HCa;,A1,BSi,O,;. (In some cases part of the Ca is replaced by Fe and Mn.)
Triclinic.
Sp. gr. 3.271-3.294.
Occurrence—A xinite occurs as a secondary constituent in basic eruptive
rocks, such as the diabases and gabbros. It is found to some extent in the
schists and gneisses, and particularly in those bearing abundant pyroxene
and amphibole. It also occurs in altered sedimentary rocks as a product
a24 A TREATISE ON METAMORPHISM.
formed in connection with the contact action of such rocks as granites,
granulites, diabases, and gabbros. In such positions the formation of
axinite is usually regarded as assisted by fumarole action.
. Alterations —Apparently the alterations which axinite undergoes in
rocks have not been worked out, as they are not recorded in the standard
text-books.
PREHNITE.
Prehnite:
H,Ca, Al, SiO».
Orthorhombic.
Sp. gr. 2.8-2.95.
Occurrence—Pyehnite is almost identical in its occurrence with the
zeolites (see pp. 831-333). It is therefore especially prevalent in the basic
and intermediate rocks, such as anorthosite, basalt, diabase, gabbro,
andesite, diorite, and syenite; also it occurs to some extent in granites and
gneisses, where it may be associated with epidote. In the igneous rocks it
is especially prevalent in the volcanics, since these are usually more
porous. Like the zeolites, it is a very frequent occupant of amygdaloidal
cavities, and also of cracks and crevices in the rocks. As already inti-
mated, the most constant associates of prehnite are the zeolites. Prehnite
occurs to some extent in the schists and gneisses, including those derived
from igneous rocks, such as the amphibolites, and from aqueous rocks,
such as the marbles. In some cases it is found in cavities in sedimentary
rocks which have been metamorphosed by granitic or granulitic intrusions.
As a secondary mineral prehnite is often derived from analcite, laumontite,
mesolite, natrolite, the plagioclases, and scolecite. Fused prehnite yields
wollastonite and ankerite.
alterations— The only alteration which I have been able to note in refer-
er. 2.71-2.725). This
f=)
change requires the substitution of magnesium for calcium. Supposing
ence to prehnite is to chlorite (monoclinic; sp.
the chlorite were amesite (hexagonal plates; sp. gr. 2.71), the change might
be expressed:
H,Ca, Al,Si,0,,--2Mg00,+H,0=H,Mg, Al,Si0,+28i0,+2CaCO,+k.
Ignoring the carbonates, the increase of volume of the chlorite and quartz
(rhombohedral; sp. gr. 2.6535) as compared with the prehnite is 3.27 per
cent. The change is one of hydration and desilication, and would be
expected to take place in the zone of katamorphism.
ROCK-MAKING MINERALS. 325
HUMITE GROUP.
CHONDRODITE, HUMITE, AND CLINOHUMITE,
The humite group includes:
Chondrodite:
[Mg(F.OH )],Mg,Si,0,.
Monoclinic.
Sp. gr. 3.1-3.2.
Humite: 2
[Mg(F.0OH)].Mg-Si,O,,.
Orthorhombic.
Sp. gr. 3.1-3.2.
Clinohumite:
[Mg(F.OH)],Mg,Si,O,,.
Monoclinic.
Sp. gr. 3.1-3.2.
In all the above the hydroxide (OH) replaces a part of the fluorine.
Occurrence—T'he humites occur in masses of magnesian limestones and
rocks bearing carbonates ejected by volcanoes. Chondrodite has a some-
what widespread occurrence in the marbles of eastern United States. In
such cases it is sometimes, at least, a contact mineral. Frequently it is
accompanied by spinel.
alterations. —The mest frequent alterations of the humites are to serpen-
tine (monoclinic; sp. gr. 2.50-2.65) and brucite (rhombohedral; sp. gr.
2.38-2.4). Inthe equations for the alterations the hydroxide will be ignored.
The reactions may be written as follows:
(1) (MgF),Mg,Si,0,+3H,0=H,Mg,Si,0)+Mg(OH),+MgF,-+k.
(2) 2[(MgF),Mg,Si,0,.] +9H,0=3H,Mg,Si,0,+3Mg (OH), +2MgF,-+k.
(3) (MgF),Mg,Si,O,,-6H,O=2H,Mg,Si,0,+2Mg(OH),+MgF,+k.
The increase in volume of the serpentine and brucite as compared with
chondrodite, from which it is derived, is 30.15 per cent; as compared with
humite, 35.53 per cent; as compared with clinohumite, 38.39 per cent.
The alterations of humite to serpentme and brucite involve hydration,
expansion of volume, and liberation of heat. They are therefore typical
reactions of the zone of katamorphism.
326 A TREATISE ON METAMORPHISM.
TOURMALINE.
Tourmaline is a complicated aluminum silicate, which may be of any
one of four different types or intermolecular growths of these types.
According to Clarke, the formule for these types are— “
Tourmaline:
NaHR,A1,B,Si,03; (R in some cases being lithium and hydrogen).
NaH,Mg,A1,B,Si,O3, (Mg frequently being replaced by Fe).
NaH,Mg,Al,B,Si,O31- :
NaH,Mg,Al;B,8i,Os).
Rhombohedral.
Sp. gr. 2.98-3.20.
Occurrence—Tourmaline rather frequently occurs in the marbles and in
the calcareous schists. It also has a rather widespread occurrence, although
generally not as an abundant mineral, in granites, gneisses, schists, and
eranulite. In these rocks it frequently occurs in such relations to dikes of
igneous rocks, especially of pegmatites, as to suggest that its development
is promoted by contact action. Because of the boron, tourmaline has
eenerally been regarded as evidence of fumarole action. Certain it is that
boron is not usually a constituent of the ordinary sediments, and to
account for this element, especially where the tourmaline is abundant, as it
oceasionally is in the schists, would seem to require its introduction from
an outside source, either by gaseous or by aqueous solutions.
Aiterations.—Mineral specimens of tourmaline are recorded as altering
into mica, chlorite (monoclinic; sp. gr. 2.71-2.725), and steatite (massive;
sp. gr. 2.794). However, in rocks tourmaline is one of the more permanent
minerals, and the chemical additions and subtractions which occur in the
alterations are so little known, and the exact nature of the tourmaline from
which individual minerals are derived is so uncertain, that it is not thought
advisable to attempt to write all the reactions representing these changes.
If one assumes a definite tourmaline and a definite mica as being produced
from it, it is easy to write a reaction. For instance, supposing that normal
biotite (monoclinic; sp. gr. 2.90) is derived from a tourmaline of the
composition of the last of the four formule given, that the additional
alkalies are added in the forms of carbonates, that the free boric acid
aClarke, F. W:, The constitution of the silicates: Bull. U. 8. Geol. Survey No. 125, 1895,
pp. 56-57. An alternative form has been proposed by Penfield.
OCCURRENCE AND ALTERATIONS OF STAUROUITE. oot
passes into borax, and that the excess of alumina separates as gibbsite
(monoclinic; sp. gr. 2.35), the reaction is—
4NaH,Mg,A1;B,SigOs,-+-4K,CO,-+-Na,CO,=
8HK Mg, Al,Si,0,,+3Na,B,0,+4Al(OH),+500,+k.
The decrease in volume of the biotite as compared with the tourmaline is
6.75 per cent; but if the gibbsite be included the increase of volume
is 3.96 per cent.
STAUROLITE.
Staurolite:
HFeA1,Si,0,;
Orthorhombic.
Sp. gr. 3.65-3.77.
Occurrence.
Staurolite is similar in its occurrence to garnet, but apparently
requires more intense metamorphic action for it to begin to form. Its most
widespread occurrence is in the schists and gneisses of sedimentary origin.
It also develops in profoundly metamorphosed rocks of eruptive origin, but
it is not known as an original constituent in any eruptive rock. Like
garnet, it may be abundantly developed in the zone of anamorphism in
rocks which are cut by intrusives. The conditions favorable to its formation
are therefore similar to those which produce garnet (see pp. 300-302) and
such minerals as tourmaline, andalusite, sillimanite, and cyanite, with
which it is associated. It is evidently a mineral which derives its materials
from various other minerals, the elements being recombined into the more
compact form of staurolite under deep-seated conditions.
Alterations—The only alterations recorded for staurolite are to tale
(orthorhombic or monoclinic; sp. gr. 2.75), to chlorite (monoclinic; sp. er.
2.71-2.725), and to muscovite (damourite) (monoclinic; sp. gr. 2.76—-3.0).
The first two minerals are essentially magnesian ones, although if the
chlorite be aphrosiderite (massive; sp. gr. 2.90) a considerable amount of
iron may be present. It is therefore clear that in the change to tale and
chlorite magnesium must be derived from some other compounds. As the
alterations are those which occur in the zone of katamorphism, it may be
supposed that the magnesium is in the form of carbonate, since magnesium
carbonate is an almost universal constituent of ground waters in the upper
physical-chemical zone. The alteration to muscovite requires an entire loss
of the iron and the addition of potassium. It is therefore clear that some
328 A TREATISE ON METAMORPHISM.
potassium mineral must also be concerned in this alteration. Staurolite
rocks usually contain orthoclase, or at least some potash feldspar. It may
be supposed that these potash feldspars break up into kaolin at the same
time, thus furnishing the potassium necessary for the change.
The change of staurolite to tale may be written as follows:
(1) 2HFeAl,Si,0,,+3MgC0,+15H,0-+0=H,Mg,$i,0,,+ Fe,0;+10Al(OH),+3C0,+k.
The decrease of the volume of the tale as compared with the staurolite is
44.02 per cent; but if the gibbsite (monoclinic; sp. gr. 2.35) be included
the inerease in volume of the two is 90.96 per cent.
In the change to chlorite the most aluminous one is chosen, amesite
(hexagonal plates; sp. gr. 2.71), since staurolite is so heavily aluminous
Moreover, it is supposed that the iron in the chlorite is to the magnesium as
1:3. On these suppositions the reaction may be written:
(2) 2H¥eA1,Si,0,,+10H,0-+6MgCO,=2H.Mg,FeAl,Si,0,.+-2A1l(OH),+6CO,+k.
The increase of volume of the chlorite and gibbsite as compared with the
staurolite is 103.58 per cent
The change of staurolite to muscovite may be written:
(3) 3HFeAl,Si,0,;+K,CO,+14H,0-+-O=2H,KAl,Si,0,,+Fe,0,+9Al (OH),+CO0,--k.
The decrease in volume of the muscovite as compared with the staurolite
is 24.96 per cent; but if the magnetite (isometric; sp. gr. 5.174) and gibbsite
be included the increase in volume would be 68.08 per cent, and if hema-
tite (rhombohedral; sp gr. 5.225) or limonite (amorphous; sp. gr. 3.8)
form, instead of magnetite, the increase would be still greater.
In the above equations it is entirely possible that the magnesium may
be added in some other form than that given, and the resultant compounds
be different. The same thing may be said of the potassium. It is uncer-
tain what becomes of the excess of aluminum. In the equations the
aluminum is regarded as passing into the gibbsite. However, the presence
of abundant gibbsite is not recorded among the alterations of staurolite,
although frequently corundum (rhombohedral; sp. gr. 4.025) occurs in
connection with it; but it is by no means certain that this corundum is one
of the results of the alteration of the staurolite; indeed, it is more probable
that the corundum formed from gibbsite at the same time the staurolite
developed.
ZEOLITE GROUP. 329
In short, the above reactions are probably as unsatisfactory as any
that have been written, because the text-books do not record what minerals
accompany the talc, chlorite, and muscovite as a result of the transforma-
tion of the staurolite. It is certain that in each case some other minerals
must be produced.
ZEOLITE GROUP.
The zeolites are a great group of hydrous silicates about which there
seems to be no consensus of opinion as to the species in the group, as to
the composition of the species, or as to their classification. Since Groth
and Clarke are among the latest authors to discuss this group, their
formulz are used, Groth’s being placed first and Clarke’s second when he
differs from Groth. Groth’s formule are put into an empirical form, and
the subordinate constituents which may replace the chief bases are omitted.
The differences between the formule given and Dana’s also are pointed
out. The important rock-making zeolites are as follows, ranged from basic
to acid:
THOMSONITE, HYDRONEPHELITE, NATROLITE, MESOLITE, SCOLECITE, ANALCITE, APOPHYLLITE, EPISTILBITE,
HEULANDITE, STILBITE, PHILLIPSITE, HARMOTOME, GISMONDITE, CHABAZITE, GMELINITE, AND
LAUMONTITE.
Thomsonite:
(CaNa,) Al,Si,0,.23H,O (Dana agrees with Groth.)
CazA1,Si,0.,.7H,0
Orthorhombice.
Sp. gr. 2.3-2.4.
Hydronephelite:
HNa,A1,Si,0,,.3H,O
HNa, Al,Si,0,,.3H,O
Hexagonal.
Sp. gr. 2.263.
Natrolite:
Na,Al,Si,0,).2H,O (Dana agrees with Groth. )
Orthorhombic.
Sp. gr. 2.20-2.25.
Mesolite:
H,Na,CaAl,Si,O.,.4H,0. (Dana makes all water that of hydration. )
H,Na,CaAl,Si,O,,.H,0.
Monoclinic and triclinic.
Sp. gr. 2.29.
Oo
A TREATISE ON METAMORPHISM.
Scolecite:
H,CaAl,Si,0,,.2H,O. (Dana agrees with Groth. )
Monoclinic.
Sp. gr. 2.16-2.4.
Analcite:
Na,Al,Si,Oy).2H,O. (Dana agrees with Groth. )
Isometric.
Sp. gr. 2.22-2.29.
Apophyllite:
H,KCa,Si,0.,.43H,O. (Dana agrees with Groth. )
H,,Ca,S8i,0,3.
Tetragonal.
Sp. gr. 2.3-2.4.
Epistilbite:
H,Ca,Al,Si,,033.7H,O. (Dana varies from both, but is nearer Groth. His formula is
H,CaAl,Si,O,5.3H,0. )
Cag A1,SisO4g. 16H, O.
Monoclinic.
Sp. gr. 2.25.
Heulandite:
H,CaAl,$i,0,.-3H,O. (Dana agrees with Groth. )
Ca, A1,Si,,0,;. 16,0.
Monoclinic.
Sp. gr. 2.18-2.22.
Stilbite (desmine) :
Ca,A1,Si,gO4¢-18H,O. (Dana’s formula is nearly the same as Clarke’s:
H,(Na,Ca) Al,Si,0,3.4H,0. )
Caz A1,Si;gO045-18H,0.
Monoclinic.
Sp. gr. 2.094-2.205.
Phillipsite:
(K,Na,Ca),A1,Si,o0s..12H,O.. (Dana’s formula is nearly like Clarke’s:
(K,Ca) A1,8i,Oj..43H,0. )
K,Ca,A1,Sij,055-14H,0.
Monoclinic.
Sp. gr. 2.2.
Harmotome:
Ba,A1,Sijg03)-12H,O. (Dana varies from each. His formula is (K,Ba)A1,Si;0,,.5H,0. )
Ba,A1,Si,,O49. 14,0.
Monoclinic.
Sp. gr. 2.44-2.50.
OCCURRENCE OF ZEOLITES. Spill
_ Gismondite:
Ca,A1,Si,O,,.12H,O. (Dana’s formula is one of the molecules given. )
Ca, A1,Si,O,,.12H,0.
Monoclinic.
Sp. gr. 2.265.
Chabazite:
CagzA],Si,903,.16H,O. (Dana agrees with Clarke as to the amount of Si. His formula
is (CaNa,) Al,Si,O,,.6H,0. )
Ca, A1,8i,.055.18H,0.
Rhombohedral.
Sp. gr. 2.08-2.16,
Gimelinite:
Na, Al,Si,o03..16H,O. (Dana varies from each. His formula is (Na,Ca) Al,Si,O,,.6H,0. )
Na,A1,$i,0.9.9H,0.
Rhombohedral.
Sp. gr. 2.04-2.17.
Laumontite:
H,CaAl,Si,0,,.2H,O. (Dana agrees with Groth.)
Ca,Al,Si,pOg5-12H,0.
Monoclinic.
Sp. gr. 2.25-2.36.
Occurrence—he zeolites are not known as original pyrogenic constitu-
ents of igneous rocks. As secondary minerals they are most abundantly
found in basic lavas and allied rocks, including both glassy and crystallized
kinds. The zeolites occur in these rocks, both in the ordinary feldspathic
varieties, such as basalt and trachyte, and in those containing leucite,
nephelite, sodalite, ete., and especially in those which are somewhat vesicu-
lar. ‘The zeolites also develop abundantly in the deep-seated equivalents
of the basalts in such rocks as the diabases, gabbros, ete. The zeolites are
less prevalent, although far from rare, in the diorite and syenite families,
including both the ordinary syenites and nepheline-syenites. Finally, the
zeolites are far from uncommon in the more acid rocks, such as those of
the granite family. In short, the zeolites may occur in almost any variety
of the igneous rocks, but, as already said, are most prevalent in the basic
group. In the sedimentary rocks the zeolites may be allogenic constituents
separate from other minerals, or an alteration product of partly decomposed
minerals. Also, after the deposition of material in the fragmental rocks
the zeolites may develop as alteration products. Hence the zeolites are
constituents of the altered sedimentary rocks, of the semimetamorphosed
Bae) A TREATISE ON METAMORPHISM.
sedimentary rocks, and of the schists and gneisses of sedimentary. origin,
which, after becoming schists and gueisses, have been subjected to agencies
of alteration in the zone of katamorphism. /
The zeolites develop from many minerals, but especially from the
plagioclase feldspars and from the leucites, sodalites, nephelites, ete. From
the plagioclases many of the zeolites are produced. The following may be
regarded as derived from anorthite: Thomsonite, gismondite, laumontite,
phillipsite, heulandite, epistilbite, stilbite, chabazite, and scolecite. The
following may be regarded as derived from albite: Analcite and natrolite.
Mesolite may be regarded as derived from albite and anorthite together.
Since the intermediate plagioclases contain both the anorthite and the albite
molecules, all of the above minerals may be derived from oligoclase,
andesine, labradorite, and bytownite, as may also mesolite. So far as
recorded the derivations of the zeolite minerals from the nephelites, leu-
cites, and sodalites are as follows: Thomsonite from nephelite and sodalite;
hydronephelite from nephelite and sodalite; natrolite from nephelite,
sodalite, haiiynite, and noselite; analcite from leucite, nephelite, and soda-
lite; stilbite from haiiynite and noselite; chabazite from hatiynite and
noselite. The zeolites are also derived from other minerals as follows:
Analcite from laumontite, natrolite from apatite and chabazite, etc.
It is hardly worth while to consider the occurrence of each of the
zeolites. It may be said, however, that the calcium-bearing zeolites
are most apt to form in the calcareous rocks, and the soda zeolites in
the rocks rich in soda. Thus stilbite, scolecite, and similar minerals are
likely to form in the calcareous rocks and limestones, while hydronephelite,
natrolite, and analcite, and similar minerals, are especially likely to form
from the rocks containing soda feldspars and nephelites, leucites, and
sodalites. The sodium-caleium zeolites, such as thomsonite, mesolite, and
phillipsite, may occur in the calcareous rocks, such as the limestones, in
the igneous soda rocks, such as the nephelite rocks, and in the basalts and
similar rocks.
In the rocks in which they occur the zeolites may be found (1) within
the mass of the rock as alteration products of the minerals; (2) in amyg-
dules, filling the vacuoles of the igneous rocks; and (3) in other openings
of all kinds, such as fractures, the pores of sedimeuts, ete.
ALTERATIONS OF ZEOLITES. D900
The development of the zeolites in nearly all cases requires hydration
and expansion of volume, as shown under the discussions of the particular
minerals from which they form. Their formation, therefore, tends to fill
up the crevices and cracks in rocks, even if no material be furnished from
an extraneous source. It may be that the zeolitization combined with other
alterations furnishes sufficient material to entirely fill the vacuoles of many
amyedaloids without material being furnished from an extraneous source.
(See pp. 631-634.) However, it is doubtless the case that much of the mate-
rial of the zeolites which is deposited in the belt of cementation is derived
by solution from the belt of weathering. As shown under the individual
minerals from which the zeolites develop, the conditions for the formation
of these minerals are those of the zone of katamorphism, both in the belt
of weathering and in that of cementation.
In the belt of cementation, in which hydration is perhaps the most
characteristic reaction and alterations can take place with expansion of
volume, the zeolites form on a great scale. Conforming with these state-
ments are the observations made by Daubrée® that zeolites can be formed
experimentally in the presence of abundant water at temperatures of about
50° C. Pointing in the same direction is the fact stated by Renard’ that
phillipsite has extensively formed at the bottom of the sea at temperatures
not far from 0° C.
While the zeolites develop chiefly in the belt of cementation, it is
certain that in very humid regions they form in the belt of weathering.
But it is also certain that to a great extent the zeolites are also destroyed in
the belt of weathering. This is especially the case in hot arid regions.
Alterations — The most comprehensive statement as to the alterations of
the zeolites is that given by Clarke.’ His statements are as follows: (1)
Natrolite alters into prehnite (orthorhombic; sp. gr. 2.875); (2) mesolite
alters into prehnite; (3) scolecite alters into prehnite; (4) analcite alters
into albite (triclinic; sp. gr. 2.635), and (5) orthoclase (monoclinic; sp. gr.
2.57) and prehnite; (6) apophyllite alters into pectolite (monoclinic;
sp. gr. 2.73); (7) heulandite alters into albite, and (8) into orthoclase;
«Daubrée, A., Etudes synthétiques de géologie expérimentale, Paris, 1879, pt. 1, pp. 199, 205-207.
» Murray, John, and Renard, A. F., Report of the scientific results of the voyage of H. M. S.
Challenger, 1873-1876; Deep-sea deposits, London, 1891, pp. 490-411.
¢ Clarke, F. W., The constitution of the silicates: Bull. U. 8. Geol. Survey No. 125, 1895, pp. 32-45.
Dd4 A TREATISE ON METAMORPHISM.
(9) stilbite alters into albite, and (10) into orthoclase; (11) chabazite
alters into natrolite (hexagonal-rhombohedral (Hintze), orthorhombic
(Dana); sp. gr. 2.225); (12) laumontite alters into albite, (13) into ortho-
clase and prehnite, and (14) into analcite Gsometric; sp. gr. 2.255).
These alterations may be expressed by the following equations, the
numbers of the equations corresponding to the numbers of the alterations.
In writing these equations, whether Groth’s or Clarke’s formula is used
depends on which is more nearly analogous to the formula of the mineral
produced.
(1) Na,Al,Si,0,9.2H,0+2CaCO,=H,Ca,Al,Sis0,.+Na,CO,+CO,+H,O—k.
(2) H,Na,CaAl,Si,0,,.4H,0+3CaCO,;—2H,Ca,Al,Si,0,.+Na,CO,+2C0,+3H,0—k.
(3) H,CaAl,Si,O,,.2H,O+CaC0,=H,Ca, Al,Si,0,.+2H,0+C0,—k.
(4) Na,Al,Si,0,).2H,0+2Si0,=2NaAl8i,0,+2H,0—k.
(5) 3(Na,Al,Si,0,).2H,O)+4CaCO,+K,CO,=
2K AlSi,O,+2H,Ca,Al,Si,0,,+3Na,CO,+2C0,+4H,0—k.
(6) Hy,Ca,Si,O,-+Na,CO,=2HNaCa,$i,0)--6H,0-+CO,—k.
(7) H,CaAl,Si,O,,.3H,0+Na,CO,=2NaAlSi,0,+CaCO, +5H,0—k.
(8) H,CaAl,Si,0,3.3H,0-+K,CO,=2K AlSi,O,+ CaCO,+5H,0—k.
(9) CazA1,Si,,0,s-18H,O-+8Na,CO,—6NaAlSi,O,+3CaCO,+18H,0—k.
(10) Cag A1,Si,,0,s-18H,0-+3K,CO,=6K AlSi,0,+3CaCO,+18H,0—k.
(11) Cag A1,Si,,0g,-18H,0-+-2A1 (OH),+4Na,CO,=
4H Na, Al,Si,0,, +3CaCO,+CO,+13H,0—k.
(12) Ca3A1,Si,,Og¢.12H,0+2Na,CO;+CO,=4NaAl8i,0,+ Al,0,+3CaCO,+12H,0—k.
(18) Ca, A1,8i,,055-12H,0+CaC0,+K,CO,;=
2K AlSi,0,+2H,Ca,Al,Sig0,,+-2C0,-+-10H,0—k.
(14) Ca,A1,Si,,04,-12H,0-+3Na,CO,=3(Na,Al,Si,0,,.2H,O) +3CaCO, +-6H,O0—k.
The decreases in volumes are as follows: Prehnite as compared with
the natrolite, equation (1), 16.12 per cent; prelhnite as compared with meso-
lite, equation (2), 15.05 per cent; prehnite as compared with scolecite,
equation (3), 16.66 per cent; albite as compared with the analcite and
quartz, equation (4), 17.25 per cent; orthoclase and prehnite as compared
with analcite, equation (5), 14.09 per cent; pectolite as compared with
apophyllite, equation (6), 19.48 per cent; albite as compared with heulan-
dite, equation (7), 25.03 per cent; orthoclase as compared with heulandite,
equation (8), 18.44 per cent; albite as compared with stilbite, equation (9),
31.67 per cent; orthoclase as compared with stilbite, equation (10), 25.66
per cent; natrolite as compared with chabazite, equation (11), 4.58 per
ALTERATIONS OF ZEOLITES. 335
cent; albite as compared with laumontite, equation (12), 34.92 per cent;
orthoclase and prehnite as compared with laumontite, equation (13), 17.75
per cent; and analcite as compared with laumontite, equation (14), 4.30 per
cent.
In calculating the volume relations the carbonates, and in equation
(11) the aluminum hydrate, are ignored. If these compounds were taken
into account the decreases in volume would in some cases be somewhat
more, in others somewhat less. To ignore these side compounds seems the
best course, since the added and subtracted salts may be in other forms
than those given.
For the most part the alterations, so far as the bases are concerned,
are remarkably simple, involving only the interchange between the alkalies,
sodium and potassium, or between sodium and the alkaline earth calcium,
or the addition of the bases sodium or calcium. They are all reactions of
dehydration, partial or complete. Many of them are reactions of decar-
bonation and one, equation (4), is a reaction of silication. Presumably
they all take place with the absorption of heat. While nothing can be
ascertained as to the actual conditions under which the changes take place,
one would expect them to occur in the zone of anamorphism, for a number
of the reactions reverse those of the zone of .katamorphism. In the latter
zone the alteration of the feldspars into the zeolites is well known. The
reverse changes, those of analcite, heulandite, stilbite, and laumontite into
albite and orthoclase, for which equations are written, would be hardly
likely to take place in the same zone. At any rate the alterations of the
feldspars into the zeolites and the zeolites into the feldspars present a very
interesting case of reversible reactions discussed subsequently. (See
pp. 366-369.)
It is further certain that the zeolites as extensively formed in the belt
of cementation, in the belt of weathermg break up, by carbonation and
hydration, into the simpler compounds, such as the carbonates of the
alkalies and the alkaline earths, diaspore or gibbsite, kaolin, and quartz.
Hypothetical reactions could readily be written for these changes similar
to those worked out for zoisite and epidote (pp. 322-323); but since so
little is known as to the definite minerals which are formed from each
zeolite, this is hardly worth while at the present stage of knowledge.
Oo
(Su)
oP)
A TREATISE ON METAMORPHISM.
MICA GROUP.
MUSCOVITE, PARAGONITE, BLOTITE, AND PHLOGOPITE.
The mica group includes the following rock-making species:
Muscovite:
(H,K) AlSiO,. (Normal muscovite KH, A1,8i,0,». )
Monoclinic.
Sp. gr. 2.76-3.0.
Paragonite:
H,NaA1l,Si,0,..
Monoclinic.
Sp. gr. 2.78-2.90.
Biotite:
(H,K),(MgFe),A1,Si,0,.. (Dana.). (Proportion of Mg:Fe varies widely. Normal
biotite: KHMg,Al,Si,0,,. (Clarke. ) )
Monoclinic. :
Sp. gr. 2.7-3.1.
Phlogopite:
KH, Mg, AISi,Oyp.
Monoclinic.
Sp. gr. 2.78-2.85.
MUSCOVITE.
Museovite, as already noted, is hydrogen-potassium-aluminum silicate.
Occurrence —Muscovite is an abundant constituent in the plutonic rocks,
but is rather rare as a constituent in the voleanic rocks. It is one of the
most abundant constituents of the metamorphosed rocks, being a chief
mineral in many metamorphosed sedimentary and many metamorphosed
igneous rocks. As a secondary constituent, it is derived from many other
minerals. The more important of these are feldspar, including both ortho-
clase and plagioclase, nephelite, sodalite, leucite, the scapolites, spodumene,
topaz, andalusite, and cyanite. It is also recorded as a pseudomorph after
tourmaline, garnet, beryl, and cordierite. There is little doubt also that
muscovite in the metamorphosed rocks is largely formed from the materials
of the zeolites. Some of the minerals, such as nephelite, sodalite, and
leucite, from which the muscovite is derived, occur only in the igneous rocks.
Others of them, such as the zeolites, occur only in rocks of altered or sec-
ondary nature. Others of the minerals from which muscovite is derived,
such as topaz, cyanite, and andalusite, are chiefly metamorphic constituents.
ALTERATIONS OF MUSCOVITE. 33
Still others from which muscovite is derived, such as the feldspars, may
be original constituents of the igneous rocks, or they may be original or
secondary constituents of the sedimentary rocks. It is therefore clear that
muscovite has an unusual variety of sources, and consequently it may be
expected in almost any variety of rock except the volcanics. It is, how-
ever, a more characteristic constituent of the acidic and intermediate rocks
than of the basic rocks.
In summary, muscovite is derived from anorthoclase, diaspore, gibbsite,
leucite, microcline, nephelite, orthoclase, plagioclase and orthoclase, scapo-
lites, sodalite, and spodumene. The muscovite damourite is derived from
andalusite, corundum, cyanite, sillimanite, staurolite, and topaz.
Alterations—'["he minerals to which muscovite alters are not nearly so
abundant as those from which it is derived. One of the most frequent
alterations is that of hydration, a part of the potassium being replaced by
hydrogen; or at the same time it may take up other bases and thus the
mineral may pass into vermiculite, a somewhat indefinite compound to
which no formula can be assigned. Muscovite also alters into serpentine
(monoclinic; sp. gr. 2.50-2.65) and into the steatitic form of tale (massive;
sp. gr. 2.7-2.8). Probably simultaneously with the formation of these
minerals gibbsite (monoclinic; sp. gr. 2.3-2.4) or diaspore (orthorhombic;
sp. gr. 3.3-3.5) forms, although the contemporaneous formation of these
minerals is not mentioned. Muscovite also may alter into the soda-mica
paragonite (monoclinic; sp. gr. 2.78-2.90).
The reactions by which muscovite passes into serpentine and tale are
very uncertain. If the magnesium were supposed to be derived from a
carbonate and all of the silica of the muscovite went into the resultant
compounds, the reactions may be written as follows:
For serpentine:
(1) 2KH,Al,Si,0,.+9MgC0,+13H,0=3H,Mg,Si,0,+6Al (OH),;-+K,CO,+8C0,--k.
For tale:
(2) 4KH,Al,Si,0,,+9MgCO,+17H,0=3H,Mg,Si,0,,+12Al(OH),+2K,CO,+7C0,--k.
The increase in volume of the serpentine as compared with the muscovite is
16.56 per cent, and the decrease of the tale 25.23 per cent. But if the
magnesium carbonate be contributed by solutions, and the gibbsite remains
as a solid with the serpentine and talc, the increase in volume of the ser-
22,
MON XLVII—(04
338 A TREATISE ON METAMORPHISM.
pentine and gibbsite as compared with the muscovite is 88.44 per cent, and
of the tale and gibbsite 46.69 per cent.
The change of muscovite to paragonite merely requires the substitution
of sodium for potassium, and may be written as follows:
(3) 2H,KAI,Si,0,,--Na,CO,=2H,NaAl,Si,0,,+K,CO,+k.
The decrease in volume is 2.67 per cent.
Muscovite under deep-seated conditions is a mineral which is practi-
cally permanent. In fact, under these conditions, as already indicated, it is
produced by the alteration of other minerals. The above alterations of
muscovite, resulting in the formation of vermiculite, serpentine, and tale,
with gibbsite all occur in the zone of katamorphism, and especially in the
belt of weathering. Even under the conditions of the surface belt the proc-
esses of change are exceedingly slow. Corresponding with this position,
the changes take place with increase of volume and liberation of heat.
PARAGONITE,
Paragonite is hydrous sodium-aluminum silicate.
Occurrence—Paragonite is not certainly known as an original pyrogenic
constituent in igneous rocks. It is found especially in the metamorphosed
igneous rocks and in the semimetamorphosed and completely metamor-
phosed sedimentary rocks. In many so-called sericite rocks it is probable
that a portion of the micaceous mineral is paragonite rather than muscovite.
Paragonite is especially likely to occur in the metamorphic rocks, instead
of muscovite, where the original rocks, either igneous or sedimentary, bear
a considerable amount of sodium. Very frequently associated with para-
gonite are the heavy metamorphic minerals, such as cyanite, staurolite,
garnet, tourmaline, ete. In certain places muscovite has been noted as pass-
ing to paragonite, and thus the potassium mica is a source for the soda mica.
In summary, paragonite as a metamorphic mineral is derived from
anorthoclase, muscovite, and plagioclases.
Alterations. —Alterations of paragonite are not recorded in the standard
text-books. However, there can be little doubt that this mineral undergoes
a set of alterations in the zone of katamorphism, and one would expect
that these alterations would be analogous to those which take place with
muscovite.
MICA GROUP.
(Se)
oo
iS)
BIOTITE.
Biotite is hydrogen-potassium-magnesium-aluminum silicate, a part of
the magnesium frequently being replaced by iron.
Occurrence. —Biotite is an original chief constituent of many of the igneous
rocks, both plutonic and volcanic, and ranging from those which are ecid
to those which are basic. It is a very abundant secondary constituent
in the slates, schists, and gneisses, developing on a great scale in the
metamorphosed rocks, both igneous and sedimentary. As a secondary
constituent it seems usually not to be derived from a single mineral, as is
frequently the case with muscovite, but is produced from material furnished
by two or more minerals. For instance, it is frequently a reaction product
between magnetite and other minerals, the magnetite furnishing the iron
for the biotite, the other constituents being derived from such minerals as
the pyroxenes, amphiboles, and feldspars. A frequent case is the formation
of biotite from the pyroxenes, feldspars, and magnetite. The feldspars and
feldspathoids frequently furnish the potassia, parts of the alumina, and silica.
The pyroxenes and amphiboles frequently furnish a part of the magnesia,
alumina, and silica. Dolomite is often a source of the magnesia. The
oxides and carbonate of iron are the most frequent sources of this element.
In summary, as a metamorphic mineral, biotite is derived from anortho-
clase, augite, hornblende, microcline, orthoclase, and the scapolites.
Alterations—Perhaps the most frequent alterations of biotite are to
hydrobiotite (probably monoclinic; sp. gr. 2.90, average of biotite) and to
chlorite (monoclinic; sp. gr. 2.80). It also alters into epidote (monoclinic;
sp. gr. 3.25-3.50); rarely it alters into hypersthene (orthorhombic; sp. gr.
3.40-3.50) and sillimanite (orthorhombic; sp. gr. 3.23—-3.24); and in some
cases it apparently alters into serpentine (monoclinic; sp. gr. 2.575). — Its
alteration into the above minerals may be accompanied by the separation
of quartz (rhombohedral; sp. gr. 2.6535), and if the biotite be ferriferous,
by the formation of magnetite (isometric; sp. gr. 5.174), or other iron oxide.
The alteration of biotite into serpentine probably requires the simulta-
neous production of kaolin (monoclinic; sp. gr. 2.615) and gibbsite (mono-
clinic; sp. gr. 2.35). Supposing that all the magnesium of normal biotite
340 A TREATISE ON METAMORPHISM.
goes into the serpentine, and that all the silica not required for the pro-
duction of this mineral passes into the kaolin, the reaction is as follows:
(1) 6KHMg,Al,Si,0,,+-18H,0-+3C0,=
4H,Mg,$i,0,+5H,Al,Si,0)+2Al(OH),+3K,CO,+k.
Supposing all the serpentine, kaolin, and gibbsite to remain as solids, and
the potassium carbonate to go into solution, the increase in volume is 14.26
per cent.
The change of biotite into hydrobiotite may be written:
(2) 2HKMg,Al,Si,0,.+7H,O-+CO, =2(H,Mg,Al,Si,0,).3H,O)-+K,CO,+k.
The increase in volume is 3.8 per cent.
The alteration into chlorite, supposing all the alumina and _ silica
to remain in the altered mineral, and the additional magnesia to be added
in the form of a carbonate, may be written as follows:
(3) 2KHMg,Al,Si,0,)+4MeC0,+-5H,0=2[H,Mg,Al,8i,0,,.4(OH)]+K,CO,+3C0,+k.
The increase in volume, supposing the magnesium carbonate is added in
solution and the potassium carbonate goes into solution, is 22.92 per cent.
The reaction by which biotite passes into epidote is uncertain. If the
ferrous iron of the biotite be changed to sesquioxide during the alteration,
and if the proportion of magnesium to iron be supposed to be 3:1 in the
biotite, and of aluminum to iron 4:1 in the epidote, the reaction may be
written:
(4) 6H,K,Mg,FeAl,Si,O,,+-20CaCO,+4C0,+30=
2 (H5CajoAl,,Fe,Si,;O¢5) +6810, -+18MgCO,+6K,CO,+-H,O+k.
The ratios assumed of the magnesium and iron for the biotite, and of
aluminum and iron for the epidote, are near means. If it be assumed that
the iron of the biotite is not changed to sesquioxide, but that the sesquioxide
of iron for the epidote must be derived from another source, the reaction
takes a very different form. Under such an assumption the epidote may
be produced from normal biotite, and the equation stand as follows:
(5) 30KHMg,Al,Si,0,,+6Fe,0,+40CaC0,-++35C0, +H,0O=
4(H,Ca,pAl,,Fe,Si,,0g;) +30Si0,+-12A10(OH)+60MgC0,+15K,CO,+k.
ALTERATIONS OF BIOTITE. 341
By adding thirteen molecules of water instead of one, twelve molecules of
gibbsite instead of twelve molecules of diaspore (orthorhombic; sp. gr. 3.40)
will be produced.
In equation (4), supposing the calcium carbonate to be added in
solution and the magnesium carbonate and the potassium carbonate to be
removed in solution and the silica to remain as a solid, the decrease in
volume is 14.86 per cent.
In equation (5), supposing the biotite and iron oxide to be solids, the
calcium carbonate to be added in solution, the magnesium and potassium
carbonates to remain in solution, but the epidote, silica, and diaspore to
remain as solids, the decrease in volume is 18.45 per cent. If gibbsite
instead of diaspore be produced the decrease in volume will not be so much.
If it be supposed that the aluminum passes into spinel (isometric; sp.
gr. 3.8) instead of diaspore or gibbsite, and spinel is known to form in con-
nection with biotite, the number of molecules of magnesium carbonate
would be reduced by six in equation (4); that is, to 54. 6MgAl,0, would
replace the 12Al10(OH). No water would need to be added, and five
molecules of water would be produced. Finally, only twenty-nine mole-
cules of CO, would need to be added. Therefore the equation would be:
(6) 30KHMg,Al,Si,0,,+6Fe,0,+-40CaCO,+2900,—=
4H,CayyA];,Fe,Si,,055-+-308i0,+ 6MgAl,0,+-54MgCO,--15K,CO,--5H,O-Lk.
In this case the volume of the resultant epidote, spinel, and silica, would
be 14.71 per cent less than that of the biotite and 18.15 per cent less than
that of the biotite and hematite.
It is a well-known fact that chlorite secondary to biotite is usually
accompanied by epidote and quartz. Comparing the equation (3) for the
formation of chlorite with equation (4) for epidote, we see why these two
minerals with quartz are frequently formed at the same time. For the
formation of chlorite from biotite additional magnesium is needed. For
the formation of epidote additional calcium is necessary and magnesium is
left over. If instead of magnesium and calcium carbonates being added,
as suggested in equations (3) and (4), only calcium carbonate were avail-
able, the excess of magnesium produced by the passage into epidote may go
into the chlorite, and thus epidote and chlorite be simultaneously produced.
a42 A TREATISE ON METAMORPHISM.
Combining these equations, (3) and (4), and supposing the iron oxide to be
furnished by hematite, the reaction may be written as follows:
(7) 60KHMg,Al,Si,0,.+6Fe,0, +40CaCO,-+ 76H,0=
30[H.Mg,Al,Si,0.).4(OH) ]+4(H;Cayo A], Fe,8i,;0¢5) +-308i0,+12A10(OH) +
30K,CO,+10C00,+k.
The increase in volume of the epidote, chlorite, quartz, and diaspore
together, as compared with the biotite and hematite, would be 1.81 per
cent.
If biotite alters to hypersthene and sillimanite it may be presumed, in
order to furnish the necessary iron for the hypersthene, that the biotite is
an iron-bearing one. If the magnesium be to the iron as 3:1, and the
hypersthene be one in which the same ratio prevails, the reaction may be
written as follows:
(8) H,K,Mg,FeAl,8i,0.,+CO,=Mg;FeSi,O,.+2A1,Si0;+H,0+K,00,+k.
The decrease in volume of the hypersthene and sillimanite, as compared
with the biotite, is 24.68 per cent.
In the majority of the above reactions a formula for biotite is used
which contains no iron. The majority of biotites im nature do contain
some iron. If this material be present, simultaneously with the formation
of other minerals magnetite (isometric; sp. gr. 5.174), hematite @hombohe-
dral; sp. gr. 5.225), and the other oxides of iron may be produced. The
abstraction of the iron oxides is accompanied commonly by a change in
color of the altering biotite from brown to green. The presence of these
compounds, however, in subordinate quantities will not alter the main
conclusions as to the volume relations above given.
The alterations of biotite, serpentine, kaolin, and gibbsite, into hydro-
biotite and chlorite, equations (1) to (3), are all reactions which are
known to occur in the zone of katamorphism, corresponding with which
position they are all reactions of hydration, and the first two also of car-
bonation. Where chlorite and epidote form together, the reaction is that
of hydration, and doubtless this change also takes place in the zone of
katamorphism. The formation of hypersthene and sillimanite from. biotite
usually occurs in connection with contact reactions of igneous rocks; it is,
therefore, a reaction requiring high temperature. Also the minerals
OCCURRENCE AND ALTERATIONS OF PHLOGOPITE. 343
silimanite and hypersthene do not form at the surface, but at depth.
Corresponding with these physical-chemical facts, the reaction is one of
dehydration and reduction of volume.
PHLOGOPITE.
Phlogopite is potassium-hydrogen-magnesium-aluminum silicate.
Oceurrence—Phlogopite has an occurrence which is somewhat different
from that of biotite. It is especially characteristic of metamorphosed
impure carbonates, such as dolomitic marbles. In these rocks it is often
associated with pyroxene, amphibole, ete.
alterations— The most frequent alterations of phlogopite are to hydro-
phlogopite (monoclinic; sp. gr. 2.303, kerrite) and chlorite (monoclinic;
variety of penninite; sp. gr. 2.649). It is also said to alter to tale (orthor-
hombie or monoclinic; sp. gr. 2.7-2.8). In these last two alterations
gibbsite (monoclinic; sp. gr. 2.8-2.4) or diaspore (orthorhombic; sp. gr. 3.40)
must simultaneously separate.
The reactions for these changes are as follows:
For hydrophlogopite:
(1) 2H,KMg, AlSi,0,,+-7H,0-+C0,=2(H,Mg, AISi,0,).3H,0)-+K,CO,+k.
For chlorite:
(2) 2H,KMg,AISi,0,,+6MgCO,+-7H,O=2[H;Mg,AlSi,0,..6(OH)]+K,CO,+5C0,+k.
For tale and gibbsite:
(3) 4H,KMg,AISi,0,.+-6H,0+4C0,=3H,Mg,Si,0,,+4A1 (OH) ,+3MgC0;+K,CO,+H,0-+k.
(4) 2H,KMg, AlSi,0,,+CO,+4H,0 =H,Mg,Si,0,,+H,Mg,8i,0)+2A1(OH),+K,CO,+k.
In equations (3) and (4) if diaspore instead of gibbsite were produced
less water would be needed.
Disregarding the carbonates, the increase in volume for hydrophlogo-
pite, equation (1), is 26.89 per cent; for chlorite, equation (2), is 41.02
per cent. The decrease for tale and gibbsite, equation (3), is 7.79 per
cent, and for tale and diaspore 18.27 per cent. The increase in volume of
the serpentine, talc, and gibbsite, equation (4), is 5.23 per cent.
All of these reactions are those of hydration and solution. They are
characteristic of the zone of katamorphism.
44 A TREATISE ON METAMORPHISM.
According to Clarke, penninite, one of the chlorites, is composed of
one molecule of biotite-chlorite and one molecule of phlogopite-chlorite;
and clinochlore is composed of two molecules of biotite-chlorite and one
molecule of phlogopite-chlorite. It is therefore easy to combine the equa-
tion given under biotite for the production of chlorite with the one under
phlogopite producing chlorite, and thus produce penninite (pseudorhombo-
hedral and monoclinic; sp. gr. 2.6-2.85) and clinochlore (monoclinic; sp.
er. 2-2.5). However, as the alterations for the production of chlorite from
biotite and of chlorite from phlogopite, reactions of hydration, carbona-
tion, and liberation of heat occur in the zone of katamorphism, it may
be said that where penninite and clinochlore are produced from biotite and
phlogopite the physical-chemical reactions are of the same class as those
which have been given for hydrobiotite and hydrophlogopite.
CLINTONITE GROUP.
MARGARITE, CHLORITOID, AND OTTRELITE.
The clintonite group includes the following rock-making minerals:
Margarite:
H,CaA1,Si,O,>-
Monoclinic.
Sp. gr. 2.99-3.08.
Chloritoid:
H,(MgFe) Al,SiO,.
Monoclinic (G) or triclinic (D).
Sp. gr. 3.52-3.57.
Ottrelite:
H,(FeMn) Al,Si,O..
Monoclinic or triclinic.
Sp. gr. 3.3.
Occurrence. —'"he most common development of margarite is in connection
with corundum. In a number of cases it is recorded that the alumina of the
margarite is directly furnished by the corundum. Margarite is also found
as a metamorphic mineral in schists and gneisses, associated with the heavy
minerals staurolite, tourmaline, etc. As a metamorphic mineral, margarite
is also recorded as being derived from diaspore and gibbsite.
CHLORITE GROUP. 34
On
Chloritoid and ottrelite both occur in the slates, schists, and gneisses
which are derived from the argillaceous sediments as a product of or
connected with deep-seated, and especially deep-seated regional metamor-
phism, and often contact action. They are thus heavy minerals which
develop from the simpler constituents in the argillaceous sediments in the
zone of anamorphism, their formation resulting in condensation.
Atterations —The only alteration of the clintonite group recorded is that
of margarite to dudleyite. However, as no definite formula for this mineral
is given, it is not practicable to write an equation representing the
transformation.
While no other alterations of the clintonite group are mentioned, there
is no doubt that in the upper zone of metamorphism, especially in the belt
of weathering, the chloritoids, ottrelite, and margarite are decomposed into
simpler compounds, as are the other silicates.
CHLORITE GROUP.
AMESITE, CORUNDOPHILITE, PROCHLORITE, CLINOCHLORE, AND PENNINITE.
The minerals of the chlorite group, according to Tschermak, may be
regarded as isomorphous mixtures of amesite (H,Mg,Al,SiO,) and serpen-
tine (H,Mg,Si,O,) molecules, although Clarke dissents from this conclusion.
T’schermak gives the range of the various orthochlorites as follows:
Amesite: At to At,Sp.
Corundophilite: At,Sp to At-Sps.
Prochlorite (ripidolite): At,Sp, to At,Sps.
Clinochlore: At,Sp, to At Sp.
Penninite: At Sp to At,Sp;.
These would correspond to the following compositions:
Amesite:
H,Mg,Al,SiO, to H,)>Mg,,A1.S8i,0,;.
Monoclinic.
Sp. gr. 2.71.
Corundophilite:
HyMg},A1,8i,0,; to Hy Mgo3A1,,8i,30g0-
Monoclinic.
Sp. gr. 2.90.
wo
os
or)
A TREATISE ON METAMORPHISM.
Prochlorite:
HyypMgy3A],,8i,3099 to Hyp Mgy.AlgSi,O,5.
Monoclinic.
Sp. gr. 2.78-2.96.
Clinochlore:
H,MgyAl,Si,0,; to HsMg,A1,Si,0,5.
Monoclinic.
Sp. gr. 2.65-2.78.
Penninite:
H,.Mg,Al,Si,0,, to HyyMg,;A1,Si,0,;.
Pseudorhombohedral and monoclinic.
Sp. gr. 2.60-2.85.
A considerable part of the magnesium, as shown by the analyses,
may be replaced by iron, the analyses of corundophilite showing as high
as 15 per cent of monoxide of iron ; of prochlorite, from 15 to 25 per cent,
running even higher than the magnesia. The percentage of monoxide of
iron in clinochlore and penninite is usually much less. The alumina may
be replaced in part by sesquioxide of iron, although the proportion of this
replacement is not nearly so great as that of the magnesia by the iron mon-
oxide, the iron sesquioxide generally not running beyond 2 or 3 per cent.
It is apparent that the specific gravities of the chlorites do not regularly
grade from lower to higher, as in the feldspars. Doubtless such a regular
gradation would occur provided the chlorites were pure magnesium min-
erals, corresponding to the formulee above given. The high specific gravities
which the intermediate minerals in the group, corundophilite and prochlo-
rite, may have are doubtless explained by their frequent high content of
iron monoxide.
Occurrence—Chlorite is the most abundant and widespread of all the
secondary silicates. As a secondary mineral it is probably subordinate
only to quartz. Chlorite is nowhere known as a pyrogenic constituent of
igneous rock. It is very abundant in many of the altered igneous rocks,
both plutonic and volcanic, including Javas and tuffs, being especially
abundant in the so-called green-schists. Also it is very abundant in many
amphibolites. In the altered igneous rocks which are changed under mass-
static conditions it is one of the most abundant secondary constituents,
being especially prevalent in the basic rocks, such as the greenstones. It
is also found in the acid rocks. Chlorite occurs as a plentiful allogenic
constituent in all kinds of mechanical sedimentary rocks. It develops as a
ALTERATIONS OF CHLORITE. D47
very abundant secondary constituent in the metamorphosed sedimentary
rocks, such as slates, schists, and gneisses. Frequently chlorite may occur
more abundantly adjacent to intrusive rocks than elsewhere. The most
characteristic associated secondary minerals are epidote, serpentine, tale,
zeolites, kaolin, magnesite, iron oxides, aluminum oxides, ete.
In the discussion of the individual minerals it has been shown that
chlorite is one of the abundant derivation products of the following minerals:
Almandite, augite, garnet, hornblende, iolite, prehnite, pyrope, staurolite,
tourmaline, and vesuvianite. Being essentially a magnesium-aluminum
silicate, it is especially likely to be derived from the heavily magnesian
minerals, of which the oliyine, pyroxene, amphibole, mica, and garnet
groups are the more important. As already seen, corundophilite and pro-
chlorite may contain a large percentage of iron monoxide, and therefore
one would naturally expect these chlorites to form from the minerals which
also contain a large percentage of iron monoxide, as, for instance, olivine,
actinolite, ete. In many cases the mineral from which the chlorite is
derived does not contain a sufficient amount of magnesium. In such cases
this substance is derived from adjacent minerals, or is brought in in solution.
It has been supposed in such cases that the maguesium is transported as a
carbonate. However, the principles of its development would be in no
way changed if any other salt of magnesium, such as magnesium chloride,
were substituted for the carbonate.
In the discussion of the individual minerals it is shown that chlorite
develops especially in the upper physical-chemical zone, and particularly in
the belt of cementation. Under conditions of quiescence it develops at
very considerable depth; but in proportion as interior movement occurs it is
likely to develop in smaller quantity or not at all, its place as a metamor-
phic mineral being taken by the magnesian mica biotite.
Alterations— "he alterations of chlorite, like those of other minerals, are
largely dependent upon the zones or belts in which the mineral is located.
The only definite alteration products of chlorite which are recorded are
those which Tschermak has called enophite and berlanite. The first is said
by him to be a serpentinous variety of chlorite. No formula for either has
been determined, and therefore it is not possible to write equations repre-
senting the transformation. Rosenbusch says that the last stage of the
alteration of chlorite is into an ageregate of limonite, carbonate, and quartz.
This degeneration is especially characteristic of the belt of weathering. As
348 A TREATISE ON METAMORPHISM.
usual, no attempt is made to write equations for these degenerative changes;
but if one knew definitely the composition of the original mineral and that
of the minerals which were produced in a given case, it would be easy to
write equations for the change and to calculate the volume relations.
While the alterations of chlorite in the zone of aaamorphism are not
recorded, it is certain that the chlorite of chloritic rocks under the condi-
tions of the lower physical-chemical zone pass into other constituents, since
chlorite is almost always rare or absent in both the sedimentary and the
igneous rocks which have recrystallized in the lower zone and have not
been later affected by changes in the upper zone.
Therefore in the lower zone chlorite and some of the material of the
associated minerals recombine and reproduce minerals from which chlorite
was originally derived, or other minerals. There is little doubt that chlorite
furnishes a considerable part of the elements for such minerals as the micas,
feldspars, amphiboles, pyroxenes, and even the olivines, which develop in
the zone of anamorphism, and also it is probable that the chlorite furnishes
a part of the constituents for certain of the heavy metamorphic minerals,
such as garnet, clintonite, staurolite, tourmaline, ete.
SERPENTINE-TALC GROUP.
SERPENTINE AND TALC.
The serpentine-tale group includes:
Serpentine:
H,Mg,Si,O,. (A part of the Mg may he replaced by Fe, and where the amount of Fe is
considerable this mineral is called bastite. )
Monoclinic.
Sp. gr. 2.50-2.65.
Tale:
H,Mg,8i,O)».
Orthorhombic or monoclinic.
Sp. gr. 2.7-2.8.
Serpentine and talc, like chlorite, are both hydrous magnesium silicates.
Indeed, as has been pointed out, Tschermak regards the serpentine mole-
cule with the amesite molecule (H,Mg,Al,SiO,) in variable proportions to
constitute the chlorites. Serpentine is more hydrous and more basic than
tale. Since the serpentine molecule is similar to some of the chlorites,
one would expect that the occurrence of the two would be very similar,
and such is the fact.
OCCURRENCE AND ALTERATIONS OF SERPENTINE. d49
SERPENTINE.
Occurrence— Serpentine occurs in substantially all the varieties of rocks in
which chlorite is found, but it is most abundant as a secondary constituent
in the igneous rocks which are very heavily magnesian, especially the
pyroxenites, peridotites, and similar rocks. Locally it may be so abundant
as a secondary constituent in rocks of this class, especially those bearing
olivine, as to form serpentine rocks. Serpentine develops very abundantly
in the sedimentary rocks which are rich in magnesian constituents, both in
detrital material from basic igneous rocks, and in limestones, and in various
transition varieties. Serpentine is a product of the zone of katamorphism,
including both the belt of cementation and the belt of weathering.
As shown under the discussion of the various minerals, it is secondary
to actinolite, biotite, bronzite, chondrodite, clinohumite, diopside, enstatite,
hornblende, humite, hypersthene, muscovite, olivine, pyrope, sahlite, and
spinel. Of these the most important is olivine, and of second importance
are the pyroxenes and amphiboles. In many cases the constituents out of
which serpentine is formed are derived not from a single mineral, but from
various minerals, in which case the serpentine may replace nonmagnesian
minerals, as feldspar, or may form in veins.
Alterations—Serpentine, where long exposed to the conditions of the belt
of weathering, is likely to break up into various minerals, of which brucite
(rhombohedral; sp. gr. 2.39), magnesite (rhombohedral; sp. gr. 3.06), opal
(amorphous; sp. gr. 2.15), and quartz (rhombohedral; sp. gr. 2.6535) are
the more important. By hydration and loss of magnesia it passes into
webskyite (amorphous; sp. gr. 1.771).
The reaction by which serpentine passes into magnesite, brucite, and
quartz may be written thus:
(1) H,Mg,8i,0,+CO,=MgC0,+2Mg(OH),+-28i0,+k.
The increase m volume is 13.02 per cent. In case opal or hydromagnesite
were formed the increase in volume would be somewhat greater, and the
reaction would involve hydration as well as carbonation. It is of course
possible that both brucite and magnesite may not always be formed simul-
taneously. If brucite and not magnesite be formed the equation is—
(2) H,Mg,Si,0,+H,0=3Mg(OH),+28i0,-+k.
300 A TREATISE ON METAMORPHISM.
The volume of the brucite and quartz is 9.82 per cent greater than the
serpentine. If magnesite and not brucite be formed the equation is—
(3) H,Me,§i,0,+3C0,=3MgC0,+28i0,-+-2H,0-+k.
The volume of the magnesite and quartz is 18.84 per cent greater than that
of the serpentine.
a
Brauns“ gives. the formula for the development of webskyite as follows:
(4) 3(H,R,Si,0,)—RO-+12aq. =2 (H,R,Si,O,,-+6aq. )
Where serpentine contains iron as a base, partly replacing the magne-
sium, the iron is oxidized and may be hydrated, thus producing hematite
or limonite.
The breaking up of serpentine occurs especially in the belt of weath-
ering, the transformation representing one of the final changes in the
degeneration of the silicates. Alterations of serpentine in the zone of
anamorphism are not recorded. But the general absence of serpentine in
the schists and gneisses of sedimentary origin profoundly metamorphosed
in the zone of anamorphism is conclusive evidence that the serpentine which
once was in these rocks, and the associated secondary minerals, have
recombined to produce heavy minerals of the classes from which serpentine
and those other secondary minerals were originally produced. One could
readily form equations for such alterations by reading the equations by
which serpentine is formed from right to left. (See Table C, pp. 875-394.)
TALC.
Occurrence—T'ale is practically coextensive in its occurrence with chlorite
and serpentine, but in its distribution is more nearly allied to serpentine
than to chlorite. Therefore it is found in almost every variety of rock long
subjected to alterations in the belt of weathering; but it is especially
abundant in the heavily magnesian rocks. Steatite, which is nearly pure
tale, is usually derived from the pyroxenites or peridotites. However, tale is
so abundant in many schists as to give them the name talcose, or even tale-
schists. Also, like serpentine, it ocewrs abundantly in the dolomite-bearmg
rocks and in dolomite.
«Brauns, R., Studien tiber den Palaeopikrit von Amelose bei Biedenkopf und dessen Umwand-
lungsprodukte: Neues Jahrbuch, supp.-vol. 5, Stuttgart, 1887, p. 322.
OCCURRENCE AND ALTERATION OF GLAUCONITE. ay)
Tale forms in the upper zone of metamorphism. In this respect it is
like chlorite and serpentine. It is especially likely to form under condi-
tions of weathering. The minerals from which tale is derived are as
follows: Actinolite, andalusite, anthophyllite, bronzite, cyanite, diopside,
enstatite, gehlenite, hypersthene, muscovite, olivine, phlogopite, pyrope,
sahlite, scapolites, sillimanite, spinel, staurolite, topaz, and tremolite. The
manner of formation is given under the various minerals. Of these minerals
the more important are the nonaluminous amphiboles and pyroxenes, both
orthorhombic and monoclinic. It also forms rather abundantly from olivine,
mica, and garnet.
Alterations —Alterations of tale are not recorded. It appears to be one
of the end products of rock alteration in the belt of weathering. However,
T have no doubt that when the tale-bearing rocks are buried so deeply as to
pass into the zone of anamorphism and there alteration takes place, tale,
like chlorite, serpentine, and other minerals, is destroyed, and that from it
alone, or from it and other minerals, the classes of heavy minerals from
which the tale was originally produced are again formed.
GLAUCONITE.
Glauconite:
Essentially a hydrous silicate of iron and potassium. Definite formulaunknown. The
potassium ranges from 1.85 to 6.56 per cent.
- Amorphous.
Sp. gr. 2.2-2.4.
Occurrence —Glauiconite occurs in sediments of many kinds and ages.
Where it is so abundant as to give the rock a green color it is known as
greensand. Greensands are especially prevalent in the Cretaceous.
Atterations.—Since no definite formula for glauconite can be given it is
impracticable to write equations representing the transformations. But
since glauconite is almost, if not quite, unknown in the schists and gneisses
formed in the zone of anamorphism it seems certain that under the condi-
tions of that zone the glauconite is broken up, its constituents passing into
other minerals.
A TREATISE ON METAMORPHISM.
(5)
Or
bo
KAOLIN GROUP.
Kaolinite is the only important rock-making mineral of this group.
Kaolinite:
H, Al, Si, Oy.
Monoclinic.
>
Sp. gr. 2.6-2.63.
Occurrence—Kaolinite is a secondary product in all classes of igneous
rocks and occurs as an important constituent in all sedimentary rocks except
the pure sandstones and the pure limestones. Kaolinite and quartz are the
chief constituents of the clays, and kaolinite is a very abundant constituent
of muds and grits.
Kaolinite is a product which forms extensively in the zone of katamor-
phism in the belt of cementation and in the belt of weathermg. It is likely
to be produced as a result of the decomposition of any of the aluminous
minerals. It has been noted as having been produced from the following
minerals: Andalusite, anorthoclase, biotite, cyanite, epidote, leucite, micro-
cline, nephelite, orthoclase, plagioclases, scapolites, sillimanite, sodalite,
topaz, and zoisite. Of these, undoubtedly the most important are the feld-
spars, and especially the acid feldspars.
No alterations of kaolinite are recorded. It is certain, how-
Alterations.
ever, that where the kaolin-bearing sediments are deeply buried the mineral
becomes dehydrated, that such bases as the alkalies and alkaline earths and
iron replace the hydrogen, and that various anhydrous silicates or silicates
low in hydrogen are produced. It is certain that in the zone of anamor-
phism the minerals which in the upper physical-chemical zone have broken
up into kaolinite as one of the products may recombine to a large extent
and reproduce the original minerals.
SUMMARY OF ALTERATION OF SILICATES.
While the important groups of the rock-forming silicates have been
treated separately, it may be well in closing the section to class together
the groups of the original minerals which have a somewhat similar chemical
composition and therefore alter into somewhat similar products.
These classes are called by the petrographers (1) the feldspathoid
class, (2) the transition class, and (3) the ferromagnesian class. The felds-
pathoid class includes the feldspar, nephelite, sodalite, leucite, and
wernerite groups. The only rock-forming minerals belonging to the
SUMMARY OF ALTERATION OF SILICATES. BoD
transition class are those of the muscovite group. The ferromagnesian
class includes the pyroxene, amphibole, chrysolite, biotite-phlogopite, and
clintonite groups.
(1) In the upper physical-chemical zone, that of katamorphism, the
more common alteration products of the feldspathoid class are the kaolins,
the zeolites, the epidotes Gncluding both zoisite and epidote proper), diaspore,
gibbsite, and quartz. Probably all of these minerals form in both belts of
the zone, but the development of the zeolites and the epidotes is more
characteristic of the belt of cementation than of the belt of weathering.
Indeed, as pointed out under these minerals, under lone-continued condi-
tions of the belt of weathering these minerals break up into carbonates
of the alkalies and alkaline earths, hydrous and anhydrous, oxides of
aluminum and iron, quartz, and, probably, also kaolin. In the zone ot
anamorphism the more common alteration products of the feldspathoid
class are muscovite (damourite) and scapolite. The nephelite, sodalite, and
leucite groups alter into the feldspars.
(2) In the zone of katamorphism muscovite alters into serpentine,
tale, and vermiculite (hydromuscovite). In the belt of weathering the
serpentine and vermiculite may break up into simpler compounds of the
same character as those which form from the zeolites and epidotes.
In the lower physical-chemical zone, that of anamorphism, muscovite
is one of the minerals commonly produced, and therefore does not usually
alter. But by profound and deep-seated metamorphism, the material of
muscovite may pass into the heavy ferromagnesian minerals, such as garnet,
staurolite, ete.
. (3) The ferromagnesian silicates may be divided into two great
divisions—those which are nonaluminous, and those which are aluminous.
In the zone of katamorphism the most common alteration products of the
nonaluminous ferromagnesian silicates are tale and serpentine. The
nonaluminous pyroxenes and amphiboles ordinarily pass into tale; the
chrysolites ordinarily pass into serpentine. The transformations in these
directions are explained by the fact that the pyroxenes, amphiboles, and tale
are metasilicates, while the olivines and serpentines are orthosilicates. The
metasilicates naturally pass into metasilicates, and the orthosilicates into
orthosilicates. In the zone of katamorphism the heavily aluminous ferro-
magnesian silicates alter into chlorites and epidotes. The pyroxenes and
amphiboles which are not heavily aluminous frequently split up into a com-
MON XLVII—04——238
354 A TREATISE ON METAMORPHISM.
bination of chlorite and tale, the aluminous part of the original molecule
going to the chlorite and the nonaluminous part into the tale. The
development of epidote is largely if not wholly confined to the belt of
cementation. But chlorite apparently forms both in the belt of cementa-
tion and in the belt of weathering, especially where there is abundant
vegetation. Under extreme and long-continued conditions of the belt of
weathering the serpentines, chlorites, and epidotes are likely to further
degenerate, breaking up into carbonates of the alkalies and alkaline earths,
‘anhydrous or hydrous oxides of aluminum and iron, quartz, and kaolin.
Or in the belt of weathering these end products may directly develop from
the metasilicates without the serpentines, chlorites, and epidotes as inter-
mediate products. In the zone of anamorphism the pyroxenes change to
amphiboles; the pyroxenes and amphiboles both alter to biotite; the
olivines change to the amphiboles, anthophyllite, tremolite, and actinolite.
The biotite group does not ordinarily alter. But by profound metamor-
phism the material of the biotites, amphiboles, pyroxenes, etc., may pass into
the still heavier class of minerals represented by the garnets, staurolites, ete.
THE TITANATES.
TITANITE AND PEROVSKITE.
As rock-makine constituents only two titanates of importance, titanite
to) 5)
and perovskite, are here treated, ilmenite being given under the oxides.
Titanite:
CaTiSiO,.
Monoclinic.
oy
Sp. gr. 3.4-3.56.
Perovskite:
CaTiOs.
Isometric or pseudoisometric.
Sp. gr. 4.017-4.039.
TITANITE.
Occurrence.—Titanite occurs as a rather subordinate but widespread min-
eral as an original pyrogenic constituent of igneous rocks, and also in the
schists and gneisses. So far as observed, titanite as a secondary constituent
is derived from ilmenite and rutile. These alterations are discussed under
those minerals.
ALTERATIONS OF TITANITE. BO
Alterations —Tjtanite alters into rutile (tetragonal; sp. gr. 4.18-4.25),
octahedrite (tetragonal; sp. gr. 3.82-3.95), and perovskite (isometric or
pseudoisometric; sp. gr. 4.017—4.039).
Rutile and octahedrite may be supposed to be produced by the follow-
ing reaction:
(1) CaTisi0,+CO,=TiO,+CaCO,+Si0,+k. -
The expansion of volume is 39.22 per cent for rutile, provided all of the
compounds separate as solids, and 42.07 for octahedrite.
Perovskite may be supposed to be produced by the simple breaking
up of titanite, according to the reaction:
(2) CaTiSi0O;=CaTiO,+SiO,
The expansion of volume is 0.14 per cent provided the silica also separates
as a solid.
Information as to the natural conditions under which these changes
take place is not obtainable from the papers giving the above minerals as
secondary to titanite. From the character of the first change one would
expect, however, that it would occur in the zone of katamorphism, and
especially in the belt of weathering. Under such conditions there would be
a reason for the change, for there carbonation of the silicates, with libera-
tion of heat and with expansion of volume, is the rule. As so frequently
indicated before, the freed silica may be taken into solution, and if this
occurs the volume is decreased. Under what conditions the second reaction
is likely to have taken place I can only conjecture from its nature. I
should expect it to occur in the zone of katamorphism.
PEROVSKITE.
Oceurrence.—Perovskite occurs as an original constituent in eruptive
rocks, and also in the metamorphic rocks, such as the schists and gneisses.
Asa secondary mineral it has been observed as a product secondary to titan-
ite. It may be suspected that in the schists and gneisses it forms in the
zone of anamorphism by the union of rutile and octahedrite or brookite,
with calcium carbonate; but this is a pure conjecture, as the details of its
formation are not found in literature.
Aiterations.— The mineral does not readily alter into other compounds,
although it has been observed to alter into an undetermined substance, and
it is said to alter into ilmenite (hexagonal-rhombohedral; sp. gr. 4.75).
306 A TREATISE ON METAMORPHISM.
THE PHOSPHATES.
APATITE.
The only important rock-making mineral among the phosphates is
apatite.
Apatite:
CaF.Ca,P,0,,, or CaCl.Ca,P;0,, or a mixture of the two.
Hexagonal.
<
Sp. gr. 3.17-3.23.
Occurrence —Apatite is one of the most widespread, if not the most wide-
spread, of all the subordinate constituents of rocks. It is a common, if not
an almost universal, constituent of the plutonic rocks, occurs almost as
broadly in the volcanic rocks, and is found in many varieties of unaltered
or little altered, sedimentary rocks, such as limestones, shales, sandstones,
etc.; and, finally, it is almost everywhere found in the metamorphosed
igneous and sedimentary rocks.
aiteration.—The only alteration which is recorded for apatite is to osteo-
lite, which is reported as having the same composition as apatite, except
that there has been a loss of part or all of the fluorine or chlorine.
It is certain, however, that in the belt of weathering of the zone of
anamorphism apatite is slowly dissolved. This is shown by comparative
analyses of the weathered with the unweathered varieties of the same
rock. his fact has been frequently noted in reference to the iron ores,
because here the presence or absence of phosphorus is of such great impor-
tance. It may be stated that in the iron ores it is the general rule that those
parts of deposits which have been long subjected to weathermg bear a
smaller proportion of apatite than the continuations of these same deposits
in the belt of cementation.
The depletion of the surface rocks in apatite would seem to furnish an
adequate source for the apatite in veins, this mineral being taken into
solution near the surface and redeposited deeper down, thus being trans-
ported from the belt of weathering to the belt of cementation
ANHYDRITE AND GYPSUM. 357
THE SULPHATES.
ANHYDRITE AND GYPSUM.
The only important rock-making sulphates are anhydrite and gypsum.
Anhydrite:
CaSO,
Orthorhombie.
Sp. gr. 2.899-2.985.
Gypsum:
CaSO, .2H,O
Monoclinic.
Sp. gr. 2.314-2.328.
ANHYDRITE,
Oceurrence.—As explained below, the main source of anhydrite is by the
alteration of gypsum in the zone of anamorphism. Although I do not
know the facts, I conjecture that the anhydrite deposits of Switzerland
have had such a history.
Alterations.— The chief alteration of anhydrite is to gypsum (monoclinic;
sp. gr. 2.314-2.328). The reaction is:
(1) CaS0,+2H,0=Ca80,.2H,0+k
The increase in volume is 60.30 per cent. This alteration is one which
takes place in the zone of katamorphism. An interesting case is that of
Bex, Switzerland, where the transformation from anhydrite to gypsum has
taken place completely to a depth of from 18 to 30 meters, and where
below this depth the material is anhydrite. The change of anhydrite to
gypsum is with liberation of heat, expansion of volume, hydration, lowering
of specific gravity, and lessening of symmetry, and thus stands as a rare
example of all the tendencies of the upper physical-chemical zone.
GYPSUM.
Oceurrence— [he most important source of gypsum is as a chemical
precipitate, especially in salt lakes having no outlets. It therefore natu-
rally occurs in association with halite, calcite, and mechanical detritus.
Gypsum also is produced in a subordinate way through fumarole action.
The calcium sulphate for the gypsum in either case is produced by the
reaction of sulphuric acid or sulphates upon calcium-bearing salts. Com-
308 A TREATISE ON METAMORPHISM.
monly the sulphate is formed by the oxidation of a sulphide. A common
method is the production of iron sulphate by oxidation of pyrite, marcasite,
or pyrrhotite, which reacts upon calcium carbonate, thus producing gypsum.
The reaction is—
CaCO,-+FeS0,+2H,0=CaS0,.2H,0+FeCO,-+k.
The development of gypsum by this method is illustrated at many mines.
Finally gypsum may be formed by the hydration of anhydrite. All
these methods of formation of gypsum are characteristic of the zone of
katamorphism, and especially of the belt of weathering.
Alteration.— An Important alteration of gypsum is to anhydrite (ortho-
rhombic, sp. gr. 2.899-2.985). The reaction is—
(1) CaS0,.2H,O=CaS0,+2H,0—K.
The decrease in volume is 37.62 per cent. The other important alteration
of gypsum is into calcite (rhombohedral, sp. gr. 2.713-2.714). The
reaction 1s—
(2) CaS0,.2H,0+C0,=CaC0,+H,80,+H,0-+k.
The H,SO, produced may simultaneously react upon some other compound.
The decrease in volume is 50.29 per cent.
Unless beds of gypsum have been deeply buried the alteration to
anhydrite has not extensively occurred. It is a reaction of diminution of
volume, absorption of heat, dehydration, increase in specific gravity, and
increase in symmetry, and therefore is one of the very rare cases which
illustrate all the tendencies of the lower physical-chemical zone. The
change of gypsum to calcite is a reaction with liberation of heat and con-
densation of volume. The change takes place near the surface, especially
in the belt of weathering, where carbon dioxide is abundant, and may also
occur in the lower zone. It therefore stands, in its physical-chemical
relations, in the same position as dolomitization. (See p. 240.)
MINERALS. ; 359
SECTION 4.—GENERAL STATEMENTS.
PHYSICAL-CHEMICAL FACTORS ON WHICH NATURE OF ALTERATIONS
DEPENDS.
As inferences from the foregoing treatment it may be said that the more
important physical-chemical factors on which the alteration of an individual
mineral depends are (1) the chemical composition of the mineral, (2) the
chemical composition of the adjacent minerals, (3) the chemical composi-
tion of the circulating solutions, (4) the specific gravity, (5) the symmetry,
(6) the heat effect of the reaction, and (7) the pressure and volume.
CHEMICAL COMPOSITION.
Certain chemical compounds are stable under a great variety of
conditions; others are stable only under certain definite conditions; and thus
the chemical composition influences the stability of minerals. As an illus:
tration of minerals which have stability under widely varying conditions
may be mentioned quartz, which forms alike from a magma and from water
solutions, and also at the surface and at great depth. Nephelite and soda-
lite are examples of minerals which can exist only under a comparatively
narrow range of conditions.
CHEMICAL COMPOSITION OF ADJACENT MINERALS.
It has been seen that mineral particles may react upon one another,
either through the medium of contained solutions or by direct rearrange-
ment under the influence of pressure. ‘Therefore, it is clear that the nature
of a mineral which is mainly secondary to another mineral is influenced
by the chemical compositions of adjacent minerals.
CHEMICAL COMPOSITION OF CIRCULATING SOLUTIONS.
It has already been shown that the secondary minerals are dependent
not only upon the adjacent minerals, but upon the material carried by the
underground solutions. The amount of such material is dependent upon
the vigor of the circulation. As explained on pages 507-518, 655-656,
764-766, the material added or abstracted may be great in the zone of
katamorphism, but is usually rather limited in amount in the zone of
anamorphism.
360 A TREATISE ON METAMORPHISM.
SPECIFIC GRAVITY.
Apparently high specific gravity is favorable to stability. This is what
one would expect, for high specific gravity involves a comparatively close
arrangement of the atoms. Where the atoms are near together the
molecular attraction is great, and in order to break up the combination
this foree must be overcome. This principle is illustrated by dimorphous
and trimorphous compounds. Diamond (av. sp. gr. 3.52) is more stable than
graphite (av. sp. gr. 2.16), and graphite is more stable than carbon (sp. gr.
1.9, charcoal). Pyrite (ay. sp. gr. 5.025) is more stable than marcasite (av.
sp. gr. 4.870). Cyanite (av. sp. gr. 8.615) is more stable than sillimanite
(ay. sp. er. 3.235), and sillimanite is more stable than andalusite (av. sp. gr.
3.18). Quartz (av. sp. gr. 2.6535) is more stable than tridymite (av. sp. gr.
2.305). This last instance well illustrates the principle; for the symmetry
of quartz and tridymite is the same, and this variable factor included in
the previous illustrations is excluded. The same is true of andalusite
and sillimanite of the aluminum-silicate series. As poimted out on page
112, the more condensed a compound, or, in other words, the higher the
specific gravity, the less energy is potentialized. In the change from a
lower to a higher specific gravity energy is liberated. In this we have the
physical explanation of the greater stability of minerals of high specific
gravity. To form minerals of higher specific gravity from those of lower
specific gravity releases energy. ‘To reproduce minerals of lower specific
gravity from those of higher specific gravity requires the expenditure of
energy. An exception to the above rule as to increase of stability with
increase of specific gravity is furnished by calcite and aragonite. Calcite
(ay. sp. gr. 2.7135) is more stable than aragonite (av. sp. gr. 2.94), but in
this case the factor of symmetry enters, which is discussed under the next
heading.
SYMMETRY.
Apparently the greater the symmetry the more stable is the mineral
likely to be. This principle is illustrated by substances which are dimor-
phous or trimorphous.
Pyrite Gsometric) is more stable than mareasite (orthorhombic).
Diamond (isometric) is more stable than graphite (hexagonal), and this
is more stable than amorphous carbon. Kelvin suggests® that soft
«Lord Kelvin, Popular lectures and addresses, Macmillan & Co., London, 1894, vol. 2, p. 428.
SPECIFIC GRAVITY AND SYMMETRY. 361
phosphorus as compared with red phosphorus, and prismatic sulphur as
compared with octahedral sulphur, contain potential energy. When the
change from the first to the second takes place, energy is evolved, and
consequently the second form is more stable. These changes are in the
direction of higher symmetry, and Kelvin’s argument applies equally well
to all the changes in which minerals pass from a lower to a higher degree of
symmetry. To reproduce minerals of lower symmetry would require the
expenditure of energy. Therefore we have an energy cause why minerals
with high symmetry are more stable. They contain less potential energy.
Their formation is under the apparent law of the universe of dissipation of
energy.
SPECIFIC GRAVITY AND SYMMETRY.
Where specific gravity and symmetry work together, as in a number
of the illustrations mentioned, there seem to be no exceptions to the rule
of increase of stability with increase of specific gravity and increase in
symimetry.
But in those instances in which the specific gravity and symmetry are
opposed to each other it can not be predicted which will be the domimant
factor. For instance, calcium carbonate crystallizes as calcite (hexagonal-
rhombohedral; sp. gr. 2.7135) and aragonite (orthorhombic; sp. gr. 2.94).
The former is the more stable. In this case it seems that symmetry is the
dominant factor. In the alumimum silicate which crystallizes as andalusite
(orthorhombic; sp. gr. 3.18), sillimanite (orthorhombic; sp. gr. 3.235), and
cyanite (triclinic; sp. gr. 3.615), the latter is the most stable. In this case
it appears that the specific gravity is the determining factor.
It is believed that when the energy relations of these changes! become
known it will be found that in each of these cases the more stable molecules
contain less potential energy. If this be true, calcite, considering both its
specific gravity and its symmetry, contains less energy than aragonite, and
eyanite less than andalusite or sillimanite. If this conjecture be true, all
compounds are subject to a common law. That mineral forming from a
compound is most stable in which the minimum energy is contained.
The relations of symmetry and specific gravity raise some very
interesting questions as to the arrangement of the molecules in minerals.
Pressure undoubtedly tends to produce the most compact arrangement.
362 A TREATISE ON METAMORPHISM.
(See pp. 182-186.) According to Slichter, the most compact possible
arrangement of spherical molecules is that which gives a rhombohedral unit
having face angles equal to 60° and 120°." One might therefore conclude,
other things being equal, that minerals having hexagonal crystallization would
be those which have the closest arrangement of molecules and therefore the
highest specific gravity; but plainly there are other factors entering into
the problem, for, as already pointed out, aragonite (orthorhombic) has a
higher specific gravity than calcite (hexagonal), and cyanite (triclinic)
has a higher specific gravity than sillimanite and andalusite (orthorhombic).
On the other hand, diamond (isometric) has a higher specific gravity than
graphite (hexagonal). This is an especially interesting case, since the
cubical arrangement of molecules, the one ordinarily appealed to to
explain isometric symmetry, is the most open of all possible arrangements.
From the foregoing it appears perfectly clear that besides the manner of the
arrangement of the molecular particles the distance of the molecules from
one another enters as a very important factor. Also the shape of the
molecules, the closeness of the arrangement of their atoms, and the com-
plexity of the molecules themselves doubtless enter as important factors
into the density of minerals.
HEAT REACTIONS.
Other thing
which give the greatest liberation of heat. This law is best illustrated
s being equal, within the lithosphere reactions take place
at or near the surface, where the reactions usually occur in accordance
with it. The reactions of oxidation, hydration, and carbonation are there-
fore dominant However, the law of reactions with liberation of heat
becomes less and less able to control as the pressure becomes con-
siderable. Where the pressure is great, as noted under the next heading,
it determines the reaction without respect to whether heat is absorbed or
liberated, and in many cases the reactions take place with the absorption
of heat, so far as the chemical factors are concerned. If all the physical
factors also were included, all reactions would take place with the dissipa-
tion of energy. (See p. 57.)
“Slichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1899, pp. 306-310.
~PHYSICAL-CHEMICAL FACTORS. 363
PRESSURE AND VOLUME.
Pressure lessens the volume and therefore tends to preserve and to pro-
duce minerals which have a high specific gravity. Where the pressure
is small this factor is relatively inefficient and consequently other factors
usually control, and many minerals of low specific gravity form. But even
where the pressure is small it is not unimportant, at least retarding reactions
which would otherwise occur. This is illustrated by a partially altered
rock described by Merrill,“ which seemed solid when confined by the
surrounding rock, but which when brought to the surface from a depth of
a few feet, and thus freed from pressure, rapidly decomposed and disinte-
erated. Where the pressure is great it is likely to be a determinative
force, controlling the reactions. At the depths of the zone of anamorphism
the uniform production of anhydrous or shghtly hydrous minerals of higher
average specific gravity than those formed in the zone of katamorphism is
clear evidence of the dominance of pressure.
In this connection it will be well to mention the mineral groups, with
their specific gravities, which are more characteristic of the zones of
katamorphism and anamorphism.
The characteristic produets of the zone of katamorphism are: Of the
oxides, (1) those of silicon, opal, chalcedony, and quartz (sp. gr 2.1 to
2.654); (2) those of iron, including the hydrous and anhydrous ferric-
oxides (sp. gr. 3.80 to 5.225); (3) the hydrous aluminum oxides, gibbsite
and diaspore (sp. gr. 2.35 and 3.40); of the carbonates, calcite and’ dolo-
mite (sp. gr. 2.7135 and 2.85); of the silicates, (1) the epidote-zoisite group
(sp. gr. 3.25 to 4.20); (2) zeolite group (sp. gr. 2.04 to 2.40); (8) chlorite
group (sp. gr. 2.60 to 2.96); (4) serpentine-tale group (sp. gr. 2.50 to 2.80);
and kaolin group (sp. gr. 2.6 to 2.63). (See pp. 519-520, 621-627.)
The characteristic important mineral groups formed in the zone of
anamorphism are as follows: Of the sulphides, pyrite, and pyrrhotite (sp.
er. 5.025 and 4.61); of the oxides, (1) those of silicon, chert, chalcedony,
and quartz (sp. gr. 2.6 to 2.654); (2) those of iron, hematite, magnetite, and
ilmenite (sp. gr. 5.225, 5.174, and 4.75); (3) those of aluminum, corundum
(sp. gr. 4.025); (4) those of titanium, rutile, octahedrite, and brookite (sp.
gr. 4.215, 3.885, and 3.975); of the silicates, (1) the feldspar group (sp. gr.
«Merrill, George P., Rocks, rock weathering, and soils, Macmillan Co., New York, 1897, pp.
252-253.
064 A TREATISE ON METAMORPHISM.
2.54 to 2.76); (2) pyroxene group (sp. gr. 2.68 to 3.58); (8) amphibole
group (sp. gr. 2.9 to 3.713); (4) garnet group (sp. gr. 3.41 to 4.30); (5) chrys-
olite group (sp. gr. 3.2 to 4.1); (6) scapolite group (sp. gr. 2.566 to 2.74);
(7) aluminum silicate group (sp. gr. 3.16 to 3.67); (8) humite group (sp. gr.
3.1 to 3.2); (9) tourmaline (sp. gr. 3.09); (10) staurolite (sp. gr. 3.71); (11)
mica group (sp. gr. 2.7 to 3.1); (12) clintonite group (sp. gr. 2.9 to 3.57).
The average specific gravity of the mineral groups above mentioned
as products of the zone of katamorphism is 2.948. The average specific
gravity of the mineral groups. of the zone of anamorphism is 3.488. It
thus appears that the average specific gravity of the minerals which develop
in the zone of anamorphism is 18 per cent greater than that of the
minerals in the zone of katamorphism. This comparison is of course very
roughly approximate, since the various minerals are not present in equal
quantities. Probably the percentage is too great, since the heavy sulphides
and the very heavy silicates are given equal weight with the abundant but
lighter quartz, feldspars, pyroxenes, amphiboles, and micas. The com-
parison, however, shows beyond question that a given mass of material
occupies much less space in the lower physical-chemical zone than in the
upper physical-chemical zone.
It is shown under the next heading that many of the reactions written
for the minerals of the zone of katamorphisin may be read in reverse order
when the resultant minerals are buried so deep as to be in the zone of
anamorphism. The lighter minerals characteristic of the zone of kata-
morphism reunite to produce heavier minerals of the zone of anamorphism,
such as the feldspars, the micas, the pyroxenes, the amphiboles, the chryso-
lites, andalusite, etc. Furthermore, where the pressure is great enough
these minerals rearrange themselves again in whole or in part so as to
produce still heavier minerals, such as garnet, staurolite, tourmaline,
sillimanite, eyanite, etc. This great change takes place within the narrow
range of less than 10,000 meters.
Since in this mere outer film of the earth a great diminution in the
volume of the minerals has taken place, it is thought to be highly probable
that, even if the average chemical composition of the interior of the earth
be supposed to be the same as the crust, the pressure is such that the min-
erals may further rearrange themselves into still more compact products,
thus probably producing minerals of a different kind and higher specific
PRESSURE AND VOLUME. 369
gravity than any with which we are acquainted. Indeed, the interior
pressures increase so rapidly with depth that rearrangement might occur
again and again. Therefore, even if the average chemical composition be
the same deep within the earth as at the surface, in the centrosphere, in
consequence of high pressure, there may be a set of silicate minerals which
have as high a specific gravity as the average density of the earth, viz,
5.67. If the accepted theory as to the distance between molecules be cor-
rect, viz, that molecules of ordinary liquids at the surface of the earth do not
occupy more than one-third of the total volume,” there is ample room
between them for the condensed rearrangement suggested. From the fore-
going it appears that we do not necessarily appeal to a great preponderance
of heavy metals deep within the earth to explain its average high specific
eravity. It may be very largely explained by the condensation of the
material due to pressure. If, as suggested by Chamberlin, the average
specific gravity of the material of the earth be that of meteoric falls, the
average change in specific gravity would be from 3.69 to 5.67 as a result of
pressure. The great increase in the average specific gravity of minerals
with increase of pressure in the crust of the earth would seem to make the
estimate of the change in average specific gravity of the minerals from 3.69
to 5.67, as a result of the very great pressures deep within the earth, a very
modest one.
While I have no doubt that the condensation of the earth material into
heavier compounds as a result of pressure is a partial explanation of the
high specific gravity of the earth, | by no means urge this as the sole cause.
Indeed, it is probable that the segregation of heavy material toward the
center and lighter material toward the surface has steadily continued
throughout geological time, and therefore the difference in composition is a
very important factor in the difference in density at the surface and the
center. But I do not venture even a guess as to the relative importance of
the two factors of condensed compounds and segregation of material in
explaining the increase in density of the material of the earth with increase
of depth. :
«Nernst, W., Theoretical chemistry, trans. by C. 8. Palmer, Macmillan & Co., London, 1895,
p- 196.
366 A TREATISE ON METAMORPHISM.
REVERSIBLE REACTIONS.
On the foregoing pages numerous reactions have been written by
which the minerals characteristic of the zone of katamorphism are pro-
duced; very few reactions have been written by which the minerals of the
zone of anamorphism are reproduced. It is certain that when the minerals
formed in the belts of weathering and cementation are altered under the
conditions of the zone of anamorphism the minerals characteristic of that
zone develop; therefore it is believed that many of the reactions for the
development of the minerals of the zone of katamorphism are reversible.
To illustrate, in the zone of katamorphism olivine may alter into the min-
erals serpentine, magnetite, magnesite, and quartz, according to the fol-
lowing equation:
3Mg,FeSi,0,+3C0, +-4H,0+0=2H ,Mg,Si,0,+Fe,0,+3MgC0,+28i0,-+k.
It is believed that when these four minerals are brought together in proper
proportions under favorable conditions in the deep-seated zone the reverse
reaction occurs, and that» the equation may be read from right to left
instead of left to right, thus reproducing the olivine.
The above illustration is chosen because the change from left to right
involves carbonation, desilication, hydration, and oxidation; and the change
from right to left involves silication, decarbonation, dehydration, and deoxi-
dation. Of course, where deoxidation takes place in the zone of anamor-
phism some reducing agent must be present to utilize the abstracted oxygen.
The principle of the reversibility of the reactions in the two opposing zones
is actually illustrated in a few cases where the products of the zone of kata-
morphism have been observed to alter in the zone of anamorphism. For
instance, it is recorded (p. 261) that analcite is derived from albite according
to the following equation:
2NaAlSi,0,+2H,0=Na,Al,$i,O,).2H,O-+28i0, +k;
whereas we find (p. 334) that analcite alters to albite by the reaction:
Na,Al,$i,0,-2H,O-+2Si0,=2NalSi,O,+2H,O—k.
In other words, the reaction is exactly reversible; for while the k is plus
in the first equation and minus in the second, it is on opposite sides in the
two equations. The feldspars alter into many zeolites, and a number of
REVERSIBLE REACTIONS. 367
the zeolites alter into the various feldspars. The above reaction chances
to be the only one given for these groups which is exactly reversed. This
is a consequence of the. fact that reactions are written only for recorded
alterations. There can be no doubt that practically all the equations
representing the recorded alterations of the feldspars (pp. 261-263) into
the zeolites, and all the reactions representing the recorded alterations of
the zeolites (pp. 833-334) into the feldspars, are reversible. For instance,
we have anorthite altering into gismondite as follows (p. 262):
3CaAl,Si,0,+12H,0 =Ca,Al,Si,0»,.-12H,O-+k.
Can one doubt that if gismondite passes into the zone of anamorphism
dehydration may take place and anorthite be reproduced?
Another line of evidence pointing to the reversibility of the reactions
in the two zones is the frequent recorded association of corundum with
diaspore and gibbsite, the latter minerals being secondary to the corundum.
Can it be doubted that these hydrates may be dehydrated in the zone of
anamorphism and reproduce corundum? Of course this particular change
may not occur alone. At the same time the dehydration takes place the
alumina may unite with silica and form andalusite, sillimanite, or cyanite,
or the alumina may enter into some other silicate.
Bearmg in the same direction are the experiments made by Daubrée
upon serpentine.* It is well known that both enstatite and olivine alter
into serpentine. Daubrée found that by the fusion of serpentine it split up
into enstatite and olivine, according to the following equation:
H,Mg,Si,0, +Heat=MgSi0, + MgSiO,+2H,0 +k.
Finally, my chief reason, in addition to those already given, for belief
in the reversibility of the reactions in the two zones lies in the actual
compositions of the unmetamorphosed sediments and their metamorphosed
equivalents. The unmetamorphosed pelites are composed largely of the
lighter hydrous minerals of the belt of weathering and the belt of cementa-
tion. It is true that with these, as already explained, there are also
considerable, or even dominant, quantities of residual undecomposed
anhydrous minerals; but it is certain that the metamorphosed equivalents
of these pelites contain none of the minerals which are characteristic of
«Daubrée, A., Expériences synthétiques relatives aux météorites: Comptes rendus des séances
de l’académie des sciences, vol. 62, Paris, 1866, p. 661.
368 A TREATISE ON METAMORPHISM.
the belts of weathering and cementation, and the only possible conclusion
is that these minerals have recombined and reproduced the heavier minerals
of the lower physical-chemical zone. That this is so is shown by the fact
that, barring the water and the carbon dioxide which are liberated in the
process of alteration, the average chemical compositions of the unaltered
pelites and their metamorphosed equivalents are nearly the same.
While it is held that the reactions are reversible, it is not supposed
that this is often exactly the case for a given rock. In order that this
should even approximately take place, it would be necessary that there be
no change of average composition in the zone of katamorphism, and this is
never the case. The minerals formed in the zone of anamorphism depend
not only upon the minerals of the zone of katamorphism present, but upon
their proportion and many other factors. What is meant by the reversi-
bility of the reactions is that, when compounds produced in the zone of
katamorphism from a given mineral are together im proper proportions
and conditions in the zone of anamorphism, the original mineral may be
reproduced.
If this law of the reversibility of reactions in the two zones be true,
the question naturally arises why so few of the reversing reactions im
the zone of anamorphism have been recorded. The answer les in the
difference in the readiness with which observations may be made in the
two zones. The reactions of the belts of weathering and cementation of
the zone of katamorphism have been more fully described, because they
are constantly taking place at or near the surface under conditions of
ready observation. Many of the reverse reactions have not been fully
described, because they occur at depth, and because in areas of strong
metamorphie action they have been complete. Usually gradation from
practically complete reactions to very incomplete reactions in the zone of
anamorphism is comparatively rapid. But notwithstanding the very imper-
fect observations of the zone of anamorphism, the general reversibility of
the reactions in the two zones seems as certain as if it were established by
observation, and it is believed that it will be established by observation.
If the conclusions of the foregoing paragraphs be correct it is evident
that there is an almost entirely neglected field of observation in metamor-
phism—that by which the minerals of the zone of anamorphism are produced
from the minerals of the zone of katamorphism.
TABLES. 369
For the reasons given above, I conclude: It is believed that most of
the equations which represent the reactions in the zone of katamorphism are
reversible in the zone of anamorphism; and so far as there is expansion of
volume and liberation of heat in the wpper zone, just so far is there condensation
of volume and absorption of heat in the lower zone.
SECTION 5. TABLES.
In order to present compactly the essential facts as to the alterations
of each mineral, a set of tables is here given.
Table A gives the mineral sources of each of the minerals.
Table B gives the minerals to which each mineral alters.
Table C gives the equations representing the alterations of each of the
minerals into other minerals and shows the volume changes.
Table D classifies the alterations of the minerals under processes and
gives their various combinations, with volume changes.
Taste A.— Sources of minerals.
ACINIILE NSIC eV. COMROM 22 Sacer es teen eee arfvedsonite.
Actinolite is derived from......-...---------- ankerite, bronzite, hypersthene, olivine, parankerite,
sahlite.
Alpitensidertvedsttomes sea nse =e em analcite, heulandite, laamontite, plagioclases (with or-
thoclase), sodalite, spodumene, stilbite.
Allophaneis derived:from)------ —~---..------ anorthoclase, microcline, orthoclase.
Amesite is derived from -...-..-------------- pyrope.
Anal cites denycdirOnMee eee see ease = laumontite, leucite, nephelite, plagioclases, sodalite.
Anhydrite is derived from .....-.-.---.--.--- gypsum.
Anthophyllite is derived from .-.--.-.-.---.- bronzite, hypersthene, olivine.
Aphrosiderite is derived from -----. ---.------ garnet.
ibeanG ii Comahyeel movin So coe aa eeeeaee noes hornblende.
IBashitenis derived Moms saee aes ee ese oe actinolite, anthophyllite, bronzite, cummingtonite,
hypersthene, sablite.
Berlanite is derived from ......--.-----------chlorite.
Beta-spodumene is derived from ------- Sees spodumene.
iBIOtiLens GeriyediroOmynes eens ssec ese ee anorthoclase, augite, hornblende, microcline, ortho-
clase, scapolites.
Biotite-chlorite is derived ------+------------- biotite.
iBrucitens:deriveduinomMpas=ssse-- ssa chondrodite, clinohumite, humite, serpentine.
Breunerite is derived from ---.-.--.---------- olivine.
Calcitetisiderived omer ss 2255-55524 -25- ce = actinolite, ankerite, anthophyllite, aragonite, augite,
diopside, dolomite, epidote, fluorite, garnet, grossu-
larite, gypsum, haiiynite, hornblende, noselite, paran-
kerite, sahlite, scapolites, tremolite, zoisite.
Chabazite is derived from -..---------------- haiiynite, noselite, plagioclases.
Chalcedony is derived from-_--.--- Pa See augite, sahlite.
Chionitensderivedmromy 2. se22 22+ s22 22-5. almandite, augite, biotite, garnet, hornblende, iolite,
phlogopite, prehnite, pyrope, staurolite, tourmaline,
vesuvianite.
MON XLVII—04——24
370 A TREATISE ON METAMORPHISM.
Taste A.—Sources of minerals—Continued.
Cimolitensrderivedtirom sere ssses eae anorthoclase, microcline, orthoclase.
Chlorophyllite is derived from .....-.-------- iolite.
Chromite is derived from ---.....-.---------- olivine.
Clinochlore is derived from....-..---.------- biotite (with phlogopite).
Corundum is derived from..-.._-.--:----.--- diaspore, gibbsite.
Cyanitejis derived:inomes a= s)2. 4 5-ee e = andalusite, corundum, diaspore, gibbsite.
Cymatolite is derived from ........--.------- spodumene.
Damourite is derived from--...--------.----- andalusite, corundum, cyanite, microcline, orthoclase,
sillimanite, staurolite, topaz.
«Diaspore is derived from.-.-.-.--.----------- biotite, corundum, garnet (conjectural), gibbsite, hatyn-
ite, muscovite, nephelite, noselite, phlogopite, scapo-
lites, sodalite.
Diopside is derived from..........----------- dolomite.
Dolomitenisideniveditrom! s) 5-2 -e see see eee ankerite, calcite, parankerite.
Dudleyite is derived from : margarite.
Enophite is derived from .....--...---------- chlorite.
Enstatite is derived from -.-...------------ -pyrope.
Mpidoteisyderivedyiroms ses eee ee are) anorthoclase, augite, biotite, garnet, hornblende, micro-
cline, orthoclase, plagioclases, scapolites.
Epistilbite is derived from .....-..----------- plagioclases.
Eucryptite is derived from_.......----------- spodumene.
Hassaite.is!\derived! from .2 20322 2-2 ee gehlenite.
Garnetusiderivedtiromvsesasseeeee ese e saree vesuyvianite.
Gibbsitens)derivedtfromeo soos. 2 sc seein anorthoclase, andalusite, biotite, cancrinite, corundum,
cyanite, epidote, garnet (conjectural), hatiynite, mi-
crocline, muscovite, nephelite, noselite, orthoclase,
phlogopite, plagioclases, pyrope, scapolites, silliman-
ite, sodalite, topaz, tourmaline, zoisite.
Gismondite is derived from ....-.------------ plagioclases.
Grossularite is derived from ..-...--.-------- gehlenite.
Gruneriteis derivediiromies-en oss. --eerret siderite.
Gypsum) isiderived\ irom) =a —j22 22a eons anhydrite.
Halloysite is derived from .....-------------- anorthoclase, microcline, orthoclase.
Hematite is derived from .........-----.----- actinolite, ankerite, anthophyllite, biotite, bronzite,
garnet, greenalite, griinerite, hornblende, hyper-
sthene, ilmenite, limonite, magnetite, marcasite, oli-
vine, parankerite, pyrite, serpentine, siderite.
Hercyniteis derived from... -5. 255-2 4--5- olivine.
Heulandite is derived from ..-....-.---------- plagioclases.
Hornblende is derived from......------------ augite, garnet.
Hydrobiotite is derived from.......-..--.---- biotite.
Hydromagnesite is derived from -..--..------ brucite.
Hydromuscoyite is derived from ..----. ------ nephelite, scapolites, sodalite.
Hydronephelite is derived from ..-...-------- nephelite, sodalite.
Hydrophlogopite is derived from...-.-.------ phlogopite.
Hydrotalcite is derived from .-..-...--.------ olivine. ;
Hypersthene is derived from.......---------- almandite, biotite, garnet.
Tlmenite:isiderivedifromeee ses. e-serr-- ese perovskite, rutile.
Kaolinuisderivedstromysascceccesceeessceeicl andalusite, anorthoclase, biotite, cyanite, epidote,
garnet (conjectural), leucite, microcline, nephelite,
orthoclase, the plagioclases, the scapolites, silliman-
ite, sodalite, topaz, and zoisite.
Latmontite is derived from........---------- anorthite.
TABLES. aii
Taste A.— Sources of minerals—Continued.
Lepidomelane is derived from..-.....-------- arfvedsonite.
Limonite is derived from actinolite, ankerite, anthophyllite, arfvedsonite, biotite,
bronzite, chlorites, epidote, garnet, greenalite, griin-
erite, hematite, homblende, hypersthene, ilmenite,
magnetite, marcasite, olivine, parankerite, pyrite,
pyrrhotite, serpentine, siderite.
Magnesite is derived from.--.--....-.-------: garnet, olivine, pyrope, serpentine.
Magnetite is derived from__..-...-----.----+- actinolite, ankerite, arfvedsonite, augite, biotite, bronz-
ite, diopside, garnet, greenalite, griinerite, hematite,
hornblende, hypersthene, ilmenite, marcasite, olivine,
parankerite, pyrite, pyrrhotite, sahlite, siderite.
Malacon (hydrous zircon) is derived from... --. zircon.
Marcasite is derived from. -- hematite. nf
iMarcanitensro eniviedmnoneeae -eaate= see see corundum, diaspore, gibbsite.
Meionite is derived from...---..---:.-.------ grossularite.
Mesoliteisiderivedstromm = sees. sseees cece plagioclases.
Wicansiderivedstromsamer penn seer e ene spinel, tourmaline, vesuvianite.
Microcline is derived from spodumene.
Muscovite is derived from.........-..-------- anorthoclase, diaspore, gibbsite, leucite, microcline,
nephelite, orthoclase, plagioclase and orthoclase,
scapolites, sodalite, spodumene.
See also Damourite.
Natroliteusderivedtinome eas 2--\ ssc s ee ace apatite, chabazite, haitynite, nephelite, noselite, plagio-
clases, sodalite.
Nephelite is derived from...-....-.---------- leucite, sodalite (conjectural).
Newtonite is derived from _-..--...------.---- anorthosite, microcline, orthoclase.
Octahedrite is derived from ...............--- ilmenite, titanite.
LO paleisderivedstronlpersee semere cece cece olivine, serpentine.
Orthoclase is derived from_-..._-._...-.------ analcite, heulandite, leucite, laumontite, stilbite.
@steolitejisiderived fromeeess-4-5-ce- eee cee apatite.
aragonitensiderivedttrombee soe sae eee seer anorthoclase, muscovite, plagioclases.
Pectolitelisideniwed fromeeeeep ae 2 - seeeee apophyllite.
Penninite is derived from..........-..------- biotite (with phlogopite).
Peroyskite is derived from. . ..--titanite.
Phillipsite is derived from _....--.--.-------- plagioclases.
Phlogopite-chlorite is derived from...-.-.----- phlogopite.
Pinitensiaerivedsinompse nee essa ence seis iolite.
Prehnite is derived from....-........-------- analcite, laumontite, mesolite, natrolite, plagioclases,
scolecite.
ymitebistd eriyedstronmees eer iavterslerel ee cleielee t= marcasite, pyrrhotite.
Pyrophyllite is derived from..--.------------ anorthoclase, microcline, orthoclase.
Quartzis\derivedtinommesmeeee eee seer eecne ee actinolite, anorthite, anorthoclase, anthophyllite, augite,
biotite, bronzite, chalcedony, chlorites, cummington-
ite, diopside, enstatite, epidote, garnet, grossularite,
hornblende, hypersthene, microcline, olivine, opal,
orthoclase, plagioclases, prehnite, pyrope, sahlite,
scapolites, serpentine, tridymite, zoisite.
Rutilewsiderivedstrompseeee seas cesece re oeeee brookite, ilmenite, octahedrite, titanite.
Salhlitesishd erivedwino meee t= (1 sees easier ate ankerite, parankerite.
Scapolites are derived from plagioclases.
Scolecite is derived from_-.-----.------------- plagioclases.
Serpentine is derived from...-....-.--------- actinolite, biotite, bronzite, chondrodite, clinohumite,
diopside, enstatite, hornblende, humite, hypersthene,
muscovite, olivine, pyrope, sahlite, spinel.
372 A TREATISE ON METAMORPHISM.
Taste A.—Sources of minerals—Continued.
Sideritens|derived frome sss as eee se eee arfvedsonite, garnet, hematite, hornblende, limonite,
magnetite, olivine.
Sillimanite is derived from-....-.-.---------- andalusite, biotite, corundum, cyanite, diaspore, gibbsite.
Smaragdite is derived from. ....--.----------- diallage.
Sodalite is derived from........-------------- nephelite.
Spinel is derived from ....... -.-------------- almandite, biotite, corundum, diaspore, garnet, gibbsite,
olivine, pyrope.
Steatite is derived from-_-...-.--.--2--=---2-- andalusite, cyanite, muscovite, sillimanite, topaz, tour-
maline.
Stilbiteisiderived, frome 323s eae see ae haiiynite, noselite, plagioclases.
Talensidenivedurompasee Sse ee eee eee eee actinolite, andalusite, anthophyllite, bronzite, cyanite,
diopside, enstatite, gehlenite, hypersthene, musco-
vite, olivine, phlogopite, pyrope, sahlite, scapolites,
sillimanite, spinel, staurolite, topaz, tremolite.
Titaniteis derived frome ee secs e a= een ilmenite, rutile.
Thomsonite is derived from .....-.----------- nephelite, plagioclases, sodalite.
Tremoliteisiderived: fromeeso2 222 esce eee sl diopside, dolomite, olivine.
Vermiculite is derived from-.-.-....-.-------- muscovite.
Webskyite is derived from-...--...---------- serpentine. |
Wollastonite is derived from ......----------- calcite, dolomite.
Zoisite is derived from..-.------------------- corundum, diaspore, gibbsite, grossularite, plagioclases.
Taste B.— * 8S8BlVOUILIO ‘OY JLOUB ‘UIGLY is x nee
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(GaEORKADER oy Np) |PORS SoeRe eC REPS oRnS FE UTLOT SB [TOPANGA Sec rare eae gcpaieae esa eae eee ae (pe7B[NULOJ JON)
BHBSOHSEoH +++ -oqorg
SSCOOOaS 4 ---ooprdy
Sebe| ssivsines = *-91OUL 10 OUQ, BITAOOSN[ fe" reee LTO UL AN |e Sa Spacey nea Shane Nh ca ch ae ae a eg AI ( pd} B[NULIOF JON)
SACOOOTRGO seeee ore,
So6coN0NS ----UTpoRy
“quad hag
‘osuByo
auINn[OA
“syonpoid
*a0anl0g
"SUOTJOBaL [BOTUTOYD
‘ponuTyUoy—sahunyo aunjow PUY SwOL Dad [avUlay) —\) ATA,
kh
TABLES. 395
TaBLE D.— Classification of alterations, with volume changes.
INDEX TO CLASSIFICATION.
Paz.
CHA DOR BINIOM ooosccacadas cenapausesenaouoqouEKeus Ke bosLooessbonuocouRseHokodasoucooesdedaad 396
Canbonatiompan dice fuori al aitio ayers seperate yes sree tapos a De 396
Car bonationan dideliy drat omen ep es er ee es erate aaa ee ae encrypt 396
Carhonationsdehydrationandidesulphation=e=-aemecsseee see ane eee eee es ee ee 396
Carbonation mdehydrationandkdesilicationise. 02 -cin0 222-55 oe oo ee ee eee naan ones 396
Canbonationkandideoxidationverrerecrer terre nists ce incre eee ne oe sere ee ee eee eee ee 396
Canbonationydeoxidationssandidelnydiatlomee sae ces ea eye eet aln ee 396
Canbonationgandkdlesilicatiom serene ete cee tere eee eee eae re ee ee ee 396
Carbonationgan dilny cation pesarseea sss rests cee aes mer aay ee pe ras ay SU rey ae (ape le 397
Carbonation hydration yandidechloridatione j=. ss-2e eee a a= oce semen me cineeess sete ee nee 398
Carbonation shy drationwandsdesilicationiees secrets asses eee see eee eee eee eee eeee 38398
Carbonation hydration sandidesulphationics=-ceseesseoe scenes nen aecte soccer eee eee noe eene 399
@arbonation shy. dration soxidationsandidesilication 2242222 snasesseee ese ee ese oes e ee eeeeee eee 399
Carponationgmydraton wand: silicatloneeene ssn ee sce see ese eseeece seca es eceesee ce eee eee ae 399
Carbonation, oxidation, dehydration, and desilication.....................-.---------------- 399
Chanveroisymmetrygandemoleculanichangebm sammy eet ser ae ete eee ae eet nen es 399
Chiloridatio neers se rete ee ee Eee ciate eas feiss o sinye ie eye a Sate else eles eielarsaie Aa eee ae ie eee ener 400
Meborationvandidecarbonatloneerssas ease eases seen reeset eee seen ase ee ee ee ee eee eeee none 400
DeCAT bona ti One rym ert ree mereer iat iar iets asia se sei nee Slate Soe CASSIE Etre Se ener eeee 400
Wecanbonationcandyiitan alone cee cctee oat eens sce ef SERIE eA eee Eee eee eae 400
Wey dina hl one er eer eer eerie eee eee eee Pe eee as emer meee nee Ree e Se See eee ree eee 400
Mehydrationandyd ecanhon atone seme een esee sees ce) eee cela eee ceeeesee teeter eens 401
OSG AUG HANGS Sef Se eS SR SE esc ORC a 401
MD CSTITCALIONE see Serna pine cis stasis ise t Sas ES ae ciliate mae ee Se See aoe eeaeeee 401
IslyChEIMNON ccoosascosesososaosce ous and ceR css uETasemedsesuabs souosoBBeedeeobsesodesacccse 402
invdrationvandid ecanbonatlonmeenaere esse ee eeeet eerie cence ene reee eee eee eee eae 402
Eby drationrandudechlondationeeses Herter eee esena eee ncicnseeeee tee een eee eee eee eee ee 403
Hydration, dechloridation, carbonation, and desilication ....-....-...-.--------------------- 403
Eby dration-dechilonridationvandydecarbonationme- mes caterees ecesereeeee eeeceine neo ee 403
Eby drationgandece Hn Orie atime eters a sels etree aie yeeaale eee eave ee alee ae elec epee jee 403;
Ely dra tioniandidesilicatloneermatneser rrr cr pecs sec ser tnt ese sae ene e tense n(n eee a eeee 404
Hydration adesilicationyandudecarbonationyssasj--seceamaseeriea ss eciciseeaeeee eeeeie nee eeene 401
is dratronuan dyOx1datlOnwrer ret eco -e eeeielse tele cece sineiace eines cist ae selene 404
iv dration woxidatlonwandudesilicationeessseeree recite aeaceinccecm aceise esas eoes eee rece 405
Eby dratloniandesilt catlo mmr yar ersciate sista()Seisaseineitale aici ee ecient heise eer era 405
Wieleoulke Gsm ooo o.ssdessdb4 eben nsbanRsaeosSaeReE Scar Seas aeboecoeoHenedsesascunacedsue 405
Oxidationmassee ea: Ba ou ORaueaEd boo bab oopRbebaoa benno ado soouD Suc Sd o4osoaacbobbodenneUodeEdaT 405
Oxidationandid ecanbonatlonmerree sete ssa [en (ers scerte sea eeeeee tesa ee cesar eine se reee 405
Oxidation decarbonationyandidesulphidation 22. s.25-s see ae esas eee ene ee eeeteeceeeceee 406
Oxidationsandsd esulphidationbessner maces ese cers sclera eee ee oe eee eee reer 406
Oxidationwhydrationwandyd ecarbonationerecrr cease eease eee asiiaeseeee eee eer eeeeee 406
Oxidation shy drationsvandidesulphidation) jssss- ese --se eee si aeesaae eee eee eeeeeeeaee 406
Oxidationvandytitanationeere eer errret cere secrisece aac erisece er caer oer erase cise 405
SUNG sed sooodasosonusemadadqanandeaddsussauusdosuRgnuobESEsenaconuaspoSSEcdoonbdausce 406
Silicationvandkdecarbonanonjsser sere reece teeters cee eice rece cescerseeecieseeiseecer sere cee 407
Silicationzandadehydrationmeseerrsmsse. se ere eee a eee ee eicineer reer ereae Cerra ee eer eree 407
Silicationsdehydrationyandidecarbonationsrr.2= a see eiacecincee aes eee ee eaeee 407
Silicationmhydration sand) decarbonation ease ssee cee ee ese ae eee en ereeaEeee=eeeee eee 408
Silicationmoxidationwand decarbonation =.= -- == see ee series eine elaine ele ee eee 408
Sulbstitutromolpoases epee yatepererete teva eee ee ee aoe ae esd eee eee eLetter vereietaleeteiie ae eee 408
SulhooUiCeMON...sosscdoossenbeabode sooauMadedco bee Pan essa sesao0 sed osdoeokosescadcecdase 408
Sulphidation, deoxidation, and carbonation.-...-.--------------------------+----------+---5-- 408
396 A TREATISE ON METAMORPHISM.
Taste D.— Classification of alterations, with volume changes—Continued.
CARBONATION.
Source. Products. ane
Per cent.
IBEU Cite A saree ce te ite Ae ee ae ere sos eee bivdromacnesite sss ssee eo eeeee nace re + 73.08
CARBONATION AND DEFLUORIDATION.
TBO Ibe ees Se eee eee oe re eee Calcite oe saree ae reece ese ae + 47.66
CARBONATION AND DEHYDRATION.
Bi Otte eee eee eee eee eee Hypersthene, sillimanite --.....--..------ — 24.68
GAUL ONLL Ce ae See eat ace enon Al biteee ace seme Fs eh AN Op se eae — 34.92
CARBONATION, DEHYDRATION, AND DESULPHATION.
Gry OSU ENA es ae ls EU ger lage ete | Calcite; ist eciln as ane cee sae eee — 50.29
CARBONATION, DEHYDRATION, AND DESILICATION.
BiG iterate ae ns Sale a ee faa NEY cy | Hpidote;ispinel quartz) soos oseceeseceeece — 14.71
Biotiteshematiteere seen eee eee Cee ener fees LON SOE ie ae ene ME eee te SUES — 18.15
Serpentine Jaseceesenieck sce eee ease | Macnesites quartz sseneeoseeeeeaceeeet + 18. 84
CARBONATION AND DEOXIDATION.
Mapnetitiers 32353252 G2 Cee eae Sideritel was soecesse cease eee anes +101. 30
CARBONATION, DEOXIDATION, AND DEHYDRATION.
|
WE OMIGe eae te ie os aes Nee nee SIG STILE ee aes Shee ee a err ae + 22.27
CARBONATION AND DESILICATION.
Almandite, melanite, and pyrope-.---.---- | Hornblende, calcite, quartz -.-.-..-------- + 24.55
Grossularites=) eee eee Se ee cee eee | Meionitesicalcitemquantzeeeeases ass c cs ee + 54. 62
Melanite (see Almandite).
Pyrope (see Almandite). |
Serpentine: os soaks ee a a Ae acetone Magnesite, brucite, quartz -....-.--------- + 13.02
Titamitenyece anne ka cy cunne Tee ke Raa bad Octahedrite;, caleite;quartz: 7-222 422. 22).2-- + 42.07
DB Xo pele tac rere ea eet I ea ee A Rutilescaleites quartz was see sess a + 39. 22
TABLES.
397
Taste D.— Classification of alterations, with volume changes—Continued.
CARBONATION AND HYDRATION.
Source.
Products.
Volume
change.
Per cent.
Al bite manorthite=e= pee e sere reese ee sess iH pistilbiteycibbsitelsse ese Hence eee eee ees +37. 14
VD Ye) es ea AREAS Wet Cche 2) 8 Oe a wElculanditecibbsitemnas-ee aaa eae ee +37. 14
DOR sen me Cees oe mee acieme tas eeee| Stillbite wor losite ys yam eee nen terrains +43. 50
Albite, anorthite, leucite.........-.-.--..- 15) oO BT ofsTh Reset epee em Shy ae ya ca eS Ce +31. 98
Albite, anorthite, leucite......-...-.--.-- Ebillipsiteeibbsiteean spear e een aera +40. 61
PAMOTLHILE Boer ea inlets cleinelseleunneee Maumontitecibbsitems-ssee heen eee +33. 65
DORR nas neem nee lace resol Sara Scolecite teh lcite pyaar sa nyse eee ree +35. 23
UNDYING el er et ee i ESTO tte oie sl te eas ha teas om tu epee ieee anya +17. 26
IBeta-spodumien cesar reer rea aae sree ne sa: IMuscovate allies eens ape ee ae meee = 5 HB
Bio tite meee sere ee eeie eee eh Gee ces iBiotite-chlonitel ase eee eee eee eae +22. 92
1) OVe severe sic aces eee nose cee mss Ehydrobiotitesss. sane eee ene ema ences + 3.80
DOE se aeeee es eiassae Societe ereiisinieizie/ae Serpentine, gibbsite, kaolin...............-- +14. 26
Fn statl Leper eee eee mae ae BRAG a enone als yates teak Speer eR ag et + 9.93
IDG Hee Hoos eee e rater Gas eee eee ees Orthoclase: 42: 2))s suey ete eae ae re —38. 57
IDG eS eee ee ceoU SoU L ECAC EE So See aoeeS Orthoclase kaolinaseese rer reer Ente oeeetene —10.58
DOS SS ou on SSE O ne ere Soe Soe eoe aeeee Oxrthoclasesmuscovite sss ss eee ne eeeee eae —12. 43
Leucite, albite, anorthite (see Albite).
IMCL OmIte eee eee sR Oe ne Sections ee iaolinscalciteseasee pe eee Pee eee Eee +35. 40
IDOE ep coussue SEB Seen po oee OS eee IMuscovitewcalcitc ms eee eer eee ater eee +29. 42
Nephelitepeass sacs seen oo seeeemsse Anal citeydiasponc sass seme sei ey ea + 5.49
1X0) a bedas Sp du ASBeE Sas aoe eS eee Amal cite wcibDsitcpere ee ae eee eee +19. 68
ID Chie tetas BABA SRG aa eee Err eee Ely dronepheclites se sse see a era as +23. 49
1D) SSeS hes SoC eS CESS oer Re eee eae eEee IMUsCOVItee Aes as se eee See ae —38. 46
Woe tee seecedce Oss at eee eae eee Miuscowate ty kaolinite as =e eae eee —16. 50
IDO) Se eee oaeber seneeSen see ees eeeEeeeee INatrolitediaspores=ne= see see eee eee eee +15. 00
IDO) 5 Aa dceuse add oudacte See eee eeee Natrolitey cil bsite mea = saa eee +24. 46
IDO) pei eoaasoncee bee ao See Re aS eEeeees ein tesa cero Linas se eerste ee eet ree! —13.00
Phlogopitersees ayes ee ene Naas | Eby drophilogopites a iee eee ree nea eee ee -+26. 89
IDO won oa SHees Rau Gas US CE EOE Eee Tale; dias porezs 22 ewe eyes ey eens aoe —18..27
TDS. Bea ec ca IiPalor gibbsite sate: meted cali Margate en puteNc = 2
Ti) O Parente commun rreuistietaea ty ANT eager | Talc, gibbsite, serpentine _._......_......-. i + 5.23
AVTOD Chee ete ae a ee ee Sayan AEA Talc, magnesite, gibbsite...........-..-..-.- +75. 91
Spodumiene met ess tee eee teste eo Muscovite:tmi croclimessse ese seen eeeerene +381. 74
ERE TAO ite memen ay set neers ae een ee ns at GS eK ha RRS oe eC pla or ye eI es =) ote)
IDO) he 6 Sa GSO Ns ree ae Taille: call cites siya ical CL pap Where all Bes echo Dl! +25. 61
398
A TREATISE ON METAMORPHISM.
Taste D.— Classification of alterations, with volume changes—Continued.
CARBONATION, HYDRATION, AND DECHLORIDATION.
Source. Products. | Nowe
Per cent.
Sodaliters sesceet cea ee eee ee ome aaa Anal cites diaspore sec nee sean ee eeeee eee eee OX), 70/
DOS ees ane harem cisisisines sie mere ees Analciteyoibbsiteseesentesee eases reese anes —10. 11
Dow sre nes ssa se ee seneaeececaesese = Hydronephelitessssssseee ree ee teeta eee aenee — 7.25
1D Yay eh seaea lions ete evi et ae pens ts ene Muscovites kaolin) Was asa ae oe eee emenie essere —37.07
Doe waka nee eee eee eee Natroliteydiasporesas se ssemenceincsemceenise ai —13. 62
1D Yaya Soe ee eter eee ri oe Sees ee Natrolite: cibbsites esses seeece eee eeeeeeeee — 6.52
CARBONATION, HYDRATION, AND DESILICATION.
Actinoliteyessecer sasce ee eeee eee eeeee IBastite: © = ).cie een eos ee ee ee eee —18. 06
DOs asses sc cee mes eeetaceee se ee eee iBastitevcalcitevquantzjasse ane enee eee eee +38. 67
UA bitelse seer sa See ee eee em ne InGubbsites quantzpesute messes mene ree + 1.58
DOP Rctss ceame erase ea eeeeec eee IReaoliniquantzi 2am eee sce tae tape ererrtert ine — 4.89
Albitesanorthiter-e---5-s-eeseeseeeee see Chabazite, gibbsite, quartz ......------------ +46. 76
ID) Olea oe eee Ree Mesolite, gibbsite, quartz......-------------- +24. 96
1D Yo) ge he oN ete ae sels See Gu Mesolite, gibbsite, quartz, calcite. .....--.---- +30. 19
An orthoclasenassemcsicrseeeeeeere sneer Gibbsites eect es ea ose se eres —68. 02
Dose se eect eeeet cess ee eee Gibbsite¥quartzeess=sscseeeeee cess eeeseeees — 3.30
DOM eecred enesiee ee see eeenaes Ria o lim panera erat eiaia acts St aicins cise See ee eee —52.19
ID}o) GacerEeeouanEecHonacouasesacccos Kea olinequantziasepsocereeeseccceaeteeeerre — 9.56
Biotite hematite a.--.c ess -scceesee eee Kpidote;quartz, diaspores--o-.2e2-eeeee ease —18. 45
Diopsid elses eeesewse esses ee eee Seeeeeee Serpentine; quartz <-m.c--eis-eeeeee = seeeeae + .44
MO 2 Soe el tS se esas eee eae Serpentine; quartz, calcite 2. 2 ss2ees-see +56. 32
WO'seiaceinceg eee eack oseeeee eases MAN Ge rel iiats eteisialla alowiadn se osisen soci seieees —30. 13
1B Yo pe ae mr ess Bey re rs arses Malcicalcitevquartzen-essses-eecee secon ase +48. 74
pid otewses=sceeseciceen eee eee eee ees Calcite, gibbsite, kaolin, limonite, quartz..---- +69. 08
Grossularitejeessceseessseccoeeese eee: Aoisite;.calcite: quartz eseessseseseeeeeeesee +40. 49
Grossularite; melanite 2.22. 22-s-<02--" Hpidotescalcite;quartzes see cess sae eee +40. 88
Grossularite, melanite, pyrope --.-------- Epidote, calcite, quartz, magnesite -...-.---- +39. 53
Horn blenders: conse ssccreses ene eee Chlorite, epidote, calcite, siderite, quartz, +25. 39
hematite.
Melanite (see Grossularite).
Orthoclase or microcline..:-.-..--------- Gibbsitey quartzes no.esereeenao seas eee eeee — 6.61
DO eteu sean aeee cise sme esos hs A OLIM esas oe meet eles net ee eee —54, 44
Dots ney gle les Ce ee ate Kaolin’iquartzie. ase a temtteael ea ecioe eee Oi
DO es Soe eae eer ous aed IMISCOVAteR quartz a \cle cee c eee a eeral aren Cre —15.58
TABLES.
Taste D.— Classification of alterations, with volume changes—Conutinued.
CARBONATION, HYDRATION, AND DESILICATION—Continued.
Source. Products. aes
Per cent.
PY TOPO esse less elejacins ice secu seceeine ciseen Amesite, magnesite, quartz -.--.-.......--.- +62. 26
Pyrope (see Grossularite).
Salita see eae se eee ee eae et re Bastitewquartsrerten ee eae sae seek mete + 1.93
IDO Gee R Saco SEOUL B SASS Ane Ee ae ae Bastitequantzscal cites eevee el ree +56. 41
VATED): OMS SS ROO S SESE SE EO See ares Calcite, gibbsite, kaolin, quartz .........-_-- +66. 22
CARBONATION, HYDRATION, AND DESULPHATION.
ieTany mite yee ey eats seid eee eno aoe ee 2 Chalbazitewoibbsitese sss aan eae — 7.46
DXG): Si es aes ee tee pene aes ene Reta Ns Natrolite, gibbsite, calcite...........-......- + 4.99
1 DYG) ek athens cue eae eRe cP oe Ore Og Stilbiteyeibbsitekcal citem= sss. aee eee eee + .46
INOselite iss de seepse een sia oe ace oi. IpNatrolite Veil bsite decane esse eee eae eee —16. 44
CARBONATION, HYDRATION, OXIDATION, AND DESILICATION.
PATCLIN OLICC Berita ntrsie ee eee ene cine ae Pek ees Sao aes pec Coe Sones SASesese tee aeee —36. 51
IDG thease occbasSoredece sees et mate | Pale; calcite; hematite; quartz. 2-4-4222 42- +20. 33
IAI GIL eisai ine erm ain re Sac ee ete ss eens ene | Chlorite, epidote, quartz, hematite ......._-- + 8.58
IDO geeaoe SOSeRBOUSS ESE EEE eee see eee | Chlorite, epidote, quartz, hematite, magnesite - +15. 43
Olivin estes seeeseeeccerece sete sece sce Serpentine, magnetite, magnesite, quartz... _- +37. 13
Salalitewmeess scene aawcoeee ces wise eee Serpentine, magnetite, calcite, quartz......-- +37. 50
1D) Oy Seda ood een ROse See ace Sena en sae Talc, magnetite, calcite, quartz ............-- +27. 88
CARBONATION, HYDRATION, AND SILICATION.
Eom blendesquantzpeeee eset ee ee Teer eee. IBIOtite kcal citeywieap ee sete Mee tera eee aaa +41.13
CARBONATION, OXIDATION, DEHYDRATION, AND DESILICATION.
AB LOUILE gegevens state eget ote mises Saye penaae DOG OWE), CENA = ses 5ascdobasuessseSeooscsas —14. 86
CHANGE OF SYMMETRY AND MOLECULAR CHANGE.
AMdalusiteese mete cn cele wesc eeseshecs Cyanite: se. 22 ae celeste earn eeacen ee —12. 03
PATrag oni tew res -aeemscceetiee- = -issesnsose Calciter: 32528 jasee sae Cee eee RES eEC ECan + 8.35
Bronzite or hypersthene--...-...-------- Anthophylitter ese ree nee se eeee ee eeee a+ 8.70
Marcasitesesenaseccmacen cece ease sees By Tite eee eyes SaR Shoe a see oaeneeoesesace — 2.98
asp. gr. hypersthene.
400
A TREATISE ON METAMORPHISM.
TasLeE D.— Classification of alterations, with volume changes—Continued.
CHLORIDATION.
Source. Products. aes:
Per cent.
MAMIE) Ge paresis Sait eo ea aay I ey Marialite se ssa sa ee eee ae ree ae ee -+10. 29
UA berehaliteyerc: icteric eee ee pe S| ee (a ova eR rome LA LE Onn ere TAS + 1.84
Nephelitey sg 5. Sees sane nace esseere SodaliteyOn geass scat ae se eee ees ee Soares +33. 14
Nephelite,haliteeaces ster ecee= saree eee lene Cows eases ake Spee eae eat cece -+-15. 64
DEBORATION AND DECARBONATION.
Nourmialin esse Aas eea tees e eee eee | Biotite ya: (eee ee Saas eo aes aes — 6.75
1D Yo yeeaieeei eres Ren ot ote SERRA ye Biotiteycibbsitesesser ees ese sees eee seas + 3.96
|
DECARBONATION.
Anorthitercalcitems-se esa eee essere Meionite cathe Seti is see eam ee eeee — 3.78
Corundums magnesite: sees e- eee eee PSS oS Uan=) aaa ae int ae AS ee Sn eer at en —29.17
DECARBONATION AND TITANATION.
Rutilevisidenitesacarececasereeecre ceases Mmeniten. ies Nee aac anton tae noes ieteeeees —34. 77
: |
DEHYDRATION.
Albite, silbsiters Sashes eal ae se enters aragonite ce a neutrals tei jie cee eee oar —18. 85
Anorthoclase; gibbsite: {22-2225 288222422 Muscowite;paraconite sas. sss) emcee eee —20. 04
Diaspore ye eee oe eee eis ee Corundum teas seenee sae nearer eee ae —28.18
Gil bsitet sstece Soe ce ee eeiee se aeereed eser Oy BSS ae ME Eps yal Sep os tee seu —61. 81
1D Yopeta wena Aye Veta sta ee ae DIASPOLO ie annie ae Cee eee eee —46. 82
Gypsum ase sa seen mee see he ane eet Anhydrite. 35-23 eees oe ae Soe syste ueeeios —37. 62
eulanditeyss en Oa aos Se ee see eee Albite...-- Hepa rae CPB ed Roe ee ee a eae —25.03
Oo es SP ake ae se es tee ar Orthoclase 2232 sj eect anes Maree —18. 44
Maumontite.acses esse asec eee eee PATA CL HS ese ante Gayo s) au Aled ss ee Taio aces — 4.30
TaAmiOnIte ps ae jo em ate me wate Neto ce EVO TMS CIC aes se ee yaa ies yates pepe ae ee —87. 78
OpalPns ean sae oes Heer ears Chert,chalcedomy2e ee steieae sees ease Cee eee
I Dee ee Ae ae ee eee aR a Quartzsite es ee ei = eee tel ache eee —22.81
Orthoclase or microcline, gibbsite... -.-.-.- Muscovite (damourite) --....-.--..------.-- —20. 81
Stilbites sae e ase eee eee ce ee eee aes IAN Dite Bz iet sees Sarre ste ispaye tte ares ecient ore —31. 67
Do esse Ses eee eee se ae Orthoclases: 22s seessscceeeee este ae ea ne —25. 66
TABLES.
401
Taste D.— Classification of alterations, with volume changes—Continued.
DEHYDRATION AND DECARBONATION.
Source. Produets. ae
Per cent.
Amal cite mean same soos ese ccetietiaceciescis Orthoclase; prehnitesae----------2---5----- —14. 09
Anorthoclase, gibbsite: -2-.2----2-52---2- ISON, OURAROMUND 5 = Saco oosooosnocesouseoEs —10. 91
pophyINteves asas-n er eecc lock ocims sees Bectolitexti <2 300 eBay ae ee eee epee —19. 48
Chalbazitemeesstsenteciggees soos. lassseee NENG KONG 3 ee Cae Seema cere Res Sees — 4.58
Wiasporessmacnesiteses=sse= -see-- +- == == FS) 08 012) Ue sce eee erm eet ees era HEAD A Fe —40. 39
Gallslostis, MEINEM = 6 = oo 555556586 cscooselseaue CLO Bese ey aie aces —60. 12
Waumi on titem=npacss-aee ease aoase eee cise Oxthoclasetprehnitessssseaa eee eee eeeeeeee —17.75
WieSsOlitee res sest ace a= cece cicee sats Prehmitesae 2 oo icemys eerste sisse eee Sacre —15. 05
INGUROWNIG) Sass oases Soccer tose Ree eee eens DO aes so Gee eeytaw Seer see ane poor er —16. 12
Orthoclase or microcline, magnesite, sider- | Biotite ._...............--.--....---------- —22. 33
ite, gibbsite.
COLE CITC Ree eee Sascmeties smemenaoesian ea Brehnitesse- see asses aes eae Se Eee —16. 66
HYDRATION, DESILICATION, AND DECARBONATION.
Bei:
Anorthoclase, calcite, hematite. .-....---- Bpidoterquantzjeeae eames sat a oesaee ere eeee —28. 30
BVO nites hematite eee ee Biotite-chlorite, epidote, quartz, diaspore---- + 1.81
Orthoclase or microcline, calcite, hema- | Epidote, quartz .......---..-.-------------- | —-33. 73
tite.
Orthoclase or microcline, magnesite, sid- | Biotite, quartz. .......--------------------- —22. 64
erite.
DEOXIDATION.
i}
elem ative mas aerial ce ate sos DAN ok Mage tite rae ai eesub te ee deal acral eyes ep ee te = 2.38
I
DESILICATION.
: | ae
Allman ditesspiyiRO pCrermeeeere see acee ees Hypersthene, spinel, quartz.-..........----- +12. 66
IEMARO) CO dan Su qua ncae us Cone oC eae E aaa leBinstatitesspinelsquantzessseereeer = eeeeeee +13. 51
RR aan Ss ar ee Perovskitequartzeseeenses eeeeeemeeeace ee + .14
|
402 A TREATISE ON METAMORPHISM.
Taste D.— Classification of alterations, with volume changes—Continued,
HYDRATION.
Source. Products. eae
Per cent.
Andalusites Ss ese hess as sees See eee Ka olineae eas Ay ee es ine as Blatant Se at — 3.15
BT) Oy repeat SE nn ee er ee Kaolin teibbsitessseseseees seen eee + 61.87
Aminydritesestceccteeceneeececoeecasare GiypSUuMiiers eset eee eels oe Te + 60.30
rAmorth ites science ace eee ace em eee Gismiondites aioe ae oe aera a yee + 52.76
Woe tases Sibits Sete oem apes esas eho mSOn ite ly =tats sn ey eyes aes eee are) te + 34. 65
TO ee coe RSS aU RE TINS [sZoisites: Kao mies =i seen ee toa a A
Anorthite=hematites ==). s--oseee seca pidotes kaolin, eibbsites:2ss2-=--242s52255- =+- 3.60
Cancrinite esate aaee ee ecene eee eee Natrolite, gibbsite, calcite: .-...........-.--- + 8.64
Corundum 2225522 Wks Se oar seats aimee DIAS POTS Su ey craiesee eee P ete ees es apart + 39. 25
TB Yo peace WU eal: Sale Nee VE (Sri o OSI eyegee eee ese ee hes ee a conde +161. 83
Cyaniteras eae senses sas ee oe --------| Rea olan Suey Bae ees eee es + 10.11
DO Bes Soe eee oe oe eae eee ete Kaolin: oil sites sso. ee a eee oer +- 84.02
VTA Gers ee eas eee Wlbiioniten t uco ist aka ality aN ec ineeRmeyias fo Se 0b 2
Tolitei(cordierite) ieee sseee eee eee | Clalkoro) \ow bits. secon ceveenaessouneeszocsce — .86
WMO cosoodgdouuonace Cet at Seeker eee |,aAtrn al Cite fate stage 5 ecre se spay ea RO etn anearis sect Se 10 74
Meroniteshematites 2 =-s-n- eee aera cee | INV CLOEy (AO TINE a ob oaeseonodeasoouuecGssa = LG
Nephelite o.oo oak ae eee zoe PhoOmsonite As sess arya oh any sey + 24. 60
IP VEO PCa sehen aes cee Noe eta eee | Malevspinel Vorbbsitews esses esa eres + 36. 84
Serpentinesih-tsseses cee see aes eee | Wiebskyite Sesct ad Sas alae ee eelae womens eae es ees
Sillimaniteng sss See sae ee Kaolin) imei es San soe ee eee eee = ee
TT) OYE SSS asthe on Shs plaudits | IVa, Gallo} oie sha eas Gace saeosusece |. —- 64.67
TAY COTE Ae ars Re Naas Or Malacon (hydrous zircon) soedaceosSobosance |} + 24.05
HYDRATION AND DECARBONATION.
Amdalusites. Sem sae soe cee ee Maled(steatite) ieee ees see setae eee — 32.37
STB) f= era are a eR RE a pe eee Talesscilb Deiter ses eee ees saan eee + 97.67
DYE Bereta are ete meyers SE ete Muscovite (damourite) .........--..--.----- Ieee 9.55
1D Yoyo be ee ins cape ale doh cl aa IMiUScowite solo DSite masa eo fo Se Bit, oe
Biotite aise Wee coms uae Gee e@inloritepes st teers ere nt ne hg e ye peat | + 22.99
G@yanites ease Sanaa cites ane See | Tale (Sheatite) pats e See IS eS Shae ce aenee — 23.12
Dole Sib aee Sesesses oases eee eee Pall cise psite eles saeco sence sse sso | +124. 71
ND Xo yates ets Her od ee ae eee es EN | Muscovite (Gamounite) se aqesecsee eee + 2.83
DO SS a oases seeps ae meaner ae lp Miuscomiteyolbbsiterse vse see oea nem | + 76.74
Muscovitessstsaenae cece eeseseeeeenes IRSerpentine cet ascent ae Saxe ee ey a eae | + 16.50
TE) OFS ape aie ee Sn eee eae eae Serpentine: jcibbsite fo. sass oe 2 eee | + 88. 44
EO yo papal a amen e tera NUD par a AHCI Fe ccinh gn ee Rene at Sead iy Re | = oaon
TABLES.
403
TaBLe D.—Classification of alterations, with volume changes—Continued.
HYDRATION AND DECARBONATION—Continued.
Source. Products. ee
| Per cent.
Ebloropiteke aa setemee eee eee ates C@hilonite gee ey SR Ss aps El aaa a + 41.02
Silliman te Meee es ayes a Se Seam 5 Oh iniViuscovites (dantourite) assy" 4aee senses | = O08
IDO Seisd Sa Gese reas Cease See ee eerie IVs covate tol losite sees) ase ery erenae |; + 58.16
AD OPER sen eae ace esis eme ye | MWANKG (SUCHNIES)) 5 os poadeadodesanocecseoodae lo S120
100) eee Shae b anes SARS ae ee erase atal coulis te seen aaah eee einen NN Sai oe | +101..09
Staurolite Reena ema Se ee seo. | Chlorite (amesite), gibbsite........-.....-.- | +103.58
HYDRATION AND DECHLORIDATION.
Sodallite betes Semen arama cios See | Thomsonite---.--- Sas SAR eye apa eI | — 6.41
Beas, AIS EEO seeds See bb =i)
HYDRATION, DECHLORIDATION, CARBONATION, AND DESILICATION.
Marialitems spasm acme cree ao saee tees IMIMSCOWIbe eq UartZ ers aero eee — 16.74
HYDRATION, DECHLORIDATION, AND DECARBONATION.
Menieliie . cbsade de see cee aes ee aes Sao limeatal ceee tree be aun ares scree aan + 7.69
HYDRATION AND DEFLUORIDATION.
(Chondrodite rte se see ee ters ra cia leSexrpentine bru cites sess e ee eee
Clin ohumiite hares e pone eater eee (6 KOVR se eases ee aD mr a eS at
JehihinhWe? oe oa cou Se aes ee Sous TESTO eeee Somes (OO ee ee a ao Ke eh ett EE
404
A TREATISE ON METAMORPHISM.
Taste D.— Classification of alterations, with volume changes—Continued.
HYDRATION AND DESILICATION.
Source. | Products. | vos
| Per cent.
YAU SIKK aa Eee io eee ms Anal citemquantzeeeeeseeeeeee tener eae eee +20. 82
dD Yoya ee Ree SR ER Gomera es Natroliteequantzisseseeoeee ase ee eee +19. 95
Albite, anorthite, orthoclase -...--------- Albite, zoisite, muscovite, quartz.......-----|-----------
Almandites pynOpes-ssseee eee meee Atphrosidenite; Quanta meses eeeree eee +50. 98
Nm onthiteaaeee nese eee eee eee Zoisites cibbsites quartzemasec sees saa — 4.58
nD Yoyo gala eR EI rere aU erecvalae ae sie, Prehnitescibbsite quantzeseaas essere alan +14. 85
Anorthites hematite snes sss =e = Epidote, gibbsite, quartz .......-.-..------- + 6.57
Amnthophyllitepeseeeese eee eee erence Bastitesou isc Neeser UN sae eeee season as +12. 09
HD Yoyo sessed a Beal See 8 cee ae ra ses Bastites quartz 22252 se ae seacct ense erse -+34. 09
Bronzite or hypersthene...-..----------- Bastite: s3.to) sits (fo aeseissee Seseocseees &+22.77
b+15. 65
ND Ya es ee ei anaealne Rincry Nn Pe eeea ters, Aupt US beBastite, quartZs os a0 Gass so eee aes | a@ +46. 87
@ummingtonitepes see eeeneetee eee ‘Bastitere sescic ese ssiae ee See ees eee eee | -+14. 20
1D Yo\e ieee eens ayn yea eae Bastiter quartz cscs ase cece ess sean | -+36. 76
Hinstatiters siete secs cnts so epeiae eee ees Serpentine Wear tema ase se see eee 44,25
FLD) OF ee oer ane ee elias oe lSerpentimes quartz) cee enema +38. 36
Orthoclase (sce Albite). | |
Prehmite tse ese ee Na ee ee ee It@hlorite\quantzisse = ee eee eee aes | + 3.27
IPARWO) lq aoaaoonsHeneesdosuoosassanosaded|beans OO sons cbsesacdsacnsounesuscopeadcacooe +56. 02
IBYoned en Rn Caen vi re are Cee Se | Serpentine, gibbsite, quartz......-.-.---.---- +81. 61
Pyrope (see Almandite). |
Senpentin essen ee ase eeeseetene es eae Brucitey quartz: = sees seas see eee + 9.82
HYDRATION AND OXIDATION.
Amith op hry lite mya ee eee aaa Male Sheri ait te wae ae eae ere ee een eter +11. 41
ID Yop sae ee Betsy eae es erie eae eee est imam Orn tre eae Se eee te rc ee retype
Bronzite or hypersthene.....----------- -||\ GUNG MEMEO. <5 25 .s5sccescaccacaceeudecee |s cheeseaeeeatteye
Yaseen eco teeter Oe pe Malcwhimonitei essen emeeeeene eee Sasser [aS Tha
IDO ped ese ease ee eee eee see ae Malcamacn etitekese see seas eee ee eae aeee | 0414.68
| | 4421.78
| ¢4-18. 20
Maomnietite tase sais em etre Heese neat Mimonibersa seco hess py Se reyes alae raee aes +64. 63
Olivine s, = eee emer ee crore tes Serpentine, magnetite............-2--------- +29. 96
aSp. gr. hypersthene.
bSp. gr. bronzite.
c Average sp. gr. bronzite and hypersthene.
TABLES.
405
TaBLE D.— Classification of alterations, with volume changes—Continued.
HYDRATION, OX
IDATION, AND DESILICATION.
Source. Products. yous
|
Per cent.
Bronzite or hypersthene-----...----.---- Serpentine wh enatiteeeese nee erse eee a— 2,21
I) OPE eee ae oe eer Bae tae isis Serpentine, hematite, quartz.....:.....-.---- a +33. 94
Eby PEnsbneneseeereccsiacencrticc ce acisdcic halesmaen etiterquantz sees sae eeaEAeee eee +12. 84
ID) OW eee te oie Aye ices att Serpentine, magnetite, quartz.............-.- +20. 24
Olivine te seen sseeeeecem scene aueet| Sersec C0 (Oyen nee eae aI Re ere bene Noe +12. 48
IDO) a doenctetoogdaa se cGce en eaee eas ene GO eS Spree ery ENO ete MRC cs em een +15. 19
HYDRATION AND SILICATION.
iormblendeyquartzss22ss-2s+225ec4-5- | Biotitevepldote eee eee ERE See eee eee —30. 05
pis se SESE
MOLECULAR DIVISION.
Spodumene (beta-spodumene) -..-.------ | OKA Tae} ClO Ganoodcooussecocecouadess + .05
OXIDATION.
imenitewee saan mcsmee ine sans ee eee] Octahedriteshematiteresees: ea eeseee ee eer +12. 20
IDO) ee a ARTS Oe et nee een | Octahedrites macnetitem=s=e-eeeeee eee eee eee +11. 07
IDG Ses GSR SOR AS oe seo cose oe eesanee | Rutileshematite ase e sees eee eee + 7.16
IDG Soogecesedon See Oooe eee Rutilewmacnetiteleeess pe eee eee eee eee eee + 6.02
Maen ctitey sarasota maccacmateaessecase FLeniatitereennc ace casa aoe ot aes + 2.44
OXIDATION AND TITANATION.
TRUS, MEAN NID os aa Sone enaoqneeosse imenitewhematiteress seer eee teeters eee = 188
OXIDATION AND DECARBONATION.
yAtmiilc Orsi Le pays reer rs sae rerctal etnies tics lvematites 36. So. ha ace cee eh ee ee oars
11) OG tet eee seers craieinises eieemiser Mapnetites sac sckoatee mace eetecnle Ss sae is ses Cee seer
iIRaran keniteaeesesen eee eeeeo= cece oasis Hematite: -i7e esse sia sincct c ceeccc coon ee nes ceeee
1000) 2b ndeke Sau H GEN co BticEe ae ame Magnetiteiosee sayssone at semioes cue aes ati el enero
Sidenitepesesce seer cee cies nce cis wiaciss Hematite: sos ssteSanssene once seme caer —49.11
1D) OPE eee Nee eis omeeis cece ssitieise Magnetite. 2.34 set aannnee ce ee oeeetaee —50, 32
a
Sp. gr. hypersthene.
406 A TREATISE ON METAMORPHISM.
TaBLe D.
OXIDATION, DECARBONATION, AND. DESULPHIDATION.
Classification of alterations, with volume changes—Continued.
Source. Products. eos
Per cent.
Sidenitesmancasiteys sashes eee Mia ome bite se se pee eg ite rate a eaten —47.14
Sidenite wpykite ere eee eee rer ledead COpsase Sa aaa Ss ew oc ae ee ee eee —46. 67
OXIDATION, HYDRATION, AND DECARBONATION.
VAM enite si ees ae ee eS le Fae clare ey MOM Ite ee as ee ree eee aes | Ae eee
Meionite: soo 2ee ace ee eee Mpidoteweib bsites=aesss ees seen e sar asee ee + 7.55
‘Parankerite see Sik West ee ee eee Mein Ora Sse et ie RR ha ae pe GD ee
Siderites sass ee ee ere ar ees ee ee GO (Aa AEE eS lee a hier ieee aay | --18. 22
Staurolite: seca ase ate seasons Muscovates(damourite) essen escent | —24. 96
DO) ie es ee Se Laas. nei Muscovite, magnetite, gibbsite -......--..--- +68. 08
DOs se eae eee oaeeece Mall CM ey et es Serta ee ee eee ce eee —44, 02
DO se Se eeet mace ee tao e one Malecib bsiterscsesee case ee eee eee eee +90. 96
OXIDATION, HYDRATION, AND DESULPHIDATION.
Marcasite take nate emai saetaeisee Thimonite ysis as ee ee eee ee ae — 0.14
Paysite yee Sesser eles Sere eee eee ee Os SEES Po Se ERS Feo ee ee uel + 2.93
Pyrrhotite nese seeeeecee ane cere ere tae eee GOn2 Laie ocean patec oak Meee se sees +24. 68
OXIDATION AND DESULPHIDATION.
IMaircasi teats seeerrstrs CO; ----=-42222---- | aahee DOP see u eel ae tee ae see eae + 1.62
SILICATION, OXIDATION, AND DECARBONATION.
|
Menthe Sees eee ee sere eee eee eee ee ee (erbanite 2 og eee eee nen Gi yee crc ae + 76.35
Dinenitescaleitesquarntziesem--e cee eee itanite; magnetite eee sae ese hesa= eee eee — 22.35
SUBSTITUTION OF BASES.
Ausite sc5- sees seule ee esses se | Eiorn blend els 2S 2s see eee eis ee eae + 4.30
Augite, siderite, magnesite. .-..-...---- == | Elornbblendejcal cite Sse na ene ese aes Seeree + 6.14
Cal cite eee eae eee eee eee Bolom terse: hw keer eee ond scapes senes et otk — 12.30
Diopsides. eee e ees esas ose ects Trem olitess. asseeetns oases eee ae + 5.68
Diopsidessmagnesitel=—- es -ees- se se cece erenvolitencal citess essen san ase eee + 10.55
Hornblendesacsecec ae eee ase eeeeee IATL DUGG 2a esse eyes ep eeats pene aes, eee ea pee — 4.13
eucite sso Chee 3) Sse ces eee eee eae eee Orthoclase;mephelitess===eeree eee — 7.59
Muscoviters teeta ancy eee eee eeeet cee Paragoniten ee asen mane see eae see eaee — 2.67
Olivinesanorthitesss essen ss eee eee Actinolitey spine lasses ae ese ee 7A iG}
Sabilitesc ceyoere tee ee ee eee aeons FACE ihe Bee ary ee ey Gee eee aN OAS
Sahlite, siderite, magnesite .......-.-.--- AG HNO, GCI soos gcescsnsccasoaslaeaose + 10.81
Spodumene presses eee eee sere err Beta-spodumiencyaaeeey ey. ar ere eae ees + 24,72
SULPHIDATION.
Pyrrhotite st ie. oiccieewe causes sees sacs IP\ AU sooo doonGEsebonasessseusossbnesoocane | + 21.18
|
SULPHIDATION, DEOXIDATION, AND CARBONATION.
Hematiten cosas sens ee ee sa ee eee eee Marcasitiegsid eritesea= hep eeeeea- nee ne sea aee + 78.78
Doe eicenasesoe wee seauspesieereaeaee itPyrite! sideritetz =. s5ceeses arcs eee + 76.12
ChAT IBIS. WAL.
THE BELT OF WEATHERING.
In Chapter IV general statements were made as to the nature of the
alterations of the belt of weathering and the relations of these to alterations
in the belt of cementation and the zone of anamorphism. The statements
were mainly from the physical-chemical point of view, and no attempt was
made to give in detail the facts upon which they were based. It is the
purpose of the present chapter, first, to consider fully the phenomena of the
belt of weathering from a geological point of view, and, second, to interpret
these phenomena in terms of the physical-chemical principles which have
been developed in the previous chapters. However, the two are not
separately considered, but are interwoven.
BELT OF WEATHERING DEFINED.
The belt of weathering has been shown to be a part of the zone of
katamorphism. (See Chapter 1V.) From a physical-chemical point of view
this belt is one in which the reactions take place with liberation of heat,
and with expansion of volume provided all the compounds formed remain
in situ.
At a variable depth below the surface of the earth ground water is a
connected body which fills all of the openings. The position of the surface
of this body of water may be called the level of ground water. Above the
level of ground water the openings are ordinarily not filled with water.
From a geological point of view the belt of weathering is the surficial belt
extending from the surface of the earth to the level of ground water. The
thickness of this belt varies greatly. At or near streams, lake, or ocean,
and in areas where the surface is not much higher than the adjacent bodies
of water, the level of ground water may reach near or to the surface, and
thus there be for these areas either a very thin upper belt or even none. In
regions of average precipitation, moderate elevation, and moderate irreg-
ularities of topography the level of ground water is usually from 3 to 30
409
410 A TREATISE ON METAMORPHISM.
meters from the surface. It is especially likely to be near the surface in
regions where there is abundant precipitation and a thick layer of drift
or a thick layer of disintegrated rocks. In elevated and irregular regions,
especially those in which the precipitation is rather small, the level of
eround water is frequently 30 to 90 meters below the surface, and in high
desert regions the level of ground water may be 800 or more meters below
the surface.
The above general statements may be illustrated by various regions of
the United States. In the humid regions of eastern United States the level
of ground water varies from 0 to 27 or 30 meters below the surface.
Throughout the greater part of the drift-covered region the sea of ground
water is penetrated at a depth of less than 15 meters, although on the
higher drift hills the level of ground water may be from 15 to 30 meters
Fic. 5.—Relations of level of ground water to topography and to surface drainage. Lines with arrows are lines of flow.
After King.
below the surface. For the disintegrated regions of the southern Appa-
lachians, water is generally reached at a depth of less than 30 meters. For
the greater part of the Coastal and Gulf plains water is ordinarily found at
a depth of less than 15 meters. In the high limestone region of Kentucky
and ‘Tennessee the level of ground water may be 60 to 90 meters below the
surface. For the major portion of the western part of the great plateaus
east of the Cordilleras the level of ground water is from 30 to 75 meters
below the surface, although adjacent to the streams it is less than this.* In
the arid regions cut by deep canyons the level of ground water may be
far below the surface. In portions of such regions it is so deep that some
authors have concluded that water wholly fails. But that this is not so is
shown by the numerous springs which issue in these regions along the
« Darton, N. H., Preliminary report on the geology and water resources of Nebraska west of the one
hundred and third meridian: Nineteenth Ann. Rept. U. S. Geol. Survey, pt. 4, 1899, pl. ex.
LEVEL OF GROUND WATER. 411
streams at the base of the canyons. For instance, in the Grand Canyon of
the Colorado many springs issue near the river and along the side canyons.
Some of these are large and are loaded with calcium carbonate.” The rule
is here, as elsewhere, that the level of ground water starts at the level
of the river and rises toward the divide. but in this arid western country,
dissected by deep, steep canyons and broken by innumerable fractures, the
level of ground water in some places is as much as 1,000 meters or more
below the surface.
While there is great variation in the depth of the level of ground water,
and ground water is found only at a considerable depth in many regions, on
the average the depth of ground water in the United States is not great—
probably in the neighborhood of 30 to 50 meters.
FORM OF LEVEL OF GROUND WATER.
From the foregoing it follows that the form and position of the level
of ground water are largely dependent upon the topography, upon the
character of the openings in the rocks, upon the precipitation, and upon
other factors. In general, the more accentuated the topography the greater
is the difference between the elevation of the surface and the level of ground
water. Where from a lowland a steep ridge rises, the level of ground
water also rises, but less rapidly and with considerable deviation between
the two. Where from a lowland there is a gentle rise, the level of ground
water also rises gently, and it more nearly corresponds with the surface
than in the case of an abrupt rise. The position of the level of ground
water is largely dependent upon precipitation. In humid regions it is apt
to be near the surface; in arid regions it is farther from the surface.
The relations of the level of ground water to topography and to surface
drainage are illustrated by fig. 5.2 Where the openings in the rocks are
numerous and large, there are much greater differences between the surface
and the level of ground water than where the openings are few and small.
For this there are two reasons: First, the precipitation in a region of
large openings more readily makes it way through the openings to the
drainage level than in a region where the openings are small, for resistance
to movement increases as the openings become small, and therefore on
«Powell, J. W., Explorations of the Colorado River of the West, 1875, p. 94.
> King, F. H., Principles and conditions of the movement of ground water: Nineteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1899, p. 99.
412 A TREATISE ON METAMORPHISM.
account of the slow movement a greater head is necessary in order that
equilibrium shall obtain in the sea of ground water between additions
from precipitation and subtractions by flowage. Second, the level of the
ground water is also dependent upon capillarity. The effect of capillarity
is to raise the water surface in the small openings of the rocks above its
natural level as found in wells. The amount of this rise is inversely
as the diameter of the openings This amount, as shown on page 151,
varies from nothing in the largest capillary tubes to 166 meters in
circular tubes of limited size between capillary and subcapillary openings,
and to 83 meters in similar sheet openings. In circular capillary openings
between 1 mm. and 0.001 mm. in diameter the rise varies from 3.32 em. to
33.2 meters, and in similar sheet openings one-half of this amount. A
comparison of the above numbers shows that in openings limiting those of
capillary and subcapillary size the rise is five times these larger amounts.
When it is remembered that in large classes of sediments, such as fine sands
and clays, the openings approach subeapillary size, or even are of subcapillary
size, it is evident that the amount which the level of ground water may be
raised as a result of capillarity may be a very important factor in determining
its position.
While the level of ground water in rocks in which the openings are
capillary or subcapillary may be greatly affected by capillarity, in the
supercapillary and larger capillary openings this is not an appreciable
factor. It follows that there may be great changes in the level of ground
water in passing from places where the openings are supercapillary to where
they are capillary, and from places where the openings are capillary to
where they are subcapillary.
Illustrations of these principles are found in all countries. In fractured
limestone regions containing caves the level of ground water may follow
approximately that of the drainage of the district, and thus there be a great
difference between the topography of the surface and that of the ground-
water level. Where aregion is covered with a thick mantle of fine material,
as drift, the topography of the ground water may very closely follow that
of the surface. Newell, F. H., Irrigation in the United States, Crowell & Co., New York, 1902, p. 27.
¢ Powell, J. W., Second annual report of the irrigation survey: Eleventh Ann. Rept. U. 8. Geol.
Survey, pt. 2, 1891, p. 27.
@ Powell, cit., p. 27.
€ Newell, F. H., Water supply for irrigation: Thirteenth Ann. Rept. U. 8S. Geol. Survey, pt. 3, 1893,
pp. 13-14.
SOURCE OF GROUND WATER. 415
This rule is well illustrated by the arid and semiarid regions, where a
very large portion of the run-off is that of the precipitation of the moun-
tain regions. Whitney estimates that in the level districts of the arid region
of the United States, even where the rainfall is as high as 50 em., only 10
per cent, or 5 cm., of the rainfall passes into the run-off.”
The cultivation of the soil is another of the secondary factors which
has an important influence in the amount of run-off. Proper soil cultivation
produces a rough surface having many minor depressions arranged in such
a way as to have the channels follow the contours. Also, cultivation
produces numerous large openings in the upper 5 to 30 em. of the soil.
As a result, when precipitation comes the depressions hold the water.
From these depressions it makes its way easily into the large openings.
The big openings give a large surface from which the water can make its
way into the smaller openings of the soil. Therefore, in cultivated soil a
much larger portion of the water makes its way into the soil than under
natural conditions. The run-off is greatly decreased and the water for
circulation in the belt of weathering is greatly increased.
It will be seen later, in considering the belt of cementation, that the
run-off includes both the water which never goes below the surface of the
ground and that which issues from the belt of cementation. Much the
larger part of the water of the belt of cementation passes through the belt
of weathering on its way to the belt of cementation. All of the water not
included in the run-off has its entire circulation in the belt of weathering.
In the United States it follows, from the figures above given, that the
entire circulation of about 50 per cent of the precipitation in the more
humid regions is confined to the belt of weathering. In the less humid
regions from 53 to 75 per cent is confined to this belt. In the semiarid
regions from 75 to 90 per cent is confined to this belt. In the arid regions
from 90 to 100 per cent of the precipitation has its only ground circulation
in the belt of weathering. It is therefore clear that the proportion of the
water of precipitation concerned in the circulation of the belt of weathering
is much greater than that concerned in the circulation of the belt of cemen-
tation; for the belt of weathering has exclusive control of from 50 to 100
per cent of the total precipitation, and nearly all of the water of the belt
of cementation first circulates in the belt of weathering.
«Whitney, Milton, Conditions in soils of the arid region: Yearbook of the Dept. of Agric., 1894,
pp. 157-159.
416 A TREATISE ON METAMORPHISM.
However, it does not follow that the water present in, the belt of
weathering at any one time is as great as the amount in the belt of cemen-
tation. Indeed, the amount of water in the belt of weathering is only a
small fraction of that in the belt of cementation. The explanation of this
is that the water contributed to the belt of weathering remains in the
belt only a short time—a few minutes, a few hours, a few days, or possibly
a few years; while much of the water of the belt of cementation remains in
that belt for many years. In the case of the deeper and longer circulations
of the latter belt, as shown on pages 585-586, much of the water must
have remained underground for centuries. Therefore, while a much larger
percentage of the water of precipitation takes part in the circulation of the
belt of weathering than in the belt of cementation, at any given time the
amount of water at work in this belt is much less than that in the belt of
cementation.
THE CIRCULATION.
It has already been seen (pp. 146-152) that the chief factors which
control the movement of the water in the belt of weathering are (1)
gravity, (2) mechanical movement, (3) molecular attraction, and (4) plant
roots. The special action of these factors in the belt of weathering needs
consideration.
The force of gravity is vertical and downward, and therefore, so far as
this force is concerned, there is a continual tendency for water to be drawn
from above the surface of the ground into and directly through the belt of
weathering to the belt of cementation. Where the different parts of the
vater interfere, as they do in the belt of cementation, there may be super-
imposed on this vertical movement very important lateral movements, as
seen on pages 572-576. But the interference due to hydrostatic pressure
comes only when the rocks are saturated. In general the rocks of the belt
of weathering are not saturated; and hence the lateral movements in this
belt due directly to gravity are unimportant, except locally and for short
periods of time.
In the belt of weathering the effect of mechanical movement controlled
by gravity is to steadily make the soils more compact, and thus to decrease
the size of the openings and so to promote capillary action. Also, to some
extent, mechanical consolidation tends to squeeze out the moisture and so
to force it to the surface or to the belt of cementation.
CIRCULATION OF GROUND WATER. 417
The influence of molecular attraction is to draw the water from areas
of more moisture to areas of less moisture. Since the water falls upon the
surface, the first tendency of this force is, like that of gravity, to draw the
water from the surface into the soil and downward. However, there are
many modifications of this general tendency. Immediately after precipi-
tation molecular attraction works very effectively with gravity in carrying
the water downward, but after rainfall has ceased, evaporation near the
surface very frequently results in the surface soil containing less moisture
than the subsoil or the deeper parts of the belt of weathering. Under these
circumstances the force of molecular attraction draws the water from below
the surface toward the surface, in opposition to gravity. Also, whenever
the belt of weathering is somewhat deficient in moisture, molecular attrac-
tion tends to draw water from below the level of ground water into the belt
of weathering. These movements are either upward or downward; but
whenever local conditions produce lateral variations in moisture, molecular
attraction tends to give the water a horizontal component from places of
more moisture to places of less moisture.
Wherever roots are present they absorb water from the immediately
adjacent soils, and carry it surfaceward. The roots extend both vertically
and laterally, and hence the movement of water due to roots is both lateral
and vertical. Where roots are present, on account of the absorption of
water the soil adjacent to the roots is deficient in amount. As soon as this
deficiency appears the force of molecular attraction carries the water
from places of more moisture to those places of less moisture, and thus
replenishes the supply.
The downward and upward movements of water controlled by the
above forces are of such consequence that they are entitled to separate
consideration.
DOWNWARD MOVEMENTS OF WATER.
As has been seen, the general forces producing downward movement
of water are gravity and molecular attraction; with these forces mechanical
movement may cooperate. The results which these forces accomplish are
very different under different circumstances. Some of the most important
of the variable circumstances affecting the downward movement of water
are the irregularities in precipitation and the amount of water in the soil
and subsoil.
MON xLvII—04 27
418 A TREATISE ON METAMORPHISM.
If precipitation comes suddenly and abundantly an upper layer of the
soil may become saturated, and this layer be separated from the belt of
saturation below by a layer in which the pore spaces are largely occupied
by air. The air below the surface layer of saturation prevents the ready
passage of the water downward, and if precipitation continues a large part
of the falling water may be prevented by the air from easily making its
way into the soil, and consequently joins the run-off. But, as explained
later, where the soil is cultivated deeply, so as to leave a considerable
percentage of large openings near the surface, and from this supply a
temporary reservoir is furnished, a considerably larger proportion of the
precipitation may get into the ground than in uncultivated areas.
The water in an upper saturated layer may make its way downward
in two ways, (1) the confined air below slowly escapes, either by upward
creep or through the rare larger passages which are not filled with water,
and the water consequently moves downward; and (2) if the soil particles
below the layer of saturation are moistened or contain water of imbibition,
the water driven by gravity and capillarity slowly creeps downward along
the surfaces of the particles.
Tf, however, the particles below the upper layer of saturation be entirely
dry, this condition exerts a strong retarding influence. Says Whitney:
“Water does not readily spread through a previously dry soil, because the
tension or contracting power of the surface of the water is greater than the
attraction of the soil grains, which tends to cause its diffusion through
the mass. One may see, therefore, a nearly saturated layer closely adjacent
a
to a perfectly dry and dusty mass.”* The explanation of this retardation
is expressed differently by Wolff, who attributes it to the elastic meniscus
at the front of the moving water.’ (See p. 141.)
The above condition of affairs is especially likely to occur in the arid
and semiarid regions, where a large part of the belt of weathering is very
dry. According to Whitney, even where the annual rainfall is as much as
50 cm., nearly the entire amount may remain within a few meters of the
aWhitney, Milton, Conditions in soils of the arid region: Yearbook of the Dept. of Agric., 1894,
p. 160. King, F. H., Principles and conditions of the movement of ground water: Nineteenth Ann.
Rept. U. S. Geol. Survey, pt. 2, 1899, p. 98. Merrill, George P., Rocks, rock-weathering, and soils,
Macmillan Co., New York, 1897, pp. 379-380.
b Wolff, H. C., The unsteady motion of viscous liquids: Trans. Wisconsin Acad. Sci., Arts, and
Letters, vol. 12, pt. 2, 1900, pp. 552-653.
CIRCULATION OF GROUND WATER. 419
surface, and separated from the belt of cementation by a belt of dry soil.
This holding of the water of precipitation near the surface, according to
Whitney, explains the greater efficiency of water in the production of crops
in the semiarid regions than in the humid regions.“
In opposition to the above, where the subsoil is moist and where the rate
of precipitation is moderate, the water may make its way downward, drawn
by gravity and by molecular attraction, without forming an upper barrier
of saturated soil which prevents the escape of the air below and the down-
ward passage of the water. Where precipitation continues long and a
considerable part of the water makes its way downward into the soil,
through the belt of weathering to the belt of cementation, locally the belt
of saturation or the level of ground water may rise almost or quite to the
surface.
The water which passes to the belt of cementation is carried down-
ward, as already explained, by gravity and molecular attraction. The
proportion of the water of the belt of weathering which on the average
passes into the belt of cementation is small. In the level arid regions the
proportion which thus passes downward may be almost zero, and even in
the humid regions only a small fraction of the water which gets into the
belt of weathering passes through this belt into the belt of cementation.
UPWARD MOVEMENTS OF WATER.
The water which enters the belt of weathering and does not join the
belt of cementation is brought to or above the surface (a) by molecular
attraction or (b) by vegetation. In either case this water is evaporated.
At times of abundant precipitation the evaporation is very small, but at
intervals between precipitation, especially at times of sunshine and in warm
climates, the water near the surface rapidly passes into the atmosphere.
The water thus brought to the surface, as already intimated, is derived
mainly from the belt of weathering, but a portion may come from the sea
of ground water.
MOLECULAR ATTRACTION.
The manner in which the water is brought to the surface by molecular
attraction, thus overcoming gravity, is fully discussed on pages 150-152.
However, the proportion of the precipitation which is thus brought to
«Whitney, cit., pp. 160-161.
420 A TREATISE ON METAMORPHISM.
the surface is very difficult to estimate. It is everywhere an important
force, but is more important in proportion as there is lack of vegetation.
In the complete desert regions this force acts alone in returning the water
to the surface. In such regions practically the entire precipitation may be
returned to the surface by capillarity. This is likely to occur where, on
account of sudden and abundant precipitation, a saturated layer forms
above a layer not saturated, and especially where the layer below is dry.
It has already been explained that under such conditions the water works
its way downward with great difficulty, and, remaining near the surface, it
is rapidly evaporated.
While much the larger part of the water returned to the surface by
molecular attraction is the water of imbibition in the belt of weathering,
a portion of the water may be derived from the belt of cementation, and
thus pass upward entirely through the belt of weathering in reaching the
surface. The effect of capillarity in raising the level of ground water above
the normal level is spoken of on page 412 and on pages 150-152. It is also
explained that, due to the attraction between the molecules of rock and
water, water is drawn along the walls of the openings of the mineral
particles beyond the height of the free surtace.
The amount of water which may be transported upward a given dis-
tance by molecular attraction is a function of the absolute amount of pore
space and the size of the openings. With a given pore space the amount
increases with fineness of subdivision to a certain limit. The amount ot
surface of the particles increases as subdivision increases. Therefore, in
finely subdivided material the wall space along which the water can creep
is very much greater than in coarse material. To illustrate, it will be
readily appreciated that the amount of water which capillarity would carry
up from the free surface through a distance of 1 meter in a coarse grained
sandstone is small as compared with that which would be transported
upward for the same distance through a soil having the same absolute
amount of pore space. But where the openings are so small as to be subeap-
illary the water adheres to the walls, and the amount of water transmitted
decreases. Thus, through a fine clay having the same pore space as an
ordinary soil, the amount of water transported upward would be less than
in the soil.
The quantity of water which may thus move upward from the belt of
saturation has been experimentally determined by King for some materials
CIRCULATION OF GROUND WATER. 421
under certain conditions. “In an experiment carried on for forty days in
coarse sands, in which the free surface of water was 15, 30, 45, 60, and 75
em. below the surface, King found that the mean evaporation per day was
0.2895, 0.282, 0.208, 0.0864, and 0.0495 em., respectively.” In the cases of
the largest and the smallest numbers, those in which the level of ground
water is 15 and 75 em. below the surface, the amount of evaporation in a year
would be 105.6675 and 18.0675 cm., respectively. “Where the movement is
vertically upward through a distance of 1 foot [30 em.] it has been found
by experiment that the rate for a fine sand was 2.37 pounds per square foot
[1.16 grams per sq. cm.] per day of twenty-four hours; when the lift was
increased to 2 feet [60 em.] the movement became 2.07 pounds [1.01 grams
per sq. cm.]; at 3 feet [90 cm.] it was 1.23 pounds [60 grams per sq. cm. ],
and at 4 feet [120 cm.] only 0.91 pounds per square foot [0.444 gram per
sq. cm.]. A similar trial with medium clay loam gave a movement of 2.05
pounds [1 gram per sq. cm.] for a lift of 1 foot [80 em], 1.62 pounds
[79 grams per sq. em.], for 2 feet [60 em.], 1 pound [0.488 gram per sq. em.]
for 3 feet [90 cm.], and but 0.9 pound [0.439 gram per sq. em.] where the
lift was 4 feet [120 cm.].”
“The observations show that it [capillary movement] is very rapid at
4 feet [120 cm.]; so rapid, indeed, that were it maintained throughout the
year it would deliver at the surface the equivalent of 63.85 inches [11.2
cu. em. per sq. em.] of water.”” With finer but equally porous soils the
amount of water thus derived from below the level of ground water may
be considerably greater.
By “capillary movement,” as used by King, is meant the process,
already described, of creep along the walls to the surface above the free
surface of ground water, not the raising of the surface in consequence of
capillarity, as that term is ordinarily used. (See pp. 150-152.)
Cultivation greatly retards the process of the upward movement of
water by molecular attraction. The upper part of the soil is in blocks
separated by supercapillary openings. ‘The continuous capillary openings
in the soils and the close contacts are broken up. The amount of wall
area upon which the water may creep upward is greatly reduced. The
conditions prevent molecular attraction from readily bringing water to the
surface, and evaporation is retarded. The intense heat from the sun during
@ King, cit., p. 92. > King, cit., p. 85.
422 A TREATISE ON METAMORPHISM.
the day mainly affects the cultivated layer. This contains but little mois-
ture, and consequently evaporation is much slower than in areas where the
capillary openings extend unbroken to the heated surface. Consequently
soils cultivated as soon after precipitation as the water has had time to
make its way to the subsoil may retain considerable amounts of water
a short distance below the surface during long periods of drought.
VEGETATION.
In proportion as vegetation is present, this becomes an important
influence working in conjunction with molecular attraction in the circula-
tion of water in the belt of weathering. In another connection it will
be shown that roots commonly permeate the soil both laterally and
vertically. Ordinary herbaceous plants frequently have roots extending
to a depth of 1 to 2 meters, and the roots of trees penetrate to a
depth of 5 to 6 meters, or even 9 to 12 meters. The lateral extent of
many plant roots is even greater than their vertical extent. In areas of
abundant vegetation the upward movement of water is much more largely
the result of plant roots than of molecular attraction. According to
Merrill, in forested areas the total amount of evaporation and transpiration
from the trees amounts to about 75 per cent of the total precipitation. In
areas covered by other vegetation the total varies from 70 to 90 per cent,
depending upon the character of the plants." If these figures be correct,
they show how important an agent the roots of plants are in the upward
transfer of ground waters to the surface. In proportion as the region
is arid the volume of the roots decreases. But even in the deserts the
amount of plant life is surprisingly great. Moreover, in deserts the roots
are often several times as extensive as the parts of the plants above
ground, so that in such regions roots are very important factors in the
circulation of the small amounts of water of the belt of weathering. It
is only in the comparatively small areas where the deserts are absolute
that roots are altogether wanting.
On the average, much the larger quantity of water brought to or above
the surface by plants is derived from the belt of weathering. But it is
clear that in many areas, and especially in low-lying areas, where the level
“Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, pp.
280-281.
CIRCULATION OF GROUND WATER. 433
of ground water is near the surface, the roots tap this water stratum and
carry water from it to the surface.
In summary, while the lateral movements of water in the belt of weath-
ering are of consequence, it is clear that the upward and downward
movements are those of dominant importance. Gravity is always at work
carrying the water downward; the roots are always at work carrying it
upward. Molecular attraction following periods of precipitation works with
gravity in carrying the water downward, and at intervals between precipi-
tation works with the roots in carrying the water surfaceward, in opposition
to gravity.
Thus, a large proportion of the water of precipitation takes a downward
journey for a certain distance into the belt of weathering, and then returns
to the surface. The chief impelling forces for the first part of the journey
are gravity and molecular attraction, and the impelling forces in the return
journey are roots and molecular attraction working against gravity. Where
vegetation is sparse or wanting, the upward movement by which water is
brought to the surface is chiefly or wholly molecular attraction.
VARIATION IN LEVEL OF GROUND WATER.
Variation in the level of ground water is of great importance in rock
alteration, since due to this a certain horizon is alternately under conditions
of the belt of weathering and under those of the belt of cementation. It
follows that the contrasting reactions of these two belts are superimposed
upon each other. Variation of the level of ground water has an exceptional
importance in connection with ore deposits. (See Chapter XIT.)
Variation in the level of ground water depends upon many factors, of
which (1) variable precipitation, seepage, and evaporation, (2) uplift and
subsidence, (3) denudation and valley filling, and (4) man, produce impor-
tant results, and (5) barometric pressure and (6) temperature produce
unimportant effects.
PRECIPITATION, SEEPAGE, AND EVAPORATION.
The surface of the ground water rises and falls (a) with climatic change
resulting in variations in precipitation, (b) with variations in precipitation
running through several years, (¢) with seasonal variations in precipitation,
and finally (d) with the cyclonic and diurnal variations in precipitation.
424 A TREATISE ON METAMORPHISM.
(a) As a result of climatic changes there may be very great variations
in precipitation in different epochs in the same region. For instance, there
is every reason to believe that in the Great Basin during Glacial time pre-
cipitation was abundant, whereas at the present time precipitation is sparse.
At the epoch of abundant precipitation there were in this region vast
fresh-water lakes, such as Bonneville and Lahontan. At this time of
humidity it is highly probable that the level of ground water was compara-
tively near the surface, while at the present time for much of the region it
is a considerable distance below the surface. Similar climatic changes
affected much of the western part of the United States and have affected
other parts of the world at various periods of geologic time.
(b) Besides these great climatic changes there are in many parts of the
the world alternating cycles of several years or a decade, during a part of
which there is more than an average precipitation, followed by other years in
which the precipitation is less than normal. At times of abundant precipi-
tation the level of ground water rises; at times of deficient precipitation it
falls. The rise and fall in the level of ground water due to such cycles
may amount to several meters. It not infrequently happens that wells sunk
during the humid part of a cycle have to be sunk deeper during the part of
the cycle in which the precipitation is small. During the closing half dozen
years of the nineteenth century California experienced the dry part of one
of these cycles. As a consequence of the deficient precipitation vegetation
very greatly suffered. During these years in southern California the level of
ground water markedly fell at various places from 5 to 10 or even 20 meters.
For instance, in the Los Angeles Basin, in the area above where the river
emerges from underground, during the years 1895-1899 the level of ground
water fell at the rate of between 0.3 and 0.4 meters per kilometer per annum
in passing upward from the source of the river. But in southern California
the fall in the level of ground water is very largely due to the influence of
man. (See pp. 427-428.) Pumping of ground water to the surface for
irrigation, and not variation in precipitation, is one of the mam causes of
this fali.
(c) The greater part of the earth has either one or two seasons of
relatively abundant precipitation, followed by one or two less wet or dry
seasons; therefore the cycle of seasonal variation is semiannual or annual.
VARIATION IN LEVEL OF GROUND WATER. 425
The variation in the level of ground water due to this cause may be excep-
tionally 2 or 3 meters, but in general is but a fraction of a meter.
(d) The cyclonic intervals of varying precipitation in the north
temperate zone are usually from three to seven days. However, in various
regions, at various times, several of the cyclonic periods may rapidly follow
one another, and give conditions of almost continuous rainfall. In certain
regions, as in the Tropics, a chief variation in precipitation is diurnal, there
being in the region of the doldrums abundant showers almost daily.
Ordinarily the variation in the level of ground water due to the cyclonic
period is but a few centimeters; but locally, and where there is nearly
continuous precipitation for a day or more, the level of ground water may
rise, due to this variation in rainfall, by amounts as great as a meter, or,
very locally, several meters.
The rise of ground water for a given amount of precipitation in any
eyele depends not only upon the quantity of the rainfall, but upon the
amount of seepage into the soil, and this depends upon many factors, For
instance, low declivity, vegetation, rough surface, and large openings are
favorable to entrance into the soil; while steep declivity, lack of vegetation,
and small openings are favorable to large run-off and evaporation. Where
there is a high percentage of seepage a comparatively small precipitation
may raise the surface of ground water a considerable amount, for the pore
space at the level of ground water may be small. In case the pore space
were 10 per cent, a fall of 1 cm. would produce a rise in ground water of
5 em., provided one-half of it reached the level of ground water through
seepage. In many districts increased seepage, due to irrigation, has mark-
edly raised the level of ground water.
In so far as the water evaporated is derived from the belt of cementa-
tion, this tends to lower the level of ground water. The amount of this
lowering, due to the upward transportation of water through molecular
attraction and vegetation, is likely to be great in proportion as the level of
eround water is near the surface. Where the level of ground water is
many meters below the surface the variation due to this cause is probably
small. The variations of the level of ground water due to evaporation are
considered in connection with precipitation and seepage; for in many
regions the abundant precipitation occurs at the same periods of the year
426 A TREATISE ON METAMORPHISM.
at which the evaporation is the most rapid. These two factors frequently
tend to neutralize each other so far as the level of ground water is
concerned, for the effect of precipitation is to raise the level of ground
water and the effect of evaporation is to lower that level.
The variations in the level of ground water due to climatic variations
in precipitation are the greatest, but the variations of the level of ground
water due to seasonal variations in precipitation, or even to cyclonic and
diurnal variations in precipitation, are far more important than might at
first be thought, for these lesser variations largely or wholly compensate
for their smaller magnitude by their much greater frequency.
UPLIFT AND SUBSIDENCE.
Vertical movements of the earth’s crust take place as a result of either
epeirogenic or orogenic movements, or the two combined. Uplift tends to
lower the ground water, and subsidence tends to raise it, im reference to a
fixed plane in the rocks. Only regions which have a considerable elevation
above the sea have the ground water far below the surface. Regions which
are near the level of the sea usually have the ground water near the surface;
but this may not apply to interior low-lying deserts.
DENUDATION AND VALLEY FILLING.
Consequent upon denudation there is a steady downward movement
of the level of ground water. This continues, unless compensated for by
uplift, until base-level is reached, when the ground-water level approxi-
mately corresponds, on the average, with the ocean. The result of denuda-
tion is continuously to transfer the upper part of the belt of cementation
into the belt of weathering, and thus to change the conditions of alteration
from those of one belt to those of the other. This steady downward migra-
tion, in reference to a fixed plane in the rocks, of the boundary between
the belt of weathering and the belt of cementation is the most important of
all the changes in the variation of the level of ground water, leading to
profound geologic and economic consequences, which are fully discussed
elsewhere. Variation in the level of ground water may also result from -
valley filling. When a valley is filled to a considerable depth, this may
result in corresponding rise in the level of ground water. If the valley
be again excavated, this may lower the ground water to its former level.
VARIATION IN LEVEL OF GROUND WATER. 427
INFLUENCE OF MAN.
Man influences the level of ground water in various ways. Probably
his most important effect upon this level is by the denudation of the forests,
by irrigation, and by cultivation. It is well known that commonly the
removal of the forests increases the run-off. It has been explained (p. 415)
that bad cultivation may increase the run-off and that good cultivation
diminishes it. But, also, cultivation increases greatly the amount of evap-
oration. All changes in the quantity of run-off, in the redistribution of the
run-off by irrigation, and in the amount of evaporation from the surface,
produce an effect upon the level of ground water. In general the effect of
cultivation is to lower the level of ground water. This is illustrated in
hundreds of localities within the area of the lake plains. In many places
where before cultivation began there were shallow lakes, marshes, or
swamps in which the ground-water level was at the surface, the ground
water is now 5 to 10 meters below the surface. For instance, in central
Wisconsin, southwestern Portage County, in the vicinity of Almond and
Bancroft, in the gravelly and sandy outwash plains bordering the Wisconsin
drift, Dr. Weidman states that in consequence of cultivation the eround-
water level has been lowered from 6 to 12 meters, and the level of water in
the ponds and lakes lowered from 3 to 4 meters, so that the area of lake
and pond waters is not more than 25 to 50 per cent of what it was fifty
years ago, when settlement of this vicinity first began. In Clark County,
in the area of thick old drift overlying sandstone and crystalline rock,
since settlement began forty years ago the level of ground water has been
lowered from 3 to 4 meters.
Man has further modified the level of ground water by drawing upon —
the sea of ground water. He has sunk innumerable wells from which
large quantities of water have been taken. This drawing of the water to
the surface lowers the level of ground water. However, the effect was
probably comparatively slight until the developments of the last century.
During that period great numbers of artesian wells were bored, from
which very large quantities of water are taken. The inevitable result of
the opening of the numerous ready passages for the ground water to
again reach the surface must be the lowering, at least to a small extent, of
the level of ground water at the feeding areas. But probably the most
important lowering of the level of ground water is in mining and irrigation
428 A TREATISE ON METAMORPHISM.
districts, where within comparatively small areas vast quantities of water
are raised to the surface. The ground-water level in many mining areas
has been locally lowered scores or even hundreds of meters. From the
Comstock lode enormous quantities of water were pumped.* From a single
iron mine in the Lake Superior region 19,000 liters per minute have been
raised for considerable periods of time. In the lead and zine district of
Missouri many small pumps located close together have lowered the level
of ground water at various places from 30 to 50 meters. Drafts upon
underground water by pumping for irrigation and for city supplies have
lowered the level of ground water to a very considerable extent in many of
the semiarid regions. An excellent illustration is furnished by southern
California, where during the last ten years the level of ground water has
been lowered at various places over extensive areas from 10 to 30 meters.
Depression of the level of ground water due to pumping rapidly
transfers a considerable horizon from the conditions of the belt of cementa-
tion to those of the belt of weathering, and thus accomplishes the same
effects which are slowly produced by the downward migration of the level
of ground water due to other causes.
BAROMETRIC PRESSURE.
King has shown that barometric pressure may slightly raise and lower
the level of ground water. The fluctuations due to this cause are, however,
usually but a fraction of a centimeter.’
TEMPERATURE.
It has already been seen that increase in temperature decreases the
viscosity of ground water. At times of increase of temperature, therefore,
the escape of the underground waters through springs and seepage is pro-
moted, and the level of ground water is lowered. At times of decrease
of temperature the escape is stayed. Also rise of temperature produces
increased pressure, due to the expansion of the gases confined in the soil
above the ground water, and King holds this to be the chief cause of
increase of flowage with increase of temperature.” However, the effect
of the temperature factor upon the level of ground water is slight, its most
important effect being slight daily oscillations.
«Lord, Eliot, Comstock mining and miners: Mon. U. 8S. Geol. Survey, vol. 4, 1883, pp. 231-232.
>King, F. H., cit., p. 76.
¢King, F. H., cit., p. 75.
VARIATION IN LEVEL OF GROUND WATER. 429
GENERAL STATEMENTS.
The variations in the level of ground water where they are consid-
erable are very slow, but in amount they are of great consequence. Of the
factors which separately, or more commonly by combination, have
produced marked variations in the level of ground water, uplift, subsidence,
denudation, valley fillmg, and the influence of man are the most important.
The lead and zine districts of the Mississippi Valley afford excellent illus-
trations of variations in the level of ground water. In the lead and zine
districts of the upper Mississippi Valley, near the close of the Glacial epoch,
there was a subsidence and valley filling to the amount of 60 to 90 meters
or more. This produced a corresponding rise in the level: of ground water,
and the transfer of a considerable belt which had been in the belt of
weathering to the belt of cementation. Possibly the same thing has hap-
pened in the lower Mississippi Valley lead and zine region. For instance,
in the Joplin district of Missouri, below the level of ground water, there
were large caves before mining began. ‘These caves may have been
formed when the level of ground water was at a lower horizon, so that the
surrounding ground was in the belt of weathering. At a time when a
horizon now in the belt of cementation was in the belt of weathering
solution was the dominant process. When the level of ground water rose,
this horizon passed into the belt of cementation and deposition and conse-
quent filling of the openings began. As a result in the Joplin district the
caves are gigantic geodes lined with great crystals of calcite, some of them
from 10 to 50 or more centimeters long. By the artificial lowering of the
level of ground water by mining processes this belt has been again trans-
ferred to the belt of weathering, and reactions characteristic of this belt
have been set up.
METAMORPHISM IN THE BELT OF WEATHERING.
VARIABLE MATERIALS AND CONDITIONS OF BELT OF WEATHERING.
Within the belt of weathering there is the greatest variety of materials
and conditions. The reactions are therefore of the most extraordinarily
complex character. In the belt of rock weathering there is the greatest
possible variety. of rocks. In fact, every known variety of rock which
exists anywhere upon the earth may be present. ‘There are in this belt
43 A TREATISE ON METAMORPHISM.
original igneous rocks of every variety, from plutonic to volcanic, including
both lavas and tuffs; rocks varying from the most dense to the most porous,
such as pumice; rocks varying from the most basic to the most acid, and
having the most variable mineralogical composition. A second class of
rocks which has even a more widespread occurrence in the belt of weath-
ering than the igneous rocks are the sedimentary rocks. These include
- mechanical sediments of all kinds, chemical and organic sediments of all
kinds, and all possible gradations between them. A third class which is
present in the belt of weathering is the metamorphosed rocks of igneous
and sedimentary origin. These rocks may have been altered either in the
zone of anamorphism or in the belt of cementation. Moreover, the amount
of alteration is variable, ranging from rocks which have been but. slightly
affected to those which have been mashed and granulated or recrystallized
throughout. Fourth, there is every variety of surficial rocks produced by
the transporting agents of wind, water, and ice. Moreover, all these four
classes of rocks are present in the belt of weathering in various stages
of alteration under the conditions of that belt, so that the products vary
from those in which the forces of weathering have produced but slight
effects to those in which they have produced final results.
Within the belt of weathering all the agents of metamorphism are
present. These include gaseous solutions, water solutions, and organic
compounds. Oxygen, carbon dioxide, water, nitrogen, and other gases are
present in variable proportions. Of these the first three are of greatest
consequence. In humid regions, at times of abundant precipitation, the
belt of weathering becomes nearly saturated with water, and these gases
may be nearly or quite excluded except as they are contained in the water
solutions. In desert regions there may be an almost complete lack of water,
even that held by imbibition being evaporated by the heat of the sun near
the surface. At such places the gases dominate. In the lower part of the
belt, in consequence of the variation of the level of ground water, is a layer
which has alternately conditions of saturation and conditions in which water,
carbon dioxide, and oxygen are together in important amounts and in various
proportions. Therefore the conditions are those ranging from saturation by
water solutions to saturation by gaseous solutions.
It follows from the above that the amount of gaseous solutions is
inversely as the amount of water solutions. When the water solutions
CONDITIONS IN BELT OF WEATHERING. 431
occupy much of the space, this leaves little space for the gases. On the
other hand, in so far as water does not occupy the openings in the rocks,
this space is taken up by the gases which may exist anywhere in the belt
of weathering. Varying amounts of water solutions and gaseous solutions
furnish favorable conditions for the gases to pass into the water solutions,
in which form they are most effective in rock alteration.
Further, in the belt of weathering there is a great variety of plant and
animal life, and consequently the complex action of organic bodies upon the
inorganic compounds. These organic forms vary in magnitude from the
smallest bacteria to large trees. Moreover, the organic bodies act both
while alive and after death. Therefore reactions of many kinds result from
them, involving gases, liquids, and solids.
Furthermore, in the belt of weathering there are great variations in
temperature and other physical conditions. The temperature varies from
far below 0° C. in polar regions to 50° C. or even 60° or 70° C. in the sun
in some of the arid regions of the Tropics. For much of the globe, as a
result of varying temperature, the surface water is alternately in the form
of liquid and of ice. Also the conditions are those of variable pressure.
Finally, there are all variations in topographic conditions. Some areas
show very slight relief, while others have the most abrupt slopes. Some
areas are near sea level, others are 3,000 or more meters above sea level.
The work in the belt of weathering is accomplished by mechanical and
chemical agencies. The mechanical work is largely that of disintegration;
the chemical work is largely that of decomposition and solution. Disinte-
gration may entirely separate the constituent minerals from one another, or
even subdivide the mineral particles. Decomposition is complete when all
of the original minerals have changed to other minerals. Solution is com-
plete only when all the original minerals are dissolved. The ultimate result
of the action of the mechanical and chemical agencies is to produce soils.
MECHANICAL WORK.
Mechanical work subdivides the rocks or disintegrates them.
Mechanical work is accomplished by the following: (1) Water, (2) ice,
(3) wind, (4) change in temperature, (5) change from water to ice, (6)
plants, and (7) animals.
452 A TREATISE ON METAMORPHISM.
WATER, ICE, AND WIND.
In the broadest sense wind, water, and ice above the surface are
included under the agents of the belt of weathering. But this subject is so
large that it is ordinarily given a separate treatment under the terms erosion,
land sculpture, or physiography. Therefore the work of these three agents
will not be here considered or even summarized. However, it may be well
to recall that these agents are not efficient in the mechanical disintegration
of the rocks except as they use rock material as their tools; that is, wind,
water, and ice are incapable of rapidly cutting the rocks. When the wind
bears sand it may do a considerable amount of cutting; when water bears
rock material varying in size from bowlders to silt it may accomplish a vast
amount of work. The same statement is true of ice. All three of these
agents, therefore, furnish their energy of motion to rock material as tools of
cutting, and thus accomplish mechanical destruction of the rocks. his
energy is indirectly derived from the sun and gravity. It should also be
said that there appears to be a limit beyond which water, wind, and ice are
incapable of finer subdivision of material. Thus Daubrée found that in
water there was a limit beyond which mechanical trituration for the hard
minerals like quartz ceased, “owing to the buoyant action of the water,
which in the form of a thin film between adjacent particles acted as a cushion
and prevented actual contact to the extent necessary for mutual abrasion.”*
Shaler appeals to the same principle in explaining the endurance of sand
grains on the seashore. He says that the grains are held apart by films of
interstitial water (this would be water of adhesion), and are therefore pre-
vented from strongly impinging against one another.” Judd says that the
small particles of the alluvial deposits of the Nile which have been trans-
ported great distances are angular, while the larger particles of the same
deposits are subangular or rounded. Judd attributes the rounding of the
larger grains to eolian rather than river work ;° but this does not lessen the
force of the argument as to there being a lower limit beyond which the
particles are not rounded by river action.
«Merrill, cit., p. 197. See Daubrée, A., Géologie expérimentale, Paris, 1879, p. 268 et seq.
bShaler, N. S., Phenomena of beach and dune sands: Bull. Geol. Soc. America, vol. 5, 1894, pp.
208-209.
eJudd, J. W., Report on a series of specimens of the deposits of the Nile Delta, obtained by the
recent boring operations: Proc. Royal Soc. London, vol. 39, 1885; pp. 213-220.
MECHANICAL WORK OF WATER. 433
Abrasion by wind, and especially by ice, may give a finer subdivision
of material than abrasion by water. For instance, it is well known that
glacial débris varies in coarseness from great bowlders to a silt so fine that
the streams issuing from the glaciers are of whitish color, and it takes many
days for the exceedingly finely divided material to settle when a stream has
passed into a lake. Chamberlin and Salisbury found, as a matter of obser-
vation, that the particles of loess of the Mississippi Valley smaller than
0.1 mm. in diameter are angular.*. This accords with the conclusion of
Daubrée that particles smaller than 0.1 mm. in diameter are not reduced
in size by their mutual trituration, since the particles of the loess have
been carried for long distances in water. However, the particles as origi-
nally formed by glacial abrasion may be very much smaller than this.
Indeed, Chamberlin and Salisbury state that only 1.3 per cent of the
particles of loess of the Mississippi Valley are as large as 0.005 mm. in
diameter, and that probably 90 per cent of the particles do not reach half
this size.’ The loess of the Rhine gave similar results, there being here
only seven-tenths of 1 per cent of material over 0.01 mm. in diameter, only
3 per cent between 0.005 and 0.01 mm., while 85 per cent is less than
0.0025 mm.’ The lower limit of size of the particles of the loess are not
given by any of these writers, but nothing is said which intimates that it
goes below the limit of visibility of the microscope.
It thus appears tolerably certain that, however fine the subdivision by
any of the mechanical agents, the finest subdivision of material is only
accomplished by chemical agents.
For instance, in the residuary soils of the driftless area of Wisconsin—
material largely subdivided by chemical agents—less than six-tenths of 1
per cent of the particles are as large as 0.005 mm. in diameter, less than 1.5
per cent are between 0.005 and 0.00285 mm., and about 98.5 per cent are
smaller than 0.00285 mm.” In this case much of the material is so finely
divided that it is stated that ‘‘since the visible proportion of exceedingly
fine particles was greatly increased with each increase of magnifying power,
“Chamberlin, T. C., and Salisbury, R. D., The driftless area of the upper Mississippi Valley:
Sixth Ann. Rept. U. 8. Geol. Survey, 1885, p. 246.
>’ Chamberlin and Salisbury, cit., pp. 279-280.
¢Chamberlin and Salisbury, cit., p. 280.
@ Chamberlin and Salisbury, cit., pp. 248-249.
MON XLVII—04 28
434
the number at the limit of vision was always great.
A TREATISE ON METAMORPHISM.
a
It thus appears that
in the case of chemical subdivision it is doubtful if the microscope used
fully resolved the material into its constituent particles.
(a)
Fic. 6.—Effect of unequal heating of the
surface of a rock. (a) shows the con-
dition of a block of uniform temper-
ature. (b) illustrates the manner in
which the upper portion of a rock
surface expands when heated above
average temperature; where the differ-
ence in» temperature is sufficiently
great, this results in the splitting off
of the upper layers. (c) illustrates the
contraction of the upper surface by
cooling below the average tempera-
ture; where the difference in temper-
ature is sufficiently great, this results
in the splitting off of the upper layers.
CHANGE IN TEMPERATURE.
Change in temperature results in the disin-
tegration of the rocks by producing differential
When the
surface is affected by any change in tempera-
expansion and contraction. rock
ture, the heat is only slowly conducted and there
is differential expansion between the outer part
of the rock and the inner part. In other words,
powerful shearing stresses are set up, as a result
of which flakes of the rock are disrupted from
the deeper located solid material (see fig. 6).
The surface temperature may be greater or
less than the average temperature of the rock,
and the proce:: i: especially effective when the
surface temperature is ‘alternately lower and
higher than the average temperature.
The expansion of different rocks per degree
centigrade varies from less than 0.000023 em. to
about 0.000046 When these
numbers are multiplied by the number of degrees
cm. per meter.
of change during any period—as, for instance,
one day—we have the increment of change in
the lengths of the rocks at the surface during the
period.
be 40° C., and the length of the rock surface be
Suppose the change for a given day to
taken as 1 meter at the mean temperature, then
its length at the coolest period will be from 0.99908 to 0.99954 em., and at
the hottest period from 1.00046 to 1.00092 cm. A few inches below the
surface it may be assumed that the temperature of the rock is near the
mean; consequently, at the coolest part of the day the surface of the rock
is shorter than the rock below, and at the hottest part of the day it is
@ Chamberlin and Salisbury, cit., p. 248.
DISINTEGRATING WORK OF HEAT. 435
longer than the rock below. When the surface is shorter than the main
mass of the rock below, if the elastic limit be surpassed tensile fracture
will take place nearly at right angles to the surface. When during the
hot part of the day the surface is longer than the average of the rock
below, powerful shearing stresses are set up parallel to the surface. If
these surpass the elastic limit of the rock, shearing rupture will take
place roughly parallel to the surface. When once a rupture is started,
it will extend until the rupture feathers out at the surface or until the flake
or spall has become long enough to permit the stresses to be relieved by
buckling. '
The surface sealing of rocks, and parting parallel to the surface
exfoliation, are attributed to differential expansion and contraction caused
by change of temperature, because the fractures are peripheral. In massive
rocks, whatever the topographic forms and shapes, the scaling produced by
temperature changes should be roughly parallel to the surface. For
instance, if there be a great dome of granite, the fractures which form
should circumscribe it (see Pl. I) as they would circumscribe a sphere,
being at any given place parallel to it. In the United States such fracturing
is well exhibited by the granite of Dunn Mountain, in North Carolina; by
the granite domes of central Missouri; and above all by the great granite
domes of the West, such as those in the Sierra Nevada. The scales are
ordinarily but a few centimeters in thickness, and are rarely more than 60
em. thick. Of course, in none of these cases does every fracture occur
precisely parallel to the surface; rather, they are inclined to it at small
angles in various ways, thus frequently intersecting. However, the corre-
spondence is, in a general way, as given, and therefore the positions of the
fractures furnish evidence of their production in consequence of shearing
stresses due to changing temperature.
Another reason for believing that the peripheral or zonal scaling and
parting are due to changing temperature is the fact that the ruptures occur
nearly parallel to the surface without reference to previous structures.
Thus it often happens that great slabs, 2 em. to 5 em. thick, of schistose or
gneissic rocks form, which are diagonal or even at right angles to the
structures.
456 A TREATISE ON METAMORPHISM.
The changes in temperature are important in proportion as the rate
of change and the amount of change are great. It is rapid change of
temperature which mainly results in differential expansion and contraction,
and therefore this is the factor of final consequence. The temperature
changes are (a) diurnal, (b) cyclonic, and (c) seasonal. The amount
of the diurnal change is considerably less than that of the cyclonic, and
that of the cyclonic change is considerably less than that of the seasonal;
but the rate of change is far greater for the diurnal than for the cyclonic,
and this is far greater than the seasonal rate. Therefore the diurnal
changes are the most important in the disintegration of rocks, the cyclonic
changes of next importance, and the seasonal changes are of least importance.
The diurnal changes range from 0° to 50° C. or more. For instance,
in Texas the diurnal changes in temperature in the shade are frequently 30°
to 40° ©.“ Branner states that in Brazil the diurnal change of temperature
from the minimum at night to the maximum in the sun frequently amounts
to 50° C., the average appearing to be not far from 40° ©.’ In some
desert regions diurnal changes even greater than this have been recorded.
Probably even these ranges are not great enough; for the sun beating upon
the surface of a rock will raise it to a higher temperature than it will a
thermometer in the air. In order to obtain the real effect of the diurnal
change of temperature, one ought to sink a thermometer in the exposed
rock surface and ascertain its range from the coolest part of the night to
the hottest part of the day.
While the diurnal range is very great, probably the diurnal change of
temperature and the resultant fracturing is limited to a few inches in depth.
The changes in temperature due to the cyclonic period are greater than
those due to the diurnal period, but the period is longer and the rate of
change is much less. The cyclonic period produces a variation of tempera-
ture which runs over several days; its effect may therefore be felt more
deeply than the diurnal range, and it is not improbable that the fractures
from a few centimeters to several meters below the surface and parallel to
the surface in many massive rocks are largely the result of the cyclonic
change of temperature. Such fractures are well exhibited in many granite
quarries. The scaling probably mainly due to the diumal and cyclonic
«Fourth Ann. Rept. Geol. Survey of Texas, 1892, p. 144.
> Branner, J. C., Decomposition of rocks in Brazil: Bull. Geol. Soc. America, vol. 7, 1896, p. 286.
PL. I
MONOGRAPH XLVII
U. 8. GEOLOGICAL SURVEY
FAIRVIEW DOME, SIERRA NEVADA, FROM THE NORTH,
ion due to changes in temperature
a result of expansion and cont
DISINTEGRATING WORK OF HEAT. 437
changes in temperature has usually been called exfoliation. Walther,
referring to the agency of the sun in producing change of temperature of
rocks, has named the process insolation, and of it he gives many instances."
The annual ranges of temperature in various parts of the United States
run from 30° C., as at San Francisco, to 67° C., as at Moorehead, Minn.
Gannett says that in the interior of Alaska the annual range is sometimes as
great as 80°C.’ Hahn agrees that the annual range of temperature in some
regions is as high as 70° to 80° C.°.. While the seasonal changes of tempera-
ture are therefore greater than the diurnal or cyclonic changes, the times of
the changes are far greater, and therefore the rates of the changes far less.
While the seasonal change undoubtedly produces some effect in disinte-
erating the rocks near the surface, probably the more important effect of
the seasonal change is at a greater depth than the diurnal or cyclonic
changes. At adepth of 1 or 2 meters, or at most several meters, the diurnal
or cyclonic effect must be obliterated. Since the seasonal effect some
meters below the surface is produced very slowly, probably this effect
is not determined by the maximum range of temperature, but by the range
of the hottest season of summer compared with the coldest season of winter,
for a long period of time is required in order to produce an effect of differ-
ential expansion more than a meter or two below the surface. If January be
compared with July, the change in temperature in the arctic regions would
be from about 45° to 60° C., the change in the temperate regions would be
from about 20° to 45° C., and the change in the tropical regions would be
from about 10° to 20° C. If the comparison were made of January and
February together with July and August together, these numbers would be
but little different. Since the annual changes of temperature are small in
the tropical regions, are much greater in the temperate regions, and are
very great in the arctic regions, it follows that, so far as the annual changes
in temperature are concerned, the effect is a minimum in the tropics and a
maximum in the arctic regions. The seasonal changes of temperature
produce an effect to a depth of 12 to 15 meters. Probably the effect which
can be discriminated from the diurnal and cyclonic changes is between 3
or 4 to 12 or 15 meters. It seems to me probable that many of the joints
aWalther, Johannes, Die Denudation in der Wiste und ihre geologische Bedeutung: Abhandl.
Math.-phys. Classe Gesell. Wiss. Leipzig, vol. 16, 1891, pp. 448-453.
> Gannett, Henry, The general geography of Alaska: Nat. Geog. Mag., vol. 12, 1901, p. 191.
¢Hahn, Julius, Handbuch der Klimatology, J. Engelhorn, Stuttgart, 1883, p. 497.
438 A TREATISE ON METAMORPHISM.
of massive rocks parallel to the surface which may be noted between these
limits are the result of seasonal changes in temperature.”
The disintegrating effect of changes of temperature is most marked in
those regions in which the precipitation is small for at least a portion of
each year. The reason for the great activity of the process during dry
periods is plain. On account of the low humidity the sun is very effective.
For the same reason the radiation is rapid at night. Consequently there
are very rapid changes of temperature, and hence very great exfoliation.
These conditions of lack of humidity are ideally illustrated in the desert
regions. In such areas it is well known that insolation is very rapid.’
However, probably insolation is not so rapid in those deserts in which there
is no precipitation as in those in which there is occasionally abundant
precipitation. The conditions for insolation are furnished by the dry periods
extending from one to several years. When the very abundant. precipita-
tion, perhaps in the form of cloud-burst, comes it transports the loosened
material away from the parent rock and leaves it bare for further insolation.
This condition of affairs is well illustrated in the desert regions of the
United States. Also insolation may be very rapid in regions which are
considered humid, provided a period of each year is dry. This is well
illustrated by Brazil, where there are alternately wet and dry seasons.
During a dry season insolation loosens the material; during the wet season
the water transports the loosened material to the lower ground, and thus
exposes the fresh rocks to msolation during the next dry season.
In regions in which the precipitation is plentifully distributed over all
parts. of the year, insolation is small, for the rocks become covered by
vegetation, which prevents the direct action of the sun. In the humid
temperate parts of the world are many regions in which insolation is at a
minimum, such as the southern Appalachians and the Olympic Mountains.
“Since the above was put in type Gilbert has said: ‘‘In many dome-like granite hills the rock
is divided into plates by curved joints approximately parallel to the surface. Some observers call the
structure exfoliation; others regard it as an original structure of the granite. Under one view the
surface forms determine the structure; under the other the structure determines the surface forms.
A study of the High Sierra of California in the summer of 1903 has led me to accept the former view,
and to believe that the forms of the parting planes are conditioned by the forms of the topography.
As to the cause of the phenomenon the following hypothesis is advanced: Formed deep within the
crust, the granite was initially subject to compressive stress, which was balanced by internal expansive
stress. As the unloading involved in subsequent denudation reduced the compressive stress, the
unbalanced expansive stress caused strains which eventually resulted in exfoliation.’’ Gilbert’s view
has much in its favor for many of the deeper joints parallel to the surface, but it does not exclude the
formation of ruptures as above described. Indeed the two causes may work together.
> Merrill, Rocks, rock-weathering, and soils, p. 283. Walther, cit., pp. 448-453.
DISINTEGRATING WORK OF HEAT. 439
Topography is also an important element in insolation. Where the
slopes are steep gravity pulls off the flakes formed, and new surfaces are
exposed to the process. Also theoretically the greater the elevation the
more the insolating effect, for the sun does not have to pass through so
much of the atmosphere and should be more effective in heating, and the
heat gained in the day should be more rapidly radiated at night. Thus
one would expect greater changes in the temperature of the rocks at high
elevations than at low elevations, but I am not aware that observations
have been made on this point.
Rocks vary greatly in their power to resist the effect of insolation.
Sandstones are refractory; limestones and shales are somewhat less so; crys-
talline rocks, especially those having a large amount of readily cleavable
feldspar, are most affected. I suspect the cause of the varying resistance
of rocks is a function of their porosity. In proportion as the rocks have
many pores, such as sandstones, the expansion and contraction is taken up
between the grains, and therefore rupture is not likely to take place.
Where, however, the rock is a solid continuous mass, with little or no pore
space, when the temperature of the surface is higher than that of the rock
below, the expansion can be taken up only by elasticity. When the stresses
accumulate beyond the elastic limit, ruptures are produced. (See p. 434.)
Frequently insolation results in burying the crystalline rocks, except
the sharpest peaks, beneath débris. Regions which well illustrate the dis-
integration largely due to change of temperature are southern California and
the peninsula of Lower California, described by Merrill* and by McGee;?
Egypt, described by Walther;*° and Brazil, described by Branner.*
Where the rocks become covered with flakes and spalls, this of course
protects the solid rock below, and thus stays the process of insolation.
Disintegration as a result of insolation can be certainly discriminated
in the low-lying regions of the Tropics, in which the temperature never falls
below 0° C. But in all areas in which the temperature frequently falls
below 0° C. the disintegrating effect of insolation is conjoint with that of
o, next considered.
freezing and thawing,
@ Merrill, George P., Sketch of Lower California: Bull. Geol. Soc. America, vol. 5, 1894, p. 499.
b McGee, W J, The formation of arkose: Science, new ser., vol. 4, 1896, pp. 962-963.
¢ Walther, cit., pp. 347-569.
@Branner, J. C., Decomposition of rocks of Brazil: Bull. Geol. Soc. America, vol. 7, 1896,
pp. 281-294.
440 A TREATISE ON METAMORPHISM.
CHANGE FROM WATER TO ICE.
When water changes to ice it expands 9 per cent. According to
Dewar, the decrease in the freezing temperature of water corresponding
to an increase of pressure of one atmosphere up to 700 atmospheres is
0.0072° C. per atmosphere. It follows from this that in order to prevent
the change from water to ice taking place at temperatures below 0° C.
very great pressures must be exerted. These pressures are shown by the
following table :
Pressures required to keep water liquid at temperatures below 0° C4. 5
Atmospheres.
—1° | 128. 888
=) ~ | 277.777
—3 416. 666
—4 555. 55
—) 694. 44
—18 to —20| 13, 000
It follows that where water is in rocks and the temperature is below
0° ©., in general the rocks are not strong enough to resist the change of
the water to ice; therefore an expansion of 9 per cent takes place. When
this expansion occurs one of two things happens. The water obtains the
additional space by pushing aside the material of the soil or rock or by
expanding into openings not fully occupied by the water. The latter is
especially likely to occur in rocks in which the water is that of imbibition
and which therefore are not saturated.
In such eases the space between the particles of the rocks, as, for
instance, grains of soil or grains of sand in a sandstone, is so great that
the expansion of freezing need not move the grains with reference to one
another. In general it may be said that, in proportion as the rocks approach
saturation, disruption is likely to oceur; in proportion as the amount
of contained water becomes small, disruption is not likely to take place.
«See Mousson, A., Einige Thatsachen betreffend das Schmelzen und Gefrieren des Wassers: Ann.
der Physik und Chemie, J. C. Poggendorff, vol. 105, Leipzig, 1858, p. 161.
DISINTEGRATING WORK OF FREEZING WATER. 44]
As shown by Buckley,* the efficiency of the change of water to ice in
disintegrating the rocks is largely dependent upon the readiness with
which the water from a saturated rock can escape. If the pore spaces are
large and continuous, as in sandstones, even where the rocks have been
recently saturated, as by a heavy rain, a large part of the water readily
and quickly escapes, so the water held long enough to freeze is mainly
that which adheres to the grains or is the water of imbibition. If the pore
spaces are not nearly full when the small amount of water is frozen
there is room for expansion without disrupting the grains, and consequently
there is very little effect in the way of disintegration. Hence a very porous
rock where drainage is easy may be comparatively little affected by
repeated freezing and thawing. On the other hand, if a rock be fine
grained, so that the pore space, while of the same volume as in a sandstone,
is very much subdivided, the water of imbibition and that of saturation
approximate to each other; the water escapes much more slowly; the pore
spaces may be nearly full for some time, and freezing is therefore much
more likely to disintegrate the rock. Such rocks are illustrated by the
chalks.
Further, in rocks which have a very small amount of pore space, and
in which the pore spaces are more or less discontinuous or are subcapillary,
so that the water is held firmly, the destructive effect of the freezing of the
small amount of water may be very great, because the pore space is full
and expansion must cause disruption.
The relation of size and continuity of pore space to disintegration by
freezing is well illustrated by the resistance of sandstones as compared with
granites and limestones. The Dunnville sandstone of Wisconsin has the
remarkably large pore space of 28 per cent. Buckley repeatedly saturated
specimens of this stone with water and immediately exposed them to tem-
peratures below 0° C., but the water so rapidly escaped that the strength
of this rock was ‘proportionally less affected by freezing and thawing than
the strong granites and limestones, having low percentages of pore space.”
In contrast with this is the behavior of Cleopatra’s Needle, a granite mon-
olith in Central Park, New York, which, after having resisted for thousands
«Buckley, E. R., Building and ornamental stones: Bull. Wisconsin Geol. and Nat. Hist. Survey
No. 4, 1898, p. 382.
bBuckley, cit., p. 382.
442 A TREATISE ON METAMORPHISM.
of years the weathering forces in Egypt, where diurnal temperature changes
are great, began so rapidly to disintegrate under conditions of freezing and
thawing that it had to be coated with paraffin to exclude water and save it
from complete destruction;® but in this case it should be remembered that
the effect of change from water to ice is superimposed upon the effect of
past centuries of insolation. Perhaps its rapid disintegration was due to
minute openings between the grains, formed by insolation, which the water
could enter. It is rather probable that stone fresh from the Egyptian
quarry would not disintegrate so rapidly.
When water is in large, deep clefts, such as joints, and freezing takes
place, the result is to widen the cracks. Furthermore, it is to be noted that
at times of alternate freezing and thawing the process is cumulative. After
the openings have been somewhat enlarged and the ice melts, the rock does
not fully return to its original position, and when freezing again occurs the
widening again takes place, and so on. Thus the process may begin with
the subcapillary openings and by repeated freezing and thawing widen them
to large openings. At each time of expansion the ice acts as a wedge, and
besides widening the cracks it may also extend them, and thus the process
of freezing and thawing enlarges cracks already formed and also extends
them.
In addition to the size and extent of the openings and the amount of
contained water, further factors affecting disintegration by freezing and
thawing are topography and latitude. In proportion as the slopes are
sharp and the elevation is great the effect is likely to be great. The dis-
rupting effect is especially strong on cliffs and mountain sides and is slight
where the topographic variation is slight. It has already been seen that
the depth to which the effect may be produced varies from nothing to a
number of meters, and thus it is confined, under the most favorable circum-
stances, to the mere outer skin of the earth. The great effectiveness of the
change from water to ice in disintegrating rocks on steep slopes is due to
the fact that the material disrupted is quickly pulled away from the parent
rock by gravity or is transported elsewhere by moving water or ice and a
new surface is exposed to the action of freezing water.
Illustrations of rapid disintegration due largely to freezing and thaw-
ing are afforded by many of the higher mountain slopes, if not by nearly
aFor discussion of relations of size of pores to destructive effect by freezing and thawing, see Buck-
ley, cit., pp. 20-25, 382-384.
DISINTEGRATING WORK OF FREEZING WATER. 445
every mountainous region of the world the peaks of which extend above
the snow line. For instance, the top of Grays Peak, in the Front Range of
Colorado, is a mass of disrupted fragments, the solid rock being nowhere
exposed. Mount Dana, in the Sierra Nevada, furnishes an illustration of
the disintegration of a schistose rock. Here also the top of the mountain is
amass of disrupted blocks, the solid rock protruding only here and there.
As to latitude, it is certain that in the lowlands of the Tropics, where
the temperature does not fall to 0° C., the effect of freezing and thawing is
nil. But in the north temperate and frigid regions where there is a long
season of alternate freezing and thawing, the effect is very great. The
rapidity of the disintegration of rocks in areas like Greenland and Spitz-
bergen is largely explained by the alternate freezing and thawing. Cushing
has described the argillites of the region of Glacial Bay as sual spain
with amazing rapidity.”
It is impracticable wholly to discriminate the above-described disinte-
erating work of freezing and thawing from that of insolation. _ Insolation
is everywhere at work, although, as has been explained, with variable
power. It is common to attribute the work of disintegration in temperate
and arctic regions wholly to freezing and thawing, but it is plain that the
disintegration in such regions is partly the result of insolation.
A further effect of freezing and thawing, fully described by Merrill, is
the movement of materials already disintegrated. The movements may
affect pebbles and bowlders as well as fine material. As the soil below a
pebble or bowlder freezes it expands upward, this being the direction of
least resistance. When the soil thaws, the fine material falls back more
readily than the bowlders, and thus leaves the pebbles and the bowlders
slightly higher than before.’
Oftentimes in saturated tough clay soils, after the surface layer of water
and soil has frozen, and thus forms a congealed sheet, as the process of
downward freezing continues the necessary space required by the expansion
in the change from water to ice is obtained by the water breaking through
the crust at very numerous points. As the water is slowly squirted up
through these pipes it congeals at the points of issue, just as it does when
«Cushing, H. P., Notes on the Muir Glacier region, Alaska, and its geology: Am. Geol., vol. 8,
1891, p. 224.
>Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, pp.
393-394.
444 A TREATISE ON METAMORPHISM.
water is frozen while confined in a deep, strong vessel. This squirting
upward at the various openings produces innumerable ice pipes. Where
a surface sheet of ice had formed before the restraining frozen sheet of soil
and water together were produced the ice pipes may raise it above the
surface. In a similar manner frequently a sheet of the congealed surface
soil and vegetation separates from the frozen material below less knit together
with roots. This is frequently very destructive to vegetation. As the
solidified surface of soil containing the roots of the various plants is pushed
upward the roots are broken and a crop of grain may thus be completely
destroyed. The ice, soil, and vegetation may be raised 5 or 10 cm. from
the surface. Thus we have a sheet of frozen material above the ground,
supported by innumerable minute pillars of ice. When first seen the
phenomena appear almost magical. All of the above phenomena are
strikingly illustrated in southern United States.
PLANTS.
The mechanical effects of plants in disintegrating rocks is mainly
accomplished by means of the roots, which first penetrate the soil and later,
at any given place, expand in size. They thus exert very considerable
pressure against the surrounding material, and universally, in the case
of soil, and frequently, in the case of rocks, push the material aside. In
some instances where the roots find lodgement in a crevice of a strong rock
they are unable to exert sufficient pressure to widen the openings, and as
the roots grow they become flat. Tree roots under such conditions may
have become so flat that their breadth is many times their thickness. How-
ever, in such cases, it is very certain that the roots exerted great pressure
against their confining walls. The plants which first take possession of the
rock surface are usually of the smaller kinds. Later larger plants appear,
and finally, plants of the largest size may obtain a foothold. The depth to
which the action of the roots extends is a function of the size of the plant.
The plants may, perhaps, be divided for the present purposes into (a) lichens,
mosses, and other small plants; (b) cacti; (¢) grass, grain, and vegetables;
and (d) shrubs and trees.
Lichens, mosses, ete—Lichens, mosses, and other small plants may take
possession of the bare rock, sending their tiny rootlets into the minute joints
or other fractures, or even into the pores between the grains. They thus
DISINTEGRATING WORK OF PLANTS. 445
exert a powerful effect in the disintegration of the rocks and help to form a
thin soil, which is gradually taken advantage of by the larger plants.
cactiCacti and similar plants may be very effective in breaking up
the rocks and producing a soil, because able to exist with a small amount
of moisture. Storer states that the lava beds of Etna are planted with a
prickly pear. The roots of the cacti soon crack the lava and in a few
years it breaks up to a sufficient depth to allow vineyards to be planted.
By the same sort of action cacti are breaking up the amygdaloidal rocks of
the pyramids of old Mexico.“ The cacti are very abundant in the semi-
arid and arid regions, and consequently in such countries produce their
most important effect.
Grasses, grains, and vegetables.—(Grasses and grains may exist on a very thin
soil, but under favorable conditions the roots penetrate to considerable
depth. Many of the grasses send their roots into the soil several meters.
The roots of the red clover are known to extend a meter into the ground;
those of barley, wheat, and oats may penetrate 2 or more meters. In
the case of corn, careful experiments have shown that for the depth of 2
meters or more the soil may be permeated with roots, so as to constitute
a remarkable tangle, fully occupying all the distance between the hills.
But as to size and extent of roots probably the most remarkable of the
plants cultivated is alfalfa. It is well known that the roots of this plant
often extend to a depth of 6 meters, and the soil is commonly a tangle of
alfalfa roots and rootlets to the depth of 8 to 5 meters. Cases are recorded
where the roots have been found in sandy soils at river banks at a depth
of 15 to 18 meters below the surface.’
Shrubs and trees—Shrubs and trees may extend their roots several or many
meters from their base, both laterally and vertically. Of the shrubs,
mesquite is one of the more remarkable. This plant in the arid regions
sends a tangle of roots several meters below the surface; indeed, the amount
of vegetable fiber below the surface is so much greater than that above the
surface that the roots are frequently dug out for fuel, as furnishing a better
supply than the growth above the surface. Second-growth black oak has
been known to extend its roots laterally more than 9 meters beyond its
«Storer, F. H., Agriculture in some of its relations with chemistry, Charles Scribner’s Sons,
New York, vol. 1, 1887, pp. 130-151.
Smith, J. G., Leguminous forage crops: Yearbook of the Dept. of Agric., 1897, p. 494.
446 A TREATISE ON METAMORPHISM.
base and 1 meter below the surface. Apple trees have been known to
send roots 14 meters from their bases. In one case a long-leaf pine in
Alabama was found to have extended its roots to a depth of 9 meters, and
at this depth the taproot had a diameter of 20 cm. It is to be presumed
that the gigantic trees of western America and the Tropics extend their
roots even deeper.
In this connection it is to be remembered that various kinds of plants
work together. For instance, in warm humid regions trees are very
numerous. Between the trees are other plants of many kinds. Thus we
are lead to the conclusion that wherever the vegetation is luxuriant above
the surface of the earth there is almost an equally luxuriant growth below
the surface. Wherever there is a cornfield, a wheat field, or a clover field,
corresponding with the plexus of stems and leaves above the ground there
is another plexus of roots below the ground almost equaling that above the
ground in extent, and pervading it throughout with roots varying in size
from the most minute hair-like rootlets to the taproots of the plants.
Wherever there is a forest the great bulk of trunks and limbs and twigs
above ground is almost paralleled below ground by the tangle of roots,
grading from the mighty taproot to the minute rootlets.
As the roots grow they thrust the soil or the rock aside. As they
decay, openings are left which are taken advantage of by downward-
moving water. Gravity tends to pull the soil or rock together again, and
thus finally close the larger openings. But in the meantime others are
made by the growing vegetation. Thus where there is abundant plant
life the rock material to the depth to which the roots extend is being
constantly moved about.
Therefore, if we would understand the entire work of the plants,
wherever we see the foliage of field or forest above the ground we must in
our mind’s eye see a nearly equal volume of roots below the surface of the
earth, prying the soil apart, splitting the rocks, and, as explained below,
acting upon them chemically.
Finally, when large plants, especially trees, fall a further mechanical
effect follows. It has already been seen that trees may extend large roots
into the ground to a depth of a number of meters, both laterally and
vertically. When a tree is overthrown by the wind, or in some other way,
a large mass of the soil, subsoil, contained bowlders, and even the solid
DISINTEGRATING WORK OF PLANTS. 447
rock, may be lifted 1, 3, or 5 meters above the surface level. When the
trees decay the inorganic material again joins the soil, producing a mound.
The material of the mound is in a much disintegrated and decomposed
state; for it has been in a position to be effectively acted upon by all forces
of weathering. How important this effect is can be appreciated only when
one travels through the original forests. In such places there are seen
almost everywhere hollows where trees have been uprooted and mounds
where the material has fallen to the surface. Where tornadoes have swept
through the forests, all the trees in their paths have been overthrown at once,
and it seems as if almost the entire mass of soil and rock to a depth ot
several meters had been upturned. This process is well illustrated in the
Lake Superior region. The paths of the tornadoes vary from 30 meters
to 2 kilometers or more in width. In traveling through the forests of this
region one may find the paths of recent tornadoes, those a few years old,
and those many years old. Where tornadoes have recently cut through
the forests the giant trees have been uprooted and thrown down in a tangled
mass like jackstraws. Where the tornadoes occurred a few years ago the
fallen trees are dead and a tangle of briers, brush, and saplings is between
the tree trunks, presenting a smooth surface above, but the whole making
an interwoven mass of live and dead vegetation, which presents all but an
impassable obstacle to travel. Still later the fallen trunks show marked
decay, and at this time the saplings have become small trees. After many
years the great tree trunks and roots are ridges of rotten wood, so decayed
that one’s foot may pass through them, and the inorganic material held by
the roots has fallen to the earth again. The saplings have become trees,
but they may be discriminated from the older trees of the adjacent forest.
In the final stage of decay one may recognize the path of a tornado by
numberless earth mounds, where the soil and rocks have been lifted up by
the roots and fallen back, but the second-growth trees can scarcely be
discriminated from those of the adjacent forest.
ANIMALS,
Burrowing animals are very numerous, and they, like the plants, have
an important mechanical effect upon the rocks in the belt of weathering.
The effect of animals is, however, somewhat different from that of plants, in
that the animals frequently move the soil in a definite direction rather than in
448 A TREATISE ON METAMORPHISM.
various indefinite directions, as do the plants. The main mechanical effect
of the burrowing animals is to bring material from below the surface to the
surface. But some animals throw the soil behind them or push it up as
they move along. The animals which are mechanically effective upon
soil are (a) earthworms, (b) ants, termites, and other insects, (c) larger
burrowing animals, and (d) man.
Barthworms—As shown by Darwin, one of the most important of all the
animals in movements of the soil, if not the most important, is the earth-
worm.” Earthworms are distributed throughout the world in great numbers.
According to Darwin there are at work in many places over 50,000 upon a
single acre of soil. The amount of material which the earthworms trans-
port to the surface of the earth in a year is very great. Darwin estimates
it as over 18 tons [16 tonneaus] per acre.” Davison states that the amount
of earth turned up by lobworms is sometimes 3,147 tons [2,832 tonneaus]
peracre.°
Ants, termites, and other insects —According to Branner, of the burrowing work
of insects that of ants and termites is most important. In tropical regions
these “are vastly more important as geologic agents than the earth-
Ӣ However, this statement should doubtless
worms of temperate regions.
be restricted to the mechanical work of turning over the soil and bringing
it to the surface. In chemical work the earthworms are probably more
important. (See p. 456.) The ant-hills of the Tropics are of great size
and incredibly numerous. Branner states that the ants of Brazil live in
large, often enormous, colonies. They excavate in the earth chambers with
galleries which radiate and anastomose in every direction, and into these
chambers and galleries they carry great quantities of leaves. One can get
some idea of the extent of these openings from the heaps of earth brought
up by the insects. These mounds are often from 15 to 30 meters long,
from 8 to 6 meters across, and from one-third to over 1 meter high, and
contain tons of earth.’
In another place Branner states in reference to the ant-hills in the
forests: “These mounds are from 3 to 14 feet [1 to 45 meters] high and
«Darwin, Charles, The formation of vegetable mould, D. Appleton & Co., New York, 1888,
pp. 1-313.
b Darwin, cit., p. 165.
¢ Davison, Charles, Work done by lobworms: Geol. Mag., new ser., vol. 8, 1891, p. 491.
ad Branner, J. C., Ants as geological agents in the Tropics: Jour. Geol., vol. 8, 1900, p. 152.
e Branner, J. C., Decomposition of rocks in Brazil: Bull. Geol. Soc. America, vol. 7, 1896, pp.
255-314.
DISINTEGRATING WORK OF ANIMALS. 449
from 10 to 30 feet [3 to 9 meters] across at the base. The new ones are
steeply conical and the old ones are rounded or flattened down by the
weather. In many places these mounds are so close together that their
bases touch each other. About the Uructi station the ant-hills are so thick
that the country looks like a field of gigantic potato hills.”"7
The termite nests, above the average surface, are said by Branner to
be from one-third to 34 meters in height, and from one-third to 3 meters in
diameter. He further says that along the upper Paraguay he has seen
“laces where the nests are so close together that one could almost walk
upon them for several hundred yards [one-half a kilometer] at a time, while
over many acres of ground no one of the nests was more than 10 feet
”> Branner further states that innumerable anas-
[3 meters] from another.
tomosing galleries are made underground from a depth up to 8 meters or
more. ‘The underground galleries of the sattbas penetrate the soil to
great distances. These ants are very injurious to vegetation, and one of
the methods used by the planters to kill them is to blow poisonous fumes
into their burrows. I have seen these fumes, blown into one of these
openings, issue several hundred, even 1,060, feet [312 meters] away.” °
Even in the temperate regions in favorable locations the number of
ant-hills on an acre is very great, and each hill may vary from one to
several feet in diameter, and from a few centimeters to a meter in height.
Shaler calculates that the ants in a certain field in Massachusetts transfer
annually half a centimeter of material from the subsoil to the surface. He
explains the freedom from pebbles of certain sandy soils of New England
resting upon a subsoil containing pebbles as due to the upward transporta-
tion of the soil by the ants in making their mounds.*
Besides ants and termites there are many other small burrowing
animals, such as beetles and wasps, but these are comparatively unim-
portant.
The larger burrowing animals—'The larger burrowing animals are very numer-
ous. Of these some of the more important are the prairie dog, rabbit,
mole, badger, woodchuck, gopher, and ground squirrel. In many regions
« Branner, J. C., Ants as geologic agents in the Tropics, cit., p. 151.
> Branner, J. C., Decomposition of rocks in Brazil, cit., p. 299.
¢ Branner, J. C., Decomposition of rocks in Brazil, cit., p. 296.
“Shaler, N. 8., The origin and nature of soils: Twelfth Ann. Rept. U. S. Geol. Survey, pt. 1,
1891, p. 280. Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897,
pp. 389-390.
MON XLVII—04 29
450 A TREATISE ON METAMORPHISM.
of the West, especially in the great plateaus, colonies of prairie dogs
occupy areas extending each from less than an acre to many acres,
Throughout one of their towns are burrows and corresponding hillocks
in every direction, at intervals from less than a meter to a few meters.
Different varieties of gophers and ground squirrels are very widespread.
They occur from the Atlantic to the Pacific. In some areas in the West
their burrows and mounds are only less numerous than those of the prairie
dogs in the dog towns. The mole is scarcely less effective than the
prairie dog and ground squirrel. In many areas the ridges produced by
his work as he makes his way underground are everywhere intersecting.
He works alike in field and in forest; in the South and in the North; in
the lowland and high up in the mountains under the snow. In the forests
of the mountains, just after the snow has melted, the intersecting ridges are
often so thick that a person can scarcely step without crushing one of them.
The rabbits in Australia have multiplied in an amazing way. Their
burrows are innumerable. In Cape Colony there is said to be almost
constant movement of the ground due to the various animals. In various
parts of the West, especially in the semiarid regions, the ground has been
so penetrated with burrowing animals that in riding rapidly great care
must be taken lest one’s horse break through the crust into the holes.
Blake says that in Tulare Valley, California, ‘mules often break through
the thin crust and sink to their shoulders in these holes.” “
man.—Finally, mari is the most important of all the mechanical organic
agents. He has stripped the soil of its protective vegetation, including
both grasses and forests, for large parts of the world. As a consequence of
this the transporting power of running water has been multiplied for these
areas many fold. Wherever there are fields the rivulets run turbid to the
brooks. Not only is the denuded soil transported seaward with many fold
the speed of natural conditions, but in many fields where cultivation is care-
lessly carried on or is neglected there form gulleys and deep ravines which
cut through the subsoil or to the very bottom of the belt of disintegration,
or even into the solid rock. A plexus of ravines once formed rapidly
deepens and extends, and thus carries on the process of transportation with
ever-accelerating speed.
@ Merrill, cit., p. 394.
DISINTEGRATING WORK OF ANIMALS. 451
Marsh,” Shaler,’ Merrill,° and others have strongly emphasized the
ravages upon the soil as the result of man’s work. They show how even
from man’s point of view the present careless methods of cultivation will in
the future work to his disaster. However, for the present purpose I wish
to emphasize the fact that as the soil is transported to the streams and to
the ocean new material is exposed to the forces of disintegration and decom-
position, and thus the process of weathering is accelerated far beyond the
speed under natural conditions.
General statements—]n conclusion, we see that the animals, from the earth-
worm, ant, and termite to the larger burrowing animals, throughout all parts
of the earth which are thickly inhabited by animals, are constantly working
over the soil to a considerable depth. By the work of these animals there
is a constant migration or movement of material from the soil or subsoil to
the surface, where the material is directly exposed to all the weathering
agencies. Man has denuded a large part of the surface of the earth of its
protecting vegetation. Thus, as a consequence of the work of all the
animals, the soilis more ready to be caught up by running water and trans-
ported to the streams. This has other far-reaching consequences in furnish-
ing increased amounts of sediments and salts to the sea, and consequently
greatly accelerating the speed of upbuilding of the sedimentary rocks.
_ Hence animals perform a very important mechanical function in weathering
the rocks, in promoting their transportation, and in exposing new surfaces
to the weathering forces; and consequently they greatly accelerate denuda-
tion and deposition.
CHEMICAL WORK.
Chemical work is accomplished by all the agents of metamorphism,
viz, (1) plants, (2) animals, (3) water solutions, and (4) gaseous solutions.
These agents will first be considered and then their joint work.
“Marsh, George P., Man and nature, or physical geography as modified by human action, Chas.
Scribner & Co, New York, 1869, pp. 576-577.
Shaler, N. 8., The origin and nature of soils: Twelfth Ann. Rept. U.S. Geol. Survey, pt. 1, 1891,
pp. 329-339. :
¢Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, pp.
396-398.
452 A TREATISE ON METAMORPHISM.
THE AGENTS.
PLANTS.
Chemical work is done by plants both while alive and when dead.
PLANTS, ALIVE.
The most important chemical action by the nonbacterial plants is that
of the abstraction of carbon dioxide from the atmosphere and its reduction
and combination with other elements so as to produce the varicas vegetable
tissues. The amount of carbon dioxide in the air is exceedingly small, not
more than from about 2.76 to 3 volumes in 10,000. Therefore the air in the
belt of weathering where plants do not exist can not be presumed to contain
more than this proportion of carbon dioxide, except for the very small
amount which is brought down by the rain, estimated by Branner in Brazil
to be 0.0065 in 1,000 parts by weight.“ However, by the action of the
chlorophyll-bearing plants, which include all of the large and many of the
small plants, and by the action of the ‘‘red, brown, and blue-green algze,”
and by one class of bacteria,’ carbon dioxide of the atmosphere is reduced
and the carbon is concentrated in the plants mainly as cellulose (nC;H,.O;)
or woody fiber. This contains 44.4 per cent of carbon. The cellulose
produces no marked geological effect so long as the plants are alive; but
when dead and acted upon by bacteria, oxygen, and moisture the cellulose
furnishes a number of active acids which produce very important geological
results. (See pp. 461-465.)
Plants contain combined nitrogen, but the great majority of plants are
unable to obtain their nitrogen from the air; they must use that which is
already in a combined state in the soil. A large part of this combined
nitrogen is that furnished by the decay of earlier plants and animals. (See
pp. 465-466.) But some combined nitrogen is brought to the soil from
the air. For instance, in manufacturing, combined nitrogen often escapes
in the form of ammonia, and ammonia may also evaporate from the ocean.
But this combined nitrogen was derived from earlier plants or animals, and
therefore we have as yet no original source for combined nitrogen.
But it is well known that the nitrogen of the air is to some extent
combined with other elements by electric discharge, and thus we have an
a Branner, J. C., Decomposition of rocks in Brazil, cit., p. 305.
» Fisher, Alfred, The structure and functions of bacteria, translated by A. Coppen Jones, Claren-
don Press, Oxford, 1900, p. 107.
CHEMICAL WORK OF PLANTS. A538
original source of combined nitrogen. The combined nitrogen of the air
may possibly be absorbed directly from the air by plants, but it is mainly
useful when brought down by rain, snow, dew, fog, ete. However, the
amount of nitrogen precipitated from the air, including both that which is
secondary to plants and animals and that which is originally produced, is
very small, only 1 or 2 kilograms per annum. per acre."
We have therefore yet to account for the main original source of
combined nitrogen. Recent discoveries have shown that there are certain
bacteria that are capable of utilizing the nitrogen of the air. The Legumi-
nose, such as peas, beans, etc., form numerous nodules upon their roots,
which become the hosts of these bacteria,’ and finally abstract the nitrogen
fixed by the bacteria. Thus the bacteria abstract and the leguminous
plants store nitrogen compounds in the fruits, stalks, and roots. Experi-
ments show that where leguminous plants are grown and turned under, the
gain in nitrogen for the soil is rapid and the accumulation of combined
nitrogen great; and this gain is certainly largely due to the capacity of the
bacteria and leguminous plants together to use the free nitrogen of the
atmosphere.
Certain bacteria are able to fix free nitrogen without the assistance of
other living plants... Whether the plants obtain their nitrogen from the
nitrates already in the soil, or, like the leguminous plants, help to produce
the combined nitrogen, the material is mainly built into albuminous
compounds or proteids.
From the foregoing it follows that there are two original sources of
combined nitrogen in the soil—that combined with other elements through
the agency of plants, and that combined with other elements through the
agency of electricity. Material from each of these sources is built into the
body of the plants, as already seen, mainly in the form of proteids. The
material so long as it is in this form has no importaut geological effect, but
when plants decay these compounds become the source of the nitrogen
acids and salts which are important geological agents. (See pp. 465-466.)
Besides concentrating combined carbon and nitrogen in the belt of
weathering, live plants have a direct chemical effect upon the rocks. It is
« Aikman, ©. 1., Manures and manuring, Wm. Blackwood & Sons, London, 1894, p. 119.
» Fischer, Alfred, The structure and functions of bacteria, trans. by A. C. Jones, Clarendon Press,
Oxford, 1900, pp. 88-97.
¢ Aikman, cit., pp. 96-97.
454 A TREATISE ON METAMORPHISM.
well known that plants abstract certain ingredients from the soil, and there-
fore exert a soluble effect. The imorganic materials which are most abun-
dantly abstracted are potash, soda, magnesia, lime, phosphoric acid, and
silica. The relative proportions of these substances in grain and straw, as
given by Johnson, are as follows: “ ;
Grain. | Straw.
» — = |
Potas hitc ss ob aia lau ee ee 18.5 | 12.0
Sod appa aera ed ce ales erie | 335 8) | 4.6
Magnesia {22a eae eaeee asec 7.0 | 3.0
Tein Se Saee ee ere Senet Phil | (ee
Rhosphoricicid@esss sere ee eeeeeeeaee 32.4] 6.0
[price ses soem an tei s eters Oe ment 31.1] 59.7
Besides these substances, the plants also abstract such compounds as
iron, sulphur, and chlorine in small amounts. The sulphur is absorbed as
a sulphate.
Finally, living plants have a further effect upon the rocks. Fresh roots
give acid reactions, and as a consequence, where they are in contact with
the rocks they corrode them chemically. This is illustrated by the action
of lichens and creepers and by the corroded surfaces of rocks where roots
are in contact with them. In some cases where the rocks are rather readily
soluble, as, for instance, limestone, actual furrows may be produced in the
rocks as a result of the solution.’ In places where lichens occur the rocks
may be softened to a depth of 2 to 4 centimeters.’ Johnson states that “on
the Schwalbenstein, near Glatz, in Silesia, at a height of 4,500 feet [1,368
meters], the granite is disintegrated under a covering of lichens, the feldspar
being converted into kaolin or washed away, only the grains of quartz and
mica remaining unaltered.” @
Positive evidence that the chemical action of the plants is important is
furnished by experiments showing that soils upon which plants have been
grown are more soluble after all the parts of the plants have been removed
than are similar soils in which no plants have been grown.’
a@ Johnson, S. W., How crops feed, Orange Judd & Co., New York, 1870, p. 364.
>Storer, F. H., Agriculture in some of its relations with chemistry, Charles Scribner’s Sons,
New York, vol. 1, 1887, pp. 187-188.
¢ Storer, cit., pp. 130, 131.
@ Johnson, cit., p. 142.
eStorer, cit., p. 186.
CHEMICAL WORK OF PLANTS. 455
PLANTS, DEAD, AND BACTERIA.
Thus far the work of plants during their upbuilding has been consid-
ered. However, there is a class of plants the chief geological work of
most of which is the destruction of other plants and animals. Of these
the bacteria are by far the most important. The number of bacteria in
the soil is almost imeredibly great. “Ordinary earth may yield anything
from 10,000 to 5,000,000 per gram; whilst from polluted soil even
100,000,000 per gram have been estimated.”* Certain authors include
some of the .fungi, and especially the molds and yeasts, under plants
which assist in the destruction of other plants and animals. This assistance
seems to be in the way of absorbing compounds of other plants and ani-
mals, and building them into new forms which may be more readily
attacked by the bacteria. Certain it is that the process of oxidation, both
of the chlorophyll-bearing plants and of the fungi, is a bio-chemical one,
the main organic agents in which are the bacteria. In this matter the
bacteria are one of the most potent factors of all the organic geological
agents. Since this process of oxidation of plants and arfimals is mainly
accomplished after death by the living bacteria, the geological work of the
bacteria is mainly considered in connection with dead plants and dead
animals.
When plants die they fall to the earth, and the part above the surface
may become buried to a greater or less extent in the soil. The roots of the
plants below the surface, nearly as great in mass as the parts above the
soil, are buried to a depth, as already seen, from a few centimeters to 10
meters or more. After this material dies it decomposes or is oxidized.
The oxidation of the carbon, nitrogen, and hydrogen of plants is the
conjoint result of bacteria, water, and oxygen. The process of the decom-
position of the plants might go on in the presence of moisture and oxygen
even if bacteria were not present. However, experiments show that the
process of oxidation would be very slow indeed if it were not for the
oxidizing bacteria. We have already seen that the bacteria are present
in the soil in enormous numbers. The oxidizing bacteria act in different
ways. They oxidize the carbon, and with water convert it into carbonic
acid; and they oxidize the combined nitrogen of plants into ammonia,
nitrous acid, and finally into nitric acid.
@Newman, George, Bacteria, Puttfam’s Sons, New York, 1899, p. 137.
456 A TREATISE ON METAMORPHISM.
Bacteria, water, and oxygen decompose the iron and other sulphides,
They further oxidize the iron of carbonates. While oxidation is the rule for
the belt of weathering, under exceptional conditions bacteria, moisture, and
vegetation, or the two latter, may deoxidize instead of oxidize compounds,
and this may result in the reduction of ferric salts and in denitrification.
These processes of oxidation and deoxidation are aully considered on
pages 461—473.*
ANIMALS.
The chemical work of animals is accomplished while alive and while
dead the same as that of the plants.
ANIMALS, ALIVE.
In chemical work probably the most important of the animals while alive
are the earthworms. It has already been seen how great is the number of
these animals, and how great is the mechanical work which they accom-
plish. These animals differ from other animals in that they pass the soil
directly through their alimentary canals, so that the active compounds of
the body act upon and decompose the inorganic materials. It is impossible
to estimate the effect of this repeated passage of the soil through the
alimentary canals of the earthworms, but it can not be doubted that it
results in dissolving a vast quantity of earth materials and in rendering
undissolved parts of the materials more soluble.
The ants and termites carry a large amount of vegetable material from
above the surface of the ground to below the surface, and thus introduce it
into the soil, so that when it decomposes the products formed will be most
effective in their chemical action. Branner states: ‘The quantities of vege-
table matter carried into their burrows is almost beyond belief. I have
seen a full-grown orange tree completely stripped of its foliage within a
few hours. In the coffee regions the damage done by them is so serious
that the Brazilian Government at one time offered a large premium for a
successful formicida or ant exterminator.””
A second effect of living animals comes from their constant excreta,
Ay mere consist in lange part of active chemical compounds, of which perhaps
«See Branner, J. C Gexcens ae the econ of rocks: Am. Jour. Sci., 4th ser., vol. 3,
1897, pp. 438-442.
> Branner, J. C., Decomposition of rocks in Brazil: Bull. Geol. Soe. America, vol. 7, 1896, p. 297.
CHEMICAL WORK OF ANIMALS. 457
the most important is uric acid. All over the world, above ground and
Pp ) g
below ground, excreta of all kinds are being added to the soil, or intro-
duced by the burrowing animals below its surface, and the chemical effect
) g )
upon the inorganic material of the soil can not but be very great.
Man has promoted chemical work in various ways. It has been
pointed out that cultivation exposes the soil to the transportational agencies,
and this continually exposes new material to chemical agents. Crops are
planted and grown. When the crop matures it is removed, and the soil is
again free from its active living plants. Thus there is continual alternation
fo} foo)
between abundant and sparse plant life, and consequently conditions very
favorable for chemical reactions. Further, it is explained on pages 463-465
that man has oxidized great quantities of the organic material entombed in
the earth during past geological ages, and thus increased the amount of the
active chemical agent, carbon dioxide, in the atmosphere; and this of course
has accelerated to an unknown extent plant erowth and chemical action.
2 >
ANIMALS, DEAD, AND BACTERIA.
Animals die both below and above ground. When dead their bodies
decompose and ultimately produce water, carbon dioxide, nitrates, some
free hydrogen and free nitrogen, and some sulphates. This process of
change to the ultimate products is accomplished by the bacteria, fungi,
oxygen, and water, precisely the same as in the plants. The process is
one of oxidation and is considered under that heading. (See pp. 461-466.)
As with the plants, the process concentrates carbon dioxide, nitrates, and
other compounds in the belt of weathering, thus placing important chemical
agents in a very favorable position to do active geological work.
WORK OF SOLUTIONS.
It has already been explained that in the belt of weathering there are
gaseous solutions, water solutions, and various mixtures of these which are
at work upon the rocks. The very important gases are oxygen and carbon
dioxide. It has been pointed out (pp. 76-81) that the activity of water
solutions is dependent upon (a) the compounds present, (b) the temperature,
and (c) the pressure.
(a) The water solutions are likely to contain considerable amounts of
the salts formed by the union of the bases and acids which ordinarily occur
458 A TREATISE ON METAMORPHISM.
in rocks. In the ground water they are ordinarily composed of com-
pounds consisting of the bases sodium, potassium, calcium, magnesium,
iron, and aluminum, and the acids, carbonic, hydrochloric, nitric, hydro-
sulphuric, sulphuric, and phosphoric, and also colloidal silicic acid. Nitrous,
sulphurous, and organic acids are also present. These bases and acids unite
in various ways to produce many salts, the majority of which are, however,
simple salts, such as the carbonates of the alkalies and alkaline earths.
Of these salts the dominant ones are those of sodium, potassium,
calcium, and magnesium in the form of carbonates, sulphates, and chlo-
rides. Furthermore, there are present other bases and acids which are
omitted because of their subordinate quantity. For instance, in some
cases various salts of manganese, copper, silver, gold, ete., are present,
but the quantities of such compounds are so minute as to have no sig-
nificance except in connection with ore deposits.
The water, moreover, contains oxygen, carbon dioxide, and other
gases in solution. Where the rocks are not saturated by water gaseous
solutions also are present and active. Of these oxygen and carbonic acid
are the most important active chemical agents.
While it is well established, as explained (p. 63), that pure water acts
upon all of the compounds which occur in nature, the activity of solutions
is very greatly increased by the mineral content. This is true of bases, of
acids, and of salts alike. Where bases or acids are present it is well known
that the activity of solutions is greatly increased; but it has not been always
fully appreciated that the same is true in reference to normal salts.
Therefore all of the above compounds, in all their forms, are actively
at work in decomposing the rocks. The amount of work which any one
compound does depends upon the strength of its bases or acids, and also
upon its quantity, or upon the law of mass action. While if the compounds
are present in equal quantity the stronger bases or acids do more work, it
is frequently the case that a weaker compound more than compensates for
this weakness by its great abundance. Illustrating this is carbonic acid,
which, while very weak, is on account of its abundance, one of the most
potent agents in the alterations of rocks in the belt of weathering.
(b) The temperature is of very great importance in the activity of
the water and gaseous solutions.
CHEMICAL WORK OF SOLUTIONS. 459
The average annual temperature near the Arctic Cirele in northern
North America and northern Asia is about —15° C.; and from this temperature
the average temperature varies to about —18° C. in northern Greenland. In
the Temperate Zone the average temperature varies from about —15° C. in
its northern part to about 22° C at its southern part. The Tropical Zone
yaries in temperature from about 20° C. to about 27° C. In central North
America and central Asia the normal average temperature is about 5° C.,
in southern North America and southern Asia it is from 15° to 20°C.
For the present purpose the average temperatures, rather than the changes
of temperature or the extremes of temperature, are the data of first
consequence; for the average action of solutions upon the rocks, extending
through years, is controlled by these average temperatures. In the arctic
and north temperate regions, during the time the temperature is below 0° C.,
it is highly probable that the decomposition of the rocks practically ceases;
as the temperature rises above 0° C. decomposition begins, and this goes on
with increasing speed in proportion as the temperature is high. Therefore,
in the above facts as to temperatures we have in large part the explanation
of the slowness of the decomposition of rocks in the arctic regions and
the rapidity of the decomposition in the tropical regions.
It is to be noted that the activity of the solutions in the warm climates,
as compared with the cold climates is much greater than would be inferred
from the absolute temperatures. The temperature of northern Greenland
(—18° C.) is 255° absolute; that of the Tropics (27° C.) is 300° absolute.
Thus the ratios of the absolute temperatures between the extreme cold of
the arctic regions and the warmth of the Tropics is as 255 to 300, a
difference of less than one-fifth; but it is certain that the activity of the
solutions in the Tropics is manyfold greater than in the arctic regions.
While the average temperature is the matter of most significance in this
connection, the changes of temperature have an important effect upon the
chemical decomposition of the rocks as well as upon the mechanical disin-
tegration. From the tables given above it appears that while the average
temperatures in the arctic regions are very low for certain seasons of the
year the temperature for a whole month may average as high as 10° C.,
and during the day the temperature may rise as high as 20° C. At such
temperatures it is certain that the process of decomposition of the rocks
460 A TREATISE ON METAMORPHISM.
takes place. Thus it appears that at certain seasons the temperature is
high enough for decomposition to affect the rocks in the most northern
and the most southern latitudes. But im polar regions the temperature is
such that decomposition takes place for only a small fraction of each year;
in the temperate regions the temperature is high enough for decomposition
to take place during more than half the year; while in the Tropics decom-
position is continuous. It therefore appears that in passing from the polar to
the tropical zones we pass from a minimum to a maximum of chemical
decomposition.
(c) As to the pressure in the belt of weathering, since the ground is
not ordinarily saturated by water there is commonly no hydrostatic pres-
sure; and the pressure at which the chemical reactions take place is there-
fore that of the atmosphere. At sea level the average pressure is that
of a column of mercury 760 mm. high, or 1.0333 kg. per sq. cm. From the
pressure at sea level the pressure diminishes as the altitude increases, and
at the tops of the higher mountains is not more than about one-half of the
amount at sea level. Not only is there change of pressure due to elevation,
but there is change of pressure due to variable meteoric conditions. As a
storm sweeps over an area the pressure is ordinarly low; at the clear inter-
vals between the storms the pressure is usually high. Commonly the
change in barometer from a low to a high is not more than 2 em. and the
average change is probably not more than 1 cm. Infrequently the change
is 5 em., or about one-fifteenth of the, total pressure. Very exceptionally
the change of pressure may be much greater than this. Probably these
changes in pressure are so slight as to produce little effect upon the water
solutions. But, as already shown (p. 61), the quantity of atmospheric gas
acting is directly as the pressure. To illustrate, if the barometric pressure
‘rises from 715 mm. to 760 mm., the proportional increase in the amount of
gas action is about one-fifteenth. More frequently, however, the variations
are from one-thirtieth to one-fiftieth, or even less.
Since the amount of gas is directly as the pressure, it is evident that,
so far as this factor goes, the higher the barometer the greater the activity
of the gases at work in the belt of weathering. But since the variations in
pressure due to this cause are ordinarily but a small fraction of the total
pressure, the variations in effect are not sufficiently great to be appreciable.
Since, in reference to the cyclonic periods, the times of high pressures are
OXIDATION OF ORGANIC COMPOUNDS. 461
usually those of low temperatures, and vice versa, so far as increased pres-
sure produces an effect it compensates to a slight extent for the low
temperature, and decreased pressure detracts to a slight extent from the
chemical activity resulting from the high temperatures.
JOINT WORK OF AGENTS OF WEATHERING.
As a matter of observation, we know that the most important chemical
reactions which take place in the belt of weathering, as a result of the
action of the various agents, are (1) oxidation, (2) carbonation, (3) hydra-
tion, (4) solution, (5) deposition.
OXIDATION.
One of the reactions of first importance in the belt of weathering is
oxidation. Oxidation is the addition of oxygen to other compounds. The
source of the oxygen is the atmosphere, of which it composes 23.12 per
cent by weight. This oxygen of the atmosphere acts to some extent
directly as a gas, but to a far greater extent through water solutions, and
to the greatest extent through water solutions and organisms combined. Of
these organisms bacteria are of the greatest consequence, but the molds
and fungi are important.
OXIDATION OF ORGANIC COMPOUNDS.
The chief elements oxidized in the organic compounds are carbon,
hydrogen, and nitrogen.
Oxidation of carbon and hydrogen.—' ‘he most abundant organic compound is
cellulose (nC,;H,,O;). Other compounds which contain carbon and hydrogen
are the proteids, carbohydrates (starch, sugar, etc.), organic acids, fats, ete.
The ultimate oxidation products of all these compounds are carbon dioxide
and water. This work of oxidation is almost wholly accomplished through
the joint work of bacteria and other microbes (such as the fungi), oxygen,
and water. The numbers of bacteria engaged in this work are enormous.
Estimates by Wollney, Adametz, Koch, Fullus, and others, of the number
of bacteria in one gram of soil connected with the formation of carbonic
acid gas alone vary from one-half million to one million.
During the oxidation of cellulose a large number of organic acids are
produced. Of these organic acids humic acid (Cy)H;,02,, Detmer; C2H.O¢,
462 A TREATISE ON METAMORPHISM.
Thenard) is the most important in the soil. Some of the laboratory prop-
erties of humic acid are as follows: It is amorphous; it decomposes at
145° C.; after drying, it requires 13,784 parts of boilmg water to dissolve
it; when undried, it requires 8,333 parts at 6° C. and 625 parts at 100° C.
to dissolve it. With humic acid in the soil are various other acids, such as
ulmic acid (CygHyO¢ or CisH,,0;, Berthelot and André), crenic acid
(C,.H,.0;, Mulder), and apocrenic acid (C,,Hy,0.., Mulder).”
The humie acids are first produced by the oxidation of the cellulose.
The other organic acids appear to be produced by further oxidation of
the humic acid. Finally, as the process of decomposition continues,
humic and the other organic acids by their decomposition are further
oxidized and broken up, so that the ultimate products are carbon dioxide
and water.
At all stages in the process the acids are active. The humic acid and
other organic acids act upon the various inorganic compounds, especially
the silicates, forming salts. However, the acids of these salts are broken
down to carbonic acid. Also the breaking down of the free humic acid
and other organic acids produces carbon dioxide. Therefore the most
important result of the decomposition of the cellulose is the production of
carbon dioxide. By the process of plant decay carbon dioxide is thus
concentrated in large quantities in the belt of weathering—that is, in the
place where it can do its most active work. The process of carbonation
(explained on pp. 473-480) is consequently largely due to this concentration.
Besides their direct chemical activity, humic and other organic acids have
various other important effects in the belt of weathering. As a result of the
dark color of humic acid it is an absorbent of the sun’s heat. Moreover, it
has a higher specific heat than soil, and hence is able to retain a large
quantity of heat. Thus, in this way, the presence of humic acid promotes
chemical activity. Furthermore, humic acid is hydroscopic, and in sandy
soils greatly increases the holding power for water; and it has been noted
that the chemical activity in soils is very largely dependent upon the
amount of water they hold. Finally, humus in the soils holds ammonia,
nitrites, nitrates, and soluble sodium and _ potassium compounds, in other
words, plant foods. These plant foods may be held in some cases as
«See also Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York,
1897, pp. 189-190.
OXIDATION OF ORGANIC COMPOUNDS. 465
humates or other organic salts These soluble materials thus held by the
presence of the humic acid are important plant foods. Therefore, humic
acid for many reasons furnishes favorable conditions for further luxuriant
plant growth, and these plants again react in a powerful way in promoting
the alterations of the belt of weathering.
The process of decomposition of the other carbon- and hydrogen-
bearing compounds, aside from cellulose, especially the decomposition of
the carbohydrates, is described by Fischer as a fermentation. In this
work, as in the decomposition of cellulose, the mold fungi, as well as
bacteria, are at work in connection with oxygen and moisture.”
While during the history of the earth by far the larger portion of the
cellulose formed by organic agencies has been oxidized as above described,
from time to time to the present, and in various areas, districts, and regions,
this process has been only very partial. As a consequence, the unoxidized
cellulose has been buried below later sediments, and thus immense quantities
of carbon compounds have been entombed within the earth. This is the
main source of the coals, oils, peats, and carbonaceous sediments. It is
therefore evident that the total quantity of this entombed organic matter is
enormous. By metamorphism of the cellulose these carbon compounds
pass into the various forms of coal. After these carbon compounds are
formed, as a result of denudation they may again pass into the belt of
weathering, and here they are subject to the same oxidizing agencies which
are at work upon the original cellulose. But they are now in a more
refractory form than the original cellulose, and while the process is doubtless
very slow, there is no doubt that the oxidation of the coal and other similar
carbon compounds takes place to some extent in the belt of weathering
under natural conditions and thus produces carbon dioxide. But, so far as
I know, this process has uot been studied, and I can make no definite
statements in reference to-it. It is probably more rapid than is commonly
supposed, and the changes are perhaps accelerated by oxidation of sulphides. .
Since man began to use coal, peat, and oil, artificial oxidation of the
cellulose and the entombed carbon compounds has taken place upon an
immense scale. While this method of oxidation of these carbon compounds
was trivial until the middle of the eighteenth century, when coal was
first applied to the manufacture of iron, it has since that time steadily
«Fischer, Alfred, The structure and functions of bacteria, trans. by A. Coppen Jones, Clarendon
Press, Oxford, 1900, pp. 107-115.
464 A TREATISE ON METAMORPHISM.
increased in importance. But it 1s only during the last half of the
nineteenth century that the quantity of carbon compounds artificially
oxidized has become of importance. In the year 1899 the amount of coal
mined and oxidized amounted to 723,287,454 metric tons. At the begin-
ning of the last decade of the nineteenth century, 1890, the production
was only 511,518,358 metric tons.“ This shows how rapid the merease
in the use of coal has been, and therefore a combustion of 1,000,000,000
metric tons a year is probably very conservative as the estimated average
for the present century. Taking 1,000,000,000 metric tons as the amount
of coal oxidized per annum for the future, and supposing the amount of
carbon in this coal to average 80 per cent, the quantity of CO, which
passes into the atmosphere would be 2,933,333,000 metric tons per annum.
This is 0.1233 per cent of the total amount of CO, at present in the
atmosphere. (See p. 972.) If this rate of consumption of coal were
continued eight hundred and twelve years the amount of CO, in the
atmosphere would be doubled.
It therefore appears probable that within a comparatively short time
in the future, as compared with a single geological period, or even an
epoch, the amount of CO, in one of its great reservoirs, the atmosphere, will
be increased to an important extent. From this fact various geological
consequences are likely to follow. One of the most important of these is
a higher average of temperature for the globe.’ According to Arrhenius, ‘‘if
the carbon dioxide is increased 2.5 to 3 times its present value, the tem-
perature in the arctic regions must rise 8° to 9° C. and produce a climate
as mild as that of the Eocene period.”* According to the above com-
putation, the CO, would be increased by the oxidation of coal alone to three
times its present amount in one thousand six hundred and twenty-four
years. Certain it is, if Arrhenius be correct, and the coal supplies of the
world are sufficient to meet the demands of man for thousands of years, that
a most profound change will tale places in the climate of the world.
«Parker, E. W., Mineral meee: of he United States, 1899; Coal: Twenty Lats Ann. Rept.
U.S. Geol. Survey, pt. 6, 1901, p. 369.
> Chamberlin, T. C., An attempt to frame a ) uO oes hypothesis of the cause of the Glacial
periods on an atmospheric basis: Jour. Geol., vol. 7, 1899, pp. 545-584.
¢ Arrhenius, Svante, On the influence of han acid in the air upon the temperature of the
ground: Philos. Mag., 5th ser., vol. 41, 1896, pp. 237-276. Summary in Jour. Geol., vol. 7, pp.
625-625.
OXIDATION OF NITROGEN. 465
A further consequence which would follow from an increase in the
amount of CO, in the atmosphere and the warmer climate would be a much
more abundant and widespread vegetation, and, as pointed out (p. 476),
more vegetation means that when oxidized more CO, will be concentrated
in the soil, and this concentration will lead to an acceleration in the rate
of carbonation. Furthermore, the increase in average temperature of the
globe will accelerate all other chemical reactions of the belt of weathering.
It therefore appears probable that the artificial oxidation of coal will result
in some of the most profound and far-reaching geological consequences
which are due to the agency of man.
Oxidation of nitrogen —As already noted, combined nitrogen occurs in various
organic compounds, of which the proteids and albuminoids are the more
important. The oxidation of this nitrogen, like that of carbon and hydro-
gen, is a bio-chemical process, being the joint work of microbes, oxygen,
and water. Of the microbes, bacteria are by far of the greatest conse-
quence. In the decomposition of the complex nitrogen compounds the
nitrogen passes into ammonia, nitrites, nitrates, and, to some extent, free
nitrogen. The first stage of the process is the transformation into ammonia.
In the case of urea, Fischer gives the reaction as follows:
CO(NH,),+2H,0 =CO,(NH,),
The second stage of the process is the transformation of the ammonia into
nitrites, and the final stage is the transformation of the nitrites into nitrates.
Each of these stages of work is accomplished by certain bacteria which
take no fart in the other stages. At the same time the nitrogen is oxidized
the carbon and hydrogen of the nitrogen compounds are oxidized into
carbon dioxide and water, precisely as in the case of cellulose.
So far as the nitrogenous compounds are concerned, the ultimate
geological products which remain in the soil are the nitrates, although in
the process some small part of the nitrogen is freed and lost, as already
noted. ”
Apparently nitrates are produced on a far vaster scale in the tropical
regions than in the temperate regions. Miintz and Marcano state that in
“Fischer, cit., p. 103.
» Fischer, cit., pp. 98-106. See, also, Conn, H. W., The story of germ life, D. Appleton & Co.,
New York, 1897, pp. 104, 118; and Aikman, C. M., Manures and manuring, Wm. Blackwood & Sons,
London, 1894, pp. 167-170.
MON XLVII—04——30)
466 A TREATISE ON METAMORPHISM.
places in the valley of the Orinoco the amount of nitrates in the soil
amounts to 30 per cent of the mass. The vast amount of these nitrates
is doubtless explained by the very great abundance and activity of the
bacteria; for it is well known that the high temperature of the Tropics,
combined with the high humidity of these regions, is very favorable to the
action of bacteria. In this connection Schloessing and Miintz state that
the maximum activity of the bacteria is at 30° C., or approximately that of
the Tropies.’
Much of the combined nitrogen is lost to the belt of weathering in the
following ways. In so far as nitrogen is set free by the action of the bac-
teria and by the passage of the ammonia into the air, it is lost. There are
further great losses in the nitrogen compounds by the transportation of the
nitrates to the sea by the streams. The quantity of nitrates thus lost has
been greatly increased in recent years by the introduction into the streams
of sewage containing much combined nitrates. Further, as pointed out by
Conn, in so far as the combined nitrogen of the soil is manufactured into
powders which are exploded, the combined nitrogen is freed and passes into
the atmosphere. The ammonia which passes into the atmosphere may be
brought back in part to the soil by the rain. The other losses must, how-
ever, be compensated by the synthesis of nitrogen compounds from the
nitrogen of the air by bacteria and leguminous plants combined. Until
very recently the latter process has preponderated, and the crust of the
earth has gained in combined nitrogen. Man in recent times has
undoubtedly increased the loss in combined nitrogen, and it is possible,
perhaps probable, that the balance is now on the other side, but by care in
cultivation, and possibly by manufacture, it will doubtless be possible to
continue the process of adding combined nitrogen to the soil faster than it
is lost from it.
OXIDATION OF INORGANIC COMPOUNDS.
The most important of the inorganic compounds oxidized are iron and
carbon. Other substances subordinate in quantity are also oxidized, but
these have small importance from a purely geological point of view. How-
ever, from the point of view of ore deposits, and therefore in reference to
«Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, p. 372.
5 Aikman, cit., p. 52.
¢Conn, cit., p. 106.
OXIDATION OF INORGANIC COMPOUNDS. 467
the needs of man, the metals in subordinate quantity, such as manganese,
copper, zinc, lead, ete., have great importance.
Iron— Since ore deposits are treated in another place, iron is the only
metal which will here be considered. The iron oxidized occurs in the
minerals mainly in the form of monoxide. Considerable quantities of it
occur as sulphides, and unimportant amounts as arsenides, ete.
Ferrous oxide occurs in the following classes of minerals: Oxides,
carbonates, and silicates. Of the oxides, magnetite is the most important.
This may be oxidized without hydration into hematite. This change is
very well illustrated by the martite ores of the Lake Superior region
and by the pseudomorphs of hematite after magnetite in the martite-
bearing schists. Simultaneously with the oxidation of magnetite hydration
may take place and thus produce hydrated hematite, limonite, or the other
hydrated oxides of iron. The change of magnetite to hematite involves an
increase in volume of only 2.5 per cent, but where magnetite is changed to
limonite the mcerease in volume is large—64 percent. Tron in the form of
carbonate may occur as pure iron carbonate or as iron carbonate in combi-
nation with various proportions of magnesium and calcium. The oxidation
of the ferrous iron of the iron carbonate may be accomplished by oxygen
and moisture. However, in the soil the process usually takes place with
the assistance of bacteria. A certain group of bacteria requires various
carbonates for its development.*| The oxidation of the iron carbonate,
whether with or without bacteria, changes it to ferric oxide. Where the
oxidation takes place without hydration hematite is formed; where with
hydration, limonite, or some of the other oxides of iron. Where bacteria
are an agent the common product is imonite. The oxidation of the carbon-
ates involves decarbonation, and on account of this fact there is decrease
in volume—in the case of hematite 49 per cent, and in the case of limonite
18 per cent. The transformation of the ferrous oxide of the silicates to
ferric oxide, either. anhydrous or hydrous, occurs simultaneously with
processes of carbonation and hydration of the other bases. So far as
there is oxidation and hydration of the iron, this would tend to-increase the
volume; but the amount of increase can not be calculated independently
of the conjoint processes.
aQLafar, Franz, Technical Mycology, translated by C. T. C. Salter, London, Lippincott, 1898, pp.
360-362.
468 A TREATISE ON METAMORPHISM.
Of the sulphides of iron, those of marcasite, pyrite, and pyrrhotite are
the most important. The oxidation of the iron of these sulphides may be
accomplished by oxygen alone, by oxygen and moisture, and finally by
oxygen, moisture, and bacteria. With oxygen alone the process is slow;
with oxygen and moisture together it is rapid; but oxygen, moisture, and
bacteria together furnish the most favorable conditions.
Where oxidation is the only process the compound produced is mag-
netite or hematite. More commonly, however, hydration occurs simulta-
neously with the oxidation; and under these circumstances limonite and
other hydrated oxides of iron are produced. So far as the oxides are formed,
this involves desulphidation. :
Sulphur—At the same time the iron is oxidized the sulphur united with
it may also be oxidized. Where the two are oxidized together and remain
united the result is to form iron sulphate. Frequently, however, at the
time the iron is oxidized the sulphur or some part of it separates as hydro-
sulphuric, sulphurous, or sulphuric acid. The various reactions resulting
in these compounds are given on pages 214-216, and need not here be
repeated. By reference to the reactions there written it will be seen that
certain of them, as already stated, result in the formation of hydrosulphurie
acid. Indeed the reactions producing hydrosulphuric acid are very
common during the oxidation of pyrite, marcasite, and pyrrhotite. In a
similar manner the oxidation of the monosulphides of the other metals may
result in the production of hydrosulphuric acid.
The sulphur of the hydrosulphuric acid may be later oxidized to sul-
phurous or sulphuric acid. While the oxidation of hydrosulphuric acid may
be accomplished by water and oxygen without the assistance of bacteria,
often bacteria are the inciting cause of the change. Where hydrosulphuric
acid is abundant the hydrogen only is first oxidized and the sulphur stored
in the cells of the bacteria, according to the reaction:
H,S+O=H,0+8
But finally the stored sulphur is oxidized by the bacteria to sulphuric
acid, which reacts upon the bases present and forms sulphates.
In the transformation of the sulphides the volume change on account
of desulphidation produces considerable diminution in volume, provided
@Latfar, cit., pp. 370-374.
RELATIONS OF OXIDATION AND DEOXIDATION. 469
the oxide of iron is not hydrated—with magnetite, 24 to 39 per cent; but
in case both oxidation and hydration occur together there is increase in
volume of but 3 per cent in the case of pyrite and 25 per cent in the case
of pyrrhotite.
GENERAL STATEMENTS.
It might be inferred from the foregoing statements in reference to the
volume relations that, on the average, there is a decrease in volume in
oxidation rather than an increase, as one would naturally expect. In
support of this view it has been stated that there is generally a decrease in
volume in the processes of oxidation of the carbonates and sulphides, since
the process of oxidation simultaneously results in decarbonation or desul-
phidation. However, much more important than the oxidation of iron in
the form of carbonate and sulphide is the oxidation of ferrous iron in
silicates, and this process does not necessarily involve subtraction of any
material. Since it involves addition of oxygen, the result is to produce an
increase in volume provided the separated silica or a considerable part of
it remains in situ.
As is well known, and as has been pointed out heretofore, the process
of oxidation involves great liberation of heat; but the data are not at hand
by which the amount of heat liberated can be determined. Concluding in
reference to volume and heat relations, it may be said that the process of
oxidation perhaps illustrates better than any other the principle that in the
zone of katamorphism combinations which take place with the liberation of
heat control the reactions rather than the volume relations. Heat is
liberated in all the cases of oxidation, and whether there is an increase or
decrease of volume is a subordinate factor.
Where the amount of oxygen is sufficient to cause the iron compounds
to be transformed to ferric oxide, there are red or yellow soils and sub-
soils. Excellent illustrations of such regions are the southern Appalachians
and the Piedmont Plateau, where the crystalline rocks have deeply
weathered. As shown by Russell, the red color of this soil is due to a
ferruginous clay containing both ferric oxide and alumina, which incrusts
the grains of the rocks.“ The ferric oxide is produced during the decom-
«Russell, I. C., Subaerial decay of rocks, and origin of the red color of certain formations: Bull.
U.S. Geol. Survey No. 52, 1889, pp. 14-15.
470 A TREATISE ON METAMORPHISM.
position of the parent rocks. Another illustrative region, described by
Hayes, is that of Nicaragua east of the continental divide. The red color
in this belt is from 3 to 10 meters deep.” Both of these regions are those
of rather high temperature and abundant precipitation throughout the year,
and therefore abundant vegetation. They therefore well illustrate the con-
ditions under which decomposition of the rocks, including oxidation, takes
place. The very abundant transpiration by the luxuriant foliage (see p. 422)
doubtless is a very important factor in preventing the soil from becoming
saturated with moisture for any considerable period, and thus the conditions
for oxidation are maintained.
One would suppose that in the soil, where oxidation of organic and
inorganic matter is active, the amount of oxygen in the gases would be less
than in the atmosphere, and observations by Fleck, Letts and Blake and
others confirm this conclusion. The amount in the atmosphere is 20.92 per
cent by volume, whereas Fleck, as cited by Letts and Blake, finds that at
a depth of 2 meters the minimum is 16.33 per cent and the maximum 19.39
per cent; at 4 meters the minimum is 15.67 per cent and the maximum
16.79 per cent; at 6 meters the minimum is 14.94 per cent and the maximum
14.85 per cent.” These last numbers are only a little more than two-thirds
the full amount of oxygen of the atmosphere. It appears that as depth
increases, and therefore as the air of the soil is further removed from its
source of supply—the oxygen of the atmosphere—there is a steadily
decreasing amount, and this fact must be explained by the consumption of
oxygen by the oxidation of organic and inorganic matter.
In some.cases the amount of oxygen is little more than half as great
as that in the air. The deficiency varies directly, although not in simple
ratio, with rapidity of oxidation and with the depth below the surface where
the process takes place.°
While oxidation is the normal process in the belt of weathering, under
certain conditions deoxidation may take place. Under such circumstances
gray or white soils and subsoils are formed. This is illustrated by the
western division of Nicaragua, where Hayes states that the clays are blue,
“Hayes, C. W., Report of the Nicaragua Canal Commission, Appendix II, Geologic Report,
1899, pp. 128-129. }
>The last number must be a misprint. Letts, E. A., and Blake, R. F., The carbonic anhydride
of the atmosphere: Scientific Proc. Royal Dublin Soc., vol. 9, new ser., pt. 2, p. 215.
¢ Aikman, C. M., Manures and manuring, Blackwood & Sons, London, 1894, p. 100.
RELATIONS OF OXIDATION AND DEOXIDATION. 471
the iron being in the ferrous form.“ This is a region of less humidity
than the eastern region, and here dry seasons alternate with very wet
seasons. According to Hayes, during the dry season each year the soil
cracks deeply, and a large amount of organic material falls into these cracks.
During the wet season the soil swells and fills the cracks, and the organic
matter becomes incorporated with the soil. This material acts as a reduc-
ing agent and prevents the further oxidation of the iron, or reduces any
part of the iron oxide which has become ferric oxide to the ferrous state.
While this explanation is plausible, it seems incomplete. In the east-
ern division the soil is.continuously in contact with numerous dead and
dying roots of the abundant vegetation, and the question naturally arises
_whether the difference is not due to saturation of the soil of the western
division, as explained below, the cracks being utilized by the water.
Deoxidation of the iron may occur in regions where the water of
the soil is so abundant as to approach saturation, and plants are abundant.
Under such conditions the large amount of oxygen required to decompose
the plants is not able to enter from the air with sufficient rapidity, and
hence the oxidation of the plants abstracts oxygen from ferric oxidé, and
this reduces the iron to the ferrous condition. This reduction of ferric salts
results from the demands of the abundant oxidizing bacteria for oxygen
where organic matter is being rapidly decomposed. Consequently the
process of deoxidation of iron compounds commonly takes place only by
the oxidation of organic constituents. So far as the morganic constituent is
concerned, there is absolute loss of oxygen by the process.
Examples of deoxidation of iron compounds are furnished by the
so-called cumulous deposits, where abundant vegetation near the water level
accumulates and does not completely decay. Instances of such deposits
are the sphagnum mosses of marshes, bogs, lakes, and the margin of the
ocean, which result in the accumulation of peat; and the swamps, for
example, cedar, spruce, mangrove, etc. Probably the same conditions
obtained in the past, especially at the coal-forming periods, for the under-
clays of coal beds are almost always gray or white, the iron having been
almost completely reduced to the ferrous form.
The iron reduced to the ferrous condition and that already in the
ferrous condition in the silicates is in the most favorable form to unite
«Hayes, cit., pp. 130-132.
472 A TREATISE ON METAMORPHISM.
with carbon dioxide, as explained under ‘‘Carbonation;”
and thus may be
produced a great quantity of iron carbonate, which may join the sea of
eround water and furnish the material for chalybeate springs and for
siderite deposits, as explained on pages 824-829. he clay ironstones and
the siderite, often associated with the coal beds and frequently in the coal
itself, are illustrations of such siderite deposits. Thus the association of
deposits of iron carbonate with coal deposits is explained, and also—as
developed on pages 842—846—there is in this iron carbonate thus formed
the source from which other iron-ore bodies are concentrated.
Another example of deoxidation is furnished by the nitrates, which may
be reduced to nitrites, or even to free nitrogen when the conditions of the
belt of weathering are those of abundant vegetation and very high humid-
ity.” The reduction of the nitrites is effected by definite bacteria. It is
highly probable that the oxygen abstracted in the deoxidation of nitrates,
like that abstracted from ferric iron, is used by the oxidizing bacteria in
decomposing the organic matter.
Under conditions similar to those in which ferric iron is reduced to
the ferrous form and nitrates are reduced, sulphates may also be reduced
to sulphides. This is usually accomplished mainly by organic material
where abundant to serve as a reducing agent; but also in this reduction, as
in various other processes, bacteria may play a part.’
Finally, where the moisture is too abundant the oxidation of the plants
themselves is greatly delayed, and may be permanently stayed. The best
illustration of the lag in decay of plants and deoxidation of the ferric salts,
nitrates, and sulphates is found in the marshes where water entirely covers
the soil, and thus makes the oxidation very slow indeed. Where the plants
fall below the surface of the water the plant decay may be very partial,
and hence there may be produced peat or coal beds. But in this connec-
tion it should be recailed that where, as a result of great humidity, the
decay of plants is slow and deoxidation takes place the conditions approach
those of the belt of cementation. In swamp areas the thickness of the belt
of weathering is practically reduced to zero; the roots of vegetation reach
below the level of ground water, and under these circumstances the reactions
which take place in connection with organic matter are those of the belt of
cementation rather than those of the belt of weathering.
« Aikman, cit., pp. 177-178. » Lafar, cit., pp. 363 et seq.
RELATIONS OF OXIDATION AND DEOXIDATION. 475
It has been shown that in the belt of cementation oxidation is much
less prevalent than in the belt of weathering, and indeed that, on the whole,
deoxidation, especially where organic material is prevalent, is the rule.
Therefore, the reversal of the ordinary process of oxidation in the belt of
weathering usually takes place only where this belt is grading into or is
under conditions which are approaching those of the belt of cementation
and below.
It has already been stated that the direct source of the oxygen for the
process of oxidation is the atmosphere. It is apparent from the foregoing
that a vast amount of oxygen is now being demanded for this process, and
the amount of oxygen which has thus been consumed during geological
time is beyond computation. But it will be seen under “The zone of
anamorphism” that there are also processes which restore oxygen to the
atmosphere. Which of these processes, oxidation or deoxidation, is, on
the whole, preponderant and what are the possible sources of supply for
oxygen can best be considered after all the reactions in each of the belts are
considered, and therefore this subject is taken up in Chapter XI.
CARBONATION.
The process of carbonation may be defined as the union of carbonic
acid with bases, forming carbonates. Since the carbonates are not known
as original minerals in the igneous rocks, we must look in other directions
for the source of the carbon dioxide for the process of carbonation. The
immediate reservoir for this carbon dioxide is undoubtedly the atmosphere,
but the amount in the atmosphere is very small, only 0.045 per cent by
weight, and the quantity now in the atmosphere is, as will be seen subse-
quently, insignificant as compared with that which must have been abstracted
from the atmosphere during past geological ages by the process of carbona-
tion. Kither the atmosphere must have once contained vastly more carbon
dioxide than at present, or else it must have been continuously replenished
in this compound, or partly both. These questions can be best discussed
after the reactions of both the zone of katamorphism and the zone of ana-
morphism have been considered, and their consideration is deferred to
Chapter XI.
It has been explained that from the atmospheric reservoir the plants
absorb carbon dioxide, reduce it, and build it into their bodies. Further-
AT4 A TREATISE ON METAMORPHISM.
more, it has been seen that oxidation of organic materials by bacteria and
oxygen in the belt of weathering produces carbon dioxide abundantly. This
process, therefore, concentrates in the upper part of the belt of weathering a
large amount of carbon dioxide, and this carbon dioxide is available for the
process of carbonation. The fact of the concentration of carbon dioxide in
the upper part of the crust of the earth was noted by Bischof many years
ago. In the waters of his laboratory well at Bonn he found three times as
much carbonate of lime as in the Rhine near by — Also, free carbonic acid
often collects above the water of wells. Facts like these, and the large
amounts of carbonic acids in mines Bischof explained by the oxidation of
organic matter, including coal.”
The importance of the process of concentration of carbon dioxide in the
belt of weathering through the oxidation of organisms can be appreciated
only by comparing the amount of this material in the atmosphere with the
amount in the gases of the belt of weathering where vegetation is present.
The amount of carbon dioxide in rain water, according to Fischer,
yaries from 0.22 to 0.45 per cent of the volume of the water, or only 0.00044
and 0.00089 per cent by weight.’
It has already been noted that the amount of the carbon dioxide in the
atmosphere by weight is about 4.5 in 10,000. The amount present in
gases of the soil is far greater than this, as shown by the following table
by Boussingault and Lewy: °
Table showing the amount of carbon dioxide in the air of belt of weathering.
| COs in 10,000
parts by
Weight.
Air from sandy subsoil of forest..-..----- 38
Air from loamy subsoil of forest---.------ | 124
Air from surface soil of forest........--.- 130
Air from surface soil of vineyard. -....---- 146
Aindrom: pasture soils eee ce eee eee ee 270
Air from soil rich in humus..---..------- 543
“Bischof, Gustay, Elements of chemical and physical geology, Harrison & Sons, London, 1854,
yol. 1, p. 239. (Translated by Paul and Drummond. )
>Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, p. 179.
¢Merrill, cit., p. 178.
CARBONATION AND VEGETATION. A475
Fleck, as cited by Letts and Blake, found in the soil at a depth of 6
meters by volume a minimum amount of carbon dioxide of 4.22 per cent,
and a maximum amount of 7.96 per cent; at 4 meters, a minimum amount
of 4.11 per cent and a maximum amount of 5.56 per cent; at 2 meters, a
minimum amount of 2.99 per cent and a maximum amount of 2.91 per cent
(the last number must be a misprint).
These figures show that the amount of carbon dioxide in the ordinary
air is insignificant in comparison with the amount in soils in regions of
luxuriant vegetation. In such regions the carbon dioxide is from thirty to
more than one hundred times more abundant than in the atmosphere.
This large amount is mainly furnished by the decomposition of vegetation,
although, as shown by Briggs, even dry soils have the power to absorb
carbon dioxide from the atmosphere.’
The process of carbonation is dominantly accomplished by the substi-
tution of carbonic for silicic acid. The negative side of this process is
desilication. To a very small extent carbonation is accomplished by the
substitution of carbonic acid for other acids—for instance, phosphoric acid.
Also carbonates are produced on a considerable scale by the union of
carbon dioxide with the oxides which were not united with other acids, as,
for instance, ferrous oxide in magnetite.
Mueller has experimentally ascertained that carbon dioxide in water at
ordinary temperatures and pressures 1s capable of attacking many minerals.
Of the oxides, he experimented with magnetite; of the silicates, he experi-
mented with orthoclase, oligoclase, hornblende, olivine, serpentine, and
muscovite. He also experimented upon a number of phosphates, including
apatite. While all of the silicates experimented with were attacked to
some extent, there was great variation in the rate of action. For instance,
orthoclase is attacked more readily than oligoclase; hornblende and serpen-
tine are attacked more rapidly than the feldspars, and olivine is the most
readily attacked of all the silicates tested. The process of carbonation
formed carbonates of sodium, potassium, calcium, magnesium, and iron.
Alumina also went into solution. The liberated silica partly went into
solution, but partly also separated as quartz. Magnetite was the most
“Letts, E. A., and Blake, R. F., The carbonic anhydride of the atmosphere: Sci. Proc. Royal
Dublin Soc., vol. 9, pt. 2, 1900, p. 215.
bCameron, Frank K., Soil solutions, their nature and functions, and the classification of alkali
lands: Bull. U. S. Dept. Agric. No. 17, 1901, p. 17.
476 A TREATISE ON METAMORPHISM.
resistant of all the minerals and rocks tested. Apatite was found to be as
readily soluble as the more soluble silicates. Perhaps one of the most
interesting results in reference to the silicates is the comparative readiness
with which the hydrous silicate, serpentine, is attacked. From this fact
Mueller makes the point that this silicate does not represent an end product
of alteration.”
Johnstone later experimented upon the micas, including muscovite,
biotite, and lepidomelane, and found that they are attacked by water solu-
tions of carbon dioxide.’ As one would expect from the above reactions,
Struve® found that water containing carbon dioxide at ordinary pressure
attacked basalt, phonolite, gneiss, granite, clay slate, and porphyry.
Combining the experimental fact that carbon-dioxide solutions decom-
pose the silicates and the observed fact of the abundance of CO, where
vegetation is plentiful, one would expect that the process of carbonation
would be more rapid in regions of abundant vegetation than elsewhere.
Belt many years ago made observations which confirm this expectation.
He says that the decomposition of rocks in tropical America is largely con-
fined to the forest regions, and ascribes it to the action of water charged
with acids derived from the decomposing vegetation.” Where vegetable
matter is abundant it has also been observed that the amount of dissolved
silica contained in underground water is much greater than where veg-
etation is sparse or absent.’ This observation is direct evidence that the
reactions of carbonation and desilication are correlative, and are very largely
due to the concentration of carbon dioxide in the belt of weathering by the
oxidation of organic matter.
Since the reaction of carbon dioxide upon the silicates and other com-
pounds forms carbonates, in a soil there may be present both sedimentazy and
secondary carbonates. Now, it has been shown experimentally by Bischof
that the alkaline carbonates are capable of decomposing the silicates at
«Mueller, Richard, Untersuchungen ueber die Hinwirkung des kohlensiiurehaltigen Wassers
auf einige Mineralien und Gesteine: Tschermak’s mineral. Mittheil., vol. 7, 1877, pp. 25-48, especially
pp- 39, 46-48. See Merrill, G. P., Rocks, rock-weathering, and soils; Macmillan Co., New York, 1897,
pp. 192-193.
b Johnstone, A., On the action of pure water and of water saturated with carbonic-acid gas on
the minerals of the mica family: Quart. Jour. Geol. Soc. London, vol. 45, 1889, pp. 363-368.
¢Struve, F. A. A., Ueber die Nachbildung der nattirlichen Heilquellen, Pogg. Ann. vol. 7, pp.
341-372-429-450.
@ Belt, Thomas, The naturalist in Nicaragua, 1874; cited in Merrill, p. 175. ~
eHunt, T. Sterry, Chemical and geological essays, 1875, pp. 149-152.
CARBONATION AND VEGETATION. ATT
ordinary temperatures and pressures.“ A great variety of silicates were
thus decomposed. The alkaline silicates and carbonates of sodium, potas-
sium, calcium, magnesium, and iron were formed. Not only do the alkaline
carbonates decompose silicates in the belt of weathering, but other carbon-
ates accomplish the same result. This has been shown by observations on
the alterations in the soil by Hilgard.’ Of these other carbonates, that of
calcium has been found to be the most important. Hilgard states that
the decomposition of the silicates is much more active in calcareous soils
than in noncalcareous soils that are otherwise similar. He says “ this is
seen when we contrast the analyses of calcareous clay soils of the humid
region with the corresponding noncalcareous ones of the same. In the
former the proportions of dissolved silica and alumina are almost invariably
much greater than in the latter, so far as such comparisons are practicable
without assured absolute identity of materials.”°
Since the carbonate sediments are mainly or solely produced by the
process of carbonation of the silicates of an earlier period, Hilgard’s state-
ment is equivalent to saying that the process of carbonation to-day is
promoted by the carbonation of past geological ages.
It is well known that soils containing carbonates are fertile, and
therefore are favorable to abundant vegetation. This is well illustrated by
the limestone soils. As already seen, vegetation promotes carbonation,
and thus produces carbonates. It therefore appears that there is constant
action and reaction through vegetation and carbonation in promoting the
process of carbonation. Vegetation results in the process of carbonation,
and this produces the carbonates. Carbonates in turn result in further
carbonation and furnish favorable conditions for vegetation. Carbonation
provides carbonates, and therefore promotes vegetation. The relations of
the three may be represented by the followmg diagram, the arrows
indicating the directions of action and reaction.
Vegetation.
eS
Carbonation.————_ > Carbonates.
« Bischof, G., Elements of chemical and physical geology, translated by B. H. Paul and J. Drum-
mond, Harrison & Sons, London, 1854, vol. 1, pp. 8-11.
> Hilgard, E. W., Report on the relations of soil to climate: Bull. Weather Bureau, U. S. Dept.
of Agric., No. 3, 1892, pp. 36-38; cited, Merrill, p. 371.
¢ Hilgard, cit., p. 37.
478 A TREATISE ON METAMORPHISM.
The process of carbonation with the liberation of silicic acid is contin-
uous, cumulative, and takes place on a vast scale. Notwithstanding this,
there is, as already seen (p. 474), only a comparatively small amount of
carbon dioxide in the belt of weathering at any one time—from 38 to 500
parts in 10,000. Carbonates may be present; if so, they represent the
action of earlier carbon dioxide. The continuous process of carbonation
may be understood when the law of mass action and the time factor are
considered. Under the law of mass action an acid anywhere existing will
unite with some of the bases present. The compounds thus produced are
in part transported elsewhere by underground and overground drainage.
The carbon dioxide used is resupplied by the oxidation of organic material,
and thus the reacting agent continues its work; and so the process goes
on unceasingly through geological time.
While this cumulative process goes on without cessation in the humid
regions where the carbonates are largely removed by solution, im the semi-
arid and arid regions the carbonates, especially the alkaline carbonates,
may become so abundant in the soil as to be unfavorable to vegetation, and
thus check the process. (See p. 543.) Where the alkalies, especially
sodium carbonate, exceed a certain very small amount, vegetation can not
exist, and the region becomes a desert."
“The amount of soluble salts which plants can stand depends upon
the character of the salt, the character of the soil, and the kind of plant:
Hilgard states that few plants can bear as much as 0.1 of 1 per cent of
sodium carbonate, or about 38,500 pounds per acre to a depth of 1 foot
[about 3.9 ke. per sq. meter to depth of 30 em.]; of sodium chloride, about
0.25 per cent; and of sodium sulphate most plants can grow with 0.45 to
0.50 per cent present, and are affected by even less salts in the sandy lands
than on heavy clay or gumbo lands.”
The process of carbonation with desilication just described may take
place without other reactions with a number of minerals. (See p. 396.)
The change involves an increase in volume ranging from about 13 per cent
«Means, Thos. H., and Gardner, Frank D., A soil suryey of the Pecos Valley, New Mexico:
Field operations of Div. of Soils, U. 8. Dept. of Agric., 1899, No. 64, 1900, pp. 53-58.
b Whitney, Milton, and Means, Thos. H., The alkali soils of the Yellowstone Valley: Bull. Div.
of Soils, U. S. Dept. of Agric., No. 14, 1898, p. 10.
-
SOURCE OF CARBONATES. 479
to over 50 per cent. But carbonation with or without desilication occurs
on far the most extensive scale in connection with hydration. When the
great majority of the abundant silicate minerals are carbonated they are
also hydrated. (See pp. 397-399.) Finally carbonation with desilication
occurs rather extensively in connection with both oxidation and hydration.
(See p. 399.) In all these combinations, provided all the compounds
formed remain as solids, there is an increase in volume. This increase
rarely falls below 5 per cent; it runs as high as 75 per cent, or even higher,
but the more common range is between 15 and 50 per cent. It is well
known that the processes carbonation, carbonation combined with hydra-
tion or oxidation,- and carbonation combined with both hydration and oxi-
dation, liberate much heat, but the data are not at hand from which the
amount can be calculated.
The process of carbonation just considered is one of paramount impor~
tance in the belt of weathermg. Although the process is not so extensive
as hydration, if one were to pick out a single chemical process especially
characteristic of this belt and of great significance in geology it would be
carbonation. Carbonation has continued through all geological time since
land areas first arose above the sea. Moreover, it has continued in all the
land areas of the globe; but the process is very slow in the frigid zones,
becomes of importance in the temperate zones, and is of great consequence
in the torrid regions. In the warm regions the process is rapid in propor-
tion as there is high humidity and consequently abundant life. In the
arid regions carbonation is comparatively slow.
There is no reason to suppose that the carbonates existed as original
rocks or as original minerals of the igneous rocks. If this be so, all the
carbonate formations which now exist and which have existed at any time
in the past have been produced by carbonation. The precipitation of
carbonates in the sea is accomplished largely through organic agencies.
These carbonates are essential to the existence of great classes of sea ani-
mals, and certain it is had not these carbonates formed by the process of
carbonation and been contributed to the ocean, the evolution of life upon
the globe would have followed entirely different lines from those that have
been followed.
°
480 A TREATISE ON METAMORPHISM.
Notwithstanding the vast scale and dominant importance of the
process of carbonation, it is mainly accomplished, as has already been seen,
through the small amount of carbon dioxide continuously concentrated in
the belt of weathering by means of organic material. It is therefore clear
that a very slow but continuous action, extending over the globe throughout
geological time, has produced stupendous results. But the positive side
of the process of carbonation is of scarcely less importance than the nega-
tive side. It has been pointed out that carbonation largely takes place
through the decomposition of the silicates or by desilication. The silica
set free, as already noted, largely separates as colloidal silicic acid. The
amount of silica thus liberated approximates to the molecular equivalent of
the carbon dioxide which unites with the bases combined with silica; or,
since the molecular weights of carbon dioxide and silicaare 44 and 60.4, there
is about one and one-third times as much silica released from the silicates
as there is carbon dioxide combimed in the carbonates. The stupendous
results of the process of carbonation are therefore matched by the results
of the process of desilication. As will be seen subsequently, while a por-
tion of the silica of the colloidal silicic acid is precipitated in the belt of
weathering, probably by far the greater part of it is carried by the ground
waters to the belt of cementation, and is there largely precipitated.
While carbonation, with correlative desilication, is of such fundamental
importance in the belt of weathering, under exceptional conditions silication
may occur. Where silica is very abundant and in a readily soluble form,
the law of mass action may be so effective as to result in the formation of
abundant silicates rather than carbonates. This has not been observed as
a general process in the belt of weathering, doubtless because of the lack
of close observation. However, in many cases in ore deposits above the
level of ground water the silicates of the metals are formed. So far as I
know, this is better illustrated in the lead and zine district of Missouri than
elsewhere. Here silica as chert, partly amorphous, is very abundant. The
zinc was originally in the form of sulphide. As the level of ground water
descended owing to denudation, and the sulphide arose into the belt of
weathering, the zine sulphide was decomposed, the zinc being oxidized.
Simultaneously with this process it united, upon a somewhat extensive
scale, with the silica, producing silicate of zine.
AGENTS OF WEATHERING. 481
HYDRATION AND DEHYDRATION.
By hydration is meant the union of water with chemical compounds,
thus producing hydrous minerals. The water for hydration is derived
mainly from the hydrosphere. Hydration stands as the most extensive
reaction in the belt of weathering. In its importance in this belt as a
geological process it is second only to carbonation. Indeed, it has been
supposed by some geologists that hydration is the dominant reaction of the
belt of weathering, the process of carbonation being wholly ignored. The
list of important hydrous minerals formed comprises many silicates and
oxides and some carbonates, sulphates, ete.
As a matter of observation, all of the so-called anhydrous silicate
minerals of the igneous, sedimentary, and metamorphic rocks which have
long remained in the belt of weathering are shown by analysis to have
become more or less hydrated. But more important than this, the new
minerals which develop in the belt of weathering are usually strongly
hydrated. Of these in the silicate class the kaolin, serpentine-tale, chlorite,
and zeolite groups are examples. Of the oxides the most important are
those of aluminum and iron, the former occurring as gibbsite or diaspore;
and the latter commonly as limonite, but not infrequently as gothite or
some other hydrated oxide.
In many cases in the dense rocks hydration goes on to a certain stage
aud then ceases or at least becomes very slow. This is due to the fact that
the process of hydration involves expansion of volume and therefore makes
it necessary that the superjacent material be elevated if the process continues.
Such partly hydrated rocks below the surface when brought to the surface,
and therefore relieved from pressure, may continue to rapidly hydrate nearly
or quite to the completion of the process with great expansion of volume.
In many cases the process is so rapid that the term slaking is applicable.
This slaking has been observed by Merrill in the granitic rocks of the
District of Columbia,“ and by Derby in the sedimentary rocks from rail-
way cuttings of Brazil.’ In both of these places the rocks when exposed
at the surface soon break into powder, although in position they are perhaps
so strong as to require blasting. These facts make it clear that the process
of hydration is largely dependent upon the pore space in the rocks. Where
“Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, p. 188.
> Derby, O. A., Decomposition of rocks in Brazil:. Jour. Geol., vol. 4, 1896, pp. 529-540.
MON XLVIr—04——3 1
482 A TREATISE ON METAMORPHISM.
there is large pore space and abundant room, so that hydration may take
place without lifting the rocks, it is probable that with sufficient time the
process may go on to completion in all parts of the belt of weathering. How-
ever, in proportion as the rocks are dense and contain small pore space the
process of hydration at an early stage is likely to be retarded or altogether
stayed, because of the necessity demanded for more space and because the
chemical affinity of rocks for water is not strong enough, at least in the later
stages of the process, to overcome considerable pressure.
While hydration is the rule for the belt of weathering, in regions in
which the temperature is high and which are not continuously humid dehy-
dration may take place. Dehydration probably occurs on the largest scale
in regions of high temperature in which dry and wet seasons alternate.
During the wet season hydration occurs, and at the dry times dehydration
occurs. ‘This process is well illustrated by iron. As is well known, ferric
iron in the belt of weathering is ordinarily hydrous, and this gives a yellow
color. In regions of high temperature where the humidity is low for at
least a portion of the year the soil is likely to be red, the iron being in the
form of hematite rather than géthite or limonite. Such regions are illus-
trated by the Desert Ranges of southern California, in which dark red is the
dominant color. But dehydration also takes place to some extent in such
humid regions as the southern Appalachians. Crosby, Dana, and Russell
attribute to dehydration the bright-red color of the surface soil in this
region as compared with the less brilliant color of the subsoil.“ This region,
although one of large precipitation, is apt to be dry in late summer and
autumn, the season when conditions are favorable to dehydration. In the
subsoil the process is naturally less marked. The dehydration of iron is
well known, because the process involves a change of color. Under the
conditions in which dehydration of iron takes place, it is certain that many
of the other hydrates which form in the belt of weathering are also dehy-
drated to a greater or less extent. It is well known that the process of
dehydration of many minerals begins at temperatures lower than 110° C.
Indeed, it is certain that the dehydration of limonite and other hydrous
oxides of iron begins as low as 100° C., and that if this temperature be
RELATION OF HYDRATION AND DEHYDRATION. 485
earried on in a laboratory, dehydration is very appreciable.* These tem-
peratures are not reached under natural conditions, but in hot regions
where water is not plentiful it is probable that partial dehydration of
such compounds as the zeolites, colloidal silicic acid, etc., takes place on a
considerable scale. Doubtless the process in reference to these minerals
has not been commented upon because ordinarily there is no accompany-
ing change in color. To illustrate, the table on pages 375-394 shows that
aluminum hydroxide, as gibbsite and diaspore, forms on a vastly greater
scale in the belt of weathering than ferric hydroxide. It is highly proba-
ble that, under the same conditions in which dehydration of iron occurs,
dehydration of aluminum hydroxide also takes place to some extent. The
same statements may be made in reference to the other hydrous minerals
which lose their water or a part of it at a low temperature, as, for instance,
the zeolites.
It has already been stated that hydration is the most extensive
reaction of the belt of weathering. This is at once found by reference
to the classified tables of alterations, page 402. It is there seen that a
number of the important oxides and many of the more important silicates
may be altered by simple hydration. The process of hydration, as
explained in Chapter IV, from a physical-chemical point of view, involves
expansion of volume and the liberation of heat. The amount of heat
liberated is great, as shown in Chapter V. Whether hydration occurs
alone or occurs combined with the other processes, there is an increase in
volume. In simple hydration the volume increase ranges from a very
small per cent to as high as 160 per cent, as in the alteration of corundum
to gibbsite. Commonly the increase in volume is less than 50 per cent.
The quantity of heat liberated in the process of hydration is great, but
the average amount can not be quantitatively stated.
OXIDATION, CARBONATION, AND HYDRATION.
As already noted, oxidation, carbonation, and hydration may each
take place separately, but commonly two or three of these processes are
simultaneously at work on the same rocks, and even on the individual
mineral particles. No average can be given as to the amount of increase
«Leith, C. K., The Mesabi iron-bearing district of Minnesota: Mon. U. 8. Geol. Survey, vol. 43,
1903, p. 262.
484 A TREATISE ON METAMORPHISM.
in volume as a consequence of the combination of these three reactions,
but doubtless in most cases the increase in volume, where the processes are
complete, is 15 to 50 per cent or even more.
It is of importance to note that the oxygen, carbon dioxide, and water
added to the rocks in the belt of weathering are not directly derived from
other rock materials, but are in large measure derived from the atmos-
phere and hydrosphere. As already noted, the oxygen is directly derived
from the atmosphere; the carbon dioxide comes from the atmosphere mainly
through the intermediary action of vegetation and other organic matter,
and water is derived directly or indirectly from the hydrosphere. The
depletion of the atmosphere and hydrosphere in oxygen, carbon dioxide,
and water is continuous, and through geological ages would doubtless
seriously encroach upon the supplies of these materials were it not for
the compensatory reversing reactions largely occurring in the zone of
anamorphism. (See pp. 366-369.)
SOLUTION.
Concurrently with the processes of oxidation, carbonation, and hydra-
tion, the underground waters of the belt of weathering take compounds
into solution. The process of solution is not only concurrent with these
other processes, but is promoted by them. ‘This follows from the fact that
the dominant processes of carbonation and hydration transform the com-
pounds into more soluble forms. The change of ferrous to ferric iron by
oxidation has a reverse effect, but the quantitative value of this reaction is
small compared with that of carbonation and hydration.
It has already been explained that all natural compounds are soluble in
pure water, but the solution would be slow if this were the only solvent. It
has been seen that in the underground solutions of the belt of weathering
there are continually produced organic acids, carbonic acid, nitric acid, sul-
phurie acid, and hydrosulphurie acid. These produce corresponding salts—
carbonates, nitrates, sulphates, and sulphides. Also the presence of these
salts favors solution. Besides these salts chlorides are present. The active
acids of the solutions unite with the bases of the minerals. But in this process
the acids with which the bases were united in the minerals are displaced, and
these acids may also pass into the solutions and become active constituents.
The oxygen, carbonic acid, nitric acid, sulphuric acid, and hydrosulphuriec
IMPORTANCE OF SOLUTION. 485
acid have been accounted for by processes in the belt of weathering. But
the source of the hydrochloric acid, producing the chlorides, has not
been explained. This subject is considered on pages 789-790.
The amounts of the compounds which are taken into solution by
ground waters depend very largely upon the number and ainounts of
the above active chemical agents or solvents. Also the amounts of the
compounds which are taken into solution depend upon the relative propor-
tions of the elements present, and the manner in which they are com-
pounded. If all the important bases occurring in the rock-making minerals
were equally abundant the stronger bases would be taken into solution to
a larger extent than the weaker bases. Therefore there would be dissolved
more sodium and potassium than calcium and magnesium, more calcium or
magnesium than iron, more iron than aluminum. And as a matter of fact,
the percentages of the compounds dissolved are in this order. But the
amount of alkalies in the rocks is much less than that of the alkaline earths,
iron, or aluminum, and consequently the total amount of the former
elements dissolved may be less than the latter—indeed, is commonly much
less than the amount of the dissolved alkaline earths. Further, the greater
the proportion of bases present as compared with the acids—i. e., the more
basic the compounds—the more readily the minerals are decomposed and
the greater the amount of bases which are dissolved. If the acids were
present in equal quantity there would be dissolved a greater amount of the
salts of the stronger acids than of the weaker acids. Thus there would be
a greater quantity of sulphates, nitrates, and chlorides than of the car-
bonates. Since, however, in the solutions carbonic acid is so much more
plentiful than any other active acid, carbonates greatly predominate. The
only acid in solution which can be compared with carbonic acid in
abundance is silicic acid, produced by the decomposition of the silicates.
However, the liberated silicic acid, as explained on pages 115, 176, sep-
arates in a colloidal state, and in that form is exceedingly inactive and,
notwithstanding its great quantity, does comparatively little work. The
great power of the weak acid, carbonic, in the solutions shows that the law
of mass action is of great consequence in the relative amounts of the
compounds formed and taken into solution.
The above general statements are very well illustrated by T. Mellard
Reade’s estimate of the amount of salts which are abstracted from each
486 A TREATISE ON METAMORPHISM.
square mile of area. He calculates that throughout the entire globe there
is removed annually in solution 96 tons (about 86 tonneaus) of material
per square mile, which he divides as follows: Caleium carbonate, 50 tons
(45 tomeaus); calcium sulphate, 20.tons (18 tonneaus); sodium chloride,
8 tons (7.2 tonneaus); silica, 7 tons (6.3 tonneaus); alkaline carbonates and
sulphates, 6 tons (5.4 tonneaus); magnesium carbonate, 4 tons (3.6 ton-
neaus); oxide of iron, 1 ton (0.9 tonneaus). The order of the larger of
these amounts is much what one would expect. The strong, abundant
base, calcium, is largely united with the weak abundant acid, carbonic.
The strong acids, such as sulphuric and hydrochloric, are largely united
with the alkaline metals, sodium and potassium, but a residuum is left over
which is supposed to be united with the calcium.
We now know that in solutions all the bases are united with all the
acids except so far as dissociation occurs, according to the various factors
of strength, mass, and solubility, and we can see that it would have been
better to have estimated the various compounds as oxides or elements rather
than as salts. But it would still be true that the most abundant salt is
calcium carbonate, that that standing next in abundance is probably cal-
cium sulphate, and that those standing next in abundance are the alkaline
sulphates and chlorides.
We conclude from the foregoing that in the belt of weathering all the
elements in the minerals are being dissolved all the time, but with greatly
varying rates, depending upon the strength and abundance of the active
compounds in solution and upon the solubility of the various minerals upon
which the solvents are acting. While the more readily soluble substances
are dissolved many times more rapidly than those which are usually spoken
of as insoluble, even those substances which are least soluble may be taken
into solution on a large scale. For instance, silica in the form of quartz is
regarded as exceedingly insoluble, yet it is certain that in the iron-bearing
formations of the Lake Superior region quartz has been dissolved on
an enormous scale. Also the evidence is conclusive that such extremely
refractory substances as hematite and limonite are dissolved. (See pp.
548-549.) While all minerals in the belt of weathering are soluble, after
a sufficient length of time the constituents remaining undissolved are, of
course, those which are relatively insoluble.
«Reade. T. Mellard, Chemical denudation in relation to geological time. Also Merrill, cit., p. 194.
RELATIONS OF SOLUTION AND DEPOSITION. 487
Consequent upon these various reactions, combined with solution,
material is continuously abstracted from the belt of weathering, and thus
the openings tend to increase in size. This does not appear in the case of
the unconsolidated materials at the surface, for gravity brings the particles
together as fast as material is dissolved; but in the rocks which have
sufficient strength to hold together, the openings are often numerous and
large, and constitute a considerable percentage of the volume of the rocks.
The best illustrations of rocks with large openings are the limestones,
which, above the level of ground water, are commonly intersected with
numerous open joints, are often porous throughout, and not infrequently
contain caves. On account of the continuous abstraction of material by the
ground waters the belt of weathering might be called the belt of solution.
DEPOSITION.
Later it will be seen that concurrently with solution, favored by it
and in direct proportion to it, deposition is constantly going on. Thus
it will be seen that the substances dissolved in greatest quantity are those
deposited in greatest quantity. As just noted, in limestone regions solution
takes place on an enormous scale. But while this is going on deposition
of CaCO; is occurring on a large scale; for instance, in caves travertine,
stalactites, and stalagmites are forming. Where evaporation is not easy,
solution will be the rule; where evaporation is easy, deposition. Thus a
cave may be simultaneously enlarged at the bottom and decreased in size
at the top. Solution and deposition are correlative processes, but in the
belt of weathering the first is dominant. The absolute amount abstracted,
estimated by Reade at 96 tons per annum per square mile, represents the
difference between the results of solution and deposition.
GENERAL STATEMENTS.
On the preceding pages it has been seen that the processes of oxidation,
carbonation, hydration, and solution are characteristic of the belt of weath-
ering, and that all take place on a great scale. However, it has also been
seen that to some extent the reverse of these processes—deoxidation,
silication, dehydration and deposition—also occur. This is what one
would expect from the laws of physical chemistry. All chemical reactions
are reversible; and while the first set of the above processes greatly
488 A TREATISE ON METAMORPHISM.
dominates over the reverse set, all the above sets of reactions take place
on an important scale in the belt of weathering, although the belt is
characterized by the one set rather than the other.
CONTACT METAMORPHISM.
Contact metamorphism is a term used to cover the mutual effects of
intrusive and intruded rocks. The effect upon the intruded rocks is known
as exomorphic and that upon the intrusive rock as endomorphic. The first
is of greater consequence. In considering the exomorphic effect, ordinarily
there has been no well-defined attempt to estimate the relative importance
of the direct and indirect action of the igneous rocks. The direct effect is
due to the heating of the intruded by the intrusive rock. The indirect
effect is due to the increased activity of the gaseous and liquid solutions
caused by the intrusive rocks.
The gaseous and liquid solutions adjacent to igneous rocks differ from
ordinary solutions in two respects. First, at the time of the crystallization
of magmas, gases and liquids emanate from the igneous rocks. The chief
constituent of these emanations is, of course, water, but associated with the
water are other compounds. By the circulation of the gaseous and liquid
solutions the material emanating from the igneous rocks may pass into the
surrounding rocks. A second and still more important indirect effect is
that of heating the solutions of the surrounding rocks. In Chapter III, on
“The agents of metamorphism,”
to) b)
it has been explained that solutions gain
marvelously in their metamorphic power by heat, and adjacent to igneous
rocks the temperature may reach or even surpass that of the critical
temperature of water. While by conduction high temperature progresses
from the magma, by convection, where the circulation is vigorous, solutions
having a high temperature are carried over wide areas. In this treatise
contact metamorphism is considered in three places, for the nature of the
contact. action, like that of other forms of metamorphism, is very different
in the different belts and zones. Therefore the subject of contact metamor-
phism is considered under the belt of weathering, the belt of cementation,
and the zone of anamorphism. In this chapter only the first part of the
subject is considered; the other parts may be found under the heading
“Injection,” on pages 646-652.
CONTACT METAMORPHISM. 489
In the belt of weathering contact metamorphism comprises (1) the
direct contact effect and (2) the indirect effect through gaseous solutions.
This second is ordinarily called the action of fumaroles and solfataras.
DIRECT CONTACT EFFECT.
The direct contact effect is produced by conduction of the heat of the
magma into the surrounding rocks. The temperature of the rocks
immediately adjacent to the igneous rocks may become very high—indeed,
approach that of fusion, and possibly reach a fusion temperature locally.
Under these circumstances the complex mixtures of minerals characteristic
of the belt of weathering, including the hydrates, silicates, and other forms,
are likely to be baked, and thus indurated. The hydrous minerals may be
dehydrated, and thus to this extent the ordinary reactions of the belt of
weathering are reversed. The zeolites, serpentines, tales, chlorites, ete.,
where dehydrated, are destroyed. Limonite may be changed to hematite,
gypsum to anhydrite, ete. From the carbonates the carbon dioxide may
be driven off or calcination take place, and thus lime and magnesia be
produced. This gives the rocks an alkaline reaction, so that contact action
on such rocks in the belt of weatherimge is sometimes called ‘caustic”
action.
The general contact effects of dry heat have been called, depending
upon the degree of action, baking, fritting, and vitrification. Irving gives
the following illustrations of these processes:
Sandstones are decolorized and often fritted to a glistening enamel-like or por-
celanic mass; in other cases, where the cement is of a calcareo-argillaceous nature,
this is melted into a glass; clay and mud are converted into porcelanite or brick,
with marked change of color in many cases; tufts and phonolites are so far vitrified
as to acquire a character resembling that of obsidian; brown coal is altered into seam
coal or anthracite, and these in other cases into a substance more resembling graphite,
while in others (probably under less pressure) the coal is converted into coke, the various
shades of metatropic change of brown coal into anthracite, carbon-glance, bituminous
coal, and black coal being observed in some cases in the same section through several
meters of the mass; a prismatic structure is developed, not only in clays and marls,
but even in sandstones, in brown coal, in seam coal, and in dolomite; limestones are
altered into crystalline marble, often with complete effacement of their stratification
and even of all traces of their fossils; the finer varieties of grauwacke and its asso-
ciated shales are converted into hornstone, as in the classical region of the Brocken.
490) A TREATISE ON METAMORPHISM.
Of these metatropic changes by the action of dry heat under pressure that of the
formation of marble has been experimentally verified years ago by G. Rose and more
recently by Richthofen and others.“
Usually all of the direct dry heat contact effects are very limited in
amount, extending only short distances from the igneous rocks. Ordinarily
the direct effect is modified by indirect effects In many of the cases men-
tioned by Irving, where the material when modified was not in the belt of
weathering, it is highly probable that the results were not accomplished by
dry heat alone, but with the assistance of gases and vapors. Commonly
the rocks which are being baked, fritted, or fused are in the midst of gases
and vapors, including, of course, abundant water vapor and oxygen. Also,
as in the cases mentioned by Irving, in many instances which have been
given of the effect of dry heat alone the material when altered was below
the belt of weathering; but as the different cases of metamorphism attrib-
uted to dry heat have not been studied in reference to the different zones
and belts of metamorphism, and it is therefore impossible to discriminate
with certainty the cases which are truly due to dry heat alone from those
in which heat works in conjunction with gases and vapors, or even in con-
junction with water, in so far as: the gases and vapors are present, we have
the indirect effects considered below mingled with the direct contact effects.
INDIRECT CONTACT EFFECT, OR WORK OF FUMAROLES AND SOLFATARAS.
It has been said that the indirect contact action is due to gaseous
solutions. The action of such solutions may extend much farther than the
direct contact effect. The belt of weathering may be permeated locally
with hot gaseous solutions. The work of these gaseous solutions is
essentially of the same nature as that of ordinary gaseous solutions,
discussed on the previous pages (see pp. 59-63); but the gaseous solutions
adjacent to igneous rocks usually contain a greater quantity of the active
chemical agents than do ordinary solutions; and, moreover, their tempera-
ture is much higher than normal. This gives a combination of conditions
which results in much more rapid alteration than the average of the belt of
weathering and alteration of a different kind. The exceptional regions in
which these unusual conditions obtain are volcanic In volcanic regions
« Trying, A., Chemical and physical studies in the metamorphism of rocks, Longmans, Green & Co.,
London, 1889, p. 76.
CONTACT METAMORPHISM. 491
the so-called fumarolic and solfataric® actions may oceur on an extensive
scale. In such regions fumarole is generally applied to the gases at the
higher temperatures, and solfatara to the gases at moderate temperatures,
although this distinction is not everywhere applied. Fumarolic and solfa-
taric action is therefore largely the work of gaseous solutions, although
liquid solations are also at work to an important extent. The gases may
be widely dispersed through the porous rocks, but in many cases they
move chiefly along large definite channels, from which they spread for
various distances along smaller and less definite openings.
Of the gases of fumaroles and solfataras, water vapor is dominant. In
many districts it is emitted in enormous quantities from the orifices. Prob-
ably the gas next in abundance to steam is sulphurous oxide (SO,). Other
important gases are chlorine (Cl,), hydrochloric acid (HCl), hydrofluoric
acid (HF 1), hydrosulphuric acid (HS), sulphuric acid (H,SO,), carbon
dioxide (CO,), oxygen (O), and hydrogen (H,). Boric acid (H,BO,), is
sometimes plentiful; nitrogen (N.), is abundant; and hydrocarbons occur.
The proportions of these gases are very different in different localities. In
many cases the differences seem to be a function of the temperatures of the
fumaroles and solfataras. In the very hot fumaroles chlorine, hydrochloric
acid, and hydrofluoric acid are likely to be abundant. At lower temperatures
sulphurous oxide and hydrosulphuric acid are very common, and at still
lower temperatures oxygen, carbon dioxide, and nitrogen are abundant. In
the hot fumaroles, associated with the strong acids characteristic of them, are
also found many of the compounds characteristic of fumaroles and solfataras
of lower temperatures, although these may be overlooked. With all classes
of fumaroles and solfataras water vapor is dominant, but it is frequently
more markedly dominant at the lower than at the higher temperatures.
The source of the gases of fumaroles and solfataras is a complex ques-
tion of which I shall not attempt to give an adequate discussion. It is to
be presumed, as pointed out on pages 931-937, that the ultimate source of
the various products formed in the belt of weathering is the material of
the original magmas. However, later it will be more fully explained that
several of the rarer elements of the igneous rocks are locally concentrated
in the belt of weathering by various processes. ‘To what extent the active
gases of the voleanoes are derived from the original magmas and what
part from the later concentrations of these I shall not here consider.
@ Latin fumus, smoke, and Italian solfo, sulphur.
492 A TREATISE ON METAMORPHISM.
The source of the most abundant vapor emitted during volcanic action
has been much discussed. Some have held that the water is derived
from the original magmas, never having been at the surface, while others
have held that it is derived from the water of the surface and underground
circulation. It may be in part derived from each. Chlorine, hydrochloric
acid, hydrofluoric acid, and hydrosulphurie acid are undoubtedly largely
formed by the action of hot water upon chlorides, fluorides, and sulphides.
Sulphurous and sulphuric oxides are produced by the action of the oxygen
upon the sulphides. These may unite with water and produce sulphurous
and sulphuric acid. To what extent the above chlorine, fluorine, and sulphur
compounds are derived directly from the magmas and to what extent from
later segregations containing an unusual amount of these elements it is not
my purpose here to discuss. The hydrogen of fumaroles and volcanoes is
usually regarded as due to the decomposition of water." This may be
accomplished by ferrous oxide according to the following equation:
H,O+2FeO=H,+Fe,03.
The immediate source of a part of the hydrogen is probably hydrosul-
phurie acid, the hydrogen being liberated as this acid decomposes and the
sulphur separates. But since the H,S is probably produced, as already
explained, by the action of H,O on sulphides the hydrogen is indirectly
derived from the water. It is shown that carbon dioxide is extensively
liberated by the process of silication in the zone of anamorphism. (See pp.
677-679.) It is natural to suspect that this liberated carbon dioxide largely
makes its way to the surface at places where there is active upward circu-
lation. Voleanic districts are certainly such places. Hence it is believed
that much of the carbon dioxide issuing in connection with fumaroles and
solfataras is that liberated by silication. Where carbonates are present as
rocks adjacent to volcanoes another portion of the carbon dioxide is doubt-
less derived by the action of the strong acids upon these carbonates, salts
of the strong acids being produced and the carbon dioxide liberated. No
opinion is expressed as to the relative abundance of these two sources of
carbon dioxide.
As would be expected, the ordinary reactions of the belt of weathering
take place in regions of fumarolic and solfataric action the same as else-
«Geikie, Sir Archibald, Text-book of Geology, Macmillan & Co., London, 1893, 3d edition, pp.
193-197.
CONTACT METAMORPHISM. 493
where. Indeed, these reactions probably take place at a greatly acceler-
ated rate. ‘Therefore in areas of fumarolic and solfataric action oxidation,
carbonation, hydration, and solution are important reactions, but ordinarily
this is not recognized because of the more striking action of the exceptional
compounds which are present in unusual abundance. That oxidation
takes place on an extensive scale is shown by the yellow, red, or brown
colors of the altered rocks due to limonite, hematite, and magnetite which
extensively form from the monoxide of iron. Hydration in connection with
fumaroles and solfataras has been to a considerable extent overlooked.
Delesse found that dry steam at a pressure of five atmospheres had no
appreciable action upon minerals. Barus found that the diabase of the
Comstock lode, containing fresh feldspar, was not kaolinized by the long-
continued action of steam at 100° C.* However, it is rare that steam is
dry in the actual case of fumaroles and solfataras, and therefore, as would
be expected at high temperatures, the process of hydration goes on at a
great rate. Carbonation also takes place on a scale scarcely less extensive
than hydration. In consequence the formation of sodium carbonates is
very common and in some places it is found in large quantities.
However, it is not by these common products but by the exceptional
products that fumarolic and solfataric action is especially distinguished.
The chlorine and hydrochloric acids form chlorides, of which sodium chlo-
ride (NaCl), potassium chloride (KCI), ammonium chloride (NH,Cl), iron
chloride (FeCl,), copper chloride (CuCl,), manganese chloride (MnCl,),
and other chlorides have been observed. Through the action of sulphurie
acid various alums are formed, of which potassium-aluminum sulphate
and sodium-aluminum sulphate are the more common. By action upon
calcium-bearing compounds sulphuric acid forms gypsum. By action
upon the alkali-bearing compounds the sulphuric acid forms Glauber’s
salt, or sodium sulphate, and potassium sulphate. The action of hydro-
sulphuric acid upon the various compounds may form various sulphides,
or it may be decomposed without action upon other substances, in which
case it liberates hydrogen and deposits sulphur. The hydrofluoric acid
attacks the various compounds, forming various fluorides, of which fluorite
is the best known. The boric acid forms borates. Of these various sub-
«Barus, C., On the thermal effect of the action of aqueous vapor on feldspathic rocks: School of
Mines Quart., vol. 6, 1884, pp. 1-23.
494 A TREATISE ON METAMORPHISM.
stances formed, some are volatile and may be sublimed. Among these
ammonium sulphate, sulphur, and boric acid are well known.
While any of the minerals present may be acted upon, on the average
the silicates are decomposed on a far greater scale than other compounds; for
silicates are the dominant salts present which the active agents find to work
upon. Besides producing the various compounds above mentioned, which
are especially characteristic of fumarolic and soltataric action, the
relatively insoluble residual products of the silicates are extensively
produced, such as kaolin, ete. While these chemical reactions are taking
place, concurrently with and depending upon them the rocks are softening,
and the result is to produce a whitish or yellowish earth, frequently stained
red where iron oxide is abundant. So long as the fumarolic and solfataric
action continues, mingled with this residual material are many of the soluble
compounds mentioned above; but after fumarolic and solfataric processes
cease, the soluble salts are rapidly leached out by the ground water circu-
lation and there remains the residual, relatively insoluble compounds char-
acteristic of the belt of weathering.
In summary it may be said that fumarolic and solfataric action is not
so different from the ordinary reactions of the belt of weathering as might
be supposed. The reactions are essentially the same. All of the reactions
characteristic of the belt of weathering occur, but at a more rapid rate
than ordinarily. Moreover, the local concentration of numerous strong
acids makes their action of relatively greater importance than under the
ordinary conditions of the belt of weathering. Finally, in the case of fuma-
roles and solfataras the process of decomposition is wholly chemical,
whereas ordinarily in the belt of weathering organic compounds play an
important part.
In a strict sense fumarolic and solfataric action, so far as it is the
action of gases, is that of mineralizers, as defined by Naumann. However,
fumarolie and solfataric action as here considered is not that which is ordi-
narily known as the action of mineralizers; hence I have avoided the term.
RELATIONS OF DISINTEGRATION TO DECOMPOSITION AND SOLUTION.
The mechanical work of weathering is the physical subdivision of the
material; in other words, is disintegration. The chemical work of weather-
ing is decomposition and solution. The process of disintegration promotes
the chemical work of decomposition and solution, because, as disintegration
RELATIONS OF DISINTEGRATION AND DECOMPOSITION, 495
goes on, the surfaces exposed to the action of the chemical agents become
larger. If a mass of material, of whatever size, be divided into any
number of parts with similar forms, the total area of the new surfaces will
be equal to that of the original surfaces multiplied by the square root of
the number-of parts. For instance, if a given mass of rock be divided into
a hundred parts, with forms similar to the original mass, the surface is
increased ten times, and therefore the area subjected to chemical action is
increased tenfold. Hence, in so far as the rocks are broken mechanically,
they give greater surface of action, and this surface of action increases
directly as the comminution increases. It follows that the finely com-
minuted material is much more readily decomposed than the coarser mate-
rial. Loughridge has shown that the amount of soluble material in soils is,
on the average, larger in proportion to the comminution, thus confirming
the conclusion that increase in the surface is favorable to chemical action.”
But decomposition reacts upon disintegration. In proportion as the
material becomes decomposed it is more readily broken up. In so far
as the material is chemically altered, and especially in proportion as it
is dissolved, the rock is weakened and thus the forces of disintegration
rendered more effective. The mechanical and chemical forces therefore
act and react upon each other, each one increasing the effectiveness of the
other; hence disintegration greatly accelerates decomposition and solution,
and decomposition and solution accelerate disintegration.
This action and reaction between the mechanical and chemical work is
well illustrated by the experiments of Daubrée,’ who found that various
minerals and glass when mechanically triturated in a rotating cylinder are
very readily acted upon by pure water, carbonated water, and water con-
taining sodium chloride. In this way he produced a very fine, impalpable
mud, which when dried resembled argillite and contained a high percentage
of alkalies. It is clear from the above that the greatest effects in the way
of weathering are produced by combination of the mechanicaland chemical
processes. Where, by theaction and reaction of the mechanical and chemical
agents, the subdivision has gone to such a stage that the particles are
“«Loughridge, R. H., On the distribution of soil ingredients among the sediments obtained in
silt analysis: Am. Jour. Sci., vol. 7, 1874, p. 17. Merrill, George P., Rocks, rock-weathering, and
soils, Macmillan Co., New York, 1897, pp. 365-366.
bDaubrée, A., Géologie Expérimentale, Paris, 1879, pt. 1, pp. 268-279. Also, Merrill, George
P., Rocks, rock weathering, and soils, Macmillan Co., New York, 1897, p. 197.
496 A TREATISE ON METAMORPHISM.
exceedingly minute, the surface exposed to action is enormous. As illus-
trating this, Whitney has calculated that there are 22 billion grains of sand
and clay in a gram of residual subsoil from a limestone, and that the surface
of these particles has an area of 5,000 square centimeters. This is an
extreme case, but Whitney estimates the number of grains in ordinary soils
to vary from 2 billions to 15 billions per gram."
In an early stage of the process of subdivision disintegration usually
plays the dominant part. As the material becomes finer chemical action
becomes more important. When the particles become minute, chemical
action is dominant in producing subdivision. As shown on pages 432-434,
it may well be doubted whether the exceedingly minute subdivision of the
residual clays from limestones could be accomplished by the mechanical
agents of weathering alone.
Tt has been seen that the mechanical work of weathering is accom-
plished by wind, water, ice, change frem water to ice, change in tempera-
ature, plants, and animals. The chemical work is accomplished by plants,
animals, and solutions. The various factors in this mechanical and chemical
work interlock in a most complex fashion. An increase in the amount of
work of one of the factors may increase or decrease the amount of work
of another factor. Moreover, the kind of work accomplished by one factor
is so different from that of another factor that it is difficult or impossible
to make quantitative comparisons between them. Thus it is impossible to
compare the mechanical work of disintegration with the chemical work
of decomposition.
The particular combination of agents and forces most actively at work
at a given place is largely dependent upon humidity, latitude, elevation,
and life. There are very numerous permutations and combinations of these,
so that the number of variations in the rate of weathering are indefinitely
great.
REGIONS FAVORABLE TO PROMINENCE OF DISINTEGRATION.
The regions favorable to prominence of disintegration are those of (a)
aridity, (b) high latitude, (c) marked topographic relief, (d) sparseness of
plants and animals, and (e) nearness to the sea.
Arid regions—It has already been seen (pp. 438-439) that in arid regions
the mechanical stresses due to rapid change in temperature and to the wind
«Whitney, Milton, The soils of Maryland: Bull. Maryland Agr. Exp. Sta. No. 21, 1893, pp. 8, 83-07.
RELATIONS OF DISINTEGRATION AND DECOMPOSITION. 497
are potent influences in breaking up the rocks. By insolation the rocks
may be coarsely subdivided; by the wind, bearing sand, they may be
finely subdivided—both with very little chemical action. In many dry
regions the rainfall is largely concentrated into a small part of the year.
In some regions this rainfall may occur as severe local storms. Under
these circumstances the disintegrated products are stripped from the rocks
by the running water, thus exposing new surfaces to weathering. The
transported material may be carried to the sea or may be accumulated in
the lower levels between the steep slopes. The former case is illustrated
by the sedimentary rocks which are now forming in the Gulf of California.
McGee describes the granitic and other rocks of southern: California as
being simply split apart and broken into their individual, grains, which are
transported at rare times of abundant rainfall by the process which he
calls sheet-flood erosion and deposited in the Gulf of California with
scarcely any chemical change Such rocks may have very nearly the
average chemical composition of the original rocks from which they are
derived.’
Very frequently the disintegrated material of arid regions does not reach
the sea, but accumulates in the lower areas. Hilgard notes that the soils of
such regions are “‘predominantly sandy or silty, with but a small propor-
tion of clay, unless derived directly or indirectly from preexisting forma-
’ Moreover, he notes that the soil and the
tions of clay or clay shales.”
subsoil are substantially the same. The deficiency of clay—that is, kaolin—
is explained by the lack of the decomposition of the feldspars. The
deficiency of clay is no less marked in these soils than the abundance of
alkalies and alkaline earths. Hilgard finds the proportions of these
elements in arid and humid regions to be as follows:
]
Arid region | Humid region |
| (per cent). (per cent).
|
TBOtAS TBR eee are as | 0. 825 | 0. 187
Sod aeuest Ss ase aesscese=" 251 | O71 |
{Ii Gee ease eet se Sei ee eee | 1. 645 112
|peMiarnesia. ae Asas ose senses | 1. 384 . 209
| | |
aMcGee, W J, The formation of arkose: Science, new ser., vol. 4, 1896, pp. 962-963.
dHilgard, E. W., Relations of soil to climate: Bull. Weather Bureau No. 3, U.S. Dept. Agric., 1892,
S
p. 17.
¢ Hilgard, cit., p. 30.
MON XLVIT-—04 32
4938 A TREATISE ON METAMORPHISM.
The soils of the arid regions investigated by Hilgard therefore contain
from four to fourteen times more of the elements potassium, sodium,
calcium, and magnesium, than do those of the humid regions. This
difference is doubtless due in part to lack of decomposition of the original
compounds; but it is also due in part to the fact that the decomposed
materials of the arid regions have not been leached as they have been in
the humid regions. As shown in another place (pp. 541-543), the soluble
constituents resulting from the decomposition of the original minerals are
largely retained.
In conclusion, we see that aridity or low humidity is generally favorable
to mechanical rather than to chemical work. In arid regions the process of
disintegration is rapid, and that of decomposition and solution is slow. The.
total rate of weathering in arid regions, including mechanical and chemical
work, is probably, on the average, less rapid than m humid regions.
Regions of high latitude —First, regions of high latitude are those of low
temperature. It has already been seen that the disrupting effect of change
from water to ice is an important factor in the disintegration of rocks. This
process is applicable only to those regions in which the temperature falls
below 0° C., and to such regions for only those parts of the year in which
the temperature alternately passes above and below the zero mark. These
conditions at low altitude do not occur at all in the Tropics. They occur
in winter only in the parts of the temperate zones near the Tropics. They
occur in spring and autumn in the cooler parts of the temperate zones and
in the frigid zones. However, in passing from low to high latitude the
length of time during each year in which the conditions are those of
alternate freezing and thawing increases; also the range above and below
zero increases. Hence the disintegration due to freezing and thawing
increases as the latitude increases from those parts of the temperate zones
near the Tropics to the far northern and southern lands. Second, regions
of high latitude are those of extensive glaciation. The glaciers are most
powerful disintegrating agents, producing débris ranging in size from great
bowlders to the minute particles which whiten the waters issuing from
them, and all this with scarcely a trace of decomposition. Third, in regions
of high latitude there are great changes in temperature which work by
differential expansion, as explained on pages 438-439, and thus help to
disintegrate the rocks. Absence of decomposition in connection with disin-
RELATIONS OF DISINTEGRATION AND DECOMPOSITION. 499
tegration at high latitudes also follows from the low temperature, because
very unfavorable to chemical action and to abundant organisms.
Therefore in regions of high latitude the rocks are ordinarily broken
down and not greatly changed chemically. The exceedingly rapid and
extended disintegration of the rocks in regions of high latitude is well
illustrated by such countries as Greenland, Spitzbergen, Franz Josefs
Land, ete. As in the arid regions, the sedimentary rocks formed in regions
of high latitude may be composed of. disintegrated materials, chemically
essentially like the materials from which the rocks are derived. As in the
arid regions, where mechanical materials from different sources are mingled,
they represent the average composition of the original rocks rather than
any one original rock. :
Regions of marked topographic relief _—Reoions of marked topographic relief are
usually regions of considerable elevation. In such regions the mechanical
forces work under most favorable conditions. Gravity pulls the material
down the steep slopes. The range in temperature is great; water alter-
nately freezes and thaws; glaciers are prevalent. Consequently material
is split or ground from the solid rocks and is carried by gravity in great
quantity to lower levels.
The remarkable effect of elevation and sharp relief in disintegration
is especially well illustrated in high mountain ranges. In such regions
great and small masses are split from the cliffs by the disintegrating forces
and agents or are carved from the valley floors by the glaciers, and are
transported to a lower level by water or by ice with scarcely appreciable
chemical change The cirques at the heads of glaciers give excellent
illustrations of the very great rapidity of disintegration due to the combi-
nation of the above causes. The Matterhorn, with its sharp triangular
peak and the great cirques at its base, is a most impressive illustration of
such disintegration. The peaks of the High Sierras and the Canadian
Rockies afford many fine illustrations of the same process.
Penck suggests that the average altitude of mountains in any given
region for a given kind of material is limited by the fact that disintegration
increases so rapidly as the height increases that for any given area there is
a height where the forces of disintegration degrade the mountains as fast as
the forces below are able to lift them. :
«Penck, A., Morphologie der Erdoberfliiche, J. Engelhorn, Stuttgart, 1894, pt. 2, pp. 334-335.
D00 A TREATISE ON METAMORPHISM.
Also the low temperature is unfavorable to chemical action and to life,
and transportation follows closely upon disintegration. The result, there-
fore, of marked topographic relief is rapid disintegration with comparatively
little decomposition.
Regions of sparse plants and animals— "hat sparseness of plants and animals is
favorable to disintegration without decomposition has already been shown,
since it has been made clear that decomposition is largely dependent upon
the action of plants and animals and their by-products. The sparsenrss of
life may be due to low temperature, low humidity, or other causes.
Regions near the sea——Nearness to the sea or to a large river is favorable to
disintegration with subordinate decomposition, since the material is trans-
ported only a short distance before it is deposited below the water. But
this relation is not so important as one might at first think. The amount
of decomposition is far more dependent upon whether the conditions are
favorable for this process before the material reaches the large streams than
upon distance from the sea.
When the material once gets permanently below the surface of the
water, it passes to a considerable extent from the conditions of the Delt of
weathering to those of cementation. While oxygen and living organisms
may still act upon the material, they are not nearly so effective as under the
conditions of the belt of weathering; hence, the process of decomposition is
much retarded. This is well illustrated by the material brought down by
the Nile and deposited in its lower reaches. According to Judd, the deposits
of the Nile delta consist of fresh, unaltered, rounded, angular and subangular
grains of the original minerals of the rock, such as the feldspars, hornblende,
augite, quartz, etc. This material, he says, has been derived from the desert
sands, which lie on either side of the Nile Valley and are swept into it by
the wind.” As has been seen (pp. 496-498), desert conditions are favorable
to disintegration without decomposition; hence the sands contributed to the
upper Nile are little decomposed, and the long river journey does not
ereatly increase the amount of decomposition.
While decomposition largely ceases when the material is transported
by streams, disintegration does not cease, for the particles grind against one
another or over the bottom, and thus comminution goes on; and therefore the
«Judd, J. W., Report on a series of specimens of the deposits of the Nile delta, obtained by the
recent boring operations: Proc. Royal Soc. London, vol. 39, 1885, pp. 213-220.
RELATIONS OF DISINTEGRATION AND DECOMPOSITION. 501
longer the journey the farther this process continues. However, as shown
on pages 432-434, there appears to be an inferior limit in size, beyond
which attrition ceases; that is, the particles become so fine that they are
floated and have not sufficient weight and momentum to materially grind
one another. This conclusion is further supported by the fact, given by
Judd, that the small particles of the alluvial deposits of the Nile which
have been transported great distances are angular, while the larger particles
of the same deposits are subangular or rounded. However, he attributes
the rounding of the larger grains to eolian rather than river erosion; but this
does not lessen the force of the argument as to there being a lower limit
beyond which the particles are not rounded by river action. Further
evidence in the same direction is the angular character of the loess of the
Mississippi Valley. Chamberlin and Salisbury found that the loess of that
valley, much of which must have been floated for long distances, probably
hundreds of miles, is wholly composed of angular particles which show no
evidence of wear. For the most part these loess grains are less than 0.005
mm. in diameter, although in some places 1 per cent or more exceed this
diameter, and a few grains are nearly 0.1 mm. in diameter.
REGIONS FAVORABLE TO PROMINENCE OF DECOMPOSITION.
The regions favorable to decomposition are those of humidity, low
latitude, moderate topographic relief, abundance of plants and animals,
and remoteness from the sea.
Humid regions—I]t is clear that humidity is one of the requisite conditions
for regions of rapid decomposition, since chemical action is very slow in
the absence of water. In humid regions, as already shown, the belt of
weathering always contains a large and variable amount of water, and
water is essential for vigorous chemical action. Hydration is one of the
chief reactions of the belt of weathering, and in humid regions there is
abundance of the necessary compound for the process.
High humidity may occur at any latitude and at any elevation. At
low latitudes and low elevations the temperature is high and life is abun-
dant, and these conditions are especially favorable to chemical work.
As evidence of decomposition in the humid regions, Hilgard contrasts
the soils of such regions with those of the arid regions, and finds that in the
humid regions the soils are usually loams with a clay subsoil. The presence
502 A TREATISE ON METAMORPHISM.
of clays, of which kaolin is the chief constituent, proves the extensive
decomposition of the silicates, especially the feldspars.
It is rather probable that decomposition is most rapid in regions of
moderate humidity, where there is moisture sufficient to carry on decom-
position, solution, and transportation of the disintegrated material with
considerable rapidity, and for animals and plants to be reasonably abundant,
but not sufficient to produce extraordinarily luxuriant vegetation, which
may practically stop mechanical transportation on all but the steepest
slopes, and thus produce a protective covering of decomposed materials.
(See p. 503.) j
While humidity is very favorable to decomposition, it must be remem-
bered that running water is the greatest of the eroding agents, and there-
fore that humidity is also very favorable to disintegration.
Regions of low latitude —Reoions of low latitude are those of high tempera-
ture. Warmth is favorable to chemical decomposition. The great increase
in the activity of chemical agents in consequence of increase in temperature
has already been explained on pages 61-62, 78-81, and hence in warm
regions the chemical agents which are at work in the belt of weathering
have a potency which they do not approach in cold regions. Therefore
the usual deep belt of decomposition in the tropical and semitropical
regions and the general absence of such a belt in the colder regions-is
partly explained by high temperature.
Consequent upon high temperature as one essential factor is abundant
life, which still further favors decomposition, as explained below. In
regions of high temperature the range of temperature may be great,
and therefore disintegration due to this cause be a concurrent process.
But in such regions the temperature does not often fall below zero, and
hence the disintegrating effect of freezing and thawing is nil, or nearly so.
Regions of moderate topographic relief _— Regions of small topographic relief gener-
ally have gentle slopes, and gentle slopes are favorable to retention of
moisture and unfavorable to rapid transportation. Moreover, regions of
small relief usually have low elevations and are favorable to high tempera-
ture, and consequently to abundant life. Hence, regions of small elevation
are those of marked decomposition.
If the elevation be slight the level of ground water may be at a very
small depth below the surface, or even reach the surface, and thus decom-
RELATIONS OF DISINTEGRATION AND DECOMPOSITION. 503
position be stayed. But areas of very low relief are favorable to decom-
position of the narrow belt extending to and somewhat below the level of
ground water. his is well illustrated by Florida. In that region the
original refractory minerals have been decomposed and the soluble material
abstracted; and the relatively soluble minerals, such as the calcium car-
bonate of the limestones, have been wholly abstracted, leaving for a large
part of the region a mantle of imsoluble residual material, corresponding in
composition almost perfectly with the ultimate products of weathering.
(See pp. 520-521.)
In contrast with low regions are regions where the altitudes are great
and the slopes steep. Disintegration and transportation may go on so
rapidly as to give littie opportunity for decomposition.
Thus the most favorable conditions for most rapid but not complete
decomposition’ are limited both by very low and by very high altitudes.
Therefore, the most favorable topographic conditions for rapid decompo-
sition are those of moderate relief. Excellent illustrative regions are
southeastern United States, Nicaragua, and large parts of Brazil. These
are all regions of moderate elevation and continuous moderate slopes.
Regions of abundant plants and animals.— Pentiful plant and animal life is very
favorable to decomposition. Abundance of plants and animals is condi-
tioned by high humidity and high temperature. Regions of abundant life
are therefore the warm, moist regions of the Tropics and the sub-Tropics.
It is not too much to say that abundant animals and plants and _ their
by-products are the sources of most of the active compounds, which,
working through abundant water solutions favored by high temperature,
do the great work in the decomposition of the rocks. As already pointed
out, the process of carbonation, one of the chief reactions of the belt of
weathering, is largely conditioned on the carbonic acid concentrated by
the decomposition of plants and animals.
While the work of life is fundamental in the decomposition of the
rocks, where life is too abundant this may stay the progress of weathering.
Abundant plant life gives a protective covering to the decomposed rocks,
which is effective in proportion to its luxuriance. The plant covering,
where very luxuriant, as in many parts of the warm regions, very greatly
retards, and may almost wholly stop, the mechanical work of disintegra-
tion and transportation. Therefore, under these conditions, in very humid
504 A TREATISE ON METAMORPHISM.
regions decomposition frequently outruns transportation, and a thick layer
of decomposed material accumulates. Finally, there may be produced a
thick, nearly permanent, belt of weathered material which is protected
from mechanical transportation by vegetation and protects the rocks below
from decomposition. Such appear to be the conditions in Nicaragua along
portions of the proposed route of the ship canal. Hayes says that in parts
of Nicaragua, notwithstanding the complete decomposition of the rocks
and the abundant rains, the streams are clear.“ The process of weathering
must therefore be very slow, being almost wholly limited to solution.
Regions remote from the sea—I]n so far as the material is remote from the sea it
is long exposed to the forces of weathermg before it is finally floated or
deposited in the sedimentary rocks, and thus in proportion as the material
is transported far before permanently reaching the water it is likely to be
decomposed.
General statements.— While in the previous pages an attempt has been made
to consider separately humid regions, regions of low latitude, regions of
moderate topographic relief, regions of abundant plants and animals, and
regions remote from the sea, in speaking of one it has been necessary to some
extent to consider one or more of the others. In regions which combine
several of these factors decomposition is ata maximum. It is only when
the materials of the belt of weathering in regions of high latitude, high
altitude, and aridity, in which life is sparse, are compared with the materials
of the belt of weathering in humid regions, regions of low latitude, low
altitude, and abundant life, that the efficiency of the latter combination of
conditions in the decomposition of rocks can be fully appreciated. It has
been seen that where the former set of conditions obtain, weathering con-
sists in rapid change of temperature, change of water to ice, and the work
of glaciers splitting or grinding the rocks into fragments; but that the
particles in chemical and mineral composition are substantially the same as
the original rock, although, of course, decomposition does always take place
to some extent. But in warm humid regions, in which plant life and animal
life are abundant, the solid rocks are not only disintegrated, but the material
is rapidly decomposed. This decomposition may extend from a few meters
to a depth of 90 meters in the subtropical and tropical regions. A result
of high humidity and high temperature are the wonderfully luxuriant
«Hayes, C. W., Report of the Nicaragua Canal Commission, Appendix II, Geologic Report,
Baltimore, 1899, p. 112.
PROFOUND IMPORTANCE OF LIFE. DOD
forests of the Tropics, such as those of Brazil and central Africa. The
difficulties of traversing such a region are almost incredible. The forests
are a tangle of vegetation, which it is possible to penetrate only by cutting
one’s way. Under such conditions the rocks in the belt of weathering are
continuously acted upon in the most powerful way, under the most favorable
conditions of humidity and temperature, by the chemical products furnished
by the plants and animals, alive and dead. No one can realize the
abundance of plant and animal material, dead and alive, both above and
below the soil, in a dense forest region without traveling through one. ‘The
soil for many feet below the surface seems to be mainly composed of vege-
tation. The individuals vary from great trees, through plants of medium
size, to the smaller plants, such as the mosses, fungi, and finally the
bacteria. To a large extent the living plants are feeding on the dead and
decomposing ones. At the surface the quantity of organic material is at a
maximum. Normally the quantity steadily decreases from the surface to
a depth of 12 to 15 meters, where it becomes unimportant. Thus there is
every gradation between places at the surface where the organic material
occupies the larger part of the space to a place below the surface where
the inorganic material occupies all the space.
One who has studied regions where life is dense will readily believe
that great geological consequences follow from the chemical activity of
compounds produced by animals and plants, both alive and dead. It is
not too much to say that life is an essential factor in far the larger part of
the chemical work of decomposition and solution of the rocks.
Of the plants, the minute bacteria are among the most important. Of
the animals, earthworms are perhaps the most important. The prodigious
numbers of these compensate for their smallness. They furnish a striking
illustration of the principle that small numerous agents may be more potent
than large and more conspicuous but less numerous ones.
From the foregoing it is plain that living and dead plants and animals
interact with one another and with the rocks in the production and main-
tenance of the soil. As shown on pages 452—453, chlorophyll-bearing plants,
and especially one group of them, the leguminose, working in conjunction
with bacteria, are the agents which collect carbon and nitrogen from the
atmosphere and place them in the soil in a combined state. Dead plants
and animals are the materials upon which living bacteria, oxygen, and water
work to produce carbonic, nitric, and sulphuric acids. These form carbon-
506 A TREATISE ON METAMORPHISM.
ates, nitrates, and sulphates in great abundance within the soil, and thus
the processes of carbonation, nitration, and sulphation of the minerals of
the rocks are rapid. The carbonates, nitrates, and sulphates furnish food
for the chlorophyll-bearing plants. The plants are food for the animals.
After a time these plants and animals die, decompose, and thus supply
additional material to carry on the process of decomposition of the rocks.
Therefore there is action and interaction between the chlorophyll-bearing
plants and the bacteria, and between these and the animals alive; interac-
tion between these plants and animals dead and bacterial plants alive, and
interaction between all live and dead plants and animals and the rocks.
Decomposition of rocks produces plant food; the food builds the plants:
the plants build the animals; these in turn, both while alive and after
death, produce decomposition of the rocks, and so round indefinitely.
It is now clear why it is so difficult to produce a good soil where such
a soil does not exist, and why a soil once formed may be retained indefi-
nitely by proper tillage. Where the soil is. good and vegetation is
abundant, the complicated machine is in full motion and is able to produce
new soil as fast as the old soil is transported elsewhere by wind or water.
The cycles above considered do not include the whole case. The full
geological cycle does not occur in the belt of weathering alone. As fully
explained elsewhere (pp. 538-539, 612 et seq.), a portion of the salts dis-
solved during the decomposition of the rocks in the belt of weathering is
transported to the belt of cementation and there precipitated; another
portion goes to the sea. It is only when these portions are followed that a
full comprehension of the complicated and far-reaching geological effect of
weathering through plants and animals may be obtained. Here it will only
be remarked that of the salts of the sea, calcium carbonate is produced by
carbonation of the silicates, but this process is mainly dependent upon the
plants. Thus the work of the plants furnishes the material for the hard
parts of sea animals; these hard parts are the source of the great limestone
formations; as a result of geological revolutions limestone formations built
below the sea are raised and become land areas; upon these limestones
soils of unusual fertility are formed, and this results in luxuriant plant and
animal life, thus completing a cycle, one among the many geological cycles
connected with plants and animals.
While life is especially potent in chemical work, the power of life in
mechanical work is considerable.
CHEMICAL CHANGES DURING DECOMPOSITION. DOT
CHANGE IN CHEMICAL COMPOSITION OF THE ROCKS.
So far as the change in the average chemical composition of rocks in
the belt of weathering is concerned, solution is largely the final determina-
tive factor; although, as has been seen, the amount and nature of the
material dissolved are dependent upon all the many factors heretofore con-
sidered. By comparing the original chemical composition of rocks with the
chemical composition of the partly disintegrated and decomposed equivalent
rocks, one is able to obtain an approximate idea of the relative losses of the
elements. In order to compare the losses which the various elements have
undergone as a result of solution and transportation, it is necessary to
suppose that some one element is insoluble and is not abstracted at all.
Usually iron and aluminum have been chosen as the insoluble constituents.
With these as a standard in. most cases, but in some cases other elements,
Merrill made the following calculations as to the losses of the elements for
various rocks: “
Supposing the amount of iron to remain constant, for a disintegrated
and partly decomposed granite in the District of Columbia the original
composition and the losses of the constituents are as follows:
Taste 1.— Analyses of fresh and altered granite.
coe Loss for
Constituent. compenition, iheon
| °
Per cent. Per cent. |
SiG yoo. Sen Naame aerate 69.33 | 14.89 |
PAM carat ae eae al or 14. 33 3.23 | |
Berri CkOX1d Ce sae aera eee \ Ban aan 2
INET NOUSLO XG Casey ee J | |
Inleimces semis Melee. donates 3 2h) bea.
| Magnesia_-_____-.--... Rarer ee erie 2. 44 | 1.49 |
OCR) tb ne neuoneneuamaarces (eee 2.70'| 28.625 |
mePotashiees: 2. ee eee 2.67 | 31.98 |
Phosphoric anhydride. ..-..-.-- 0.10 40.00 |
[ Upptbilon soe Bocesteannoaosesqters | 122 0.00
otal eho se eenee G2) G0 |secesoedac
@Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, pp.
209-232. !
908 A TREATISE ON METAMORPHISM.
Supposing the amount of aluminum to be constant (except in Table
III, analysis 2, fresh diabase from Venezuela, in which iron is supposed to
be constant), the original composition and the losses of the various elements
for the following rocks are as follows:
Tasie Il.—Analyses of fresh and altered gneiss and syenite.
«3 98 Fresh
Constituents. ‘Wine | (cigs
Original composition.
| Per cent. | Per cent.
Silica soe ewe ee eee 60. 69 59. 70
ATuminants doa die hee ene ce aie | 16.89] 18.85
Herric/oxid Chrssse-eraeeeee eee | 9. 06 4.85
Tuimie teats yauuenatiese Saas ace 4.44 1.34
Maenesiay ara ssa aa ae eee 1.06 . 68
Ptashne wlan ak em iest | 4.255] * 5.07
Soda ie bees Teele ee 2.82 6. 29
Phosphoric acide-s: a--e seceeeece | ZO eevee eee taas
Tomi tionts= 22 oaeee ae ec esas eens 62 1.88
Teo tall ses eR es eRe aS | * 100.08 99. 56
= = | — ——
Decomposed ‘Decomposed
gneiss. | syenite.
Constituents.
Loss for each constituent.
Per cent. Per cent. ,
Silicas fess ese aee eee eee se ee 52. 45 62.18
AS] amin a) eee re a eee ena 0. 00 0. 00
Herric oxi dem emse tess see seeee 14. 35 86. 17
ACs Ye Rar ae ae tar ee see 100. 00 87. 90
Magnesia sian see senee cece eeee 74. 70 82. 10
Potashaeereey ae a emer ue eee 83. 52 81. 85
Soc aiy aie eae ict ape 95. 03 97.11
Phosphoricsacids seme serseesee ee ONOO) aa
Tomitioneseecascsc saeceee sees O00; Saaseeeene
CHEMICAL CHANGES DURING DECOMPOSITION.
Taste II].—Analyses of fresh and altered diabase and basalt.
Fresh dia- | Fresh dia- |
base (Mass-| base (Vene-| (Bonemia). | (France).
Original composition.
Per cent. Per cent. | Per cent. Per cent.
Silicatnemepeeeeerenene see eaes 47, 28 49. 35 43.61 48. 29
JNO S o 5 ooo cadonaesbeadsas 20. 22 15. 30 12. 26 13. 25
Rend Cloxid Glee per eeraseac see SAGGMuleayyaertsctte 3.51 0, 00
Ferrous oxide....--.----.-.-.-- 8. 89 12. 28 12.16 16. 66
IVS gS beone dosaSbsbeeoSbonOoS 7.09 9. 60 Thile B7/ 7. 33
WEGOESO), -cocodeoseesenbooouo 3.17 7.38 9. 14 7.03
Manganese oxide.-..-..------- Ul- |lodaatesnsdlsascotosocusccdllsscacscec
Potashterecesce reece eer 2.16 85 .81 1.81
SioGky Sau ceacuosesccleaSssrees 3. 94 1.98 2,72 Beal
iPhosphoriciacideesseeserrccsa- aS peaseossen| SHacoseusonos6|soanacoses
lenitionesseesere eee ee eeeaae 2.13 3. 25 H,O 4.42 4.92
Motalee ace cece. sos cess 100, 59 100. 00 100. 00 100. 00
grsted dia. [Pegommpored| Decomgered Pecomposed
Loss for each constituent.
Per cent. Per cent, | Per cent. Per cent.
Silicamerees ee sere ease a See 18. 03 42.40 32399 65. 56
Atm nae ee ee 2 See eects ceases 0. 00 21.38 0. 00 0. 00
Bera onde acres si ies eer CH10 0.00 50.17 | 88.84
IREMMOWE OPCS socdsadstauceue J ;
SIME eae eet eS Nena eas 25. 89 83.23 | 84.53 47. 24
Meemesiay aes eso 21.70 | 61.37 | 74.10 | 96.38
Manganese oxide.-....--..-.-- ANGE? -\sedocsssoc Ieee ema Wersertoseiets
IPotashaomesage pence sascccecices 29.15 45. 88 \ a 9 { 83. 34
Sod aston ene eee eee nce 12. 83 95.37 J ie 74.41
iPhosphoricjacideesse=eeeeeeeee TSO len eae saree crete aeeneere | eerie
Tonitloneeeee eee eceene 0. 00 OX O03) 5 Saas ere ee 0. 00
509
510 A TREATISE ON METAMORPHISM.
Taste 1V.—Analyses of fresh and altered diorite, pyroxenite, and argillite.
| Es
Heat alone) Purensnte | ite (Mary.
; Original composition.
Per cent. Per cent. Per cent.
Silica 2 ate ee eee 46.75 38.85 --}---44.15
jMluminases hee DA SIGS 9 ew ae erp , 30.84
Ferric oxides s22.222-- 16.79 |-- 12.-86 | 14. 87
TE Line Wesyense sere ea ener 9. 46 .-b2 - -| .48
lmeMaon estate —ceeeeess 5.12 22.58 | 121
Rotashessee= screenees | .55 | 5/8) | 4.36
eiSinglr wensecacuasscneee lees DUS ie sai 51
Phosphoric¢ acid..-----) 626. |ecerscseeclzcesecces<
le mitionee seems | 92 6.52 4.49
otal este ae mare 100.01 | 100.00 | 99.97
=
Raeeaiozl (eval iperoxenitollaaereaitee
Loss for each constituent.
Per cent. | Per cent. Per cent.
Silica (Sous saeee ee oe 87-310) 4358 abi
ANU ae eee eee 0. 00 | 0.00 | 0. 00
Ferric oxide 2: 225-25: 21. 03 41.48 | 8. 78
Lime so 228: Benes 97. 30 | 44.45 | 100.00
Magnesia sees see Sieve aes |e OL 28. 16
Potashigeecn = Pia ay aad 38.75 | 47.05 77.95
Sodaee eee: | 84.87 | 0.00 99. 64
Phosphorie acid. -.----- 19. 87 | Swear So aeS See eee
omit neta eee a | 0.00 | 20. oad See aeesss
|
One of the best cases of complete decomposition known to me is that
of the diabase of the iron-bearing formation of the Penokee-Gogebic district.
In this rock in its most altered form not one vestige of any original mineral
was discovered with the microscope. While the analyses of this rock are
here inserted, it was largely altered below the level of ground water, but at
a point where abundant descending waters from the belt of weathering were
converged, and where therefore the conditions were intermediate between
those of the belt of weathering and those of the belt of cementation. The
CHEMICAL CHANGES DURING DECOMPOSITION. 511
analyses of the fresh diabase and the decomposed rock are given in columns
1 and 2. Supposing the alumina to have remained constant, the losses of
the other constituents are as given in column 3.
Taste V.—dAnalyses of fresh and altered diabase.%
| 1 | 2 3
| |
H,O at 105°_......--.-| 0.15 | 0.29 c
H,0 at red heat ....-.- | 234 | 13.54 } G00.
(CORR aia Sane | 38. | 88 58. 06
SOROES tems as nos: | (Sane eee teal Retire
PY Ouatneers ee cue es ns | 13 14 | 54.83
SiO Waseem mae | .47. 90 41. 60 63. 57
BT ©) eee te esos oe .82 3.79 0. 00
PAN Osuceerye ete eh 15.60 | ¢37. 20 0. 00
fH Oumenenttar aes Su | BEG) ah Bh om 63.51
CHO peter aeee ease WeeTrace sen eeeeece tat 100. 00
HeO eaeee teas 8.41 30 98. 51
RHO(COO) = Sees s5eSsc ING) a pretenses. oe 100. 00
WOO) Rees senaeaeenees | 17 . 08 | 80. 26
Ba@ Waieete snc 05 Trace | 99. 00+
CaO easeee eee he nae 9°99) 28 99. 04
Me ORE rae seewss--ccse ere pleliaet| 02 99. 90
K,0..2:--s/00see = | D9= lees oe 100. 00
Nal Ome een ees, 2.05 .07 98.57
| Totals nee TO0s15 0 e100! coi | eee
| { 7 |
alrying, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin: Mon. U. Ss. Geol.
Survey, vol. 19, 1892, p. 357.
bSOs; calculated from BaO found, as this latter probably exists as BaSO..
c AleOs is probably a little high, owing to alkali retained by titanic acid.
512 A TREATISE ON METAMORPHISM.
Upon the supposition that no potash is lost in the case of a phonolite,
the original composition and the losses are as follows:
Taste VI.—Analyses of fresh and altered phonolite.
Fresh
phonolite
(Bohemia).
| Original com-
| position.
| Per cent.
Silica 42545 Sees es ei cise ae See eseeee 55. 67
Alumina Sema crc teicese a sein cieaane ce ane 20. 64
ErriClOxi dees esere cease seas eee ee eeeeee 3.14
isha yeereee sa Sa Uma acters a 1. 40
Mapniesiassiiesai ras See nclee aeeaoeiseteeiee . 42
Potashe ieiSesaisccesee sec oe see eseeioaee 5. 56
Sod ab ce erice eeae eacen ane ee ecieee eee Up ile)
Toni tiontesiraseeceroe cc ose a c ease cece 4,33 ‘
Total sseoe fe Sets Se See sees see | 98. 28
Decomposed
phonolite.
Loss for each
constituent.
Per cent.
STH ey ss es a ae Ne arate ape aie ot 8. 54
Alumina sSooh 2 eke a Soe oss sees sins 1.60
Ferricioxidem=erua es secsacs cceeeeeee 100. 00
DSTA Oa Sa a ae eR 16. 34
Mapnesiauose sees ore see eat ee eiceetoe 4.35
Potash::- see eseuce ve cence see e seecnases - 00
SOd a srsees seine vee cee ewee een eneees 65. 99
Tenitions ee ss ease se oe ee eesees a eeces 00
CHEMICAL CHANGES DURING DECOMPOSITION. 138
Supposing the amount of silica remains constant in a limestone, the
original composition and the losses are as follows:
Taste VII.—Analyses of fresh and altered limestone.
Fresh
Garena
| Original com-
| position.
Per cent.
SULT Cae ees cee eat eareie wns tans Pee oe eee 4.13
AN UIA aly Peete repre amine laa eae eae ee 4.19
EGE CATON ase Seem sae ons soe Scie 2.35
Wancanicroxid Caeser ene eeet eee aes 4.33
Terrie Nee seed ete onet nee ence. wee |e eedaitrg
Magnesiazece sasccetssoe p= Ssceeneccceses 30
Ro tas hig erie iis tek ras Mee ed Drees 35
GEL Sia fe Sese eee Ree 16 |
iWiateTeye neh, eisai ceca sera Scene amie secese 2.26
Carboniciacideessassheseesseeseeeesee eee 34. 10
Iphogphoricjacidemeacemaseeeee eee eee 3. 04
100. 00
Residual clay.
Loss for each
constituent.
Per cent.
SHUG SS eee eee Bococtonacdaeae 0. 00
Alluminate sis: ae osteises cece eee ee 11. 35
Berni Cyn OMe een ee sete e erie ercteisite ese meee 89. 56
Weiner @xiGlso 8 seceucsssesacbads sass 57.59
IL) ae ee eee SOC ae bee seme een ' 98. 93
IMiaom es] ate mae ae ene eae are eres 89. 38
Botas iets Sse sae seN nas Aone eae | 66. 37
SO Be emseee ane = alae octet eee cane 53. 26
Waterss sees coker cece eee ccm ents 41. 63
Carboniciacideesassseeeeee eee eee ee ee aac | 100. 00
phosphor cia Gi dees sese nee sae eee ee 89. 76
While in the foregoing tables, with the exception of one rock spoken
of as disintegrated, the altered forms of the rocks are classed as decom-
posed, it must be understood that these terms are relative. In the so-called
MON XLyII—04——33
514 A TREATISE ON METAMORPHISM.
decomposed rocks the processes of decomposition have gone to difterent
stages. In but few of the rocks analyzed has the process nearly approached
completion. Also the rock which is spoken of as disintegrated has been
partially decomposed. This is conclusively shown by the fact that there is
ereat variability in the calculated losses for the various elements. If the
‘rock had simply been mechanically disintegrated the composition of the
altered rock would have been the same as that of the original rock.
In the tables the various elements—iron, aluminum, potassium, and
are chosen in different cases as constant in amount. This shows
silicon
that the supposition that any element is fixed im amount is erroneous.
Evidently in each case in making the calculations the method has been to
choose the element in which the loss has been the least, and by this to
gauge the loss of the other elements. For any one of these elements it is
highly probable that the loss im most instances is as great as or greater
than the loss of this element where other elements are chosen as the
standard.
For instance, when aluminum is taken as fixed the loss of the potassium
ranges from 29 to 83 per cent. It is highly probable that in the case of the
phonolite, where potassium is taken as fixed, the loss of this element is at
least as great as the minimum, and it may equal the maximum. Therefore
the percentages of loss of the other elements of the phonolite, given on page
512, are greater than estimated. On the average aluminum is the least
soluble of the elements, and in those eases where it can be used as the
standard the underestimates for the other elements may not be large; but
the oceasional very considerable loss of alumina in some instances—for
example, a limestone (Table VII, p. 513), where the loss is over 11 per cent—
shows that even where this element is taken as fixed there may be very
considerable losses of it, and consequently corresponding underestimates
of the losses of the other elements. For instance, in a diabase (column 1 of
Table III, p. 509) the aluminum is taken as fixed and the loss of the iron
oxide is calculated at 18.1 percent. In column 2 of the same table the iron
oxide is supposed to be fixed and the loss of the aluminum is calculated at
21.38 per cent. The facts probably are that there were heavy losses of
iron and aluminum in both rocks, and that in each case the calculated loss
is much too small.
CHEMICAL CHANGES DURING DECOMPOSITION. SUS)
It is therefore clear that even the large losses of the elements repre-
sented by the above tables are much too small. The amount of the error
in a given case for a certain element is large in proportion as the element
is relatively insoluble. For instance, in the phonolite, where the potassium
is considered fixed, the loss in alumina is given as 1.60 per cent; but if the
loss of the potassium is, for instance, 830 per cent—and it is probably more
than this—the loss of the alumina was more than 30 per cent. But in the
ease of the sodium, the loss of which is calculated as 66 per cent, this
amount may be considerably too small, but it can not be more than 100
per cent; hence the relative error is much less in this case. For instance,
in the diabase (Table V) the loss of sodium, 98.57 per cent, is almost, and
of potassium, 100 per cent, is quite, correct. Where the losses are given
as small in the foregoing tables, therefore, it is probable that the calculated
amounts are but a fraction of the real losses; but where the losses are given
as large, the errors, while very considerable, are probably not so great
proportionally, and the calculated amounts give a rough approximation to
the minimum percentage of depletion of the rocks in these elements.
In order to show the caiculated comparative losses of the six important
elements, aside from aluminum, the following table is compiled for those
rocks in which the calculation has been made upon the supposition that the
aluminum is fixed.
Table showing calculated percentage losses of important elements on the supposition
that the aluminum is constant.
| Potash. | Soda. | Lime. | M88” | sitica. | [mon | Alu”
Gneiss (column 1, Table II, p. 508) --.----- 83.52 | 95.03 | 100.00 | 74. 70 | 52.45 | 14.35 0. 00
Syenite (column 2, Table II, p. 508)----..-- 81. 85 | 97.11 |- 87.90 | 82.10 | 62.18 | 86.17 0. 00
Diabase (column 1, Table III, p. 509) ------ 29.15 | 12.83 | 25.89 | 21.70 18. 03 | 18.10 0. 00
Diabase (column 3, Table V, p. 511)---.---- | 100.00 | 98.57 | 99.04 | 99.90 | 63.57 | 87.81 0.00
Basalt (column 3, Table III, p. 509) -.-.---- 61. 69 84.53 | 74.10 | 32.99 | 50.17 0. 00
Basalt (column 4, Table III, p. 509).--.---- | 88.34 | 74.41 | 47. 24 | 96. 38 | 65. 56 | 88.84 | 0.00
Diorite (column 1, Table IV, p. 510) ..----- | 88.75 | 84. 87 | 97.30 | 97.17 | 37. 31 | 21.03 0. 00
Pyroxenite (column 2, Table LV, p. 510) El 47.05 | 0. 00 | 44.45 | 76.19 | 43.58 | 41.48 0. 00
Argillite (column 3, Table IV, p. 510) ..---- | 77.95 | 99. 64 | 100.00 | 28.16 | 57.57 | 8.78 0. 00
crave eteneer es AMP. ol Ps | 67.70 | 70.31 | 76.26 | 72.27 | 48.14 | 46.30 | 0.00
a Averages of potash and soda made from eight tables, asin the fifth it is impossible to state relative amounts of K
and Na.
516 A TREATISE ON METAMORPHISM.
From the above table it appears that the order of loss, reckoned by
quantity, beginning with the greatest, is: Lime, magnesia, soda, potash,
silica, iron oxide, alumina. Of the two alkalies, more of the soda is dis-
solved than the potash. Of the alkaline earths, while on the whole the lime
is dissolved in greater amount, in some rocks more magnesia is dissolved
than lime. One of the unexpected things shown by this table is the fact
that the percentage loss in silica is more than one-half as great as the average
losses of the alkalies and alkaline earths. When it is remembered that the
average amount of silica in the original rocks is 48.7 per cent, and that
the average of the alkalies and alkaline earths together is but 17.5 per cent
of the total, it is clear that the loss of the silica is about the same as the
entire losses of the alkalies and the alkaline earths together.
This is a very interesting conclusion, since it shows that a very
erroneous impression prevails in reference to silica. It has been generally
supposed that the rocks in the belt of weathering are not much depleted in
this compound. On account of the extreme insolubility of crystallized
silica or quartz, it has been supposed that the silica of a rock is exceedingly
insoluble. For instance, Merrill says: ‘Silica, even in its most soluble form,
requires 10,000 times its weight of water for solution.”* Doubtless, under
ordinary circumstances, quartz is but sparingly soluble, although, as
shown in another place (see p. 848), even this form of silica is dissolved
to some extent. But in the majority of instances a large part of the silica
is present as a silicate, although of course quartz is also abundant. When
the silicates are broken up by carbonation there is every reason to believe
that the greater portion of the freed silica separates as silicic hydroxide
or colloidal silicie acid, which is readily soluble. That this reaction
takes place has already been shown (see p. 480) and that the silicic
hydroxide is dissolved and abstracted from the rocks is shown by the
tables already given. In the rocks in which the silica is largely as
silicate and the alumina is taken as constant the calculated losses of
silica are high—in gneiss, 52.45 per cent; in syenite, 62.18 per cent; in
decomposed diabase and basalt, 18.03 to 65.56 per cent; in diorite, 37.31
per cent; in pyroxenite, 43.58 per cent, and in argyllite, 57.57 per cent.
The average of these numbers is 48.14 per cent. It should also be
remembered that these are underestimates, because they are based upon the
“Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, p. 238.
CHEMICAL CHANGES DURING DECOMPOSITION. SILT
erroneous supposition that the alumina is not dissolved at all. Since in
some of these rocks a part of the silica was present as quartz and the
rocks are not completely decomposed, it is certain that much more than
half of the silica of the silicates decomposed in the belt of weathering went
into solution and was carried elsewhere by the underground solutions.
The number of instances given is large enough to make it probable that
they represent approximately the average fact as to the soluble nature of
the freed silica in the belt of weathering. It is therefore no inference, but
a fact proved by analyses, that the freed and dissolved silica of the belt
of weathering furnishes an enormous and adequate supply of this compound
for the induration of the rocks of the belt of cementation, explained on
pages 617-640.
Another notable thing shown by the table on page 515 is the small
range in solubility of the various elements. With some exceptions the
usual range for all the important elements except iron is not greater than a
ratio of 1 to 2. But in the case of iron the amount dissolved varies greatly
and irregularly. Disregarding the wider variations and considering only the
thoroughly decomposed rocks in which the variations in the amounts of the
other elements are not great, the amount of iron dissolved varies from 14.35
to 88.84 per cent, a ratio of more than 1 to 6. Doubtless this great varia-
bility in the amount of iron dissolved is dependent upon the fact that the
iron occurs in both the ferrous and the ferric forms. The former is much
more readily soluble. When the iron is mainly ferrous one would expect
that a large proportion would be dissolved; where ferric, a small proportion.
In conclusion it may be said that, in order to comprehend the amounts
of the losses of the various elements in the belt of weathering, it is necessary
that the amount of loss of the supposed fixed substances should be known,
and the methods of calculation used, although a line of attack giving approx-
imate results fails to convey a correct impression. Doubtless in many
cases the supposed fixed element is reduced to one-half or even to one-third
of the original amount. If this be true, it is evident that the percentages
of losses given fall far below the facts. It is, of course, impossible that more
than 100 per cent of any constituent shall be lost; but from this it does not
follow that the volume of the residual material may not be an exceedingly
small fraction of the original material. For instance, if in a given rock the
losses of all the various elements were exactly proportional, the conclusion
from the analyses would be that there had been no loss, and yet 0.90 or 0.99
O18 A TREATISE ON METAMORPHISM.
of the constituent might have been abstracted, and thus the amount of
residual material left be but 10 per cent or 1 per cent of the original amount.
Doubtless even the latter shrinkage sometimes occurs in the case of the
limestones. It is therefore manifest that all that the foregoing tables show
is that the losses calculated for the elements are at least as great as given,
aud in many cases they are in all probability much greater. Probably
more accurate calculations than have been made are possible by taking as
the constant constituents the very difficultly decomposable accessory min-
erals. For instance, where zircon is present and is uniformly distributed
through the rock the assumption that this mineral is constant would
undoubtedly lead to closer approximation to truth than has been obtained on
the hypothesis that the iron or aluminum, or both, are constant.
But in order fully to solve the problem of the amount of losses of the
various elements, in addition to the facts given in the tables, we need to
know the original mass of the material as compared with the resultant mass.
If we could combine these data we could give a quantitative estimate of the
loss of each of the elements due to solution in the belt of weathering. But
as long as one of the two necessary factors of the problem is not ascertained,
we know only that the losses of material are much greater than any method
of calculation yet made would lead us to believe.
ORDER OF DECOMPOSITION OF THE MINERALS, AND THE END PRODUCTS.
From the relative amounts of losses of the various elements, as given on
page 515, one may infer the relative rate of decomposition of the minerals.
The alkalies sodium and potassium commonly occur together; the same is
true for the alkaline earths calcium and magnesium; but neither of these
rules is invariable. One would expect the heavily alkaline minerals to be
decomposed most rapidly; next in order the alkaline earth minerals; next
in order the ferromagnesian minerals; and lastly and least rapidly the
heavily aluminous minerals. Of course this statement is only of the most
general kind, since most of the abundant minerals may have as bases more
than one of these groups. But in proportion as the alkali end of the series
dominates, the minerals are soluble; in proportion as the aluminous end of
the series is dominant, they are insoluble.
We thus have the explanation of the order of decomposability of the
silicates. Among the common anhydrous minerals, nephelite-leucite min-
erals are the most readily decomposed Second in order of decomposability
MINERALS OF BELT OF WEATHERING. 519
are the olivines and similar minerals. The third group is the pyroxenes
and amphiboles. Fourth is the biotite-muscovite group. The fifth and
most difficultly decomposable important group is the feldspars. Still more
difticultly decomposable are some of the subordinate silicates. Of these
garnet, staurolite, tourmaline, andalusite, fibrolite, cyanite, and zircon are
probably the more important. The decomposition of these minerals is a
relatively slow process. The order given is that of relative decomposability.
Garnet alters somewhat readily; zircon very slowly indeed.
While the above statements hold in a general way, besides the con-
stituent elements, another very important factor enters into the rate of
decomposition, viz, the acidity of the mimeral. Thus the feldspars vary
from orthosilicates to trisilicates. On this account the orthosilicate anorthite
decomposes more readily than the trisilicates albite and orthoclase, not-
withstanding the fact that alkalies are absent in the first and abundant in
the second. For the same reason some of the basic feldspars decompose
under some circumstances more readily than certain of the more acid
pyroxene-amphibole and biotite-muscovite groups
The more abundant relatively insoluble products of the first stage
of the alteration of the anhydrous silicates may be classified into hydrous
silicates and oxides. The hydrous silicates comprise (1) the kaolin group,
(2) the serpentine-tale group, (3) the chlorite group, (4) the hydro-mica
eroup, (5) the zeolite group, and (6) the epidote group. The oxides
include (1) the gibbsite-corundum group, (2) the quartz group, and (3) the
ferric iron group.
Under long-continued favorable conditions of the belt of weathering
many of these hydrous silicates, which form in the belt of weathering or
which migrate into the belt of weathering from the belt of cementation,
are further modified and changed to simpler compounds. The hydrous
silicates thus further altered comprise serpentines, chlorites, zeolites, and
epidotes. These silicates, it may be noted, are minerals very characteristic
of the belt of cementation. The serpentine alters into magnesite, quartz,
iron oxide, and other compounds. The chlorite passes into gibbsite,
magnesite, iron oxide, and quartz. The epidote passes into gibbsite and
quartz. Itis probable that the zeolites pass into kaolin, gibbsite, carbon-
ates, and quartz. Of course, under the conditions of the belt of weathering,
the carbonates, including most of the magnesite, are dissolved and
removed.
520 A TREATISE ON METAMORPHISM.
Of the oxides in the original rocks, quartz is that of dominant impor-
tance; but hematite and magnetite are of consequence. Quartz is only
slowly dissolved. Hematite may become slowly hydrated. Magnetite may
be oxidized and hydrated. All of the iron oxides are very slowly soluble.
It therefore appears that the important end products in the belt of
weathering, whether the original compounds are silicates, hydrous or anhy-
drous, or are oxides, are included in the following groups: (1) Kaolin
group, (2) tale group, (3) gibbsite-corundum group, (4) quartz group, (5)
ferric iron group, hydrous or anhydrous. ‘These are the minerals which
have become adapted to their environment and- may therefore persist
indefinitely, so long as the conditions remain those of the belt of weathering.
The elements which constitute this group are but six in number. In order
of probable abundance they are as follows: Oxygen, silicon, aluminum,
ivon, magnesium, hydrogen.
A sixth unimportant group is formed by the very difficultly decom-
posable minerals already mentioned—staurolite, tourmaline, andalusite,
cyanite, fibrolite, and zircon, and to these should probably be added the
additional rare difficultly decomposable minerals, such as diamond, casit-
erite, gold, silver, xenotime, monazite, ete.
The quartz is largely that which was originally present in the rocks as
quartz. Watson notes this fact for the granitic rocks of Georgia. While
the total loss of silica in the rocks varies from 10 to nearly 80 per cent, the
residual products contain quartz granules similar to those in the original
rocks, with little or no evidence of corrosion.” The loss is due to the
decomposition of the silicates and the abstraction of the greater part of the
liberated silica, as explained on pages 516-517. For the most part the
other important minerals of the end series are secondary.
The foregoing minerals, by the processes of decomposition and solu-
tion, are concentrated in the belt of weathering in various proportions.
Under one set of conditions one group of them may be dominant and others
entirely subordinate. Ordinarily, for instance, the rare difticultly decom-
posable minerals mentioned are not noticeable, but if the original rocks
contain an unusual amount of them they may become rather important. In
some cases a single one of these minerals may become important or domi-
nant. An excellent illustration of such a case is that of Iron Mountain,
«Watson, Thos. L., Weathering of granitic rocks of Georgia: Bull. Geol. Soc. America, vol. 12,
1901, p. 99.
MATERIALS OF BELT OF WEATHERING. a21
Missouri. Here the country rock is quartz-porphyry, in which are veins of
iron oxide. The quartz-porphyry was disintegrated, decomposed, and
largely removed, while the iron oxide was concentrated on the surface of
the mound, producing a residual mass composed largely of nodules and
bowlders of ferric oxide in a matrix of residual clay.
Where the conditions for solution and removal of the soluble material
are not favorable, in addition to the end products above given there may
be found important amounts of many other relatively soluble minerals.
This condition of affairs, as explained on pages 497-498, frequently occurs
in arid and semiarid regions, where in the residual soils are found consid-
erable quantities of carbonates, sulphates, chlorides, and nitrates of the
alkalies and alkaline earths.
These ultimate products of weathering are found to the exclusion of
other compounds only where the process is complete, and this is very
exceptional. Hven under very favorable conditions of weathering the limit
of disintegration is never attained and decomposition is usually far from
complete. The end products of weathering are found as preponderant
constituents only in the soils and subsoils in humid warm regions, which
are permeated by plants; and even in these soils it is rare indeed to find all
the constituents fully changed to the ultimate products above given. In
proportion as the process of weathering is incomplete, other minerals and
compounds are present; and it has already been seen that all kinds of rocks
may exist in all stages of disintegration and decomposition. Hence the
chemical composition of the rocks of the belt of weathering may vary
most widely. Ordinarily the undecomposed or partly decomposed materials
are prominent or even predominant. The undecomposed minerals are
likely to be present in the order of their refractoriness. Therefore, the end
compounds above given adapted to the belt of weathering may vary from
a very subordinate quantity to an amount which vastly preponderates over
all other constituents. While this is true, it is perfectly clear that the sum
total of the processes of weathering tends toward mineralogical and chem-
ical simplicity. The end products of weathering comprise but a few of
the mineral species, and these species are simple compounds of a few
elements.” Hence the propriety of the assignment of this belt to a zone
of katamorphism.
@Merri!l, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, N. Y., 1897,
pp. 265-266. ; Ses
922 A TREATISE ON METAMORPHISM.
TOTAL GAINS AND LOSSES IN WEATHERING, AND CHANGES IN VOLUME.
The processes of oxidation, carbonation, and hydration involve great
additions of material and expansion in volume, provided all of the compounds
formed remain. Of these expansions in volume, that due to hydration is
the greatest. The added material is of course oxygen, carbon dioxide,
and water. The increases in volume due to the processes of oxidation,
carbonation, and hydration and their various combinations have been
worked out for each of the minerals (see Chapter V), and the results are
brought together in Table D on pages 395-408. It is seen there that the
expansion of volume with most of the silicates due to these processes and
their combinations usually varies from about 20 to 60 per cent, but in some
cases 1s 80 per cent or more.
However, the processes of oxidation, carbonation, and hydration are
accompanied by solution of great quantities of materials, and consequently
the amount and the volume of the residual solid compounds are usually
far less than those of the original compound because of the abstraction of
a large proportion of the majority of the elements originally present and
their transfer to the surface waters or to the belt of cementation.
As already fully noted, the calculations as to losses of elements thus
far made have been upon the supposition in a given case that one element
is constant; but in most cases, as already explained, it is highly probable
that a considerable portion of these elements also has been dissolved.
On this hypothesis Merrill has brought together a number of computa-
tions upon the total losses of various rocks at different stages of weathering.
The calculated losses vary from 13.47 to 97.635 per cent. Supposing the
iron constant, disintegrated and partly decomposed granite from the Dis-
trict of Columbia has lost 13.47 per cent; supposing the aluminum con-
stant, a disintegrated and partly decomposed diabase from Medford, Mass.,
has lost 14.93 per cent; a diorite from Albemarle County, Va, 37.51 per
cent; an argillite from Harford County, Md., 40.83 per cent; a diabase
from Spanish Guiana, Venezuela, 39.51 per cent; a basalt from Kammer
Bull, Bohemia, 43.96 per cent; an altered pyroxenite, now soapstone, from
Harford County, Md., 52.46 per cent; a syenite from Arkansas, 55.28 per
cent; a basalt from Crouzet, France, 60.12 per cent; an altered pyroxenite,
LOSSES OF MATERIAL BY SOLUTION. 523
now soapstone, from Fairfax County, Va., 77.95 per cent; and supposing
the silica constant, a Carboniferous limestone from Arkansas has lost 97.635-
per cent.
Watson “ has calculated the losses of various disintegrated and decom-
posed granites from Georgia, including granite, porphyritie granite, and
eranite-gneiss, and has found that the loss for the entire rock varies in
different cases from 7.68 to 71.82 per cent’ The losses are in. all cases
higher on the assumption that the Fe,O, is constant than on the assump-
tion that the Al O, is constant.’
So far as I am aware, computations have not been made as to the
addition of the constituents oxygen, carbon dioxide, and water in the proc-
esses of weathering. Until this is done it is impossible to make approxi-
mately accurate quantitative statements as to the total amounts of original
material and weathered material. It is still less practicable to make quan-
titative statements as to the relative volumes of the original and altered
materials, since in this matter are involved not only the additions and sub-
tractions of materials, but the specific gravities of the original and second-
ary materials and their states of aggregation. Undoubtedly the specific
gravities of the resulting minerals are less, on the average, than the specific
gravities of the original minerals. Also, the state of aggregation is in almost
all cases less compact. Therefore, for a given amount of material the
volume of the weathered products is greater than that of the original
material. But notwithstanding the .addition of material. the lessened
specific gravity, and the looser state of aggregation, the losses of the
original elements are so great, where decomposition has gone far, that
there is no question that the volume of the resultant material is often
diminished.
Among the cases of diminished volume, that of the formation of
residual clay from limestone undoubtedly represents the maximum. The
dominant constituent of the rock, calcium carbonate, has been almost
wholly dissolved and removed. The few feet of residual material at the
surface of limestones represents the alteration products of the noncalcareous
«Watson, Thos. L., Weathering of granitic rocks of Georgia: Bull. Geol. Soc. America, vol. 12,
1901, pp. 93-108.
> Watson, cit., p. 101.
¢ Watson, cit., pp. 106-108.
524 A TREATISE ON METAMORPHISM.
impurities. Whitney estimated that to make 1 meter of residual clay in
Wisconsin requires 35 to 40 meters of limestone and shale.* This involves
a diminution of volume of more than 95 per cent. The shrinkage of lime-
stone is scarcely less strikingly shown by underground solution, as a
result of which the surface becomes uneven and pitted and frequently
honeycombed by caves. The process may go so far as to result in the
irregular sinking of the surface. This condition is especially well illus-
trated by the limestone of Kentucky and by the honeycombed limestones
of Lake Huron, described by Bell.’
In line with the above is the explanation by Rutley of the slight
thickness and lenticular character of many beds of Paleozoic limestone.’ He
holds that the underground waters have dissolved much the greater portion
of the limestones, and that these slight layers or lenses are but remnants of
the original formations.
From this maximum diminution of volume represented by limestone
there are of course all gradations to no diminution, and doubtless in some
cases there is actual expansion of volume. If expansion of volume occurs it
would be likely to be in rocks where the minerals are mainly silicates.
From the figures given by Merrill and by Watson, it appears that the losses
of the original elements in acid, intermediate, and basic igneous rocks vary
from 14 to nearly 80 per cent. ‘These losses, as already pointed out, would
be partly compensated for by the addition of oxygen and water. More-
over, on account of the lower specific gravity of the secondary minerals
and the looser state of aggregation, there would be considerable expansion
of volume. Hence it appears probable that the decrease in volume in the
case of the weathering of these igneous rocks commonly does not exceed
50 per cent, and from this the decrease may run down to zero, or even
expansion of volume may take place, especially in early stages of alteration.
EMPHASIS AND RETENTION OF STRUCTURES AND TEXTURES.
The first effect of weathering upon structures and textures is to
emphasize them so that they may be readily seen. Structures which may
not be visible in fresh rocks may be strongly marked upon the weathered
«Hall, James, and Whitney, J. D., Geology of Wisconsin, vol. 1, 1862, pp. 121-125.
> Bell, Robert, Honeycombed limestones in Lake Huron: Bull. Geol. Soc. America, vol. 6,
1895, pp. 297-304.
¢Rutley, F., The dwindling and disappearance of limestones: Quart. Jour. Geol. Soc., August,
1893, pp. 380-381.
EMPHASIS OF STRUCTURES AND TEXTURES. 525
surfaces. Textures which the eye can not discover become clearly apparent.
Any variation in the hardness or solubility of the rock is shown by a ridgy
structure. As a consequence, every variation in original and secondary
structures and textures may be shown on a weathered surface. So nice is
weathering in its selective power in disintegration and solution, that when
the different unaltered rocks can not be discriminated the weathered forms
are easily separable.
Some cases will be mentioned illustrating the above general statements.
Frequently the rocks which when unweathered appear to be massive, in the
belt of weathering develop a strongly marked cleavage or fissility—cleavage
in the lower part of the belt of weathering and fissility in the upper part.
This effect is so marked that often it is difficult to believe that the weathered
and unweathered portions of the rock belong to the same formation. The
unaltered rock may be so massive that it has only a rift which can scarcely
be called cleavage. The weathered portion may be split up into leaf-like
layers, scores of them within the breadth of an inch, so that the rock at once
would be called a schist, while one would not think of applying the term
schist to the massive phases.
In connection with the emphasis of schistosity, micaceous minerals are
likely to become very conspicuous. In the weathered portion of some
rocks it seems as if the mica is one of the most abundant of the minerals,
while in the unweathered portion it is scarcely noticeable. Excellent
illustrations of this are found in various places on the Piedmont Plateau.
Perhaps one of the best is at Washington, D.C. Here the unweathered
rock at various places is a very massive appearing, slightly cleavable granite
or a somewhat dense schist, whereas the partly weathered portions are
fissile mica-schists.
Where mechanical forces have not displaced the particles with reference
to one another, a rock may be completely weathered in the sense that none
of the original minerals remain, and yet perfectly retain the original struc-
tures and textures. This is beautifully illustrated by the deeomposed
diabase dikes in the Penokee district, which still perfectly retain the textures
of diabases, and yet in composition some of them approach very closely to
kaolin. Every original mineral has been completely altered. Other
similar instances described by Merrill” are the decomposed gneisses and
«Merrill, George P., Rocks, rock-weathering and soils, Macmillan Co., New York, 1897, p. 264.
526 A TREATISE ON METAMORPHISM.
schists of the Piedmont Plateau which, where so soft as to be readily
removed with the pick and shovel, still show all their textures and struc-
tures. Russell describes the Triassic conglomerate near Wadesboro, N. C.,
as showing every detail of the original rock, and yet it is so ‘““completely
decomposed that when moist it can be cut with a pocketknife through
pebbles and matrix alike, as easily as so much potter’s clay.”* Potter
describes the feldspar-porphyry of Iron Mountain as ‘“‘so soft that it can
easily be whittled away with the penknife or scratched with the finger
nail; at the same time the original porphyritic structure of individual
crystals scattered through the mass is beautifully preserved, and is even
frequently more distinctly visible than in the original rock, owing to
stronger contrasts of color in the kaolinized material.”’ The so-called
tallow rock of the lead and zine district of southwestern Missouri shows all
the textures and structures of the original chert, yet is easily pared like
tallow with the penknife.
OBLITERATION OF STRUCTURES AND TEXTURES.
In proportion as the mechanical forces are important in connection
with decomposition the original structures and textures are apt to be
destroyed because of the differential movement of the particles. Structures
and textures are likely to be long preserved in cases of decomposition
alone,’ but are destroyed by decomposition and disintegration combined,
since these act together at and very near the surface.
The final stage in the process of weathering is the destruction of all
textures and structures and the production of soils and subsoils. Near the
surface, after the various minerals are broken apart they are mixed with
one another indiscriminately by the mechanical forces of freezing and
thawing and by the work of plants and animals (see pp. 440-451), and they
are decomposed by the chemical forces. Finally no definite structures and
textures remain. The character of the surface soil is modified by its
organic content. It may contain from a small percentage of organic mate-
rial to a predominant amount of that material, as in the case of the peat
«Russell, I. C., Subaerial decay of rocks, and origin of the red color of certain formations: Bull.
U.S. Geol. Survey No. 52, 1889, p. 16.
» Potter, W. B., The iron-ore regions of Missouri: Jour. United States Assoe. Charcoal Iron
Workers, vol. 6, 1885, p. 25. ;
¢Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897,
pp. 264-265.
IRREGULARITIES OF SURFACE. 527
deposits. There is every gradation from the fresh rocks deep below the
surface, which do not well exhibit their structures and textures, through
those in which structures and textures are brought out with great clearness
by the process of weathering, to completely disintegrated and partly
_ decomposed rocks constituting the soils, in which no structures and textures
remain. The gradations of the process may be very well seen at many
localities where there are sufticiently deep cuts on the Piedmont Plateau of
the United States.
SURFACES OF WEATHERING.
Weathering is only one of the complex processes which have an
influence upon the surface features of the land. Transportation and ero-
sion are quite as Important in this respect as weathering, or even more
important. But certain features of the surface are largely due to difference
in the rate of weathering. Wherever the rocks are heterogeneous they
differ in their power to resist weathering. The more resistant rocks
commonly constitute the elevations, and the less resistant rocks the minor
features. This is true of both the major and the minor features. Illustra-
tions of this principle of various kinds will be mentioned. One of the most
common illustrations are elevations or ridges formed by dikes, which are
often more resistant than the surrounding rocks. In other cases dikes are
less resistant than the intruded rocks, and where the dikes once were may
be found deep, narrow-walled depressions or chasms, which may be occu-
pied by streams or, if along the shore, by the water of the ocean or lake.
Basic dikes in the resistant gneisses furnish frequent instances of this.
Beautiful illustrations are seen on the coast of New England, especially
Maine. *
Another interesting case of an irregular surface due to weathering is
produced by bowlders of disintegration. The process of rock decay
follows along the planes of weakness, and especially along fractures, such
as joints. When a rock is cut by various sets of intersecting joints the
process of weathering goes on more rapidly at the corners of the rectan-
gular blocks than on the sides, thus producmg round masses which
aLord, E. C. E., On the dikes in the vicinity of Portland, Me.: Am. Geologist, vol. 22, 1898, p.
338. Bascom, F., On some dikes in the vicinity of Johns Bay, Maine: Am. Geologist, vol. 23, 1899,
pp. 275-280.
528 A TREATISE ON METAMORPHISM.
frequently simulate to a remarkable degree bowlders produced by water or
ice erosion. “
On the Piedmont Plateau, at many places where there are good
sections exposed, as in railroad cuts, one may see hundreds of these roundish
bowlder-like masses in the midst of soft disintegrated material. Erosion
carries away the soft material faster than the bowlder-like masses. The
latter therefore accumulate at the surface, and finally produce a surface
which presents the appearance of a ‘Felsenmeer.” Such a surface has
strewn over it innumerable bowlder-like masses, small and large, from
those less than a foot to those many feet in diameter.. Some of the largest
masses are dome-like and of such magnitude that the name ‘“‘mountain” has
been applied to them; for instance, Dunn Mountain, in North Carolina.
These masses, small and great, are at the surface simply as a consequence
of fracturing, differential weathering, and differential erosion.
Bowlders of disintegration and mountains of disintegration have been’
described by Branner’ as also occurring on a most extensive scale in Brazil.
They are beautifully illustrated by the granitic mountains of southern
California and other arid regions of the United States.
The innumerable bowlders in the till of the glacial deposits of America
and Europe are believed by some to be largely bowlders of disintegration
which were produced before the advance of the ice sheets. This explana-
tion premises that before the time of the continental ice sheet the glaciated
regions were weathered in a manner somewhat similar to that of the
Piedmont Plateau.
In minor details the nature of the minerals present controls the
character of the surface. In general, any surface long exposed to weathering
is roughened, owing to difference im the solubility of the minerals, or to
imperfect cementation, or to these and other causes combined. The
minerals which protrude are usually those which are relatively insoluble.
The order of solubility, and therefore of disappearance, is that given on
pages 518-521. Very numerous illustrations might be given of the rough-
ening of the surface of rocks as a result of the variable rate of solution of
the constituents. One of the best illustrations is furnished by the nepheline
“Becker, G. F., Geology of the quicksilver deposits of the Pacific slope: Mon. U. 8. Geol.
Survey, vol. 13, 1888, pp. 68-72.
>Branner, J. C., Decomposition of rocks in Brazil: Bull. Geol. Soc. America, vol. 7, 1896, pp.
269-280.
DEPTH OF WEATHERING. 529
syenites. The nepheline-syenite of the Wausau district of central Wis-
consin has a peculiar, deeply pitted appearance, due to the complete
solution of the nephelites at the surface.
A further case of irregular surface is furnished by the cherty lime-
stones; the calcite is dissolved more rapidly than the quartz, and thus the
weathered surface presents a series of ridges and hollows, giving a very
rough, ridgy surface.
DEPTH AND DEGREE OF WEATHERING.
The belt of weathering has already been defined as extending from
the surface to the level of ground water, and locally to some distance below
the level of ground water. (See pp. 409-411.) Furthermore, it has been
seen that the level of ground water varies from 0 to more than 300 meters
below the surface.
Whatever the thickness of the belt of weathering, the conditions
throughout this belt are favorable for oxidation, carbonation, hydration,
and solution, and therefore for the chemical reactions of decomposition.
But the conditions are far more favorable for decomposition at the surface
than at the lower part of the belt of weathering. Disintegration is much
more limited in extent. It is at a maximum at the surface, but decreases
rapidly with depth, and at 10 meters below the surface is scarcely appre-
ciable. While it is clear that reactions of weathering extend throughout
the belt as defined, it is equally certain that the processes are at their maxi-
mum efficiency at or near the surface, and decrease in efficiency as depth
increases. But in some cases the processes are very important to the
bottom of the belt, even where of very considerable thickness. This is
illustrated by limestone regions, in which minor openings or even extensive
caves may form abundantly quite to the level of ground water, although
this may be 100 or 200 or more meters below the surface.
As to degree or stage of weathering, rocks are ordinarily spoken of as
‘““weathered” where they are very appreciably affected by the peculiar weath-
ering processes. As already explained, there are all gradations, from no
weathering to complete weathering. Both extremes are very rare, although
it is common for geologists to speak of ‘‘complete weathering” without telling
what they mean thereby. In regions where the conditions for weathering
MON XLYII—O4 34
530 A TREATISE ON METAMORPHISM.
are favorable and the forces and agents of weathering have been long at
work, greater or less thicknesses of material in various stages of weathering
have accumulated.
Some of the regions will be mentioned in which the weathering pro
cesses are far advanced. Perhaps one of the most striking illustrations of
advanced weathering is furnished by various limestone regions. The
destruction of limestones located in the belt of weathering occurs both at
the surface and below the surface. Below the surface the process is mainly
that of solution, and in many regions the same statement applies to the
surface reactions. Solution below the surface is the chief cause of the
porous, cellular, and cavernous character of limestone formations above the
level of ground water. As a result of solution at the surface, residual clays
are formed. Residual clays in limestone regions vary in thickness from a
ry
few centimeters to 7 meters or more. Chamberlin and Salisbury give the
average thickness of the residual clay of the driftless area of the upper
Mississippi Valley as more than 2 meters. Whitney estimates the thick-
ness of the clay in this same area as 3 meters” This he believed to corre-
spond to an original thickness of from 105 to 120 meters of limestone and
shale. :
In regions of noncaleareous rocks advanced decomposition may
extend to the depth of a score or more of meters. The comparatively late
coast ranges of California at many places are much weathered. The
weathering affects all the rocks, but to a different depth. The Montara
granite in many places is disintegrated to a depth of 6 to 10 meters. The
complete disintegration and advanced decomposition of the rocks of the
Piedmont Plateau, from Peunsyivania to Georgia, is well known to have
extended from a depth of 2 or 3 meters to nearly 80 meters. In Georgia,
according to Spencer,’ decomposition has gone on to some extent ata
depth of 60 meters. Merrill says that in many places the Sierra Nevada
granodiorites are disintegrated and partly decomposed to a depth of 60
meters.’ Belt states that the igneous rocks ef Nicaragua are decomposed
aChamberlin, T. C., and Salisbury, R. D., Driftless area of the upper Mississippi Valley: Sixth
Ann. Rept. U. S. Geol. Survey, 1885, p. 254.
bHall, James, and Whitney, J. D., Geology of Wisconsin, vol. 1, 1862, pp. 121-125.
eSpencer, J. W., The Paleozoic group of northwestern Georgia: Geol. Survey Georgia, 1893, pp.
22-24,
d Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, p. 274.
See also Merrill on extent of weathering, pp. 276-278.
VARIABLE AMOUNT OF WEATHERED MATERIAL. ays)
to a depth of 60 meters.“ The Transvaal granites, according to Furlonge,’
are decomposed to a depth of 60 meters.
From maximum amounts of weathered material illustrated by the
above regions there are all gradations to amounts almost inappreciable.
Nxeellent illustrations of such regions are the areas of the Northern
Hemisphere over which the latest ice incursion has taken place. Glaciated
quartzites may retain the glacial striations with marvelous delicacy, micro-
scopic striz being perfectly preserved. Such igneous rocks as the granites,
gabbros, and peridotites, and other families usually show only a mere skin
of appreciably decomposed material, ordinarily but a fraction of an inch
in thickness. The readily soluble limestones usually present planed,
grooved, and striated surfaces which show comparatively little evidence of
modification. The minimum weathering effects are found on the solid
rocks which have a thin veneer of drift or soil. Where this veneer is thick
enough to bury the rock surface below the frost line, the only weathering
effects ordinarily produced are slight stains of decomposition on the surface
and along the joints and other fractures. Where the topographic forms
are more rugged, so that the rocks are more exposed to weathering, there
has been pronounced disintegration, and the decomposition has also gone
farther. While the latest drift shows much more evidence of weathering
than the solid rocks, below the soil the weathering effects are surprisingly
slight. The pebbles and bowlders commonly show planing, grooving,
and striz, and scarcely any evidence of decomposition. The marvelous
freshness of this drift, which has been exposed to the weathering forces
under favorable circumstances for thousands of years, perhaps tens of
thousands of years, is to me the best evidence of the slowness of the
complex process of weathering.
A comparison of the small amount of weathering in the glaciated
regions with the great amount in some other regions gives one an idea of
the vast amount of time required for deep and advanced weathering.
Even if the most conservative estimate as to the length of time since the
last ice invasion were used as a measure of the rate of weathering, it would
follow that to have accomplished such weathering results as are exhibited
«Belt, Thomas, The naturalist in Nicaragua, p. 86.
> Furlonge, W. H., Notes on the geology of the Dekaap gold fields in the Transyaal: Trans. Am.
Inst. Min. Eng., vol. 18, 1890, p. 337.
532 A TREATISE ON METAMORPHISM.
in the southern Appalachians, Nicaragua, and Brazil must have taken
several or many hundreds of thousands of years. The process of decom-
position was of course more rapid in these regions than in the colder
northern regions, but even allowing for this the time required for the work
accomplished must have been great.
RATE OF WEATHERING.
The rate of weathering depends upon all the factors that have here-
tofore been given. These factors may be classified under the forces and
agents at work in weathering, the materials weathered, the thickness of the
belt of weathering, and the stage of weathering.
FORCES AND AGENTS AT WORK IN WEATHERING.
The rate of weathering is rapid in proportion as the temperature is
high; the humidity is great; erosion by wind, water, and ice is active; and
plants and animals are abundant. All these statements have been so fully
developed on previous pages that they will not be further amplified here.
MATERIALS WEATHERED.
The rate of weathering as dependent upon the material involves a con-
sideration of the chemical composition of the rocks, the mineral composition,
and the closeness of aggregation.
CHEMICAL COMPOSITION.
In general the more basic a rock the more rapid the rate of weathering,
and the more acid a rock the less rapid the rate of weathermg. As to indi-
vidual elements, the following statements may be made: The richer a rock
is in alkalies the more rapid is the rate of weathering. With a given quan-
tity of alkalies, the greater the proportion of sodium the more rapid the
alteration, and the greater the amount of potassium the less rapid the altera-
tion. The ereater the amount of alkaline earths the more rapid is the
alteration. With a given quantity of alkaline earths, the greater the pro-
portion of calcium the more rapid the alteration, and the greater the propor-
tion of magnesium the less rapid the alteration. The greater the amount
of iron the more rapid is the alteration. The greater the amount of
alumina the less rapid is the alteration. Where the iron and aluminum
RATE OF WEATHERING. 533
~ mutually replace each other, the greater the proportion of aluminum the
more rapid the alteration. The greater the amount of silica the less rapid
is the alteration.
MINERAL COMPOSITION.
The readiness of the alteration of minerals is largely a direct conse-
quence of the character and the chemical composition. With a given
chemical composition, the greater the proportion of glass the more rapid
the alteration, and the greater the proportion of crystallized minerals the
slower the alteration. Among the crystallized minerals, the greater the
proportion of the easily decomposed minerals the more rapid the alteration.
The order of decomposition of the common anhydrous minerals from
the easiest to the hardest, as given on pages 518-521, is (1) nephelites and
leucites, (2) olivines, (3) pyroxenes and amphiboles, (4) biotite-muscovite
group, (5) feldspars, (6) garnets, staurolite, tourmaline, andalusite, cyanite,
and (7) hematite, magnetite, and quartz.
STATE OF AGGREGATION.
Fine-grained rocks weather less rapidly than coarse-grained ones.
This is a consequence of the closer interlocking of the mineral particles of
the fine-grained rocks, and of the fact that the differential expansion and
contraction by change of temperature is less with fine particles than with
coarse particles. The more open the aggregation the more rapid the alter-
ation; the closer the aggregation the less rapid the alteration. This follows
from the fact that openings give ready access to the agents of alteration, and
the border of every opening is a place of attack for the chemical agents, and
therefore all classes of openings promote decomposition. The openings in
rocks are divided into pore spaces between the mineral particles, sheet open-
ings, and irregular openings (see p. 129). Pore spaces are generally in the
mechanical sediments, the lavas, and the tufts; therefore mechanical sedi-
ments, lavas, and tuffs alter much more rapidly than the plutonic igneous
rocks and the compact metamorphic rocks. Sheet openings are of various
kinds, such as faults, joints, bedding partings, and fissility. All these open-
ings serve as channels through which the waters may readily pass and extend
the decomposing effect. In general, along a fault or a joint, decomposition
extends more rapidly downward than in the adjacent solid rock. Where
D3 A TREATISE ON METAMORPHISM.
the openings are fine and close, as in the case of fissility, the process
extends downward uniformly and much more rapidly than where this struc-
ture is absent.
As examples of very fine- and close-grained rocks which weather less
rapidly than other rocks of similar chemical and mineral composition may
be mentioned the Berlin rhyolite-gneiss and the Niagara limestone of Wis-
consin. ‘The former has a maximum crushing strength of 3,304 ke. per
sq. cm.'and the latter a crushing strength of 2,812 ke. per sq. cm." These
are probably the highest crushing strengths yet recorded for such rocks.
While altogether unlike in composition and origin, they are alike in their
remarkable uniformity, fineness, and closeness of grain. Many of the
marbles are not more porous than the limestone mentioned, but the mineral
particles are much larger, and in this difference probably lies the explanation
of the very unusual strength and slow weathering of this limestone.
From the above there follows the conclusion that in proportion as the
ageregation is coarse and open the rate of weathering is rapid; in propor-
tion as it is fine and close the rate of weathering is slow. These general
statements need modification with reference to disintegration through
freezing and thawing (see pp. 441-442).
THICKNESS OF THE BELT OF WEATHERING.
Since the entire belt of weathering is affected by the reactions charac-
teristic of that belt, the greater the thickness of the belt the more effective
the process of weathering. It has already been explained that, other things
being equal, the level of ground water is likely to be a considerable depth
below the surface in proportion as the elevation is great. It therefore
follows that the rate of weathering is greater in elevated than in low regions.
This is partly a direct consequence of the fact that the weathering agencies
have a greater volume of material upon which to work. But probably
even more important than this is the rapid removal of weathered material
in elevated regions. Material disintegrated and partly decomposed by the
weather is removed by erosion as rapidly as formed, and therefore contin-
uously exposes new surfaces to the agencies of weathering.
«Buckley, E. R., Building and ornamental stones of Wisconsin: Bull. Wisconsin Geol. and Nat.
Hist. Survey No. 4, 1898, pp. 360-363.
RATE OF WEATHERING. 539
While the above is true, if one were to confine his attention entirely
to the decomposing effect of the weather and exclude disintegration, it is
probable that there is a limit beyond which increase in elevation decreases
the decomposing effect. As explained on pages 499-500, 502-5038, great
elevation gives conditions unfavorable for the existence of life, and life is
one of the necessary agents in rapid decomposition.
While the reactions of weathering are taking place throughout the
entire belt above the level of ground water, the process is at its maximum
efficiency near the surface and rapidly decreases in efficiency with depth.
This is largely due to the fact that life is confined to the upper few meters
and therefore the decomposing effects of life rapidly diminish with depth.
It is a corollary from the relation of the thickness of the belt of
weathering and the rate of the process that geological periods in which
there are extensive land areas at a high elevation above the sea are periods
in which the processes of weathering are most rapid. These processes are
essential to the rapid formation of all classes of sediments, including
the limestones. The relations between weathering and sedimentation are
discussed on pages 555-560.
STAGE OF WEATHERING.
Where the rocks in the belt of weathering are fresh, and therefore have
not been adapted to their environment, the processes of weathering are
active. And, per contra, where the processes of weathering have trans-
formed the rocks to minerals adapted to the belt of weathering the processes
are slow. For instance, the conditions are favorable for rapid weathering
in glaciated regions where all the weathered products have been recently
removed, and where, therefore, the rocks and minerals are not adapted to
the belt in which they are found. In such areas as the Piedmont Plateau,
on the contrary, where in some places the minerals have been very largely
transformed to those characteristic of the belt of weathering, and are there-
fore permanent under the conditions of that belt, the processes of weathering
are very slow.
Probably where weathering is in an early but not the earliest stage the
processes are at their maximum activity. Where by changes of physical
conditions rocks have been brought into the belts of weathering, at first the
conditions are very unfavorable for the existence of life, and the great réle
536 A TREATISE ON METAMORPHISM.
which life plays in weathering has already been pointed out. But when
the processes of weathering have gone so far that a good soil has formed
and abundant life has taken possession of the little weathered rocks, the
conditions for weathering, including disintegration and decomposition, are
probably most favorable.
DISTRIBUTION OF DISSOLVED MATERIALS.
We have seen that all the elements of the rocks are dissolved, but
with exceedingly variable rates, and that in consequence of this there is a
concentration in the belt of weathering of a few minerals and fewer
elements. Where only the end products of weathering are present the
important elements there concentrated are but six in number—oxygen,
silicon, aluminum, iron, magnesium, and hydrogen. Before taking up the
question of what becomes of the dissolved materials, it is advisable to recall
the elements dissolved, their relative rates of solubility, and the fact that
under ordinary conditions of weathering and erosion but a portion of any
of the elements in the belt of weathering are rapidly dissolved.
Tt has been seen (p. 518) that the common bases are taken into solution
in the following order:
(1) The alkalies. Of these the sodium minerals are more readily
decomposed than the potassium ones and hence a much greater amount of
sodium than potassium is dissolved.
(2) The alkaline earths. Of these the calcium minerals are certainly
more readily decomposed than the magnesium minerals, and therefore
calcium is taken into solution in much greater quantity than magnesium.
(3) Iron, especially that in the ferrous form.
(4) Aluminum, which, while dissolved to a less extent than the other
bases, is dissolved in large amounts.
It has also been seen (p. 485) that the dominant acids which go into
solution are silicic and carbonic. Common subordinate acids in solution
are hydrosulphuric, sulphuric, nitric, hydrochloric, and phosphoric. In vol-
canic districts hydrofluororic and hydrobromic acids are frequently present.
Within the zone of observation by far the greater quantity of the
substance is carried as carbonate, and, as is well known, the salts are usually
not carried as normal carbonates but as bicarbonates.*
«Letts, BE. A., and Blake, R. F., The carbonic anhydride of the atmosphere: Sci. Proc. Royal
Dublin Soec., new series, vol. 9, pt. 2, 1900, p. 160.
DISTRIBUTION OF DISSOLVED MATERIAL. Ba 0
It must also be recalled that the dissolved silicic acid is in the colloidal
state, and in the belt of weathering is an inactive chemical agent, and
therefore is not united with the bases in solution. The acids in the solutions
in combination with the bases therefore do not inelude silicic acid. Of the
remaining acids carbonic is that of dominant importance.
It is further to be recalled that the amounts of the various bases and
acids which may be taken into solution are dependent not only upon their
relative strengths, but also upon the amounts present and upon the combi-
nations in which they exist.
We are now prepared to take up the question as to what becomes of
the materials dissolved in the belt of weathering. ‘These materials are
abstracted by plants, transported to the belt of cementation, permanently
abstracted by the run-off, and dissolved, transported, and reprecipitated in
the belt of weathering.
MATERIAL ABSTRACTED BY PLANTS.
Considerable quantities of the soluble constituents produced in the belt
of weathering are absiracted by the plants and built into their bodies. Of
the bases, the plants take greater quantities of the alkalies than of the alkaline
earths. They take oniy minute amounts of iron and aluminum. The acids
abstracted by plants in important amounts are nitric, phosphoric, and silicic.
In this connection it should be remembered that under ordinary conditions
the amount of alkalies available is less than that of the alkaline earths, and
that the amounts of nitrates and phosphates available are far less than the
amount of silicic acid. Therefore, even it the amounts of these compounds
abstracted by the plants were the same, this would result in the abstraction
of a considerable portion of the alkalies and the nitric and phosphoric
acids, while the major portion of the alkaline earths, of the iron and alumi-
num, and of the silicic acid would remain.
The distance of the transfer of material in the belt of weathermg by
means of plants is limited by the depth to which roots extend. It has been
noted (pp. 445-446) that the roots of ordinary grains and grasses extend into
the soil 1 or 2 meters, and that the roots of larger plants may extend to a
depth of 10 meters. The vertical transfers are therefore to be measured by
these distances. The amount of material transferred by plants is a direct
function of the luxuriance of the vegetation. Luxuriant vegetation occurs
538 A TREATISE ON METAMORPHISM.
where humidity is high, and hence it is in the humid regions that vertical
transfers of material through the agency of plants is greatest in quantity.
It will be remembered that by far the greater portion of the plants
either decay or are used by animals, which in turn ultimately decay. At
the time of decay the mineral material dissolved by the plants either passes
into the run-off and is transported to the sea or else joins the soil, to be there
again redistributed. Probably the latter is the larger portion. Therefore
much of the soluble material abstracted by life from the belt of weathering
is but temporarily held. The only permanent loss to that belt is the portion
which gets into the streams and thence into the sea.
Since the part of the vegetation above ground to a large extent decays
at or near the surface, the permanent effect of the vegetation in the trans-
portation of the dissolved material is to bring it from a depth of one to
several meters below the surface near or to the surface.
The roots decaying in situ to some extent redistribute the material and
transfer it toward the surface, but not to so great a degree as the portion of
the plants above the ground with the help of the roots.
MATERIAL TRANSFERRED TO BELT OF CEMENTATION.
A large amount of material which is dissolved in the belt of weathering
is carried downward by the descending waters and joins the subterranean
sheet below the level of ground water. It will be shown below and in
the following chapter (pp. 612-617) that the material which joins the belt
of saturation is again subdivided, a part being precipitated in the belt
of cementation and a part joining the surface waters and thus being
transported to the sea.
MATERIAL PERMANENTLY ABSTRACTED BY RUN-OFF.
A considerable amount of the soluble material of the belt of weathering
joins the run-off. In times of abundant rain water flows over the surface.
This water takes up the soluble material with which it comes into contact.
The amount dissolved is especially likely to be considerable if abundant
rains follow a time of drought, for it will be seen below that in times of
deficient rain capillarity brings the soluble salts to the surface where
they can be readily abstracted by the run-off. From seepage waters and
springs the streams receive large quantities of water from the belt of
DISTRIBUTION OF DISSOLVED MATERIAL. 539
cementation, containing soluble material derived directly or indirectly from
the belt of weathering. In so far as the streams empty into lakes with no
outlet the material is not permanently lost to the belt of weathering, but
rejoins it, as will be shown below. But the vast majority of streams reach
the ocean and the dissolved material of the run-off is contributed to the
mother water. The material thus contributed is the source of the salts of
the sea.
MATERIAL DISSOLVED, TRANSPORTED, AND REPRECIPITATED IN BELT OF
WEATHERING.
A portion of the compounds dissolved in the various ways in the belt
of weathering is transported to other parts of the belt and precipitated.
The salts which are reprecipitated depend upon (a) the abundance of the
compounds, (b) their relative and changiny solubility, and (c) the humidity
of the region.
(a) The more abundant a compound is in a solution the more likely is it
to be reprecipitated. This law is almost self-evident, for the more material
there is in solution the more likely, under the changing conditions which
obtain in the belt of weathering, are the solutions to become supersaturated
and thus precipitation take place. The supersaturation may result from
any of the causes given on pages 113-123. This principle of precipitation
of the abundant compounds is one of the utmost importance in metamor-
phism. It applies not only to the belt of weathering but to all belts and
zones, and is correlative with the principle already explained, that the more
abundant a substance is the more likely it is to be dissolved. Both are
but special cases of the law of mass action. (See p. 94.)
Both of these principles are well illustrated by the solution and precip-
itation of calcium carbonate in the limestone regions, although the relative
solubilities of the compounds present also enter into the result. Where
the dominant material is calcite or dolomite, or any combination of these,
these materials preponderate in the underground solutions. Also to an
equal degree they preponderate in the precipitates. ‘The extensive solution
of limestone has already been considered. (See pp. 485-486). But wher-
ever there is a minute cavity, a cleft, a cave, or an opening of any other
kind in limestones in the belt of weathering, there calcite, aragonite, or
dolomite, or some combination of these, may be precipitated. The small
940 A TREATISE ON METAMORPHISM.
cavities may be lined with crystals of nearly the composition of the sur-
rounding rock, thus producing geodes; the joint, fault, and irregular open-
ings may be partly or completely filled with carbonates, producing veins;
but geodal filling and veins commonly form in the belt of cementation. In
the caves the floor is ordinarily covered with stalagmites and from the roof
stalactites depend. 'The precipitation of the carbonate in the openings is
commonly caused by the lessened pressure, by the escape of some of the
dissolved carbon dioxide, or by evaporation, any one of which or any
combination of which may produce a condition of supersaturation and
consequent deposition.
Another compound which is very abundantly precipitated in the belt
of weathering is silica, as opal, chert, or quartz. This again is largely a
consequence of its abundance, but also is a function of its solubility. (See
p- 480.) By the decomposition of the silicates silicic acid, probably
largely as colloidal silicic acid, is set free in almost incredible quantities.
(See pp. 480, 516-517.) The larger amount of this material probably
joins the belt of cementation, and thus is a source of the silicification which
is so dominant a process of that zone. (See pp. 622-623.) But by chemical
change, which is a factor in its precipitation as well as in its abundance,
a very considerable amount is partly or completely dehydrated and thrown
down in the belt of weathering as opal or quartz. Like calcite and dolomite,
it is deposited in the belt of weathering in geodal cavities, in caves, and in
veins, and as siliceous sinter.-
(b) The more insoluble a compound is, the more likely is it to form
a precipitate in the belt of weathering. This is almost self-evident, and
only needs cases illustrating the principle. The best illustrations are
furnished by the iron and aluminum oxides and their mixtures. It has
already, been seen that these are the two compounds which are most
frequently taken as constant in weathered rocks, and on this hypothesis are
used to determine the amount of loss of the other constituents. Much of the
iron in the original compcunds is in the ferrous form, but under the con-
ditions of the belt of weathering it is oxidized to the ferric form, and much
of it simultaneously precipitated. On account of the insolubility of the
oxide of iron and oxide of aluminum, as compared with other constituents, a
large amount of iron, as hematite, limonite, and other hydrated forms, and
of alumina, both hydrous and anhydrous, and of mixtures of iron oxide
and aluminum oxide, bauxite, accumulates at the surface. It is well known
DISTRIBUTION OF DISSOLVED MATERIAL. 541
that iron is abundantly transported as a carbonate, and aluminum as a
sulphate, as in the natural alums. Also, both the alumina and the ferric
oxide may be partly transported to the places of precipitation as aluminates
and ferrates.”
What proportions of these materials have been dissolved and precipi-
tated and what parts are simply residual undissolved substance is probably
not usually determinable, but in many places solution and reprecipitation
appear to be the chief cause of the concentration. Not infrequently these
concentrates are so rich in iron and alumina as to be ores. This phase
of the matter is much more fully developed on pages 842-846 and pages
983-989. 4
As much less abundant important insoluble compounds which accu-
mulate under the same principle as iron oxide may be mentioned the
manganese oxides, pyrolusite, manganite, etc. The abundant precipitation
of quartz in the belt of weathering, as explained on page 540, while largely
due to its plentifulness, is also controlled in an important degree by its
relative insolubility.
Still another set of compounds which well illustrate the principle are
the phosphates. It is well known that the manufactured soluble phosphates
used as fertilizers largely “revert” or are transformed to relatively insoluble
phosphates, and as such-are precipitated in the soil.’
(c) To a certain point.the less humid a region the more likely are
precipitates to form in the belt of weathering. The reason for this is
obvious. If the amount of rainfall be so small that it is largely brought to
the surface by capillary and other forces, as explained on pages 419-423,
and evaporated, the material it holds in solution will be precipitated; but
if a region be so arid that there is not sufficient precipitation to dissolve
material abundantly, it can not be abundantly precipitated. Indeed, it has
been seen (pp. 496-498) that in regions of extreme aridity disintegration is
the rule and decomposition takes place only to a small extent. Therefore
we conclude that the climatic conditions for abundant precipitation of
minerals in the belt of weathering are those of moderate rainfall for a
certain period of the year, with aridity for the remainder of the year.
a Cameron, F. K., Application of the theory of solutions to the study of soils: Rept. Div. of Soils,
U.S. Dept. Agric., No. 64, 1899, p. 169.
Wyatt, Francis, The phosphates of America, Scientific Pub. Co., New York, 1892, pp. 22-26.
542 A TREATISE ON METAMORPHISM.
In the arid regions not only are the relatively insoluble compounds
precipitated in the belt of weathering, but the readily soluble compounds
may likewise be thrown down, such as the salts of the alkalies and alkaline
earths. These elements may be combined with any of the acids which
exist in the soils. Thus there may be precipitates of such soluble salts as
the alkaline carbonates, and even the alkaline chlorides and nitrates. The
more abundant salts precipitated are sodium, potassium, calcium, and mae-
nesium carbonates, sulphates, and chlorides. Nitrates and borates of the
bases mentioned are also locally abundant. In any given case the bases
and acids unite in such a manner as to produce the most insoluble com-
pounds. Therefore the carbonic acid first unites mainly with the alkaline
earths and is thrown down as calcium and magnesium carbonates. Where
sulphuric acid is present it unites mainly with calcium, producing gypsum.
The hydrochloric and nitric acids are left for the alkalies. Frequently the
salts and acids may not be present in proportion to combine as above. For
instance, after the alkaline earths are exhausted by the sulphuric and
carbonic acids there may be residual carbonic acid which will necessarily
unite with the alkalies. Or again, on the contrary, the sulphuric and
carbonic acids may not be sufficiently plentiful to exhaust the alkaline
earths, and under such circumstances chlorides of calcium and magnesium
may form. This is well illustrated by the calcium chloride deposits of the
borders of Great Salt Lake, where locally there is as much as 40 per cent
of this material.“ :
The compounds are likely to be precipitated in the openings of the
rocks, where evaporation may occur. Such openings are, of course, by far
the most abundant in the soils at and near the surface, and decrease in
abundance with depth. Therefore it is in the soils that the greatest amount
of precipitation takes place, but below the soils are found joint cracks,
bedding partings, caves, etc, and in such openings deposits of the various
compounds mentioned are built up. While in these openings the precipi-
tation is largely dependent upon the abundance of the solutions aud their
evaporation, and also upon the abundance of the various salts and their
relative solubilities, in exceptional cases the reactions may not be simple
chemical ones, but be largely or wholly dependent upon animal life. This
is illustrated by certain caves where abundant phosphates and nitrates are
«Gardner, F. D., and Stewart, John, A soil survey in Salt Lake Valley, Utah, Rept. Div. of Soils,
U.S. Dept. Agric., No. 64, 1899, p. 113.
DISTRIBUTION OF DISSOLVED MATERIAL. 5438
attributed to the excrement of bats.“ But, as already said, the preponderant
precipitates of the belt of weathering form in the soil.
The accumulation of particles at and near the surface in the soil is a
matter of great geological and agricultural importance, and therefore this
case will be especially considered.
CONCENTRATION AT AND NEAR THE SURFACE.
The abundance of alkalies throughout the soils of the semiarid regions
of the United States and other parts of the world is well known. The soil
for large areas is impregnated with considerable amounts of alkaline salts.
Of these salts sodium and potassium chlorides, carbonates, and sulphates
are the most important. Where the amount of alkaline carbonates and sul-
phates in the soil exceeds one-half of 1 per cent crops can not be grown?
Where more than one-tenth of 1 per cent is as carbonate or as “black
alkali” crops can not be grown. But locally in arid regions, as, for
instance, at places near Salt Lake, the amount of sodium carbonate is as
much as 10 per cent.’ In some regions nitrates of the alkalies are not less
important than the chlorides, sulphates, and carbonates. The country
most noted for abundance of nitrates is Chile, where there are extensive
deposits. Muntz and Marcano believe that these nitrates were produced
by the action of nitrifying bacteria in the humid regions, that the salts have
been transported by water to the area where now found, and that the
evaporation of this water has resulted in the deposition of the nitrates.?
The concentration of soluble products in the soil is accomplished both
by underground and by overground circulations.
CONCENTRATION BY UNDERGROUND CIRCULATION.
The concentration of considerable quantities of soluble material at and
near the surface may be accomplished by the circulation of the ground
water. This circulation may be confined to the belt of weathering or may
include the circulation in the belt of cementation.
«Hess, Wm. H., Origin of nitrates in cavern earths: Jour. Geol., vol. 8, 1900, pp. 129-134.
Nichols, H. W., Nitrates in cave earths: Jour. Geol., vol. 9, 1901, pp. 236-243.
> Means, T. H., and Gardner, F. D., A soil survey in the Pecos Valley, New Mexico: Rept. Div.
of Soils, U. 8. Dept. Agric., No. 64, 1899, pp. 53, 74.
¢ Gardner, F. D., and Stewart, John, A soil survey in Salt Lake Valley, Utah: op. cit., p. 113.
@ Comptes rendus Acad. Sci., Paris, vol. 101, 1885, pp. 65-68. See, also, Merrill, George P., Rocks,
rock-weathering, and soils, Macmillan Co., New York, 1897, pp. 372-373.
544 A TREATISE ON METAMORPHISM.
CONCENTRATION BY CIRCULATION MAINLY CONFINED TO BELT OF WEATHERING.
It has been seen (pp. 419-423) that of the water which enters the
belt of weathering a considerable portion returns to the surface through
molecular attraction and vegetation and is evaporated. During the journey
of water downward and upward, in the outer belt of water circulation, it
takes material into solution.
When the water, the circulation of which is wholly confined to the belt
of weathering, or which is derived from the belt of cementation by mole-
cular attraction, nears the surface evaporation begins, and finally a state
of saturation is reached. Ordinarily saturation occurs only within a few
inches of the surface, and especially in the outer inch or half inch. For
instance, in a case of segregation of saline incrustation at Washington, D.C.,
Cameron states that practically all of the salt was in the surface inch, the
larger part of it being in the top eighth inch.“ As a result of the above
processes the soluble materials in the outer part of the earth are taken into
solution in large quantities and brought near or to the surface and there
deposited. (See pp. 539-543.)
It has been seen (pp. 417-419) that the downward movement of the
water is due both to gravity and to molecular attraction, and that the upward
movement is due largely to molecular attraction, working against gravity.
It is well known that the soil, even in humid regions, is relatively rich in
soluble constituents. This is undoubtedly in large part explained by the
fact that the soluble products are mainly produced by the process of decom-
position, and are more largely manufactured within the soil than elsewhere,
as so fully set forth on previous pages. The material there manufactured
is strongly held by the water of imbibition. But Means suggests in this
connection that the failure of the downward-moving water in humid regions
to remove the excess of soluble material is due to the fact that the down-
ward movement is mainly in large openings, the force being gravity, and
is therefore rapid, and consequently only a relatively small amount of
material is taken into solution, while the upward movements are in the
smaller openings, mainly under the influence of molecular attraction, and
consequently this wpward-moving water is loaded with material.”
«Means, Thos. H., On the reason for the retention of salts near the surface of soils: Science, new
series, vol. 15, 1902, p. 33.
bMeans, cit., pp. 33-35.
DISTRIBUTION OF DISSOLVED MATERIAL. 545
The upward transfer of material, while largely due to the water which
does not pass beyond the belt of weathering, is also partly due to water which
under the influence of molecular attraction passes upward from the sea of
ground water. The amount of such water increases in proportion as the sur-
face of ground water is near the surface. It has already been pointed out
(pp. 419-422) that when the ground water is comparatively near the surface
a very large amount may thus migrate upward and be evaporated. This
material carries with it all the salts which it had in solution in the belt of
cementation and such as it may take into solution during its upward move-
ment above the level of ground water. Locally, as a result of over-irrigation,
seepage from irrigation ditches or unusual precipitation for several years,
the level of ground water may be raised. A layer of the belt of weathering
may be thus encroached upon. This belt is likely to contain a large amount
of soluble material. To illustrate, Hilgard states that ‘The investigations
of the California station have shown that in the arid region few uplands
normally contain less than from 2,000 to 2,500 pounds [900 to 1,300 kilo-
grams] of soluble salts per acre in 4 feet [1.22 meters] depth, and much
more has been found in the silty substrata of the Salton Basin in southern
California even to 22 feet [6.7 meters] depth.” When the level of ground
water is raised this material is abundantly dissolved by the water. If the
rise continues so that the level of ground water is within 3 meters or less of
the surface, a very large amount of salts may be carried upward to the
surface from the belt of cementation by molecular attraction.”
The solutions carry more of the readily soluble compounds than of
the difficultly soluble ones, and therefore the substances transferred in
greater amounts are the alkalies and the alkaline earths. In humid regions
the material thus transferred to the surface layer is largely taken into solu-
tion at times of abundant precipitation and carried downward again. There-
fore, if there be no change in the composition of the compounds, there is little
accumulation of such material near the surface. But in the semi-arid and
arid regions, where the belt of weathering is comparatively rich in alkalies
and alkaline earths, the material carried upward during the long periods
of drought may not be compensated by downward transfer at the rare
occasions of abundant precipitation. Consequently the upper part of the
soil may become exceedingly rich in salts of the alkalies and alkaline earths,
“Hilgard, E. W., The rise of alkali salts to the soil surface: Science, new series, vol. 15, 1902, pp.
314-315, : :
MON XLVII—04——35
546 A TREATISE ON METAMORPHISM.
especially the former. Indeed, the surface may thus become saturated
with these salts.
The process of migration of alkalies to the surface is well iliustrated
in various irrigated districts. Before irrigation the soil may have contained
only a sufficient amount of alkalies to be very fertile. In many districts,
by irrigation for some years, so large a quantity of alkalies has been brought
to or near the surface as to injure vegetation or even to make crop growing
impossible. Where the water used is sweet, these effects follow from over-
irrigation. Where more than a sufficient amount of water is used a part-of
it passes to some depth below the surface. Between the times of flooding
a part of this is brought to the surface by capillarity, carrying with it
various dissolved salts. Also by continual seepage from canals at higher
levels water passes below the surface, and by capillarity is brought to the
surface in the cultivated areas, carrying with it dissolved salts. As the
water evaporates these salts are precipitated in the soil. This process is
cumulative, and after a number of years, as already stated, the quantity of
alkalies may become so great as to lead to the abandonment of formerly
productive tracts. This process is well illustrated in San Joaquin Valley,
California, and in Salt Lake Valley, Utah. At the latter place alone
Whitney states that of the 50 square miles once irrigated about 10 square
miles, or one-fifth, has become unfit for agriculture.“
Where the waters used for irrigation are themselves very rich in
alkalies, as, for instance, in Pecos Valley, New Mexico, the accumulation
of alkalies in the soils is prevented in a different way, namely, by very
abundant use of water with considerable intervals between and by thorough
underdrainage. By this method the salts which are precipitated between
the periods of irrigation by evaporation are dissolved by the abundant
waters at the time of irrigation, and thus accumulation of the alkalies in the
soil is prevented.”
While the more readily soluble constituents are of far more importance as
precipitates at or near the surface than the less soluble salts, the latter are pre-
cipitated in the soils. Of these difficultly soluble compounds silica and ferric
oxide are perhaps the only ones of sufficient importance to require mention.
Many sandstones which have long been exposed to the weather have a
“Whitney, Milton, Field operations of the Division of Soils: Rept. Diy. of Soils, U. 8. Dept.
Agric., No. 64, 1899, p. 25.
U Whitney, Milton, cit., pp. 22-24.
CASEHARDENING AND DESERT VARNISH. 547
thin outer layer or crust of quartzite. Sections of such material show that
it is a true cementation quartzite. (See pp. 865-868.) Casehardening also
occurs In many other kinds of rocks, for instance, granite. In the produe-
tion of casehardening the waters have penetrated to a greater or less depth,
and through capillarity have been brought back to the surface as a result
of drying of the outer surface.
During the journey of the water silica is dissolved, probably mainly in
the colloidal form. This H,SiO, is doubtless largely that liberated from
the silicates during the process of carbonation. (See pp. 473-480.) Prob-
ably the solutions do dissolve some silica from chert and quartz and bring
it to the surface, but the amount of this is probably small as compared with
that transported in the colloidal form. It therefore appears that the process
of surface cementation, so far as the cement is silica, is a correlative of the
process of carbonation.
Very near the outer surface of the rock the water evaporates, the silica
is dehydrated, and quartz is deposited. In this case chemical change takes
place. The silica is thrown down in a very difticultly soluble form; so that
even in humid regions the falling water is unable to dissolve the quartz and
carry it downward. This process may continue until the sandstone, which
may be loose or almost incoherent below the surface, is a quartzite at the
surface. The casehardened layer may be a mere outer film but a fraction
of a centimeter in thickness, perhaps one-half to one-fourth of a centimeter.
In other instances the casehardening may extend to a depth of a centi-
meter, or even several centimeters. Ordinarily the maximum induration is
at the surface, and the transitions between the indurated and soft parts of
the rocks are somewhat rapid. This fact gives positive evidence that
evaporation and consequent deposition are at a maximum at the surface.
Casehardening has been observed in many parts of the world. It is
beautifully illustrated at many points in the arid region of western United
States. In arid regions the hardened film has frequently been smoothed by
the wind-blown sand, so as to present a polished surface. Such polished
hardened films are known as ‘desert varnish.” While more common in
arid regions, casehardening has been found in humid regions. For instance,
the phenomena are well exhibited by the Potsdam and St. Peter sandstones
of Wisconsin.“
a@Wadsworth, M. E., Some instances of atmospheric action on sandstone: Proc. Boston Soe.
Nat. Hist., vol. 22, 1883, p. 202.
548 A TREATISE ON METAMORPHISM.
Surface induration, while most common as the result of the deposition
of silica, is not limited to this compound. The upward-moving solutions
carried by molecular attraction may, under favorable circumstances, con-
tain iron salts in solution. When the solutions approach the surface evap-
oration takes place and the iron salts are thrown down, usually as limonite
or hematite. If precipitated as limonite, this compound may be later
dehydrated and hematite be formed. Where the iron salts are transported
as carbonate, oxidation takes place at the time of precipitation. While it is
not necessary to suppose that the iron is always transported as carbonate,
since limonite and hematite are so insoluble it is natural to suppose that in
most cases the iron is transported in some other form than the oxide, and
is chemically changed when precipitated. By the above processes we have
surface induration due to iron oxide cement. This is beautifully illustrated
at various places in the desert regions of western United States. One of
the best localities known to me is that of the voleanic rocks of the Colorado
Desert east of Needles along the line of the Santa Fe Railway. The desert
is strewn with bowlders of disintegration, of a light-colored lava. The part
of these bowlders embedded in the sand has its normal color; the part
above the sand line is a rich reddish brown, due to hematite, the iron of
which has been brought to the surface in solution and has there been
oxidized and precipitated.
Another interesting case of the segregation of iron oxide at the surface
is furnished by quartzite bowlders in the Potsdam sandstone of the driftless
area of the Baraboo district of Wisconsin. Here, locally, in consequence
of weathering processes. the sand matrix has been carried away and the
quartzite bowlders have accumulated at the surface. The quartzite is
ferruginous, containing hematite, which gives purplish and brownish tones
to the unweathered rock. The centers of the bowlders still show these
colors; the borders are rich yellow-brown, or red, due to limonite and
hematite, the color being much more marked than in the centers of the
bowlders or in the massive quartzite, and plainly showing an unusual
proportion of the iron oxide. Between the outer surfaces of the bowlders
aud the central cores of unaltered quartzite are frequently bands, from two
to five centimeters broad, almost white, which fade off into the heavily
ferruginous outer parts of the bowlders and to the less ferruginous cores.
It is very clear in these cases that the iron oxide once in this lighter middle
CASEHARDENING AND DESERT VARNISH. 549
band has been taken into solution, transferred to the surface, and there
deposited. It is a depleted area, and correlative with it is the surplus of
iron oxide at and near the surface. This case is interesting, since the iron,
both in its original form and where segregated, is an oxide. It is therefore
clearly shown that iron in the form of hematite is soluble in the belt of
weathering. Whether actually carried in that form or whether reduced
and changed to carbonates or other salts at the time of transfer is uncertain.
In a similar manner other cements besides silica and iron oxide may
be brought to the surface and deposited. These processes, while most
prominent at the surface, are not limited to it; they take place to a consid-
erable extent along the major openings of the rocks near the surface, such
as jomts. Crosby has noted the induration of Pikes Peak granite along
the joints. Surface induration of granite, rhyolite, and other rocks has
been noted, the casehardening materials ordinarily being the three most
common ones—silica, iron oxide, and calcite.”
In connection with casehardening it is interesting to note the influence
of the so-called quarry water contained in stones taken from the quarry.
This is the water of imbibition. As this water is evaporated the material
in solution is deposited between the grains near the surface and thus helps
to cement them. The very considerable induration thus produced by the
quarry water is evidence that this water is rich in mineral solutions. In
this deposition of material we have the explanation of the great advantage
of dressing sandstones and other porous rocks before the quarry water is
lost by evaporation.
From the previous pages it appears that capillarity is most effective in
transferring soluble material from below the surface to the surface in semi-
arid and arid regions. But if in regions which are ordinarily known as
humid there are seasons of drought, great quantities of soluble material
may be segregated at the surface by capillarity. So far as this material is
thrown down in a readily soluble form it is likely to be largely taken into
solution at times of abundant rainfall and again carried below the surface.
But if at the time of precipitation chemical change takes place, so that the
material is transformed to a relatively insoluble form, as, for instance, the
dehydration of colloidal silicic acid forming quartz, or the oxidation of iron
“Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, pp.
254-256.
590 A TREATISE ON METAMORPHISM.
carbonate, forming hematite or limonite, its solution is very slow, and thus
the upward transfer of the compounds at the time of drought may more
than compensate for the solution at periods of humidity, and actual
segregation of certain ‘materials at the surface take place.
Furthermore, it is to be noted that in humid regions vegetation is
abundant, and therefore the upward transfer of material through the
agency of life is at a maximum. This compensates in part for the relative
inefficiency of capillarity in humid regions as compared with that in arid
regions. It therefore appears that whether a region be arid or humid the
vertical transfer of soluble material through the agency of life and by
capillarity takes place effectively.
CONCENTRATION BY CIRCULATION EXTENDING INTO. BELT OF CEMENTATION,
It has been pointed out, on pages 128, 156, that waters continuously join
the belt of cementation and continuously issue from it. In general the
waters which issue from the belt of cementation are not saturated, but not
infrequently the waters of springs as they near or reach the surface pre-
cipitate material by lowering of the temperature, by evaporation, or by
chemical change.
Perhaps one of the most common deposits thus produced is limonite. -
The iron precipitated is usually carried underground as carbonate, but when
it reaches the surface, where oxygen is abundant, oxidation takes place, the
carbon dioxide is liberated, the ferric oxide unites with water, and limonite
is thrown down. Thus bog deposits of iron ore are built up. Another
deposit which is not infrequently produced is calcium carbonate. This may
be thrown down as a result of lessening pressure and temperature, or by
evaporation. Such deposits are known as tufa or travertine. They are
found in many parts of the world. One of the best illustrations is fur-
nished by the travertine deposits of the Yellowstone National Park, at
Mammoth Hot Springs and on Terrace Mountain. The latter represents
the deposit of springs now dead. A third class of deposits thus built up
by issuing spring water is siliceous sinter. These are best illustrated by
the deposits of geyserite in the Yellowstone National Park. It is inter-
esting to note that the travertine and the siliceous sinter deposits in the
Yellowstone Park are in the same region, and one in which the spring waters
are hot. To the latter fact may be attributed the magnitude of the deposits.
CONCENTRATION OF DISSOLVED MATERIAL AT SURFACE. 551
Such waters were capable of taking more calcium carbonate and silica into
solution than ordinary cold underground waters. An jnteresting fact in
connection with these deposits is the correlation of their character with the
rocks adjacent. As pointed out by Hague, the calcareous deposits of
Mammoth Hot Springs and Terrace Mountain are adjacent to sedimentary
rocks which contain abundant calcium carbonate to serve as a source of
supply.” The geyserite deposits of siliceous sinter, on the other hand, are
in aregion of volcanic igneous rocks, mostly rhyolite, and where there are no
sedimentary rocks. Here calcium carbonate is not available, but. silica is
very abundant in natural glass and in crystallized minerals. These sili-
cates are decomposed by the processes of weathering. Colloidal silicic
acid is thus formed, which is readily taken into solution by the hot waters
and abundantly brought to the surface. There dehydration, partial or
complete, takes place and the geyserite deposits are built up.
In these cases it is clear that the major portion of the material deposited
was derived from the solid rocks through which the hot solutions circulated,
rather than from the magmas which heated the water, as maintained by
some. (See pp. 1033-1034, 1071-1072.) :
CONCENTRATION BY OVERGROUND CIRCULATION.
The waters of streams and lakes always contain soluble material
derived from the soil dissolved by the run-off and from the lower part of the
belt of weathering and the belt of cementation by the issuing spring waters.
The material dissolved by the run-off may form in the soil by the processes
of weathering or be brought to the surface by means of life or by the
underground circulation. The amount of this material is comparatively
small in humid regions, but is relatively large in arid regions. In some arid
regions the amounts held by the surface waters may be so great as to unfit
them for domestic purposes, or even for purposes of irrigation, since, as
already noted, a comparatively small amount of alkali is sufficient to
prevent plant life.
The best illustrations of overground concentration are furnished by
basin regions, where there isno permanent run-off. In the United States the
Great Basin is the one of dominant importance. Southern California also
contains basins of great extent. Into the Great Basin a large number of
«Hague, Arnold, Geological history of Yellowstone National Park: Trans. Am. Inst. Min. Eng.,
vol. 16, 1888, pp. 795-796.
5o2 A TREATISE ON METAMORPHISM.
streams flow, some of them of considerable size, for instance, the Bear,
Ogden, and Weber rivers. The smaller streams which flow into the basin
for the most part rise on the mountain slopes of the Wasatch and Sierra
Neyada, which wall the Great Basin on the east and the west. Within the
Great Basin there are many mountain ranges, some of them of great size.
The greater of these are the Humboldt and the Inyo, ranges. On these
basin ranges many streams rise and flow down upon the floor of the basin.
The precipitation in the Great Basin region is mainly in the winter and
spring. At times of abundant precipitation and rapid melting of the snow
many ephemeral streams form upon the floor of the basin itself. The
streams, small and great, all fail to reach the ocean; the larger of them
flow into the permanent lakes of the basin, such as Great Salt Lake, Mono
Lake, and Winnemucca Lake, but by far the greater number flow into the
ephemeral lakes. These lakes, which are numbered by hundreds, are
shallow; they may be many miles across in early spring and entirely
disappear before autumn. Also, the permanent lakes, like Great Salt Lake
and Winnemucca Lake, greatly expand at the time they receive the large
contributions from the streams and shrink during the summer and autumn.
All the streams bring their contributions of soluble materials to the lakes
and, in addition, the usual amounts of mechanical sediments. Hence there
is mingled in the Great Basin lakes the greatest variety of materials.
The salts of alkalies, the salts of the alkaline earths, and lesser amounts of
other soluble salts are all commingled with one another and with the
mechanical sediments.
When the waters of the ephemeral lakes are evaporated, all the
materials held in solution and in suspension are thrown down. In the
larger of the ephemeral lakes there is a distinct tendency for the chemical
and mechanical sediments to be deposited in alternate layers. Within a short
time after the flood season of spring the mechanical sediments are largely
laid down. During the dry season the lake evaporates and the chemical
sediments are precipitated. These sediments, while perhaps partly dissolved
by the waters of the lake of the succeeding flood season, are largely buried
under the mechanical sediments of that year. Upon these mechanical
sediments follows the next layer of chemical sediments, and so on.
It is difficult for one who has not traveled in the Great Basin region to
appreciate the vast amount of alkaline material deposited by the ephemeral
CONCENTRATION IN BASIN LAKES. Doe
lakes. In journeying across the desert of Nevada in summer or autumn a
person is rarely out of sight of one or more areas, small or great, varying in
size from a fraction of an acre to hundreds or even thousands of square
miles, covered with the almost snow-white chemical deposits of the evapo-
rated lakes, which glare with intense light under the brilliant sun.
The permanent lakes, hike Great Salt Lake, continually receive supplies
of material in solution and are constantly being evaporated. When such
lakes were first formed the salts steadily accumulated until the water became
saturated. After astate of saturation is reached each year at the flood season
chemical deposition may and usually does cease and solution even may
take place to some extent. After the flood season the mechanical sediments
quickly subside. During summer and autumn evaporation greatly exceeds
the influx of water; the lakes shrink; the solutions become supersaturated and
chemical precipitation follows. Along the borders of the permanent lakes
chemical and mechanical precipitates are interstratified the same as in the
ephemeral lakes; but in the central areas of these lakes, beyond the depth to
which plentiful mechanical sediments are carried, the chemical precipitates,
comparatively little contaminated with mechanical material, are steadily
built up. It is well known that the annual rainfall is not uniform, but that
there are periods extending over a number of years of more than and less than
the average rainfall. It is in the dry parts of the cycles that the thick deposits
of chemical sediments are largely built up. The formation of such deposits
is now also promoted in many lakes by the use of the water in irrigation.
As already noted, the abundant chemical sediments are the alkalies
and alkaline earths, carbonates, sulphates, and chlorides. The order in
which the materials are deposited depends upon the relative amounts of
these substances and upon their relative solubilities. In consequence of
difference in solubility there is a more or less marked tendency for the
bases first to unite with the acids so as to produce the most insoluble com-
pounds. Thus the calcium and magnesium unite with carbonic acid to
produce marl and tufa. The calcium unites with sulphuric acid to produce
gypsum. But always before one substance is completely precipitated one
or more others begin to be thrown down, and consequently there is a min-
gling of precipitates. This is especially true of those compounds which are
closely allied chemically, such as the calcium and magnesium carbonates,
the sodium and potassium chlorides, etc. While there is a tendency for
5do4 A TREATISE ON METAMORPHISM.
the most insoluble compounds first to be thrown down, these relatively
insoluble compounds may be very sparse in quantity, and therefore more
soluble salts be first precipitated. While all the above complications
make the result very uncertain, still there is usually a marked tendency
to stratification, each stratum being composed dominantly of one compound
or of two closely allied compounds. Thus a stratum may consist mainly of
marl, another mainly of gypsum, another mainly of common salt, and so on.
By great changes in the humidity of a region and some changes
perhaps in the topographic conditions a basin lake may become smaller or
wholly disappear, thus baring its deposits to the ordinary forces of erosion.
Such lakes are Lake Bonneville, described by Gilbert, and Lake Lahontan,
described by Russell. The chief chemical deposit of Lake Bonneville,
according to Gilbert, was a white calcareous marl. However, this marl is
very impure, containing a large amount of silicates and silica, in one case
as much as 74 per cent. The chief chemical deposits of Lake Lahontan,
according to Russell, are a calcareous white marl and tufa. The white
marl, like that of Bonneville, is impure, containing about 30 per cent of
silica, alumina, and iron, of which 22 per cent is silica. The tufa deposits
are mainly calcium carbonates, although small amounts of magnesium
carbonates and other constituents are contained.’ It appears that Lakes
Bonneville and Lahontan did not become saturated with the salts of the
alkalies. In the case of Bonneville this stage was not reached until it had
shrunk to Great Salt Lake.
By still more radical topographic revolutions and changes in meteoro-
logical conditions than those experienced by Bonneville and Lahontan,
deposits of salt lakes may become deeply buried below other sediments,
and thus we have the explanation of the salt, the gypsum, and other similar
deposits of various parts of the world.
DISTRIBUTION OF RESIDUAL MATERIALS.
The distribution of the dissolved material in the belt of weathering is
of very great importance, but the distribution of residual material which
at no time goes into solution is of no less importance. The concentration
aGilbert, G. K., Lake Bonneville: Mon. U. 8. Geol. Survey, vol. 1, 1890, p. 202. j
> Russell, I. C., Geological history of Lake Lahontan: Mon. U. 8. Geol. Survey, vol. 11, 1885,
pp. 152-153, 203.
DISTRIBUTION OF RESIDUAL MATERIAL. 550
of this material and its transportation, separation and deposition by the
epigene agents of erosion ought, perhaps, to be considered here; but this is
a subject so large and complicated that it is ordinarily given a separate
treatment in physiography; therefore no attempt will be here made even
to summarize this part of the cycle of the movements and alterations of the
material of rocks. The residual undissolved material of the belt of
weathering is a source of the mechanical sediments and the partial source of
the combined mechanical and organic or chemical sediments.
This residual material is deficient in the elements which have been
taken in solution in large amounts; it is therefore complementary to the
dissolved material. It follows that the residual material which is the source
of the mechanical sediments is likely to be deficient, to a varying extent,
in the alkalies and the alkaline earths, and of these it is much more likely
to be deficient in sodium and calcium than in potassium and magnesium.
This is true without taking into account the mechanical sorting. So far as
there is mechanical sorting, there may be deficiencies or excesses of other
elements. But it may here be suggested that the deficiency in certain
elements, as compared with the original rocks, furnishes an adequate
reason why the mechanical sediments, when mietamorphosed, do not
reproduce mineral combinations like those of the original rocks. The
metamorphosed equivalents of the mechanical sediments will naturally be
deficient in minerals rich in sodium and in calcium, such as sodalite,
nephelite, anorthite, ete.
RELATIONS OF BELT OF WEATHERING TO SEDIMENTARY ROCKS.
BELT OF WEATHERING THE SOURCE OF SEDIMENTARY ROCKS.
The materials transported to the sea in suspension and in solution by
overground and underground waters, and subordinately by the wind and
ice, supply the materials for the sedimentary rocks. These materials are
almost wholly derived, directly or indirectly, from the belt of weathering.
The main source of the sedimentary rocks is therefore the belt of weathering.
The material derived from the belt of weathering is transported in suspension
and in solution.
506 A TREATISE ON METAMORPHISM.
MATERIAL TRANSPORTED IN SUSPENSION.
The dominant constituents of the sedimentary rocks transported in
suspension are the minerals produced in and adapted to the belt of
weathering and the slowly decomposable minerals. The first class com-
prises the kaolin group, the serpentine-tale group, the chlorite group, the
zeolite group, quartz, iron-oxide minerals, and aluminum-oxide minerals.
Of the second class the minerals of dominant importance are quartz and
the acid feldspars. Quartz is included in both classes, and is the mineral of
greatest abundance in the sedimentary rocks; second to it is feldspar. With
the abundant minerals there are present, of course, very subordinate amounts
of the very difficultly decomposable minerals, such as garnet, staurolite,
tourmaline, zircon, ete.
The readily alterable minerals are usually rare or subordinate in
quantity in the sediments. The group of minerals most readily alterable,
the feldspathoids, including nephelite, leucite, and sodalite, are rarely,
if ever, found in the sedimentary rocks. Minerals of the olivine group are
also rare in the sedimentary rocks. The ferromagnesian group of minerals,
including the pyroxenes, amphiboles, and biotites, are more difficultly decom-
posable and are frequently found; still, they alter with such readiness that
they are generally subordinate in quantity, although locally abundant.
While where decomposition has been important the above are the dominant
minerals in the mechanical sedimentary rocks, in many sedimentary rocks
the materials are but slightly decomposed, disintegration being the chief
process of destruction of the original rocks. In mechanical sediments
derived from disintegrated material, as already pointed out, all or nearly all
the original minerals may be present. Where disintegrated materials are
dominant the rocks produced by their consolidation are conglomerates,
arkoses, and grits. As would be expected, there is every gradation between
rocks in which decomposed material is important and rocks in which disin-
tegrated material is dominant. Therefore, sedimentary rocks may be built
up of materials which, in chemical composition, stand anywhere between
decomposition products and disintegrated products, and which vary from
the mineralogical simplicity of the former to the mineralogical complexity
of the latter. Since the chemical analyses of such materials vary greatly,
as shown by the analyses given on pages 507-515, it is usually possible, by
the analysis of a sedimentary rock, to obtain a rough idea as to the stage of
MATERIAL OF THE SEDIMENTARY ROCKS. 557
decomposition. In cases where both the decomposed and the undecom-
posed materials are abundant the mineralogical complexity is even greater
than in the ordmary original rocks.
Kyven where decomposable minerals are largely altered and the
abundant minerals are few, by different combinations of these few minerals
a wide variety of sedimentary rocks may be produced. This is due to the
opposing processes of mingling of the materials in streams and separation
of them by waves and currents. Where material is deposited at the mouth
Olmal great and rapid river all the minerals are intermingled; where the
material is contributed to the sea no faster than it can be sorted by
the waves and the currents the different minerals are separated. Between
thorough assorting and no assorting there are all gradations. In proportion
as the material is unassorted the combinations of different minerals and of
different-sized particles built into a deposit is varied; in proportion as the
process of assorting is advanced a deposit is likely to be built up of a single
mineral or of a combination of two or more minerals having approximately
the same size and the same specific gravity, or at any rate which are
floatable to the same degree. Where the materials are slowly contributed
to the sea and are long subjected to the waves, the nearly perfect assorting,
both as to mineral material and as to size, isremarkable. As an illustration
of nearly perfect assorting may be mentioned the St. Peter sandstone of
Wisconsin. ‘This sandstone is almost wholly composed of quartz grains,
analyses showing it to have 96.74 per cent of silica” Moreover, the varia-
tion in diameter of the great majority of the grains is less than by ratios
of 2:3 (pp. 861-862).
MATERIAL TRANSPORTED IN SOLUTION.
It has been poimted out that the material transported in solution is
mainly composed of the more soluble compounds, viz, salts of the alkalies
and the alkaline earths. The ocean as a whole is not saturated with the salts
derived from the belt of weathering, nor is there any evidence that it has
been so at any time in the past, although locally inclosed seas and lagoons
do become saturated, and in this case chemical precipitation may occur
precisely as in inland lakes, described on pages 551-553. But the domi-
nant precipitation of material from solution in the ocean is not through the
process of chemical precipitation, but through the agency of life. As is
«Geology of Wisconsin, vol. 2, 1878, p. 680.
908 A TREATISE ON METAMORPHISM.
well known, by far the most abundant of all the constituents thus precipi-
tated is calcium carbonate. The corals and other shell animals abstract
this material from the water and build it into their hard parts. As the
animals die this material is deposited, and thus great limestone formations
are built up, and doubtless have been in the past. No adequate reason
has been given for the belief that great seas connected with the ocean have
become saturated by calcium carbonate so as to build up limestone forma-
tions by chemical precipitation, although, as already intimated, chemical
precipitation of calcium carbonate has locally occurred. While the chief
precipitation of calcium carbonate is through the agency of life, chemical
replacement works in conjunction with organic precipitation. No sooner is
a deposit of calcium carbonate formed than the other salts of the sea begin
to act upon it, under the laws of heterogeneous systems, and thus modify
the deposit. Calcium is taken in solution and is replaced by the other
bases, especially by magnesium. (See pp. 798-802.)
MATERIAL TRANSPORTED IN SUSPENSION AND SOLUTION..
Commonly the material transported in suspension and that transported
in solution are built into deposits which are largely separated from each
other; but not infrequently both classes of material are laid down together,
and this causes additional variety in the sedimentary rocks. We thus have
sediments built up of all proportions of materials mechanically transported
and materials chemically transported. It therefore appears that the sedi-
mentary rocks may have a wider variety of chemical composition and of
mineral composition than any of the igneous rocks. But while this is
possible, the rule for the great sedimentary formations is simplicity, and
this is due to the tendency toward simplicity in the belt of weathering and
to the processes of assorting. The first great process of assorting in the
belt of weathering is the subdivision of material into insoluble compounds
transported in suspension and soluble compounds transported in solution.
As already pointed out, the mechanical sediments are commonly assorted
by waves and currents, and the chemical sediments are assorted by life;
and thus the dominant formations of the sedimentary rocks are simple, both
chemically and mineralogically. This is especially true of the two
kinds of formations, sandstones and limestones. The shales, arkoses,
eraywackes, and conglomerates represent the varieties of sedimentary
rocks in which there is greater complexity.
MATERIAL OF THE SEDIMENTARY ROCKS. 5o9
ROCKS PRODUCED FROM MATERIAL OF BELT OF WEATHERING WITHOUT
TRANSPORTATION TO THE SEA.
While by far the larger part of the material produced by the processes
of weathering is transported to the sea, either in suspension or solution, the
weathered material may be buried under other deposits without transpor-
tation to the sea. The weathered material not carried to the ocean may
be classified into residuary and transported material. Such deposits may
be from a few meters to hundreds of meters, or, in the case of the trans-
ported material, even a thousand meters in thickness. After a deposit of
residuary or transported material has accumulated, the sea may transgress
over the region so quietly as not to disturb the larger part of the weathered
rock. As an example of the burial of residuary material under marine
deposits may be mentioned the Coastal Plain of the United States. Here
the deeply disintegrated pre-Cretaceous rocks have been overridden by the
ocean, and upon them have been laid down the Coastal Plain deposits. If
the sea should now advance over the Piedmont Plateau somewhat rapidly
part of the deeply disintegrated and decomposed belt there occurring might
be buried under marine deposits without disturbance.
Weathered material may be transported a greater or less distance, but
not to the sea, and thick deposits be built up. This is well illustrated by
the deposits of the Great Basin region of the United States. Here the
weathered material, instead of being transported to the sea, continuously
accumulates in the lower areas. This has gone on until there are hun-
dreds or a thousand or more meters of weathered rock material, which has
accumulated between the mountains. Some of this material is deposited
by the streams; other parts are deposited by the ephemeral lakes; others
by the permanent lakes. (See pp. 551-554.) The result is the building of
great sedimentary deposits, in which there is local assorting. When the sea
next encroaches upon this Great Basin area there may be buried below
the marine deposits a great mass of weathered material, which differs
radically from the ordinary marine deposits in that there has been com-
paratively little abstraction of soluble salts, and which therefore must have
nearly the same average chemical composition as the origimal rocks from
which they are derived, but not the same mineral composition.
The residuary or transported material, when sufficiently deeply buried,
whether below the sea or by upbuilding, as in the Great Basin area, passes
560 A TREATISE ON METAMORPHISM.
into the belt of cementation, and may even pass into the zone of anamor-
phism. It may be metamorphosed in those positions, and produce rocks
different in character from ordimary metamorphosed sedimentary rocks,
because the material has a different chemical composition. It is pointed out
in another place (see pp. 831-833) that the jaspilites of the Lower Huronian
of the Marquette district represent a weathered belt, which has been
overridden by the Upper Huronian seas, deeply buried by later sediments,
and metamorphosed under the conditions of the zone of anamorphism.
TRANSITION BETWEEN BELT OF WEATHERING AND BELT OF
CEMENTATION.
Before taking up ‘The belt of cementation,” it will be well to call atten-
tion to the fact that the belts of weathering and cementation are not sharply
separated, but there is a transition between them. In many cases the
explanation of the transition is partly that locally strong downward currents
carry solution as a preponderant process well below the level of ground
water, and partly that a considerable quantity of oxygen may be carried
some distance below the level of ground water and produce reactions charac-
teristic of the belt of weathering. In other cases the level of ground water
rises and falls, as explained on pages 423-429, and therefore there is a
belt in which the conditions are alternately those of the belt of weathering
and the belt of cementation. While in general the transition from one belt
to the other is somewhat gradual, in some instances it is rather abrupt. An
excellent illustration of an abrupt change is that given by Culver in the
case of the diabase in Minnehaha County, 8. Dak.“ This rock is thoroughly
disintegrated and apparently much decomposed to a depth of 6 or 8 meters,
that is, to the bed of the stream which marks the limit of ground water.
Says Culver: “The limit of decomposition seems to be marked by the
position of the stream; the rock in its bed is firm and apparently unaltered.”
As an illustration of a transition belt of considerable width may be cited
the iron-ore deposits of the Lake Superior region. Many of these deposits
extend from the surface to a depth of 200 to 500 or more meters, but at the
greater depths they usually become gradually smaller and less rich in
hematite. The level of ground water is rarely deeper than 30 meters.
«Culver, G. E., and Hobbs, William H., On a new occurrence of olivine-diabase in Minnehaha
County, South Dakota: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 8, 1888-1891, pp. 206-207.
DEPTH OF OXIDATION AND SOLUTION. 561
These iron-ore deposits consist mainly of hematite with some limonite, and
occasionally some magnetite, with silica as the main impurity. In passing
downward the silica is likely to become somewhat more abundant, and
finally the ores become too lean for working. As explained in another
place, these ores have been precipitated in pitching troughs on impervious
basements by downward-percolating waters which bear oxygen, often at
places where the rocks have been much fractured by orogenic movement
and are therefore very open and porous. Simultaneously with the pre-
cipitation of the iron oxide silica is dissolved. (See pp. 1193-1197.) It
appears, therefore, in the case of these ore deposits, that oxidation and solu-
tion, both reactions characteristic of the belt of weathering, have locally
extended for 300 meters or more into the belt of cementation.
MON XLVII—03 36
Cio I ek WII
THE BELT OF CEMENTATION.
BELT OF CEMENTATION DEFINED.
The belt of cementation, like the belt of weathering, belongs to the
zone of katamorphism. The belt has been discussed from the physical-
chemical point of view in Chapter IV. From this point of view its”
definition is very similar to that of the belt of weathering. It is a belt
in which the reactions take place with liberation of heat and expansion
of volume, or come under the first part of van’t Hoff’s law. From a
geological point of view the condition of affairs is very different in the belt
of cementation from that in the belt of weathering, and it is primarily the
purpose of this chapter to consider the belt of cementation from this point
of view, but, as heretofore, the geological treatment is subject to the
general principles which have been developed in the previous chapters.
Geologically the belt of cementation may be defined to include that
part of the zone of katamorphism which is below the belt of weathering.
It has as its lower limit the zone of anamorphism, the zone in which
permanent openings, whether produced by fracture, original sedimentation,
or any other cause, are of subcapillary size. It is bounded above by the
belt of weathering, the lower limit of which is the level of ground water.
All of the classes of openings described on pages 129-146 are found in
this belt. But whatever the nature of the openings, whether cracks and
crevices produced by mechanical action, such as those of joints, faults,
bedding partings, and fissility, or the openings originally present in the
rocks, such as pore spaces of the mechanical sediments and the vacuoles in
voleanic rocks, they are usually filled with water.
The belt under discussion is named the ‘belt of cementation” because
cementation is the most obvious and probably the most important single
process of the belt. But it is by no means the only process; it will be seen
562
BELT OF CEMENTATION DEFINED. 563
that metasomatism, injection, consolidation, and fracturing are important,
and some of them scarcely less so than cementation. The geological
evidence clearly shows that no sooner are openings produced in this belt, or
rocks containing openings transferred to this belt, than deposition of material
in the openings begins and continues until the separated parts are firmly
cemented. Since cementation is only one of the processes which occur, the
name ‘‘belt of cementation” is less fortunate than the name for the belt at
the surface, the belt of weathering, for weathering is a general term properly
covering many, if not all, the processes which take place within the belt
to which it is applied; but as there is no comprehensive term available,
the name “belt of cementation” has been chosen, with hesitation, on the
basis of naming the belt by the most obvious and characteristic of the
processes which take place within it. In many respects the name ‘“ belt
of induration” would be a good one, since under quiescent conditions the
rocks are hardened by the various processes at work. The process of
cementation also hardens or indurates many of the rocks. This name
would cover both of the chemical processes, cementation and metasomatism,
and the igneous process of injection, but it is contradicted by the mechanical
process of fracturing. It is more comprehensive than the name “belt of
cementation,” but is maccurate in that induration is not always characteristic
of the belt. The name “belt of saturation” well covers the fact that the
openings are commonly filled by water solutions, but this is also true of the
exceedingly minute openings of the zone of anamorphism. Furthermore,
the term ‘belt of saturation” would be likely to be understood to imply that
the solutions in the belt are always saturated with the compounds with which
they are in contact, and this is far from the truth. Certainly the term ‘‘belt
of cementation” is the one which seizes as distinctive the most characteristic
and obvious feature, evidences of which are seen in the field in all
major openings, and which are equally evident through the microscope in
minor openings.
The geological evidence that cementation is the universal law for the
part of the zone of fracture in which the rocks are usually saturated with
water seems to be conclusive. In mining operations the world over it has
been found that there is a tightening of the ground when the level of ground
water is reached. Above that level the rocks are likely.to be open and
porous, giving the freest circulation to the waters of the belt of weathering;
064 A TREATISE ON METAMORPHISM.
but below that level the rocks are usually much less porous, although it does
not follow that the water circulation may not be important and rapid. Fine
illustrations of the sudden lessening of the pore space at water level ‘are fur-
nished by the lead and zine districts of the Mississippi Valley. This lessen-
ing of the pore space is partly due to the solution of material ‘above the level
of ground water, rather than to cementation below it. But observations
below the level of ground water show that cementation is as certain as is
solution above. Sandstone formations, as is well known, are cemented
mainly by silica, although calcium carbonate, iron oxide, ete., are subordi-
nate cementing constituents. Wherever sandstones are found which have
long been below the level of ground water, cementation has taken place,
either by enlargement of the old grains or by independent deposition between
the grains, or by the two combined. All stages of this process are seen,
from those in which the grains have simply built out crystal facets that
sparkle in the sun, to rocks so firmly cemented as to be perfect quartzites,
in which fracture breaks through the cement and across the old grains
rather than around them.* The pore spaces of arkoses, tuffs, and other
rocks which have long been below the level of ground water, are shown by
microscopical examination to have been also closed; but often the cements
are more variable than in the case of quartzites, frequently including
feldspar, pyroxenes and amphiboles, calcite, and various other minerals.
That cementation is the universal rule for the porous rocks below the
level of ground water was ascertained only by microscopical study, but in
the field may be seen evidence of cementation of the larger fractures. The
filling of ancient joimts, recementing the separated joint blocks, the filling
of fault openings, thus producing true veins, and the cementing of the frag-
ments of breccias are all well-known phenomena. Where rocks have been
broken and buried deep enough to be below the level of ground water and
have remained there long, it is found to be an almost universal rule that the
filling of these larger openings has begun; and usually, so far as the older
openings are concerned, the process has been practically completed, although
recent earth movements may have produced openings which have not been
closed.
The openings which are being filled in the belt of cementation vary in
size from minute pores between the grains to great caves. Interesting
“Irving, R. D., and Van Hise, C. R., On secondary enlargements of mineral fragments in certain
rocks: Bull. U. 8. Geol. Survey No. 8, 1884, pp. 1-56.
UNIVERSALITY OF CEMENTATION. 565
instances of very large openings below the level of ground water are fur-
nished by the caves in the lead and zine district of Missouri. Here, in
consequence of extensive pumping operations, the level of ground water has
been lowered from a few meters to 45 to 60 meters. At various places below
the level of ground water caves of considerable magnitude have been found,
some nearly 60 meters in length and with rooms 12 to 15 meters in width.
These caves are lined with crystals of calcite. The faces are perfectly
clear. It is almost certain that the erystals continued to grow until the
level of ground water passed below them. The caves are, in fact, like
gigantic geodes with great scalenohedral crystals, some of them a half meter
or more in length, projecting everywhere from the walls. No stalactites
or stalagmites are found, or any of the other peculiar phenomena so
characteristic of caves above the level of ground water in the belt of
weathering. These caves, before transferred from the belt of cementation
by the lowering of the level of ground water, were being filled, and had
the process continued sufficiently long there would have been formed great
masses of crystallize calcite analogous to the masses of that mineral which
are sometimes elsewhere found.
Concluding from the geological facts observed, it seems perfectly clear
to me that the fundamental fact of the part of the zone of fracture between
the level of ground water and the zone of anamorphism is that cementation
is the most characteristic process, and therefore the one which properly
gives its name to the belt.
BOUNDARIES OF BELT OF CEMENTATION.
It has been explained (pp. 409-411) that in different areas the level of
ground water is at different depths from the surface, varying from zero to
300 meters, and exceptionally to a thousand meters.
It has been shown that the level of the upper surface of the ground
water is not horizontal, but undulating, and that the undulations of the
level of eround water roughly follow the topography, as shown by the fact
that upon many hills and mountains wells reach water of saturation at the
very moderate depths of a few meters to 50 meters. A topographic map
of a region is to a certain extent a map of the level of ground water, but
the latter shows less accentuated contours. The elevation of the contour
of the ground water at a given place is less than the elevation of the
e
566 A TREATISE ON METAMORPHISM.
surface contour by the depth of the level of ground water. It has also
been seen (pp. 411-413) that the surface of ground water rises and falls
from a few centimeters to a number of meters, and that therefore the apper
boundary of the belt of cementation is somewhat variable.
The lower limit of the belt of cementation is the lowest horizon at
which there may exist abundant openings in the rocks of supercapillary and
capillary size. It has already been pointed out (pp. 190-191) that to this
limit the rocks are self-supporting, but below it the rocks are not sufficiently
strong to support themselves, so that if openings were supposed to be
produced they would be closed by flow.” It has been calculated that for
the strongest rocks the bottom of the belt of cementation may be as deep
as 10,000 or possibly 12,000 meters, although for most rocks under ordinary
conditions the bottom of the belt of cementation is believed not to be nearly
so low. With the same kind of rock the bottom of the belt of cementation
is at different depths under different conditions. For instance, when earth
movements are rapid the belt might extend considerably deeper than when
they are slow. The belt of cementation, extending as it does from the level
of ground water to the bottom of the zone of fracture, is much broader
than the belt of weathering.
CONDITION OF WATER IN BELT OF CEMENTATION,
The question whether hydrostatic pressure increases sufficiently fast
with depth to prevent the water from passing into the form of gas needs to
be answered. If the average temperature at the surface be assumed to be
0° C.—and in the arctic regions the average temperature is probably lower
rather than higher than this—and if the increment of increase of tempera-
ture be taken as 1° C. for every 30 meters, the critical temperature of water,
365° C., is at a depth of 10,950 meters. If the average temperature at the
surface were supposed to be 25° C., about the maximum for the tropical
regions, in order to reach a temperature of 865°, the critical temperature of
vater, a depth of 10,200 meters would be required.
At any given place the water is subject to the pressure of the super-
incumbent column of water. Supposing the temperature of the water were
100° C., of just at the boiling point, at the surface of the earth (the most
a@Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann.
Rept. U. S. Geol. Survey, pt. 1, 1896, pp. 591-554.
CONDITION OF WATER IN BELT OF CEMENTATION. 567
unfavorable assumption to holding the water as a liquid), the water would
be a liquid in the belt of cementation, as is shown by the following table
based upon this supposition, column (1) being temperatures, column (2)
being pressures necessary to hold water as a liquid at these temperatures,
column (3) bemg depth in meters at which the pressures would be pro-
duced, column (4) being the depth which would be required to produce
the temperatures on the supposition that the increment of the increase of
temperature is 1° C. for every 30 meters, and column (5) being the actual
temperatures which exist upon this supposition at the depths represented
by column (3).
lations essures, temperatures, and depths in ground water.
Relations of pressures, temperatures, and depths in ¢ 7 wat
(1) (2) | (3) | (4) (5)
Pressures cor-| Depth neces- | eee) Pecuialiyy ex
Temperatores| ESPONGHNE To | sary tO PPO. ghee tempera. | UDE at Toe
of column 1. | of column 2. area Th depths ohcer:
OO Atmosnheres. Meters. Meters. | o¢:
120 2.0 20 600 100, 66
180 10.0 100 2, 400 103. 33
225 25.0 250 3, 750 108. 33
265 51.0 510 4,950 117. 00
310 99. 0 990 6, 300 133. 00
340 | 148. 0 1,480 | 7, 200 149. 33
365) |) 205.5 2,055 | 7, 950 168. 33
A TREATISE ON METAMORPHISM.
Or
ion}
(0)
A fuller table, showing the relations of temperature and pressure
between 225° and 365° C., at intervals of 5° C., is given below.”
Relations of temperatures and pressures of water.
Tempera- |PEESORS PeEE™) Tempera. PLEssUre neces
H water as liquid. wateras liquid.
©) (6h Atmospheres. © oh | Atmospheres.
225, Dara 300 | 86.2
230 27.5 | 305 | 92.2
235 30.0 | 310. | 99.0
240 32.8 |} 315) 106.1
245 Rs ht SD. Ph TheS
250 39.2 325 121.6
255 42.9 | 330 130.0
260 46.8 | 335 138. 8
265 | 50.8 | 340 147.7
270. | 55.0. || 345 1E7.5
275 59.4 | 350 | 167.5
280 | 643 | 355 Biamelzene,
| 285 | 69.2 || 360. | 188.9
soso | 74.5 | 365 200.5
295 | 80.0 |]
|
From these tables it is seen that the hydrostatic pressure at various
depths is normally far in excess of that required to hold the water in the
form of a liquid, or, looked at in another way, for any given depth the
temperature is not sufficiently high to allow the water at that depth and
pressure to exist in the form of a gas.
It therefore appears perfectly clear that where the increase in tempera-
ture with depth is normal the water remains as a liquid to its critical
temperature, i. e., 865° C., and that the depth of the zone of water circu-
lation is, therefore, about 10,000 or 11,000 meters. In another place (pp.
189-190), it has been shown that this approximates to the greatest depth
at which continuous crevices and cracks can long exist in the earth. It
therefore follows that in the belt of cementation the water is normally in the
form of a liquid to the bottom of the zone. Where magma is intruded in
the lithosphere the temperature may become so high that this statement
will not hold. But this is the exceptional, not the usual case. Furthermore,
«Preston, Thomas, The theory of heat, Macmillan & Co., London, 1894, p. 385.
CONDITION OF WATER IN BELT OF CEMENTATION. 569
it is conceivable that as a result of deformation itself the temperature of
the rocks may become so high as to convert the water present into the form
of gas, but, from investigations upon metamorphism, it is believed to be
probable that this condition of affairs rarely obtains, since, as shown on
pages 690-692, long before the critical temperature of water is reached
solution and deposition of rock material, or recrystallization, readily takes
place, and in this change the work converted into heat is far less than in
mechanical granulation.
In conclusion the general statement may be made that in the belt
of cementation water is commonly in the form of liquid, and only excep-
tionally in the form of gas.
AMOUNT OF WATER IN BELT OF CEMENTATION.
It has already been suggested that the belt of cementation might be
called the belt of saturation. his implies that the openings in the Delt of
cementation normally are filled with water solutions; therefore, the answer
to the question as to the amount of openings in the belt of cementation
approximately answers the question as to the amount of water there
contained. It has been seen that in the belt of weathering the openings
vary from a small amount to 40 or 50 per cent or more. In the belt of
cementation the amount of pore space varies from zero to a maximum as
high as 28.28 per cent, the actual amount of pore space found by Buckley
in the Dunnville sandstone of Wisconsin.* ‘The mechanical sediments,
especially sandstones which have not been much cemented, contain very
large amounts of pore space. The volcanic fragmental rocks also furnish
considerable pore space. The very dense rocks, especially the plutonic
igneous rocks, have a very small amount of pore space unless they have
been fractured. Wherever the rocks have been deformed above the zone
of anamorphism fractures of all classes are produced, including joints,
faults, bedding partings, fissility, and brecciation fractures. The amounts
of openings which are thus produced may vary from a fraction of a per
cent to as large a per cent as in the porous sandstones, as, for instance, in
breccias. But the openings which originally existed in the rocks or which
have been produced by deformation are in all stages of cementation. Also
“Buckley, E. R., Building and ornamental stones of Wisconsin: Bull. Geol. and Nat. Hist. Survey
Wisconsin No. 4, 1898, table v, p. 403.
570 A TREATISE ON METAMORPHISM.
in all probability the amount of pore space gradually decreases in passing
from the upper to the lower part of the belt of cementation. All of the
foregoing uncertain factors make any estimate of the water of the belt of
cementation little better than a guess. On page 128, assuming there is
Fic. 7.—Ideal horizontal section of the flow of ground water through a homogeneous medium from one well to another.
present only one-fifth as much as Dana’s estimate of 2.67 per cent by
weight as the average amount of water in the upper part of the belt of
cementation, supposing that from this amount it gradually decreases to
zero at the bottom of the belt, and assuming the belt to extend to a depth
of 10,000 meters, I have calculated that the amount of water in the belt, if
concentrated in a stratum, would make a sheet 69 meters thick. How-
AMOUNT OF WATER IN BELT OF CEMENTATION. Hel:
ever, it may be that this estimate is too small, and that the amount of
water of the belt of cementation may be two or three times this amount.
CIRCULATION OF WATER IN BELT OF CEMENTATION.
VIGOR OF CIRCULATION.
The vigor of the circulation in the belt of cementation is best shown by
the amount of work which has been accomplished in cementation of rocks
A B
Fic. 8.—Ideal vertical section of the flow of ground water through a homogeneous medium from one well to another.
by ground waters. It is shown in another place that great sandstone for-
mations, which must originally have had a pore space of from 32 to 40 per
cent, have been completely cemented by the deposition of silica. (See pp.
865-868.) It is further shown that tne amount of silica thus deposited is
572 A TREATISE ON METAMORPHISM.
enormous—to be measured in tens of thousands, and probably hundreds
of thousands of cubic kilometers. The amount of silica which the waters of
mineral springs carry is, on the average, less than 2 parts in 100,000. In
only a few cases is the amount greater than 10 parts in 100,000, although
in one case, that of San Bernardino spring of California, the amount runs
as high as 22 parts in 100,000. Supposing the amount of silica deposited
by underground waters to be as high, on the average, as 1 part in 100,000
(and the amount deposited is invariably less than that in solution), it would
be necessary to believe that over 260,000 times as much water must have
circulated through the sandstone formations as the enormous amount of
quartz which has been deposited. The amount of silica assumed to be
deposited by underground waters may be too great or too small, but it is
certain that many thousand times as much water as deposited quartz has
circulated through the rocks.
CHARACTER OF CIRCULATION.
It has already been explained that the complex movements of ground
water may be resolved into two components, horizontal or lateral move-
ments and vertical movements. In order to understand the work done by
underground water in its journey, it is first necessary to know the path
which it follows. On this point the recent analytical work of Prof. C. 8.
Slichter gives the desired information.” According to Professor Slichter’s
analysis, the flowage of water from one place to another through a
homogeneous medium, say from A to B, is not by a direct path, but by a
large number of diverging paths from A during the first part of the journey,
and by a large number of converging paths to B during the latter part of
the journey. This may be illustrated by supposing the water to be poured
into a well, A, and to flow to a well, B. The horizontal course of the water
is represented by fig. 7, and the vertical course by fig. 8. These conclusions
apply equally well to any porous rock, such as a soil or sandstone in which
the spaces are distributed in a somewhat uniform manner.
It is apparent that Slichter’s conclusions have far-reaching consequences
as to the flowage of ground water. In the passage of the water from the
«Peale, A. C., Mineral springs of the United States: Bull. U. S. Geol. Survey No. 32, 1886, pp.
156, 187, 192, 195, 212, 213.
DSlichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1899, pp. 297-384.
CIRCULATION OF WATER IN BELT OF CEMENTATION. oe
top or slope of a hill to a point of issue at the foot of a hill, supposing these
to be the only points of entrance or issuance of the water, and supposing
the spaces to be uniform, the vertical course would be represented by the
lines of fig. 9 and the horizontal course would be represented by the lines
of fig. 7.
Fic. 9.—Ideal vertical section of the fiow, through a homogeneous medium, of ground water entering at one point
on a slope and issuing at a lower point.
- In an actual case of ground-water flowage the water does not enter the
ground at a single point, but at every point of a slope. As a simple case,
we may suppose that the water entering on a slope reaches the surface again
at the level of a stream in an adjacent valley. To get an idea of the com-
plexity of the flow in this ideal case we may arbitrarily select a number of
points where the water enters and trace out its course. We may plat by
574 A TREATISE ON METAMORPHISM.
different kinds of lines the vertical components of the flowage of the water
which enters at each place independently of the water that enters at other
places. (Fig. 10.) In the figure we have a series of intersecting lines
representing the vertical components of movement.
It is not supposed that water actually follows paths similar te those
represented by the figure, for there is mutual interference of the water
out act
F ground water
We Fi ;
IN
Fic. 10.—Ideal vertical section of the flow, through a homogeneous medium, of ground water entering at three points
and issuing at a single point, each system of flow being independent of the others.
entering at the various points. As a result of this the water entering the
opening nearest the exit would take a more direct course than the average
of that platted; but, as a consequence of this, the water from the next
opening up the slope would take a more indirect course, on the average,
than that platted, and so on. The total result would be to give an average
course for the water which may be represented by combining the inde-
CIRCULATION OF WATER IN BELT OF CEMENTATION. 575
pendent curves. (Fig. 11.) The effect, so far as the geological action of
the underground water is concerned, would be approximately the same
whether the course of the water were that represented by fig. 10 or that
represented by fig. 11. This statement, applicable to a few points of
entrance and one of exit, is equally applicable to a great number of points
of entrance and one of exit. The statement can be further extended to
Fie. 11.—Ideal vertical section of the flow, through a homogeneous medium, of ground water entering at many points
along a slope and issuing at a single point at a lower elevation.
an indefinitely great number of points of entrance distributed along the
contours of the slope as well as up the slope and to many points of exit at
or near the level of the valley.
Disregarding the lateral movement after the water reaches the level
of ground water, the sea of ground water at a given place might be con-
sidered as a column moving downward as rapidly as the increment of ground
water is added from above. However, the lateral movement which is super-
576 A TREATISE ON METAMORPHISM.
imposed upon the vertical movement carries it sooner or later to some point
where upward movement is taking place. Thus the amount which con-
tinues downward is an ever-decreasing fraction of the amount of precipita-
tion which joins the sea of ground water.
Thus far the discussion of the flowage of ground water has been
carried on as if it were through a homogeneous porous medium extending
indefinitely in all directions in which the pressure and the temperature are
the same throughout. It is needless to. say that such are not the conditions
of natural systems of underground flowage. Under natural conditions there
are many other factors which very greatly modify the nature of the flowage.
Among these are limiting formations, gravity, increase in temperature with
depth, the relative lengths of the vertical and horizontal components, and
preferential use of large and continuous channels.
LIMITING FORMATIONS.
It has been shown (pp. 190-191) that the bottom of the belt of cemen-
tation corresponds with the bottom of the zone of fracture. It has been
explained that to the bottom of the zone of fracture supercapillary or
capillary openings may exist. Below the bottom of the zone of fracture the
openings in the rocks are subcapillary and therefore practically impervious
to arapid circulation. The bottom of the zone of fracture is the lowest pos-
sible boundary of efficient circulation. But there is no theoretical reason —
why a ground-water system may not utilize the entire zone of fracture to its
lower boundary. Indeed, the well-known hydrodynamical principle that
the entire available cross section is utilized by flowing currents demands
that the circulation extend to the bottom of the zone of fracture. This
generalized statement conforms well with the lines of flow in a homo-
geneous medium as determined by Professor Slichter. (See figs. 7 and 9.)
It therefore appears highly probable that, in any system of ground-water
circulation, where there is no impervious rock nearer the surface than the
bottom of the zone of fracture the entire zone of fracture is searched,
although waters joming and departing from the underground sea disappear
and appear at its surface. While this is true, other things being equal, the
more direct route is utilized to a greater degree than the more indirect
route, and therefore the remoter corners of available space have relatively
LIMITS OF CIRCULATION OF WATER. BT
small circulations The principle of the distribution of the flowage of water
over the entire available area is well illustrated by the case of water flowing
horizontally into a beaker from one side and overflowing the beaker on the
other side. The movement of the water is not confined to the liquid near
the surface, but all portions of the water in the beaker, from the top to the
very bottom, take part in the flowage, although, of course, the rate of
movement is much more rapid at the top than at the bottom.“
While the circulation may extend to the very bottom of the zone of
fracture, in many cases before that depth is reached a stratum is met all
the openings of which are subeapillary. As shown on pages 143-146, such
a stratum is practically impervious, and therefore becomes a practical limit
of circulation in that direction. Such impervious strata may be only a few
meters below the level of ground water, and thus the circulation be exceed-
ingly shallow. From such a position an impervious stratum may be at
various depths to the bottom of the zone of fracture.
While one system of circulation may thus be of very limited depth, it
Fre. 12.—Ideal section illustrating the chief requisite conditions of artesian wells. A, a porous stratum; B and C,
impervious beds below and above A, acting as confining strata; F, the height of the water level in the porous bed A, or, in
other words, the height of the reservoir or fountain head; D and 5, flowing wells springing from the porous water-filled
bed A. After Chamberlin.
does not follow that other systems of circulation are not utilizing the other
parts of the zone of fracture below the first circulation; for very frequently
a belt below, which contains openings and is available for flowage, may
elsewhere reach the surface and have a feeding area. Indeed, such are the
circulations of many systems which are limited above by impervious strata.
One of the simplest of such systems is represented by fig. 12. There are
an indefinite number of variations of the systems of underground cireula-
tions, dependent upon various combinations of pervious and impervious
strata. In some instances there may be several superimposed or adjacent
systems, the relations of which are exceedingly complex. Some of the
cases are of general geological interest. A number have been discussed by
7 Fa +
@$lichter, C. 8., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1899, p. 331:
MON XLVU—O4 Bi
578 A TREATISE ON METAMORPHISM.
Chamberlin and others in connection with artesian waters;” but the best
illustrations of complex systems have been worked out in connection with
ore deposits, and in the chapter on that subject a number are discussed.
GRAVITY.
Thus far the full effect of gravity upon the character of the circulation
has not appeared. The discussion has been carried on as if the movement
of the ground water were controlled by simple pressure. But gravity is
ever pulling the water downward. This force, in the early part of the
journey, accords with the downward movement produced by head, and the
result is that the water follows a deeper course than it would if moving
through a homogeneous medium, in a manner similar to that of an electric
current.
INCREASE OF TEMPERATURE WITH INCREASE OF DEPTH.
It has been repeatedly stated that the normal increase of temperature
with increase of depth is 1° C. for each 30 meters. It has been pointed out
(pp. 140-141) that the viscosity of water decreases as the temperature
increases, and that the flowage of water is relatively rapid in proportion as it
has low viscosity. Indeed, in capillary openings it has been shown (p. 139)
that the flowage is inversely as the viscosity. When it is remembered that
the pore spaces of the ordinary mechanical sediments are of capillary size,
it is seen that this principle is very important in the flowage of ground
water. At a depth of 1,350 meters, supposing the increment of temperature
to be normal, the ground waters would have a temperature of 45° C. greater
than at the surface. Ata depth of 2,700 meters the ground waters would
have a temperature of 90° C. greater than at the surface. The viscosity
of water at 45° C. is only about one-third as much as at 0° C., and
at 90° C. is only one-fifth as much as at 0° C. (See p. 141.) In the lower
part of the zone of fracture the temperature of water varies between
100° C. and the critical temperature of water, 365° C. (See pp. 566-569.)
If we suppose that at temperatures above 90° C. the viscosity decreases at~
«Chamberlin, T. C., Artesian wells: Geol. of Wisconsin, 1873-79, vol. 1, 1883, pp. 690-691; also
vol. 2, 1878, pp. 149-152. Leverett, Frank, The water resources of Illinois: Seventeenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1896, pp. 802-805. Hill, R. T., Geography and geology of the Black and
Grand prairies, Texas: Twenty-first Ann. Rept. U. 8. Geol. Survey, pt. 7, 1901, pp. 398-401.
VISCOSITY AND DEPTH OF CIRCULATION. 579
the same average rate as between 0° C. and 90° C., at a depth of 9,000
meters, where the temperature is probably about 300° C. greater than at
the surface, the viscosity would be only about one-twentieth of that at the
surface. From the foregoing it appears that in capillary openings in the
part of the zone of fracture below 1,350 meters, with a given head, the
flowage of water for capillary tubes of a definite size would vary from
about three times to about twenty times faster than at the surface. These
numbers are not accurate, because water at the surface usually has a higher
temperature than 0° C., but the error is not so great but that the conclusion
follows that increase of temperature with depth is a very important factor,
tending to promote a circulation in the deeper parts of the zone of fracture,
for water follows the lines of least resistance; therefore, ground water, on
the average, follows a deeper path than it would were the temperature and
viscosity uniform.
-RELATIVE LENGTHS OF VERTICAL AND HORIZONTAL COMPONENTS.
The vertical component of the journey of ground waters in the belt of
cementation may be considered as confined to the zone of fracture, and is
probably measured by 10,000 meters, or at most by 12,000 meters. The
lateral component, on the other hand, may vary from a few meters to
hundreds or even a thousand or more kilometers. There can be little doubt
that, on the average, the horizontal component is much greater than the
vertical component. No limit can be assigned to the horizontal movement
of water. It is known that the nearest source for artesian waters may be
many kilometers from the wells. For instance, in the James River Valley,
Dakota, the Dakota sandstone yields abundant water. The nearest place at
which this sandstone outcrops is several hundred kilometers distant, and it
therefore appears highly probable that the horizontal movement of the
ground water is measured by hundreds of kilometers, while its vertical
journey is probably less than 1,000 meters. Doubtless in this case the
horizontal distance which the water has journeyed is far greater than the
average, but still the average journey is probably one of considerable
length.
The length of the horizontal component of the journey has much to do
with the depth of the circulation. Where the horizontal component is
5&0 A TREATISE ON METAMORPHISM.
comparatively short, not many times longer than the vertical component,
there is a strong tendency for the circulation to be relatively shallow.
Where, however, the horizontal component is very great, the relatively short
distance between the bottom and top of the zone of fracture of a porous
stratum is of comparatively little consequence. In such cases the circulation
through the deeper channels, provided they are equal in area and _ size to
the shallow channels and the water is of the same temperature, might be
practically the same as through the shallow ones. For instance, in the case
already cited of the Dakota sandstone of the James River Valley, its thick-
ness, about 100 meters, is insignificant compared with the horizontal journey
of hundreds of kilometers. Therefore, in such a case, if the sandstone were
equally porous throughout and of the same temperature, for the greater
part of the journey there would be no appreciable difference between the
amount of flow in the upper and that in the lower part of the sandstone,
although in the very early and very late stages of the journey the upper part
of the sandstone would have a greater flow. Where the horizontal journey
is long, it is possible, on account of increase of temperature, giving decreased
viscosity, that the circulation is more rapid through the lower part of a
formation than through the upper part. This would almost certainly be
true if all parts were equally porous. But if this were so, sooner or later
the more rapid flowage would carry the process of cementation of the lower
part of the formation farther than the higher part; and as the openings
became partially closed, this would tend to lessen the amount of the deeper
circulation.
PREFERENTIAL USE OF LARGE CHANNELS.
A very important factor in the flowage of ground water is the great
variation, in any natural system of underground flow, of the area of avail-
able space and the rate of movement. In nature the points of entrance for
ground water are indefinitely numerous, and the places of exit compara-
tively few. The water falls upon the ground everywhere and enters the
innumerable pores. After a longer or shorter underground course, perhaps
passing under many subordinate hills and valleys, it escapes to the surface
as a spring or by seepage, nearer the drainage level than where it entered
the ground. The water began its journey through an almost infinite num-
ber of openings; it issues at many openings, but these are few compared
with the vast number of those at which it entered.
PREFERENTIAL USE OF LARGER CHANNELS. 581
This conclusion is based on the following facts: Openings in rocks are
never of uniform size. It has been seen that the resistance to flow in
capillary openings is far greater than in supereapillary openings. In small
supereapillary openings the resistance is greater per unit of flow than in
larger ones. Thus there is a strong tendency for the water starting through
innumerable small openings to converge into larger and larger openings,
because these are the lines of least resistance. Of course, water may go
long distances underground without finding larger openings than those
near the surface, as in some sandstones; but if large openings exist, they
are fully utilized. Finally, when a single opening or a group of openings
larger than the average reach the surface at a lower altitude than the
average level of entrance of the water, there is a spring.
This reasoning is confirmed by experimental work of King, who finds
that the flow in the Amherst sandstone of Wisconsin is faster along than
across the bedding planes. The openings along the bedding planes are
larger than those between the grains. The first are largely utilized in
flowage along bedding and the second must be utilized in flowage across
bedding planes.“ !
From his experimental work King holds “that the movements of
ground water across long distances must take place in considerable
measure through passageways larger than those which depend upon the
pore space fixed by the diameters of the grains which constitute the beds
themselves.” ”
While I believe that in proportion as openings are large they are
much more fully utilized than a similar area of distributed openings of
small size, we must remember that the movement of the widely dispersed
deep water is often excessively slow, and that under these circumstances
the resistance in the capillary tubes per unit distance is reduced to a very
small, almost an infinitesimal, amount. (See pp. 141-142.) Therefore
capillary openings between the grains in the cases of great sandstone and
similar formations may be the chief channels of circulation for large quan-
tities of water and for long distances. That such openings are the chief
channels through which the water actually flows in the deeply buried sand-
stones bearing artesian waters is indicated by the fact brought out by
«King, F. H., Principles and conditions of the movement of ground water: Nineteenth Ann.
Rept. U. S. Geol. Survey, pt. 2, 1899, p. 126.
b King, cit., p. 249.
582 A TREATISE ON METAMORPHISM.
Slichter, that the pressure a short distance from the bottom of a well is com-
monly as great while the well is flowing as it is at the mouth of the well
when the flow is stopped.” If the flowage was mainly, or even largely,
through supercapillary openings, this would not be the case.
Large openings are favorable to a somewhat direct course; small
openings are favorable to a circuitous route. As the openings decrease in
size, a more circuitous route must be taken, for to pass a given volume
of water from one point to another it is necessary that a wide range of open-
ings be utilized. It has been pointed out (pp. 569-570) that the openings
of the belt of cementation are relatively large in its upper part and probably
diminish in magnitude as depth increases. So far as this is true, therefore,
large channels for trunk circulation are, on the average, most numerous near
the level of ground water, and diminish in number and size as depth increases.
From this, and the principle that the ground water tends to utilize the
larger openings to the greatest extent, it follows that the abundant cir-
culation tends to follow a relatively shallow course.
The direct course of water in large openings is illustrated by limestone
regions where there are numerous large joints and sometimes caves within
which the water is quickly concentrated. In such instances the flowage
of water is very largely in the upper part of the zone of fracture.
RESULTANT CIRCULATION.
Summarizing, it has been seen that the movement of water in a
homogeneous medium is similar to the movement of a current of electricity
through a homogeneous medium, in that it utilizes the entire available
cross section. But by the water the more direct course is utilized to a
much greater extent than the less direct course: For the actual underground
circulation this general statement must be modified in various ways. The
circulation is limited wherever a formation is met in which the openings
are subeapillary. Gravity and increase of temperature with depth tend to
cause the water to take a deeper course than it otherwise would. On the
other hand, the preference of water for the larger openings, and the greater
abundance of openings of large size near the surface, tend to give the water
a shallow course. Therefore the influence of gravity and temperature and
that of large openings oppose each other. Which of these two opposing
“Slichter, C. 8., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1899, pp. 363-364.
SYSTEM OF CIRCULATION SIMILAR TO A TREE. 583
factors is the more important is uncertain, but probably the existence of
larger fractures near the surface is more important than the effect of gravity
and temperature.
It has been seen that during the first part of the underground journey
of water the vertical component is downward, and that during the latter
part of its journey the vertical component of much of it is upward. It
follows that, on the average, the downward movements of water are through
the smaller, and the upward movements through the larger, openings in the
rocks. Of course, where large openings are available for the downward-
moving water they are fully utilized, and therefore to a greater extent than
a similar area of smaller openings. But even if this be true, the statement
still holds that, on the average, the larger openings are more extensively
used by the upward-moving water than by the downward-moying water.
From the foregoing it appears that a system of circulation of ground water
has a very close analogy to a tree of peculiar character.
The points of entrance of water are the ends of the indefinite number
of twigs; these twigs unite into a branch; the branches unite to produce a
larger branch; the larger branches unite into a trunk; and at the end of a
trunk is a spring. The analogy of an underground drainage system to a
tree is even closer than that of a surface system; for in a system of under-
ground water circulation three dimensions are concerned to an important
extent, while in a surface system of drainage the movement of the water is
approximately confined to a plane. But, from what has gone before, it is
clear that the tree of ground water has a peculiar shape. The twigs and
branches have an important downward component; the larger branches
of the tree may be considered as approximately horizontal; and the trunk
usually has an upward component, which may be slight but is often
important. Thus twigs, branches, and trunks together ordinarily make a
ereat U. Where the water issues near the places of entrance the sides
ot the U are rather close together. Where there is great lateral move-
ment of the water the sides of the U are very far apart. Such a system
of ground water is somewhat similar to that of a surface system of drainage.
Ordinarily where the water first enters the underground sea the area
of available space is exceedingly great and the rate of movement is very
slow. As the water gathers in the larger openings the speed increases, and
in the final trunk channels the water may have a very rapid movement, as
584 - A TREATISE ON METAMORPHISM.
shown by springs. These facts are important in connection with the varia-
tion in the size of the openings. The earlier movements are in the capillary
openings, where the resistance to rapid flow would be great; but here the
movement is slow, and consequently the friction is very small. (See p.
581.) As the water passes into the supercapillary branch and trunk
channels, the important factor in friction is not that between the moving
water and the adherent films, but the internal friction. “he total friction
in these larger channels is almost indefinitely small as compared with the
friction of the same amount of water moving at the same rate through the
same area subdivided among capillary openings; for the area of contact,
and therefore the friction between the moving liquid and that fixed to the
walls, is inversely as the size of the openings. (See pp. 136-137.)
An underground circulating system is ideally illustrated in many arid
and semiarid regions. In the United States it is especially well illustrated
in the Great Basin and in southern California. In these regions the early
parts of the courses of many streams are in mountain gorges, where
usually considerable streams are above ground. As the streams leave
the
deposits, consisting of great alluvial fans of material, coarse and fine,
gorges and pass out to the plains their courses are over their own
which is called ‘‘wash.” The streams at these places commonly pass
underground, and there, as shown by drilling for water, follow some-
what definite channels. Lower in the course of the drainage systems a
large part of the underground water frequently issues in a series of springs
or by seepage on a large scale, and continues its course above ground.
The gravel and coarse sands furnish a close approximation to a homoge-
neous medium and ideally illustrate the laws of flowage given on pages
129-152, 572-576. Such an underground circulation as the above differs
from an overground system in that the boundaries of the water courses are
indefinite, in that the movement as compared with the movement over-
ground is exceedingly slow, and in that the underground cross sections
are necessarily much larger. The work of Professor Slichter shows that
the ground water of the Arkansas River flows in gravels at a rate not
greater than 3 to 5 meters a day.* Supposing the rate of movement of the
overground streams to be 10 kilometers an hour, or 240 kilometers a day,
and supposing the rate in the underground streams in the Great Basin or
“Slichter, C. S., The motions of underground waters: Water-Sup. and Irr. Paper No. 67, U.S.
Geol. Survey, 1902, p. 43.
GREAT SIZE OF UNDERGROUND STREAMS. 585
in southern California to be four times as great as the most rapid rate in
the Arkansas sands—that is, 20 meters a day—the overground rate would
be 12,000 times as fast as the underground rate. When it is remembered
that underground about three-fourths of the space is occupied by sand
and gravel, and that the openings present are more or less irregular, at
the maximum the available openings are not more than one-fourth the
entire space. It follows that, in order to accommodate the underground
water, the channel avould have to be 48,000 times larger than at the
surface. This number may be an overestimate, but certainly an under-
ground channel through sand and gravel to accommodate a certain amount
of water must be many thousand times larger than an overground channel
with the same slopes which accommodates the same amount of water.
While many streams in the semiarid and arid regions normally have
an underground course as above described, at occasional times of flood,
which may be for a brief season each year or may occur only once in a
number of seasons, the mountain streams are of such increased volume
that the underground circulation is not sufficient to dispose of the water, in
which case there is for a time also an overground circulation, which usually
follows .approximately the same general course as the ground waters. It
is at these times of flood that the fans of the wash are built up and the
overground channels determined. Such circulations are ideally illustrated
by the Santa Ana, San Gabriel, San Antonio, and other streams of southern
California.
Excellent illustrations of underground flowage are furnished by
artesian systems. For instance, in Wisconsin the Potsdam sandstone is an
artesian water-bearing stratum. This sandstone is 200 to 250 meters thick.
The annual precipitation in the district of its outcrop is approximately 50
centimeters. Supposing that one-half the rainfall enters the sea of ground
water in the sandstone, this would give an additional increment every
year of 25 centimeters. If the pore space of the sandstone be supposed
to be 163 per cent, in order that the increment added shall find space in
the sandstone without raising the water level it is necessary to suppose
that the water of the previous year shall have moved downward 150
centimeters. In other words, the vertical flowage per annum is only
150 centimeters, or 14 meters. Suppose the dip of the sandstone to be 2
meters per kilometer; it would follow that the lateral movement of the
586 A TREATISE ON METAMORPHISM.
water, in order that the new water shall have room to enter the sandstone
without raising the water level, should be three-fourths of 1 kilometer per
annum. If the dip of the artesian-bearing strata were steeper the lateral
movement would be much less. The above case, however, is approxi-
mately that of the Potsdam sandstone of Wisconsin. Since the average
lateral movement of the water can not be supposed to be more than
three-fourths of 1 kilometer per annum, the water which enters the artesian
circulation at a distance of 150 kilometers from Chicago should, on the
average, issue from the wells at Chicago 200 years after it enters the sea
of ground ‘water in Wisconsin.
In so far as the openings in the porous stratum are not of uniform size,
the movement will be more rapid through the larger openings, and so far
as this takes place the water rising in the artesian wells would remain
underground for a less time than calculated; but, on the other hand, the
remainder of the water which did not follow the larger openings would
travel at a slower rate than calculated, and thus remain underground a
longer time; and thus, so far as difference in porosity is concerned, the
average would be as calculated.
The average length of time during which the water remains in the
ground would probably be longer, perhaps much longer, than calculated,
for all the premises are made so as to give the minimum time. For
instance, the calculation is made upon the supposition that the porous
stratum is homogenous and that there is no leakage through the overlying
impervious structure, whereas it is certain that there is leakage. In so far
as there is leakage the lateral movement would be slower and the calculated
length of time should be increased.
Also the annual rainfall which enters the sea of ground water in the
belt of cementation is not so great as 25 centimeters, for a considerable
portion of the water which yvoes underground is evaporated from the belt
of weathering. In so far as the amount is overestimated the leneth of time
calculated—two hundred years—should be increased.
The above result as to length of time which water remains underground
in an artesian circulation is not so surprising when we compare it with the
exceedingly slow flow already mentioned for the underground water in the
sands of the Arkansas River and of the alluvial cones of southern Cali-
fornia, in which the descent of the level of ground water is many times
TIME THAT WATER REMAINS UNDERGROUND. 587
greater per kilometer than the gradient for the artesian water, as figured by
dividing the difference between the head and the point to which the water
would rise by the horizontal distance. Where the sandstone is perforated
by artesian wells near Lake Michigan the water rises with a pressure which
is a large fraction of the pressure due to head.
If, for instance, the city of Chicago be taken, the nearest point at
which the Potsdam sandstone outcrops is about 150 kilometers, and the water
for the Chicago wells is probably mainly derived from points 150 to 250
kilometers from Chicago. The average elevation of the catchment area
above Chicago is 80 meters. The water of the early artesian wells at Chi-
cago, before they were so numerous as mutually to interfere when allowed
to flow, rose to an altitude of about 30 meters above the surface:” or, there
is a loss, in the entire distance of 150 kilometers or more, of only about 50
meters of pressure. This small loss of head, which may be attributed to
friction between the water and the sandstone, and within the water, is to
be distributed through this entire distance. It follows that in the capillary
openings of this sandstone the friction of the water is almost zero, else the
pressure at the wells would not so nearly equal the pressure due to head.
Of course the moment the wells are allowed to flow, and especially if there
be a number of contiguous wells which are allowed to flow, the pressure
rapidly drops. This is not due to friction through the major course of the
water, where the movement is slow, but is due to friction adjacent to the
wells, where the water is moving rapidly toward them. This is shown by
the fact that at a distance of from 100 to 200 meters from a well the pressure
is commonly as great while the well is flowing as it is at the mouth of the
well when the flow is stopped.’ Indicating the same thing is the fact that
the flow of a well may be greatly increased by shattering the rock at the
bottom of the well, thus producing supercapillary openings into which the
water may issue from the capillary openings. If the area of these super-
capillary openings be many times that of the cross section of the well, the
chief source of friction is that of moving water in the well itself; for the
water has a chance to slowly ooze from the capillary into the supercapillary
tubes without rapid motion, and to make its way to the well tube through
aLeyerett, Frank, The water resources of Illinois: Seventeenth Ann. Rept. U. S. Geol. Survey,
pt. 2, 1896, pp. 805-806, 811.
bSlichter, C. §., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1899, pp. 363-364.
588 A TREATISE ON METAMORPHISM.
many supercapillary channels of much greater cross section than the well.
The rapid movement is restricted to the section of water in the well, and
there the friction is great. This Wisconsin illustration is typical of natural
systems of ground water flow; for, as shown on page 583, a natural system
of flowage simulates a tree. At the outset there are an indefinite number
of minute openings through which the water slowly moves to the branch
openings. In the branch openings the movement is faster. The branches
unite into the trunk channels, and in these the movement of the water is
rapid.
The analogy to a tree has been suggested in order to get definitely
in mind the general character of the circulation of ground water. But the
analogy must not be pushed too far. A tree commonly has but a single,
continuous, solid trunk, although willows and other trees have many trunks.
Very frequently, indeed commonly, the trunk channels of ground water
circulation are very complex. While a main watercourse may exceptionally
occupy a single open passage, ordinarily it occupies a number of inter-
locking passages. ‘These may be the parallel openings of a complex fault,
the smaller numerous openings of a zone of fissility, or the more open places
of sandstones or conglomerates. In short, a trunk channel of ground water
differs from other channels only in that because the openings are larger
than the average, they are places where there is more circulation.
Thus far the discussion has not taken into account the geological
work of the circulation itself. But the character of the circulation is being
continually affected by the material deposited.
Later in this chapter it will be explained that under quiescent condi-
tions the process of deposition of mineral material in the openings of the
belt of cementation continues until the openings are practically closed. In
proportion as this process is advanced the openings become smaller and the
circulation slower. In many great, almost perfectly cemented formations,
as the process neared completion, the movement of the ground water must
have become exceedingly slow, and finally practically ceased. After a
formation has been once cemented, orogenic movements may again produce
fractures within it, and thus form new openings through which an under-
ground circulation may again be set up.
CIRCULATION LARGELY SHALLOW. 589
GENERAL STATEMENT.
In conclusion of this part of the subject, while the lessened viscosity
with depth, slow movement, and long journey are all very favorable to a
deep circulation, I have little doubt that in porous strata large openings
near the surface and the more direct course of a shallow journey result, on
the average, in much larger flowage in the upper part of the belt of cemen-
tation than in the lower part.
TEMPERATURE OF ENTERING AND ISSUING WATER, AND TRANSFER OF HEAT.
Before taking up the work of ground water in the belt of cementation
it is necessary to consider the relative temperatures of the water which
enters and departs from the sea of underground water. Is the water which
joins the belt of cementation colder or warmer than that which issues
from it?
At a certain definite distance below the surface, in most regions from
12 to 15 meters, seasonal variations in temperature are eliminated; the
thermometer discloses no difference between the temperature of winter
and that of summer. The thermal conductivity of the rocks is so low and
the amount of water which passes to the belt of cementation is relatively
so small that the great variations of temperature of the atmosphere, of
the rocks at the surface, and of the meteoric water, emphasized on pages
433-444, 458-460, at the depth of a few meters all merge into a temper-
ature uniform for a given locality, bui of course variable in different
localities, and especially with latitude.
When the rain falls upon the soil it may be warmer or colder than the
surface of the earth, but the part that sinks into the soil is subdivided
between the pores of the rocks and comes into most intimate contact
with them, so that when the water at the surface is warmer than the rocks
it is cooled; when colder than the rocks it is warmed. Thus the two
quickly assume nearly the same temperature. Doubtless downward-moying
water is one of the most important of the agents which determine the fixed
average annual temperature of the rocks a short distance below the surface.
The reason for this belief is the high specific heat of water as compared
with rocks. However, the relative influence of the heat given by water
and that due to other climatic factors, such as latitude, altitude, ete., need
590 A TREATISE ON METAMORPHISM.
not here be discussed. For the present purpose the important point is that
for any locality at a very moderate distance below the surface the water
and the rocks have a common and practically invariable temperature.
It is highly probable that the point at which there is no appreciable
annual change of temperature is not deeper, on the average, than the level
of ground water. If this be so we may consider the ground water as
entering the belt of saturation at a fixed temperature. Assuming this, in
order to answer the question whether the water is colder or warmer when
it issues from the belt of cementation than when it entered the belt, it is
necessary to consider in what manner the water may gain or lose heat
during the interval.
For the following reasons it appears highly probable that, on the
average, the water gains heat during its underground journey:
(1) Earth movements are general for the lithosphere, and in many
regions these are of the most intense character. So far as earth movements
take place, to a very large extent the energy of the mechanical action finally
passes into heat. Where the movements are forceful the amount of heat
thus produced is considerable. The circulating underground water in
contact with the rocks necessarily absorbs some of this heat.
(2) It has been shown that the dominant chemical reactions that take
place in the belt of cementation are those which liberate heat. Of all the
reactions characteristic of the belt of cementation it has been pointed out
that hydration is the most important, and this process liberates a large
amount of heat. The chemical reactions therefore furnish heat which the
percolating waters must certainly absorb, in part at least.
In the lower zone, that of anamorphism (see p. 167), it has been
pointed out that the dominant chemical reactions take place with absorption
of heat. But where these reactions take place it is only through the
expenditure of mechanical energy which liberates heat to a greater extent
than that absorbed by the chemical reactions. That is, as a result of
mechanical action and chemical action together, as fully developed on
pages 110-113, there is a residuum of heat liberated to be absorbed by
the water, and therefore if the reactions of the zone of anamorphism
affect the temperature of the water of the belt of cementation at all it
must be by addition of heat.
(3) Very large quantities of heat are brought into the belt of cementa-
tion by intrusive igneous rocks. The intruded rocks are partly cooled by
ISSUING WATER TRANSPORTS HEAT. 591
tue transmission of their heat to the surrounding rocks and by conduction
to the surface, but the underground circulating water in contact with the
igneous rocks and in contact with the other rocks heated by them absorbs
a large amount of the heat, probably a much larger amount than that
transmitted to the surface through conductivity.
(4) After the water joins the sea of ground water its vertical com-
ponent is at first downward. Because of the normal increment of increase
of temperature—1° C. in 30 meters—the descending water gradually
absorbs heat from the rocks. During the ascent the water tends to give
heat to the rocks, because it is passing from rocks of higher to those
of lower temperature. But the upward movement of the water is of
shorter duration, is in larger openings, and is more rapid than its downward
movement (see pp. 582-584); therefore it seems probable that in conse-
quence of. normal increase of temperature due to depth, more heat is trans-
mitted to the water than is abstracted from it.
When we combine all four of these factors it follows to a certainty
that the water in its journey absorbs heat, on the average, and therefore
issues from the belt of cementation at a higher temperature than it enters it.
Hence it appears perfectly clear to me, from general reasoning, that during
the circulation of ground water it abstracts heat from the rocks.
So far as I know, no attempt has been made to prove this general
statement by observation; but it appears to be highly probable that it could
be proved by careful experiments. At the crests of elevations in various
regions temperatures should be taken at the level of ground water. In the
same regions the average temperature of the water issuing from springs
should be observed. The two sets of data should be compared. While no
general observations have been made along these lines, in some regions of
recent orogenic movements and volcanism observation does clearly show
that the temperature of the issuing water is higher than that of the entering
water. This is well illustrated by the Cordilleran region of western United
States. This is a region of comparatively recent epeirogenic and orogenic
movements and of volcanism. Gilbert and others” have shown that,
scattered throughout this vast region, occupying nearly one-third of the
@Gilbert, G. K., Volcanic rocks and mountains, with localities of thermal springs in the United
States: U. S. Geog. Surv. W. 100th Mer., vol. 3, 1875, pp. 145-155. Howell, E. E.. Thermal springs:
ibid., pp. 256-257. Stevenson, J. J., Mineral springs: ibid., pp. 478-487.
592 A TREATISE ON METAMORPHISM.
United States, are many springs the temperatures of which vary from
30° to 100° C. For instance, in the Yellowstone Park are a great
number of hot springs and geysers, the temperatures of which approximate
100° C. That a vast amount of heat is brought to the surface by the
underground circulation of this district can not be doubted. The number
of springs in the Cordilleran region which are known as hot is very small
as compared with those which are simply warm. The warm springs,
according to Gilbert,“ may be considered as those haying a temperature
between 18.3° and 37.7° ©. (65° and 100° F.) or practically blood heat
According to Gilbert,’ the water of all the foregoing springs exceeds the
mean annual temperature of the air by 8.3° C. (15° F.).
Although we have no data by which to verify the statement, I have
no doubt whatever that the number of springs the temperature of which is
slightly but measurably above the mean annual temperature exceeds many
times the total number of all springs the temperatures of which are so notably
above the normal temperature of the region as to be called warm or hot.
Finally, it is highly probable that the amount of water which escapes
through notable springs is small as compared with that which reaches the
drainage of the valleys through small openings and seepage. The moye-
ment of this water is relatively slow, and it may be presumed that it is at
but a shghtly higher temperature at the point of issue than the average
temperature of the rocks of the surface. But it is to be remembered that
a slightly higher temperature of issuing water over that of the entering
water through the vast number of springs of very moderate temperature,
and through seepage, is probably of far greater quantitative importance
than the marked increase of temperature in the comparatively few warm
and hot springs. This illustrates the old principle that widespread small
forces and agents may be of more importance, sometimes incomparably
more, than the more conspicuous but more circumscribed forces and agents.
In other regions of recent orogenic movements and voleanism essen-
tially the same facts prevail as in the Cordilleran region of America; and
therefore observation, so far as it has gone, confirms the general reasoning
given on pages 590-591, that water issues from the belt of cementation at
a higher temperature than it enters it.
Aside from the matter of metamorphism, this conclusion has a bearing
upon the refrigeration of the earth. The heat of the earth produced by
“Gilbert, cit., p. 148 > Gilbert, cit., p. 149.
COMPLICATION OF HEAT PROBLEM OF THE EARTH. Sola)
mechanical action, produced by chemical action, transferred to within the
zone of fracture by volcanism, and derived from the interior by conduction,
so far as it escapes is largely transferred by means of water. If this con-
clusion be true the question immediately arises as to the effect upon the
temperature gradient.
Barus states that, because of the chemical action of water in the pores
of rocks, heat is liberated; and that we may consider the zone of circula-
tion as afurnace. From this he concludes that the temperature gradient
observed is probably not high enough, on the average, or the ‘ observed
rate of increase of temperature with depth is too large.”* The reasoning
applied to heat developed by chemical reactions would apply to that pro-
duced by mechanical action in the zone of fracture and to that introduced
by volcanism. But in Barus’s statement the fact that the water is in cireu-
lation is altogether neglected. This makes the case much more complicated
than Barus supposed. The water is m circulation, and, as already shown,
is constantly abstracting heat from the earth. Therefore it appears that,
aside from volcanism, from the zone of fracture heat is being transferred to
the surface in two ways—by conduction and by the conyectional circula-
tion of water. I suspect that the heat convectionally transferred by
circulating waters is as great as or greater than that lost by conduction.
The amount thus transferred by convection may more than compensate for
that developed within the zone of fracture by mechanical and chemical
action and by volcanism.
The above suggestions are offered to show that the problem of the
heat gradient is much more intricate than has been supposed, and that tlie
heat problem of the earth can not be treated as a simple one in which con-
duction is the only factor to be handled. The problem is so complicated
by chemical action, by mechanical action, by igneous intrusions and
extrusions, and by the water circulation in the zone of observation, that
any conclusion as to the loss of heat of the earth and the depth to which
cooling extends which ignores these disturbing factors is of very little value,
to say the least.
«Barus, Carl, The compressibility of liquids: Bull. U. 8. Geol. Survey No. 92, 1892, pp. 83-84.
MON XLVII—04——38
594 A TREATISE ON METAMORPHISM.
VARIABLE MATERIALS AND CONDITIONS OF BELT OF CEMENTATION.
Within the belt of cementation the materials and conditions are variable,
but not nearly so variable as in the belt of weathermg. The materials
within the belt of cementation include all the classes which exist in the
belt of weathering (see pp. 429-430); that is, igneous rocks, sedimentary
rocks, surficial rocks, metamorphosed rocks, and their variations and
gradations; but ordinarily the alteration products are not nearly so com-
plex in the belt of cementation as in the belt of weathering, and therefore
the alteration forms of the belt are much less varied than those of the belt
of weathering.
Water solutions are the main active agents at work in the belt of
cementation. Gaseous solutions and organic compounds, which are so
important within the belt of weathering, play a comparatively small part.
The water solutions may contain any of the bases and acids which occur in
the belt of weathering. (See pp. 457-458.) They may also contain gases
in solution, of which oxygen and carbon dioxide are the more important,
but others are of considerable consequence. In the belt of cementation the
temperature varies with depth and also varies greatly in consequence of
mechanical action and igneous intrusions. A great range of temperature is
much more common than in the belt of weathering, but the speed of change
at any place is slow, so that for a long time at any one place the tempera-
ture is relatively constant. The work of the belt of cementation is
accomplished by mechanical and chemical agents, precisely as in the belt
of weathering, but the character of the work performed is very different
from that of the belt of weathering.
WORK IN BELT OF CEMENTATION.
The rocks in the belt of cementation are modified by mechanical work,
by chemical work, and by igneous work.
MECHANICAL WORK.
The mechanical work of the belt of cementation occurs in connec-
tion with deformation. The mechanical work of wind, water, and ice, of
changes in temperature, of plants and animals, so important in the belt of
weathering, plays no part. The chief force producing deformation is gravity
MECHANICAL CONSOLIDATION. 595
acting directly or indirectly through orogenic movements and intrusives.
The direct pressure due to gravity increases with depth. It is therefore
increasing at areas where loading is taking place by sedimentation or igneous
accumulations, and is decreasing in areas where denudation is going on.
Heat is also a force producing mechanical effects. As a result of denudation,
by which process the rocks approach nearer the surface; by sedimentation,
by which process rocks pass deeper below the surface; by mechanical and
chemical action, and by the intrusion of igneous rocks, the temperature is
changed. Increase of temperature expands the material affected, and
therefore induces compressive strains and tends to decrease the openings
within the rocks. Decrease of temperature produces the opposite effect.
The mechanical work of deformation includes the processes of consoli-
dation, strain within the elastic limit, and strain beyond the elastic limit.
Each of these processes is considered in turn.
CONSOLIDATION.
Mechanical consolidation is a process by which the separated parts of
a rock are brought closer together through pressure. The pressure may
be due to the weight of the superincumbent rocks, to the lateral thrust of
orogenic movements, to the hydrostatic pressure of igneous rocks, and
to mineral growth during cementation and metasomatism. The very
idea of the zone of fracture, to which the belt of cementation belongs,
implies that the rocks and mineral particles are sufficiently strong to be
self-sustaining.
In proportion as the rocks show openings, and especially in proportion
as they are incoherent, mechanical consolidation is likely to produce an
important effect. The incoherent rocks comprise the mechanical sediments
and tufts in their original condition. The coherent rocks comprise both of
the above classes which have been more or less cemented and the massive
rocks which may have openings due to various causes.
For the present purposes the mechanical sediments are of two general
classes, the silts and muds, and the coarse sands and grits. The first of
these classes is the one which is most affected by mechanical consolidation.
In many eases the mineral particles of the fine-grained silts and muds appear
to be separated by water. In such a rock pressure gradually squeezes
out this water, bringing the mineral particles into contact. At the same
596 A TREATISE ON METAMORPHISM.
time the mineral particles are mechanically readjusted in reference to one
another. In consequence of the two the volume of the material may be
considerably decreased. The process of consolidation may continue until
finally the particles are brought so close together that they cohere. As
explained by Becker,“ when the films of water between the particles
become very thin, they may become an important factor in the coherence
of the rocks. The molecular attraction of the water films and the adjacent
particles, or their adhesion, and the cohesion of the molecules of the films
may be sufficient to give the rocks a certain amount of strength. As the
process of squeezing out the water goes on, many of the particles are
brought so close together as to be within the sphere of molecular attraction
independently of the separating films of water. So far as this is true the
particles are welded by the pressure. The welding and the adherent
influence of the water films may give the muds and silts marked coherence.
During the process of consolidation, as the water escapes the pressure
to which it is subject gradually becomes less. Therefore if saturated or
nearly so, during its escape it may deposit a portion of the material in
solution, and hence cementation dependent upon mechanical consolidation
may promote induration.
In proportion as the particles of the mechanical sediments become
large and clean the process of mechanical consolidation becomes less
important, for the points of contact rapidly decrease in number as the
sediments become coarse, and where they are very coarse such contacts are
so few that the rocks gain little strength as a result of close contact and the
adherent effect of the films of water. Moreover, as shown by Professor
Slichter,’? coarse mechanical sediments deposited by water are usually
arranged in a somewhat compact manner. ‘This natural arrangement,
while not the most compact possible, is so compact that experiments
show that subsequent mechanical disturbances sufficient to displace the
grains in reference to one another ordinarily result in decreasing the com-
pactness of the packing, and therefore in increasing the volume of the rocks.
The tuffs are somewhat intermediate in character between the coarse
and the fine mechanical sediments. They ordinarily include both classes
« Becker, Geo. F., The torsional theory of joints: Trans. Am. Inst. Min. Eng., vol. 24, 1895, p. 131.
bSlichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept.
U. S. Geol. Survey, pt. 2, 1899, p. 305.
GENERAL STATE OF STRAIN IN ROCKS. 597
of material in the same rocks. If originally arranged under air, they have
a very much less compact arrangement than they or the ordinary sediments
do if arranged under water. Thus many tuffs are in a condition in which
mechanical adjustment of the particles by pressure may give a considerably
more compact arrangement. Also, since between the coarse particles there
are many fine particles, the number of points of contact between the
particles may be sufficient to give considerable coherence by welding, and
the strength is further increased by the adherent power of the water films,
precisely as in the case of the muds and silts. Therefore it is probable that
to a considerable extent tuffs may be consolidated by pressure.
In general in the zone of fracture the coherent rocks are sufficiently
strong to support themselves, and mechanical consolidation is relatively
unimportant. However, the openings produced by rather rapid movements
may be subsequently gradually closed by the steady pressure of gravity or
thrust. In the massive rocks containing no openings mechanical consolida-
tion has no appreciable effect.
STRAIN WITHIN ELASTIC LIMIT.
All substances subjected to stress are more or less strained, and the
amount of the strain is proportional, although not directly proportional to
the stress. The rocks throughout the belt of cementation are all subjected
to stress, and therefore to strain; for none are free from the weight of the
superincumbent material, and in general they are subjected to compressive
or tensile stresses parallel to the surface of the earth. It has been seen
that in the belt of cementation the direct stress of gravity, and its indirect
effect through orogenic movements, is sufficient to mechanically consolidate
the rocks and to fracture them in the most extensive manner. It is clear
that in a given case of rupture, before fracture took place the rocks must
have been strained to the elastic limit. Therefore the amount of the strain
within the belt of cementation varies from an insignificant quantity to
strain to the elastic limit of the rocks under the conditions in which they
exist. It would be generally agreed that the intensity of the strain is very
important at moderate depths. But it is commonly supposed that rocks
near the surface, and especially in little disturbed regions, are not under
considerable strain; but that this conclusion is erroneous is shown by
observations which have been made by a number of men. Niles has
598 A TREATISE ON METAMORPHISM.
shown” a high state of strain in the granite quarry at Monson, Mass.
Before the quarry was opened it would not have been supposed that the
rocks were subject to any unusual amount of stress, but when the quarry
was opened the rocks were found to be in such a state of strain that fre-
quent ruptures spontaneously took place, or occurred with very slight
assistance by the quarrying operations. . Before quarrying began the rocks
were prevented from lateral expansion, but after an excavation was made
they were freed from pressure on one side, and this loss of restraint led to
the rupturing already mentioned. That the Monson granite is elastically
compressed is shown by the fact that when a great mass is cut off from the
sides of the quarry it expands so that the bisected drill holes do not match.
Also elastic compression is shown by the expansion at various times during
the quarrying, anticlinal arches thus being formed by the elevation of lay-
ers along horizontal planes of rupture. Johnston had previously described”
similar spontaneous movements in the sandstone strata at Portland, Conn.
Niles has also given’ evidence of very important strain in the sandstone at
Berea, Ohio, in the limestone of Lemont, Ill, and in quarries in Connecticut.
At various places in the Galena limestone of the Fox River Valley,
Wisconsin, after excavation has been made, the layers have been found to
be under such elastic strain that they have released themselves by expan-
sion, forming anticlinal arches, in places 40 cm. in height.” . Gilbert has
described’ post-Glacial anticlinal arches at various places in the Devonian
shale of New York and northwestern Ohio and in the Trenton limestone of
New York. It thus appears that the rocks in the upper part of the belt of
cementation may be in astate of strain near or even to the elastic limit at
many places throughout extensive regions. The condition of mechanical
strain can be discovered only by quarrying processes, and by observations
such as are given above. Whena specimen is taken from a quarry in which
a@Niles, W. H., Peculiar phenomena observed in quarrying: Proc. Boston Soc. Nat. Hist., vol. 14,
1871, pp. 1-8.
> Johnston, John, Notice of some spontaneous movements occasionally observed in the sandstone
strata in one of the quarries at Portland, Conn.: Proc. Am. Assoc. Ady. Sci., 8th meeting, 1854, pp.
283-286.
¢ Niles, W. H., The geological agency of lateral pressure exhibited by certain movements of rocks:
Proc. Boston Soc. Nat. Hist., vol. 18, 1876, pp. 272-284.
dCramer, Frank, On a recent rock flexure: Am. Jour. Sci., 3d ser., vol. 39, 1890, pp. 220-225.
Also, Cramer, Frank, On the rock fracture at the combined locks mill, Appleton, Wis.: ibid., 3d ser.,
yol. 41, 1891, pp. 432-434.
e Gilbert, G. K., Post-Glacial anticlinal ridges near Ripley and Caledonia, New York: Am. Geol.,
vol. 8, 1891, pp. 2380-231.
GENERAL STATE OF STRAIN IN ROCKS. 599
the rocks are much strained, by that process it is largely relieved from
strain, and a slide of it placed under the microscope may show little or no
evidence of its former state of strain. i
It therefore appears probable that a high state of strain is the common
condition for the great mass of the rocks constituting the belt of cementa-
tion. It has been explained on pages 95-98 that in proportion as min-
eral particles are in a state of strain they are likely to be acted upon
chemically by the ground solutions. Therefore the very general and
marked state of strain in the rocks of the belt of cementation is probably a
very important factor in their alteration. This is especially true of meta-
somatism. By that process, as explained on pages 690-692, the rocks
may be released from strain, the mechanical energy producing the strain
being utilized in promoting recrystallization. It is difficult or impossible
to prove the importance of state of strain in metasomatism except by
general reasoning, based upon the experimental facts referred to in Chapters
IT and VIII. But it is my confident belief that the general state of strain
is one of the most important of the inciting causes of alteration.
STRAIN BEYOND ELASTIC LIMIT. =
Strain beyond the elastic limit results in fracture. The belt of cemen-
tation belongs to the zone of fracture. It has been seen in the previous
sections that deformation may be accomplished to a considerable extent by
differential movement of the particles of incoherent rocks and by strain
within the elastic limit. Where the rocks are coherent marked deforma-
tion in the belt of cementation is largely accomplished by fractures and
by movements along the fracture planes. In the very partially indurated
rocks fracturimg may consist in the rupturing of the particles from one
another, and adjustment come by differential movements between the par-
ticles. But in the coherent rocks the ruptures are wider spaced than the
individual grains. In the weaker coherent rocks the surfaces of rupture
are more or less irregular, ordinarily breaking around rather than cutting
through the grains. ‘his form of rupture is well illustrated by sandstones.
The strongly coherent rocks usually show clean-cut fractures which cut
the mineral particles.
When rocks in the belt of cementation are strained beyond the elastic
limit there is a marked tendency for the ruptures to follow planes more or
600 A TREATISE ON METAMORPHISM.
less closely, and for the planes to be in parallel sets. In a given case there
may be one or more of these parallel sets; commonly there are as many as
three. In such instances the rocks are broken into parallelopiped blocks.
In some instances there are as many as six or more parallel sets of ruptures,
and by them the rocks are cut into various irregular forms.
The spacing of the different sets of ruptures is very variable. For
instance, if there be three sets of ruptures, the distances between the frac-
tures in one set may be several or many times as great as between those of
another set. The spacing of all the sets in one case may be much wider
than that of any set in another case. The fractures may be very far apart,
as with widely separated faults or jomts, or they may be very close
together. In some instances the different sets of ruptures are so close
together that it is scarcely possible to find an unruptured parallelopiped a
centimeter in diameter. Ruptures of this kind are very well illustrated in
the slates and graywackes of the Wausau district of Wisconsin. However,
in the belt of cementation the ruptures are not so close as to cut all, or even
any considerable proportion, of the mineral particles. Between the amount
of rupturing necessary to produce particles a fraction of a centimeter across
and that necessary to affect every mineral particle there is a very wide gap.
In general, near the surface the ruptures are likely to be widely spaced,
with considerable differential movement along the fractures. As depth
increases, so that the rocks are subject to the weight of the superincumbent
material, the ruptures are likely to be closer together. When the depth
become so great that the fractures affect the individual mineral particles,
the zone of anamorphism, or rock flowage, has been reached.
The fractures produced in rocks must take place as a result of tensile or
compressive stresses.” Where the rocks are homogeneous and the fractures
are by tensile stresses, they take place normal to the force. Where the
fractures are by compressive stresses—and this is probably much more com-
mon—the ruptures are inclined to the direction of maximum compressive
stress. Ignoring the effect of the stress, and supposing the rocks to be
homogeneous, the ruptures would occur in sets at angles of 45° to the
maximum stress. However, since the stress itself gives additional strength
“Becker, G. F., Finite homogeneous strain, flow, and rupture of rocks: Bull. Geol. Soc. America,
vol. 4, 1893, pp. 48-49. Van Hise, C. R., Principles of North American pre-Cambrian geology: Six-
teenth Ann. Rept. U. 8. Geol. Survey, pt. 1, 1896, pp. 668-678.
COMPLEXITY OF RUPTURES IN ROCKS. 601
in the direction of compression, the inclinations of rupture are inclined at
angles less than 45° to the direction of greatest compressive stress.
The actual direction of fracture depends not only upon the direction
and nature of the stresses, but upon the character of the rocks. The
directions of rupture are greatly influenced by the variable strength of the
rocks, by the rock structures and textures, and by the difference in size and
strength of the constituent particles. Because of these varying factors
force is transmitted with unequal efficiency in different directions. Conse-
quently the direction of the maximum stress is not the same throughout a
rock mass, but varies from place to place. This variability both in the
nature of the rock and in the direction of the stresses gives very great
irregularity of rupturing. Because of these variable factors no comprehen-
sive statement can now be made as to the directions of ruptures in rocks;
but certain special conclusions are evident. Where the rocks are hetero-
geneous the ruptures are likely to take place to a greater extent in the
weaker rocks. Where there are contacts of strong and weak rocks ruptures
are likely to take place at the contact planés, for these are planes of weak-
ness. Where there are regular structures, such as bedding, gneissosity,
schistosity, cleavage, the ruptures are likely to follow these planes of weak-
ness. Wherein a rock the particles have variable strength, weaker particles
suffer more than the stronger ones. In the rupture of mineral particles
fractures are likely to follow cleavage planes, since these are planes of
weakness. All these factors result in giving ruptures which vary greatly
from positions at right angles to tensile stresses and from 45° to the
maximum compressive stresses.
It follows from the foregomg that the determination of the precise
direction and nature of the forces which produced rupture in any individual
rock is a most difficult matter.
After ruptures are produced differential movements may result in the
rubbing of the parts adjacent to the fractures against one another. At such
places the individual mineral particles may be granulated. However, this
fracturing of the mineral particles is largely confined to thin layers along
the walls of the openings. This fracturing of the particles is in many
respects like the fracturing of mineral particles in the zone of anamorphism.
(See pp. 673-675.) The matter is not further considered here.
From the foregoing it follows that deformation accomplished by frac-
ture in the belt of cementation results in breaking up the rocks into great
602 A TREATISE ON METAMORPHISM.
masses by faulting, into blocks by jointing, into layers by fractures along
bedding planes and other planes of weakness, into narrow slices by fissility,
and into irregular fragments by brecciation. It has furthermore been
seen that deformation produces comparatively unimportant changes in the
size of the masses between the fractures. In these masses the original
structures and textures are unaffected. But it will be explained under
cementation, metasomatism, and injection, that the ruptures furnish condi-
tions very favorable for these processes. Everywhere the ruptures give
ready passages for the circulating ground water; everywhere they give
ready entrance to intrusives. In consequence, the openings are closed
by cementation and injection. Moreover, from the openings the water,
frequently heated and loaded with material, is able to make its way for
short distances in the subcapillary openings between the grains, and thus
metasomatic processes are greatly promoted. Where the spacing of the
fractures is small metasomatism may modify the unbroken masses through-
out. Where the spacing of the ruptures is somewhat wide the metasomatic
alterations may not extend all the way from one passage to another, and
thus leave the interiors of the masses unaffected. Therefore there is very
great variation in the amount of metasomatic alteration in passing from the
walls of the openings to the interiors of the large unbroken masses. This
hhas been fully worked out in connection with ore bodies, which commonly
form along widely spaced, simple or complex, large openings. Lindgren
especially has shown” how metasomatic processes greatly affect the walls
of the fissures. The effect gradually dies out in passing away from the
openings, but may be marked for some distance.
CHEMICAL WORK.
The chemical work in the belt of cementation needs consideration
from two points of view—the chemical changes and the resulting processes.
CHEMICAL CHANGES.
The chemical work within the belt of cementation is accomplished
mainly by water solutions. Water is here the great agent of metamor-
phism, but the igneous rocks are second only to water solutions. They
«Lindgren, Waldemar, Metasomatic processes in fissure veins: Trans. Am. Inst. Min. Eng., vol. 30,
1901, pp. 578-692.
CHEMICAL CHANGES IN BELT OF CEMENTATION. 603
are of vastly greater importance than in the belt of weathering. At the
surface or within the belt of weathering, igneous rocks rapidly lose their
heat, and thus produce a comparatively short-lived effect. Within the belt
of cementation the igneous rocks retain their heat for a very long time.
By their introduction they produce both a direct effect, due to their own
action, and—far more important—an indireet effect by heating the
solutions, and thus very greatly increase their efficiency both as to speed
of reaction and as to quantity of material which can be held, as is fully
explained elsewhere. (See pp. 79-81.)
The range of temperature at which the solutions work is from 0° C. to
the critical temperature of water, which, as we have seen (pp. 566-569), is
365° C. The amount of work which the solutions accomplish in a given
time at very high temperatures is almost indefinitely greater than that
which is accomplished at lower temperatures. (See p. 79.)
The pressure at which the solutions work varies from that of an
atmosphere to the pressure of a column of water to the bottom of the
zone of fracture. Supposing the bottom of this zone of fracture to be at a
depth of 10,000 meters, the maximum pressure would be 1,000 kilograms
per square centimeter. Therefore the range of pressure upon the solutions
within the belt of cementation is very great.
By observation we know that in the belt of cementation the important
chemical reactions which take place as a result of the action of the various
agents are oxidation, carbonation, hydration, and solution and deposition.
These reactions are the same as those in the belt of weathermg. Just as in
the belt of weathering, the reactions of oxidation, carbonation, hydration,
and possibly solution preponderate over the reverse reactions. However,
the relative importance of these various reactions in the two belts is very
different.
All of these reactions, as has been seen, are of very great consequence
in the belt of weathering. Oxidation and carbonation are of much less
consequence in the belt of cementation; but this statement can not be made
of hydration, solution, and deposition. Hydration, because of the decreased
prominence of oxidation and carbonation, becomes the chief one of these
three processes in the belt of cementation. Solution and deposition are both
of fundamental importance; but the first, solution, does not dominate as
it does in the belt of weathering. All of these reactions have been fully
604 A TREATISE-ON METAMORPHISM.
considered in Chapter VI on “The belt of weathering,” and they will here
be taken up only so far as their work is different in the belt of cementation
from that in the belt of weathering. The modifications are easily made for
oxidation, carbonation, and hydration and their reversals, but are complicated
for solution and deposition.
In the upper part of the belt of cementation, that near the level of
ground water, the active agents in the solutions are very similar to those
in the belt of weathering, and therefore the reactions are closely allied to
those of the belt of weathering. But as depth below the belt of weathering
increases, the distinctions between the reactions which are of controlling
importance in the two belts becomes more and more clear, and for the
major part of the belt of cementation the relative importance of reactions is
very different from that in the belt of weathering. The depth to which all
the reactions of the belt of weathering extend into the belt of cementation
depends very largely upon whether the waters for the particular area are
ascending or descending. At places where large amounts of descending
waters are converged, and where the variations of topography are marked,
so as to give a very high head, all the reactions of the belt of weathering
may be quantitatively important to depths of 800 or 1,000 meters or more,
although such depths as these are very unusual. But where the descend-
ing currents are driven by a low head, some of the reactions of the belt
of weathering may extend only to a very moderate depth into the belt
of cementation. In areas where the water currents are ascending, and so
continue to the belt of weathering, some of the reactions of the latter belt
may extend scarcely at all into the belt of cementation. Oxidation, car-
bonation, hydration, and solution and deposition commonly do not take
place separately, but occur at the same time upon the same materials;
although, as pointed out on page 611, in the upper part of the belt of
cementation, carbonation and oxidation are to some extent exclusive of
each other. But for the purposes of analysis it is necessary to treat each
separately.
OXIDATION.
Ordinarily when water passes from the belt of weathering into the belt
of cementation it contains oxygen in solution. The amount of this oxygen
may be as great as can be dissolved under atmospheric conditions of pres-
sure and temperature, or the water may be saturated. From this the
OXYGEN SOON EXHAUSTED. 605
amount varies to almost nothing. Also, in consequence of atmospheric
pressure, oxygen may be forced into the sea of ground water to some
extent. The favorable conditions for abundant oxygen in the water when
it joins the belt of cementation are a very porous belt of weathering and
absence of luxuriant vegetation. Under such conditions the atmosphere
circulates somewhat freely through the’ belt of weathering, and as oxygen
is exhausted in the belt by the processes of oxidation it is resupplied to the
solutions by the atmosphere.
It is seen (pp. 609-610) that the sources of carbon dioxide are abun-
dant, and therefore that the process of carbonation is almost universal in
the belt of cementation. The situation is very different with oxygen.
Practically the only oxygen supplied is that which is carried down by
solutions from the belt of weathering and that which makes its way into the
water of the belt of cementation at the level of ground water in conse-
quence of atmospheric pressure. It naturally follows that, as the waters
move downward and the process of oxidation continues, the amount of
oxygen available becomes less and less. Commonly the oxygen is prac-
tically exhausted at very moderate depths. Therefore, while oxidation
takes place within the belt of cementation wherever oxygen is available,
for much of the belt there is no oxygen available for this process and hence
oxidation does not occur. This general reasoning is plainly confirmed by
observation. The waters of the upper part of the belt of cementation are
commonly oxidizing, while the deeper waters of the belt rarely contain
oxygen. According with this general observation, Lepsius has made exact
experimental tests of the amount of oxygen in waters derived from bore
holes, and he finds that with increase of depth there is a gradual and some-
what uniform decrease of oxygen contained in the ground waters.” Bischof
many years ago verified the depletion of oxygen as the result of oxidation
by determinations of the relative amounts of oxygen and nitrogen in springs.
He found in cold springs that the oxygen is not so great in proportion to
the nitrogen present as in the atmosphere, and in warm springs this defi-
ciency is even more marked, and he correctly explains this deficiency as
due to the “partial combination of this gas with oxidizable substances.”’
@ Berichte Deutsch. chem. Gesell., vol. 18, 1885, p. 2487.
» Bischof, Gustav, Elements of chemical and physical geology, translated by Paul and Drum-
mond, Harrison & Sons, London, 1854, vol. 1, p. 234.
606 A TREATISE ON METAMORPHISM.
Under ordinary circumstances oxygen is practically exhausted in the
early part of the journey, so that it is rather unusual for marked oxidation
to extend more than a few meters below the level of ground water. But
where, because of marked relief, there are large and strong downward-
moving currents through open and porous rocks, the process of oxidation
may extend to a very considerable depth into the belt of cementation.
This has been especially noticed in mining regions, where ore deposits are
commonly located along joints, faults, or other trunk channels where there
are numerous and large openings. One of the best illustrations of oxidation
extending to a considerable depth is that furnished by the iron ores of the
Lake Superior region. Here, as fully explamed in the chapter on ore
deposits, the process of oxidation has extended on a very great scale to the
depth of 100 meters, in mines has produced great ore bodies to a depth
of 300 meters, and exceptionally has gone on to an important extent to a
depth of 500 to 700 meters. As an instance in which oxidation has
extended to a great depth may be mentioned the San Juan district of Colo-
rado. Here in some of the mines oxidation is marked to a depth of 600
meters, and is occasionally noticeable to a depth of 1,000 meters. In
contrast with such regions as these are others in which the water ascends
almost or quite to the surface. In such instances oxidation scarcely extends
below the level of ground water. An excellent illustration of this is fur-
nished by the Missouri-Kansas lead and zine district.
The oxygen is chiefly utilized:in the oxidation of iron, sulphur, and
organic material. In the upper part of the belt, where oxygen is abundant,
large amounts of hematite and limonite may be produced by the oxidation
of the ferrous particles, but for the greater part of the belt of cementation,
where oxygen is somewhat deficient, the ferrous oxide is oxidized only to
the form of magnetite, since this requires, per unit of iron, only two-thirds
as much oxygen as to produce hematite and limonite.
The sulphur is mainly united with the iron as pyrite and marecasite. At
the same time the sulphur is oxidized the iron also is oxidized. Where the
oxygen is abundant there may be produced ferrous sulphate, or ferric oxide
and sulphuric acid; but where oxygen is not abundant, as in the major
portion of the belt, magnetite and sulphurous acid are more likely to be
formed.
Where the oxygen is utilized in the oxidation of organic material
carbon dioxide and water are produced, and these compounds join the
OXIDATION AND DEOXIDATION. 607
circulating ground water. In many cases the major portion of the oxygen
is thus utilized. In so far as this takes place the process leaves no
direct evidence of itself by the existence of oxides. So far as suphuric acid
is formed the same is true. The carbonic acid and the sulphuric acid may
remain in part in the belt of cementation in carbonates and sulphates. So
far as this occurs the oxidized compounds remain in the belt, but not as
substances which we ordinarily think of as evidences of the process of
oxidation.
After the oxygen of the waters of the belt of cementation is exhausted
organic and other compounds, such as sulphides, are likely to be taken into
solution, which renders the waters reducing, as explained on page 165.
The waters under impervious strata which take a long underground journey
are especially likely to be in this condition, for such a journey furnishes
ample time for the complete consumption of the oxygen contained in the
solutions. The above is illustrated by the deep artesian waters of many
parts of the world. Excellent cases are those furnished by the artesian
waters of the Potsdam sandstone of Wisconsin and the Dakota sandstone
of the James River Valley, Dakota. The waters which issue from the
Potsdam formation along Lake Michigan are of a reducing .character,
containing, as noted by Chamberlin, some hydrogen sulphide.“
Where the waters become reducing deoxidation of the compounds with
which they come into contact may take place.
One form of deoxidation is of sufficient consequence to require men-
tion. While of no great quantitative importance, it is important from the
point of view of the metallic ores. It has been explained under ‘‘The belt
of weathering” (pp. 468-469) that the sulphides are continuously oxidized
to sulphites or sulphates. A part of the sulphurous and sulphuric acid
united with the original elements, or with other elements, joins the belt
of cementation. After the oxygen is exhausted and organic compounds are
present the sulphites and sulphates may be reduced to sulphides by the
direct action of the organic matter, or by these compounds after passing
into the solutions.
The process of reduction, or deoxidation, of the sulphates involves
the oxidation of the organic material, and thus this reaction, from one point
of view only, is reduction. From the other point of view it is oxidation.
«Chamberlin, T. C., The ore deposits of southwestern Wisconsin: Geol. of Wisconsin, vol. 4, 1882,
p. 547.
608 A TREATISE ON METAMORPHISM.
Thus the process of deoxidation or reduction, so far as it takes place,
involves the passage of the oxygen abstracted to the reducing agents or
their oxidation. Moreover, the sum total of the heat effect, taking into
account both the reduction and the oxidation, is to liberate heat. Hence
the reaction is one normal for the zone of katamorphism. The sulphides
formed may be precipitated and serve as one of the subordinate cementing
substances. The formation and precipitation of sulphides will be much
more fully considered in Chapter XII, on “Ore deposits.”
We have just seen that deoxidation, so far as it takes place, is explained
by oxidation of some other compound. But oxidation by the oxygen origi-
nally derived from the surface and present in the solution does not involve
the deoxidation of some other compound. It therefore follows that the
process of oxidation in the belt of cementation overbalances that of reduc-
tion in the belt by the amount of oxygen which is carried into the belt of
cementation by the entering waters, and is abstracted from it by the union
of the oxygen with the compounds encountered during its underground
journey.
It has been explained on page 467 that the process of oxidation of
inorganic compounds involves increase of volume varying under ordinary
circumstances from a small amount to as much as 64 per cent. But by the
oxidation of the carbonates carbon dioxide is liberated, and this may give
a considerable decrease in the volume of the product, amounting in some
instances to as much as 50 per cent.
In summary, it is clear that oxidation is a reaction characteristic of
the belt of cementation, but that in this belt it has no such importance
as in the belt of weathermg. In the belt of cementation oxidation goes
on but to a very limited extent. While in the upper part of the belt great
masses of highly oxidized rocks are locally found, as in the case of iron
ores, for the major part of the belt the evidences of oxidation are not
apparent, it beg evidenced perhaps to the greatest extent by magnetite.
CARBONATION.
The water which passes from the belt of weathering to the belt of
cementation carries dissolved carbon dioxide precisely as it does oxygen.
The quantity thus carried varies from an exceedingly small amount to
sufficient to produce saturation.
SOURCE OF CARBON DIOXIDE FOR CARBONATION. 609
It has been fully explained on pages 473-475 that carbon dioxide is
produced in large quantities in regions of abundant vegetation, where
the process of oxidation of carbon goes on on a great scale in the belt of
weathering. Therefore, the conditions for abundant carbonic acid in the
waters joining the belt of cementation are those of luxuriant vegetation.
It has just been explained that the conditions favorable for the transporta-
tion of abundant oxygen to the belt of cementation are those of open soil
and lack of vegetation. Hence, the waters which carry carbonic acid
plentifully to the belt of cementation are not likely to carry oxygen plen-
tifully. Where one is abundant the other is likely to be deficient It has
just been explained that the process of oxidation may take place upon
carbonates. Also, by oxidation of organic matter which has been so deeply
buried as to be within the belt of cementation, carbon dioxide is formed.
In so far as these occur carbon dioxide is produced by the reactions within
the belt of cementation itself, and thus is added to the ground solutions.
It thus appears that the occurrence of one of the chemical processes
within the belt of cementation, that of oxidation, by its very action
produces another active agent, carbon dioxide, and therefore promotes
the second of the important reactions of the belt of cementation, that of
carbonation.
The third source of carbon dioxide for the belt of cementation, as is
fully explained in the following chapter (pp. 677-679), is the process of
silication, with simultaneous decarbonation as a prominent and often a
dominant reaction. Some of the liberated carbon dioxide is permanently
retained in the zone of anamorphism as occluded gas and liquid. (See
p. 678.) But the temperatures of the zone of anamorphism are above the
critical temperature of carbon dioxide. This compound is therefore origi-
nally a gas, and is capable of making its way through very small openings.
Doubtless a considerable amount of it escapes from the zone of anamor-
phism and enters the belt of cementation. Thus is largely explained the
abundant carbonic acid which is frequently found in the waters of deep
mines rising from a still deeper source.
It therefore appears that the waters of the belt of cementation have
four important sources of carbon dioxide—from the belt of weathering,
from the decomposition of carbonates of the belt of cementation through
oxidation, from the oxidation of organic material within the belt of cemen-
MON XLYVII—04——39
610 A TREATISE ON METAMORPHISM.
tation, and from the silication of carbonates in the zone of anamorphism.
Therefore, the carbon dioxide for carbonation in the belt of cementation is
contributed by the belt of weathering above, by the zone of anamorphism
below, and by the belt of cementation itself.
Bischof fully realizea the very great abundance of carbonic acid in
‘‘a numerous class of springs which are abundantly distributed over the
surface of the earth, especially in districts where extinct volcanoes or
basaltic rocks occur.”* He also notes that ‘carbonated springs which
evolve abundance of this gas are likewise met with in the midst of sedi-
mentary formations.”?
The locations of a very large number of carbonated
springs scattered through extensive regions are mentioned.“ Bischof
further says that the carbonated springs ‘‘are always situated at the lower
part of mountain declivities or at the deepest points of valleys, gener-
ally near brooks;” while ‘fresh-water springs issue at points still higher
above the bottom of the valley, and sometimes at tolerably considerable
heights.”* He further fully realized that the springs bringing the carbonic
acid abundantly to the surface are deep-seated waters which have arisen
from considerable depths.’ Finally, Bischof attributes the abundance of
carbonic acid in the waters of deep springs and those which come from
deep borings to the silication of the carbonates. However, he recognizes
that the carbonic acid found in the waters of wells is also produced in part
by the oxidation of organic material’ Bischof therefore many years ago
correctly gave two main sources of carbon dioxide for the water of the
belt of cementation.
It has been fully explained on pages 160-161 that, under the conditions
of the belt of cementation, where the rocks support themselves and open-
ings exist, the reactions take place which liberate heat. Carbonation of
the silicates and oxides belongs to this class. (See pp. 475-480.) It there-
fore follows from general reasoning that the carbon dioxide abundantly
supplied to the waters of the belt of cementation will react upon various
compounds and produce carbonates. Of the compounds upon which it acts
«Bischof, Gustay, Elements of chemical and physical geology, trans. by Paul and Drummond,
Harrison & Sons, London, 1854, p. 217.
b Bischof, cit., vol. 1, p. 217.
¢ Bischof, cit., vol. 1, pp. 218-241.
@ Bischof, cit., vol. 1, p. 228.
é Bischof, cit., vol. 1, pp. 228-230.
J Bischof, cit., vol. 1, p. 239.
}
CARBONATION GENERAL IN BELT OF CEMENTATION. 611
the silicates are undoubtedly those of dominant importance. The action of
carbonic acid upon the silicates results in the production of carbonates of the
bases and the liberation of silica, which may remain in situ as quartz or
go into solution as colloidal silicic acid. Monoxides of iron and manganese
originally present in this form or produced by the oxidation of sulphides
may unite with the carbon dioxide and form carbonates.
Since the carbon dioxide of the belt of cementation is derived from
above, from below, and, by reactions of oxidation, from within the belt,
one would expect carbonation to take place throughout the belt of cemen-
tation. In the upper part of the belt of cementation the main source of
supply of carbon dioxide is that from the belt of weathering; but, as has
been noted, where the carbon dioxide is abundant oxygen is apt to be
deficient, so that where one finds active carbonation in the upper part of
the belt of cementation one would not expect to find active oxidation.
The majority of the carbonates formed, and those originally present in
the belt of cementation, are rather soluble. Commonly, therefore, the
waters issuing from the belt of cementation contain very large amounts of
carbonates and carbon dioxide. Average amounts of carbonates given by
Peale from many mineral springs from various portions of the United States
are as follows: %
Grains per gallon. Milligrams per liter.
INoxthwAtilanticiS tates Saas eeeeee masseter eee 32. 88 56. 27
SouthpaAtilantieys tates = ess syste ae eee ars 13.70 23.45
INOTthEC Cnitrallls tales meee eee ee as err 45. 35 77. 62
Sovuii on Orrin! Syresig es eco ecseeaseouscossecosese 18. 30 ail, BY
Wiestemnistateseep saree = eee Oe nae ce ee ere 139. 59 238. 90
Carbonie acid is a very much less active agent than oxygen. The
process of carbonation therefore takes place slowly, and the carbonic acid
does not all sueceed in getting united with the bases. In many instances
the amount which has failed to unite with the bases is large. This is
illustrated by the abundance of uncombined carbon dioxide m the waters
of many of the famous springs of the world, as those of Carlsbad and
Shasta, and also by the great abundance of uncombined carbon dioxide
noted in connection with the rising waters of many mines.
It has been explained under “The belt of weathermg” that the
increase in volume in the process of carbonation of the silicates, provided
@Peale, A. C., Lists and analyses of the mineral springs of the United States: Bull. U.S. Geol.
Survey No. 32, 1886, pp. 1-235.
612 A TREATISE ON METAMORPHISM.
the liberated silica remains in situ, varies in most cases from 15 to 50 per
cent. If, therefore, the carbonates produced and the silica liberated be
deposited, there is great increase in volume.
While in the belt of cementation carbonation unquestionably domi-
nates over.decarbonation, doubtless it frequently happens during earth
movements that the pressure is so great that silication and decarbonation
take place.
HYDRATION.
It has been stated that the belt of cementation might almost equally
well be called the belt of saturation. Water is everywhere present. The
process of hydration is one which involves a great increase of volume, but
openings exist which may be utilized by the process without overcoming
the strength of the rocks or gravity. Therefore the conditions are ideal for
hydration, and this everywhere occurs. Indeed, it is in the belt of cemen-
tation that the great group of hydrous silicates form most abundantly.
The belt of cementation is the home of the hydromicas, of the chlorites, of
the zeolites, of serpentine, of the epidotes, and of limonite and gibbsite.
Kaolin and tale also form there, although they are more especially char-
acteristic of the belt of weathering. The increase in volume in these
processes of hydration, provided all the compounds remain in situ, varies in
most cases from 20 to 50 per cent: While, as already explained, oxidation
and carbonation are important in the belt of cementation, the process of
hydration occurs on a vastly greater scale, and the chief change in volume
in this belt is due to this process.
Just as decarbonation may take place under conditions of exceptional
pressure in the belt of cementation, so dehydration may, and certainly does,
oceur, although this is so subordinate to the process of hydration as almost
to be negligible.
SOLUTION AND DEPOSITION.
Both solution and deposition are processes of great importance in the
belt of cementation. We have already seen that deposition occurs on so
ereat a scale as to make the filling of the cavities and consequent cementing
of the rocks the rule. While at first thought it might be supposed that
solution is subordinate, a close analysis shows that this process is not less
important than deposition, and, indeed, it may more than balance deposition.
SOLUTION AND DEPOSITION. 613
QUANTITATIVE RELATIONS BETWEEN SOLUTION AND DEPOSITION.
It has been fully explained that the belt of weathering at any given
time represents the partly disintegrated and decomposed material above |
the level of ground water. During the long-continued erosion of a region
the belt of weathering steadily migrates downward. Thus the forces of
weathering continually find new material at the bottom of the belt upon
which to work. Therefore, as denudation goes on, there is always a belt
of a certain thickness in which weathering processes are taking place.
These include solution as a dominant process. It has further been noted
that the material abstracted from the belt of weathering is divided into two
parts, one of which goes to the overground circulation and thence to the
sea, the other of which passes through the belt of weathering into the belt
of cementation below. Each unit of water which passes downward
through the belt of weathering into the belt of cementation carries with it
in solution a certain amount of material. As a consequence of continuous
downward migration of the belt of weathering, it is certain that an incre-
ment of material is continuously added to the belt of cementation from the
belt of weathering. :
If this increment which the sea of underground water continually
receives were deposited in the belt of cementation, it would furnish a
sufficient supply of material for the cementation of that belt. Much of this
material is certainly deposited in the belt of cementation, but in that belt
solution is also taking place, and the question therefore arises whether or
not more material is deposited than is dissolved in the belt of cementation.
This question is not easy to answer, but we know certain data which have
an important bearing upon it.
One of the fundamental conclusions worked out in Chapter III, on
“The agents of metamorphism,” is that the quantity of water which passes
through the belt of weathering and enters the sea of underground water is
substantially equal to that which emerges from this sea through springs
and through seepage and joins the run-off. If we knew the relative
amounts of material contained in solution in the waters entering and issuing
from the belt of cementation, we could answer the third question—i. e.,
whether more material is dissolved or deposited in the belt of cementation.
But the only possible way to get this information is by numerous analyses.
614 A TREATISE ON METAMORPHISM.
In various regions the amounts of salts should be determined in the solutions
near the bottom of the belt of weathering and in the solutions issuing from
the belt of cementation. Such a comparative analytical study has not been
made, and we must therefore have recourse to general reasoning for a
probable answer to the question of the relative amounts of the salts entering
and issuing from the belt of cementation.
It might be concluded that the actual fact of widespread consolidation
due to filling the openings of the belt of cementation, so fully emphasized
in the opening pages of this chapter, is evidence that the amount of material
contained in the water issuing from the belt is not so great as that which
joins it through percolation. But there are considerations which render
this view very doubtful. As shown by numerous analyses, it is certain
that large amounts of materials are contained in the spring and seepage
waters which issue from the belt of cementation. From general considera-
tions, if the character of the compounds transported be ignored, one would
expect that this amount would be greater than that which entered the belt.
These considerations are as follows:
As water passes through the belt of weathering and enters the belt of
cementation it may not have been sufficiently long in the belt of weathering
and in close enough contact with the various compounds to become sat-
urated. Doubtless in many cases of fine soils containing plentiful soluble
compounds, where vegetation is abundant and percolation is slow, saturation
for many of the compounds may be approached or even attained. But
where there are coarse and thin soils underlain by fissured or porous
material, it is highly probable that the descending waters are far from
saturated when they leave the belt of weathering and join the belt of
cementation. After the waters join the belt of cementation they take a
longer or shorter journey before issuing at the surface. It has been
explained (pp. 584-586) that the general movement of the ground water is
exceedingly slow. In many instances also its journey is long—in some
areas hundreds of kilometers. In this connection it may be recalled that
it was calculated that the waters which issue at Chicago probably entered
the ground in central Wisconsin somewhere from one hundred and fifty to
two hundred and fifty years before ‘This factor of time, therefore, is of
fundamental consequence in the work of the water of the belt of cementa-
tion, and it would be strange if the water which has taken a considerable
RELATIONS BETWEEN SOLUTION AND DEPOSITION. 615
journey in a formation, even if but a small fraction of that in the case men-
tioned, should not become saturated with the compounds with which it is
in contact. =
The journey of the water in the belt of cementation involves a com-
ponent parallel to the surface and a vertical component. As a result of the
component of the movement of ground water parallel to the surface there
is a tendency for material to be taken into solution and to be abstracted
by the water. For a given depth the pressure and the temperature are, on
the average, the same. The longer this course the nearer will the water
approach to saturation with the compounds with which it is in contact,
because of the time factor. If the journey be long, the state of saturation
may be attained at a comparatively early stage, after which the additions
and subtractions of material may be presumed, on the average, to neutral-
ize each other, although in a given instance the total amount of material
held in solution would vary greatly, depending upon the character of the
rocks with which the water is in contact.
Under normal conditions, so far as the vertical component is concerned
there is steadily increasing pressure and temperature during the downward
movement, and steadily decreasing pressure and temperature during the
upward movement. If the places of entrance and issue were at the same
level, these two factors might be considered to neutralize each other, but
the descending column is necessarily the longer; therefore the balance of
the two processes is in favor of solution rather than deposition. Also, unless
the water has become saturated during its descent solution will continue
during its ascent until saturation is attained, and if the vertical component
be short, this may not occur before the entire ascent has been made.
Finally, the temperature of the entering and issuing water is of great
consequence in the matter of relative quantities of dissolved and deposited
materials. Even if the temperature were precisely the same at the enter-
ing and issuing points this would favor solution, for increase in temperature
as the water descends very greatly accelerates the process of solution, and
as the water ascends precipitation takes place only when saturation is
reached, and then only to an extent sufficient to prevent supersaturation.
But into this matter of temperature another very important factor
enters. It has been shown on pages 589-592 that, on the average, the
temperature of the issuing water is higher than that of the entering water.
616 A TREATISE ON METAMORPHISM.
It has been fully explained on pages 79-81 how important a slight increase
in temperature is, not only in respect to the speed with which material is
dissolved, but in respect to the quantity which may be taken into solution
before saturation is reached. This small average additional increment of
heat in the issuing water over that of the entering water is very favorable
to the presence in the issuing water of larger amounts of material than was
contained by the entering water.
Therefore it appears that, in proportion as the underground journey
of the water in the belt of cementation is slow, in so far as it has a hori-
zontal course, in so far as it enters at a higher elevation than it issues, and in
so far as it issues at a higher temperature than it enters, it should contain
more material in solution per unit mass when it issues than when it enters.
From all these points of view it is to be expected that ground waters
contain more material in selution when they issue from the belt of
cementation than they contained when they entered that belt, and that
_waters which have taken a journey of considerable length in a uniform
formation will have reached a state of almost complete equilibrium between
themselves and the surrounding rocks, or will be saturated.
But another important factor enters into the matter. It is not to be
supposed that during the underground journey the materials which are-held
in solution are the same. Throughout the journey there are various chemical
interactions. Much of the material brought from the belt of weathering is
deposited; there is solution of material at a certain place, and later depo-
sition of it elsewhere; there is interaction between the solutions and solids;
there is interaction between the mingled solutions from different sources.
It is the uncertainty as to the average effect of these various chemical .
reactions that leaves us in doubt as to the end result; that is, whether or not
the solutions of all systems of circulation are, on the average, richer in
material when they emerge from the belt of cementation than when they
entered it. For instance, if a solution containing oxygen were mingled with
a solution in which iron was the chief base, hematite or limonite would be
precipitated, and the solution would be depleted in material even if the
temperature of the water were higher at the end than at the beginning
of the journey. Again, colloidal silicic acid is readily soluble even at low
temperatures, but the acid has a marked tendency to decompose into water
and silica, though the reaction is a slow one. So far as it takes place in the
RELATIONS BETWEEN SOLUTION AND DEPOSITION. 617
belt of cementation, there is precipitation of quartz and reduction of the
amount of material held in solution, even if the temperature be high.
Indeed, it is not improbable that high temperature is favorable to the
dehydration of silicic acid.“ Because of the uncertainty of the effect of
the chemical reactions, one can not say whether more material emerges
with the solutions than enters with them. Were it not for the enormous
quantity of colloidal silicic acid contributed by the belt of weathering and
the dominant importance of quartz as a cement (see pp. 622-623), probably
solution due to the causes above assigned would overbalance deposition.
But in the present state of knowledge I must leave unanswered the question
as to which of the processes, solution or deposition, is preponderant in the
belt of cementation. Both are of the utmost consequence.
Concluding, we now see that the unity of the belt of cementation and
the belt of weathering from a physical-chemical point of view is perfect.
Both are belts of oxidation, carbonation, hydration, and solution and
deposition. Both are belts of reactions with liberation of heat and
expansion of volume. ‘The contrast between the two belts is largely due to
the variable quantitative value of each of these processes in the two belts
and the resulting condition in which the rocks are left—disintegrated and
softened in the one and cemented and indurated in the other.
RESULTANT PROCESSES.
The processes resulting from the chemical changes are cemertation and
metasomatism.
CEMENTATION.
By cementation is meant the binding together of the rock particles by
deposition of material as minerals in the interstices of the rocks. In the
previous chapter on “The Belt of Weathering,” we ascertained the source
and character of the materials which joined the sea of ground water and
which are therefore available for cementation in the belt of induration. It is
clear that there are constantly being added from the belt of weathering to
the continuous sheet of water in the belt of. cementation the following
compounds: Sodium, potassium, calcium, magnesium, iron, aluminum, as
carbonate, sulphate, nitrate, phosphate, chloride. With these there are also
4Mendeléeff, D., Principles of chemistry, trans. by George Kamensky, Longmans, Green & Co.,
London, 1897, vol. 2, p. 112.
618 A TREATISE ON METAMORPHISM.
abundant colloidal silicic acid and lesser proportions of the rarer bases and
acids, such as manganese, hydrofluoric, and boric. In the following section
on metasomatism it will be shown that during the alterations of the materials
of the belt of cementation equal or greater amounts of these compounds are
added to the solutions. A portion of the great amount of material which
joins the sea of ground water from the belt of weathering and from the
alterations of the minerals of the belt of cementation is there precipitated
as minerals and the rocks are cemented. Hence the name, “belt of
cementation.”
It will be remembered that the openings in rocks comprise those
between the mineral particles, and especially the grains of sediments, the
vacuoles of igneous rocks, the regular openings of fissility, joimts, and
faults, and the irregular openings from those of the fractures of the
individual mineral particles to those of breccias. Further, it will be
recalled that the amount of openings varies from a small fraction of 1 per
cent to 75 per cent, that in the mechanical sediments and the porous lavas
a pore space of 20 to 40 per cent is common, and that the quantitative
importance of the fault, joint, and fissile openings is great. (See pp. 124—
131.) Therefore the amount of material which is required to fill the open-
ings of great formations, thousands of meters in thickness and extending
over areas of hundreds or thousands of square kilometers, is vast. Yet it
is rather rare, if not unknown, for a rock which has been deeply buried
and approaches the surface by denudation not to have the older openings of
all kinds almost entirely filled.
In proportion as the process of cementation advances it necessarily
follows that the openings become smaller, the circulation of the water is
retarded, material is transported in smaller quantity, and the process
becomes slower. This lessening of speed continues from the first, and
when cementation is near completion it must be exceedingly slow.
Notwithstanding this the process, as has been explained, has been
practically completed for great thicknesses of rocks over extensive areas.
The time required for the work must have been great, and yet for
completion under favorable conditions it is certain that it does not require
geological eras. This is shown by the complete cementation of formations
of Eocene age In such instances the process must have been completed
and a sufticient time have elapsed for erosion to bring the cemented rock
COMPLETENESS OF CEMENTATION. 619
to the surface since early in Tertiary time. The best illustration known to
me of a completely cemented great formation of Tertiary age is the San
Juan breccias of Colorado. This formation is 1,500 meters thick; it was
very porous, and yet every ancient opening not of microscopic size, from
great fissures to pores between the particles of ash, is completely filled.
Of course, after cementation has been partially or wholly completed
orogenic movements may occur which produce a new set of openings, and
a vigorous circulation be set up in the new openings. This would give
new trunk channels for circulation and thus assist in the cementation of
partially closed old openings.
In judging of the nearness to completion of the process of cementation
when the rocks were at a considerable distance below the surface, openings
which have been produced by later orogenic movements when the rocks
were nearing the surface must be ignored. Commonly such openings are
not closed.
It is explained on pages 595-597, 646-648 that the openings in the
rocks are to some extent closed by pressure, and locally are closed by
injection. So far as these processes take place the space left to be filled by
the process of cementation is decreased; but the residual actually observed
to have been filled by the process of cementation is enormous.
In the cementation of the openings between the mineral particles the
new mineral material may be added in two different ways. It may attach
itself to the old grains of like mineral character, or it may be deposited as
independent interstitial material. If mineral particles be fractured, it may
hea] them with the same or some other mineral.
When interstitial mineral material attaches itself to an old mineral of
like character, the two being in optical continuity, the mineral is said to
have been enlarged. The principle explaining the enlargement of old min-
eral particles rather than the development of new smaller mineral particles
is essentially the same as that explained on pages 74—76, that large indivi-
duals grow at the expense of small ones. Where there are old nuclei which
can be used the solutions deposit material upon these; for if independent
particles begin to form, these under the principle above referred to would be
likely to be again dissolved and deposited upon the old larger particles.
The enlargement process is far more important for quartz than for any
other mineral, although enlargements of feldspar, hornblende, augite, biotite,
620 A TREATISE ON METAMORPHISM.
calcite, garnet, and tourmaline have been observed, some of them frequently,
In a clastic rock the process of enlargement means that the component
grains are minerals still and have the structure and potency of minerals:
These minerals, originally produced in igneous or metamorphic rocks, are
taken from their original positions and deposited in a secondary rock. The
beds are buried by overlying formations. Mineral-bearing solutions pass
through the new rock and each mineral fragment chooses from the solu-
tions material like itself. This it attaches to itself in optical continuity,
even though the time interval between the first and second growths be *
indefinitely long. In the early stages of this renewed growth crystal faces
are often rebuilt. (Pl. IX, 4.) If the growth continues, the enlarge-
ment of a particle meets similar enlargements from other grains. By
further growth these interlock and finally fill up
_all the interspaces and perfectly indurate the rock.
(GED, 765.)
This process of enlargement is of greatest
importance in the mechanical sedimentary rocks.
Its magnitude in these rocks is fully explained on
pages 865-868. The process is also important in
the volcanic fragmental rocks, including both coarse
Fig. 13.—Part of a thin section of a tuffs and ashes; and, finally, the process does take
quartz-schist showing liquid- and
gasfilled cavities of a secondary Place, although it is of subordinate importance, in
oc tw. ag oMmassive) aeneous, rocks, the) cracks withingand
between the grains may be cemented by substances the same as or different
from the materials cemented. If the cementation be imperfect secondary
gas- or liquid-filled cavities may be formed. Often the broken fragments
are somewhat displaced, but occasionally, in the case of parallel fractures,
the particles are broken apart and left very nearly in their original relative
positions, in which case the healing may be so perfect that the fact that the
rock has been fractured may be shown only by the gas- or liquid-filled
cavities and by the secondary inclusions. Thus are explained many of
the inclusions in cavities which are so frequently noticed to occur along
regular parallel planes. (See fig. 13.)
In all cases where the process of cementation has gone far the depos
ited minerals interlock, and in the larger spaces they may have a coarsely
crystalline texture. The quantity of cementing material varies in different
COMPLEXITY OF CEMENTATION. 621
cases from a relatively unimportant amount to predominance. The
cementing minerals may permeate the rocks through and through, and
fill all the imterspaces, microscopic and macroscopic. The result is in
many instances to give a most extraordinarily complex structure, the places
between the original minerals being filled, the parallel fractures being
emphasized by the parallel impregnation, and the whole being intersected
by larger masses of holocrystalline interlocking mineral materials, some of
which are parallel to the original structures, some of which are diagonal to
them, and some of which are parallel for a certain distance and then eut
across them. The material filling the larger crevices often has distinct vein-
like forms. (PIII, 4.) When examined carefully the material deposited
parallel to laminated rocks may be found to follow the folia very closely, or
it may be found to follow along them for some distance, then break across
one or more, and then follow them again. In proportion as the cementing
material follows the laminze the cement bands are likely to be of approxi-
mately the same width. In proportion as there is a tendency for the material
to break across the folia the veins usually are of unequal size.
The cementing minerals fillmg the openings of fissile rocks are usually
different or in different proportions fron: the minerals of the adjacent lam-
ine, and hence there is a tendency to preserve and emphasize the laminated
arrangement. Where sets of parallel joints are filled, for a similar reason
a parallel structure is produced. Where there are intersecting sets of par-
allel joints the parallel structures of cementation may form simultaneously
in two or three directions. It therefore follows that where the openings
are of the sheeted parallel kind, in general the process of cementation results
in a regular alternation of parallel layers of different kinds, the structures
of which have no definite relations to the original bedding.
ULEMENTING SUBSTANCES.
The more important cements of rocks may be divided into oxides,
carbonates, silicates, and sulphides.
The most important oxides are silica, iron oxide, and aluminum oxide.
The more important carbonates are calcite, dolomite, and siderite. The
silicates include both the hydrous and anhydrous silicates, but the former
are by far the more important. The only sulphides which are geologically
622 A TREATISE ON METAMORPHISM.
important are marcasite and pyrite, but from the point of view of ore
deposits many other sulphides are of great consequence.
In addition to these, there are many subordinate minerals deposited in
the belt of cementation; indeed, almost every mineral which occurs in rocks
may there form, but the amounts are so small that they will not be
considered here.
OXIDES.
silica —Silica may be deposited as opal, chalcedony, or quartz. In the
process of precipitation it may fill any of the classes of openings which exist
in rocks. But the greatest quantity of this material is deposited between
erains of sediments, and especially between the grains of quartzose sand.
Sandstone formations are one of the most abundant of the sedimentary
deposits. It has been shown on pages 124-126 that the original pore space
of such formations probably varied from one-fifth to two-fifths of the volume,
with a probable average of approximately one-third. The ordinary quartz-
ites which have not been modified by mechanical action are completely
indurated by the deposition of quartz between the grains, in orientation
with the original grains, or as independent material, or the two combined.
(Pl. IX.) The volume of quartzite of this class now existing upon the
earth is very great. (See pp. 865-868.) In reaching a judgment as to the
amount of quartz which has been precipitated in sand, it must be remem-
bered also that many sandstone and quartzite formations produced in
past geological times have been destroyed by the forces of erosion and
redeposited.
One of the reasons why quartz is so very extensively deposited in
quartzose sandstone is furnished by the principle (see pp. 120-122) that,
other things being equal, material in solution is precipitated where there
are nuclei of the same composition upon which it may be deposited.
The innumerable rounded grains of quartz in the sands furnish the neces-
sary nuclei which promote the precipitation of silica.
But quartz is deposited not only between the grains of quartzose sands;
to some extent it is deposited between the grains of all other porous
mechanical sediments, although the amount thus precipitated is subordinate.
A vast amount of quartz is also deposited in the vacuoles of volcanic rocks
as amygdules. Further, silica is deposited in the innumerable openings of
the fissile rocks, in the numberless joints, and along the numerous faults
CEMENTING SUBSTANCES. 623
and openings in the breeciated rocks. The amount thus deposited is only
second to that deposited in the sediments. Finally, as shown in the
succeeding section on metasomatism, there are very extensive replacement
deposits of quartz. There is no means by which the amount of replace-
ment silica can be measured, but the quantity is certainly very great.
The foregoing facts give us some idea of the vastness of the amount
of siliceous material which is deposited in the belt of cementation.
Certainly the amount is to be measured in thousands and probably in
_hundreds of thousands of cubic kilometers. An adequate source of this
silica is that liberated by the process of carbonation in the belt of weath-
ering. (See p. 480.)
Iron oxides.—Iron oxide is extensively deposited between the grains of
mechanical sediments, and therefore is an important cementing agent Tron
oxide is also deposited in the other classes of openings in rocks, and also as
a replacement product. ‘Iron oxide is deposited as magnetite, hematite,
limonite, and other hydrated oxides.
A very large amount of the iron is precipitated as hematite or limonite
or other hydrated oxide. This precipitation is likely to occur where oxygen
is abundant. These conditions are likely to obtain where solutions from
different sources are united in some main channel of descending ground
water. No better illustration of precipitation of iron oxide by this process
can be given than the great hematite deposits of the Lake Superior region,
which are precipitated by the mingling of waters bearing iron carbonate
and those bearing oxygen. (See pp. 1194-1197.)
Magnetite is deposited in the belt of cementation as a very widespread
constituent. The explanation of the precipitation of some magnetite is: In
the belt of cementation there is usually a deficiency of oxygen. If the
iron be supposed to be carried in the form of carbonate, the reaction for
the precipitation of magnetite may be written as follows:
3FeCO;+0=Fe,0,+3C0,.
Other methods by which magnetite is precipitated are considered on
pages 845-846. Where iron is thrown down as magnetite it is ordinarily
found in erystals. As illustrations of this may be mentioned formations in
the Lake Superior region, some of which are of economic importance, as,
for instance, the magnetite of the Michigamme, Republic, and other mines
624 A TREATISE ON METAMORPHISM.
along the Republic trough. However, the magnetite thus deposited in
solid bodies is entirely subordinate to the vast amount which is dissemi-
nated through the rocks of the adjacent formations—the quartzites, slates,
and tuffs. Extensive deposition of magnetite is illustrated by the Michi-
gamme schist of the Marquette district and by the Hemlock olan) forma-
tion of the Crystal Falls district.”
It will be remembered that hematite and limonite are very abundantly
precipitated in the belt of weathering as well as in the belt of cementation,
but magnetite very rarely forms in the belt of weathering. It therefore
appears that magnetite cement is especially characteristic of the belt of
cementation.
Aluminum oxides—The aluminum oxides which are deposited as cementing
substances are gibbsite, diaspore, and perhaps corundum. Both of the
hydrous minerals are known to be rather frequent cementing constituents
where the rocks are of an intermediate or basic character. Usually they
have not been regarded as important cementing minerals, but I suspect
they are rather more plentiful than has been supposed. My reason for
this belief is the frequency with which aluminum hydroxide is a by-product
in the common alterations of such minerals as the feldspars, feldspathoids,
micas, and other minerals. (See pp. 375-394.)
CARBONATES.
The important carbonate cements are calcite, dolomite, and siderite.
Calcite and dolomite—Oalcite and dolomite are treated together, since without
chemical analyses it is usually impracticable to discriminate between the
two where deposited in rocks. Also, there are various gradations between
calcite and dolomite. In general, it may be said that these minerals are
deposited in all the kinds of openings in rocks. They are more likely to be
extensively deposited in the caleareous rocks than in the siliceous rocks, since
in such rocks the solutions are sure to contain abundant carbonates, and
there are crystals of calcite and dolomite to serve as nuclei for deposition.
The calcium carbonate and magnesium carbonate may be deposited upon
calcite and dolomite, respectively, thus enlarging these minerals precisely
«Van Hise, Cc R., and Bey ieee W. S., The eae iron-bearing district of Nichiooe Mon.
U. 8. Geol. Survey, vol. 28, 1897, pp. 444452. Ceara J. Morgan, and Smyth, H. L., The Crystal
Falls iron-pearing district of Michigan: Mon. U. 8. Geol. Survey, vol. 36, 1899, pp. 150-152.
CEMENTING SUBSTANCES. 625
as quartz is enlarged. The deposition of magnesium carbonate in many
cases involves the solution of an equivalent amount of calcium carbonate,
in which case the process is that of metasomatism, fully considered on
pages 640-646.
The quantity of calcite and dolomite deposited between the grains of
sediments, the vacuoles of igneous rocks, the openings of fissility, joints,
and faults, while very great, is vastly less than that of quartz.
Siderite—The iron in solution may be partly or largely precipitated in the
form of siderite, ankerite, or ferrodolomite. These compounds all form
somewhat extensively in the minor interstices in rocks, and occasionally
they may form veins of such magnitude as to be worked as iron ore.
SILICATES.
The silicates which are most abundantly precipitated as cements are
hydrous, but anhydrous silicates are also deposited. The important
cementing silicates include, in order of abundance, the (1) zeolites and
prehnite, (2) chlorites, (3) epidotes, (4) serpentine and tale. The zeolites
and prehnite are sodium-aluminum silicates, calcium-aluminum silicates,
sodium-calcium aluminum silicates, and potassium-calcium silicates. The
chlorites are magnesium-aluminum silicates when pure, but ordinarily the
magnesium is partly replaced by iron. The epidotes comprise zoisite
(calcium-aluminum silicate) aud epidote (calcium-aluminum.-iron silicate).
Serpentine and tale are magnesium silicates. As to the degree of hydra-
tion, the minerals may be divided into three classes: The zeolites and
prehnite, the chlorites and serpentine, and epidote and tale. The bases
of all of the above compounds are those which abundantly enter into the
solutions. Hydration, as already shown, is one of the most important of
the chemical processes which result in the liberation of heat. Probably
the dominant factor in the precipitation of these hydrous compounds is the
chemical law obtaining in the zone of katamorphism that, other things
being equal, those compounds form by which the greatest amount of heat
is developed and liberated.
The anhydrous silicates which are somewhat abundantly deposited in
the belt of cementation are feldspar, hornblende, and mica, the order given
being that of relative abundance. These materials may be deposited as
- independent constituents in the interstices of the rocks, and they may also
MON XLVII—04——Y40)
626 A TREATISE ON METAMORPHISM.
be deposited in optical orientation upon nuclei of like minerals. Feldspar
enlargements have been found in many arkoses. Perhaps the best known
instance is that of the Keweenawan sandstone of Lake Superior, the cemen-
tation of which is mainly accomplished by the deposition of feldspar upon
worn grains of that mineral. The grains are of
different kinds of feldspar, orthoclase, and various
plagioclases. The material deposited in each case
is in optical continuity with the old material, even
to the extension of the twinning lamelle. (See
fig. 14.) An excellent istance of the enlarge-
ment of hornblende is that of the hornblende
crystals of the volcanic tuffs of Kekekabic Lake,
in northeastern Minnesota.” The relations of the
cores and additions of hornblende are identical
with those of the feldspars, even to the extension
Fie. Li.—Enlargement offelaspar Of the twinning lamellee. (See fig. 15.)
oe The enlargement of feldspar and hornblende
furnish the best illustrations known of the principle so strongly emphasized
on pages 120-122, that mineral nuclei already present are able to abstract
from solutions materials like themselves, and thus control
the combinations of the elements when precipitated. In
this principle probably lies the partial answer to the ques-
tion why, in the cases cited of feldspar and hornblende
enlargements, the material was not deposited between the
grains as hydrous minerals. The formation of hydrous
compounds would have developed more heat, but the
power of the nuclei of old minerals to control the precipi-
tation appears to more than have overbalanced the chemi-
cal law that in the belt of cementation reactions commonly — ls. 15.—Enlargement of
i i hornblende fragment.
take place which liberate the greatest amount of heat.
But another factor which frequently enters into the precipitation of such
anhydrous minerals as feldspar, hornblende, and mica is the abundance
and the proper proportions in the solutions of the elements out of
which they can be made. Rocks in which feldspar, hornblende, ete.,
«Van Hise, C. R., Enlargement of feldspar fragments in certain Keweenawan sandstones: Bull.
U.S. Geol. Survey No. 8, 1884, pt. 2, pp. 44-47.
» Van Hise, C. R., Enlargements of hornblende fragments: Am. Jour. Sci., 3d ser., vol. 30, 1885,
pp. 231-235.
CEMENTING SUBSTANCES. 627
are deposited usually contain these same minerals in abundance, and in
many cases the formations extend to the surface. The materials for the
solutions may have been derived from the same minerals in the belt of
weathering, or from a part of the belt of cementation above the places
of deposition in case the waters are descending, and below the places of
deposition in case the waters are ascending.
SULPHIDES.
The only important sulphide cement is that of iron. Iron is somewhat
extensively thrown down as pyrite or marcasite. This may be precipitated
by means of hydrosulphuric acid; it may be produced by the reduction of
iron sulphate by means of organic reducing agents in the solutions or the
rocks. These and other methods of precipitating sulphides are fully con-
sidered on pages 1108-1118. The quantitative importance of the cement-
ing sulphides in the belt of cementation, from a geological point of view, is
small, but from the point of view of ore deposits they are of the greatest
consequence. (See pp. 1104-11065.)
DISTRIBUTION OF ELEMENTS IN CEMENTING MINERALS.
From the foregoing it appears that the elements added to the solutions
from the belt of weathering and from the belt of cementation itself are
distributed between the various minerals as follows:
The sodium passes into the silicates as zeolites, and may produce any
one of the followimg sodium-bearing minerals of this group: Thomsonite,
hydronephelite, natrolite, mesolite, gmelinite, analcite, phillipsite. The
potassium passes into silicates as zeolite, being a constituent of apophyllite.
The calcium passes into the carbonates and silicates. As a carbonate it is
a constituent of calcite, dolomite, ankerite, and parankerite. As a silicate
it enters into the zeolites and epidotes. The zeolites containing calcium
are thomsonite, mesolite, scolecite, apophyllite, stilbite, phillipsite, gis-
mondite, chabazite, and laumontite. Both epidote proper and zoisite con-
tain calcium. The magnesium may enter into carbonates or silicates. As
a carbonate it is a constituent of dolomite, ankerite, and parankerite. As a
silicate it is a constituent of all the chlorites, of serpentine, and of tale.
The iron may pass into oxide, carbonate, silicate, or sulphide. As an oxide
it is a constituent of magnetite, hematite, and limonite. As a carbonate
it is a constituent of ankerite, parankerite, and siderite. As a silicate it is
a constituent of epidote and chlorite. As a sulphide it is a constituent of
628 A TREATISE ON METAMORPHISM.
pyrite and mareasite. The alumina passes into the hydroxides and silicates.
As a hydroxide it is a constituent of diaspore and gibbsite. As a silicate it
isa constituent of all the zeolites except apophyllite, of all the chlorites, and
of the epidotes.
The carbon of carbonic acid enters as a constituent of the carbonates
above mentioned. The silicon of silicic acid enters into quartz, opal, and
the silicates above mentioned. The sulphur of hydrosulphuric acid enters as
a constituent of the sulphides.
DISTRIBUTION OF CEMENTING MINERALS.
It is very noteworthy that the cementing minerals have a strong
tendency to be like the compounds which are cemented. Thus the sands
are transformed to sandstones and finally to quartzites by siliceous cement.
Where nearly pure limestones and dolomites have been fractured, the dom-
inant cementing minerals are calcite and dolomite. If, however, the lime-
stone be a very cherty one, the cementing material may be largely chert.
Where the porous rocks are largely silicates, as in the case of the amygda-
loids, the cementing materials are mainly silicates, including the zeolites,
epidotes, chlorites, etc., but also with these are usually associated important
amounts of quartz and carbonates, one being correlative with the other; for
carbonation of the silicates, forming carbonates, liberates silicic acid, which
may separate as quartz. .
These phenomena are beautifully illustrated by the amygdaloids of
the Keweenawan series described by Pumpelly“ and Irving,’ and by the
Crystal Falls voleanics described by Clements.’
In many cases not only are the silicates cemented by silicates, but the
cementing minerals are like the dominant minerals cemented. For instance,
the feldspathic sandstones may be cemented by feldspar, as in the case of
the Keweenawan sandstones of Eagle Harbor.“ Where hornblende is
abundant the cementing material may be largely hornblende, as in the case
of the Cacaquabic tuffs of northeastern Minnesota.
“Pumpelly, Raphael, Metasomatic development of the copper-bearing rocks of Lake Superior:
Am. Acad. Arts and Sci., vol. 18, 1878, pp. 253-309.
0 Irving, R. D., The copper-bearing rocks of Lake Superior: Mon. U. 8. Geol. Survey, vol. 5, 1883,
pp. 87-91, 134-139. ’
¢Clements, J. Morgan, and Smyth, H. L., The Crystal Falls iron-bearing district of Michigan:
Mon. U. 8. Geol. Survey, vol. 36, 1899, pp. 73-154.
dVan Hise, cit., Bull. 8.
WORK IN BELT OF CEMENTATION. 629
CAUSES OF CEMENTATION,.
It has already been explained (see p. 156) that the amount of ground
water which reaches the surface by springs and by seepage for a given
period is approximately equal to the amount which is added to the sea of
ground water. It is well known that all issuing waters contain material in
solution; and it has been shown that the amount of such material may
be greater than that which the solutions contained when they entered the
belt of cementation. Notwithstanding the foregoing conclusion, it is an
undeniable fact, as fully shown in the previous pages, that cementation is
the rule for the part of the zone of fracture below the level of ground
water. If more material be dissolved in this belt than is deposited by the
circulating waters, what is the explanation of the apparently contradictory
fact of deposition to the extent of complete cementation?
Before attempting to answer this question, it is advisable to recall the
amount of openings which may be present in the belt of cementation. It
has been shown that the coarse mechanical sediments frequently have an
origimal average pore space as great as one-third; that in the thinly bedded
lavas this amount is often equaled, and that the openings produced by
mechanical action are great. It follows, as already clearly shown, that the
amount of deposited cementing material is vast. Therefore we must not
only furnish a cause which will result in the deposition of material in the
pore spaces, but we must show that the causes assigned are quantitatively
adequate to perform the great work.
In this connection it is well to restate the principles of precipitation
worked out on pages 113-123. Mentioned in order of probable impor-
tance, precipitation is due (1) to the mingling of solutions from different
sources, (2) to reactions between the solutions and the wall rocks, (3) to
decrease in temperature, and (4) to decrease in pressure. The compounds
are precipitated from the solutions in proportion as they are relatively
insoluble and in proportion as they are abundant.
The law of chemistry that when compounds of different kinds come
together substances form (if possible) which are insoluble in the liquids
present, explains the importance of the first and second causes of precipita-
tion. It has been shown that decrease in temperature is also very important,
and that probably the importance of this factor in connection with decrease
630 A TREATISE ON METAMORPHISM.
of pressure has not been sufficiently appreciated. The experimental work of
Barus” shows that at temperatures above 185° C. water and soft glass are
miscible in all proportions. Further, it is held that water and rock are mis-
cible in all proportions at proper temperatures and pressures. (See p. 723.)
These conclusions throw new light upon the importance of the third
and fourth causes of precipitation. If these conclusions as to the misci-
bility of water and rock be true, at the high temperatures and pressures
which prevail in the deep seated circulation the water must be very rich in
mineral content. It is clear that ascending waters which continuously fall
in temperature and pressure must throw down great quantities of this
material before they reach the surface. It is notable that large channels
are most favorably situated with reference to all of these causes of
g of solutions is most
D
precipitation, for in the large channels minglin
likely to take place, along them reactions with the wall rock are important,
and in them solutions are most likely to be ascending. (See pp. 582-584.)
Therefore all the causes for precipitation combine to explain the great
amount of material deposited in the large openings.
While all of these statements are undoubtedly true, in view of the fact
already pointed out, that waters issuing from the belt of cementation
may contain more material in solution than those entering that belt, none
of them, nor their combinations, explain the general cementation in large
and small openings alike. Before we can satisfactorily assign causes for
general cementation it is necessary also to recall the cementing minerals
and to indicate their relative importance. Of the substances deposited in
the belt of cementation quartz is undoubtedly the one which dominates
over all others. The one great process in the belt of cementation is
silicification. Of very great importance is the deposition of the silicates
and carbonates. Both of these classes of minerals are deposited on a
great scale, but which of these is quantitatively the more important it is
hard to say. It has been noted that of the various silicates deposited the
important ones are the zeolites, the chlorites, the serpentines and tales, and
the epidotes. The deposition of oxide of iron, as hematite, limonite, ete.,
and of aluminum, as gibbsite and diaspore, is rather important, but quan-
titatively very subordinate to that of the classes of minerals already
mentioned,
“Barus, C., Hot water and soft glass in their thermo-dynamic relations: Am. Jour. Sci., 4th ser.,
vol. 6, 1898, p. 270; and vol. 9, 1900, pp. 167-168.
CAUSES OF CEMENTATION. 631
We are now prepared to suggest causes which may combine to explain
the apparently contradictory facts that a vast amount of openings in the
belt of cementation is cemented and that emerging waters may contain
more material in solution than entering waters; or that, of the two processes
of deposition and solution, the latter is the preponderant one.
EXPANSION REACTIONS.
The first cause which will be given for cementation—and I believe the
one of greatest importance—is that of expansion reactions. It has been
explained (p. 603) that the chief reactions of the belt of cementation are
oxidation, carbonation, and hydration, and that of these three the latter is
of greatest consequence. Further, it, has been seen that, provided all of the
compounds formed remain as solids, the average volume increase in con-
sequence of these reactions varies from 15 to 50 per cent or more. So far
as these expansion reactions take place—and they undoubtedly occur on a
most extensive scale—they tend to fill the openings, and thus cement and
consolidate the rocks.
In the following section it is explained that the process of metasoma-
tism, or change of the minerals within the body of the rocks, has taken place
upon a vast scale in the belt of cementation. Indeed, in extensive ancient
formations, especially in rocks of a porous character, it has frequently hap-
pened that, with the possible exceptior of quartz, scarcely a vestige of any
original mineral remains. Many of the rocks, especially the igneous rocks,
are largely or almost wholly composed of silicates. Even in the acid rocks
the amount of quartz and other compounds, aside from the silicates, together
is rarely as great as 50 per cent, leaving 50 per cent or more of silicates.
We shall now consider the expansion due to alteration of a rock which
contains 50 per cent of silicates. If we suppose the silicates to be com-
pletely altered by the processes of oxidation, carbonation, and hydration,
and that in these processes the expansion is 25 per cent—which is less rather
than more than the average amount—if all of the materials were deposited
the resultant expansion would be sufficient to furnish material to fill pore
spaces to the amount of 124 per cent of the total original solids of the rock.
Therefore, in the case of an igneous rock of exceptional acidity and excep-
tional porosity, the expansion reactions would be adequate to entirely fill
the openings of a rather porous rock. But it may be said that many of the
652 A TREATISE ON METAMORPHISM.
rocks, especially the lavas, contain more pore space than 10 or 12 per cent.
This is especially likely to be true of the basic igneous rocks, of which the
basalts may be taken as illustrative. But in basalt there is ordinarily no
quartz. Practically the entire mass of the rock is composed of silicates, and
the complete alteration of these compounds would furnish a sufficient amount
of material, supposing the oxidation, carbonation, and hydration to produce
an expansion of volume of 25 per cent, and all the compounds formed remain
in situ, to fill a pore space of one-fifth the volume of the rock.
As an illustration of the very considerable amount of enlargement in
volume by metasomatism in the belt of cementation a specific case may be
given. For this purpose we may take a rock of intermediate composition.
The amphibole-gabbro of Beaver Creek, Big Trees quadrangle, California,
the mineralogical composition of which has been determined by Cross,
Iddings, Pirsson, and Washington, will serve the purpose.* Columns (1)
to (3) are taken from these authors:
Table showing relative volumes of orignal and altered rocks in belt of cementation.
a) | (2) | @) (4) (5)
Mineral. Formula. | Per cent. volume eee
| s products.
es: im | | | i
Orthoclases-eeee eee ASAE Sie O pew eee ee re ate eee 1.11] 0.8748 0. 9705
AMIS tG kN eee, a | IRE INUSE Ole cnn Sononsacctasuonoucsaseae | 23.06} 1.2082) 27. 8611
AMortniteyeeeeeeee aS CavATEGIS Ogee see ee ee eae tare IRA ROD | 1.3465 | 59.1383
Diopside.- mien ame ee CaN ReSisOnre uur ot cote ec mem ie | 18.10] 1.2788) 16. 7523
Hypersthene ----------- Wika NISHHO)s. 5. SocooaamopobaesoueHeoeeesse 3. 92 1. 1284 4, 4233
Olivine see seaecee eee WKH ISIE Oe eennounnuEaboaeue SsebacassS 7.75 | 1.1519 8. 9272
Maonetite= === sence | WEO conoteceseseseoosocesacessessooscs 2.55 | 1. 0000 2.55
Ten ites eee seen |CReTi Og: 2 See eve Ste ean eee be ene 1.67 | 1.0000) 1.67
iA patiten sauces earns ISCAS O sear e ac tr ere een eet eke 1.55} 1.0000 1.55
Byritesus. eee Beas rr MOA sR ERS CO cna ae anor 20 1.0000 20 |
Windetenmaimed ist Skee aa kere vee erates eds I, AYR ee mpi mE 3% | 1. 0000 US oye
Totals cPLA ecb NI eee ae tee eam ee ea 1004201) Meee ne 125. 4127
Supposing that the orthoclase by hydration and carbonation passes into
kaolin and quartz, the decrease in volume is 12.57 per cent (p. 389). Sup-
posing that the albite passes into analcite and quartz by simple hydration,
“Cross, Whitman, Iddings, J. P., Pirsson, L. V., and Washington, H. 8., Quantitative classifica-
tion of igneous rocks, Univ. of Chicago press, 1903, p. 199.
CAUSES OF CEMENTATION. 633
the increase in volume is 20.82 per cent (p. 375). Supposing that the
anorthite by hydration passes into thomsonite, a zeolite with only a mod-
erate amount of water, the increase in volume is 34.65 per cent (p. 376).
Supposing that the diopside by carbonation, hydration, and oxidation passes
into tale, quartz, magnetite, and calcite, the increase in volume is 27.88
per cent (see sahlite, p. 391). Supposing that the hypersthene by hydration
and oxidation passes into tale, magnetite, and quartz, the increase in volume
is 12.84 per cent (p. 385). Supposing that the olivine by hydration and
oxidation passes into serpentine, magnetite, and quartz, the increase in
volume is 15.19 per cent (p. 388). Since the percentages of the other
constituents are small, they will be supposed to remain unaltered. In the
above table, column (4) is the velume ratio and (5) the volume of resultant
products.
Tt thus appears that the supposed reactions of hydration, carbonation,
and oxidation of the amphibole gabbro result in an increase of volume of
25.21 per cent. It is to be noted also that the calculation is made upon a
conservative basis. It might be supposed, for instance, that the anorthite,
the most abundant mineral in the rock, passed into gismondite, which
would increase its volume by 52.76 per cent rather than 34.65 per cent.
Also, if the conditions were very favorable for oxidation the iron would
pass to hematite and perhaps to limonite. If all these suppositions were
made in favor of getting the maximum volume, this would make the volume
increase considerably greater than calculated.
While the scoriaceous upper parts of lava beds may contain a pore
space greater than 20 per cent, such a proportion of openings is compara-
tively rare for any considerable thickness of lava beds. It is well known
that the amygdules of amygdaloids are ordinarily filled with quartz, with
the zeolites, with chlorites, with epidotes, serpentine, and other hydrous
silicates, and with carbonates; in other words, with the minerals mentioned
on pages 621-627 as especially likely to be formed in the belt of cementa-
tion. That from the anhydrous silicates, hydrous silicates should form and
fill the openings is to be expected. hat the bases should unite with carbon
dioxide is natural. But what is the source of the quartz? So far as car-
bonation of the silicates has taken place—and the presence of plentiful
carbonates shows that this is an important reaction—silica, probably mainly
in the form of colloidal silicic acid, is liberated. The precipitation of this
534 A TREATISE ON METAMORPHISM.
silica from the solutions forms the abundant chalcedony and quartz of the
amy eduies.
It therefore appears probable that, in the alteration in the belt of
cementation of the porous rocks containing abundant silicates, expansion
reactions characteristic of the belt of cementation may be entirely adequate
not only to account for the filling of the openings, but possibly to furnish
additional material to the grouud solutions.
CONTRIBUTIONS FROM IGNEOUS ROCKS.
Expansion reactions, however, are not the only cause of cementation.
Tn a subsequent section upon igneous rocks it is seen that injections locally
produce profound metamorphic effects. It is there explained that the chief
of these is the increased action of solutions due to the heat of the magma.
But, as is well known, magma when solidifying gives off water. This
water is sure to be very rich indeed in mineral content. The material thus
contributed to the ground water solutions is certainly a source of material
for cementation. This source is of importance, for volcanism is a widespread
geological process. So far as material of this origin is available it explains
the cementation of rocks notwithstanding the probability that solutions
emerging from the belt of cementation contain more material than when
entering this belt.
SELECTIVE PRECIPITATION.
But it is clear that the explanation thus far offered by no means fully
covers the matter of cementation for all rocks. Indeed, it does not fully
include the most important of the cases of cementation. It has been seen
that the dominant cement of the belt of cementation is quartz. In many
instances (see pp. 622, 865-866) the transformation of the great sand forma-
tions to sandstones and quartzites is wholly, or almost wholly, accomplished
by the deposition of quartz. Probably 90 per cent or more of the fractures
of rocks produced by deformation, such as joints, faults, bedding partings,
etc., are cemented by quartz. The cementation by silicates, carbonates, ete.,
may be fully accounted for by the expansion reactions above given; but
what is the source of the great quantity of silica for silicification? In order
to answer this question we need to recall that the most distinctive process
of the belt of weathering is carbonation; that there, by the process of
carbonation of the silicates on a vast scale, colloidal silicic acid is set
CAUSES OF CEMENTATION. | 635
free. This material is taken into solution by the descending waters and
transmitted to the belt of cementation. As already pointed out, by the
processes of denudation and downward migration of the belt of weathering,
an abundant increment of silicic acid is ever contributed to the belt of
cementation. If this material could be there deposited, this would account
for the very general cementation by quartz. It has been seen that the
silica largely separates from silicates as colloidal silicic acid. This com-
pound is unstable, and tends to slowly decompose into water and silica.
During the long and very slow journey of the water in the belt of
cementation there is ample time for this process. It is likely that the
principle that insoluble products are apt to be precipitated has a bearing
at this point. Quartz is an extremely insoluble compound, and therefore
tends to separate from the solutions bearing colloidal silicic acid. In the
precipitation of the silica from the solutions the previous almost universal
presence of quartz is also of great importance; for it has been shown (pp.
120-122) that the presence of a mineral of a certain kind favors the abstrac-
tion of like material from a solution and the addition of that material to
the nuclei. Hence a large part of the most abundant substance contributed
by the solutions from the belt of weathering is precipitated in the belt of
cementation. As already pointed out, an additional supply of silica is fur-
nished by the carbonation of the silicates within the belt of cementation.
Finally, silica is furnished by emanations from intrusive rocks. Thus
is explained the universal dominance of quartz as a gangue and cementing
material, and the formation of great masses of sandstones and quartzites by
this cementation process, as fully explained (pp. 864-868).
It is highly probable that the solutions are partly or fully compensated
for the silica abstracted by the addition of calcium, magnesium, sodium,
potassium, iron, etc., obtained by alteration of the minerals within the belt
of cementation. It has been noted (pp. 609-610) that carbon dioxide is
added to the belt of cementation in four ways. This carbonic acid unites
with bases, producing carbonates. Further, it has been explained (pp.
624-625) that carbonates are very important compounds added to the belt
of cementation. Thus the solutions of the belt of cementation have two
sources of these compounds. It is therefore to be expected that undergound
solutions which issue at the surface and in springs would bear as their most
abundant compounds the carbonates of the alkalies, alkaline earths, and iron,
636 A TREATISE ON METAMORPHISM.
and with this expectation analyses of waters of springs fully accord. With
the carbonates are also salts of other acids, such as hydrochloric, hydro-
sulphuric, and others, and also various salts of other subordinate metals.
From the foregoing it appears that during the circulation of water in
the belt of cementation the processes of precipitation and solution are selec-
tive. Quartz and the hydrous silicates are the dominant precipitates.
Carbonates of the alkalies and alkaline earths are the dominant salts which
come to the surface. The above precipitations and solutions are precisely
what should be anticipated from the laws of chemical precipitation given
(pp. 118-123). The compounds which, on the average, are thrown down
to the greatest extent are those which are least soluble and most abundant.
The compounds which are retained in solution to the greatest extent are
those which are most soluble and least abundant. However, of the more
soluble and less abundant compounds a portion is precipitated. The
conditions under which we should expect partial precipitation of these
compounds are those of lessening temperature and pressure. These are the
conditions of the ascending columns of water. It has already been seen
that the ascending columns are likely to be the main water’ channels.
Hence is explained the precipitation of the comparatively soluble carbon-
ates of the alkaline earths and other minerals which form in the trunk
channels.
By these various processes the larger openings are filled with deposits,
some of which contain a sufficient quantity of valuable minerals to
constitute ores. But where one large opening contains valuable minerals
in sufficient quantity to be of service to man, millions are filled with quartz,
hydrous silicates, calcite, dolomite, and other gangue minerals. It thus
appears that the deposition of much of the ores is but a special phase of
a general geological process of great consequence. This idea will be fully
developed in Chapter XII.
DIFFUSION.
Other factors besides those already considered enter into the cemen-
tation of openings. Of these, diffusion is important. Before considering
the influence of diffusion, it is necessary to recall some of the points
already developed. After the water enters the belt of cementation it first
has a downward movement, and usually later an upward movement.
RELATIONS OF DIFFUSION TO CEMENTATION. 637
Superimposed upon these vertical movements are lateral movements.
During the downward course under normal conditions the temperature and
the pressure steadily increase; during the upward course they steadily
decrease. But there are irregular variations in temperature and pressure
due to orogenic movements and to igneous intrusions. Increase of tem-
perature and pressure increases the capacity of water to hold material
in solution. The amount of material required to produce saturation is
therefore exceedingly variable for different portions of the underground
circulation. Equilibrium in the solutions extending between two places
depends not upon the absolute quantity of material contained, but upon
the relative approach to saturation at the various intermediate points.
Material migrates by diffusion from places where the approach to saturation
is nearest to places where it is less near, and this might be from a cool
dilute solution to a warmer stronger solution. The conditions of equilibrium
are therefore not those of uniform concentration, or in many cases even
approximately so, for often the temperature and the pressure are different
in horizontal columns from those in vertical columns, and vary greatly
in vertical columns if they are of great altitude. Low temperatures
and pressures obtain near the surface, and high temperatures and pressures
obtain at depth. Consequently as depth increases more material may
be held in solution before reaching a state of saturation; and because
of this the conditions of equilibrium are those of greater concentration
in the lower than in the upper parts of the columns. Moreover, the exper-
imental work of Barus,“ showing the wonderful dissolving power of water
on glass at high temperatures and pressures, but still at temperatures which
obtain in the lower part of the zone of fracture, shows that the differences
in the amount of material held in solution may be very great—may, indeed,
vary by ratios as great as 1: 100 or 1: 1,000.
From the foregoing it follows that if there were uniform concentra-
tion, with variable temperatures and pressures in the underground solutions,
the material would diffuse from high levels to low levels and thus produce
equilibrium. Where the currents are moving downward, but with decreas-
ing slowness on account of the lateral movement of the water, which
continually carries a part off to one side (see pp. 572-576), the lower
« Barus, Carl, The compressibility of hot water and its solvent action on glass: Am. Jour. Sci., 3d
ser., vol. 41, 1891, pp. 110-116.
63 A TREATISE ON METAMORPHISM.
part of the column, because of its contact with the rocks for a greater time,
would be more likely to be near the point of saturation than the higher
parts; and thus there is a tendency for upward diffusion against the current.
Diffusion is so slow that this movement might be thought to be of so little
consequence as to be negligible; but it has been seen that the average
downward movement of water in the belt of cementation is exceedingly
slow, the rate of movement frequently not being more than a meter per
annum. Therefore it is rather probable that upward diffusion against the
downward-moving currents is a matter of consequence in maintaining the
approximate equilibrium of the underground solutions, and therefore in
promoting continuous and uniform precipitation throughout the belt.
During the upward movements which are characteristic of the later
parts of the journey of the water currents diffusion may also work against
the movement. This would be true if at the place where the upward
movement began saturation had not been attained. At some higher point,
as a result of the lessening temperature and pressure, saturation would be
reached, and thus the coefficient of saturation would be higher. Hence
there would be diffusion downward, or from places where there is less
material in solution to places where there is more material in solution.
So far as the lateral movement of water is concerned, if variations in
temperature and pressure be ignored, there is diffusion from areas of greater
concentration to areas of less concentration. But the farther the water has
gone, and the longer therefore it has been in contact with the rocks, the
nearer are the solutions to saturation; hence there is a tendency for diffusion
to take place against the currents. But where the temperature and the
pressure are unequal, due to igneous rocks or orogenic movements or both,
this condition of affairs may be reversed.
This factor of diffusion in connection with movements of the under-
ground currents apparently explains some of the anomalous features of the
belt of cementation. The downward movement may be presumed to be
generally so slow that diffusion keeps the solutions approximately at
equilibrium, and the process of cementation goes on subject to the laws of
the expansion reactions and selective precipitation. But at places where
the downward currents move so fast that diffusion does not maintain
RELATIONS OF DIFFUSION TO CEMENTATION. 639
equilibrium by movement of the dissolved material in a direction against
the current, the descending waters continuously take material into solution;
or, if not that, at least deposit little or no material; and thus a formation
or an area in a formation below the level of ground water may not be
cemented or indurated. In this manner may be explained the streaky
cementation and induration of sandstones and quartzites, and very frequent
preponderant solution below the level of ground water where there are
strong descending currents (see p. 604). In places where the downward
movement was slow and regular, cementation and induration are in an
advanced stage or complete; while at places where there was rapid descend-
ing movement of the waters there was little deposition, and the material
is but feebly or not at all cemented.
Besides the greater movements of diffusion connected with general
ewrents of ground water, diffusion is unquestionably of great importance
in the short movements of the ground water in the subcapillary openings
between the single mineral particles or aggregates of mineral particles
and between the adjacent capillary and supercapillary openings in which
the main water currents travel. In the subcapillary openings, during the
metasomatic processes, the water becomes saturated with material. Even
if the water within the subcapillary openings itself does not move, the
material in solution slowly migrates by diffusion toward the larger open-
ings containing circulating waters, and vice versa. Therefore the materials
of the small openings and those of the main water currents meet. The
latter may carry not only recently acquired material, but material obtained
long before, perhaps at great distances, or even from the belt of weathering
above. Asa result of this meeting selective precipitation takes place.
CONCLUSION.
From the foregoing pages I conclude that cementation is caused by
the expansion reactions, by contributions from magmas, by selective precipi-
tation, and by diffusion, and that these processes are adequate to explain
the process, notwithstanding that the waters issuing from the belt of cemen-
tation contain large quantities of material in solution. In the actual precip-
itation in the openings of rocks these different factors work together, not
separately.
640 A TREATISE ON METAMORPHISM.
The material precipitated is derived from three sourees—that carried by
the waters passing from the belt of weathering into the belt of cementa-
tion, that contributed by igneous emanations, and that which passes into
solution within the belt of cementation. While at any one time the belt of
weathering may not be of great thickness, as a consequence of denudation,
there is steady addition of new material to this belt, and therefore a con-
stant supply of material. If all of this material could be deposited in the
belt of cementation it would undoubtedly be adequate for the work. The
same is true of the material which passes into solution within the belt of
cementation itself. But doubtless if the belt of cementation had only one
of these sources, cementation would be very imperfect. It is only by the
combination of the important sources of material that an adequate supply
is obtained to furnish the issuing solutions the abundant materials which
they carry and yet leave a sufficient residuum for cementation,
METASOMATISM.
DEFINITION.
Metasomatism may be defined as the process by which original minerals
are partly or wholly altered into other minerals, or are replaced by other
minerals, or are recrystallized with or without mineral changes, or one or
more of these together. In the alteration of a mineral into other minerals,
or in its replacement by other minerals, there may be addition or subtraction
of certain constituents. As a result of the changes, the new rock may gain
or lose variety in the minerals composing it. Either by the substitution of
constituents, or by the loss of constituents, and therefore concentration of
other constituents, a resultant rock may have a predominant mineral.
EXTENT OF PROCESS.
In rocks changed by metasomatism under mass-statie conditions all
stages of alteration may be seen, from comparatively fresh rocks, in which
the changes are incipient in the minerals most readily alterable, to those
rocks in which all alterable minerals have been transformed by metasomatism
into others which are permanent under the prevailing conditions.
Throughout extensive areas important formations are so altered that
no original mineral remains. That the recrystallization of great masses of
EXTENT OF METASOMATISM. 641
sedimentary and igneous rocks may go far toward or quite to completion
under mass-static conditions is so well known that the fact need not here
be emphasized. One of the best illustrative American localities is that of
the Keweenawan series of the Lake Superior region. As shown by
Pumpelly® and Irving,’ the more porous lavas of this series have in many
cases largely recrystallized. The less porous ones show extensive alterations.
In various regions even great dense igneous masses have been profoundly
affected or completely recrystallized throughout by metasomatie change.
However, as subsequently seen, the rocks which have recrystallized
under mass-static conditions are easily discriminated from those which have
been recrystallized under mass-mechanical conditions.
CONDITLONS FAVORABLE TO METASOMATISM.
Since the changes are produced mainly through the medium of water,
the presence of a considerable amount of water, and especially conditions
favorable to its circulation, are very favorable to metasomatic changes. It
follows that in proportion as the rocks are porous metasomatism is likely to
be rapid. But if there be sufficient time, in order to produce profound
changes, there is no necessity for circulation of ground waters beyond
that necessary to provide water for the development of the hydrated
minerals which form in the belt of cementation so as to keep intact the
minute amount of water in the small openings. If these conditions obtain,
a very small amount of water may be the medium through which the rocks
are completely altered and recrystallized. Even the subcapillary openings
may be penetrated by solutions and the unstable minerals transformed
throughout. But in dense rocks under mass-static conditions, where the
spaces bearing water are all subecapillary, the changes are exceedingly
slow. Even in the pre-Cambrian rocks, in the larger dense masses, such
readily alterable minerals as nepheline and olivine are found. Olivine
occurs extensively in the pre-Cambrian rocks of the Lake Superior region.
Throughout extensive -masses in this region the less alterable minerals,
augite and basic feldspar, are apparently almost pertectly fresh. The
«Pumpelly, Raphael., Metasomatic development of the copper-bearing rocks of Lake Superior:
Proc. Am. Acad. Arts and Sci., vol. 13, 1878, pp. 253-309.
bIrving, R. D., The copper-bearing rocks of Lake Superior: Mon. U. 8. Geol. Survey, vol. 5, 1883,
pp. 87-91.
MON XLVII—04——4]
642 A TREATISE ON METAMORPHISM.
small masses of dense rocks, especially those which are in the midst of
porous rocks, are much more altered.
Porosity is also favorable to metasomatism because the necessary space
is available for the expansion reactions of the belt of cementation without
lifting the rocks. This necessity for more space is an important restraining
factor in the alteration of the dense rocks.
In so far as the minerals are in a state of strain, this is favorable to
solution and redeposition, and therefore, in proportion as this condition
obtains, metasomatism is rapid. In so far as the mineral particles are finely
divided, they furnish a large area for the attacking solutions, and therefore,
in proportion as this condition obtains, metasomatism is rapid. Other
things being equal, the higher the temperature the more rapid is metaso-
matism. Since normally the temperature increases 1° C. for 30 meters in
depth, metasomatism is likely to be much more rapid in the middle and
lower parts of the zone of fracture than in the higher parts. The process
of metasomatism may be greatly promoted by increase of temperature due
to intrusives, as explained on pages 648-649.
Metasomatism is rapid in proportion as the rocks are composed of
readily alterable minerals. Of these leucite, nephelite, olivine, ete., are
readily alterable; the pyroxenes, amphiboles, and micas oceupy an inter-
mediate position; and quartz and the acid feldspars are very slowly
alterable under the conditions of the belt of cementation, although, as has
been explained in a previous chapter (see p. 519), the feldspars are some-
what readily attacked in the belt of weathering.
Finally, the extent to which metasomatism has taken place is a direct
function of the age of the rocks. The older a formation the more likely
are the changes to be far-reaching.
MINERALS PRODUCED.
The dominant minerals produced by metasomatism are the same as the
cementing minerals (see pp. 621-627), and this is what should be expected;
for it is to be remembered that the solutions pass through the rocks slowly,
and that the minerals deposited in the openings and those deposited in the
body of the rock are alike the joint result of the reactions between the
solutions and solids.
METASOMATISM. 643
If this be so, it can not but follow that the minerals precipitated from
the solutions in openings will be essentially the same as those produced by
the changes within the mineral particles, although their relative proportions
may be very different; for it is certain that the changes in the body of the
rock are largely accomplished by solution and redeposition, although the
material may be deposited very close to the place at which it was taken
into solution.
GROWTH OF LARGE INDIVIDUALS WITH PRESERVATION OF TEXTURES.
One of the most distinctive features of metasomatism in the belt of
cementation is the growth of large mineral individuals with the preservation
or emphasis of original textures and structures.
The formation of large individuals is a result of the physical-chemical
law explained on pages 74-76, under which large individuals form at the
expense of smaller ones. In rocks altered
by metasomatism in the belt of cementation
the more than average growth of certain
individuals may be recognized in the very
unequal size of the mineral particles and in
the enlargement of the old individuals. The
general unevenness in the magnitude of
mineral particles in rocks altered by meta-
somatism in the belt of cementation is so well Fre. 16.—clastic quartz penetrated by serpentine.
5 z 3 After Becker.
known that the point need not be emphasized.
While certain individuals may grow to much more than average magnitude,
many others break up into very numerous smaller individuals of different
kinds; for instance, the change of feldspar to quartz and mica. (See PI.
Ill, A.) The growth of large individuals at the expense of small ones
explains the numerous interpenetrations of minerals in the recrystallized
rocks. As a result of the disturbance of equilibrium from any cause, a
change may take place. One mineral may grow. At the same time the
adjacent mineral may be dissolved. ‘The growth of one in many cases is
apparently conditioned by the solution of the other. Cases of this are the
growth of magnetite into quartz, and the secondary penetration of needles
of actinolite and serpentine into quartz. (See fig. 16.)
644 A TREATISE ON METAMORPHISM.
The enlargement of mineral particles has been described as occurring
in quartz, feldspar, hornblende, augite, garnet, tourmaline, and other
minerals, by Sorby, Becke,’ Irving,’ myself,” Williams,’ Hobbs,’ Whittle,’
and others. (See figs. 14 and 15.)
However profound the alterations of metasomatism by molecular
mechanical action under mass-static conditions, the original textures and
structures of the rock may not be greatly affected. It may be that all of
the original minerals composing a rock are completely changed and yet
the original igneous or other textures be perfectly preserved. he case is
parallel to that of petrifaction of a wood in which no particle of the woody
fiber remains and yet the textures of the original organic tissue are almost
perfectly preserved. The modifications are mainly changes of substance,
not changes of form. Thus all the textures characteristic of igneous rocks,
such as granolitic, ophitic, porphyritic, ete., may be almost completely
preserved in a rock which has altered throughout. This is illustrated by
the dolerite dikes in the iron-bearing formation of the Penokee series of
Michigan and Wisconsin. ‘These dikes in the black impervious slates are
little altered dolerites, but their continuations in the iron-bearing formation
do not contain one vestige of any original mineral, but are ferruginous,
hydrated, aluminum silicates, which in composition correspond very closely
to kaolin.” Yet the texture in the altered rock and in the dolerite is the
same.
Indeed, not only may there be no tendency to destroy textures and
structures which were originally present, but there may be a tendency to
“Sorby, H. C., On the structure and origin of noncalcareous stratified rocks: Proc. Geol. Soe.
London, 1880, p. 62.
> Becke, F., Eruptivgesteine aus der Gneissformation des niederésterreichischen Waldviertels:
Tschermaks mineral. Mittheil., vol. 5, pt. 2, 1883.
¢Trving, R. D., and Van Hise, C. R., Enlargement of quartz fragments and genesis of quartzites:
Bull. U. S. Geol. Survey No. 8, pt. 1, 1884, pp. 11-43.
d Van Hise, C. R., Enlargements of feldspar fragments in certain Keweenawan sandstones: Bull.
U.S. Geol. Survey No. 8, pt. 2, 1884, pp. 44-47. Van Hise, C. R., Note on the enlargement of horn-
blendes and augites in fragmental and eruptive rocks: Am. Jour. Sci., 3d ser., vol. 33, 1887, pp. 385-388.
e Williams, G. H., The greenstone-schist areas of the Menominee and Marquette regions of Mich-
igan: Bull. U. 8. Geol. Survey No. 62, 1890, p. 173.
f Hobbs, Wm. H., Phases in the metamorphism of the schists of the southern Berkshire: Bull.
Geol. Soc. America, vol. 4, 1893, pp. 173-176.
g Whittle, C. L., Some dynamic and metasomatic phenomena in a metamorphic conglomerate in
the Green Mountains: Bull. Geol. Soc. America, vol. 4, 1893, pp. 156-158.
Irving, R. D., and Van Hise, C. R., The Penokee 1ron-bearing series of Michigan and Wisconsin:
Mon. U. 8. Geol. Survey, vol. 19, 1892, pp. 357, 358.
RELATIONS OF METASOMATISM TO TEXTURE. 645
emphasize them. This emphasizing of old textures and structures results
from the fact that solutions work along openings and surfaces of weakness.
At any place in which water is present in more than the average amount,
or is more than usually active, there may be greater than average solution
and deposition, and thus emphasis of the old texture or structure. Common
cases are the emphasis of perlitic cracks and bedding planes.
However, where large individuals alter to many small particles, or
undergo a secondary enlargement with needle-like terminations, or are
altered in various other ways, the original textures may become much less
definite than they were originally, although the process of modification
rarely goes so far as to obliterate original textures.
As a result of the preservation or emphasis of original textures and
structures during metasomatism in the belt of cementation, it may happen
that somewhat extensive changes in a rock are\ overlooked or ignored.
Those who are most familiar with the recent little modified rocks are
inclined to explain the phenomena they see in them as original. Those
who have been working among the ancient and therefore more modified
rocks are inclined to explain similar phenomena as the result of alteration.
In each case the phenomena must be studied in the field and in the labora-
tory, taking into account all the evidence, in order to ascertain the actual
truth; for it is certain that such phenomena as amphibole surrounding
pyroxene cores and pegmatitic textures may be due to primary crystalliza-
tion or to secondary alteration, and the appearance in the two cases be
much the same, if not identical.
SEGREGATION OF INDIVIDUAL MINERALS.
Under exceptional conditions, by alteration or substitution or the two
combined one mineral may very largely or wholly replace the minerals
previously occupying a certain space. The space thus taken may vary
from an insignificant amount to a considerable area for the entire thickness
of a formation. In the latter case the process of metasomatism is often
given a name dependent upon the mineral which replaces the other
minerals, or that into which the other minerals alter. If, for instance, the
segregating compound be silica, the process is called silicification; if it be
serpentine, it is called serpentinization; if it be chlorite, it is called
chloritization, ete.
646 A TREATISE ON METAMORPHISM.
Probably the most common of the various segregations is that of
silica. This perhaps most frequently takes place in limestones, pro-
ducing chert or quartz masses. According to Hinde,” silicification has
extensively occurred at Spitzbergen and Axels Island, where formations
once largely carbonates, aggregating 250 or more meters in thickness, are
composed almost entirely of chert.
Cases of extensive serpentinization are those of the metamorphosed
sandstones and igneous rocks in the Coast Range of California, described
by Becker, where serpentine is said to replace quartz, feldspar, and apatite
on an extensive scale.”
IGNEOUS WORK.
The igneous work of the belt of cementation is all comprised under
‘njection.”
INJECTION.
By injection is meant the penetration of a rock by a molten magma.
In the belt of cementation the injecting rocks make their way chiefly by
following fractures, such as faults, joints, bedding planes, fissility planes,
irregular fractures of brecciation, etc. The injected material therefore fills
the larger crevices, such as those produced by faulting, jointing, bedding,
fissility, or brecciation, and to some extent it may penetrate the interspaces
between the individual grains—for instance, those of the ordinary sedi-
mentary rocks. But it is probable that pure igneous injection between the
mineral particles of the dense rocks does not usually penetrate a great dis-
tance from a continuous mass of the magma. Not only do the injecting
masses utilize the openings already formed, but they force the walls apart
and extend the openings, thus making possible intrusive masses of large size.
The intrusive masses vary in size from great laccoliths many kilometers in
extent and thousands of meters in thickness, through numerous great dikes
or sills, many of which may be a hundred or more meters in thickness and
many kilometers in extent, through dikes and sills of small size, to minute
laminze between the fissile leaves or to stringers between the grains.
« Hinde, G. J., On the chert arid siliceous schists of the Permo-Carboniferous strata of Spitzbergen:
Geol. Mag., new ser., dec. 3, vol. 5, 1888, pp. 241-251. Reviewed in Am. Jour. Sci., 3d ser., vol. 36,
1888, p. 73.
» Becker, G. F., Geology of the quicksilver deposits of the Pacific slope: Mon. U.S. Geol. Survey,
vol. 138, 1888, pp. 122-125.
WORK OF INJECTION. 647
Injection by occupying the openings of the rocks indurates them. In
these respects the process is similar in its results to cementation. The
injection magma is a solution which crystallizes. This is a freezing of the
solution. (See p. 113.) In cementation the solids crystallize from the solu-
tions. It is believed, as will be seen (pp. 723-728), that the process of
injection passes by gradation into cementation. The forms which injec-
tions take are often similar to those of the veins produced by cementation,
and rock masses formed by either process may have the same relations to
the previous structure of the rock.
Injecting materials comprise magmas of all kinds. Injections follow
parallel and irregular fractures alike. Where the injections are in plane
parallel openings, such as those of faults, joints, and fissility, the rock may
take on a banded character. These bands may be large and wide apart, as
is often true of imjections along faults; smaller and closer together, as is
frequent along joints; or very narrow and very close together, as is occa-
sionally the case along planes of fissility. The banded rocks produced by
injection may have an added complexity, due to the fact that many of the
dikes follow one or more sets of fractures diagonal to the previous structure
as well as parallel to it. Where the injections are in rocks with original
irregular fractures, such as brecciation, this results in great complexity of
structure.
The Sierra Nevada granite furnishes an admirable example of a rock
which has been injected in a complicdted manner. This granite is cut by
several sets of intersecting joints. Many of these joints have been taken
advantage of by the later injecting granite; and in the magnificent
exposures in the region the dikes may be seen in parallel sets, intersecting
other parallel sets of dikes. One of the localities in which the phenomena
may be particularly well seen is the Yosemite Valley. Here there are at
least six sets of intersecting joints. At some places two or three of these
sets of joints have been taken advantage of by the entering material.
In some regions, after one set of fractures has been taken advantage of
by injection material, succeeding orogenic movements have produced other
sets of fractures, which have again been injected, and in some regions there
may be found evidence of several distinct periods of fracturing and injec-
tions Complicated injection is particularly likely to occur adjacent to
great laccoliths within the zone of fracture.
648 A TREATISE ON METAMORPHISM.
Not only do igneous intrusions follow the various openings of rocks,
- such as those of joints, faults, fissility, brecciation, etc., and thus close them,
but they produce a metamorphic effect upon the injected rocks. This
contact effect may be confined to a centimeter or two adjacent to the
intrusive or it may extend to a distance of several kilometers, depending
upon many factors considered below. Furthermore, the contact effect,
whether confined within narrow limits or affecting an extensive area, may
be either slight or profound.
As already stated on page 488, the contact effect of the injecting upon
the injected rock is known as the exomorphic effect, and that of the injected
rock upon the injecting rock as the endomorphic effect. As has been fully
explained on pages 489-494, the exomorphic effect is produced in two
ways—by the direct contact action of the igneous rocks due to rise of
temperature, and by indirect action through heated solutions. The exomor-
phism is ordinarily at a maximum immediately adjacent to the intrusive,
and it decreases in amount, either rapidly or gradually, with distance from
the intrusive. The change immediately adjacent to the injecting rock is
often very marked. The rocks may be greatly indurated; the colors may
be changed, ete; and these physical changes are commonly accompanied
by changes of chemical and mineral compositions.
The modifications of the intruded rocks immediately adjacent to the
intrusive rocks are commonly attributed to direct contact action, but the
change in chemical composition shows that solutions are also an important
factor in the result, and therefore that there is indirect action. Indeed, in the
belt of cementation the two are never independent, and it is impracticable
to separate direct and indirect exomorphism and measure their quantitative
importance. While this is true, it will be seen that contact metamorphism
in the belt of cementation is only in small measure direct, and in very large
measure indirect.
As fully explained on pages 490-494, in the belt of weathering the
exomorphic effect is accomplished through gaseous solutions; in the belt
of cementation the exomorphice effect is accomplished mainly through liquid
solutions. In the latter belt, as in the belt of weathering, the solutions are
modified in two ways—by the actual emission of water and associated
materials from the igneous rocks to the solutions, and by the simple
CONDITIONS FAVORABLE TO INJECTION. 649
heating of the solutions by the igneous rocks. In the belt of weathering
the range of action of the gaseous solutions is somewhat limited. (See
pp: 480-431.) In the belt of cementation the range of action of the liquid
solutions may be far more extensive and the total result vastly greater.
Indeed, I believe it would be difficult to overestimate this indirect effect.
The solutions are the great agents which permeate all the openings of
the rocks, great and small, travel great distances from the source whence they
derived their heat and dissolved materials, and in their journey everywhere
alter the rocks.
To illustrate, the waters of hot springs, such as those of the Yellowstone
Park, derive their heat from igneous rocks at considerable distances from
the points where the waters issue, and throughout the journey the activity
of these solutions is very greatly enhanced, and therefore exomorphism is
greatly accelerated.
The amount of contact action and the distance to which it extends
depend mainly upon (a) the porosity of the intruded rock, (b) the efficiency
of the water circulation, (¢) the composition of the intrusive and intruded
rock, (d) the size of the intrusive masses, (e) the length ef time of intrusion,
(f) the temperature of the intrusive, and (@) the depth of the intrusion.
(a) A most important factor in the extent and amount of contact
metamorphism is the porosity of the intruded rock. If the rock be
nonporous the contact effect is usually limited to a somewhat narrow belt
adjacent to the intrusive. If the intruded rock be broken by faults, joints,
planes of fissility, and especially if the individual masses between the larger
openings are porous, as in the case of sandstones, the contact effect for a
given intrusive mass may extend many times farther and be many times
more effective than in a nonporous rock.
Chemical change is greatly facilitated by the perviousness of the
intruded rock. A porous rock, such as a sandstone or a fissile rock, has
openings in which may be deposited great additions of materials different
from the unmodified rock, and thus change its chemical composition. For
instance, a quartzose sandstone may have large additions of feldspar or
amphibole, as in the Wausau district of Wisconsin. On the other hand, an
impervious rock, such as a mudstone or shale, may be but little changed, as
the solutions can not readily penetrate this rock and carry with them
650 A TREATISE ON METAMORPHISM.’
constituents of a different composition. Of course an impervious rock may
be considerably changed in chemical composition immediately adjacent to
the intrusive, but the change is not likely to extend far. (See e.)
(b) If circulating underground waters are in rapid motion in the
intruded rock—and this of course depends, among other things, upon the
porosity—the contact effect will extend widely and the alterations be great,
for the rapidly moving percolating waters derive both heat and soluble
material from the igneous rocks, or the heated rocks adjacent to them, and
this material and the heat are carried far and therefore produce great
effects.
(c) The amount of contact action depends upon the compositions of
the intrusive and intruded rocks. The general law in this connection is that
the exomorphism and endomorphism tend to make the rocks approach each
other in chemical composition. Therefore the amount of change is likely
to be great in proportion as the difference in composition of the intrusive
and intruded rocks is great. If they be practically the same in chemical
composition, the interchange of constituents through the solutions is not
likely to greatly modify the composition of either. But if, on the other
hand, the two rocks differ greatly in composition, each will be likely to gain
constituents from the other and be modified,in chemical and mineral compo-
sition. Each furnishes to the other materials in which it is deficient.
Thus, if an intrusive rock in which feldspar and bisilicates are abundant,
but in which quartz is absent, intrudes a rock rich in quartz, but one in
which feldspar and bisilicates are absent, the intruded quartzose rock will
be likely to gain feldspar and bisilicates, while the intrusive rock will be
likely to gain silica, and may gain an amount sufficient to cause the silica
to separate as quartz. An excellent example of the addition to the intruded
rock from the intrusive is furnished by the adinoles and spilosites of the
Crystal Falls district of Michigan described by Clements.* These metamor-
phosed slates contain very much more potassium than the average slate of
the district. The igneous rock is rich in potassium and this element, there-
fore, must have been derived from that source.
In producing the above effects mtrusive rocks are likely to cause
important change in proportion as they are mobile and in proportion as
they contain a large amount of occluded water. Indeed, to a certain extent
«Clements, J. Morgan, A contribution to the study of contact metamorphism: Am. Jour. Sci., 4th
ser., vol 7, 1899, pp. 85-90.
CONDITIONS FAVORABLE TO INJECTION. 651
these two factors are mutual; for the greater the amount of occluded water
the greater the mobility of the magma. The greater the mobility the more
extensive the intrusion is likely to be and the more minute the openings
which may be entered. Further, magmas which contain a large amount
of occluded water at the time of crystallization emit this water loaded with
salts, and this material is added to the intruded rocks and increases the
exomorphism.
(d) Other things being equal, the greater the mass of the intrusive the
farther its effects are likely to extend and the more potent they are likely
to be. While there isa tendency for the metamorphism to be proportional
to the size of the intrusives, the small metamorphic effect upon the adjacent
rocks which large intrusive masses, such as the great dikes or even the
laccoliths, frequently show is astonishing. In many cases the intruded
rocks seem to be scarcely modified at all by masses of the largest size in
the belt of cementation. An excellent illustration of great intrusive masses
with little or no apparent metamorphism is furnished by the extensive sills,
from 10 to 100 or more meters thick, in the Animikie slates northwest of
Lake Superior. But, as explained below, comparatively small masses of
intrusive rocks may, for various reasons, result in profound modification.
(e) An important factor in the extent of metamorphism is the length
of time through which flow continues along a certain opening. If mag-
matic flow continues long through an opening, the quantity of passing
magma is great and the contact effect is likely to extend far and to be
profound. This is a general consequence of the large amount of heat and
emanations which may be furnished to the surrounding rocks and solutions
by the long-continued intrusions. Long-continued flow of lava to higher
levels or to the surface may take place through small openings, and a large
intrusive mass may quickly form. Therefore a comparatively narrow
igneous mass may be accompanied by profound contact action, because
the changing mass furnishes to the intruded rock a continuous supply of
heat and material to the solutions for a long time; while accompanying
great masses of igneous rocks which simply occupied an area, compara-
tively little contact action may occur. The latter case is illustrated by the
Henry Mountain laccoliths* and the great dolerite sills, some of them
«Gilbert, G. K., Report on the geology of the Henry Mountains: U. 8. Geog. and Geol. Sury.,
Rocky Mt. Region, 1880, pp. 51-98.
652 A TREATISE ON METAMORPHISM.
hundreds of feet in thickness and many miles in extent, in the Animikie
series of the north shore of Lake Superior.
(f) The higher the temperature of the intrusive the greater the
amount and extent of the contact effect. The temperatures of lavas as
they reach the surface vary greatly, ranging from 700 to 1,100° C.?
Other things being equal, it is evident that those magmas which have the
higher temperatures are more effective in metamorphism, for two reasons.
In the first place, they furnish a larger amount of heat to the surrounding
rocks and solutions; and in the second place, they have greater mobility
and therefore are more likely to extend far and enter minute openings.
(g) Other things being equal the greater the depth of the intrusion
the greater the contact effect. One of the chief reasons for this is that
rocks are very poor conductors of heat, and therefore the direct and
indirect effect upon the intruded rocks continues for a longer time with a
gradually broadening zone of action. Indeed, it is highly probable that
the larger masses of the intrusive rocks in the middle and lower parts of
the belt of cementation, of Tertiary age, have not yet cooled. The solu-
tions of very greatly increased metamorphic efficiency on account of this
heat have been at work for millions of years.
Finally, the pressure is much greater where the intrusives are deep
seated, and this promotes metamorphism, This matter of the increase of
the metamorphic power of intrusives with depth can not be too strongly
emphasized. Indeed, in cases of widely extended and profound exomorphie
effect the intrusives are almost invariably deep seated. As the depth of the
intrusives increases, we gradually pass from the belt of cementation, which
we are now considering, to the zone of anamorphism, to be considered in
the following chapter. It will there be seen (pp. 711-736) that in that zone,
where the intrusives are of still greater depths than in the belt of cemen-
tation, the contact effects are of the most profound character. But it can
not be too strongly insisted upon that these effects are not mainly due to
the direct heating effects of the intrusive rock, but to the indirect effect
upon the solutions in heating them and furnishing materials to them.
I repeat, the solutions are the important direct agents which produce
far-reaching and profound metamorphism in the belt of cementation.
«Lawson, A. C., The laccolitie sills of the northwest coast of Lake Superior: Bull. Geol. and Nat.
Hist. Surv., Minnesota, No. 8, 1893, pt. 2, pp. 24-48.
> Geikie, A., Textbook of geology: Macmillan & Co., London, 1893, pp. 225-226.
COMBINATION OF PROCESSES. 653
COMBINATIONS AND RELATIONS OF MECHANICAL WORK, CHEMICAL WORK,
AND IGNEOUS WORK.
For the sake of clearness of analysis, each of the mechanical processes
of consolidation—strain within the elastic limit, and fracturing; the chemical
processes of cementation and metasomatism; and the igneous process of
injection—has been treated separately. As a matter of fact, any one of
the processes may be dominant in an individual case, as, for instance, the
cementation of a sandstone to quartzite. Commonly, however, they do not
occur independently in the rocks, but two or more take place simultaneously.
The joint work of different combinations will be briefly considered in order
to make clear the interaction of the processes.
The mechanical, chemical, and igneous processes often occur together,
one frequently promoting the other. The chemical processes are promoted
by the mechanical processes. Fracturing gives water a much larger surface
of contact; it promotes water circulation; it allows easy injection. The
mechanical processes induce a state of strain in the mineral particles where
it does not fracture them. Therefore fracturing very notably promotes
cementation, metasomatism, and injection. Indeed, in the belt of cementa-
tion, where the rocks are massive and originally contained no openings,
cementation, metasomatism, and injection would take place with difficulty
and slowly if it were not for mechanical action.
It has been explained that the processes of oxidation, carbonation, and
hydration, characteristic of the belt of cementation, can take place only
with expansion of volume. Where there are no openings available, in
order that these reactions shall occur, it is necessary that much material
shall be dissolved to compensate for this expansion; and in the massive
rocks the water circulation is so slow that the material can be dissolved and
transported from the belt of cementation only very slowly indeed. This,
therefore, is the explanation of the small amount of alteration in the belt
of cementation where the rocks are not porous and have not been fractured.
However, where the massive, originally nonporous rocks have been
fractured, the fracturing increases the total volume of the rocks by the
amount of the openings produced. Later the openings are closed by
cementation, metasomatism, and injection, and thus there is absolute
increase in the volume of the material. This increase has been estimated
654 A TREATISE ON METAMORPHISM.
by Shaler, as already noted (p. 127), to be 3 to 5 per cent of the superficial
area at certain places along the New England coast,* and this corresponds
to an increase in volume from 0.52 to 1.12 per cent. Where one set of
openings which originally existed or has been produced by fracturing has
been closed by cementation, metasomatism, and injection, a new set of
ruptures may form, thus further increasing the volume of the rocks, and
these openings may again be closed by cementation. Of course, in such
rocks as were originally very porous—for instance, the sandstones and
amygdaloids—the alterations of the belt of cementation may take place
with no gross expansion of the volume of the rocks, the necessary spaces
being found in the original openings.
The conditions favorable to cementation and metasomatism are usually
the same; therefore the two processes naturally occur together. In some
cases, such as cementation of a pure quartz sand, cementation may take
place without marked metasomatism; but this is exceptional. Metasoma-
tism is almost invariably accompanied by cementation, and usually where
the former process is important the other is sure to be of consequence.
Where injections occur it has been seen that, besides closing the openings,
an extended and profound exomorphic effect is produced. Further, it has
been seen that this is especially likely to be true when the exomorphism is
indirect or the work of solutions. But the alterations accomplished by the
solutions as an agent are cementation and metasomatism. Therefore, where
injection is an important metamorphosing influence, cementation and meta-
somatism are exceedingly active and widely extended. The underground
waters are heated. These heated waters dissolve a large amount of the
material of the injected and injecting rock; they receive a further contribu-
tion furnished by the solutions emanating from the injected rock. Solu-
tions thus enriched travel for a greater or less distance, but ultimately a
portion of the material is deposited, thus cementing the rocks. Just as
injection promotes cementation it promotes metasomatism, and for the same
reasons. The solutions are heated and rich in mineral materials, and there-
fore the mineral particles present in the injected rock are modified. It
follows from the foregoing that the chief indirect exomorphic effects are
accomplished by cementation and metasomatism.
«Shaler, N. S., The crenitic hypothesis and mountain building: Science, vol. 11, 1888, p. 281.
COMBINATION OF PROCESSES. 655
Irving further pots out that, after an igneous rock has solidified, as
it continues to cool it contracts, and therefore at the contact of the igneous
rock with the intruded rocks openings are likely to be produced. Fre-
quently the contraction openings are not confined to the contacts, but
extend into both the injecting and the injected rock. Such openings, being
within the intrusive rocks or near the contacts of the intrusive and intruded
rocks, furnish trunk channels which are likely to receive solutions from both
the injecting and the injected rocks. Thus are explained the very frequent
trunk channels for circulation and vein formation within and adjacent to
igneous rocks, The openings along the borders of igneous rocks and
within them are of very great consequence in connection with ore deposits.
(See p. 1116.)
Concluding, it is clear that while one or two of the processes of cemen-
tation, metasomatism, and injection may occur without the other one or
two, it is rarely that one works alone. Of the processes, that which is most
likely to occur without the others is cementation. Metasomatism rarely, if
ever, takes place without cementation. Cementation and metasomatism
may take place on an extensive scale without injection. Injection is invari-
ably accompanied by cementation and metasomatism.
CHANGES CF CHEMICAL COMPOSITION.
The amount of change in the chemical composition of the rocks in the
belt of cementation is likely to vary with the porosity. Where the rocks
are dense, chemical analyses appear to show that the average composition
of the rocks does not greatly change, except by oxidation, carbonation, and
hydration. The oxygen and carbon dioxide are added directly or indirectly
from the atmosphere, and the water from the hydrosphere. In the opening
pages of Chapter II it has been explained that all chemical changes involve
molecular mechanical action, even where the average chemical composition
remains the same. There may be interchange on a great scale between the
minerals within short distances (for mstance, glasses may wholly deyitrify),
but in the dense rocks the migrations of material are ordinarily confined
within somewhat narrow limits and the average change in chemical
composition is not great.
«Trying, A., Chemical and physical studies in the metamorphism of rocks, Longmans, Grcen & Co.,
London, 1889, pp. 82-84.
fer)
TU
(oP)
A TREATISE ON METAMORPHISM.
In the porous rocks, such as sandstones, extrusive igneous rocks,
especially the vesicular lavas and porous tuffs, and much fractured rocks,
where there is comparatively rapid circulation of water, the migration of
materials for considerable distances may be important. The addition or
subtraction of material at any place may be large; for instance, simply
filling the pore spaces may require the addition of 20 to 40 per cent of the
volume of the rocks. (See pp. 569-570.) Also alterations extend through
the minerals originally present. Therefore the chemical compositions of the
rocks for extensive areas may be much changed. Excellent illustrations of
the transportation of material for long distances and its replacement of other
material, thus changing the composition of the rocks, are furnished by the
segregation of individual minerals, described on pages 645-646. One of the
best illustrations of extensive change in chemical composition is furnished
by the silicification of limestone formations. (See pp. 646, 816-820.)
Perhaps there is no more conclusive case of replacement than the very
humerous instances of partial and complete substitution of copper for
porphyry pebbles and bowlders, described by Pumpelly,* in the Calumet
and Hecla conglomerate. Other illustrations of replacement are furnished
by many of the ore deposits.
“Pumpelly, Raphael, The paragenesis and derivation of copper and its associates on Lake Superior:
Am. Jour. Sci. for Sept., Oct., and Noy., 1871, vol. 2, pp. 188-198, 248-258, 347-355.
Claes
THE ZONE OF ANAMORPHISM.
DEFINITION OF ZONE.
In Chapter IV the zone of anamorphism has been treated from a
physical-chemical point of view. It is there shown that this is a zone in
which there is great pressure in all directions, and that this is the dominant
factor controlling the reactions, so that changes take place which diminish
the volume of the rocks. The chemical iaw of energy is subordinate to
the dominant physical demand of pressure for reactions which lessen the
volume. Chemical reactions therefore take place with liberation or absorp-
tion of heat, as demanded by the pressure; but in large measure pressure
demands that chemical reactions take place with absorption of heat, such
as silication, dehydration, deoxidation.
It is the purpose of the present chapter to consider the zone of anamor-
phism from a geological pomt of view. The zone of anamorphism is
bounded above by the belt of cementation of the zone of katamorphism.
It has no assignable boundary below. The depth of the upper surface of
the zone of anamorphism is very variable, depending upon many factors,
among which the strength of the rocks and the speed of deformation are
very important. It is not practicable to assign a minimum limit to this
depth, but it is highly probable that in many cases the upper surface of this
zone is not at greater depth than 1,000 to 2,000 meters, for rocks have been
modified by the reactions characteristic of the zone which have apparently
not been buried to a greater depth than this. By making assumptions so
as to give a maximum depth for the belt of cementation, the upper surface
of the zone of anamorphism is calculated to be at a depth not greater than
10,000 to 12,000 meters.” (See pp. 189-190.)
“See Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept.
U. 8. Geol. Survey, pt. 1, 1896, p. 593.
,
MON XLVII—04——-42 657
698 A TREATISE ON METAMORPHISM. ~
It has been said that no lower limit can be given for the zone of anamor-
phism. The alterations certainly obey the physical laws of the zone of
anamorphism to the depth at which observation is possible as a result of
deformation and denudation. How the rocks behave at greater depths is
only to be judged by inference. So far as one can foresee the action of
the laws of energy, it appears highly probable that the reactions of the
zone of anamorphism extend to a very great depth. But in this treatise
the zone of anamorphism is restricted to the lower part of the lithosphere
of which we have observational knowledge, and, following Powell, I apply
to the unknown depth below it the term ‘‘centrosphere.”
The zone of anamorphism differs in an important respect from that of
katamorphism, in that the openings are in general those of subcapillary
size. This follows from the fact that the pressure in all directions is
greater than the crushing strength of the rocks, and if large openings could
be supposed to exist they would be closed by the flowage of the rocks.
While the general fact of the subcapillary size of the majority of the open-
ings in the zone of anamorphism is beyond doubt, it can not be stated that
somewhat larger openings do not exist. So far as there are openings
which are filled with liquid that can not escape, the above reasoning does
not apply, for the inclosed liquid successfully resists the closing of the
cavities. By general reasoning one is not able to determine how large such
liquid-filled openings are, but observation indicates that they are small, in
general so smail that they are observed only by the microscope, and in no
cases known to me are they larger.than capillary size.
One modification of the above statements should be made in reference
to openings in the rocks in which the alterations are mainly those of the zone
of anamorphism. During periods of very rapid deformation large fractures
may extend much deeper than under ordinary conditions, for whether
rupture takes place by major f ractures or by flowage is largely dependent
upon the speed of the movement. This principle can not be better illus-
trated than by the deformation of marble at the surface. Where a marble
slab is rapidly deformed at the surface, it is ruptured. Where, however, a
similar slab is laid horizontally, being supported only at the ends, so that
eravity steadily tends to bend it, the slab bends very slowly with permanent
set. This is illustrated in cemeteries by slabs long suspended in the manner
described. Julien cites a number of instances of the bending of marble
CONDITION OF WATER IN ZONE OF ANAMORPHISM. 659
slabs at the surface without rupture, the most notable being that of an
upright marble slab, 3.85 meters long, 22.8 cm. wide, and 6.35 cm. thick, in
one of the doors of the Alhambra, Spain, which, through the settling of the
material of the wall, had attained a curvature of 76.2 mm.”
From these illustrations it appears that a strong force rapidly applied
may deform a rock by fracture, whereas a very moderate force slowly
applied may deform it by flowage. This principle probably has great
geological significance in metamorphism. Where the deformation is very
rapid the zone of fracture and the alterations of the zone of katamorphism
may extend to great depth. Under such circumstances igneous injection
and cementation may go on, but as soon as the earth movements cease, or
lessen in speed, the openings are gradually closed by rock flowage under
the laws of the zone of anamorphism. Thus a great zone or belt of rocks
is alternately under conditions of the belt of cementation and under condi-
tions of the zone of anamorphism, according as deformation is fast or slow.
This subject will be further considered on subsequent pages, especially in
the section on pegmatization. (See pp. 720-728.)
CONDITION OF WATER.
In considering the condition of water in the zone of anamorphism the
temperature is of the utmost importance. It has been explained on page
566 that above a temperature of 365° C., the critical temperature of water,
water can exist only as gas. Supposing the increment of increase of tem-
perature with depth to be 1° C. for 30 meters, this temperature would be
reached at a depth of 10,950 meters, even if the temperature be 0° C. at
the surface. It has been explained that the upper surface of the zone of
anamorphism is probably in most cases at a depth not greater than one-half
of 10,950 meters. It therefore is clear that in the upper part of the zone of
anamorphism the water may be in the form of heated liquid, not as water
gas. Disregarding igneous rocks and orogenic movements, where the
critical temperature is not reached, even if the water were at temperatures
far above the boiling point at the surface, the water would be held in the
form of liquid by the pressure, as fully explained on pages 566-569. In
so far as the water is in the form of liquid, nothing further need be said
«Julien, A. A., The durability of building stones, etc.: Tenth Census of United States, 1880, vol.
10, 1884, pp. 366-367.
660 A TREATISE ON METAMORPHISM.
in reference to it, for the laws controlling its movements and metamorphic
action are precisely the same as those governing the heated water of the
belt of cementation. (See pp. 566-646.) But at depths greater than about
11,000 meters the water must be supposed to be gas. Moreover, in conse-
quence of igneous intrusions and orogenic movements, the critical tempera-
ture is probably frequently attained over extensive areas at depths much
less than this. Therefore it is highly probable that for much of the zone ot
anamorphism the water is in the form of gas, not liquid.
The general statement may be made that, for the greater part of the
zone of anamorphism, water is in the form of water gas, but that for con-
siderable portions of the upper part of the zone the water is in the form
of liquid. Thus, in this matter the zone of anamorphism is in direct oppo-
sition to the zone of katamorphism, where the water is dominantly in the
form of liquid and only exceptionally in the form of gas. .
The probable action of water gas in the zone of anamorphism should
be considered. What are the differences between the circulation and work
of water in the form of liquid at high temperatures and under great pressures
and the circulation and work of water gas at such temperatures and
pressures? At the pressures existing in the zone of anamorphism it is
certain that the gas would be dense. At the depth of 10,950 meters the
pressure per square centimeter of a hydrostatic column of water to the
surface would be 1,095 kilograms, or 1,060 atmospheres.
The density of water gas at atmospheric pressure and 100° C. is
0.0006 of the density of water at 4° C. Assuming, for purposes of caleu-
lation, that, under the temperatures and pressures given, water gas acts as
a perfect gas, H. C. Wolff has calculated for me that at a pressure of 1,060
atmospheres and at the critical temperature of water (365°), the density of
the gas would be 2.32 times the density of water at 4° C., a result almost
certainly too great, but still indicating that the gas is very dense.
In a very dense gas the molecules attract one another,” and in so far
as this is true the gas approaches a liquid. It is certain that superheated
water at high temperature has a low viscosity and remarkable chemical
power, as shown on pages 79-81, 105-110, 140-141. In so far as the water
gas in the openings of the zone of anamorphism is less dense than the liquid
“Nernst, W., Theoretical chemistry, trans. by C. 8. Palmer, Macmillan & Co., London, 1895,
p. 185.
WATER OF ZONE OF ANAMORPHISM. 661
in the openings, this would reduce its chemical activity; but the fact that
the viscosity of the gas is probably less and its temperature higher may
render a small quantity of water in the form of gas as potent or even more
potent in the transformation of rocks than a much larger amount as liquid.
QUANTITY OF WATER.
The quantity of water in the crystallized rocks of the zone of anamor-
phism is certainly small as compared with that in the belt of cementation.
It has been explained that the openings of the zone are normally of sub-
capillary size, although some of them are as large as capillary openings.
The quantity of water is therefore limited to the space which may be
furnished by these openings, unless it be supposed that under the extra-
ordinary pressures and high temperatures which prevail in this zone the
water gas finds a place between the molecules of the crystallized minerals,
and this hypothesis can not be proved. Ignoring this possibility, it is
probable that the amount of free water in the forms of liquid and gas in
the zone of anamorphism is in most cases less than 1 per cent.
CIRCULATION OF WATER.
Before the question as to the circulation of the water in the zone of
anamorphism can be satisfactorily discussed, it is necessary to answer the
question as to the probable sources of the water. The certain sources of
water in the zone of anamorphism are (a) the water mechanically contained
in the rocks at the time they passed into the zone, (b) water produced by
dehydration of hydrous minerals, and (c) water from igneous intrusions.
(a) The sediments as originally formed commonly contain large
quantities of water in their interstices. As explained on pages 124-127, this
amount by volume is frequently 25 per cent or more. By deep burial the
sediments pass into the zone of anamorphism. In most cases it is highly
probable that before this takes place the larger portion of the original
water has been lost as a consequence of consolidation and cementation.
But it is not likely that all of the water is lost, and therefore the sedimen-
tary rocks as they pass into the zone of anamorphism probably carry with
them mechanically mingled water. As to the quantity of this water, no
certain knowledge is available; but I suspect that in the majority of cases
itis small, possibly less than 1 per cent. Doubtless the rocks of the centro-
662 A TREATISE ON METAMORPHISM.
sphere which pass into the lower part of the lithosphere also contain some
mechanically mingled water, but it is rather probable that the amount is
very small.
(b) It has been noted that dehydration is one of the most important
reactions of the zone of anamorphism. Water is thus set free within the
zone of anamorphism. Dehydration is therefore a certain source of water.
It is explained elsewhere (pp. 742-744, 895-896) that in muds the com-
bined water is about 8 per cent, in shales is only about 4 per cent, in slates
about 3 per cent, and in schists about 1.50 per cent. The process of
metamorphism by which a mud is transformed to a schist requires the
squeezing out of least 6.5 per cent of water by weight. If the conclusion
be true that in general im the zone of anamorphism the quantity of free
water is less than 1 per cent, the process of dehydration is capable of
producing more water than is present at any one time. Dehydration is
therefore an important source of water. Indeed, water of hydration is
probably a main reservoir of this essential agent for the recrystallization
of the rocks in the zone of anamorphism.
(ce) It will be explained under “Injection” that igneous intrusion is one
of the important processes which modify the zone of anamorphism. It is
well known that when a magma makes its way to the surface it may contain
a considerable amount of occluded water. The amount of this water is
ereater in many cases than the amount ordinarily contained in the crystal-
lized deep-seated equivalents of these magmas. If this be so, the process of
crystallization of intruded magma in the zone of anamorphism may release
water within the zone, and thus furnish another important source of this
agent within the zone itself. How important this source of supply is, is
more or less conjectural, but I suspect that it is important, as will be more
fully explained under ‘“Pegmatization.” But it is necessary to say here
that where there are great complex intrusive masses in the zone of anamor-
phism the central masses crystallize like igneous rocks. Often pegmatites
and veins form peripherally to this central mass. In their crystallization
water is believed to be a very important agent, and it is rather probable
that this agent, or at least a large part of it, is that liberated during the
crystallization of the large igneous masses.
In addition to the above sources of water, water may have passed from
the belt of cementation downward into the zone of anamorphism, or from
CIRCULATION OF WATER. 663
the centrosphere upward into the zone of anamorphism. But it is probable,
as will be seen below, that these are not important sources of water. I
suspect that, on the average, the mechanical water and the water of dehy-
dration are the main sources of water supply of the zone of anamorphism.
My reason for this belief is that rocks which, containing large quantities of
combined water, like the sediments or the porous lavas, are readily recrys-
tallized in the zone of anamorphism, as fully explained on pages 741-748,
whereas great masses of igneous rocks in which there is a deficiency of
combined water are very frequently not recrystallized, but granulated.
The fact that massive igneous rocks containing little combined water do not
readily reerystallize is rather clear evidence that sufficient water to do this
‘work is not driven downward from the belt of cementation, or upward from
the centrosphere into the zone of anamorphism.
We are now prepared to consider the circulation of the water in the
zone of anamorphism. It has been seen that the available openings are
mainly those of subeapillary size. It is through these openings that the
water must circulate, unless it be assumed that under the high pressures
and temperatures obtaining in parts of the zone of anamorphism the water
can make its way through the intermolecular spaces of the solid crystallized
minerals. This has been held by some geologists. It has been pointed
out that water is certainly occluded within magmas; but this is a very
different thing from the existence of free water between the molecules of a
erystallized substance. While the possibility of the transmission of water
in this manner is not denied, it is a mere hypothesis; and therefore the only
openings which we know to be available for water circulation are those of
subeapillary size, with possibly a relatively few of capillary size between
the mineral particles.
The movement of the water through these openings must be slow
because of their subcapillary size. It has been explained on page 148
that in openings of this kind the attraction of the mineral particles extends
from wall to wall, and the films are strictly adherent and therefore not
free water at all. But as the temperature becomes high the viscosity of
the water becomes much less, the molecular attraction between it and the
rock is decreased, the adherent film is less fixed, and its mobility is there-
fore very greatly increased. Also, in so far as the temperature of the water
exceeds its critical temperature, it is in the form of a gas; between a gas
664 A TREATISE ON METAMORPHISM.
and the rock particles molecular attraction is not so important as between
rock particles and a liquid; therefore, so far as the water is gaseous, there
are probably no strongly adherent films in the openings. The gas, as
already explained, is very dense; gas is very mobile; therefore, so far as
the water is in the form of gas, it would be likely to very effectively make
its way from places of great pressure to places of less pressure. With a
given amount of water per unit volume the water gas would move from
places of higher temperature toward places of lower temperature, for with
a given amount of gas occupying the same volume, the higher the temper-
ature the higher the pressure.
As to what extent the water is in the form of liquid and to what extent
it is in the form of gas we know so little that n> attempt is made to discuss
the movement of the two separately. The statement below is true for
both aqueous solutions and gaseous solutions of the zone of anamorphism-
Where the term water is used in the succeeding paragraphs there is no
implication as to which state the compound is in.
Disregarding the possible transmission of water through the inter-
molecular openings, all of the subecapillary and capillary openings would be
taken advantage of at every favorable opportunity, and favorable oppor-
tunities would be furnished at times of orogenic movement. In the zone of
anamorphism during earth movements the mineral particles either are
broken and move differentially with reference to one another, or else are
recrystallized, or the two combined. In any case there is a constant read-
justment of the rock material by which the positions of the minute openings
are constantly changed. Where the movements are rapid, supercapillary
openings temporarily extend downward from the belt of cementation. (See
pp. 658-659.) During orogenic movement the pressures vary greatly
from place to place; indeed, at the same place they vary from moment to
moment. ‘This results in very variable pressure upon the water solutions
within short distances. Therefore, during orogenic movements, when the
rock material itself is in motion, the water is in most active movement.
The activity of the water in connection with deformation would be further
likely to be promoted by the fact that locally the movements would raise
the temperature above the critical point if it were not so already, and thus
the water would be in the form of water gas, and hence have the high
mobility of that form.
CIRCULATION OF WATER. 665
As to the direction of circulation for a given depth, it can be said that
the movements would be from places of greater pressure and higher tem-
perature to places of less pressure and lower temperature. It seems
probable that the water would move laterally from places of intense orogenic
movement to places of less strong orogenic movement. It seems probable
that water would move away from places of igneous intrusions, because
these produce high temperatures and pressures.
But the water movements of greatest geological interest are the
vertical movements. Does water make its way downward from the belt
of cementation or upward from the centrosphere into the zone of
anamorphism?
It has been explained that the water at the upper part of the zone of
anamorphism is under great pressure and at high temperature. It might be
supposed, under these circumstances, that the water would make its way
downward into the rocks below. This has been a favorite hypothesis of
those who have explained the water contained in magmas issuing from
voleanoes as of surface origin. The experimental work of Daubrée has
been appealed to in support of this view. Daubrée showed that water gas
at a temperature of 160° C. penetrated through a slab of fine-grained sand-
stone 2 centimeters thick, and produced a pressure upon the other side of 1.9
atmospheres (1,963.2 grains per sq. cm.).* However, it isto be remarked that
an essential point in this experiment is that the amount and pressure of water
gas are very much less upon the side of the rock to which the water makes
its way than upon the other side, for to hold water as liquid at a temperature
of 160° C. a pressure of about six atmospheres is required. If there were
no water in the zone of anamorphism there is little doubt that the water
would make its way from the belt of cementation into the zone of anamor-
phism until equilibrium were reached. he force tending to carry this water
downward is that of gravity, and is equal to the pressure of a column of
water from the bottom of the belt of cementation to the surface. But before
we can assume that this pressure is sufficient to drive the water into the
zone of anamorphism we must be certain that the pressure in that zone is
not as great as or greater than in the lower part of the belt of cementation.
We have seen that water may be present in the zone of anamorphism
from at least three sources. This water is under the high pressure of the
«Daubrée, A., Etudes Synthétiques de Géologie Expérimentale, pt. 1, Paris, 1879, pp. 236-241.
666 A TREATISE ON METAMORPHISM.
plastic rocks, and in large part above the critical temperature of water. It
is a characteristic of the zone of anamorphism that the rocks are not
sufficiently strong to sustain themselves, and that consequently no large
and continuous openings are present; therefore the imprisoned water is not
subject to the hydrostatic pressure of a column of water reaching to the
surface only, as is that of the belt of cementation, but to the weight of
the rock column to the surface, which is approximately 2.7 times as much as
the pressure of an equivalent column of water. It has been seen that in
much of the zone of anamorphism the water is above its critical temperature,
and therefore that the enormous expansive force of this gas is tending to
drive the water from the zone of anamorphism to the belt of cementation.
In the experiment of Daubrée* the water gas traveled from a place of greater
pressure to a place of less pressure. So far, therefore, as this experiment is
applicable to the case under discussion it indicates that the water would
be driven from the zone of anamorphism to the belt of cementation.
From a consideration of physical principles, therefore, we conclude that
water does not pass from the belt of cementation to the zone of anamor-
phism, but probably is driven by the superior pressure and the expansive
force of the gas from the zone of anamorphism to the belt of cementation.
This conclusion, reached by physical reasoning, is fully confirmed by
the facts of observation. It has already been pointed out that, in the zone
of anamorphism, in the transformation of a shale to a schist the amount of
combined water is reduced from 4 to 1.50 per cent. But the amount of
free water in the zone of anamorphism is only a fraction of 1 per cent.
This liberated water must have escaped either above or below. That the
relative pressures in the zone of anamorphism and the belt of cementation
are such as to make the movement upward has already been seen, and there
can be little doubt that much or most of the water freed by dehydration
goes in this direction. ‘This seems to me highly probable from the fact that
the amount of combined water in the recrystallized rocks lessens as the
intensity of the metamorphism increases. Where the rocks have been
metamorphosed under moderately deep-seated conditions, such minerals as
chlorite may be found; where the metamorphism was deeper seated, biotite
and muscovite, containing less water, are abundant; but where the
metamorphism is most profound, and where apparently the pressure has
a@ Daubrée, cit., p. 286-241.
ORIGIN OF CARBON-DIOXIDE INCLUSIONS. 667
gone to the extreme recognized by observation, muscovite and biotite may be
destroyed, with the production of such heavy anhydrous minerals as garnet,
staurolite, etc. Therefore the water apparently does not make its way
downward, or if it does it is water occluded between the solid molecules
rather than water combined with them; but as already stated, there is no
evidence that water can exist in important quantity in such positions. From
the above facts, since the amount of water apparently decreases with the
intensity of the metamorphism, or practically, disregarding local irregulari-
ties, with depth, and the freed water must escape somewhere, I conclude
that the probable general movement of the water present or produced by
dehydration and from magmas in the zone of anamorphism is upward into
the belt of cementation.
This conclusion is confirmed by other facts in connection with meta-
morphism. It has been said, and will be fully explained, that silication of
carbonates, forming silicates and releasing carbon dioxide, is one of the
chief reactions of the zone of anamorphism.* In the rocks metamorphosed
under deep-seated conditions inclusions of liquid and gaseous carbon
dioxide in the cavities are very common. When formed all of the occluded
carbon dioxide was probably a deuse gas, because above its critical tem-
perature. Under surface conditions, below critical temperature and with
the high pressures of the gaseous portions, the carbon dioxide largely
condenses to a liquid. It is a fact that waters rising from a deep-seated
source almost invariably carry large quantities of carbon dioxide. It seems
highly probable that much of the carbon dioxide brought up by the rising
deep-seated waters is a portion of that carbon dioxide liberated by the
process of silication of the carbonates. If this be true, it is further probable
that the upward movement of the carbon dioxide was accompanied by
water or water gas, for there is no reason why one should go up and the
other down.
Therefore it seems to me that the principles of physics and the facts
of observation both lead to the conclusion that, so far as the vertical
circulation of water between the zone of anamorphism and the belt of
cementation are concerned, the transfer is upward, from the former to
the latter.
«See Le Conte, Joseph, Genesis of metalliterous veins: Am. Jour Sci., 3d ser., vol. 26, 1883, p. 9
-d.
668 A TREATISE ON METAMORPHISM.
The next question of importance is the probable movement of water
between the zone of anamorphism and the centrosphere. We know nothing
as to the average comparative amount of water in the openings of the lower
part of the zone of anamorphism and in the openings of the centrosphere
below, nor do we know the pressures and temperatures to which the water
is subjected. If the amount of water in the centrosphere be as great as it
may be supposed to be from the quantity in the extrusive lavas, it is press-
ing upward toward the surface, driven by the enormous pressure and high
temperature to which it is subjected. Are the quantities of water in the
crystallized zone of anamorphism, combined with the pressures and tem-
peratures there, suflicient to give to the water an equally strong tendency
to move downward? Whether water makes its way into the zone of ana-
morphism from the centrosphere below, or moves in the reverse direction,
would depend upon the relative force of these two tendencies. Since we
know nothing of the quantity of water in the centrosphere, but little as to
the condition of the material of that zone, little of the temperatures there
prevailing, and absolutely nothing as to a change in the condition of the
material in passing from the lithosphere to the centrosphere, I do not
venture to express an opinion as to whether or not water travels upward
from the centrosphere into the zone of anamorphism. So far as the analogy
between the zone of anamorphism and the belt of cementation has any °
weight, it indicates an upward rather than a downward movement. The
permanence of the ocean might be adduced as evidence of the gradual
although slow movement of water from the centrosphere to the zone of
anamorphism, and from the zone of anamorphism to the belt of cementation,
and thence to the surface, rather than a reverse movement. But this move-
ment may be accomplished mainly by volcanism rather than by circulation
through the solid rocks.
VARIABLE MATERIALS AND CONDITIONS.
The rocks of the zone of anamorphism include various classes: (1) All
kinds of plutonic igneous rocks may be found in the zone\of anamorphism.
Indeed, the lower zone is especially the zone of the plutonic igneous rocks.
It is in this zone that the great batholiths form. Chemically they vary
from the most basic to the most acid, and mineralogically they are
correspondingly variable. (2) All kinds of sedimentary and all kinds of
voleanic rocks may pass into the zone of anamorphism by deep burying.
MATERIALS OF ZONE OF ANAMORPHISM. 669
As sediments are buried under later sediments, the earlier sediments join
the lower zone. Volcanic rocks may join the lower zone either by burying
beneath later volcanic rocks or by burying below sediments, or both
combined. During their downward movement to the zone of anamorphism
they undergo the alterations of the belt of cementation. (3) Any of the
products of the belt of weathering may be included within the zone of
anamorphism. This may follow as a consequence of the transgression of
the sea upon a weathered area and subsequent deep burial under sediments.
To illustrate, the rocks of the Piedmont Plateau of the United States and
of Brazil are disintegrated to a depth varying from a meter to 90 meters.
If the sea transgresses over these regions in the future, as it has in the past,
without very active erosion, the weathered zone may be buried beneath
later sediments, or voleanics may be poured over the weathered rocks
and thus protect them. By deep burying, as a result of volcanism or
sedimentation, or the two combined, the entire weathered zone may thus
join the lower zone. Finally, it is to be remembered that the weathered
rocks which join the lower zone by burying necessarily pass through the
belt of cementation, or have undergone the modifications of that belt at
least to some extent, before joining the zone of anamorphism.
From what has gone before it is clear that there are considerable
variations in the conditions of the zone of anamorphism. The pressure and
the temperature vary greatly, depending upon depth, deformation, and
igneous intrusions; the quantity of injection varies greatly, the igneous
material being here very abundant or preponderant and there altogether
absent; the quantity of water, while absolutely small, has a wide ratio of
range, being many times more abundant under some conditions than under
others. Since this agent, although present in small quantity, is a chief
agent of transformation, this variation in the quantity of water is very
important.
Therefore the conditions are exceedingly varied in the zone of
anamorphism. Excluding the effects of intrusives, the changes of con-
ditions generally take place rather gradually, and changing from one set
of conditions to another a considerable vertical depth or a greater lateral
distance is commonly required. In this respect the conditions contrast with
those of the zone of katamorphism. The contrast is especially marked with
the belt of weathering, where the variations in conditions are great and
670 A TREATISE ON METAMORPHISM.
sudden, but less marked with the belt of cementation, where the variations
in conditions are somewhat gradual, and therefore approximate in this
respect those of the zone of anamorphism.
WORK IN ZONE OF ANAMORPHISM.
Asin the zone of katamorphism, the work from a physical-chemical
point of view may be divided into mechanical work, chemical work, and
igneous work.
MECHANICAL WORK.
Mechanical work comprises welding, strain within the elastic limit, and
strain beyond the elastic limit.
WELDING.
Welding is the cohering of the rock particles in consequence of
pressure which brings them so close together that they are within the limit
of molecular attraction of one another. It has been explained that within
the zone of anamorphism the pressures in all directions are greater than the
crushing strength of the rocks, and that if openings could be supposed to
be produced they would necessarily be closed because of the incapacity of
the rocks to support themselves. This pressure is so great that the particles
are brought within the limits of molecular attraction, and therefore are
welded for the same reason that metals weld under proper conditions. That
this conclusion, based upon physical reasoning, is certainly true, is shown
by the invariably strong cohesion between the mineral particles of rocks
which have been in the zone of anamorphism; and recently this conclusion,
founded upon physical reasoning and observation, has been confirmed by
the experiments of Adams and Nicolson. They have ascertained that
when dry marble is deformed under pressure in all directions greater than
its crushing strength, no supercapillary openings form, and the deformed
rock has strength only a little short of that of the original marble. Since
the deformation was carried on while the rock was dry, the coherence of
the rock particles can not be attributed to the action of water, but must be
due to true molecular attraction or welding. The rock was therefore
deformed under conditions similar to those which often obtain in the zone
of anamorphism.
“Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble:
Philos. Trans. Royal Soe. London, ser. a, vol. 195, 1901, pp. 363-401.
MECHANICAL WORK. 671
In the zone of anamorphism, in consequence of pressure in all directions
ereater than the crushing strength of the rocks, and the resulting invariably
close contact of the mineral particles, the process of welding is of far greater
consequence than in the belt of cementation, although it has been shown
that in the latter belt welding does take place under favorable conditions.
(See pp. 595-597.) In short, while welding is an exceptional process in the
zone of katamorphism, it is a universal process for the zone of anamorphism.
The depth at which welding occurs is different for different substances.
Such plastic substances as coal and clay may be welded at very moderate
depth, while the strong and refractory rocks, such as quartzites, require a
very considerable depth.
STRAIN WITHIN ELASTIC LIMIT.
Within the zone of anamorphism all the mineral particles of all rocks
are in a high state of strain at all times. Where the conditions have been
quiescent for a long time and the mineral particles have readjusted them-
selves to the conditions, it may be supposed that the stresses are approxi-
mately equal in all directions, and therefore that the conditions approach
those of hydrostatics. Under such circumstances the pressure would be
the same in all directions within a mineral particle, and the only tendency
would be to elastically condense it. The strain would be one of simple
compression; or putting it in physical terms, negative homogeneous dilata-
tion.” The molecules of a given mineral particle, as, for instance, one of
quartz, are pressed together by virtue of the elasticity of the mineral, and a
given particle occupies less space than under surface conditions, where the
pressure is slight. Where the hydrostatic pressure is very great, recrystal-
lization may follow, by means of which the minerals pass into more con-
densed molecules, as is fully explained under ‘“ Metasomatism,” and
especially in connection with the development of porphyritic crystals. (See
pp. 699-705.)
Strain within the elastic limit in which the pressure is equal in all
directions is very exceptional, if it ever exists. Almost invariably the
pressure is unequal in different-directions. The difference in the amount
«Hoskins, L. M., Flow and fracture of rocks as related to structure, appendix to Van Hise, C. R.,
Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept. U. 8. Geol. Survey, pt. 1,
1896, p. 860.
672 A TREATISE ON METAMORPHISM.
of the pressure in the different directions varies from zero to the elastic
limit of a mineral particle under the conditions in which it exists. The
difference between the stresses in the different directions where they are
ereat in all directions has been called by Darwin the “‘stress-difference.”* It
has been assumed by Darwin that if a mineral particle or a rock be sub-
jected to stress in all directions, when the stress-difference between the
maximum and minimum pressure is as great as the strength of the particle
or rock at the surface, deformation by rupture will take place. But this
by no means necessarily follows. It is highly probable that a greater
stress-difference is required for deformation when the rocks are under
pressure in all directions than when the pressure in one direction is zero, as
at the surface. Hallock found’ that when a solid of a certain composition,
readily plastic under ordinary conditions, was subjected to a pressure of
6,000 atmospheres in cylinders on one side of which were placed coins and
tacks, the plastic substance, instead of flowing around the coins, pressed
them against the surface of the cylinder so as to fit it perfectly. Moreover,
the coins and the steel tacks were forced against the cylinder so strongly
that their impressions were left on the steel holder so as to be seen and felt.
From this experiment Hallock concludes that “in general for one and the
same substance, over considerable ranges of condition, the rigidity dimin-
ishes as the intermolecular distances increase.”’ Or, reversing this gener-
alization to accord with his experiment, the rigidity increases as the
intermolecular distance decreases. Now in the zone of anamorphism,
where great pressure obtains in all directions, the molecules are brought
closer together than at the surface, and probably therefore a much greater
stress-difference is required to reach the elastic limit of mineral particles
in rocks in the zone of anamorphism than at the surface. How much
greater the stress-difference must be it is impossible to conjecture.
This statement applies only to mechanical deformation. In so far as
change in form takes place through solution and deposition, or by recrys-
tallization, the above rule does not apply; and this is probably a fact of
@Thomson, W., and alt P. G., Treatise on natural philosophy, Cambridge Uniy. Press, London,
edition of 1890, pt. 2, p. 423
> Hallock, William, The flow of solids; or, liquefaction by pressure: Am. Jour. Sci., 3d ser., vol.
34, 1887, pp. 277-281.
¢ Hallock, cit., p. 278
MECHANICAL WORK. 673
most profound significance in the metamorphism of rocks, for it will be
fully explained under ‘‘Metasomatism” that in so far as mineral particles
are strained, and especially as they are under unequal strains, in different
directions, solution and redeposition is likely to accomplish deformation.
(See pp. 690-692.) The materials are dissolved and mineral particles of
different shapes are deposited in an unstrained condition. To accomplish
this it is not necessary that the stress difference shall equal or even approach
that of the elastic limit of the minerals and rocks.
We conclude that strain within the elastic limit, either with or without
a stress difference, but almost universally with a stress difference which for
much of the zone of anamorphism often exceeds the crushing strength of the rocks
at the surface, prevails throughout the zone, affecting every mineral particle, small
and great.
Where the stress difference surpasses the elastic limit of the rocks
under the conditions in which they exist, rupture may take place, and this
leads us to the next section.
STRAIN BEYOND ELASTIC LIMIT.
Strain beyond the elastic limit in the zone of anamorphism results in
disruptive deformation, as in the belt of cementation. But the ruptures
are very different in the two. During deformation in the zone of anamor-
phism every particle, small or great, takes part in deformation, and it is
this fact which gives a fundamental difference between fracturing in the
zone of anamorphism and in the belt of cementation, since in the latter belt
between the fractures are blocks, small or great, which do not take part in
the movements. In this belt but a small number—in many cases an almost
infinitesimal fraction—of the mineral particles are actually ruptured,
whereas in the zone of anamorphism strain beyond the elastic limit breaks
all particles. If a slide be made from rocks deformed in the zone of
anamorphism not one square centimeter can be found in which movement
has not taken place. Not only so, but no mineral particle has escaped the
effect of the deformation. Where strain has extended beyond the elastic
limit, under the microscope the deformed mineral particles are seen to give
evidence of the above facts by their undulatory extinction and their
granulation. (See fig. 17.)
This contrast in the nature of mechanical deformation between the
zone of anamorphism and the belt of cementation is of great. significance,
MON XLYI—04——43
674 A TREATISE ON METAMORPHISM.
and yet it has almost altogether escaped notice. What stronger mechanical
contrast can there be than escape from rupture of all but a very small
number of the minerals of a rock and the participation in fracturing of all
the mineral particles of a rock? But we shall see that in the zone of
anamorphism deformation is largely accomplished by chemical processes,
whereas in the belt of cementation deformation is mainly accompanied
by mechanical processes,
thus presenting an even
stronger contrast between
the two. (See pp.766—-768.)
The amount of granu-
lation varies greatly, de-
pending upon the minerals,
the size of the particles, and
the amount, rate, and other
(@)
Fic. 17.—Granulation of feldspar, and gradation between undulatory No, x y
extinction and granulation. (a) normal anorthosite; (b) granulated conditions of deformation.
Ee Wie gee Some minerals are much
more readily granulated than others. For instance, in many rocks contain-
ing quartz and feldspar the former mineral may be granulated while the
latter mineral is but little affected. (See fig. 18.) The granulation of a
particle may be peripheral or extend throughout
the particle. Ata stage of deformation, when the
smaller particles are granulated throughout, the
LES
D. y a
larger particles may suffer only peripheral granula-
tion; in a more advanced stage the larger particles
may be granulated throughout; and in some cases
of extreme deformation the largest bowlders of con-
glomerates are mashed into thin layers not recog-
nizable as clastic fragments, each being composed 1 18—Granulation of quartz in a
rock in which the feldspar is but
of a multitude of particles. The degree of sub- _ litle affected. (aa) granulated
quartz; (b) feldspar. After Adams.
division by granulation in cases of extreme defor-
mation is not usually realized. It should be recalled in this connection
that the volumes of particles are as the cubes of their diameters. Very
often the diameters of the granulated particles are not more than .1 the
diameters of the original fragments; rather frequently they are not more
than .01, and in some cases they may not be more than .001 of the originals.
This means a subdivision of the original particles into 1,000, 1,000,000,
GRANULATION. 675
and 1,000,000,000 particles, respectively. One of the best illustrations of
mechanical granulation is that of the anorthosite described by Adams.
Mr. S. H. Ball has compared the size of the grains of the original anorthosite
and the granulated anorthosite in two specimens furnished by Dr. Adams,
and found that, on the average, one feldspar grain of the original rock is
broken into 70,000 grains.
By granulation the volume is sure to be somewhat increased, for it is
not possible that the broken particles of a grain shall fit so closely as they
did when all were parts of one crystallographic unit. The subcapillary
openings between the particles may be very small, but they are not so
minute as to be ignored. Also, not infrequently small capillary openings
form between the granules, and these considerably increase the volume.
Miigge’ has shown that ice crystals may be mechanically deformed
by differential movement along gliding planes without the destruction of
the crystals. Adams and Nicolson* have shown by experiment that the
same process may occur in calcite crystals; and they thus largely explain
the deformation of many of the marbles. This process is essentially
mechanical strain beyond the elastic limit. In so far as gliding takes
place, deformation may occur without diminution in size of the mineral
particles, and hence it contrasts very strongly with granulation. It is
notable that in the experimental deformation of marble by gliding, the
mineral composing the rock is one in which there are numerous gliding
planes, and how important this process may be in reference to other
minerals is more or less conjectural. Doubtless it is of some consequence
in such minerals as have good gliding planes—for instance, the micas; but
probably it is of little consequence with the greater number of the rock-
making minerals, such as quartz, feldspar, the pyroxenes, and amphiboles.
CHEMICAL WORK.
~ The chemical work of the zone of anamorphism, like that of the belt
of cementation, must be considered from two points of view—the chemical
changes and the resultant processes.
« Adams, F. D., Report on the geology of a portion of the Laurentian area lying to the north of
the Island of Montreal: Ann. Rept. Geol. Sury. Canada, new ser., vol. 8, 1896, pt. J, pp. 31-85.
> Mugge, O., Ueber die Plasticitit der Eiskrystalle: Neues Jahrbuch fiir Mineralogie, etc., Jahr-
gang, 1895, vol. 2, pp. 211-228.
¢ Adams, F. D., and Nicolson, J. T., An experimental inyestigation into the flow of marble: Philos.
Trans. Royal Soc. London, series 4., vol. 195, 1901, pp. 363-401.
676 A TREATISE ON METAMORPHISM.
CHEMICAL CHANGES.
The chief chemical changes in the zone of anamorphism comprise
deoxidation, silication, dehydration, and solution and deposition.
It is commonly true that the reactions of deoxidation, silication,
dehydration, and solution and deposition do not occur separately, but two
or more together; yet, in order to clearly understand their effect, each is
considered separately.
DEOXIDATION.
Many of the sedimentary rocks contain organic material. In some
cases, at least, the deep-seated igneous rocks contain metallic iron, and
often they contain sulphides, among which bisulphide of iron is the most
important. The zone of anamorphism is deep below the surface, so that
oxygen can not get into it from the surface of the earth. Indeed, it has
been shown that the oxygen which passes downward with the surface
waters is usually exhausted before it has gone far into the belt of cementa-
tion. It follows that the solutions of the zone of anamorphism are reducing,
and the conditions those of deoxidation. Therefore highly oxidized com-
pounds are likely to be partly reduced. Ferric oxide is the most important
of such compounds. This compound may be reduced to the ferrosoferric
state, forming magnetite, or to the ferrous state, in which form it is ina
favorable condition for silication. The amount of reducing agents which
are present in the zone of anamorphism is very variable. For instance,
the sediments may have almost no organic matter or they may have a
great, even a predominant, quantity. Where reducing agents are abun-
dant little or none of the iron is likely to remain as ferric oxide. Where,
on the other hand, organic material is sparse and there are large quantities
of ferric oxide this may permanently remain in this state. Where deoxi-
dation occurs there is a decrease in volume, but in most cases the amount
is small.
In the above paragraph it is supposed that deoxidation takes place
only if a reducing agent be present. There is no evidence that the pressure
may be so great that oxygen is squeezed out because of the demand for
decreased volume, although such a reaction is theoretically possible if the
pressure were high enough.
CHEMICAL WORK IN ZONE OF ANAMORPHISM. 677
SILICATION.
In Chapter 1V (p. 168) silication has been defined as the union of silica
with bases so as to produce silicates. In that chapter the close analogy
between the silicates and the carbonates has been pointed out, as well
as one of the most fundamental facts of metamorphism, their mutual
interchange. (See pp. 173-177). In the chapters on the zone of katamor-
phism it has been seen that the carbonation of the silicates is one of the
processes of fundamental importance, and in the belt of weathering the one
which has the most far-reaching effects. It has further beeu explained
that in the zone of anamorphism this process is reversed, silica replacing
carbon dioxide of the carbonates and producing silicates. While silica in
the zone of anamorphism unites with bases not previously combined with
carbon dioxide, it is probable that silication of this kind is unimportant. It
is certain that much of the free silica which unites with bases in the zone
of anamorphism simultaneously drives off carbon dioxide. There are a
number of silicates which are formed by the direct silication of a single
mineral. ‘To illustrate: silication of calcite forms wollastonite; silication of
dolomite forms tremolite and wollastonite; silication of ankerite forms sahlite
and actinolite; silication of siderite forms griinerite. Frequently silication
requires two or more minerals to produce the new silicate. As instances,
we have the silication of rutile and calcite together, producing titanite; of
hypersthene and calcite, producing actinolite and anthophyllite; of olivine
and calcite, producing actinolite; and of dolomite and siderite, or ferrous
dolomite, producing anthophyllite. Silication usually does not occur alone,
but takes place in connection with deoxidation or dehydration, or both. As
a case where we have silication with dehydration may be mentioned the
silication of gibbsite, producing sillimanite and cyanite. A case of silication
with deoxidation and dehydration is the formation of griinerite from limonite.
But usually the instances of silication combined with deoxidation or dehy-
dration, or both, are so complex that it is impossible to state what particular
combinations of minerals are deoxidized, dehydrated, or both, and silicated
in order to produce a definite silicate.
It will be seen in Chapter LX, when rocks are considered, that this
process of silication takes place on a vast scale. Indeed, no sooner does a
carbonate pass from the zone of katamorphism to the zone of anamorphism
678 A TREATISE ON METAMORPHISM.
than silication of the carbonates, with decarbonation, takes place. In many
of the ancient formations the process of transformation has been complete;
in others it is far advanced, and it is uncommon to find any carbonate
formation that is buried in the zone of anamorphism in which the process
has not taken place to an important extent.
It has been explained (pp. 665-667) that a portion of the carbon
dioxide and water freed by silication probably escapes to the belt of cemen-
tation and thence to the surface. It will be seen on pages 970-971 that one
of the important sources of carbon dioxide for the process of carbonation
is the vast quantity which comes to the surface through ground waters.
Illustrating this is the startling case furnished by Lecoq, who says that the
mineral springs in Auvergne district annually give off 7,000,000,000 cubie
meters of carbon dioxide; and the immeasureably greater amount which
reaches the surface for the world as a whole is mainly carbon dioxide freed
by the process of silication in the zone of anamorphism. This idea, that
the carbon dioxide which reaches the surface from subterranean sources is
that derived from the carbonates produced in previous geological ages, was
first suggested by Bischof.’
However, a part of the carbon dioxide and accompanying water does
not escape, and this is believed to largely explain the innumerable cavities
partly filled with water and carbon dioxide which are so generally found
in the sedimentary rocks metamorphosed in the zone of anamorphism.
Such inclusions are comparatively rare, although not unimportant, in the
original igneous rocks. (See p. 969.) Silication is thus offered as a cause
which explains a large proportion of the liquid carbon dioxide inclusions
in the rocks.
Commonly when the silicated rocks have reached the surface, so that
thin sections may be cut, liquid carbon dioxide and water do not entirely
fill the cavities, a part being occupied by the gases of these compounds.
Doubtless in the deep-seated zone where the rocks were altered the carbon
dioxide and often the water were above their critical temperature, and were
altogether in the form of gas. Possibly if accurate measurements were
made of the volumes of water, liquid carbon dioxide, and water and carbon-
dioxide gas, and the total volume of many of the openings in a rock, so as
«Blake, R. F., and Letts, E. A. The carbonic anhydride of the atmosphere: Sci. Proc. Royal
Dublin Soe., new series, vol. 9, pt. 2, 1900, p. 159.
> Bischof, Gustav, Elements of chemical and physical geology, translated by Paul and Drummond,
927.
Harrison & Sons, London, vol. 1, 1854, pp. 237-241.
CHEMICAL WORK IN ZONE OF ANAMORPHISM. 679
to get average determinations, this might enable one to arrive at alternative
conclusions as to the pressures and temperatures under which the alterations
took place. Ifa given temperature were assumed, the pressure, and there-
fore the depth, could be calculated. If, on the other hand, pressure were
assumed, the temperature could be calculated.
Silication involves decrease in volume, varying from a very small
amount to 40 per cent or more. In the silication of the carbonates, if the
freed carbon dioxide be supposed to escape, the decrease in volume is
usually between 20 and 40 per cent, and averages fully 30 per cent. In
so far as there are cavities that are filled with the water and carbon dioxide
which have been liberated by the processes of silication and dehydration
but have not escaped, the above determinations as to diminution of volume
are too great. No estimates of the volumes of these inclusions in the meta-
morphic sedimentary rocks have been made, but the average amount would
probably be comparatively small, possibly less than 1 per cent, although
in certain of the schists and gneisses the amount would be much greater
than this.
DEHYDRATION.
It has been seen in the previous chapters (Chapters IV to VII) that
hydration is one of the most important and. characteristic of the reactions
which occur in the zone of katamorphism. By that process alone or com-
bined with others a large number of hydrated minerals are formed. The
most important of these comprise the hydrous silicates and the hydrous
oxides. The hydrous silicates include the kaolin group, the serpentine-tale
group, the chlorite group, the hydromica group, the zeolite group, and the
epidote group. The oxides include the aluminum-oxide group, of which
gibbsite is the most important, and the iron-oxide group, of which limonite
is the most important. In the zone of anamorphism all these minerals are
dehydrated; but in general one can not assert that from a definite one of
them some other definite mineral is produced, for commonly during the
time of dehydration other alterations also take place, and commonly the
materials of two or more minerals unite to produce a new mineral. How-
ever, in some cases the alteration is that of simple dehydration; as, for
instance, hematite is formed from limonite, anhydrite is produced from
gypsum, quartz is formed from opal, corundum develops from gibbsite.
By the process of simple dehydration there is usually a very considerable
decrease in volume, running from 20 to 40 per cent.
680 A TREATISE ON METAMORPHISM.
It is believed: that the process of dehydration is largely caused by
pressure. In other words, water is actually separated from its combination
and made free water by the pressure, and the freed water is squeezed out of
the rocks as water from a sponge. Undoubtedly, also, the increase of tem-
perature with depth promotes dehydration. Without exhaustive experi-
mental work it is impossible to give any quantitative estimate of the relative
importance of pressure and temperature in producing dehydration. While
it is certainly true that moderate pressures and temperatures together
rapidly produce dehydration of the more hydrous minerals, ordinarily
pressure and temperature are not sufficiently great to drive off all water.
Apparently liberation becomes more and more difficult as the water increases
in amount. Dehydration is always incomplete, and commonly does not
reduce the combined water below 1.5 per cent. (See pp. 742-744.)
It is probable, as explained on pages 665-667, that the larger part of the
freed water escapes upward to the belt of cementation, but some part of it
is confined in the altered rocks. Thus dehydration explains a large portion
of the water inclusions in the metamorphosed rocks. Of course the water
inclusions of original igneous rocks are explained differently, although in an
analogous manner. ‘The water, or a part of it, occluded in the magmas,
separates at the moment of crystallization, and such part as can not escape
is included by the crystallizing minerals.
SOLUTION AND DEPOSITION.
Solution and deposition are essential concomitants of the chemical
reactions of deoxidation, silication, and dehydration, as well as of all other
important chemical changes. Cementation, next considered, involves depo-
sition. In metasomatism, subsequently treated, recrystallization is the
change of greatest consequence, and this is accomplished almost wholly by
solution and deposition.
Whether solution is preponderant in the zone of anamorphism, or the
reverse process, deposition, it is difficult to say with certainty. But if intru-
sives be ignored, probably solution is slightly preponderant. Elsewhere
it is shown that there is no evidence of transfers of material into the zone
of anamorphism by the water solutions. (See pp: 665-668, 764-766.) By
deoxidation, silication, and consequent decarbonation, and by dehydration,
the solids constantly lose material. The water as a whole moyes from the
CEMENTATION. 681
zone of anamorphism to the belt of cementation (see pp. 665-667), and takes
with it the freed materials and all the other compounds it can hold. Hence,
under normal conditions, solution probably somewhat overbalances deposi-
tion; but where igneous instrusions occur the above reasoning is
inapplicable.
RESULTANT PROCESSES.
The processes resulting from the chemical changes comprise cementa-
tion and metasomatism. ;
CEMENTATION.
Cementation in the zone of anamorphism is of far less importance
quantitatively than in the belt of cementation. The openings are mainly
confined to those of microscopic size—i. e., to the capillary openings
between the mineral particles and to the innumerable subeapillary open-
ings. There are in this zone no large openings. The laws of deposition
within the minute openings are the same as in the belt of cementation, and
will therefore not be repeated. However, the minerals produced are those
characteristic of the zone of anamorphism rather than those of katamorphism.
The list of these minerals is deferred to the next section on metasoma-
tism, since that process is far more important than cementation. While
the quantity of cementing material in the zone of anamorphism is small,
it is not unimportant, at least so far as the strength of the rocks is
concerned. If after deformation the new openings between the particles
were not occupied by mineral material, the rocks would be rather weak.
When, however, these particles are cemented by deposition, they may
become as strong as or stronger than they were before deformation.
That cementation actually does take place in the minute openings
between the grains is shown by the experimental work of Adams. When
marble was deformed without water the modified rock was weaker than
the original marble, but where water was present, under great pressures
and at high temperatures, in one case the deformed rock was actually
stronger than the original marble, thus showing that cementation caused
the grains to cohere more strongly than when simple welding was the
cause of cohesion, as in the case of deformation where water was absent.”
«Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble:
Philos. Trans. Royal Soc. London, ser. a, vol. 195, 1901, pp. 370-385.
682 A TREATISE ON METAMORPHISM.
It has been seen (pp. 663-664) that the migration of water through the
discontinuous minute openings is exceedingly slow. ‘Therefore the transfer
of material in any considerable amount and for great distances is very
slow. It follows that the process of cementation is confined to the depo-
sition of material in the minute microscopic spaces, and, ignoring injec-
tions, that this material is mainly if not wholly derived from the body
of the adjacent rock. In this respect there is a marked contrast between
the zone of anamorphism and the belt of cementation. It has been pointed
out on pages 617-619, 656, that in the latter belt the material deposited
may be great in quantity and derived from points remote from deposition.
It follows that, so far as the process of cementation is concerned, the chemical
composition of the rocks is little changed in the zone of anamorphism;
whereas, as has been shown on pages 655-656, in the belt of cementation
the chemical composition may be greatly changed by this process.
METASOMATISY,
Metasomatism in the zone of anamorphism may take place in various
ways, the same as in the belt of cementation. A mineral may recrystallize
without change in chemical composition, as, for instance, the alteration of
tridymite to quartz. ‘Two or more minerals may unite to form a single
mineral, as, for instance, the union of calcite and quartz, producing wollas-
tonite. A mineral may change into two or more minerals, as, for instance,
kaolinite into andalusite and quartz, but this class of reactions is much
more common in the belt of cementation. One mineral may be replaced
by another, as, for instance, the substitution of magnetite for quartz. Two
adjacent minerals may react upon each other, producing a third mineral,
as, for instance, bytownite and olivine, producing amphibole.” But more
frequent than any of these simple reactions are complex changes by which
the materials from a number of minerals rearrange themselves to produce
more than one new mineral. These various reactions between the minerals
are mainly accomplished, as usual, through the medium of the water
solutions.
There are important differences between metasomatism in the zone of
anamorphism and metasomatism in the belt of cementation. In this zone
« Williams, Geo. H., The gabbros and associated hornblende rocks occurring in the neighborhood
of Baltimore, Md.: Bull. U. 8. Geol. Survey No. 28, 1886, p. 52.
MINERALS FORMED BY METASOMATISM. 683
the water solutions are in minute openings, in small quantity, and circulate
slowly; therefore, it can not be supposed that any considerable amount of
material is contributed from an outside source except by injection. This
subject is more fully discussed later. (See pp. 764-766.) The reactions
which occur are in the direction of deoxidation, silication, and dehydra-
tion, instead of in the direction of oxidation, carbonation, and hydration.
The minerals formed comprise practically all of the important heavy
anhydrous minerals.
MINERALS FORMED.
Among the minerals formed in the zone of anamorphism are the
following :
The carbon minerals are anthracite, graphite, or diamond.
The abundant sulphides are pyrite and pyrrhotite. Marcasite, having
lower symmetry and lower specific gravity than pyrite, is rare or altogether
absent, although abundant in the belt of cementation. :
The important oxides are those of silicon, iron, aluminum, and titanium.
Silica is found in the form of chert, chalcedony, or quartz. Tridymite is not
known. Tridymite alters to quartz because of the higher specific gravity ot
the latter mineral. Iron oxide occurs in hematite, magnetite, and ilmenite.
Aluminum oxide occurs as corundum. ‘Titanium oxide is found as rutile,
octahedrite, and brookite. The two’ latter are rare as compared with rutile.
This is probably explained by the fact that the latter mineral occupies less
volume. The absence of the hydrous oxides, such as limonite, gibbsite,
brucite, etc., is explained by the general fact of dehydration characteristic
of the zone of anamorphism.
The carbonates, aragonite, magnesite, dolomite, siderite, ankerite, and
parankerite are extensively recrystallized; but the processes as a whole
tend to destroy them, forming silicates. Ordinarily, where the carbonates
were in large volumes, the process of silication and decarbonation is
incomplete.
The most important sulphate is anhydrite. Gypsum does not occur,
because the process of dehydration destroys it. Titanate as titanite, and
yhosphate as apatite, occur.
Practically all the important anhydrous silicates are abundant. These
comprise the feldspars, the pyroxenes, the amphiboles, the nephelite-
sodalite-leucite group, the garnets, the olivines, the scapolites, the epidotes,
684 A TREATISE ON METAMORPHISM.
allanite, chondrodite, andalusite, sillimanite, cyanite, staurolite, tourmaline,
the micas, chloritoid, and ottrelite; and of course various subordinate
silicates, such as axinite, melilite, gehlinite, vesuvianite, zircon, pied-
montite, topaz, ete. The only silicates which bear appreciable amounts of
water are the epidotes, staurolite, and the micas. The ordinary com-
pression conditions of the zone are not strong enough to completely
dehydrate these minerals. It therefore appears that the most profound
conditions of metamorphism with which we are familiar are not sufficient
to drive off all of the water from all the minerals.
It has been explained that the hydrated, carbonated, and highly
oxidized minerals produced in the zone of katamorphism may, when buried,
pass into the zone of anamorphism. It has also been explained on pages
366-369 that in these two zones the reactions are reversed. However,
it by no means follows that when a rock altered in the belt of weathering
or the belt of cementation has passed into the zone of anamorphism the
minerals originally present in the rock will be reproduced in their original
proportions. Indeed, it is certain that this will rarely, if ever, occur. The
reason for this is that in the zone of katamorphism, and especially in the
belt of weathering, there may have been great changes in the chemical
composition of the rocks. For instance, it has been pointed out that the
rocks are very much depleted in alkalies and alkaline earths in the belt of
weathering, and that in the belt of cementation the addition of silica is
3?)
very common. It is therefore clear that when rocks of changed chemical
composition pass into the lower zone the minerals which were originally
present before the rocks were altered in the zone of katamorphism may not
all be reproduced, nor will those there formed develop in the same propor-
tion as originally. Furthermore, the sediments are likely to be deficient in
certain elements as compared with original igneous rocks. One element
which is especially deficient in these is sodium. It naturally follows
that when a sedimentary rock is transferred to the zone of anamorphism
by burying, such soda minerals as leucite, nephelite, and sodalite are
rarely, if ever, produced, simply because of a deficiency of one of the
elements out of which they may be made. But even where unchanged
igneous rocks are recrystallized in the zone of anamorphism the minerals
are not preserved in the same proportion, for it has been explained that
pressure tends to produce molecules which are heavy, and consequently
HEAVY MINERALS FORMED DEEP IN LITHOSPHERE. 685
such minerals as garnet, staurolite, and other heavy minerals form by the
metamorphosis of many of the igneous rocks without any necessary change
in average chemical composition.
Not only may such changes as those given occur, but there may be
repeated changes. These are especially likely to occur with changes in
the pressure conditions. Under the law that the greater the pressure the
heavier the minerals formed, there may be repeated recrystallizations of the
rocks. Minerals produced at an early stage under conditions of moderate
temperature and pressure are destroyed and minerals of higher specific
eravity are produced. Thus a mud may change to shale, thence to slate,
thence to mica-slate, thence to andalusitic micaceous schist, thence to gar-
netiferous, staurolitic, and cyanitic micaceous schist or gneiss. In propor-
tion as the pressure is great and the temperature high, the tendency is to
produce heavier and heavier minerals. Thus a rock which had become
more and more deeply buried may be recrystallized, or partly so, a number
of times, minerals of higher and higher specific gravity successively
appearing. It is entirely possible that in the deeper part of the lithosphere
and within the centrosphere unknown minerals are produced which are
heavier than any formed in the part of the lithosphere which has reached
the surface as the result of denudation. In this connection it is noteworthy
that the majority of the heavy silicate minerals developed in the zone of
anamorphism are those which, so far as they have been artificially produced,
have been formed either under igneous conditions or, if water were present,
under conditions of very high pressure and temperature. The latter is
notably the case for amphibole, pyroxene, quartz, and adularia, which
were obtained by Chrustschoff from aqueous solutions heated to a tempera-
ture of 550° C.*
Metasomatism in the zone of anamorphism may take place under
mass-mechanical or mass-static conditions. The modifications under these
two sets of conditions are so different that it will be necessary to consider
them separately.
ALTERATIONS IN CONNECTION WITH MASS-MECHANICAL ACTION.
It has been seen that where deformation in the zone of anamorphism
is accomplished by rupture alone the result is ever to subdivide the rock
particles. (See pp. 673-675.) It was there indicated that under other
«Chrustschoff, K. von, Ueber kiinstliche Hornblende: Neues Jahrbuch, vol. 2, 1891, pp. 86-90.
686 A TREATISE ON METAMORPHISM.
circumstances deformation is mainly accomplished, not by mechanical
subdivision, but by the chemical action of recrystallization. The funda-
mental idea of this process is that as the rock is deformed it takes the new
form necessary by means of solution and redeposition of the rock material.
But this process is accompanied by strain within and beyond the elastic
limit of the minerals. In considering recrystallization these facts also are
necessarily taken into account. In order to make this clear the facts of
recrystallization will first be considered, and then the theory.
* RECRYSTALLIZATION.
Facts of recrystallization—Qne would expect, from the section on strain beyond
the elastic limit, that the more profound the kneading the finer would be
‘the granulation of the altered rock, but this is not the case. Many of the
most profoundly deformed rocks, instead of being extremely fine-grained,
are somewhat coarsely crystalline.
This anomaly was long a puzzle to me. In examining the deformed
rocks, I found that under certain conditions the more profound the defor-
mation the finer the granulation; but in tracing the process to the extreme,
I found that there was always a limit beyond which the particles did not
become more finely granulated. On the contrary, at a certain stage a
reverse tendency appeared, and the particles, instead of becoming smaller,
eradually became larger. ‘This increase in coarseness of the mineral
particles may be followed through all stages to the coarse schists and
eneisses.
In the granulated rocks the mineral particles everywhere show strongly
the strains of undulatory extinction, but the mineral particles of many of
the coarse schists and gneisses show no more than slight strain shadows.
The coarse, perfect schists and gneisses, nearly free from strain shadows,
are always found to be those which have been deeply buried and pro-
foundly deformed or which are adjacent to great intrusive masses, or both.
It is therefore clear that those rocks represent the most advanced stages of
metamorphism.
It is generally agreed that the schists and gneisses of this character
have been recrystallized throughout, and therefore strongly contrast with
those rocks which have been granulated. However, the granulated and
recrystallized rocks are not separated sharply from each other (see pp.
FACTS CONCERNING RECRYSTALLIZATION. 687
766-768); on the contrary, there is every gradation between the two.
The original rock may have varied greatly in the coarseness of its con-
stituent particles. If one passes from a place of granulation to a place of
recrystallization, one may find that recrystallization of the matrix begins
while granulation of the larger particles is still going on. In an inter-
mediate stage the matrix may have completely recrystallized and the
granulation of the coarser particles be still incomplete. As a consequence,
the mineral particles of the matrix are increasing in size at the same time
the larger particles are decreasing in size.
At a certain stage the larger grains are granulated into particles which
average about the same magnitude as those which have crystallized out of
a fine-grained and perhaps irresolvable matrix, and, moreover, the grains
which have formed from the matrix approximate uniformity of size. Thus
there is a marked tendency toward uniformity in the size of the grains of
the metamorphosed rocks, and this tendency is ordinarily dominant in the
schists and gneisses so long as mass deformation continues. (Pl. XI, C.)
This statement is more nearly accurate in reference to the particles of each
mineral than to particles of different minerals. This tendency toward
uniformity controls notwithstanding the principle that under ordinary
conditions large minerals grow at the expense of smaller ones (see Chapter
Ill, pp. 74-76); for under mass-mechanical conditions a large grain,
whether original or produced by uneven growth, is especially exposed to
the mechanical stresses, and therefore is granulated in part or put into a
state of strain, and thus is more readily attacked by the solutions. Some
of the properly oriented smaller particles may themselves grow at the
expense of the larger ones or of the small ones not properly oriented or
happily placed. Thus is explained the characteristic uniformity in the size
of the particles of the schists and gneisses which have not been modified
since mass-mechanical action ceased. However, in some cases, where the
mineral particles are properly oriented, the tendency for large individuals
to grow at the expense of smaller ones may control, and porphyritic
textured schists and gneisses be produced. (Pl. III, C, D.)
Such an occurrence is beautifully illustrated by the albite-schist of
Hoosac Mountain, Massachusetts, described by Wolff* Here there are
«Pumpelly, Raphael, Wolff, J. E., and Dale, T. Nelson, Geology of the Green Mountains in
Massachusetts: Mon. U. S. Geol. Survey, vol. 23, 1894, pp. 59-63.
688 A TREATISE ON METAMORPHISM.
numerous porphyritic, simple twinned albites, with a close approximation
to definite orientation, the two greater dimensions of the crystals lying in
the planes of schistosity. Similar phenomena in reference to feldspar are
shown by some of the augen-gneisses. Not infrequently porphyritic mica
and chloritoid show approximately similar orientations. In some cases
porphyritic staurolites have their greater dimensions arranged approxi-
mately in the schistose planes. In such cases as these the fortunate
positions of the porphyritic crystals are such that the tendency for large
individuals to grow at the expense of small ones is sufficiently strong to
prevent the usually dominant tendency to destroy large individuals, and
thus to prevent the production of the even-grained texture which is normally
characteristic of the schists produced during mass-mechanical movement.
In the case of an isometric mineral, such as garnet, which has no
cleavage and an isometric habit, it may be possible for porphyritie crystals
to develop during mass-mechanical movement, although it is believed not
to be common at least for undistorted crystals, for such growth would imply
that minerals of this kind are able to grow as far against the greatest pres.
sure as in the direction of least pressure.
The second characteristic feature of the recrystallized slaty, schistose,
and gneissose rocks is that the mineral particles show a marked tendency
toward regular orientation. This orientation may consist in the particles
having major, mean, and minor diameters in approximately common direc-
tions, or in certain species having their crystallographic axes in nearly
common directions, as a result of which the like cleavages of all the
particles of a given mineral are approximately in the same plane, or in
the two combined. (PI. XI, C.) Orientation, where marked, gives the
rocks a cleavage.
The most important of the minerals the particles of which show
similar crystallographic orientation are the micas, especially’ biotite and
muscovite. With these minerals similarity of orientation is usual. Another
set of minerals the particles of each of which frequently show a marked
tendency toward similar crystallographie orientation are chlorite, amphi-
bole, and feldspar. Other less important minerals are known to show +he
same phenomena. Of course, it is understood that the crystallographic
orientation is in no case perfect, but with the micas it may approach per-
fection. From the extreme of regularity of orientation shown by mica in
FACTS CONCERNING RECRYSTALLIZATION. 689
the typical schists to the random orientation of some of the minerals in the
same rocks there are gradations; also there are gradations from the schists
recrystallized under mass-mechanical conditions to rocks recrystallized
under mass-static conditions where none of the minerals show a marked
tendency to similar crystallographic orientation.
In many cases the similar orientation of mineral particles in a typical
schist or gneiss may have been greatly disturbed by subsequent deformation
near the surface, and therefore in the zone of fracture. Under such
conditions shearmg fractures may be produced parallel to the slatiness or
schistosity, and the ‘shearing motion between the layers may largely destroy
the original regularity of the orientated particles.
The particles of some of the mineral constituents of igneous rocks
which have not been recrystallized show a tendency toward parallel crys-
tallographic orientation. With this structure are other structures charac-
teristic of rocks crystallized from a magma. I know ot but few instances
where unaltered igneous rocks so closely resemble the recrystallized schists
and gneisses that there is great trouble in distinguishing them.
In the production of the characteristic textures and structures of the
slates, schists, and gneisses, the original textures and even the structures
may be destroyed, whether they be those of sedimentary or those of
igneous rocks. In passing from an area metamorphosed under mass-static
conditions to an area altered during mass-mechanical action, often all stages
of destruction of the original textures and structures and the development of
new textures and structures may be seen. In an intermediate stage the
larger particles or more refractory minerals may show the textures of the
original rock, the matrix of the same rock, however, having the texture of
a slaty or schistose rock. In instances of extreme alteration under mass-
mechanical conditions no trace of the original textures remains, even where
the rocks were coarse conglomerates or coarse porphyritic, igneous rocks;)
and the secondary structures may traverse the directions of the original
structures and the latter may be wholly obliterated.
Thus metamorphism under mass-mechanical action stands in sharp con-
trast to metamorphism during mass-static conditions, in so far as textures
and structures are concerned. In metamorphism during mass-mechanical
action there is a tendency to destroy old textures and to produce a charac-
teristic texture, the more important features of which are mineral particles
MON XLVII—04——44+
690 A TREATISE ON METAMORPHISM.
of uniform size and parallel orientation, and there is a tendency to destroy
old structures and to produce a characteristic slaty or schistose structure.
In metamorphism under mass-static conditions original textures and struc-
tures are usually preserved, although they may be somewhat modified or
emphasized by the unequal size and lack of orientation of the newly-
developed mineral particles.
Theory of recrystallization— Where recrystallization occurs in the deep-seated
zone the temperature is considerably higher than at the surface, because of
the increase of temperature due to depth, because of heat resulting from
mechanical action, and in many districts because of heat derived from
intrusive igneous rocks. Water occupies all the openings, including those
of subeapillary size. Moreover, this water has about the temperature of
the adjacent rocks, and is therefore extremely active. Taking the ordinary
gradient, the temperature at a depth of 3,000 meters would be 100° C.; at
6,000 meters, 200° C.; and at 9,000 meters, 300° C. At these temperatures
the material would ordinarily be water and not steam, for, ignoring the rock
pressure, the pressure of the supermeumbent column of water is more than
sufficient to prevent it from passing into the condition of a gas. But in
consequence of the heat of mechanical action or of igneous intrusion, or
both, the temperature at a given depth may be so high that the water may,
at least locally and for short times, be in the form of a gas. During the
mass movements of rocks water as liquid makes its way between the rock
particles much more readily than under conditions of quiescence. (See
p. 664.) Therefore the water, on account of high temperature and com-
paratively free movement, is in a most favorable condition for work.
It has been pointed out (p. 98) that during orogenic movements the
rock pressures vary from place to place and from moment to moment.
This results in great variation in the pressure upon the contained water.
When the pressure increases, solution takes place; when it decreases, depo-
sition occurs. Therefore, in consequence of changing pressure during
orogenic movement the conditions are favorable for alternate solution and
deposition. Since the pressure continuously varies throughout long periods
of orogenic movement, it is probable that this is a factor of very consider-
able importance in the recrystallization of the rocks.
Another factor which is of great importance in recrystallization is the
potentialized energy which exists in mineral particles in consequence of a
THEORY OF RECRYSTALLIZATION. PaGoll
state of strain. It has been seen on pages 95-98 that state of strain is
very favorable to chemical action. This follows from the principle of
the conservation of energy. So far as minerals are in a state of strain,
energy is potentialized. This conclusion has been fully verified by Barus,
who showed experimentally that when metals are strained a large amount
of energy is potentialized; and, finally, Hambuechen’ has shown experi-
mentally that strained metals are much more easily acted upon chemically
than unstrained metals. Therefore the experimental work of Barus and
Hambuechen together has completely demonstrated that a state of strain in
substances is favorable to chemical action.
It has been shown (pp. 671-673) that all mineral particles in the zone of
anamorphism are in a high state of strain. It has been further noted that
the stresses may vary from equality in all directions to those so unequal as
to approach or surpass the elastic limit of the rock under the conditions in
which it exists. Therefore, the mineral particles may be strained within
the elastic limit or to the point of granulation; and with the latter the
former occurs, for even where the original mineral particles are broken the
individual granules ordinarily show strain shadows in polarized light. The
condition of unequal stress and strain is especially characteristic of mass-
mechanical action. (See pp. 670-672.)
It follows from Barus’s and Hambuechen’s work that in this interior
state of strain of the mineral particles, and especially in unequal strain,
we have a cause for recrystallization. The simplest illustration of the
effect of a state of strain is perhaps furnished by glass. It is well known
that unannealed glass is in a strained condition. This is best illustrated by
Prince Rupert’s drops. When a point is broken the glass explodes, break-
ing into powder, showing that a large amount of energy is potentialized.
Unannealed glass, even in the laboratory and without the presence of water,
slowly releases itself from strain by recrystallization. Corresponding with
this fact it is to be expected that ancient natural glasses, because of their
unannealed condition, would have released themselves from strain by
recrystallization; and such are the facts.
We conclude from the above that the state of strain, and especially
unequal strain combined with high temperature in the presence of water, is
«Barus, C., The mechanism of solid viscosity: Bull. U. 8. Geol. Survey, No. 94, 1892, pp. 107-108.
> Hambuechen, C., An experimental study of the corrosion of iron under different conditions: Bull:
Uniy. of Wisconsin No. 42, 1900, p. 255.
692 A TREATISE ON METAMORPHISM.
a sufficient cause to produce recrystallization of rocks. As soon as move-
ment begins, equilibrium is disturbed and the processes of solution and
deposition or recrystallization set to work rapidly to adjust the minerals.
The amazing power of heated water in solution and deposition, or in
recrystallization, has already been poimted out on pages 79-81, and it
may be recalled that Barus has shown that above 200° C. glass and water
are miscible in all proportions.* At temperatures as high as or higher
than this, which undoubtedly prevail in the deep-seated zone of deforma-
tion, recrystallization can go on with comparative rapidity. At any
moment the substances are present almost wholly as minerals. However,
superheated water is in the capillary and subcapillary spaces between the
particles, and through this as a medium adjustment by solution and deposi-
tion goes on continuously during the deformation. At any given moment
only an exceedingly small part of the material is in solution; but under the
molecular theory of solids all materials in a state of strain, or subject to
unequal pressure, or not in a compact state, will be more ready to part
with their molecules than the minerals not so conditioned. Thus, from all
mineral particles which are under one or all of these conditions, particles
are filed off or solution is constantly taking place. Simultaneously with
this, from the solutions there is deposition of material in more compact
molecules than those dissolved at the places where the pressure on the
mineral particles is less than the average.
Two minerals that excellently illustrate the process are quartz and mica.
The first reerystallizes somewhat readily and the second develops on an
extensive scale in the schists and gneisses. That quartz occurs abundantly
in flat individuals in the schists is well known. Moreover, it is known in
some cases that the flat individuals are largely the equivalent of individual
erystals which have had a nearly spherical form. As illustrations of flat
grains of this mineral are the quartzes of the quartz-porphyries described by
Futterer’ (Pl. III, B) and of the schists from the Black Hills* (PI. XI, @),
which I have described. The many flat particles have exactly the appear-
ance they would have had if the material could have been pressed out and
«Barus, C., Remarks on colloidal glass: Am. Jour. Sci., 4th ser., vol. 6, 1898, p. 270. See also
Am. Jour. Sci., 4th ser., vol. 9, 1900, pp. 167-168.
>Futterer, Karl, Die ‘‘Ganggranite’? yon Grosssachsen, und die Quartzporphyre yon Thal im
Thiiringer Wald. Heidelberg, 1890, pp. 27-47.
e Van Hise, C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1,
1890, pp. 222-226, 244.
THEORY OF RECRYSTALLIZATION. 695
had reerystallized anew as a single individual and subsequently had been
somewhat strained. In some cases the flat individuals have a somewhat
curved form. (PI. III, B; also, see p. 753.) The phenomena are believed
to be due» to solution and deposition, or recrystallization, as already
explained. he particles of the quartz not fortunately oriented or at places
of great strain are taken into solution and transported to fortunately ori-
ented individuals or those less strained, and redeposited; or, the material
dissolved from the more strained part of a grain may be deposited on another
part of the same grain where the strain is less. Thus the quartz of a given
flat granule may be largely the same quartz as that of the original grain,
but it has been dissolved and redeposited in part, perhaps repeatedly.
Were the quartz grains to be granulated the volume of the rock would
be increased. (See pp. 674-675.) But by the process of solution and
deposition above described the form of the grain is changed and no increase
in volume results. Therefore, in the zone of anamorphism, where pressure
is the dominant force, recrystallization rather than granulation takes place
where it can, for by so doing the volume remains the same even if the
grains remain of the same average size, and heavier minerals are not
formed; and the volume is decreased where many grains merge, as cited
below, and where heavier minerals are produced.
Micas, especially biotite and muscovite, are very abundant in the slates,
schists, and gneisses. Moreover, in proportion as the rocks approach typical
schists and gneisses the particles of these minerals are large, of approximately
uniform size, and. oriented crystallographically. (See Pl. XI, C.) In the
original pelites, from which the micaceous schists most extensively form,
the micas are not abundant constituents. Even in the psammites, where
allogenic micas occur, the particles are large, more or less irregularly
arranged, often somewhat decomposed, and are readily discriminated from
the regularly arranged, fresh micas of the schists. These facts are so well
known that nearly all petrographers who have studied thin sections of the
schists have regarded the micas as authigenic. Chemical analyses show
that soils, muds, clays, and shales contain the elements out of which mica
may develop." Many of these elements occur in hydrated compounds, such
as kaolinite, zeolite, chlorite, and limonite. In the schists which develop
@Clarke, F. W., and Hillebrand, W. F., Analyses of rocks and analytical methods: Bull. U. 8.
Geol. Survey No. 148, 1897, pp. 277-301.
694 A TREATISE ON METAMORPHISM.
from such sediments these minerals may be altogether absent, their places
being largely taken by the micas and other minerals. It is clear that
during the metamorphism of the rocks these minerals are taken into solu-
tion, and from such solutions the new minerals, containing little water, are
deposited. The solution and deposition give the material a less hydrated
and more compact form. During the process, at numerous places mica
nuclei oriented by the ‘differential stresses (see pp. 671-673) begin to form.
The minute nuclei, once formed, serve as cores upon which the material
which is continuously taken into solution may be deposited. The mineral
particles grow somewhat uniformly, being subject to the same laws in this
respect as original particles. (See pp. 120-123.) By studying a series of
thin sections from any of the districts in which the rocks of a formation
vary from little altered material to coarse schists, all stages of the process
may be seen, from that in which the original hydrated minerals are abundant
and mica is absent to that in which the former are absent and mica is
abundant.
In the foregoing we apparently have the explanation of the large
average size of the mineral particles which constitute the schists formed at
considerable depth during mass-mechanical action. They are continuous
growths during deformation by solution and redeposition.
As excellent illustrations of rocks showing all or many stages of recrys-
tallization of quartz and the development of mica may be cited the schists
and gneisses which [ have described in the Penokee-Gogebic and Marquette
districts of Michigan and in the Black Hills of South Dakota.* (PI. ITI,
AAD Rl xB. 30.)
As a beautiful illustration of the transition from finely crystalline to
coarsely crystalline rocks may be cited the iron-bearing formation of the
Marquette district of Michigan.’ The deformation of this formation was
mainly by reerystallization. In the eastern part of the district granulation
and widely spaced fractures occurred to some extent, but the temperature
was not high enough for recrystallization, or else some other essential .con-
“Irving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin:
Mon. U.S. Geol. Survey, vol. 19, 1892, pp. 305-345. Wan Hise, C. R., and Bayley, W. 8., The Mar-
quette iron-bearing district of Michigan: Mon. U.S. Geol. Survey, vol. 28, 1897, 444-459. Van Hise,
C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1, 1890, pp. 222-229.
bVan Hise and Bayley, cit., Mon. 28, pp. 336-375.
THEORY OF RECRYSTALLIZATION. 695
dition was lacking. In the western part of the district, while the rocks
were probably not more deeply buried, the deformation was much more
profound, and probably because of this the temperature reached 180° C.
or more. As a consequence the mineral particles grew to a large size. At
places in the eastern part of the district, where the conditions were least
favorable for recrystallization, the quartz granules in the jaspilite average
about 0.01 mm. in diameter. In the western part of the district, where the
conditions were most favorable, the quartz particles in the coarsest jaspilite
average about 1 mm. in diameter. Moreover, they show little strain.
These particles therefore average about a million times greater than those
of the eastern part of the district, and hence to form one new individual the
material of a million old particles was utilized. This illustration gives
conclusive evidence of the capacity of quartz to accommodate itself to the
most intense deformation by recrystallization.
The explanation suggested by Adams* for the deformation of the quartz
of the leaf gneisses of the original Laurentian district is movement along
gliding planes, as advocated by Miigge” in reference to ice crystals. How-
ever, this explanation is inadequate to explain the phenomena above
described, and similar phenomena for other minerals, for two reasons: First,
the greater dimensions of the flat new individuals always corresponding to the
secondary structures in the slates and schists, are wholly independent of the
orientation of the original particles, and therefore independent of their glid-
ing or other definite planes. If gliding had taken place, it must have occurred
along definite crystal planes. Second, as shown on pages 686-688, the par-
ticles of the metamorphosed schists are very frequently, and in the case of
the metamorphosed sediments commonly, much larger than the original
particles. In many instances the particles average so much larger that a
multitude of old particles are built into a single new particle. In those
cases where different mineral particles merge to form new particles of larger
size, gliding along any set of definite planes can not possibly explain the
process; this can be due only to solution and deposition, or recrystallization,
as already explained. But it does not follow from the above that where
« Adams, F. D., Report on the geology of a portion of the Laurentian area lying to the north of
the Island of Montreal: Ann. Rept. Geol. Sury. Canada for 1895, vol. 8, pt. J, 1897, p. 48.
bMiigge, O., Ueber die Plasticitiit der Eiskrystalle: Neues Jahrbuch fiir Mineralogie, ete., Jahr-
gang 1895, vol. 2, p. 212.
696 A TREATISE ON METAMORPHISM.
recrystallization is the dominant or preponderant process, glidmg may not
occur as a subordinate simultaneous process, especially with such minerals
as calcite, which, as Adams“ has shown, is especially likely to be deformed
by movement along gliding planes.
In the theory of recrystallization we have an explanation of the
general uniformity in size of the particles of any definite mineral in a rock
metamorphosed at depth during mass-mechanical action: The larger grains
of any mineral have smaller areas of contact for the solutions to work on,
and therefore granulation plays a large part; the smaller particles have,
larger areas of contact for the solutions to work on, and consequently
recrystallization merges them, producing larger particles. . Therefore, the
tendency of granulation and recrystallization together is to produce
uniform-textured rocks.
Recrystallization lags behind deformation —In the deep-seated zone adjustment may
not lag far behind the disturbing forces. However, in all cases there is
apparently some lag. In the most regularly laminated of the schists, close
examination usually reveals a shght undulatory extinction, and therefore a
state of unequal strain in the minerals, showing that recrystallization has
not exactly kept pace with deformation, or else that the schists have been
somewhat deformed since recrystallization.
Where such subsequent deformation has not taken place, the amount
of strain shadows and granulation is thought in many cases to be a measure
of the amount that molecular readjustment lags behind the disturbing
movement. In the typical schists strain is in many cases scarcely percep-
tible. In other cases all of the mineral particles show marked strain
shadows. In still other cases the strain shadows are accompanied by more
or less of granulation, and this phase of the rocks grades into the ordinary
eranulated rocks. Thus there are all gradations between molecular read-
justment or recrystallization almost pari passu with deformation, and
readjustment almost wholly by granulation.
Evidence that reerystallization does nearly keep pace with deformation
in the case of the schists consists partly in the absence of marked strain
structures, for it is to be supposed that if recrystallization did not nearly
keep pace with deformation the result would be that the mineral particles
« Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble:
Philos. Trans. Roval Soc. London, ser. a, vol. 195, 1901, pp. 363-401.
RECRYSTALLIZATION LAGS BEHIND DEFORMATION. 697
would show important strain shadows or even granulation. The texture
characteristic of the schists (described on pp. 688-690) is itself further
evidence of continuous recrystallization during deformation. It is a tex-
ture peculiar to the schists. If the minerals were not readjusted in a
continuous fashion they must have become granulated by the mechanical
forces. If they had become fused into a magma, from that state the
material would have recrystallized with textures peculiar to the igneous
rocks. The regular arrangement of the mineral particles, with their
longer axes in definite planes is just what would be expected if the con-
tained water were taking material into solution and depositing it largely
at the borders of the mineral particles, and thus continuously building
them out laterally. j
Further evidence that recrystallization may nearly keep pace with
deformation is found in the porphyritic minerals which frequently occur in
the schists Some of the more common of these minerals (mentioned on
page 700) are garnet, staurolite, andalusite, feldspar, hornblende, chlori-
toid, chlorite, and mica. Such porphyritic minerals ordinarily show no
perceptible stram. They frequently lie with their longer axes or readiest
cleavage across the schistosity. This is true even of mica and chloritoid,
the cleavage of the porphyritic constituents cutting directly across the
cleavage of the abundant small individuals of mica which accord with
the schistosity. It is maintained (p. 702) that such minerals have
developed mainly under static conditions after mass movement ceased.
These porphyritic minerals seem to be evidence that the differential
stresses of static conditions are ordinarily not sufficient to control the
orientation of the mineral particles; that in order to do this the differential
stresses must be sufficient to produce actual movement throughout the
inass of the rocks. If this be so we must suppose that the orientation of
the minerals producing schistosity occurred during the movement. itself,
or, in other words, that recrystallization nearly kept pace with the
movements. ;
During movement, in some cases the tendency for large individuals to
grow at the expense of smaller ones may control, and properly oriented
individuals grow to a porphyritic size. For instance, porphyritie feldspars
may show a marked tendency toward crystallographic orientation, the
cleavages of the feldspars corresponding with the cleavages of the rocks.
698 A TREATISE ON METAMORPHISM.
This is beautifully illustrated by the albite-gneiss of Hoosac Mountain,* by
augen-eneiss of the French Broad River, and at various other localities.
The argument that orientation of mineral particles in rocks is primarily
due to unequal stress in different directions during crystallization applies
equally to the cases of the parallel crystallographic orientation of individ-
uals which occasionally occur in rocks crystallizing directly from magmas.
Feldspar is not infrequently oriented in such rocks, and the phenomena is
known in reference to other minerals.
conclusion F'rom the foregoing it is concluded that the development of
the schists is to be explained as a process of chemical action induced by
mechanical action, resulting in the constant solution and deposition of the
material, or its recrystallization, so as to atcommodate it to the changing
form of the mass.
ALTERATIONS UNDER MASS-STATIC CONDITIONS.
It has just been shown that metasomatic recrystallization largely
induced by mass-mechanical action is the most important process in the
development of the schists. However, recrystallization may and does take
place under mass-static conditions, by means of which minerals are
produced characteristic of the zone of anamorphism. The cause fer
recrystallization under mass-static conditions is the demand of the pressure
for less volume. If the minerals be recrystallized or made over into those
which have a higher average specific gravity, the volume is lessened and
work is done. Changes are likely to be important in proportion as the
temperature is high and the pressure great. It naturally follows that
recrystallization under mass-static conditions is important where the rocks
are very deeply buried, or where orogenic movements have occurred, or
where igneous rocks have been intruded, and especially where two or all of
these are combined. If a rock has recrystallized during mass-mechanical
action or igneous intrusion, and therefore has developed into a schist, under
succeeding mass-static conditions recrystallization is particularly likely to
recur, for the orogenic movement or the intrusive produces a higher
temperature in the rocks than would obtain at the same depth under static
conditions.
“Pumpelly, Raphael, Wolff, J. E., and Dale, T. Nelson, Geology of the Green Mountains in
Massachusetts: Mon. U. 8. Geol. Survey, vol. 23, 1894, pp. 59-63.
TEXTURES FORMED UNDER MASS-STATIC CONDITIONS. 699
Under mass-static conditions the textures which are produced by
recrystallization are very different from those formed by crystallization
during mass-mechanical action. It has been seen that under mass-mechan-
ical conditions schists form, the distinctive textures of which are uniformity
of size of the mineral particles, and especially the similar orientation of
certain minerals. Under mass-static conditions, where the pressure con-
ditions approximate to hydrostatic, schists do not ordinarily develop. The
rearrangement takes place so as to form more condensed molecules, without
producing either uniformity of size or regularity of arrangement of the
mineral particles. During the development of the mineral particles they
ordinarily interfere with one another and interlock, producing a complex
texture often more intricate than that formed by the mutual interference of
minerals when crystallizing from a magma.
Commonly, the old mineral particles, if large, break up into a great
number of other mineral particles of varying size. If, on the other hand,
the original particles are very small, they are apt to coalesce so as to
produce larger mineral particles; but usually the magnitude of the resultant
mineral particles is not great. To the above extent there is a tendency
toward uniformity of size, but it does not go so far as to result in approxi-
mate uniformity. Indeed, a very notable characteristic of the texture of
rocks recrystallized under mass-static conditions is the unequal size of the
mineral particles. This results from the fact that there is no movement to
prevent the full operation of the law of growth of large individuals at the
expense of small ones. In this particular the conditions contrast with those
of mass-mechanical action in which large individuals are especially exposed
to strain and rupture, and are therefore likely to be destroyed.
This process of the growth of large individuals at the expense of small
ones develops a porphyritic texture. This process is of such significance
that it needs to be especially considered, as does also another process closely
allied to it, that of the regeneration or rejuvenation of old mineral particles
of large size which are adapted to their environment.
DEVELOPMENT OF PORPHYRITIC TEXTURES.
Where the mass-static conditions favorable to recrystallization continue
long, some of the mineral particles may grow to great size as compared
with their fellows, and thus a porphyritic texture is produced. The pressure
700 A TREATISE ON METAMORPHISM.
conditions demand diminution of volume, and the large mineral particles
are apt to be those of high specific gravity. The more important dense
porphyritic minerals are feldspar, garnet, staurolite, tourmaline, andalusite,
sillimanite, cyanite, mica, chloritoid, and hornblende. The minerals also
exhibit a marked crystal habit, and not infrequently as they grow they
maintain crystal forms. Often the porphyritic minerals show a distinct
zonal texture, comparable in every way tothe zonal texture sometimes
shown by minerals in igneous rocks. The zonal texture is frequently seen
in minerals having a varying composition, as, for instance, garnet; and is
rare or has not been observed in minerals having a rather definite chemical
composition, as, for instance, staurolite. It is believed that the zonal texture
of the minerals is often due to varying composition, during their growth, of
the mineral solutions or adjacent minerals, or both. In some cases there
may have been actual cessation of growth and later a renewed growth. In
such instances the renewed growth is analogous to the process of enlarge-
ment of fragmental mineral particles.
Single porphyritie individuals may be larger than hundreds of individ-
uals of the background. Those a centimeter in diameter are very abundant,
while those two or even several centimeters in diameter are not infrequent,
and occasionally they are many centimeters across.
As the large porphyritic individuals form at the expense of many small
mineral particles, they either absorb the material of which the surrounding
minerals are composed, and thus grow by their destruction, or else absorb
a part and inclose a part. The minerals mentioned differ in capacity to
absorb the other minerals which before occupied the space they now
occupy. In proportion as they lack capacity to absorb the constituents of
prior minerals, they have the capacity to grow around and inclose them, so
that a single individual may be a large, reticulating, honeycombed mass,
which incloses a large amount of other material. (PI. III, C.) In some
cases the inclosed material may be several times as great in quantity as the
host. As minerals conspicuous for their capacity to include other minerals
may be mentioned andalusite and staurolite; occupying an intermediate
position are garnet and feldspar; while tourmaline usually does not contain
any considerable quantity of the prior minerals.
A large variety of minerals may be included by the porphyritic con-
stituents, but the dominant inclusion is quartz. Andalusite and garnet are
U. S. GEOLOGICAL SURVEY
A. PARALLEL VEINS OF CALCITE, GREAT BASIN.
B.
LENTICULAR AREAS DEFICIENT IN |IRON-BEARING MINERALS,
MONOGRAPH XLVII
PL.
BIOTITIC GRANITE SHOWING GARNET SURROUNDED BY
DEVELOPMENT OF PORPHYRITIC CRYSTALS. 701
orthosilicates; staurolite and tourmaline are subsilicates. They are there-
fore unable to absorb a large amount of silica, and hence the explanation
of the abundant inclusions of quartz. The inclusion of quartz is finely
illustrated by some of the garnetiferous and staurolitic schists. In the
staurolites the quartz frequently seems as abundant as in the background,
but the micas and other minerals are usually absent, although they may be
abundant in the background. The garnet and staurolite have partly or
wholly absorbed the micas, chlorites, feldspars, and other minerals, which
are largely composed of the same elements as themselves, and have built
them into their bodies, thus making denser minerals.
The evidence of this consists in the absence of inclusions of the iron-
bearing constituents of the schists in garnet and staurolite, and the presence
of abundant quartzose particles. While frequently the iron-bearing min-
erals extend without apparent diminution in amount to the garnets and
staurolites, in some cases around the garnets are aureoles or lenticular areas
of material which are markedly deficient in the iron-bearing minerals.
This I first observed at Bristol, Conn. (See Pl. I], B.) Here is a biotitic
granite blotched by white spots, consisting of quartz and feldspar with
no mica, the larger of them 2 cm. wide by 8 em. long, in the cores of
which are large crystals of garnet The same thing is beautifully illus-
trated by the almandite in a mica-schist of Hampshire County, Mass. The
acid “minerals, such as quartz and feldspar, outside or inside the garnets
which could not be absorbed have arrangements characteristic of the schists.
The above occurrences give beautiful illustrations of the principle of
the development of the large heavy mineral particles from the material of
small and light ones. They also give positive evidence that the work is
done by solution and redeposition. The transportation of the material of a
mica flake to a garnet or staurolite must be done by solutions
Commonly the porphyritic minerals do not show any orientation. But
in the schistose rocks containing porphyritic minerals it is usually true that
the minerals constituting the background have laminar forms, and often
similar crystallographic orientation. These minerals of the background
often stop abruptly at the junction with the garnets, staurolites, tourma-
lines, ete., without any deviation whatever or tendency to peripheral
arrangement about the porphyritic minerals. The micas may be seen with
perfect arrangements extending to the exact junction of the porphyritic
702 A TREATISE ON METAMORPHISM.
individuals with undiminished transverse dimensions, but with various frac-
tions of the average length of those of the background which have not been
‘nterfered with by the porphyritic minerals. In short, the appearance, so
far as the minerals of the background are concerned, is precisely that which
would be produced if one could cut out from a fully developed schist an
outline for a porphyritic mineral and subsequently insert that mineral with
extreme nicety. (PI. III, D.)
The minerals included in the porphyritic individuals have the same
appearance, and have their granules arranged according to the same sys-
tem, as the like minerals in the background of the schist.
Where in the schists the porphyritic constituents have crystal habits,
and are not arranged with their greater dimensions parallel with the schis-
tosity of the groundmass, the evidence seems conclusive that such
porphyritic crystals developed under mass-static conditions. The case
seems especially conclusive when readily cleavable porphyritic minerals,
such as mica, chlorite, and chloritoid, in well-defined crystals, occur with
their cleavages diagonal or perpendicular to the dimensional arrangement
and cleavage of similar minerals of the background. Such cases are
beautifully illustrated by the Hudson schist of New York and by the
Michigamme schist of the Upper Huronian of the Lake Superior region.
Such porphyritic constituents with random orientation formed after the
movement ceased during which the minerals with parallel orientation were
formed and the schistose structure developed.
The conditions favorable to the development of porphyritie crystals
are those already mentioned as favorable to recrystallization under mass-
static conditions, viz, the presence of water, high pressure, and high temper-
ature. Where those conditions obtain to an exceptional degree porphyritic
constituents are common. These conditions are likely to obtain to an
unusual degree after either powerful orogenic movements or great batho-
lithic intrusions, or, still more commonly, the two combined. This is
illustrated by the porphyritic crystals of nearly every variety, including
mica, chloritoid, andalusite, garnet, staurolite, and tourmaline, in the schists
1890, pp. 222-230.
IP oA AB ALI
703
Pala Wiese lel
PHOTOMICROGRAPHS OF METAMORPHIC TEXTURES.
A. Uneven texture developed by metasomatic alteration of feldspar to quartz and mica. From
biotite-slate of Penokee-Gogebic district, Michigan. :
B. Thin section of mashed quartz-porphyry, showing the quartzes elongated by recrystallization,
while the adjacent feldspar is little affected. After Futterer.
C. Albite-schist, showing secondary porphyritic albites which include other mineral constituents
of the rock and their longer axes parallel to the prevailing schistosity. From Hoosac Mountain,
Massachusetts. After Leith.
D. Porphyritic chloritoid developed after rock flowage has ceased. The chloritoid includes the
other constituents of the rocks with their longer diameters parallel to the prevailing cleavage. From
Black Hills, South Dakota. After Leith.
704
U. S. GEOLOGICAL SURVEY
MONOGRAPH XLVII PL. Il
PHOTOMICROGRAPHS OF METAMORPHIC TEXTURES.
coe
Seukate
REGENERATION OF MINERAL PARTICLES. 705
porphyritic constituents in the schists and gneisses adjacent to the great
granite batholiths of western Massachusetts.”
Occasionally there may be present in schistose rocks porphyritie min-
erals which show a marked tendency toward similar crystallographic or
dimensional orientation, or both.
To instances of orientation of porphyritic constituents the arguments
above given in reference to the usual development of porphyritic minerals.
under mass-static conditions after movement has ceased are not applicable.
This case is discussed on pages 687-688.
REGENERATION OF MINERAL PARTICLES.
The regeneration’ or rejuvenation in the zone of anamorphism of large
mineral particles which have been partly altered in the belt of weathering
or the belt of cementation is closely analogous to the growth of porphyritic
minerals. Such partly destroyed large minerals are sometimes regenerated
at the expense of the small ones for the same reason that the porphyritic
minerals form. As an illustration of the process, the feldspars may be
cited. In the belt of weathering they are likely to become cloudy in
consequence of kaolimization, development of chlorite, zeolite, deposition
of iron oxide, ete. In the belt of cementation they may become cloudy,
in consequence of zeolitization or other alterations. When such altered
feldspars pass into the lower zone the reactions may be reversed; new
feldspar is produced, which is controlled in its orientation by the residual
unaltered feldspar; the alteration products are absorbed by this new
feldspar or else dissolved; thus the cloudy appearance disappears, and
the feldspar once more becomes clear and fresh—i. e., is regenerated or
rejuvenated.
This process of regeneration is especially likely to occur where the
rocks pass from the zone of katamorphism into the zone of anamorphism
a Emerson, B. K., Porphyritic and gneissoid granites in Massachusetts: Bull. Geol. Soc. America,
vol. 1, 1890, pp. 559-561.
> Wolff, J. E., Metamorphism of clastic feldspar in conglomerate-schist: Bull. Mus. Com. Zool.
Harvard College, whole ser., vol. 16, 1891, pp. 178-183. Weidman, 8., A contribution to the geology
of the pre-Cambrian igneous rocks of the Fox River Valley, Wisconsin: Bull. Wisconsin Geol. and
Nat. Hist. Surv., No. III, 1898, pp. 20-24. Whittle, C. L., Some dynamic and metasomatic phenom-
ena in a metamorphic conglomerate in the Green Mountains: Bull. Geol. Soc. America, vol. 4, 1893,
pp- 155-164. Hobbs, W. H., Phases in the metamorphism of the schists of southern Berkshire: Bull.
Geol. Soc. America, vol. 4, 1893, pp. 167-178.
MON XLVII—O04 45
706 A TREATISE ON METAMORPHISM.
by burying without orogenic movement. Where recrystallization during
orogenic movements occurs the process of rejuvenation is less likely to take
place, but may occur in the more resistant minerals which escape destruction.
As is well known, feldspar is one of the most resistant minerals under deep-
seated conditions of deformation, and hence many of the original partly
altered particles of this mineral may be rejuvenated during the time in
which other minerals are recrystallized. Of course, if the deformation is
severe the borders of the original feldspars are likely to be granulated or
recrystallized, and during extreme orogenic movements there may be com-
plete recrystallization of the feldspar.
The facts that under certain conditions feldspars are destroyed by
kaolinization, chloritization, zeolitization, ete., and under other conditions
are rejuvenated or newly developed, have been known for some time. We
can now assign reasons for the reversal of these reactions. Feldspar is
not adapted to the zone of katamorphism, and especially to the belt of
weathering, and it is destroyed by the reactions demanded by the chemical
and physical forces there obtaining; whereas feldspar is adapted to the
zone of anamorphism, and the chemical and physical forces there at work
reproduce it.
The nature of the process of rejuvenation has been illustrated by
feldspar because this is the best known case, but other minerals may be
rejuvenated in a similar manner.*
« Lepsius has anticipated a number of the points of the previous pages, as is shown by the following
quotations and summary from the Geologie von Attika (Ein Beitrag zur Lehre vom Metamorphismus
der Gesteine), Dietrich Reimer, Berlin, 1893, 196 pp. With tables and atlas.
“Die Mineralien in den Gesteinen kénnen nur umkrystallisiert werden, wenn ihre Molekitle
fliissig werden; dies kann auf dreierlei Weise geschehen:
1. Dureh Schmelzung in der Glihhitze und zwar fiir unsern Fall bei Gegenwart von Druck, also
durch die eben besprochene Druckschmelze.
2. Durch Lésungsmittel auf chemischem Wege, und zwar in unsrem Falle bei Gegenwart von Druck
und bei einer Temperatur, die héher ist als die mittlere Temperatur der Erdoberfliche, die jedoch im
allgemeinen nur bis zu einigen hundert Grad Celsius und wohl nicht wher 500° steigen wird.
3. Durch hohen Druck auf mechanischem Wege, das ist diejenige Art von Druckverfltissigung,
welche durch die Springschen Versuche illustriert wurde.
Betrachten wir jetzt die zweite Méglichkeit, die Verflissingung der Molektile durch Losungsmittel
auf chemischem Wege; diese Art der Verfliissigung und Auskrystallisierung aus der chemischen Lésung
scheint mir am besten die yon aus beobachteten Vorginge bei der Metamorphose der Gesteine
aufzukliren.’’ (P. 183.)
Holds that the schistose Silurian and Tertiary rocks when metamorphosed were buried to a depth
of about 12,500 meters, which with a specific gravity of 2.5 would give a pressure of 3,125 atmospheres.
(P. 187.)
METAMORPHISM BY IGNEOUS AGENCIES. 707
IGNEOUS WORK.
The igneous work of the zone of anamorphism is comprised under
“Injection.”
INJECTION.
Injection in the zone of anamorphism probably occurs on a much
greater scale than in the zone of katamorphism. The field evidence for this
view is found in the vast amount of intrusive igneous material in the areas
where deep denudation has exposed rocks which have been within the zone
of anamorphism. For instance, in the extensive areas of the Canadian
pre-Cambrian, in the Paleozoie of New England, and in the cores of the
great mountain masses of the world intrusive rocks constitute a large
percentage of the total material exposed. Indeed, over considerable
districts intrusive igneous material composes 25 to 50 per cent of the area.
These facts accord with what one would expect, for as magma struggles
toward the surface the quantity which is unable to accomplish the task
.naturally increases from the surface toward the source.
Holds that recrystallization takes place, producing coarser crystals, because of the fact that the
larger crystals have less surface for action than the small crystals, and therefore that, if solution and
precipitation are occurring, the small crystals will be dissolved and the large crystals grow. (P. 188.)
Holds that where the pressure is great the chemical reactions will take place in such a sense as to
produce a diminution of volume, and cites Caillete and Piaff’s work as showing that certain reactions
can be greatly hindered and finally ceased altogether by a pressure of from 40 to 80 atmospheres.
(P. 190.)
Holds that crystallization is not possible without solution, and explains that through crystalliza-
tion under pressure the large crystals grow at the expense of the small ones. In this connection states,
“ist dieser zweite Krystall kleiner als der erste, so kann er nach dem oben angefiihrten Gesetze
aufgeldst werden, und konnen alsdann seine Molekiile allmiihlich mittels des Lésungwassers zu dem
grosseren Krystall hintiberwandern.’”’ (P. 193.)
~“Neben der Zeitbauer sind die fiir die Grésse der Krystalle bestimmenden Faktoren: die Héhe
der Temperatur, mittelbar die Stirke des Druckes, sowie natiirlich auch die gréssere oder geringere
Léslichkeit der Mineralien in tiberhitztem Wasser und die absolute Quantitit, in welcher die betref-
fenden Molektile in dem ursprtinglichen Sedimente vorhanden waren. Daher wiichst die Grdsse
der Krystalle auch mit dem stiirkeren Druck; jedoch geschieht iberhaupt kaum eine Umkrystallisier-
ung, wenn ein mechanischer Druck allein ohne chemische Lésung wirken sollte; umgekehrt wird die
Lésungsfiihigkeit des Wassers erhéht durch den Druck.’’ (P. 193.)
‘‘Bei der Entstehung yon metamorphen Gesteinen wirken also vier Ursachen zusammen: Wasser
als chemisches Losungsmittel der in den Gesteinen vorhandenen Substanzen; héhere Temperatur, um
das Wasser zu erwiirmen; mechanischer Druck, um das tiberhitzte Wasser in fltissiger Form in den
Gesteinen festzuhalten und dessen Lisungsfiihigkeit zu erhéhen; endlich auch eine lange Zeitdauer,
wihrend welcher die chemischen Umsitze in den Gesteinen vor sich gehen kénnen.”’ (P. 194.)
708 A TREATISE ON METAMORPHISM.
MANNER OF INTRUSION.
The manner of intrusion in the zone of anamorphism differs in many
respects from that im the zone of katamorphism. In the latter zone it has
been explained that the igneous rocks mainly follow fractures. In the
lower zone no large fractures can be supposed to exist continuously.
In some way the magma must make its way through rocks which normally
have no large openings.
The progress of the igneous rocks is therefore a matter of far greater
difficulty. In consequence of earth movements the material may make its
way (1) en masse, without breaking across the structures of the intruded
rocks to a great extent; (2) by breaking across the structures of the
intruded rocks and by following along planes of weakness such as cleavage,
bedding, and contacts; and (3) by fusion and absorption of the intruded
material. :
(1) It is probable, indeed certain, that all three of the above processes
occur to some extent, but it is believed that the movements en masse,
without extensive breaking across the intruded rocks, are of great conse-
quence. As evidence of this may be cited the fact that im general, adjacent
to the great batholiths, the injected rocks have peripheral structures, as if
they had been driven aside en masse by the earth stresses in connection
with epeiorogenic and orogenic movements. It is believed that the magmas
transmit the thrusts they receive substantially under the laws of hydro-
statics, and consequently make their way by raising up and pushing aside
the material previously occupying the space, without necessarily breaking
through it on a large scale. Under such circumstances, if the intruded
rock be sedimentary, the bedding of the sediment is peripheral to that of
the great intrusive masses. Materials of other kinds are similarly arranged.
The intrusive acts in reference to the rocks as would a hemispherical or
lenticular mass raised against a piece of flexible but tightly stretched
leather. The leather roughly adjusts itself to the shape of the hemisphere
or lenticule.
Also, as will be seen, high pressure, temperature, and water result in the
recrystallization of the rocks adjacent to intrusive masses, and during: this
process they are shortened in a direction normal to the intrusion and
sometimes lengthened in a direction peripheral to it. Thus room is made
for the entermg mass. Often the result, so far as the relations of the
MANNER OF INTRUSION. 709
Inasses are concerned, is as if a solid body were pushed up against a sheet
of rubber. The rubber would be thinned and extended above the mass
and on the sides. Therefore a consequence of recrystallization is to carry
the entering magma nearer to the surface.
The deep-seated intrusives, producing uplift of the intruded rocks, form
mountains at the surface. Such mountains have been called subtuberant
by Russell and others.* When later erosion removes the capping rocks the
igneous masses below are seen. Intrusive igneous masses of this class are
exposed on a great scale in this country, in the Cordilleras, in the Black
Hills, northwest of Lake Superior in Canada, and in New England. The
intrusive masses may be nearly spherical, lenticular, or many times longer
than broad. The larger of them have minor diameters of many kilometers
and major diameters of 50 to 100 kilometers, or even more. When the
magnitude of these masses is appreciated it is easy to understand how the
great interior earth stresses result in concentrated upward motion at some
area, as a consequence of which the magma, either liquid or potentially
liquid, slowly but with tremendous power oozes up toward the surface,
carrying with it and pushing aside the superincumbent rocks.
(2) Next in importance to raising the intruded rocks and pushing them
aside in the lower zone is the breaking across or the following of the
structures or planes of weakness of the intruded mass. During the great
movements resulting in mass intrusion the rocks which are thrust aside are
cut and injected parallel to their structures on a large scale, as a necessary
correlative of the greater process above described. Returning to the
illustration of the leather, this material stretched against a hemisphere is
wrinkled on its sides unless it be as extensible as rubber. Similarly, as a
great mass of igneous rock is intruded and pushes the rocks up and aside,
they are wrinkled, unless we assume greater flexibility than is warranted
by the facts. Commonly, therefore, subtuberant intrusion demands close
corrugation on the sides. Further, I have held in another place that great
periods of intrusion are also periods of rapid orogenic movements.’
When the deformation of a rock mass is rapid, fractures may extend
much deeper into the earth than they would under normal conditions.
@ Russell, I. C., Volcanoes of North America, Macmillan Co., New York, 1897, pp. 103-105. Also,
On the nature of igneous intrusions: Jour. Geol., vol. 4, 1896, p. 189.
> Van Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11, 1898,
pp. 493-494.
710 A TREATISE ON METAMORPHISM.
(See pp. 658-658.) I have said that it is characteristic of the zone of
flowage that no large openings can be supposed to exist continuously. It
does not therefore follow that at times of rapid movement extensive fractures
may not be temporarily formed. If these were at a place of subtuberant
intrusion, or were adjacent to any magma, material would break away from
the central mass, intrude itself into the openings and fractures, wedge the
walls apart, and thus we should have intrusives for a considerable belt
peripheral to the central mass which are analogous in their forms to the
intrusives characteristic of the belt of cementation—that is, intrusives along
fractures; and this would be so even if the rock alterations of the place
were, on the average, those characteristic of the zone of anamorphism
rather than of the zone of katamorphism. We have here really the case of
an intermediate belt of combined fracture and flowage, fractures extending
deep at times of rapid deformation, although the normal conditions are
those of flowage. Thus from the great central masses of igneous rocks
smaller masses extend in various directions to various distances, some being
subordinate subtuberant masses, and others taking advantage of the tem-
porary fractures and thus forming dikes There is a marked tendency for
these subordinate masses to follow planes of weakness, as, for instance, the
contacts between rocks of different character, the bedding of sedimentary
rocks, or other structures such as the planes of cleavage in schistose rocks.
Intrusives of this kind usually have greater dimensions parallel to the
structures followed than in the transverse directions. The tendency of
the injections to follow cleavage is very marked indeed. Parallel to the
planes of cleavage in the slaty and schistose rocks intrusions may be very
close together and small, there being perhaps a considerable number within
the breadth of an inch; or the imtrusives along cleavage planes may be
larger and far apart, or they may be any combination of these. All these
phenomena are well illustrated by the Hudson schist of New York City,
especially at New Rochelle, and by the gneisses at many places upon the
Piedmont Plateau.
(3) Finally, the upward progress of the great masses of magma is also
doubtless made in part by the fusion and absorption of the material with
which it is in contact, although the evidence that such absorption has taken
place on a great scale is lacking. That absorption does take place in a minor
ray, however, is certain. (See ‘Fusion and absorption,” pp. 728-736.)
METAMORPHISM BY IGNEOUS AGENCIES. fell
RESULTANT METAMORPHISM.
It is natural to expect that the rocks surrounding great batholiths of
igneous rocks, and those intimately associated with the abundant and
numerous branch batholiths and dikes, would be profoundly modified, and
such is the fact. Conditions for profound and far-reaching modification are
perhaps most favorable in connection with batholithic injection and conjoint
orogenic movements, and to this fact is probably due the classification of
certain kinds of metamorphism as contact metamorphism. But it must be
remembered that, with the one exception, the forces and agents at work are
essentially the same as those in other parts of. the zone of anamorphism,
the difference being, merely, that under the conditions of batholithic
intrusion these forces and agents are particularly potent. During an
intrusion the orogenic forces and the thrust transmitted by the magma
both act upon the rocks, and thus the mechanical processes are at work.
The temperature is abnormally high; hence the solutions are especially
potent. To some extent, as pointed out on page 662, the magma itself
may furnish water, and thus the surrounding rocks contain more than an
average amount of water; and this is the most important agent in recrys-
tallization. Therefore about great batholiths are usually zones of profound
metamorphism.
FACTORS CONTROLLING METAMORPHISM.
The factors of alteration to be especially considered in this connection
are the size of the intrusive masses, the temperature, the amount of water
present, and the composition of the intrusive and intruded rocks.
SIZE OF INTRUSIVE MASSES.
The size of the intrusive masses is of the first importance in the
amount of metamorphism. The smaller of the deep-seated irregular masses
are ordinarily called stocks; those of intermediate size, bosses; and those
of the largest size, batholiths. We know of no downward limit for any of
these. Oftentimes they undoubtedly increase very rapidly in magnitude
with depth, although this fact can not be directly observed. Erosion may
have extended only deep enough to expose a very small area of a bath-
olith, and yet for long distances about that place the subtuberant mass may
be only a short distance below the surface, and the rocks exposed therefore
be well within the limit of its influence. This must be a common ease, for it
ale, A TREATISE ON METAMORPHISM.
is certain that the batholiths of great magnitude which 1ow show a large area
must at an earlier stage of erosion have been in the position just described
with reference to the surrounding rock. Therefore, while metamorphism is
a function of the size of the intrusive masses, it is not a function of the
size of the masses exposed. When the masses are small and do not rapidly
expand in size below, the appreciable metamorphic effect may extend only
a few centimeters or a few meters; but the metamorphosing effect of the
great batholiths, some of them scores of kilometers in diameter, may
extend from a kilometer to many kilometers.
THE TEMPERATURE.
The temperature effect depends largely upon the size of the intrusive
mass, and to a less extent upon its temperature. The greater the size of
the intrusive mass and the higher its temperature the larger the amount
of heat it is able to furnish to the surrounding rocks and the farther that
heat penetrates by conduction and convection. In the matter of heat the
intrusives in the zone of anamorphism are in a vastly more advantageous
position than those of the belt of cementation. In the latter belt the rocks
are comparatively near the surface. There is a somewhat rapid circulation
of the water, so that the heat is dispersed, slowly to be sure, but with rela-
tive speed as compared with the zone below; but in the zone of anamor-
phism the great store of heat of an intrusive mass is so far below the
surface, and the circulation is so slow, that it is a sustained factor in
increasing the temperature of the surrounding rocks, certainly for geo-
logical epochs, and frequently in the case of the larger masses for geological
periods. Thus there is ample time during high temperature for the altera-
tions to take place. Moreover, the heat through conduction and convection
slowly disperses itself from the central mass over a very wide area and
produces a metamorphic effect upon the rocks far beyond the distance to
which the materials of the magma can possibly penetrate.
AMOUNT OF WATER PRESENT.
The amount of water, as fully explained in other connections (see
pp. 741-748), is of the utmost importance in determining whether the meta-
morphism is by granulation or by recrystallization. In proportion as water
is abundant recrystallization is likely to take place. Where the intrusive
INTERACTION OF INTRUSIVE AND INTRUDED ROCKS. (ls
masses are introduced into sedimentary, volcanic, or other rocks, which
have passed through the zone of katamorphism, they already contain a
reservoir of water in hydrated minerals which can be released by dehy-
dration. This supply is usually ample to entirely recrystallize the rocks.
But as nearly as we can ascertain from observation, the magmas themselves
contain a considerable store of water, as pointed out on page 662. At the
time of crystallization of a magma a portion of this water escapes and passes
slowly into the surrounding rocks, and thus in magmas we have a second
reservoir, which supplements the first; so that adjacent to masses of igneous
material in the zone of anamorphism there is commonly an amount of water
such that recrystallization readily takes place.
COMPOSITION OF INTRUSIVE AND INTRUDED ROCKS.
The absolute and relative compositions of the intrusive and the intruded
rocks are both of importance in reference to the alterations which take
place. So far as observation can determine, it seems to be a fact that the
alterations are more widespread in connection with batholiths of granite
and syenite and other acid rocks than with basic rocks. Possibly the
apparently greater contact effect may not be due so much to the chemical
composition of the crystallized intrusive as to a greater than average
amount of occluded water (see pp. 720-728), and to the high temperatures
which are frequent accompaniments of acid magmas.
The character of the intruded rock has much to do with the nature of
the alteration. In so far as rocks are composed of minerals which are easily
recrystallized, this process is more likely to take place; whereas rocks
which are difficult to recrystallize under the same conditions may be com-
paratively little altered. The presence of hydrous minerals which may
be dehydrated and furnish water, and fineness of grain are very favorable
to recrystallization.
As to the mutual effect upon the chemical composition of the intrusive
and the intruded rocks, the law in the zone of anamorphism is the same as
that in the belt of cementation, viz, that the joint endomorphic and exo-
morphic effect is to make the intruded and intrusive rocks approach each
other in composition. Under the law of mass action each gains from the
other elements in which it is deficient. One of the commonest illustrations
of this law is furnished by the large amounts of feldspar which frequently
714 A TREATISE ON METAMORPHISM.
develop in a schist adjacent to an intrusive, when the continuations of the
schist formations remote from. the intrusive contain little or no feldspar.
Such are the facts about the Black Hills batholiths (see p. 724) and at
many places about the New-England granite batholiths.
Another excellent illustration is furnished by the Rib Hill quartzites of
central Wisconsin, adjacent to a great intrusive batholith of augite-syenite.
The center of the quartzite mass contains no feldspar. As the masses of
syenite are approached feldspar and hornblende appear in the quartzite.
These minerals increase in amount as the syenite is approached, and close
to that rock the quartzite is so thoroughly impregnated with feldspar which
has developed in and between the grains as to make many of the hand
specimens of the rock difficult to discriminate from a granite.
There is much difference between the zones of anamorphism and kata-
morphism in the extent to which interchange of material takes place between
the injected and the injecting rocks. It has been seen that in the belt of
cementation, in consequence of the porosity of that zone, the material of
the magma, both by direct injection and by transmission through water,
may profoundly affect the average chemical composition of the imtruded
rock for great distances from the intrusive mass. While these changes do
not take place on an extensive scale to any such distance from the intrusive
in the zone of anamorphism, we must not go too far im minimizing the
importance of the exomorphic effect of intrusive rocks in this zone. The
short distance to which magma can furnish material to the injected rock is
compensated to a large extent by the great scale of the intrusions. As has
been explained, there are in the zone of anamorphism innumerable batho-
liths, bosses, and stocks, and in temporary fractures intimate intrusions for
extensive areas. Hence the conditions favorable for endomorphic and
exomorphic effects without transportation of material to a great distance
from an intrusive are widespread and important. The quantity of solutions
is small, and they make their way very slowly through the subcapillary
openings. The consequence is that, where there is a great simple mass
of intrusive with few offshoots, the chemical compositions of the two rocks
are affected for only a short distance from each other; but where the
intrusive and intruded rocks are intimately mingled there may be a pro-
found modification in the chemical composition of both rocks over an
extensive area, each approaching the chemical composition of the other.
This general statement requires some amplification.
INTERACTION OF INTRUSIVE AND INTRUDED ROCKS. 715
As just noted, where the contacts are sharp one does not aeed to go
far in order to obtain rocks the chemical composition of which corresponds
to that of the intruded and intrusive masses, although the intrusive may be
affected te a greater distance by the absorbed material than are the intruded
rocks, simce im a magma the elements are more rapidly distributed than
they are in the solid rock, in which the water is very small in amount and
moves very slowly. However, even in the case of the intruding magma,
Becker“ and others have shown that the process of diffusion is compara-
tively slow, a very long time being required for the chemical composition of
the intrusive to become greatly affected at a considerable distance from the
contact with the intruded rock.
Where, however, the intruded and intrusive rocks are intimately min-
eled there may be profound modification of the chemical composition of
both rocks. A case of this already given is that of parallel injection into
slates, schists, and gneisses. In such a case the modification may be so
ereat that the chemical composition of the intruded rock, if sedimentary,
may vary greatly from that of ordinary sediments, and the intruded rock
may differ considerably in composition from the main mass of inirusives.
Such rocks as these are the so-called injection gneisses (See pp. 725-727.)
These rocks are usually coarsely or finely banded, and may be aqueous
and igneous in any proportion, or igneous rocks cf two kinds in any pro-
portion, provided the intruded rock was a schistose igneous one.
Another case of intimate mingling of the two rocks is that where
numerous fragments of the intruded rock are caught by the intrusive. By
subsequent movement such fragments may make their way into the magma
for some distance from its border, and their partial or complete absorption
may affect the chemical and mineral composition of the intrusive for con-
siderable or even long distances from the outside of the intrusive.
In closing this part of the subject it should be noted that the above
statements as to the rather short distances to which chemical changes are
usually limited does not contradict the. statement already made, and more
fully developed later, that the metamorphic effect of great intrusives often
extends far from the intrusive masses.
«Becker, George F., Some queries on rock differentiation: Am. Jour. Sci., 4th ser., vol. 3, 1897,
Sie
pp. 27-31.
716 A TREATISE ON METAMORPHISM.
METAMORPHIC EFFECTS.
The metamorphic effects accomplished in connection with intrusions
are both structural and mineral. It has already been noted (p. 712) that
the effects of great masses of intrusive rocks may extend for several or many
kilometers. It is by the study of areas adjacent to batholiths that the
contact effects are best appreciated. About a great batholith there are
commonly various zones of metamorphism which grade into one another.
This is true both of the structures which are formed and of the minerals
which develop.
STRUCTURES.
It has already been indicated that as batholiths are intruded the rocks
are deformed by being pushed aside. The detormation is mainly accom-
plished by recrystallization, as described on pages 690-696. During
the process the majority of rocks take on a slaty or schistose structure.
Cleavage developing normal to the pressure is peripheral to the batholith
or circumseribes it. The contact effect naturally dies out as the distance
increases from the intrusive. Near the intrusive the recrystallized rock may
be a coarse-grained gneiss or schist; farther away from the intrusive it may
become an ordinary slate; and still farther away a slaty cleavage may
not be found unless it had been previously developed in some other way.
Between these three zones there are gradations.
In America peripheral structures in gneisses, schists, and slates are
illustrated by the Carboniferous batholiths of New England, described by
Emerson;* by the Algonkian batholiths of the region northwest of Lake
Superior, described by Lawson;’ by the granite batholith of the Black Hills,
described by Van Hise;’ and by the batholiths of Vancouver, described by
Dawson.@
If a previous regional cleavage exists in an area intruded by a bath-
olith, this older cleavage is reenforced if it correspond to the new direction
of cleavage, and tends to be obliterated if it be diagonal or per pendicula:
a8 merson, B. K, Augen ate and ameter granites in Maas husetts: Bull. Geol. Soc. America,
vol. 1, 1890, pp. 559-561.
> Lawson, A. C., The laccolitic sills of the northwest coast of Lake Superior: Bull. Geol. and Nat.
Hist. Surv. Minnesota, No. 8, 1893, pt. 2, pp. 24-48.
¢ Van Hise, C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1,
1890, pp. 206-212.
@ Dawson, George M., Report on a geological examination of the northern part of Vancouver Island
and adjacent coasts: Rept. Geol. and Nat. Hist. Sury. Canada for 1886, pt. B, 1887, pp. 1-129.
MINERALS PRODUCED IN CONNECTION WITH INTRUSION. 71%
to it. All of the above relations may obtain about a single batholith, as in
the case of the Black Hills granite already mentioned. At places where
the new cleavage develops at right angles to the old cleavage, the older
may be completely obliterated near the batholith. Farther away from
it the old and new cleavage may both be present and intersect each other,
and still farther away the old cleavage may be dominant.”
THE MINERALS.
The minerals which form depend upon the intensity of the metamor-
phism. To illustrate, closest to the Black Hills granite batholith is a gneiss
containing abundant quartz, mica, feldspar, garnet, staurolite, hornblende,
and tourmaline. Farther from the granite is a schist containing quartz
and mica, comprising both muscovite and biotite, with staurolite and garnet.
Farther out is a schist in which the staurolite is absent and garnet is present.
Still farther away is an ordinary slate in which there are small flakes of
mica, but in which there are no garnets or staurolite. Similar illustrations
are furnished by the batholiths of Massachusetts, described by Emerson;?
but these have, in addition to the minerals above mentioned adjacent to
the granite, cyanite; somewhat farther out in the schists, sillimanite; and
still more remote in the ordinary slate, andalusite. The three minerals
mentioned furnish a particularly good illustration of the metamorphic
effect with reference to position, since they have precisely the same chem-
ical composition, Also these minerals furnish a perfect illustration of the
principle, expounded in Chapter IV (pp. 182-186), that in proportion as the
metamorphic effect is profound and the pressure great, minerals of high
specific gravity develop. The specific gravities of andalusite, sillimanite,
and cyanite are, respectively, 3.18, 3.235, and 3.615, and this is their order
observed in approaching the intrusives.
In a given case the different mineral zones are not sharply defined,
real
but grade into one another. Moreover, the most intensely metamor-
phosed area may contain all the minerals developed by the feebler meta-
morphic processes This may be explained on the supposition that as
the metamorphic wave passes away from the central batholith the condi-
tions were first those of mild metamorphism, later those of moderate meta-
morphism, and finally those of severe metamorphism. Also adjacent to the
«Van Hise, cit., pp. 232-234. > Emerson, cit., pp. 559-561.
718 A TREATISE ON METAMORPHISM.
intrusive mass as it cools, following the severe metamorphism, the conditions
are those of moderate and mild metamorphism. As the metamorphic wave
passes out from the newly intruded batholith the minerals first developed
may later be transformed into other minerals requiring more intense
metamorphic power, and thus early minerals may be partly or wholly
obliterated by transformation into the minerals of more intense metamor-
phism. Under such circumstances the facts might be discovered by partial
change or by pseudomorphs. The phenomena are finely illustrated about
the batholiths of western Massachusetts. According to Emerson—
The zonal character of the contact metamorphosis around these batholites is
interesting, especially in aluminous sediments. The first wave of heat develops the
easily formed minerals, fibrolite and chiastolite; stronger heat, staurolite and garnet;
then the first influx of the alkaline waters from the granite forms pseudomorphs of
these in muscovite, and with increasing heat feldspars develop. So the highly altered
rocks nearest the intrusive mass have often passed through all the stages one passes
over in going from the outer zone inward. Thus, in the Carboniferous argillite in
Harvard one finds masses of interlaced prisms of andalusite, of the largest size and
finest pink color, inclosing crystals of fibrolite in abundance (the two not orientated
to each other), and the whole in every stage of change to coarse muscovite. This
preserves three stages which were plainly passed over in succession, and nearer the
granite large feldspars are interspersed. In the Hattield argillite, a zone of delicate
chiastolite is succeeded inwardly by a zone where the chiastolites are changed to a
mixture of muscovite and minute twins of staurolite (the mass still retaining the
shape and black cross of the chiastolite) by the influence of greater heat and alkaline
solutions; and nearer the granite the whole changes to sericite schist, chlorite schist,
and finally hornblende and feldspar appear near the contact with the hornblende
granite.” Q
In Europe at many localities a zonal arrangement of the metamorphic
minerals peripheral to intrusive masses has been observed. The minerals
commonly developed and the order of their arrangement are in general
similar to the instances above given. Four well-known instances are those
furnished by the Vosges, the Erzgebirge, the Hartz, and the Tyrol. In
the Vosges, granite intrudes the Steiger Schiefer.’ Says Rosenbusch:
Die gesammte Contactzone der Steiger Schiefer an den Granititen liisst sich
demnach gemiiss der wesentlichsten Entwicklungsglieder in 3 annihernd concen-
trische Partialzonen zerlegen, welche etwa zu bezeichnen wiiren als:
(1) die Zone der Fleck-, Frucht- oder Knotenschiefer mit unveriinderter Schie-
fergrundmasse oder Anotenthonschicfer:
“Emerson, B, Kk., Porphyritic and gneissoid granites in Massachusetts: Bull. Geol. Soc. America,
vol. 1, 1890, pp. 560-561.
> Rosenbusch, H., Die Steiger Schiefer und ihre Contactzone an den Granititen yon Barr-Andlau
und Hohwald, R. Schultz & Co., Strassburg. 1877. pp. 178-250.
MINERALS PRODUCED IN CONNECTION WITH INTRUSION. 719
(
(2) die Zone der Fleck-, Frucht- oder Knotenschiefer mit deutlich gréber Hage s-
tallin entwickelter Schiefergrundmasse oder Anotenglimmerschicfer;
(3) die Zone der islemse ikea, speciell der weitaus vorwiegenden Andalusithorn-
felse.”
In the Erzgebirge granite intrudes phyllite. Dalmar summarizes as
follows:
Sonach lassen sich also innerhalb der fortschreitenden Reihe von Umwandlungen,
welche die Phyllite im Contact mit den Granitstécken erlitten haben, folgende vier
Stadien unterscheiden:
(1) Das Stadium der /ruchtschiefer mit unverinderter Schiefermasse.
(2) Das Stadium der /puchtschiefer mit krystallinisch verinderter Schiefermasse.
(8) Das Stadium der schicferigen Glimmerfelse.
(4) Das Stadium der Andalusitglimmerfelse und Andalusithornfelse.”
In the Hartz granite has intruded the sedimentary rocks, and here
Lossen finds three contact zones. Says Lossen:
Im Allgemeinen aber doch sehr wohl ausgepriigte, concentrisch um den Granit
verlaufende Steigerungszonen der nach dem Eruptivgestein hinwachsenden Umbil-
dung: die Aeussere Knotenschieferzone, die mittlere Hornfelszone und die innere
one der Glimmerschiefer-iihnlichen Hornfelse gegliedert werden.°
Near Monzoni in the Predazzo district of Tyrol isa set of zones charac-
terized in passing from the intrusive by a succession of garnet, augite,
serpentine, and brucite.* This is a very interesting case, because the first
two minerals are those which ordinarily develop in the zone of anamorphism,
whereas the third and fourth minerals commonly develop in the zone of
katamorphism. So far as I know this is unique. It suggests, if all these
minerals be original, that near the intrusive mass the pressure was sufficient
to require reactions giving decrease in volume, and therefore the condi-
tions were those of the zone of anamorphism; whereas at a greater distance
the pressure was not so great but that reactions could take place with
increase of volume, and therefore the conditions were those of the zone of
katamorphism. However, before this can be regarded as established it
must be shown that the serpentine and brucite developed simultaneously
with the garnet and augite, and are not later alteration products.
“ Rosenbusch, cit., pp. 177-178.
>Dalmer, K., Erliiuterungen zur geologischen Specialkarte des Konigreichs Sachsen: Section
Kirchberg: Leipzig, 1884, Blatt 125, pp. 26-27.
¢Lossen, K. A., Erliiuterungen zur geologischen Specialkarte yon Preussen und den Thiringischen
Staaten: Blatt Harzgerode, Berlin, 1882, p. 45.
@ Teller, F., and John, C. von, Geologisch-petrographische Beitrage zur Kenntniss der dioritischen
Gesteine von Klausen in Sudtyrol: Jahrbuch k.-k. geol. Reichsanstalt, 1882, XXXII, 589-684.
Irving, A., Metamorphism of rocks: Longmans, Green & Co., London, 1889, p. 80.
720 A TREATISE ON METAMORPHISM.
One additional American illustration is given. Grant describes the
development of contact minerals adjacent to the great basal gabbro of
northeastern Minnesota.* This is an interesting case, since the intrusive
rock is basic and since it is in contact with several different formations—
Animikie slates, Animikie iron-bearing member, green-schists, greenstones,
and granites. The prominent contact minerals developed in the slates are
feldspar, biotite, and muscovite, with occasional graphite, cordierite, and
hypersthene. The important contact minerals in the iron-bearing member
are magnetite, amphibole, augite, hypersthene, and olivine. In the meta-
morphosed greenstones augite and hypersthene are found in considerable
amounts. Besides the distinctive minerals developed, all of the rocks are
completely recrystallized for a distance varying from a hundred meters to
several hundred meters from the intrusive rock.
Concluding this part of the subject, it is evident that the development
of the metamorphic minerals occurs largely while intrusion is taking place
and movement is still going on, which explains the marked tendency for
the development of slates, schists, and gneisses. Following the intrusions
of the great batholiths and the orogenic movements connected with them
the mass conditions are static, yet the conditions may be very favorable
for recrystallization. The temperature may still be high and the solutions
active, and under such circumstances the conditions are very favorable for
the development of porphyritic crystals which have no relation in their ori-
entation either to the batholiths or to the slates, schists, or gneisses. Hence
it follows that within these rocks peripheral to the batholiths we find the best
illustrations of secondary porphyritic crystals. Moreover, the porphyritic
crystals which appear are dependent upon the tensity of the metamor-
phism. The order of the heavy porphyritic crystals in relation to the bath-
oliths is that above mentioned, viz., near the batholiths are apt to be the
heavy silicates, tourmaline, staurolice, and cyanite; more remote from them,
garnet and sillimanite; and still more remote, andalusite. Of course these
overlap and are intermingled, as already explained.
PEGMATITES.
The complex processes in connection with the intrusive batholiths
and their offshoots may, and indeed do, produce masses of rocks which
«Grant, U. 8., Contact metamorphism of basic igneous rock: Bull. Geol. Soc. America, vol. 11,
1900, pp. 503-510.
ORIGIN OF PEGMATITES. 721
may be called pegmatites. So far as my observation goes, many of the
more complex phenomena of pegmatization which can not be explained
by igneous injection alone are in regions in which the rocks have been
buried to a very considerable depth. They commonly form in the lower
part of the zone of fracture, in the zone of combined fracture and flowage,
or in the part of the zone of flowage in which continuous fractures may
exist temporarily The phenomena occur to a great extent in the lower
zone in connection with offshoots from the deep-seated batholiths—i. e.,
injection dikes which have made their way along planes of rock weakness,
such as contacts or cleavage planes. Probably a strictly logical arrange-
ment would require that the treatment of pegmatites be deferred until the
relations of the zones of anamorphism and katamorphism are considered,
but the relations of pegmatites to deep-seated intrusives are so close that,
for the sake of continuity, the subject is dealt with here.
Pegmatization has been variously explained as the result of true igneous
injection, of aqueo-igneous action, and of water impregnation or cementa-
tion. Brogger has strongly enforced the idea that many pegmatite veins
are true igneous injections. In support of this idea he cites the undoubted
frequent association of pegmatitic ves with intrusive masses of acid rock,
the fact that many of the pegmatitic veins behave like other eruptives, and
that their structure is that of igneous rocks.
Williams is in substantial agreement with Brogger in assigning an
essentially eruptive origin for the greater number of the pegmatite dikes of
the Piedmont Plateau.’ In favor of this view he cites (1) the likeness in
chemical and mineralogical composition of the granite masses to the pegma-
tites, although it is stated that the pegmatites are usually somewhat more
acid; (2) the fact that the size and abundance of pegmatites are directly
proportional to their nearness to eruptive granite masses; (8) that in com-
position the pegmatites are independent of the rocks which surround them,
and (4) that they do not show the drusy or symmetrical character of veins.
While Williams thus holds to the igneous origin of the pegmatites, he
thinks they show a greater activity of mineralizing agencies than the normal
fiir Kryst., vol. 16, 1890, pp. 215-235.
bWilliams, G. H., The general relations of the granitic rocks in the middle Atlantic Piedmont
Plateau: Fifteenth Ann. Rept., U. S. Geol. Survey, 1895, pp. 675-684.
MON XLVII—(04 46
T22 A TREATISE ON METAMORPHISM.
interprets those pegmatites which by their mode of occurrence and associa-
tion strongly indicate an igneous character as the products of the residual
and therefore most acid portion of a granite magma highly charged with
water and other mineralizing agents. Such a siliceous material, in a state
intermediate between fusion and solution, has been injected into fissures and
there crystallized into very coarse-grained aggregates, not necessarily
through any great slowness of this process, but rather in virtue of the aid to
crystallization afforded by the abundance of mineralizers present.” *
While Williams and Broégger strongly emphasize the igneous side of
pegmatization, they both agree that some of the pegmatites which show a
comb structure are essentially the result of aqueous processes.”
Standing at the other extreme is Hunt, who maintains a strictly
aqueous origin for pegmatites.° In an intermediate position, ascribing an
aqueo-igneous origin to pegmatites, are Elie de Beaumont, Scheerer, Leh-
mann, Credner, Reyer, and Crosby and Fuller. Scheerer in 1847, according
to Hunt, ‘‘Conceives the congealing granitic rocks to have been impreg-
nated with ‘a juice,’ which was nothing else than a highly heated aqueous
solution of certain mineral matters. This, under great pressure, oozed out,
penetrating even the stratified rocks in contact with the granite, filling
cavities and fissures in the latter, and depositing therein crystals of quartz
and of hornblende, the arrangement of which shows them to have been of
”@ Reyer, following Scheerer, regards the pegmatites as
“‘Hixsudate der erstarrenden Granitmasse.”* But of all the authors writing
successive growth.
upon pegmatites Lehmann,’ followed by Crosby,’ makes the closest approx-
imation to the view which I hold in reference to pegmatites. Lehmann
conceives that as a result of crystallization of the parent mass and the
concentration of the water in the residual uncrystallized part, a gelatinous
magma rich in silicais formed. ‘‘ Between such a gelatinous magma and a
saturated aqueous solution a large number of consecutive intermediate
stages can be imagined.”” However, Lehmann insists that no part of the
aWilliams, G. H., The general relations of the granitic rocks in the middle Atlantic Piedmont
Plateau: Fifteenth Ann. Rept. U. 8. Geol. Survey, 1895, p. 684.
> Williams, cit., p. 679.
¢Hunt, T. Sterry, Chemical and geological essays: Osgood & Co., Boston, 1875, pp. 191-219.
@ Hunt, cit., p. 189.
¢ Reyer, E., Theoretische Geologie: Schweizerbart’sche Verlagshandlung, Stuttgart, 1888, p. 101.
fLehmann, Johannes, Untersuchungen iiber die Entstehung der altkrystallinischen Schiefer-
gesteine: Bonn, 1884, pp. 24-58.
gCrosby, W. O., and Fuller, M. L., Origin of pegmatite: Tech. Quar. vol. 9, 1896, pp. 326-356.
h Crosby, W. O., and Fuller, M. L., citing Lehmann, cit., p. 345.
ORIGIN OF PEGMATITES. 123
water is atmospheric, but is wholly eruptive water. Crosby and Fuller,
however, think it is possible that a portion of the water required “for the
more perfect hydration and liquefaction of the residuum of the magma may
be derived from extraneous sources.” ”
It seems to me that to explain adequately all the facts of pegmatization
described in various regions of the world, we must conclude that all three
processes have been at work—in some cases igneous injection, in some
cases aqueo-igneous action, in other cases pure water cementation, and in
still other cases combinations of two or all of these processes. It is further
believed that there is no sharp separation between these processes, but that,
on the contrary, there are all gradations between the three. That is, it is
thought highly probable that, wnder sufficient pressure and at a high tem-
perature, there are all gradations between heated waters containing mineral
material in solution and magma containing water in solution. In other words,
under proper conditions water and liquid rock are miscible in all proportions.
This possibility, first published by me in 1896,’ in precisely the above
words, has received almost conclusive experimental verification at the
hands of Barus. As noted on page 80, Barus has shown that at temper-
atures below 200° C., and at high pressure, soft glass and water are actually
miscible in all proportions.
From the water solutions true cementation takes place; from the rock
solutions, true injection. Pegmatization comprises these and the interme-
diate processes. It is not to be expected that under great pressure and at
high temperatures there is any sharp line of demarcation between the
processes of aqueous cementation and igneous injection. At the surface
it is usually easy to sharply separate aqueous from igneous action, but
deeper within the earth even the strongest rocks are latently plastic. At
great pressure heated waters must have power to absorb a quantity of
material far beyond that at the surface of the earth. Truly liquid rock is
highly impregnated with water. It therefore is probable that at consid-
erable depths we have, on the one hand, material which all would call water
solution, and on the other hand material which all would call liquid rock,
with no sharp division line between the two. If this be so, there are all
«Crosby, W. O., and Fuller, M. L., Origin of pegmatite: Tech. Quar., vol. 9, 1896, p. 348.
bVan Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept.
U.S. Geol. Survey, 1894-95, pt. 1, 1896, p. 687.
724 A TREATISE ON METAMORPHISM.
stages of eradation between true igneous injection and aqueous cementation,
and all the various phases of pegmatization may thus be fully explained.
This idea of continuity was first suggested to me by the phenomena
observed in the schistose rocks surrounding the intrusive core of the Black
Hills. Remote from the intrusives the sedimentary rocks are slates; adja-
cent to it they are schists and gneisses. The core rock is a great batholith
of granite, 11 km. broad and 18 km. long. Besides this central mass there
are, to the southwest, a number of smaller masses, from 2 to 6 km.
long, which may be connected below with the greater mass. From the
central mass great quartz-feldspar dikes radiate. In passing away from the
core the dikes become smaller and have a less typical form; at the same
time the material assumes the appearance which we ordinarily denominate
pegmatitic. ‘These veins might be considered true igneous injections. — Still
farther away the pegmatitic masses begin to have vein-like characters—
that is, there is a rough concentration of the material in different layers par-
allel to the walls. Still farther away a true banded-vein structure is found.
Yet farther away feldspar becomes less and less important in the veins and
the quartz more abundant, until remote from the granite the impregnating
material is mainly quartz. Within the intruded rock, adjacent to the
granite, there is also an extensive development of feldspar, giving the rock
a banded appearance and changing it from a schist to a gneiss. If one
examined only the outer zone, in which the quartz cementation series
occur, one would not doubt that they are ordinary deposits from ground
water; if one examined the dike-like masses of the inner zone adjacent to
the great granite mass, one would not doubt that they are true igneous
injections; but in passing back and forth between the two, one observes that
there is every gradation between them, and is driven to the conclusion
that true igneous injection was predominant adjacent to the granite, that in
the central zone aqueous and igneous agencies were about equally impor-
tant, and that in the outer zone aqueous agencies were predominant. It is.
impossible to believe that the larger dikes—plainly offshoots of the central
batholith—are not igneous injections; it is equally impossible to believe
that quartz veins remote from the granite are dikes, or that the ordinary
granite magma has penetrated to considerable distances between the indi-
vidual grains of the schist and thus impregnated it with feldspar.
From the foregoing it is clear that under the general term pegmatite
ORIGIN OF PEGMATITES. 12
on
are included the results of three somewhat distinct phases of action, which,
however, grade into one another; first, pegmatization mainly aqueous;
second, pegmatization mainly igneous, and third, aqueo-igneous pegmati-
zation.
The first case is illustrated in the Marquette district of the Lake
Superior region,” where at certain places in the Michigamme formation
there is a great development of feldspar within the interstices of the schists.
In the crevices minute pegmatitic veins of quartz and feldspar occur. No
adjacent intrusive in this formation has been discovered even by the closest
detailed study. In this case it seems clear that pegmatization has taken
place during the metamorphism of the rocks, in connection with mechanical
action, without the assistance of any extraneous igneous material, and is
therefore essentially aqueous.
The second of these cases is illustrated by the pegmatites described by
Brégger in southern Norway,’ in. which igneous injection satisfactorily
explains all the phenomena. It is to be noticed that in this region the
process of pegmatization occurred at a maximum depth of 600 meters,
while much of it occurred at a depth varying from 30 to 100 meters.
The third phase of pegmatization, which combines both aqueous and
igneous agencies, is far the most extended and important. By it are formed
the great class of the rocks which are usually spoken of as pegmatized.
This phase may affect massive igneous rocks, massive sedimentary rocks,
and their metamorphic equivalents. It occurs in both acid and basic rocks.
In all, the effects may be most complex, ranging from the formation of
great pegmatite dikes to the development of individual crystals of feldspar,
quartz, and other minerals anywhere within the intruded masses.
But this phase of pegmatization is most extensive and best illustrated
by rocks in which there is a gneissic or schistic structure, since, as pointed
out on page 710, cleavage furnishes planes of weakness which are readily
taken advantage of by the igneous rocks. The relations are best studied
where intrusive and intruded rocks are of different colors; for instance,
where there is a dark-colored schist intruded by granite. In such rocks
will be seen a background of schist. Parallel to the folia are immumerable
a@Van Hise, C. R., and Bayley, W.S., The Marquette iron-bearing district of Michigan: Mon.
U.S. Geol. Survey, vol. 28, 1897, pp. 447-448.
b Broégger, W. C., Syenitpegmatitgiinge der stidnorwegischen Augit- und Nephelinsyenite: Zeitsch.
fir Kryst., vol. 16, 1890, pp. 215-235.
126 A TREATISE ON METAMORPHISM.
cementation-injection bands of lighter color. These bands vary from those
as thin as leaflets, bemg perhaps but a single row of crystals, to those of
considerable width. There may be many such bands within the space of
a centimeter, or a single one may be many meters across. Frequently
parts of the injected material are in dike-like masses of varying size, which
cut the schistosity at various angles. At numberless places the leaf-like
bands of pegmatitic-looking material parallel to the schistosity are found
to be connected directly with the dike-like masses cutting the schistosity.
The dark schistose bands are found to show exomorphism in various
degrees by the injecting material. Besides recrystallization, the modifica-
tion consists in the development of various new minerals, the material of
which is derived in part at least from the intrusive. Clear evidence of
this is the frequent addition of minerals, or a large increase in the amount
of minerals, in the pegmatized rock of the kinds found in the adjacent
intrusive. Not only may the main mass of unpegmatized schist show a
deficiency of certain minerals as compared with the pegmatized areas, but
if within the pegmatized area there be any considerable belt of. schist not
affected by the pegmatite masses it may exhibit a similar deficiency. The
endomorphic effect upon the injection bands follows the same rule. The
more intimate the intrusion and pegmatization the greater the endomor-
phism. The smaller bands of injection material gain proportionally more
material from the schist than do the larger ones.
The intrusive rock, which is the final inciting cause of the third phase
of pegmatization may vary from acid to basic, but pegmatization in connec-
tion with granite and other acid intrusives has been most closely studied.
‘The process of granitization described by Michel Lévy* comprises the par-
ticular form of pegmatization produced by this rock. The pegmatization
known as granitization has been studied more closely than other forms
because of its prevalence, because the light color of the granite renders
it easily distinguishable, and perhaps because acid magmas supply more
water and heat than basic ones.
It seems to me that the third phase of pegmatization described above
can be conceived of only as an aqueo-igneous process. In many instances
the larger masses of cutting material are truly igneous dikes containing a
«Lévy, A. Michel, Sur l origine des terrains cristallins primitifs; Congrés géologique international,
4th sess., London, 1888, pp. 117-129.
ORIGIN OF PEGMATITES. 727
little water. On the other hand, the intimate penetration of all parts of the
rocks, down to the openings between the mineral particles, by the pegma-
tizing material can be accomplished only by very mobile solutions which
are more nearly allied to water than to magma; but such water is very
rich in mineral material. Between these extremes are all gradations.
The aqueo-igneous pegmatites, especially the peematitic schists and
gneisses, in all their phases may be seen in any of the regions of America
in which there are great batholithic intrusions. They are illustrated equally
well in the Cordilleras, the Lake Superior region, Canada, and eastern
United States from Maine to Alabama. All these regions afford innumer-
able perfect illustrations of the process. ven the most extreme pegma-
tization is illustrated at many places. One of the best instances known to
me is that of Rib and Mosinee hills, central Wisconsin. These hills of
quartz rock are surrounded by a great batholith of augite-syenite many
kilometers in diameter. Within the quartz rock adjacent to the syenite
feldspar has so extensively developed as to change the rock from a nearly
pure quartz rock to one in which feldspar is an important constituent. A
more extensive and no less perfect illustration of pegmatization, as well as
of almost every other phase of the process, is furnished by the Hudson
schist in the upper part of Central Park, New York, and even better at New
Rochelle, adjacent to Long Island Sound. Williams’s accurate description
of the phenomena exhibited in Maryland shows this to be an admirable
case of pegmatization.”
In summary, pegmatization, when it occurs on a great scale, usually is
found in connection with great intrusive masses in which there have been
long-continued composite intrusions. No great batholith is the result of a
single simple intrusion. ‘The introduction of such masses went on irregu-
larly through a very long time. Pegmatitic masses are not the result of a
distinct epoch of eruption, but usually are produced in connection with the
closing phases of igneous activity. The pegmatites very frequently cut the
igneous rocks intruded in an earlier stage of the igneous epoch. After the
main masses of igneous rock have crystallized they continue to contract as
they cool, and are thereby fractured. This occurs while they are still very
hot, and gives ready access to the pegmatizing material. Also the surround-
«Williams, G. H., The general relations of the granitic rocks in the middle Atlantic Piedmont
Plateau: Fifteenth Ann. Rept. U. 8. Geol. Survey, 1895, pp. 657-684.
728 A TREATISE ON METAMORPHISM.
ing rocks which have been long affected by the batholiths become highly
heated as a result of direct conduction and in consequence of convection
through water. The occluded water which undoubtedly exists in the parent
mass, at the moment of crystallization is in large measure expelled. As
crystallization continues the residual magma gradually becomes more
watery. Thus a considerable part of the water, the presence of which is
evidenced by the character of the pegmatites, is derived from the magma
itself. Doubtless the pegmatite dikes in immediate connection with the
great parent masses of igneous material, which closely resemble ordinary”
igneous dikes, largely and perhaps predominantly derive their water from
the igneous rock itself.
As the pegmatites close to the central mass solidify, a large portion of
the water is expelled and travels outward to help form the pegmatitic rock
having a more distinctive vein character; but in many and perhaps most
eases the water in the outlying pegmatite dikes and veins, which more
and more assume the character of ordinary veins, has been largely derived
from the surrounding rocks. We thus have completely explained the
variations from the true igneous dikes by imperceptible gradations to
materials which have the unmistakable characters of aqueous mineral veins.
This injection-cementation theory of pegmatization gives an adequate
explanation of the extreme coarseness of crystallization which the pegma-
tites frequently exhibit. The rocks in which the pegmatites form, both the
parent crystallized igneous mass and the surrounding rocks, are highly
heated, and hence during the closing pegmatitic stages, even if the masses
be small, the temperature falls so slowly that there is ample time for the
formation of very coarse crystals. Also it is well known that there is no
limit to the size to which the crystals may grow from water solutions. The
separation of the crystalline material in many pegmatite veins is more
nearly analogous to crystallization from a solution than to the solidifica-
tion of a solvent (see p. 113), and this analogy, so far as it exists, is
favorable to very coarse crystals.
FUSION AND ABSORPTION.
In connection with metamorphism in the deep-seated zone where igne-
ous rocks are among the agents, the question of the fusion of the intruded
rocks should be considered. It has been held by Mallet and by others
that heat produced by mass-mechanical action is a sufficient cause to pro-
FUSION AND ABSORPTION. 729
duce aqueo-igneous fusion, and that such supposed fused material is the
source of molten material for volcanoes.“ Mallet supposes that the material
is mechanically divided so fine that a sufficient amount of heat is developed
by this work and by friction between the particles to fuse the rock. How-
ever, he does not tell how fine this must be, although he speaks of reducing
a rock to an absolute powder. What is meant by an absolute powder is
not apparent, but one might suppose it means a powder the particles of
which are of molecular size.
The conclusions herein contained concerning recrystallization and rock
flowage have an important bearing upon the hypothesis of aqueo-igneous
fusion. It has appeared that if water be present when the material, as a
result of the mechanical subdivision, or for any other cause, reaches the
very moderate temperature of 185° C., the adjustment is accomplished
mainly by recrystallization, and that fusion is not necessary to account for
the plasticity of the rocks. Probably a much higher temperature is required
for aqueo-igneous fusion than for recrystallization. Barus has failed to
secure aqueo-igneous fusion of the silicates at a temperature of 600° C.,?
and at temperatures much lower than this it is certain that recrystallization
goes on very rapidly.
So far as the typical schists and gneisses themselves are concerned, it
is certain that they are not the products of aqueo-igneous fusion. They
have the peculiar textures and structures characteristic of these rocks. (See
pp- 779-783.) Every magma crystallizes according to the laws of magmas,
and produces textures and structures which are characteristic of such
crystallization. The textures and structures of the two classes of rocks
are very different. There is no evidence that the great masses of the
magmas are formed by the fusion of the crystallized rocks in consequence
of mech nical action. But it does not follow that various rocks, including
the schists and gneisses, may not be fused by contact with intruded
magmas. It has been seen in connection with injection that profound
metamorphic effects are produced upon the intruded rocks, the ordinary
metamorphic forms being slates, schists, and gneisses. It has been held
that the phenomena of pegmatization may be explained by crystallization
«Mallet, Robert, Volcanic energy; an attempt to develop its true and cosmical relations: Philos.
Trans. Royal Soc. London, vol. 163, 1874, pp. 147-227.
> Powell, J. W., Report of the Director: Fourteenth Ann. Rept. U.S. Geol. Survey, pt. 1, 1893,
pp. 161-162.
730 A TREATISE ON METAMORPHISM.
from materials between water solutions and true magmas. During pegma-
tization, in proportion as the conditions approach those of a magma,
the textures and structures may depart from those of slates, schists, and
gneisses and approach those of igneous rocks. Thus there may be all
gradations between the textures and structures characteristic of the slates,
schists, and gneisses and those characteristic of the igneous rocks. In some
regions there occur remarkable combinations of textures and structures
characteristic of various rocks; for instance, the clastic rocks, the schists,
and the igneous rocks.
When once a rock, whether a sediment, a crystallized igneous rock,
or a metamorphic rock, has become a magma by fusion, whether it be
by aqueo-igneous fusion or by dry fusion, the resultant material is a true
magma. The crystallized rock which forms from it by cooling has all the
characteristics of an igneous rock. . The textures and structures formed are
those of the igneous rocks. The textures and structures of the fused
sedimentary, original igneous, or metamorphic rocks are totally destroyed,
and the newly formed rock is to all intents and purposes a new igneous
rock. If rocks derived by the fusion of previous solid rocks have again
crystallized on a large scale, it is most difficult to prove this. The contacts
between the fused and unfused portions of the material would be similar to
those between ordinary intruded and intrusive rocks. The only way which
I can suggest to show fusion and reerystallization on a large scale is to
prove that the rock supposed to be fused and recrystallized possesses the
chemical composition of the rock from which it is believed to have been
derived. For instance, a fused and recrystallized shale should possess the
textures and structures of an igneous rock, but the essential chemical
composition of a sedimentary rock. This criterion of chemical composition
as applied to the red granular rock (meta-rhyolite) of Pigeon Point leads
to the conclusion that it is probably an independent intrusive, rather than the
result of the fusion of the Animikie rocks by the gabbro.* (See pp. 732-733.)
But while it has not been shown that fusion and recrystallization have
taken place on a great scale, various cases have been described in which
the evidence seems clear that to some extent intruded rocks have been fused.
There can be no question that fusion of fragments included within
igneous rocks does take place. Very numerous inclusions which are found
a Bayley, W.S., The ae and sedimentary rocks on Pigeon Point, Minnesota, and their con-
tact phenomena: Bull. U. 8. Geol. Survey No. 109, 1893, p. 118.
FUSION AND ABSORPTION. 731
adjacent to great batholiths show various stages of absorption. Frequently
the partly absorbed residual fragments are profoundly metamorphosed,
being perhaps completely recrystallized, and frequently greatly changed in
chemical composition from that of the parent rock.
It is certam that the absorption of fragments must react upon the
chemical composition of the absorbing rock. In so far as rocks fused and
absorbed by a magma differ from it in chemical composition, they must
change the chemical composition of that magma. Where the rock absorbed
is igneous, and therefore has the composition of igneous rocks, the
modified magma has a composition intermediate between two magmas.
Where the material has the composition of a sediment, this changes the
chemical composition of the magma toward that of the sediment. For
instance, if a considerable portion of limestone or dolomite be absorbed,
the rocks become very rich m calcium or calcium and magnesium. If a
considerable portion of shale or slate derived from mudstone be absorbed,
the igneous rock would be likely to be deficient in the alkalies, especially
sodium; it might be deficient in the alkaline earths; and it would probably
be high in aluminum, thus becoming intermediate in chemical composition
between igneous rocks and the mudstone series. If a sediment intermediate
between the limestone series and the mudstone series, as, for instance,
caleareous shale or slate, were absorbed by magma, while there might not
be any deficiency in the alkaline earths, there would be a deficiency in the
alkalies, and probably the aluminum would be abnormally high.
If a rock were fused by contact with a magma without absorption by
it or mingling with it, it would, of course, have approximately the compo-
sition of the original solid rock, and therefore that of a sedimentary, igneous,
or metamorphic rock, as the case might be. Doubtless the compositions of
such rocks would be somewhat modified by the exomorphic effect of the
igneous rocks. (See pp. 713-714.) If any considerable mass of solidified
rock were fused as a result of contact with igneous rocks, it is natural to
suppose that for a zone of variable width the two would become mixed, and
thus there would be a gradation in chemical composition between the fused
rock and a normal magma.
While it is beyond dispute that all of the above cases occur, petrog-
raphers have been inclined to believe that none has taken place on a great
scale. It may well be doubted whether the excess of heat in molten magma,
beyond that required to keep it liquid, is sufficient to perform the vast
oO, A TREATISE ON METAMORPHISM.
amount of work required for the liquefaction of a great mass of solidified
rock. Work must be done in fusing the material, and work must be done
in expanding the material. Besides raising the temperature to the fusion
point, all the latent heat of fusion must be supplied. The heat required
for the process of fusion of rocks in a furnace is very great. Ordinarily a
magma has a temperature only slightly in excess of that required to hold
it in the liquid form. Therefore, that it could furnish a sufficient amount
of heat to liquefy immense masses of solid rocks seems highly improbable.
And it has already been seen that it can not be assumed that the necessary
heat is furnished by orogenic movements, although this may be a helpful
factor.
While fusion of solidified rocks of the lithosphere on a great scale,
either by magma or by orogenic movement, or the two combined, is to be
proved, it is certain that fusion has locally occurred. One of the clearest
instances of fusion or semifusion in America is that on Pigeon Point,
described by Bayley.” At this place is a metarhyolite, called keratophyre,
on one side of which is a great mass of gabbro, and on the other side of
which are the slates and quartzites of the Animikie series. The keratophyre
has all the characteristics of an eruptive rock younger than the gabbro.
In a position between the keratophyre and the gabbro is a coarse-grained
red rock, which is intermediate in character between the two and grades
into each of them. It is therefore regarded as a product formed by the
intermingling of the gabbro and rhyolite magmas, the gabbro having been
actually fused for a short distance from the keratophyre. For a distance
of about 9.5 kilometers, with a few interruptions, the contact rock has a
width of from 30 to 90 meters, and for one continuous stretch of nearly
3 kilometers it has an average width of fully 60 meters. If this rock,
which is intermediate in composition between the metarhyolite and the
gabbro, and which grades into each, and yet has the textures and struc-
tures of an igneous rock, really represents the result of the action of the
quartz-keratophyre on the gabbro, as it appears to do, it is one of the best
known cases of actual fusion of one rock by another.
Between the metarhyolite and the slates and quartzites of the Animikie
there is another contact belt which extends for about the same length as the
“Bayley, W. 8., The eruptive and sedimentary rocks on Pigeon Point, Minnesota, and their
contact phenomena: Bull. U. S. Geol. Survey No. 109, 1893, pp. 1-121.
FUSION AND ABSORPTION. (33
contact between the quartz-keratophyre and gabbro, and has an even greater
width, averaging perhaps 90 meters, and being in some places as wide as
150 meters. But only a comparatively small part of this belt, that nearest
the rhyolite, has the textures and structures of an igneous rock, and seems
to have been completely fused and recrystallized into slates and quartzites.
The fusion metamorphism produced by the metarhyolite is correlated with
the enormous masses of intrusive granite which occur in the Keweenawan
series above the Animikie. The Pigeon Point dike of this rock doubtless
is located at one of the passages through which the material of some of
these larger masses made their way toward the surface. In this connection
it is notable that subsequent intrusive trap dikes cut all of the other rocks
of Pigeon Point with sharp contacts, and no sign of metamorphosing effects:
Bayley states that the metarlyolite itself may be the product of the
action of the gabbro upon the slates and quartzites. If this suggestion
is correct the Pigeon Point locality is a still more remarkable case of the
production of a rock by fusion of sediments. But that the rhyolite has
been produced from the slates can not be considered as determined. Indeed,
the analyses of the rhyolites and of the slates and quartzites are different
from each other in various important respects. While the amount of silica
and alumina in the two rocks is substantially the same, the alkalies are
nearly twice as abundant im the rhyolite as in the sedimentary rocks. Also,
the iron is more than twice as abundant in the sedimentary rocks as in the
igneous rocks. Nor can it be supposed that the surplus of alkalies and the
deficiency of iron in the rhyolite can be explained by the gabbro, it modi-
fying the composition of the fused slates and quartzites so as to produce a
magma of the composition of the rhyolite; for if any considerable portion
of gabbro magma had been added the silica content would have fallen and
the alumina would have gone up. Furthermore, the gabbro is scarcely
richer in alkalies than the slates and quartzites; hence, on the theory that
the red rock was formed by the fusion of the slate and quartzite by the
gabbro, one would have to explain why the quantity of alkalies in the red
rock is about twice as great as in either of its possible sources. Thus the
analyses seem to stand in the way of the conclusion that the granular red
rock (metarhyolite) is really a result of the fusion of the Animikie slates
and quartzites by the gabbro.*
“Clarke, F. W., and Hillebrand, W. F., Analyses of rocks and analytical methods: Bull. U. S.
Geol. Survey No. 148, 1897, pp. 106-110.
734 A TREATISE ON METAMORPHISM.
Cases of claimed fusion of vastly greater magnitude than that at
Pigeon Point are described by Lawson, Barlow, and other members of the
Canadian survey. Lawson“ has held that the great batholithic masses of
granite and gneiss, some of them many kilometers in diameter, in the
region of Rainy Lake and Lake of the Woods, northwest of Lake Supe-
rior, have been produced by the fusion of the sedimentary and volcanic
series, which he has called Keewatin and Coutchiching. He has called
this a case of subcrustal fusion. Similar statements have been made by
Barlow in reference to the batholiths of granite in the Original Huronian
district;’ and the same thing has been said with reference to the granite and
gneiss of the Original Laurentian district. However, no adequate evidence
has been offered to support any of these conclusions. Composite chemical
analyses have not been made of the Keewatin and Coutchiching rocks and
the granites and gneisses supposed to be formed by their fusion, in order
to compare their compositions and thus to determine whether the granites
could possibly have been derived from the Keewatin and Coutchiching
rocks by subcrustal fusion. All of the phenomena along the contacts of
the granitic rocks and the Keewatin and Coutchiching are such as may be
found at various parts of the world where batholiths of igneous rock have
intruded other rocks. Many fragments of the mtruded rocks are included in
the intrusive. At many places there is a border belt in which the two are
intermingled in the most confused way, and, furthermore, in the intruded
rocks pegmatization has occurred adjacent to the batholith. The phenom-
ena might possibly lead one to infer the derivation of the intrusive rocks
from the intruded recks by the fusion of the latter, and subsequent action
as an intrusion of the magma thus formed, but I do not so interpret them,
I have gone over the Rainy Lake and Lake of the Woods region in
the field, and have visited and closely studied the specific localities described
by Lawson as giving evidence of subcrustal fusion. At every one of these
localities the phenomena are precisely those which generally occur where
great intrusive batholiths penetrate other rocks. For instance, at the Nar-
rows of Kaiarskons Lake, which is cited by Lawson’ as one of the best
«@ Lawson, A. C., The internal relations and taxonomy of the Archean of central Canada: Bull.
Geol. Soc. America, vol. 4, 1890, pp. 185-186. 2
+ Barlow, A. E., Relations of the Laurentian and Huronian rocks north of Lake Huron: Bull
Geol. Soc. America, vol 4, 1893, pp. 313-332.
¢ Lawson, A. C., Report on the geology of the Rainy Lake region: Ann. Rept. Geol. and Nat.
Hist. Survey of Canada, for 1887-88, new ser., vol. 3, pt. 1, 1889, p. 32.
FUSION AND ABSORPTION. 735
instances giving evidence of subcrustal fusion, the granite cuts the horn-
blendic schist in a most intricate fashion. However, in most cases the con-
tacts between the two rocks are knife-like in their sharpness, and this is true
whether the granite dikes and veins are large or small. At one place a few
feet square the granite and schist are so confused as to make it possible that
the hornblende-schists were locally softened. The granite near the contact
contains numerous fragments of the schist. The hornblendic schist gener-
ally retains its typical texture and structure directly to the contact with the
granite. The phenomena are clearly those of complex intrusion, with no
evidence whatever that the granite has crystallized from the fused horn-
blendie schist. Similar phenomena were found at other localities cited by
Lawson as evidence of suberustal fusion.
This case has been dwelt upon because it is the one in America where
it has been most strongly claimed that immense masses of igneous rocks
have been produced by the fusion of older solidified rocks. The relations
seem to me, however, to be clearly those which are characteristic of great
batholithic intrusions in the rocks of the zone of anamorphism.
Another instance where in America it has been claimed that fusion meta-
morphism has taken place on a great scale is that described by Emerson.
it is well known that in eastern Massachusetts there are numerous great
batholiths of granite intrusive in the Paleozoic strata. Concerning these
batholiths Emerson makes substantially the following statements:
The Quincy granites of eastern Massachusetts do not alter the Cambrian schists
and do not absorb any material from them. Several other granite bands extending
across Worcester County contrast with the Quincy band in the following particu-
lars: They are often coarsely porphyritic, while the Quincy granites are not. They
are microcline granites. The Quincy granites are orthoclase granites. They contain
biotite, or biotite and muscovite instead of biotite and hornblende or glaucophane.
These granite batholiths are also contrasted with the Quincy rock in haying a
broad peripheral layer, which has all the peculiarities of pegmatite in some cases, and
grades into black albitic granites, or even quartz-diorites.
These differences are largely due to the fact that in the Worcester district the
granites have fused much of the surrounding schist into their composition. This
was proved by finding characteristic inclusions of the schist in great numbers and of
every size in the granite, and also by tracing these inclusions into smaller and smaller
filaments, until they faded from sight, and finding with the microscope far beyond
this point in the fresh granite clear traces of the schists. Where the schist contains
pyrite, garnet, fibrolite, cordierite, or graphite, the granite becomes more ferruginous
and garnetiferous. The amber coarse fibrolite of the schist appears dissolved and
736 A TREATISE ON METAMORPHISM.
recrystallized as a white, silky bucholzite, or fazer-kiesel in the granite, and the gra-
phite scales are inclosed in all the constituents of the granite over many square miles.
Over the whole surface of the great Hubbardston batholith of perfect coarse
porphyritic granite, 51 kilometers long and 10 kilometers wide, it was possible to
map the areas once occupied by the different schists, which formerly mantled over
the granite mass, by means of the indestructible constituents of the former schists,
by the portions which had melted into the mass of the granite, by the filaments still
remaining unabsorbed, and by the different aspect of the granite, dependent largely
upon the great increment of iron. Using the first two criteria especially, this double
mapping of the region was carried out in the sheets prepared for the United States
Geological Survey.
The region extending for several kilometers south from Mount Wachusett, and
that north and south of Brookfield, in the Hubbardston batholith, furnish abundant
exposures for the study of the phenomena here described.“
The statement of facts which Emerson makes seems clearly to lead to
the conclusion that the granites of the Worcester district have actually
fused the metamorphosed schists to some extent. However, the changes
in chemical and mineral character of the granites adjacent to the schists
may be partly due to the transfer of material from the injected rock by
solutions. Until the facts for this district are given in detail, including
chemical analyses of the different phases of the granites, it will be impos-
sible to say how far the intruded schists have been absorbed by the granite;
and certainly the assumption that the differences between the Worcester
granites and the Quincy granites are due to absorption of schistose material
by the former requires much more evidence than has yet been offered.
COMBINATIONS AND RELATIONS OF THE VARIOUS PROCESSES.
The processes of welding, strain within the elastic limit, strain beyond
the elastic limit, cementation, metasomatism, and injection rarely occur
separately. Commonly several of them take place at the same time, and in
many instances all of them unite in the metamorphism of a rock. In the
previous pages, for the sake of clearness in analysis, each process has been
considered by itself, so far as this was possible, so that the effect of each
could be clearly appreciated. This natural tendency to consider each
process separately, and the tendency to regard a certain process under
consideration as the sole cause of metamorphism, have led to the artificial
classification into dynamic metamorphism, contact metamorphism, ete
«Emerson, B. K., Difference in batholithic granites according to depth of erosion: Bull. Geol. Soe.
America, vol. 10, 1899, pp. 499-500.
RELATIONS OF GRANULATION AND RECRYSTALLIZATION. 737
The mechanical process of welding can not take place without strain within
the elastic limit. Strain beyond the elastic limit can not occur without
strain within the elastic limit and welding. Therefore, where the former
occurs all the mechanical processes are involved. The chemical processes
of cementation and metasomatism do not occur separately; where one takes
place the other is sure to be active. These processes, together, go on with
or without injection. But injection, which involves both mechanical and
chemical factors, never takes place without involving one or two or all of
the other processes. Where it takes place on a large scale it is sure to
include all the mechanical processes. Moreover, under such circumstances
the exomorphie alterations involve the chemical transformations of cemen-
tation and metasomatism on a great scale.
From the foregoing it follows that in most instances of alteration in the
zone of anamorphism it is impracticable to state the relative importance of
the mechanical processes among themselves or of the chemical processes
among themselves. However, the chemical processes, as a group, and the
mechanical processes, as a group, can be better compared. As has been
noted, the mechanical processes where important usually produce granula-
tion, and the chemical processes where important result in recrystallization;
but also mechanical action promotes chemical action. The importance of
the relations of these two classes of processes demands full consideration.
RELATIONS OF GRANULATION AND RECRYSTALLIZATION.
In a rock the same mineral may be partly granulated and partly
recrystallized. Even the individual grains of a mineral may exhibit the two
processes in various proportions. In this case the fracturing may be along
the borders of the individuals, may extend entirely across them, or may
granulate them throughout. ‘The simultaneous solutions and depositions
may occur along the borders of the original or the secondary granules, or
within the spaces produced by the fracturing, or may regenerate the old
mineral particles throughout. Any of the deposited material may occur as
independent individuals or as enlargements of original grains or mechan-
ical granules. Thus, in the different particles of the same mineral in a
given rock granulation or recrystallization may be the dominant. process.
Tf this be so, the same is true, to an even greater extent, of the particles of
different minerals.
MON XLVi1—04——47
798 A TREATISE ON METAMORPHISM.
Whether granulation or recrystallization is preponderant in a given
place in the zone of anamorphism depends upon many factors. Some of
these factors are, the character of the material and water content, tempera-
ture, pressure, and rapidity of deformation.
CHARACTER OF MATERIAL.
The degree to which granulation and recrystallization takes place
depends to a considerable extent upon the character of the material. The
character of the material involves both mineral composition and coarseness.
In the same rock mass certain minerals may be mainly recrystallized
and others mainly granulated or even retain their integrity. For instance,
it is well known that quartz suffers granulation and recrystallization much
more readily than feldspar. This is illustrated by the recrystallization of
the flat individuals of quartz in the quartz-porphyry described by Futterer®
(Pl. III, B), in which the feldspars have been little affected.
I shall make no attempt to compare the various minerals with one
another with reference to ease of recrystallization. There are all gradations,
from calcite, which can be easily recrystallized in the laboratory by the
passage of water through finely powdered material under very moderate
pressure, to the more refractory minerals, such as feldspay.
Since some minerals when strained recrystallize much more readily than
other minerals, it follows that a formation composed chiefly of one class of
minerals may be deformed mainly by granulation, and an adjacent formation
composed of another set of minerals may be deformed mainly by recrys-
tallization. One formation may thus show complete granulation or other
important strain effects, while the recrystallization of the adjacent formation,
because of the greater mobility of its mineral particles, nearly keeps pace
with the deformation. One rock may recrystallize so as to show the tex-
tures and structures of the schists and gneisses with more or less residual
strain effects, while an interlaminated rock recrystallizes so readily as to
take on a granolitic texture after movement has ceased. As an illustration
of this are the closely associated gneisses and marbles of the Adirondacks
and of the Hastings series of Canada. The gneisses, besides having their
characteristic textures and structures, show marked residual strain, while at
«@¥Futterer, Karl, Die ‘‘Ganggranite’’ yon Grosssachsen, und die Quartzporphyre yon Thal im
Thiiringer Wald, Heidelberg, 1890, pp. 27-47.
RELATIONS OF GRANULATION AND RECRYSTALLIZATION. 739
various localities interstratified with these are coarse marbles with granolitic
textures showing no other strain effects than those of polysynthetic twinning
and similar phenomena, which may have been developed by the slight
stresses to which the material was subjected in section cutting in the
laboratory.
The coarser the particles the more likely is granulation to be of
importance. ‘The finer the particles the more likely is recrystallization to
be dominant. The finer the particles originally or by granulation the
greater is the surface exposed to the action of the solutions. he dissolving
power of water, when not nearly saturategl, is almost directly in proportion
to the area upon which it can act. If the grains of a rock be broken by
granulation into particles having radii 0.1 of those of the original grains,
each small grain will have 0.001 the volume of an original grain, and the
total surfaces of the fewer original grains will be to the total surfaces of
the more numerous grains as 1:10. If the granulation goes so far as to
give the granules radii averaging only 0.01 of those of the original grains,
each small grain will have 0.000001 of the volume of the original grains,
and the total surfaces of the original grains will be to the total surfaces of
the granules as 1:100. A good illustration of mechanical granulation is
furnished by the anorthosites described by Adams.* Mr. 8. H. Ball has
compared the size of the grains of the original anorthosite and the granu-
lated anorthosite in two specimens, the result showing that, on the average,
the ratio of the diameters of the feldspar grains of the original rock to
the diameter of the granulated grains is 1:40, or one feldspar grain of the
original rock is broken into about 64,000 grains.
Ultimately, even under conditions otherwise favorable to granulation,
finer subdivision does not continue, for so large a surface is exposed to the
action of the solutions that the process of adjustment is accomplished by
recrystallization at least to such an extent that the granules do not, on the
average, decrease in size. In this fact of increased surface of action for
the solutions by granulation we have the explanation of the fact that
granulation is not known to produce particles the average of which is near
the limit of observation with a moderate power of the microscope.
An excellent example of the influence of the character of the material
« Adams, F. D., Report on the geology of a portion of the Laurentian area lying to the north of the
Island of Montreal: Ann. Rept. Geol. Sury. Canada, new ser., vol. 8, 1895, pt. J, pp. 85-134.
740 A TREATISE ON METAMORPHISM.
upon the gradation and relations between granulation and recrystallization
is furnished by the Algonkian rocks of the Black Hills. There conglom-
erates, quartzites, slates, schists, and gneisses occur in intimate relations to
one another, due largely to varying character of material. Nearly every
phase of deformation, from that in which granulation is the dominant
process to that in which recrystallization is the dominant process, is there
represented.
TEMPERATURE.
Low temperature is favorable to granulation; high temperature is
favorable to reerystallization. Ifs.Barus’s experiments upon the solubility
of glass’ be a guide as to the silicates, the temperature of 185° C. is
more nearly erucial between the processes than any other. Below 185°
© granulation is likely to be prevalent, especially if the deformation be
rapid. Above 185° C., if sufficient water be present, recrystallization is
probably so rapid that the mechanical strains do not go far before they are
largely obliterated by recrystallization.
Temperature increases with depth: the less the depth the greater
the tendency to deformation by granulation, and the greater the depth the
greater the tendency to recrystallization. But it must be remembered
that the mechanical work of deformation itself develops heat, which can
escape only by conduction or convection. Therefore, during mountain-
making periods, temperatures suflicient for recrystallization may exist
much nearer the surface than under quiescent conditions, and consequently
recrystallization takes place rapidly at no great depths.
Furthermore, the temperature is raised by imtrusive igneous rocks.
The heat of the intrusives is conveyed to the adjacent rocks, both by
conduction and by convection through water. Hence the presence of
igneous rocks is favorable to recrystallization. This has already been
dwelt upon. The numerous broad zones of schists and gneisses in which
recrystallization is complete about great batholiths, with schistosity every-
where parallel to the sides of the intrusives, have been described. (See
pp. 716-717.)
«Van Hise, C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1,
1890, pp. 214-230.
> Barus, C., Hot water and soft glass in their thermo-dynamic relations: Am. Jour. Sci., 4th ser.,
vol. 9, 1900, pp. 161-175; vol. 6, 1898, p. 270.
IMPORTANCE OF WATER IN RECRYSTALLIZATION. 741
The identical character of the schists surrounding batholiths and those
produced in connection with regional mechanical action is explained by the
foregoing pages. ‘The necessary conditions for the production of recrystal-
lized schists are movement under sufficient pressure, moderate temperature,
and presence of water. These conditions are produced in the two cases in
different ways.
PRESSURE AND RAPIDITY OF DEFORMATION.
The less the pressure the more likely is the deformation to be accom-
plished by granulation. The greater the pressure the more likely is the
deformation to be accomplished by recrystallization. The pressure increases
with depth, with mechanical action, by igneous intrusions, and possibly —
from other causes.
To a certain point, the more rapid the deformation the more likely is
the adjustment to be by granulation. The limit beyond which this does
not apply is reached when the mechanical process develops such an amount
of heat that the readjustment is by recrystallization rather than by granu-
lation. The slower the deformation the more likely is the readjustment to
be by recrystallization.
WATER CONTENT.
Absence of water is favorable to granulation; presence of water is
favorable to recrystallization. If a series be so dense, or is of such origin,
that it contains comparatively little water, even if other conditions be favor-
able, deformation by granulation rather than by recrystallization may occur.
Another series, which is in every other respect under similar conditions, but
which contains a fair amount of combined water that may be liberated
by dehydration to serve as a medium for the process, may be adjusted by
recrystallization and form coarse schists. This principle is believed to
explain the difference in the character of the deformation of the different
formations for many districts. It is a well-known fact that in the same
district different rock masses composed of similar minerals deformed in the
zone of anamorphism vary greatly in the character of the alteration, some
formations yielding by granulation, others by reerystallization. In general
the sedimentary rocks contain a considerable percentage of combined water,
and therefore when deformed are recrystallized. The same is true of the
porous igneous rocks, such as lavas and tuffs. In contrast with these are
742 A TREATISE ON METAMORPHISM.
the massive igneous rocks, and especially the plutonic rocks, which contain
little combined water, and when deformed are apt to be granulated rather
than recrystallized. One of the best districts to illustrate the principle is
the Original Laurentian area described by Adams." In this district are the
anorthosites, the Grenville sedimentary series, and the Basement gneiss of
igneous origin. Adams’s careful descriptions show that the most funda-
mental point of difference between the three classes of rocks in their response
to deformation is in reference to recrystallization and granulation. The
sedimentary rocks of the Grenville series have been completely recrystal-
lized and are typical schists.’ The igneous gneisses are largely deformed
by granulation, but “the granulation has perhaps been effected, in part at
least, by reerystallization.”’ The anorthosites have been almost wholly
deformed by granulation.” Corresponding exactly with these facts are the
amounts of contained water. Analyses of three recrystallized Grenville
eneisses give an average water content of 1.46 per cent.’ An analysis of
one partly granulated and partly recrystallized gneiss of igneous origin
gives 0.70 per cent of water’ An analysis of granulated anorthosites gives
only 0.51 per cent of water.’
The frequent granulation of the massive igneous rocks does not con-
tradict the conclusion (pp. 712-713) that in many cases water as a source of
recrystallization may be derived from the plutonic rocks during recrystalli-
zation. The very fact that water is given off during this process leaves the
plutonic rocks deficient im combined water. When later mass movements
take place with only a very small amount of combined water present,
eranulation is likely to occur, and, as already noted, somewhat in pro-
portion to the amount of the residual water; for, as shown by the illustra-
tions already given, the anorthosites, contaiing a small amount of water,
under mass movement have been granulated to a greater extent than the
granites, which contain somewhat greater amounts of combined water.
As examples of recrystallized schists of igneous origin may be men-
a Adams, F. D., report on the geology of a portion of the Laurentian area lying to the north of
the Island of Montreal: Ann. Rept. Geol. Sury. Canada, vol. 8, 1895, pt. 5, p. 184.
b Adams, cit., pp. 49-66.
¢ Adams, cit., p. 45.
@ Adams, cit., p. 106.
e Adams, cit., p. 58.
J Adams, cit., p. 43.
g Adams, cit., p. 130.
IMPORTANCE OF WATER IN RECRYSTALLIZATION. 743
tioned the schists and gneisses of the Basement Complex of the Marquette
district of Michigan. Two analyses of the Kitchi schists showed, respec-
tively, 2.51 and 2.70 per cent of water above 100° C.* A micaceous schist
gave off 2.04 per cent of water above 100° C.’? A Palmer gneiss gave oft
2.33 per cent of water above 100° C.° Partial analyses of two other
micaceous schists from the same locality as the micaceous schist above
referred to, which show a cataclastic or granulated structure, unfortunately
do not include water determinations.
The foregoing analyses of recrystallized schists from the Archean and
Algonkian may be taken as typical of the recrystallized schists of all ages
the world over. This is shown to be evident by running through the
various analyses of the recrystallized schists in any of the published tables
of analyses. Such tables show that the recrystallized schists average more,
rather than less, than 1.50 per cent of combined water, and in many cases
that they contain more than 2 per cent of water.
In Clarke and Hillebrand’s book of analyses,” besides those mentioned,
are found three other analyses of recrystallized gneisses. The analyses are
of typical gneiss, derived from basic granite at Washington, D. C., with a
water content (at and above 110° C.) of 1.97 per cent, and two plagioclase
gneisses from the Sierra Nevada, with a water content (at and above
110° C.) of 1.71 and 1.47 per cent, respectively. These analyses, with
those already quoted, cover all the schists and gneisses cited in Clarke and
Hillebrand’s book which, from the available descriptions, can be ascertained
to be certainly recrystallized. They are here included to show that in this
bulletin there is no exception to the rule laid down, viz, that when rocks are
deformed in the zone of anamorphism, which contain a sufficient amount of
combined water to serve for recrystallization, this process is dominant rather
than granulation.
It is to be noted that the water contents of the schists and gneisses
mentioned are amounts given off above 100° C., and in most cases above
110° C.; in other words, the water is combined water. This point is of
importance as showing that, after the metamorphism of the rocks to schists
aVan Hise, C. R., and Bayley, W. 8., The Marquette iron-bearing district of Michigan: Mon. U. 8.
Geol. Survey, vol. 28, 1897, p. 168.
bMon. 28, cit., p. 202, anal. vi.
¢Mon. 28, cit., p. 217.
dClarke, F. W., and Hillebrand, W. F., Analyses of rocks and analytical methods: Bull. U. S.
Geol. Survey, No. 148, 1897, pp. 88, 215.
744 A TREATISE ON METAMORPHISM.
or gneisses, 1.50 per cent or more of water remained. The percentages of
combined water after metamorphism are, of course, no measure of the
amount of water, free and combined, contained by the rock before and
during the metamorphosing process. It is highly probable that the amount
of combined water in the later stages of the process, in the case at least of
the sedimentary rocks, is lower than in the earlier stages of the process,
consequently water is driven off during the process, thus renewing the water
films in the subeapillary spaces, and furnishing a medium for solution
and redeposition. (See Chapter III, p. 145.) This is inferred from the
analyses of pelites in their various stages of alteration from soils and clays
to shales, slates, schists, and gneisses. Sixteen analyses given by Clarke
and Hillebrand of soils and clays from Pennsylvania, Florida, and Colorado
gave an average loss of water at or above 100° C. of 7.15 per cent. Six
analyses of clays and soils from Virginia gave an average loss of water
above 110° C. of 8.61 per cent. Forty-four analyses of clays and soils
from Massachusetts, South Carolina, Alabama, Missouri, Colorado, Nevada,
and California gave an average loss of water upon ignition of 8.10 per
cent.” ‘Twenty slates and shales from Vermont, Colorado, ard California
gave an average of 4.42 per cent of water above 100° C. Fourteen slates
and shales from Vermont, New York, Kentucky, Georgia, and Alabama
gave an ayerage of 4.34 per cent of water above 110° C.’ The original
mudstones from which the shales and slates were produced may be pre-
sumed to have contained at least as much water as do soils and clays. The
shales and slates are rocks partly metamorphosed during mass-mechanical
action, and they certainly lost about one-half of the combined water during
the change.
It is well known that shales and slates, when profoundly metamor-
phosed during mass-mechanical action, produce schists or gneisses. Since
most of the schists contain less than 2 per cent of water, this agent was
again reduced to one-half or one-third of the amount present when the
rocks were in the stage of shales and slates. Therefore the pelites in their
original form contain in combination not only a sufficient amount of water
to satisfy the requirements as to combined water in the completely meta-
morphosed schists, but an excess of water, which may be steadily given off
«Clarke, F. W., and Hillebrand, W. F., Analyses of rocks and analytical methods: Bull. U. 8.
Geol. Survey, No. 148, 1897, pp. 287-301.
>Clarke and Hillebrand, cit. pp. 277-286.
IMPORTANCE OF WATER IN RECRYSTALLIZATION. 745
during the process of metamorphism, and thus constantly furnish a supply
of the medium through which reerystallization can take place. The same
is certainly true of many other rocks the materials of which have been
subject to the processes of alteration in the zone of katamorphism, and
which therefore contain hydrous minerals. The most important exceptions
are the pure silica and the pure carbonate rocks.
The steadily lessening amount of combined water with increasing
metamorphism is illustrative of the facts already explained on pages
178-180, that the deep-seated zone of recrystallization is one of dehydra-
tion. In opposition to this, the alterations under conditions of small depth
are those of hydration. Many of the rocks metamorphosed under such
conditions contain as high or higher percentages of water than the soils
and clays the water contents of which have been quoted. If later such
hydrated rocks, whether of igneous or of aqueous origin, were subjected
to alterations in the zone of anamorphism, recrystallized schists or gneisses
would be developed.
In the above no account is taken of the mechanically mingled water.
This is undoubtedly always present in important amount in all of the
sedimentary and in many of the igneous rocks, and is an additional supply
of the agent of transformation. The amount of this mechanical water has
already been discussed. (See Chapter III, pp. 124-127.) It varies from a
fraction of 1 per cent of the volume in the densest rocks to as high as 50
or more per cent in the more open soils and clays. At the outset of the
process of change the mechanically included water is often an abundant
source of the agent of metamorphism; but when the rocks become compacted
by pressure, or cemented, or both, this water is largely removed. This is
likely to take place in large measure in the zone of katamorphism;
consequently, when the rocks reach the zone of anamorphism the amount
of mechanical water is usually much reduced. It follows that the originally
mechanically mingled water, while probably of importance, can not be
depended upon as an adequate supply for recrystallization in the zone of
anamorphism. Indeed, it is believed that the water freed by dehydration
is the most reliable source of water for recrystallization im the zone of
anamorphism. (See pp. 662-663.)
The facts presented on the preceding pages give strong support to the
idea that presence of water at the beginning of the process of metamorphism
746 A TREATISE ON METAMORPHISM.
is necessary in order that recrystallization shall readily occur. If the
massive original rocks at the beginning of the mass movements do not
contain water sufficient for recrystallization to take place, it does not appear
that they are likely under normal conditions to gain sufficient water from
an outside source. But where intrusives are introduced on an extensive
scale, this source of water may make good the deficiency, so that recrystal-
lization may occur. Apparently the amount of water present during
recrystallization must be great enough for at least 1 per cent to be left as
combined water; for in the deformed igneous rocks containing less than 1
per cent of water granulation is frequent, and the schists and gneisses
formed by recrystallization usually contain between 1 and 2 per cent of
water. But doubtless in proportion as rocks are deep seated, so that the
pressure is great and the temperature high, a
decreasing amount of water suffices for recrystal-
lization.
While the minerals of the rocks are steadily
dehydrated during the process of recrystalliza-
tion, not all of the water driven off escapes; a
small part of it is caught within the recrystal-
lizing material; and thus are explained the almost
infinite number of minute microscopic water in-
Fic. 19.—Liquid-filled cavities extend-
ing across several quartz individuals
without change of direction, From Tp some instances the water bubbles are arranged
Black Hills quartz-schist. fo)
clusions contained in the schists and gneisses.
along planes of fracture, in which case they appear
in the sections as rows of bubbles which frequently extend continuously
across many individuals. (See fig. 19.) The arrangement of continuous
rows of water bubbles across many minute individuals has been taken as
evidence that the minerals are original, whereas it is plain that the correct
interpretation is that the bubbles themselves are probably secondary inclu-
sions, as I have fully explained in another place.” |
Liquid carbon dioxide is frequently associated with the water inclu-
sions. It is believed that in most cases this material is a portion of the
carbon dioxide given off by the decomposition of the carbonates during sili-
cation. Another and probably larger portion of the carbon dioxide escapes
«Van Hise, C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1,
1890, p. 218.
IMPORTANCE OF WATER IN RECRYSTALLIZATION. 747
with the freed water, and is squeezed upward to the belt of cementation.
(See pp. 177, 667.)
The foregoing facts seem to show that whether granulation or recrystal-
lization occurs in the zone of anamorphism in a given district in rocks of a
certain chemical composition does not depend upon whether the rocks are
igneous or aqueous, but, other things bemg equal, upon whether sufficient
water is present, by means of which recrystallization can’occur. As this is
more frequently the condition in the sedimentary rocks than in the igneous
rocks, the sedimentary rocks are more frequently recrystallized than the
igneous rocks, though in many instances recrystallization, rather than gran-
ulation, has been the process of modification for the igneous rocks.
The experimental work of Adams and Nicolson * upon the deformation
of marble seems to me to be fully confirmatory of the above conclusions
as to the influence of water in determining whether granulation or recrys-
tallization occurs. They deformed marble under conditions of the zone
of flowage, both dry and wet, at a temperature of about 300° C. The
marble deformed while dry, “if not quite as strong, is at least very nearly
as strong as the original rock,”’ as shown by tests of its crushing strength,
whereas the marble deformed at the same temperature in the presence of
steam at a pressure of 460 pounds per square inch was “not weaker, but
actually somewhat stronger than the original rock.”’ While they conclude
that “the presence of water was not observed to exert any influence,”” they
say: “It is just possible, however, that there may have been a deposition
of infinitesimal amounts of calcium carbonate along very minute cracks or
fissures, thus contributing to maintain the strength of the rock. No signs
of such deposition, however, are visible.”’
It seems to me that the increased strength of the rock deformed in
the presence of steam is positive evidence of the influence of water.
When it is remembered that the mineral experimented upon was one
which is readily deformed by movement along gliding planes, and there-
fore one in which for deformation without granulation recrystallization
is not necessary, and that the time through which the experiment was
a Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble: Philos.
Trans. Royal Soc. London, series 4, vol. 195, 1901, pp. 363-401.
b Adams and Nicolson, cit., p. 379.
¢ Adams and Nicolson, cit., p. 385.
d Adams and Nicolson, cit., p. 399.
e Adams and Nicolson, cit., p. 385.
748 A TREATISE ON METAMORPHISM.
continued was but a minute- fraction of the time of the action of water
under natural conditions, the experiment seems to me to furnish the
strongest possible confirmation of the conclusion already expressed as to
the importance of the presence of water during the recrystallization of
rocks.
ROCK FLOW AGE.
MEANING OF ROCK FLOWAGE.
If the conclusions of the foregoing pages be true, the real meaning of
rock flowage, at least as deep as observation extends within the earth,
follows as a corollary. It is apparent that the process of rock flowage is
very different from the flowage of a liquid or that of a malleable solid,
although it involves elements which occur in these processes.
In discussing the subject of rock flowage it will be necessary to repeat
the substance of certain things which have already been said. This is nec-
essary in order to bring the different elements which enter into rock flowage
into juxtaposition in proper relations to one another. I have previously
maintained that deformation at considerable depth is accomplished by rock
flowage.* However, | made no attempt to explain in detail the nature of
the interior movements.
All of the different processes which have been described are involved
in rock flowage. That is to say, the mechanical processes of strain within
the elastic limit, gliding, granulation, and welding, and the chemical proc-
esses of cementation and metasomatism resulting in recrystallization, all
enter into the result. In the foregoing pages each of these processes has
been separately discussed and their relations have been pointed out. Vari-
ous combinations of the foregoing mechanical and chemical processes
explain rock flowage. Commonly in rock flowage the chemical process of
recrystallization is dominant; but this, as has been explained (pp. 690-692),
is promoted by the simultaneous mechanical strains. But in many cases of
rock flowage the evidences of chemical and mechanical processes are about
equally marked, and in some instances of considerable importance mechan-
ical processes have been dominant.
I shall first consider the slates, schists, and gneisses, which are typical
examples of rocks that have been deformed by rock flowage, and are also
“Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept.
U.S. Geol. Survey, pt. 1, 1896, pp. 593-595, 636-643.
MEANING OF ROCK FLOWAGE. 749
the class of rocks in which the process has taken place on the most exten-
sive scale. It will be remembered that slates, schists, and gneisses are
rocks in which many of the mineral particles show parallel orientation and
in which the major portion of the mineral particles now show slight
or no strain effect. (Pl. XI, C.) It is evident that these are rocks
which have nearly perfectly accommodated themselves to the deformation
through which they have passed. The accommodation, as already
explained, is accomplished by continuous solution and deposition, or by
continuous recrystallization. While the adjustment during deformation at
any moment was nearly as complete as though the rock were a magma,
and while it may nowhere show even a microscopic opening, it is evident
that the flowage is wholly different from that of a liquid. At no time was
the rock a liquid; on the contrary, it was at all times almost wholly a
crystallized solid. At no time was more than an almost imappreciable
fraction of it im liquid form—that is, dissolved in water—yet at all times it
was adjusting itself by means of the small percentage of water contained
in the capillary and subcapillary openings, this being the chief agent of
deformation working through solution and deposition, or recrystallization.
In order that such a continuous process shall Le adequatc to explain rock
flowage, it is necessary only that it shall be sufficiently rapid to keep pace
with the deformation. Probably one’s first thought is that it is not possible
that the process can be sufficiently rapid to account for the phenomena.
However, the experiments of Barus on the solution of glass give us a basis
upon which we can make a quantitative calculation.
Barus has shown“ that a temperature of 185° ©. is critical so far as the
solution of glass by water is concerned. At temperatures lower than this
the rate of solution by water is very slow.” However, at temperatures of
185° ©. and above, solution of the silicates of glass goes on with astonishing
rapidity. Barus dissolved, in capillary tubes, a sufficient amount of glass
to cause an apparent contraction of volume of the water amounting to 13
per cent of the water present in 42 minutes, and 18 per cent in an hour.
This illustrates the fact that the activity of water at a very moderate
temperature is amazing and one need not be surprised at its potency in the
alteration of rocks deep below the surface of the earth. ‘Temperatures
higher than 185° C. exist at moderate depth, and therefore it is reasonable
@Bana, C, The compressibility of liquids: Bull. U. 8. Geol. Survey No. 92, 1892, pp. 78-84.
750 A TREATISE ON METAMORPHISM.
to suppose that a small amount of water may be the medium of rapid and
most profound modification of the rocks.
We have already seen (pp. 690-691) that during the process of defor-
mation the material, if not dissolved, may be strained even to the point ot
granulation by the mechanical processes; also it has been seen that, so far as
strain occurs, or the particles are small, the minerals are in a state in which
solution is easier than for unstrained or larger mineral particles. But it is
probable that the solution of mineral particles and the deposition of the
material in an unstrained crystallized condition is considerably slower
than the solution of amorphous glass; for, among other reasons, it can not
be supposed that the same amount of energy is potentialized in the mineral
particles as in the glass. But the further the strain goes before fracture
the more energy is potentialized, or if fractures occur smaller particles are
produced. Moreover, the contained water is in small capillary or subeapil-
lary spaces, and therefore’a given volume is acting upon a much larger
surface than in the capillary tubes used by Barus in his experiments. In
so far as granulation occurs, the surface of action is still further increased.
All these conditions are favorable to solution and redeposition; therefore,
the greater the straining and resultant granulation, the more rapid the
process of recrystallization; hence, in the deep-seated zone, when water is
present in sufficient amount, mechanical disintegration never gets far in
advance of solution and redeposition. (See pp. 696-698.)
At temperatures above 185° C., if it be supposed that in the capillary
and subeapillary spaces within the rocks the speed of solution of minerals
is 0.1 of that of glass, as ascertained by Barus, water would dissolve its own
volume of minerals in about five hours. If the deep-seated rocks be
supposed to contain 2 per cent of water by volume—that is, less than 1 per
cent by weight—the entire mass of rocks might be dissolved in about 250
hours, or little more than ten days, provided deposition went on at the
same rate as solution. The percentage of water premised is known to be
lower than the amount ordinarily found in the recrystallized schists and
gneisses (see pp. 742-743), and the rate suggested seems reasonable; but
if the speed of solution and deposition be decreased to 0.1 of that sug-
gested, or to 0.01 of that of the solution of glass, still the entire mass of
the rock might be dissolved and redeposited in about 100 days. Make the
rate 0.1 of this, or 0.001 of that of the solution of glass, and still recrystal-
MEANING OF ROCK FLOWAGE. Tol
lization might be complete in about 1,000 days, or three years. If it be
supposed that a mountain-making period occupied 150,000 years—and this
is probably less rather than more than the time required for most mountain-
making movements—during this period at the slow rate suggested the
rocks could be recrystallized 50,000 times by 1 per cent of water, and this
number certainly seems adequate to fulfill the requirement that at any given
moment the rock shall exhibit but a slight strain effect.
Of course it is not thought probable that any rock has completely
recrystallized 50,000 times. Indeed, it is well known that many of the
rocks in which recrystallization is complete, in so far as the finer particles
are concerned, contain many larger particles which have not been completely
recrystallized. Perhaps one of the best instances of this is furnished by
the schist-conglomerates. The typical schist-conglomerates contain a
schist matrix, embedded in which are numerous large fragments. In many
of these the matrix is completely recrystallized, but the fragments, unlike
the matrix, show important strains, which not infrequently pass to the
point of partial granulation with partial recrystallization. To explain
the phenomena exhibited by the perfect schists which h-ve developed
during a continuous process of deformation it does not seem necessary to
suppose that complete recrystallization is necessary. If the éase of a large
grain of quartz or feldspar in a recrystallizmg rock be taken, we may
suppose the process to go on somewhat as follows: Because of the lack of
homogeneity of the rock the stresses are irregularly distributed. At the
most exposed places upon the mineral particles the conditions are favorable
for solution, for the following reasons: The particles are there greatly
strained, perhaps to the point of granulation; and, so far as strain exists, or
small granules are formed, these conditions are favorable to solution. At
the places of great strain the material is therefore taken into solution and
transported to the parts of the particles less strained or to other particles.
At such places the conditions are favorable to deposition, on account of the
relatively large size of the residual original grains as compared with the
granules. (See pp. 74-76.) The mineral particles, where least strained,
separate from the solution materials like themselves and attach them in
orientation with the cores in an unstrained or little strained condition. The
process of growth is analogous to that of mineral growth by secondary
enlargement. The entire process is similar in several respects to that of
752 A TREATISE ON METAMORPHISM.
tho continuous solution and deposition of calcium carbonate in the chemical
laboratory when water is passed through a layer of this material under
pressure. Where the pressure is greatest, in the upper part, the grains are
taken into solution; at the place of escape, where the pressure is least, the
material is deposited from solution and the grains increase in size, or grow.
During the deformation of the rocks this process of solution and
deposition of the mineral particles is continuous.
If it be supposed to go on to a stage in which an original particle is
one-half or one-third as thick as it was originally, it is not necessary to
suppose that the central part of the mineral particle has been recrystallized.
This is illustrated by fig. 20. A spherical grain is supposed to have changed
; to the superimposed spheroidal grain. The
WEN. common portion C, or a large portion of it,
may be an uncrystallized part of the old grain,
c but the material AA has been dissolved and
added to the borders at BB. Corresponding
ee to this explanation, some of the flat quartz
Fig. 20.—The possible relation of oldandnew oyains of the slates and schists of the Black
grains of recrystallized rocks. ‘S
Hills of Dakota show residual cores.”
Since in most cases the stresses are unequal in three dimensions, there
is a direction of greatest pressure; at right angles to this is a direction of
mean pressure; and at right angles to these two, a direction of least pressure.
During the process of recrystallization, at any given moment there is maxi-
mum shortening in the direction of greatest stress, maximum elongation in
the direction of least stress, and shortening or elongation in the direction of
mean stress. Consequently the shape of the modified particle may be that
which would be produced if a plastic grain were rolled out, the sides being
confined in one direction, but with liberty to elongate in another direction
in the same plane; or it may be that which would be produced if a roundish
cake of dough were flattened between two boards, and consequently elon-
gated in all directions at right angles to the direction of greatest pressure;
or, finally, the mean stress may approach so closely to the maximum
stress that there is shortening in two directions and elongation in a single
one only, in which case a fibrous. structure is produced. But from my
«Van Hise, C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1,
1890, p. 224.
MEANING OF ROCK FLOWAGE. 753
study of the slates, schists, and gneisses I am inclined to believe that
shortening in one direction and unequal elongation in the direction at right
angles to this is the most common case, though my thin sections give
illustrations of all the cases. The particles are arranged with their greater,
mean, and minor diameters corresponding to least, mean, and greatest
pressures. In proportion as the movement involves shearing motion or
scission,® the mineral particles are rotated from a position in which the
direction of greatest elongation is at right angles to the direction of ¢reatest
pressure, although at any given time the mineral particles tend to develop
with their longer axes at right angles to the maximum pressure.
In some cases the direction of greatest pressure varies within exceed-
ingly short distances. This is illustrated by large rigid particles, such as
feldspar, garnet, and other refractory minerals, which act as transmitters
of pressure to more than an average extent. Adjacent to the rigid
mineral particles the direction of greatest pressure is modified from point
to point, thus deviating from the average for the rock. New minerals
forming may curve about the rigid granules. This is beautifully illus-
trated by the flat, curved quartzes adjacent to feldspar in the quartz-
porphyry described by Futterer. (PI. III, B; see p. 738.) In this case
the flat individuals lack orientation and show undulatory extinction inde-
pendently of subsequent strain.
The above process of flattening by recrystallization is general; and
therefore we conclude that while recrystallization is constantly occurring
in the deformation of rocks, at any given time a large number of the
mineral particles retain their integrity, and are nuclei which at any moment
orient the material being deposited. In many rocks evidence may be
seen that this has happened. The old mineral particles, represented by the
cores, are partly altered, and in consequence of this may be discriminated
from the freshly added material; or the cores show a border of iron oxide
or other mineral; or the old and new materials have slightly different
compositions, and this may be discovered by a ditference in the color,
refraction, or extinction, or in some other way. Finally, all of the old
mineral particles may be regenerated or recrystallized.
«Becker, G. F., Finite homogeneous strain, flow, and rupture of rocks: Bull. Geol. Soc. America,
vol. 4, 1893, pp. 24-25.
MON xXLy11—04———48
Ta4 A TREATISE ON METAMORPHISM.
Therefore a given portion of a definite mineral particle in a. slate,
schist, or gneiss may not have been recrystallized at all, or, on the other
hand, may have recrystallized several or many times. It is believed that
ordinarily the reerystallization is far advanced or complete for all parts of
a typical schist, although this is far from the case in the semicrystalline
schists or imperfectly schistose rocks. Where deformation is mainly accom-
plished by reerystallization the process may be called recrystallization
flowage or chemical flowage.
Of course, during this rearrangement it is not supposed that the
identical molecules which are taken from the more severely stressed parts
of a grain are necessarily deposited at the places of less stress upon the
same grain. Undoubtedly there is great interchange of material between
the particles by means of the solutions. It is, however, thought probable
that in many cases of deep-seated deformation, where the passage of solu-
tions is difficult and slow, much of the identical material which is taken
from a grain at one place is added to it at another place.
When new individuals are produced in any way, as by granulation, or
by deposition as new mineral particles, perhaps as different species from
any originally in the rock, they are subject to the same laws as the original
mineral particles. Many have a tendency to form with dimensional orienta-
tion, which usually carries with it similar crystallographic orientation. How-
ever, it is only rarely that the orientation of the particles of a given minéral
approximates exactness. One mineral—for instance, mica—may be well
oriented, whereas such minerals as quartz or calcite may not be oriented.
In proportion as the minerals readily respond to the forces of recrys-
tallization, or are mobile, they do not gain or retain a regular arrangement.
After mass movement has ceased the temperature may be high enough
and the heat be held long enough for the solutions to completely recrystal-
lize the minerals under mass-static conditions, and therefore orientation
may be lost. In proportion as minerals do not readily recrystallize or
stubbornly resist the force of recrystallization, the minerals once oriented
retain their regularity of arrangement.
The most mobile of the important minerals is calcite. As explained
by Adams and Nicolson,* the mobility of this mineral is doubtless partly
«Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble:
Philos. Trans. Royal Soc. London, series a, vol. 195, 1901, pp. 363-401.
MEANING OF ROCK FLOWAGE. 7995
due to movement along gliding planes. Quartz is also somewhat mobile.
Therefore these minerals in marble and in recrystallized quartzite frequently
lack regularity of arrangement. However, in some cases even calcite may
show well-developed dimensional and to some extent similar crystallographic
orientation. The usual almost complete lack of regular arrangement for
calcite is illustrated by most of the marbles from the Laurentian Mountains
to Alabama. The complete recrystallization of quartz to a coarse grano-
litic-textured rock, the individuals wholly lacking orientation, is illustrated
by the quartzites of the Wausau district of central Wisconsin.
In the process of recrystallization of rocks it is not supposed that every
large mineral particle retains a nucleus for lateral growth; indeed, it is
certain that in a rock some large particles of a mineral may retain the
modified integrity above described, while other particles of the mineral
may be wholly destroyed. It is believed that orientation of the mineral
particles in reference to the varying stresses has an influence upon their
preservation. If the original particles happen to be in such positions that
they would develop as authigenic minerals under the differential stresses,
this is thought to be favorable to the preservation of their nuclei and to
growth. It is believed that in proportion as the particles vary from such
positions the mineral particles are likely to be destroyed. The effect of
position with reference to the principal stresses upon the persistence of a
given particle is probably great in proportion as the mineral has a tend-
ency to be influenced in its dimensional arrangement and crystallographic
orientation by the stress differences which exist during deformation. To
illustrate: The position of the axes in reference to the greatest pressure in
mica, which shows a marked tendency to parallel arrangement, would be a
more important factor in its preservation than in quartz, which only rarely
shows regular arrangement.”
In the foregoing nonrotational distortion has been assumed. In case
the deformation includes a rotational element, during the recrystallization
each of the particles would be similarly rotated, as well as flattened or
recrystallized, or both; and consequently the direction of the maximum
compression, and therefore the most favorable direction of elongation of
the mineral particles, would change in reference to them. At any stage
«Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept.
U.S. Geol. Survey, pt. 1, 1896, p. 635.
756 A TREATISE ON METAMORPHISM.
of the process there would be a tendency to retard the growth of the min-
eral particles which had formed at an earlier stage. Another effect of
rotation would be to decrease the regularity of the similar arrangement of
the mineral particles as compared with those produced during nonrotational
distortion; for at any stage the new mineral particles would form in
positions somewhat different from those of the particles formed at an earlier
stage. I suspect that in rotation with reference to the forces during devel-
opment lies the principal cause of imperfect arrangement of the cleavage-
making mineral particles of the slates, schists, and gneisses. This point is
more fully developed by Leith.“
While in the slates, schists, and gneisses many mineral particles are
able to maintain themselves, a vastly greater number are probably destroyed
by the process of recrystallization. This is certainly true in those very
numerous instances in which the average size of the mineral particles of
the metamorphosed rocks is much greater than the average size of the
particles in the unaltered rocks. It has been pointed out (pp. 674-675)
that in many instances hundreds or thousands or even a million or more
of particles may be required to produce a single individual. From such
cases there are all gradations to those where the particles of the recrys-
tallized rocks are not larger than those of the unaltered rocks, and through
these to those in which the particles of the recrystallized rocks are smaller
than those of the unaltered rocks. In proportion as the particles of the
original rocks are large, an increasing proportion of them, or of parts of
many of them, is likely to be preserved in the recrystallized rock. In
proportion as the particles of the unaltered rock are small, a decreasing
proportion of them is likely to be preserved in the recrystallized rock,
although in the latter case the absolute number of mineral particles preserved
and enlarged may be as great as or greater than in the former case.
On pages 751-753 the modifications of the forms of the original particles
have been considered. Substantially the same principles apply to newly
formed mineral particles. With or without a nucleus, a mineral particle
begins to grow. Because of the differential stresses the molecules which
first segregate to form minerals that have strong crystal habit orient them-
selves in such positions that their shortest axes are in the direction of maxi-
mum pressure, their longest axes are in the direction of minimum pressure,
«Leith, C. K., Rock cleavage.
MEANING OF ROCK FLOWAGE. 757
and their mean axes are in the direction of mean pressure. When once
the mineral particles have oriented themselves in this way, they continue
to grow according to their crystal habits, the most rapid growths being in
the direction of greatest pressure. This dimensional arrangement control-
ling the orientation of mimeral particles forming in rocks during conditions
of unequal stresses has been formulated by Leith as a result of his. detailed
studies of rock cleavage." The amount of mass shortening which results in
the regular arrangement may be comparatively slight, and yet the differ-
ences in the dimensions of the mineral particles be great. For instance,
Leith’s measurements show that the commen ratio between the least and
greatest diameters of the mica flakes of mica-schists is, on the average,
about 1:10.’ It does not follow, as has been thought by some, that this
involves the shortening of the rock mass by 0.9. Indeed, it is perfectly
conceivable that the rock mass may be but slightly shortened, perhaps by
0.1, and yet produce mineral particles the shortest dimensions of which are
only 0.1 as great as their longest dimensions. During recrystallization of
a rock the material of those minerals which have strong crystal habit
rapidly attaches itself to the borders of the particles and but slowly to the
sides.
The above conclusion seems to be completely confirmed by the con-
stancy of the ratios between the greatest and least diameters for a given
mineral. Leith has ascertained that in mica the ratio varies from 100: 65
in the case of biotite to 100:10 in muscovite, with an average of 100: 20
for biotite and 100:14 for muscovite; in hornblende from 100: 40 to
100: 25; in feldspar from 100: 75 to 100:50.° In the case of each mineral
these ratios are practically the same whether the rock be a slate, a fine-
grained schist, or a coarse-grained gneiss. The full range of variation in
the ratios of a mineral can be found in the same rock in each of the dif-
ferent stages of metamorphism. If the ratios between the least and greatest
diameters of the mineral particles were taken as evidence of the amount of
shortening of a rock, we should reach a different conclusion for each
mineral and the same conclusion for all stages of metamorphism. Such a
conclusion is absurd, and the independence of the dimensional ratios of the
orienting minerals and the amount of rock shortening is perfectly estab-
lished. Therefore, so far as new mineral particles are produced with similar
a Leith, C. K., Rock cleavage. b Leith, C. K., cit. ¢ Leith, C. K., cit.
708 A TREATISE ON METAMORPHISM.
dimensional arrangement, these give no basis to estimate the amount of
shortening of the rock.
The case of flattened original fragments is very different. In so far
as the resultant material or flattened ovules can be identified with original
forms, as, for instance, where conglomerate pebbles and fossils are deformed,
these distorted objects give a means of determining the amount of the mass
shortening. ‘
In proportion as the conditions are unfavorable for recrystallization—
that is, as mineral particles are refractory, as they are coarse grained, as the
deformation is rapid, as the depth is small, as the temperature is low, as the
water content is small—granulation occurs instead of recrystallization. This
is sufficiently evident, without amplification, from the discussion already
given. (See pp. 737-748.)
In the imperfect slates, schists, and gneisses, which are widespread
rocks, adjustment during deformation is accomplished in part by the
process of recrystallization just described, but also in part by the mechan-
ical processes. Of these, granulation with differential movement of the
granules and welding of the granules is of first importance. Strain within
the elastic limit is of very considerable moment. The deformation of the
mineral particles by movement along gliding planes may be of occasional
importance, especially in certain classes of rocks, such as marbles.
The cataclastic rocks do not usually show slaty or schistose structures.
In these rocks the process of flowage is mainly mechanical. It is chiefly
accomplished by the multitudinous fracturing of the solid particles, by
differential movements between the minute particles, and by the welding
of the differentially moved bodies. With this process as a main one, there
is sure to be marked strain within the elastic limit in all the particles, both
original and secondary; and thus this is a rather important factor in the
process. Finally, gliding is more important than in the case of recrystalli-
zation-flowage. But it may perhaps be doubted whether even in the typical
granulated rocks the process of flowage is wholly mechanical. In all rocks
there is a small amount of water present, and even in the instances of
apparently perfect granulation probably solution and deposition, or recrys-
tallization, has taken place to some extent. Where deformation in the zone
of anamorphism is mainly accomplished by granulation and differential
movement of the particles, the process may be called granulation-flowage,
or mechanical flowage.
MEANING OF ROCK FLOWAGE. 759
The illustrations of the perfect schists produced mainly by reerystalli-
zation, and of the imperfect schists produced by recrystallization and
mechanical strains combined, are so well known that they need not be
mentioned. ‘The best illustration known to me of flowage mainly by
granulation is afforded by the anorthosites of the Original Laurentian,
described by Adams.”
So far as the mass deformation is concerned, the rock flowage may be
mainly by recrystallization, mainly by granulation and other mechanical
processes, or by any combination of them. In all alike there is minute
interior adjustment of the particles so as to change the mass form of the
rock. It makes no difference, so far as the resultant form is concerned,
whether the change is accomplished mainly by the differential movements
of the molecules, as in the recrystallized schists, or by the differential
movements of granules, as in the cataclastic rocks, or by the two combined.
CONCLUSION.
Rock flow is mainly accomplished through continuous solution and
deposition—that is, by reerystallization of the rocks through the agency of
the contained water. The recrystallization is largely induced by mechanical
strains. Rock flow is partly accomplished by direct mechanical strains.
At the beginning of the process of flow, during the process, and at the end
of the process, the rocks, with the exception of an inappreciable amount,
are crystallized solids. At any moment only an infinitesimal quantity is
fluid—that contained in the minute quantity of water present. Yet the
solids respond to deformation like plastic bodies, not only not losing erys-
talline character, but usually acquiring mineral particles of fair magnitude,
because the water is ever changing the position of the crystallized material,
dissolving a substance here and depositing it there. By this process,
combined with the mechanical strains, there is continuous adjustment or
adaptation of the mineral particles to their environment, as demanded by
the deformation of the rock.
This is rock flow. In this conclusion nothing is said as to the condition
of material below that part of the earth which is called the lithosphere, nor
as to the meaning of flow in this part of the globe—i. e., the centrosphere.
«Adams, F. D., Report on the geology of a portion of the Laurentian area lying to the north of
the Island of Montreal: Ann. Rept. Geol. Sury. Canada, new ser., vol. 8, 1895, pt. 3, pp. 103-131.
760 A TREATISE ON METAMORPHISM.
MEANING OF ROCK CLEAVAGE.
The conclusions of the foregoing pages show clearly the meaning of
rock cleavage. I have already held that this structure is largely accord-
ant with the cleavage planes of minerals the particles of which have
crystallographic orientations; that these particles are mainly authigenic,
and therefore that rock cleavage is a capacity to part largely due to the
actual cleavage of similarly oriented mineral particles.“ As the cleavage of
mineral particles has long been known to be a molecular structure, it
follows that the cleavage of rocks is also largely a molecular structure. I
have also explained that the similar crystallographic orientation is fre-
quently, indeed usually, accompanied by an arrangement of the mineral
particles with their longer diameters in the same plane as the cleavage, and
that this dimensional arrangement is a factor in rock cleavage, although
less important in most cases than that of the crystallographic orientation of
the mineral particles.’ Leith has shown that the dimensional arrangement
dependent upon mineral habit is the factor which controls the similar
crystallographic arrangement.’
Where similar dimensional and crystallographic orientation prevails
with a number of the important cleavable minerals, and a similar dimen-
sional arrangement without crystallographic orientation prevails with other
important minerals, cleavage is perfect. The ruptures take place by taking
advantage of the cleavage of the mineral particles or by partings between
their longer dimensions, and both these factors give easier rupturing than
transverse to the cleavage. Since rock cleavage is fully discussed by Leith
in the publication referred to above, no detailed discussion is here made,
but it is mentioned in order to put this part of the subject in proper relation
to rock flowage.
EFFECT OF ROCK FLOW ON TEXTURES AND STRUCTURES.
As soon as rock flowage, combining mass and molecular movements,
is inaugurated, the interior movements begin the destruction of the original
textures and structures. With comparatively little flowage the original
«Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept.
U.S. Geol. Survey, pt. 1, 1896, pp. 633, 635.
bVan Hise, cit., p. 635.
¢ Leith, C. K., Rock cleavage.
EFFECT OF ROCK FLOWAGE ON TEXTURE AND STRUCTURE. 761
textures of some rocks may be wholly destroyed. For instance, such
rocks as quartzose sandstones, composed dominantly of a single mineral,
which retain their structures for indefinite periods if there be no marked
deformation even when buried under a thousand or more meters of other
rocks, when deformed in the zone of anamorphism, rapidly lose all clastic
textures. In place of the original textures, whether those of sedimentary or
those of igneous rocks, in consequence of flowage there appear the peculiar
textures of the slaty, schistose, gneissose, and cataclastic rocks.
But while there is a marked tendency to obscure and finally to oblit-
erate textures and structures by flowage, the extent to which this goes is
very variable. The larger the texture or structure the less likely it is to be
destroyed. Since structures involve larger masses than textures, the
former may not be much obscured at a stage when the textures are
wholly gone.
During rock flowage, even if there be perfect granulation or recrys-
tallization, or some combination of the two, the resultant rock may preserve
the larger textural and structural units of the original rock. This is more
likely to be true of granulation than of recrystallization. In rocks com-
posed of a number of minerals, even where granulation is complete, the
aggregate of mineral particles resulting from each original mineral particle
is commonly preserved as an elongated or flattened disk, the different
granules not being mingled to any considerable extent with those of the
granules of the associated minerals. For instance, the many granules
from a large feldspar grain of anorthosite may constitute an oval area little
or not at all mingled with the granules of the adjacent pyroxene. Again,
the innumerable granules of a flattened pebble of a conglomerate may be
sharply separated from the granules of the adjacent matrix, so that the
pebble is easily discriminated.
In proportion as recrystallization takes place the textures are likely to
become more rapidly obscured. The solutions transport material for short
distances from one mineral particle to another, and thus the products
become intermingled. For instance, adjacent feldspar and pyroxene may
produce reaction minerals different from each. But even where recrys-
tallization occurs the process must go to an extreme before the flattened
pebbles of a coarse conglomerate are wholly lost. Even when the matrix
is a coarse schist or gneiss the flattened disks of the pebbles are often
762 A TREATISE ON METAMORPHISM.
recognizable in a section transverse to the greatest elongation. But where
the metamorphism is extreme the pebbles may be so greatly flattened as to
allow the solutions to mingle the materials of the pebbles and matrix, and
thus the pebbles be wholly lost.
But even with most extreme metamorphism by reerystallization. the
larger structures are usually preserved, although the process may occasion-
ally go so far as to make the strata or even the beds indistinguishable; but
it rarely, if ever, goes so far as to obliterate formations. Excluding igneous
action, interstratified sedimentary formations of, for instance, sand, mud,
and limestone, have rarely been so profoundly metamorphosed as to make
them indistinguishable. The composition of a formation is the most promi-
nent feature by means of which the original nature of a rock can be
recognized. Where there are metamorphic formations having the approxi-
mate chemical compositions of sands, muds, and limestones, we may be
sure that such formations are sedimentary. Rock flowage may wholly
obliterate the clastic textures, and even the bedded structures; but there is
not sufficient kneading and intermingling of the materials to obliterate
major structures. ‘That this should be so is precisely what we would
expect when we understand that rock flowage is accomplished not by
recrystallization from fusion, but by recrystallization and by granulation of
the individual mineral particles, the rocks remaining solids throughout the
transformations.
ROCK FLOWAGE AND MASHING.
In another place I have proposed the term ‘‘mashing” to describe
the process of mass deformation in the zone of rock flow.” It has been
seen that rock flow involves the universal participation of the mineral
particles. The preceding study shows that this participation may be by
recrystallization or by granulation, or by the two combined; but in the
field, where microscopical work is, not usual, it is diffeult or impracticable
in many cases to discriminate between these two processes. Therefore it is
very convenient to have a word which will cover deformation by rock
flowage without reference to the detailed effects upon the mineral particles,
and for this purpose it has seemed to me that the word “mashing” expresses
“Van Hise, C. R., Principles of North American pre-Cambrian Geology: Sixteenth Ann. Rept.
U.S. Geol. Survey, pt. 1, 1896, pp. 694-696.
ROCK FLOWAGE AND MASHING. 763
better than any other the macroscopical point of view of the zone of flow.
Without further exposition, which would involve repetition, I propose the
term ‘‘mashing” to cover mass deformations of all kinds in the zone of rock
flow.
iher term
mashing,” thus used, includes much of the process which
has usually been described under the terms “dynamic metamorphism” and
“shearing.” The term ‘dynamic metamorphism” is objectionable for
many reasons; it will here simply be said that fracturing in the belt of
cementation is equally dynamic metamorphism; but the effects in the two
zones of flowage and fracture contrast so sharply that they should not be
confused. The term “shearing” has been used in a very loose and most
objectionable manner. Most authors who have used it mean differential
movement along a certain set of parallel planes, but apparently most
geologists who have thus used the term do not recognize that shearing
parallel to one plane is invariably accompanied by shearing in other planes.
Nor is it generally understood that when rocks are compressed by short-
ening without rotation this is possible only by shearing along all sets of
diagonal intersecting planes. In short, in all cases which have been
described as shearing and as shortening there is maximum shearing along
two sets of intersecting planes and maximum shortening and elongation in
the two directions half way between tho two sets of shearing planes.
Further, as' the term ‘‘shearing” is ordinarily used, it seems to be assumed
that there results a structure parallel to the one set of shearing planes,
whereas this may or may not be true.* Therefore, to avoid errors implied
by the terms “dynamic metamorphism” and “shearing,” the term ‘“mash-
ing” is introduced, and from this term the implications which attach to the
others have been excluded.
In this connection it should be noted that mashed rocks and their
constituent portions, such as pebbles and minerals, are frequently spoken
of as stretched. For the most part, this term can not be applied to rocks
in the same manner that it is appled to india rubber or steel. In general,
the facts upon which the statements as to stretching are made are simply
that the rocks show evidence of being longer in a certain direction than
they were originally. However, it does not follow from this that the rocks
@ Hoskins, L. M., Flow and fracture of rocks as related to structure; appendix to Van Hise, C. R.,
Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 1,
1896, pp. 860-866.
764 A TREATISE ON METAMORPHISM.
have been stretched. Most rocks and minerals when stretched quickly pass
their elastic limit, rupture takes place, and the dissevered parts are drawn
away from each other, often without deformation of the separated masses.
The elongations of rocks and portions of rocks are in most cases explained
not by pulling or stretching, but by compression or mashing in a direc-
tion transverse to that of the elongation, but this process does produce
tensile stresses, which often dissever the mineral particles. But the use of
the word “stretching” in reference to such rocks implies that tensile stress
is the dominant force. The term “stretching” should be dropped, except
in those instances where it is shown that tensile stress has been the primary
cause of deformation.
CHANGES IN CHEMICAL COMPOSITION.
If injection be excluded, changes in the chemical composition of the
rocks during metamorphism in the zone of anamorphism are not nearly so
great as they are in the zone of katamorphism. It has been explained that
in the latter zone, both in the belt of weathering (see Chapter VI, pp. 507-518)
and in the belt of cementation (see Chapter VII, pp. 655-656), the chemical
composition of the rocks may be very greatly modified. Indeed, the
changes of chemical composition are most profound in the belt of weather-
ing, and are very important in the belt of cementation. The great changes
in these belts are due to rapid and continuous circulation of water, by
means of which large quantities of material are transported from one place
to another. It has been explained that in the zone of anamorphism the
amount of water present is very small and its circulation exceedingly slow.
It follows that the water is not capable of transporting large quantities of
material considerable distances and thus making great changes in the chem-
ical composition of the rocks through extensive areas. In this we have the
explanation of the relative constancy of the chemical composition of the
equivalent unmetamorphosed and metamorphosed rocks of the zone of
anamorphism. While chemical changes do not greatly modify the average
composition of masses of rocks, it has been explained (pp. 682-685) that
there is important migration of material for short distances, and therefore
profound interior readjustment of the elements in different mineral combi-
nations.
Excluding injection, the chief chemical changes applying to the rock
masses as wholes are those of dehydration and decarbonation. Deoxidation
CHANGES IN CHEMICAL COMPOSITION. 765
also occurs. The water and carbon dioxide eliminated from the chemical
combinations are largely squeezed out of the zone of anamorphism (see
pp. 665-667), but a subordinate amount may remain as inclusions in the
minerals developed. (See pp. 667-678.) The absolute losses of these
compounds is considerable. The loss of hydrogen, oxygen, and carbon
dioxide increases somewhat the percentage of the other elements, but does
not affect their relative proportions. Excluding hydrogen, the relative pro-
portions of the bases are substantially the same before and after alteration.
The replacement of carbon dioxide by silica reduces the amount of carbon-
ates, Increases the quantity of silicates, and lessens the amount of free
quartz; but the absolute quantity of silica remains the same. In discrimi-
nating the metamorphosed sedimentary from the metamorphosed igneous
rocks, it will be shown on pages 914-915 that in many cases the pro-
portions of the important chemical elements furnish the best criteria for
separating the two classes of rocks.
While in the absence of intrusives there is small change in chemical
composition of the rocks in the zone of anamorphism, there may be great
changes in the chemical composition where injection is an important factor
in the metamorphism (see pp. 711-720), for by injection material different in
chemical composition from the injected rock may be intruded in various
amounts and in various degrees of division for extensive areas. (See pp.
708-711.) The changes in chemical composition of the rocks where injec-
tion is important may be as great as or greater than in the belt of
cementation.
The difference in the amount of chemical changes in the rocks in the
belt of cementation and in the zone of anamorphism under ordinary condi-
tions, and the difference in the amount of change in the latter zone where
injection is absent and where it is present, explain some of the differences
of opinion which are prevalent among geologists. One geologist says that
metamorphism does not alter the composition of the rocks; another says
that metamorphism greatly alters the chemical compositions of the rocks.
Each has a part of the truth. Probably the facts with which each is familiar
verify his point of view, but each makes a mistake in supposing that a nar-
row set of facts covers the entire field. The-truth of the matter is, as
explained, that there are profound changes in the chemical composition of
the rocks in the belt of weathering, great changes in the belt of cementa-
tion, and considerable changes where intrusion is important in the zone
766 A TREATISE ON METAMORPHISM.
of anamorphism. But the alterations in the zone of anamorphism where
intrusives are absent do not result in great changes in the chemical
composition.
RELATIONS OF ZONE OF ANAMORPHISM TO ZONE OF KATAMORPHISM.
It. has already been intimated that the zones of anamorphism and
katamorphism are not sharply separated, and that there is a gradational belt
between them. This gradational belt follows of necessity from the varying
character of the rocks and from the varying forces and agents at work.
One rock is of a chemical and mineral composition which readily alters;
another is of a composition which alters with difficulty. One rock is weak;
another is strong. At one place the conditions are mass-static, or those of
quiescence; at another they are mass-dynamic, or those of movement. In
one place igneous rocks may be absent; in another, abundant. In one
place the temperature at a given depth may be low; at another, relatively
high. The amount of water has the widest range. Hence it is imevitable
that there should be no sharp bounding plane between the zones of
katamorphism and anamorphism.
The weak or readily alterable rock may be changed by the reactions
of the zone of anamorphism, while a stronger and more refractory rock
below may be altered by the reactions of the zone of katamorphism.
Moreover, the same rock in the same position, because of varying condi-
tions, may be altered by the reactions of the zone of katamorphism and at
a different time by those of the zone of anamorphism. It follows that there
is a belt of considerable width in which we may have at one time, under
one set of conditions, the reactions of hydration, carbonation, and oxidation,
with expansion of volume; and at another time, under another set of
conditions, the reactions of dehydration, silication, and deoxidation, with
diminution of volume. Neither oxidation nor deoxidation is prominent in
the intermediate belt.
We therefore have in an intermediate belt the metamorphic results of
the belt of cementation, or those of the zone of anamorphism, or various com-
binations of the two. In one place we may find the metamorphic effects of
the belt of cementation superimposed upon those of the zone of anamorphism;
in another we may find those of the zone of anamorphism superimposed
upon those of the belt of cementation.
In order to understand more fully the phenomena in this transition
RELATIONS OF ZONES. 767
belt, we may consider the metamorphism of an imaginary rock of homo-
geneous character, composed of a single mineral which extends from the
surface to an indefinite depth. Near the surface the rock is broken into
blocks by faults and joints. There is no marked deformation of the indi-
vidual particles, except in thin layers along the fractures. The textures of
the rocks are for the most part preserved. Deeper down the fractures are
closer together, and at sufficient depth the layers may be no thicker than
leaves. Still deeper down every particle takes part in the deformation.
This is the belt in which granulation is prominent, although with it there
may be some recrystallization. Still deeper down recrystallization becomes
important, and finally dominant. In the intermediate belt fracture and
flowage do not exclude each other; both occur to varying extent in different
positions at the same time.
The transition above described for a single formation composed of a
single mineral takes place at different depths for different formations and
for different minerals of the same formation, and hence it is that in hetero-
geneous formations all the phenomena discussed under both the zone of
katamorphism and the zone of anamorphism may occur together.
At a given depth the stronger or less readily recrystallized rocks may
be largely deformed by fracture, and the weaker or more readily recrystal-
lized rocks be largely deformed by flowage. The result is that original
textures and structures may be more or less preserved in the former, while
in the adjacent layers original textures and structures may be entirely
destroyed and the rocks become slates or schists. It very often happens
that the alternating beds which show original textures and structures and
those in which they are obliterated are not more than a few inches thick.
In the intermediate belt many of the beds are deformed by combined
mass fractures and fractures of the individual mineral particles, so that in the
same rock in which joints, faults, fissility, ete., and the alterations attending
these phenomena occur, there are also found, between the major fractures,
all grades of deformation by interior movement, from the earliest stages of
peripheral granulation of the grains to complete granulation or recrystal-
lization, extending throughout the mineral particles. Thus we have all com-
binations of macroscopic and microscopic fractures and recrystallization.
In some places the intermediate belt is broad; in other places it is
narrow. The phenomena of the belt may be seen in aqueous and igneous
rocks alike. Many illustrations of each might be given.
768 A TREATISE ON METAMORPHISM.
As typical cases of sedimentary formations altered in the intermediate
belt may be mentioned the Wewe and Siamo slates of the Lower Marquette
series, the Goodrich quartzite, and the eastern half of the Michigamme
formation of the Upper Marquette series, all in the Marquette district of
Michigan.* Within these formations almost every one of the multifarious
reactions described, both within the zone of katamorphism and within the
zone of anamorphism, are beautifully illustrated. Indeed, it was a study of
these formations which first suggested to me the idea of a combination of
the phenomena of different alterations near the surface and at depth, and
the very great difference in the alterations which often occur at the same
depth under mass-static conditions and under mass-dynamie conditions.
In Calaveras Creek, a short distance below Calaveras Valley, south of
of
transition between a brecciated igneous rock and a schist. The first
San Francisco, in the Coast Ranges of California, may be seen all stages
was deformed under the conditions of spaced fractures. The second was
deformed by granulation and recrystallization. In passing from the breccia
to the schist one first finds about the blecks of igneous rock which have
their characteristic textures mere films of schist. Farther toward the schist
is found an intermediate stage in which unmashed blocks lie in a schistose
background or matrix. But a short distance from this place is the com-
pletely altered schist, in which no unmashed fragments remain. Every stage
of the transition is seen. The alterations within the blocks are those of the
zone of katamorphism. (See pp. 160-167, 187-191, 599-602.) Within
the films and layers of schists constituting the matrix in which the blocks
rest and in the main mass of schist the alterations are those of the zone
of anamorphism. (See pp. 167-170, 187-191, 657-659.)
These cases of the combination of the phenomena of metamorphism of
the zone of katamorphism and those of the zone of anamorphism are but
typical of almost innumerable illustrations. The phenomena in any one
case are very complex and intricate. Detailed consideration of any single
instance would require much space, and it is yet too early to attempt to
classify the different cases of various combinations of the alterations of the
zones of anamorphism and katamorphism.
7Van Hise, C. R., and Bayley, W. 8., The Marquette iron-bearing district of Michigan: Mon. U.S.
Geol. Survey, vol. 28, 1897, pp. 1-608.
ENERGY REQUIRED FOR MECHANICAL SLICING. 769
COMPARATIVE ENERGY REQUIRED FOR DEFORMATION IN ZONES OF
KATAMORPHISM AND ANAMORPHISM.
The question of the amount of energy required to produce deformation
in the zone of katamorphism, in the intermediate zone, and in the deep-
seated zone of anamorphism is of great importance.
The energy for rock deformation may be divided into two parts—
energy for mechanical work and energy for chemical work. The mechan-
ical work is of three kinds—the subdivision of the rocks, the transfer of the
material in order to produce a changed form, and the friction between the
parts of the subdivided rocks during the transfer.
The most useful comparison as to the amount of energy spent in the
different zones is upon the basis of average mass deformation. By average
mass deformations I mean the strains necessary to change the shape of unit
masses of rock in a nearly similar way, so that the exterior forms are prac-
tically the same. ‘To illustrate:
A cubic foot of rock may be sup-
posed to be divided into ten hori-
zontal slices and sheared parallel
to these slices, so as to produce,
ignoring the minor corners, a a neers mere GORA
roughly rhomboidal mass (fig.
21). If instead of ten there were a hundred slices, the approximation
to a rhomboidal mass would be closer; if a thousand, closer still; and so
on, until the slices became of infinitesimal thickness, when the mass would
be rhomboidal. In all of these strains the mass deformation averages about
the same.
It is perfectly clear in the case of this hypothetical deformation that
the amount of work in rupturing is directly as the number of slices. The
average mass deformation is substantially the same, and the energy required
for change of form—in other words, for transfer of material—is nearly
constant. The total amount of differential movement or shear is practically
the same in all cases, and therefore the friction is nearly constant. Hence,
in the case of the illustration, the energy for the deformation is almost
directly as the number of slices. But in the case of the crust of the earth,
supposing the fracturing to become closer as depth increases, the energy
MON XLVII—04——49
770 A TREATISE ON METAMORPHISM.
required for a given mass deformation would increase with depth for two
reasons: (1) More energy is required for the finer subdivision; and (2) the
load increases with depth, and therefore the energy required to overcome
friction also increases with depth. The energy required for the similar
transfers of material remains practically the same at all depths.
It has already been seen that near the surface the dominant deforma-
tions are relatively wide-spaced faults and joints; that with increase of
depth the spacing between the faults and joints decreases until the fractur-
ing is that of fissility and finally of granulation. :
It is therefore clear that the amount of energy required for fractures
a considerable distance apart, such as prevail where faults or joints, or
both, are the dominant deformations, is much less than where the fractures
are close together—as, for instance, in fissility. Furthermore, it is clear
that the amount of energy required for the slicing of fissility is much less
than that required for granulation of the individual particles, for in the
latter case a mass equivalent to a fissile leaf must be broken into a
multitude of particles. Probably the ratio between the energy required
for breaking a rock into fault or joint blocks near the surface and that
required for producing fissile leaves deeper down is not greater than the
ratio between the energy réquired to produce fissility and that required to
produce granulation throughout at a still greater depth. No general ratio
between the amount of energy spent in deformation by faults and joints
and deformation by granulation can be given, but it is certain that the
amount of energy used in extreme cases of the latter may be indefinitely
greater than that required for the former.
Since it is certain that in passing from the surface to considerable
depth there is a passage from deformation by faulting or jointing, or both,
to deformation by granulation, it is certain that to a depth of many hundred
meters there is a steady and very rapid increase in the amount of energy
required for a given mass deformation.
At sufficient depth, as has been seen, granulation is more and more
replaced by recrystallization, and finally this process is dominant. It would
be very interesting to know exactly the relative amounts of energy required
for the two processes of granulation and recrystallization.
As already noted, the energy required for granulation is wholly
mechanical, and includes three factors—(1) that required for the subdivision
ENERGY REQUIRED FOR DEFORMATION. C
“1
—
of the rocks, (2) that required for transfer of material, and (3) that required
to overcome friction.
The energy factors in recrystallization are four'in number:
(1) Energy is continuously used in straining minerals during defor-
mation, but it is impossible to determine the amount of straining which
takes place, for evidence of the strain is continuously obliterated by
solution and deposition. If the mechanical stresses did not continuously
produce a state of strain, and thus disturb the equilibrium, it is probable
that the rate of the process of solution and deposition would be very slow.
It is this constant mechanical work in producing strain that keeps the
process of recrystallization going.
(2) Energy is required for the transfers of materials by solution.
(3) Energy is required to overcome the viscosity of the solutions, or,
stated in a different way, energy is required to overcome the friction of the
molecules against the water during their movements.
(4) As a result of solution and deposition in the lower zone, the
minerals produced are, on the average, more compact than before the
process. In so far as a more compact condition results energy is liberated.
On the other hand, the dominant chemical reactions of deoxidation, silica-
tion involving decarbonation, and dehydration all demand a large amount
of energy. The energy thus consumed is probably ereater than that
liberated by condensation. It is therefore thought to be probable that the
process of solution and deposition consumes energy. In the rare cases in
which the minerals are equatly compact before and after the process, and
no chemical change takes place, as in the recrystallization of a limestone,
the energy of solution and that of deposition balance.
We may now compare the energy demanded for each of the different
factors in the two processes of granulation and recrystallization. The three
factors entering into granulation are paralleled by the first three of the four
factors mentioned below entering ito recrystallization. (1) It appears
probable that the energy required to produce granulation is greater than
that required to produce a state of strain during recrystallization. (2) The
energy required for the actual transfer of the material by granulation and
by solution may be supposed to be the same. (3) The energy required
to overcome friction during granulation is certainly vastly greater than the
energy required to overcome the friction of the molecules against the water
T12 A TREATISE ON METAMORPHISM.
during the transfer of the material. (4) As a result of the solution and
redeposition, energy is consumed.
Therefore the energy required to accomplish granulation, on account of the
greater work of subdiwision and the much greater work necessary to overcome
friction, is almost certainly greater than the energy required for recrystallization.
If one premises that when the conditions are such that either granu-
lation or recrystallization might occur the process takes place which
requires the less expenditure of energy, this furnishes additional support
to the above conclusion; for wherever the conditions are such that recrys-
tallization can replace granulation, this occurs.
In the artificial deformation of dry marble in the experiments per-
formed by Adams and Nicolson," the deformation was accomplished by
gliding and granulation. When the deformation was made in the pres-
ence of steam, the acjustment was, to a small extent, by recrystallization.”
(See pp. 747-748.) If the conditions could be so varied as to accomplish
the deformation by recrystallization mainly, it would be interesting and
important to compare the amount of work done upon the mass during
deformation under these different conditions; for if this could be done it
would be possible, at least in this case, to make an estimate of the relative
energy demanded by deformation through granulation and through recrys-
tallization. Doubtless this would be a difficult task. It would be necessary
to separate the total work done in the machine into the parts which were
required for the deformation of the rock mass and that required for the
deformation of the surrounding iron, and in one case also to estimate the
energy furnished by the water. If it were possible to make the determi-
nation, I anticipate from the analysis of the previous pages that the energy
required for deformation through recrystallization would be less than that
required for deformation through granulation.
The question naturally arises, If less energy is required for recrystalli-
zation than for granulation, why did the latter process occur extensively
during mountain making in various regions? The answer is plain. Recrys-
tallization can not take place except where the proper conditions of temper-
ature and moisture are present. If the nucleus of the earth be shrinking,
the lithosphere must be reduced in size to accommodate itself to this nucleus.
“Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble:
Philos. Trans. Royal. Soc. London, ser. a, vol. 195, 1901, pp. 376-382.
bAdams and Nicolson, cit., pp. 3882-385.
ENERGY REQUIRED FOR DEFORMATION. CUS
This implies crustal deformation which extends to the surface, and therefore to
places where the conditions are not such that recrystallization can take place.
From the foregoing considerations, I believe that the amount of work
done, in order to produce the same mass deformation of the rocks, increases
to a certain depth and then decreases until the belt of the perfect schists is
reached. How far down this holds I am unable to conjecture, but believe it
is probable that it does so at least as deep as the zone in which the schists
formed by recrystallization develop, and that it may continue much farther.
The deformation of this deep-seated zone may or may not require the
elevation of the superincumbent mass. Where the superincumbent mass is
not elevated it is concluded that the energy required for deformation by
recrystallization per unit mass is probably less than that required for defor-
mation by granulation, and may be less than that required to produce the
spaced fractures which occur near the surface. Where the deformation is
of the kind which requires the elevation of the superincumbent material,
energy is needed, not only to do interior work of deformation, but to elevate
this material. Where these conditions obtain it may be that the amount of
energy required to produce the deformation steadily increases with depth
on account of the energy required for lifting the load in addition to that
required for the interior deformation.
So far as I know, the region in America which best illustrates all the
phenomena from deformation by widely spaced thrust faults and by joints to
the interior deformation of recrystallization is the Southern Appalachians.
In the Great Valley the Paleozoic rocks are little deformed except by thrust
faults and joint folds. Eastward, deeper into the mountains, the faults and
joints become closer together, and are finally replaced by numerous closely
distributed fractures. At the same time the rocks show more and more
evidence of metamorphism, first by granulation, and second by reerystalliza-
tion. In the cores of the mountains are rocks which have largely or
completely recrystallized, being slates, schists, and gneisses, with well-
developed cleavage. In the valley we find the alterations to affect but a
small part of the mass of the rocks now exposed. The deeper we go into
the mountains the larger is the proportion of the material which was affected
by the alterations, and in the schists in the core the entire mass was modi-
fied, both in a mechanical and in a chemical way. In this passage it
becomes clear that for a certain distance the amount of energy required for
deformation per unit mass increased, but this tendency may have been
T714 A TREATISE ON METAMORPHISM.
reversed in the deepest seated zone. However, since this latter zone is a
region of uplift, and the work required for the raising of the superimcumbent
strata must be added to that required for the interior deformation of the
rocks which we now see, no positive statement can be made as to whether
the total energy increased or decreased per unit mass in the deformation
of the deeper seated rocks.
The conclusions of the previous pages concerning the energy required
for a given mass deformation at different depths gives a possible explana-
tion of the concentration of superficial deformation found in mountain
ranges. If the energy of deformation be less at the depth at which the
slates, schists, and gneisses develop than in more superficial belts, it is
possible that the more rigid outer shell of the earth may shear over the
nucleus in the zone at which the schists develop, the deformation being
widely distributed. Such shearing for a considerable area may require
less expenditure of energy than would be demanded for the similar defor-
mation of the rocks above; but during the earth movements, as a result
of cooling and other changes, the superficial material must certainly be
deformed and shortened,* and) at such places deformation is concentrated
and mountain ranges are formed. ‘This subject is, however, better dis-
cussed in Chapter X, under the heading “ Relations of rock flowage to
mountain making.”
CONCLUSION.
The energy requi.ed to produce a given mass deformation increases
downward to the bottom of the zone where deformation is chiefly by
fracture. In deformation by recrystallization the energy required to pro-
duce a given mass deformation is probably less and may be much less than
that in the lower part of the zone of fracture. By observation we see that
recrystallization does take place wherevér that process can occur. We have
shown that the amount of water present in the rocks wherever recrystalliza-
tion takes place is adequate to accomplish the process. In the fact that less
energy is required for recrystallization than for granulation lies the most
fundamental answer to the question why recrystallization rather than defor-
mation by fracture takes place wherever the conditions are such that the
former process can occur. Nature is a great economist, and expends the
minimum amount of energy to accomplish her work.
«Van Hise, C. R., Estimates and causes of crustal shortening: Jour. Geol., vol. 6, 1898, pp. 41-64.
CiaeUe IB IR Je
ROCKS.
In the preceding chapters 1 have considered the forces and agents of
metamorphism, the divisions of the lithosphere into zones of metamorphism
upon a physical and chemical basis, the alterations of each of the important
rock-making minerals in reference to the zones in which they occur, and
the general nature of the alterations of rocks in the belt of weathering,
the belt of cementation, and the zone of anamorphism. It now remains
to consider the metamorphism of individual rocks, but for a number of
reasons it is impossible to do this part of the work satisfactorily at the
present time.
In papers upon metamorphism there has in general been no attempt
whatever to consider the subject from the points of view set forth in the
previous chapters. The usual practice has been to describe the metamor-
phism of a given region or district and give various conjectures as to the
causes of the alterations, often with little or no reference to the different
belts and zones of metamorphism. ‘To some extent the belt of weathering
has been recognized as having distinctive- reactions. The general neglect
of the principles of physics and chemistry in the consideration of meta-
morphism renders it impossible in most cases to interpret the descriptions
of the phenomena from the points of view discussed in this volume. If
the principles of metamorphism set forth in the foregoing chapters are well
founded they should be considered in the study of individual instances.
If these principles be recognized it will be comparatively easy to describe
a given case of metamorphism in reference to them. When this work is
done for a sufficient number of districts it will then be possible to write a
satisfactory chapter upon the metamorphism of individual rocks.
A second insuperable difficulty preventing a systematic treatment of the
metamorphic igneous rocks is the status of the classification of these rocks.
At the present time there is not only no consensus of opinion concerning
Ber
(19
776 A TREATISE ON METAMORPHISM.
a classification of such rocks, but there is not even an approximation to a
consensus of opinion as to the principles upon which a classification should
be based. Until the original igneous rocks are satisfactorily classified it is
quite impossible satisfactorily to consider their metamorphosed equivalents.
The situation in reference to the original sedimentary rocks is somewhat
more fortunate. There is rather general agreement as to the principles
upon which the classification of the sedimentary rocks is to be based, and
upon the main classes, orders, and families of rocks which shall be
recognized. I shall therefore attempt to consider the metamcrphism of
each of the main divisions of the altered sedimentary rocks, but shall not
make a similar attempt for the igneous rocks.
USE OF SOME GENERAL TERMS APPLIED TO METAMORPHIC ROCKS.
The more important of the terms generally applied to metamorphic
rocks are the prefixes meta and apo, and the general names ‘‘slate,” “schist,”
and ‘‘oneiss.”
META.
The prefix meta is used in this treatise in a general way to indicate
any kind of alteration of any kind of rock. This usage of the prefix
corresponds with the definition of the term ‘‘metamorphism” as given in
Chapter I, page 32, where metamorphism is defined as meaning all kinds
of alterations of all rocks by all forces, agents, and processes. In the sense
proposed we may say meta-sandstone, meta-shale, meta-arkose, meta-
dolerite, etc. We may even apply the prefix meta to a rock already
metamorphosed which has undergone a second set of changes, as, for
instance, meta-graywacke and meta-quartzite. ‘Thus used it means that a
rock which had first been transformed to a graywacke or quartzite, respec-
tively, was afterwards again metamorphosed.
APO.
The prefix apo is here used as a qualifier to indicate metasomatic
cnanges in rocks in which the original textures and structures are largely
preserved. This usage of apo accords with the underlying idea of the term
as proposed by Doctor Bascom." She proposed to call a rhyolite which has
“Bascom, Florence, Aporhyolite of South Mountain, Pennsylvania: Bull. Geol. Soc. America,
vol. 8, 1897, pp. 393-396.
DEFINITION OF TERM APO. CUE
all the textures of glass, but which has been completely devitrified, apo-
rhyolite. In her original definition she restricted the term to this single
alteration, but later she enlarged the meaning” so as to include not only
devitrified glasses but alterations of all rock in which the original textures
and structures are preserved.
The prefix apo thus used discriminates rocks which have been meta-
morphosed under mass-static conditions: and retain their original textures
and structures from those which have been metamorphosed under mass-
dynamic conditions, so as to destroy or partly destroy the original textures
and structures and to produce slaty, schistose, or eneissic structures. A
rock to which apo is prefixed may differ greatly from the original rock in
both chemical and mineral composition.
The term epi was proposed by Giimbel” as a prefix for rocks which have
undergone a change in mineral composition, and which by this change have
come to have the same mineral composition as another rock. ‘Thus a dia-
base the pyroxene of which has changed to amphibole, and which, therefore,
has the mineralogical composition of diorite, he calls an epidiorite. The
term diabase itself was originally applied to an altered dolerite, which has
as an important constituent secondary chlorite. Applying the method which
is proposed to be here followed, the rock which Giimbel calls epidiorite
would be called apodolerite. This name is much more satisfactory than
that of Giimbel, since it gives the original nature of the rock, and tells
that a part or all of its minerals have altered, but that it retains its original
textures and structures. Therefore the name apodolerite gives a better
understanding of the history and relations of the rock than does the name
epidiorite.
In a manner similar to the above apo may be prefixed to any rock
altered under mass-static conditions, when it is desired to call attention to
the rock from which it was derived. Thus quartzite is an aposandstone;
graywacke is an apogrit, etc. The term apo thus used supplements the
terms schist and eneiss, its usage being structural in a negative sense—
that is, applied to metamorphic rocks in which the textures and structures
have not changed.
aBascom, Florence, Voleanics of Neponset Valley, Massachusetts: Bull. Geol. Soc. America,
vol. 11, 1900, pp. 121-122.
> Giimbel, K. Wilhelm yon, Geologie yon Bayern, Kassel, 1888, vol. 1, p. 125.
Utes A TREATISE ON METAMORPHISM.
SLATE AND SCHIST.
Slate and schist are here used as general terms which are applicable
to all rocks having a well-defined cleavage in which the cleaved plates
are essentially like one another. I have explained in another connection
that cleavage in rocks is due to the arrangement of the mineral particles
with their longer diameters or readiest cleavage, or both, in a common
direction, and that this arrangement is caused, first and of most impor-
tance, by parallel development of new minerals; second, by the flattening
and parallel rotation of old and new mineral particles; and third, and
of least importance, by the rotation into approximately parallel positions
of random original particles. Subsequently Leith has shown that parallel
slicing may also be a subordinate factor. In a slate or schist the cleavage
may or may not be parallel to an original structure such as bedding,
but usually it intersects the original structures. Cleavage is fully dis-
cussed by Leith.’ His work shows that cleavage ultimately rests upon a
parallel dimensional arrangement of the mineral particles, but for some
minerals this dimensional arrangement carries with it mineral cleavage, and
the cleavage of these minerals is usually the controlling factor in rock
cleavage. Moreover, he argues that the dimensional arrangement is mainly
caused by recrystallization. His work while advancing in an important
way the theory of cleavage also confirms my own view in showing that
rock cleavage is mainly due to the actual cleavage of mineral particles
produced by recrystallization, and subordinately due to the easy separation
between mineral particles in the direction of their greater dimensions.
The production of slate and schist requires recrystallization during
mass-dynamic action. Previous textures and structures are partly or wholly
obliterated. In these particulars slate and schist contrast with those rocks
to which the term apo may be prefixed.
SLATE.
Slate is defined to include those cleavable rocks the cleavage pieces of
which are like one another and the mineral particles of which are for the
most part so small as to be invisible to the naked eye. The typical example
is furnished by the roofing slates, which, so far as the eve can see, are gray
aVan Hise, C. R., Principles of North American pre-Gambrian geology: Sixteenth Ann. Rept.
U.S. Geol. Survey, pt. 1, 1896, p. 635.
> Leith, C. K., Rock cleavage.
DEFINITIONS OF SLATE AND SCHIST. CUD
or black, homogeneous, aphanitic rocks, which may readily be parted into
thin plates which are indistinguishable from one another. The microscope
shows that the cleavage of these rocks is due to the causes above given.
The foregoing definition of slate is purely structural. It involves no
mnplication as to the minerals which compose the slate. It says nothing as
to whether the rock from which a slate is derived is sedimentary or igneous.
If it be desired to ignore the origin of a slate and to indicate its mineral
composition, this may be accomplished by prefixing mineralogical qualifiers.
For instance, if a slate be composed of mica and quartz as the chief con-
stituents, it is a mica-quartz-slate. If it be desired to emphasize the slaty
structure, and at the same time to indicate the original rock from which the
slate is derived, the name of this rock may be prefixed. For instance, if a
slate be derived from mud, it may be called a pelitic slate or pelite-slate; if
a slate be derived from dolerite, it is a doleritic slate or dolerite-slate, ete.
If it be desired to emphasize the original rock from which the slate is
derived, and only secondarily to indicate that the rock has a slaty structure,
this may be done by placing the word slate in the first position; as, for
instance, slaty pelite or slate-pelite, and slaty dolerite or slate-dolerite.
Finally, mineralogical qualifiers may be prefixed to the above compound
names, as mica-quartz-pelite-slate or mica-quartz-slate-pelite.
SCHIST.
Schist is defined to include those cleavable rocks the cleavage pieces
of which are like one another and the mineral particles of which are for
the most part so large as to be visible to the naked eye. The thin cleavage
plates of a schist which are like one another are called folia, and the rock
is spoken of as foliated. The most important of the cleavage-making
minerals is mica, and a typical example of a schist is one composed of mica
and quartz in which mica is the chief cleavable mineral. Such arock may
therefore be called a mica-quartz-schist. There is great variation in the
approach to perfection of the arrangement of the minerals. For instance,
the plates of mica may be almost perfectly straight and parallel or they
may be curved or even crenulated. The closest approximation to the above
definition of schist is that given by Geikie. According to this author, “A
rock possessing a crystalline arrangement into separate folia is in English
termed a schist.”“ It is to be noted that the definition of schist given is
@ Geikie, Archibald, Text-book of Geology, Macmillan & Co., London, 3d ed., 1893, p. 178.
780 A TREATISE ON METAMORPHISM.
purely structural, and to its structural meaning the term will be rigidly
confined in this treatise.
However, the term schist has been widely used both in a structural
sense and as the name of definite rocks; and indeed this double usage is
found even in Geikie, whose definition restricts the term to a structural
meaning. Ilustrations of the use of the term schist both as the name for a
definite rock and with a structural signification are furnished by the terms
mica-schist, chlorite-schist, and hornblende-schist as generally used. In the
‘ase of mica-schist, the term means that the rock is a schistose mica-quartz
rock. As here proposed such a rock should be called a mica-quartz-schist.
The omission of the term quartz from the name of this rock arose naturally,
since this is one of the most abundant varieties of schists, and since in it mica
is So conspicuous and quartz so Mconspicuous. After it was discovered that
quartz is usually equaily or more important than mica in most schists,
usage came to imply the presence of quartz in the rock called mica-schist.
But as the study of rocks continued, other schists were found in which the
conspicuous constituent is not mica, but chlorite or hornblende; and such
rocks were immediately called chlorite-schists or hornblende-schists, the
implication being that quartz was the remaining chief constituent How-
ever, when close microscopical studies were begun, they showed that in the
chlorite-schists and hornblende schists quartz might or might not be an
abundant constituent, its place being taken by feldspar or other minerals, and
thus chlorite-schist or hornblende-schist, as used by most English-speaking
geologists, means a schistose rock containing chlorite or hornblende and
other unnamed minerals. Thus the original scheme broke down. The
German petrographers soon saw this, and where schistose rocks were found
in which feldspar was an important constituent they proposed to call the rock
a gneiss, since feldspar was recognized as an important constituent in the
rocks which had before been called gneiss. When the English and American
geologists ascertained that a schist, as they use the term, might contain
various combinations of minerals, they were inclined to return strictly to a
structural sense, as indicated by the quotation from Geikie already made.
But as yet this plan has not been earried out consistently in reference to
individual rocks, and the usage of combining a structural meaning with
mineralogical implications is still common. It seems to me that clear dis-
crimination can be obtained only by restricting the term schist to the
DEFINITION OF SCHIST. 781
structural meanimg as advocated. Illustrating the usage, if a rock which
has the structural characters of schists as here defined be composed of horn-
blende and plagioclase as chief constituents, it is a hornblende-plagioclase-
schist. If the plagioclase be definitely determined, as, for instance,
labradorite, such a rock is a hornblende-labradorite-schist; and of course
the classical mica-schist is mica-quartz-schist, as already stated.
The schists im which ‘it is not desired to direct the attention to their
origins, or the origins of which are not known, are thus satisfactorily dis-
criminated by mineralogical qualifiers. If the origin of a given schist be
known, and it is desired to indicate this, the term is used in precisely the
manner in which it is proposed to use slate, viz, by combining schist with
the names of rocks from which it has been derived. In doing this either
the schistose character or the origin of the rock may be emphasized. If it
be desired to emphasize the schistose character, and at the same time to
indicate the original rock from which the schist is derived, the name of this
rock may be prefixed. For instance, if a schist be derived from arkose, it
is an arkose-schist. If a schist be derived from a gabbro, it is a gabbro-
schist. If it be desired to emphasize the original rock from which the
schist is derived, and only secondarily to indicate that the rock has a
schistose character, this may be done by placing the word schist in the
first position; as, for instance, schistose arkose or schist-arkose, schistose
gabbro or schist-gabbro. Finally, if it be desired to combine the chief
minerals with the compound names which indicate the schistose character
and the origi of the rock, this may be done. For instance, a rock may be
said to be a mica-quartz-feldspar-arkose-schist.
The usage above advocated is very advantageous in handling the
metamorphosed rock masses, since the mineral character of a given schist
may be correctly indicated without any reference to its origin, or the origin
of the schist may be very accurately indicated. If the term schist be used
with a combined structural and mineralogical meaning, as is ordinarily
done, such usages are impossible. For instance, if schist means the pres-
ence of a certain mineral, as quartz, it can not be applied to a rock which
has a schistose character as here defined and which does not contain quartz.
For example, one could not say peridotite-schist. Considering all the fore-
going facts, the advantage of restricting the term schist to a purely struc-
tural usage is so great that all mineral implications for the term should be
dropped.
782 A TREATISE ON METAMORPHISM.
GNEISS.
The term gneiss is defined to apply to a banded rock the bands of
which are petrographically unlike one another and consist of interlocking
mineral particles. The bands in different gneisses are of variable thickness,
ranging from a fraction of a centimeter to many centimeters. Also there
is a similar range in thickness of the different bands of the same gneiss.
The lithological dissimilarity of the bands of gneiss constitutes a funda-
mental distinction between gneisses and the slates and schists, which are
comparatively homogeneous. Usually the gneisses have a cleavage par-
allei to the banding, but this cleavage is by no means so general or
distinctive as in the slates and schists, and not infrequently a parallel
arrangement of the mineral particles resulting in cleavage is almost wholly
lacking. This is especially likely to be the case with the gneisses of
igneous origin. It has been explained that the slaty and schistose struc-
tures are mainly dependent upon recrystallization during mashing. (See
pp: 688-690, 748-759.) Where a parallel arrangement of the mineral
particles is marked in gneiss this is likely to be largely due to the same
cause. But the parallel orientation of some of the mineral particles of a
part of the original gneisses formed from magmas is due to differential stress
during the primary crystallization of the rocks.
The use of the term gneiss advocated in the preceding paragraph
approximates closely to the practice of American and English geologists in
recent years. But gneiss, like schist, has been extensively used with a dual
meaning, comprising structural and mineralogical factors. For instance, in
Germany the word gneiss has been used generally to designate rocks which
have the structure either of schists or of gneisses, as defined in the foregoing
pages, and which have a quartz-feldspar background with one or more
other constituents. In this sense the terms mica-gneiss and hornblende-
eneiss mean rocks having either a schistose or a gneissose structure, as here
defined, and a quartz-feldspar groundmass in which are respectively mica
and hornblende. ‘The dual significance of the term gneiss has arisen natu-
rally from the fact that many of the gneisses, especially the abundant ones
first studied, have as chief constituents quartz and feldspar, and with these
minerals one or more other constituents. The minerals quartz and feldspar
common to them were taken as distinctive of the gneisses, and the names of
the other minerals were prefixed as qualifiers. But precisely as is the case
DEFINITION OF GNEISS. 783
with schists, the term gneiss was extended to banded rocks many of which
do not have this definite mineral groundmass. This became apparent only
when the microscope was applied. to thin sections of rocks.
Since, as a matter of fact, it has been found that the rocks which have
been called gneiss have a very great variety of mineral constituents, and not
necessarily any of the constituents originally supposed to be implied by the
name, it is no longer possible to use the term gneiss with a dual significa-
tion and at the same time to have clear discrimination. I therefore propose
to confine the term gneiss strictly to its structural sense, including all finely
banded crystalline rocks, whether of igneous or of aqueous origin. Thus
defined, the term gneiss may be applied precisely as are the terms slate and
schist. Mineralogical qualifiers may be prefixed; the name of the rock from
which the gneiss is derived may be prefixed; the word gneiss may be pre-
fixed to the name of the original rock; and, finally, names denoting the
mineral composition, the name of the original rock, and the term gneiss may
be combined. For instance, we may say mica-quartz-feldspar-eneiss, diorite-
oneiss, granite-gneiss, gneissoid diorite, gneiss-diorite, gneissoid granite;
oneiss-eranite, and finally mica-quartz-feldspar-gneiss-granite, ete.
GENERAL STATEMENTS.
It has been seen that all of the terms which have been discussed—
meta, apo, slate, schist, and gneiss—may be united with petrographical names
and with mineralogical qualifiers in various ways. For additional refined
discrimination geographical qualifiers may be used.
Under the usages advocated the character of a metamorphosed rock
may be designated very loosely and broadly either in a structural or in a
petrographical sense, may be discriminated with a moderate degree of
accuracy, or may be designated very accurately. The following usages
illustrate the various stages of gradation from loose to accurate designation:
An original unaltered rock may be granite, diorite, syenite, limestone,
sandstone, ete. If we wish to indicate that the rock has been altered
without saying anything as to the nature of the alteration, we may prefix
meta to these terms. If alteration has taken place, but the original texture
of the rock is retained, we may prefix apo to any of the names. If
alteration has taken place so as to produce new structures, we may prefix
to the names the words slate-, schist-, or gneiss-, or the adjectives slaty,
784 A TREATISE ON METAMORPHISM.
schistose, or gneissose. If we wish to designate the minerals of which the
rocks are composed, we may prefix the names of the minerals to any of
the previous names, as, for imstance, mica-hornblende-schist-syenite, or
mica-hornblende-syenite-schist. Still further accuracy is obtained by pre-
fixing a geographical name, as Aurora granite. A final stage of accuracy is
obtained by combining all of these devices, as Aurora mica-quartz-feldspar-
schist-granite, or Aurora mica-hornblende-syenite-gneiss. Of course ordi-
narily the rock will be designated by the simpler terms, but somewhere in
a paper all of the qualifiers can be put together, so as to give in a single
compound name its most accurate designation.
The elasticity of this plan, by which it is possible to make loose,
approximate, and very fine discriminations, is noteworthy. In the field it
may not be possible to give the mineral composition or the origin of the
rocks and an expression on these points can be avoided. But so far as the
facts are known they may be expressed in the notes if desired. Thus all
grades of knowledge of the metamorphic rocks can be indicated without
implying more than is known. When additional knowledge is gained and
further refinement is possible, these refinements may be expressed.
In preparing reports in the office it is very often advantageous to be
able to throw large groups of rocks together which have common structures
or textures without reference to their mineral composition or origin. In
another part of the text it may be desirable to indicate the mineral character
of the different rocks. In other parts of the text it may be desirable to
indicate exactly the origin of the rocks. All these wants are very nicely
met by the proposed usage of the terms meta, apo, slate, schist, and gneiss.
SEDIMENTARY ROCKS.
The sedimentary rocks may be divided in accordance with the ordinary
classification into nonfragmental or nonclastic and fragmental or clastic.
The nonfragmental rocks may be divided into five orders—nitrates,
sulphates, chlorides, carbonates, and oxides. The only important nitrate
is niter. The important sulphates are gypsum and anhydrite. The only
important chloride is rock salt. The carbonates may be divided into two
families—the calcium-magnesium carbonate family and the iron-bearing
carbonate family. The oxides are divided into two families—the iron-oxide
family and the silica family.
CLASSIFICATION OF SEDIMENTARY ROCKS. 785
Following Naumann and Haiiy,” the fragmental rocks are divided into
three orders—psephites (dy@os, a pebble), psammites (lauuds, sand), and
pelites (adc, clay). The psephites include the fragmental rocks which
contain abundant fragments coarser than peas. Frequently the matrix
of the psephites is psammitic. The psammites, following Rosenbusch,’
are defined as including the sedimentary rocks which are composed of
particles under the size of peas and larger than dust. The -pelites are
composed of the minute particles, from those of the size of dust down.
The unconsolidated psephites, depending upon the coarseness, may be
called pebble, gravel, or bowlder deposits. Since the psephites are
unassorted material, the order includes but this one family and their
metamorphosed equivalents.
The psammite order comprises three families—quartz sands, quartz-
feldspar sands, and ferromagnesian sands, and the metamorphosed
equivalents of each.
The pelite order includes but one family—the muds, and their
metamorphosed equivalents.
The foregoing classification may be represented in tabular form as
follows:
Classification of sedimentary rocks.
4 ye Y
Class. Order. Family.
Nitratests sess a= Niter.
Sulphates.-......-- | Gypsum and anhydrite.
Chiloridessaee eee | Rock salt.
Nont Jalcium-magnesium cart es.
Nonfragmental_....----- | Gio Se el jc alcium-magnesium carbonates
| Iron-bearing carbonates.
Quid el | Tron oxide.
| Silica.
Psephiteses2-=--=/ Pebble, gravel, and bowlder deposits and their meta-
morphosed equivalents.
| Quartz sands. ‘
Hragmentall-\-2ss5----- : 1} And their metamorphosed
Psammites.-..----- Feldspar-quartz sands. ;
s ; equivalents.
Ferromagnesian sands.
Pelites)=.2+-25522-- Muds and their metamorphosed equivalents.
@ As cited by Zirkel, F., Lehrbuch der Petrographie, Leipzig, 1893, vol. 1, pp. 493-504.
bRosenbusch, H., Elemente der Gesteinslehre, Stuttgart, 1898, pp. 386-389.
MON XLVII—04 50
786 A TREATISE ON METAMORPHISM.
The foregoing classification is made upon the basis of giving family
names to the abundant kinds of the sedimentary rocks. If names were
provided for kinds which are not abundant, a multitude of names would
be necessary. The intermediate varieties may be provided for by com-
pounding the various names within either the fragmental or the nonfrag-
mental class. If the materials of each of the two classes be about equally
abundant, the names may be hyphenated; if one is subordinate, this may
be used in the adjective form. The following illustrates the usage: If a
rock is about halfway between shale and limestone, it may be called a
shale-limestone or limestone-shale. If the fragmental material be subordi-
nate, it may be called shaly limestone. If the nonfragmental material be
subordinate, it may be called a calcareous shale. In a similar manner, if
a rock be intermediate between the carbonate and siliceous rocks, one may
say limestone-chert, chert-limestone, or calcareous chert, cherty limestone.
If it be desired to indicate still more closely the character of the rock,
geographical names may be prefixed.
It thus appears that the classification of the sedimentary rocks is
comparatively simple, since, as explained on pages 555-560, the sorting
of the sedimentary material, both mechanically and chemically, is along
definite lines, and consequently large masses of material are produced
which have comparatively narrow ranges in composition, and these abun-
dant rocks are selected for the family names. However, the fact must
not be forgotten that there are all gradations between all varieties of
the sedimentary rocks. The law that gradation is the rule in nature is
uo better illustrated than by the sedimentary rocks. In biology, while
gradations have once existed between various forms, in many cases the
intermediate forms have been destroyed. In the early days of petrog-
raphy it was not supposed that between the various igneous rocks gradation
existed. But gradation varieties are known between the more important
facies, although gradational varieties between all kinds have not yet been
discovered. But the fact that each kind of sedimentary rock grades into
the related kinds has been recognized since the rise of geology, and thus
there has been no such confusion and multiplication of names for the
sedimentary rocks as for the igneous rocks. While gradations between
the different varieties of the sedimentary rocks exist, it is believed that
under the families above named probably 90 per cent or more of the
THE NITER FAMILY. 787
material of the sedimentary rocks may be placed, and, as already
explained, the remaining 10 per cent is provided for by compound names.
We are now prepared to consider the manner of formation of the
various families of original sedimentary rocks, their transformation, and
resultant alteration forms.
NONFRAGMENTAL CLASS.
NITRATE ORDER.
NITER FAMILY.
Natural niter is mainly soda niter (NaNO), although potash niter
(KNO,) does occur in subordinate quantity. The sodium and potassium
of the niter are mainly derived from the many alkaline-bearing silicates by
their decomposition im the zone of katamorphism, and chiefly in the belt of
weathering, through the process of carbonation. The original source of the
nitrogen of the niter is the atmosphere. This is oxidized in the zone of
katamorphism, mainly in the belt of weathering and very largely in the
soil, to nitric acid. (See Chapter VI, pp. 465-466.) The nitric acid unites
for the most part with the alkalies, forming the nitrates. The nitrates,
once formed, are dissolved from the soils by the circulating ground water
and carried to the sea or to lakes with no outlets. In the lakes niter is
chemically precipitated in consequence of continual addition of salts in
solution and evaporation of water. Niter is the most readily soluble of the
important natural salts, and therefore is the last of the series of chemical
precipitates to separate. It forms in the final stages of desiccation of the
lakes as the top deposit. Therefore niters are wholly the products of the
zone of katamorphism, and mainly of the belt of weathering.
The most extensive known deposits of niter are those in the extremely
arid regions of Chile. Niter deposits also occur in Bolivia, in Nevada, and
in southern California.
The alterations of niter are those of recrystallization and solution.
The process of recrystallization need not be dwelt upon, as it is in no way
different from the recrystallization of other readily soluble compounds.
Niter is so readily soluble that, unless the region be one of extreme aridity,
the rock, after having been precipitated, is redissolved and transported else-
where. Even where niter becomes buried under other rocks, it is likely
to be dissolved by the ground waters and again brought to the surface.
Consequently niter as a deeply buried rock is rare.
788 A TREATISE ON METAMORPHISM.
SULPHATE ORDER.
GYPSUM AND ANHYDRITE FAMILY.
Gypsum is hydrated calcium sulphate (CaSO,.2H;0). Anhydrite is
calcium sulphate (CaSO,). The calcium for gypsum is released from other
combinations, mainly silicates, in the zone of katamorphism, and especially
in the belt of weathering. Sulphur, as sulphide, and especially as pyrite,
is a widespread but not abundant mineral in the original igneous rocks.
In the zone of katamorphism, and especially in the belt of weathering,
the sulphur is oxidized to sulphuric acid. The sulphuric acid unites with
bases and forms sulphates. Sulphates formed in the belt of weathering are
transported by the underground circulating waters to the sea or to lakes
without outlets. In these lakes, by the continual addition of sulphates and
evaporation, supersaturation is reached, and the sulphates are thrown down
as calcium sulphate, because this is the most insoluble of the sulphates of
the bases which abundantly occur in such bodies of water. Where locally
barium and strontium are found, these sulphates form in preference.
Gypsum is less soluble than niter and sodium chloride, and more soluble
than calcium carbonate; therefore it is mainly precipitated after the tufas
and before the rock salts. Gypsum is now forming on a somewhat exten-
sive scale in lakes with no outlets. In the past it has formed on a very
extensive scale in such lakes and has been buried under later deposits.
Some of the more important gypsum deposits of the United States are those
of Grand Rapids, Michigan, and Fort Dodge, Iowa, and the widespread
gypsums which occur in connection with the Red Beds (Permian and
Triassic) of western America.
Gypsum, like rock salt, may by recrystallization become coarsely
crystalline. In many localities, because of its somewhat ready solubility,
it has formed crystals of great size. The most famous of the localities for
large gypsum crystals is that of the. Paris Basin. However, the most
gigantic crystals known have recently been discovered in Utah, some of
them measuring 150 cm. in greatest dimension.“
Gypsum, like niter and rock salt, is a product of the zone of kata-
morphism, and mainly of the belt of weathering.
When gypsum beds become so deeply buried that pressure is dominant,
and the heat is somewhat higher than at the surface, the process of dehy-
«Talmage, J. E., A remarkable occurrence of selenite: Science, vol. 21, 1893, pp. 85-86.
THE ROCK-SALT FAMILY. 789
dration occurs, and thus gypsum passes into anhydrite, with a decrzase of
volume of 38 per cent. Anhydrite is then a result of the dehydration of
gypsum, often in the zone of anamorphism. But it is to be remembered that
after a deposit of gypsum has been changed to anhydrite in the lower zone,
in order that it shall reach the surface by denudation it must pass through
the upper zone, and under these conditions hydration occurs to a greater
or less extent, and it is thus transformed to gypsum in part or in whole.
At various localities the beds of calcium sulphate are in part composed of
gypsum and in part of anhydrite. Doubtless in places where the deposits
are deep the change toward anhydrite is taking place, and in other places
near the surface the change toward gypsum is taking place.
CHLORIDE ORDER.
ROCK-SALT FAMILY.
Rock salt, impure sodium chloride; or simply salt, is the only abundant
chloride. But potassium chloride, calcium chloride, and magnesium chlo-
ride are found as impure compounds. The sodium is derived mainly
from the silicates. It is liberated from these compounds in the zone
of katamorphism, mainly in the belt of weathering, by the process of
carbonation. The chlorine is more difficult to account for. Of the min-
erals in the original rocks, only sodalite, wernerite, and apatite contain
chlorine. But sodalite is abundant in the soda rocks, and thie last two
minerals are very widespread. Therefore these minerals are an impor-
tant source of chlorine. It is well known that hydrochloric acid and
other chlorine compounds are emitted in large quantities in connection
with volcanic action. In this connection it should be noted that at
periods of regional volcanism the quantity of chlorine emitted is vastly
greater than during periods of local volcanism, like the present.’ It has
been supposed that emissions of chlorine during volcanism are evidence
that sea waters from the surface, or the salts of underground solutions,
have penetrated to the roots of voleanoes, and thus contributed chlorine
to magmas. Doubtless this is true in some instances, but it can not
be assumed from océasional cases of this kind that the chlorine very
generally emitted in volcanic regions is thus derived from the zone of
«Van Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11, 1898,
pp. 496-497.
790 A TREATISE ON METAMORPHISM.
katamorphism. I believe that much of the chlorine emitted from volea-
noes is not thus derived, but is an original constituent of the deep-seated
magmas, and it follows that before the crystallization of the magmas a
portion of the chlorine escapes. Consequently the quantity of chlorine in
the crystallized igneous rocks is less than originally existed in the magmas.
Probably the original chlorme minerals and the emissions i connection
with voleanism are sufficient to account for the chlorine of the chlorides.
Sodium chloride formed in the soil is taken into solution and is
transported to the sea or to lakes with no outlets, which are prevented
from expansion by evaporation. As the process continues salt accumulates
to the point of saturation, when further evaporation leads to chemical
precipitation, precisely as with niter.
Sodium chloride ranks in solubility next to niter, and is abundantly
precipitated before the latter compound is. Deposits of rock salt are
forming in various desert regions at the present time by the evaporation
of the waters of inclosed seas, such as the Dead Sea, Great Salt Lake, ete.
Extensive deposits formed under similar conditions during past geological
periods have been buried under later formations, and these are the rock-
salt beds. Illustrations of such deposits are the thick rock-salt formations
of Poland, of central Germany, and of Louisiana. From the foregoing it
is plain that rock-salt deposits are strictly products of the zone of kata-
morphism, and mainly of the belt of weathering.
The alterations of rock salt are those of recrystallization and solution.
In some cases the material has become coarsely crystalline, as in the case of
the Polish deposits. In order that a rock-salt formation shall be preserved
it is necessary that it be buried under relatively impervious formations,
in order that the underground circulation shall not be so rapid as to
dissolve the material and bring it to the surface. That solution of many
ancient deposits is taking place is shown by the briny waters which are
found at many localities. Where such ground waters issue at the surface
we have salt springs. Such salt springs were known in America in early
days as “salt licks,” because the animals frequented these places in order
to lick the salt which was deposited. As illustrations of deposits which are
being dissolved and which are artificially abstracted from the earth in the
form of brine, those of the Saginaw district of Michigan and the Syracuse
district of New York may be mentioned.
ORIGIN OF LIMESTONES. Us)
CARBONATE ORDER.
CALCIUM-MAGNESIUM CARBONATE FAMILY AND METAMORPHOSED EQUIVALENTS.
The calcium-magnesium carbonate family, with its metamorphosed
equivalents, comprises limestone, dolemite, marble, cherty limestone, cherty
dolomite, and cherty marble, silicated marble, and silicate rocks. Each of
these will be considered in turn.
LIMESTONES.
Limestones comprise all those rocks which are mainly composed of
calcium carbonate (CaCQOs).
SOURCE OF MATERIAL OF LIMESTONES.
The original dominant source of the calcium and magnesium of the
calcium-magnesium carbonates is mainly the many alkaline earth-bearing
silicates. The silicates are decomposed in the zone of katamorphism, and
very largely in the belt of weathering, by the processes of carbonation,
hydration, and oxidation. So far as the process is due to carbonation the
carbonates are formed directly. In so far as the bases are set free by
other reactions they may later unite with the carbon dioxide. The carbon
dioxide immediately concerned in the process of carbonation is mainly
derived directly from the atmosphere, from that produced by the oxidation.
of the organic compounds, by the liberation from rocks during their decom-
position of inclosed carbon dioxide, and from the deep-seated waters rising
from the zone of anamorphism. (See pp. 461-465, 473-475, 608-610, 667,
677-678.) The relations of these sources of carbon dioxide to one another
and the replenishment of the supplies in order to carry on the process of
carbonation are fully discussed in Chapter XI, under the heading ‘‘ Carbon.”
The calcium and magnesium unite with the carbonic acid formed and
form carbonates. The original carbonates are solely produced by the
reactions of the zone of katamorphism, and chiefly by reactions of the belt
of weathering.
The calcium carbonate formed as above described, after transportation
to the sea or to lakes with no outlets, is thrown down as organic precipi-
tates and as chemical precipitates, and thus limestone is formed.
792 A TREATISE ON METAMORPHISM.
ORGANIC PRECIPITATES.
Animals of the ocean and lakes abstract calcium carbonate from solution
and build it into their hard parts, The number of kinds of organisms which
are doing this work is great. Of these the more important classes are the
corals, mollusks, and crinoids, the relative importance probably being
in the order given. In each class are many species, and the number of
individuals of each species is beyond computation. In order that a great
quantity of material may be abstracted from the water by organisms it is
necessary that abundant material shall be furnished to it, and also that the
conditions shall be favorable for the existence of abundant animal life of
the right kind. The material for the building of limestone is furnished
to the ocean mainly by the streams. Therefore, adjacent to great bodies
of land from which large rivers enter into the sea there is a continuous
and abundant supply of calcium carbonate. Sluggish rivers which have
meandered through limestone regions furnish an especially abundant
supply. An essential condition for abundant animal life in the sea is a
warm climate, and hence the limestones are likely to be formed most
plentifully in the tropical or subtropical regions. Tropical and subtropical
regions are favorable for limestone building for the further reason that
evaporation is great and concentration of calcium carbonate is thus pro-
duced. his is of great importance in shallow, nearly inclosed seas, and
lagoons. The areas which combine the two conditions—abundance of
material and abundance of life—are those in which the great limestone
building of the present time is going on. Such regions are illustrated by
Florida and the Bahamas, the sea adjacent to which is constantly fed by
the currents from the Gulf of Mexico, which receives the great contri-
butions of calcium carbonate of the Mississippi and many other rivers; and
by the regions adjacent to the Yellow Sea, which receives the contributions
of the Yangtze, Hoangho, and many other important rivers. But the
immediate contributions of great rivers are not necessary for extensive
limestone formations, for the ocean currents distribute material throughout
the seas, and the water contains a sufficient amount of calcium carbonate,
so that where other conditions are favorable extensive limestone formations
may be built. This is illustrated by the Great Barrier Reef of Australia
and the numerous coral reefs about the tropical islands of the Pacific.
ORIGIN OF LIMESTONES. 193
The material deposited by animals may be amorphous or crystalline
aragonite or calcite. The material deposited by corals and crinoids is
erystalline, and is in the form of aragonite; the material deposited by the
mollusks is in the form of calcite, and to a less extent of aragonite. The
continuous precipitation of calcium carbonate of the sea by organisms, and
the building up of organic deposits through geological periods, combined
with mechanical rearrangement and with recrystallization (see pp. 795-797),
result in the great limestone formations.
CHEMICAL PRECIPITATES.
Chemical precipitates of calcium carbonate include the deposits of
springs and streams; inland seas with no outlets, like the Dead Sea, Great
Salt Lake, etc.; and possibly chemical precipitates in the ocean or in seas
connected with the ocean.
Springs and streams.—Tufa deposits are frequently formed by streams just
after issuing from underground, and they are formed from water in caves
as it issues from the confined passages into the open caverns. In both
these cases the causes of the supersaturation and precipitation are the
same—release of pressure, with escape of carbon dioxide, and evaporation.
Spring and stream deposits of tufa are usually small and unimportant.
Inland seas with no outlets——In inland seas with no outlets the calcium car-
bonate contributed by the streams steadily accumulates. On the average,
evaporation balances the additions of water. Concentration of the calcium
carbonate is the result, until supersaturation follows, and finally precipita-
tion. These deposits have generally been called tufa, and they are finely
illustrated by the deposits of the ancient lakes Lahontan and Bonneville.
Such deposits are forming at the present time in Salt Lake, Pyramid Lake,
Walker Lake, and Winnemucca Lake.” The tufa forming in lakes and
inclosed seas is likely to be rather impure im consequence of the simul-
taneous precipitation of other compounds, especially sodium chloride and
gypsum.
Possible chemical precipitates in the ocean or in seas connected with the ocean. Willis holds it
to be extremely probable that extensive limestone formations have been
chemically precipitated in the sea.’ At the present time the ocean and the
«Russell, I. C., The geological history of Lake Lahontan: Mon. U. 8. Geol. Survey, vol. 11, 1885,
pp. 188-223. Gilbert, G. K., Lake Bonneville: Mon. U. 8. Geol. Survey, vol. 1, 1890, pp. 167-169.
b Willis, Bailey, Condition of sedimentary deposition: Jour. Geol., vol. 1, 1893, pp. 519-520.
794 A TREATISE ON METAMORPHISM.
connected seas are not saturated with calcium, or even approximately so.
According to Mendeléeff,* 1 kg. of water saturated with carbon dioxide will
dissolve 3 grams of calcium carbonate. The amount of calcium carbonate
in the ocean at the present time, per kilogram of water, is .11869 gram, or
3.956 per cent of the above amount;’ but since sea water is not saturated
with CO,, the amount of calcium carbonate which could be held is less
than 3 grams per kilogram. It is certain that under present conditions the
precipitation of calcium carbonate from the ocean is mainly accomplished
by organisms, as explained on pages 792-793. But in early geological
periods, before life was abundant and great quantities of calctum carbonate
were extracted from the sea by animals, it is barely possible that super-
saturation occurred and chemical precipitation resulted. Supersaturation
could have been brought about by two factors: First, steady additions of
calcium carbonate from the land in connection with evaporation from the
sea produced concentration. Second, it is possible that the amount of
carbon dioxide in the atmosphere, and therefore in the ocean, was greater
then than at present. As the carbon dioxide in the atmosphere was used
up by the processes of carbonation, carbon dioxide would pass from the
ocean to the atmosphere, and a decrease in the amount of this compound
in solution would result in chemical precipitation of the calcium carbonate.’
The two causes for precipitation considered apply to the entire body
of the ocean. Precipitation caused by them alone would be very wide-
spread, if not universal. The theory of chemical precipitation in the ocean
has greater plausibility if there be combined with the two previous causes
the existence of partly inclosed tropical mediterranean seas, which received
great contributions of materials from large rivers and lost great quantities of
water by evaporation. Under these circumstances chemical precipitation
might occur without premising saturation of the entire ocean. It is therefore
theoretically possible that extensive limestone deposits were produced by
chemical precipitation in early geological ages. Butif the ocean as a whole
did at any time contain enough calcium carbonate to saturate it, or even
enough to nearly saturate it, the subsequent process of limestone building by
a Mendeléeff, D., The principles of chemistry, trans. by Geo. Kamensky, Longmans, Green «& Co.,
London, 1897, vol. 1, p. 592.
> Dittmar, William, The composition of ocean water: Report of the Scientific Results of the
Exploring Voyage of H. M. 8. Challenger, 1873-76; vol. fT, Physics and Chemistry, 1884, pp. 2, 204.
¢Mendeléeff, cit., vol. 1, p. 592.
METAMORPHISM OF CALCIUM CARBONATE DEPOSITS. 795
organic precipitation must have more than equaled the additions of calcium
carbonate from the land, and thus have reduced the total amount in solution.
In the present state of knowledge it must be said that the precipitation
by chemical means of great limestone formations in the ocean, or in medi-
terranean seas connected with the ocean, is a speculation based upon the
premised inadequacy of life in early times to precipitate such amounts of
materials, and is required to explain only the limestones of pre-Paleozoic
times, the rocks of which contain very scanty fossils. But even these lime-
stones may have been precipitated by organisms and the fossils destroyed
by the processes of recrystallization and metamorphism subsequently
described, just as they have been in many later formations.
METAMORPHISM OF ORGANIC AND CHEMICAL CALCIUM CARBONATE DEPOSITS.
During the time that the various forces and agents are abstracting the
calcium carbonate from the sea the mechanical and chemical forces are at
work. No sooner is a deposit formed than the waves may break it into
detritus. All the organic and chemical deposits within the reach of the
wayes and currents are handled by them. Thus the material may be
widely distributed according to the laws of the distribution of mechanical
deposits. (See Chapter VI, pp. 555-560.) To illustrate: The débris of
the coral reefs is carried seaward until the water reaches such a depth that
the undertow has lost its power. Thus conglomerate, sand, and _ silt com-
posed of limestone may be deposited in concentric belts. (Pl. IV, 4.)
Any of the shore deposits described by Gilbert may be built up.* But
the mechanical work does not go on alone. The moment an animal dies the
calcium carbonate is subjected to the action of solution and deposition by
the sea. When the material is exposed to the open sea, or when there are
continuous currents, the water is not saturated with calcium carbonate, and
there is continual solution of the material at the rock surface. Within the
body of the rock the particles may be cemented by deposition. (PI. IV, B.)
In inclosed or nearly inclosed areas, as, for instance, locally between the
islands and reefs encircling Australia, the evaporation may more than compen-
sate for the influx of fresh water, and thus the tendency, on the whole, be to
precipitate additional material from the sea, rather than to dissolve material.
At the same time limestone is forming there may be a substitution of
magnesium for calcium. The processes of solution and redeposition and of
aGilbert, G. K., Lake Bonneville: Mon. U. 8. Geol. Survey, vol. 1, 1890, pp. 23-89.
796 A TREATISE ON METAMORPHISM.
substitution are fully considered below, in treating of “Recrystallization”
and ‘Dolomitization.” The amount and speed of the recrystallization
depend, of course, upon many factors. Rather ancient limestones may
have been only partially recrystallized. On the other hand, the limestones
now forming may locally be coarsely crystalline.
Commonly the mechanical and chemical forces work together in the
rearrangement of the rock. This is finely illustrated among the lagoons of
Florida and Australia. ~In these regions abundant life is building up calcium-
carbonate deposits. The deposits are being broken up by the waves; they
are being taken into solution by the sea; evaporation is going on; and
redeposition is occurring within the deposits. Thus in these regions, while
the first precipitation takes place as a consequence of the action of the
animals, the mechanical and chemical forces immediately begin to rearrange
the calcium carbonate into a detrital rock cemented by calcite, or into
coarsely crystalline rocks entirely dissolved and redeposited by means of
the chemical forces, or into a combination of these.
Saville-Kent describes the main mass of the material upon which the
Australian corals are at present building their deposits as limestone con-
glomerates, largely built up of the cemented debris of corals and mollusks.”
(Pl. IV, A.) The breaking of the Barrier Reef is due to the waves; the
cementing of the material he attributes to evaporation during the low tide,
when the surface is above water, at which times the sun evaporates a large
amount of the interstitial water, and thus deposits material. It may be
supposed that this process is continuous, material being added to the solu-
tions at each time of high water and deposited at each time of low water.
But it may be doubted whether it is necessary for the reefs to be exposed
in order that consolidation shall occur. It seems highly probable that in
the ocean menstruum the limestone débris are consolidated by the chemical
processes of solution and deposition, even if the material remains continually
below the surface of the water.
The process of reerystallization does not cease when the limestone
formations are raised above the sea. When these formations constitute
land, recrystallization continues under both mass-static and mass-mechanical
conditions. This is accomplished by the action of the underground waters
It has been fully explained that in its movement water is continually taking
material into solution and is continually Geno ae material oun solution.
“Saville-Kent, W., The Gra one Reef of Neill. London, 1893, pp. Aap
U, S. GEOLOGICAL SURVEY MONOGRAPH XLVII PL. IV
TEXTURES OF LIMESTONES AND MARBLE,
A. Coral rock conglomerate developing from the breaking up and cementation of coral shells. After Saville-Kent. B. Oolitic limestone from
Beaver, Ind C. Coarsely crystalline marble from the Adirondacks.
Ss a
ne
rei
METAMORPHISM OF CALCIUM CARBONATE DEPOSITS. 797
Further, it has been explained that in the belt of weathering solution
is the rule, and that in the belt of cementation solution does not so far surpass
deposition but that cementation is the rule. No other rock so well illus-
trates these principles as limestone, for it is the most readily soluble of the
extensive formations. Under the law of dominant solution in the belt of
weathering the openings are enlarged, and thus the very numerous cracks,
crevices, and caves, great and small, so characteristic of limestone, are
formed. But while solution dominates in the belt of weathering, as shown
by these openings, deposition is, as explained on page 487, an invariably
accompanying process. Thus are made the stalactites, stalagmites, and
travertine deposits. These deposits may be either aragonite or calcite.
In the belt of cementation below the level of ground water in lime-
stones the openings formed in various ways are as generally decreased in
size by deposition as they are enlarged in the belt of weathering. The
material here deposited is generally in the stable form of calcite.
But the process of solution and deposition in the belt of weathering
and the belt of cementation does not affect the walls of the openings only.
Solution and deposition are going on continuously throughout the entire
mass of limestone formations in the zone of katamorphism. As a conse-
quence of this the rock is continuously recrystallized. This process, even
under mass-static conditions, may advance so far as to transform a limestone
into a uniformly crystalline rock, to which the term marble is applicable.
However, where the recrystallization goes so far as to transform the rock
to a marble, there has usually been more or less mechanical action. But
the consideration of the marbles is deferred. (See pp. 808-816.)
In modern limestones fossils are likely to be abundant; in ancient
limestones they are comparatively sparse. In general it may be said that
they are less abundant as the rocks are old. Also, in proportion as the
rocks are old they are likely to be well crystallized. The increased sparse-
ness of fossil remains and the crystalline character of the limestone may both
be regarded as evidence that recrystallization has taken place. Also they
might be interpreted as evidence that life was not so abundant in the past
and that the material had been deposited as a chemical precipitate. But cer-
tainly the latter argument does not apply to rocks later than the Algonkian.
Since fossils so commonly constitute but a small fraction of the present
mass of limestones, it is concluded that reerystallization has extensively
occurred in Paleozoic and post-Paleozoic limestones. (See Pl. VI, A.)
798 A TREATISE ON METAMORPHISM.
DOLOMITE.
ORIGIN OF DOLOMITE.
Dolomite is calcium-magnesium carbonate (CaCO;.MgCO,). In consid-
ering the origin of dolomite the questions immediately arise: Are magnesian
limestones and dolomites originally deposited as such, or are they the results
of replacement of calcium by magnesium? and, if the latter, how and when
did the process take place? 2
Dolomite due to replacement of calcium by magnesium.— [hat the magnesium carbonates
are due to replacement of magnesium for calcium, and therefore to dolo-
mitization, seems to be conclusively shown by the following considerations:
(1) The first of the various lines of evidence in favor of dolomitization
is the composition of the hard parts of sea animals. We do not know any
sea animals which deposit more than a small amount of magnesium car-
bonate in their hard parts. Forchhammer analyzed a large number of
corals and shells of marine animals. In bivalves he found the amount of
magnesium to vary from 0.5 to 1 per cent; in cephalopods he found it to
be less than 0.5 per cent. In most corals the amount of magnesium is less
than 0.5 per cent, although in one species it is 2 per cent and in another
species 6.4 per cent.“ Sharples analyzed seven species of corals,’ and does
not report magnesium carbonate, although it is to be supposed that a small
amount of that compound is present. Forchhammer found® in a number of
late deposits, which are mainly composed of the remains of animals, that
the amount of magnesium is less than 1 per cent. From these results,
if it be supposed that formations now dolomitic limestone were originally
organic precipitates, it must be concluded either that the magnesium is
mainly introduced by a secondary process or that sea animals in early
periods used a larger proportion of magnesium for their hard parts than do
the present animals. ‘The latter conclusion would be a pure unverified _
assumption.
(2) Some of those who hold that dolomites are original deposits, in
order to escape the difficulty involved in the supposition that animals have
a@¥Forchhammer, Georg, Bidrag til Dolomitens-dannelshistorie: Oversigt over det Kongelige Danske
Videnskab. Forhandlingar, Copenhagen, 1849, p. 89. See also Bischof, Gustay, Chemical and physical
geology, London, 1855, vol. 2, pp. 48-49.
>Sharples, 8. P., On some rocks and other dredgings from the Gulf Stream: Am. Jour. Sci., 3d
ser., vol. 1, 1871, p. 169.
¢Forchhammer, cit., p. 89. Also, Bischof, cit., p. 48.
ORIGIN OF DOLOMITE. 799
at some past time secreted magnesium in greater quantities than at present,
have held that magnesium limestones and dolomites as such are original
chemical precipitates. While the sea is not now saturated with calcium
and magnesium salts, it is held that this might have been the case before
life became abundant, and that consequently magnesium limestone and
dolomite formed directly. This is again a pure, unverified assumption,
which seems to be negatived by the following considerations: (a) Since the
living animals extract so little magnesium, on this hypothesis it is difficult
to explain why the sea is not now saturated or nearly saturated with
magnesium salts; (b) Bischof* found by experiment that when solutions
become saturated with carbonates of magnesium and calcium the caleium
carbonate is largely precipitated before the magnesium carbonate begins to
be thrown down in appreciable quantity. Hence, by chemical precipitation,
there would be produced separate layers composed mainly of carbonate of
caleium and carbonate of magnesium, rather than calcium-magnesium
carbonate. According with Bischof’s experiment, the alkaline earth ecar-
bonates precipitated in Lakes Lahontan and Bonneville are essentially
calcium carbonate, containing not more than 2.14 per cent of magnesium.’
The deposits of inland lakes in which the salts accumulate until precipita-
tion occurs should be analogous to those of the ocean in ancient times, on
the supposition that betore life existed abundantly chemical percipitates
formed. The fact that tufas deposited in such lakes contain so little
magnesia seems to bear against the hypothesis that the dolomites are
original chemical precipitates.
(3) But perhaps the most decisive of the various lines of evidence in
favor of secondary dolomitization are observed facts of occurrence which
seem to be explicable only upon the basis of replacement. While, as
already noted, coral is nearly pure calcium carbonate, Dana found that the
limestone of the elevated coral island Metia is heavily magnesian, one
specimen containing as much as 388 per cent of magnesium carbonate.’
Prestwich states that in the Carboniferous limestones of Kilkenny and Cork
the upper surface and parts of the rock along the bedding and joint planes
@ Bischof, Gustav, Elements of chemical and physical geology, trans. by B. H. Paul: Cavendish
Society, London, vol. 3, 1859, pp. 169-170.
> Russell. I. C., Geological history of Lake Lahontan: Mon. U. 8. Geol. Survey, vol. 11, 1885,
p. 208. Gilbert, G. K., Lake Bonneville: Mon. U. 8. Geol. Survey, vol. 1, 1890, p. 168.
¢Dana, Jas. D., Corals and Coral Islands, 3d ed., Dodd, Mead & Co., New York, 1890, pp. 393-394.
800 A TREATISE ON METAMORPHISM.
are often dolomite. Where the rock is dolomite the fossils and the stratifi-
cation are obscured or destroyed. The deeper lying rock, as well as the
rock remote from the joints, is ordinary limestone." Geikie says that
essentially the same facts are true on a large scale in the Carboniferous
limestone in the north of England.’
Spurr describes the limestone of Aspen Mountain, Colorado, as dolomite
near the faults. The magnesium becomes less and less plentiful as the
distance from the fractures increases. He further states that dolomite
almost invariably accompanies the ore.’ Bain describes precisely similar
phenomena in the limestone carrying lead and zine ore in Iowa.“ Spurr
further finds in the limestone of Glenwood Springs, Colo., old water
channels and zones of fracture; and adjacent to these he finds the rock
to approach dolomite. Adjacent to the fractures and the underground
water channels there is a gradation of the dolomite to the ordinary lime-
stone.’ Bain states that the Carboniferous limestones of Missouri bearing
lead and zinc ore are dolomite in the vicinity of the ore deposits, and
elsewhere are largely limestone. He further finds that dolomite is a most
important gangue mineral in the openings of the rocks. The fissures con-
nect with the magnesium-bearing Cambro-Silurian limestones below, and he
infers that rising waters have transported the magnesium and dolomitized
the Carboniferous limestone adjacent to the water channels.’ Other
instances of rapid gradation in the amount of dolomite are furnished by
limestones where cut by intrusives. Finally, there are gradual variations
in the amount of magnesium in great limestone formations; as, for instance,
the great Cambro-Silurian limestone of North America. The Appalachian
Valley Cambro-Silurian limestone is a true dolomite, while at various places
in the Mississippi Valley the limestone contains so little magnesium as to be
merely a magnesian limestone.
The different cases of occurrence of magnesian limestone above given
«Prestwich, Joseph, Geology:
113-i14.
>Geikie, Archibald, Textbook of Geology, 3d ed., Macmillan & Co., London, 1893, p. 321.
¢Spurr, J. E., Geology of the Aspen mining district, Colorado: Mon. U. S. Geol. Survey, vol. 31,
1898, pp. 210-211.
@ Calvin, Samuel, and Bain, H. F., Geology of Dubuque County: Iowa Geol. Survey, vol. 10, 1900,
pp. 492-498, 572-575.
e Spurr, cit., pp. 212-216.
f Bain, H. F., with Van Hise, C. R., and Adams, G. L., Preliminary report on the lead and zine
deposits of the Ozark region: Twenty-second Ann. Rept. U. 8. Geol. Survey, pt. 2, 1901, pp. 208-210.
chemical, physical, and stratigraphical, Oxford, 1886, vol., 1, pp.
ORIGIN OF DOLOMITE. 801
may be gathered together under a general principle, viz: On the average,
limestones in regions of strong orogenic movements, and consequent frac-
turing, are more strongly magnesian than limestones of equivalent age in
less disturbed regions. To illustrate, the Tertiary limestones of the Coast
Range of California and of the Alps are more strongly magnesian than
the undisturbed limestones of the same age. When the cases of variable
magnesium are brought together under this rule, the reason for the
variation is plain. Limestones are likely to be magnesian in proportion as
there were favorable conditions for the entrance of solutions bearing
magnesium, and these conditions are of course furnished by orogenic
movements and fractures causing openings. The variations in magnesium
content, rapid and slow, above cited, and other similar instances, can
apparently be explained only on the theory of secondary replacement.
(4) Taking the world as a whole, the older a limestone formation is
the further advanced is dolomitization. Thus the ancient limestones,
especially those of Cambrian and Silurian age, are very generally strongly
magnesian, and the pre-Cambrian limestones are usually completely
dolomitized. This is partly a direct consequence of (3). For in propor-
tion as formations are old they are likely to have been subjected to orogenic
movement and to fracture. Also in proportion as they are old sufficient
time has elapsed for the other favorable conditions to have occurred. But
the principle of increase in dolomitization with age must be broadly applied,
for, as already seen, a young formation may be strongly dolomitic in one
region, and a much older formation be only slightly dolomitized in another
region; also, in various districts heavily magnesian formations occur above
limestones containing little magnesium.
(5) It has been shown on page 239 that the change of calcite to
dolomite involves a contraction of 12.30 per cent. It is a well-known
fact that dolomites which have not been subjected to strong orogenic
movement, deep below the surface, are very porous. This is beautifully
illustrated by the dolomites of the Mississippi Valley, especially the
so-called magnesian limestone series. In this respect the dolomites contrast
strongly with the pure limestones. The natural explanation is that the
porosity is due to the contraction of the volume as a result of dolomitization.
Where the change has taken place in the zone of anamorphism, or where
the dolomite has been in the lower zone since the change, mashing and
recrystallization have closed the openings.
51
MON XLVIT—04
802 A TREATISE ON METAMORPHISM.
Conclusion.— From the foregoing facts it is concluded that the major part
of the magnesium in dolomitic limestones and dolomites is due to replace-
ment. In the instances of rapid local variation in the amount of magnesium
in the limestones of coral islands and in the limestones of England, Ireland,
and America, where the amount of magnesium is greatest near the surface
or adjacent to fractures, there seems to be no other conclusion. And there
is no reason to doubt that fhe magnesian character of the great dolomite
formations is due to replacement.
HOW AND WHY DOLOMITIZATION OCCURS.
The magnesium for dolomitization is primarily derived from magnesium-
bearing minerals of the original igneous rocks. The more important of
these are the micas, pyroxenes, amphiboles, and olivines. But at the
present time a large part of the magnesium for dolomitization is derived from
secondary minerals, such as garnet, staurolite, tourmaline, chondrodite,
chlorite, the zeolites, and even from previously formed dolomite. Indeed,
the magnesium concentrated in earlier dolomite is one of the most impor-
tant sources for present dolomitization. From these various sources the
processes of alteration of the zone of katamorphism, and especially of
the belt of weathering, produce soluble magnesium compounds, such as
carbonate, hydrate, sulphate, ete. The magnesium compounds formed or
dissolved in the belt of weathering are transported in part to the belt of
cementation and in part to the sea, either before or after an underground
journey. This process has been continuous since the continents were
divided from the seas, consequently the amount of magnesium in the sea has
probably continuously increased since early geological times, for it can not
be supposed that, on the average, more magnesium has been precipitated
from the sea than has been added to it during any geological period. It is
seen in Chapter X that the amount of magnesium now in the sea is more
than three times as great as the amount of calcium.
The process of dolomitization may take place while the limestone is
still in the sea or after it has been raised above the sea and constitutes a
part of the land. is
Dolomitization before limestone emerges from the sea— The carbonate originally depos-
ited by animals is almost wholly calcium carbonate. The sea contains
abundant magnesium salts. Thus we have in the sea a heterogeneous
DOLOMITIZATION BEFORE LIMESTONE EMERGES FROM SEA. 803
system (see Chapter II], pp. 90-91), the sea not being saturated with
either magnesium or calcium salts, but containing more than three times as
much magnesium as calcium, and having below it in various regions solid
calcium carbonate. Under these circumstances there is a tendency for some
of the caleium of the solid carbonate to change places with the magnesium
in solution, and thus dolomitization may take place beneath the sea.
If the sea were saturated with magnesium salts it is probable that the
replacement would go on rapidly. Since, however, the sea is far from sat-
urated with such compounds, and since, as already shown (pp. 798-799), it
can not be supposed that the sea has ever been richer in magnesium salts
than at present, the change has taken place slowly. But the substitution
is continuous; for, in consequence of the currents, the water depleted in
magnesium is replaced by water containing a normal amount of that
element. But the solutions of the ocean also contain calcium. Therefore
the conditions are approximately as if one ran magnesium-calcium solutions
like those of the ocean through solid calcium carbonate until equilibrium
‘under the laws of chemistry (see Chapter III, p. 90) had been reached.
But in the ocean equilibrium is probably but rarely reached on an extensive
scale because of the slow and imperfect manner in which the solutions make
their way through the calcium-carbonate deposits at the bottom of the sea.
Occasionally in the arms or shut-off portions of the sea, where the conditions
are favorable for unusual concentration, the amount of magnesium in the
water may be even greater than that in the normal waters, and under these
circumstances the reaction would go on more rapidly. This is illustrated
by the coral reefs of Metia, which are heavily magnesian. This case
furnishes positive evidence that dol~mitization may locally, under favorable
circumstances, go very far while a limestone is below the sea. Dana’s
explanation of the unusual amount of magnesium is that “the sand or mud
may have been that of a contracting and evaporating lagoon, in which
magnesian and other salts of the ocean were in a concentrated state.” ”
Dana compares concentration of magnesium in lagoons mainly shut off
from the sea to concentration in salt pans.’
While it is clear that dolomitization below the sea may locally go
far, the usual facts of observation correspond with the conclusion above
«Dana, Jas. D., Corals and coral islands, New York, Dodd, Mead & Co., 3d ed., 1890, p. 394.
> Dana, Jas. D., Manual of geology, American Book Company, 4th ed., 1895, p. 133.
804 A TREATISE ON METAMORPHISM.
given, that dolomitization below the sea is usually very partial, and that
the Metia instance is exceptional. It has already been seen that Foreh-
hammer found that late limestone deposits mainly composed of the remains
of animals usually contain less than 1 per cent of magnesium,” and that
a small content of magnesium is the ordinary thing for late limestones
which have not been greatly disturbed by orogenic movements. In view
of these facts the question naturally arises as to the time when the dolomiti-
zation mainly took place in those extensive dolomite formations in which the
magnesium is moderately uniform for a given district, although slowly vary-
ingasa whole. The typical formations of this kind in America are the great
Cambro-Silurian limestones of the United States, which in the Appalachian
region, where deformation is great, are nearly pure dolomites, and which in
the Mississippi Valley are often heavily magnesian but rarely dolomite,
and show, moreover, rather sharp variations in the amount of magnesium..
The hypothesis which one naturally favors, because of its simplicity, is that
such formations as these were mainly dolomitized below the sea. But the
more closely the facts are examined the less certain does the conclusion
appear to be. It is entirely possible that locally the Cambro-Silurian lime-
stone was dolomitized more extensively below the sea than is the case, on the
average, for the later limestone formations. Indeed, it has been supposed
that this limestone was deposited in a mediterranean sea, and this entire sea
may have had to some extent the concentrated conditions at Metia described
by Dana. But it appears certain to me, even if the dolomitization was
further advanced in the case of some of these formations while below the
sea than can be paralleled by recent extensive formations, that they have
subsequently been much further dolomitized and the magnesium extensively |
rearranged since the limestones emerged from the sea.
Dolomitization after limestone emerges from the sea—— While it is clear that the process
of dolomitization may take place below the sea, often it is there only
initiated. Frequently dolomitization takes place in the main after the lime-
stone becomes a part of the land area and is subject to the forces of meta-
morphism beneath the air. It is believed that dolomitization in land areas
largely occurs in the zone of katamorphism rather than in the zone of
anamorphism. The basis of this belief is furnished by the conclusion, fully
«Forchhammer, Georg, Bidrag til Dolomitens-dannelshistorie: Oversigt over der Kongelige Danske
Videnskab., Forhandlingar, Copenhagen, 1849, p. 89. See also, Bischof, Gustay, Chemical and physi-
cal geology, London, 1855, vol. 2, pp. 48-49.
DOLOMITIZATION AFTER LIMESTONE EMERGES FROM SEA. 805
explained on pages 655-656, 764-766, that transfer of material and change
in chemical composition are extensive and great in the zone of katamor-
phism and are trivial and small in the zone of anamorphism. Further, in
the zone of katamorphism it is believed that dolomitization mainly occurs
in the belt of cementation rather than in the belt of weathering, although
the process undoubtedly takes place in both belts.
We have seen that rocks contain magnesium mainly as a silicate or
carbonate. If above the level of ground water the magnesium be as silicate,
the process of carbonation changes it to a carbonate or other compound,
and as such it is taken into solution and carried downward to the sea of
ground waters. If the magnesium be as carbonate, the process of carbona-
tion is not necessary. In any case the waters of the belt of weathering
steadily carry downward magnesium and calcium carbonate and other salts
to the belt of cementation. ‘These solutions pass into limestones and other
formations.
When the solutions pass into limestones which contain little magnesium,
the process of replacement begins. The substitution of magnesium for cal-
cium is controlled by various factors, among which time, the absolute and
relative amounts of*calcium and magnesium in the solutions and in the
rocks, the change in volume in consequence of the reactions, and the
direction of the movement of the solutions are important.
It is certain that time is an important factor. The underground
solutions are continually circulating, and there are ever-renewed supplies
of magnesium to carry on the process in the solutions which join the belt
of cementation. At the outset calcium is greatly predominant in the
limestone; but as the substitution goes on, magnesium becomes more and
more important, and may finally become molecularly equivalent to cal-
cium. But to accomplish this must require much time, for the solutions
at any moment occupy but a small percentage of the volume of the rock,
and they contain but a very small amount of magnesium. Therefore the
magnesium held at any one time by the solutions is but an inappreciable
fraction of the total amount which is finally introduced into the rock.
The absolute and relative amounts of calcium and magnesium in the
solutions and the limestones is a matter of the greatest consequence in
dolomitization. The law of mass action requires that where solid calcium
carbonate is in contact with magnesium- and calcium-bearing solutions a
806 A TREATISE ON METAMORPHISM.
part of the calcium of the solid be repiaced by the magnesium. In a
limestone which has been slightly dolomitized the conditions are particularly
favorable for dolomitization, for under such circumstances under chemical
laws substitution goes on rapidly. But the further the process continues
and the nearer equilibrium is approached the slower it becomes. Since, on
the whole, calcium is a more abundant base in the sedimentary formations,
and also in the ground solutions, than magnesium, and since it is a more
energetic base, it follows that at low pressure one could not expect that
one-half of the calcium would be replaced by the magnesium, for when the
substitution had gone to a certain stage calcium would replace the deposited
magnesium as fast as the magnesium replaced the calcium. This stage
would usually be reached before the magnesium was molecularly equal
to the calcium. In the foregoing we have the explanation of the fact that
but very rarely is the magnesium so abundaut in extensive formations as to
have the precise composition of dolomite.
In determining the stage at which the static condition of affairs is
reached pressure is very important. It has been seen that as a result of
the substitution of magnesium for one-half the calcium in calcium ear-
bonate there is a contraction in volume of 12.30 per cént; hence pressure
tends to promote the change. But pressure increases with depth; therefore
the deeper a rock is, the greater is the tendency for magnesium to replace
the calcium. Hence inthe deeper buried rock, there is a strong force which
tends to substitute magnesium for calcium. But the complete substitution
is resisted, under the law of mass action, by the greater abundance of
calcium, and by the more energetic character of that base. Further, as
already pointed out, where the pressure is very great, as in the zone of
anamorphism, circulation is very slow, and it is not possible for the solu-
tions to supply sufficient magnesium to accomplish this, and probably in
most cases not even enough to change the rock to dolomite. Usually these
factors prevent the substitution from going so far as to produce equimo-
lecular quantities of the two elements; but in the case of the pure dolomites
this occurs. In such instances it may be that pressure is an important
factor. Since few carbonates contain more than an equimolecular amount
of magnesium as compared with the calcium, it may be supposed that this
compound gives the limiting effect of such pressures as have obtained in the
rocks which are within the zone of observation. As a corollary from the
DOLOMITIZATION BY GROUND WATERS. SOT
foregoing, one would expect to find dolomites among those rocks which
have been deeply buried and strongly deformed, but at the time of dolo-
mitization not so deeply buried as to prevent deformation from producing
fractures for circulation, for there, if anywhere, the process of dolomitiza-
tion would have been nearly completed. With this expectation the facts of
occurrence of dolomite as given on pages 798-802 fully correspond.
As to whether dolomitization takes place more rapidly while the solu-
tions are descending or ascending no definite answer can be given without
experimental work. It is believed that the process probably occurs
throughout the journey of the mnderground water, as explained on pages
636-639. Doubtless in many cases where there is local dolomitization
adjacent to the main underground circulation channels, the process has
taken place while the waters were ascending, for, as shown on page 583,
the trunk channels of underground waters are more often ascending than
descending. Excellent illustrations of local dolomitization by ascending
waters are those already referred to at Glenwood Springs, Colo.,
described by Spurr,” and in the Carboniferous limestones of Missouri,
adjacent to the lead and zine areas described by Bain.’ But in many cases
of dolomitization along joints and other fractures the water was doubtless
descending. The process may have been accomplished by waters which
were ascending at one time and descending at another time. For instance,
when the rock was somewhat deeply buried, a channél may have been
occupied by ascending waters, and when brought nearer to the surface by
erosion this same channel may have been occupied by descending waters,
and dolomitization continued throughout the process.
Various regions could be mentioned which illustrate the above-
mentioned conditions for dolomitization. One of the best of these is that
portion of the limestones of the Mississippi Valley which is in the belt of
cementation. It is well known that these limestones have been broken at
various times so as to produce joint fractures and local faults, and thus
furnish channels for ready circulation. In the belt of weathering the great
Cambro-Silurian limestone, which contains magnesium, and the magnesium
of the other rocks is continuously being dissolved and carried downward
aSpurr, J. E., Geology of the Aspen mining district, Colorado: Mon. U.S. Geol. Survey, vol. 31,
1898, pp. 212-216.
> Bain, H. F., Van Hise, C. R., and Adams, G. I., Preliminary report on the lead and zine deposits
of the Ozark region: Twenty-second Ann. Rept. U. S. Geol. Survey, pt. 2, 1901, pp. 208-210.
808 A TREATISE ON METAMORPHISM.
into the openings of the belt of cementation. After a given period of
deformation the process of dolomitization continues until the openings are
closed and circulation is checked, to begin again as soon as new openings
are formed by earth movements. The process goes on with intermittent
speed until a state of equilibrium is approximately reached between the
calcium-magnesium solutions and the adjacent rock.
Other excellent illustrations of areas in which the conditions are
favorable for dolomitization are furnished by various districts in the
Cordilleras. Here in many areas recent orogenic movements and intrusion
and extrusion of magma have taken place. The movements open new
cracks and crevices in the limestones. The voleanic rocks, intruded or
extruded, raise the temperature of the rocks and promote the activity of
the solutions. Furthermore, the increase in temperature promotes the
process of carbonation in the belt of weathering. In areas where there are
limestones, recent volcanism, and orogenic movement, the conditions are
especially favorable for the transfer of magnesium from the belt of
weathering to the belt of cementation, with resultant dolomitization.
MARBLE.
In describing the processes of recrystallization and dolomitization in
limestones, marbles have been accounted for in the main. Marble is the
term which is applied to evenly granular, finely or coarsely interlocking
calcite, dolomite, or intermediate minerals. (PI. IV, C.) In the process of
recrystallization it is common for a change to take place in the color of the
rocks, as a result of which marbles are ordinarily white or pinkish white,
rather than blue or gray, although marbles may, of course, have various
other colors.
As already mentioned, marble results mainly from limestone in conse-
quence of recrystallization or dolomitization, or the two combined. The
absolutely conclusive evidence on this point is the increase of size of the
mineral particles in marbles.as compared with limestones. (See Pl. VI, A
and B.) The formation of one mineral particle of marble of average size
required a multitude of particles of the limestones, probably thousands;
and in those cases where the grains of marble are coarse and the original
limestone was fine perhaps many thousands were required. There is no
mechanical process which alone can merge a great number of mineral
MARBLE. 809
particles into a single mineral. This can be done only by solution and
deposition, or reerystallization. It is notable in this connection, as explained
below, that in proportion as the rocks were deep seated and as orogenic
movements were severe the marbles are likely to be coarse grained,
and were therefore thoroughly recrystallized. If the deformation were
mechanical, the greater the severity of the movement the greater would be
the strain effects mentioned below.
The development of marble is frequently assisted by the intrusion of
igneous rocks. If the masses be large they may furnish the conditions
of heat necessary for the solutions to be able to recrystallize the limestone
for a considerable distance about the intrusive. Of course, in many regions
orographic movements and volcanism are synchronous, and in such cases
the conditions furnished by the movements and those furnished by the
mtrusive combined are particularly favorable to the process of recrystalli-
zation. Reerystallization may have been completed in regions of com-
paratively little disturbance, and thus occasionally in unfolded regions
caleareous rocks are locally found to which the term marble is applicable.
But the great belts of marble occur in regions in which orographic move-
ments have taken place, and especially in rocks which have been so deeply
buried as to be in the zone of anamorphism. This is shown by the asso-
ciation of the marbles with slates, schists, and eneisses—rocks which have
been metamorphosed in the lower zone. Where the forces and agents
were sufficiently potent to recrystallize the more refractory minerals, the
recrystallization of the calcite and dolomite is probable, even if the marbles
do not give such clear evidence of the fact as does slate, schist, or gneiss.
Calcite and dolomite are most ready to respond to stresses by solution
and deposition, and therefore to recrystallize, and also mechanically by
development of twinning structures, by movement along gliding and
cleavage planes, and by granulation. (See Pl. VI, C and D.) Indeed,
they are the most mobile of all the minerals which make up large masses
of rocks. The minerals, halite, niter, ete., are even more mobile, but their
rock masses are relatively small.
In consequence of the great mobility of calcite and dolomite, marbles
generally differ in structure from the associated metamorphosed slaty,
“See Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble:
Philos. Trans. Royal Soc., London, ser. 4, vol. 195, 1901, pp. 363-401.
810 A TREATISE ON METAMORPHISM.
schistose, or gneissose rocks, in that they are massive and evenly granular,
although the mineral particles commonly show other strain effects men-
tioned below. The explanation of the general lack of slaty and schistose
structures is believed to lie largely in the fact that the process of recrys-
tallization under favorable conditions goes on almost pari passu with the
deformation, and thus at any given moment the minerals lack but little of
adjustment to their environment. In so far as the calcite and dolomite are
left with residual strain there is a tendency to recrystallize. Their mobility
is such that this tendency often prevails, and the minerals are largely
released from strain. But in many instances release from strain is not per-:
fect, for when thin sections are examined in many of the apparently com-
pletely erystalline marbles strain effects remain. Cataclastic structures
eccur in variable amounts; strain shadows are usual; twinning bands are
very abundant, often are exceedingly fine, and
Eee frequently the lamelle are curved, in such in-
stances showing conclusively the existence of
residual strain effects. And it is not always true
that during the development of marble the pro-
cess of crystallization so nearly keeps pace with
Ow, sf
Se
movement as to prevent the formation of a sec-
in: caldite with longer Ginmetae par ONGary structure. In such cases the marbles
BUS OTR eee ren Na have a well-developed cleavage. This ‘is
well illustrated in some of the pre-Cambrian
cleavable marbles of the Laurentian Mountains, by the cleavable Cambro-
Silurian marbles of western Massachusetts, described by Dale,” and in the
marbles of Alabama at Talladega Mountain. (See fig. 22 and Pl. V, 4.)
At this latter locality the marble cleaves as evenly and smoothly as a slate.
Where marbles have a cleavage it is found to conform to the cleavage of
the surrounding rock. The cleavage is found to be wholly due to a
dimensional arrangement of the mineral particles. (See Chapter VIII,
p. 760.) The fact that the cleavage of the mineral particles has nothing
to do with the cleavage of the rock is explained by the rhombohedral
cleavage of calcite resulting in fracture with equal ease in three directions.
In this respect calcite and dolomite contrast strongly with the minerals
“Pumpelly, Raphael, Wolff, J. E., and Dale, T. Nelson, Geology of the Green Mountains in
Massachusetts: Mon. U. 8. Geol. Survey, vol. 28, 1894, pp. 181-182.
. S. GE GICAL SURVE
uU. S OLOGICAL SURVEY MONOGRAPH XLVII PL. V
TEXTURES OF METAMORPHOSED MARBLES.
A. Schistose marble from Talladega Mountain, Alabama B. Tremolitic marble from Canaan, Conn. After Hobbs.
DEFORMATION OF MARBLE. 811
such as mica, the cleavage of which often controls the cleavage of rocks.
The rare existence of cleavable marbles in connection with marble forma-
tions, the larger parts of which lack cleavage, as in the case of the marbles of
the Appalachian regions just mentioned, seems to be conclusive evidence that
the granular character of ordinary marbles is due mainly to the processes
of ready solution and deposition, or recrystallization, as above explained.
Adams and Nicolson have experimentally deformed Carrara marble.“
Among the various reasons for selecting this marble was its evenly granular
character and absence of strain effects. The deformation was accomplished
under the conditions of pressure of the zone of anamorphism. Cylinders
of marble confined on their sides by strong steel bands were shortened
by pressure, and under these circumstances the shape of the marble was
changed without perceptible cracks being produced. The process was
carried on under various conditions.
First, the dry marble was deformed at ordinary temperatures. The
rock thus deformed was found to be weaker than the original rock, but
sufficiently strong to show that the particles were partially welded. The
thin sections of the marble showed a cataclastic structure “Gdentical with
that seen, in the feldspars and many gneisses.”” Many of the grains were
flattened, and these grains showed strain shadows and fibrous structures
due to minute polysynthetic twinning.
Second, the dry marble was very slowly deformed during a period of
about four months at a temperature of 300° C. The deformed marble was
found to be strong, but not quite so strong as the original rock, showing a
nearer approach to perfect welding than in the case of the marble deformed
at ordinary temperatures. The thin sections showed a tendency toward
rock cleavage. ‘‘Cataclastic structure is absent, but almost every grain
shows an exceedingly fine fibrous structure. When examined under a
high power this fibrous structure resolves itself into an extremely narrow
polysynthetie twinning—the whole grain consisting of slightly sinuous
twin lamelle, extinguishing in alternate sets. Each individual is usually
twinned throughout, the lamelle passing from end to end, although a
Philos. Trans. Royat Soc. London, ser. a, yol. 195, 1901, pp. 363-401.
b Adams and Nicolson, cit., p. 375.
812 A TREATISE ON METAMORPHISM.
calcite grains, which in the original rock are practically equidimensional,
are now often distinctly flattened, some of them being three or even four
times as long as they are wide. Some grains can be seen to have been
bent around others adjacent to them, the twin lamellee and the extinction
curving with the twisted grain. In other twisted individuals the twin
lamellze only extend in to a certain distance from the margin of the grain,
leaving a clear untwinned portion in the center; and other crystals again
show not only the fibrous structure due to twinning in one direction, but
broader lamella crossing this obliquely. As the twinning in all cases is
probably parallel to—SR—, this is due to the appearance of a set of twin
lines parallel to a second face of the rhombohedron.”® ..... ‘There
has been no breaking—the rock has not been crushed in the ordinary
sense of the term. The movement has been brought about partly by
twinning, but chietly by a deformation of the grains due to a slipping on
their gliding planes.”’ (PI. VI, C)
Third, the dry marble was deformed during eight and one-fourth
hours—much more rapidly than in the second case—at a temperature of
400° C. The result in this case was very similar to that in the previous
case. From this experiment it was concluded that ‘quick deformation at a
high temperature shows therefore that calcite has freer movement in its
gliding planes at a high temperature and breaks less readily than when
OGL Ge WAL, 705)
Fourth, the marble was slowly deformed during a period of fifty-four
days, at a temperature of 300° C., in the presence of water gas. This
rock after deformation “is not weaker, but actually stronger, than the
original rock.”“ The thin sections of the rock showed substantially the
same textures and structures as in the case of the dry marbles deformed at
300° or 400° C. The “granulated material is so trivial in amount that
the deformation may be said to be due exclusively to movements on the
gliding planes of the calcite, accompanied by potysynthetic twinning. It
is thus identical in character with that seen in the case of the marble when
deformed while dry, either at 300° ©. or 400° C”’ |... 2 2“ There’ are
no signs of solution and redeposition of calcium carbonate even in this
iron-stained portion of the rock. The presence of water, therefore, did
not influence the character of the deformation. It is Just possible, however,
«Adams and Nicolson, cit., p. 379. DOp. cit., p. 380. ¢ Op. cit., p. 382. @ Op. cit., p. 385.
Pape Vee
PHOTOMICROGRAPHS OF LIMESTONE AND MARBLES.
A. Limestone from Wauwatosa, Wis., showing the evenly granular character of a compact lime-
stone. After Buckley.
B. Marble trom Cockeysville, Md. After Diller.
©. Carrara marble after haying been slowly deformed during 124 days at a temperature of 300° C.
The calcite has yielded by gliding.
D. Carrara marble deformed at 400° C.
grains free from all fracturing or cataclastic structure.
After Adams.
The slide shows a uniform mosaic of somewhat flattened
After Adams.
814
U. S. GEOLOGICAL SURVEY MONOGRAPH XLVII_ PL. VI
PHOTOMICROGRAPHS OF LIMESTONE AND MARBLES,
fy
aun
on)
ae
STRUCTURE OF DEFORMED MARBLE. 815
that there may have been a deposition of infinitesimal amounts of calcium
carbonate along very minute cracks or fissures, thus contributing to
maintain the strength of the rock. No signs of such deposition, however,
are visible.” ®
It has been pointed out in another place (see pp. 747-748) that the
greater strength of this sample, as compared with those in which water
was not present, is almost conclusive proof that solution and deposition,
or recrystallization, did take place to some extent.
Adams and Nicolson compare the structures produced in the Carrara
marble artificially deformed with limestones and marbles in various regions
where the rocks have been metamorphosed under deep-seated conditions.
In 21 rocks from different regions, 10 of which are marble and 11 of which
are limestones, they find all the structures which they have artificially pro-
duced to be paralleled.’ But an equal number of rocks from other regions,
only 3 of which are limestones, 2 dolomites, and 16 marbles,* which, as
shown by the greater number of marbles, have undergone more profound
metamorphism, they say ‘‘do not present any undoubted evidence of
movement under pressure. Their structure is that of a mosaic, apparently
resulting in each case from the recrystallization of a previously existing
finer-grained limestone. This process, as described by Lepsius in the Attie
marbles,® consists of the enlargement or growth of certain of the constit-
uent grains at the expense of others until finally a coarse-grained mosaic is
produced.”
In these instances it is apparent that recrystallization is the
dominant process, for only by this process can large individuals be produced
by the merging of small ones. (See Chapter VIII, pp. 690-696.) Adams
and Nicolson conclude: ‘‘While, therefore, recrystallization undoubtedly
plays an important, and in many cases probably a chief part, in the great
movements which are observed to have taken place in the limestones of
contorted districts, this process is by no means the only one by which such
movements are brought about. Many limestones under pressure in the
earth’s crust flow precisely as metals do, by deformation of the com-
« Adams and Nicolson, cit., p. 385.
bOp. eit., pp. 387-390.
cOp. cit., pp. 389-390; Nos. 22-42.
@ Lepsius, R., Geologie von Attika; ein Beitrage zur Lehre yom Metamorphismus der Gesteine.
Dietrich Riemer, Berlin, 1896, p. 186.
¢ Adams and Nicolson, cit., p. 397.
816 A TREATISE ON METAMORPHISM.
pressed grains, and without the intervention of water or any other solvent.”“
If this last sentence were written with the word may before ‘‘flow,” no
exception to the conclusion would be taken. While the process of flatten-
ing by movement along gliding planes undoubtedly occurs in marbles, it
by no means follows, even in the 21 rocks im which the residual strain
effects are described, that this was the chief process in their metamorphism.
Indeed, in the production of the marbles from the original limestones,
in order that the rocks could change from the very fine-grained condition
of limestone to that of marble, recrystallization must have occurred, as
explained on pages 808-809. Therefore to explain the metamorphism of
the 21 rocks showing residual strain effects, we must place recrystallization
as the chief process. During or after this, mechanical deformation supple-
mented recrystallization and gave the various strain effects described.
Indeed, the history of these 21 rocks is very nearly analogous to the com-
plete history of the Carrara marble experimentally deformed by Adams
and Nicolson. Nature, in producing the Carrara marble free from strain
effects, utilized recrystallization as the chief process. Later Adams sup-
plemented this process by mechanical deformation. This later artificial
deformation was an unimportant episode as compared with the recrystalli-
zation by nature.
Therefore, while I have no doubt that during the deformation of marble
all the mechanical movements which Adams and Nicolson have described
occurred, I hold that these, in the case of the production of marble from
limestone, are entirely subordinate to, although they assist, the dominant
process of solution and deposition, or recrystallization. And if this be true
for marbles it can not be doubted that for those rocks composed of minerals
which are less mobile than calcite and dolomite, and which do not have
ready gliding planes in various directions, recrystallization was the process
of paramount importance. It is certainly yet to be proved that, in such
rocks as the slates, schists, and gneisses, movement along gliding planes is ¢
process of consequence in their metamorphism.
CHERTY LIMESTONES, CHERTY DOLOMITES, AND CHERTY MARBLES.
Chert is a term used to include all forms of finely crystalline nonfrag-
mental silica, including opaline, semicrystalline, and completely crystalline
varieties. It differs from quartzite and novaculite in being nonfragmental.
a Adams and Nicolson, cit., p. 398.
ORIGIN OF CHERT IN LIMESTONES. 817
It is well known that in many limestones, dolomites, and marbles are
nodules and belts of opal, semicrystalline opal, chaleedony, finely crystal-
line quartz, or coarsely crystalline quartz, or various combinations of these
materials. Such rocks are called cherty limestones, cherty dolomites, and
cherty marbles. The siliceous material may constitute separate nodules.
The nodules may unite at their ends so as to form layers of varying thick-
ness within the limestone. The chert may be in bands with no evidence of
a nodular character. Bands of chert usually have a lenticular form. The
lateral extent of the lenses is likely to be in direct proportion to their
thickness. The layers, which are a few centimeters in thickness, are usually
only a few meters in lateral extent. The less numerous layers, which are
several meters in thickness, may have a lateral extent of a number of
kilometers. At the horizon at which one lens dies out others may appear,
and thus the large lenticular bands at a definite horizon be analogous in
their relations to magnified nodules. Rarely the masses of chert are many
meters thick and many kilometers in length, constituting considerable
formations. So far as such formations occur, they are treated on a subse-
quent page. The limestone or marble formations containing nodules or
thin layers or bands of chert as subordinate members, and usually having
no great lateral extent, are known as cherty limestones or cherty marbles.
The majority of those who have worked on chert in the cherty lime-
stones, including Wallich, Sorby, Sollas, and Hinde, regard the silica of
the chert as having been deposited by organisms simultaneously with the
limestone. Of these organisms siliceous sponges appear to be the most
important, although radiolaria and diatoms are of much consequence. All
these animals live under essentially the same conditions as the limestone-
building animals. On the whole, in proportion as rocks are old, the evidence
of the organic origin of the silica becomes less and less. Oftentimes in the
Paleozoic and pre-Paleozoic cherty limestones no remnants of sponge,
diatom, or radiolarian can be found. But this does not show that organisms.
were not as important in these formations as in the later formations, since
there has been a longer time for their obliteration through recrystalliza-
tion. On the whole, I incline to the belief that the silica of the cherty
limestones and marbles is-mainly due to organic precipitation, although
this source is supplemented by contributions furnished by ground waters,
as explained below.
MON XLVII—04 52
818 A TREATISE ON METAMORPHISM.
The question now arises why the chert occurs so generally segregated
in nodules, thin layers, and bands in the limestone. The silica as deposited
by organisms is in the amorphous, and therefore readily soluble, form. It
has been explained on pages 120-123 that during the rearrangement of
minerals by solution and deposition there is a marked tendency for those of
the same kind to segregate, and this tendency is strong in proportion as the
minerals are readily soluble. Ata point where there is more than an average
amount of silica, especially large grains of quartz, this silica draws to it
silica from the surrounding rocks through the solutions, under the principle
expounded on pages 74-76, that large masses and grains grow at the
expense of smaller ones. In this principle we have the explanation not
only of the segregation, but of the lenticular character which is so charac-
teristic of the masses. The principle under which larger masses or grains
grow at the expense of smaller ones is that the change tends to give the
total mass the smallest surface area. ‘This would be ideally accomplished
by the spherical form of the masses; but since openings are numerous and
circulation is comparatively easy along the beds, the lenticular masses,
especially those which are more than a few centimeters in diameter, have a
greater lateral than vertical extent. But in proportion as the chert masses
are small they are likely to approach the spherical form, and this gives proof
of the correctness of the principle. It is therefore believed that the segre-
gation and. form of nodules and thin lenticular layers of chert in limestones
are adequately explained by concentration through solution and deposition.
Where, however, there are bands of chert of considerable thickness,
so as to constitute definite members of a formation, and especially where
there are formations of chert, the above explanation is not adequate. It is
probable that many such deposits were originally dominantly siliceous.
The principle of the natural groupings of animals of like kind into a
colony is well known. Where there was a colony of silica-secreting
organisms a major band or formation of silica would form. Modern deposits
composed of nearly pure silica formed by siliceous organisms show how
lenticular bands of chert may have originated. For instance, the well-
known diatom deposit of chert used as tripoli at Richmond, Va., is about
10 meters thick. Such deposits may have been enlarged by segregations
«Merrill, George P., The nonmetallic minerals of the U. 8. National Museum: Rept. Smithsonian
Inst. for 1899, Washington, 1901, p. 219.
ORIGIN OF CHERT IN LIMESTONES. 819
from the surrounding limestone, for in regions in which there are local
colonies of silica-secreting organisms there are sure to be similar organisms
among the limestone-secreting organisms.
Therefore I hold that the nodules and minor bands of chert in the
limestones are mainly due to segregations by solution and deposition, but
that many of the larger and more persistent bands of chert are at places
where the silica was originally deposited in large part by the siliceous organ-
isms, although the silica of such bands has usually been rearranged. Both
original and secondary masses of chert may occur in the same formation.
For instance, in the lead and zine district of southwestern Missouri, in the
Carboniferous limestone are one or more large and continuous bands of
chert which can be mapped as separate members of a formation, and these
are believed to be chiefly original. But in this limestone are innumerable
minute bands and lentils, from a fraction of a centimeter to several centi-
meters in thickness, and extending from a few centimeters to a few meters,
to which the colonization theory can not be fully applied, and which were
doubtless largely segregated as above described. Bands of intermediate
width are doubtless due in varying degrees to original deposition and to sec-
ondary segregation. Where limestone containing chert reaches the surface, it
is well known that the amount of the cherty material in the belt of weather-
ing is relatively great This is largely due to the more ready solubility of
the limestone than of the chert, in consequence of which the percentage of
chert is increased. At or near the surface the removal of the limestone
may be almost or quite complete, and thus produce a partly coherent or
even incoherent belt of almost pure chert along the surface. In many
cherty limestone regions, where the soil is residual, one might think that
nearly the entire rock was chert, so prominent does this material appear in
the soil and in the rock near the surface.
The silica originally present in a given mass of limestone in consequence
of organic precipitation is not the only source of silica for nodules and bands
of chert. The silica of the limestone in the belt of cementation may be
greatly increased. This may be done in two ways. First, where the
limestones reach the surface, and the segregation above mentioned takes
place in the belt of weathering, a part of this silica is being continuously
dissolved and transported downward into the belt of cementation This
process is especially active where the silica has not become wholly
820 A TREATISE ON METAMORPHISM.
crystallized. Second, limestones in the belt of cementation may receive
important additions of silica even where they do not reach the surface but
are overlain by other rocks. It has been explained that the process of
carbonation of the silicates liberates colloidal silicic acid. This is taken
into solution, and passes downward into the belt of cementation. The
material transported to this belt, either from another part of the limestone
or from the silicates, is selectively precipitated as explained on pages
634-636, the limestone being simultaneously dissolved, and thus silicifica-
tion takes place.
This process of segregation of silica, both within the belt of weathering
of the limestones and in the belt of cementation, is steadily cumulative.
Therefore it is possible for limestones which originally were not especially
siliceous to contain considerable amounts of silica. However, it is supposed
that usually those limestones which are heavily siliceous contained originally
a considerable quantity of silica as an organic precipitate.
SILICATED MARBLES.
The silicated marbles are products intermediate between marbles
and the silicate rocks. The processes by which they are formed are a
combination of those producing marble and those resulting in silication of
the carbonates. These processes will not be described in detail here, since
they are given under the immediately preceding and succeeding headings.
The conditions for their formation are intermediate between those ruling in
the production of evenly granular pure marble and those rulmg in the
complete replacement of the carbon dioxide by silica.
Like the marbles and the silicate rocks, the silicated marbles form in
the zone of anamorphism, and especially in connection with mechanical
action or igneous intrusion. The most abundant silicate minerals of the
marbles are pyroxene and amphibole, but many other silicates may form.
Of these, olivine, chondrodite, vesuvianite, tourmaline, and mica are the
most important. In any given case one or two silicates may be the
preponderant ones, and under such circumstances a more definite name
may be given to an individual rock or formation. For instance, if tremo-
lite is the dominant silicate it may be called a tremolitic marble (Pl. V, B);
if diopside is the dominant silicate it may be called a diopsidic marble. If
two silicates occur together, as, for instance, diopside and olivine, the rock
SILICATED MARBLES. 821
may be called a diopsidic and olivinitie marble. In a similar manner, other
appropriate names may be given to individual formations, more accurately
defining their character than the general term silicated marbles.
Of the two most prevalent groups of minerals, pyroxene and amphi-
bole, calcium is more abundant in the former. Therefore from the nearly
pure calcium carbonate rocks the pyroxenes are likely to develop, rather
than the amphiboles. Thus from the rocks which have not been strongly
dolomitized wollastonite and diopside form, rather than tremolite, because
the latter mineral requires a larger proportion of magnesium. Where the
rocks have been dolomitized before silication the amphibole tremolite is
especially likely to develop.
In the calcareous rocks in which there is a considerable amount of
fragmental material mingled, and consequently aluminum is plentiful, the
aluminous pyroxenes and amphiboles are likely to form.
In so far as the process of silication takes place by the combination of
solid silica with the carbonate, and the liberated carbon dioxide escapes,
the volume of the rock is decreased. The decrease in volume varies from
avery small percentage to very considerable amounts as they approach
the silicated rocks, which have a decrease in volume varying from 20 to 40
per cent.
The silicated marbles grade into the pure marbles on the one hand and
into the silicate rocks on the other. The term silicated marble is properly
applied to rocks which range from those in which the silicate minerals are
unimportant to those in which the carbonates are subordinate. Those rocks
in which the silicates are not abundant should be classified with the
marbles, and those in which the carbonates are not abundant should he
classified with the silicate rocks. Thus limited, the silicated marbles occur
at very numerous localities among the rocks profoundly metamorphosed
under deep-seated conditions. This state of affairs naturally exists to a
ereater extent among the old than among the new formations, and therefore
this class of rocks is especially abundant among the Paleozoic and _pre-
Paleozoic sedimentary rocks. Illustrative localities of the silicated marbles
occur in almost every country. The best representatives in this country
are found in the pre-Cambrian formations of Lake Superior and Canada
and in the Paleozoic formations of the Appalachian region.
1
822 A TREATISE ON METAMORPHISM.
SILICATE ROCKS.
In certain localities the limestones and dolomites may be wholly
replaced by silicates. Of these silicates the most important are the pyrox-
enes and amphiboles. Of the pyroxenes the most common are wollastonite,
diopside, and sahlite. Of the amphiboles the most important are tremolite
and actinolite. Of course other pyroxenes and amphiboles may be present,
such as the augites and hornblende. Other silicates which rather frequently
replace limestones are olivine, mica (including muscovite, biotite, and
phlogopite), chondrodite, vesuvianite, and tourmaline. In fact, almost any
of the dense silicates may develop. The limestones are transformed to
silicate rocks only in the zone of anamorphism. The silicates are the
ultimate products of alteration under the conditions there obtaining. The
process is one of simple silication ; the silica originally present in the lime-
stone, or contributed to it by ground waters, replaces the carbon dioxide
and forms the silicate.
If the rock be pure limestone, wollastonite forms, according to the
following reaction:
CaCO,+SiO,-+-nH,0=CaSiO,+CO,+nH,0,
with a decrease in volume of 31 per cent, provided the silica is a solid
and the carbon dioxide escapes.
If the rock be magnesian, diopside or tremolite forms, according to the
following reactions, respectively :
CaMz(CO,),-+2Si0,=CaMgSi,0,+-2C0,,
3CaMg (CO;),+48i0,=CaMg,$i,0,,+2CaCO,+4C0,,
with a decrease in volume of 40.11 and 25.20 per cent, respectively,
provided the silica is a solid, the calcium carbonate remains as a solid
when tremolite forms, and the liberated carbon dioxide escapes.
If the limestone be one which contains many bases in important
amounts, other more complex silicates develop, with variable decreases in
volume; and thus we have the explanation of subordinate amounts of
augite, hornblende, mica, chondrodite, vesuvianite, tourmaline, ete., which
are so common as replacement products of marble.
The localities in which the caleium-carbonate formations have been
completely changed to silicate formations are comparatively few, and at
IRON-BEARING CARBONATES. 825
such localities the formations are usually not extensive. These deposits
are well illustrated by the kalk-silikat-hornfelse of the Harz Mountains,
described by Lossen and Rosenbusch.“
GENERAL STATEMENTS.
The processes of crystallization, dolomitization, silicifi¢ation, and silica-
tion of the limestones and the resultant rocks have been separately described
as if each occurred alone. As a matter of fact, in the field all the processes
may occur together in various proportions, and hence we may have all
eradations between limestone and dolomite, between limestone and marble,
between pure limestones, dolomites, and marbles and siliceous limestones,
dolomites, and marbles, and between these rocks and those in which the
process of silicification is complete. Finally, we may have all gradations
between the various forms above mentioned and those in which the sili-
cates have developed in subordinate, important, or dominant amounts.
Therefore, within the limestone series there are complete gradations between
the various rocks of the series.
IRON-BEARING CARBONATE FAMILY AND METAMORPHOSED EQUIVALENTS.
The iron-bearing carbonate family and metamorphosed equivalents
comprise siderite, ankerite, and parankerite; ferruginous shales, ferruginous
cherts, and jaspilites; actinolitic and griineritic marbles; and actinolitic
quartz rocks and griineritic quartz rocks.
SIDERITE, ANKERITE, AND PARANKERITE.
The original forms of the iron-bearing carbonates include siderite,
ankerite, parankerite, and gradations between them. (Pl. VII, 4.) With
these carbonates, as with the less ferriferous carbonates, chert is an almost
universal associate. It varies in amount from a minute quantity to an
important amount, and occasionally to dominance. ~The alterations of the
above carbonate compounds as minerals have been considered on pages
242-245. As rock masses they may be considered under the terms siderite
and ferrodolomite, including under the latter term all gradations between
siderite on the one hand and dolomite on the other.
The iron-bearing carbonates occur mainly in layers in the stratified
rocks, and are therefore of sedimentary origin.
“Rosenbusch, H., Physiographie der Mineralien, Stuttgart, 1896, vol. 2, p. 97.
824 A TREATISE ON METAMORPHISM.
The bedded iron-bearing carbonates vary from nearly pure carbonate
material through siliceous and clayey iron-bearing carbonates to carbon-
aceous, siliceous, and argillaceous rocks. The most widespread of the iron-
bearing carbonates are of Carboniferous age. Other extensive deposits of
iron-bearing carbonates are those of Algonkian age in the Lake Superior
region. A subordinate amount of the iron-bearing carbonates occurs as
veins, but so far as this is true the material will not be here considered.
ORIGIN OF SIDERITE, ANKERITE, AND PARANKERITE.
So far as siderite, ankerite, and parankerite are composed of calcium,
magnesium, and carbonic acid, the sources of the materials are the same as
for the same materials in the calcium-magnesium carbonate family just
considered. But we must account for the iron. As the iron-bearing car-
bonates are aqueous sedimentary rocks, the iron must have been deposited
from water. The formations containing iron carbonate plentifully are gen-
erally local. Often they are associated with carbonaceous rocks. Many of
them are closely associated, with the coals which are known to have formed
in lagoons and estuaries, some of which possibly were only imperfectly
connected with the sea. Deposits from which coal is derived are formed at
times of very luxuriant vegetation near the level of the sea, where the soil
is very moist. It is believed that the physiographic conditions for the
deposition of iron-carbonate deposits are similar to those which obtain in
flat-lying areas near water level, where vegetation is very luxuriant. It
may be recalled that under such conditions carbonation of the silicates
goes on very rapidly. (See Chapter VI, pp. 476-477.) The iron silicates
are decomposed precisely as are the others. It has been fully explained on
pages 470-471 that where vegetation is very abundant, in very humid
areas, the oxygen of the soil is not sufficient for the oxidation of the
vegetation. It was seen that under such conditions ferric oxide oxidizes
some of the residual vegetation, and is thereby reduced to the ferrous oxide.
The oxidation of the organic compounds produced abundant carbon
dioxide. The ferrous oxide originally present and that formed by reduction
unites with the carbon dioxide and produces iron carbonate.
Where soluble ferrous compounds are formed abundantly, as above
described, it is probable that the rocks which are being destroyed in the
ORIGIN OF SIDERITE, ANKERITE, AND PARANKERITE. 825
belt of weathering are rather rich in iron; for instance, intermediate or basic
igneous rocks. If rock of this favorable composition were scoriaceous
volcanic rocks, the conditions would be exceptionally favorable to the pro-
duction of such salts by the process of carbonation of the silicates, the
partial reduction of the ferric iron, and its union with carbonic or sulphuric
acids.
Such conditions are illustrated by the Lake Superior region, where the
iron for the iron-bearing formations was largely derived from basic volcanic
rocks. In that region in each of the important iron-bearing districts below
the iron-bearing series are found greenstones, often ellipsoidal, in many
places porous and amygdaloidal, in many places schistose, all rich in iron.
Below the Mesabi, Penokee, Marquette, and Menominee series are great
masses of basic volcanic rocks. In the Penokee and Crystal Falls districts
basic voleanic outflows were contemporaneous with or immediately preceded
the deposition of the iron-bearing formation. In the Vermilion district of
Minnesota and in various districts of Canada, including the Michipicoten
district, the iron-bearing formations immediately overlie enormous masses of
ellipsoidal and often amygdaloidal basic volcanic formations.
The ground waters take iron compounds abundantly into solution
wherever the rocks are of a composition and texture favorable to furnishing
iron. The waters take a longer or shorter journey before issuing at the
surface. ‘The issuing waters, aside from the iron compounds, contain, among
other abundant compounds, the calcium and magnesium carbonates. Where
extensive iron-carbonate deposits have formed, it is probable that the sources
of supply were adjacent to shallow standing bodies of water, such as arms of
seas, lagoons, estuaries, etc. Under such circumstances the ground waters
usually issue at the lower places a short distance above or below the level
of the water. As has been explained, such bodies of water are generally
rich in vegetation. In these bodies the iron salts are precipitated.
As to the form in which the iron salts enter the seas, we can judge only
by analogy, but if the present be a guide to the past, the iron was chiefly as
a carbonate and to a subordinate extent as a sulphate, although it might
have been in part in the form of other salts. When the iron salts reach the
lagoon, they are precipitated under favorable conditions as ferric hydrate or
826 A TREATISE ON METAMORPHISM.
possibly in part as basic ferric sulphate. Supposing the iron salt to be car-
bonate, it would be precipitated according to the following reaction:
4FeCO,+3H,0-+20=2Fe,05.3H,0+4C0).
Where this process goes on, on an extensive scale, limonite bodies (considered
on pages 842-843) are built up.
It was formerly supposed that this reaction took place as a result of
the work of oxygen and moisture alone, and this is true to some extent.
But recent observation has shown that where in lagoons iron carbonate is
abundant the oxidation is largely performed through the agency of a class
of bacteria called the iron bacteria. It has been found that these bacteria
are unable to exist without the presence of iron carbonate or manganese
carbonate, but the iron carbonate is the chief compound used. This material
they absorb into their cells. There the iron carbonate is oxidized and the
limonite is precipitated. Says Lafar:
The decomposing power of these organisms is very great, the amount of fer-
rous oxide oxidized by the cells being a high multiple of their own weight. This
high chemical energy on the one hand, and the inexacting demands in the shape of
food on the other, secure to these bacteria an important part in the economy of
Nature; the enormous deposits of ferruginous ochre and bog iron ore, and prob-
ably certain manganese ores as well, being the result of the activity of the iron
bacteria.@
Evidence is furnished of the precipitation of the limonite of bog iron-
ore deposits in this manner by the discovery in some of them of large
numbers of the sheaths of the iron bacteria.’ Further evidence of the
importance and activity of these bacteria is furnished by their partly or
completely closing water pipes of cities where the water contains a consid-
erable amount of iron carbonate.’
The iron part of the salts carried down to the sea as a sulphate would
be likely to be thrown down as basic ferric sulphate,” according to the
following reaction:
12FeSO,+60-+ (x+9) H,O=Fe,(SO,)s.5Fe,0s.xH,0+9H,80,.
aLatar, F., Technical mycology, Lippincott & Co., 1898, vol. 1, p. 361.
» Fischer, A., The structure and functions of bacteria, trans. by A. Coppen Jones, Clarendon
Press, Oxford, 1900, p. 69.
¢ Lafar, cit., p. 361.
@ Pickering, 8S. P. U., On the constitution of molecular compounds; the molecular weight of basic
ferric sulphate: Jour. Chem. Soc. London, vol. 48, 1883, p. 182.
ORIGIN OF FERROUS SILICATE ROCK. 827
The material thrown down as a hydrated ferric oxide and basic ferric
sulphate is mingled with more or less of organic material, and a deposit of
considerable thickness may thus be built up. This deposit is below the
level of ground water, and is therefore in the zone of incomplete oxidation, or
is under the conditions of the belt of cementation. The oxygen required
for the partial oxidation of the organic material is derived in part from the
ferric oxide, and the iron is reduced to the ferrous form; but probably this
reaction does not take place on an important scale at the surface. The
reducing agent may be regarded as carbon, carbon monoxide, or some
of the carburetted hydrogens, such as methane. The result is the same in
any case. The oxygen and the carbon produce carbon dioxide, and thus
the conditions are reproduced for the production of iron carbonate. A
representative reaction may have been as follows:
2Fe,0,.3H,0+3C0,+ C=4FeC0,+3H,0.
In the Mesabi district of Minnesota Leith has shown that a compound,
which he has called greenalite, exists on an extensive scale as an important
original mineral of the iron-bearing formation.“ (Pl. VII, B.) The exact
composition of greenalite has not yet been determined, but according to
Clarke it appears to be either
Fe//’,Fe/’’,(SiO,)3.3H,O or else FeSiO,.nH,0,
with the probability somewhat in favor of the latter composition.” Where
silica in a colloidal form is especially abundant, with limonite produced as
explained above, and with organic matter to serve as a reducing agent, the
silica may unite with the ferrous oxide produced by the reduction of the
ferric oxide to ferrous oxide through the organic matter, and thus with
hydration produce hydrous ferrous silicate. The formation of the hydrous
silicate under such circumstances, rather than the carbonate, may have
been dependent upon the law of mass action.
The principles are illustrated by the conditions under which the oxidized
compounds of zinc occur in the Wisconsin and southwestern Missouri
districts. In the Wisconsin district silica is not especially abundant, and
where zine sulphide is oxidized the zine oxide unites with carbon dioxide
“Leith, C. K., The Mesabi iron-bearing district. of Minnesota: Mon. U. 8. Geol. Survey, vol. 43,
1903, pp. 100-168.
> Leith, cit., p. 246.
828 A TREATISE ON METAMORPHISM.
and forms smithsonite (ZnCO,). But in Missouri silica, partly in the
amorphous form, is very abundant; and there, when the zine sulphide is
oxidized, the oxide of zine largely unites with the silica, forming calamine
[((ZnOH),SiO;]. Both smithsonite and calamine oceur in both districts, but
calamine occurs abundantly only where silica is abundant. Similarly,
where in lagoons the iron is reduced to the ferrous form, it would unite
with the silica on a large scale, provided that compound were abundantly
present in a form suitable for union. Hence the hydrated ferrous silicate
of the Mesabi district is regarded as a product of the chemical reactions of
the zone of katamorphism.
Another iron compound, besides ferrous silicate, which is associated
with iron-bearing carbonates is iron sulphide. It is believed that this
compound results from the reduction of the basic ferric sulphate. It has
been pointed out that, so far as the iron goes into lagoons as a sulphate, it
is thrown down as basic ferric sulphate [Fe,(SO,);.5Fe.0;.xH,0]. This
compound in the belt of cementation in the presence of organic matter
which strongly demands oxygen is reduced, the oxygen being abstracted
from the sulphur, and so far as possible from the iron, thus producing
bisulphide of iron. The excess of ferrous iron unites with the carbon
dioxide, or exceptionally in part with the silica, forming ferrous carbonate
or ferrous silicate. Supposing ferrous carbonate were formed, and the
reducing agent to be CO, the reaction may be written:
2[Fe,(SO,)5.5Fe,0,.xH,0] +3300 =3FeS, +21 FeCO,+12C0,+2xH,0.
In an analogous manner the reaction may be written for ferrous
silicate as follows:
2[ Fe, (SO,),.5Fe,0.xH,0]+33C0+218i0, =3F eS, +21 FeSi0,+33C0,+2xH,0.
So far as there is water in the silica or the ferrous silicate, this may be
added to both sides of the equation. The reducing agent may be supposed
to be C or H or some combination of the two. This would modify the
equation, but would not change the principle involved.
Perhaps the formation of the compounds, ferrous silicate and bisulphide
of iron, can not logically come under the caption ‘“‘iron-bearing carbonates;”
but since they occur with the iron-bearing carbonates as subordinate con-
stituents, and only in a few districts in sufficient masses to be regarded as
independent formations, they are here considered.
JASPILITES AND FERRUGINOUS SHALES AND CHERTS. 829
It is concluded from the foregoing that the main sources of the iron-
bearing carbonates are the iron-bearing silicates of various rocks, and
especially intermediate and basic igneous rocks which are porous. From
these, iron carbonate and other soluble iron salts are produced by the
reactions of the belt of weathering. They are transported to inclosed or
partly inclosed bodies of standing water by the underground solutions.
The material is there thrown down, mainly as ferric hydrate. By the
action of the organic matter upon the ferric iron it is reduced to ferrous
oxide and unites with the carbon dioxide simultaneously produced, or with
silicic acid, forming iron carbonate or iron silicate. So far as the iron is a
sulphate it is reduced wholly or in part to sulphide.
In what manner the calcium and magnesium are thrown down with
the iron is uncertain. In the inclosed lagoons, which must have been of
exceptional character, as shown by the precipitation of the iron itself, it is
possible that calcium and magnesium might so accumulate as to be precipi-
tated chemically, and thus the carbonate be originally an iron-bearing
calcium-magnesium carbonate, in which the iron varies from a subordinate
to a dominant constituent. This suggestion is the more plausible because
where iron is abundantly precipitated as hydroxide and remains in that
form life is usually somewhat sparse. While this may be a partial explana-
tion of the precipitation of the calcium and magnesium, the calcium may
have been mainly thrown down through the instrumentality of life, in the
same manner as were the ordinary limestones, and the substitution of
magnesium for calcium may have occurred subsequently, precisely as in
the case of the limestones, thus forming the ferrodolomite.
FERRUGINOUS SHALES, FERRUGINOUS CHERTS, AND JASPILITES.
In the zone of katamorphism ferruginous shales or ferruginous cherts
are produced from the iron-bearing carbonates, and by modifications of
these in the zone of anamorphism jaspilites are formed. By ferruginous
shales are meant rocks which have a bedded structure and are composed
mainly of oxide of iron, but which include a greater or less quantity of
finely disseminated quartz. By ferruginous cherts are meant those rocks
which consist of two sets of bands, one of which is composed mainly of
iron oxide and the other mainly of chert.
830 A TREATISE ON METAMORPHISM.
FERRUGINOUS SHALES.
The ferruginous shales are produced from siliceous siderite, ankerite, or
parankerite, by the decomposition of the iron carbonate and the partial or
complete oxidation and hydration of the ferrous oxide, combined with the
rearrangement of the particles, and frequently the introduction of silica.
In the zone of katamorphism the first alteration is the oxidation and partial
or complete hydration of the ferrous oxide, with the liberation of carbon
dioxide, thus producing limonitic and hematitic slate and various gradations
between them. The reaction for hematite is as follows:
2FeCO,+0=Fe,0;+2C0).
For limonite it is:
4FeCO,+3H,0+20=2Fe,03.3H,0+4C0,.
Where the constituents of the rock are not extensively rearranged the
original regular stratiform arrangement of the siderite is preserved, and the
rock is called a ferrugmous shale. Where the iron oxide is mainly
hematite the rock may properly be called hematitic shale; where mainly
limonite, a limonitic shale; and where both are abundant, hematitic and
limonitic shale. Simultaneously with the oxidation of the iron carbonate,
calcium and magnesium carbonate, so far as they are present, may be
partially or completely dissolved.
FERRUGINOUS CHERTS.
At the same time the iron carbonate is altered the silica, which is
usually present in greater or less quantity, may become concentrated in
layers by exactly the same process as it becomes concentrated in layers in
limestones, and thus there may be produced a hematitic and limonitic chert.
The rock may be somewhat regularly banded, or the bands may be very
irregular and broken. As the rearrangement goes further, oxide of iron is
apt to become concentrated in bands, alternating with bands which are
predominantly chert. Also in this stage a concretionary nodular or geodal
character is apt to be developed, and then the material may be called
banded iron oxide and chert, or chert with bands and shots of iron oxide.
(PL VII, C.) In the earlier stages frequently the silica, as in the original
rock, is partly opaline, and as the rearrangement becomes more nearly
complete the silica takes the form of finely crystalline, interlocking
JASPILITES. 831
quartz. Each grain may become coated with a film of hematite, thus
giving it a red color. Some of the more irregularly banded rocks have been
called cherts with bands and shots of ore. In these rocks minute openings
exist between the particles, and minor geodal cavities are very common,
thus giving the rock a porous character. This porosity results in part from
the diminution in volume in the change from iron carbonate to hematite or
limonite. Also it is partially caused by the solution of the calcium and
magnesium carbonates and silica more rapidly than iron oxide is substituted
for these materials. During the process the dissolved iron carbonate may
be transported a greater or less distance. In openings precipitation may
take place, and thus veins of hematite or limonite be produced. Under
favorable conditions, which are more fully discussed in Chapter XII, on
“Ore deposits,” the excess of silica and other material may be dissolved
and nearly pure hematite or limonite bodies be formed. Thus from the
iron carbonate large iron-ore deposits are produced. The details of the
process of transformation I have fully discussed elsewhere. *
In closing this part of the subject it may be noted that to the stage
where the ferruginous shales and ferruginous cherts are produced the iron
has been twice in the form of the carbonate, or once as a carbonate and
once as a silicate, and twice in the form of an iron oxide, and, therefore,
that at least four chemical transformations have occurred to that time.
These were: (1) When the material was first taken into solution as car-
bonate, (2) when it was precipitated in the sea as hydroxide, (8) when
it was transformed to an iron carbonate or hydrous iron silicate, and (4)
when by the underground waters it was again chemically changed to
limonite or hematite.
JASPILITES.
The jaspilites are banded rocks, the bands being composed of hematite
and chert in various proportions. The jaspilites differ from the ferruginous
cherts in that the siliceous bands have a bright-red color, due to the fact
that each granule of quartz is coated with and includes innumerable minute
flakes of blood-red hematite. The bands in which the iron oxide is pre-
dominant are composed chiefly of specular hematite.
“Trying, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin:
Mon. U.S. Geol. Survey, vol. 19, 1892, pp. 268-295. Van Hise, C. R., The iron-ore deposits of the
Lake Superior region: Twenty-first Ann. Rept. U. 8S. Geol. Survey, pt. 3, 1901, pp. 326-328.
832 A TREATISE ON METAMORPHISM.
The hematitic and limonitic shales produced in the zone of katamor-
phism, and especially in the belt of weathering, are so unlike the original
rocks from which they were derived that one would not be inclined to
believe that they were derived from ferriferous carbonates if the actual
transitions had not been traced at many localities.“
After the ferruginous shales and ferruginous cherts have developed
they may undergo still further alterations. For instance, the hematitic and
limonitic shales may, after having been formed, become deeply buried, and
thus pass into the zone of anamorphism. In this position they are in the
zone of dehydration, and this process will occur, especially if mass-
mechanical action takes place. Thus there are produced the banded
hematitic siliceous rocks known as jaspilites, which are merely banded
hematite and quartz rocks. In some of the bands the quartz is dominant,
in others the hematite is dominant, and in still others both are abundant.
In the quartz bands innumerable minute grains of quartz are stained by
still more numerous microscopic flakes of red oxide of iron. ‘The iron oxide
bands usually include much quartz. In proportion as mass-mechanical
action is severe the rocks take on a strongly developed schistose structure,
due to the similar dimensional arrangement of the hematite flakes. From
the nearly pure iron-oxide bands, bands of hematite-schist are produced, the
hematite being in brilliant parallel flakes resembling flakes of mica. These
bands of hematite are similar to the solid specular ore bodies.
Excellent illustrations of hematitic schists and jaspilites are furnished
by the upper horizon of the Lower Huronian of the Lake Superior region,
especially in the Marquette district. In this district, after the original iron
carbonate had formed, the upper part of the formation was subjected to
alteration in the belt of weathering on an extensive scale, and thus was
transformed to hematitic and limonitic shales. It was afterwards buried
under the great mass of deposits of the Upper Huronian, and thus passed
into the zone of anamorphism. Later with that series the ferruginous
a@Trving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin:
Mon. U. 8. Geol. Survey, vol. 19, 1892, pp. 198, 201-202, 205, 253, 294. Clements, J. Morgan, and
Smyth, H. L., with W.S. Bayley and C. R. Van Hise, The Crystal Falls iron-bearing district of
Michigan: Mon. U. 8. Geol. Survey, vol. 36, 1899, p. 62. Van Hise, C. R., with Bayley, W.S., and
Smyth, H. L., The Marquette iron-bearing district of Michigan: Mon. U. 8. Geol. Survey, vol. 28,
1897, pp. 336-375. Bayley, W. S., The Menominee iron-bearing district of Michigan: Mon. U.S.
Geol. Survey, vol. 46, 1904, pp. 397, 466-468.
ORIGIN OF ACTINOLITIC AND GRUNERITIC MARBLES. 833
shales were subjected to profound mass-mechanical action and then were
transformed to jaspilites. Simultaneously with the process of jaspilitization
of the upper part of the iron-bearing formation the basal conglomerate of
the Upper Marquette series, largely composed of the weathered material
of the iron-bearing formation, was transformed to a jasper-conglomerate.
It has been noted that the formation of the ferruginous shales and ferrugi-
nous cherts required four chemical transformations. Where jaspilites occur
a fifth chemical change has taken place—dehydration. —
ACTINOLITIC AND GRUNERITIC MARBLES.
In the zone of anamorphism the alterations of the iron-bearing carbon-
ates are entirely different in character from those in the zone of katamorphism.
In the lower zone the iron carbonates go through a set of transformations
which are analogous to those of limestones under similar conditions. Fur-
thermore, magnetite forms. Therefore the rocks which are first produced
are actinolitic or grtineritic magnetitic marbles. In these rocks the chief
constituents are dolomite, magnetite, and actinolite or griinerite, often also
quartz.
The development of the dolomite is the same as in the marbles. By
the silication of the carbonate either actinolite or griinerite is produced,
with diminution of volume—the former in case the bases present are calcium,
magnesium, and iron, and the latter in case the carbonate was originally
siderite or the original rock was greenalite (hydrous silicate of iron).
Where some iron sulphide or oxygen is available a part of the ferrous
iron may change to magnetite; and rarely a very little hematite may form.
Very frequently there is greater or less rearrangement and concentration of
the silica, so that bands or nodules of this material are formed. If the
metamorphism takes place under mass-static conditions, there is little
tendency for parallel orientation of the actinolite and griinerite. In propor-
tion as the alteration occurs under mass-mechanical conditions there is a
tendency for parallel orientation of the developing amphibole. Since
actinolite-magnetite rocks or griinerite-magnetite rocks, the end products of
the alteration of the siderites, ferrodolomites, and greenalite rocks are much
more common than the intermediate rocks treated under this heading, the
reactions producing the silicates are more fully discussed under the next
heading.
MON XLvII—04—53
834 A TREATISE ON METAMORPHISM.
ACTINOLITE-MAGNETITE-QUARTZ ROCKS AND GRUNERITE-MAGNETITE-QUARTZ ROCKS.
The actinolite-magnetite-quartz rocks consist of interlocking actinolite,
magnetite, and quartz, with or without other iron oxides, and accessories of
various kinds. (Pl. VII, D.) The griinerite-magnetite-quartz rocks consist
of interlocking griinerite, magnetite, and quartz, with or without other iron
oxides, and accessories of various kinds. In a given case the magnetite or
actinolite or griinerite may drop out, or nearly so. Where the magnetite
becomes subordinate the rocks are actinolite-quartz rocks and griinerite-
quartz rocks; where the actinolite or griinerite drops out the rocks become
magnetite-quartz rocks.
The conditions under which these rocks form are those of the zone of
anamorphism. In many cases the actinolite and griinerite have a random
or radial fibrous arrangement to their bands, but in other cases they are
arranged with their longer axes in a common direction, with a marked ten-
dency toward crystallographic orientation. Where the parallel arrange-
ment of the amphibole occurs the quartz and magnetite also have a tendency
to have their longer dimensions in the same direction as that of the actinolite
and magnetite. Where this arrangement is marked—and this is very com-
mon—the term gneiss is properly applicable to the rocks; and thus we may
have actinolite-quartz-gneiss, magnetite-quartz-gneiss, etc. The amount of
parallel arrangement of the particles is a measure of the amount of orogenic
movement during the process of recrystallization. Where there is little
tendency to parallelism the rocks were probably crystallized by metasoma-
tism under mass-static conditions. Where the arrangement is well devel-
oped the recrystallization occurred during powerful orogenic movement.
This relation is well illustrated by the Marquette district of Michigan.
In the central part of the district the folding is only moderately close, and
there the parallel arrangement of the mineral particles is very imperfect.
At the west end of the district the folding is of the closest and most intense
character, and there are beautiful coarse actinolitic and griineritic gneisses,
many of the mineral particles of which are arranged with almost perfect
parallelism.
The most important change of the iron carbonate is to a. silicate.
Where the carbonate is nearly pure siderite, griinerite is produced, accord-
ing to the following reaction:
FeCO,+Si0,=FeSi0O,+CO,,
Je Tea Dy WIE IL
PHOTOMICROGRAPHS OF IRON-BEARING ROCKS.
. Sideritic rock from Penokee-Gogebic district of Michigan and Wisconsin.
. Greenalite or ferrous silicate rock from Mesabi district of Minnesota. After Leith.
. Concretionary chert resulting from the alteration of iron carbonate. From Penokee-Gogebic
district of Michigan and Wisconsin.
. Actinolite-magnetite-quartz rock resulting from the alteration of iron carbonate. From Penokee-
Gogebic district of Michigan and Wisconsin.
835
U. S. GEOLOGICAL SURVEY
MONOGRAPH XLVII PL. Vil
PHOTOMICROGRAPHS OF |RON-BEARING ROCKS.
oi : valle : JESS) z _ : cai elles:
Be Sa : ne Suet 5 ace ri ge
Gola ie en ae Cs : ERs:
Neurone
ag bf Tea
METAMORPHISM OF IRON-BEARING CARBONATES. 837
with a decrease of volume of 32 per cent, provided the silica be a solid
and the carbon dioxide escape. Where the original material was hydrous
ferrous silicate, greenalite, simple dehydration only is necessary to form
the griinerite.
Where the iron-bearing carbonate bears calcium and magnesium in
considerable quantity, imstead of griinerite being produced sahlite or
actinolite may be formed. Supposing the carbonate to be normal ankerite,
sahlite is produced, according to the following reaction:
CaFeC,O0,.CaMgC,0,+-4810, =Ca,MeFeSi,O,,-+4C0,,
with a decrease in volume of 37 per cent, provided the silica be solid and
the carbon dioxide escape.
From ankerite actinolite may be produced, according to the following
reaction:
3(CaFeC,0,.CaMgC,0,)-+88i0, =Ca, Mg,Fe,Si,0,,+8C0,+-4Ca00,,
with a decrease in Volume of 23 per cent, provided the silica be a solid, the
CaCO, formed remain as a solid, and the carbon dioxide escape.
If a more ferriferous and less caleareous iron-bearing carbonate be
taken, it would not be necessary to suppose any calcium carbonate to have
separated.
The iron-bearmg carbonates may be very impure, just as limestones
may be impure; and in this case there may develop various other minerals.
In proportion as impurities are mingled with the carbonates, other amphi-
boles and the pyroxenes, micas, garnets, and other heavy minerals such as
olivine, may abundantly develop; and thus there may be produced a ereat
variety of rocks, such as garnetiferous magnetite rocks, micaceous eriinerite
rocks, ete. As the impurities become abundant and the silicates other than
griinerite, sahlite, and actinolite more prominent, the alterations beeome
nearly those of the fragmental rocks, considered on page 853 et seq.
Between the two there are of course all gradations.
But as a matter of fact, the two silicates which most extensively form
by the alterations of the iron-bearing carbonates in the zone of anamorphism
are actinolite and griinerite. Where these reactions are complete we may
have, in place of the iron-bearing carbonate, actinolite rocks, griinerite
rocks, and all gradations between them.
838 A TREATISE ON METAMORPHISM.
With the griinerite, actinolite, sahlite, or other silicates, magnetite
usually develops to some extent, probably as follows:
2FeCO,+-FeS,-+-2H,0=Fe,0,+2H,S-+2C0,.
The following reactions are also possible:
3FeCO,=Fe,0,+C0-+2C0,;
or where there is sufficient oxygen:
3FeC0,-+0=Fe,0,-+-3C0,.
The first reaction produces a decrease in volume of 47 per cent, and the
second and third a decrease of 50 per cent, provided the hydrogen sulphide,
oxygen, carbon monoxide, and carbon dioxide be ignored.
Observation in the field shows beyond question that the change from
iron carbonate to magnetite takes place on an extensive scale. Which of
the above reactions is the more important may be an open question. Since
the change takes place in the zone of anamorphism, where the conditions
are reducing instead of oxidizing, and since, as has already been explained
(p. 828), pyrite is a very common associate of iron carbonate, I think it
probable that the first of the three reactions written is the dominant one.
The reactions of the second and third equations are identical with those
which take place when. iron carbonate is heated in a closed tube in the
laboratory where oxygen is absent or deficient in amount. It is explained
below that the development of magnetite from iron carbonate takes place
on the most extensive scale where igneous rocks have been intruded. This
suggests at first that the reactions may be those of dry heat, but this idea
is probably not warranted, for the magnetite which forms from carbonate is
usually segregated into crystals or clusters of crystals, and this leads me to
believe that the change is accomplished through the agency of water, with
heat as a promoting force and iron sulphide as one of the active agents.
In the group of rocks under consideration the relative proportions of
the silicates, of magnetite, and of quartz are very variable. In some
instances the actinolite or the griinerite is predominant, with quartz as a
subordinate constituent, and we have a quartzose actinolite or quartzose
griinerite rock. In other instances the predominant constituents are mag-
netite and quartz, and in still other cases quartz is subordinate in amount;
hence we may have a magnetite-quartz rock, or a quartzose magnetite
METAMORPHISM OF IRON-BEARING CARBONATES. 839
rock. In still other instances all three classes of minerals are abundantly
present, and we have actinolite-magnetite-quartz rocks or griinerite-
magnetite-quartz rocks. Hach of these different combinations of minerals
may be of sufficient extent to constitute a member of a formation, or even
an entire formation.
Which of the above rocks develops at a given place depends not only
upon the original composition of the rocks, but upon the nature of the
alteration. For instance, where in the original rock silica is subordinate
and nearly pure siderite abundant, a quartzose-magnetite may develop, as
at various places in the Lake Superior region. Where the conditions are
such that the silicates form, the development of the actinolite or griinerite
uses up both the iron carbonate and the silica, and an actinolite rock or a
griinerite rock may be produced. Where silica was originally an abundant
constituent both magnetite and the silicates are likely to develop. Thus
we have various proportions of all the minerals, producing the magnetite-
quartz rocks, the actinolite-magnetite-quartz rocks, the griinerite-magnetite-
quartz rocks, the actinolite-quartz rocks and the griinerite-quartz rocks.
Usually a given formation, or member, does not show a_ perfectly
homogeneous arrangement of the mineral particles. The original sedi-
mentary rock is banded, and the different bands have different compositions.
Naturally the transformation of these bands produces different combinations
of minerals. Moreover, during the recrystallization there is a tendency,
as explained in Chapter III (pp. 120-123), for minerals of the same kind to
segregate. Hence, in any of the above cases, where as a whole a certain
set of minerals are dominant within a rock, a single mineral, or two com-
bined, may be largely segregated in bands; and in the alternate bands the
other minerals be largely segregated. Thus a banded rock, consisting
mainly of magnetite and quartz, may have a banded appearance. as the
result either of the segregation of the quartz and magnetite in separate
bands or, more commonly, the segregation of more quartz and less magnet-
ite in one band and less quartz and more magnetite in another band. In
a similar manner alternate bands may be made up of actinolite or griinerite
with quartz in various proportions, and of actinolite or griinerite with
magnetite in various proportions. In still other instances the banding may
be due to the combining of actinolite or griinerite, magnetite, and quartz
in various proportions. In general, therefore, the alterations of the rock do
840 A TREATISE ON METAMORPHISM.
not destroy the original sedimentary banding, but, on the contrary, empha-
size it. The striking banded appearance of actinolitic and griineritic rocks
is one of their most characteristic features.
The development of the actinolitic and griineritic marbles and of the
actinolitic, griineritic, and magnetitic quartz-rocks is very greatly promoted
by the presence of igneous rocks. Indeed, in the Lake Superior region,
where iron silicates have been produced in large amounts from the iron-
bearing carbonates and hydrous iron silicates over extensive areas, intrusive
rocks, in large masses, seem to be invariably present. At the west end of
the Penokee range in Wisconsin, and at the east end of the Mesabi district
in Minnesota,” the entire iron-bearing formations have been changed to
silicate-bearing rocks, and directly in contact with them—indeed, intrusive
in them in a complex manner—are great batholithic masses of the basal
gabbro of the Keweenawan. Also in the Mesabi district granite has been
intruded in great quantities. In the Marquette district of Michigan, east of
Negaunee, dolerite is a very abundant intrusive within the iron-bearing
formation, and there in the iron-bearing formation the silicate rocks are
again found, although not so abundantly as in the Mesabi and Penokee
districts.” Not only do the great areas of the silicated rocks occur where
there are great masses of intrusives, but intrusives are sure to be found
where the silicated rocks appear locally abundant, as, for instance, adjacent
to Humboldt, Mich., in the Marquette district. Apparently in the Lake
Superior region the iron-bearing formations were not sufficiently deeply
buried, or subjected to sufficiently strong orogenic movements to cause the
silication of the carbonates to take place on an extensive scale for those
reasons alone; but where these metamorphic conditions were reenforced
and intensified by intrusive rocks, there the transformation took place. If
any case of metamorphism ought to be called contact metamorphism, the
development of the silicated rocks from the iron-bearing carbonates should
aVan Hise, C. R., and Irving, R. D., The Penokee iron-bearing series of Michigan and Wisconsin:
Mon. U. S. Geol. Survey, vol. 19, 1892, pp. 257-260. Grant, U. 8., Sketch of the geology of the east-
ern end of the Mesabi iron range in Minnesota: Eng. Year Book, Uniy. of Minnesota, 1898, p. 58.
Van Hise, C. R., and Leith, C. K., The iron-ore deposits of the Lake Superior region; section on the
Mesabi district: Twenty-first Ann. Rept., U. S. Geol. Survey, pt. 3, 1901, pp. 359-360. Leith, C. k.,
The Mesabi iron-bearins district of Minnesota: Mon. U. 8. Geol. Survey, vol. 48, 1903, pp. 159-164,
182-188.
> Van Hise, C. R., and Bayley, W. S., The Marquette iron-bearing district of Michigan: Mon.
U.S. Geol. Survey, vol. 28, 1897, pp. 380-381.
METAMORPHISM OF IRON-BEARING CARBONATES. - 841
be so designated, since in the Lake Superior region the presence of the
igneous rocks seems to have been an essential condition for the process to
take place on an extensive scale.
After the griineritic or actinolitic rocks have developed in the zone of
anamorphism, in consequence of denudation they may pass into the zone
of katamorphism, or even into the belt of weathering. Then there will
begin the processes of oxidation, hydration, and carbonation, as a result of
which the magnetite is changed to hematite or limonite, and the actinolite
and eriinerite decompose into carbonate. However, since magnetite and
the iron-bearing amphiboles are very refractory, this process is exceedingly
slow, and usually has affected only comparatively thin layers of material
adjacent to the surface or adjacent to openings in the rock. Indeed, the
reagents of the belt of weathering and the upper part of the belt of cemen-
tation, which, as pointed out (pp. 830-831), may produce large iron-ore
bodies where they have the original iron-bearing carbonates or the hydrous
ferrous silicates to work upon, have nowhere in the Lake Superior region
formed large ore bodies where they are working upon the griineritic and
actinolitic rocks. This follows directly from their refractory character,
since the average amount of iron of the iron-bearing formation is essen-
tially the same in both cases. In the refractory character of these rocks
we have the explanation of the absence of rich workable iron-ore deposits
for extensive areas in each of the iron-bearing districts of the Lake Superior
region where the alterations of the original rocks in the zone of anamor-
phism developed actinolitic or griineritic magnetite rocks.
GENERAL STATEMENTS.
From the foregoing pages it is apparent that the iron-bearing carbon-
ates and the hydrous iron silicates are metamorphosed along two main
lines, depending upon whether they are in the zone of katamorphism
or the zone of anamorphism. Where they are altered in the zone of
katamorphism, and especially in the belt of weathering, the ferruginous
shales, ferruginous cherts, and ores result. Where they are altered in the
zone of anamorphism the actinolitic and griineritic marbles, or the actin-
olitic rocks and griineritic rocks, are produced. Further, where the
metamorphosed rocks produced from the iron-bearing carbonates and
hydrous iron silicates change from one zone to another they are further
842 A TREATISE ON METAMORPHISM.
altered. The ferruginous shales and ferruginous cherts, products of the
zone of katamorphism, when transferred to the zone of anamorphism are
transformed to jaspilites. The actinolitic and griineritic rocks transferred
to the zone of katamorphism are altered. to ferruginous shales and ferrugi-
nous cherts, but this change takes place so very slowly that these products
have not been thus produced on a very large scale.
OXIDE ORDER.
TRON-OXIDE FAMILY.
The iron-oxide family comprises a number of oxides—anhydrous and
hydrous. Of these limonite, hematite, and magnetite occur in sufficient
masses to entitle them to consideration as rocks. Limonite is hydrated
ferric oxide (2Fe,0;3H,O). Hematite is ferric oxide (Fe,0;). Magnetite
is ferrosoferric oxide (Fe,O,). Many of the larger formations are inter-
mediate between limonite and hematite. Some are partly hematite and
partly magnetite. Usually subordinate minerals are associated with the
iron oxides. Of these, quartz is almost universal. But all the carbonates,
many of the hydroxides and silicates, and some of the sulphides are often
associated. The carbonates and hydroxides other than of iron are apt to be
associated with the limonites, and less commonly with the hematites. The
silicates and sulphides are usually associated with the magnetites, and often
with the specular hematites.
LIMONITE.
Limonite may compose considerable rock deposits. Ordinarily such
deposits are not very thick, although they may have considerable extent.
The deposits are commonly of very irregular thickness, or bunchy, being
locally perhaps 3, 6, or 10 meters thick, and within a short distance almost
disappearing.
Recent deposits of limonite are extensive. These occur in marshes
and bogs. They have been worked to a greater or less extent as iron ores
throughout the civilized world. Probably the most extensive deposits of
limonite known are those of the Lower Silurian, which have been worked
for ore in many countries.
The Silurian limonites and the limonites of many other horizons are
largely replacement and concentration products of limestones, being there-
fore a result of metamorphism. In some districts the limestones originally
FORMATION OF HEMATITE. 843
contained a small amount of iron carbonate. This carbonate has a history
identical with that of the iron of the iron-bearing carbonates, discussed on
pages 824-829. Locally the iron carbonate of the limestones has been
picked up by the percolating waters. At various places these waters have
been converged at localities favorably situated. At such localities the iron
is precipitated from the carbonate according to the reaction:
4FeC0, +20 -+3H,0=2Fe,05.3H,0+4C0).
Often simultaneously with this process calcium carbonate has been dis-
solved. Thus, at the surface of a limestone formation there may be pro-
duced a layer of limonite which passes downward into the limestone.
In still other cases the iron of limonites associated with limestones
has been in large part derived from adjacent formations. In such cases
the association is, in part at least, a consequence of the ready solution of
the limestone, which makes it easy for the substitution to take place. Such
substitution is likely to occur where underground waters from different
sources unite, as, for instance, where the limestones are fractured, so as to
be open and porous; where there are impervious basements, ete.
HEMATITE,
Hematite occurs in much larger masses than limonite. ‘The hematite
is not wholly anhydrous, but contains a variable amount of water. For
instance, in the Mesabi hematite, the most extensive deposits of this oxide
known, the combined water varies from 2.09 to 8.23 per cent.” Thus, there
are all gradations between limonite and hematite, and the majority of the
ereat soft iron-ore deposits contain limonite, hematite, the intermediate
oxides, and various combinations of them. The more extensive iron-ore
deposits of the United States, which are largely hematite, are those of the
Algonkian and Archean of the Lake Superior region and the Clinton ore
horizon of the Silurian. These masses of limonite and hematite were partly
deposited in place as original sediments, but to a larger extent are due to
subsequent segregations.
The limonitic hematites which occur as original sediments in place are
identical in their development with the oxides of iron which are the original
source of the iron carbonates. (See pp. 824-826.) In summary, iron salts,
“Leith, C. K., The Mesabi iron-bearing district of Minnesota: Mon. U. 8. Geol. Survey, vol. 43,
1903, pp. 214-217.
844 A TREATISE ON METAMORPHISM.
as iron carbonate mainly, but subordinately iron sulphate and other com-
pounds, are brought by underground solutions to shallow bodies of water,
such as lagoons. By oxidation and hydration the material is thrown down as
hydrated hematite or limonite, with a greater or less amount of basic ferric
sulphate. But the larger masses of limonite and hematite are usually mainly
produced by segregation subsequent to the formation of the original
sediments. These, like the limonites, are largely replacement deposits of
limestone or of some other carbonate rock. The Clinton hematite ore
deposits are partly replacements of limestone which inay have contained a
small amount of iron carbonate, but according to various authors the hema-
tites, as such, in large measure were deposited with the associated lime-
stones. But the most extensive deposits of hematite known, those of the
Lake Superior region, are segregations in formations which were originally
in large measure iron-bearing carbonate or hydrous ferrous silicate. For
these deposits the iron as carbonate has been transported to the places of
deposition; there it meets other solutions bearing oxygen, and in conse-
quence of the mingling of such solutions the iron is thrown down as a
partially hydrated hematite, the reaction being:
2FeCO,+-0-+nH,0=Fe,0,+-nH,0-+2C0,.
It may be said, therefore, that limonitic hematite is either a direct
chemical precipitate in the zone of katamorphism or is due to aqueous
concentration in that zone.
Ordinarily, so long as limonite and limonitic hematite remain im the
horizon in which they form, they are not further altered, since they are end
products of metamorphism for the zone of katamorphism. But at or near
the surface in arid regions, or in times of drought in humid regions, the
more hydrous forms of the oxide may be dehydrated and thus pass into
hematite.
The more important alterations of limonite and hydrated hematite take
place when these ores pass into the zone of anamorphism. In that zone
dehydration occurs. The limonites and more hydrated hematites are grad-
ually decreased in amount, and where dehydration is complete hematite is
formed. Where the change is from pure limonite to pure hematite a
decrease in volume of 38 per cent is involved.
Where the rock is subjected to strong orogenic movements, not only
does dehydration occur, but the iron becomes specular, and a laminated
FORMATION OF MAGNETITE. 845
texture is produced. Thus are formed the specular hematites. Where the
movements are of extreme intensity the little flakelets of hematite may
become flat and parallel, and thus resemble mica. Such a rock is a
hematite-schist. Where more or less of a reducing agent is present at the
time of the dehydration, magnetite may form and be associated with the
hematite.
MAGNETITE.
Magnetite, with quartz and various silicates as subordinate constitu-
ents, may compose considerable rock bodies. Such material occurs as
important members of the iron-bearing formations of the Lake Superior
region, and in the gneisses of the eastern metamorphic region of the United
States—in the Adirondacks, in the highlands of New York and New J ersey,
and in the southern Appalachians, notably at Cranberry, N.C. But the
greatest of the eastern magnetite deposits is that of Cornwall, Pa., in lime-
stone of Lower Silurian age, underlain by Triassic intrusive trap. It is
uncertain whether or not the gneisses with which many of these magnetitic
rocks are associated are of sedimentary origin, but presumably many of
them are. If these rocks be sedimentary it is more than probable that the
magnetite members are also sedimentary. The most natural hypothesis as
to the origin of these rocks is that they were iron-bearing carbonates meta-
morphosed as described on pages 834-841. The deep-seated metamorphism
changed the original iron-bearing carbonates to magnetite, with the simul-
taneous development of quartz and silicates.
The production of magnetite from siderite may be supposed to be that
of imperfect oxidation, the reaction being:
3 FeCO,-+ O = Fe,0, + 3C0,.
But it has been seen (p. 828) that where iron carbonate is produced
iron sulphide also is often found. Magnetite and pyrite, also, are often
associated. In the deep-seated zone of anamorphism it can not be supposed
that oxygen is usually present. It therefore appears to me that the more
frequent reaction is that producing magnetite from siderite and pyrite.
The change may be written thus:
2FeCO,-+ FeS, + 2H,0 = Fe,0, +2H.S + 2C0,.
These changes would result in a decrease of volume of 50.32 per cent
for siderite to magnetite, or of 46.67 per cent for siderite and pyrite.
846 A TREATISE ON METAMORPHISM.
This dimunition of volume furnishes an entirely adequate cause for the
transformation.
Also magnetite may be produced under conditions of partial oxidation
of iron sulphide, such as pyrite and pyrrhotite, according to the following
reactions:
Le Conte describes the formation of magnetite by the imperfect
oxidation of iron sulphide at Sulphur Bank, California.“
Some magnetites are undoubtedly formed by the alterations of limonite
or hematite or intermediate compounds. The change of these oxides of iron
into magnetite is likely to take place in the deep-seated zone where organic
or other material is present which can take away the oxygen. At the same
time dehydration occurs. The reaction may be written as follows:
3(Fe,0,).nH,0O—O=2Fe,0,+3nH,0.
The oxygen abstracted unites with the carbon to form carbon dioxide,
or with hydrogen to form water. Where the limonite or hematite is asso-
ciated with iron sulphide it is not necessary that organic matter be present
in order that the change shall take place; the sulphide may serve as the
reducing agent. In this case the reaction may be written:
22(Fe,0,.nH,O)-+FeS,-+-2H,0=15Fe,0,+2H,80,+22nH,0.
Supposing the original oxide were hematite, the volume of the
magnetite is 3.58 per cent less than that of the hematite and pyrite. So far
as water was present dehydration took place and the percentage of decrease
in volume is greater than given. Where the amount of the reducing agent
is not sufficient to change all of the limonite or hydrated hematite to
magnetite, dehydration only takes place for the remainder of the material,
and consequently with the magnetite a variable amount of hematite is often
associated. The segregation of iron oxide into rich deposits is further
considered in Chapter XII, on ‘Ore deposits.”
progress at Sulphur Bank, California: Am. Jour. Sci., 3d ser., vol. 24, 1882, p. 33.
OCCURRENCE OF CHERT OF ORGANIC ORIGIN. 847
SILICA FAMILY.
CHERT.
The silica deposits are all included under the general term chert.
This term, as here applied to siliceous layers and formations, includes
amorphous, partly amorphous, and crystallized silica. Often in the same
mass may be seen very minutely spotted quartz grains separated by partly
amorphous material, cryptocrystalline silica of various kinds, such as
chalcedony, and completely crystalline quartz. In almost every extensive
chert formation all these varieties are mingled in the most complex manner.
Chert comprises both organic and chemical deposits.
The most extensive deposits of chert are those of organic origin, the
silica being taken by certain animals and built into their hard parts,
precisely as is calcium carbonate by other animals. The animals which
absorb the greatest amount of silica are sponges of a certain class. Other
than the sponges, radiolaria and diatoms appear to be the more important.
These and other animals may build up considerable deposits of chert.
As illustrations of chert formations of organic origin may be mentioned
the Cretaceous flints described by Wallich,” the flints of the Trimmingham
chalks described by Sollas,’ and extensive deposits of chert in Ireland,
England, Wales, Spitzbergen, and Axels Island, described by Hinde.’
Hinde says that the chert beds in Yorkshire have an estimated thickness
of about 30 meters, and in North Wales of 100 meters, and these, he says,
can be proved to be due to sponge remains.*
But probably the most extensive formation of chert which is known to
be of organic origin is that of Axels Island, where the beds of cherty
material, according to Hinde, aggregate 260 meters in thickness.’ This is
not a continuous mass of chert, but consists of a number of beds inter-
stratified with limestone.
«Wallich, G. C., A contribution to the physical history of the Cretaceous flints: Quart. Jour. Geol.
Soc. London, vol. 36, 1880, pp. 68-91.
> Annals Mag. Nat. Hist., 5th ser., vol. 6, 1860, p. 438. See also Walcott, C. D., Fossil meduse:
Mon. U.S. Geol. Survey, vol. 30, 1898, p. 18.
¢ Hinde, G. J., On the organic origin of the chert in the Carboniferous limestone series of Ireland,
and its similarity to that in the corresponding strata in North Wales and Yorkshire: Geol. Mag.,
London, new ser., dec. 3, vol. 4, 1887, pp. 485, 486. Hinde, G. J., On the chert and siliceous schists
of the Permo-Carboniferous strata of Spitzbergen, and on the character of the sponges therefrom,
which have been described by Dr. E. von Dunikowski: Ibid., vol. 5, pp. 241-251.
4 Hinde, cit., vol. 4, pp. 485-446.
€ Hinde, cit., vol. 5, p. 243.
848 A TREATISE ON METAMORPHISM.
The source of the silica for organisms is probably in part that dissolved
in sea water. Since, however, when filtered the pure sea water contains
only one part in 250,000, Murray and Renard ‘consider the probability of
the pelagic organisms which secrete silica obtaining it from the hydrated
silicate of alumina or clay held in suspension as well as the silica held in
solution. This might explain the fact that these organisms abound in
brackish waters, and waters of low salinity and low temperature, where the
clay is more abundant than in the warmest and saltest waters of the ocean.
In the case of siliceous sponges, which are rooted for the most part in the
oozes and clays, Messrs. Murray and Renard think that the silica of their
skeletons may be derived from the silica in solution in sea water, or from
the colloid silica set free during the decomposition of the feldspathic rock
fragments and minerals in the deposits.” “
Chemical deposits of silica may be formed when underground solutions,
especially hot solutions, reach the surface. The silica, in consequence of
the decrease in pressure and temperature, is precipitated. Such deposits
are known as siliceous sinter. The best known examples are the geyserite
formations of the Yellowstone Park.
The silica for chemical deposits is believed to be mainly derived from
that contributed to the underground waters by the process of carbonation
in the zone of katamorphism, and especially in the belt of weathering, as
fully explained in Chapter VI (pp. 475-480). But it is certain that a por-
tion of the silica of the underground waters is directly derived by solution of
amorphous and semicrystalline silica of organic deposits and by the solution
of quartz itself. It is frequently supposed that crystallized quartz is not
dissolved by the ground waters, but, as shown by Hayes,’ quartz crystals
at the surface are dissolved and corroded, and observations in many mines
show that the solutions have dissolved quartz in large quantities from the
wall rocks. This is illustrated by the granite of the Portland mine, at
Cripple Creek, Colo., where the quartz and mica are dissolved and the
feldspar is comparatively untouched. While, therefore, silica for chemical
precipitation is derived directly from the solution of quartz, a vastly
greater amount is the colloidal silicic acid liberated by the process of
«Walcott, C. D., Fossil medusze: Mon. U. 8. Geol. Survey, vol. 30, 1898, p. 20.
> Hayes, C. W., Solution of silica under atmospheric conditions: Bull. Geol. Soc. America, vol. 8,
1897, pp. 213-220.
CRYSTALLIZATION OF AMORPHOUS CHERT. 849
carbonation of the silicates and that furnished by the solution of amorphous
and semicrystalline quartz.
Rearrangement of chert——Chert, as deposited by organisms or chemically,
may be hydrous and amorphous, semicrystalline, or completely crystalline.
In proportion as it is amorphous or semicrystalline, it is likely to be easily
and extensively dissolved and rearranged. Usually the opaline, semi-
crystalline, and completely crystalline materials, are mingled in an intricate
manner. Very often a single thin section shows all varieties of material.
In many sections the opaline or amorphous silica constitutes a background
in which there are innumerable polarizing spots of chaleedony and quartz.
The process of rearrangement may go on until the material, whether of
organic or chemical origin, becomes wholly crystalline quartz.
If crystallization be complete, there may be produced a finely
crystalline rock composed of closely fitting, minute particles of quartz,
many of them with crystal outlines, which average only a small fraction
of a millimeter in diameter. Such rocks may be called finely crystalline
quartz rocks. Representatives of them are the whetstones of Arkansas and
the quartz rock of the Penokee district of Wisconsin. In many cases such
rocks are stained by oxide of iron, and these are often called jasper.
The crystallization may go further, so as to produce coarsely crystal-
line quartz rock. ‘This process of crystallization of the amorphous
material so as to produce first, chert; next, finely crystalline quartz; and
next, coarsely crystalline quartz, occurs under the laws of physical
chemistry. In so far as the silica passes from a hydrous to an anhydrous
condition the volume is lessened. In so far as it passes from an amorphous
to a crystalline condition the volume is lessened. The change in volume
in the passage of one of the ordinary opals to quartz has been calculated
on page 221 to be 22.81 per cent. All these processes may occur at any
depth, precisely the same as the process of dolomitization. But they are
promoted by depth of burying and by high temperature, and therefore the
finely crystalline and coarsely crystalline quartz rocks derived from organic
silica or siliceous sinter have usually been buried to a very considerable
depth.
If in addition to depth of burying there be profound orogenic move-
ment, the rock may be further recrystallized and mechanically sliced, and
MON XLVII—04-——_54
850 A TREATISE ON METAMORPHISM.
thus the quartz particles be so rearranged that their maximum and mean
diameters are in a common plane, producing quartz-schist.
It has already been explained (pp. 818-819) that where silica is
rearranged it is sure to be segregated, because any area of chert attracts
materials of like kind to itself, and large masses grow at the expense of small
ones. Even in the cases where the cherts are undoubted replacements the
source of the silica may largely have been sponge spicules, diatoms, etc.,
which were disseminated through the limestones. This material can not be
discriminated from that furnished by the ground waters coming from some
other formation. The essential fact is that such a deposit is a replacement
deposit, whether the silica be derived from the decomposition of silicate, by
the solution of sponge spicules, or in some other manner.
In order that the silica may be concentrated, it is of course necessary
that the material previously occupying the place shall be dissolved. Such
concentration and solution are especially likely to occur in those formations
which originally contained a considerable amount of cherty material, and
which were readily soluble. The best representatives of rocks of this class
are the carbonates—caleareous, magnesian, and ferruginous. At the time
the chert is segregated the carbonate is dissolved. It therefore follows that
in the calcium-magnesium carbonate family and the iron-bearing carbonate
family the most extensive deposits of chert should be expected; and with
this the facts correspond.
Observation shows that many carbonate formations have been exten-
sively silicified. Positive evidence of the replacement of carbonates by
chert is furnished by formations which contain numerous silicified fossils
which were originally caleareous, such as the mollusks and corals. Where
fossils of this kind, now chert, are contained in a mass of chert, it is natural
to suppose that the formation was in the main originally calcareous. A
good illustration of extensive replacement of calcareous formation by silica
is that of the Tampa Bay chalcedony pseudomorphous after coral.
Where replacement has been extensive, the average. compositions of
considerable masses of rocks have been greatly changed—indeed, where
pure limestone is replaced by chert, entirely changed. ‘There is no means
by which one can estimate the amount of silica that has been introduced -
‘from an outside source in the silicification of great carbonate formations.
Doubtless the amount of such material is great, but probably in most
REARRANGEMENT OF CHERT. 851
instances the process of silicification of the carbonate formations is largely
that of segregation of the originally dispersed silica of the limestone
formations themselves.
No better illustrations of extensive cherts are known to me than those
furnished by the iron-bearing formations of the Lake Superior region.
Here silica in its various forms and iron oxide constitute almost the entire
weathered belt of the cherty iron-carbonate formations. Locally in this
- region the chert is segregated almost entirely free from iron oxide. Such
occurrences are illustrated by the chert near Amasa, in the Crystal Falls
district of Michigan.* Within the limestones extensive chert beds are also
found at many localities. Some of the best known in this country are those
of the Boone chert formation, of Lower Carboniferous age, in the lower
Mississippi Valley, and the thick bed of chert in the Carboniferous limestone
of southwestern Missouri.”
In none of these formations are organic remains found from which
the chert is derived. Where the rearrangement has gone so far that the
siliceous organisms, if they once existed, would have been destroyed, it .
becomes difficult or impossible to certainly discriminate between chert
deposits which are mainly organic and those which are mainly chemical.
It is natural to suppose that where chert beds are somewhat persistent they
are largely organic, although doubtless they may have been extensively
rearranged and have had great additions of silica. On the other hand,
where the chert masses are small, nodular, and bunchy, and especially
where they are exceedingly irregular, it may be supposed that they are
mainly substitution products. But reasoning along either of these lines is
of very doubtful value.
For instance, the rather persistent nature of the chert formations above
mentioned in the Lake Superior region might be taken as evidence that
they were mainly organic deposits in their present position. However, a
close study of this district leads to the conclusion that these formations by
substitution for carbonate now occupy the place of cherty iron-bearing
carbonates and silicates, and that they are mainly explained by segregation
«Clements, J. Morgan, and Smyth, H. L., with Bayley, W. S., and Van Hise, C. R., The Crystal
Falls iron-bearing district of Michigan: Mon. U.S. Geol. Survey, vol. 36, 1899, pp. 62, 177.
> Bain, H. F., Van Hise, C. R., and Adams, Geo. I., Preliminary report on the lead and zinc
deposits of the Ozark region: Twenty-second Ann. Rept. U.S. Geol. Survey, pt. 2, 1901, pp. 86-87, 129.
852 A TREATISE ON METAMORPHISM.
from the parts of the formation once above the plane of denudation. The
cherty formations of the Lake Superior region are usually steeply inclined,
and therefore descending water rapidly and easily segregates silica on a
large scale from material widely disseminated. Where such substitution
oceurs throughout the outcrop of a tilted formation it does not at all follow
that it has taken place throughout the formation. Indeed, in the majority
of the districts of the Lake Superior region it is probable that the segrega-
tion of chert extends to only a very limited depth. Where the chert is
thus segregated by descending waters the source of the material may still
be mainly chert of organic origin originally disseminated through the
carbonate.
Where there are rather extensive formations of chert in a horizontal
position, or nearly so, interstratified with beds of limestone or shale, the
transportation of silica from material once above the plane of denudation by
descending waters is not so likely to have been important. This is espe-
cially true of those cherts which are interstratified with shales, and therefore
are protected from the surface waters by impervious formations. Hence
such formations are more likely to be in large part the direct products of
organic precipitation. While bunchy deposits of chert having no great
lateral extent may often be produced by chemical precipitation, it is certain
that such deposits may also be produced by direct organic precipitation.
This is illustrated by the chert deposits of the Franciscan series of
California. These are regularly bedded deposits, locally of considerable
thickness. They are lenticular and have an extremely narrow lateral
extent. Often a deposit having considerable thickness disappears within a
short distance. Ifa bunchy character be regarded as evidence of chemical
origin it would be concluded that the Franciscan cherts are chemical
deposits, and this was Lawson’s conclusion.“ Yet Lawson states that these
cherts contain very numerous radiolaria; but this fact does not prevent
him from suggesting that they are mainly due to local precipitation, m the
bed of the ocean, of silica from silica-bearing springs. Lawson explains the
abundant radiolaria as a coincidence. He says, in these deposits ‘‘radio-
laria remains became embedded as they dropped to the bottom.” It
@Lawson, A. C., Sketch of the geology of the San Francisco Peninsula: Fifteenth Ann. Rept.
U. S. Geol. Survey, 1895, pp. 420-426.
> Lawson, cit., p. 426.
PEBBLE, GRAVEL, AND BOWLDER DEPOSITS. 853
seems to me that this is hardly a satisfactory explanation of the very
abundant radiolaria which compose a considerable proportion of the
deposits. If Hinde’s argument as to the organic origin of the cherts
which contam sponge spicules be correct, it probably follows that the
radiolarian cherts of the Franciscan series of California are also organic
deposits.
Another very interesting occurrence of chert is that of the whetstones
of Arkansas. These are associated with clay shales® rather than lime-
stones. They were regarded by Griswold as fragmental,’ and therefore
not belonging to the class of cherts at all, but to the novaculites. But
Rutley has shown that these rocks contain no evidence of clastic origin, and
have all the characteristics of cherts.’ He regards them as replacement
deposits of dolomite or dolomitic limestones. As this chert formation is
somewhat persistent, and is between shale beds which are rather impervious,
I am inclined to believe that it may be largely an organic precipitate,
although now completely recrystallized, so as to be composed of perfectly
fitting granules of quartz.
FRAGMENTAL CLASS.
PSEPHITE ORDER.
The psephite order includes pebble, gravel, and bowlder deposits,
conglomerates, schist-conglomerates, and gneiss-psephites. (PI. VIII.)
PEBBLE, GRAVEL, AND BOWLDER DEPOSITS.
The psephites form below bodies of water and also on the land.
The chief body of water which deposits psephites is, of course, the ocean,
although lakes and inland seas are by no means unimportant. The
psephites in standing bodies of water are mainly formed along shores,
where the wave action is vigorous, but to some extent they occur at the
mouths of streams which are rapid until they reach the standing bodies of
water. ‘The psephites formed on the land are the till deposits of glaciers,
especially the extensive formations of continental glaciers, and the coarse
@ Griswold, L. S., Whetstones and the novaculites of Arkansas: Ann. Rept. Geol. Sury. Arkansas
for 1890, vol. 3, 1892, pp. 205-206.
> Griswold, cit., pp. 168-194.
¢ Rutley, Frank, On the origin of certain novaculites and quartzites: Quart. Jour. Geol. Soc. Lon-
don, vol. 50, 1894, pp. 377-392.
854 A TREATISE ON METAMORPHISM.
deposits of streams. These stream deposits are especially well illustrated
in semiarid regions. For imstance, at the present time large masses of
psephites are found at the mouths of canyons adjacent to the mountains of
the Great Basin, southern California, ete.; and Davis, Emmons, and Cross
regard it as probable that other extensive psephite formations of the
Cordilleras are fluviate.”
The material composing the psephites is not at all, or but roughly,
assorted. Consequently, with the coarse material, there is a considerable
amount of rather fine material to which the term psammite is applicable, and
of still finer clayey material belonging to the pelite order. Commonly,
however, the major part of the pelitic material is abstracted from the
psephitic material by the water. This takes place to a greater degree in
those deposits laid down under water than in those laid down under air.
The material of the psephites is but poorly sorted mineralogically.
This is a necessary consequence of the large size of the fragments. The
majority of the larger fragments are composed of two or more minerals.
If the rock against which the waves are beating is granite, the pebbles
contain all the minerals of the granite. The same statement is true in
reference to all other classes of igneous and sedimentary rocks which are
not readily broken into fine débris. But in some instances the majority of
the pebbles me7y be composed of a single mineral. The most frequent
illustration of this is furnished by quartz. Finally, any of the original
igneous or sedimentary rocks may be in any stage of alteration, and such
complex mineral material constitute the pebbles, provided the alteration-is
not of such kind that the material breaks down into its individual mineral
particles.
It follows from the foregoing that the psephites have the greatest pos-
sible variation in mineral and chemical composition. An organic or chem-
ical precipitate or an indurated clastic rock may be a chief constituent,
quartz may be a chief constituent, any igneous rock may be a chief con-
stituent, any metamorphic rock may be a chief constituent, or any of these
various materials may be united in any proportion.
As rock masses, individual deposits of psephites are very common.
Ordinarily these masses are not extensive in area, although sometimes they
«Davis, W. M. (with discussion by 8S. F. Emmons and Whitman Cross), Continental deposits of
the Rocky Mountain region: Bull. Geol. Soc. America, vol. 11, 1900, pp. 596-604.
t
CONGLOMERATES. 8d:
~~
are. Generally they are of but moderate thickness. The psephite deposits
made by standing bodies of water are usually in narrow bands adjacent to
the shore, and, moreover, occur only locally along the shore. Where,
however, a body of water steadily transgresses over the land, as, for instance,
the ocean, a continuous formation of psephite over an extensive area may
be deposited. Such a formation, while continuous, is not of the same age
throughout. The earlier portion of a formation is usually buried under
other sediments by the time the later part of the formation is deposited.
Usually such psephites are not very thick formations—that is, hundreds of
meters—for, as the land subsides and the sea transgresses, the waves can
not transport the coarse material back to the areas where earlier psephites
were deposited; consequently they are buried under finer material. The
psephite deposits of continental glaciers may be very extensive and of
considerable thickness. But psephite deposits of this class found among
the older rocks are not very numerous. This may be partly due to their
destruction by later geological action. The psephite deposits of fluviate
origin, while usually of rather local extent, may be of considerable thick-
ness, often from one hundred to several hundred meters. How far deposits
of this origin are represented among the older formations which have been
buried by later deposits it is difficult to say.
CONGLOMERATES.
Pebble, gravel, and bowlder deposits, by consolidation, cementation,
and metasomatism, are transformed to conglomerates. (Pl. VIII, 4.) These
processes take place in the belt of cementation. The rocks here considered
are so coarse that consolidation due to pressure in -the belt of cementation
produces comparatively little effect, since the coarse fragments have com-
paratively few points of contact. The process of cementation is identical
with that of the psammites, described below. Since the psammites are
more important than the psephites, the process of cementation is more
fully discussed under that head. Because of the very great variety of
material composing the psephites upon which the ground waters are acting,
there may be a great variety of minerals deposited in the interstices. But
the most abundant cement is usually quartz. Next in importance to quartz
are iron oxide and the carbonates. In some cases hornblende, feldspar,
etc., may be of consequence.
856 A TREATISE ON METAMORPHISM.
The amount of material required to cement the psephites is usually
not nearly so great as that required to transform pure quartzose sands to
quartzite. The reason for this is that the material is of such varying sizes
that the spaces between the larger blocks are largely filled by finer
materials. Also the fine material varies greatly in size. Consequently the
amount of original pore space is frequently but a fraction of that of the
even-grained sandstones. But in some instances the amount of fine material
is not great, and the pore space is correspondingly large. Such deposits
are illustrated by the psephites at the mouths of canyons of the West, which
are so porous as to permit the absorption and transmission of vast quantities
of water.
During the process of cementation metasomatism may take place. As
already noted, there is great variety in the composition of the psephites,
and any rock-making mineral may be present. From these there may be
produced a great variety of secondary minerals—in fact, any of the minerals
mentioned as characteristic of the belt of cementation. (See pp. 621-627).
These minerals may be mingled with secondary minerals formed before
the material was built into a psephite. It is frequently difficult or
impossible to discriminate the alterations which occurred while the minerals
were in the primary rock from those which took place after the minerals
were in the secondary rock.
The question now arises as to where the processes of transformation of
pebble, sand, and bowlder deposits to conglomerates take place. May a
considerable part of the work be done for those rocks which are deposited
below water before they emerge from it, or is the work in all cases mainly
accomplished when the rocks are below land areas? It is, of course,
possible, and indeed probable, that cementation and metasomatism, the
important processes in induration, take place to some extent below the sea.
However, it is not thought that these processes are there important. One
reason for this belief is the certainty that circulation of the waters of the
ocean must be comparatively slow through the sediments at the bottom; and
it has been explained on pages 571-572, 866-868, that to dissolve and trans-
port a large quantity of mineral material requires a vigorous circulation.
A second reason for this belief is that while psephite deposits are still below
the sea they have above them no belt in which weathering is taking place,
and therefore no belt from which solutions steadily supply material for
METAMORPHOSED PSEPHITES. 857
cementation. But it should be stated that the psephites are the most
favorable of all mechanical deposits for alteration below the water. This
follows, first, from their coarseness, which is ordinarily such as to furnish
openings of supercapillary size in which the resistance to water circulation
is comparatively small; and, second, from the great variety of readily
alterable minerals which they contain The alteration of these minerals by
oxidation, hydration, and carbonation, resulting in increase of volume,
might supply considerable quantities of material to cement the interstices
of the rocks without additions of any material from an outside source, and
therefore not require vigorous circulation for its accomplishment. But, as
already stated, it is believed that the main work of induration is accom-
plished after the rocks have emerged from the sea and erosion has begun.
After arising from the sea, with differences of elevation, a vigorous under-
ground circulation is set up; material is steadily contributed from the belt
of weathering; consequently the processes of cementation and metasomatism
take place with comparative rapidity, and thus transform the unconsolidated
pebbles, gravels, and bowlders to conglomerates.
Where indurated conglomerates pass into the zone of anamorphism
they may be further altered by the reactions of that zone. Where the
conditions are mass-static the alterations do not obliterate previous textures
and structures, and therefore the rocks remain conglomerates. But recrys-
tallization of matrix and pebbles alike may take place on an extensive scale
by silication and dehydration, with the production of the heavy anhydrous
minerals characteristic of the zone of anamorphism.
SCHIST-CONGLOMERATE AND GNEISS-PSEPHITE, OR CONGLOMERATE-SCHIST AND PSEPHITE-GNEISS.
Where any of the previously described psephites—i. e., pebble, gravel,
and bowlder deposits, or conglomerates—pass into the zone of anamor-
phism and are there subjected to mass-mechanical action, recrystallization
and granulation, and the development of a schistose structure, take place
under the principles described in Chapter VIII. (See pp.685-696.) Because
of the great variety of minerals which may be present in the psephites, all
the minerals which develop in the lower zone under mass-dynamic action
may form, and they may be most intimately intermingled. In general, the
process of recrystallization and granulation is complete for the matrix at a
stage when the pebbles are still very distinct, although, of course, in such
858 A TREATISE ON METAMORPHISM.
cases they are usually flattened and more or less granulated and reerystal-
lized. At the same time a schistose structure develops in the matrix, and a
schist-conglomerate or conglomerate-schist is thus produced. (PI. VIII, B.)
Where the mass-mechanical movement is very severe the pebbles may
be flattened into laminz. Since material does not readily migrate in the
zone of rock flowage, each pebble passes into minerals which may be
derived from it. If there are different kinds of pebbles, it follows that
there are various layers composed of different combinations, or at least
different proportions, of the same minerals. Large bowlders may be
observed in different stages, from oval forms, through greatly elongated
and widened fragments, to forms in which the greatest diameters are several
times the normal. (PI. VIII, 4 and B.) Such a rock, when broken par-
allel to the direction of greatest flattening, appears to be a perfect schist
or gneiss, but at right angles to this direction still plainly shows its con-
glomeratic character. As the process goes on the pebbles and bowlders are
flattened more and more, until they become thin, lenticular masses, perhaps
scarcely thicker than cardboard, or even like sheets of paper of great size.
Wherever there was a quartz pebble there is apt to be a lamina composed
of a quartz aggregate. Wherever there was a granite pebble there is
usually a lamina in which mica has abundantly developed, and therefore
one composed of mica, quartz, and feldspar. In many cases in which the
process of transformation is nearly complete one may fail altogether to
recognize the laminze as pebbles in masses split parallel to the direction of
greatest elongation; but when a specimen is cut in a direction transverse to
this the pebbles distinctly appear. When the process has gone as far as this
the material of the pebbles becomes so mingled with that of the matrix that
it is difficult to exactly define the outlines of the fragments. Finally, as
the process continues, no evidence of the pebbles and bowlders is left. In
their places are thin laminze of material of a mineralogical character differ-
ent from that of the adjacent lamine.
At the same time the bowlders are being transformed the matrix is also
being recrystallized. At various steps of the process the matrix may be, in
turn, slate, schist, and foliated schist. Before the pebbles are destroyed
the folize of the matrix wind in and out, remaining approximately parallel
to the nearest pebble, which acts as a transmitter of force. In proportion
as the process continues, and the pebbles and bowlders approach oblitera-
U. §. GEOLOGICAL SURVEY MONOGRAPH XLVII_ PL. VIII
A. UNALTERED NEWARK CONGLOMERATE FROM VIRGINIA. AFTER KEITH.
B. SCHIST CONGLOMERATE FROM FELCH MOUNTAIN DISTRICT, MICHIGAN.
oh
at
jell
oi
METAMORPHOSED PSEPHITES. 859
tion, the mineral particles more nearly approach a parallel orientation,
instead of a part of them winding in and out about the more refractory
particlés. (See pp. 760-762.) Thus, by the granulation and recrystalliza-
tion of the pebbles and matrix alike, with the consequent development of
schistose and banded structures, a rock is produced to which the term
gneiss-psephite or psephite-gneiss is applicable. (See pp. 782-783.)
Such a rock may be composed in various proportions of any of the
minerals which form in the zone of anamorphism, as listed in Chapter V
(pp. 363-3864). This follows from the fact that the matrix and the pebbles of
the psephites may be derived from all kinds of rocks, from the most basic
to the most acid, from the volcanics and the plutonies, and from all varieties
of metamorphic rocks. Therefore the rocks may have chemical composi-
tions which vary from those of the original igneous rocks to those of mud,
in which certain elements, and especially alkalies and alkaline earths, have
been depleted. From the great variety of chemical compositions of the
psephites it follows that any of the minerals may develop in them which
are subsequently spoken of as forming in the schists and gneisses which
belong to the psammite and pelite orders. While there is a very great
variety of secondary minerals in the schists and gneisses of the psephites,
in any given case two or three of these minerals are apt to be preponderant;
and a given psephite-gneiss may be exactly designated by prefixing the
names of the chief mineral constituents, as explained on page 783.
A very characteristic feature of the gneiss-psephites is the parallel
orientation of the mineral particles, which gives the rocks cleavage. This
characteristic is almost, if not quite, universal with the schists and gneisses
which are derived by the metamorphism of sedimentary rocks, and thus
serves in many cases as a criterion by which one may separate the original.
gneisses of igneous origin from those produced by metamorphism.
Some of the best known illustrations of schist-conglomerates and
gneiss-psephites are those described by Hitchcock* many years ago, those
of the Hastings district of Canada,’ those of various places in the Lake
« Hitchcock, Edward, Hitchcock, Edward, jr., Hager, A. D., and Hitchcock, Charles H., Report on
the geology of Vermont, vol. 1, 1861, pp. 28-45. Whittle, C. L., The occurrence of Algonkian rocks
in Vermont, and the evidence for their subdivision: Jour. Geol., vol. 2, 1894, pp. 422429.
b Adams, F. D., and Barlow, A. E., On the origin and relations of the Grenville and Hastings
series in the Canadian Laurentian, with remarks by R. W. Ells: Am. Jour. Sci., 4th ser., vol. 3, 1897,
pp. 173-180.
860 A TREATISE ON METAMORPHISM.
Superior region, and those of Hoosac Mountain, western Massachusetts.’
In each of these localities, where the mashing was not so severe as the
average for the formation, the rocks are not metamorphosed beyond the
stage of schist-conglomerate, and thus give evidence that the gneiss-
psephite continuations of these rocks have been produced from sediments.
One of the very best illustrative localities is that of the Vermont
formation of Hoosae Mountain, described by Wolff. In this formation
every stage of gradation from a distinct and unmistakable schist-conglom-
erate to gneiss may be seen. Another excellent illustration which shows
intermediate stages between the conglomerate and the gneiss is that of
the Sturgeon River tongue of the Crystal Falls iron-bearing district of
Michigan.“ Here the matrix is completely recrystallized, and if it were
not for the pebbles would be unhesitatingly called a gneiss. But the
pebbles are in an intermediate stage of destruction, being considerably
flattened, though still very distinct. The upward continuation of this for-
mation, in which the fragments were small, shows no evidence whatever,
by texture or structure, of its original clastic character, either in the field
or under the microscope. It is a completely recrystallized quartz-feldspar
sand which has become a psammite-gneiss. (See pp. 875-876.)
PSAMMITE ORDER.
QUARTZ-SAND FAMILY.
The quartz-sand family (Pl. IX.) includes quartz-sand rock, sandstone,
quartzite, and schist-quartzite.
QUARTZ-SAND ROCK.
Quartz-sand rock is an unconsolidated deposit or formation composed
mainly of grains of quartz. Quartzose sands are one of the most common
and widespread of the mechanical sediments. The origin of such sands is
so well understood that little need be stated as to this part of the subject.
«Van Hise, C. R., and Bayley, W.8., The Marquette iron-bearing district of Michigan, with a
chapter on the Republic Trough, by H. L. Smyth: Mon. U.S. Geol. Survey, vol. 28, 1897, pp. 294-298,
434-437, 477-479. Clements, J. Morgan, and Smyth, H. L., with Bayley, W. 8., and Van Hise,
©. R., The Crystal Falls iron-bearing district of Michigan: Mon. U.§. Geol. Survey, vol. 36, 1899,
pp. 474-476.
>Pumpelly, Raphael, Wolff, J. E., and Dale, T. Nelson, Geology of the Green Mountains in Mas-
sachusetts: Mon. U. 8. Geol. Survey, vol. 23, 1894, pp. 48-59.
¢ Wolff, cit., pp. 48-59.
dMon. U. 8. Geol. Survey, vol. 36, cit., pp. 474-476.
QUARTZ-SAND ROCK. 861
Wherever coarse mechanical sediments are contributed to the sea by streams,
quartz-sand grains are present. Wherever the sea is at work upon the shore,
quartz sand is one of the products. At all places where the bodies of
water are of a large enough size to produce waves of considerable power—
that is, in large bays and along the border of the open ocean—the process
of mechanical sorting takes place, by means of which the quartz-sand grains
are separated from other minerals. This follows from the difference in
specific gravity, size, and shape between the grains of quartz and those of
other minerals. Not only are the quartz-sand grains sorted from feldspar,
hornblende, and similar minerals, but the quartz sands are sorted according
to size, as a result of which in one place the grains of sand are rather large,
perhaps averaging 1 to 5 millimeters in diameter, while in other places they
are very small, averaging as low as 0.1 to 0.05 millimeter. Of course there
are all gradations between deposits of different coarseness, and also between
pure quartzose sand deposits and those in which other minerals are impor-
tant or dominant.
However, the frequent almost perfect sorting of the quartz grains from
other material, and the arranging of the quartz grains of nearly uniform size
together, is a matter of constant surprise. Great formations of sand rock
are produced which have not more than 2 or 3 per cent of other minerals
besides quartz, and some which have not so much as 1 per cent of any other
mineral. Moreover, in some formations, the ratios of the diameters of but
few of the grains is as great as 1 to 2, and the majority of the grains do not
vary by as much as a third of a diameter. Both the advanced condition of
the sorting and the uniformity of the size of the grains are well illustrated
by the St. Peter sandstone of Wisconsin. Specimens of this sandstone
show but few particles of any other mineral than quartz.
Mr. Sydney H. Ball made for me a considerable number of measure-
ments of the grains of this sandstone from two different localities. From
one locality 25 grains were measured, which gave as the average greatest
diameter 0.3388 mm., and as the average least diameter 0.2964 mm. Of
these grains 21 have major diameters ranging between 0.20 and 0.50 mm.,
of which 14 are between 0.25 and 0.45 mm. Of the grains measured, 21
have minimum diameters between 0.20 and 0.50 mm., and of these, 14 are
between 0.25 and 0.45 mm. The maximum and minimum lengths and
breadths for individual grains are 0.54 and 0.16 mm., and 0.50 and 0.14 mm.,
862 A TREATISE ON METAMORPHISM.
respectively. From another locality 11 measurements of sandstone grains
gave an average greatest diameter of 0.2009 mm., and an average least
diameter of 0.1463 mm. At this locality all but 2 of the major diameters
are between 0.15 and 0.37 mm., and all but 2 of the minor diameters are
between 0.16 and 0.23 mm. The maximum and minimum lengths and
breadths of any individual grain are 0.37 and 0.10 mm., and 0.32 and 0.08
mm. ‘The above measurements for each of these rocks show a remarkable
approximation to a spherical form for most of the grains, and an almost
astonishing uniformity of size for the great majority of the grains, although
an occasional grain varies considerably from the average in size.
Besides sand formations which are deposited below the water, sand
formations are built up below the air. The most important of these are
dunes, the material of which is derived from the sands sorted by the water
along the shore. The wind picks up this sand, carries it inland, and thus
builds up along large bodies of water, especially such bodies as the Great
Lakes and the ocean, very considerable deposits which are similar in mate-
rial and arrangement to the sands deposited by water. Such sand forma-
tions are, however, more variable in thickness and more limited in extent
than sands deposited under the water. In the interior, especially in desert
regions, formations of quartz sands may occasionally be built up independ-
ently of water, but, in general, interior wind-deposited sands are not well
sorted, and consequently contain as abundant constituents other materials
than quartz. Therefore such deposits ordinarily do not belong in the quartz-
sand family, and they are considered on pages 877-878.
Since quartz is second in abundance only to feldspar as a constituent
of the original igneous rocks, since it is extensively manufactured from the
silicates by the processes of metamorphism, and since, unlike many other
minerals, it does not readily dissolve or unite as a solid with other minerals,
one would expect that very extensive formations of quartz sand would be
produced; and to such expectation the facts correspond. Almost every
inland lake is building up a belt of quartz sand of greater or less width along
the shore. The width of this belt is, of course, dependent upon the size of
the lake and upon the depth of the water. The greater the lake the wider
is the sand belt, the coarser sands being near the shore and the finer
ones farther from the shore. Gradual deepening of the water also is
QUARTZ-SAND ROCK. 865
favorable to a broad belt, for in that case the undertow is effective farther
from shore than where the waters are deep. The rule as to gradual increase
in depth applies to the ocean where the shore is swept by the full force of
the waves. In bays the building up of quartzose sands is largely dependent
upon the amount of material contributed to it. If, for instance, a great
river flows into a bay of moderate size, the amount of material of all kinds
is so great that the wave action is not sufficiently vigorous to sort the
material, and pure quartz-sand deposits are not formed. It appears proba-
ble that in the past the most extensive quartz-sand formations have been the
deposits of moderately shallow waters in great mediterranean seas, as, for
instance, the Cambrian sands of the interior basins of the United States.
These sands constituted thick deposits in the Appalachian Mountain system,
the southern part of Canada, and throughout the entire northern part of the
United States. In the western part of the United States almost equally
extensive sand formations have been produced In order that thick sand
formations shall be built up below the water, it is, of course, necessary that
subsidence shall take place concurrently with the deposition. In this respect
one rock formation does not differ from another.
If the sand grains be supposed to be of uniform size and spherical, and
to be arranged in the most compact system geometrically possible, the
space occupied by the sand grains is 74 per cent, and the space between
the grains is 26 per cent, of the entire space occupied by the formation.
(See Chapter III, p. 125.) As shown by Buckley,’ in the Dunnville sand
rocks, which have been partially indurated, the space is found to be as
great as 28 per cent. The explanation of this lies in the fact, discovered
by Professor Slichter, that under natural conditions sand grains are never
arranged in the most compact manner possible. Ordinarily this fact more
than compensates for the varyimg size of the grains and their imperfect
spherical shape (see p. 126); and it may be regarded as certain that the
quartz grains of well-sorted pure-sand formations, such as the St. Peter
sandstone of Wisconsin (see p. 861), occupy not more than two-thirds
to three-fourths of the space.
«Buckley, E. R., Building and ornamental stones of Wisconsin: Bull. Wisconsin Geol. and Nat
Hist. Sury. No. 4, 1898, p. 225.
864 A TREATISE ON METAMORPHISM.
SANDSTONE.
Sandstone is indurated quartz-sand rock. The induration is sufficient
to make the grains of sand rather strongly coherent, but not so great that
the grains break across when the rock is fractured; on the contrary, the
fractures follow the cement around the grains. The change of a sand
formation to a sandstone is due to consolidation and cementation. These
processes are characteristic of the belt of cementation. The process of con-
solidation by pressure is of relatively little importance in sandstones. Slich-
ter has shown that sands as naturally deposited by water are likely to have
a tolerably compact arrangement,” and that mechanical disturbance of such
sands is likely to result in a less compact arrangement. Sand grains, while
having many more points of contact than psephites, have few as compared
with the finer grained sediments. Doubtless the superincumbent pressure
does bring these grains into juxtaposition, and may possibly weld them to
some extent at the points of contact, but sands in which this process alone
has occurred are sure to be very weak.
The main process of induration is that of cementation. (See PI. IX,
A.) Silica is dissolved in the belt of weathering, either by the decomposi-
tion of silicates or by the solution of silica, either amorphous or ecrystal-
lized. The solutions join the sea of ground waters, and there the silica
is deposited upon the sand grains, usually in optical orientation with
them. ‘The result of this process is to partially fill the spaces between the
sand grains. These filling materials after a time interfere and thus cement
the sand grains together. In so far as they do not interfere, the added
material is likely to develop crystal faces, and thus there are produced
crystal-faceted sand grains, which together make up the sandstone. Sand-
stones the world over are mainly thus indurated, and ordinarily show
innumerable crystal faces when a specimen is held so that the light of the
sun may be reflected. While quartz is the dominant cement, iron oxide and
calcite are often important cements, and many other minerals are unimpor-
tant supplementary cements. Rocks indurated by cementation are usually
called sandstones from the time the cementation is sufficient to make the
erains weakly cohere to the stage in which the cementation is far enough
advanced to cause the rock to break across the sand grains rather than
around them. At this later stage they become quartzite.
“Slichter, C. 8., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept.
U. 8. Geol. Survey, pt. 2, 1899, p. 305.
QUARTZITE. 865
The question as to where the process of cementation from sand to
sandstone’ takes place is discussed under the next heading, ‘‘Quartzite,”
since the problem is precisely the same with both, and it can be better
understood atter the facts relating to the cementation of quartzites have
been considered.
QUARTZITE.
One of the most inportant rocks produced by the cementation process
is quartzite. (PI IX.) The term ‘‘quartzite” is here restricted to quartzose
sand rocks which have been so firmly indurated by the cementing processes
that when broken the fractures pass through the original grains and not
around them. The process of cementation continues, until the enlarged
sand grains or the independent quartz deposited interfere and interlock so
as to nearly or quite fill the interspaces. (Pl. IX, B.) In the production
of indurated sandstone and quartzite the quartz may be deposited either as
enlargements of old grains or as independent interstitial material. In either
case the induration may be so nearly perfect that a vitreous quartzite is
produced. If composed of very fine grains of sand, so that the indurated
rock may serve as a whetstone, the rock has been called novaculite. Novac-
ulite is therefore no more than an even- and fine-grained quartzite. In
general, there is a strong tendency for the quartz to deposit upon the old
grains rather than as independent mineral particles. The reason for the
deposition of the quartz from the solutions and the optical orientation of
this quartz with the original sand grains has been discussed on pages 75-76,
121-123. It may here be said in summary that the explanation lies in
the power of a mineral to abstract from solutions material like itself, for
large particles to grow rather than new ones to develop, and to the fact
that silica is one of the most abundant of the materials which are transported
by ground waters. Many of the cemented quartzites show little or no
evidence of strain, which proves that mechanical action has not been
potent in the induration. Neither is there any evidence that a high degree
of heat is requisite to dissolve the silica or to deposit it as quartz upon or
between the old grains. The sands from which quartzites develop contain
various impurities. During the transformation of the sands to quartzite
these impurities are rearranged to a greater or less extent by metasomatic
changes and new minerals develop, as described in the cases of the minerals
in the feldspathic sands and the grits. (See p. 870 et seq.) As with sand-
MON XLVII—O04 D5
866 A TREATISE ON METAMORPHISM.
stones, so in quartzites, iron oxides and calcite are subordinate cements, and
frequently other minerals to some extent perform a similar function. In
many parts of the continents the process of cementation has transformed
sandstone to quartzite for extensive areas, many such formations of widely
differing geological ages covering hundreds of thousands of square kilo-
meters. These quartzite formations vary in thickness in different places
from a few meters to as much as 3 or 4 kilometers.
It has been seen under ‘‘Quartz-sand rock” (p. 863) that ordinarily the
material of well-sorted, pure sand formations does not occupy more than
two-thirds to three-fourths of the space. This requires that in the cementa-
tion of asand to a quartzite from one-fourth to one-third of the formation be
added by the ground waters. It is therefore certain that the amount of
secondary silica required to indurate such great quartzite formations as
occur in the Paleozoic and pre-Paleozoic of the continent is enormous.
That required for vein fillings, while vast, is probably inconsiderable as
compared with this.
For instance, if a quartzite formation were supposed to have a volume
of 100 cubic kilometers, to change this rock from a sand to a quartzite would
require the addition of from 25 to 33 or more cubic kilometers of quartz.
It would be very interesting to make an estimate as to the amount of quartz
which has been added by the ground circulation to the quartzites which
are now known to exist, but the descriptions of quartzite formations are
not sufficiently accurate to admit of even an approximate estimate of their
volume. However, certainly many thousands, and probably hundreds of
thousands, of cubic kilometers of quartz must have been added by the
ground solutions in order to indurate these formations. It has been fully
explained on pages 480, 516-517 that the siliea liberated from the
silicates by the process of carbonation in the belt of weathering is entirely
adequate to account for much or all of the silica supplied by the ground
waters.
There remains to be considered the question where the process of
cementation of sand to sandstone and quartzite takes place. If the argu-
ment given on pages 856-857 in reference to the minor importance of the
cementation of psephites below the sea be correct, it is still more applicable
to the cementation of sands. Murray and Renard have estimated that sea
water contains in solution, on the average, only one part of silica in 250,000
QUARTZITE. 867
parts of water.* The openings between sand grains are of capillary size.
The circulation of the sea water through sandstone formations below the
sea where there is hydrostatic equilibrium must be exceedingly slow;
indeed, is probably negligible. In the case of sandstones there is no way
by which recrystallization may cement the material, as with the psephites,
which contain minerals capable of expansion reactions. The sand grains
are composed of quartz, and quartz is required for cementation. There is
no belt of weathering, the material of which is being continuously dissolved
and transported to the belt of cementation below. For the above reasons
it is thought that the cementation of sandstones, while still below the waters
in which they were deposited, is negligible. If this be true, the cementation
of sands must take place after they have been raised aboye the sea and an
underground circulation has been established. ven after these conditions
exist, and the conditions are favorable to a vigorous underground circula-
tion which steadily derives from the belt of weathering large quantities of
silica, cementation is still very slow.
It has been seen that the amount of silica required to indurate the
great quartzite formations is enormous. ‘The vastness of the geological
work of this kind probably gives us the best measure of the extent to which
ground waters have circulated and the enormous amount of water which
must have passed through these formations before the cementation is
complete.
The numerous analyses of mineral spring waters, more than ordinarily
rich in soluble compounds, show that such waters do not contain, on the
average, more than one part by weight of silica in 100,000 to one part in
10,000. This latter amount is exceeded by only a few wells or springs,
such as the Humboldt salt well.’ Supposing that the underground water
which passes into the belt of cementation steadily carries one part in 50,000
of silica by weight, and that ali of it is deposited, the filling of a pore space
amounting to one cubic kilometer would require the circulation of 50,000
times. 2.6, or 130,000, cubic kilometers of water.
Even for the cementation of a single formation, such as that of the
quartzite of the Penokee-Gogebic iron-bearing district of Michigan and
“Murray, John, and Renard, A. F., Deep-sea deposits: Report on the scientific results of the
voyage of H. M. S. Challenger, 1873-1876, London, 1891, pp. 286-288.
> Peale, A. C., Lists and analyses of the mineral springs of the United States: Bull. U. S. Geol.
Survey, No. 32, 1886, p. 159.
868 A TREATISE ON METAMORPHISM.
Wisconsin, where there is a quartzite formation 130 kilometers long and 120
meters wide, the amount would be enormous. Supposing the induration
to have extended only to the depth of 2 kilometers along the dip, the
amount of water which must have passed through this formation in order
to have cemented it would be 4,056,000 cubie kilometers. Thus the
cementation of this single formation required the circulation of almost
incredible quantities of ground water; and the addition of the almost indefi-
nitely greater quantity of quartz cementing the quartzite formations now
existing would require a correspondingly greater amount of water.
Even this point of view does not give a conception of the amount of
ground circulation required to cement the quartzites; for during geological
history probably a great many more quartzite formations than now exist
have been produced and destroyed by the processes of erosion. And in
the quartzites we have but a single phase of the work of ground water.
It therefore follows that the amount of ground water required to cement a
single formation is infinitesimal as compared with the amount which has
circulated through the rocks during geological ages. Indeed, it is probably
not too much to say that the same water has circulated through the
ground more than once, for it is doubtful whether the entire volume of the
water of the ocean is sufficient to have done the work of cementation and
metasomatism by use a single time.
SCHIST-QUARTZITE OR QUARTZITE-SCHIST.
A schist-quartzite or quartzite-schist is a quartzose rock derived from a
sandstone or quartzite which has a more or less well-developed schistose
structure. These rocks are produced in the zone of anamorphism under
mass-mechanical conditions. Granulation or recrystallization, or both, are
the dominant processes, but with these, weldmg and cementation are at
work. In order to produce a schist-quartzite it is not necessary to suppose
that cementation has filled the entire spaces between the sand grains; for
mashing, resulting in granulation, recrystallization, and welding may so
rearrange the material as to close the openings and lessen the volume of
the rock, with no added material from an outside source. Therefore it
can not be assumed that any considerable amount of material has been
added to schist-quartzites, unless evidence remains in the enlargements of
the quartz grains. Not infrequently in the schist-quartzites such evidence
may be seen. In the least mashed phases of schistose quartzites the quartz
METAMORPHOSED QUARTZITES. 869
grains may be but shghtly fractured. In passing to more mashed varieties
regular lines of fracture occasionally appear at right angles to each other.
As the mashing becomes more advanced, granulation begins. This first
affects the exteriors of the grains, but as the process continues the grains
may be granulated throughout. (Pl. IX, D.) Generally, however, before
granulation is far advanced recrystallization begins. As a result of this, in
the interstices new quartz and mica develop (the latter from the impurities)
and the recrystallization of the old grains of quartz occurs, as described on
page 865. Reerystallization checks, and finally reverses, the tendency of
granulation to produce finer and finer particles. Where recrystallization
becomes important, and especially where it becomes dominant, the mineral
particles may grow to a relatively large size. In proportion as a mass is
deep seated and the mashing is extreme there is a marked tendency to a
parallel dimensional arrangement of the newly. crystallized particles (see
pp. 688-689, 760), and the final process is a schist-quartzite or quartzite-
schist which shows no sign of fragmental origin. In many instances the
coarser quartzite-schists have particles averaging 7 mm. in length and
4 mm. in breadth. The common ratio between minor and major diameters, _
as ascertained by Leith, varies from 20:100 to 100:100, with an average of
perhaps 50:100.° vl
The schist-quartzites and quartzite-schists which are largely readjusted
by recrystallization beautifully illustrate the principle that this process
lags behind mechanical deformation. (See Chapter VIII, pp. 696-698.)
Residual strain effects, as shown by strain shadows, are very general.
Occasionally, however, the entire rock has largely recrystallized under
static conditions after mashing ceased, and there has been produced an
interlocking granular quartz rock in which the quartz particles have no
similar dimensional orientation and in which there is little residual strairi
effect. This peculiar variety is beautifully illustrated by the quartz rocks
of Rib and Mosinee hills, near Wausau, Wis. (PI. TX, C.) After complete
recrystallization has taken place, producing a granular quartz rock, when
the rock is again deformed granulation may occur. This further change
has locally taken place at Rib Hill. (See Pl. IX, D.)
In proportion as the sands, sandstones, and quartzites are impure the
schist-quartzites and quartzite-schists forming from them will be impure.
«Leith, C. K., Rock cleavage.
870 A TREATISE ON METAMORPHISM.
From the impurities are apt to develop some of the heavy minerals of the
zone of anamorphism. Of these minerals mica is by far of the greatest
consequence, and of the micas muscovite is the most abundant. The
secondary mica always shows a weil-developed parallel arrangement, with
its greater dimensions and readiest cleavage parallel to the greater dimen-
sions of the quartz. These mica-bearing rocks are schistose, micaceous
quartzites and micaceous quartzite-schists. While mica is the most frequent
of the minerals which form in connection with the schist-quartzites, other
minerals, such as tourmaline, actinolite, ete., may form, and if they become
important they may give mineralogical qualifiers to the names schist-
quartzite and quartzite-schist
In some districts all stages of gradation from quartzites, through schist-
quartzites and quartzite-schists, may be seen. These stages may be best
observed in a region not too strongly folded on the crests and in the troughs
of the folds. At such places there is comparatively little readjustment, while
on the limbs of the folds, especially between the beds, the differential
movements are much more marked. Therefore on the crests and in the
troughs of the folds and in the center of the beds the less changed rocks
are found, while at the outer parts of the beds and on the limbs of the folds
schists may be formed.
QUARTZ-FELDSPAR-SAND FAMILY.
The quartz-feldspar-sand family includes quartz-feldspar sands, arkose,
and schist-arkose or eneiss-arkose.
QUARTZ-FELDSPAR SAND.
Some fragmental formations are composed mainly of feldspar and
quartz, and such deposits may be called quartz-feldspar sands. ‘The most
favorable conditions for their formation are those of disintegration of acid
feldspathic rocks, with comparatively little decomposition, and contiguity
to the sea. (See Chapter VI, pp. 496-501.) The more prominent rocks
furnishing material for the quartz-feldspar sands are the granite and syenite
families among the igneous rocks, and the-acid gneisses and schists among
the metamorphic rocks. The volume of these rocks is very considerable,
and the acid feldspars, which are among their essential constituents, are
slow to decompose. It follows that the quartz-feldspar sands occur in
considerable volume. Feldspars from intermediate and basic rocks are in
Ws Sa
Ls
EU
= i
ris
5/
a
aan:
Pe eAC ae Ble Xe
PHOTOMICROGRAPHS OF SANDSTONE AND QUARTZITES.
A. Sandstone from Arlington, Columbia County, Wis., showing enlargement of quartz grains in
optical continuity, and crystal facets so common in sandstones which have been partly cemented.
B. Quartzite from Penokee-Gogebic district of Michigan and Wisconsin. The grains have been
thoroughly cemented by enlargement, haying grown by adding new quartz material to them-
selves in optical continuity until they met adjacent grains growing out in a similar manner.
The rock is a thoroughly crystalline quartzite.
C. Completely recrystallized quartzite in which all trace of rounded grains is lost. From Rib Hill,
Wis. The rock has been so thoroughly recrystallized that the quartz grains form an irregular
mosaic, like vein quartz or recrystallized chert.
D. The same, granulated. In the field, in hand specimens, and under the microscope all stages of
the mashing ofthe rock are to be observed. Each of the granules is a minute part of the quartz
individual from which it was derived.
872
U. S. GEOLOGICAL SURVEY
MONOGRAPH XLVII PL. IX
AND QUARTZITES.
PHOTOMICROGRAPHS OF SANDSTONES
QUARTZ-FELDSPAR SANDS. 873
some instances important supplementary sources of material. Quart and
acid feldspars have very nearly the same specific gravity, and unless such
material be gradually contributed to the water, so that it can be handled by
strong wayes and currents for a long time before final deposition, quartz
and acid feldspar will not be separated. The basic feldspar is more apt to
be separated. These conditions for imperfect separation especially prevail
in bays and eulfs where the full power of the ocean is not felt. The only
other important constituent of ordinary granite and of many of the acid
schists and gneisses, beside quartz and feldspar, is mica. This mineral,
because in flakes, is carried farther than the quartz and feldspar, and thus is
easily and rapidly separated from them and transported to deeper water.
Consequently, in the comparatively shallow waters along the shore many
nearly pure quartz-feldspar sands may be built up.
One of the best illustrations of the deposition of quartz-feldspar sands
at the present time is that in the Gulf of California, described by McGee."
The rocks of this region consist largely of granites and granitoid schists and
gneisses, although mingled with them are various igneous rocks and tufts.
The region is one of great aridity and rapidly varying temperature. The
small amount of rainfall is concentrated in thunder storms, in some cases of
exceptional violence. The result is that the rocks which are disintegrated
but not decomposed are transported by the process which McGee has
called sheet-flood erosion. (See Chapter VI, p. 497.) The undecomposed
material is piled up in the ravines of the mountains or is transported to the
gulf by storms of exceptional violence. ‘The waves of the gulf are at work
upon such disintegrated material and the salients of the solid rock. Thus
a large amount of material is furnished by the waves and streams, to be
distributed by the shore currents and undertow. The final result is that
the quartz and feldspar, with some mica, are deposited as a quartz-feldspar-
sand formation. McGee notes that where there are prominent salients the
quartz-feldspar sand is coarsest and cleanest, and that where there are
reentrants there is mingled with the coarse quartz and feldspar a considerable
proportion of finely comminuted materials.”
Doubtless this case of the formation of quartz-feldspar sands in the
Gulf of California points to the conditions under which similar sands were
«McGee, W J, The formation of arkose: Science, new ser., vol. 4, 1896, pp. 962-963.
bMcGee, cit., pp. 962-963.
874 A TREATISE ON METAMORPHISM.
deposited in the past, although other combinations of conditions may have
produced similar results. For instance, where the sea has rapidly trans-
eressed over a region of granitic rocks the material may be broken down
by wave aetion and incompletely assorted, thus producing quartz-feldspar
sands. Or great deposits formed in an interior basin may be overridden.
Thus if by subsidence the sea should advance over the Great Basin of
western United States (see Chapter VI, p. 559) at various places stratified
feldspathic sands similar in character to those of the Gulf of California
would be buried, since in this region there are various granitic and acid
schistic, gneissic, and granitic mountain ranges about which the fieldspar
and quartz débris of the mountains is being built up as stratified rocks by
the streams and ephemeral lakes and by sheet flood erosion. But from
such deposits neither mica nor any other mineral is more than imperfectly
separated. Therefore, unless the original rocks were enormously rich in
quartz and feldspar, such deposits are likely to contain with the quartz-
feldspar sands considerable quantities of ferromagnesian sands, considered
on pages 877-879.
ARKOSE.
Arkose is cemented quartz-feldspar sands. The arkoses are formed in
the belt of cementation. The cementing material is largely quartz, just as
with the quartz sands. Other cementing materials are calcite, ferrite, ete.,
and in some cases the cementing material is largely feldspar. This is well
illustrated by the arkoses of the Keweenawan of the Lake Superior region.
The rocks from which these arkoses were derived were the volcanic rocks
of the Lower Keweenawan. In the Keweenawan arkoses the cementing
material is mainly quartz, but this is occasionally supplemented by feldspar,
and in some instances where the arkose is a nearly pure feldspar rock the
feldspar interstitial material is the main cement. This feldspar is largely
added to the feldspar grains in optical continuity, precisely as is the quartz
to the quartz grains, thus enlarging them. Where the feldspars are twinned
plagioclases, the enlargements are twinned in similar fashion.” (See fig. 14,
p- 626.) These arkoses give one of the best illustrations of the capacity of
mineral particles to abstract from the solutions material like themselves and
add it to themselves. (See pp. 121-123.)
“Trying, R. D., and Van Hise, C. R., On secondary enlargements of mineral fragments: Bull.
U.S. Geol. Survey, No. 8, 1884, p. 46.
METAMORPHOSED ARKOSE. 875
The process of cementation may be partial, as with the sandstones, or
may continue until the material has practically filled all the interspaces, as
with quartzites, when the rock becomes completely indurated arkose.
Where cementation is very partial, the rock fractures around the grains
rather than across them, just as in a sandstone. Where cementation is
complete or nearly so, the fractures are across the grains rather than
around them, just as in quartzite, to which the rock is then analogous.
During cementation the feldspars in the arkoses may undergo meta-
somatic changes. ‘The dominant feldspars are orthoclase, microcline, and
the acid plagioclases. Any of the alteration products of these minerals
characteristic of the belt of cementation may form, but the most abundant
products of their alteration are quartz, chlorite, the zeolites, and the
epidotes.
Where by deep burying the arkoses pass into the zone of anamorphism,
the original feldspars or their alteration products are likely to recrystallize,
producing heavy anhydrous minerals. Under such circumstances the acid
feldspars are most likely to pass into quartz and mica. The alteration
products of the feldspars produced in the belt of cementation may
reproduce feldspar or may unite to form quartz and mica. In proportion
as the quartz-feldspar sands were impure, the heavy minerals are likely to
form. So long as the conditions are mass-static the original textures and
structures of the rock are likely to be preserved; but if in the deep-seated
zone the conditions are mass-mechanical, schist-arkoses or gneiss-arkoses,
considered under the next heading, are likely to form.
The induration of the arkoses is believed to be mainly accomplished
while the deposits are land areas. The reasons for this belief are the same
as in the case of the sandstones. (See pp. 866-867.)
SCHIST-ARKOSE AND GNEISS-ARKOSE, OR ARKOSE-SCHIST AND ARKOSE-GNEISS.
After the arkoses are formed and have become buried deep enough to be
in the zone of anamorphism, mass-mechanical action may take place. In
consequence of this a schistose or gneissose structure is developed, and the
rocks become schist-arkose and gneiss-arkose, or arkose-schist and arkose-
gneiss. There are, of course, all gradations between the rocks here con-
sidered and the arkoses. Where the mechanical action is rather weak,
a schistose structure is not marked. Where mass-mechanical action is
876 A TREATISE ON METAMORPHISM.
profound, a schistose structure is strongly developed. Where the original
sands were of the same general character, schists are likely to be produced ;
where the sands were in alternate bands, some of which were mainly
quartzose and others strongly feldspathic, a gneiss is likely to form.
Such a gneiss is beautifully illustrated in the Sturgeon River tongue of the
Crystal Falls district of Michigan, adjacent to the conglomerate-gneiss
already mentioned.” (See p. 860.)
During the transformation the quartz is granulated and recrystallized,
precisely as in the schistose quartzites. Since the feldspars of the arkoses
dominantly belong to the acid end of the series, they usually break up into
mica and quartz. So far as the feldspar is orthoclase or microcline, musco-
vite and quartz form. Where the arkose is impure, and magnesium and
iron are present in considerable quantity, biotite is likely to develop. At
the stage i: wuich the alteration of the feldspar is partial there is an inter-
locking .uxture of quartz, feldspar, and mica. In other words, at this
stage the rocks have a composition which, under the German usage of the
verm, would be called mica-gneiss. Where among the feldspars interme-
diate varieties, such as labradorite and bytownite, are present, simultane-
ously with the development of the other minerals, albite, oligoclase, and
andesine may form, but of these albite is by far the most prevalent. Thus
out of the old feldspar a great deal of new and usually acid feldspar is
produced. The development of new feldspar is particularly. likely to take
place waen adjacent igneous intrusions have occurred, especially granitic
intrusions. Doubtless the hot mass of granite promotes the formation of
solutions which carry abundant feldspathic material, and out of this, in part
at least, the new feldspar develops. (See pp.713-715.) In the schist-arkoses
the micas have well-developed parallel orientation, with their longer axes
aud cleavages in a common plane. The new feldspars may also have a
sinilar orientation. Such an arrangement is beautifully illustrated in the
Heosae schist, described by Wolff.’ In this rock, mainly composed of
quartz, mica, and feldspar, the latter mineral, mainly albite, is wholly
recrystallized and locally shows a very marked parallel orientation.
«Clements, J. Morgan, and Smyth, H. L., with Bayley, W. 8., and Van Hise, C. R., The Crystal
Falls iron-bearing district of Michigan: Mon. U. 8. Geol. Survey, vol. 36, 1899, pp. 463-464.
>Pumpelly, Raphael, Wolff, J. E., and Dale, T. Nelson, Geology of the Green Mountains in
Massachusetts: Mon. U. 8. Geol. Survey, vol. 23, 1894, pp. 59-63.
FERROMAGNESIAN SANDS. 817
PFERROMAGNESIAN-SAND FAMILY.
The ferromagnesian-sand family includes the ferromagnesian sands,
grits, graywacke, and slate-graywacke, schist-graywacke, or gneiss-
eraywacke.
FERROMAGNESIAN SANDS.
At many localities ferromagnesian sands are built up which are
mainly composed of quartz, the feldspars, and the ferromagnesian minerals.
With these dominant minerals there are subordinate quantities of various
other minerals—in fact, any of the minerals which are important as rock
constituents, with the possible exception of such readily decomposable
soda-bearing minerals as sodalite, nephelite, ete.
The conditions for the deposition of the ferromagnesian sands are those
of disintegration, with little decomposition. These conditions are fully
discussed on pages 496-501. It is here only necessary to say that
the conditions are substantially the same as those for the formation of the
feldspar-quartz sands, except that the rocks from which the minerals are
derived contain the ferromagnesian minerals abundantly. In proportion as
the climate is arid, in proportion as the material has to be transported only
a short distance, in proportion as it is coarse, in proportion as there is great
change of temperature by insolation in the warm regions and freezing and
thawing in the cold regions, the various igneous rocks are broken down.
Thus, sands composed of individual grains or aggregates of grains of the
same or unlike mineral composition are formed. The rocks from which
the ferromagnesian sands are derived must contain abundantly the ferro-
magnesian minerals. These ferromagnesian sands are deposited under
physical conditions very similar to those under which the quartz-feldspar
sands are deposited.
The formations may be built up on land or below water. Where built
on land in arid regions, thick deposits are likely to result, especially where
there is no drainage to the sea. Such deposits are illustrated in the Sahara
and in the Great Basin of western United States. The conditions for the
formation of sands in such districts have been fully discussed by Walther,’
«Walther, Johannes, Die Denudation in der Wiuste und ihre geologische Bedeutung, 8. Hirzel,
Leipzig, 1891, pp. 448-461. Walther, Johannes, Das Gesetz der Wistenbildung in Gegenwart und
Vorzeit, Dietrich Riemer, Berlin, 1900, pp. 31-52.
878 A TREATISE ON METAMORPHISM.
and have been alluded to in this treatise. (See Chapter VI, pp. 496-497.)
Nothing further will be said upon their manner of formation. The essential
point in this connection is that such sands are likely to be not at all, or
very slightly, sorted; therefore they contain practically all the minerals
of the original rocks from which they are derived, and hence include
the ferromagnesian minerals. While such formations are of very great
thickness in various parts of the world, and doubtless are now being
indurated in their lower parts, as described below, it is difficult to ascertain
how far the ferromagnesian sands, and their altered equivalents thus
formed, have been preserved among the rocks of past geological ages.
Ferromagnesian sands are being extensively deposited below the waters
of inland seas and of the ocean, where the sorting is very imperfect. This
is likely to be the case in the estuaries or near the mouths of strong rivers
which are rapid to their mouths. All these conditions are well illustrated
by some of the rivers which flow into the estuaries of the Atlantic, such as
the Susquehanna. ‘The conditions for the deposit of ferromagnesian sands
are still more nearly perfect along the Pacific coast. The rapid streams of
the Coast Range carry the extremely varied material of the mountains to
the bays, such as San Diego, San Francisco, and Puget Sound, or to the
ocean. But in the open ocean the ferromagnesian sands are likely to be
sorted into their constituent minerals. =
While the perfect conditions for the formation of the ferromagnesian
sands are those of disintegration with no decomposition, of course decompo-
sition does everywhere occur to a greater or less degree. In the case of the
rivers of the Piedmont plateau, which are emptying into estuaries, the
decomposition of the sands is considerably advanced. But the decomposi-
tion of the sands of the Coast Range is not nearly so far advanced.
Since decomposition everywhere occurs to some extent, the ferromag-
nesian minerals and the feldspars have been changed more or less to
minerals into which they commonly alter in the belts of weathering and
cementation. The feldspars may have been changed in part to kaolin, mica,
quartz, zeolites, etc. The pyroxenes, amphiboles, and biotite may have
been changed to a greater or less extent to chlorite, epidote, serpentine,
tale, ete. Thus in the sands under consideration there may be present not
only original minerals, but any of their alteration products in various
proportions. In so far as alterations have taken place the proportion of the
GRITS. 879
elements in the secondary rocks as compared with the original rocks is
likely to be changed. For instance, alumina may be relatively increased
in amount, and sodium greatly depleted, as compared with the potassium,
etc., as explained on page 507 et seq. his process of alteration is likely
to have proceeded far, in proportion as the material is fine grained. By
increase in amount of decomposition and decrease in size of grains, the
group of sands under consideration passes by gradation into the muds or
pelites. By decrease of the ferromagnesian minerals they pass into the
arkose sands. By decrease of the ferromagnesian and feldspathic minerals
they pass into the quartz sands. Therefore there are all gradations between
the ferromagnesian sands and the pelites, the quartz-feldspar sands, and the
quartz sands.
GRITS.
In the belt of cementation, by consolidation, cementation, and meta-
somatism the ferromagnesian sands pass into grits. The grits occupy the
same place in the family that sandstone and arkose bear in the quartz-
sand. and quartz-feldspar-sand families, respectively. In the induration of
the grits consolidation by pressure and welding is comparatively unimpor-
tant. The principles of cementation applicable are identical with those of
the sandstones and arkoses. A much greater number of cementing minerals
are important in the grits than in either the sandstones or the arkoses.
Where a ferromagnesian sand formation extends to the surface in the belt
of weathering there may be contributed to the underground waters all the
important elements which make up the great masses of the rocks. Also
im tke belt of cementation like diversified material inay be added to the
solutions by alteration of the minerals. Therefore, from the waters per-
colating through the ferromagnesian sands almost any of the common
rock-making minerals which develop in the belt of cementation may be
deposited. While this is true, the fact remains that the most abundant
cement is quartz. Next in importance to this, in all probability, is calcite,
and occupying a third place are the iron oxides. However, feldspar, horn-
blende, and other silicate minerals may be enlarged by each mineral
selecting from the solutions appropriate materials for this purpose, precisely
as with quartz. Also there may be deposited in the interstices such
minerals as the zeolites, epidote, and chlorite, precisely as in the vacuoles
of lavas, which have an equal variety of materials.
880 A TREATISE ON METAMORPHISM.
During cementation of the grits, metasomatism in the feldspars, ferro-
magnesian minerals, and constituents other than quartz is taking place,
This process is precisely the same as in the metasomatism of the gray-
wackes, next taken up; and since in these rocks the process is of greater
consequence, it will not be here considered further than to remark that the
expansion reactions which take place contribute material to the solutions
for cementation.
It is believed that the change of the ferromagnesian sands to grits is
mainly accomplished on the land areas. ‘The argument is the same as with
the graywackes. Since it can better be given after these rocks have been
considered, the statement on this point is deferred. (See p. 882.)
Grits of Carboniferous age are rather extensive formations in various
parts of the world. This is natural, since coal is known to have formed in
lagoons and partly inclosed seas, and it has already been noted that such
places, where the sorting power of the waves is small, are favorable ones
for the building up of the ferromagnesian sands. But the grits are not by
any means confined to the Carboniferous system; they occur extensively
in the Cretaceous, Tertiary, and other systems.
GRAYWACKE,
Graywackes are produced from grits in the belt of cementation.
Where the cementation is advanced so far that the rock when fractured
breaks across the original grains rather than around them the rock is a
oraywacke. Ger XA) Gray wacke, therefore, occupies the same posi-
tion in the family under consideration that quartzite and completely indu-
rated arkose occupy in the quartz-sand and quartz-feldspar-sand families.
Nothing further need be said of the process of cementation, since it is mainly
the completion of the work which cements a ferromagnesian sand into a
grit. While the process of cementation is going on metasomatic changes
may also be occurring in the body of the rock. (See Chapter VII, pp.
640-646.) The clastic particles may alter in many ways. To describe
them in detail would be to give a treatment of the alteration of minerals.
The final result of the alteration is a consolidated rock which may have a
very great variety of mineral constituents. One of the most characteristic
features of the graywackes is, however (as indicated by the name), their
gray color. While the new minerals may have something of a tendency
~
GRAYWACKE. 881
to similar orientation, they usually do not become regularly parallel. The
slight tendeney toward a parallel arrangement usually accords with bed-
ding, but in some cases it may not do so. The banded character of the
original rock is ordinarily preserved, even if all of the decomposable detrital
minerals are altered into new ones. Hach mineral may change into any of
the products which form in the belt of cementation, as listed. (See pp.
621-627.) To illustrate, the feldspars may alter into quartz, chlorite, the
zeolites, and the epidotes; the pyroxenes and amphiboles may alter into
quartz, serpentine, calcite, dolomite, magnesite, magnetite, and the epidotes;
the micas may alter into serpentine, chlorite, and quartz. These secondary
minerals vary from a subordinate to a dominant amount, and in some
instances but little if any of the original feldspars, pyroxenes, amphiboles,
and micas remain. But in general it is difficult to discriminate the meta-
somatic changes which take place in grits and graywackes from the partial
alterations of the minerals before the material was deposited as a ferro-
magnesian sand. Where a ferromagnesian mineral of a certain kind is very
abundant the alteration products from such a mineral are correspondingly
plentiful.
Secondary minerals which form from several important primary min-
erals are likely to be especially abundant; as, for instance, serpentine.
This mineral very often develops on a considerable scale in the graywackes.
Indeed, it sometimes becomes so prominent that the metamorphism of gray-
wackes has been called serpentinization. The secondary serpentine acts in
a manner similar to that of silica. It may occupy the crevices and cracks,
the spaces between the constituent minerals, and, finally, the place once
taken by other minerals. Exactly as in the case of silica, material for the
serpentine may be furnished in part or in whole by the minerals present, or
the material of the serpentine may come from an extraneous source, espe-
cially from the belt of weathering. Widespread formations may be exten-
sively serpentinized, so as to give for considerable areas almost solid masses
of serpentine. The serpentinized graywackes are especially well illustrated
in the Coast Range of California.* (Fig. 23.)
Excellent illustrations of the graywackes may be found in the Huronian
of the Lake Superior region, and perhaps the best illustrations of all are
« Becker, G. F., Geology of the quicksilver deposits of the Pacific Slope: Mon. U.S. Geol. Survey,
vol. 13, 1888, pp. 120-128.
MON XLVII—04
56
882 - A TREATISE ON METAMORPHISM.
the so-called slate-conglomerates of Logan in the Original Huronian area’
on the north shore of Lake Huron.* Here there are extensive formations
the matrix of which is typical graywacke, and in which are scattered frag-
ments, great and small, of the various rocks from which the finer débris
was derived. By an increase in the amount and size of the coarse material,
the graywackes pass into psephite-conglomerates. Almost equally good
illustrations of graywackes are locally found in the great slate formation of
the Penokee and Animikie series, although they are here not nearly so
extensive as north of Lake Huron. Geikie describes graywacke as a
rather common rock in Great Britain.”
The change from ferromagnesian sands to grits and graywaekes is
believed to take place in an important degree only when the deposits are
below land areas. The argument in this connection is essentially the same
as that applicable to the transformation of sands to sandstone and quartzite.
The openings of the rocks are capillary, and it can
not be presumed that in the rocks while below the
water there is any rapid circulation by which material
may be introduced. Indeed, there is no belt of
weathering available which can furnish such material.
ANS Bilin:
Fig. 23.—Grayw
ing serpentinization along the ferromagnesian sands. In the latter expansion
cracks. After Becker. o
ccke undemo. Dut there is one difference between quartzose sand and
reactions may occur while the rocks are below the
water. So far as metasomatism there takes place, the processes of hydra-
tion, carbonation, and oxidation, all producing increase of volume, would
provide material which could be deposited between the grains and thus
cement the rocks. To what extent this process has actually taken place
below the water must, for the present at least, be a matter of conjecture.
But, as first stated, it is believed that the major part of the cementation
and metasomatism of the graywackes occurred while the rocks were parts
of land areas, for there the conditions are favorable for cementation, viz,
vigorous circulation of the ground water and steady contribution of mate-
rial to the belt of cementation from the belt of weathering.
After the graywackes have become cemented by deep burying
they may pass into the zone of anamorphism. There metasomatic
changes may take place in the rocks, in consequence of which the heavy
“Logan, W. E., Geology of Canada, 1863: Montreal, 1863, pp. 50-54.
>Geikie, Archibald, Text-book of geology: Macmillan & Co., London, 1893, p. 132.
METAMORPHOSED GRAYWACKES. 883
minerals characteristic of that zone are formed. These minerals and their
manner of development are, however, identical with those of the graywacke-
schists and graywacke-gneisses next to be considered, and they will there-
fore not be here discussed.
SLATE-GRAYWACKE, SCHIST-GRAYWACKE, AND GNEYSS-GRAYWACKE; OR GRAYWACKE-SLATE, GRAYWACKE-
SCHIST, AND GRAYWACKE-GNEISS.
Where graywackes are buried deep enough to be in the zone of ana-
morphism and are subjected to mass-mechanical action, a schistose or
banded rock is produced, to which the terms slate-graywacke, schist-
graywacke, and gneiss-graywacke, or graywacke-slate, graywacke-schist,
and graywacke-gneiss, are applicable. Where the movement is moderate
the slate-graywackes or graywacke-slates are likely to form, but where the
mass-mechanical movements are severe schists and gneisses develop.
The rearrangement of the minerals under mass-mechanical conditions
combines the processes of recrystallization and granulation fully described
in Chapter VIII (pp. 673-675, 690-696). The minerals which form are of
course the heavy anhydrous minerals characteristic of the zone of anamor-
phism. The average composition of the minerals in a given case approxi-
mates that of the chemical composition of the original ferromagnesian
sands, except so far as they have been changed in composition in the belt
of anamorphism by dehydration and decarbonation or by injection
Since one of the essential conditions for the formation of the ferro-
magnesian sands is disintegration with but little decomposition, within these
rocks the various elements, including the alkalies and alkaline earths and
iron, are likely to approximate the average original proportions of the
rocks from which the sands were derived. It therefore follows that the
ferromagnesian sands are likely to be richer in alkalies, alkaline earths, and
iron than the quartz sands or even the quartz-feldspar sands. Since the
rocks may contain nearly a full quota of all the elements, almost any of
the minerals may form which are characteristic of the deep-seated zone,
with the exception of the minerals rich in soda, such as sodalite, nephelite,
ete., and the heavy aluminum silicate minerals. Appareritly there has
always been sufficient depletion in soda to prevent the development of the
former. As pointed out on page 900, the latter are likely to develop
only where there is an excess of aluminum beyond that required for the
formation of the minerals containing the alkalies and alkaline earths.
884 A TREATISE ON METAMORPHISM.
The original quartz is granulated and recrystallized, as in the other
varieties of sands. As the feldspars recrystallize, the micas are likely to
form abundantly, especially biotite, since iron and magnesium are plentiful.
(Pl. X, B.) But while the feldspar may in part pass into other minerals,
it in large part recrystallizes as feldspar, with perhaps a change in species.
Such new feldspar is apt to be an abundant constituent in the recrystallized
rocks. Frequently the ferromagnesian, minerals are so plentiful that the
amphiboles develop extensively. In the abundance of amphibole these
rocks differ from the schists and gneisses of the quartz-sand and quartz-
feldspar-sand families. The particular amphibole which develops in a given
formation depends, of course, upon the composition of the rock, but actino-
lite and common hornblende are very abundant. (Pl. X,CandD.) Rarely,
where the metamorphism is of a very extreme nature, the pyroxenes may
also develop to some extent, but any considerable quantity of these min-
erals is rather unusual. It is probable that the greater abundance of
amphibole is due, in part at least, to the fact that in this group of min-
erals the ratio between the calcium and the magnesium is 1:1, whereas
in the pyroxenes it is 3:1. So far as decomposition has taken place in
the original sands, the calcium is likely to have been extracted in greater
proportion than the magnesium (see pp. 515-516); therefore many of the
ferromagnesian sands may have been so depleted in calcium that pyroxenes
can not readily form. Associated with the more abundant minerals, the
heavy metamorphic minerals, such as garnet, staurolite, and tourmaline,
may appear. However, these minerals are not likely to form so abundantly
as in the pelite family, and the aluminum-silicate group of minerals, so com-
mon in the pelites, is rarely found. So far as hydrous minerals have been
produced in the belt of cementation, they are decomposed. The material
of the chlorites may largely go into the micas, amphiboles, garnets, stauro-
lite, and other minerals. The zeolites may pass into the feldspars, especially
into albite or oligoclase. In short, any of the combinations of hydrous
minerals which have been formed by the alteration of a heavy original min-
eral may recombine to reproduce the original heavy mineral; for instance,
serpentine, gibbsite, and kaolin may unite to produce biotite; kaolin and
quartz, or gibbsite and quartz, with sodium-bearing minerals, may unite to
produce albite; serpentine and quartz to produce enstatite, ete.
METAMORPHOSED GRAY WACKES. 885
The newly developed minerals show a marked tendency to similar
dimensional arrangement. This tendency is very strongly marked with the
micas, is important with the hornblendes, and applies to a considerable extent
to quartz and the feldspars. Corresponding with the dimensional arrange-
ment of the mica there is an eminent cleavage. There may be a cleavage
corresponding to the dimensional arrangement of the hornblendes and the
feldspars. This dimensional arrangement and mineral cleavage combined
give the rocks a slaty or schistose structure, depending upon the coarseness
of the mineral particles. (See pp. 778-779.)
Where the original ferromagnesian sands were banded in consequence
of alternation of coarse and fine material, or of alternation of different
combinations of the mineral constituents, this banding may be preserved
and thus a gneissic structure be produced in the recrystallized rocks.
Where there was a band in which quartz was very abundant, the laminz
are composed largely of quartz; where there was a quartz-feldspar band
there is likely to be a lamina composed of quartz, feldspar, and mica;
where there was a band in which were abundant ferromagnesian minerals
there are likely to be laminze in which hornblende, feldspar,*mica, and
quartz are all plentiful. Of course, the bands are not sharply separated
from one another, and there is likely to be subordinate amounts of all the
minerals which occur in the reck in each of the bands. The preservation
of the banded structure is due to the fact, fully explained in Chapter VIII
(pp. 764-766), that during the alterations which take place in the zone of
anamorphism migration of large quantities of material does not take place
for more than exceedingly short distances.
It will be seen later that the gneisses of igneous origin usually contain
hornblende. The sedimentary gneisses belonging to the family under
consideration usually contain hornblende, and therefore are more likely to
be confused with the gneisses of igneous origin than with the oneisses of
the quartz-sand or quartz-feldspar-sand families. (Pl. X, C.) However,
in many cases the average chemical composition of the rocks and the
arrangement and structures of minerals enable one to discriminate gneisses
of the family here considered from those derived from the igneous rocks.
But, apparently, where the sedimentary rocks are adjacent to great
intrusive igneous masses of hornblende-granite the sedimentary rocks may
886 A TREATISE ON METAMORPHISM.
gain hornblende in consequence of the exomorphic effect, and in that way
produce a graywacke-schist containing much hornblende and very similar
to hornblendic-schists of igneous origin. (See Pl. X, D.)
The graywacke-slates, graywacke-schists, and graywacke-gneisses, of
course, grade into the graywackes from which they are derived. Further,
they grade on the one hand into the arkose-schists and arkose-gneisses, and
on the other hand into the pelite-schists and pelite-gneisses.
PELITE ORDER.
MUD FAMILY.
Mud is the finest of the mechanical sediments. The greatest deposits
of mud form at the mouths of large rivers. Where very great rivers empty
into the sea deltas of immense thickness and size are built up of mud, as,
for instance, the delta of the Mississippi. Great deposits of muds are also
forming in the estuaries into which the rivers enter, such as the estuaries of
the Atlantic. These delta and estuary deposits, besides being extensive, are
of immense thickness. In mediterranean seas great deposits of mud may
be built up-as the products of many rivers. Thus Willis explains the Ham-
ilton shale of the Devonian of the Appalachian region, a formation extending
from New York to Maryland, and having a thickness of from 350 to 420
meters.” Also in the open sea, as a result of the process of sorting, the
muds are carried beyond the psammites, and so extensive formations of
mud of moderate thickness are built up.
Muds deposited in mediterranean seas or in the open ocean very fre-
quently are interstratified with or grade into the psammites toward the
shore. Seaward they frequently are interstratified, intermingled, or grade
into the calcareous deposits. To a less extent similar transitions are found
associated with delta and estuary deposits. Consequently we have deposits
intermediate between muds and sandstones, and between muds and lime-
stones. From the foregoing it follows that muds vary in composition from
that of the various psammites to that of the carbonates. The rocks inter-
mediate between the muds and the carbonates usually contain a large
percentage of calcium, magnesium, and carbon dioxide. ;
On the average the material of which mud is composed has been
«Willis, Bailey, Paleozoic Appalachia, or the history of Maryland during Paleozoic time: Mary-
land Geol. Survey, vol. 4, pt. 1, 1900, pp. 57-61.
PayAMIU Xe:
PHOTOMICROGRAPHS OF GRAY WACKES.
A. Graywacke from Penokee-Gogebic district of Michigan and Wisconsin. The orthoclase is in most
cases separable from the quartz, in that it lacks the perfect clearness and uniformity of color
which each grain of that mineral shows. The striated feldspars are nicely shown.
B. Biotite-graywacke from Mesabi district of Minnesota. This is the normal phase of Lower Huronian
graywacke, consisting of quartz and feldspar grains, mainly the former, very imperfectly rounded,
and a considerable amount of secondary biotite and muscovite or sericite. All the constituents
have a dimensional parallelism, and the micas have also a crystallographic parallelism. After
Leith.
C. The same, showing development of new hornblende near contact with intrusive granite. After
Leith.
D. The same, showing development of new hornblende to the practical exclusion of mica at contact
with intrusive granite. After Leith. 5
888
U. S. GEOLOGICAL SURVEY
MONOGRAPH XLVII PL. X
PHOTOMICROGRAPHS OF GRAYWACKES
COMPOSITION OF MUDS. 889
exposed to the weathering processes under more favorable circumstances
than any other mechanical deposit. Consequently mud is material in
which decomposition is farther advanced than in any of the previously
considered classes of mechanical sediments. It follows from this that
muds are likely to be deficient in the more readily soluble compounds.
Of these the alkalies stand first, and of the alkalies sodium is more largely
dissolved, since a large proportion of sodium in the original igneous rocks
occurs in minerals which are more readily decomposed than the minerals
which bear potassium—that is, sodium occurs largely in the nephelites,
sodalites, and basic feldspars, which, as shown on pages 252, 260, 292,
295-299, are readily soluble; whereas the great sources of potassium are
orthoclase and microcline, difficultly decomposable minerals. The mate-
rials are also apt to be depleted in calcium and magnesium, since the
alkaline earths are so readily soluble. (See analyses A, B, C, of shales in
Chapter XI, under ‘Composite analyses of sedimentary rocks.” The
depletion in calcium usually has gone farther than the depletion in mag-
nesium, since in the belt of weathering much of the magnesia is retained
in the serpentines and tales. The material may or may not be depleted
in iron. While aluminum and silica also have been dissolved in the belt
of weathering, the solution of these substances is less rapid than of the
others, and thus there is usually an increase in the relative amounts of
these elements. The relative rate of solution of the elements is more fully
considered on pages 507-518.
The processes which lead to the solution are those of carbonation,
hydration, and oxidation, so frequently mentioned. From the muds the
carbonates are mainly removed in solution, the hydroxides are removed
to some extent, and the highly oxidized iron largely remains. All the
foregoing facts clearly appear when the analyses of original igneous rocks
are compared with those of muds. I reproduce from Clarke the first
column of a table giving the average of analyses of igneous and crystalline
rocks, made up from 830 analyses from various parts of the world, which
may be regarded as closely approximating the composition of the original
material from which the muds must have been derived, except that a con-
siderable number of the 830 analyses are from metamorphic rocks, a portion
of which may be metamorphic sedimentary rocks. So far as there was
890 A TREATISE ON METAMORPHISM.
depletion in any of the elements of such metamorphic sedimentary rocks,
this would somewhat reduce the amount of these elements and correspond-
ingly increase the amounts of the other elements.
Average of analyses of 830 igneous and crystalline rocks.4
| Per cent. | Per cent.
|
SiO etree semen eecer enn | HOR TAC a | SAKE Ogee ees Se eer ee 02. 80
Wes Ogee SERS et bee [ies AH 2 see eee ae eee 1.52
wR eO5 rot s= ness | 226353) eli Omee a ate ators ee 60
He Oss eaaee Maser | 3.52 PIO Re Senet eee es 22
MeO eka Seca ees >
TO #86 Motal ace va 99, 22
CaO ee ME Sess 4.90
Nas OSes ees 3.55 |
Probably the best obtainable estimate of the composition of the muds
is the average of analyses of 78 Cenozoic, Mesozoic, and Paleozoic shales
given by Clarke. The individual shales are given weight proportional to
the mass of the formation, as nearly as this could be done. His results,
by combining the two composite analyses, one for 27 Mesozoic and Ceno-
zoic shales and the other for 51 Paleozoic shales, are as follows:
verage of analyses of 78 shales.”
Average of analyses of 78 shales.”
| | Percent. |] | Percent.
pas [Raise |
S1O sess hee ane os | 58. 38 Ps O geese sie aera 0.17
JAMS CQ vemaarsetliCa nate th bye T5SA PE SOs ven eben t cman Se eta | 65
Me; Og: cigsk eke eee ae nee ASO Sipe Clee aN ee rere ey ss ae Aces eae
Re@iac re sees sone eee 2.46 PS Raat et Siesta ates Ser bs reste HS
MeO) ssieeiaeeeeeeeere 2,45 MnOjs ass seescemenssee Trace
CaQecasaeestcee ese eee 3. 12 Basu ooasais Sekt ee 05
Na; Oi Bees oe acest cas 1.31 SLO: aes eS ae eae None
Ke Om as has cesar eee eee 38. 25 ip Oe cee NaS see heme Trace x
ET Of atoll 0 2 eer 1, 34 | Carbon (of organic origin), 81
Eee aboverll0e 2 ss5-= | 3. 68 Mota eee 100. 46
TiO; ee haces oreo 65
COp ea ite thee eer 2.64 || |
@Clarke, F. W., Analyses of rocks, laboratory of the U. 8. Geol. Survey, 1880-1899: Bull. U.
Geol. Survey No. 168, 1900, p. 14.
bClarke, cit., Bull. 168, p. 17,
va
ANALYSES OF SHALES AND CLAYS. 891
Probably the chief respect in which these shales differ in composition
from muds is that water is less abundant in them. To compare the above
analyses with the analyses of the original rocks, the water ought to be
omitted. Omitting the water, and increasing the other elements in propor-
tion, the analysis of shales would be as follows:
Analysis of shales on basis of 100 per cent excluding the water.
Per cent. | | Per cent.
SO GRC NAS Oy Sees eae 0.68
PANS Reena eases 16. 29 | Ol eeenacotadeececoonaued sececaacss
TREO eae A ee 4.24 Geast tis rerun Brera ae als |
Be Osh see ee wa 3 ee 2.59 | Min @) Asters sores aie een Trace
IMG Oise eet tea ata 2.58 RaQ nee ee seek oe teases 052
CaO east ae ase eee BER) I SSO) occ askeseas. cesta None
Nas OR Seem onisiao aoe Us6i)- lb IWYO) oc scconcaceeassose Trace
IKE Ose Sats RL 3.42 Carbone) veo a rene 85
1G se re Rae te Severe BS ——— sl
Le: g MG tale ee oe Moe | 100. 460
(QO) <2 ates egen eee 2.78 |
TES) fis ie at ee han 178 |
| Il |
For comparison there is inserted the average of 12 analyses of clays
and soils from Illinois, lowa, Minnesota, and Wisconsin.
Average of 12 analyses of clays and soils.®
| iT ]
Per cent. || | Percent.
|
|
SiO cia ey eee ae eae [eG SMO eeemauen er oee 0. 42
ATO ene eee he he 14.51 (COMES Were eae iiia 3.53
Hes Ope eaten stents 6. 25 | PIO: Sas eee seas eeeee . 09
He Ojeraaeeesine a sec tan Uo SNPS O ens pare ere eee | 08
Mig Oates ee marae 2299) | iC lees eae eee gees 02
CHORES pee B04 cies Ons sealers Satya nae 08
Nai ORS ae nese aac 1. 21 Carbon, organic. -------- 24 |
|
AO ESCH St eae eee 2.12 |
EO Total eos see 100. 04
Ele Omeerecee cee mere ena 8.41 |
|
aClarke, F. W., Analyses of rocks, laboratory of the U. 8. Geol. Survey, 1880-1899: Bull. U.S.
Geol. Survey No. 168, 1900, pp. 296-297.
892 A TREATISE ON METAMORPHISM.
The secondary minerals are important constituents or the muds. Of
these the most abundant are those which especially form in the belt of
weathering, as given on pages 518-521. With these there are, of course,
almost all the minerals which form in the belt of cementation, as given on
pages 621-627, and finally with both of these classes are residual unaltered
minerals, especially refractory ones, such as quartz, orthoclase and micro-
cline, albite, etc., and the heavy refractory minerals. Not infrequently
the quartz and feldspar constitute the major portion of the material. From
the foregoing it is clear that in proportion as alteration is advanced the
minerals forming the muds tend toward the very end products of alteration—
quartz, kaolin, tale, ferric oxide, and gibbsite. (See p. 520.) Some of the
clayey muds are composed almost wholly of quartz and kaolin, with some
tale and ferric oxide. It therefore appears that the muds have a wide
variety of composition, both chemical and mineral.
SHALE FAMILY.
By consolidation, metasomatism, and cementation in the belt of
cementation the muds pass into shales. Shale stands, in the pelite order, in
the same place in which sandstone, arkose, and grit stand in the psammite
order. ‘The processes of alteration are the same in all, but their relative
importance is very different in the pelites and psammites. In the induration
of the psammites consolidation is of subordinate importance, and cementa-
tion by material derived from an outside source is of the first importance.
In induration of the muds consolidation due to pressure is of the first impor-
tance. The particles are very small. According to Whitney, the finer
soils, which in reference to subdivision may be taken as approximately the
same as mud, contain from 10,000,000,000 to 20,000,000,000 particles per
gram.” Ina fine sand, such as the St. Peter sandstone of Wisconsin, the
number of particles per gram, as determined by Mr. 8. H. Ball, are about
115,000, and in a somewhat fine grained and better sorted beach sand about
167,000 particles per gram. Therefore in a mud, after the steady pressure
has squeezed out the water so as to bring the particles in contact with one
another, the number of points of contact are from 87,000 to 174,000 times
as great as in such a sandstone as the St. Peter, and from 60,000 to 120,000
«Whitney, Milton, Some physical properties of soils in their relation to moisture and crop
distribution: U. S. Dept. Agric., Weather Bureau Bull. No. 4, 1892, pp. 73-74.
ANALYSES OF SHALES AND CLAYS. 893
times as great as in the beach sand. Consequently, where the pressure is
sufficient to bring the particles, or a fair proportion of them, so close together
that they are within the limits of molecular attraction, welding is an
important factor in the coherence of the rock. It is well known that when
mud is subjected to very moderate pressure and the water can escape, this
is sufficient to make the particles cohere. There is therefore every reason
to believe that the weight of the superincumbent material accomplishes the
same thing for mud formations.
Metasomatic processes are important in the induration of the muds.
The mechanical water within the interstices of the muds and that liberated
by dehydration are agents through which changes may be made within the
mineral particles and between the adjacent mineral particles. That dehy-
dration occurs is shown by comparison of the analyses of slates and
shales with those of muds. It has been seen on pages 742-744 that the
water of the slates and shales is, on the average, about 50 per cent of that
of the muds.
Where rocks are so strongly hydrated as are the shales, some of the
vater is combined with the hydrous minerals which most readily part with
it, such as the zeolites. Under such conditions the moderate pressures
and temperatures which produce the shales are sufficient forces to begin
dehydration.
Where organic matter is present in muds—and this is very common—
the process of deoxidation is likely to begin. This is shown by the relative
proportions of the ferric and ferrous oxides in the muds and shales, as
exhibited in the analyses given on pages 890-891. It is seen that the ferric
oxide in the clays and soils averages 6.25 per cent, and in the shales is
reduced to 4.03 per cent; whereas the ferrous oxide in the clays and soils is
0.77 per cent and in the shales is increased to 2.46 per cent. These facts
accord with the statement in Chapter IV (p. 165; also see Chapter VII,
pp: 607-608) that deoxidation, while especially characteristic of the zone of
anamorphism, takes place in the belt of cementation where the conditions
are favorable. It thus appears that in the change from muds to shales we
have the beginning of reactions which for most rocks are not important
until they are deeply buried. Of the three reactions, hydration, oxidation,
and carbonation, which are characteristic of the zone of katamorphism for
the majority of rocks, two begin to be reversed for the pelites in the belt of
894 A TREATISE ON METAMORPHISM.
cementation. This simply means that the pelites begin to pass into the
zone of anamorphism at less depth than do other rocks.
Cementation is of consequence for the pelites, but not of such dominant
importance as in the psammites. The muds are practically impervious to
percolating waters, because the openings between the particles are sub-
capillary. (See Chapter II], p. 143.) Hence very little material can be
brought in from an outside source to fill the minute interspaces of the
particles of mud; but by the metasomatic processes within the minute
mineral particles the solutions obtain material which may be deposited
between the grains. As the water is squeezed out by pressure, not only
are the particles brought closer together, but as the water moves toward
the place of escape the pressure lessens and material from solution is
precipitated, and thus cements the particles. (See p. 114.)
The change of mud to shale is therefore largely a mechanical process
of consolidation, resulting in welding; but the chemical processes of meta-
somatism and cementation also are important.
Where the shales pass into the zone of anamorphism and the conditions
are mass-static, the processes of that zone are inaugurated. Dehydration
may be carried far. The silication of the subordinate amounts of carbo-
nates which are so generally present in the shales may take place. In the
shales there is very frequently organic material, and this results in deoxida-
tion. By metasomatism the heavy minerals develop. The rocks thus
altered may become compact, strong rocks, composed of anhydrous minerals.
The reactions for shales in the zone of anamorphism are those which pro-
duce the rocks next to be considered.
SLATE-PELITE, SCHIST-PELITE, AND GNEISS-PELITE,; OR PELITE-SLATE, PELITE-SCHIST, AND PELITE-GNEISS.
Where shales are buried so deep as to pass into the zone of anamor-
phism and mass-mechanical action takes place, slate-pelites, schist-pelites,
and gneiss-pelites, or pelite-slates, pelite-schists, and pelite-gneisses are
produced. Where the depth is not great and the mass-mechanical action is
not very severe slates are likely to form. Where the depth is greater and
the mass-mechanical action is severe schists or gneisses are likely to
develop. As the shales are very weak rocks, the depth at which the
reactions of the zone of anamorphism are inaugurated is not great. It is
certainly less, on the average, than for any of the previous families.
ANALYSES OF SLATES. 895
The rearrangement of the mineral particles under mass-mechanical
conditions is mainly that of reerystallization. The process of granulation,
which is so important in connection with recrystallization of the psephites
and psammites, is of relatively less consequence in the deep-seated metamor-
phism of slate, although by no means unimportant. The particles of the
original rocks are so small that they can readily slip over one another with-
out being broken finer, so that granulation is mainly restricted to the large
particles. The dominance of recrystallization is shown by the fact that
the moment the change from shale to slate, schist, or gneiss commenced
the small particles begin to merge and a coarser-grained rock is produced.
(G2 2XIb)
The alterations which take place of course come under the general
classes so frequently mentioned as characteristic of the zone of anamor-
phism—dehydration, silication, and deoxidation.
In order to get a better idea of the chemical change from mud to
shale and then to slate and schist, analyses of slates and schists are here
inserted. The average of 12 analyses of clays and soils is given on page
891. The average of composite analyses of 78 shales is given on page 890-
The following is an average of the analyses of 9 slates from Vermont:
Average of analyses of 9 slates from Vermont.”
Per cent. | Per cent.
SiO wememe iene a 61. 25 ADO See eae Ap a | 0. 83
IANIE @ Su ea yore aero! 16. 61 (CO) le save eas | 82
TRS (OBS Ss iets aes Sere ee 2.01 HOB Guat oubeteemaee 12
INSO) GSe So eas ose Oe Anes 4.83 WENO sae ses wes cee 14
IVIg OS lence ic 2. 80 BAO Beaks ebesey es eee iain 06
(OO Sige areata HOS He Ges sia alte Nay eet aans 22
Nay OR tend oe ARE, 154 I). Chopin Sosaccoso sone 05
ee Feats orc chase ot ana ae l Totaleeeasean sc as 100. 07
\eeEIeOratsl00S% Sores 27 |
aH O/above 10022552 3.57 |
The following is the average of 22 analyses of pelite-slates from
various localities in the United States, taken from the publications of
@Clarke, F. W., and Hillebrand, W. F., Analyses of rocks and analytical methods, U.S. Geol.
Survey, 1880-1896: Bull. U. 8. Geol. Survey No. 148, 1897, pp. 277-278.
896
A TREATISE ON METAMORPHISM.
the United States Geological Survey. Analyses were taken only from
specimens which were clearly of pelitic origin.
'
Average of analyses of 22 pelite-slates.
Per cent. Per cent.
|
SiO scale been fide ORs B00; elk ieiOpeenery anos 0. 044
INS © Siig Sore tae anes aed 16. 538 SOS esa ener . 025
IH OR a oesesueScaee seas Za Pet Ole ecessosassccossenase | IE.)
A IKEA O Jie ane ecs ra eran 3. 634 TB is aie erate oes lta ee Trace
Mie Oi kee he Saas sy a 2 OBS All Re ea Selecta leer sale None
Ca0 ee eo 10660 Io MnOe a ce. se | Trace
NaOneee ee 2ib66) (|lsBaQ ss esas ets ae eo
Gt 0 eR ee ere aie eo 3. 146 SrOM ee sass tones Trace
EL, Okatnlil 02S eSeee eee 533 Teg OE eising 2 aie ee einer 5 Trace
He OjabovestlO2 ss se5—- 3. 311 Is SaAaaoatose sea seesE 112
WMOs ase seers . 82 Cee i eee ieee siete 222
OO, ---------2-2-+-++--- 20 TLotalsescese ee eae “100.231 |
1 |
|
The following is the average of 5 analyses of pelite-schists from vari-
ous localities in the United States, taken from the publications of the
United States Geological Survey.
mens which were clearly of pelitic origin.
Analyses were taken only from speci-
Average of analyses of 5 pelite-schists.
Per cent. Per cent,
Sige ale wie sae @5.740 «|W PiOn scree anes ee 0. 122
ANE O gaan aimee eee 17348 NGS © gases eae eee . 032
Be: Ogee Rak Sena ee: 1896: xe sleeps ane ee Trace.
Hie @ epee ste ales earewas BABB Us| BeBe seers hapa ene pae eee . 068
Mio @ sates eae wi eet See F896 | [Sei Si ere meee eee oe eat een gers | ees retrd
CaO tect ee eas 1. 252 Vii ©) soo eee ee ea 034
Na One eee cu aerate 1. 784 Ba @ Ses Ne eee a lo eesae 046
KG O25 Bane Reece 3. 280 | STOR eae ae geo neers Trace.
EE OlatilOgne eee eee . 195 Li Og era ae aS Trace
He Osabovewl0e a oses. TEeseei Kaye [PON Obie peat enc crete ea eae oh . 582
Og oe errs be fase s LOtal Sees ene 99. 992
COS Ns Paes sersane None.
METAMORPHISM OF SHALES: 897
By comparing the analyses of the shales, slates, and schists it is seen
that the chief chemical changes are those of loss of water and of carbon
dioxide, and transformation of the ferric iron to the ferrous form. The
first of these changes is evidence of the process of dehydration, so char-
acteristic of the zone of anamorphism. It is notable, as pointed out in
Chapter VIII (p. 744), that in the change from clays and soils, which
certainly do not contain more water than muds, to shales and slates the
combined water is reduced by one-half, and that in the further transforma-
tion to schists the combined water is again reduced by one-half or one-third.
The disappearance of the carbon dioxide of the carbonates is due, of course,
to the process of silication, the silica uniting with the bases of the carbonates
and forming’ silicates, and the carbon dioxide slowly escaping with the
squeezed-out water. It has already been noted that deoxidation begins
in the change from muds to shales. The average amount of ferric iron
is further decreased as the process of metamorphism goes on, as shown
by the fact that in the shales the ferric oxide is 4.03 per cent, in the
slates is 2.726 per cent, and in the schists is 1.896 per cent. Correlative
with this process is increase of ferrous oxide from 2.46 per cent in the
shales to 3.634 per cent in the slates and 3.348 per cent in the schists. (See
pp. 890, 896.) In the analyses of shales from eight different States, given in
the publications of the United States Geological Survey, carbon is reported
in six of them. The carbon in the shales is an adequate reducing agent for
this work. While the amount of carbon in the shales is not great, it is per-
sistently present. The average amount of carbon in 78 shales is 0.81 per
cent.” (See p. 890.) It is to be remembered that this amount of carbon
is much more than sufficient to reduce all of the ferric iron to the ferrous
state, as is shown by the following equation:
2 Fe,0,;+C0=4 FeO-+CO,.
Since the molecular weight of two molecules of Fe,O, is 320, and the atomic
weight of carbon is 12, this equation means that 12 parts of carbon is
theoretically sufficient to reduce 320 parts of the ferric oxide to ferrous
oxide. It follows, therefore, that 0.81 per cent of carbon would theoret-
ically reduce 20 per cent of ferric iron to the ferrous state. Since the total
«Clarke, F. W., Analyses of rocks, laboratory of the U. S. Geol. Survey, 1880-1899: Bull. U. S.
Geol. Survey No. 168, 1900, p. 17, col. C.
MON XLVII—O4 57
898 A TREATISE ON METAMORPHISM.
amount of ferric oxide is 6.25 per cent in clays and soils and 4.03 per cent
in the shales,” it is clear that the small amount of carbon in the shales
furnishes, by its reducing action, an entirely adequate cause for the partial
change of the ferric to ferrous iron as metamorphism of the pelites con-
tinues from muds to shales, slates, schists, and gneisses.
The fact that, with the exceptions of the chemical changes mentioned,
the compositions of the pelitic gneisses, schists, and slates, where unmodified
by igneous intrusions, correspond with those of the shales and muds is
positive evidence of the general conclusion given on pages 145, 764-766,
that circulating ground waters are unable to transport material from an
outside source through rocks the particles of which are so small as to
furnish only subcapillary openings. Of this principle the pelite family
affords the best illustration.
The development of the minerals will first be considered at the stage
where the rocks are slates, and second where they have been transformed
to schists or gneisses.
Development of minerals of slates —Out of the constituents which are present in
the irresolvable background of the shales, innumerable flakes of mica and
small particles of quartz develop. The feldspar, with the addition of other
constituents, also largely alters into quartz and mica. (PI. XI, 4.) From
a single fragmental grain of feldspar are produced many individuals of
interlocking quartz and mica. (Pl. XI, Bb.) The minerals formed in the
interstices interlock with one another and with those produced from the
feldspar grains, so that an approximately uniform interlocking groundmass
of quartz and mica is produced. If fragmental grains of quartz are not
present to indicate the clastic character of the rock, the secondary inter-
locking quartz and mica do not betray this. The most abundant mica. is
usually biotite. Frequently muscovite accompanies the biotite. The
sources of the material for the micas of the slates are the same as for the
micas of the schists and gneisses. (See pp. 899-900.)
While in the ordinary slates the process of recrystallization as above
described is inaugurated, it is far from complete, and many of the original
mineral particles of the mud usually remain partly changed or wholly
unchanged.
@Clarke, cit., Bull. 168, p. 17, col. C.
METAMORPHISM OF SHALES. 899
Development of minerals of schists and gneisses—'The processes which result in the
formation of pelite-slate from shale when carried to completion produce
pelite-schist (Pl. XI, C’) or pelite-gneiss. All stages of the change may
be seen. We see, besides the replacement of the feldspar by quartz and
mica, the granulation and recrystallization of the quartz and the feldspar.
The process is essentially the same as the development of slate, except that
the destruction of the clastic quartz and feldspar grains is greatly promoted
by mashing. The process takes place in perfection in proportion as the
rock is fine grained. Thus a shale may completely change to pelite-schist,
while an interbedded coarse-grained grit may still reveal evidence 'of its
fragmental origi. In the same way a conglomerate may have its matrix
completely changed to a schist and the resistant pebbles be merely
deformed; but in the case of extreme alteration the most resistant and
largest pebbles and bowlders are entirely destroyed. (See pp. 858-859.)
In the typical schist-pelites and gneiss-pelites the chief minerals present
are quartz, muscovite, and biotite. (Pl. XI, C.) This is a natural con-
sequence of the composition of the shales. The orthoclase and microcline
feldspars, with the probable addition of alumina from the background,
in most cases recombine into quartz and muscovite. (See pp. 254-255.)
This uses up much of the potassa, alumina, and silica. Where magnesium
and iron are also available biotite is formed. (See p. 255.) Usually some
residual feldspar remains or is produced by recrystallization, and the
amount of this material is likely to be great in proportion as soda was
present in undecomposed plagioclase.
Since silica and alumina are abundant, an acid plagioclase is most
likely to form. Consequently albite is the most frequent plagioclase of
the schist-pelites and gneiss-pelites. If the amount of soda be considerable,
the albite may be so abundant as to be a chief constituent. Such an albite-
pelite-gneiss is beautifully illustrated by the Hoosac formation of Hoosae
Mountain, Massachusetts, described by Wolff.* (Pl XI, D.) In some
cases where the soda is unusually plentiful, a part of the soda with alumina
may pass into the soda-mica paragonite. Indeed, soda may more frequently
go into paragonite in the schist-pelites and gneiss-pelites than has been
supposed, for the close microscopical and chemical examinations of recent
«Pumpelly, Raphael, Wolff, J. E., and Dale, T. Nelson, Geology of the Green Mountains in
Massachusetts: Mon. U. S. Geol. Survey, vol. 28, 1894, pp. 59-64.
900 A TREATISE ON METAMORPHISM.
years have shown that the sericite of many of the so-called sericite-schists
is probably paragonite rather than muscovite; but the paragonite-schists
in many cases are not pelite-schists, but are derived from the alteration of
igneous rocks. As already seen, analyses of original muds show that, on
the average, the potassa is several times more abundant than soda, and this
fact explains why muscovite is so much more common in the slate-pelites,
schist-pelites, and gneiss-pelites than is paragonite.
While the greater part of the potassium probably passes into muscovite,
where the potassa is abundant a portion of it in many cases undoubtedly
passes into feldspar, producing orthoclase and microcline, since these
minerals are rather abundant in many of the schist-pelites and gneiss-
pelites. All the important elements which go to make up the muds are
fully accounted for in the minerals quartz, biotite, muscovite, paragonite,
albite, orthoclase, and microcline.
Where there is an excess of alumina beyond that required for the
formation of the minerals already mentioned, since silica is always in excess,
aluminum-silicate minerals form, and thus is produced the andalusite-
sillimanite-cyanite series. . The development of these minerals accounts for
a part of the material of the kaolin, which by dehydration and separation
of silica may pass directly into any of the aluminum-silicate minerals,
according to the reaction:
H,Al,Si,0, = Al,Si0, + SiO, + 2H,0,
with a decrease in volume of the quartz and aluminum-silicate mineral of
from 25.40 to 26.28 to 31.61 per cent, depending upon whether andalusite,
sillimanite, or cyanite, respectively, be formed. Ordinarily the andalusite,
having the lowest specific gravity, forms when the metamorphism is
moderate, as in the slates. Sillimanite, having a higher specific gravity, is
likely to form where the metamorphism is more intense; and where the
metamorphism is most intense the heaviest of the aluminum-silicate minerals,
cyanite, may develop. This fully conforms to the principle explained on
pages 363-365, 683-685, that in proportion as a rock is deeply buried and
metamorphism is profound, minerals of high specific gravity form. These
three minerals in the inverse order mentioned are described by Emerson
as occurring peripherally to great batholiths of granite in western Massa-
chusetts. (See pp. 717-718.)
uA Xt
12 byl 1d) SCI,
PHOTOMICROGRAPHS OF PELITES.
A. Slate from Mesabi district of Minnesota. This is the normal, fine-grained Virginia slate, consisting
of quartz and feldspar, chlorite, muscovite, biotite, and iron oxide, all the constituents arranged
with their longer diametérs roughly parallel, though the chlorite and iron oxide are arranged to
a less extent than the other constituents. After Leith.
B. Biotite-slate from Penokee-Gogebic district of Michigan and Wisconsin. Shows the alteration of
the feldspars to the micas. The crystalline character of the developing rock is well shown.
There remain, however, areas of feldspar which act as a unit, although they are cut in every
direction by the alteration products, quartz and mica.
C. Muscovite-biotite-quartz-schist, showing uniformity of texture. Black Hills, South Dakota.
D. Albite-gneiss from Hoosac Mountain, Massachusetts. Shows no trace of clastic origin. After
Wolff.
902
PL. XI
MONOGRAPH XLVI
U. S. GEOLOGICAL SURVEY
ROGRAPHS OF PELITES.
PHOTOMIC¢
i
%
a
METAMORPHISM OF SHALES. 903
Simultaneously with the development of the aluminum-silicate series,
garnets and staurolites may be produced. The garnet usually marks a
stage of alteration less advanced than the staurolite, and thus it is that very
- often schist-pelites and gneiss-pelites are garnetiferous and not staurolitic,
but in almost every case staurolitic schists and gneisses are garnetiferous.
The development of the garnets may use parts of any of the constituents,
calcium, magnesium, aluminum, iron, depending on the kind of garnet.
Common garnet, as explained in Chapter V (p. 302), is usually an isomor-
phous mixture of grossularite, pyrope, and almandite, and occasionally
melanite. The particular garnet which develops in a given case largely
depends upon the elements which are present in the original rock. In case
iron is rather plentiful and the metamorphism is intense, staurolite is also
likely to form. The development of these minerals disposes of a large
amount of iron and alumina, iron being derived from the oxide, and alumina
in many cases being derived from the kaolin or from gibbsite or diaspore.
If exceptional elements are present, as, for instance, boracic acid, tourmaline
and other exceptional minerals may form.
While in the majority of the schist-pelites and gneiss-pelites feldspar
is rather subordinate, in certain of them, as already noted, it is plentiful.
In proportion as the feldspar becomes abundant, the material from which
the rock was derived grades toward the quartz-feldspar-sand family of the
psammite order.
During the metamorphism of the shales to slates, schists, and gneisses
the new mineral particles which develop have a very strong tendency to
dimensional arrangement. Indeed, the pelite order is the one which shows
this dimensional arrangement to the best advantage. The rocks here
belonging are the types of rocks of this class. The mineral of most
fundamental importance showing the dimensional arrangement is, of
course, mica, although some of the other minerals which develop show a
marked dimensional arrangement. That the eminent cleavage of mica
corresponds with its dimensional arrangement has been frequently men-
tioned heretofore, and is dealt with in detail by Leith.* Consequent upon
the correspondence of the dimensional arrangement and cleavage of the
newly crystallized mica, the eminent cleavage of the slates, schists, and
4 Leith, C. K., Rock cleavage.
904. A TREATISE ON METAMORPHISM.
eneisses of the pelite order results, thus affording the best examples of
rock cleavage.
Where the shales are rather homogeneous in texture, slates and schists
form. Where, however, the original muds had alternations of coarse and
fine material, or alternations of bands of material of different chemical
compositions, these differences are apt to be maintained, to some extent at
least, in the metamorphosed rocks, and thus a banded gneissic structure
be produced. Each band of such a gneiss corresponds approximately in
chemical composition to that of the original rock, and is mainly composed
of the particular minerals, in the approximate proportions, which may be
produced from the band of mud.
Since the muds are the most extensive of mechanical sediments, it
naturally follows that there are great formations composed of their meta-
morphosed equivalents—slate-pelites, schist-pelites, and gneiss-pelites. As
illustrations of coarse pelite-schists having a very widespread occurrence
may be mentioned the Hudson schists of New England and New York, of
Silurian age.* Other equally good illustrations of very extensive schist-
pelite formations are those of the Upper Huronian formation of the Lake
Superior region.’ Many other illustrations might be mentioned, but it is
hardly necessary to take the space to do this.
IGNEOUS ROCKS.
As yet we have no classification of the igneous rocks to which there is
general agreement. In this respect the igneous rocks stand on a basis
different from that of the sediments. As no classification of the igneous
rocks is agreed upon, since petrographers themselves have not decided as to
which rocks shall have specific names, and as it is agreed that there are
«Merrill, F. J. H., Metamorphic crystalline rocks in New York City district: Geologic Atlas U.S.,
folio 83, U. 8. Geol. Survey, 1902, p. 4.
bTrving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin:
Mon. U. 8. Geol. Survey, vol. 19, 1892, pp. 296-345. Van Hise, C. R., and Bayley, W. 8., with
H. L. Smyth, The Marquette iron-bearing district of Michigan: Mon. U. 8. Geol. Survey, vol. 28,
1897, pp. 444-459. Clements, J. Morgan, and Smyth, H. L., with W. 8. Bayley and C. R. Van Hise,
The Crystal Falls iron-bearing district of Michigan: Mon. U.S. Geol. Survey, vol. 36, 1899, pp. 164-174.
Bayley, W. S., The Menominee iron-bearing district of Michigan: Mon. U. 8. Geol. Survey, vol. 46,
1903, pp..320-321, 462-488. Leith, C. K., The Mesabi iron-bearing district of Minnesota: Mon. U. 8.
Geol. Survey, vol. 48, 1903, pp. 168-177. Clements, J. Morgan, The Vermilion iron-bearing district of
Minnesota: Mon. U. 8. Geol. Survey, vol. 45, 1903, pp. 391-396.
CLASSIFICATION OF IGNEOUS ROCKS. 905
gradations between many kinds of rocks to which names have been assigned,
it is evident that it is impossible at the present time to trace out the stages
of alteration of the various rock species. All that is now possible is to indi-
cate how metamorphic igneous rocks may be handled in order to lead at
some future time to a systematic treatment similar to that applied in the
present treatise to the sedimentary rocks.
A series of names has been proposed (pp. 776-784) to designate the
different kinds of alterations to which rocks are subjected, and their usage
pointed out. But it seems desirable here to apply these usages to the igneous
rocks, even at the risk of repetition. If it is desirable only to express the
general fact of alteration, without indicating its character, the term ‘‘meta”
is prefixed. If the alterations have taken place under mass-static conditions,
so that textures and structures are preserved, the term ‘‘apo” is prefixed. If
the alterations have taken place under mass-mechanical conditions, so that
a slaty, schistose, or gneissose structure is produced, the terms schistic
or gneissic may be prefixed. If it is desired to emphasize the structure
rather than the kind of rock, the name of the rock may be placed before
the structural name, as gabbro-schist. Finally, the dominant metamorphic
minerals not implied by the name of the rock may be prefixed, as, for
instance, chlorite meta-dolerite. Another illustration is hornblende-dolerite.
This is the proper name for rocks which were originally dolerite but in
which the pyraxene has changed to amphibole. Such rocks occur exten-
sively in the Lake Superior region, where they have been generally called
diorite. But they do not ordinarily have the same chemical composition
as the diorites, in which the hornblende is original, nor is the feldspar
usually the same. If accompanying the alteration a secondary structure
is produced, the altered dolerite may become hornblende-schist-dolerite or
hornblende-dolerite-schist.
Each of the igneous rocks has a certain range in chemical composition
and is composed of certain important minerals. The alterations to which
the constituent minerals of rocks are subject in the zones of katamorphism
and anamorphism and in the belts of weathering and cementation of the
former zone have been fully given (see Chapter V), and the variations in
the alterations in each of these zones and belts have been fully discussed.
(Chapters IV, VI, VII, and VIII.) Supposing the zone and conditions to
906 A TREATISE ON METAMORPHISM.
be definite for a given rock, one can specify what alterations are likely to
occur, so far as each mineral is concerned, and also the alterations which
are likely to take place from a particular combination of minerals. It is
believed that this information will be of great service in a future systematic
study of the metamorphism of the various igneous rocks in the different
belts and zones of metamorphism, for it is a great advantage to have definite
suggestions as to what may occur. Such suggestions in effect ask the
question, when one is making observations, whether or not the facts of
alteration for each rock in the different zones and belts correspond to the
expectation. If they do the suggestions are confirmed, if not, advance has
still been made upon the problem as to the physical conditions under which
the alterations observed actually took place.
Once a fair degree of agreement is reached as to the orders, families,
and species of the original igneous rocks, it will be possible to trace out
their alterations in the various zones and belts in a manner somewhat similar
to that in which the alterations of the sedimentary rocks have been
imperfectly traced out. This, however, is plainly a work which will not
be accomplished for many years. In the meantime this chapter furnishes
guideboards telling how a great territory of petrography, only peered into
from the borders, may be occupied.
Cine EIN x
THE RELATIONS OF METAMORPHISM TO STRATIGRAPHY.
INTRODUCTORY.
It is sufficiently evident from the previous chapter that rocks which
have very similar lithological characters may have had very different
origins. This point can probably best be appreciated with the aid of a few
illustrations.
(a) It is well known that often it is difficult or impossible to discrim-
inate limestones of organic and chemical origin.
(b) Sometimes true water-deposited conglomerates and volcanic tufts
closely resemble each other, and often they grade into each other. This
last occurs wherever volcanoes are adjacent to the sea and the fragmental
material falls partly upon the land and partly in the water.
(c) A disintegrated and partly decomposed granite or other massive
rock in situ is often very similar to an arkose which is formed from it. For
instance, it has been pointed out that the arkoses which are forming in the
Gulf of California are practically identical in mineral and chemical char-
acter with the rocks from which they are formed. In western United
States at various places, even where there is very little alteration, in the
field it is difficult to delimit sharply the massive rocks, such as granite,
from the arkoses built up from and resting upon them.
(d) It is frequently difficult to determine whether igneous rocks are
tuffs or are lavas broken during flowage. Gradations between these two
classes of rocks occur where tuffaceous material falls upon unconsolidated
lavas.
If rocks comparatively little modified and of different origin may so
closely resemble one another that they are difficult to discriminate, it is
plain that after profound metamorphism they may be still nearer alike. In
907
908 A TREATISE ON METAMORPHISM.
fact, in many cases rocks of diverse origins have become so similar that as
yet no criteria have been devised by which they can be discriminated from
one another.
It has been fully explained that many of the minerals produced by
metamorphic processes are the same as those produced by crystallization
from a magma and have identical characters. It has further been seen that
during metamorphism secondary textures and structures are formed in
rocks of all kinds. Some of the secondary textures are enlargement, cata-
clastic, laminated, porphyritic. Some of the secondary structures are joint-
ing, faulting, bedding partings, cleavage, fissility, schistosity, gneissosity.
At the time these textures and structures are produced the original textures
and structures are usually obscured, and they are not infrequently obliter-
ated. And even if the original textures and structures are not greatly
modified they are often overlooked because of the prominence of the
secondary textures and structures.
Finally, where rocks of different origin but similar chemical composi-
tion are strongly metamorphosed with the production of secondary textures
and structures, it is scarcely possible to discriminate them, for, as has been
fully shown, upon the original chemical composition of the rocks and upon
the conditions of their alterations depends the character of the resultant
product. Two metamorphosed rocks of different origin but similar chemical
composition may have practically identical mineralogical, textural, and
structural characters.
DISCRIMINATION BETWEEN METAMORPHOSED SEDIMENTARY AND
METAMORPHOSED IGNEOUS ROCKS.
For the purposes of stratigraphy, it is very desirable that, so far as
possible, the criteria be given by which metamorphosed sedimentary rocks
may be discriminated from metamorphosed igneous rocks. So long as the
sedimentary rocks retain little modified clastic textures, either macroscopical
or microscopical, they can be discriminated as such. So long as the igneous
rocks retain the igneous textures, these may readily be recognized in thin see-
tions. Also, it has been fully explained on pages 644-645, 689-690, that,
in so far as the alterations are those of mass-static conditions, the textures and
structures are not rapidly destroyed, so that it is ordinarily easy to recognize
with the microscope the original textures of such rocks, even if complete
METAMORPHOSED SEDIMENTARY AND IGNEOUS ROCKS. 909
recrystallization has taken place. A very good illustration of this is the
ready recognition of the aporhyolites as volcanic rocks by the presence of
spherulitic and other igneous textures. Further, it has been pointed out
that where the conditions are mass-mechanical for rocks in the zone of frac-
ture, they are merely broken up into blocks or slices and that the textures
in the integral masses are preserved. Therefore, the origin of rocks altered
under mass-mechanical conditions in the zone of fracture may usually be
determined. Stated in another way, one may say that when the alterations
are of a kind to which the prefix apo is applicable, under the significance
given to it on pages 776-777, it is easy to discriminate between altered
igneous and altered sedimentary rocks.
Tn contrast with the above, it has been explained that where the rocks
are mashed in the zone of anamorphism previous textures and structures
are rapidly destroyed. This may go so far as to obliterate not only the
textures and minor structures, but even the major structures of the sedi-
mentary and igneous rocks, such as bedding and differentiation. Nat-
urally there are all grades of this process of destruction. It is only when
the mashing has gone so far as to have granulated and recrystallized the
rocks in the zone of anamorphism, and has thereby transformed them
to schists or gneisses, that original textures and structures are wholly
destroyed.
CASES OF CONFUSION.
The following are some of the classes of igneous and sedimentary
rocks which are most likely to be confused:
(1) The schistose and gneissic sedimentary rocks are particularly
likely to be confused with the schistose and gneissic tuffs. Both classes of
these rocks in their original form may have a lamination or banding which
is roughly similar; and if the alterations go far enough to destroy the
textures, the rocks frequently assume a very similar aspect.
(2) Metamorphosed sedimentary rocks derived from arkoses and gray-
wackes—i. e., derived from material in which the original minerals have
been but little sorted—are likely to be with difficulty discriminated from
metamorphosed igneous rocks. This follows, first, from the fact that the
chemical composition of such rocks does not necessarily vary from that of
some igneous rocks; and, second, from the fact that arkoses and gray-
wackes are frequently very coarsely banded or even massive, so that the
910 A TREATISE ON METAMORPHISM.
sediments have somewhat the same homogeneity of structure that igneous
rocks possess. Where there is any considerable sorting of the sedimentary
materials, so that they are arranged in layers of different composition, this
fact tends to prevent their change to forms which appear similar or identical
with metamorphosed igneous rocks.
(3) A third case which has occasioned difficulty is that of an igneous
rock which has been intruded by another igneous rock in a very compli-
cated fashion, so that the mtrusive contains very many fragments of the
intruded rock. These fragments are oftentimes partly absorbed, and thus
take on a pseudo-conglomeratic aspect. While it is comparatively easy in
most cases to separate such a rock from a true conglomerate when in the
unalterated condition, the difficulty of the discrimination increases in
proportion as the metamorphism is advanced. The difficulty may be
appreciated from the fact that cases are known where unmetamorphosed
pseudo-conglomerates of intrusion and true conglomerates which happen to
be side by side have been confused by geologists. In various cases included
blocks in gneissoid granites have been taken as evidence of the sedimentary
character of these rocks. This mistake has been made in reference to the
Cottonwood granite of the Wasatch, which contains rounded fragments, or
else segregations in situ, which have distinctly the aspects of bowlders.
Similar phenomena have led to like conclusions in the Lake Superior
region. In some instances the imtruded rock and included blocks are
schist. In such cases parallel injections are also likely to occur. Thus we
have a pseudo-conglomerate, below which are alternating layers of different
kinds. These phenomena in the Lake Superior region have led certain
geologists to the conclusion that the explanation of the phenomena is
downward progressive metamorphism; but it has been shown that the
phenomena are due to intrusion. These difficulties are greatly increased
where the rocks have been mashed subsequent to the intrusions.
(4) Another case in which it may be difficult to determine the origin
of the metamorphosed rocks is that of the porphyritic schists. By the
mashing of a porphyritic igneous rock the matrix may become schistose.
The phenocrysts may become rounded by the interior movement, and
closely resemble residual fragmental material. In many cases the porphy-
ritic schists of sedimentary origin are derived from material which contains
large grains or crystals in a fine-grained matrix. The matrix may be
METAMORPHOSED SEDIMENTARY AND IGNEOUS ROCKS. 911
recrystallized and the larger particles, being more resistant, only flattened.
Thus schists derived from porphyritic igneous rocks and from sediments
containing materials of varying coarseness may closely resemble each other.
Another way in which close resemblance between porphyritic schists
of sedimentary and of igneous origin may arise is the development of
porphyritic constituents during metamorphism. The new porphyritic
minerals may show random orientation if formed under mass-statie condi-
tions, or parallel orientation if formed under conditions of mashing. The
augen-gneisses are cases of this kind. For instance, the augen of one
gneiss may be the partly mashed original porphyritic feldspars of an
igneous rock, while those of another gneiss may be newly developed
porphyritic constituents in a metamorphosed sediment. In such eases it is
difficult or impossible to determine from the appearance of the rock whether
an augen-gneiss is of igneous or of sedimentary origin.
(5) After igneous rocks have been deformed in the zone of fracture
the openings may be filled by cementation. Subsequent mashing may
break the fillmgs into fragments, and shearing may give them roundish
forms which very closely simulate pebbles. This happens in more instances
with quartz veins than with other minerals. Such metamorphosed schists
bear numerous apparent pebbles of quartz, and are very difficult indeed to
discriminate from metamorphosed sedimentary rocks which contain true
waterworn pebbles of quartz.
(6) A final class of cases in which it is very difficult to determine what
part of a rock mass is igneous and what part is sedimentary is that in which
a sedimentary rock has been injected in a complicated fashion by one or
more igneous rocks, and later the combined mass is metamorphosed. Such
a schist is derived partly from a sedimentary and partly from an igneous
rock. Some cases are as follows:
A metamorphosed fissile sedimentary rock, such as mica-schist or mica-
gneiss, may be injected in a complicated way parallel to the schistosity, and
thus produce a banded gneiss, part of which is igneous and part sedimentary.
The rock may be predominantly of either one of these materials. If the
injected sedimentary rock be subsequently folded, this will produce differ-
ential movements parallel to the banding, and the igneous and aqueous
bands may be merged into one another and have structures so similar that
it is impossible to determine what part of the rock is igneous and what part
92 A TREATISE ON METAMORPHISM.
aqueous. The Hudson schists and gneisses of southeastern New York,
especially near Long Island Sound, are a perfect illustration of rocks
produced by the extreme metamorphism of a pelite and the subsequent
parallel and cross injection of granitic material.
Another case is that in which an intrusive rock enters a fractured
sedimentary rock in such a manner as to give the intrusive masses roundish
forms. When later metamorphosed the roundish forms are dissevered, and
they may resemble bowlders. The frequent inference is that the rock is a
metamorphosed conglomerate, whereas it is a metamorphosed sediment in
which the pseudo-bowlders are introduced igneous material.
From the foregoing it is clear that an imseparable schist or gneiss
formation may be produced from altered intrusive rocks, from altered lavas,
from altered tufts, from altered sediments, and from any possible com-
bination of two or more of these. Doubtless in many regions in which the
schists and gneisses are of a very complex character a number of the
processes mentioned in the previous pages, and possibly others unknown,
must be united in order to explain all of the phenomena.
While similar schists or gneisses may be derived from sedimentary
rocks or from igneous rocks, or partly from each, one who has had much
experience in the field is apt to have a bias toward the one or the other
origin in a particular instance. But if a given schist or gneiss formation
can at no place be traced into a form characteristic either of a sedimentary
rock or of an igneous rock, it is extremely hazardous to make positive
assertions as to its origin.
CRITERIA FOR DISCRIMINATION.
While the schistose igneous rocks may have foliations, bandings, and
lithological characters which very closely simulate those of metamorphosed
sedimentary rocks, it is still true of the rocks belonging to the Algonkian
and later periods that in many districts the majority of the metamorphosed
igneous rocks can be discriminated from the metamorphosed sedimentary
rocks.
The more important criteria for discrimination between the metamor-
phosed sedimentary and igneous rocks are as follows:
(1) The much-mashed igneous rocks are apt to have a very regular,
fine foliation and great uniformity of lithological character. If an igneous
METAMORPHOSED SEDIMENTARY AND IGNEOUS ROCKS. 913
mass gradually varies in composition in its different parts, as is so often the
case with differentiated magmas, the mashed varieties grade into one
another. The complete absence of minor plications, indicating the absence
of a parallel structure before the schistosity was produced, is most striking.
It is therefore concluded that the finely, regularly laminated, homogeneous
schists are usually of igneous origin. It should perhaps be remarked that
this is a reversal of the interpretation frequently made at the present time,
and which was almost universally made a few years ago.
The converse proposition, that the banded and heterogeneous schists
and gneisses are of sedimentary origin, can not be made; for the igneous
rocks, intrusive and extrusive, in a given area may have had an original
banded character, or may have had great complexity in their parts, as a
result of which their mashed equivalents show a banding. This is particu-
larly likely to be true of lavas and tuffs, and may be true even for intrusive
masses. Further, it has been seen that the mashed, originally homo-
geneous, igneous rocks may have become cleaved or fissile, and that along
the secondary structure impregnations or injections may occur and the rock
thus become banded. If a new mashing now takes place which develops a
tertiary structure cutting the secondary one, the minor plications of the
secondary structure may simulate similar plications of sedimentary strata.
(2) The metamorphosed sedimentary rocks are not usually so changed
as to obliterate all evidence of their original condition. This is true even
of the metamorphosed pelites. Minute variations in the coarseness of the
laminze are usually sufficient to produce minor plications which somewhere
will be found to intersect the secondary structure. Moreover, in a larger
way, thick pelite formations are apt to contain some beds of grit, sandstone,
“or limestone. These give schists of different kinds, and each of these is
different from the metamorphosed mudstone. :
Variations in original lithological character are still more prominent
when great formations are considered. It has been seen that a sandstone
formation may be transformed into a quartz-schist or a micaceous quartz-
schist. Limestone may be transformed into marble, bearing greater or
less quantities of silicates, depending largely upon its original purity.
The arkoses and shales may be transformed into schists and gneisses.
Widely disseminated graphite and the presence of ferruginous beds have
been regarded as indicating a sedimentary origin. There is no evidence
MON XLy1I—04—\58
914 A TREATISE ON METAMORPHISM.
that any transformations through which sedimentary rocks go are sufficient
to obliterate the original differences in thick formations of greatly varying
lithological character.
These principles are well illustrated by the Paleozoic metamorphosed
sedimentary rocks of the Green Mountains of New England. In this area
the Paleozoic rocks have almost completely lost all interior evidence of
clastic characters. It is only very rarely, where the rocks were conglom-
eratic, that any particles can be recognized as fragmental. However, the
shales and grits are transformed to mica-schists and mica-gneisses, the
sandstones to quartz-schists, and the limestones to marbles. The schists
are not evenly homogeneous and regularly schistose with a secondary
structure in a single direction, as is so characteristic of metamorphosed
igneous rocks. Each formation is composed of minor beds, these beds of
minor laminz. Notwithstanding the metamorphism, these retain to a
greater or less degree their integrity. At many places the bedded struc-
tures intersect the secondary schistose structures. In a larger way the rela-
tions of the formations to one another are those of sedimentary rather than
igneous rocks. No one skilled in work in the metamorphic rocks would
fail to recognize this Paleozoic series as of sedimentary origin. For the
most part these formations can be discriminated from the pre-Paleozoic
eneissoid granites.
In regions of extreme metamorphism, where both igneous and aqueous
rocks have had long and complex histories, including the development of
secondary and perhaps tertiary structures, and sometimes impregnations
and injections, no criterion as to texture or minor plication or regular band-
ing is sufficient to discriminate the two. Even in regions in which the great
formations may indicate with a high degree of probability that a consider-
able portion of the material is of sedimentary origin, it may be true that
another considerable portion is igneous and can not be discriminated from
the aqueous part of the series.
(3) A third criterion of great importance in the discrimination of meta-
morphosed sedimentary and igneous rocks is chemical composition. It has
been shown on pages 555-558 that the materials for sedimentary rocks
are sorted, that in general there is depletion in certain of the elements as
compared with the igneous rocks, and that the proportions of the elements
in the sedimentary rocks are therefore different from those in the igneous
METAMORPHOSED SEDIMENTS AND IGNEOUS ROCKS. 915
rocks. Furthermore, it has been shown that in the zone of anamorphism
the chemical composition of rocks is not greatly changed during the process
of metamorphism, and it has already been seen that this is the only zone in
which metamorphism is likely to result in the confusion of the two classes
of rocks. Therefore the metamorphosed sedimentary and igneous rocks
which are likely to be confused have the compositions which are charac-
teristic of their class: the metamorphosed sedimentary rocks, with minor
modifications, have the chemical composition of muds, grits, sandstones, ete.;
the metamorphosed igneous rocks have the compositions of granites, diorites,
etc. For both sedimentary and igneous rocks there are wide variations in
chemical composition, but in general the proportions of the elements are
markedly different, in the two classes, as may be seen by comparison of the
composition of the metamorphosed sedimentary rocks and that of the meta-
morphosed igneous rocks. The criterion has great value in some cases
where the criterion of banding fails, for instance, in discriminating between
metamorphosed sedimentary rocks and metamorphosed tuffs. The meta-
morphosed sediments have their characteristic compositious, while the
metamorphosed tufts, notwithstanding the fact that they may show banding,
and thus closely resemble metamorphosed sediments, have the composition
of igneous rocks. This criterion has been successfully used by Adams in
discriminating between the Grenville gneisses of the Original Laurentian
area and the lower granitoid gneiss. The former have the composition, as
he has shown, of pelites, whereas the lower gneisses have the composition
of granites.
While this criterion of chemical composition is one of the most impor-
tant and has a wide application, it is not infallible. It has already been
seen on pages 497, 870-874 that in such places as the Gulf of California,
and certain other localities, the sediments are little decomposed or sorted, and
thus there is produced a sedimentary rock which has essentially the same
composition as the original igneous rocks from which it was derived.
(4) Closely connected with (3), in fact dependent upon it, is a fourth
criterion, that of mineral composition. On account of the difference in
chemical composition which has already been mentioned, when the rocks
are metamorphosed there results different mineral compositions. It has
been seen that the depletion of certain elements in the sediments and
the sorting of the material produce comparatively few kinds of original
916 A TREATISE ON METAMORPHISM.
mechanical sediments. The alterations of these sediments have already
been traced out, and it has been seen that from them the common charac-
teristic rocks produced are quartzites, quartz-schists, mica-schists, and mica-
gneisses, which are occasionally more or less pyroxenic or amphibolitic,
but with these latter materials there is apt to be a considerable quantity
of caleareous material. Also, staurolite and andalusite, sillimanite and
cyanite are very characteristic minerals. Therefore, where certain single
minerals are dominant in the schists and gneisses it seems to be a fairly
safe conclusion that the rocks are sedimentary in origin. Thus, where
great rock formations are composed chiefly of quartz, chiefly of calcite or
other carbonate, or, with these, silicates which may readily develop by the
process of silication, it is comparatively safe to assume that the materials
from which the schists are derived are of sedimentary origin. Also, schists
composed almost wholly of mica and quartz are likely to be of sedimen-
tary origin.
Just as there are mineral combinations which are generally charac-
teristic of sedimentary rocks, so there are others which are generally
characteristic of the igneous rocks. From the tuffs and other rocks which
develop structures similar to those in the metamorphosed sediments, and
are therefore likely to be mistaken for metamorphosed sedimentary rocks,
hornblende is likely to develop instead of biotite and muscovite, and thus
hornblende-schists and hornblende-gneisses are produced from the metamor-
phism of the tufts instead of mica-schists and mica-gneisses. In the igneous
rocks with the hornblende, feldspar is apt to be abundantly present, and
frequently this feldspar may be near the orthosilicate end of the series.
In such rocks, also, there is likely to be present a considerable quantity of
titanium, which passes into menaccanite, leucoxene, or titanite. In the
igneous schists and gneisses there is likely also to be formed large amounts
of epidote and zoisite. The prominence of the various minerals mentioned
is due to the fact that the igneous rocks have not been depleted in alkalies
and alkaline earths.
But it can not be positively asserted that a metamorphic rock containing
metamorphic minerals especially characteristic of igneous rocks may not
be of sedimentary origin, for, as already noted, sediments exceptionally
contain the unsorted constituents of the original igneous rocks.
In discriminating between the metamorphosed sedimentary and meta-
VARIATION IN METAMORPHISM. BUG
morphosed igneous rocks it is plain that all the criteria above mentioned
should be applied. But where there are intricate mixtures of metamorphosed
igneous and sedimentary rocks, as a result of injection, all these criteria
may fail to a greater or less extent, for the rocks are not wholly sedimentary
or igneous, but a combination of the two.
It is therefore clear that there is frequently very great difficulty in
determining the origin of the schists and gneisses, and that the criteria
which are at present available are inadequate. However, it is believed
that a close study of the character of the changes in each of the well-
recognized igneous and sedimentary rocks will in the future furnish other
important guides which may be used to discriminate between the two great
classes of metamorphosed rocks.
RELATIONS OF METAMORPHIC SEDIMENTARY ROCKS TO STRATIG-
RAPHY.
VARIATION IN METAMORPHISM.
In working out the stratigraphy of a metamorphosed region the fact
that the metamorphism varies in degree under different conditions is of the
utmost importance.
UPON WHAT VARIATIONS ARE DEPENDENT.
The variation in the amount and kind of metamorphism is dependent
upon many factors (see pp. 89-44), but from the point of view of structure
it is necessary to consider only variations depending upon the amount of
orogenic movement, upon the character of the rock, and upon the depth
of burying.
That the degree of metamorphism is largely dependent upon orogenic
movement has been sufficiently shown on previous pages.
It has been seen on pages 760-762 that where rocks consist of alternate
layers of different character, these are slow to be obliterated, and especially
is this so if the formations be thick. To illustrate, it may be said that it is
very difficult, and perhaps impossible, to completely destroy a great lime-
stone or quartzite formation by metamorphism. Also, coarse and strong
sedimentary formations are likely to retain their original features to a
recognizable degree unless the metamorphism be extreme. The existence
of such formations enables one to determine the sedimentary character of
918 A TREATISE ON METAMORPHISM.
the rocks and often the true strike and dip. In a metamorphosed series the
contacts between formations of different character are the most reliable
criteria for correct structural observations.
It has been explained (Chapter IV, p. 187) that a formation in the zone
of katamorphism is merely fractured, instead of being mashed throughout,
and therefore if rocks are deformed under conditions of moderate burying
it is usually easy to determine the origin of the rocks with which one is
dealing. However, where rocks are deformed at a small depth, faulting is
likely to occur, and thus while the problem of stratigraphy is made easier,
in that the formations are not so greatly altered, it is made more difficult in
that faulting is always a possibility which has to be taken into account.
Rocks which were deeply buried when deformed are mashed, and
therefore under.these conditions the profoundest kind of metamorphism
occurs and all the difficulties which result from extreme metamorphism are
introduced. On the other hand, where mashing is the typical kind of
deformation faulting is not likely to occur. But it is to be remembered
that a series which is mashed before it reaches the surface must pass through
the upper zone, and during this passage may again be subjected to dynamic
moyements and thus be broken and faulted. Consequently, combined with
extreme metamorphism we may have also displacement, and in such cases
the difficulties of stratigraphy are at a maximum.
RESULTING VARIATIONS.
In handling the various difficulties m stratigraphy which result from
metamorphism the variations in amount and kind are of great assistance.
Such changes in the nature of metamorphism are to be considered both (1)
along the strike, and (2) across the strike.
(1) Aseries or formation which is profoundly metamorphosed in one part
of a district or region may be but slightly metamorphosed in another part.
The change in metamorphism is likely to be gradual along the strike. Con-
sequently, by following a formation along the strike, one may solve the
problem of the origin of completely metamorphosed formations, for some-
where he may find a locality where it is so little metamorphosed that its
original character may be determined. In the less altered area the true
succession may be made out, and thus it may be possible to interpret an
VARIATION IN METAMORPHISM. 919
extremely metamorphosed district or area by connecting it with areas which
are less metamorphosed.
These principlés are well illustrated at various places. The Penokee
series of the Lake Superior region is very little metamo:phosed in the east-
ern part, but is much more metamorphosed in the western part. A still more
striking illustration is found in the Marquette district. In the eastern and
central part of the district the rocks are so little metamorphosed that there
is no difficulty whatever in determining the origin of the formations. But in
the western part of the district, especially in the closely mashed Republic
tongue, the three unconformable series there existing—the Archean, the
Lower Huronian, and the Upper Huronian—are so mashed and metamor-
phosed as to appear completely conformable, and one might conclude that
here is an inseparable series. However, by tracing the formations along
the strike until they connect with the less metamorphosed portions in the
central and eastern parts of the district, one is certain that in this tongue
there exist three uncontformable series.
A third excellent illustration of the change of metamorphism along
the strike is furnished by the Ocoee series, which in its northeastern part, in
east Tennessee, is but semimetamorphosed, and there it is easy to recognize
the original character of the formations. As these formations are traced to
the southwest they become more and more metamorphosed, until they are
schists and gneisses, and then it is difficult to discriminate between the
sedimentary and igneous parts of the series and to separate the Ocoee
series from the older series unconformably below.
(2) Rapid change in metamorphism across the strike is frequently as
characteristic as is gradual change along the strike. Of course in some
cases the change across the strike is gradual, but in many cases it is very
abrupt. Thus the rocks on the crown of an arch or at the bottom of a
trough may be only partly metamorphosed, while the same formations on
the limbs of the folds may be profoundly metamorphosed. ‘This is due to
difference in the amount of shearing in different parts of the folds.* In
other cases the abrupt change in amount of metamorphism is due to the
fact that orogenic movement may die out rapidly across the strike or
change in its character. As a consequence, rocks on a hill may be folded
a@Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept.
U. &. Geol. Survey, pt. 1, 1896, pp. 598-600.
920 A TREATISE ON METAMORPHISM.
and in the next valley be unfolded. They may be very closely folded in
a valley and on the mountain beyond be in a horizontal position. The
rocks exposed may be strongly metamorphosed by mashing in a mountain
mass, because deeply buried during the movement, and along the adjacent
area merely be fractured, because in the zone of katamorphism during the
movement. Since the amount and kind of metamorphism are so directly
a function of the deformation and the depth, the rocks where greatly
deformed or deeply buried may be profoundly metamorphosed, and those
which are little deformed or near the surface may be comparatively little
altered. Failure to understand these principles has led to many mistakes
in stratigraphy. Some illustrations of the rapid change across the strike,
and of the mistakes in stratigraphy which have resulted, are as follows:
(a) The Taconic Mountains are immediately adjacent, across the
strike, to the Hudson River series of New York and Connecticut. The
latter formations are little metamorphosed. The rocks in the Taconic
Mountains are schists and gneisses. It was therefore concluded by
Emmons and by others that the rocks of the Taconic Mountains were
metamorphosed before the Hudson River rocks were deposited, and are
therefore much older. Hence Emmons gave the Taconic rocks the name
Taconic series. The entire Taconic controversy, which has cut such a
figure in America, arose from this mistake. It was only when Dana,
followed by Pumpelly and those who worked with him, appreciated that
there might be very rapid changes in metamorphism across the strike that
it was ascertained that the unmetamorphosed Hudson River rocks
immediately to the west of the completely metamorphosed schists and
eneisses of Greylock and the other Taconic Mountains are parts of the
same sedimentary formations.
(b) Another region which very well illustrates the rapid change in
metamorphism across the strike is that of the Great Valley and the Unaka
Mountains, in the southern Appalachians. The Unaka Mountains are
largely composed of the Ocoee series, which is of lower Paleozoic age, and
which in the sonthern part of the region is completely metamorphosed.
The metamorphism is much greater than in the rocks of the Great Valley
immediately adjacent, also Paleozoic. In the southern part of the region
the much metamorphosed Ocoee is thrust over the little altered rocks of the
Great Valley.
VARIATION IN METAMORPHISM. 921
It is plain from the foregoing that a series of rocks may be profoundly
metamorphosed locally and that its metamorphism in an adjacent area
may be unimportant. Not only is this so, but different parts of the same
fold are often metamorphosed differently, depending upon their positions
within the fold. Still further, one set of beds may be more readily
metamorphosed than another, so that one formation may be a perfect schist
while the adjacent formation may still be plainly fragmental; for instance,
a shale may be changed to a mica-schist and an interbedded quartzite be
but little affected by any process except cementation. As pegmatization
is so frequently dependent upon the presence of an intrusive rock, an
extreme phase of metamorphism engendered by volcanism, combined
with other processes, may have a very local character.
Notwithstanding the variations of metamorphism, the amount and char-
acter of the metamorphism of a series of formations may be an important
guide in structural work. It may serve to separate one series from another
and assist in determining the structure within a series. The criterion has
limitations, which are readily deducible from the facts as to the variation
in metamorphism just considered. But if a set of formations be superim-
posed upon another set and the lower has become schistose throughout, while
the upper shows little or no change, it is probable that the lower series
underwent a period of metamorphism before the upper series was deposited.
In order to make this conclusion at all certain, however, the superior for-
mations should be of kinds which are equally likely to be metamorphosed,
further faulting must be excluded, and it must be certain that it is a case of
superposition, for laterally the metamorphosing forces may die out rapidly
and the altered rocks pass quickly into those but little changed. Still fur-
ther, if the metamorphism be due to pegmatization as a consequence of the
intrusion of great batholiths, the process may die out rapidly in a vertical
direction, so that the extremely metamorphosed lower formations gradually
pass into the unmetamorphosed beds higher up, with no structural break.
Also, it has been seen that metamorphism is a direct function of depth of
burying. But probably the change in metamorphism as a result of varying
depth of burial is so slow that it ordinarily involves no practical difficulty
in separating series.
922 A TREATISE ON METAMORPHISM.
RELATIONS OF IGNEOUS ROCKS TO STRATIGRAPHY.
Before considering the relations of igneous rocks to stratigraphy it is
necessary to recall the forms which they usually take and the manner in
which they may be expected to die out. The intrusives are ordinarily
known as batholiths, bosses, laccoliths, sills, and dikes. The extrusives
are either lavas or tuffs. Frequently dikes radiate from a batholithic or
laccolithic mass, become less and less abundant in passing away from the
igneous mass, and finally disappear. Not infrequently the intrusives and
extrusives are closely associated, intrusives being in one part of a region
or district and equivalent extrusives in another part. But whether the
rocks are intrusive or extrusive, if the quantity of igneous material be
large and in various forms the igneous rocks usually do not die out at
once, but gradually. Ordinarily the distance required for their disappear-
ance is considerable, but in certain cases there are exceptions to this.
For the most part the ordinary masses of volcanic rocks do not
profoundly affect the beds with which they are associated. On the other
hand, large intrusive masses may metamorphose surrounding sediments
for miles. q
As a consequence of the gradual disappearance of eruptives under
ordinary circumstances, the abundance of eruptives in one horizon and
their absence in an adjacent horizon indicate that a structural break
probably exists between the two, or that a fault has displaced the strata.
The absence of phenomena which indicate faulting renders the first
alternative probable. The sudden disappearance of eruptives is a particu-
larly valuable criterion in separating adjacent series in cases in which
parallel secondary structures have been induced in both. These secondary
structures may have obliterated or nearly obliterated the original bedding,
so that there may be none of the ordinary evidences of structural breaks;
but if a set of dikes passes to a certain horizon and is suddenly cut off,
this is very suggestive of an erosion interval between the two series. In
cases of this kind the eruptives themselves are likely to have been
extensively altered by the forces which produced the secondary structures
in the sedimentary rocks; they may have been changed from their original
condition to schists, and then may be shown to be dikes or intrusives by
their structural behavior, by microscopical examination, or by chemical
analysis.
RELATIONS OF IGNEOUS ROCKS TO STRATIGRAPHY. 923
When a series is cut by intrusives it sometimes happens that there is a
difference in the erodibility of the dike and of the containing rock. In case
the dike be more resistant, and the two were subjected to eroding agencies,
it protrudes from the underlying formation, while in the opposite case there
is a hollow. The latter is the condition of affairs described by Pumpelly
at Hoosac Mountain, where an ancient dike cuts through the egneissoid
granite. After a hollow of differential erosion was formed a sedimentary
series was deposited. In approaching the dike the members of the sedi-
ments thicken somewhat and pass into the depression. ‘The greater thick-
ness of the lower beds at the hollow finally compensates for it, and the
thickened beds gradually pass up into ordinary regularly stratiform beds.
(Fig. 24.) In this case later orogenic movements produced consonant
secondary structures both in the granite and in the overlying formations,
and this relation of the dike to the two series was the first evidence found
Fic. 24.—Conglomerate deposited in depression produced by erosion of basic dike through gneiss. After Pumpelly.
C, conglomerate; c, lower layers of conglomerate rendered schistose by admixture of material from the altered dike; d,
diabase of the dike rendered schistose by metamorphism; é, altered dike material; g, pre-Cambrian granitoid gneiss.
of a structural break between them. They had hitherto been considered
as conformable, although a very close study had been made of them.
Later other evidence confirmed this inferred unconformity.
In a different case the contacts between two unconformable series
may not be found, but one series may be cut through and through by dikes
and contain bosses of igneous material and interstratified intrusives or
extrusives, with perhaps also volcanic fragmentals, while the adjacent set of
formations may be wholly free from igneous rocks. Relations of this kind
have force in propertion to the extensiveness of the phenomena. If one
series is rich in igneous rocks for many miles, while another contiguous
series is entirely free from them, and the irregular distribution of the two is
such as not to be explained by faulting, the evidence of a structural break
between the series is very strong.
In any of the above cases the time break between the two series must
‘
have been long enough for a full cycle of igneous activity.
924 A TREATISE ON METAMORPHISM.
In proportion as the igneous rocks are metamorphosed it is difficult to
apply the above criteria. Where they are no longer distinguishable from
the sedimentary rocks the criteria given for determining structural relations
are not available.
RELATIONS OF ROCK FLOWAGE TO MOUNTAIN MAKING.
The principles of metamorphism worked out in the previous chapters
have an important bearing upon theories of mountain making. Various
authors have advocated expansion theories of mountain making. Probably
the most notable of these is Reade. His explanation of the rise of moun-
tains is that the volume of the thick deposits of sediments increases in
consequence of the rise of the isogeotherms.“ It is perfectly clear from
the previous chapters that the alterations of the zone of anamorphism
result in very important contraction. Not only is this true of the deep-
seated zone, but in the zone of katamorphism the volume of one great
class of sediments, the pelites, is certainly decreased both by consolidation
and by dehydration. It may perhaps be conceded that the reactions of
metamorphism, if all of geological time be taken into account, have
somewhat expanded the volume of a very superficial belt of the earth.
But in the deep-seated zone, where pressure is dominant, the reactions are
taking place in a reverse sense, and as these have prevailed throughout
geological time and to unknown depths, it can hardly be doubted that
the contraction due to these deep-seated reactions is of vastly greater
importance than the sum total of expansion due to superficial causes.
Perhaps it will be well to recall the facts bearing on the amount of
contraction which takes place in consequence of the reactions of the zone
of anamorphism.
It has been seen on page 364 that the average specific gravity of the
minerals formed in the zone of anamorphism is about 18 per cent greater
than that of those which develop in the zone of katamorphism; but for
the rocks of the zone of anamorphism not all the minerals have passed to
the products characteristic of that zone. Therefore, one can not premise
that in the metamorphism of sedimentary rocks there is a diminution of
volume as great as this. However, it is probable that the average of the
chemical reactions of dehydration and silication and mechanical consolida-
“Reade, T. Mellard, Origin of mountain ranges: London, 1886, pp. 1-359.
RELATIONS OF ROCK FLOWAGE TO MOUNTAIN MAKING. 925
tion give a decrease in volume of at least 10 to 15 per cent for the majority
of the sediments. This is not believed to be an overestimate for the
average of the pelites, psephites, and limestones, but the decrease in
volume for the sandstones is probably not so great as this. It is therefore
certain that the decrease in volume in consequence of reactions controlled
by compression in the deep-seated zone is vastly greater than any expan-
sion which can result from rise in temperature. And it is certain that
from the top of the zone of anamorphism to the greatest depth to which
observation extends the reactions of metamorphism tend to produce min-
erals of higher specific gravity. However, the entire zone of observation
is very narrow as compared with the distance to the center of the earth.
Tf the law observed continues to great depth, it would follow that deeper
within the earth than observation extends minerals of specific gravity higher
than any known are produced. There is every reason to believe that the
density of the most densely crystallized minerals with which we are familiar
is far from the maximum possible density. Hence, as pointed out on pages
364-365, deep within the earth, where the pressures are vastly greater
than any with which we are familiar, it is certainly possible, and perhaps
probable, that there exists a class of heavy minerals of complex molecules
with which we are entirely unfamiliar.
but whether this speculation be well founded or not, there is every
reason to believe that during geological time there has been steady conden-
sation of the volume of the earth as a result of crystallization and recrys-
tallization. For eras at least the earth has steadily lost heat, and magma
has changed to crystallized rock, which has less volume. Probably also
the pressures within the earth have increased in amount in consequence of
decreasing rotation period, and increasing pressure promotes recrystalliza-
tion, with the formation of heavy minerals. It seems to me highly probable
that the decrease in volume due to the crystallization and recrystallization
of rocks, with the production of minerals of high specific gravity, is prob-
ably one of the most potent of the processes which have resulted in dimin-
ished volume.” If this be so, it appears that the sum total of metamorphic
processes is subordinate to the general law of gravity which steadily demands
decreasing volume for the earth. Recognizing this law, I have elsewhere
held that it can not be maintained that masses of the earth locally rise
«Van Hise, C. R., Estimates and causes of crustal shortening: Jour. Geol., vol. 6, 1898, pp. 59-60.
>
926 A TREATISE ON METAMORPHISM.
unless an equal or a larger mass elsewhere subsides—in short, that the
center of gravity of the moved mass is nearer the center of the earth after
the movement than before.*. The expansion theory of mountain making is
wholly controverted by the facts of metamorphism.
But mountain systems have been formed in some way, and during their
building the material of the mountains has been uplifted even if material
elsewhere has subsided. The uplifted areas are local. Indeed, it is be-
cause of localization that uplift can be recognized. As the geoid responds
to the stresses within it, at least near the surface, the folding and faulting
is largely concentrated in certain belts. It follows that the outer crust of the
earth must have moved in some way over the interior. This fact of the
localization of mountain masses and the resultant necessity that the crust
should shear over the interior is one of the reasons which led the early
geologists to adhere strongly to the existence of a liquid substratum.
However, the physicists who have studied the earth with reference to tidal
deformation. insist that its interior is rigid; that if it were a liquid the
attraction of the moon and sun would result in tides which would be shown
in the crust of the earth.
It appears to me that the explanation of rock flowage offered on
pages 748-759 shows how mountain masses may be segregated, the upper
crust of the earth shearing over the deeper seated material for a long
way without a liquid substratum. It has been explained that im the zone
of anamorphism, under the deforming stresses of the earth, rocks flow
by recrystallization. They are solid at the beginning of the process,
throughout the process, and at the end of the process; and yet they have
accommodated themselves to the form demanded by continuous solution
and redeposition. It has further been somewhat fully explained (pp.
769-774) that the energy required for such deformation in the zone of
flowage is not nearly so great as that required for deformation of an equal
mass producing a similar form in the zone of fracture. In other words, the
outer shell of the earth, the zone of fracture, is more rigid under slow
deforming stresses than is a deeper seated zone which may be deformed
by recrystallization. If this be so, we are able to see how the outer part
of the lithosphere may shear over the zone below, as if it were a plastic
aVan Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11, 1898,
pp. 512-514.
SEGREGATION OF MATERIALS OF MOUNTAINS. +. 927
stratum, and thus the necessary crustal shortening demanded for a large
segment of the earth may be concentrated in a single mountain mass.
This theory of the deformation of the zone of flowage by recrystallization
meets the needs of the geologist who demands a substratum sufficiently
plastic to be deformed under long-continued moderate stresses, and at the
same time meets the demands of the physicist for a material so strong as to
resist sudden stresses.
But it may be said that this hypothesis is a mere speculation,
unsupported by any observation and with no possibility of support by
observation. This is true if we consider the existing mountains, the last
uplift of many and perhaps the majority of which occurred in Tertiary
time. But before these mountains existed more ancient mountains existed,
and denudation since early geological time has exposed many parts of the
lithosphere over vast regions which were once thousands of meters below
the surface. For instance, the Appalachian system of the United States,
from Maine to Georgia, including the New England and Piedmont
plateaus, has undergone deep erosion since Paleozoic time. Vast regions
in Canada and in northern Europe have been subject to enormous, although
perhaps interrupted, erosion since Cambrian time. In such regions obser-
vations show that there now exist at the surface extensive areas of rock
which were deformed in the zone of rock flow.
Is there any evidence in such regions, from the character of the
deformation and the structures which have been produced, that shearing
has taken place in the same direction for considerable areas? Such
evidence would support the view that equivalent shearmg may have taken
place in the present zone of rock flow, and thus have segregated the
existing mountains. In another place I have shown that monoclinal
cleavage may result from shearing motion in rocks parallel to the surface
of the earth, there being no elongation or shortening in consequence of it.
This is illustrated by fig. 25, where a mass represented by abcd in the
diagram at the left is sheared into the form shown in the diagram at the
right, the lines a—b, c—d in both diagrams being supposed to be parallel to
the surface of the earth. The position of the cleavage is indicated by the
longer diameters of the flattened ellipsoids. If this sort of deformation
should take place over an extensive area in the zone of flow, it might
aVan Hise, C. R., Estimates and causes of crustal shortening: Jour. Geol., vol. 6, 1898, pp. 29-31.
928 A TREATISE ON METAMORPHISM.
transport the superjacent material of the zone of fracture with it, and thus
carry forward the upper crust of the earth toward the mountain mass and
allow the segregation of all of the superficial deformation in one belt.
It is to be noted that in the theoretical case illustrated by the diagram
there is no crustal shortening of the belt in the zone of fracture or of the
material of the zone of flowage. The material has merely changed its
position with reference to the surface of the earth
From the foregoing it is not to be supposed that monoclinal cleavage
has generally been produced without any plications or crustal shortenings.
Indeed, in areas in which monoclinal cleavages are prevalent there may
be numerous folds, as, for instance, in the New England region and in the
Blue Ridge area. No doubt in cases where the deformation is consider-
Fracture
Ce
QO en OO), Lm AG
Fic. 25.—Diagrams illustrating the manner in which deformation in the zone of flowage may concentrate crustal shorten-
ing in the zone of fracture.
Fracture
able there are all gradations, from very little or no crustal deformation of
the superjacent rocks to that of the complex kind which is characteristic
of the local zones of intense deformation. And even if the suggestion
be true in general, it is believed that in any instance there is likely to be
more or less of monoclinal folding in connection with the monoclinal cleay-
age, and consequently more or less of crustal shortening. However, it is
thought that the hypothesis affords a possible explanation of the concentra-
tion of the greater part of the crustal shortening in local zones.
The question now arises whether, in the regions of the earth in which
the past zone of flowage is now at the surface, monoclinal cleavage exists.
That such cleavage does exist for some extensive regions is well known.
For instance, the crystalline area of the Appalachian system from Maine to
Georgia shows a dominant easterly dipping cleavage.
SEGREGATION OF MATERIALS OF MOUNTAINS. 929
Where mountain systems exist with monoclinal folds or with thrust
faults the segregations may be explained by shearing motion in the zone of
flow over an extensive adjacent area to the east of the mountain mass in
case the monoclinal folds have eastward-dipping axial planes, and to the
west of the mountain mass in case the monoclinal folds have westward-
dipping axial planes. (Fig. 25.) Where the folds of a mountain system are
symmetrical it would be necessary to suppose that the shearing motion
toward the mountain mass occurred on both sides of it.
To illustrate, we may consider one of the large, simple anticlinoria,
such as those of the Park Ranges. We may suppose that such an anti-
clinorium was raised mainly in consequence of deep-seated flow toward the
mountain mass from either side. In this case the cleavage on either side
of the mountain would dip away from it. It is well known that the present
Park Ranges are comparatively recent uplifts along axes of old uplifts.
Some of the recent uplifts are partly or wholly covered by Paleozoic and
later strata. In the cases of these simple anticlinoria of the West the
cleavages of the old rocks on both sides of the anticlinal masses dip away
from the cores. Indeed, because of this fact, in the days when cleavage
was not discriminated from primary structures the cleavage was supposed
to be bedding, and upon the basis of this structure the mountains were
described as anticlmal. The present mountains we may properly describe
as anticlinal with reference to the Paleozoic and post-Paleozoic rocks which
rest upon their flanks. But smce we know that mountains rise again and
again along the old belts, the cleavage, dipping away from the cores of
these mountain masses, is very suggestive that when the ancient higher
mountains were developed the material was segregated in consequence of
deep-seated flow by recrystallization toward the mountain masses, there
being produced at this time the divergent cleavage existing in the pre-
_ Cambrian rocks which are now at the surface, but which were well below
the surface when the cleavage was produced.
The cores now exposed were covered by rocks deformed in the zone
of fracture and showed the deformation of joints, jomt folds, faults, ete.,
characteristic of the superficial zone. The deep-seated flowage extended
for an unknown distance on either side of the mountains, and thus brought
to the place of uplift the necessary material. The covering rocks belonging
to the zone of fracture on either side of the mountain range may be
59
MON XLVII—04
930 A TREATISE ON METAMORPHISM.
conceived as riding quietly forward toward the uplift without marked
deformation.
Applying this theory, we may imagine that the Kaibab Plateau, for
instance, on either side flanked by a monocline, represents an uplift which
is produced by deep-seated shear from either side toward the plateau,
producing simultaneously a cleavage or a tendency toward a cleavage in
the zone of flowage. Such cleavage would dip away from the plateau on
each side of it. The joint and fault deformations exhibited by the mono-
clines in the zone of fracture represent the equivalent necessary superficial
deformation.
Where the folds of a mountain range are symmetrical and upright it
would be necessary to suppose that there was an equal amount of deep-
seated shearing on each side toward the mountain mass. Where the axial
planes of the structures of the mountain masses tend to dip in one direc-
tion more than in another it is to be supposed that the preponderant
movement came from the direction in which the axial planes dip.
Possibly the suggestion of recrystallization shearing in the zone of rock
flowage also has a bearing upon the importance of the tidal forces in
producing deformation. It has been noted that in America the eastward-
dipping monoclinal cleavage tends to preponderate over the westward-
dipping cleavage. The same is true for monoclinal folding. This is
especially marked for the eastern United States and Canada. How far it
is true over the extensive areas of pre-Cambrian rocks of central Canada
is uncertain. Whether in other continents there is a preponderance of
eastward-dipping cleavage over westward, and of monoclinal folding with
eastward-dipping axial planes over the reverse, I do not know and have
been unable to ascertain; but it is interesting to note that if these easterly
structures are preponderant they are in the right direction to be genetic-
ally connected with tidal friction. The earth in its rotation upon its axis
from west to east produces a westward-moving tide. The same force which
acts upon the water acts upon the rocks below, or, in other words, tends
to slip the outer shell of the earth over the deeper shells. Such slipping,
if it takes place through the processes of recrystallization in the zone of
rock flowage, would produce an eastward-dipping monoclinal cleavage and
monoclinal folds with eastward-dipping axial planes.
RELATIONS OF ROCK FLOWAGE AND MOUNTAIN MAKING. 93
If Darwin and Peirce are correct in thinking that the moon was once
very close to the earth, and that at this early time the earth had a much
briefer rotation period than at present, the tidal force tending to produce
such monoclinal structures in the zone of flow would be vastly greater than
at present. It is worthy of note that the preponderant easterly structures
which have been noted are among the Paleozoic and pre-Paleozoic rocks
which in post-Paleozoic time have been brought to the surface in conse-
quence of denudation. If more extended and closer geological studies of
cleavage and monoclinal folding lead to the conclusion that easterly
structures do preponderate over westerly structures for the earth as a whole,
it may be plausibly argued that these phenomena in the old rocks are
geological evidence of the correctness of Darwin’s general theory in refer-
ence to the earth-moon couple, and that the tidal forces in the past were
vastly greater than those which now exist.”
IT am aware that the foregoing pages upon the relations of rock Jowage
to mountain making are speculative, but the speculations accord, so far
as I know, with the facts of the field. They are here published in the
hope that they may lead to more accurate observations: of the relations
of primary and secondary structures to each other, and of the positions of
secondary structures in various mountain regions. Concerning these points
I have found the published literature upon the metamorphic rocks woefully
lacking.
@ Darwin, G. H., On the precession of a viscous spheroid and on the remote history of the earth:
Philos. Trans. Royal Soc., vol. 170, pt. 2, 1879, pp. 530-538.
CHHUIAIP TT IB I= OC ik
RELATIONS OF METAMORPHISM TO THE DISTRIBUTION OF
THE CHEMICAL ELEMENTS.
In consequence of metamorphism the chemical elements are redis-
tributed on the face of the earth. This redistribution concerns many large
questions of geological theory. At the present time our knowledge of
metamorphism is not sufficiently advanced to treat the redistribution of the
elements except in a very imperfect manner. In the following pages an
attempt is made to consider the original source of each of the important
chemical elements composing the outer part of the earth, and the effect which
the metamorphic processes have had on its redistribution and on the character
of the secondary rocks. So faras practicable redistribution is dealt with in a
quantitative way, but a quantitative treatment is necessarily subject to the
yery imperfect hypotheses upon which it is based. Indeed, for the most
part the quantitative estimates are the roughest sort of approximations, made
more with a view of stating the various problems which in the future will
undoubtedly be satisfactorily treated quantitatively rather than with the
belief that the calculations given even approach accuracy. They are all
purely provisional, and it is hoped that they will be considered from this
point of view. Indeed, this chapter is no more than an attempt to blaze a
trail in the wilderness. It will require the work of many men for many
years to conquer the wilderness.
COMPOSITION OF THE LITHOSPHERE.
The term lithosphere as used in this chapter is rigidly restricted to that
part of the solid crust of the earth of which we have observational knowl-
edge, and which, following Clarke, is assumed to extend to a depth of only
10 miles (16.1 kilometers) below the level of the sea. According to R. 8.
Woodward, the volume of the part of the earth that is above the sea and
that extends to a depth of 10 miles (16.1 kilometers) below the level of the
932
COMPOSITION OF THE LITHOSPHERE. da
sea is 1,935,000,000 cubic miles (8,075,000,000 cubie kilometers). Of this
amount 302,000,000 cubic miles (1,260,000,000 cubic kilometers) are ocean
and 1,633,000,000 (6,815,000,000 cubic kilometers) are solid matter. The
mass of the atmosphere is equivalent to that of 1,268,000 cubic miles
(5,292,000 cubic kilometers) of water, the unit of density. ‘To sea water
we may assign a density of 1.03, which is a trifle too high, and to the solid
rocks a specific gravity not lower in average than 2.5, nor much higher
than 2.7. With these values we can get the following expression for the
percentage composition of the known matter of the globe:
Relative masses of air, sea, and earth's crust.
Per cent. Density of | Density of
crust 2.5. crust 2.7. |
INTO NVA « ssacoccosscosseosee 0. 03 0. 03 |
Oceanymer sana ae ete = cee Ta Ofss 4 6.58
= . . |
Solidkcrust=esee= eee eter eee 92.89 93. 39
100. 00 100. 00
In short, we may regard the earth’s crust, to a depth of 10 miles [16.1
kilometers], as composed essentially of 93 per cent solid and 7 per cent
liquid matter, treatmg the atmosphere as a small correction.””
A large number of elements have been discovered on the earth, but of
these, according to Clarke, only 21 each constitute as much as 0.01 per cent
of the outer part of the lithosphere, and need be considered in this chapter.
Those elements which compose more than 1 per cent may be regarded as
abundant and those which form less than 1 per cent as subordinate. Of
the former class there are only eight, viz: Oxygen, silicon, aluminum, iron,
calcium, magnesium, sodium, and potassium. Between the least plentiful
of the abundant class and the most plentiful of the subordinate class there
is a very considerable break, for potassium, the least plentiful of the
abundant elements, makes up about 2.3 per cent of the lithosphere, while
titanium, the most abundant of the subordinate elements, composes only
about 0.4 per cent of the lithosphere.
In the chemical laboratory of the United States Geological Survey a
very large number of analyses of rocks have been made. Clarke has
“Clarke, F. W., The relative abundance of the chemical elements: Bull. U. 8. Geol. Survey, No.
78, 1891, pp. 34-35.
934 A TREATISE ON METAMORPHISM.
compiled these and other analyses, with the purpose of obtaining an
approximate idea of the composition of the lithosphere. In his computa-
tions Clarke has considered only the ‘igneous and crystalline rocks.” As
these terms are used there are excluded from consideration the unmeta-
morphosed sediments and the metamorphosed sediments so far as they are
recognized as sediments. In the following table are found the analyses of
various groups of 880 igneous and ecrystallme rocks, and the mean of the
whole:
Analyses of igneous and crystalline rocks.@
A. BG Dipole F. G. H.
gee Wistert iE :
Per cent. | Per cent. | Per cent. | Per cent. | Per cent. | Per cent. | Per cent. | Per cent.
SiO ieee 61.89 | 61.89} 60.49} 60.66 | 60.50 59.80) 56.75] 58.59
INYO soca 15.71] 15.73 | 16.08] 15.46) 14.30] 14.65|-14.90| 15.04
HelOn es ee 1.81 3.18 | 2.47 2.74 | 3. 35 4.99 | 4.58 3. 94
FeQu eee 3. 65 2. 40 2. 86 2.27 4.31 PAPA NS Bh (Al 3. 48
CaQa-ee ee: 4.51 4.58 6.15 4.71 3.52 5.19 5. 79 5. 29
MRO) Soeaoes | 2.40 3.08 | 4.31 3. 35 5. 00 3.45] 5.22 4.49
UG Owen sesee 3. 54 2.70 1.80 3. 97 2.52 3. 06 2.90 | 2.90
NELisssacos 3528) Clarke, F. W., Report of work done in the Division of Chemistry and Physics, 1889-90: Bull.
U. S. Geol. Survey No. 78, 1891, p. 39. Clarke, F. W., Analyses of rocks, laboratory of the U.S.
Geological Survey, 1880-1899: Bull. U. S. Geol. Survey No. 168, 1900, p. 15.
¢ With bromine.
COMPOSITION OF THE LITHOSPHERE. 937
Amounts of the eleven most important oxides of the lithosphere, as estimated in 1891
and 1900. %
| Thiet |
| Old mean. | New mean. | Old mean. | New mean. |
= | | B
SiO RCM eE el arse, SGC) | oieyaait Wh) TO ce | 2.90 2. 80
PEO eens Ie 15. 04 IBS | KOEN “ose lestoG 1.52
TENG), eat aes ae 3. 94 DHE SIN NRT O Roe eee ae era | 55 | . 60
IRS O a noo eee eae 3.48 3.52 | DIOR asia Aeieel| 22, | 22
|
No Op sewers os sed ZO) £3614
B0 Roe Total eeey 99.66 | 99,22
CHOMP ngs e thy | 5.29 4,90 || |
Nas O emery eye eeoe20 3.55 |
| |
* Including hygroscopie water; probably 0.40 per cent.
Clarke further made a rough approximation of the proportion of the
more important minerals in 500 igneous rocks, as follows :?
Important minerals in 500 igneous rocks.
Per cent.
CONE LA yay Bas Ke tea peer o s 12.0
els parsieeer siete ce sis nels aoe ee a eee 60. 0
Pyroxenes and amphiboles .._....-.--.---- 18.0
INTL aoa ets ae a ea RR ee cla e Fo 0
FIR © Gea) PO Ee aE Sr ed AS seiyste spar iee 94.0
bee —!
The foregoing estimates are wholly based upon the analyses of ‘erup-
tive and crystalline”. rocks, and represent, according to Clarke’s point of
view, the average composition of the ‘‘primitive crust of the earth.” In these
analyses all of the unmetamorphosed and metamorphosed sediments, so far
as they are recognized as such, are excluded. As we shall see later, the
unmetamorphosed sediments probably compose as much as one-thirtieth
and the unmetamorphosed and metamorphosed sediments may together com-
prise as much as one-fifteenth of the lithosphere. The average composition
of the-sediments is of the utmost importance in reference to a redistribution
of the elements. Clarke has had made six composite analyses of groups of
“Clarke, F. W., Analyses of rocks, laboratory of the U. 8. Geol. Survey, 1880-1899: Bull. U.S.
Geol. Survey No. 168, 1900, p. 14.
» Clarke, cit., p. 16.
938 A TREATISE ON METAMORPHISM.
sedimentary rocks, in which 1,545 rocks were taken into consideration.
The table below gives his results.
Composite analyses of sedimentary rocks. “
rU oheaas G Dz E. F. G. H. | I.
| | | | |
SOassocose 55.43 | 60.15 | 58.38 | 78.66 | 84.86 | 81.76 5.19 | 14.09 9. 64
NYO nesinceae 13. 84 | 16.45 | 15.47 4.78 5. 96 5. 37 . 81 1.75 1.28 |
PaO ssokas 4.00 4.04} 4.03 1.08} 1.39 1.24 .o4 ott . 66
REO 1.74| 290] 2.46] .30) .84| 57 |Undet. | Undet: | Undet.
IMXO) Begosae 2.67 2.32 2,45 Wey 52 -80| 7.90 4. 49 6. 20
CaO see 5. 96 1. 41 ! 3.12 5.52 | 1.05 3. 29 | 42.61 | 40.60 41. 60
|) WEO Se scadse 1.80 1.01 | 1.31 45 .76 61 | .05 762 . 84
lenal Includes organic matter. ¢Of organic origin.
MASS OF THE SEDIMENTS. 939
Various estimates, which are little more than guesses, have been made
of the thickness of the sedimentary rocks upon the earth. Dana in 1875
stated that the mean thickness of the sediments for the continental areas
will not exceed 5 miles (8.05 kilometers).” T. Mellard Reade states that an
estimate of 10 miles (16.1 kilometers) is a moderate one for the thickness
of the sedimentary crust of the earth throughout.’ Later he greatly reduces
this estimate, saying: ‘I think we may with safety provisionally assume
that the actual average thickness of the sedimentary crust of the globe is
not less than 1 mile (1.61 kilometers).”° This average is for the globe as
a whole, and not for the continental areas, and therefore is not greatly
below Dana’s estimate of 8 kilometers for the continents alone. It appears
to me that even Reade’s revised estimate is much too great. At only a
very few localities can it be shown that the thickness of the sediments
approximates 8 kilometers. For the great plains and plateau areas where
there are deep borings or deep gorges, as, for instance, in the Grand Canyon,
the sediments frequently do not exceed 1 kilometer, and in general are less
than 2 kilometers. For great areas of the continents, such as extensive
Archean areas of Canada, there are no known sediments. For the greater
part of the ocean, aside from the continental shelves, all the evidence avail-
able points toward very slow deposition, and over such areas, composing
about two-thirds of the surface of the globe, the sediments are probably of:
inconsiderable thickness.
The area of the globe is approximately 500,000,000 square kilometers,
of which somewhat more than one-fourth belongs to the continents and
continental shelves. If it were supposed that the sediments which have
not been within the zone of anamorphism, but have always remained in
the zone of katamorphism, are upon an average 2 kilometers thick for the
continents, this would give 250,000,000 cubic kilometers of sediments.
This estimate is, of course, little more than a guess, made in order that a
number of problems in the distribution of the elements may be quantita-
tively stated. In comparing this estimate with that of Dana or Reade it
should also be noted that mine includes only sediments of the zone of
katamorphism—not those which have been metamorphosed in the zone
of anamorphism—while Reade’s estimate includes both.
@Dana, James D., Manual of Geology, second edition, 1875, p. 657.
b Reade, T. Mellard, Chemical denudation in relation to geological time, p. 29.
¢ Loc. cit., p. 52.
940 A TREATISE ON METAMORPHISM.
On the supposition that the sediments of the zone of katamorphism
are 2 kilometers thick, they compose about 3.668 per cent of the volume
of the lithosphere to a depth of 16.1 kilometers (10 miles) below the level
of the sea (6,815,000,000 cubic kilometers). With a specific gravity of 2.7
the weight of the sediments would be 675,000,000,000,000,000 metric tons.
In suggesting the above figures the aim has been to underestimate
rather than to overestimate the total amount of the sediments. It is thought
better to have the calculated amount of elements required for certain
changes smaller than the real amounts rather than larger. For instance,
it will be explained on the following pages that the change of ferrous to
ferric oxide in the sediments has required a vast amount of oxygen—indeed,
an amount so great as to be a large percentage of the total oxygen now
present in the atmosphere. If an overestimate of the total mass of
sediments were made, this would lead to an overestimate of the amount of
oxygen required for this change. But the purpose is to make the estimate
of the amount of oxygen and of the other elements required for this and
for other changes less than the real amounts rather than more.
In making the first rough approximation of the weight of the sediments,
they will all be supposed to have a specific gravity of 2.7. While the
specific gravity of sediments varies, and doubtless upon the average is
somewhat less than 2.7 (the average of the igneous and crystalline rocks),
the error in placing it at 2.7 is small compared with the errors in the
estimates of the amount of sediments and their relative proportion. There-
fore, for convenience in the present provisional calculations, a uniform
specific gravity for the rocks of the lithosphere is assumed. If the sediments
of the zone of katamorphism were supposed to be 0.65 shales and other
rocks largely composed of silicates (pelites and psephites), 0.30 sandstones,
and 0.05 limestones, this would give 162,500,000 cubic kilometers of shales,
weighing 438,750,000,000,000,000 metric tons, 75,000,000 cubic kilometers
of sandstones, weighing 202,500,000,000,000,000 metric tons, and 12,500,000
cubic kilometers of limestones, weighing 33,750,000,000,000,000 metric
tons. In the following pages the term shale will be understood to include
all the clastic sedimentary rocks rich in silicates.
So far as I know there has been no attempt to estimate accurately the
relative quantities of shales, sandstones, and limestones. The above parti-
tion of the sediments is the roughest sort of a guess. The only computation
MASS OF CALCAREOUS SEDIMENTS. 941
which I am aware of on the subject is inreference to limestone. T. Mellard
Reade, by comparing the amount of calcium carbonate and calcium sulphate
in solution in rivers with the sediments mechanically carried by rivers,
concluded that one-eighth to one-ninth of the transported material is com-
posed of the carbonate and sulphate of calcium. Using his estimate of a
thickness of 1 mile for the sedimentary rocks, and supposing that the mass
of limestone is one-tenth of the whole mass of the rock, he says: “This
gives us the equivalent of a zone of limestone rock 528 feet (160.9 meters)
thick, enveloping the globe, as a very rude approximation to the absolute
quantity of carbonate and sulphate of lime in the sedimentary crust of the
earth.”* Of the two compounds calcium carbonate so dominates over
calcium sulphate that the latter will be neglected and the whole will be
considered as calcium carbonate.
If the amount of calcium carbonate carried by the streams at present
is not an accurate measure of the average carried by streams through geo-
logical time, the error is likely to be an overestimate rather than an under-
estimate, since early in geological times, before the calcareous sediments
had accumulated, it can not be supposed that the streams carried as much
calcium carbonate as at present. Further, the method of computation used
by Reade for estimating the volume of the limestones is rather unsatisfac-
tory, since it is certain that a considerable portion of the calcium carbonate
passes into shales and sandstones, as is shown by the analyses (p. 938).
The shales contain 2.64 per cent of carbon dioxide and the sandstones
3.03 per cent. If most of this carbon dioxide is united with calcium oxide,
as is probable, this would give about 5 per cent, or one-twentieth, of calcium
carbonate in the shales and sandstones. Therefore, if the shales and sand-
stones were supposed to be nineteen times as abundant as the limestones,
these rocks would require one-half of the calcium carbonate, and the propor-
tion of limestone instead of being one-tenth, as estimated by Reade, would
be one-twentieth of the total mass of the sediments. This ratio of 19:1
between the sandstones and shales together and the limestones is accordant
with the relative amounts of the rocks as calculated upon the basis of
depletion of calcium oxide in the sandstones and shales as compared with
the original rocks (see pp. 990-991). It thus appears that the estimate of
one-twentieth, or 0.05, of the mass of sediments for limestone, is probably
as near the truth as can be made from data at the present time.
« Reade, T. Mellard, Chemical denudation in relation to geological time, p. 53.
942 A TREATISE ON METAMORPHISM.
As a result of the work of the Challenger expedition, the ocean is esti-
mated to have a volume of 1,283,840,000 cubic kilometers.” This estimate,
equivalent to 307,000,000 cubic miles, is somewhat in excess of Woodward’s
estimate of 802,000,000 cubic miles, given on page 933. The weight of
1,283,840,000 cubic kilometers of the ocean, with a specific gravity of 1.03
(see p. 933), is 1,322,355,000,000,000,000 metric tons. Of this 85.79 per
cent, or 1,134,448,354,500,000,000 metric tons, is oxygen, and 10.67 per
cent, or 141,095,278,500,000,000 metric tons, is hydrogen. Taking the
maximum salinity of sea water as given by Dittmar,’ 37.37 grams of salts
per kilogram of water, sea water is 96.263 per cent water and 3.737 per cent
salts. The 3.737 per cent salts is made up of the following compounds:
Composition of salts in sea water.°
Per cent.
Chl orin 62sec hea Sa ee orators 55. 292
Bromine ssi se oes eee a aaeeee ee oe . 1884
Sulphurictoxiden(SO:) esse eestee sees eeee 6. 410
Carbonidioxides(CONMesesssee tees sees . 152
Timea (CaO) shee ees Save e ep Mon cee Less 1. 676
Maonesian(Mic@) eemasee eee ee eneeere 6. 209
Potash (IK Omer Sacer san ee cepts iGeEy)
Sodan (Na; O) ssa ee ese ecseeeer meeseaee 41. 234
Oxygen equivalent of halogen.------.-.-- —12. 493
Total 2! aceae ean tear aes 100.000 |
«Dittmar, William, Composition of ocean-water salts: Narrative of the cruise of H. M.S. Chal-
lenger, vol. 1, pt. 2, 1885, pp. 951, 980.
> Dittmar, William, Report of the scientific results of the exploring voyage of H. M. 8. Challenger,
1873-1876: Vol. I, Physics and Chemistry, London, 1884, p. 201.
¢ Dittmar, Narrative of the cruise, etc., p. 954.
SALTS OF THE OCEAN.
943
Combining acids and bases, we have the following average composition
of sea salt:”
| Percentages of salts in sea water.
Per cent.
Chiorid erofisodium@ss—= esses seee asses 77. 758
Chloride of magnesium.._................. 10. 878
Sulphatejoiimagnesium= ss. s2 4-22. - sees 4. 737
Sulphatesofilim(e ess esssee ese se aeeeaee 3. 600
Sullphatetotspotashienseseesese ee seen a ee sees 2. 465
IBromideyoiumacnesinmysess ss ae see eee 227
Carbonateyolglim Cees eeeee ee eee eee eae . 3845
MO taller ese sarcte nics se cies ee 100. 000
From the above table the percentages of the different elements in the
ocean salts are calculated as follows:
Percentages of elements in sea salt.
| Per cent. | | Per cent.
|
[RCS Dh eae HDT Tall Ise ala oss A Soya 1.107
Nae tact cae al 804637 NEBr eee ES IAs ae .188
(Que eee Ateneo 5. 283 | (Chime oe AUU RA O41
Nowa aeee henna) 3. 769 ER CRET
Ig : | Total sees 100. 000
SOS Pst ea 2.562 |
[PCa teese suc det uses 1.196 | |
The total amount of these compounds as calculated by Dittmar is
Total amounts of salts in the ocean.
[Unit=10!2 metric tons.]
Ohiloraae or gocbinnin (NEON) oo shscsecSedc5s cede sasasuoboodeucscoseobees
Chioridejoipmarnesiumy (Vie Cl) pemse see esee a eee ree et eee eee eee
Sulphatevofimacnesiumy (MoSO})Passssesesese=seeeeenee eee ea seee oes
Sulphaterohcalciumy(CaSO}) Pees ee sees eaesetr seer nese esse eee eee
Sulphatetotpotashy (Ki SO) seer esemesesees sere eeeee tere earn eae eses
iBromideort magnesium (MeBrs) reeset eee ease eeEe eee eee ee eeee eee
Carbonateroficalcium) (Cat Os) See eereen eee eee eee eee eee eceeeee
46,283,000,000,000,000 tons, or 46,283 (10° 1,000) kilograms.
amounts of the different compounds are given in the table below.”
The
«Dittmar, Narrative of the cruise, cit., p. 954. > Dittmar, cit.,
p- 980.
944 A TREATISE ON METAMORPHISM.
Taking the above estimates, the amounts of the different elements of
the ocean salts are given in the following table:
[Unit=10! metric tons.]
Sparta eG Se tees Soa 2 TOOT | IRS ee eh eye hae 512
Te ae eee aes Sipe at ale ee eed TA TSUS OM SB Trek ess pains aie See ae ae 87
(peepee aeestiiael eae atte a i SP NA DAA DiI Ola bine cee ee etapa eee ao eet Nesp ate 19
iN (ea ae ope ae 2 all Bh Rees 1, 743 =
Riad aac e to Aad Re Nee 1, 187 Potala see soescoeceser 46, 283
(OP ARU? pe Sas ey Peay tie er eee Sees ay 553
Clarke, using the above results of Dittmar, computes the elemental
composition of the ocean, as already given (p. 936); but for convenience his
results are here repeated: “
Composition of ocean.
(O} GS Baie Ms an Nets ecgere CSIP ACI TIA s Sees ae Re a 0. 04
EAE VE stata aaa ANE 10. 67 | Pesan Ne wer ore ttre 09
GALES Sr arate eases spa a QMO AMER re fh Seesh A rian a Naa 008
ONE Se ar eS ea Ae een WS AR se Gite es OP Ab Seles nn EN 002
NW ep Se oe Bs eer NS ee 145)
Rapa Cis wns heals Me NA 05 Total Beeeseoasasssecce 100. 000
Excluding the subordinate constituents, air has the following compo-
sition:
Composition of air.
By volume.) By weight. |
Per cent. Per cent. |
Nitrogen Fase eee tee ea ee ee | (79.08 76.88 |
Oxey emer s Siac eS ee 20. 92 Zoul2,
The amount of carbon dioxide in the air is 0.03 per cent by volume
and 0.045 per cent by weight.’ There are also in the air minute proportions
of water vapor, ammonia (NH;), argon, ete.
According to Woodward (see p. 933) the mass of the atmosphere is
equivalent to that of 5,292,000 cubic kilometers (1,268,000 cubic miles) of
water of unit density. This is equal to a weight of 5,292,000,000,000,000
metric tons. Disregarding the subordinate elements, the weight of the
nitrogen would be 4,068,489,600,000,000 metric tons, and of the oxygen
1,223,510,400,000,000 metric tons.
«Clarke, F. W., Reladee cpanebands of the chemical alae Bull. U. Secor Samara No. 78,
1891, p. 35.
» Mendeléeff, D., The principles of chemistry, translated by George Kamensky: Longmans, Green,
& Co., London, 6th ed., vol. 1, 1897, pp. 235, 236, and 238.
COMPOSITION OF METEORITES. 945
The above data give the approximate composition of the lithosphere as
now constituted; but not of the earth as a whole. As pointed out by Far-
rington,“ we can probably get a rough approximation to the composition of
the earth by considering the nature of meteoric falls, as they are supposed to
furnish the best basis for estimating the composition of the material out of
which the world has segregated. Meteorie falls rather than meteoric finds
are chosen because “the iron meteorites are much more likely to be known
1d
and preserved than the stony.”’ The average specific gravity of meteoric
falls, as determined by Farrington, is 3.69... No attempt has been made to
estimate quantitatively the proportions of the elements which compose such
How-
ever, Farrington” recently considered the mineral and chemical compositions
meteorites, taking into account both their masses and compositions.
of various classes of meteorites, including both falls and finds, and in some
cases gave the relative proportions of a few of the elements in these, and
the percentages of a few of the minerals in some of the meteorites con-
sidered. From his article the following table is made:
Constituents of meteorites.
Percentage of total of
Name. Composition. Relative proportions. | definite meteorites!
| ESSENTIAL. ipertcents
Nockel -ironeese esse eee INI ony7 Od NHL EVEL IE Son ocosoHocosuseabanscasses Ni 6-20
| (Co .5-2)
| (Cu .006-.02)
| {Fe 10-30 |
Chrysoliteresssss-- =e oe 4
Orthorhombic pyroxenes....
(QUIO) ASO). eo coneanconobacasbsdoansonaSosdes!
All gradations between:
MgSi0O; (enstatite)
(MgFe)SiO,(bronzite)
(FeMg) SiO;(hypersthene)
|Ni almost lacking.
Monoclinic pyroxenes:
INVEGNIGS Ese as os eesecRon Dae Ca(MgFe)(Si0.)2
with (MgFe)(AlFe).Si0g
Miopsides a= eee | {CaMg(SiO3)2
||Ca(MgFe) (SiO3)o
Plagioclase: |
PAM Orthite ven sessaeerieeiae (CRYUNITISTH OV aooodpcooonouSodscodooddnonaassocdsllbaooadsoseaapossdasedcie 35 (2 meteorites).
Allpitewat veseet serine --| NaAlSi303
Oligoclase. .------- .--| AbgAn, to AbszAny;
Labradorite ....- .-| AbAn (1:1 or 1:3)
Maskelymitetseencscassaccse= Aiboutisamejaslabrad Oriteteemscseeeeraeeeieeeelseeee neice actleciacels 22.5 (1 meteorite).
“Farrington, O. C., The average specific gravity of meteorites: Jour. Geol., vol. 5, 1897, pp. 127-128.
> Farrington, The constituents of meteorites: Jour. Geol., vol. 9, 1901, p. 394.
¢ Farrington, The average specific gravity of meteorites: Jour. Geol., vol. 5, 1897, p. 130.
@ Farrington, The constituents of meteorites: Jour. Geol., vol. 9, 1901, pp. 393-408, 522-532.
MON xXLV11r—04——60
946 A TREATISE ON METAMORPHISM.
Constituents of meteorites—Continued.
‘ aie a 5 | Percentage of total of
Composition. Relative proportions. |“ qofnite meteorites.
ACCESSORY.
Iron. Stone.
63.09
20.70
8.12
7.52
c | 0.57
| 100. 00
Tron sulphide:
Mrowite ewes eneeeea Fes.
Pyrrhotite (2) secseecccce Fes.
Schreibersite 2-55-2222... (HeNICO) she assesceoras hee ee A aeneeen eee Most widely distrib-
uted constituent of
iron meteorites
next to nickel iron.
Graphite eetnene eee C.
Cohenite peeeesn see ensemeeeas (FeNiCo)3C
Glass.
@hromitem-seesesse seen eens FeO. Crp03
Amorphous carbon .......--- ¢c:
IDN CHINO VEl SSsemowasceobeosouse Cc.
Daubréelite. Fes. CrS3
Tridymite -- SiO,
Lawrencite - FeCl.
Maen etite eee aaa | SEBO n coseaSonose saacoboodeasoskcoaasceosecasas | BESSRe SSD aSAaaEeeeRosSsa | 4.57 (one meteorite).
Oldhamitestses2—-seeas seer | CaS.
Hydro-carbons: |
(a) (Cand eee noo ee | CHen | {Less than 1 in major-
(bp) iC hHivam disper eee LOWS Os eee EOS a RO HORE EMEC OReMSAASacHascn se Bacuinnocacdbneconiacmee |) ity of meteorites in
(c) C, H, and 0...--.--.- | CsH:90| which present.
| |
The list of elements in meteorites given by Farrington“ includes all
those, except fluorine, which compose as much as 0.01 per cent of the outer
part of the earth and a number of others. Farrington has given also a list
of the eight chief elements in meteorites, including both falls and finds, in
order of their abundance (column 1).’ For comparative purposes there
are placed opposite these, in order of abundance, the eight elements
which, according to Clarke, are most plentiful in the outer part of the
earth (column 2).
“Farrington, cit., pp. 393, 394. > Farrington, cit., p. 394.
RELATIVE ABUNDANCE OF ELEMENTS. 947
Order of abundance of elements in meteorites and in the outer rocks of the carth.
Meteorites. Outer crust of earth.
1. Iron. Oxygen.
2. Oxygen. Silicon.
3. Silicon. Aluminum.
4. Magnesium. Iron.
5. Nickel. Calcium.
6. Sulphur. Sodium.
7. Calcium. Magnesium.
§. Aluminum. Potassium.
Nickel and sulphur are found in column 1 and not in column 2, but
in the latter sodium and potassium appear. Farrington suggests that
because iron meteorites are preserved longer than stony meteorites, and are
therefore more likely to be found, iron probably occupies a higher place
than it would if meteoric falls only were considered. He further suggests:
“The relative excess of magnesium and nickel and searcity of aluminum
and calcium in meteoric as compared with terrestrial matter may be due to
the same cause.” “
REDISTRIBUTION OF THE CHEMICAL ELEMENTS.
We are now prepared to consider the redistribution of the important
chemical elements as a joint result of the forces and agents of metamor-
phism, including therein for this purpose the forces and agents of denuda-
tion. In order to appreciate the results it is perhaps well to state the point
of view from which the redistribution is considered. If the distribution of
any element in the igneous and sedimentary rocks be compared it will be
found that in the latter class certain formations are deficient in a given
element, and correlative with this deficiency there is a surplus in some other
formation or formations. In other words, for any element some sedimentary
formations, as compared with the igneous rocks, are likely to show marked
deficiencies and some formations marked segregations. Where the material
segregated has a value to man, either in the form in which it exists in
nature or as furnishing the source of one of the elements, it becomes an
economic product. Thus in tracing out the redistribution of the elements
we are arriving at the genesis of various economic products. In this chap-
ter only the abundant elements are considered, but where an abundant
element may be an ore we are to that extent dealing with the genesis of ore
“Farrington, cit., p. 394.
948 A TREATISE ON METAMORPHISM. '
deposits, as for instance in the case of iron. The development of the greater
number of ore deposits involves the segregation of the rarer elements, and
this aspect of the subject is especially considered in the following chapter.
OXYGEN.
Oxygen is the most abundant of the elements, both in the lithosphere
and in the hydrosphere. According to Clarke’s estimate of 1891 oxygen
composes 49.98 per cent of the lithosphere, hydrosphere, and atmosphere
together, or only 0.04 per cent short of the total amount of all the other
elements. According to Clarke’s estimate of 1891 it composes 47.29 per
cent of the lithosphere, and according to his estimate of 1900, 47.02 per
cent. Of the hydrosphere it composes 85.79 per cent, and of the atmos-
phere 23.12 per cent. While the percentage of oxygen in the hydrosphere
is greater than in the lithosphere, by far the greater proportion of the
oxygen is in the lithosphere, on account of its enormous mass, although an
immense quantity is in the hvdrosphere. The oxygen of the lithosphere
and hydrosphere is combined, while that of the atmosphere is free.
Oxygen differs from all of the other abundant elements in that a large
quantity of it is in the free state. The only other abundant element which
oceurs partly in the free state is iron, and the amount of free iron in the
lithosphere is exceedingly small. One other element, nitrogen, near the
bottom of the list of the 23 more plentiful elements, also occurs in the free
state and in even greater quantity than oxygen. Oxygen is the element
second in abundance in the meteorites. Among the gases of the meteorites
free oxygen is not included in the list given by Farrington, but Wright
reports oxygen as occluded.’ Even if oxygen exists As a gas in meteorites,
this is not evidence that this oxygen was brought from the outer space,
It may have been absorbed there or while the meteors were passing through
the atmosphere, or partly in both ways.
The source of oxygen for the atmosphere is a matter of great conse-
quence, since in the process of oxidation, one of the fundamental processes
of the zone of katamorphism, the oxygen is derived directly or indirectly
from the atmosphere. So far as known, the vast quantity of free oxygen
«Clarke, cit., Bull. 78, p. 39: Bull. 168, p. 15.
> Wright, A. W., Spectroscopic examination of gases from meteorites: Am. Jour. Sci., 3d ser., vol.
9, 1875, p. 301.
SOURCE OF OXYGEN FOR THE ATMOSPHERE. 949
within the lithosphere is combined, and there is no evidence that this is not
true of the centrosphere also.
It might be supposed, as has been suggested by various men in
reference to carbon dioxide, that oxygen has been attracted to the earth by
the force of gravity from the interplanetary spaces. To what extent this
has occurred, if at all, must long, and perhaps forever, be a matter of
conjecture.
The chief certain source of oxygen for the atmosphere is the reducing
action of organic material upon carbon dioxide. As plants grow they
decompose carbon dioxide of the atmosphere, thereby liberating oxygen,
and build the carbon into their bodies. Animals take their carbon com-
pounds directly or indirectly from plants. So far as the plants and animals
decompose, the carbon is again oxidized to carbon dioxide by the oxygen
of the atmosphere, thus consuming an amount of oxygen equivalent to that
originally liberated. Therefore, so far as life products decompose there is
no permanent gain of oxygen to the atmosphere by the cycle. But a portion
of the organic compounds do not completely decompose, and in so far as
this is true and they remain in the rocks in carbonaceous shale, graphitic
slates, schists, or gneisses, in peat, im coal, and other rocks, the oxygen
liberated by the reduction of the carbon dioxide is a permanent gain to the
atmosphere. It will be seen under carbon, pp. 962-974, that the quantity of
carbon which is thus locked up in the earth is enormous. Since no quanti-
tative estimate has ever been made of the amount of such carbon, there is
no way in which one can ascertain to what extent the air has gained in
oxygen in consequence of the reduction of carbon dioxide. But it is
suspected that a considerable percentage of the oxygen now in the atmos-
phere could be thus accounted for. Indeed the reduction of carbon dioxide
by plants and the liberation of oxygen to the atmosphere in consequence of
the formation of the rock coals, peats, etc., has so impressed Koene and
Phipson as to make them think that originally the relations of carbon dioxide
and oxygen in the atmosphere were probably reversed, but that as a result
of the continuous reducing action of vegetation and the decay of only a part
of it the atmosphere has become one in which carbon dioxide is very subor-
dinate and oxygen one of the chief constituents. “
«Chemical News, vol. 67, 1893, p. 135; vol. 68, 1893, pp. 45, 75, and 259; vol. 70, 1894, p. 223.
950 - A TREATISE ON METAMORPHISM.
But the oxidation of carbon and deoxidation through life take into
account only one aspect of the question. The inorganic carbon compounds
of the original earth stuff should also be considered, and Koene and Phipson
ignore this side of the question. It will be seen, under carbon, that this
element in various forms is a constituent of the original igneous rocks, and it
has been seen that carbon componds are also constituents of meteorites. So
far as elemental carbon, carbon monoxide, carbureted hydrogen, or other
carbon compounds are present in the original igneous rocks and have
been oxidized to carbon dioxide, demands have been made upon the oxygen
of the atmosphere. ‘These demands have certainly been large. No data
are at hand to make even an approximate estimate as to how large a
quantity of oxygen has thus been abstracted from the atmosphere during
geological ages. But, as explained under carbon, the oxidation of carbou
compounds of the original igneous rocks is believed to be one of the chief
sources of carbon dioxide. In so far as this reaction has taken place it
is a reversal of that of vegetation. Which of these opposing processes is
preponderant is a question of great importance, but one upon which not
even a qualitative guess is warranted. Therefore no statement can be
made as to whether the atmosphere has permanently gained or lost oxygen
as a consequence of the oxidation and deoxidation of carbon compounds
throughout geological time.
A second very important reaction which depletes the atmosphere in
oxygen is the oxidation of the ferrous iron of the original rocks. It has
been pointed out (Chapters VI and VIII) that oxidation of the ferrous
oxide to the ferric oxide is characteristic of the belt of weathering and
that the reduction of the ferric oxide to the ferrous oxide may occur in
the belt of cementation and is characteristic of the zone of anamorphism.
But the oxidation of ferrous iron to the ferric form clearly preponderates
over the reduction of ferric oxide to ferrous oxide, and hence oxygen is
consumed. This is shown by the following facts:
In 78 shales the percentage ratio between the ferrous oxide and the
ferric oxide is 2.46: 4.03; in 624 sandstones the ratio is 0.57: 1.24; in 843
limestones the ferrous oxide is so small that it is undetermined, but the
ferric oxide is 0.66. Giving these sets of numbers the respective weights of
0.65, 0.30, and 0.05 (the estimated proportions of these different kinds of
sediments), the ratio between the ferrous oxide and ferric oxide in the
ABSTRACTION OF OXYGEN FROM THE ATMOSPHERE. 951
sediments as a whole is 1.77:3.025. The amount of ferrous oxide is
about one-half of that of the original igneous rocks, 3.52 per cent. The
amount of ferric oxide somewhat exceeds that in the original rocks, 2.63 per
cent, but this excess falls far short of the amount required to compensate
for the deficiency of ferrous iron in the secondary rocks, for the total
amount of iron in 1.77 per cent of ferrous oxide and 3.025 per cent of
ferric oxide is 3.49, whereas the total amount of metallic iron existing
in the oxidized form in the original rocks is 4.58, the remainder of the
4.64 total iron, as given by Clarke, probably being largely accounted
for by the iron united with sulphur. It therefore appears that a large
amount of the iron of the original rocks is not accounted for in the
sediments which are ordinarily analyzed, and this difference amounts
to one-fourth of the total amount of iron in the original rocks. This vast
deficiency in iron oxide is probably largely accounted for by the segre-
gation of iron in the iron-bearing formations and in the iron-ore deposits
such as those in the Lake Superior region. The iron in these formations is
dominantly in the ferric form. Certainly the ratio of ferric oxide to ferrous
oxide in them is as great as among the sediments.
Supposing, therefore, that the ratio of ferrous oxide and ferric oxide in
the iron-bearing formations is the same as in the sediments analyzed, and
increasing the amounts of ferrous oxide and ferric oxide in the shales, sand-
stones, and limestones together in the same ratio, so that they contain an
amount of iron equivalent to that in the original igneous rocks, thereby
including the iron-bearing formations, this would give 2.35 per cent of
ferrous oxide and 4.02 per cent of ferric oxide. Upon this hypothesis
the difference between the amount of ferrous oxide present in the original
rocks, 2.35 per cent, and that now found upon the average in the secondary
rocks, 1.77 per cent, is 0.58 per cent. To change this percentage of fer-
rous oxide to ferric oxide for the 675,000,000,000,000,000 metric tons of
sediments of the zone of katamorphism would require 435,000,000,000,000
metric tons of oxygen, or 35.55 per cent of the oxygen now in the
atmosphere. (See p. 944.) This shows how enormous has been the draft
of oxygen from the atmosphere in consequence of the oxidation of ferrous
oxide alone. (See p. 1003.)
Another very large demand upon oxygen is made by the oxidation of
the metals and the sulphur united as sulphides in the original rocks. Of the
952 A TREATISE ON METAMORPHISM.
metals united with sulphur, iron of course dominates over all others com-
bined. We may therefore use FeS,, the most common sulphide, to illustrate
the process. If the iron were changed to the ferric oxide and the sulphur
to sulphuric acid, the reaction may be represented by the equation—
2F eS, +150 +4H,0=Fe,0,-+4H,S0,.
Since sulphates are not known in the original igneous rocks, nor in
meteorites, it is to be presumed that practically all the sulphates which now
exist are a result of the oxidation of the sulphur of the sulphides by oxygen.
The total amount of sulphates in the ocean is vast. The amount of
sulphur present as sulphates in the ocean is estimated by Dittmar to be
1,187,000,000,000,000 metric tons. The oxygen derived from the atmos-
phere to unite with this sulphur in order to produce sulphuric acid is
1,777,200,000,000,000 metric tons, or 145.25 per cent of that now in the
atmosphere. If the sulphur in the sulphate deposits be estimated as equal
to one-tenth that in the ocean, the amount of oxygen required for the
production of all the sulphates would be 159.77 per cent of that now in
the atmosphere. As shown by the above equation, to oxidize the iron of
sulphides to the ferric oxide requires one-fourth as much oxygen as the
oxidation of the sulphur to sulphuric acid. Thus for this part of the
process 39.94 per cent of the amount of oxygen in the atmosphere would
be required. Summing all these amounts we find that for the oxidation of
the sulphides 2,443,473,000,000,000 metric tons of oxygen, or 199.91 per
cent (i. e., twice), that now in the atmosphere, has been consumed. (See
p. 1003 for possible modification.)
It is shown elsewhere that sulphates produced by oxidation of the
sulphides are extensively reduced to sulphides by carbonaceous material,
and this cycle of reproduction of sulphates and sulphides for some of the
material doubtless has been repeated many times. The above is a first
attempt to roughly estimate the amount of oxygen required to produce the
present existing sulphates, not the total amount of oxygen which has been
consumed in the oxidation of the sulphides to sulphates through geological
time. So far as the sulphides have been changed to sulphates and reduced
again to sulphides by organic matter, a consideration of the amount of
oxygen which is drawn from the atmosphere by this process has already
been accounted for in considering the relations of oxygen and carbon.
ABSTRACTION OF OXYGEN FROM THE ATMOSPHERE. 953
Metallic iron is known as a rare constituent of certain very basic
igneous rocks. When such material reaches the zone of katamorphism the
iron is oxidized to ferrous oxide or ferric oxide, and thus oxygen is con-
sumed. At the present time the consumption of oxygen by metallic iron
is so small as to be insignificant, but it is possible that earlier in the history
of the earth, before the segregation of the oxidized products at the surface,
not only iron, but possibly other metals, may have been oxidized in great
quantity, and thus enormous quantities of oxygen abstracted from the
atmosphere. ‘The quantity of oxygen which has thus been consumed must,
so far as one can see, ever be a matter of conjecture.
The source of the water of the hydrosphere is uncertain. It may be
supposed that early in the history of the earth a large part of the elements
composing water was in a free state, although doubtless a part was also
combined. If any considerable proportion of the water of the hydrosphere
was produced by the oxidation of hydrogen during the time the sedimentary
rocks were deposited, the amount of oxygen thus consumed would have
been enormously greater than that required for the processes which have
heretofore been considered, indeed, many times greater than the amount of
oxygen now in the atmosphere. However, it is useless to speculate as to
the extent to which the oxidation of hydrogen took place in early geological
time. It is probable that during the deposition of the sedimentary rocks, and
even at the present time, hydrogen is being oxidized on a small scale. It is
seen (pp. 945-946) that hydrogen and carbureted hydrogen are constituents
of meteorites, and further that hydrogen occurs occluded in various rocks.
Hydrogen is met also in connection with fumarolic and solfataric action.
As already explained (p. 492), this hydrogen is supposed to be produced
by the reaction of the ferrous oxide upon water, according to the equation
H,0+2FeO=2H-+ Fe,0;.
So far as water is produced by the oxidation of hydrogen free or com-
bined oxygen is abstracted from the atmosphere. This abstraction certainly
occurs to some extent in volcanic districts. _Phipson also holds that hydro-
gen, so far as it is present in the atmosphere, may be oxidized by the nascent
oxygen freed by plants. He grew certain plants m an atmosphere of
hydrogen and was thus able, as he claims, to completely oxidize all of it.”
@Phipson, T. L., Vegetation in hydrogen: Chemical News, vol. 67, 1893, p. 303.
954 A TREATISE ON METAMORPHISM.
It might be supposed that in counting the oxygen consumed in the
oxidation of hydrogen to produce water, that formed by the decomposition
of water through the agency of ferrous oxide should be disregarded, since
the amount of oxygen required for the change of ferrous oxide to ferric
oxide in the change from the original to the altered sediments has already
been estimated. But the lavas are original igneous rocks and the ferric
oxide which they contain may be in part produced by the oxidation of
ferrous oxide through water in accordance with the equation above given,
and therefore the oxygen required to oxidize the ferrous oxide to the ferric
oxide as calculated (p. 951) does not include that required by the reaction
above.
Since there is so little hydrogen in the air, it might be argued that the
oxidation of hydrogen has taken place upon the earth on a great scale,
and therefore that large quantities of oxygen have been thus consumed.
But Chamberlin has shown that it is doubtful if hydrogen can be per-
manently held in the atmosphere by the attraction of the earth." If free
hydrogen can escape from the earth it can not be assumed that the amount
of oxygen required to oxidize hydrogen is important, at least during later
geological time.
Another way in which oxygen has been consumed by metamorphism
is in the oxidation of nitrogen. It has been explained (pp. 465-466)
that in various ways, but chiefly by means of the bacteria and leguminose
plants, free nitrogen is oxidized to nitric acid, and in consequence the
nitrates are formed. Since the nitrates are not known in the original
igneous rocks, nor in meteorites, it is to be presumed that all of the
nitrogen compounds upon the earth have been produced by the oxidation
of nitrogen. At the present time it is wholly impracticable to make any
estimate of the amount of nitrates upon the earth. In arid regions, as is
well known, there are large deposits of nitrates. (See p. 787.) In some
humid regions, as for instance at places in the Orinoco Basin of South
America, the nitrates compose a large percentage of the soil. As explained
(pp. 505-506), nitrates are essential constituents of all fertile soils. Hence,
while no quantitative statement can be made, the quantity of oxygen which.
must have been consumed in the oxidation of nitrogen must have been
very large.
«Chamberlin, T. C., A group of hypotheses bearing on climatic changes: Jour. Geol., vol. 5,
1897, pp. 666-667.
ABSTRACTION OF OXYGEN FROM THE ATMOSPHERE. 955
Besides the compounds already considered there are various rarer
substances which have been oxidized, such as manganous oxide. Doubt-
less, however, the consumption of oxygen by the rarer compounds is incon-
siderable.
The above calculations may be only roughly approximate, but they
seem to show that the amount of oxygen which has been abstracted from
the atmosphere during geological time in the oxidation of iron and sulphur
is enormous—apparently about twice that now free in the atmosphere.
If these results be correct, omitting any consideration of the amount which
has been required for the oxidation of hydrogen, nitrogen, and other sub-
stances, the original atmosphere must have been much more voluminous
than at present, or else during geological time there has been some source
of oxygen from which the atmosphere has been replenished.
In the previous pages only the sediments of the zone of katamorphism
are considered. While in the zone of katamorphism oxygen has been added
to the lithosphere and abstracted from the atmosphere, it is equally certain
that in the zone of anamorphism and in the belt of cementation deoxidation
takes place to some extent. Reduction is usually accomplished through
vegetation, carbon being oxidized at the same time and largely returned
to the atmosphere as carbon dioxide. So far as this is the case it involves
no correction beyond that already considered, since the end result is the
same as if this carbon had been oxidized by the process of decomposition
at the surface of the earth, and thus to that extent balanced the process of
liberation of oxygen when the organic carbon compounds were originally
formed. On the whole there is no reason to believe that the sedimentary
rocks metamorphosed in the zone of anamorphism contain more ferric oxide
than the original rocks. Until the relative amounts of ferrous and ferric
oxides in the sedimentary rocks of the zone of anamorphism are compared
with the amounts of these compounds in the original rocks, it can not be
asserted whether the sum total of the reactions of the sedimentary rocks in
the zone of anamorphism is in the direction of the consumption or liberation
of oxygen.
The oxygen added to the zone of katamorphism is not uniformly
distributed, but is segregated to a very large extent. The most important
segregation is that occurring in connection with iron. There are vast
quantities of ferric oxide in the red sandstones, and the added oxygen is
956 A TREATISE ON METAMORPHISM.
therefore distributed through these. A much greater concentration of the
added oxygen is, however, found in the rich and lean iron-ore formations,
from the Archean to the Pleistocene. Such segregation is well illustrated by
the Clinton iron ores, and is shown on a magnificent scale by the ferruginous
formations of the Lake Superior region. The gypsum and nitrate deposits,
aside from the iron, are those in which the oxygen abstracted from the
atmosphere is segregated to the greatest extent.
In summary it appears that the chief certain source of oxygen for
the atmosphere is the reduction of carbon dioxide by vegetation and the
burial of a part of this vegetation in the earth. This source is vast in
amount.
On the other hand, oxygen has been consumed by the oxidation of
the carbon compounds of various kinds in the original igneous rocks and in
the meteorites; by the oxidation of ferrous oxide to ferric oxide in the
zone of katamorphism; by the oxidation of iron in the metallic form and
as a sulphide; by the oxidation of sulphur, nitrogen, and hydrogen; and
in some small measure by the oxidation of rarer compounds, such as
manganous oxide. The sum of these gives the amounts of oxygen
consumed during geological time. Until estimates have been made of the
various amounts of the oxidized compounds, it is wholly out of the question
to make any quantitative estimate of the total amount of oxygen which has
been abstracted from the atmosphere by the process of oxidation since the
beginning of geological time, but it is certain that the amount of oxygen
thus consumed has been enormous. It probably vastly exceeds the amount
of oxygen which has been liberated to the atmosphere by the reduction of
carbonic acid through plants.
If this conclusion be correct such wild guesses as those of Koene and
Phipson (see p. 949), that the carbon dioxide of the original atmosphere
ereatly exceeded the oxygen and that the proportions of these elements
have been reversed in consequence of the reduction of carbon dioxide by
organic matter, are wholly unwarranted. This case illustrates the danger
of drawing a conclusion from the consideration of only one factor of a
complex problem.
SULPHUR IN ORIGINAL AND SECONDARY ROCKS. 957
SULPHUR.
According to Clarke’s estimate of 1891 sulphur forms 0.04 per cent of
the outer 10 miles (16.1 kilometers) of the earth, including the original rocks,
hydrosphere, and atmosphere, 0.09 per cent of the ocean, and 0.03 per cent
of the original rocks. However, in his estimate of 1900 the amount in the
original rocks is increased to 0.07 per cent. Sulphur thus has fourteenth
place in the seale of abundance.* If the sulphur were reckoned as con-
tained in SO, Clarke’s estimates for 1891 and 1900 for the original rocks
would be 0.0749 and 0.1748, respectively.
It is well known that sulphides are constituents of the original igneous
rocks. Sulphur occurs in these rocks as pyrite and pyrrhotite certainly.
Sulphur is found also in the meteorites in the form of troilite (eS), possibly
as pyrrhotite (Fe,S,,1), as oldhamite (CaS,), and also in the hydrocarbon
(C,H,.8;). It thus appears that there are various sources of sulphur in the
material of which the earth is formed.
Sulphur occurs in the shales, sandstones, and limestones as sulphates
and sulphides. The amount of SO, reported by Clarke in 78 shales is 0.65
per cent; in 624 sandstones, 0.08 per cent. No sulphur as sulphide is
reported, but Clarke writes that the above determinations include probably
both the sulphur as sulphate and as sulphide. The amount of SO, in 843
limestones is 0.06 per cent, and the amount of sulphur as sulphides in these
limestones is 0.08. The latter amount corresponds to 0.20 per cent SOs,
and thus if all of the sulphur of the limestones were reckoned as SO, the
amount would be (0.26 per cent.
It thus appears that, taking into account the molecular weights of
sulphur and SO;, sulphur is about four times as abundant in the shales as
in the original rocks, is reduced by about one-half in the sandstones, and
is increased by about one-half in the limestones. Since the volume of the
shales is so great, the amount of SO, in the shales is much more than suffi-
cient to compensate for the deficiency in the sandstones even if the excess
in the limestones were ignored. If one were to combine amounts of sedi-
ments and percentage of SO, in these rocks, and thus make an estimate of
the amount of the compound there should be in the original rocks, he would
have the following equation:
.69 X.65+.08 X.30-+.26 x .05=.4595
aClarke, cit., Bull. 78, p. 39; Bull. 168, p. 15.
958 A TREATISE ON METAMORPHISM.
Clarke’s average for the original rocks is-0.1748. It thus appears as if
there were an excess of sulphur in the secondary rocks amounting to 0.2847
per cent. To this excess, upon the theory that the salts of the ocean have
been derived from the original rocks, should be added the vast amount of
SO, in the ocean. Reckoned as sulphur, from Dittmar’s estimates this is
1,187,000,000,000,000 metric tons, which corresponds to 2,964,000,000,-
000,000 metric tons of SO;. But this is only .439 per cent of the total mass
of the sediments.
The above calculations as to the amount of SO, in the sediments take
no account of gypsum deposits. If these be supposed to contain one-tenth
as much SO, as the ocean, namely, .0439 per cent of the total amount of
sediments, there is in the sediments and the ocean together an excess of
.7676 per cent beyond the amount calculated in the original rocks.
These facts show that there is a great discrepancy between Clarke’s
estimate of 0.1748 per cent of SO, in the original igneous rocks and the
amount of sulphur in the secondary rocks. It is thought highly probable
that this discrepancy is largely explained by the actual escape of much
sulphur as a gas during periods of volcanism, as hydrogen sulphide, which
is oxidized during volcanic action, or by the direct oxidation of the sulphur
to SO, or SO;. It is certain that the amount of sulphur may be greater in
the secondary than in the original rocks, as is shown by analyses, because
the original rocks contain only the residual sulphur which separated as a
sulphide when the magma crystallized. While the igneous rocks may lose
a considerable portion of their sulphur before crystallization, this may not
fully explain the discrepancy. It is possible of course that Clarke’s estimate
of sulphur in the original igneous rocks may be somewhat too small, since
this element is in such subordinate amount, and selections for analysis were
made upon the basis of other elements rather than to get an average sample
for sulphur.
As to the geological processes through which the redistribution of the
sulphur takes place, the followimg summary statement may be made: In the
alteration of the original rocks in the belt of weathering, the sulphides are
largely oxidized to sulphates, which are for the most part taken into solution
by the underground waters. Some of this dissolved material is carried
down into the belt of cementation, and a portion is there precipitated as
sulphate in the form of barite, celestite, and gypsum; another part is again
REDISTRIBUTION OF SULPHUR. 959
reduced to sulphide by organic matter and other reducing agents, and in
this form is precipitated. These precipitates are mainly those of the heavy
metals, especially of iron. A considerable portion of the sulphates which
form in the belt of weathering and are carried to the belt of cementation
is brought to the surface by solutions which join the overground circulation.
By this circulation sulphates are transported to the sea. It has already
been noted that a large amount of sulphur is found in the sea as sulphate.
The amount now present in solution is not necessarily any measure of the
amount which has been contributed to the ocean. Indeed the relatively
large amount of sulphates in the shales suggests that where immense masses
of mud have become mingled with the sea water, as at the mouths of great
rivers, reactions take place which result in the precipitation of the sulphates,
and these salts thereby become mingled with the muds which later are
transformed to shales. While by far the larger portion of the sulphates
of the overground circulation are carried to the sea, in areas such as the
Great Basin calcium sulphate is thrown down in the lakes and gypsum
deposits are built up. Such deposits are of very considerable magnitude.
So far as these gypsum deposits formed in the zone of katamorphism pass
into the zone of anamorphism they may become dehydrated and anhydrite
be formed.
Gypsum and barite are of very considerable economic importance in
various ways. In these products a very large amount of sulphur is concen-
trated. The segregated sulphur, which, combined with the other elements,
makes these products of value, has been brought together in the condensed
form from the fraction of 1 per cent of the sulphur in the original rocks.
Thus we have another illustration of the manner in which processes of
metamorphism segregate elements, and result in the formation of deposits
which are of service to man.
SILICON.
Silicon stands second in abundance among the elements of the earth.
According to Clarke’s estimate of 1891 silicon forms 25.30 per cent of the
lithosphere, hydrosphere, and atmosphere together, and of the lithosphere
alone 27.21 per cent, a little more than one-fourth of the total. According
to Clarke’s estimate of 1900 silicon composes 28.06 per cent of the litho-
sphere.” All of this silicon is combined. In analyses silicon is usually
«Clarke, cit., Bull. 78, p. 39; Bull. 168, p. 15.
960 A TREATISE ON METAMORPHISM.
reported with its combined oxygen as silica. Stated as silica, Clarke’s
estimates of 1891 and 1900 are 58.59 and 59.71 per cent, respectively.”
This is the average, and in the original rocks the silica rarely falls below 50
per cent and rarely rises above 75. Of the elements present in meteorites
it has been seen that silicon is third in abundance. Much the larger portion
of the silicon in the original rocks occurs as silicates, although a large
amount occurs as silica in quartz.
The amount of silica in 78 shales is 58.38 per cent, in 624 sandstones
is 81.76 per cent, and in 843 limestones is 9.64 per cent. (See p. 938).
It is plain from the foregoing that within the lithosphere metamorphism,
denudation, and sedimentation have redistributed the silica to a very
important extent. In the shales the amount remains about the same as
the average for the original rocks. In the limestones the amount of silica
has been very greatly reduced; indeed, to less than one-sixth of that in the
original rocks. Complementary to this depletion of silica in the limestones
is its segregation in the sandstones. In these rocks the silica is a little
more than one-third greater than the average for the original rocks. This
small difference is due to the fact that the silica in the sandstones is mainly
in the form of quartz; while in the original rocks, as already noted, the
larger part is in the silicates, of which it composes about two-thirds.
Therefore, while the percentage of silica in the lithosphere as a whole is
two-thirds as great as in the sandstones, the amount of silica as quartz in
the lithosphere, 12 per cent (see p. 937), is only about one-sixth as great
as in the sandstones, for the majority of sandstones contain 75 per cent or
more of silica.
Considering the zone of katamorphism, there is constant solution of
silica in the belt of weathering and steady additions of silica in the belt of
cementation, already fully explained. (See pp. 473-480, 516-517, 621-
623, 634-636.) Carbonation in the belt of weathering destroys the silicates
and liberates silica as colloidal silicic acid. This is largely carried down-
ward into the belt of cementation and is there deposited in enormous quan-
tities. (See p. 618.) Hence we have constant subtraction of silica from the
belt of weathering and its steady addition to the belt of cementation. In
the belt of weathering the proportion of silica may be increased or decreased
according to the rate of solution of the other elements. (See pp. 507 et seq.).
vClarke, cit., Bull. 168, p. 14.
REDISTRIBUTION OF SILICON. 961
By erosion the belt of cementation, enriched in silica, rises into the belt of
weathering and the rocks rich in quartz are broken down. The larger
part of the quartz is not chemically modified, but mechanically disintegrated,
contributed to the streams, and by them carried to the sea. It there joins
the quartz liberated from the rocks of the shore by the action of the waves,
and the whole is segregated in sandstone formations. Many of the sand-
stone formations thus produced in early geological time later became land
areas. During later geological time streams running over sandstone areas
contributed to the sea more than an average amount of quartz. Thus the
combined processes of metamorphism and denudation have resulted in
segregating sufficient quantities of quartzose sands to produce the quartz-
sand formations and their metamorphosed equivalents, the quartzites and
quartz-schists.
In the deep-seated zone of anamorphism silica is neither added nor
subtracted in any considerable quantity, but the form of the compound is
often extensively changed. As fully explained (pp. 677-679), silica unites
with the bases, especially those of the carbonates, to produce silicates.
Whether at the present time the destruction of the silicates in the zone of
katamorphism, and especially in the belt of weathering, or the development
of the silicates and the destruction of the carbonates in the zone of
anamorphism is more important is considered under the heading ‘‘Carbon.”
(See pp. 962-974.)
The proportion of silica in the different classes of sediments gives a
criterion by which the estimates of the relative amounts of the sediments
(see pp. 940-941) may be very roughly tested. If the shales, sandstones,
and limestones compose 0.65, 0.30, and 0.05 of the sediments, respectively,
and contain the percentages of silica above given, the sum of the multiples
of the percentage of silica in each of the sediments by the quantities of
the sediments should equal the average percentage of silica in the original
rocks. ‘Thus
58.38 X.65-+81.76 X.30+9.64 x .05=62.957.
But this equation shows an excess of 3.247 per cent of silica over that
determined by analysis, 59.71 per cent.
Practically all of the silicon occurs in the lithosphere. If all classes of
sedimentary rocks were taken into account, including both their composition
and mass, an estimate of the percentage of silica based upon the sediments
MON XLv1I—04——61
962 A TREATISE ON METAMORPHISM.
ought to correspond with the percentage found in the original igneous rocks.
Therefore the discrepancy above noted indicates that there are factors left
out of and errors in the above equation. In reference to the similar equations
of most of the other elements it will be seen that the determination of the
amount of the element as made from the sediments usually shows a defi-
ciency as compared with the original rock. It will be explained that one of
the reasons for this is that determinations of the average composition of
the sedimentary rocks based upon analyses of the shales, sandstones, and
limestones neglect the particular formations in which an element is segre-
gated to the largest extent. In the case of silica the reverse is the case.
The sandstones in which silica is concentrated are included; whereas other
exceptional compounds in which there is depletion of silica are not included.
Consequently we should expect in the case of this compound that the esti-
mates made upon the basis of the sediments should be in excess of the
amount found in the original igneous rocks. While the discrepancy is thus
partly explained, I suspect that it is due more largely to an error in the
determination of the amount of silica in the shales. Few analyses have
been made of this class of compounds. It will be seen on subsequent
pages (see pp. 984-986) that alumina shows a discrepancy just the reverse
of that of silica. It appears to me probable that when we have better rep-
resentative analyses of the shales, taking into account their composition
and masses, the silica will be lower and the alumina higher than in the
analyses given.
CARBON.
AMOUNT OF CARBON.
In 1891 Clarke estimated the carbon as 0.21 per cent of the lithosphere,
hydrosphere, and atmosphere together. This corresponds to 0.22 per cent
of the original rocks, 0.002 per cent of the hydrosphere, and only .0127
per cent by weight of the atmosphere. In 1900 his estimate for the
lithosphere was reduced to 0.12 per cent. Recently Letts and Blake,’ as a
result of careful consideration of all the data and new work, conclude that
carbon dioxide is present in the atmosphere to the amount of three volumes
in 10,000, which corresponds to about 0.045 per cent by weight.’
“Letts, E. A., and Blake, R. F., The carbonic anhydride of the atmosphere: Sci. Proc. Royal
Dublin Society, vol. 9, new series, pt. 2, 1900, p. 172.
> Mendeléeff, D., The principles of chemistry, translated by George Kamensky: Longmans, Green
& Co., London, 6th ed., 1897, vol. 1, p. 288.
AMOUNT OF CARBON IN LITHOSPHERE. 963
Reckoned as an oxide, 0.12 and 0.22 per cent of carbon would corre-
spond to 0.44 and 0.81 per cent of carbon dioxide, respectively. This
places carbon dioxide tenth among the oxides, and carbon eleventh in
abundance, making it less plentiful than either hydrogen or titanium. In
Clarke’s estimate of 1900 the carbon in limestones is excluded from con-
sideration.” In his estimate for 1891 is included the carbon in a layer of
limestone 528 feet thick enveloping the globe, the amount of limestone
estimated by Reade.’
When Clarke made his estimate in 1891 he thought the analyses of
volcanic and crystalline rocks probably showed too high a percentage of
carbon dioxide, since the rocks analyzed were not perfectly fresh. But he
regards this error as offset by the undeterminable amount of carbon in coals,
shales, and petroleums, which were not considered.’ He thinks his resultant
estimate, 0.22 per cent for the lithosphere, can hardly be too low. However,
it is possible that this estimate is too small, since it is practically impossible
to make more than the roughest sort of a guess as to the amount of carbon
contained as graphite, anthracite, and hydrocarbons in the crust of the
earth. The quantity of carbon in the coal of the earth has never been esti-
mated, and doubtless the quantity present in workable beds is less than —
that present in nonworkable, widely extended, small, and thin seams. Fur-
thermore, in the carbonaceous shales associated with the coals there is a
vast amount of carbon, possibly more than in the coals. Shales as old as
the Algonkian in some places contain nearly 20 per cent carbon. Graphitie
gneisses have a widespread occurrence among the older metamorphosed
sediments. It seems to me that the carbon present in the above forms in
the lithosphere is likely to much more than compensate for the excess of
carbon dioxide due to alteration in the igneous and crystalline rocks
analyzed, since rocks which are as fresh as can be obtained are always
selected for analysis. But, on the other hand, the estimated amount of
carbon in limestone is probably too high, for it is very probable that Reade’s
estimate of the quantity of limestone is too great. While it is possible that
the amount of carbon in the lithosphere is somewhat greater than the larger
amount estimated by Clarke, even interpreting all doubtful points in favor
@Clarke, F. W., Bull. U. 8. Geol. Survey, No. 168, p. 15.
5Clarke, Bull. U. S. Geol. Survey, No. 78, p. 38.
964 A TREATISE ON METAMORPHISM.
of carbon, the amount would still be small, certainly but a fraction of 1
per cent.
Farrington’s* analyses of meteorites show that carbon is present in
these bodies in various forms. As a solid it occurs as amorphous carbon,
as graphite, and as diamond. As solid hydrocarbons it is found as CH,,,
as C,H,,8;, and as CyH,O,. As gases included in the meteorites it occurs as
carbon monoxide (CO), carbon dioxide (CO,), and carbureted hydrogen
(CH,). The total quantity of all of these compounds in any one meteorite
is small, usually less than 1 per cent, and corresponds very well with the
amount in the outer 10 miles of the earth.
The importance of carbon compounds to the lithosphere is out of all
proportion to the abundance of this element. As is well known, the carbon
compounds are the basis of all forms of life, and it has been seen that life
is one of the most potent factors concerned in the decomposition of rocks.
Hence the small percentage of carbon in the lithosphere, hydrosphere, and
atmosphere is of first importance among the elements concerned in the
metamorphism of rocks.
SEGREGATION OF CARBON.
It is evident from what has been said in considering the amount of
carbon in the lithosphere that this element has been segregated in various
ways. Carbon is segregated (1) by the process of carbonation, and (2) in
the carbonaceous deposits. It has been seen (pp. 473-474) that the carbon
now being segregated is directly derived from the carbon dioxide of the
atmosphere. The amount of CO, in the atmosphere, calculated upon the
basis of 0.045 per cent by weight, is 2,381,400,000,000 metric tons. This
figure is very close to that of Dittmar, who estimates the amount to be
2,277,000,000,000 metric tons.’ Each of the processes of segregation and
the sources of supply of carbon dioxide to the atmosphere will be considered.
Segregation by carbonation —Many years ago Hunt’ stated ‘that the carbonic
acid absorbed in the process of rock-decay during the long geologic ages,
and now represented in the form of carbonates in the earth’s crust, must
have equalled probably two hundred times the entire volume of the present
«Farrington, cit., pp. 395-408, 522-532.
> Dittmar, William, Narrative of the cruise of H. M.S. Challenger, 1873-1876, vol. 1, pt. 2, 1885, pp.
954-955.
¢Hunt, T. Sterry, The geological relations of the atmosphere: Rept. British Assoc. Ady. Science,
1878, p. 544. :
SEGREGATION OF CARBON. 965
atmosphere of our earth” The data upon which this calculation is made
are not given, and it may be a great overestimate, but that any calculation
could have given such a result shows how trivial is the amount of carbon
dioxide at present in the atmosphere as compared with that which has been
abstracted by carbonation.
In 78 shales the amount of carbon dioxide is 2.64 per cent, in 624
sandstones it is 3.03 per cent, and in 843 limestones it is 88.58 per cent.
While segregation of the carbonates is therefore the.rule for all the
rocks produced by the processes of denudation, this segregation takes
place to the least extent in the shales, to a greater extent in the sandstones,
and to the greatest extent in the limestones.
If carbon dioxide composes but 0.81 per cent of the original rocks, it
appears that as compared with the average of the original rocks about three
times as much carbon dioxide is concentrated in the shales where it is
lowest, and forty-seven times in the limestones. This shows how enor-
mously the carbon in the secondary rocks has been increased, as compared
with the original rocks, by the process of carbonation in the zone of kata-
morphism.
Combining the above figures with the masses of the sediments given
on page 940, it follows that the amount of carbon dioxide in the shales is
11,583,000,000,000,000 metric tons, in the sandstones is 6,135,750,000,-
000,000 metric tons, and in the limestones is 13,020,750,000,000,000
metric tons; total, 30,739,500,000,000,000 metric tons. From the fore-
going figures it appears that the fixed CO, in the shales, sandstones, and
limestones is 12,900 times the amount now in the atmosphere, and is 1,970
times the amount of free CO, in both atmosphere and hydrosphere, upon
the basis of 0.001 per cent in the hydrosphere, and is 660 times the total
free carbon dioxide of the atmosphere and the hydrosphere together, upon
the basis of Dittmar’s figures.”
It appears from the foregoing that in the great limestone deposits
a large quantity of carbon dioxide, 38.58 per’ cent, has been segregated
from the exceedingly small amount of carbon in the original igneous rocks
and from the atmosphere and hydrosphere. As already seen there is 47
times as much carbon in the limestone formations as in the original igneous
rocks. The importance of this process of segregation, considered from an
economic point of view, is not generally appreciated. In order to realize
a Dittmar, Narrative of the cruise, etc., p. 955.
966 A TREATISE ON METAMORPHISM.
this it is necessary to consider the relations of man to agriculture. The
limestone soils of the United States, and of many other parts of the world,
are of surprising fertility. The historian tells how settlement followed the
limestone soils of the Great Valley, of the Blue Grass region of Kentucky
and Tennessee, and of the Upper Mississippi Valley. The world over,
limestone soils have unsurpassed fertility. Since the products of the soil
are the materials of most importance to man, the greatest geologic formation
from an economic point of view is limestone produced by the segregation
of carbon and calcium.
Segregation in carbonaceous deposits—Besides the carbon which is segregated as
carbon dioxide in the lithosphere by carbonation, vast quantities have
been buried in the rocks in forms varying from nearly pure carbon, as
eraphite and anthracite, to cellulose. Various estimates have been made
of the amount of carbon in the better-known coal beds, but so far as I know
no attempt has been made to estimate the amount of carbon in all the coals,
good and poor, in the carbonaceous shales associated with them, and in the
shales and other rocks not associated with coal beds. The fact that
carbon as hydrocarbons exists in shales has been usually overlooked.
Carbon forms 0.81 per cent of 78 rocks analyzed. (See p. 938.) This
quantity seems small, but when the enormous mass of the shales is con-
sidered, 438,750,000,000,000,000 metric tons, it is seen that this percentage
amounts to 3,553,875,000,000,000 metric tons. If this amount of carbon
were oxidized to CO, it would represent 5,470 times the amount of carbon
dioxide in the atmosphere, as calculated above. If it could be consumed at
the rate of 1,000,000,000 metric tons per year, which is more rapid than
the present rate of combustion of coal, it would last over 3,500,000 years.
This great amount of carbon, in these, as well as in some other rocks, is
ordinarily overlooked. It is therefore certain that the total amount of car-
bon contained as hydrocarbons in rocks of all kinds, from the great coal
seams to the sediments containing but a very small percentage, is enormous.
Next to the limestones the economic product of vastly greater impor-
tance than any other is coal. Coal contains on an average 80 per cent
carbon, or 364 times the amount in the original igneous rocks, in which the
carbon amounts to only 0.22 per cent. The inestimable economic value of
coal is so fully appreciated that it need not be emphasized. The point to
be understood is that the forces and agents of geology have segregated
this immeasurably valuable material from a fraction of 1 per cent of
SOURCES OF SEGREGATED CARBON. 967
carbon in the original rocks and from the atmosphere and hydrosphere.
Furthermore, the segregation of carbon from widely dispersed material is
essential to life, and life has profoundly modified many geological processes.
The segregation of carbon by the various geological processes in limestone,
in organisms, and in coal explains the great importance of this sparse
element in the genesis of the earth.
SOURCES OF SEGREGATED CARBON.
It has been seen that fixed carbon exists in the salts of the ocean and in
the lithosphere as a carbonate and as hydrocarbons. The important pro-
cesses now producing these various carbon compounds are those involved
in the interaction of the carbon dioxide of the atmosphere, of organic bodies,
and of the rocks. If we apply the fundamental hypothesis of geology that
the processes now at work explain past results, we must assume that the
carbon dioxide for carbonation and that for the carbonaceous deposits is
derived indirectly from the atmosphere. It has been seen that the amount
of carbon required for this work is many thousands of times the amount
of carbon dioxide now free in the atmosphere, and many hundred times
the amount free or potentially free in the hydrosphere. The amount of
carbon dioxide in the atmosphere may have been originally vastly greater
than at present, but even if this were so the demands upon carbon have
been so great that it is probable that the atmosphere has been replenished
in that compound. There are several possible sources of carbon dioxide
for the replenishment of the atmosphere.
An important immediate source of carbon dioxide is the ocean. The
amount of carbon dioxide in the hydrosphere, computed on the basis of a
content of 0.002 per cent of carbon (Clarke’s estimate), is 96,531,915,000,000
metric tons. Dittmar’s estimate of the entire carbon dioxide of the ocean
is 70,350,160,000,000 metric tons, or considerably less than Clarke’s. Of
Dittmar’s total he estimates the loose carbon dioxide (i. e., the excess of
carbon dioxide of bicarbonates above that required for normal carbonates
and the free carbon dioxide together) of the ocean to lie between 35,200,-
000,000,000 and 52,800,000,000,000 metric tons. Supposing the true
amount to be the average of these, this would give 44,000,000,000,000
metric tons or 19.3 times the amount of carbon dioxide in the atmosphere
as calculated by Dittmar. According to Schloesing the carbon dioxide
@Dittmar, William, Report of voyage of H. M. 8S. Challenger, 1873-76; Narrative of the cruise,
vol. 1, pt. 2, 1885, pp. 954-955.
968 A TREATISE ON METAMORPHISM.
in the bicarbonates of the ocean beyond that required to balance the bases
as normal carbonates is about ten times the amount in the atmosphere.
It thus appears that the potentially free carbon dioxide of the ocean is in
much greater amount than the actual free carbon dioxide in the atmos-
phere. According to Schloesing” there is a very delicate adjustment
between the amount of carbon dioxide in the atmosphere and the hydro-
sphere. Great additions to the amount in the atmosphere would result in
absorption by the hydrosphere. Depletion in the amount in the atmos-
phere is compensated by additions from the hydrosphere. This idea of the
balance between the amount of carbon dioxide in the atmosphere and that
contained as bicarbonate in the ocean has been elaborated by Chamberlin
and Tolman.’ “They note that under the laws of physical chemistry, when
the amounts of free carbon dioxide in the atmosphere and in the ocean are
in equilibrium and this equilibrium is disturbed by a decrease of the carbon
dioxide of the amosphere, this compound will pass from the ocean to the
atmosphere until the equilibrium is restored. Chamberlin has also empha-
sized the fact that when the calcium bicarbonate of the ocean is precipitated
by organisms, one-half of the carbon dioxide is thereby liberated. Conse-
quently at periods of limestone building the amount of free carbon dioxide
in the ocean, and therefore the amount in the atmosphere, would be greatly
increased. These considerations lead to the conclusion that at various times
in the history of the world the atmosphere has been replenished in carbon
dioxide at the expense of the ocean. ;
Petrographic work of recent years shows that graphite and diamond
are original constituents of some of the igneous rocks, especially the
basalts. Further, Tilden,’ by chemical analyses, has shown that the
plutonic igneous rocks granite and gabbro, and also the volcanic igneous
rock basalt, at certain localities contain considerable quantities of carbon
monoxide and methane.
«Letts, E. Bs and Blake, R. F., The carbonic anhydride of the atmosphere: Sci. Proc. Royal
Dublin Society, vol. 9, new series, pt. 2, 1900, pp. 160-161.
> Chamberlin, T. C., A group of hypotheses bearing on climatic changes: Jour. Geol., vol. 5, 1897,
pp. 653-683. Chamberlin, T. C., The influence of great epochs of limestone formation upon the con-
stitution of the atmosphere: Jour. Geol., vol. 6, 1898, pp. 609-621. Chamberlin, T. C., An attempt to
frame a working hypothesis of the cause of glacial periods on an atmospheric basis: Jour. Geol., vol. 7,
1899, pp. 545-584. Tolman, C. F., The carbon dioxide of the ocean and its relations to the carbon
dioxide of the atmosphere: Jour. Geol., vol. 7, 1899, pp. 585-618.
¢Tilden, W. A., On the gases inclosed in crystalline rocks and minerals: The Chemical News,
vol. 75, 1897, pp. 169-170.
SOURCES OF SEGREGATED CARBON. 969
The igneous rocks containing the carbon compounds may flow out in
the belt of weathering, or, if deep seated, may reach it by denudation.
In either case, under favorable conditions, oxygen may slowly oxidize the
amorphous carbon, graphite and diamond, carbon monoxide, and the carbon
of methane, to carbon dioxide. If the theory be correct which regards
high temperature and volcanism as much more prevalent in the early stages
of the earth than at the present time, the oxidation of these compounds
may have gone on more rapidly than at present, but even now when lavas
containing carbon compounds are poured out over the surface their high
temperature in the belt of weathering affords conditions very favorable for
the oxidation of the carbon.
In this connection it should be recalled that carbon dioxide is given
off in great quantities by volcanoes. ‘‘Cotopaxi, according to Boussingault,
evolves more carbonic anhydride annually than a whole city like Paris.” “
Cotopaxi is, of course, only a single volcano, and many other volcanoes
are giving off large amounts. Moreover, at times of regional volcanism
it is to be presumed that vastly more carbon dioxide is given off than at
times of local voleanism like the present. It is probable that some, perhaps
much, or even a large part of the carbon dioxide extruded by volcanoes
is that resulting from the oxidation of the various carbon compounds
present in the original magmas, although a part of it may be occluded
carbon dioxide and some or much of it may be derived from carbonate
formations representing previous segregations.
It is to the oxidation of the carbon of the original rocks that we must
look for one important primal source of carbon dioxide.
Tilden found that in a number of rocks, including gneiss, granite,
gabbro, and basalt, the volume of occluded gases varied from one to
eighteen times the volume of the rock, and of these gases carbon dioxide
was the most abundant, varying in five cases from 23 to 78 per cent, but in
one case being as low as 5.5 per cent.’ Occlusions occur even to a greater
extent in the sedimentary rocks which have been metamorphosed under
deep-seated conditions by the process of silication of the carbonates.
When the rocks occluding carbon dioxide are disintegrated and decom-
«Letts, E. A., and Blake, R. F., The carbonic anhydride of the atmosphere: Sci. Proc. Royal
Dublin Society, vol. 9, new series, pt. 2, 1900, p. 159.
b Tilden, W. A., On the gases inclosed in crystalline rocks and minerals: The Chemical News,
vol. 75, 1897, pp. 169-170.
970 A TREATISE ON METAMORPHISM.
posed in the belt of weathering this carbon dioxide escapes. Since the
volume of the original and sedimentary rocks which have been broken up
into fine particles or decomposed so as to allow the major portion of the
gases to escape is very great, the amount of carbon dioxide given off to the
atmosphere from this source must be vast.
In order to obtain any idea of the net gain to the atmosphere from
carbon dioxide liberated from occlusion, much more work must be done.
First, an estimate must be made as to the mass of the original igneous rocks
which have been broken down, and it must be ascertained how much
occluded carbon dioxide there is upon an average in these rocks in order
to compute the amount of carbon dioxide liberated. In the second place it
must be ascertained how much occluded carbon dioxide is in the secondary
rocks. Locally the sedimentary rocks are very rich in occluded carbon
dioxide in consequence of the process of silication and of the partial escape
of the liberated carbon dioxide. This carbon dioxide is indirectly derived
from the atmosphere. Third, a part of the enormous quantity of carbon
dioxide given off by volcanoes may be that originally occluded in the
centrosphere. The quantity of carbon dioxide of this origin must appar-
ently long, if not forever, remain a matter of conjecture. The total gain
of the atmosphere in carbon dioxide from occlusions is the sum of that
liberated from the original igneous rocks and that derived from magma,
less that held in the secondary rocks.
It has been seen that various forms of carbon are subordinate original
constituents of meteorites. (See pp. 945-946.) Hence, another source
of carbon dioxide may have been the oxidation of the carbon contained
in meteorites and the exhalation from meteorities of carbon dioxide
originally contained in them in that form. However, the additions of
carbon dioxide derived from meteorites can hardly have been important
since the beginning of Algonkian time. Consequently this means of
restoring carbon dioxide to the atmosphere is of little consequence for
the period which we are considering, that in which the sedimentary rocks
were built up.
Another important source of carbon dioxide is the water ascending
from the zone of anamorphism. The amount of this carbon dioxide is
incalculable, but certainly it is very large. The gases included in hot
springs and in deep waters rising from mines are dominantly composed
SOURCES OF SEGREGATED CARBON. 971
of carbon dioxide. This fact has been noted again and again since the
days of Bischof. ‘‘Lecoq has calculated that of the mineral springs those
of Auvergne alone give off in the same time [one year] 7,000,000,000
cubic meters of the gas, an amount rather less than one-tenth of the volume
produced by the annual combustion of the coal employed throughout the
Ya
whole of Europe. It is to be remembered that Auvergne is but a single
region, and while the amount of carbon dioxide emitted is exceptional, it
is approximated in other regions where there are hot carbonated springs.
The total amount of carbon dioxide emitted in the few areas where it is
exceptionally abundant is probably small as compared with the quantity
carried by the vastly larger amount of water of the great mass of springs
of the world.
If the argument given on pages 176-177, 677-679 be correct, the
carbon dioxide furnished by deep-seated springs is largely that liberated
by the process of silication of carbonates, although some part of it may be
derived from the carbon dioxide originally occluded in the earth.
The segregation of carbon as carbonates in the sedimentary rocks is a
consequence of the reaction of carbonation, and chiefly of carbonation of the
silicates. These carbonates, as already seen, are mainly concentrated in the
carbonate formations, but to a considerable extent are found as accessory
minerals in the shales and in the sandstones. When the rocks of the zone of
katamorphism are buried so deeply as to pass into the zone of anamorphism
the silication of the carbonates occurs with the liberation of the carbon
dioxide. That this process has taken place almost completely in the pelites
thus buried is shown by the very small percentage of carbonates in the
schists, which are the metamorphosed equivalents of ancient muds. The
silication of the limestones which have passed into the zone of anamorphism
is im various stages, ranging from slight to complete. It is certain that
this process is going on at the present time on a very great scale. Indeed,
it is not impossible that the process of silication in the zone of anamorphism
is taking place with a close approximation to the speed of the process of
carbonation in the belt of weathering.
In so far as the process of silication with decarbonation is taking place
in the deep-seated zone, and the liberated carbon dioxide is carried by the
ground water to the atmosphere or to the hydrosphere, the depletion of the
“Letts and Blake, cit., p. 159.
972 A TREATISE ON METAMORPHISM.
atmosphere and the hydrosphere in carbon dioxide by carbonation is
compensated.
In the early eras of the deposition of the sedimentary rocks, before
carbonates existed in important quantity, it is certain that carbonation far
outclassed silication; for before the latter process became important,
carbonates were necessarily formed in large amounts and buried to a
considerable depth. The existence of the vast amounts of carbonates in
the sediments is evidence that carbonation has greatly exceeded silication.
The further carbonation advances the more abundant become the
carbonate formations, many of which in due time are buried to a great
depth, and thus the more important becomes silication. And finally, in the
natural course of events, the two processes will approach each other in
quantitative value. The carbono-silicic cycle will approach a_ balance.
But that silication is now keeping pace with carbonation is not asserted.
Only conjecture can be offered on that point. While silication of car-
bonates may not yet equal carbonation in amount, it is believed that the
former process directly or indirectly returns to the atmosphere a large
percentage of the carbon dioxide abstracted from it by carbonation.
Another obvious source of carbon dioxide for the atmosphere is the
oxidation of the carbonaceous materials of the earth. It has been pointed
out that nearly as fast as organisms are produced they are decomposed.
In so far as both processes occur there is neither loss nor gain of carbon
dioxide to the atmosphere. But continuously during a large part of geo-
logical time a residual portion of the organic material has not decomposed,
and thus the carbonaceous deposits have been built up, abstracting carbon
dioxide from the atmosphere. (See pp. 949, 966-967.) In so far as the
processes of the belt of katamorphism result in the storing of carbon in
the various hydrocarbons within the earth there is continuous depletion of
the carbon dioxide of the atmosphere, and during much of geological time
this has been a steady, cumulative process. But recently man has begun
artificially to oxidize these carbonaceous materials. Until the last half of
the nineteenth century this was unimportant, but in recent vears the oxida-
tion of carbonaceous material once buried within the earth has been carried
on on an enormous scale. As calculated on page 464, the combustion of
1,000,000,000 metric tons of coal turns into the atmosphere 2,933,333,000
metric tons of carbon dioxide. Thus, at this annual rate of combustion of
REDISTRIBUTION OF CARBON. Sg
coal, in 812 years the amount of carbon dioxide added by this process
would be equal to the total carbon dioxide of the atmosphere.
Possibly carbon dioxide may be steadily gathered to the earth from the
interplanetary or interstellar spaces by the force of gravity. According
to Hunt, this idea was first suggested by Sir William Grove in 1843.°
This source of carbon dioxide Hunt regards as the chief one. The pos-
sibility of se sregation of carbon dioxide from the stellar spaces has been
recently brought forward again by Chamberlin.’
If we write an equation representing the amount of carbon dioxide in
the sedimentary rocks we haye the following:
t=)
(2.641 .81 X 44/12) X .65-+ (3.03 X .30)-+ (38.58 x .05) = 6.4585.
In the above equation 2.64 is the CO, in shales and 0.81 is the carbon
contained as carbonaceous material in shales, which must be multiplied by
44/12 in order to find the equivalent amount of CO,. The above equation
does not take into account coal and other especially carbonaceous deposits,
which would increase the quantity. Even omitting the coal deposits, the
average amount of carbon in the sedimentary rocks is 8 times greater than
Clarke’s estimate of the amount of carbon dioxide in the original igneous
rocks, 0.81 per cent. As in the case of sulphur, it is not necessary to
suppose that these two amounts should balance, for there may have been
originally a large amount of carbon dioxide in the atmosphere and hydro-
sphere not derived from the crystallized rocks. Also, as has been pointed
out, it is little short of certain that the carbon originally in magma, either as
carbon, hydrocarbons, or carbon dioxide, largely escapes as carbon dioxide
at the time of the crystallization of the rocks. At this time of escape the
hydrocarbons and carbon are largely oxidized to CO, However, the
above equation shows it to be certain that the original crystallized igneous
rocks do not constitute the sole, or even the chief, source of the carbon
which has taken such an important part in the economy of the earth.
In summary it appears that the carbon of the lithosphere was originally
widely dispersed in small quantities. The operations of metamorphism
concentrate carbon in the carbonates and carbonaceous deposits, and to a
«Hunt, T. Sterry, The geological relations of the atmosphere: Rept. Brit. Assn. Ady. Sci., 1878,
p. o44.
>Chamberlin, T. C., A group of hypotheses bearing on climatic changes: Jour. Geol., vol. 5,
1897, pp. 653-683.
974 A TREATISE ON METAMORPHISM.
less extent in the carbonaceous shales. The direct source of the carbon
for this concentration is the atmosphere, which has been continuously
replenished in various ways.
The certain original sources of carbon dioxide for the replenishment
of the atmosphere were the magmas and meteorites. Silication, the
oxidation of organic material, and the ocean return to the atmosphere
carbon dioxide which las been taken from it. But all of the above
supplies of carbon dioxide are available to replenish the atmosphere. Con-
cluding, the chief processes which abstract carbon dioxide from the
atmosphere are those of carbonation and the building up of carbonaceous
deposits. All of the replenishing processes, including the reversing
processes of silication and the oxidation of buried carbon compounds,
have been barely able to keep a minute portion of carbon dioxide
in the atmosphere—0.030 per cent by volume, or 0.045 per cent by
weight. It is probable, however, that the work of man, especially during
the last half century, has returned a great volume of carbon dioxide to the
atmosphere by the artificial oxidation of carbonaceous material, and thus
has reversed the average of the processes of nature, which plainly appear
to have caused depletion of the carbon dioxide in the atmosphere. In
consequence, at the present time the amount of carbon dioxide in the
atmosphere may be increasing rather than decreasing.
TITANIUM.
According to Clarke’s estimate of 1891 titanium comprises 0.30 per
cent of the original rocks, the lithosphere, and the atmosphere together.
All of the titanium is in the original rocks, of which it composes 0.33 per
cent. In 1900 Clarke increased his estimate of titanium in the original
rocks to 0.41 per cent, thus giving it ninth place in the scale of abundance
next to potassium. In passing from sodium to titanium we go from the
elements which may be called abundant to those which are subordinate.
Reckoned as an oxide (TiO,) Clarke’s estimate of the amount in the original
rocks for 1891 is 0.55 per cent and for 1900 is 0.60 per cent.
In. the original rocks titanium occurs both as an oxide and as a titanite.
As an oxide it is found in ilmenite and in the rutile group. As a titanite
it is found in titanite and perovskite. The amount of these minerals
in the rocks is not usually large, but they are very widespread. The
REDISTRIBUTION OF TITANIUM. 975
original magnetite of many rocks is strongly titaniferous, and because of
its abundance may be the chief home of titanium.
Titanium forms 0.65 per cent of 78 shales; 0.33 per cent of 624 sand-
stones, and 0.07 per cent of 843 limestones. It thus appears that as com-
pared with the original rocks the amount of titanium is somewhat increased
in the shales, is reduced to a little more than one-half in the sandstones,
and is less than one-eighth in the limestones. If one were to estimate the
amount of titanium in the original igneous rocks by the amount in the
sedimentary rocks we would have the following equation:
-65 X .654-.33 & .380-+.07 & .05=.525.
This accords very well with the actual amount as estimated by analyses
in the original igneous rocks, 0.55 to 0.60 per cent. The minerals produced
by metamorphism are the same as those of the original rocks. The details
of the transformations with reference to the different zones and belts of
metamorphism have not been worked out. Titanium minerals have been
especially observed in the metamorphosed pelites and psephites, and to this
observation the analyses above given correspond.
PHOSPHORUS.
According to Clarke’s estimate of 1891 phosphorus forms 0.09 per cent
of the outer 10 miles of the crust of the earth, including the original rocks,
hydrosphere, and atmosphere. All of it is in the lithosphere, of which it
composes 0.10 per cent. In his estimate of 1900 Clarke reduces this amount
to 0.09 per cent.
Phosphorus is thus twelfth in the scale of abundance, ranking next to
carbon, an element of vastly greater importance. Reckoned as an oxide
Clarke’s estimates of 1891 and 1900 are both 0.22 per cent.* This gives
phosphorus oxide eleventh place in the table of oxides. In the original
rocks phosphorus is known to occur only in the mineral apatite, of which
it composes 18.47 per cent. This mineral, while usually subordinate in
amount, is very widespread. In the meteorites phosphorus is found in
schreibersite (FeNiCol,P). (See p. 946.) The amount of P,O;in 78 shales
is 0.17 per cent; in 624 sandstones 0.07 per cent; in 345 limestones not
used for building purposes is 0.04 per cent, while in 498 limestones used for
building purposes it is 0.42 per cent.’
«Clarke, cit., Bull. 78, p. 39; Bull. 168, p. 15. » Clarke, cit., Bull. 168, p. 14.
976 A TREATISE ON METAMORPHISM.
The amount of P.O; in the limestones used for building purposes rather
than the average of all limestones is taken, as probably more nearly repre-
senting the average amount of phosphorus in these rocks, since it is well
known that in the belt of weathering the phosphorus is leached out.
Therefore it appears that the amount of P.O; in the shales is about
three-fourths of that present in the original rocks; in the sandstones is about
one-third; and in the limestones is nearly doubled. If one multiplies the
amount of P,O,; in each of the sediments by their estimated quantities, and
adds them together, we have the following equation:
.17.65-++.07 X.30-+.42 .05 =. 1525.
This shows a deficiency of about one-third of P,O;, as compared with
the original rocks. A portion of this deficiency is undoubtedly accounted
for by the phosphate rock deposits, such as those of South Carolina, Florida,
and Tennessee, and by the guano deposits. These represent the economic
products of segregating processes which have increased the proportional
amount of phosphorus many fold.
All agree that the first stage of the segregation of phosphates in the
sedimentary rocks is accomplished through the agency of animals. For the
guanos this first concentration is made by sea birds. For the more exten-
sive phosphate deposits the first concentration of phosphorus was by inver-
tebrate animals, such as brachiopods and crustaceans, and by vertebrates,
such as sharks and saurians. Very commonly this first concentration is in
limestones. The further concentration of the phosphorus of guanos and
that of phosphatic limestones and other rocks is by underground water.
The circulations producing the concentration and the forms of the resultant
deposits are multifarious, but the general principle applicable to most cases
appears to be that the phosphates are dissolved by descending waters in
the belt of weathering and thrown down on reaching the belt of cementa-
tion.” Usually the latter reaction takes place in the upper part of the belt
of cementation, so that the phosphates are segregated at or just below
the level of ground water. The precipitation of the phosphates is especially
«Penrose, R. A. F., jr., Nature and origin of deposits of phosphate of lime: Bull. U. 8. Geol. Survey
No. 46, 1888, pp. 1-143. Dall, W. H., and Harris, G. D., Correlation papers—Neocene; -hosphatic
deposits of Florida: Bull. U. 8. Geol. Survey No. 84, 1892, pp. 134-140: Eldridge, Geo. H., A prelimi-
nary sketch of the phosphates of Florida: Trans. Am. Inst. Min. Eng., vol. 21, 1893, pp. 196-231.
Hayes, C. W., The Tennessee phosphates: Seventeenth Ann. Rept. U. 8. Geol. Survey, 1895-96, pt. 2,
1896, pp. 513-850.
REDISTRIBUTION OF PHOSPHORUS. Say
likely to occur in limestone. Eldridge suggests that under such circum-
stances the precipitation is brought about “by the simple interchange of
bases between the phosphate and carbonate of lime thus brought together,
or by the lowering of the solvent power of the waters through loss of car-
bonic acid. The latter would happen whenever the acid was required for
the solution of additional carbonate of lime, or when, through aeration, it
should escape from the water. The zone of phosphate deposition was
apparently one of double concentration, resulting from the removal of the
soluble carbonate thus raising the percentage of the less soluble phosphate,
and from the acquirement of additional phosphates of lime from the over-
lying portions of the deposit.” ”
The precipitated phosphate is likely to be deposited in nodules. This
material is more resistant than the containing limestone. Through erosion
by streams or ocean there may be a further concentration of the phosphates
due to the greater resistance of the phosphate to both solution and
mechanical wear as compared with limestone.
The bedded Devonian phosphates of Tennessee are in part, according
to Hayes, an important exception to the above, the phosphatic material of
the beds being regarded by him as largely concentrated when originally
laid down.’
Whether or not the sedimentary rocks rich in phosphates not considered
in the analyses of shales, sandstones, and limestones are sufficient to account
for the deficiency in these rocks in phosphorus is undetermined. In this
connection it should be remembered that the vein phosphates of Canada,
Norway, and similar phosphatic rocks, in which a large amount of phos-
phorus is concentrated probably im consequence of original igneous or
pegmatitic processes, are also excluded from the original rocks, and in so
far as these deposits occur they offset the concentrations of the sedimentary
phosphatic deposits.
The redistribution of phosphorus by the ordinary metamorphic processes
has as yet been little studied. In the iron-ore deposits in which phosphorus
is a very important economic compound such studies have been begun, but
as yet no general conclusions have been reached.
«Eldridge, Geo. H., cit., p. 216.
b Hayes, C. W., and Ulrich, E. O., Description of the Columbia [Tennessee] quadrangle: Geologic
Atlas U. S., folio 95, U. 8. Geol. Survey, 1903, pp. 4-6.
MON XLv1I—O4 62
978 A TREATISE ON METAMORPHISM.
CHLORINE.
According to Clarke’s estimate of 1891 chlorine, including bromine,
forms 0.15 per cent of the original rocks, the hydrosphere, and the atmos-
phere. Of the ocean it composes 2.07 per cent. His estimate for the
original rocks in 1891 and 1900 is the same, 0.01 per cent.
As to the source of chlorine the only minerals of the original igneous
rocks in which it is found are apatite, sodalite, and the marialite molecule
of the wernerites. Chlor-apatite and marialite are very unimportant original
minerals, and therefore the chief mineral in which it is found is sodalite, of
which it constitutes 7.3 per cent.“
It has already been explained (pp. 789-790) that chlorine is emitted
from volcanoes as hydrochloric acid and to a less extent as chlorine.
It has been further noted that a part of this chlorine may possibly have
been derived from sea water; but there is reason to suppose that much
of it is that of the original magmas. So far as this is true we have an
original source for chlorine. And when it is considered that at periods of
regional volcanism the amount of chlorine which probably issued from
volcanoes was vastly greater than at the present time of local volcanism,
it follows as a possibility that volcanism is the most important source of
that element.
In the meteorites chlorine occurs in lawrencite (FeCl,). Lawrencite
is a volatile and readily decomposable mineral. If at the time the earth
stuff segregated chlorine was contributed as lawrencite, it is certain that the
action of water in the magmas upon this compound would produce hydro-
chloric acid; this suggests a source of a part of the hydrochloric acid of
volcanoes.
In the secondary rocks the amount of chlorine is very small. In the
mechanical sediments it is so small that it can not be estimated. It is not
mentioned in the analyses of the shales, and only a trace is reported in the
sandstones. In 843 limestones chlorine composes 0.01 per cent. ‘The
sea water which was originally between the pores of the mechanical
sediments must have contained considerable chlorine. The absence of
chlorine from the mechanical sediments shows how thoroughly the sea
water has been squeezed or leached out in the transformations of the
rocks from their original forms to shales, sandstones, etc. The presence of
«Dana, E.8., A system of mineralogy, Wiley & Sons, New York, 6th ed., 1892, p. 429.
REDISTRIBUTION OF CHLORINE. 979
an amount of chlorine in the limestones somewhat greater than the average
for the original rocks is not easy to understand. The explanation may lie
in the fact that limestones are the home of the phosphates and that the
phosphate apatite is a chlorine-bearing mineral.
A large part of the chlorine which was present in the original igneous
rocks and which has escaped into the atmosphere through volcanoes has
passed into the hydrosphere. Under ordinary conditions, after the chlorine
is once taken into solution apparently very little of it is again redeposited.
It continues in the circulating waters until it reaches the sea, in which it
appears to have been segregating during geological time. Of all the
elements of the salts of the sea it is by far the most abundant. According
to Dittmar’s estimates it composes 55.292 per cent of the total salts, or
more than half of all the compounds held in solution in the sea. This
would amount to 25,557,000,600,000,000 metric tons.
While a large part of the chlorine which has been subjected to
metamorphic processes has been carried to the sea, at places where there
are inclosed basins, as for instance the Great Basin, the Dead Sea, ete.,
chloride deposits have been built up. Locally these are of considerable
magnitude. Moreover, during geological ages chloride deposits, mainly of
sodium, have accumulated in abundance, so that considerable beds con-
taining variable percentages of chlorine have been buried below later
sediments. Such material furnishes the rock salt of the salt mines, for
instance, of Poland, and the brines of many salt-producing districts, such
as those of New York and Michigan.
As yet it is entirely impracticable to estimate the amount of chlorine
which is locked up in the rocks as sodium chloride. It has been the
common impression that the major portion of the sodium chloride is in the
sea rather than in such deposits. It is subsequently suggested that this
impression may be erroneous (see pp. 997-998), and that the great amount
of segregated sodium is in the salt deposits rather than in the sea. If this
be true for that element, it would also be true for chlorine.
The chlorine concentrated with sodium in the salt deposits and the sea
is the great economic product resulting from the segregation of this element
in large proportions from material originally very sparsely disseminated.
The great importance of sodium-chloride to man and to animal life in
general is so well known that it need not be emphasized.
~
980 A TREATISE ON METAMORPHISM.
NITROGEN.
Nitrogen forms 0.02 per cent of the outer 10 miles of the crust of the earth
including the lithosphere, hydrosphere, and atmosphere.” All but an inesti-
mably small quantity of this is in the atmosphere, of which it composes
76.88 per cent by weight. (See p. 944.) It stands sixteenth in the scale of
abundance. Nitrogen is among the gases which are occluded in meteorites;
but what portion of this nitrogen is absorbed by the meteorites while passing
through the atmosphere and what portion has been brought in from the
interplanetary spaces is uncertain. The amount of nitrogen in the average
sedimentary rocks, such as shales, sandstones, and limestones, is so small
as to be inestimable.
It has been explained that in consequence of the action of bacteria and
leguminous plants nitrogen is fixed in the belt of weathering, and nitrates
are formed. These nitrates in arid regions are locally segregated in the
belt of weathering in considerable amounts, forming the nitrate deposits.
The bases with which the nitrogen is combined are almost exclusively
sodium and potassium. Since the nitrates are now forming in large quan-
tities (see pp. 452-453, 465-466) the total amount of the compounds which
have been produced through geological time must have been large. But the
nitrates readily break. up into their original elements, and the liberated
nitrogen and oxygen rejoin the atmosphere. Thus there is a continuous
cycle by which nitrogen, oxygen, and carbon are united by means of the
leguminous plants and bacteria; and by decomposition reduced to the
original elements and carbon dioxide.
While the total quantity of nitrates is inconsiderable, as compared with
the majority of the other important elements concerned in metamorphism,
nitrogen is essential for the growth of plants and animals. Hence nitrogen
is one of the necessary elements in the sequence of events by which
carbon is abstracted from the atmosphere, passes into the plants, and
thence by oxidation is concentrated so as to carry on the process of car-
bonation. Therefore a chief importance of nitrogen in metamorphism is as —
one of the aids in the chain of reactions in connection with carbon.
The nitrates in the soil and the nitrate deposits represent the economic
products resulting from the abstraction of nitrogen from the air and its
segregation in solids.
@Clarke, cit., Bull. 78, p. 39.
REDISTRIBUTION OF HYDROGEN. 981
HYDROGEN.
According to Clarke hydrogen composes 0.94 per cent of the outer 10
miles of the crust of the earth, including the lithosphere, hydrosphere, and
atmosphere. The larger part of the hydrogen is in the hydrosphere, of
which it composes 10.67 per cent. According to Clarke’s estimates of 1891
and 1900 it constitutes, respectively, 0.21 and 0.17 per cent of the litho-
sphere.“ The hydrogen of the atmosphere is inappreciable. It thus appears
that 0.8 or more of the hydrogen is concentrated in the hydrosphere,
although the volume of the lithosphere is five times as great. Hydrogen has
the tenth place in the scale of abundance. Practically all of the hydrogen
is combined. In so far as hydrogen enters into the processes of meta-
morphism it is in conjunction with oxygen as water. The source of the
water is very largely conjectural. It may be supposed that hydrogen and
oxygen, united as water, at the time the earth segregated, were with
the other ingredients in great volume. As magma crystallized the water
separated. This process has continued to the present time. By it some
explain the ocean, and hold to its continuous growth in magnitude. It is
possible that a considerable amount of the hydrogen for the water was
originally in the free state or in the form of carbureted hydrogen. This
is suggested by the fact that hydrogen and carbureted hydrogen are both
occluded in meteorites. If the water of the hydrosphere was produced in
any large measure by the oxidation of hydrogen an enormous quantity of
hydrogen was thus consumed.
Probably the most characteristic reaction of the zone of katamorphism
is that of hydration. This process, while taking place most rapidly in the
belt of weathering,
belt of cementation. According to Clarke’ the combined water in the
occurs continuously throughout the vast volume of the
igneous and crystalline rocks liberated at 110° C. is 0.40 per cent, and above
110° C. 1.52 per cent; total, 1.92 per cent. In 78 shales the water liberated
at 110° C. is 1.34, and above 110° is 3.68 per cent; total, 5.02 per cent.
In 624 sandstones at 110° it is 0.29 per cent, and above 110° is 1.40 per cent;
total, 1.69 per cent. In 843 limestones at 110° it is 0.26 per cent, and above
110° is 0.72 per cent; total, 0.98 per cent. (See p. 938.) It therefore
appears that the amount of water in the shales is more than double and in
aClarke, cit., Bull. 78, p. 39; Bull. 168, p. 15. >Clarke, cit., Bull. 168, p. 14.
982 A TREATISE ON METAMORPHISM.
the sandstones and limestones is less than the average for the original rocks.
The increase in the amount of combined water in hydrating the materials
of the shales, 0.65 of the sediments, 438,750,000,000,000,000 metric tons,
from 1.92 to 5.02 per cent, is 13,601,250,000,000,000 metric tons. The
decrease in the combined water in the sandstones, 0.30 of the sediments,
202,500,000,000,000,000 cubic kilometers, from 1.92 to 1.69 per cent, is
465,750,000,000,000 metric tons. The decrease of the water in the lme-
stones, 0.05 of the sediments, 33,750,000,000,000,000 metric tons, from 1.92
to 0.98 per cent, is 317,250,000,000,000 metric tons. The total decrease
in the sandstones and limestones is 783,000,000,000,000 metric tons, which
sum, subtracted from the increase in the amount of water in the shales,
leaves a net increase in the water of hydration of the zone of katamorphism
of 12,818,250,000,000,000 metric tons, and an equivalent loss of water to
the hydrosphere.
This estimated amount is probably too small, for the following reasons:
Throughout the immense thickness of the zone of katamorphism,
hydration is the rule. A very large portion of this zone is composed of
igneous rocks, and the minerals of these rocks have been hydrated to a
varying extent. In the plutonic rocks which have not been fractured in a
complex way the process has usually not gone far, but in the volcanic
rocks, and especially the porous ones, such as basalts, hydration has taken
place upon a vast scale. For instance, in various parts of the world, such
as in the Lake Superior region and in the Deccan, hydration has gone so
far in the basic amygdaloidal lavas as in many cases to have completely
destroyed the original minerals. One of the results of such alterations is
the filling of the pores of the vesicular rocks with various minerals of which
hydrous silicates, such as the chlorites, zeolites, etc., are especially abundant.
The process of hydration of the zone of katamorphism has continued during
geological time. So far as I can see there is no way to estimate the amount
of water thus absorbed, but I suspect that the amount abstracted from the
hydrosphere by the process is probably greater than that used in the hydra-
tion of the sedimentary rocks.
In consequence of the hydration of the original rocks of the zone of
katamorphism, it is certain that the amount of combined water in the
igneous and crystalline rocks analyzed in order to determine the compo-
sition of the lithosphere is oreater than in the recent igneous rocks. The
REDISTRIBUTION OF HYDROGEN. 983
analyses of the freshest modern rocks usually show less than 1 per cent of
combined water. It follows that, in regarding the original rocks as contain-
ing 1.92 per cent of water, the calculated amount of water for the hydration
of the sediments is too small.
In the zone of anamorphism the dehydration of the rocks which have
been hydrated in the zone of katamorphism and passed into the zone of
anamorphism, is not complete. Even the coarsest schists and gneisses
ordinarily contain from 1.5 to 2 per cent of water. In so far as the process
of dehydration of the rocks, which have passed into the zone of anamor-
phism is incomplete, water has been abstracted from the hydrosphere.
But even if the total of all the above could be estimated this would
give no idea of the amount of water which has been added temporarily to
the rocks by hydration during geological time. From the earliest time to
the present hydrated sedimentary rocks formed in the zone of katamor-
phism have passed continuously into the zone of anamorphism, and have
been steadily dehydrated. No data are available to estimate the amount
of water which has been added by hydration and liberated by dehydration.
The question naturally arises as to what extent at the present time the
process of dehydration is reversing that of hydration. It seems certain
that hydration is taking place more rapidly than dehydration. While an
estimate of the amount by which hydration exceeds dehydration would be
very desirable, | see no way in which a quantitative result on this point
can be even approximated.
While the amount of water abstracted from the hydrosphere by
hydration is large, it does not follow that the ocean is decreasing in magni-
tude; for the water continuously added to the hydrosphere by the liberation
of occluded water through volcanism and the crystallization of magma may
more than compensate for the losses due to hydration.
ALUMINUM.
Aluminum is the most abundant of the metals of the lithosphere.
Clarke’s original estimate in 1891 was that aluminum constituted 7.26 per
cent of the lithosphere, hydrosphere, and atmosphere, all of this being in
the rocks, of which it composed 7.81 per cent. Clarke’s estimate of 1900
increases the amount in the original rocks to 8.16 per cent.“ It therefore
«Clarke, cit., Bull. 78, p. 39; Bull. 168, p. 15.
984 A TREATISE ON METAMORPHISM.
appears that aluminum, the most abundant of metals, is only about one-
third as abundant as silicon, and only about one-sixth as abundant as
oxygen. The metal takes third place among the elements. Reckoned as
an oxide, in 1891 Clarke estimated the aluminum as 15.04, and in 1900
as 15.41 per cent. As an oxide it stands second only to silicon.
Aluminum has its chief source in the silicates, which occur very abun-
dantly in the original rocks. The aluminous silicates occur also in the
meteorites. By far the most important of the silicates bearing aluminum
are the feldspars, which, as Clarke calculates, compose about 60 per cent
of the mass of the original rocks. Hayes, C. W., The Arkansas bauxite deposits: Twenty-first Ann. Rept. U. S. Geol. Survey,
pt. 3, 1900, p. 461.
¢ Hayes, cit., pp. 464-466.
986 A TREATISE ON METAMORPHISM.
It therefore appears to me that we must look somewhere else to explain
the greater part of the discrepancy. It is certain that when the shales (the
sedimentary rocks into which the aluminous minerals mainly pass) are meta-
morphosed in the zone of anamorphism, the secondary minerals show an excess
of alumina. All of the chief original aluminous minerals, viz, feldspar,
pyroxene, amphibole, and mica, may be reproduced; but there also develop
other aluminous minerals which are not known in the original rocks. Of
these, garnets, staurolite, and the aluminum-silicate minerals—andalusite,
sillimanite, and cyanite—are by far of the greatest consequence. The
latter minerals are especially characteristic of rocks of the approximate
composition of the pelites metamorphosed in the zone of anamorphism.
They clearly mark an excess of aluminum and furnish almost conclusive
evidence of the segregation to a considerable extent of the aluminum if
the pelites beyond the amount which is present in the original rocks.
This argument has such weight that I believe more careful analyses of
shales with reference to their mass and chemical composition will show that
the alumina ought to be considerably higher than in the analyses given. It
has already been noted (p. 962) that correlative with the deficiency of
alumina there is apparent excess of silica in the shales, and the deficiencies
and excesses are remarkably accordant. It has just been seen that the
apparent deficiency in alumina is 3.68 per cent. A small fraction of a per
cent can be accounted for by the bauxite deposits. Theexcess of silica as
shown by the calculations (p. 961) is 3.247 per cent. Consequently the
two nearly balance. I venture to predict that when more satisfactory
average analyses are available the shales will be found to contain several
per cent more alumina and several per cent less silica than is given in the
average analyses of shales on page 938.
IRON.
According to Clarke’s estimate of 1891 iron composes 5.08 per cent of
the outer 10 miles (16.1 kilometers) of the crust of the earth, including
the lithosphere, hydrosphere, and atmosphere, and 5.46 per cent of the
original rocks alone. In his estimate of 1900 Clarke estimates that the iron
forms only 4.64 per cent of the original rocks." This gives iron fourth
place among the elements, it being surpassed only by oxygen, silicon, and
aluminum. Reckoned as an oxide, according to Clarke’s estimate of 1891,
«Clarke, cit., Bull. 78, p. 89; Bull. 168, p. 15.
REDISTRIBUTION OF IRON. 987
the amount of ferric oxide is 3.94 per cent and of ferrous oxide is: 3.48 per
cent, and in his estimate of 1900 the ferric oxide is 2.63 per cent and the
ferrous oxide 3.52 per cent. Reckoned as an oxide, iron thus has the third
place in abundance, being surpassed only by silica and alumina.
Tron occurs as an original constituent of the igneous rocks in perhaps
more numerous forms than any other element. It is found as a sulphide as
pyrite, pyrrhotite, ete.; and as an oxide as hematite, magnetite, and ilmenite.
Tt occurs in many silicates. Of these, the pyroxenes and amphiboles, the
olivines, and the micas are the more important. In the meteorites iron is
the most abundant constituent, occurring alloyed with nickel and cobalt,
and in sulphides, oxides, silicates, ete.
In 78 shales the ferric oxide is 4.03 per cent of the rock, the ferrous
oxide 2.46 per cent; in 624 sandstones the ferric oxide is 1.24 per cent, and
the ferrous oxide 0.57 per cent; in 843 limestones the ferric oxide is 0.66
per cent, the ferrous oxide being undetermined.”
The 4.03 per cent of ferric oxide and the 2.46 per cent of ferrous oxide
in the shales is equivalent to 4.73 per cent metallic iron. The 1.24 per
cent of ferric oxide and the 0.57 per cent of ferrous oxide in the sand-
stones is equivalent to 1.31 per cent metallic iron. The 0.66 per cent of
ferric oxide in the limestones is equivalent to 0.462 per cent metallic iron.
It thus appears that there is a sheht increase, less than 0.1 per cent, in the
amount of iron in the shales as compared with the original rocks. There is
ereat depletion of iron in the sandstones—which contain between one-third
and one-fourth of the amount ih the original rocks. The depletion of the
iron in the limestones is very great—there being about one-tenth the
amount in the original rocks. The slight increase in the shales is by no
means suflicient to account for the depletion of the sandstones and lime-
stones. If we multiply the percentage of iron present in the different kinds
of sediments by the estimated quantity of those sediments, and add the
three together, we have the following equation:
4.73 X .65-+1.31 X .30-+ .462 x .05 =3.491 per cent.
Since the amount of iron present in the original rocks is 4.64 per cent,
this shows a deficiency of 1.149 per cent for the entire mass of sediments.
This difference seems small, but it amounts to 7,155,750,000,000,000
aClarke, cit., Bull. 168, pp. 16, 17.
988 A TREATISE ON METAMORPHISM.
metric tons of metallic iron. The question naturally arises, What has
become of this enormous amount of iron? ‘The processes of metamorphism
have segregated from 30 to 70 per cent of iron in the various iron-bearing
formations and in the iron-ore deposits. (See pp. 842-846, 1193-1198.)
While the iron-ore deposits themselves contain much the larger per-
centage of-iron, the main mass of the segregated iron is in the iron-bearing
formations rather than in the iron ores. Indeed, the amount in the iron
ores is probably insignificant as compared with the amount in the iron-
bearing formation. For instance, in the Mesabi district of Minnesota,
where the iron-ore deposits are larger than in any other region, Doctor
Leith has calculated that in the part of the iron-bearing formation which is
exposed at the surface, including no part which passes below the overlying
slate, the amount of disseminated iron is probably one hundred times as
great as that contained in the ore deposits. And the amount of this
formation below the slates, in which there are no known ore deposits, is
certainly many times, probably hundreds of times, that exposed. This
calculation in reference to the Mesabi range shows how trivial is the
amount of iron in the ore deposits as compared with the more widely
distributed lower-grade products of the iron-bearing formations Such
formations are illustrated by the great pre-Cambrian iron-bearing forma-
tions of various parts of the world, such as those occurring in the Lake
Superior region; by the very extensive iron-bearing member and the ores
of the Clinton horizon of the Silurian; by the great iron-bearing horizons
of the Carboniferous, and by the bog deposits of the Pleistocene. It is
believed that if the iron-bearing formations and the iron ores associated
with them were represented in the above equation in proportion to their
mass and percentage of iron, the excess of iron in them beyond that of the
original rocks would be nearly or quite sufficient to account for the great
deficit of iron shown by the ordinary sediments. The segregation of the
iron in the iron-bearimg formations and the ores is treated on pages
102-130, 823-846, 1193-1198.
In the zone of katamorphism many minerals are produced by the
alteration of the original iron-bearing minerals. Of these the hydrated
oxides of iron, especially limonite, and the silicates, such as the chlorites and
the epidotes, are important. Where the rocks pass into the deep-seated zone
any of the minerals in which the iron originally occurred may be reproduced.
REDISTRIBUTION OF MANGANESE. 989
Other iron-bearing minerals also are formed. Among them garnet and
staurolite are important; but various other heavy iron-bearing silicates
develop, among which may be mentioned ilmenite, humite, clinohumite,
ottrelite, and chloritoid.
MANGANESE.
According to Clarke’s estimate of 1891 manganese forms 0.07 per cent
of the outer 10 miles of the crust of the earth, including the lithosphere,
hydrosphere, and atmosphere. In the lithosphere alone the amount
estimated is 0.08 per cent. In his estimate of 1900 he reduces this amount
to 0.07 per cent.” By this estimate manganese is thirteenth in the scale of
abundance. If these amounts of 0.08 and 0.07 per cent were reckoned as
MnO, they would be respectively 0.1265 and 0.1107 per cent.
Very small percentages of manganese are reported in the following
silicates: lavenite, arfvedsonite, spessartite, piedmontite, and astrophyllite.
The amount inno ease is large enough to make the element an essential one.
Manganese is reported among the subordinate constituents of the meteorites.
A trace of it is reported in the shales and sandstones, and in 843 limestones
manganese oxide composes 0.04 per cent of the rock. Since the mass of the
limestones is so small as compared with the shales and sandstones, manganese
in the limestones is trivial compared with that which is present in the
original rocks. The probable explanation of the deficiency of manganese in
the common secondary rocks is that the manganese is segregated in
manganese ore deposits precisely as the iron, the reactions for segregation
being analogous throughout. (See pp. 1198-1199.) ‘Thus the manganese
reported in the sedimentary rocks is found in limestones—that is, in car-
bonate rocks. Iron is, in a similar manner, to a large extent segregated
in connection with carbonate deposits.
The relative masses of the segregated manganese ore and iron ore
are interesting, since these two compounds go through analogous trans-
formations, producing ore bodies under similar conditions, and in very
numerous cases being associated in the same ore deposits. But subject to
the law of mass action the abundant element iron produces ore deposits of
enormously greater size than does the rarer element manganese. The
amounts of iron and manganese in the original rocks are 4.64 and 0.07
per cent respectively. Thus if the two elements were segregated in the
990 A TREATISE ON METAMORPHISM.
same proportion by the processes of metamorphism the iron-ore deposits
should be 66.3 times as great in magnitude as the manganese deposits, and
this ratio is certainly approximated by the facts. There can be no better
illustration of the importance of the chemical law of mass action in
geological processes.
CALCIUM.
According to Clarke’s estimate of 1891 calcium composes 3.51 per cent
of the outer 10 miles of the crust of the earth, including the original rocks,
hydrosphere, and atmosphere; of the original rocks alone 3.77 per cent, and
of the ocean alone 0.05 per cent. According to his estimate of 1900 the
amount of calcium in the original rocks is 3.50 per cent.” Calcium is fifth
in abundance among the elements. Reckoned as an oxide Clarke’s estimate
of the amount in the original rocks was 5.29 per cent in 1891, and 4.90 per
cent in 1900. This gives lime fourth place in the table of oxides.
Calcium is an abundant constituent of many of the minerals of the
original rocks. It occurs in all of the feldspars except the acid end of the
series. Itis an essential constituent of nearly all of the pyroxenes and
amphiboles, being, however, more abundant in the former than in the latter.
For instance in diopside. the ratio between the calcium and magnesium is
1:1, whereas in tremolite the ratio is 1:3. It is an essential constituent of
the scapolites, meionite and wernerite, which, however, do not form an
especially important group of minerals in the igneous rocks. While
calcium occurs in fewer minerals than magnesium and is less abundant
than magnesium in the pyroxenes and amphiboles the fact that calcium is
an essential constituent of so many feldspars which, according to Clarke’s
estimate, compose 60 per cent of the minerals of the original rocks, makes
calcium a more abundant element than magnesium. In the meteorites
calcium occurs abundantly in the feldspars and pyroxenes, and as calcium
sulphide in the mineral oldhamite.
In 78 shales the calcium oxide amounts to 3.12 per cent of the rock;?
in 624 sandstones to 3.29 per cent, and in 843 limestones to 41.60 per cent.
(See p. 938.) These numbers show that the amount of calcium oxide, as
compared with the original rocks, is reduced by more than one-third in the
shales and about one-third in the sandstones, and is increased more than
@Clarke, cit., Bull. 78, p. 39; Bull. 168, p. 15. » Clarke, cit., Bull. 168, pp. 16-17.
REDISTRIBUTION OF CALCIUM. eI
eightfold in the limestones. It therefore appears that the very considerable
deficiency of calcium in the abundant shales and sandstones is to be
accounted for by the great concentration of calcium in the limestones.
On the hypothesis that the sum of the deficiencies should equal the excess
in the limestones, we have another case by which we may test the correctness
of the estimates of the relative quantities of the sediments. For if the
above supposition be true, the percentage of CaO in each of the kinds of
sediments, multiplied by the mass of the sediments, should, added together,
equal the percentage in the origimal rocks. Putting this in the form of an
equation we have:
3.12 .65-++3.29 X .30-+41.60 < .05=5.095.
The sum, 5.095 per cent, is about halfway between Clarke’s estimate of
1891 of the amount of CaO in the original rocks and his estimate of 1900,
being slightly less than the former and slightly greater than the latter.
Therefore, so far as the criterion of analyses is applicable, the deter-
minations of calcium seem to furnish a confirmation of the estimates of the
relative masses of the shales, sandstones, and limestones.
In the above computation the amount of CaO in solution in the
ocean is not considered. According to Dittmar’s estimates this amount is
553,000,000,000,000 metric tons of calcium, which would correspond to
774,200,000,000,000 metric tons of CaO. The total amount in the sedi-
ments on the basis of the above analyses and estimates of relative volumes
would be 34,391,250,000,000,000 metric tons. Therefore the amount in
the ocean is 2.25 per cent of the estimated amount in the sediments.
While the CaO in the ocean is to be regarded as derived from the destruc-
tion of the original rocks, this amount is so small as to be negligible in
comparing the amount of CaO in the original and the secondary rocks.
The segregation of the calcium, with the greater segregation of carbon,
in the limestones, is one of the most interesting and important results of
chemical, physical, and organic processes. (See pp. 964-966). It has been
pointed out that the process of carbonation in the zone of katamorphism is
made effective by the concentration of carbon dioxide by plants in the belt
of weathering. The carbonates produced are largely transported to the sea.
In the sea the carbonates are thrown down by animals, and thus the lime-
992 ‘A TREATISE ON METAMORPHISM.
stones are formed. While much of the CaCO, produced by carbonation is
transported to the sea, a considerable part of it is transported to the belt of
cementation, and enough of this is precipitated to make CaCO; a cementing
agent second in importance to silica. (See pp. 624-625.) A very subor-
dinate part of the CaCO, in the belt of cementation enters into combination
with silica, forming the zeolites and epidotes.
From the foregoing it appears that the calcium in the secondary
minerals of the zone of katamorphism is largely in the form of carbonate,
often not a simple carbonate, but a calcium-magnesium carbonate. In this
zone also are found other secondary calcium-bearing minerals, such as the
zeolites and epidotes. ;
When the rocks containing these secondary calcium-bearing minerals
pass into the zone of anamorphism, the minerals are decomposed, the carbo-
nates by silication and the zeolites and epidotes by dehydration. In the
nearly pure calcium-magnesium carbonate rocks the silicates which form
most plentifully are wollastonite, diopside, tremolite, and actinolite, Natur-
ally, where the limestones are nearly pure calcium carbonate, by silication
the pure calcium silicate, wollastonite, forms. If magnesium is not very
abundant, there is likely to be developed the pyroxene, diopside, in which
the calcium-magnesium ratio is 1:1. Where the carbonate rocks are
strongly magnesian, tremolite forms; and where they also bear iron,
actinolite develops. In the rocks in which there is a considerable number
of bases, such as the impure limestones, the shales, and the sandstones,
other silicates form. Any of the calcium-bearing silicates occurring in
the original rocks may be reproduced, and part of the calcium usually,
passes into heavy minerals not commonly found in the original rocks,
such as garnets, melilite, the scapolites, ete.
MAGNESIUM.
According to Clarke’s estimace of 1891, magnesium forms 2.50 per cent
of the outer 10 miles (16.1 Kilometers) of the crust of the earth, including
the original rocks, the ocean, and the atmosphere. Of the ocean it
comprises 0.14 per cent, and of the original rocks 2.68 per cent. In his
estimate of 1900, Clarke reduces the amount for the original rocks to 2.62
per cent.*. Magnesium thus stands seventh in the seale of abundance. It
is about five-sevenths as abundant as calcium, the element with which it is
aClarke, cit., Bull. 78, p. 39; Bull. 168, p. 15.
REDISTRIBUTION OF MAGNESIUM. 993
most intimately associated and most closely allied. Reckoned as an oxide,
Clarke’s estimate of 1891 is 4.49 per cent, and of 1900 is 4.36 per cent.
This gives maguesia fifth place among the oxides. Magnesium occurs as a
constituent of the original igneous rocks in a number of minerals. Of
these, the pyroxenes, amphiboles, micas, and olivines are the more
important. It is only rarely present in any considerable proportion in the
feldspars, the most abundant group of mimerals. Thus, while occurring in
more minerals than calcium, it is not so abundant as calcium, because of
the great réle of the feldspars.* In the secondary rocks magnesia composes
2.45 per cent of 78 shales, 0.85 per cent of 624 sandstones, and 6.20 per
cent of 843 limestones. It thus appears that as compared with the original
rocks the amount of magnesia in the shales is reduced to about three-fifths,
in the sandstones to about one-fifth, and in the limestones is increased by
more than one-third. The sum of the products of the percentage of each
class of sediments by their proportional amounts gives the average
percentage of magnesia in the sediments, thus:
2.45 x .65+.85 x .30-+6.20 .05=2.1575.
Subtracting this 2.1575 from 4.36, the amount in the original rocks, we have
a deficiency of 2.2025 per cent. The total amount of sediments multiplied
by this latter percentage gives 14,866,875,000,000,000 metric tons.
The question now arises as to the explanation of this apparent vast
deficiency of magnesia in the sedimentary rocks. The question is not easily
answered. Possibly some part of the calculated deficiency is only apparent.
Probably the composite analyses of 345 limestones, taken at random, in
which the magnesia is 7.90 (see p. 938) is nearer the average for the lime-
stones than this amount averaged with the quantity of magnesia, 4.49 in
the building stones (see p. 938), since it is well known that the magnesian
limestones are likely to be porous and brecciated, and therefore not so
suitable for building purposes. If the average amount of magnesia in the
limestone were taken as 7.90 instead of 6.20, as given in the above
equation, we have:
But 0.085 per cent is only about one twenty-sixth of the calculated
deficiency, and therefore we must look somewhere else for the explanation
«Clarke, cit., Bull. 168, p. 16.
63
MON XLVII—O4+
994 A TREATISE ON METAMORPHISM.
of the major part of it. The natural direction to which we first turn to
account for the deficiency is the ocean. According to Dittmar’s estimates
the amount of magnesium in the ocean is 1,743,000,000,000,000 metric
tons, which corresponds to 2,887,800,000,000,000 metric tons of magnesia.
It is at once seen that the total amount in the ocean is vast, the magnesia
being between three and four times as abundant as lime. But this enormous
amount accounts for only 19.45 per cent of the deficiency.
Another large portion of the deficiency is probably accounted for by
magnesium in classes of the sedimentary rocks not considered—that is, the
saline deposits. In all salt deposits magnesium salts are important impu-
rities. In the water of Great Salt Lake the magnesium varies from 0.3 to
2.6 per cent of the total solids in solution; “ in the water of the Dead Sea
it is given as 14.41 per cent of the total solids.’ In the salt deposits of
New York and Michigan the magnesium in the brines varies from .034 to
454 per cent.° In the rock salts of Louisiana the magnesium varies from
0.003 to 0.06 per cent.” These figures may be taken as representative of
ordinary salt deposits, but in certain exceptional salt deposits, as those of
Stassfurt, the magnesium salts are much more abundant. In such deposits
a great portion of the magnesium salts with the accompanying potassium
salts is likely to be found in a more or less distinct bed above the rock salts.
As illustrating the abundance of these compounds Precht states that at
Stassfurt, from 1876 to 1880, there were mined 699,136 metric tons of car-
nallite (KX MeCl,.6H,O), kieserite (MgSO,+H.0), and kainite (MgSO,.KCI1+
3H,0) and only 96,856 tons of rock salt. A considerable amount of
magnesium occurs also, in accompanying polyhalite.* It is plain that the
total quantity of the magnesium in saline deposits is great, but what portion
of the deficiency is thus accounted for can not be stated until very careful
studies have been made of the volumes and compositions of the salt deposits
of the world.
It is believed that the explanation of a larger part of the deficiency is to
be found in the concentration of magnesia in the zone of katamorphism in
consequence of the alterations of that belt, the magnesia as is explained
«Gilbert, G. K., Lake Bonneville: Mon. U. S. Geol. Survey, vol. 1, 1890, p. 2538.
> Encyclopedia Britannica, 9th ed., 1877.
¢Tenth Census, vol. 2, p. 1017.
@ Mineral Resources, 1883-84, p. 841.
¢Precht, H., Die Salz-Industrie von Stassfurt und Umgegend, 1889, p. 12.
REDISTRIBUTION OF MAGNESIUM. 995
below, being much more largely retained in the belt of weathering and in
the belt of cementation than is calcium. But as yet no data are available
to enable us to make any statement as to the quantitative importance of
this concentration.
To what extent the magnesium in the ocean, in the saline deposits, and
segregated in the zone of katamorphism will explain the deficiency in the
ordinary sedimentary rocks can not be stated. This can be determined only
after a careful quantitative estimate has been made of these various supplies
of magnesium. The redistribution of magnesium by metamorphism, while
in many respects analogous to that of calcium, is m many respects also ditfer-
ent. It has been seen that by the process of carbonation of the silicates
by far the larger part of the calcium is released from silica and changed to
carbonates. The major part of the calcium carbonate and other carbonates,
such as sodium carbonate, find their way to the sea in solution. While the
process of carbonation also changes a considerable portion of the magnesium
of the silicates to magnesium carbonate, a very large part remains in the
silicates. The olivines change to serpentine. The nonaluminous pyroxenes
and amphiboles alter to tale on an extensive scale. The aluminous pyrox-
enes and amphiboles pass into chlorite, a magnesium aluminous silicate.
The pyroxenes extensively change to amphibole, and this involves a partial
substitution of magnesium for calcium, or else a subtraction of calcium and
partial carbonation, or both. Thus, the magnesium is concentrated in the
zone of katamorphism, and especially in the belt of weathermg. But this
concentration is largely due to the abstraction of other elements rather than
the addition of magnesia.
The magnesium of the original silicates, which is changed to carbonate
in the zone of katamorphism, goes through the same processes of distribu-
tion as the calcium carbonate. A part of it passes down into the belt of
cementation, and is there precipitated in dolomite, ankerite, parankerite,
and hydrous magnesium silicates. But a large part of the magnesium salts,
before or after an underground journey, join the surface streams and find
their way to the ocean. Magnesium is there precipitated on a great scale
as carbonate, not by the reaction of organisms, but by the reaction of
magnesium salts in solution upon the calcium carbonate previously precipi-
tated by organisms. This process inaugurated in the sea is continued in
the carbonate formations on the land. This involves a substitution of ma
or
D
996 A TREATISE ON METAMORPHISM.
nesium for calcium, or else abstraction of much calcium, or both. It is
plain that the magnesium which occurs in the sedimentary rocks as mag-
nesium carbonate is largely introduced by the substitution of magnesium
for previously precipitated calcium.
When the sedimentary rocks and the altered original rocks of the zone
of katamorphism containing the secondary magnesium minerals pass into
the zone of anamorphism the original magnesium minerals, viz, the pyrox-
enes, amphiboles, micas, and olivines, may be produced; but a part of
the magnesium goes into other minerals. Of these the garnets appear
to be the most important, and some of the subordinate minerals are
chondrodite, humite, clinohumite, tourmaline, and melilite.
It is a well-known fact that in mechanical sedimentary rocks meta-
morphosed in the zone of anamorphism the amphiboles are developed upon
a much greater scale than the pyroxenes, and that the magnesium-bearing
mica, biotite, forms very abundantly. This is a natural consequence of the
depletion of the mechanical sediments in calcium as compared with the
original rocks. The amphiboles are much more heavily magnesian than
the pyroxenes. For instance, in tremolite the magnesium-calcium ratio is
3:1, whereas in diopside it is 1:1. Consequently from the mechanical
products in which the magnesium is somewhat concentrated and the calcium
depleted there is a tendency to produce the magnesian minerals, amphi-
bole, and biotite. But in the development of amphiboles and pyroxenes
the pressure and the specific gravities of these minerals are also con-
cerned. (See pp. 278-280.)
SODIUM.
Sodium, according to Clarke’s estimate of 1891, composes 2.28 per
cent of the outer 10 miles (16.1 kilometers) of the crust of the earth,
including the original rocks, the hydrosphere, and the atmosphere. Accord-
ing to this estimate it composes 1.14 per cent of the hydrosphere and 2.36
per cent of the lithosphere. In his estimate of 1900 the amount of sodium
in the original rocks is increased to 2.63 per cent. Sodium thus has sixth
place among the elements. Reckoned as an oxide Clarke’s estimate for
1891 is 3.20 per cent and for 1900 it is 3.55 per cent, and of oxides it
stands sixth.”
@ Clarke, cit., Bull. 78, p. 39; Bull. 168, p. 15.
REDISTRIBUTION OF SODIUM. 997
Sodium is a constituent of many of the minerals of the original
igneous rocks. These minerals comprise the feldspars, the nephelites,
cancrinite, sodalite, hatiynite, noselite, and marialite. In the meteorites
sodium is reported only in the feldspars. Because of the dominating
importance of the feldspars® the larger part of the sodium occurs in them,
although the sodalite and nephelite groups of minerals are important
sources of the element. Sodium is so abundant in the acid plagioclases as
to give them the name of soda feldspars.
In 78 shales soda composes 1.31 per cent; in 624 sandstones, 0.61 per
cent, and in 843 limestones, 0.34 per cent. It thus appears that in the
shales the amount of soda is reduced to only about one-third of that in
the original rocks, in the sandstones to about one-sixth, in 845 limestones
not used for building purposes’ to a minute fraction, but in 498 lime-
stones used for building purposes to about one-sixth. (See p. 938.)
Multiplying the percentages of soda in each of the classes of sediments
by their masses and taking their sums we have the average percentage of
soda in the sediments, thus:
1.31 .65-+.61.30+-.34><.05=1.0515.
Since the soda in the original rocks is 3.55 per cent, the deficiency in the
sediments is 2.4985 per cent. It thus appears that the deficiency in the
sediments considered is about 70.38 per cent of the total amount of soda in
the original rocks. For the entire weight of the sediments this gives a
deficiency of 475,065,000,000,000,000 metric tons.
It is possible that when more analyses have been made of sedi-
mentary rocks the estimates of soda in them may be somewhat increased,
but since the analyses show the amount of potassium to accord fairly well
with that of the original rocks (see p. 1000), it does not seem probable that
much of the deficiency in soda can be explained by insufficient analyses.
The natural source to which one immediately turns to account for this
vast deficiency is the ocean. The actual amount of sodium in the ocean, as
determined from Dittmar’s estimates, is 14,180,000,000,000,000 metric tons,
which, reckoned as an oxide, would be 19,101,000,000,000,000 metric tons.
While sodium is the most abundant element in the ocean with the exception
of chlorine, and is more than five times as abundant as the magnesium,
@Clarke, cit., Bull. 168, p. 16. > Clarke, cit., Bull. 168, pp. 16-17.
998 A TREATISE ON METAMORPHISM.
potassium, and calcium together, it is seen that this amount is trivial as
compared with the total deficiency in the sedimentary rocks, being indeed
only 4.02 per cent of this amount. It is therefore plain that, as in the case
of magnesium, we must turn to some other direction to account for the
great deficiency of sodium in the ordinary sedimentary rocks. The natural
direction to which to turn is to the salt deposits of the world. Many great
salt deposits have been discovered in the United States, Europe, and other
parts of the world; such deposits are illustrated by those of Poland, New
York, and Louisiana. Doubtless many other salt deposits exist which have
not been discovered. Moreover it is known that there are extensive salt
deposits below lakes such as Great Salt Lake and the Dead Sea. No
attempt has been made to quantitatively determime the mass of the salt
deposits. Until this is done no statement can be made as to the proportion
of the deficiency of sodium in the ordinary sedimentary rocks which such
sediments will account for.
Finally, as is shown on pages 541-543, in arid regions sodium salts
are largely retained in the deposits of the belt of weathering. Since
sodium-bearing deposits are known to be very thick im various arid regions,
such, for instance, as those in the western part of the United States, the
amount of sodium thus accounted for is very great, but as yet our
knowledge is not sufficiently advanced to make any quantitative estimate
of it. .
From the foregoing it appears highly probable that we must look to
the salt deposits and to the alkaline deposits of arid regions to explain the
great deficiency of sodium in the ordinary sediments rather than to the
ocean. If this conclusion be correct, calculations upon the age of the earth
have no vaiue which are based upon the derivation of salt from the land
through weathering processes and its accumulation in the sea, and which
ignore or place as relatively unimportant the salt deposits of the land.
Such calculations furnish but another illustration of the danger of consid-
ering a single factor in a series of complex geological processes, and
neglecting to ask the question whether there are other equally or more
important factors concerned.
When the original rocks containing sodium-bearing minerals are
decomposed in the zone of katamorphism, the larger part of the sodium
passes into solution, contrasting in this respect, as will be seen, in a marked
REDISTRIBUTION OF SODIUM. 999
degree with potassium. During the underground journey of a part of the
solutions a small portion of this sodium is deposited in the belt of cementa-
tion. The sodium-bearing minerals formed by direct alteration in the zone
of katamorphism, and those there deposited as cementing minerals, are
substantially the same and are mainly the zeolites. The more important
of these zeolites containing sodium in notable quantities are thomsonite,
hydronephelite, natrolite, mesolite, analcite, stilbite, and emelinite.
So far as rocks containing sodium in the zeolites pass into the zone of
anamorphism some of the original minerals of which sodium is a constituent
may be reproduced, but manifestly the amount of such minerals will be
small as compared with the amount of the original sodium-bearing minerals
in the rocks. Corresponding with this fact the rich sodium minerals,
nephelite, cancrinite, sodalite, hatiynite, and noselite, have never been
‘reported as anamorphic minerals of the sedimentary rocks. Most of the
sodium present passes into the feldspars; but where the rocks have been so
far decomposed as not to leave a large quantity of the original sodium-
bearing minerals, the amount of sodium is not sufficient to produce any
considerable proportion of the feldspars. In consequence, as pointed out
on pages 899-904, the schists and gneisses which form from pelites only
exceptionally contain any considerable quantity of soda feldspars. The
dominant secondary minerals which are produced are quartz and mica.
However, in the psephites, in which the processes of decomposition are not
far advanced, and in which the sodium is therefore relatively abundant,
considerable quantities of soda feldspars are produced during recrystalli-
zation. But as already noted, even in those sedimentary rocks in which
residual soda is present in the undecomposed original minerals, it appears
that it is not known to have been anywhere so abundant that the peculiar
minerals known as the soda-bearing minerals, such as nephelite, sodalite,
ete., are produced.
POTASSIUM.
- According to Clarke’s estimate of 1891 potassium composes 2.23 per
cent of the outer 10 miles (16.1 kilometers) of the crust of the earth,
including the original rocks, hydrosphere, and atmosphere. Of the ocean
it composes 0.04 per cent, and of the original rocks 2.40 per cent. In his
estimate of 1900 Clarke reduces the figure for the original rocks to 2.32
1000 A TREATISE ON METAMORPHISM.
per cent. This gives potassium eighth place in abundance among the
elements. Reckoned as an oxide the amount in the original rocks, accord-
ing to Clarke’s estimates of 1891 and 1900, is 2.90 and 2.80 per cent,
respectively.” As an oxide, potassium thus has seventh place.
Potassium is an abundant constituent of a number of the silicates of
the original rocks. The more important of these are the feldspars and the
leucites. Since the feldspars probably compose more than half of all the
original rocks (see p. 937), the chief original sources of the potassium are
the feldspars. Of the feldspars the content of potassium is so great in
crthoclase and microcline as to give them the distinctive name potash
feldspars, thus discriminating them from the acid end of the plagioclase
series in which soda is the dominant alkali.
Considering the secondary rocks, the amount of potassa in 78 shales is
3.25 per cent,’ in 624 sandstones 1.24 per cent, and in 843 limestones 0.46
per cent. It thus appears that as compared with the original rocks, the
amount of potassa in the shales is increased by about one-fifth, in the sand-
stones is reduced to less than one-half, and in the limestones to less than
one-fifth. Multiplying the percentage of potassa in each class of sediments
by the volume of those sediments, and taking their sum, we have the
average amount of potassa for the sediments considered, thus:
3.95 & .65 + 1.24 & .80 + .46 x .05 = 2.5075.
Since the amount of potassa in the original rocks as given by Clarke in his
latest estimate is 2.80 per cent, we have a deficiency of potassa in the
sediments of 0.2925 per cent, or 1,974,375,000,000,000 metric tons.
According to Dittmar’s estimates the actual amount of potassium in the
ocean is 512,000,000,000,000 metric tons, which, reckoned as potassa, is
616,600,000,000,000 metric tons, or 31.23 per cent, i. e. about one-third of
the total deficiency. Potassium compounds are also important accessories
in salt deposits. For instance, analyses show that the potassium composes
from 1.1 to 7.4 per cent of the total solids of Great Salt Lake*® and 1.478
per cent of the total solids in the Dead Sea."
As is well known, above beds of rock salt there are not infrequently
overlying beds rich in potassium and magnesium. This class of salts is well
«Clarke, cit., Bull. 78, p. 39; Bull. 168, pp. 14-15. ¢Mon. U.S. Geol. Survey, vol. 1, pp. 253.
+ Clarke, cit., Bull. 168, p. 17. d Encyclopedia Brittanica, 9th ed., 1877.
REDISTRIBUTION OF POTASSIUM. 1001
illustrated at Stassfurt, where, as already noted (p. 994), from 1876 to 1880
the amount of carnallite (KMgCl,.6H,0), kieserite (MeSO,4-H.0), and
kainite (MgSO,KC1+3H,O) mined was 699,136 metric tons, whereas the
amount of rock salt was for the same years only 96,856 metric tons. Since
the salt deposits are so important, it follows that the total amount of
potassium in such deposits is large.
It seems probable that the potassium in the salts of inland seas and in
the saline deposits is sufficient to account largely and perhaps fully for the
remaining two-thirds of the deficiency of potassium in the sediments.
Where the rocks which contain potassium are altered in the zone of
katamorphism, the potassium-bearing minerals do not decompose to the
same extent as the sodium-bearing minerals. So far as the potassium is in
leucite, it readily alters; but, as already noted, the greater part of the potas-
sium is in orthoclase and microcline, and these are the most difficultly
decomposable of the feldspars. These minerals disintegrated and only par-
tially decomposed are carried in great quantities to the sea. But even
where the potassium feldspars are decomposed, it seems that the potassium
is held in an insoluble form to a large extent. Prestwich” attributes the
retained potassium to the action of alumina, although he does not explain
the reaction by which it is held. Mendeléeff’ emphasizes the absorptive
power of the soil, and especially of the vegetable mold, for potassium. It
is known that potassium compounds are used by plants to a much greater
extent than sodium compounds. But the influence of plants in retaining
potassium can not be cited to explain the apparent concentration of
potassium in the muds. It appears to me that where the feldspars have
decomposed and the potassium largely remains in the silts and muds, it is
likely that this element has passed into the zeolites, especially apophyllite.
When the rocks formed in the zone of katamorphism are buried in the
zone of anamorphism, the original minerals which contain the potassium
may be produced, but it has already been seen that the chief minerals
originally holding the potassium are orthoclase and microcline. It has
further been poimted out that the soda minerals are much more readily
decomposed and that much less of the sodium is left in the sedimentary
“Prestwich, Joseph, Geology—chemical, physical, and stratigraphical, vol. 1, 1886, p. 54.
> Mendeléeff, D., The principles of chemistry, translated by Geo. Kamensky, Longmans, Green
& Co., London, vol. 1, 1897, pp. 546-547.
1002 A TREATISE ON METAMORPHISM.
rocks. Thus it is that orthoclase and microcline are the chief feldspar
minerals which develop by the deep-seated metamorphism of the pelites—
rocks in which decomposition has been carried further than m the other
mechanical sediments. To a less extent orthoclase is likely to be the domi-
nant feldspar in the other sediments. However, since complete decompo-
sition for the mechanical sediments is very rare, the potassium for much of
the orthoclase and microcline which develop by metamorphism is derived
from the undecomposed original minerals.
BARIUM, STRONTIUM, CHROMIUM, NICKEL, LITHIUM, FLUORINE, BROMINE.
in Clarke’s estimate of 1900 of the lithosphere barium is put down as
composing 0.05 per cent, strontium as 0.02 per cent, chromium, nickel,
lithium, and fluorine as 0.01 per cent each and bromine as less than
0.01 per cent. The quantities of these elements are so small and so little
is known about the transformations through which they go that no attempt
is here made to consider the influence of metamorphism upon their
distribution.
GENERAL STATEMENTS.
At the beginning of this chapter it is stated that the attempt to apply
quantitative methods to the redistribution of the chemical elements is made
more with the idea of laying out problems to be solved than with the belief
that the calculations approach accuracy. However, when all of the equations
which compare the average composition of the sediments with the original
rocks are taken into account the impression is gained that the calculated
relative proportions of the three classes of sediments discussed—that is, the
shales (including all silicate rocks), the sandstones, and the limestones—can
‘not be far from the truth. The percentages of each of the elements in the
sediments and in the original recks are matters of quantitative determina-
tion, by imperfect methods, it is true, and so far as absolute masses are
concerned the estimates given for the sediments are but roughly approxi-
mate, but the relative proportions stated are probably nearer the truth.
Other proportions of the sediments than those used might be selected which
would answer in the equations for a single elemeut, and possibly some other
proportions than those used might be selected which would accord better
with the facts; but it seems certain that such proportions could not differ
GENERAL STATEMENTS. 1005
ereatly from those given—0.65 for the shales, 0.30 for the sandstones, and
0.05 for the limestones. While with the numbers used discrepancies have
appeared, for the most part they are of a kind which throw light upon the
processes of segregation of compounds in materials other than shales,
sandstones, and limestones.
In conclusion it is to be remembered that the equations are independent
of the estimates of the absolute masses of the sediments, and therefore they
ean not be said to give any support to the estimates of the average
thickness of the sediments for the continental areas, nor to the absolute
amounts of the elements which have been calculated to be abstracted
from certain of the sediments and segregated in certain others of them.
Certain of the results set forth in the previous pages indicate the pos-
sibility that the estimate of the mass of the sediments as 2 kilometers thick
for the continental areas is too large, but it will be noted that this estimate
is very conservative as compared with estimates made by others. (See
p- 939.) If one were to estimate the mass of the sediments as 1 kilometer
thick for the continental areas, while the calculated percentages of the
deficiencies and excesses of the various elements in the sediments as com-
pared with the original rocks would not be changed, the calculated absolute
amounts would be reduced one-half. This would reduce by one-half the
calculated percentage of oxygen of the atmosphere required to oxidize
ferrous iron (see p. 951), and to oxidize iron sulphide (see p. 952); would
divide by 2 the numbers comparing the amount of carbon in the sedi-
ments with that in the atmosphere and hydrosphere (see pp. 965-966); and
would divide by 2 all absolute estimates of the surpluses and deficiencies
of the various elements in the ordinary sediments, as, for instance, the
calculated deficiency in iron. (See pp. 987-988.) Other similar changes
would be made in the calculations. These facts may be regarded by some
geologists as evidence that even the moderate estimate for the sediments of
2 kilometers thick for the continental areas is too great, and with this view
I am inclined to agree.
Chel EK ee
THE RELATIONS OF METAMORPHISM TO ORE DEPOSITS.@
PART IL. GENERAL PRINCIPLES.
INTRODUCTORY.
The principles of metamorphism, discussed somewhat fully in the
previous chapters, have a direct bearing upon ore deposits, as it will be
shown in this chapter that the deposition of most ores is but a special case
of metamorphism of exceptional interest to man. ‘Through the preceding
chapters are scattered the principles applicable to ore deposits; but as many
persons interested in ores may not care to study in detail all the principles
which concern metamorphism, it seems advisable to give here a brief sum-
mary of the more important principles and conclusions directly applicable
to ore deposits. In order not to make the extension too great, only the
principles will be repeated; for evidence showing their correctness the reader
may turn to the previous chapters.
From this discussion are excluded all the nonmetallic economic
products which are used without reduction to the metallic form, as apatite,
clay, salt, ete., and the very rare and unimportant metals. Thus the
treatment is confined to those important ores the metals of which are
commonly used in the metallic rather than in the combined form. Thus
circumscribed, the chapter is chiefly restricted to ores of iron, aluminum,
cobalt, nickel, manganese, lead, zinc, copper, tin, mercury, silver, gold,
and platinum. Arsenic, antimony, bismuth, chromium, molybdenum, and
tungsten are not considered at all, or are only incidentally mentioned.
«This chapter ina less mature form was published in time for distribution at the Washington
meeting of the American Institute of Mining Engineers in February, 1900, as a pamphlet of 126 pages.
A revised pamphlet edition of 151 pages was published during the summer of 1900. This edition appears
as pages 27 to 177 of Volume X XX of the Transactions of the American Institute of Mining Engineers.
This paper was republished in 1902 by the Institute in the special volume upon the Genesis of ore
deposits, pages 282-432, which volume contains also a closing discussion under the same title, pages
763-781.
1004
DEFORMATION OF THE LITHOSPHERE. 1005
CLASSIFICATION OF ORE DEPOSITS.
Ore deposits may be divided into three classes upon the same basis as
are the three great divisions of rocks:
(1) Ores produced by the processes of sedimentation, or sedimentary
ores.
(2) Ores produced by igneous processes, or igneous ores.
(3) Ores produced by the processes of metamorphism, or metamorphic
ores.
Since this volume is a treatise upon metamorphism and not upon
physical geology in general, only the third class of ores, those produced
by metamorphism, come within its scope. But ores formed by processes of
sedimentation and by igneous processes will be briefly considered, mainly
in order to point out their relations to ores produced by processes of
metamorphism.
Before taking up the ores it is necessary, as already stated, to summarize
certain parts of the previous chapters. The three great divisions of the
processes of modifications of rocks which most intimately concern ore
deposits, and therefore need to be summarized, are:
The deformation of the lithosphere.
Volcanism.
The circulation and work of solutions.
DEFORMATION OF THE LITHOSPHERE.
In another place I have shown that the outer part of the crust of the
earth may be divided into three zones, depending upon the character of the
deformation—(1) an upper zone of rock fracture, (2) a middle zone of
combined rock fracture and flowage, and (3) a lower zone of rock flowage.*
In Chapter IV it has been shown that the zone of fracture corresponds to
the zone of katamorphism and that the zone of flowage corresponds to the
zone of anamorphism.
ZONE OF FRACTURE, OR ZONE OF KATAMORPHISM.
The zone of fracture is that near the surface and in it the rocks are
deformed mainly by macroscopic fracture. The ruptures are those of
faulting, jointing, bedding parting, fissility, and brecciation. The rocks
«Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept.
U.S. Geol. Survey, pt. 1, 1896, p. 589.
1006 A TREATISE ON METAMORPHISM.
are adjusted to their new positions mainly by differential movements
between the separated parts. The so-called folds in the zone of fracture
are largely the result of numerous parallel joint fractures across the strata
with small angular displacements at the joints, giving each block a slightly
different position from those on either side, and thus as a whole making
joint folds. For instance, the folds of the rigid rocks in the Alleghenies
are not in the main true flexures, but a series of slightly displaced blocks.
It is shown on pages 189-190 that, making all the assumptions in
favor of as great a thickness as possible, the maximum thickness of the zone
of fracture can not be. greater than 10,000 or 12,000 meters. In many
cases, even for the strongest rocks, deformation frequently takes place by
flowage at depths not greater than one-third to one-half of 10,000 meters.
For the weaker rocks deformation by flowage may take place at very
moderate depths. If the conclusions above given be correct, it follows that
all open fissures must disappear at moderate depths, and that the maximum
depth at which they can exist is the depth of the zone of fracture for the
strongest rocks. Illustrations of the disappearance of fissures with depth
are known at various places. In the gold belt of the Sierra Nevada, as
Lindgren says, it is ‘tan incontestable fact that many small veins close
up in depth.”
The dying out of fissures below is insisted upon in order to exclude the
hypothesis of filling of fissures from the bottom. If fissures gradually
decrease in size and finally die out, the streams which make their way into
the fissure must enter from the sides or from above. For further develop-
ment of this point see pages 1069-1072.
OPENINGS OF ZONE OF FRACTURE.
It has been shown that the zone of fracture is characterized by open-
ings. It will be seen that the nature of the openings in the rocks is of the
greatest importance in the formation of ore deposits. It is therefore
necessary to summarize the material given in Chapter III upon this subject.
It is there shown that the openings of rocks deserve consideration from
three points of view: The form and continuity of the openings; the size of
the openings; and the volume of the openings, or the pore space.
OPENINGS OF ZONE OF FRACTURE. 1007
FORM AND CONTINUITY OF OPENINGS.
The openings in rocks include those which are of great length and
depth, as compared with their width, and thus are essentially flat parallelo-
pipeds; those in which the dimensions of the cross sections of the openings
are approximately the same, and therefore resemble tubes of various kinds;
and those which are irregular.
The openings which have great length and depth as compared with
their width are those of faults, of joints, of bedding partings, and of fissility.
The order mentioned is that of continuity. As to position, bedding partings
are parallel to previous structures; while faults, joints, and fissility are at
various angles with the bedding parting, and therefore intercept the layers.
Because of this fact, these forms of openings may connect separated porous
strata. This is more likely to be true of faults than of joints, and of joints
than of fissility.
The openings in which the dimensions of the cross section are approxi-
mately the same are those in mechanical deposits, such as conglomerates,
tuffs, sandstones, and shales. From the point of view of ore deposits the
most important characteristic of the openings of this class is that they are
continuous, and therefore any part of a coarse, uncemented mechanical
deposit is connected with all other parts by openings.
Irregular openings are those of the vesicular lavas, and the irregular
fractures of rocks The openings of the lavas are usually variable in mag-
nitude, and are discontinuous. Usually single irregular fractures are of
limited extent, but many fractures may be concentrated along a zone, and
a composite zone may be continuous for long distances.
Openings of any of the above classes, whether produced by deforma-
tion, by original sedimentation, or by volcanic action, may be enlarged by
solution. This will be the case wherever the processes of solution more than
counterbalance the processes of deposition, and, as later explained, is more
likely to occur with downward-moving water than with upward-moving
water. Since downward-moving waters are dominant above the level of
eround water, and are prominent in the upper part of the belt of cementation,
it is in this area that openings are most frequently enlarged by solution.
It has been argued by Posepny that openings serving as channels for
ground water may be produced wholly by solution. That openings may
be somewhat prolonged and adjacent opening connected by solution,
1008 A TREATISE ON METAMORPHISM.
thus helping underground circulation, is more than probable, but that long
and important passages are produced wholly by solution is an assumption
which I think has not been verified.
SIZE OF OPENINGS.
Openings in rocks may be divided, upon the basis of size, into those
larger than capillary size, or supercapillary openings; capillary openings;
and those smaller than capillary size, or subcapillary openings. The mag-
nitudes of these openings are given on pages 134-137. The openings of
bedding parting, of faults, of joints, of conglomerates, and of tuffs are
frequently of supercapillary size. Many of these openings, and also many
of those of the sandstones, are of capillary size. All classes of openings
may be somewhere of subcapillary size, but many rocks, such as massive
igneous rocks and shales, may contain only subeapillary openings.
VOLUME OF OPENINGS.
The total volume of the openings is dependent upon the number and
the size. The amount of pore space in rocks varies from less than 1 per cent
to more than 50 per cent. The total volume of the openings in a rock in
which they are so small as to be almost imperceptible and all subcapillary
may be as great as in arock in which many openings are seen. For instance,
the pore space of a chalk may be as great or greater than that of a sandstone
or of a massive rock containing numerous continuous fracture openings of
large size. (See pp. 124-125.) In such cases the small size of the open-
ings is compensated by their vast numbers.
CHEMICAL REACTIONS.
As explained in the preceding chapters, especially in Chapter TV and
more fully in Chapters VII and VIII, the reactions of the zone of fracture,
or katamorphism, are those of oxidation, carbonation, hydration, solution,
and deposition. All are of great importance in connection with ore deposits.
In the belt of weathering all of these reactions, with the exception of depo-
sition, are vigorous. The sulphides, including the metals and the sulphur,
are likely to be oxidized. Oxides may be further oxidized. The metals
may be carbonated and hydrated and are likely to be dissolved upon
an extensive scale. While all of these reactions take place in the belt of
DEFORMATION IN ZONE OF FRACTURE AND FLOWAGE. 1009
cementation, oxidation is frequently confined to those parts of the belt in
which the waters come somewhat directly from the surface; whereas deeper,
especially where the percolating waters have come into contact with organic
compounds or sulphides, the waters may become reducing, and partial or
complete deoxidation of the valuable metals take place. Furthermore,
while solution probably occurs to a greater extent than deposition in the
belt of cementation, as explained on pages 629-640, material is continually
deposited in the openings of the belt, and the rocks are cemented in conse-
quence of the expansion reactions of oxidation, carbonation, and hydration,
and in consequence of selective precipitation, of intrusion, and of other
phenomena.
ZONE OF COMBINED FRACTURE AND FLOWAGE.
Deformation by combined fracture and flowage takes place in a middle
zone because the rocks of a given part of the lithosphere vary in character,
temperature, moisture, speed of deformation, etc. (See pp. 766-768.) Of
these factors producing combined fracture and flowage, the effect of the
character of the rocks is most obvious. While very considerable rock
masses, such as the limestones and quartzites, may be homogeneous,
heterogeneity is the rule. The sedimentary rocks are composed of laminze,
layers, beds, and formations, no one of which is exactly like the adjacent
-one.. The massive igneous rocks contain minerals of different resisting
power, the particles of which are of different sizes. In consequence there
is a broad zone in which the deformation may be by combined fracture
and flowage. For instance, in an interbanded slate and graywacke the
slate bands may be deformed by flowage and the more rigid graywacke
bands be deformed largely by fracture. In an interbedded limestone and
clastic series the limestones may be deformed mainly by flowage and the
clastic material mainly by fracture. In many cases the change from one
kind of deformation to the other is amazingly sharp, the more resistant
bands being intersected by innumerable fractures which stop abruptly at
the bands where the rock is deformed by flowage. The deformation of a
massive rock mainly by flowage, but in a subordinate way by fracture, is
finely illustrated by the Berlin rhyolite-gneiss described by Weidman.” The
deformation of this rock was mainly by reerystallization, but many of the
«Weidman, 8., A contribution to the geology of the pre-Cambrian igneous rocks of the Fox River
Valley, Wisconsin: Bull. Wis. Geol. Nat. Hist. Survey, No. 3, 1898, pp. 32-47.
MON XLVII—04——64
MONO A TREATISE ON METAMORPHISM.
mineral particles were fractured into several pieces, or even granulated.
Between the larger pieces of the feldspar minute openings formed. Many
minute joints and crevices also were produced and were subsequently filled
by cementations.
The marked effect which the character of the rock may have upon the
nature of the fractures is well illustrated in the Cripple Creek district, where,
aecording to Penrose, in the hard rocks the fissures are sharp, clean-cut
breaks, while in the soft rocks they are ordinarily a series of very small
cracks, constituting a displacement of a kind which I call a distributive
fault. Mines which are partly in hard and partly in soft rock illustrate this,
as the following extract from Penrose will show: °
The vein on which the Buena Vista, Lee, Smuggler, and Victor mines are located
occupies a sharp, clean-cut fissure, partly in the massive rock and partly in the hard
breccia; but when it passes into the soft, tufaceous breccia on the east slope of Bull
Hill the fissure is represented only by faint cracks occupied by no vein of importance.
In this case the force which caused the fissure overcame the cohesion of the harder
rock sufficiently to make a clean break, but in the more plastic rock it overcame
cohesion only to the extent of causing a series of faint fractures without any one
well-defined break.
It follows from the above that displacement may disappear at variable
depths. Where there are fractures with large displacement they are likely to
extend to very considerable depths, and in proportion as the displacement is
small they are likely to disappear at less depths. Thus the depths to which
displacements extend depend largely upon the character of the rocks. For
instance, in a region in which there is a shale formation at moderate depth
underlying brittle rocks, strong fissures in the higher formations may disap-
pear as they encounter the shale, being there replaced by flexures. Where
formations of shale are between brittle formations fissures may cease at the
top of the shale and other fissures appear below it. Thus may fissures not
only die out below, but maty disappear above, the fault along the fissures
being replaced by a flexure in the shale, which yields by flowage. This is
beautifully illustrated by the Enterprise mine, of Rico, Colo., described by
Rickard and Ransome (see fig. 29, p. 1208), where faulted fissures in sand-
stone and limestone disappear at the place where shale is encountered, the
shale accommodating itself to the fractures below by monoclinal flexures.
(See p. 1204.)
“Cross, Whitman, and Penrose, R. A. F., Geology and mining industries of the Cripple Creek
district, Colorado: Sixteenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1895, p. 144.
DEFORMATION IN ZONE OF FLOWAGE. 1011
From the above it appears that openings of the zone of the combined
fracture and flowage comprise all of the classes of openings characteristic of
the zone of fracture. There is, however, the great difference that upon the
average the supercapillary openings are smaller, less continuous, and less
numerous than in the zone of fracture.
Since there is such a great variation in the strength of the rocks and in
other factors, the belt of combined fracture and flowage may be of consid-
erable thickness, possibly as thick as 5,000 meters. In this zone we have
all combinations of the phenomena of fracture in the various ways above
mentioned, and of flowage by granulation and recrystallization.
ZONE OF FLOWAGE, OR ZONE OF ANAMORPHISM.
In tne zone of rock flowage deformation is chiefly by granulation and
recrystallization, few openings being produced, except those of microscopic
size. (See pp. 658-659, 673-675, 685 et.seq.) This conclusion rests upon
arguments which can not here be fully repeated.“ However, it may be
said in passing that the conclusion that a zone of rock flowage exists at
moderate depth is based, first, upon deduction from known physical
principles as to the behavior of solid bodies under pressure; and, second,
upon observation. It is well known that when a rigid body, such as a rock,
is subjected to unequal stresses in various directions and the difference in
the stresses is greater than its ultimate strength under the conditions in
which it exists, it must rupture or flow. If a rock be subjected to a
stress in a single direction greater than its ultimate strength in that
direction, and the rock is not wnder pressure in other directions, rupture
occurs. However, if we suppose that the rock be subjected to stresses
greater than the ultimate strength of the rock in all directions, and that
the difference in the stresses in different directions is greater than the
ultimate strength of the rock under the conditions in which it exists, then
if openings could be produced by rupture, they would be closed by
pressure. In other words, at a certain depth below the surface of the
earth, if we could suppose that cracks and crevices are formed by the
deformation to which the rocks are subjected, the pressures in all directions
being greater than the ultimate strength of the rock, these cracks and
«Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept.
U.S. Geol. Survey, pt. 1, 1896, p. 594 et seq.
1012 A TREATISE ON METAMORPHISM.
crevices would be closed. The rate of closing would depend upon many
factors. (See pp. 766-768.)
Since this conclusion was reached, Adams and Nicolson“ have
actually deformed marble under the conditions supposed to exist at
moderate depth, with the result that the rock changed its form with no
perceptible openings.
Before the above inductive reasoning or Adams and_ Nicolson’s
experiments were made, I had become convinced from observation that
at moderate depth rocks are deformed with fracture and differential
movements between the solid particles (granulation), and by continuous
solution and redeposition by underground water (recrystallization). :
It has been explained (pp. 189-190) that the maximum possible
‘depth of the zone of fracture for the strongest rocks under quiescent
conditions is not greater than 10,000 or 12,000 meters, and that for the
majority of rocks, and especially under conditions of movement, the zone
of flowage is probably reached at depths much less than this.
OPENINGS OF ZONE OF FLOWAGE.
The openings of the zone of flowage are dominantly subeapillary.
Capillary openings are numerous, but they are not usually continuous for
any considerable distance. Supercapillary openings may exceptionally
exist in the upper part of the zone of flowage in consequence of very rapid
deformation, but such openings are likely to be temporary, for when rapid
deformation ceases they are closed by flowage.
REACTIONS OF ZONE OF FLOWAGE.
The chemical reactions of the zone of anamorphism, as fully explained
in Chapters IV and VIII, are dominantly those of silication, dehydration,
deoxidation so taking place as to decrease volume.
RELATIONS BETWEEN ZONES OF DEFORMATION.
From the preceding pages it is clear that the zones of fracture, of com-
bined fracture and flowage, and of flowage are uot sharply separated from
one another.
«Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble:
Phil. Trans. Roy. Soc. London, ser. a, vol. 195, 1901, pp. 365-401.
RELATIONS BETWEEN ZONES OF DEFORMATION. 1013
It is highly probable that upon the average the openings of the zone of
fracture gradually decrease in number and size as depth increases, until in
the zone of flowage the openings are, as already explained, microscopic or
nonexistent. If such a gradation exists, it is a necessary corollary that the
deformations of the zone of fracture must have their equivalents in the
deeper-seated zone of flowage. This point is fully discussed elsewhere."
Tt is explained that with depth, faults are replaced by flexures, and that
any deformation of a large mass of a given rock from one form to another
by fracturing may be imitated by similar changes of form in the zone of
flowage, the result being there accomplished by granulation of the mineral
particles or by their recrystallization, or by both.
Where rocks have been deformed in the zone of flowage and are now
at the surface there is superimposed upon the effects of the deep-seated
deformation the deformation by fracture resulting from earth movements
during the time the rock was slowly migrating through the zone of fracture
to the surface.
EFFECTS OF DEFORMATION AND CHEMICAL CHANGES UPON TEMPERATURE.
It has been pointed out in previous chapters that deformation results
in the liberation of heat which may considerably raise the temperature of
the rocks. Indeed, Mallett holds (see pp. 99-160) that deformation may
actually produce enough heat to fuse the rocks. While I dissent from this
conclusion, this view illustrates the importance of the rise of temperature
which may follow from mechanical action.
It has been shown further that a moderate rise in temperature may
increase the activity of the solutions to an amazing extent. Indeed, in a
chemical laboratory a slight rise in temperature is often sufficient to inau-
gurate a reaction which otherwise would not take place, and to increase the
speed of the reaction many fold. It is therefore clear that deformation is
favorable to the segregation of ore deposits in two different ways, first, by
forming openings for a vigorous circulation, and second, by heating the
solutions and thereby greatly increasing their activity.
In the zone of katamorphism the chemical reactions liberate heat, cause
an increase of temperature in the zone in which the ores are mainly segre-
«See Chapter VIII (pp. 776 et seq). See also Principles of North American pre-Cambrian
geology, cit., pp. 674-676, 694-698.
1014 A TREATISE ON METAMORPHISM. ~
gated, and therefore help to develop the ores. Thus, in this zone, deforma-
tion and chemical reactions both favor the concentration of ores. In the
zone of anamorphism the chemical reactions absorb heat, and to that extent
work against the effect of deformation.
In the western portion of the United States there have been recent
orogenic movements and also recent development of ore deposits. But
correlative with orogenic movements is volcanism, and it is pointed out in the
following section that volcanism also, for various reasons, is favorable te the
segregation of ores. It is therefore exceedingly difficult in any given region
to determine the relative quantitative importance of orogenic movements
and volcanism in the development of the ores. Until this study is seriously
taken up for extensive regions the relative importance of the two must
remain largely a matter of speculation.
VOLCANISM.
The great importance of volcanism in reference to metamorphism has
been fully dealt with in the previous chapters. The vast extent and great
thickness of the Tertiary volcanic rocks afford evidence of the prodigious
quantities of material which may be extruded-in a single geologic period.
It has been explained that correlative with and below these extrusions
probably occurred intrusions on an even vaster scale. It has also been seen
that important volcanism and extensive orogenic movements are usually
simultaneous in the same regions. The relation between the two is that
of cause-effect. Orogenic movements produce numerous openings, which
extend deeper than those that exist under quiescent conditions. These are
taken advantage of by the intrusive rocks which are being forced toward
the surface. The enormous hydrostatic pressure of the upwelling lava is
also one of the factors in further fracturing. Consequently there is action
and reaction between orogenic movements and volcanism, each advancing
the other.
Volcanism may promote the development of ore deposits in various
ways.
First. The igneous rocks may furnish metals for the ores. In the case
of a given ore deposit the metal may be almost wholly derived from the
recent igneous rocks, almost wholly from earlier rocks surrounding the
later intrusion, or partly from the two sources. Illustrations will be given
later.
VOLCANISM AND ORE DEPOSITS. 1015
Second. The igneous rocks may furnish aqueous and gaseous solutions
which transport the metals to their places of deposition. It is very difficult
to determine the quantitative amount of the solutions of this sort. Where
the igneous rocks are intruded in the zone of fracture, or appear at the
surface in the ordinary forms characteristic of volcanism, it is impossible
to discriminate the gaseous and aqueous material which has always been
within the magma from the solutions which have derived their water from
meteoric sources or from the sea. It is certain that vast quantities of
water of meteoric origin pass into the zone of fracture. In connection
with the water phenomena of volcanic action it may perhaps be doubted if
the water from the sea is important; but there can be no question con-
cerning the effect of water of meteoric origin. Where voluminous hot
springs issue in connection with present or past volcanic action all the
evidence indicates that the major portion of this water is of meteoric origin
(pp. 1065-1069). However, it has been pointed out (pp. 661-668) that
in the zone of anamorphism the conditions are very different. It can not
be assumed that there the water is abundantly derived from meteoric sources.
In connection with batholitic action it has been shown that there is complete
gradation between strictly igneous material and material which is apparently
deposited from solutions. It has been explained that the water which does
this work is probably in considerable part derived from the magma itself,
although some is doubtless included between the mineral particles and some
is produced by processes of dehydration. But even for this zone it is
impossible to make any quantitative statement as to the relative amounts of
water concerned in the segregation of ores which is derived from emanation
of original magmas and from other sources.
Third. The heat of the igneous rocks may render the solutions vastly
more active in segregating the metals from the intruded rocks. The amaz-
ing increase in the power of solutions to dissolve material in consequence
of rise of temperature has been emphasized (pp. 79-81). The meteoric
solutions adjacent to the igneous rocks must be at a much higher tempera-
ture than normal; therefore their activity is imcreased many fold. It
follows that where the solutions in sedimentary, metamorphic, or ancient
igneous rocks may not be sufficiently active to segregate the ores even
where the metal exists in sufficient quantity, they may so gain in their
segregating power in consequence of igneous intrusions that ore deposits
1016 A TREATISE ON METAMORPHISM.
develop. This heating of solutions in the surrounding strata by igneous
rocks is believed to be a factor of the most pronounced importance in the
segregation of many ore deposits.
Fourth. The introduction of the igneous rocks may promote fracturing
and thus open channels for circulation. Also, after an igneous rock has
erystallized it continues to cool and to contract. In consequence of its con-
traction trunk channels may be opened along the contact of the intrusive and
intruded rock and within the igneous mass. The production of openings is
very favorable to the segregation of ores.
Fifth. Another important function of the igneous rocks is to furnish
impervious basements which control the trunk channels of circulation, as,
for instance, in the Lake Superior region, in the Leadville and Tenmile
districts of Colorado, and in the Mereur district of Utah.
The formation of a given ore deposit with the aid of igneous rocks
may be a consequence of any combination of the above factors, and the
relative importance of each factor is variable in different cases. In a given
district the determination of the quantitative importance of each of these
factors is a problem of great difficulty, but one of great economic impor-
tance and scientific interest.
The effect of igneous rocks in the production of ore deposits is differ-
ent in the zone of fracture and the zone of flowage. In the zone of fracture,
as has been fully pointed out, the igneous rocks follow the openings toward
the surface, and when they finally reach the surface are spread out over it.
In the zone of fracture the effects of igneous rocks in the belt of
weathering and in the belt of cementation need separate consideration. In
the belt of weathering, above the level of ground water, intruded rocks are
baked or calcined. They are steamed by the gaseous solutions. It is
possible that in this belt ore deposits may be segregated through the
influence of volcanism, but positive evidence that this is a fact is lacking.
In the belt of cementation the igneous rocks are of great consequence.
There is here the direct effect of heating the intruded rocks, but far more
important than this is the indirect effect in heating the solutions. Where
the temperature rises above the critical temperature of water the solutions
are gaseous. It is, however, believed that the gaseous solutions are prob-
ably closely confined to the borders of the intrusive masses. The chief
effect is believed to be in reference to the aqueous solutions. It will be
VOLCANISM AND ORE DEPOSITS. TROLL
seen thatif gaseous solutions are produced in the belt of cementation, where
the openings are numerous and the rocks are saturated, there is a strong
tendency for the gaseous solutions to condense (see pp. 1019-1020). In
this we doubtless have the partial explanation of the dominance in the belt
of cementation of ore deposits, which show all the evidence of having been
deposited by aqueous solutions, although many such ore deposits are con-
tiguous to igneous rocks.
In the zone of flowage the conditions for intrusions and their effects upon
ore deposits are very different from those in the zone of fracture. It has
been pointed out that in the zone of flowage the igneous rocks make their
way by elevating and thrusting the intruded rocks aside, by actually mash-
ing them so as to shorten their diameters at right angles to the intrusives,
and finally by breaking across their structures. The effect of the igneous
rocks is both direct and indirect, as in the zone of fracture. The direct
effect is the heating of the surrounding rocks, their absorption to some
extent, and occasionally even their local fusion. The indirect effect is of
far greater consequence. In this deep-seated zone, where the amount of
water is small and the openings are usually small and discontinuous, it is
highly probable that the solutions are largely gaseous. In all but the
upper parts of this zone the natural increase of temperature, due to depth
alone, is sufficient to raise the solutions above the critical temperature of
water, and thus produce gaseous solutions. Where igneous rocks are
present in large quantity and contribute heat it is little short of certain
that gaseous solutions are abundant. Therefore it is believed that gaseous
solutions, comparatively unimportant in the zone of fracture, are of great
importance in the zone of flowage (see pp. 1020-1021).
CIRCULATION AND WORK OF SOLUTIONS.
Solutions in rocks occur in two forms, gaseous solutions and aqueous
solutions. Where the underground solutions are referred to without quali-
fication there is no implication as to their state, and they may be as either
one or the other, or partly both. Oftentimes for aqueous solutions the term
water is used, but in such usage it is not meant to imply that the water is
pure. It has been shown (pp. 146-152) that the general forces producing
circulation of solutions are gravity, heat, mechanical action, and molecular
attraction. These forces act very differently and do not have the same
1018 A TREATISE ON METAMORPHISM.
relative importance in gaseous and aqueous solutions. Therefore the two
classes are considered separately. It will be seen that in the belt of
cementation aqueous solutions are probably of predominating importance
and gaseous solutions very subordinate, and that in the zone of anamor-
phism gaseous solutions are of dominating importance and aqueous solu-
tions subordinate or unimportant.
CIRCULATION OF GASEOUS SOLUTIONS.
The general circulation of gaseous solutions is a function of the
amount of gaseous solutions, of the absolute temperature of the gaseous
solutions, of the amount of aqueous solutions adjacent, of gravity, and of
the form, continuity, size, and amount of the openings.
The general principle which controls the movement of gaseous solu-
tions is that the gases move from places of high pressure to places of low
pressure. The expansive force of a given amount of gas in a given volume
is directly proportional to its absolute temperature. Other things being
equal, gaseous solutions tend to pass from places of high temperature to
places of low temperature. The higher the temperature the greater its
expansive power, and the more forceful its movement toward places of low
temperature. Therefore heat is a fundamental force in the circulation of
the gaseous solutions. Gravity, of the very greatest importance for aqueous
solutions, is of importance also for gaseous solutions. So far as gravity is
concerned, the same principles apply to gaseous solutions and aqueous
solutions. The heavier column of gas pulled by gravity descends and the
lighter column is forced to rise. Where the columns are of equal length
that of lower temperature descends and that of higher temperature rises.
The circulation of gaseous solutions in the zone of fracture and in the
zone of flowage is very different. Moreover, the circulation in the belt of
weathering is different from that in the belt of cementation.
CIRCULATION IN BELT OF WEATHERING.
In the belt of weathering the law of gravity practically controls the
circulation. Where the temperature of the gases is higher than normal
they rise, being driven up by the greater pressure of the adjacent cooler
air. Thus, above the level of the ground water, wherever heated gaseous
solutions are produced they rise in columns of steam, as in the case of
fumaroles and solfataras and in connection with voleanic action.
CIRCULATION OF GASEOUS SOLUTIONS. 1019
CIRCULATION IN BELT OF CEMENTATION.
In the belt of cementation the conditions are very different. Here the
openings in the rocks are normally occupied by aqueous solutions. How-
ever, adjacent to igneous masses gaseous solutions may be produced either
as emanations from the igneous rocks or by the heating of the solutions of
the intruded rocks above their critical temperature, or by both combined.
The surrounding rocks, being occupied by aqueous solutions, serve the pur-
pose of condensers. ‘The conditions are those which would obtain between
steam generated in a boiler connected by many openings with condensers
containing water. Almost as fast as the steam is developed it is liquefied in
the condensers, thus heating the contained water. Thus gaseous solutions
are condensed to liquids almost as rapidly as formed. It is possible that in
the zone of fracture immediately adjacent to a great mass of intrusive
igneous rocks there may be a narrow border area in which the water may
be maintained as a gas and rise toward the surface, because it is driven by
its expansive force from places of high temperature to places of low tempera-
ture, but evidence is necessary upon this point
In attempting to arrive at the probability of the importance of gaseous
solutions in the zone of fracture it is necessary to consider the manner
in which igneous rocks solidify. It is well known that in this zone the
intrusive igneous rocks ordinarily show sahlbands, giving evidence of
quick chilling and solidification along their borders. From the exteriors
erystallization extends inward. It is therefore plain that the surface of
crystallization is ever a changing one. It should also be remembered that
rocks are poor conductors of heat. It has further been pointed out that as
crystallization and condensation continue openings are likely to form along
the contacts, and thus give ready passages for water. The question is
whether in the zone of fracture, along the borders of the intrusive masses,
where there is free circulation, the temperature would be long maintained
above the critical temperature of water. It seems to me that the prob-
ability is strongly against this. This probability is confirmed by observa-
tion, which shows that minerals produced along the contacts of igneous rocks
in the zone of fracture are those characteristic of the belt of cementation,
and therefore the same as those known to be produced by aqueous solutions.
There is therefore no good reason for supposing that ores adjacent to the
igneous rocks in the belt of cementation are commonly precipitated from
1020 A TREATISE ON METAMORPHISM.
gaseous solutions. But it would not follow that the aqueous solutions
precipitating the ores do not receive contributions of gases, water, and
metals from the contiguous igneous rock as it crystallizes and continues to
cool after crystallization.
CIRCULATION IN ZONE OF ANAMORPHISM.
In the zone of anamorphism the conditions are very different. Here
the openings are normally minute and disconnected, although locally and
for a brief time of active intrusion they may be large and continuous.
The amount of water in the surrounding rocks is very small, and the
openings are so minute that it can not make its wav readily toward the
igneous masses. Cousequently there is in this zone no condenser available
to transform the gaseous solutions to the liquid condition. The temperature
is certainly high, probably often above the critical temperature of water,
because of the normal increase of temperature due to depth, and because
of the high heat of the igneous rocks. Under these conditions it is natural
to suppose that adjacent to the igneous rocks gaseous solutions are produced
in considerable quantity, probably largely as emanations from the igneous
rocks, but also to an important extent by transformation to the gaseous
condition of the free water of the adjacent rocks and of the water liberated
by the process of dehydration. Gaseous solutions would utilize the large
openings temporarily formed to the fullest possible extent. It has been
explained that the gaseous solutions, having very little viscosity and under
the great pressure of this zone, make their way through the minute open-
ings of the rocks and even between the individual mineral particles. ‘The
gaseous solutions emanating from the igneous rocks and produced from
the surrounding rocks may contain metallic material in considerable
quantity. Such gaseous solutions from various sources would deposit
minerals in the larger openings precisely as do aqueous solutions under
similar circumstances. Ores would probably be deposited even more
extensively as impregnations in the minute openings of the adjacent rocks.
The places where precipitation occurred would be controlled in part by
the strength of the rocks, rigid rocks furnishing openings of a larger size
than those which are more plastic. The localization would be very largely
controlled also by the composition of the rocks. If they were able to
take part in a reaction which would result in a precipitation, this would
CIRCULATION OF AQUEOUS SOLUTIONS. 1021
influence the result. For instance, it will be seen (pp. 1086-1087) that
caletum carbonate may be very influential in precipitating the sulphides
from aqueous solutions. It is supposed that the same reactions may take
place with dense gaseous solutions at high temperatures; indeed, it might
be argued that the calcium carbonate would be even more active with
gaseous solutions than with aqueous solutions. Jn this connection the very
frequent occurrence of the disseminated ores and deposits with irregular
boundaries, with garnet, pyroxene, tourmaline, and other heavy anhydrous
minerals as simultaneously developed gangue minerals in limestone is to
be noted. (See pp. 1052-1056 )
CIRCULATION OF AQUEOUS SOLUTIONS.
The general circulation of aqueous solutions is a function of the amount
of water in the rocks, of the form, continuity, size, and amount of the open-
ings in the rocks and of difference in head, difference in temperature, and
mechanical action.
As to the amount of water in openings, in some cases the openings are
not full and in some they are full, or the rock is saturated. Where the
rocks are not saturated the chief forces producing circulation are gravity
and molecular attraction. Gravity tends always to carry the water down-
ward. Molecular attraction, or what is ordinarily called capillarity, tends
to draw the water to places where the pore spaces have little water. It
therefore is an effective force only where the rocks are not saturated with
water. In the saturated rocks the forces producing movement are gravity,
heat, and mechanical action. Where the rocks are saturated hydrostatic
pressure is active, and therefore under these conditions flowage is usually
much more rapid than where the rocks are not saturated. The flowage is
also rapid in proportion as there is unequal head and unequal temperature
in the two columns. The movement of water caused by gravity is due to
the unequal weights of the columns of moving water. The unequal weight
may be due to difference in the lengths or in the temperatures of the vertical
columns or the two combined. That difference in length gives difference in
weight is obvious. Where two connected columns of water have unequal
temperature, the cooler column is the denser of the two, and therefore
under these circumstances gravity in connection with heat produces cireu-
lation. In regions where the increment of heat is normal, there may be
1022 A TREATISE ON METAMORPHISM.
movement due to difference in temperature, but probably it is of great
consequence only where the heat increment of one column is more than
normal on account of mechanical action or of igneous rocks, or both.
Mechanical action may squeeze out the water in the openings of rocks or
possibly even some of the combined water and thus produce circulation.
(See pp. 149, 661-665.)
Where the rocks are saturated, whatever the cause of the flow of under-
ground water, the direction of movement is from places of greater pressure
to places of less pressure. A current going in any direction is evidence of
an excess of pressure in the rear of the current. Thus, water which enters
by seepage or through capillary tubes into a larger opening, such as a
fissure, must be under greater pressure than the column of water into which
it makes its way. Whether the motive force in the movement of the water
be difference in gravitative stress, of deformation, or any other cause, the
excess of pressure resulting in movement is behind the current. In propor-
tion as the opening approaches a circular form the flow increases because
the friction between the moving water and the film of fixed water upon the
walls is less per unit volume. The more continuous the openings, the more
rapid is the flowage. Flowage increases as the size of the openings increases.
In super-capillary openings the ordinary laws of hydrostatics apply, and
therefore the flowage may be very rapid. In capillary openings the laws
of capillary flow apply, and the movement of water is slow. Where the open-
ings are subcapillary, the attraction of the mineral particles extends from wall
to wall, the water films are glued to the rocks, and flowage is inappreciable.
Flowage increases as the amount of openings or the pore space increases.
So much for the laws controllmg the general circulation of aqueous
solutions. The combinations of the various factors are so fundamentally
different in their effects in the zone of fracture and zone of combined frac-
ture and flowage that further statement is necessary in reference to them
CIRCULATION IN ZONE OF FRACTURE, OR ZONE OF KATAMORPHISM.
In the zone of fracture a great many openings are of super-capillary
and capillary size, and many of them are continuous. It is clear that in
this zone the conditions are very different in the belt of weathering above
the level of ground water where the rocks are commonly not saturated and
in the belt of cementation below the ground water where the rocks are
saturated. Each belt requires consideration.
CIRCULATION OF AQUEOUS SOLUTIONS. 1023
BELT OF WEATHERING.
The belt of weathering comprises the belt at the surface of the earth
in which the rocks are not continuously saturated with water. Its thick-
ness is ordinarily between 0 and 300 meters; it is commonly less than 100
meters, but in the arid regions it may be exceptionally 1,000 meters. In
the belt of weathering the openings in rocks are upon the average more
numerous, and larger than in any other belt. The amount of water present
may vary from a very minute fraction of the amount required to fill the
pore spaces to saturation. Therefore there is greater variation in the
amount of water in the belt of weathering than in any other belt. The water
is derived from rainfall and from the belt of saturation. At a given place,
the rainfall varies greatly from time to time and there is much variation in
the amount of water. It is shown (see pp. 416-423) that the forces
which control the movement of aqueous solutions in this belt are gravity
and molecular attraction and that these work directly and also through
plant roots and mechanical movements.
So far as ore deposits ‘are concerned, the direct actions only are
important. Gravity steadily tends to carry the water downward. Molec-
ular attraction ever tends to carry the water from places of more abundance
to places of less abundance. The movements of the water due to these two
factors are very complex in the upper portion of the belt of weathering,
but for the middle and lower portions the movement of the ground water
is mainly controlled by gravity. Therefore in these parts of the belt the
water usually steadily descends. In considering ore deposits this is the
fundamental point in connection with the belt of weathering, as the lateral
movements and the ascending movements are unimportant.
The level of ground water for a given area is not fixed, and in conse-
quence of its variation a certain layer near the boundary between the belt
of weathering and the belt of cementation, may alternately be in one belt
and in the other. There are various causes for this fluctuation, of which
changes in amount of rainfall, orogenic movements, and denudation. are
important. Where uplift and denudation are the rule, the belt of weathering
migrates. steadily downward and consequently encroaches on the belt of
cementation. Where there is subsidence and valley filling it is not uncom-
mon for the belt of weathering to be encroached upon by the belt of
cementation. These transfers from one belt to another will he seen to be
of considerable importance in the genesis of ore deposits.
1024 A TREATISE ON METAMORPHISM.
BELT OF CEMENTATION.
The belt of cementation is the belt between the bottom of the belt of
weathering and the top of the zone of flowage. Since in places the rocks
are saturated to the surface of the ground, and since in the strongest rocks
the depth of the zone of flowage under the most favorable conditions may
reach 10,000 to 12,000 meters, the possible maximum thickness of the belt
of cementation is 10,000 to 12,000 meters. The average thickness of the
belt of cementation is probably less than 5,000 meters. Since the tempera-
ture increases 1° C. for 80 meters, the water in the lower part of the belt
of cementation is above the boiling point for atmospheric pressure. Indeed,
if the belt of cementation extends to a depth of 10,950 meters, the water
at this rate of increase would have a temperature of 365°, or the critical
temperature of water. But it has been shown (pp. 556-569) that, not-
withstanding the high temperature of the water up to the critical point,
the pressure is sufficient to hold it in the form of a liquid. Where mechan
ical movements or igneous intrusions have taken place the increase of
temperature may be higher than 1° C. for 30 meters. But if the critical
temperature be not reached, the increment of temperature must be very
much greater than this amount in order that the water shall be changed to
the form of a gas. (See pp. 566-569, 659-660.) Igneous intrusions at
various times and places have doubtless raised the water above the critical
temperature, but such water does not come under the subject now being
considered.
The circulation of aqueous solutions in the belt of cementation is vig-
orous and extensive. The evidence for this is abundant. First, it is known
that a considerable percentage of all meteoric water reaches the belt of
cementation. All of this water except the inappreciable portion which
enters into combination, after a longer or shorter journey in this belt, must
somewhere again issue from it, unless a part of it makes its way downward
into the zone of anamorphism, and there is no evidence to show that it does
this. (See pp. 665-667.) The vigor of the circulation in the belt of cemen-
tation is further shown by the almost incredible number of springs carrying
a vast volume of water, all of which are supplied by water from this belt.
The water issuing from the belt of cementation in springs is probably only
a small fraction of that which passes to the overground circulation by
seepage through many small widely dispersed openings. (See pp. 413-423,
571-589.)
bo
CIRCULATION OF AQUEOUS SOLUTIONS. 1025
The strongest evidence of the vigorous and extensive character of the
circulation in the belt of cementation is the fact, so strongly emphasized
on pages 562-565, that cementation is universal in it. Wherever porous
rocks have long remained in the belt of cementation an enormous amount
of material has been deposited by underground aqueous solutions. In
most cases where the porous rocks have been deeply buried in this
belt for geological ages cementation is nearly or quite complete. Thus
great sandstone formations have been transformed to quartzite by deposi-
tion of interstitial silica. In the San Juan district of Colorado great tuft
formations of Tertiary age have been completely cemented. In order to
get an idea of the original porosity of this formation it should be compared
with the very recent porous cinder cones made up of bombs and lapilli,
such as occur at many places in the West, as for instance near Flagstaff,
Ariz. These cinder cones are so porous that rain water, however great its
amount may be, passes into them as it would into a sieve, and therefore
performs no work of erosion. Indeed, it seems as if the process of erosion
of such deposits could scarcely begin until the openings had been partly
filled by the process of cementation. Yet in the San Juan district of Colo-
rado thousands of meters of tuffs originally as porous.as these have been
so perfectly cemented that the microscope scarcely discovers an opening
except those which have been produced by recent fracture since cementa-
tion. The fillings of the major and minor openings are continuous physic-
ally and are composed of the same kind of material. No one who has
studied such a formation as this can doubt that the general cementation
and the filling of the fractures were performed by the same agent.
In general, in the rocks which have been long within the belt of cemen-
tation, the innumerable bedding partings, faults, joints, fissility and breccia-
tion openings have been closed. The complete cementation of the more
porous sandstone and tuffs requires the deposition of mineral material to
the extent of 30 to 40 per cent of the volume of the rock. It has been
shown (pp. 571-572) that to have accomplished this work the amount
of water which circulated through the rocks probably in most cases must
have been 100,000 or more times as great as the material deposited.
Further, in case of the San’ Juan tuffs, the enormous amount of water
required to deposit the immense mass of cementing material passed through
the rocks since early Tertiary time.
(ays)
MON XLVII—04
1026 A TREATISE ON METAMORPHISM.
Wherever rocks are exposed which once possessed numerous and large
openings and which have been in the belt of cementation for a long time,
their general cementation gives conclusive evidence of a vigorous past circu-
lation. In consequence of the process itself the openings become smaller and
smaller; consequently the circulation less and less vigorous until, when the
process of cementation is complete or nearly so, the circulation becomes very
feeble. ‘Thus in many regions, humid and arid alike, where there has been
vigorous underground circulation, and where great quantities of material |
have been deposited by the aqueous solutions, the circulation now may be
relatively unimportant. At many mining districts these conditions now
exist. At the time of rapid deposition of the ores there was a pervasive
and vigorous circulation which, as cementation continued, gradually became
less and less, and finally practically ceased. But after the ground-water
circulation becomes feeble or practically stops, in consequence of cemen-
tation, earth movements may again fracture the rocks, and thus a new
circulation be inaugurated which results in further cementation and perhaps
in further concentration of ore. :
In such cases the amount of the present circulation is proportional to
the fracturing which has taken place so recently that the openings formed
have not had time to be filled again. In contrast with the circulation in
formations containing numerous large openings is that in rocks which con-
tain few openings larger than subeapillary size—such as shales and the
massive igneous rocks. In rocks such as these, unless orogenic movements
produce capillary or super-capillary openings, the circulation may never be
vigorous. The existence of impervious formations, either original or pro-
duced by the processes of cementation themselves, will be shown later to
be very important factors in the general circulation of underground waters
and in the localization of ore deposits.
The aqueous circulation of the belt of cementation for homogeneous
mediums is fully discussed on pages 571-589. It is there seen that
the movement of the water may be resolved into two components—hori-
zontal or lateral movements and vertical movements. In simple cases
these components are usually combined in such a way that the lateral com-
ponent is continuously in the same general direction, and the vertical com-
ponent of the curved path is first downward and later upward. (See figs. 7
to 11.) But in areas of marked topographic relief a part of the water may
CIRCULATION OF AQUEOUS SOLUTIONS. 1027
move continuously downward from the place where it enters the rock to
where it issues from it. The form of the curved path is influenced by
many factors of which the increase in temperature with depth, limiting
formations, the preferential use of large channels, the relative lengths of
the vertical and horizontal components are the more important.
The greater the temperature the less the viscosity of water. At 90° C.
the viscosity is only one-fifth as great as at 0° C., and at 300° C. the vis-
cosity may not be more than one-twentieth of the amount at 0° C. Since
the flowage in capillary openings, which is the prevalent abundant kind in
rocks, is inversely as the viscosity, the increase of temperaine with depth
is plainly favorable to deep penetration.
The movement of any aqueous circulation is practically stopped by
strata in which all or nearly all of the openings are subcapillary. In the
most favorable case for depth this would be the bottom of the zone of frac-
ture. In many cases, however, limiting strata are found at very moderate
depths, and there may be several limiting strata bounding pervious strata.
(See fig. 12.) In all such cases a given circulation is stopped by the lim-
iting strata, and the lines of flow are thus severely confined. The influence
of limiting impervious strata upon the ground-water circulation is well
illustrated by many of the artesian systems. Of the various combinations
which result in artesian flow a limiting impervious stratum above a. per-
vious stratum is the most common.
In general it may be said for a system of circulation that the nar
pynlabls cross section is utilized, but not uniformly. Other things being
equal, the more direct route is used to a greater extent than a less direct
one, and therefore the remoter corners of a water system may have rela-
tively small circulations.
Since the vertical movement of eround water is at a maximum confined
to the zone of fracture, it can not exceed 10,000 or 12,000 meters, and
commonly is much less than this. Indeed, in many systems of circulation
the greater part of the water does not reach a depth as great as 1,000
meters.
There is no assignable limit to the extent of the lateral movement, but
commonly it is longer than the vertical moyement, in many cases hundreds
or thousands of times longer. For instance, in the artesian circulation of
the Dakota sandstone the horizontal journey of the water is at least from the
1028 A TREATISE ON METAMORPHISM.
Black Hills to the James River Valley, about 400 kilometers, whereas
the vertical circulation is limited to a depth less than 1,000 meters. In this
case the vertical journey is insignificant in magnitude as compared with the
horizontal journey. While this is an extreme case, in general the horizontal
journey is several to many times longer than the vertical journey.
Large channels are utilized to a much greater extent than small chan-
nels. This preference for large channels frequently amounts to dominance,
particularly for the latter part of the journey. Large channels may be
culled the trunk channels of circulation. The importance of trunk channels
is due to the fact that (1) large channels are likely to be more direct
than smaller channels, and (2) the friction along the walls per unit of
water is very much less than in capillary and in subcapillary channels.
In the belt of cementation, as a rule the channels are larger and more
numerous in its upper part, and steadily diminish in number and in size
with depth. Therefore large channels tend to produce a shallow circulation,
and thus counteract the influence of lessened viscosity with depth and
the effect of gravity, which tends to produce a deep circulation. (See pp.
578-582.) The preferential use of trunk-channels leads to the conclusion
that fault, joint, and bedding-parting openings, and those of conglomerates,
all of which are likely to be of supercapillary size, are largely utilized by
the ground-water circulation.
As a matter of observation, it is well known that the water which joins
the belt of cementation is distributed throughout the belt, and that it enters
at an infinite number of points. It is further known that at least a large
part of the water issuing from the belt of cernentation emerges from trunk
channels. Combining these facts with the principles controlling the circula-
tion, it follows that the circulation of the belt of cementation is analogous
to that of a tree, the twigs and branches of which have an important down-
ward component, the medium branches of which are approximately hori-
zontal, and the trunk channel of which usually has an upward component.
CIRCULATION IN ZONE OF COMBINED FRACTURE AND FLOWAGE.
Circulation in the zone of combined fracture and flowage is intermediate
in its nature between that of the zone of fracture and the zone of flowage.
It has been pointed out (pp. 766-768) that as the bottom of the zone
of fracture is approached probably the openings decrease in size, until
CIRCULATION IN ZONE OF FLOWAGE. 1029
they approach those of subeapillary size which are characteristic of the
zone of flowage. But because of variations in the nature of the rocks
this change does not take place uniformly, but irregularly. Hence one
would expect that in formations where the deformation is mainly by
fracture the movement of the water would be similar to that of the zone of
fracture, but less vigorous, and that where the deformation is mainly by
flowage the movement of the water would be unimportant.
CIRCULATION IN ZONE OF FLOWAGE, OR ZONE OF ANAMORPHISM.
In the zone of flowage, which corresponds to the zone of anamorphism,
the circulation of water is very different from that in the belt of cementa-
tion. While much of the water in the upper parts of this zone is probably.
liquid, in the deeper parts of it, and in those parts where recent orogenic
movements or great batholithic intrusions have occurred the solutions prob-
ably reach a temperature higher than 365°, and therefore are in the gaseous
condition.
The openings of the zone of anamorphism are for the most part sub-
capillary and discontinuous. They are not only small in size, but small in
total volume. In the zone of anamorphism, therefore, the quantity of free
water at any one time is very small as compared with the zone of fracture.
So far as the solutions in the zone of anamorphism are aqueous they are
mainly glued to the walls of the subcapiliary openings, and the circulation
under conditions of quiescence is extremely slow. If there were no addi-
tions to the free water, doubtless the circulation would be unimportant so
far as ores are concerned. But it is shown (pp. 662, 679-680) that one
of the most characteristic reactions of the metamorphism of this zone
is liberation of the combined water by dehydration, thus continually
adding to the amount of free water. It is certain that this excess of free
water is squeezed out of the zone of anamorphism, and it is shown
(pp. 665-667) to be highly probable that the major movement of this
liberated water is upward into the zone of fracture rather than downward
into the centrosphere. This highly heated water carries with it carbon
dioxide liberated by the process of silication and as much mineral material
as it is able to transport.
This upward movement may be largely as gaseous rather than as
liquid solutions. So far as this is true the laws of these solutions apply
rather than those of aqueous solutions.
1030 A TREATISE ON METAMORPHISM.
SOURCE OF THE METALS.
The nature of the rocks which contribute the metallic salts has been
much discussed. With Sandberger,” I have little doubt that the metallic con-
stituents of ores are in large part derived from the igneous rocks which have
been intruded into or extruded upon the lithosphere. In my paper on
‘‘Some principles controlling the deposition of ores,” as originally published,’
I strongly advocated the idea that igneous rocks are the direct source of
some ores, that they are the ultimate source of all ores, and that the heat of
the igneous rocks is of fundamental importance in the segregation of the
ores. The igneous rocks as a source of metallic ores are especially impor-
tant at periods of exceptional volcanism. At such times there rise from
the lower parts of the lithosphere, and possibly to some extent from the
centrosphere, enormous masses of igneous rocks which are injected into
the zone of fracture or brought to the surface. While, during the middle and
later portions of geological time the magmas which came into the zone of
fracture or were spread over the ‘surface of the earth were derived from
unknown depth, it is entirely possible that, during the early stages of the
earth’s history, magma very generally existed near or at the surface, and
that such magma may have furnished a portion of the metals which now
occur in ore deposits, although in such cases doubtless the metals have
been distributed and redistributed again and again. From the earliest
geological time the igneous rocks have been the original source of the
sedimentary rocks, although their immediate source may have been other
sedimentary rocks. From the sedimentary and igneous rocks metamorphic
rocks are produced. Therefore the original source of all ore deposits is
believed to be magma.
I have further held that there is every gradation between igneous action
and aqueous action, or, as I have stated it ‘under proper conditions water
and liquid rock are miscible in all proportions.”* I believe that there are
complete gradations between purely igneous pegmatitic dikes and purely
aqueous vein material. (See pp. 720-728.) Moreover, in previous papers®
«Sandberger, F., Untersuchungen tber Erzgiinge, Wiesbaden, 1885, 2d Heft, pp. 159-431.
oTrans. Am. Inst. Min. Eng., vol. 30, 1901, pp. 27-177.
¢Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept. U. S.
Geol. Survey, pt. 1, 1896, p. 687.
“Van Hise, C. R., Metamorphism of rocks and rock flowage: Bull. Geol. Soc. Am., vol. 9, 1898
», 209. Also this monograph, Chapter VII, pp. 566-569; Chapter VIII, pp. 659-660.
?
SOURCE OF THE METALS. Osu
and in this monograph I have emphasized the point that if at any time the
temperature of the solutions exceed 365° C. they are necessarily gaseous.
I have also held that there is every gradation from ore deposits produced
by pure magmatic differentiation through those deposited by gaseous
solutions to ores produced by the work of underground water at ordinary
temperature. “
But it has been explained (see pp. 1014-1016) that igneous rocks
influence the segregation of ores in other important ways than by contrib-
uting the metals, viz, by heating the solutions of the surrounding rocks
and furnishing hot solutions to them, by producing channels for circula-
tion, and by furnishing impervious formations so as to converge solutions
into trunk channels.
Le Conte pomted out many years ago that the undoubted frequent
occurrence of workable ore deposits in regions of volcanism may be
explained by the heat furnished by the igneous rocks, thus promoting the
work of underground solutions.’ That the heat furnished by the igneous
rocks is a very important factor in the production of the ore deposits, I have
no doubt. Since it is often difficult to prove that the metallic content of
an igneous rock is original, it is impossible to make any general statement
as to whether the metallic content or the heat furnished by the igneous
rocks is the more important in the production of the ore deposits. It seems
clear to me that both are very important; and equally clear that in many
cases both work together. That is to say, an igneous rock may furnish all
or a part of the metal which appears in an ore deposit, and the heat of the
same igneous rock may greatly assist its concentration by underground
raters of meteoric origin.
While the later igneous rocks are the undoubted source of some,
perhaps many, ore deposits, it is also equally certain that another large
part is derived from the sedimentary rocks and the metamorphosed, or
partly metamorphosed, igneous and sedimentary rocks. As a case clearly
illustrating the importance of igneous rocks in increasing the activity of
solutions and enabling them to derive a large amount of material from the
adjacent sedimentary rocks, there may be mentioned the Mammoth Hot
a@Van Hise, C. R., Some principles controlling the deposition of ores: Am. Inst. Min. Eng., vol.
30, 1901, pp. 174-175. ;
bLe Conte, Jos., On the genesis of metalliferous veins: Am. Jour. Sci., 3d series, vol. 26, 1883,
p. 10.
1032 A TREATISE ON METAMORPHISM.
Springs of Yellowstone National Park. At this place there issues a
vast amount of water, and great, brilliant travertine deposits, consisting of
almost pure calcium carbonate, have been built up.* In this vicinity are
Tertiary sediments which include limestones. There can be no doubt in
this case that the waters are heated by the igneous rocks, and that the
source of the material which is dissolved by the heated waters is the
intruded sediments and not the igneous rocks. The case is particularly
conclusive, because the geyserite deposits in the adjacent basins where
sedimentary rocks are not close at hand consist of silica. If the heat of
the igneous rocks can be so effective in promoting the solutions of vast
quantities of calcium carbonate in the adjacent sedimentary rocks, may it
not be equally effective in dissolving a sufficient amount of gold, silver,
and copper, so that in a deposit a fraction of an ounce of gold, a few
ounces of silver, or a few per cent of copper may be found ?
It is also certain that many ore deposits derive their metalliferous
content in part from the intrusive and in part from the intruded rocks.
Probably this is the most frequent of all cases. To give an estimate of the
relative amounts of metalliferous metals derived from the original igneous
rocks and from the secondary rocks is quite impossible.
Recently there has been a strong tendency among many geologists to
maintain that the metals in ore deposits are mainly or wholly derived from
almost immediately adjacent igneous rocks. As already intimated, this is
undoubtedly true in many cases. For instance, where there is a great
complex of aqueous and igneous rocks of many different kinds and the
ores are always intimately associated with one particular kind of igneous
rock and no other, it is very probable that the source of the metal is the
intimately associated igneous rock. Such cases are rather numerous. An
excellent illustration is furnished by the copper-nickel deposits of Sudbury,
which are invariably associated with norite. Another case in which it
is believed that the ore is mainly derived from the igneous rocks is that of
the copper deposits of the Lake Superior region, which occur in amygda-
loids and conglomerates, but a portion of the copper was probably leached
from the conglomerates. This belief is not based upon the fact that these
ores occur to a considerable extent within openings of the amygdaloids,
«Hague, Arnold, Weed, W. H., Iddings, J. P., Yellowstone National Park: Geol. Atlas U.S.,
folio 30, U. 8. Geol. Survey, 1896.
SOURCE OF THE METALS. 1033
for the conglomerate deposits are even more important, but it is based
upon the fact that copper, both in the native form and to a less extent as a
sulphide, is sparsely disseminated throughout the basic Keweenawan lavas
of the entire Lake Superior Basin. This broad relation has much greater
significance to me with reference to the source of the metal than does the
association of the ore with an igneous deposit at a particular mine.
Spurr notes substantially the same relations between the ore bodies of
the Monte Cristo district of Washington and the associated tonalite. He
says that metallic sulphides are everywhere noticeable throughout the tona-
lite; that scarcely a cross fracture occurs through which there has been
water circulation and not some concentration of sulphides; and that in the
solid tonalite itself there are local segregations of sulphides. Furthermore,
he says there are all gradations from these bunches of sulphide in the
fractured rocks through a slight dissemination of sulphides to the ore
deposits along the major fractures, such as those of the Mystery and Pride
mines. Accepting this statement of fact, there is strong warrant for the
suggestion of Spurr that the ores are derived ‘directly by concentration
from the tonalite in which they lie.”*
While in many instances it is therefore believed that a contiguous
igneous rock is the main source of the metal for ore bodies, by some this
connection is over-emphasized, and the cases in which this is not true are
often ignored. Doubtless this is a consequence of the fact that gold and
silver are usually in mind when ore deposits are considered. Gold and silver
ores may rather frequently be derived largely from recognizable igneous
rocks, but to a very large extent, even for gold and silver, the immediate
source of the metal in the deposits heretofore exploited was not the original
igneous rocks. For instance, placers, which have yielded more gold to the
world than any other form of deposit, have derived their metals from veins,
from previous placers, and from the widely dispersed gold in many kinds
of rocks. Much of the gold of many placers has been worked over by the
processes of nature several times.
If we turn to iron, a metal of vastly greater importance than gold and
silver combined, in few or no deposits now exploited can the metals be said
to be derived directly from igneous rocks. Again, the deposits of lead and
@Spurr, J. E., The ore deposits of Monte Cristo, Washington: Twenty-second Ann. Rept. U. S.
Geol. Survey, pt. 2, 1901, pp. 828-829.
1054 A TREATISE ON METAMORPHISM.
zinc of the Mississippi Valley, from which 90 per cent of the zinc and more
than 20 per cent of the lead of the United States are mined, have certainly
not derived their metals directly from igneous rocks. It is rather probable
that the metals for these deposits, originally in rocks of unknown origin,
have been taken into solution in the belt of weathering, have been trans-
ported to the sea, and there precipitated in minute quantities at the time the
magnesian limestone formations of the Mississippi Valley were built up.
The immediate source of the metals for the present deposits is agreed by
all who have closely studied them to be the Cambro-Silurian limestones.
The conclusion is reached that these rocks are really the source of the
lead and zine, because almost everywhere through the great Cambro-Silurian
limestones small amounts of these metals are found. In cracks and crevices
at innumerable localities there are very minor segregations of galena and
blende, but not in sufficient amount to constitute ore deposits. Further-
more, Robertson has made analyses of the fresh solid Cambro-Silurian
limestone where it showed no galena or blende to the eye, and very small
amounts of lead and zine were found."
Thus the reasons leading to the conclusion that the Cambro-Silurian
limestone is the source of the lead and zine ores of the Mississippi Valley
are precisely the same as those leading to the conclusion that the Keweena-
wan lava is the main source of the copper deposits of the Lake Superior
region.
If these ore deposits in limestones are derived from the sediments,
why should it be assumed, in cases of ore deposits in sedimentary rocks,
especially in limestones cut by igneous rocks—as in the copper districts
of Arizona and New Mexico, in the Leadville, Tenmile, and Aspen
districts of Colorado, in the Mercur district of Utah, in very many other
places in the United States, and at a greater number in other parts of
the world—that the associated igneoys rocks, frequently subordinate in
volume, constitute the sole source of the metals lead, copper, gold, zine,
and iron?
The foregoing facts show that the source of the metal for an individual
district is not to be ascribed a priori to igneous, sedimentary, or metamor-
phic rocks, but can be determined only after an inductive investigation of
the facts. The metal of a district may be derived from the late igneous
«Winslow, Arthur, Lead and zinc of Missouri: Missouri Geol. Survey, vol. 7, 1894, p. 480.
SOURCE OF THE METALS. 1055
rocks, from ancient igneous rocks, from sediments, from metamorphosed
rocks, or from any combination of the above. When the important eco-
nomic districts of the world are inductively studied, and certain knowledge
obtained, I believe that it will be discovered that a great number, if not the
majority, of ore deposits are not the result of a single segregation, but are
the accumulated fruits of a great interrupted process of segregation, a part
of the metals for the deposits having been worked over many times by the
metamorphic processes.
According to this view, it has been shown in Chapter XI, on the
redistribution of the elements, that a general result of metamorphism and
accompanying processes is that many of the secondary rocks are depleted
in reference to each metal, and that correlative with such depletion other
deposits are formed in which each metal is segregated. Since these
processes result in deposits in which each of the common metals is
segregated, why should we hold that the metal of a present deposit of
gold or silver or copper is derived solely from an immediately adjacent
igneous rock unless evidence for this be conclusive? The natural view is
that the metallic ore deposits of the world are, broadly, the accumulated
results of the processes of segregation carried on throughout geological
time.
While it is held that the metals for very numerous and important ore
deposits do not have their immediate source in igneous rocks, it is recog-
nized, as stated at the outset, that ultimately the metals for all ore deposits
are probably to be traced back to igneous rocks. Since leaving their
original positions the metals for many ore deposits have been transferred
and segregated here and there until they reached the places where they
are now found.
It is recognized that this general statement as to the sources of metals
for ore deposits will be unsatisfactory to many persons, as every one is more
r less influenced in his view by his own personal experience and by the
particular metals which he has studied, but it is not the purpose of this
chapter to consider individual districts, except as they illustrate principles.
It is properly the part of the geologist or mining engineer who studies an
individual district to find the source of the metals. In many cases careful
investigations can undoubtedly determine this point, as, for instance, in the
Lake Superioriron region. In other districts, however, the most exhaustive
1056 A TREATISE ON METAMORPHISM.
study may not enable the investigator to determine the source of the metals.
This is especially likely to be true of ore deposits produced by ascending
waters from a somewhat deep circulation. The underground waters may
have their sources of supply in rocks which do not reach the surface, and
have not been penetrated by the mine workings.
In concluding this part of the subject it may be suggested that in
many instances mistakes have been made in assuming that some one for-
mation, sedimentary or igneous, is the sole source of the valuable metals.
Such an assumption is particularly prevalent in papers descriptive of gold
and silver deposits. In many districts where there are a number of sedi-
mentary and igneous rocks I have no doubt that the silver and gold are
partly derived from two or several formations.
PART II. SEGREGATION OF ORES.
GENERAL STATEMENTS.
We are now prepared to consider each of the divisions of ores. It
will be seen that the different divisions, groups, classes, and subclasses
of ores are based upon the last dominant genetic process concerned in
their production. It has just been explained that there has been a steady
redistribution of the metals, each of them being taken away from some
of the formations, and, corresponding with this, segregated elsewhere.
Looked at from this point of view, an existing ore deposit may represent
the result of segregations by many processes throughout geological time.
Thus, if the full history of a metal in a given ore deposit were known it
might be found that at different times parts of it had been segregated by
various sedimentary, igneous, and metamorphic processes. Not only is
this so, but the segregations by these various processes may have occurred
and recurred. It is therefore manifestly impossible to give the full history
of ores, or to apply a genetic classification to them which does not restrict
itself to the later dominant processes which resulted in the segregation of
the ores at the particular places where they now are. Even thus restricted
it will be found that some ore deposits placed in one division or class could,
with almost equal plausibility, be placed in another.
SEGREGATION OF CHEMICAL PRECIPITATES. 1037
DIVISION A. ORES PRODUCED BY PROCESSES OF SEDIMENTATION.
Since ores produced by processes of sedimentation do not properly
fall within the scope of a treatise on metamorphism, they will be considered
only to the extent necessary to show their relation to the division of ores
somewhat fully discussed—that produced by processes of metamorphism.
The ores produced by processes of sedimentation may be divided into
two groups—those formed by chemical precipitation, and those formed by
mechanical concentration.
ORES FORMED BY CHEMICAL PRECIPITATION.
Ores produced by chemical precipitation comprise some of the iron,
manganese, and aluminum deposits.
Bog deposits of iron ore, of recent origin and doubtless in some cases
within older rocks, are the direct results of chemical precipitation. The
processes by which these develop—the leaching of the metal from the land
areas, and its transportation and precipitation as limonite in shallow bodies
of water—have all been fully treated in connection with the development
of iron carbonate and limonite (see pp. 824-829, 842-845), therefore the
process will not be here again described. Some manganese deposits are,
in part at least, the direct result of chemical precipitation, the lines of
development being the same as those of limonite.
Hayes holds that certain bauxite deposits also are the result of chem-
ical precipitation. According to his views the aluminum for some of the
bauxite deposits of Arkansas was taken into solution by underground water,
was brought to the surface by springs which issued into a shallow sea,
where the aluminum and iron oxides were chemically precipitated. (See
Chapter XI, p. 985.) Since these deposits are considered in the chapter
on the redistribution of the elements nothing further will be said here in
reference to them.
Aluminum and iron are the most abundant of the metals, while man-
ganese is rather plentiful. Again the law of mass action applies. The
abundant metals are those which are likely to be thrown down as chemical
sediments in sufficient amount to constitute ore deposits.
105 A TREATISE ON METAMORPHISM.
ORES FORMED BY MECHANICAL CONCENTRATION.
Mechanical concentrates may be divided into residual deposits, stream
deposits, and beach deposits. The term placer, originally used for stream
deposits, has been, in some cases, extended to seabeach deposits.* The
principles which result in the production of residual deposits, stream
deposits, and seabeach deposits are the same.
The valuable materials segregated have a high specific gravity and
are relatively indestructible. In consequence of their capacity to resist
mechanical wear and chemical solution they are segregated in alluvial
deposits, in the sands and gravels of streams, and on beaches of lakes
and oceans, because the other constituents are transported, worn, or dis-
solved more readily than is the resistant valuable product. In the pro-
duction of residual deposits the capacity of the valuable material to resist
solution is of dominating importance, and in the production of beach
and stream deposits the valuable material must have power to resist both
solution and wear.
The ores produced by mechanical concentration comprise both metals
and oxides. The important metals belonging to this group are gold, plati-
num, iridium, and osmium. Practically all of the last three metals which
have been exploited have been derived from placers. But of the metals
produced by processes of sedimentation gold is the one of dominating
importance. To the end of the nineteenth century a very large fraction
of all the gold extracted from the earth by man was derived from placers.
The oxides which occur as mechanical concentrates are those of tin
(cassiterite) and iron (magnetite). For the first of these compounds the
placer deposits are of great importance, but for the iron oxides are relatively
unimportant. Although much cassiterite has been taken from veins, for
many years the chief supplies of tin ore have come from the East Indies—
the so-called Straits tin—and all except a relatively insignificant amount
of this cassiterite has been derived from mechanical concentrates.” The
Malay Peninsula now produces almost 50 per cent of the tin of the world;
. @Schrader, F. C., and Brooks, A. H., Some notes on the Nome gold region of Alaska: Trans. Am.
Inst. Min. Eng., vol. 30, 1901, pp. 236-247.
DRolker, C. M., The production of tin in yarious parts of the world: Sixteenth Ann. Rept. U. 8.
Geol. Survey, pt. 3, 1895, pp. 458-492.
SEGREGATION OF MECHANICAL CONCENTRATES. 1039
the adjacent islands, Banca and Billiton, produce about 25 per cent.
Doubtless this tin is of the same origin as that of the Malay Peninsula.
Thus it appears probable that substantially 75 per cent of the tin of the
world is derived from mechanical concentrates.“
In a small way magnetite beach sands have been utilized. ‘Those of
New Zealand are best known.’
An excellent illustration of an iron-ore deposit produced by mechanical
concentration was that of Iron Mountain, Missouri, where considerable
masses consisting of well-rounded bowlders of hematite’ were found scat-
tered over the mountain as a residual deposit and buried in pre-Cambrian
ravines under Cambrian and Silurian deposits.
METAMORPHIC ALTERATIONS OF SEDIMENTARY ORES.
No sooner are ore deposits produced by processes of sedimentation
than they are subject to the metamorphic processes, and thus are modified
to a variable extent. Sedimentary deposits, whether originally formed in
the belt of weathering or in the belt of cementation, later may be in the
belt of weathering, in the belt of cementation, in the zone of anamorphism,
or partly in one and partly in another. In each case the deposit is
subject to the metamorphic processes of the horizon in which it may be.
Where a deposit is in the belt of weathering, the material may be dis-
solved to some extent and the metal carried downward into the belt of
cementation. If a deposit is in the belt of cementation valuable minerals
may be added to or taken away from it as a consequence of the work of
underground water. If the material is partly in the belt of weathering and
partly in the belt of cementation a portion of the material dissolved in the
belt of weathering may be carried downward and deposited in the belt of
cementation, and thus the lower part of the deposit enriched at the expense
of the upper part. Ifthe material reaches the zone of anamorphism it may
there undergo the alterations of that zone. After having been subject to
the metamorphic effect of a certain belt or zone a deposit may be trahs-
ferred to another belt or zone and thus the alterations of this zone be
“Penrose, R. A. F., jr., The tin deposits of the Malay Peninsula, with special reference to the
Kinta district: Jour. Geol., vol. 11, 1903, pp. 135-154.
» Birkinbine, John, The production of iron ores in various parts of the world: Sixteenth Ann.
Rept. U. S. Geol. Survey, pt. 3, pp. 29-186, 186.
¢Nason, F. L., Iron ores of Missouri: Geol. Survey Missouri, vol. 11, 1892, pp. 27-3
oO
1040 A TREATISE ON METAMORPHISM.
superimposed upon the previous alterations. These later alterations by
metamorphism, like the earlier ones, may greatly diminish or greatly
increase the value of the original sedimentary deposit. In consequence of
the varying action of the secondary metamorphic processes an ore deposit
may be chiefly due to the direct processes of sedimentation or chiefly due
to metamorphic processes. There are therefore all gradations between
deposits produced solely by the processes of sedimentation and those
formed entirely by the processes of metamorphism.
This is especially well illustrated by the gold placers. Many of these
deposits are in large part in the belt of weathering, but extend downward
into the belt of cementation. It has been repeatedly explained that in the
belt of weathering ferric and cupric sulphates and chlorides are produced.
These are capable of dissolving gold. Doubtless other solutions are
formed which have similar power. Wherever these solutions traverse
placers it is little short of certain that the gold is dissolved to some extent.
The material may be dissolved in the belt of weathering, transported
downward to the belt of cementation, and there precipitated; and thus one
part of the deposit may be enriched at the expense of another; or
solution may take place throughout a deposit and, in consequence, a
placer be impoverished; but the abstracted material may be precipitated
in another deposit, so that correlative with the depletion of some deposits
there may be enrichment of others.
The formation of placer deposits has not been restricted to the present
geological period. It is highly probable that throughout geological time
placer deposits of greater or less richness have been produced, although it
is likely that those in early geological times were comparatively small. In
many cases as denudation continues the material of placers is largely
transported to the sea, and so forms beach deposits. It at first might be
thought surprising that so few ancient beach deposits of gold have been
found, but this is partly explained by the fact that gold is undoubtedly
dissolved in sea water, since the ocean contains compounds capable of
dissolving this metal. When the gold is transported to the seabeach, it is
in a very favorable position to be finely ground by attrition, and in this
fine condition is especially subject to the solvent action of the sulphates
and chlorides of the sea. It has been estimated that in sea water there is
a greater amount of gold than has been extracted from the earth by man.
MELAMORPHISM OF SEDIMENTARY ORES. 1041
While some of this gold was undoubtedly transferred to the sea in solution
by the streams, it is believed to be probable that a major part of it was
transferred to the sea by the streams mechanically, and by the action of
the sea upon the beach deposits the gold was dissolved.
River deposits and beach deposits may in consequence of physical
changes be buried under later deposits, and thus pass far down into the belt
of cementation or even into the zone of anamorphism. They are then
subject to all the metamorphosing processes of the belts in which they
occur. In the belt of cementation the circulating underground water will
superimpose its work. As illustrating this principle may be mentioned the
difference of opinion which exists as to the great gold deposits of the Rand.
These deposits, according to Becker, are dominantly marine beach deposits,”
although he agrees that the gold has been rearranged to some extent by
underground water,’ thus explaining the crystallized character of portions
of the gold and the existence of gold in cracks within the quartz pebbles.’
But Becker holds that not more than a minute fraction of 1 per cent of the
gold has been introduced from an extraneous source. Others have held
that the Rand deposits are of alluvial origin, i. e., stream placers; and still
others believe that the gold was deposited from solution at the time the
gravels accumulated.” Upon the other hand, Curtis’ and Hammond’ both
insist that all of the gold was deposited by underground water, and there-
fore the deposits belong to the class produced by processes of metamorphism.
Recently an excellent statement of facts in reference to these deposits
has been made by de Launay.’ According to this author the gold exists
in pyrite contained in the siliceous cement, together with other metamorphic
minerals, such as chlorite and muscovite. Much of the pyrite is rounded
like rolled grains, but a larger part is well crystallized and presents sharp
angles. De Launay is unable to decide between the hypotheses (1) of
«Becker, Geo. F., The Witwatersrand banket, with notes on other gold-bearing pudding stones:
Eighteenth Ann. Rept. U. 8. Geol. Survey, pt. 5, 1897, pp. 160-177.
> Becker, cit., p. 171.
¢ Becker, cit., p. 176.
@Becker, cit., pp. 169-170.
eCurtis, J. S., The banket deposits of the Witwatersrand: Eng. and Min. Jour., vol. 49, 1890, pp.
200-201.
f Hammond, John Hays, Gold mining in the Transvaal, South Africa: Trans. Am. Inst. Min. Eng.,
vol. 31, 1902, pp. 841-845.
9 De Launay, L., Observations on the Rand conglomerate: Eng. and Min. Jour., vol. 75, No. 14,
1903, pp. 519-521. :
MON XLVII—04-——66
1042 A TREATISE ON METAMORPHISM.
placer deposits, (2) of simultaneous chemical precipitation with mechanical
accumulation of the gravel, and (3) of introduction of the gold. into the
conglomerate by underground waters since its formation. He rejects
the idea of a combination of these, saying that “this would only uselessly
complicate the hypothesis.” It seems to me that the only hypothesis
which explains the facts is a combination of the placer and metamorphic
processes.
Where two men differ so radically as do Becker and Hammond, the
truth may lie between, as is so often the case. Each is possibly partly
right and partly wrong. It is natural to think that a given ore deposit can
not be produced partly by the processes of sedimentation and partly by
underground water. These two classes of deposits are automatically
assumed not to grade into each other, whereas it is perfectly clear that such
gradation may and probably does exist. It may therefore be suggested
that a closer study of the Rand deposits will show that they are partly in
the nature of mechanical sediments and partly in the nature of deposits
seoregated by underground water. If this be the fact, study should be
directed to determine the quantitative importance of each. :
The placers of California and New Zealand furnish an instance in
which it is certain that both mechanical concentration and later chemical
modification by underground water have played a part.* In these placer
deposits, and especially in the deep placers, crystallized gold occurs not
infrequently, and it must be held that at least the rearrangement of this
gold has been due to the work of the underground water since the formation
of the placers. If this be admitted, the question at once arises as to what
extent gold has been added by the circulating waters since the placers were
formed. The added gold may have been derived from the leaner upper
portion of the placers, from the surrounding rocks, including the veins,
or in part from both sources.
It is clear from the foregoing that mechanical concentration or second-
ary metamorphic action may be the dominant factor in producing a sufh-
cient amount of segregation to make an ore deposit, or both may be
necessary in order to accomplish this work.
The chemical and mechanical ore deposits particularly well illustrate
the principle that ores can be classified only upon the basis of the last
«Gordon, H. A., Hysteromorphqus auriferous deposits of the Tertiary and Cretaceous periods in
New Zealand: Trans. Am. Inst. Min. Eng., vol. 25, 1896, pp. 292-301.
METAMORPHISM OF SEDIMENTARY ORES. 1043
process which is concerned in their segregation. Almost any one of these
deposits illustrates the idea. For instance, a sedimentary deposit of bog
iron or manganese is finally concentrated by precipitation of these materials
in a marsh or lagoon. But the earlier stages of the work of segregation are
by underground water, and if this part of the work were considered the ores
would be classified as metamorphic. Again, the mechanical concentrates
are not mainly segregated from the sparsely dispersed metal in the original
rocks, but are segregated from previous segregations, which may be the
result of igneous, sedimentary, or metamorphic processes, or some combina-
tion of the two or three. To illustrate, the gold placers of the Sierra
Nevada chiefly derived their gold from the vein deposits of the same
mountain system. The segregation of the gold within these veins is
the most important part of the process and it will be considered under the
work of underground water, but the last process—mechanical concentra-
tion—produced the placers and it must be taken as the basis of classi-
fication. Similar statements could be made with reference to the residual
cassiterite of the Malay Peninsula or the residual iron ore of Iron Mountain,
Missouri.
DIVISION B. ORES PRODUCED BY IGNEOUS PROCESSES.
Ores produced by igneous processes, like those produced by processes
of sedimentation, do not properly come within the scope of this treatise
upon metamorphism. They will, therefore, be considered only so far as
necessary to show their relation to the ores produced by processes of meta-
morphism.
Ores directly produced by igneous processes are regarded as a small
division by those who have most closely studied them. The recent advo-
cates for a direct igneous origin for certain ore deposits include Vogt, Beck,
Kemp, Spurr, and others. Emmons“ has also favored the idea of at least a
first concentration of the metallic contents of ores by processes of differen-
tiation of igneous rocks, more particularly the basic rocks. Spurr has
recently well summarized the principles which result in the segregation of
ores by magmatic processes.’
«Emmons, §. F., Geological distribution of the useful metals in the United States: Trans. Am.
Inst. Min. Eng., vol. 22, 1894, pp. 53-95. The mines of Custer County, Colo.: Seyenteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1896, pp. 470-472.
5Spurr, J. E., A consideration of igneous rocks and their segregation and differentiation as related
to the occurrences of ores: Trans. Am. Inst. Min. Eng., vol. 33, 1903, pp. 288-341.
1044 A TREATISE ON METAMORPHISM.
Since upon the whole Vogt has most closely studied ores produced by
magmatic segregation, and has been the most vigorous advocate for the
existence of such, | quote from him a paragraph giving a list of deposits
which he believes to be of igneous origin:
Ore deposits formed by simple magmatic differentiation are confessedly infre-
quent, and therefore relatively subordinate in importance to other classes. Under
this head may be named: (1) The occurrences of titanic iron ores in basic and inter-
mediate eruptives, perhaps also of iron ores in acid eruptives; (2) those of chromite
in peridotites and their secondary serpentines (and also according to J. H. Pratt,
those of corundum in the peridotites of North Carolina); (3) a number of deposits of
sulphide ores, particularly the nickeliferous pyrrhotites occurring in gabbro (at Sud-
bury, Canada, Lancaster Gap, Pennsylvania, many places in Nosar and Sweden,
and Varallo, in Piedmont); (4) according to some authorities, the auriferous pyrites
of Rossland, British Columbia; (5) according to B. Lotti, the high-grade copper
ores occurring in serpentinized peridotites in Tuscany and Liguria, Northern Italy
(for instance at Monte Catini), and analogous occurrences in other regions; (6) the
occurrences of metallic nickel-iron (without economic value) in eruptive rocks; (7)
those of the platinum metals in highly basic eruptive rocks, ete.
It may be pretty safely assumed that the foregoing list will be enlarged by
future investigations, though it can never become very extensive.”
Of these classes the first mentioned, the titanic iron ores, is probably
the largest. This class of ore deposits has been elaborately considered by
Vogt, and a full list of illustrative localities is given by him.’ Of such ore
deposits the only ones with which I am familiar are the titaniferous mag-
netites at the base of the Duluth gabbro in the Lake Superior region.
While I have not studied these deposits closely I have no reason to dissent
from Vogt’s belief that they are differentiation products of the gabbro
maema. Recently Lindgren has described a very large titanic iron ore
deposit at Iron Mountain, Wyoming, which I understand him to believe to
be derived by magmatic segregation from an igneous rock.’ The total
mass of titaniferous iron ores is large, but much of this material is of low
grade. It is for the most part high in titanium, averaging about 12 to 20
per cent of titanium oxide, but occasionally having as little as 3 per cent."
«Vogt, J. H. L., Problems in the geology of ore deposits: Genesis of ore deposits, 2d ed., pub-
lished by Am. Inst. ‘Min. Eng. 1902, pp. 642-643.
bVogt, J. H. L., Weitere Untersuchungen tber die Ausscheidungen yon Mitaneisenerzen in ba-
sischen Eruptivgesteinen: Zeit. fir prak. Geol., Jahrgang 1900, pp. 233-242, 370-382; Jahrgang 1901,
pp. 9-19, 180-186, 289-296, 327-340.
¢Lindgren, Waldemar, A deposit of titanic iron ore in Wyoming [a paper presented to. the
Geological Society of Washington]: Science, new ser., vol. 16, 1902, pp. 984-989.
dKemp, J. F., Titaniferous iron ores of the Adirondacks: Nineteenth Ann. Rept. U. 8. Geol.
Survey, pt. 3, 1899, pp. 387-388.
ORES PRODUCED BY IGNEOUS PROCESSES. 1045
In consequence of their low grade and the difficulty of smelting them,
titanium ores are at present of little or no economic importance. In the
future, when high-grade nontitaniferous ores are exhausted, or nearly
exhausted, it is possible that such ores will be an important future resource
for iron.
In connection with iron ores produced by magmatic segregation the
question naturally arises as to why they are titaniferous. The answer is
substantially that they are titaniferous because they are magmatic segrega-
tions. The magnetite in diabase, gabbro, and other basic rocks is almost
universally titaniferous. This being true, it naturally follows that where
this magnetite is segregated in sufficient quantity to be an ore such ore is
titaniferous.
So far as I know, all of the iron ores which have been shown to be
magmatic segregations are titaniferous. ‘The majority of the great iron-ore
deposits which are at present worked are known to have been produced by
the action of aqueous solutions. (See pp. 1193-1197.) In view of this fact
and the history of titaniferous magnetites, it seems to me highly probable
that none of the nontitaniferous ores of iron are magmatic segregations.
Certain corundum deposits are the result of direct igneous processes.
For instance, the corundum syenite of eastern Ontario, Canada, is to all
appearances, as shown by its textures and structures, a coarse igneous
rock.“ Apparently the magma was so very aluminous that at the time of
crystallization not all of the alumina was able to combine with the various
acids, and hence a part of it separated as an oxide, or corundum, precisely
as iron oxide, which does not unite with the acids.
As to the production of titanic magnetite and corundum ores by
magmatic processes there is no difference of opinion. But it appears to
me that a point which should be emphasized in this connection is that
aluminum and iron are the two most abundaut metals of nature, and
therefore they are the ones which are most likely during crystallization of
magmas to segregate as oxides to such an extent as to produce ores.
The extent to which magmatic segregation must take place in order to
produce an igneous ore of iron or aluminum is comparatively small. To
illustrate, the average amount of metallic iron in original igneous rocks is
4.64 per cent,’ and in the basic rocks in which magmatic iron ores occur it
«Miller, W. G., Economic geology of eastern Ontario; corundum and other minerals: Rept.
Bureau of Mines, vol. 7, pt. 3, 1898, pp. 210-215.
bClarke, F. W., Analyses of rocks, laboratory of the U. 8. Geol. Survey, 1880-1899: Bull. U. S.
Geol. Survey No. 168, 1900, p. 15.
1046 A TREATISE ON METAMORPHISM.
is more than this, being 8.66 per cent in the Duluth gabbro of Minnesota.”
Therefore to produce an ore body by magmatic segregation in such a rock
as this the iron need be increased only about seven times. The production
of ores of the less common metals by direct magmatic segregation is an
entirely different matter. In the original igneous rocks the amounts of
copper, nickel, cobalt, gold, silver, etce., are usually so small as not to be
detected by assay. In order to produce ores of these metals segregation
must take place many fold, probably in all cases more than a hundredfold,
and in many cases probably a thousand or many thousand fold. Those
who maintain that an ore deposit of any of these less common metals is
produced wholly by magmatic segregation should determine the ratios
between the amount of metal in the original igneous rock and in the ore,
and show that the igneous processes have segregated the metal to that
extent.
These considerations apply to the other cases of ore deposits held by
Vogt to be due to magmatic segregation and to the gold-ore deposits
referred by Spurr to the same processes. ‘Thus to segregate chromite from
peridotite and the sulphides of the less common metals from igneous rocks
by magmatic processes alone would require an amount of segregation
which is yet to be proved. Of the sulphides ores that of nickel is the one
which has most strenuously been held to be due to magmatic segregation.
Of nickeliterous pyrrhotites, I have seen deposits at only one locality,
near Sudbury, Canada, where they are associated with norite. I shall not
deny that a first segregation of these deposits along the border of the
norites may have taken place in part as a consequence of magmatic differ-
entiation. Walker? states that in certain cases near the center of the
norite masses there is comparatively little pyrrhotite, and that in passing
toward the borders the amount steadily increases until the ore deposits
are reached. But so far as I have seen the deposits there is a great differ-
ence between the nickeliferous norite and the deposits which are mined, a
difference between a comparatively small and a high percentage of sulphide.
It is these high-grade products which I hold were certainly to a large
extent precipitated from circulating solutions. The increase in the amount
« Winchell, A. N., Gabbroid rocks of Minnesota: Am. Geol., vol. 26, 1900, p. 374.
» Walker, T. L., Geological and petrographical studies of the Sudbury nickel district, Canada:
Quart. Jour. Geol. Soc. London, vol. 53, 1897, p. 52.
ORES PRODUCED BY IGNEOUS PROCESSES. 1047
of sulphide in passing from the center to the border of a norite area is just
what one would expect, even if the material had been segregated by circu-
lating solutions, since the trunk channels for circulation are frequently along
the borders of intrusive masses, and at such places solutions from the
igneous and adjacent rocks extensively mingle. At Sudbury almost con-
clusive evidence of segregations by solutions is furnished by distinct veins
of pure sulphides, evidently formed in openings in the norite. Both walls
of some of the veins are impregnated with ore in varying degrees. The
impregnation is greatest at the walls, and in passing from them the amount
of sulphide gradually lessens until none is visible. In short, the phe-
nomena are absolutely identical with those which Lindgren has described
as especially characteristic of metasomatic processes along the walls of
fissure veins.
It may be suspected that a close study of some of the other nickel-
iferous deposits will show somewhat similar phenomena. To what extent
circulating underground solutions have concentrated the nickeliferous
deposits of Norway and of Varallo in Piedmont I do not venture to say.
Nor do I express any opinion as to the part which magmatic segregation
has played in the production of the auriferous pyrites of Rossland, British
Columbia, and the high-grade copper ores described by Lotti; but Lind-
gren, as the result of careful studies by himself and previous studies by
King and Raymond, states that the Rossland deposits are metasomatie
replacements rather than magmatic differentiations’ It is to be noted that
the copper ores described by Lotti and tnat chromite ores- described by
Pratt’ are in serpentinized peridotites.
In those cases in which the ores are in much altered rocks the question
certainly should be asked as to what extent the ores have been modified
by the processes of alteration. In the other cases, in which no evidence of
secondary action is mentioned by Vogt, it certainly is a proper subject of
«While this volume is going through the press a paper by Dickson on the ore deposits of Sudbury
has appeared (Ore deposits of Sudbury, by Charles W. Dickson: Trans. Am. Inst. Min. Eng., vol. 34),
which gives a full summary of the views that others have held in reference to the genesis of the Sud-
bury ores and contains an elaborate discussion of the facts of occurrence of these ores. As a consequence
of his elaborate study, Dickson concludes that ‘‘while magmatic differentiation has gone on to some
extent, the sulphides are not the result of it.”’ Hesaysturther: ‘‘It might be safely stated that at present
the whole weight of the evidence points to the secondary formation of the Sudbury ore bodies as
replacements along crushed and faulted zones, with only minor indications of open cayities.”’
» Genesis of ore-deposits: Am. Inst. Min. Eng., 2d ed., 1902, pp. 564-565.
¢Pratt, J. H., Occurrence, origin, and composition of chromite: Trans. Am. Inst. Min. Eng., vol.
29, 1900, pp. 22-24. b
1048 A TREATISE ON METAMORPHISM.
inquiry as to what extent magmatic segregation has produced the deposits
and to what extent subsequent metamorphic action has been influential. I
suspect it will be ascertained, as is so frequently true of ore deposits, that
in some of the cases cited the ores, instead of being due to a single process
at one period, are due to a combination of processes. Even if a first segre-
gation were made by direct igneous action, as maintained by Vogt, such
deposits may have been further enriched by metamorphic processes after
the first concentration. (See pp. 1235-1236.)
Concerning the occurrence of metallic nickel-iron in eruptive rocks,
and platinum in highly basic rocks, it may be merely remarked that such
deposits are not ores. It is well known that not only these, but various
other metals, such as copper, occur widely distributed in igneous rocks, and,
indeed, it has been shown that in such rocks, directly or indirectly, all ores
have their ultimate source. But before the term ‘‘ore” can be. properly
applied to such rocks it should be shown that there is at least a reasonable
chance that the economic conditions will be such that some metal may some-
time be exploited with profit. It follows that the occurrence of sparsely
disseminated metals of various kinds through igneous rocks has no bearing
upon the question of the segregation of ores by igneous processes alone.
Spurr, holding that pyrrotite veins are due to magmatic segregation,
has applied the same explanation to the gold-quartz veins of the Yukon.“
He says the pure quartz veins are produced at the final stage of crystalliza-
tion in connection with igneous rocks when the residue ‘‘is little more than
hot siliceous water, which contains, besides silica, small quantities of many
other rock elements which have not been taken up by the rock-forming
minerals.”” Later, but with no further field study or additional evidence,
Spurr goes somewhat further in his views toward magmatic segregation.
Explaining the origin of the Yukon and other gold-bearing quartz veins,
he says:
Therefore it has been concluded that certain quartz veins in the Yukon district
(part, at least, of which are auriferous) have originated by a process of magmatic
segregation, which has separated them from other materials while in the state of
aqueo-igneous fusion (the condition of molten rock in general), and that they repre-
sent the siliceous extreme of that process. From this standpoint, they are a variety
of the igneous rocks. But it has been shown that as magmas become more siliceous
“Spurr, J. E., Geology of the Yukon gold district, Alaska: Eighteenth Ann. Rept. U. 8. Geol.
Survey, pt. 3, 1898, pp. 311-316.
bSpurr, cit., p. 312.
ORES PRODUCED BY IGNEOUS PROCESSES. 1049
they also contain more water; so, when the stage of quartz veins is reached, the
magma is believed to be so attenuated that it may best be described as water highly
heated and heavily charged with mineral matter in solution.@
He further states that there is no great difference between veins
origiating in this manner and those having other origins. On this point
he says: “A quartz vein originating by magmatic segregation often might
not be distinguishable from one formed in the many other ways which are
possible.”’ The only difference which he suggests between gold-quartz
veins of magmatic origin and those deposited by aqueous solutions is that
the gold ores supposed to be produced by magmatic segregations are
“without the admixture of so great proportions of the commoner metals as
is usual in ore deposits.””
Spurr, however, gives no reasons showing the
applicability of this criterion. From my own point of view, I should regard
it as applying in the reverse direction. That is to say, if the exceedingly
rare metal, gold, be segregated by magmatic processes to such an amazing
extent as to produce an ore deposit, it seems to me to be exceedingly likely
that other metals would be segregated in connection with the gold ores so
as to be present in greater amounts than usual.
Since Spurr gives no certain criteria by which gold-quartz veins sup-
posed to be produced by magmatic segregation are to be discriminated from
those formed by solutions, I take the conservative view that gold-quartz veins
have their origin in solutions, and I ask for positive evidence from those who
assert that such quartz veins are produced by magmatic segregation alone.
In general, concerning the production of ores of the metals other than
those of iron and aluminum by magmatic segregation alone, I repeat that it
seems to me that the enormous amount of segregation required should be
taken into account, and that it should be most conclusively shown that this
process alone produces the ores before the conelusion be accepted that during
the crystallization of the magma segregation took place to the necessary
extent.
While Vogt,’ Beck,” Lindgren,’ and others believe in a direct igneous
a8purr, J. E., A consideration of igneous rocks and their segregation or differentiation as related
to the occurrence of ores: Trans. Am. Inst. Min. Eng., vol. 33, 1903, p. 311.
>Spurr, cit., p. 311.
¢Voet, J. H. L., Problems in the geology of ore deposits: Trans. Am. Inst. Min. Eng., vol. 31,
1902, pp. 131-147.
@ Beck, R., Lehre von der Erzlagerstiitten, Berlin, 1901, pp. 700.
e Lindgren, Waldemar, Character and genesis of certain contact deposits: Trans. Am. Inst. Min.
Eng., vol. 31, 1902, pp. 242-244.
1050 A TREATISE ON METAMORPHISM.
origin of certain ore deposits, they hold that far more numerous and impor-
tant ore deposits” are of contact-metamorphie origin instead of the result of
direct igneous action. This view is an old one supported by von Cotta,”
von Groddeck,’ de Launay,” and many others. Still others have used the
word ‘contact after-action” apparently as marking a grade of deposit
between contact metamorphic deposits and deposits laid down by aqueous
solutions.
Lindgren, considering the phenomena of pegmatitization under the
principle already mentioned, that liquid rock and water are miscible in all
proportions, has suggested a relation between pegmatite veins and ore
deposits; but he is careful to say that the relation between the pegmatites
and quartz veins must be proved, and that this has not yet been done.
He further says that the California, Idaho, and Oregon gold-quartz veins
show no relation whatever to pegmatitic dikes. He also calls attention to
the auriferous quartz veins of North Carolina, described by Pratt, which
occur together with barren lenses of pegmatitie quartz.’
Thus Lindgren, who has studied the gold-quartz veins of the United
States more extensively than any other geologist, recognizes the fact that
so far as his observations go the productive auriferous gold-quartz veins,
even if associated with and possibly dependent upon pegmatitic action, are
not the deposits which have ordinarily been referred to as pegmatites.
Although strongly holding that the majority of fissure veins are “genetically
connected with bodies of intrusive rocks,”’ Lindgren states that in those
great fissure veins which he has studied in the West he has not been able
as yet to identify the igneous rocks with which the veins are connected
genetically. Further, he says that the waters which deposited gangue min-
erals of the gold-quartz veins and gold “are chiefly surface waters, which,
after a circuitous underground route, have found in a fissure an easy path
on which to return.”?
«Vogt, J. H. L., Problems in the geology of ore deposits: Trans. Am. Inst. Min. Eng., vol. 31,
1902, pp. 137-140.
> Cotta, B. von, Erzlagerstitten im Banat und in Serbien, Wien, 1864.
¢ Groddeck, A. von, Die Lehre yon den Lagerstiitten der Erze, Leipzig, 1879, p. 260.
dLaunay, L. de, Traité des gites minéraux et métalliféres, Paris, vol. 2, 1893, pp. 245-208.
e Lindgren, Waldemar, Character and genesis of certain contact deposits: Trans. Am. Inst. Min.
Eng., vol. 31, 1902, pp. 243-244.
f Lindgren, Waldemar, Metasomatic processes in fissure veins: Trans. Am. Inst. Min. Eng., vol. 30,
1901, p. 691.
g Lindgren, cit., p. 691.
ORES PRODUCED BY IGNEOUS PROCESSES. 1051
Vogt, although so strong an advocete for igneous action, does not
include among ore deposits formed by eruptive after-action in connection
with pegmatite ves any gold, silver, copper, lead, zinc, or iron deposits.
There is therefore agreement between Vogt, Lindgren, and myself upon the
fundamental point that the great class of ore deposits which have been
known as contact metamorphic deposits were deposited by underground
solutions. There is also agreement that the igneous rocks in various
instances constituted the partial or chief source from which the metals are
derived. There is further agreement that the waters which deposited the
materials were hot, and derived their heat largely from the igneous rocks.
It follows from this that we agree that the ores were mainly put into
their present places during the time in which the igneous rocks were
able to produce a contact effect through hot solutions. ‘There is also
agreement that the water depositing the ores was derived from two sources,
the ordinary circulating underground waters and water derived from the
crystallization of the magmas themselves. So far as the waters were
derived from the latter source, it is further agreed that in many cases they
were rich in metalliferous material, and therefore unusually effective.
Probably in some cases solutions from the igneous rock were more
important than those of the meteoric origin.
In a genetic classification of ores upon the basis of agency there is no
place for the contact deposits. It does not follow that no ore deposits are
produced in connection with contact metamorphism. Contact metamorphic
ores must be produced by direct magmatic processes and therefore be
igneous ore deposits, by gaseous solutions, by aqueous solutions, or by
some combination of the three. In other words, the terms contact meta-
morphism and eruptive after-action are too vague to give any precise meaning
with reference to the deposition of ores. That this is so is shown by the
very different usages of the words ‘‘contact metamorphism” and ‘eruptive
after-action” by different authors. It is perfectly clear that different
deposits, or even the same deposit, produced in connection with contact
metamorphism or eruptive after-action may belong partly to igneous ores,
gaseous ores, and aqueous ores. Shortly after intrusion there may be
magmatic segregation along the border of the igneous rocks; later, in the
process of cooling, additions may be made to the ore deposits by gaseous
1052 A TREATISE ON METAMORPHISM.
solutions; and still later, when the rocks have further cooled, further
additions may be made by aqueous solutions. Indeed, I suspect that a
considerable number of ores have had such a composite history, viz, first,
segregation by magmatic processes, further segregations by gaseous
solutions, and finally segregations by aqueous solutions. I hold this latter
stage to be commonly the one in which occurs the final concentration
necessary to make a deposit of value, and therefore entitled to the name of
ore. In the field the general terms ‘‘contact-action,” to cover all of these
three kinds of work, and ‘contact metamorphic ores,” to cover the total
result, may be useful; but one can not make a classification of ore deposits
in which he introduces at the same time contact metamorphic ores and the
classes of igneous ores, gaseous solution ores, and aqueous solution ores.
DIVISION C. ORES PRODUCED BY PROCESSES OF METAMORPHISM.
It is believed that ore deposits produced by processes of metamorphism
are of dominant importance. While ore deposits produced by processes of
sedimentation and by igneous processes are of great consequence, they
together are subordinate in importance to those produced by processes of
metamorphism.
Ores produced by processes of metamorphism may theoretically be
divided into two groups—those deposited in their present positions by
gaseous solutions and those deposited in their present positions by aqueous
solutions.
The second group is believed to be more important than all other ore
deposits. I hold, therefore, that the dominant agents which deposited ores
in their present positions are aqueous solutions. How important is the
group of ores deposited by gaseous solutions is uncertain, as the criteria
have not been fully worked out by which these deposits can certainly be
discriminated from those precipitated from aqueous solutions.
GROUP A. ORES DEPOSITED BY GASEOUS SOLUTIONS.
Ores deposited from gaseous solutions comprise all those ores which
are precipitated from gaseous mixtures of all kinds. They therefore include
all deposits actually formed by pneumatolytic, fumarolic, and solfataric
action. That ore deposits have been precipitated from gaseous solutions
ORES DEPOSITED BY GASEOUS SOLUTIONS. 1053
has been a favorite hypothesis ever since ores were a serious subject for
investigation. The idea goes back as far as Elie de Beaumont,” who sup-
posed that the majority of ore deposits were thus produced. This theory
has been especially applied to ores of mercury and tin. During the last
ten years the idea has again been taken up and emphasized by a consider-
able number of vigorous advocates, of whom Vogt, Beck, Lindgren, Weed,
and Kemp are well known.
Recently Weed’ gave a classification of ores in which he separates
pneumatolytie deposits, fumarolic deposits, and gas-aqueous deposits, giving
each as one of the major classes. The deposits here placed by Weed
together comprise a very large proportion of ore deposits. Spurr’ also has
given a classification of ore deposits in which contact metamorphic deposits,
deep-seated gaseous deposits, fumarolic deposits, and solfataric deposits are
recognized as classes. Weed divides his pneumatolytic, fumarolic, and
gas-aqueous deposits into subclasses and groups. Moreover, the ores of
gold, silver, and copper of many well-known districts are assigned to each
of these classes and to the various subdivisions of the classes. However,
neither Weed nor Spurr gives the criteria by which he distributes the
various ores among the pneumatolytic, fumarolic, solfataric, and gas-
aqueous deposits. Nor are the criteria even given by which deposits are
known to be formed by gaseous solutions. It has been shown that the
heavy anhydrous minerals of the zone of anamorphism probably develop
at temperatures above that of the critical temperature of water. (See pp.
182-185, 685.) This has led me to hold that where such minerals have
developed with ores formed from solutions, such solutions are gaseous.
But I am unable to formulate any criteria, except purely theoretical ones,
which subdivide the deposits of gaseous solutions into pneumatolytic,
fumarolic, and solfataric. Indeed, I hold that the entire class of ore
deposits produced by gaseous solutions, while important, is small as
compared with ores deposited from aqueous solutions.
Ores deposited from gaseous solutions above the critical temperature
of water may conceivably be produced in the zone of anamorphism in con-
«Bull. Soc. géol. de France, 2d ser., vol. 4, 1846-47, p. 1249.
>The genetic classification of ore bodies, a proposal and a discussion (including papers by W. H.
Weed and J. E. Spurr): Eng. and Min. Jour., vol. 75, 1903, pp. 256-258.
1054 A TREATISE ON METAMORPHISM.
sequence of the normal increase of temperature with depth. But probably
ore deposits belonging here are more commonly produced in connection
with eruptive rocks or orogenic movements, or both; that is to say, the
eruptive rocks and earth movements are important factors in raising the
water above its critical temperature. It has already been seen that ores
may grade from those deposited by igneous agencies to those deposited by
aqueous agencies. At one end the ores are the result of magmatic
processes; at the other end the ores are deposited by water. At an
intermediate stage it is to be supposed that ores may be deposited from
gaseous solutions above the critical temperature of water, viz, 365°.
Lindgren says: “We may assume with great confidence that at the
contacts of intrusive rocks with a sedimentary series the temperature usually
Na
exceeded 365° C., and the pressure 200 atmospheres.”“ Theoretically one
would expect that rather frequently in connection with great intrusive
masses, and especially in connection with batholitic masses, the water in
the surrounding rocks would be raised above its critical temperature, and
therefore that its action is pneumatolytic. But the practical question is con-
cerning the importance of gaseous solutions in connection with the ores.
What are the ores which were thus deposited? What are the criteria by
which they may be identified? The only metallic deposits which Vogt
regards as produced by eruptive after-action in the sense of pneumatolytic
action, are cassiterite veins.” Beck agrees in regarding the tin deposits of
Zinnwald, in Saxony, as due to pneumatolytic action.’
While it may be possible that some tin deposits may be produced by
magmatic segregation or pneumatolytie action, it by no means follows that
all of the tin or even a greater part of itis of this origin. Recently Penrose
has visited and studied the deposits of the Malay Peninsula.” Here, as is
well known, the greater portion of the tin is actually obtained from the
alluvium; but Penrose’s studies have shown that the tin of the alluvium has
its original source in yeins in the granite axis of the peninsula and in the
limestones flanking this axis. The ore in these rocks is associated with
sulphides, such as bornite and chalcocite, and occurs in veins which have
“ Lindgren, Character and genesis of certain contact deposits, Trans. Am. Inst. Min. Eng., vol. 31,
p. 238.
> Vogt, cit., pp. 1382-137.
©The origin of ore deposits (discussion by R. Beck): Trans. Am. Inst. Min. Eng., vol. 31, 1902,
p. 945.
@ Penrose, jr., R. A. F., The tin deposits of the Malay Peninsula, with special reference to those
of the Kinta district: Jour. Geol., vol. 11, 1903, pp. 135-154.
ORES DEPOSITED BY GASEOUS SOLUTIONS. 1055
all the characters of ordinary water-deposited veins, and which are substan-
tially alike both in the granite and in the limestones. Were it not for the
cassiterite there would be no reason to suggest that these deposits differ in
any way from other vein deposits produced by underground water. (See
pp- 1058-1065.) As already pointed out, about 75 per cent of the tin of
the world is produced in the Malay Peninsula and from Banea and Billiton.
The product is mainly a mechanical concentrate, but the cassiterite for
this concentrate is derived from the veins in the limestone and granite.
Therefore probably 75 per cent of the present product of tin was deposited
from aqueous solutions.
Lindgren finds from his study of metasomatic processes in fissure veins
that topaz, garnet, tourmaline, and biotite occur as important metasomatic
minerals in connection with .certain ore deposits. But the metasomatic
minerals which are found in an overwhelming preponderance in ore deposits
are those which are produced by water action in the belt of cementation,
viz., hydrous silicates, carbonates, oxides, ete.
The metallic ore deposits which contain anhydrous minerals Lindgren
divides into three classes, (1) topaz-cassiterite veins, (2) tourmalinic gold-
copper veins, and (3) biotitic gold-copper veins.“ Lindgren’s description of
these veins shows that usually they are not in the igneous rocks, but
are largely replacement deposits in adjacent rocks. Pyrrhotite and chal-
copyrite carrying gold are found as replacement products in feldspar, and
by gradual extension of the processes finally replace the whole rock. The
process of formation of the biotitic gold-copper veins is ‘‘more characteristic
of dynamic metamorphism than of ordimary fissure veins.” ?
Lindgren
describes the ores between Signal Peak and Meadow Lake as consisting of
pyrite, pyrrhotite, and sphalerite, with some galena, with a gangue con-
sisting of quartz, epidote, and tourmaline, with some mica. He says ‘‘the
ores occur intimately intergrown with this gangue.”° Lindgren divides the
copper deposits of the gold belt of the Blue Mountains of Oregon into
various ‘‘types.” The Seven Devils type, he says, is a contact deposit of
chaleopyrite and bornite between limestone and diorite, which has a gangue
of garnet, epidote, and other contact minerals. The tourmaline type has
«Lindgren, Waldemar, Metasomatic processes in fissure veins: Trans. Am. Inst. Min. Eng., vol.
30, 1901, pp. 619-645.
> Lindgren, cit., p. 645.
eLindgren, Waldemar, The auriferous veins of Meadow Lake, California: Am. Jour. Sci:, 3d series, ©
vol. 46, 1893, p. 205.
1056 A TREATISE ON METAMORPHISM.
an ore consisting of chalcopyrite and pyrite associated with the gangue
of quartz and tourmaline. He here places the Copperopolis mine and
Jessie vein. The ores are intergrown with quartz and tourmaline. At the
Jessie vein the other gangue materials are calcite, dolomite, and specula-
rite.” Lindgren says the copper deposits of the Seven Devils and tourma-
line types “are of rare occurrence.”’
In the South Mountain district of Idaho, an argentiferous galena, with
some zine blende and copper minerals, occurs in crystalline limestone and
in schist. The gangue minerals are quartz, calcite, actinolite, brown garnet,
and ilvaite, the gangue minerals and ores being intergrown.’
Weed states that the Jimenez copper deposits of Mexico follow a line
of contact between limestone and granite, the limestone being largely con-
verted into massive garnet.’ He further states that the Porvenir gold-
copper vein in the Sierra Azul mining district of Mexico occurs in granite,
and has tourmaline as an abundant gangue mineral.’
Yung and McCaffery describe the copper deposits of San Pedro, in
New Mexico, as contact deposits in limestone. The limestone has been
largely replaced by garnet. Chalcopyrite is intimately associated with the
garnet, and with subordinate amounts of specular hematite, epidote, vesu-
vianite, wollastonite, quartz, and calcite. They say ‘the ore is always
accompanied by garnet, although the garnet does not in all places carry
ore. When the garnet carries the ore the chalcopyrite is disseminated
throughout its mass, and appears to be of synchronous origin.””
In those instances in which it is clearly shown that heavy anhydrous
minerals, such as topaz, tourmaline, garnet, and biotite, develop simulta-
neously with the deposition of the ores by metamorphic processes, it seems
highly probable that they formed under the conditions of the zone of ana-
morphism and that the temperature at the time of the deposition was above
the critical temperature of water.
aLindgren, Waldemar, The gold belt of the Blue Mountains of Oregon: Twenty-second Ann. Rept.
U.S. Geol. Survey, pt. 2, 1901, pp. 629-630.
bLoe. cit., p. 629.
cLindgren, Waldemar, The gold and silver veins of Silver City, De Lamar, and other mining
districts in Idaho: Twentieth Ann. Rept. U. 8. Geol. Survey, pt. 3, 1900, p. 189.
d@ Weed, Walter Harvey, Notes on certain mines in the States of Chihuahua, Sinaloa, and Sonora,
Mexico: Trans. Am. Inst. Min. Eng., vol. 32, 1902, p. 404.
€ Loc cit., p. 440.
f Yung, Morrison B., and McCaffery, Richard S., The ore deposits of the San Pedro district, New
Onn
Mexico: Trans. Am. Inst. Min. Eng., vol. 33, 1903, p. 355.
ORES DEPOSITED BY GASEOUS SOLUTIONS. 1057
In this connection it may be recalled that beautiful crystals of the
variety of garnet known as spessartite, fayalite, and topaz occur in litho-
physz in recent volcanic rocks, especially in the openings in them. The
formation of these openings in the lavas and the development in them of
_ these minerals naturally suggest the action of gaseous solutions. It has
also been pointed out (p. 685) that so far as the heavy anhydrous minerals
have been reproduced artificially under conditions where water is present
high temperature and pressure have been necessary. For instance, Chrust-
schoff obtained from water solutions heated to a temperature of 550° C,
amphibole, pyroxene, quartz, and adularia.”
It thus appears that both observation and experiment strongly favor
the idea that such minerals as topaz, garnet, and amphibole, so far as sepa-
rated from solutions, are deposited by gaseous solutions above the critical
temperature of water. If this be so, it follows that those sulphides which
were deposited simultaneously with the development of the heavy anhydrous
minerals were precipitated from gaseous solutions.
It is evident that it is difficult to make a certaim case in favor of
gaseous solutions. It must be shown, where the ores occur in connection
with the heavy minerals supposed to be formed under conditions of gaseous
solutions, that the metals have not been introduced at a subsequent stage
when aqueous solutions were the active agent. There should be excluded
also the possibility that the ores were deposited by aqueous solutions in the
belt of cementation and were later transferred to the zone of anamorphism,
where the heavy minerals developed by later alterations in that zone.
Apparently in some of the cases above cited the latter possibility is excluded,
as all the evidence seems to be in favor of denudation since deposition rather
than deeper burial. :
Under normal conditions it might be supposed that ore deposits would
be produced by gaseous solutions in the deep-seated zone above the critical
temperature of water; that above this zone in the belt of cementation ores
would be deposited by aqueous solutions, and that above the level of ground
water other ore deposits would be produced in the belt of weathering by
gaseous solutions. The latter ores would be those to which the terms
fumarolic and solfataric are naturally applicable; but, so far as 1 know,
the actual development of ore deposits by gaseous solutions above the
«Chrustschoff, K. yon, Ueber kiinstliche Hornblende: Neues Jahrb., 1891, Bd. II, pp. 86-90.
MON XLviI—04 67
1058 A TREATISE ON METAMORPHISM.
level of ground water is only a hypothesis. If such deposits exist, | know
of no criteria which have been worked out by which they may be dis-
criminated from deposits of aqueous solutions.
The hypothesis that the vapors of the metals rise with the vapors of
water from an unknown source and deposit ore in the openings of the rocks
is exceedingly attractive, but it has been seen that where openings occur in
rocks below the level of ground water, the rocks are saturated with aqueous
solutions, therefore the locus of the deposition of ores from vapors would be
in the belt of weathermg. The view that fumarolic and solfataric vapors
have deposited ores has been especially prevalent in reference to cinnabar
and such easily sublimed deposits. Daubrée has maintained that tin ores are
formed by the sublimation of stannic chloride and stannic fluoride and their
reaction upon water.” For the production of any ore deposit by sublimation
in this sense I know no scrap of evidence.’ In Chapter VI, on “The belt of
weathering,” it has been seen that where there is fumarolie or solfataric action
the rocks are rapidly carbonated, hydrated, and oxidized, the mechanical
result being softening and disintegration. The products are colored yellow
or brown by oxide of iron. hat metallic ore deposits have been produced
in connection with such action remains to be proved.
It has already been seen that in various recent classifications of ore
deposits, which are made rhainly upon the basis of agent, there are also
introduced contact action and eruptive after-action. In so far as deposits
are precipitated from gaseous solutiuns, whether the solutions be derived
partly from the intruded and partly from the intrusive rock, or partly
from both, they belong in this class rather than in an independent class
erected on an entirely different basis and therefore an incongruous element
in a scientific classification.
GROUP B. ORES DEPOSITED BY AQUEOUS SOLUTIONS.
It has been stated that ores deposited by aqueous solutions are believed
to be more important than all others combmed. The evidence in favor of
this view will not be presented in detail, but a few of the more salient points
bearing upon it will be summarized.
«Daubrée, A., Memoir sur le gisement, la constitution, et l’origine des amas de minerai d’étain:
Ann. des Mines, 3d series, vol. 20, 1841, p. 65.
>This statement is restricted to the metallic ore deposits; it does not apply to deposits of sulphur
and similar compounds made in connection with volcanic action, in reference to which I express no
opinion.
ORES DEPOSITED BY AQUEOUS SOLUTIONS. 1059
First, it has been shown (pp. 571-572, 612-640) that the general
cementation of the belt so named is due to the work of underground water.
The evidence upon this point is briefly summarized in this chapter. (See
pp. 1024-1028.) If general cementation is correctly attributed to deposi-
tion by underground water, it can hardly be doubted that many ore deposits
are due to the same agency, since in general the majority of ore deposits differ
in no essential respect from the ordinary cementation products, except that
they contain an unusual amount of certain metals of importance to man. If
underground water carried and deposited the material in the openings of
sand, transforming them to sandstones and quartzites, in the larger openings
of conglomerates and great tuff formations, thus indurating them, and in the
innumerable openings produced by joint, fault, bedding parting, and brecci-
ation fractures, it follows that similar deposits containing a minute fraction
of a per cent of gold or silver, a small amount of copper, or a large quantity
of iron—in other words, ores—are also the results of aqueous deposition.
However, evidence that the majority of ores are deposited by underground
waters does not rest alone upon the general argument of cementation.
The second argument in favor of the deposition of the majority of ores
by aqueous solution is the nature of the accompanying gangue minerals
and their relations to the ores. The gangue minerals which accompany
the great majority of veins are the hydrous silicates, including kaolinite,
sericite, the zeolites, and chlorites; the carbonates, such as calcite,
dolomite, and siderite; and the oxides, such as quartz and hematite. In
other words, the dominant gangue minerals are the identical ones produced
on an extensive scale by metasomatic processes within the belt of cementa-
tion, and deposited upon a grand scale in the openings of that belt, thus
cementing the rocks.
But the conclusion that such minerals are deposited from aqueous
solutions does not depend upon their likeness to the minerals of the belt
of cementation. Actual observations show that these minerals are now
being deposited by aqueous solutions. This has been observed at various
places. In this cl runtry the most famous localities are Sulphur Bank,
California, Steamboat Springs, Nevada, and the Yellowstone Park.
Perhaps the best-studied instance of all is that of Boulder Hot Springs,
Montana, where the vein deposits and the alterations of the accompanying
rock have been recently very closely examined and well described by
1060 A TREATISE ON METAMORPHISM.
Weed.” The spring issues through a granite. This granite has been
extensively altered into sericite, kaolinite, and zeolite. The filling of the
vein consists of quartz, zeolite, and calcite. Weed finds that the vein
filling contains a very small amount of gold and silver, and also that
the altered granite contains a smaller amount of silver and a trace of gold.
The thin sections show the presence of pyrite and hematite. Thus there is
actual mineralization of this vein.” Substantially the same phenomena
as those at Boulder Hot Springs have been long known to exist at Steam-
boat Springs and Sulphur Bank. It is therefore certain that at those
localities, where ore material is accompanied by the minerals characteristic
of secondary action in the belt of cementation, the ores were deposited by
aqueous solutions. If this be so, it is little short of a certainty that in
general the ores associated with similar gangue minerals in the belt of
cementation are deposited from aqueous solutions. At any rate, one who
maintains that ores so intimately associated with the minerals mentioned
as to show contemporaneous deposition are not deposited by aqueous
solutions must furnish evidence upon which he reaches a conclusion
adverse to all observed facts.
It is to be observed that this reasoning is in accord with the funda-
mental hypothesis of geology mentioned at the outset of this treatise, viz,
that when a certain set of complex forces and agents is observed to produce
complex phenomena, and no other combination of forces and agents has
been observed to cause such phenomena, the conclusion is that the phe-
nomena are to be referred to the forces and agents now at work.
A third argument showing the direct connection between the recognized
work of underground water and deposits of ores has been fully stated by
Posepny. For the banded structure so characteristic of fillings of open-
ings in rocks, whether these contain metals in sufficient quantity to be
recognized as ores or not, Posepny proposes the term crustification.© That
crustified or banded or comb veins are usually precipitates from underground
waters no one doubts. It has yet to be shown that regular crustification,
so general a phenomenon in ore deposits, is produced by any other agent
than aqueous solutions.
a Weed, Walter Harvey, Mineral vein formation at Boulder Hot Springs, Montana: Twenty-first
Ann. Rept. U. 8. Geol. Survey, pt. 2, 1900, pp. 227-255.
> Weed, cit., pp. 248-249.
¢Posepny, F., Genesis of ore deposits: Am. Inst. Min. Eng., 2d ed., 1902, p. 12.
ORES DEPOSITED BY AQUEOUS SOLUTIONS. 1061
As further evidence of the deposition of ores from aqueous solutions
we may now pass to illustrations of ore deposits that exhibit phenomena
identical with those which are known to be produced as a consequence of
the action of aqueous solutions.
Upon the poimt that the iron ores are dominantly produced by the
circulation of ordinary meteoric waters, there is no difference of opinion;
there is general agreement also that these waters are mostly descending
waters at ordinary temperatures. (See pp. 1193-1197.) The importance
of waters of meteoric origin at ordinary temperatures in the genesis of ores
should perhaps be emphasized. Probably the iron ores preponderate in
volume over all other metallic ores. In the United States, Great Britain,
and Germany 66,000,000 tons of iron ore were mined in the year 1902.
It is not probable that this amount is approached by the remaining metallic
ores. Not only do the iron ores occur in largest quantity, but iron is a
metal of dominant importance. There is no question that the world could
spare all the other metals better than it could spare iron. If it be agreed
that iron is more important to the advancement of the race than all other
metals, and that iron ore occurs in greater quantity than all other ores and
is dominantly deposited by meteoric waters at ordinary temperatures, it
follows, disregarding the manner of the deposition of any other metals,
that the work of meteoric waters at ordinary temperatures is the most
important factor in metallic ore deposition.
Other metals also are dominantly deposited by aqueous solutions.
Many ore deposits are so associated with the general process and the belt
of cementation that one must hold that the work of cementation and the
deposition of ores were simultaneously caused by the same agent. For
instance, the San Juan tuff of Colorado, a Tertiary volcanic formation, at
the time of its deposition must have been exceedingly porous. It is now
so thoroughly cemented by quartz and other gangue minerals that the
microscope can discover no openings. In the cementation of this tuff
formation an immeasurably greater quantity of material was deposited
between the fragments than is now found in the veins. he filling of the
intersecting veins contains the same gangue minerals as those which cement
the tuffs. It is perfectly clear that the formation of these veins was an
incident in the general process of cementation. In some of the veins, and
to a variable extent in the same vein, a sufficient amount of mineral was
1062 A TREATISE ON METAMORPHISM.
deposited to make the material an ore. The gangue of such veins, the
barren veins, and the cement of the tuff are connected physically and are
similar mineralogically. Thus it is certain that they are the result of like
processes, and few can doubt that the general cement and vein filling were
deposited through aqueous solution.
The conglomerates and amygdaloids of Lake Superior furnish con-
clusive evidence that the deposition of the copper was a mere incident
in the general process of cementation. Throughout the Lake Superior
region the Keweenawan amygdaloids, sandstones, and conglomerates
have been cemented by the minerals which developed in the belt of
cementation. Throughout this region, coincident with this process, there was
deposited sparsely disseminated copper. Scarcely a Keweenawan district
has been studied in which such disseminated copper, associated with the
cementation minerals, may not be seen, and in many cases it is sufficiently
abundant to warrant exploration with the hope of finding ore deposits. In
only a single district, however, that of Keweenaw Point, has copper been
discovered in sufficient quantity in veins, amygdaloids, and conglomerates
to constitute ore deposits. In every respect these copper deposits are like
the remaining great volume of the Keweenawan greenstones, sandstones,
and conglomerates, being cemented in substantially the same manner and
with the same minerals. At various places on Keweenaw Point from 1 to
4 per cent of metallic copper was deposited simultaneously with the other
cementing minerals. There can be no escape from the conclusion that the
general cementation of the Keweenawan rocks and the deposition of the
copper were performed by the same agent and at the same time. I do not
know that any one has ever held that this general cementation is the result:
of other than aqueous solutions. If this general belief be true it is conclu-
sively shown that the copper was also deposited by aqueous solutions.
Other districts similar to those mentioned could be cited.
Passing to veins of lead, zine, gold, and silver it is found that all but
a comparatively small number of such veins exhibit the cementation min-
erals which have been mentioned. The ores are so intimately associated
with these gangue minerals that they must have been simultaneously depos-
ited. In many cases it can be shown that the filling of the veims was con-
temporaneous with extensive alterations of the walls within which developed
like minerals, and with the general cementation of the belt of cementation.
ORES DEPOSITED BY AQUEOUS SOLUTIONS. 1063
Generalizing in reference to districts, it may be said that, excluding sedi-
mentary deposits, probably all of the great mining camps of the world show
the phenomena which indicate deposition by aqueous solutions. Certainly
this is true for America. The ore deposits which have been shown to be
formed by gaseous solutions, or by magmatic segregation, are at compara-
tively small and little known camps.
Objection has been raised to the above general statements as to the
importance of aqueous solutions, on the ground that in many ore-bearing
districts there is not now a vigorous aqueous underground circulation.”
This may be true of deposits either in humid or in arid regions. Many
important humid districts could be mentioned in which the circulation is at
present very feeble. For instance, in the deep copper deposits of the Lake
Superior region there is little water. The same is true of the San Juan
districts of Colorado, of the deep mines of Przibram, and of many other
places in the world. Another instance in which deep workings have shown
very little circulating water is that of the Newhouse tunnel, at Idaho
Springs, Colo. This tunnel, more than 4,000 meters long, in a granite-
eneiss-schist complex of rocks, shows but an insignificant amount of water.
A small amount percolates in at different places, but the quantity is so small
that it is ignored in the drifting arrangements. or arid regions with small
circulation the copper mines of New Mexico and Arizona furnish admirable
illustrations. Indeed it not infrequently happens that in such regions, after
one reaches a moderate depth, the levels are dry, or even dusty.
But from the above facts it is a mistake to infer that when the ores
were deposited there was not a vigorous circulation. As has been so fully
explained in various places, the very process of cementation lessens the size
and number of the openings, and when the process nears completion the
openings all become subcapillary and circulation practically ceases. We
have already seen that such are the facts in reference to the Lake Superior
conglomerates, the San Juan tufts, and to the rocks of various other regions.
The original openings, and the deformation cracks and crevices have been
so thoroughly cemented, and since that time so few new fractures have been
formed, that but little water can find access from the surface.
«Kemp, J. F., The role of the igneous rocks in the formation of veins: Trans. Am. Inst. Min. Eng.,
vol. 31, 1902, pp. 184-198. Rickard, T. A., Water in veins, a theory: Eng. and Min. Jour., vol. 75,
1903, pp. 402-403.
1064 A TREATISE ON METAMORPHISM.
Under present conditions mines may be dry even when the openings
have not been closed by cementation. For example, very recently in New
Mexico and Arizona there have been important climatic changes. It is
well known that these districts were humid in the Quaternary period, during
at least two different epochs. During these times the level of underground
water was probably as near the surface as it is in the humid districts of the
west at the present time. Therefore in these districts in late geological
time there was a vigorous underground circulation at those places where
cementation was not complete.
The dryness of mines which at the time of the vein fillmg had a vig-
orous circulation may be due either to cessation of circulation by cementa-
tion or to cessation of circulation in arid regions in consequence of change
from humid to arid conditions.
It seems to me that where the gangue minerals are those known to be
deposited by aqueous solutions, and the relations of the vein fillings are
such as to show that they were deposited contemporaneously with the gen-
eral cementation material, these facts are decisively in favor of action of
aqueous solutions notwithstanding absence of vigorous circulation at the
present time. Those who argue that the ores were not deposited by aque-
ous solutions because there is not now a vigorous aqueous circulation have
equally good or better ground for stating that the ores were not deposited
by gaseous solutions; for certainly in regions of ore deposits where there
is not now a vigorous aqueous circulation there is still more notably a
deficiency in gaseous circulation. By a strange inconsistency some of the
men who have held that the main class of ores is not deposited by aqueous
solutions, and have used as an argument for this belief the absence of a
present vigorous circulation, have jumped to the conelusion that such ores
were therefore deposited by gaseous solutions, not taking the trouble to
ask whether the same argument applied in this case.
In this connection it is to be recalled that in those cases where actual
vein filling is now known to be going on, as at Steamboat Springs, Sulphur
Bank, Boulder Hot Springs, and various other localities, there is now a
vigorous aqueous circulation. Also in some cases of very recent vein
formation, as at the Comstock lode and Cripple Creek, where the openings
were not fully closed by cementation, an enormous amount of water is
handled in mining. The Portland mine at Cripple Creek, Colo., from 1898
ORES DEPOSITED BY AQUEOUS SOLUTIONS. . 1065
to 1902, pumped between 300 and 900 gallons per minute, with maxima
for short periods much higher than this.“ Could there be more decisive
evidence that here the circulation is one of extraordinary vigor?
Therefore, I hold, the fact that in many ore-producing districts there
is not now a vigorous circulation is no evidence of the nonexistence of
such a circulation at the time the deposits were formed.
Finally, while comparatively little stress is to be placed on authority,
it is certain that there has been general agreement among the majority of
the great workers upon ore deposits as to their deposition by aqueous solu-
tions. Upon this pomt Bischof, von Groddeck, von Cotta, Daubrée, and
Posepny all agree. Posepny differs from some of the authors mentioned
in such particulars as the source of the ores and the depth from which the
waters rose. Between the authors mentioned there is difference as to
whether the waters depositing the ores are mainly ascending or descending,
as to the source of the material for deposition, and as to the cause of the
circulation of the water. There is no difference between them upon the
fundamental point that the great majority of ores are precipitated from
aqueous solutions in the positions where they are now found.
My first main conclusion in reference to ores deposited by aqueous solutions
is that they form the dominant class.
I. THE SOURCE OF AQUEOUS SOLUTIONS.
Since it has been shown that considerable circulation of underground
water in the zone of rock flowage can not be assumed, it follows that
we can not suppose that the water of the zone of fracture passes into or is
derived from the zone of rock flowage on any large scale. Doubtless this
transfer does take place to some extent. Also hydration and dehydra-
tion of the rocks are constantly taking place, and these processes at any
given place to a small extent subtract water from or add it to the circula-
tion. Further, through volcanism water originally occluded in magma is
transferred from the zone of rock flowage, or possibly even from the
centrosphere, to the zone of rock fracture, and by the crystallization of the
magma is there liberated and thus becomes a part of circulating under-
«Ninth Ann. Rept. Portland Gold Mining Co., Pl. XI, 1903.
1066 A TREATISE ON METAMORPHISM.
ground water. But the water of meteoric origin is held to be dominant. It
has already been noted that the great iron-ore deposits have been concen-
trated by waters of meteoric origin. It is further believed that the waters
from which were deposited many other ores, the association of which show
they were precipitated from aqueous solutions, are also dominantly of
meteoric origin. It has been shown already that in order to produce
aqueous ore deposits the amount of water required was probably many
thousands of times the volume of the minerals deposited. If this be so,
even if all the water exuded from magmas at their time of crystallization
could be supposed to be converged into the trunk channels holding the
ore deposits, this would be but a fraction of that required for the segregation
of the ores.
It is impossible to make an exact quantitative estimate of the relative
proportions of the waters of meteoric origin and igneous origin concerned in
the production of aqueous deposits. But I have no doubt whatever that
water of meteoric origin forms more than 95 per cent of such waters, and [I
think it probable that it constitutes more than 99 per cent, if all ore deposits
produced by underground water are taken into account. Mistaken concep-
tions upon this point frequently have been due to the fact that authors in
considering this matter often take into account only ores of a part of the
metals, such as those of gold or silver, whereas a general statement with
reference to the proportion of water of meteoric origin should take into
consideration deposits of iron ore and of the other base metals. Even in
the case of ore deposits closely associated with igneous rocks, it is believed
that the water is dominantly of meteoric origin. For instance, at the
present time, ore deposits are known to be in the course of formation at
Steamboat Springs, Sulphur Bank, the Comstock lode and Boulder Hot
Springs, Mont., described by Weed.“ The same is probably also true of
the numerous hot springs of Yellowstone Park. The quantity of water
which issues from these hot springs is enormous. ‘These springs, in vol-
ume of water and in their relation to the topography and precipitation,
are in all respects like the vastly more numerous and more important
springs which have ordinary temperatures. The study of underground
circulation with reference to artesian waters has shown beyond question
that the waters discharged from the great majority of springs is of meteoric
@ Twenty-first Ann. Rept. U. 8. Geol. Survey, pt. 2, 1900, p. 227-255.
SOURCE OF AQUEOUS SOLUTIONS. 1067
origin. Numerous large springs are found both in regions in which there
has been no igneous action for a long time and in regions in which vol-
canism is now or has been recently prominent. No one would claim that
in regions of the former class the water is of other than meteoric origin.
For instance, no one would hold that the waters of the great springs of
the Appalachiaus and the Mississippi Valley are derived from any other
source. Why then should this be held in reference to the Cordilleran
region of the West?) He who maintains that the water for these springs
is largely derived from igneous rocks must show that the amount which
issues is greater than could be expected upon the meteoric theory, taking
into account the precipitation, the climate, the character of the rocks and
the topography. The only way to find the effect of an agent, such as
igneous rocks, with reference to the water is to compare regions in which
these rocks occur with others in which they are absent. Making this com-
parison, observation has as yet not shown a difference of volume, but in
regions of volcanism the waters issuing from springs are frequently hot,
and are depositing more than an average amount of mineral material.
This difference is therefore logically referred to the igneous rocks. During
its underground journey the water comes into contact with or comes
near to hot rocks, and thereby becomes heated. In consequence of this
it becomes a far more potent chemical agent, and takes into solution a large
amount of mineral matter. When gathered into and rising in trunk
channels a considerable portion of this mineral matter is deposited. This
effect may be accentuated in volcanic regions by mechanical action. As
has been pointed out in other places, the deposited material is derived from
all the rocks with which the underground water comes into contact during
its journey, but in many cases the material is dominantly derived from a
single formation or series through which the water journeyed.
While it is held that the water is dominantly of meteoric origin, in
regions of volcanism where magmas are crystallizing the heated solutions
have doubtless had accretions from this source. But those who hold that
the enormous quantities of water issuing from hot springs are mainly
derived from crystallizing magmas must furnish the evidence in favor of
excepting them from the general rule. The fact that cold springs issue
adjacent to hot springs has no bearing on the case, for often trunk channels
which issue close together have entirely different feeding areas. Nor is the
1068 A TREATISE ON METAMORPHISM.
fact that hot waters deposit more minerals and different minerals an evidence
that the hot waters are derived from crystallizing igneous rocks, although
it is possible, and rather probable, that in many instances igneous rocks
have contributed materials to the solutions. The high mineral content
of waters is adequately explained as a function of high temperature.
That this is certainly true for Mammoth Hot Springs has already been
shown. (See pp. 1031-1032.) The view that the waters of Boulder Hot
Springs and of the Yellowstone Park Springs are of meteoric origin was
held by Weed at the time he studied and reported upon these thermal
regions.”
Adjacent to great intrusive igneous masses the contributions of water
from crystallizing magma may be important. Since in many cases the ores
are deposited during periods of volcanism, the exuding waters are hot, and
may be charged with mineral material to an unusual extent. In such
cases the waters derived from igneous sources are proportionately far more
active than ordinary underground waters. In many instances the water
liberated by the magma during solidification has doubtless acted as a potent
agent in segregating the ores, but after solidification waters of meteoric
origin continue the work. Often to a large extent the solution of the metal
and its deposition takes place after the magma has crystallized, but while the -
rocks are still hot. After an igneous rock has solidified and continues to
cool the ordinary procéss of shrinkage tends to form openings within it,
and especially to form openings along its contact with the contiguous
deposits. Orogenic movements also find that the contacts between the
igneous and the adjacent rocks are planes of weakness, and are likely there
to form openings. This being the fact, it is entirely natural, indeed inevi-
table, that the hot circulating waters during the time that the rock is cooling
are most active in the segregation of the deposits. The extreme activity of
hot water in general metamorphism and in the production of ore deposits
has been repeatedly emphasized. Thus in various instances the water
liberated by the magma, at solidification may have played an appreciable
part in the segregation of ores, but very often the importance of this part
has been overemphasized because it has not been appreciated that after
solidification takes place the solutions continue the work of segregation of
the ores under most favorable circumstances.
«Weed, Walter Haryey, Mineral vein formation at Boulder Hot Springs, Mont. : Twenty-first
Ann. Rept. U. 8. Geol. Survey, pt. 2, 1900, pp. 249-252.
SOURCE OF AQUEOUS SOLUTIONS. 1069
Summarizing, the conclusion is reached that so far as the main work
of aqueous ore deposition is concerned the water is that of meteoric origin,
which makes its way from the surface into the ground, there performs its
work, and issues to the surface again. The amount of water coming from
the deep-seated zone of rock flowage, emanating from crystallizing lava
and contributed by dehydration, is believed to be relatively small upon
the average, although exceptionally important in proportion to its mass.
It is held that, at any given time, the meteoric water entering the crust
substantially balances that issuing from it, although there may be a slight
continuous surplus in favor of the latter.
My second main conclusion concerning ores deposited by aqueous solutions
ts that the major part of the water performing the work is meteoric.
II. SOURCE OF METALS FOR ORES DEPOSITED FROM AQUEOUS SOLUTIONS.
It is believed that the greater part of the metals for ores deposited by
water is derived from the zone of fracture. If the reasoning thus far
be correct, viz, that ores deposited from aqueous solutions are trans-
ported to their present positions by underground waters, that in order to
transport this material an abundant circulation is required, and that in the
deep-seated zone of anamorphism the circulation is a minimum, it follows
that the waters derive their metals from the rocks of the zone of fracture.
If any one asserts that the metalliferous materials of mineral veins are
derived by water circulation from the centrosphere, or are derived from the
lithosphere below the zone of rock fracture, I hold this to be a pure
unverified assumption, for which there has not as yet been adduced one
particle of evidence, and opposed to which stand well-known principles
of physics concerning the movement of water in minute openings, and the
condition of water in the deep-seated zone.
The conclusion that the waters derive their metals from the zone of
fracture was reached as outlined on the foregoing pages, but since the pub-
lication of this conclusion the observational work of Lindgren upon metaso-
matic processes in fissure veins has fully confirmed it. Lingdren has made
a careful study of the metasomatic changes in the wall rocks while ores
and gangues are being deposited. He finds that the abundant minerals
produced by metasomatic processes are quartz, fluorite, calcite, magnesite,
dolomite, siderite, muscovite, chlorite, kaolinite, zeolites, and the sulphides.
1070 A TREATISE ON METAMORPHISM.
He does not include epidote among the abundant metasomatic minerals of
fissure veins, but it is certain that this mineral is important in certain deposits
which are essentially the same as fissure veins so far as the circulating water
is concerned. For instance, epidote is very important in connection with the
Lake Superior copper. All of these minerals are shown (Chapters VI and
VII) to be~characteristic of alterations in the zone of katamorphism and
especially of the belt of cementation. The carbonates are produced by the
process of carbonation of the silicates, and the separation of quartz is the
correlative process. The hydrous silicates are produced from the anhy-
drous silicates by a process of carbonation and hydration For instance,
Lindgren“ says the muscovite is produced from orthoclase and microcline,
and he thus writes the reaction:
3K AlSi,O, + H,O + CO,=KH,Al, (SiO,),-+ K,CO, + 6Si0,
: It therefore appears that this reaction is one of hydration and carbona-
tion, and at the time the muscovite is formed silica is liberated, to be
deposited in the walls or in the veins.
According to Lindgren the kaolin is mainly formed by the alteration
of feldspars, and especially the potassium and sodium feldspars. For potas-
sium feldspar Lindgren writes the reaction:’
6K AlSi,0,-+ 6H,0 + 3CO,=3H,Al,Si,0, + 3K,CO, + 12Si0,
Thus this reaction, like that of muscovite, is one of hydration and
carbonation, but takes place to a greater extent, the resultant minerals
being kaolin and quartz. It is not necessary to write here reactions for
the zeolites, chlorites, and other hydrous minerals, for these may be found
in Chapter V. An examination will show that the reactions producing
them are those of hydration and carbonation of anhydrous silicates with
the liberation of silica.
Lindgren mentions also, as subordinate minerals which form by meta-
somatic processes, rutile, anatase, garnet, orthoclase, albite, tourmaline,
topaz, scapolite, and apatite. These minerals are those which commonly
form in the zone of anamorphism. It has already been seen that in those
cases in which it is shown that these minerals have developed at the same
«Lindgren, Waldemar, Metasomatic processes in fissure veins: Trans. Am. Inst. Min. Eng., vol.
30, 1901, p. 608.
» Lindgren, cit., p. 614.
SOURCE OF THE.METALS. 1071
time the ores were deposited as the dominant metasomatic minerals, the ores
were probably formed in the zone of anamorphism or rock flowage, prob-
ably through the influence of gaseous solutions. The ore deposits which
show these minerals as accompanying metasomatic products are few in
number and unimportant in amount as compared with the ore deposits in
which the metasomatic minerals are those of the first group mentioned.
The point to be enforced at present is that the great mass of metasomatic
minerals formed at the time the ore deposits are produced shows that
these ores were deposited within the zone of fracture, or the zone of
katamorphism.
This conclusion is confirmed by the close relations of the gangue
minerals to the country rock. Where a vein runs through a quartzite or
very siliceous rock, the gangue mineral is likely to be dominantly quartz.
Where a vein runs through basic igneous rocks, the gangue minerals are
likely to be largely zeolites and other hydrous silicates, carbonates, and
quartz, all decomposition products of such rocks. Where the veins run
through limestone the gangue material is likely to be mainly carbonate.
It is therefore clear that the gangue minerals are largely segregated from
the immediately adjacent rocks traversed by the veins. If this be so for
the dominant minerals, there is every reason to believe that the same con-
clusion applies to the very subordinate amounts of valuable metals which
constitute a small or almost an inappreciable part of the filling of an ore
deposit. Numerous examples could be given illustrative of this principle.
One of the best known to me is that of the Lake vein in the San Juan
district. This vein, running through the volcanic breccias, extended into
the limestone below, and from this into sandstone still lower down. Where
the vein was in the breccia it was a quartzose vein carrying metals; when
followed into the limestone it was seen to become a calcite vein, and when
followed into the sandstone it gradually changed again into a quartz vein.
In the limestone and sandstone the vein was barren. In this case it seems
little short of certain that the vein filling was dominantly derived from the
adjacent rock, and there is no reason to doubt that in the San Juan tuff the
metalliferous material and the quartz alike were derived from the breccias.
While it is held that the waters derive the metals, as their immediate
source, from the zone of fracture, it does not follow that the metal for the
ores deposited from aqueous solutions may not have been derived from
1072 A TREATISE ON METAMORPHISM.
greater depths; for, as fully explained (pp. 1030-1036), magma, carrying
various metals with it, does rise from unknown depths into the zone of
fracture upon an enormous scale, and there yields its metallic contents to
circulating waters.
My third main conclusion is that the metals for ores deposited by aqueous
solutions are derived from rocks within the zone of fracture.
IIT. WORK OF AQUEOUS SOLUTIONS IN SEGREGATING ORES.
The work of aqueous solutions in segregating ores is most complex.
The ores produced by aqueous solutions may be divided into three sub-
classes—(1) ores precipitated from aqueous solutions by ascending waters,
(2) ores precipitated from aqueous solutions by ascending and descending
waters combined, and (3) ores produced by precipitation from descending
waters. As the processes of segregation of each of these classes of ores are
distinctive, each class will be considered in turn. In order to produce any
of the ores of all these subclasses three stages of work need to be consid-
ered—the solution of the valuable metals, the transportation of tltem to the
places where they are deposited, and their precipitation.
SUBCLASS 1. ORES PRECIPITATED FROM ASCENDING AQUEOUS SOLUTIONS,
There have been endless discussions as to whether ore deposits are pro-
duced by descending, lateral-secreting, or ascending waters. It is believed
to be a corollary from what has gone before that the first concentration of
many ore deposits is the result of descending, lateral-moving, and ascending
waters. I say first concentration, for it will subsequently appear that many,
if not the majority, of the workable ore deposits precipitated from aqueous
solutions have undergone a second concentration. If the waters which
deposit ores are mainly meteoric, such waters at the outset are descending.
In most cases before they again issue at the surface they must ascend.
During the journey they have a lateral movement.
Thus, the larger, more complete idea of the genesis of aqueous ore
deposits comprises all of the old ideas, and shows that instead of being
contradictory, as supposed by many, they are mutually supporting. Com-
bined, they furnish a much more satisfactory theory than any one of them
alone. How true these statements are will appear more clearly later.
SOLUTION OF THE METALS. 1073
While the subclass is named ‘Ores precipitated by ascending aqueous
solutions,” possibly a more exact heading would be either ‘‘Ores precipi-
tated from solutions deficient in oxygen,” or ‘Ores precipitated from deep
solutions.” From the chemical point of view the fundamental fact is that
the solutions are deficient in oxygen. It follows from this that the solu-
tions must be sufficiently deep or sufficiently protected from the surface
to be free from oxidizing effects. Thus the subclass corresponds rather
closely to Posepny’s ‘Ores of the deep circulation.”” But since the great
majority of ores belonging to the subclass under discussion are deposited
from -solutions, the vertical element of which at the time of precipitation
is ascending rather than descending, the heading chosen is given as the
name of the subclass.
SOLUTION OF THE METALS.
In the first stage of the concentration of many deposits the waters are
descending. They move slowly downward, are widely dispersed in small
passages, have an exceedingly large surface of contact with rocks, and are
subject to increasing temperature and increasing pressure. All of these con-
ditions favor solution to the point of saturation. The various metalliferous
elements present in exceedingly small quantities in the rocks, as well as
many other compounds, are picked up. This follows from the law of phys-
ical chemistry, that a solution holds some part of all the elements with which
it is in contact. y
In the work of solution of the material for the ores, and in the deposi-
tion of it by the water, the two physical factors, temperature and pressure,
are of great consequence. It is shown (Chapter III) that increase of
temperature increases the activity of the water in two ways—first, the speed
of the solution is very greatly increased by rise of temperature, and at
temperatures of 100° C., and especially at a temperature of 185° C. and
above, the activity of water is no less than amazing (p. 79). The effect of
increase in temperature upon the activity of underground solutions can
not be too strongly emphasized. It is also shown that as the temperature
rises in general a larger quantity of material may be held in solution, at
least for moderate increases of temperature, probably as high as 100° C.
(pp. 79-81).
«Posepny, F., The genesis of ore deposits: Am. Inst. Min. Eng., 2d ed., 1902, pp. 1-72.
638
MON XLVII—O4
1074 A TREATISE ON METAMORPHISM.
Under normal conditions of increase of temperature of one degree for
30 meters, the temperature would be 100° C. at a depth of 3,000 meters;
therefore, where the circulation is deep, increase in temperature with depth,
even under normal conditions, is of great consequence. But in many
regions, during the segregation of ore deposits, igneous rocks have been
intruded or extruded, or important orogenic movements have taken place,
or both. Wherever igneous rocks have been intruded in the upper part of
the lithosphere, or poured out upon the lithosphere during the time when
ore deposits are forming, the temperature of the underground water is
higher, and may be much higher than normal, so that even where ore
deposits have been produced by a relatively shallow circulation the waters
may have had the advantage of a high temperature. The temperature may
increase with more than normal rapidity in consequence of mechanical and
chemical action. Where there is no evidence of igneous intrusion during
the time of ore deposition, but mechanical or chemical action has taken
place upon a great scale, the temperature of the underground water may
be raised considerably, and thus the deposition of the ores even in such
cases be accomplished by solutions at higher temperatures than normal.
For the underground solutions which occur in nature, pressure usually
promotes solution. (See Chapter IIL.)
The particular metals and the amounts of them which are taken into
solution also depend greatly upon the nature of solutions. ‘To illustrate,
where the solutions contain strong acids they are likely to dissolve the
metals; where oxygen is abundant the sulphides are likely to be oxydized
into sulphates and taken into solution; abundant carbonic acid forms car-
bonates; where alkaline sulphides and carbonates are present and the
solutions come into contact with sulphides, these are somewhat readily
dissolved as such. j
While all of these compounds favor solutions, in the early part of the
journey the most important single factor in the process of solution of the
- valuable metals is the presence of oxygen in the water. This is of great
consequence, because many of the valuable metals, both as original com-
pounds in the igneous rocks and as secondary products, are as sulphides.
Where oxidizing waters come into contact with sulphides they are trans-
formed to sulphates, and thus sulphates of most of the valuable metals may
be formed, as, for instance, those of silver, copper, mercury, lead, zine, iron,
SOLUTION OF THE METALS. 1075
arsenic, and antimony. All of these compounds are readily, or somewhat
readily, soluble, with the exception of lead sulphate, which is soluble only
to the extent of one part in 31,500 parts of water at 15° 0.“ But even this
degree of solubility is entirely adequate to account for the transportation of
the lead as a sulphate.
While the effect of the oxygen is to transform the sulphides to sul-
phates, it is not supposed that the metals necessarily travel as such salts.
The well-known principles of physical chemistry make it certain that each
of the metals present in solution is combined in part with each of the acids.
For instance, if silver sulphide be oxidized to sulphate, and sodium car-
bonate be present in the underground solutions, as is sure to be the case,
then a part of the silver will be transformed to silver carbonate and the
sodium will travel in part as sodium sulphate. In this connection the point
is that the sulphides get transformed through the agency of oxygen to
compounds which are much more readily soluble.
It is further to be remembered that in the upper part of the course of
descending water, where oxygen is abundant, ic salts are produced. Of
these the sulphates are most abundant, but with them are chlorides also.
Of these ic salts, ferric sulphate and chloride are very prevalent.
In salts of this class metallic gold and silver are dissolved. The solution
is controlled by the law of mass action and by the temperature. The
greater the amount of the salts present, and the higher the temperature,
with a given amount of salts, the more gold and silver may be dissolved.
Thus, descending solutions are those in which ic salts are abundantly
formed and in which the temperature is increasing. Both the abundance
and the increasing temperature are favorable to the solution of gold and
silver.
Therefore we conclude that the solutions which perform the first work in
the genesis of ore deposits, the dissolving of the metals, are descending.
TRANSPORTATION OF THE METALS.
Superimposed upon the downward movement of the waters is a lateral
one which, combined with the vertical movement, carries water sooner or
later to the trunk channels. The amount of water taking part in the lateral
movement is greatest near the surface of ground water, and from that
«Comey, A. M., Dictionary of chemical solubilities: London, 1896.
1076 A TREATISE ON METAMORPHISM.
surface on the average decreases to the bottom of the zone of fracture. It
has been explained that all fissures and other openings gradually die out
below as the zone of rock flowage is neared. (See pp. 187-191, 766-768.)
Therefore, for a given fissure, the waters enter it mainly from the side or
top, not from the bottom. Furthermore, the water does not enter the
fissure at a single place, but at numberless points all the way along its
course, from the deepest parts to the surface. Somewhere, however, the
water which enters a fissure must flow from it. This place may be at the
Fig. 26,—Ideal vertical section of the flow of water entering at a number of points on a slope and passing to a valley
below through a homogeneous medium interrupted by two open vertical channels, one on the slope and one in the
valley.
surface or at a considerable depth below the level of ground water. (See fig.
26.) The streams entering the fissure at high levels may have a downward
movement and contribute water abundantly. Below the level at which
water escapes laterally from a channel of given size the water contributed
to it decreases on the average with increase of depth, until in the deeper
Posepny, F., The genesis of ore deposits, Am. Inst. Min. Eng., 2d ed., 1902, pp. 242-243.
TRANSPORTATION OF THE METALS. 1077
calls attention to the frequently observed fact of the decreasing amount of
water contributed laterally as depth increases. As a specific instance of
this, he mentioned the Przibram district, in which the amount of water
entering the fissures below a depth of 300 meters is so small as to be
insignificant, but this may not have been the fact at the time the lode was
forming. (See pp. 1063-1065.)
While the amount of water laterally entering a fissure decreases
from near its top to the bottom, the amount of mineral material per unit
volume in all probability increases on the average; for the waters entering
at a low level take a longer journey through smaller openings and at higher
temperatures and pressures than the waters entering at a high level. There-
fore it is clear, if the rocks with which the deeper water comes in contact
can furnish metalliferous materials, that such water will be heavily loaded.
It follows from this, even if the amount of water which is furnished in a
short time to a fissure be small, that such water may furnish from the
country rock mineral material in solution much more than sufficient to
entirely fill a fissure during its long life.
We now understand that, on the average, the amount of water entering
a fissure decreases from the level of ground water to its bottom, but that the
amount of mineral matter brought into the fissure by the water (but not
necessarily deposited) increases per unit volume from top to bottom. It is,
therefore, impossible to make a general statement as to whether more mineral
material is contributed to a trunk channel in its upper portion or in its lower
portion. Doubtless this varies in different cases. Other conditions than
amount of water or depth may be controlling factors. For instance, it
igneous rocks be intruded at high or low levels only, such rocks may
furnish conditions which determine the amount of metalliferous material
contributed by the waters.
While the foregoing paragraphs imply that the lateral moving waters
are also downward moying, this is meant only as a general rule. The
lateral movement may be accompanied by no downward movement.
Not only this, but lateral movement may be accompanied by an upward
component. Indeed, this is believed to be very frequently the case, espe-
cially so far as the main branch streams in the deeper parts of the zone of
fracture are concerned. In so far as there is an upward component in these
branch streams, the reactions which obtain are the same as those of the
trunk channels to be considered below.
1078 A TREATISE ON METAMORPHISM.
During the lateral journey of the water, before trunk channels are
reached, the transported metals may be largely precipitated. Frequently
such precipitates are very widely dispersed and are not sufficiently rich to
constitute ore deposits. Exceptionally the amounts of precipitated material
during the lateral movements in the dispersed channels may result in the
formation of a product of sufficient richness to constitute an ore. Pre-
cipitation may result from any of the causes or combinations of causes
which are spoken of (pp. 1081-1088; see also pp. 118-123) as producing
precipitation from ascending waters in trunk channels. For the purpose
of illustration, one class of compounds may here be mentioned. Salts
traveling as sulphates may be reduced to sulphides and precipitated by the
direct reactions of carbonaceous materials, by the reaction of previously
precipitated sulphides, or, in the case of copper, silver, ete , by the reaction
of abundant ferrous compounds. The Crystal Falls voleanies * furnish an
illustrative case where rocks have been so profoundly altered by metaso-
matic changes as to leave scarcely an original mineral present. In them
there are sparse, widely disseminated secondary sulphides. In this forma-
tion there is no organic material, and the natural cause to assign for the
precipitation is the reducing action of the ferrous compounds which are
abundantly present. The possible reactions are given on pages 1111-1112.
From the foregoing it appears that ores are carried to trunk channels by
laterally moving waters. Lateral secretion is, therefore, an essential step in the
first concentration of ore deposits, although I use the term lateral secretion in
a broader sense than did Sandberger.
The places where the ore deposits themselves are found will now be con-
sidered. As already noted, these occur mainly in or adjacent to the more
continuous larger openings. These openings are occupied by the trunk
streams of circulating waters, and therefore the water is in the latter part of
its course. Hence these trunk streams, as has already been shown (p. 583),
have in general an upward rather than a downward vertical movement.
The waters reaching the trunk channel at any point immediately begin their
ascent. At any given cross section of a channel there must pass all of the
water contributed below. At great depth this amount has already been
seen to be small. From a small amount the waters steadily increase in
«Clements, J. Morgan, and Smyth, H. L., with W. 8. Bayley, The Crystal Falls iron-bearing
district of Michigan: Mon. U. 8. Geol. Survey, vol. 36, 1899, pp. 73-154.
TRANSPORTATION OF THE METALS. 1079
volume to the point where they begin to escape laterally. (See fig. 26.)
Hence in a trunk channel of a definite size the circulation is slow below and
increases in speed above. Near the bases of the channels from which the
Mammoth Hot Springs and geysers of the Yellowstone Park issue the
amount of water contributed may be small and the movement of the water
may be exceedingly slow. Even if true, as held by some, that rapid
movement of water is unfavorable to deposition of ores, it is wholly possi-
ble that at moderate depth, especially in the deeper parts of a channel from
which the flow at the surface is rapid, the conditions are those of slow move-
ment and rapid precipitation of ore deposits.
As the water passes upward the variety of solutions as well as the
amount increases, for each stream differs in its salts from every other, since
no two streams can possibly have had exactly similar histories. Moreover;
the character of the wall rock may vary from place to place. The pressure
and the temperature also lessen. These conditions are favorable to precip-
itation. Therefore many ores in their first concentration are precipitated by
ascending waters.
It is now clear that a satisfactory account of the genesis of ores includes
ascending waters. Many ores in their first concentration are actually pre-
cipitated from the ascending waters, and therefore emphasis has been placed
upon this part of the work of circulating waters.
The broader statement of the genesis of a great class of ore deposits is
that the water after penetrating the earth is widely scattered in contact with
rocks in innumerable minor openings. ‘These waters travel downward with
steadily increasing pressure and temperature. They take up the constitu-
ents of the ore deposits. The downward movement of the waters has
superimposed upon it a lateral component, as a result of which the waters
are carried to the larger openings. During this process also the waters
continue to take material into solution. In the larger openings where the
waters are congregated they are upon the average ascending with decreasing
temperature and pressure, and the ores are precipitated.
While it is beheved that in the great majority of instances the journey
of the underground water involves first a descending and later an ascending
movement, it is recognized that this is not invariably the case. Upon the
descending movement may be superimposed a lateral movement which
brings the water_to the surface again at a lower level, there being at no
1080 A TREATISE ON METAMORPHISM.
time an important ascending movement. This is especially likely to oceur
in districts of great topographic relief, where the descending waters strike
an inclined impervious plane dipping toward a valley. (See fig. 27.) In
the case illustrated the water falling on the mountains percolates downward
through the rocks until it comes ito contact with an impervious stratum,
which it follows toward the valley until it issues above the stream, having
nowhere during its journey an ascending component.
While it is held, as explained above, that upon the whole the descending
waters are dissolving material and the ascending waters are depositing
Fic. 27.—Ideal section showing underground circulation in which no water anywhere ascends before issuing at the surface.
material, it is not meant to imply that materials are not deposited by
descending and lateral-moving waters, nor that materials are not dissolved
by ascending waters. Indeed, it is certain that solution and precipitation
are taking place at all times throughout the entire course of all the branches
of the underground circulation. This is a necessary consequence of the
laws of physical chemistry. It is meant only to imply that in the first con-
centration of one class of ore deposits, solution so far as the ores are
concerned is the rule for the descent and deposition for the ascent, although
there is no doubt that there are many local exceptions to this.
TRANSPORTATION OF THE METALS. 1081
Tt is, of course, understood that the underground circulation in any
actual stance is much more complex than that given in the simple ideal
case which has been considered. For instance, it is certain that, in the same
mineral-bearing area, immediately adjacent trunk channels may have had
very different histories. This is especially well shown by the deposits at
Butte, Mont., where there are two parallel main zones of mineralization, only
a short distance apart, the mineral wealth of one of which is mainly copper,
while that of the other is mainly silver. Many of the other special factors
which modify the simple general statement above given are discussed on
pages 1199-1222.
PRECIPITATION OF THE METALS.
The precipitation of metals in the trunk channels by ascending waters
is of so much importance in the concentration of ores that this process needs
further consideration.
Precipitation may take place (1) by change in temperature and pressure,
(2) by mingling of solutions, and (3) by reactions between solutions and
solids. (See pp. 113-123.)
PRECIPITATION BY DECREASE OF TEMPERATURE AND PRESSURE.
The general relations of solution and precipitation as a consequence of
varying temperature and pressure have been already considered. (See pp.
114-116.) Where the increase of temperature with depth is normal, it has
been seen that decreasing temperature and pressure due to the ascension
of waters from a depth of at least 3,000 meters are favorable to precipitation.
Furthermore, the same statement holds even if the increment of tempera-
ture be greater than normal, provided the temperature does not greatly exceed
100° C. Cases in which water issues at the surface at such temperatures
are very rare. The probably decreasing pressure and temperature of solutions
rising from depths greater than 3,000 meters are also favorable to precipita-
tion. Since it has just been shown that ascending waters are likely to be
in trunk channels, lessening temperature and pressure are likely to produce
precipitates in the openings of faults, joints, and bedding partings, and the
more open places in sandstones, conglomerates, and amyedaloids.
Since upon the average ascending solutions are those in which the
pressure and temperature are decreasing, precipitation is more likely to
«Emmons, 8. F., Notes on the geology of Butte, Mont.: Trans. Am. Inst. Min. Eng., vol. 16,
1888, p. 54.
1082 A TREATISE ON METAMORPHISM.
eceur from them in consequence of change of temperature and pressure
than from descending solutions.
When one attempts to apply these general statements to specific metals,
experimental data are lacking. It is undoubtedly true that decreasing tem-
perature and pressure are much more influential in the precipitation of some
metals than of others. Until experimental work has determined how the
various economic metals respond to changing temperature and pressure, it
is impracticable to specify the ores in which precipitation is more strongly
favored by decrease of temperature and pressure. One would expect that
precipitation as a consequence of changing temperature and pressure would
tend to give a somewhat orderly vertical distribution of the various metal-
liferous ores.
As an illustration of the influence of a decrease of temperature we
may take the cases of gold and silver. It is well known that gold is
precipitated by cuprous chloride, according to the following reaction:
AuOl,+3CuCl=Au-+3CuCl,
Silver is precipitated by ferrous sulphate according to the following
reaction:
Ag,SO,+2FeS0,=2Ag+Fe,(S0,)3 ~
Under given conditions, if a sufficient amount of time be allowed, these
reactions will proceed until equilibrium is reached. Stokes has shown that
if the temperature is decreased, after equilibrium is reached, these reactions
will proceed further, but if the temperature is increased the reactions
reverse and the gold and silver are dissolved. Therefore, where for a
given temperature these solutions are saturated with silver and gold, and
the temperature is decreased, precipitation of the metals will result.
PRECIPITATION BY MINGLING OF SOLUTIONS.
Precipitation in the trunk channels is produced by reactions caused by
the mingling of various solutions. The mingled solutions may be gaseous
and aqueous, or all aqueous. Precipitation by reactions between aqueous
solutions and gases is especially likely to take place near the level of ground
water, where gases from the belt of weathering mingle with the solutions
of the belt of cementation.
Precipitation by reactions between aqueous solutions is especially likely
to take place at the intersections of trunk channels, where aqueous solutions
PRECIPITATION OF THE METALS. 1085
from different sources meet. Such precipitation takes place under the
general law that when solutions of two or more kinds are mingled, if a
substance can form which is insoluble in the liquids present, this compound
will be produced and precipitation take place. This mingling of solutions
is probably the most important of the causes of precipitation of ores.
It is evident that solutions from different sources enter a given trunk
channel at many places and thus a multitude of streams with different
composition mingle in a trunk channel. Each of the incoming streams is
different from any of the others, although in many cases the difference may
be slight. As a case of certain considerable difference may be mentioned
ascending and descending streams. (See pp.1175-1177.) If in a chemical
laboratory a multitude of solutions be taken at random and thrown together
precipitates will be almost certain to form, and in an underground channel
the same effect is likely to be produced when the various solutions come
together. This mingling of solutions is one of the most important of all
the factors which results in the deposition of the ores. I haye little question
that the wide variety of solutions which enter a given channel explain in a
large measure the exceedingly irregular richness of ore deposits. Where
a metal is found abundantly in a fissure the explanation in many cases is
certainly that at or near that place there entered a stream which carried
either the precipitated metal or an agent capable of precipitating it from a
solution already in the trunk channel. For instance, it is believed that at or
near the place where the great bonanza of the Comstock lode was found,
there entered either solutions rich in gold and silver, or a solution having a
compound which precipitated the gold and silver already traveling upward
within the lode. Perhaps the former hypothesis is the more probable.
Ore shoots, or chimneys of ore of exceptional richness, occur very
frequently im veins. They are sometimes parallel with the dip and at
other times pitch to the right or left of it. The locations of these ore shoots
in many instances I believe were controlled by cross fractures or joints
through which entered waters, carrying either metalliferous material or
solutions capable of precipitating the metalliferous mineral in the trunk
channel at the place where the lateral streams of water entered.
The lead and zinc deposits of the Mississippi Valley, according to
Jenney, are larger at the crossings of two sets of fissures than elsewhere.
This may be explained partly by the greater abundance of the solutions
1084 A TREATISE ON METAMORPHISM.
furnished by two sets of fissures, but, as suggested by Jenney,” partly
by the mingling of two different kinds of waters, thus giving conditions
favorable for precipitation.
In the Enterprise mine, at Rico, Colo., described by Rickard,’ the ore
bodies are in vertical veins and in flats under shales. While a set of cross
veins is barren, ‘‘the rich ore bodies overlie them in the contact zone.’
Below the shale it is common to find ores of more than average grade in
the pay veins where they are broken by the cross veins. It is believed the
explanation of these relations is the reactions resulting from the mingling of
the solutions of the ‘‘verticals” with the inclined cross veins. With this
Ransome’s recent studies accord. He says: “Large bodies of workable
sulphide ore occur only where the solutions in the lode fissures have had
opportunity to mingle with laterally moving solutions in the blanket ”?
The silver-lead deposits of the Aspen district of Colorado, described
by Spurr, furnish an instance of very probable precipitation of rich ore
shoots by the mingling of solutions. Spurr states that generally an ore
body is ‘found at the intersection of two faults, one of these faults usually
dipping steeply, while the other is much flatter.” For this ‘‘the explana-
tion is offered that by the mingling of solutions which had previously
flowed along different channels the precipitation of metallic sulphides was
brought about.” °
Probably the rich shoots of gold ore in the Sierra Nevada, which,
according to Lindgren, pitch to the left as one looks down the vein, further
illustrate the principle of precipitation by mingled solutions. For the most
part, Lindgren makes no statement as to the relations of ore shoots and
lateral seams. However, on Canada Hill vein there are ‘occasional rich
bunches at the intersections” of the two systems of veins’
aJenney, W. P., The lead and zinc deposits of the Mississippi Valley: Trans. Am. Inst. Min.
Eng., vol. 22, 1894, pp. 189-190, 224.
> Rickard, T. A., The Enterprise mine, Rico, Colo.: Trans. Am. Inst. Min. Eng., vol. 26, 1897, pp.
906-980.
¢ Rickard, cit., p. 977.
@ Ransome, F. L., The ore deposits of the Rico Mountains, Colorado: Twenty-second Ann. Rept.
U.S. Geol. Survey, pt. 2, 1901, p. 301.
e Spurr, J. E., Geology of the Aspen mining district, Colorado: Mon. U. 8. Geol. Survey, vol. 31,
1898, p. 234.
f Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California:
Seventeenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1896, p. 195.
PRECIPITATION OF THE METALS. 1085
It is believed that the Cripple Creek deposits likewise illustrate this
principle. Penrose” notes that many of the rich ore shoots occur at cross
fissures. It is thought probable that the main cause for the formation of
ore shoots at such places is the reaction of solutions furnished by one set
of fissures upon those furnished by the other set. It is but fair to say,
however, that Penrose makes the explanation, the ‘‘mechanical one, in
deflecting the course of the ore-bearing solutions.”
What are the reactions which result in precipitation from the mingling
of solutions? It has been pointed out (p. 1075) that in the upper part of
the journey of descending waters ic salts are likely to be present. In the
deeper parts of the belt of cementation, where organic matter and ferrous
compounds are abundant, these ic salts are commonly changed to ous salts,
and such salts are abundant. Such solutions are likely to make their way
into the trunk channels. If they there meet salts of gold, silver, or copper,
these compounds may be thrown down. This is likely to take place if the
reducing solutions are abundant. That is, the reaction is under the law of
mass action. It has already been seen that this reaction is promoted by
decreasing temperature. Another reaction frequently resulting in the
precipitation of sulphides follows from the mingling of solutions, one of
which bears hydrogen sulphide and the other sulphates or other oxidized
salts. In trunk channels the mingling of solutions of these kinds must be
very common, the oxidized solutions coming perhaps somewhat directly
from the surface, whereas the hydrogen-sulphide solutions have usually
taken a longer journey in the belt of cementation, and thus have taken on
a reducing character.
While the above explanation may in many cases account for apparent
irregularities in the kinds and percentages of metals, other principles are
needed to explain the fact that metals occur in a definite order from the
surface downward and that many valuable metalliferous ores grow poorer
at a depth of 1,000 meters or less. Varying temperature and pressure are
important in this connection; but more influential in many instances, as
will be shown subsequently, is a second concentration produced by
descending waters.
«Penrose, R. A. F., jr., Mining geology of Cripple Creek, Colorado: Sixteenth Ann. Rept. U. S.
Geol. Survey, pt. 2, 1895, pp. 164-165.
1086 A TREATISE ON METAMORPHISM.
PRECIPITATION BY REACTIONS BETWEEN SOLUTIONS AND SOLIDS.
Precipitating reactions between the solutions and solids with which
they are in contact often take place. The solid wall rock frequently pro-
duces precipitations of metalliferous ores from the solutions in the trunk
channels in the following ways: (1) It has already been explained that a
solid, when placed in contact with a liquid, may precipitate some compound
previously held in solution, some part of the solid going into solution at
the same time. Thus, the wall rock may precipitate ores. (2) The wall
rock furnishes the trunk solutions with precipitating solutions, which may
precipitate metals already in solution within the trunk channels. (8) The
wall rock itself may supply the ore deposit with metalliferous material,
which, when it reaches the trunk channel, may be precipitated by the
solutions there contained. Where the wall rock is easily soluble, enlarge-
ments of the openings occur readily and afford places for the deposition of
the metalliferous material. (See pp. 1212-1216.)
The reactions due to the country rock are likely to be effective in
proportion as it is porous and soluble and therefore allows solutions to
permeate and to dissolve it. Thus the country rock is likely to be especially
effective in its reactions where the trunk channel is a complex one and
gives a large surface of action. It is believed that the effect of the wall
rock in these various ways is of great importance in the production of many
ore deposits. ‘
As an illustration, may be cited the very general association of lead,
zinc, and copper ores, and the accompanying gold and silver, with limestone.
It is well known that in many cases ore deposits in limestone are large and
rich, and in the associated rocks are small and barren. This is illustrated
by the lead and zine deposits of the Mississippi Valley, which occur almost
exclusively in limestones, although these rocks are interstratified with sand-
stones. Where the openings pass into sandstones the ore deposits become
small and poor or die out altogether. Precisely the same relation is illus-
trated in the copper deposits of the Southwest. Here the ores are very
largely in the limestone, and when they pass into the acid associated rocks,
either sediments or porphyries, become small, and often fail.*
aDouglas, James, The Copper Queen mine, Arizona: Trans. Am. Inst. Min. Eng., vol. 29, 1900,
pp. 512-513. Ransome, F. L., The copper deposits of Bisbee, Arizona: Eng. and Min. Jour., vol. 75,
1903, pp. 444-445, x
PRECIPITATION OF THE METALS. 1087
Many other similar illustrations could be given. It is recognized that
these relations are explained in part by the ready solubility of the lime-
stones as compared with the acid rocks, thus giving openings for the depo-
sition of ores, but also largely, it is believed, by the reaction of limestone,
or its solutions, upon the solutions which contributed the metals. This
conclusion, already announced from the geological relations, without refer-
ence to experimental work, has been confirmed by laboratory work by
Mr. H. N. Stokes. Mr. Stokes has shown that when soluble zine, lead,
copper, or silver salts are heated with iron bisulphide, in the presence of an
alkaline carbonate, the metals are thrown down as sulphides. The reactions
are given on p. 1117. It was suggested that calcium carbonate might be
substituted for the sodium carbonate and similar reactions take place, and
this Mr. Stokes found to be the fact. It is therefore certain that where
limestone is the wall rock it often has a very important influence in the
precipitation of the ores.
The supposition that the limestones are a cause of precipitation does
not preclude the possibility that they may not also contribute the metals,
as in the case of the lead of the Mississippi Valley. But in such cases the
precipitation is probably due more to the mingling of solutions in trunk
channels than to the effect of the wall rock. Of course, in some eases, the
association of ore with limestone may be due to more than one of the factors
mentioned—the ready solubility of the limestone, its power to cause direct
precipitation, to furnish precipitating solutions, and to supply the metals or
some part of them.
Another case of precipitation resulting from the influence of the wall
rock is the well-known occurrence of metallic copper in the openings of
sandstones, conglomerates, and amygdaloids of Keweenaw Point... The
metallic copper between the particles was doubtless precipitated by ferrous
solutions furnished by the wall rocks, which in many cases are basic
volcanics.
A particularly clear illustration of the effect of wall rock is furnished
by ores in which the sulphides are confined to strata containing organic
matter, as in some copper deposits’ and some of the gold reefs of Australia.
@Trving, R. D., The copper-bearing rocks of Lake Superior: Mon. U.S. Geol. Sury., vol. 5, 1883,
pp. 419-430.
>Posepny, F., The genesis of ore deposits, and Cazin, F. M. F., discussion of same: Trans. Am.
Inst. Min. Eng., vol. 23, 1894, pp. 316, 606-607.
1088 A TREATISE ON METAMORPHISM.
In the case of the copper deposits the organic matter has in all probability
reduced sulphites or sulphates to sulphides. The function of the organic
matter in the case of the gold may have been to reduce it to metallic gold,
or to produce ous salts, for instance, ferrous sulphate, which reduced the
gold. (See pp. 1093-1095.) :
GENERAL STATEMENTS.
In conclusion, it may be said that the precipitation of metallic ores
by the mingling of various solutions is probably the most important single
factor which results in the first concentration of ores. Probably next in
importance to this are the reactions upon the trunk streams, due to the wall
rocks. As lateral streams from beyond the wall rocks must pass through
the latter, many of these streams produce an effect due partly to materials
more remote than the wall rocks and partly to the wall rocks. Thus in
many cases the effect of solutions originating beyond the wall rocks and
that of solutions furnished by the wall rocks may not be discriminated.
Diminishing temperature and pressure, while probably subordinate in
their effect to the mingling of streams and reactions due to the wall rocks,
are in many instances undoubtedly important, and in some instances
dominant, factors. In general, the tendency of writers has been to
emphasize the effect of diminishing temperature and pressure, and to
minimize or even disregard altogether the effects of mingling solutions
or the wall rocks, or both.
Precipitation in many cases is not produced by a single one of the
factors, but by two or three of them. For instance, precipitation may be
produced by the joint effect of a change in pressure and temperature, by
the reaction between gases, solutions, and solids, and by the combination
of change in pressure, change in temperature, and the mingling of solutions.
In short, all possible combinations of the various causes of precipitation
may and do occur in connection with ore deposits.
COMPOUNDS DEPOSITED BY ASCENDING SOLUTIONS.
Of the metallic ores those of iron, copper, lead, zinc, nickel, silver,
gold, and mercury are the more important. These metals vary greatly in
the forms in which they are precipitated. The deposits formed by ascend-
ing water occur in the metallic form, as sulphides, tellurides, oxides, car-
bonates, and silicates. The metals deposited in the metallic form in impor-
COMPOUNDS DEPOSITED BY ASCENDING SOLUTIONS. 1089
tant amounts by ascending water at the first concentration are silver, gold,
and copper. Metallic gold, copper, and silver are also produced in connec-
tion with the reactions of descending water, but such deposits are not here
considered. Their development is treated on pages 1158-1174. All of
the metals mentioned, with the exception of gold, occur in the form of
sulphides. The sulphides are either simple binary salts or are ternary
salts, such as sulpharsenites, sulpharsenates, sulphantimonites, and sulph-
antimonates. The sulpharsenical and sulphantimonical compounds will
not be considered separately from the simple sulphides. The important
tellurides are those of silver and gold. An important oxide deposited by
ascending water is magnetite, and possibly franklinite and zincite belong
here. Of the carbonates that of iron is of the greatest consequence. Of
the silicates that of zine may belong here. Of course many other oxides,
carbonates, and silicates are found in ore bodies; but so far as these are
in sufficient abundance to constitute workable ore deposits, it is believed,
as will be fully explained later, that these compounds do not belong to the
class of ores deposited by ascending water.
Why compounds deposited by ascending waters are, for the most part,
not oxidized compounds, but metallic compounds, sulphides or tellurides,
is easily explained. The widely disseminated, downward-moving water,
bearing oxygen, is robbed of this constituent early in its course. Ferrous
compounds are abundantly present in the rocks in the forms of magnetite
and silicates. Iron is a strong base; and where ferrous compounds are
present they continue to abstract the oxygen of the downward-moving
waters until it has practically disappeared. Moreover, buried organic
matter takes oxygen from underground waters. Hence oxidizing com-
pounds do not exist in the deep-seated ascending water.
METALS.
GOLD.
Solution—It has been stated that gold is deposited by ascending waters
in important amounts. The manner in which gold is dissolved and trans-
ported to the greatest extent in underground solutions is not well known.
As a matter of experiment it has been long known that gold is soluble in
ferric chloride and in cupric chloride. Doelter has shown that gold is some-
what readily soluble in a 10 per cent solution of sodic carbonate, and also
MON XLVII—04——69
1090 A TREATISE ON METAMORPHISM.
in an 8 per cent solution of sodic carbonate containing an excess of carbonic
acid and containing sodic silicate.* Becker has shown that gold is easily
soluble in sodie sulphide and in sodic sulphydrate.’ It has also been
held that gold is soluble in ferric sulphate. Recently Stokes has experi-
mentally investigated the solubility of gold. He finds that gold is
somewhat readily soluble at a temperature of 200° C. in solutions of ferric
chloride and cupric chloride according to the following reactions:
Au+8FeCl,—AuCl,+3FeCl,
Au+3CuCl,— AuCl,+3CuCl
It thus appears that when gold is dissolved by ferric chloride or cupric
chloride it is transformed to a chloride. The greater the amount of these
chlorides and the stronger the solutions the more gold may be dissolved.
With a given amount of ferric chloride or cupric chloride, with constant
temperature, the reactions cease when equilibrium has been reached.
Stokes’s work has further shown that when this condition exists a rise in
temperature causes the reaction to continue. That is to say, with increasing
temperature a given amount of ferric or cupric chloride in a given solution
is capable of taking an increasing amount of gold into solution.
Stokes finds that gold is not appreciably dissolved in ferric sulphate
unless chlorides are present at the same time, thus furnishing ferric chloride
and making the solution really that by the reaction already given. His
experiments have not been carried to sufficient refinement to prove that
ferric sulphate may not dissolve gold to an extent to be of consequence in
the segregation of the metals under natural conditions.
Lenher, in advance of publication, has kindly given me the results of
experiments which show that gold is soluble in sulphuric acid, phosphoric
acid, and various other acids, if a compound be present which liberates
oxygen, as, for instance, manganese dioxide. The reaction takes place at
0° C., but is greatly accelerated by heat. This principle may be of con-
siderable importance in the solution of gold, since all of those compounds
occur rather plentifully under natural conditions. The frequent association
of gold ores with manganese minerals strengthens this suggestion.
«Doelter, C., Einige Versuche tiber die Léslichkeit der Mineralien: Tschermaks Mineral. Mittheil.,
vol., 11, 1890, p. 329.
> Becker, G. F., Geology of the quicksilver deposits of the Pacific slope: Mon. U.S. Geol. Survey,
vol. 13, 1888, p. 433.
SOLUTION OF GOLD. 1091
The conclusion that gold is soluble in ferric and cupric chlorides, in
acids where oxygen is liberated, and is possibly soluble in ferric sulphate,
and that these processes are promoted by increasing temperature, is of the
greatest importance in reference to the segregation of gold. It has been
pointed out that the zc salts are those which are extensively produced in
the belt of weathering by descending solutions. Iron sulphide is the most
abundant sulphide. By the reaction of oxygen upon it ferric sulphate is
produced, and by its decomposition sulphuric acid. Ferric chloride would
form wherever chlorine is present. If copper sulphides occur, these would
be transformed to similar salts. Thus in the belt of weathering considera-
ble quantities of the solvents of gold are formed. In connection with the
production of a large amount of these reagents where the solutions are
descending, the law of mass action leads to the conclusion that the gold
will be dissolved somewhat in proportion to the mass of these compounds
produced. Furthermore, where the solutions are descending there is a
rise in their temperature, and the work of Stokes shows that this further
icreases their activity. Hence, under normal conditions in a region of
mineralization where there are sulphides and where gold exists, one would
expect that the dispersed descending solutions would dissolve gold in con-
siderable quantity. =~
Precipitation—In whatever form gold is carried it is known to be pre-
cipitated in the first concentration as metallic gold or as a_ telluride.
Whether it is also precipitated as a sulphide is uncertain.
It is easy to suggest various causes for the precipitation of metallic
gold from its solutions. First, it is well known that gold solutions are
immediately precipitated by the more basic metals. Of these metals there
is likely to occur underground those of copper, silver, and tellurium. Ocea-
sionally also iron may occur. The rapid precipitation of gold from its
solutions by contact with iron, copper, and silver are well known.
Recently Hall and Lenher have shown that tellurium completely
precipitates gold from its solutions according to the following reactions:"
4AuCl,+3Te=4Au-+ 3TeCl,
They state: “Time is a considerable factor in bringing about complete
precipitation, from two to three hours being necessary with continued
«Hall, R.D., and Lenher, Victor, Action of tellurium and selenium on gold and silver salts:
Jour. Am. Chem. Soc., vol. 24, 1902, p. 919.
1092 A TREATISE ON METAMORPHISM.
heating, or several days at room temperatures. The only conditions to be
observed, in order to obtain quantitative precipitation of the gold by the
tellurium, are sufficient time and the direct contact of all the tellurium with
the gold solution.”“ Hall and Lenher have also shown that selenium
precipitates gold from its solutions in a similar manner according to the
reaction:
4AuCl,+3Se=4Au+38eCl],
They say: ‘From six to eight hours of continued boiling are necessary to
insure complete precipitation, or better, from two to three days at a tem-
perature of 70° to 80°.”’
Not only is gold precipitated from its solutions by various metals, but
it is precipitated by many ous salts and oxides; for instance, it is well known
that ferrous sulphate readily reduces gold to the metallic condition. Stokes
has recently shown also that ferrous salts in silicates are sufficient to pre-
cipitate copper from its solutions, and if this be so, it is certain that gold may
be precipitated in like manner. There is little doubt, also, that ferrous
oxide in magnetite is adequate to precipitate gold from its solutions. It is
to be noted that Stokes writes the equations
AuCl,-+-3FeCl,2@Au-+3FeCl,
AuCl,+3CuCl2@Au+3CuCl,
as reversible reactions. He states that “with rising temperature the equi-
librium moves from left to right, with falling temperature from right to
left.”°
It follows that where there is lessening temperature and pressure,
ferrous chloride and cuprous chloride reduce gold solutions, throwing down
the metallic gold. Normally, the conditions for falling temperature are
those for ascending solutions, and therefore, so far as gold is precipitated
by these reactions, it is more likely to take place where the waters are
ascending.
So far as I know, experiments have not been made in the precipitation
of gold by cuprous oxide and cuprous salts other than cuprous chloride;
but in general it may be stated with a considerable degree of certainty that
any of the ferrous and cuprous compounds which occur in nature are ade-
quate precipitating agents for gold solutions. From the descriptions of
@ Hall and Lenher, cit., p. 920. > Hall and Lenher, cit., p. 921. ¢ Stokes, manuscript, p. 7e
PRECIPITATION OF GOLD. 1095
various gold deposits it seems to me very probable that in many instances
the ferrous salts have been important reducing agents, and the same may
be true of cuprous compounds.
Tt has been seen on a previous page that the ic compounds capable of
dissolving gold are produced extensively in the early part of the journey
of underground water. It has further been fully explained (p. 1085), that
deep in the belt of cementation, where organic matter and sulphides are
abundant, ¢c salts are changed to ous salts, which are the prevalent com-
pounds deep within the belt. Therefore the trunk channels which carry
the gold in solution are likely to receive contributions from other solutions
which carry ous salts in large amount. Here the conditions are just the
reverse 0. ‘hose existing in the early part of the water’s journey. The
law of mass action now demands the reduction of the gold to its metallic
condition. For instance, if the gold were a chloride, and ferrous sulphate
were présent, the following reaction would take place:
2AuCl,+6FeSO,=2Au-+-2Fe, (SO,),+2FeCl,
Similar reactions are written for ferrous chloride and cuprous chloride on
the preceding page. It is also to be remembered that in trunk channels the
solutions are likely to be ascending. Therefore the temperature is falling,
and as pointed out by Stokes all of these reactions are promoted by this
condition. ‘Thus an abundance of reducing solutions and the temperature
both work together to produce precipitation in the trunk channels.
In connection with these reactions the question naturally arises as to
which of the ows salts is probably the most important. Since iron is the
most abundant of all the metals carried in underground solutions, such sul-
phates would be more likely to be sulphates of iron than any other. If the
salts formed in the belt of weathering were ferric sulphates, they would be
likely to be reduced to the ferrous condition at depth by organic matter or
by sulphides. Indeed, analyses of mineral waters which bear sulphates also
ordinarily show ferrous iron.” Therefore ascending waters bearing ferrous
sulphate or other ous salt might be brought into a lode by side streams and
there precipitate the gold. Such side channels entering through lateral
cracks may, in many cases, cause the extreme irregularity in the distribution
of the gold.
@Peale, A. C., Mineral waters of the United States: Bull. U. 8. Geol. Survey No. 32, 1886.
1094 A TREATISE ON METAMORPHISM.
Although Lindgren argues to the contrary with regard to the Sierra
Nevada, the suggestion that a part of the gold there has been reduced by
ferrous sulphate has extreme plausibility. The gold associated with the
pyrite is native. From that district are given two analyses of the waters of
feeding streams (the only analyses reported) entering the lodes at a depth
of 125 meters. Both of these analyses show that sulphates and iron are
present.’ According to the analyses the iron is reported as ferric; but
apparently no precautions were taken, when the waters were collected, to
prevent the oxidation of ferrous to ferric iron. Indeed, the precipitates of
yellow material, which is partly ferric oxide, made by underground springs
renders it highly probable that ferrous salts were contained in them before
oxygen came into contact with the solutions. The clean vein quartz itself,
which holds a large number of fluid inclusions, contains sulphates,’ showing
that sulphate-bearing waters were present at the time the lodes formed.
The ore shoots have great irregularities in richness, for which Lindgren
offers no explanation. The suggestion above made that the gold is precip-
itated in the metallic form by the reducing action of ferrous sulphate
explains all of these facts. The deposits are rich where the side springs
issued from cross fissures and furnished the ascending waters with ferrous
sulphate. The gold is in the metallic form because reduced by the ferrous
sulphate.
Organic material is capable of precipitating gold from its solutions.
At many places the precipitation of the gold has been ascribed, in part
at least, to the influence of the organic matter. Rickard’ calls atten-
tion to the frequent association of metallic gold with sedimentary
rocks bearing organic matter in California, New Zealand, Australia,
and Tasmania. ‘The most remarkable case is the concentration of gold
in veins where they-eross strata of carbonaceous shale, called indicators.
Says Don,’ “Away from the indicator, the greater part of the vein quartz
is absolutely barren; but at the intersection with the indicator large
«Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California: Seven-
teenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1896, p. 181. See also Pl. V, p. 134.
» Lindgren, op. cit., pp. 121-123.
¢ Lindgren, cit., pp. 130-131, 260, 261.
@ Rickard, T. A., The origin of the gold-bearing quartz of Bendigo Reefs: Trans. Am. Inst. Min.
Eng., vol. 22, 1894, pp. 314-815. Rickard, T. A., The indicator veins of Ballarat, Australia: Eng.
and Min. Jour., vol. 60, 1895, pp. 561-562.
eDon, J. R., The genesis of certain auriferous lodes: Trans. Am. Inst. Min. Eng., vol. 27, 1898,
p. 569.
PRECIPITATION OF GOLD. 1095
masses of gold (often more than 100 ounces in one piece) have been
obtained, and the greater part of the gold extracted from this belt has
come from those parts of the quartz veins near some one of the indicators.”
Furthermore, Rickard* describes experiments in which the black carbon-
aceous shale of Rico was placed in silver solutions and in solutions
containing Cripple Creek gold ore. Both metallic silver and gold were
abundantly precipitated upon the shale in a short time. In the instances
above mentioned it can hardly be doubted that the organic material was
an important or controlling factor in the reduction and precipitation of gold.
The argillite with which many of the gold ores of the Sierra Nevada
are associated is carbonaceous,’ but the chief influence of this carbonaceous
material may have been to assist in the production of the ous salts which
ultimately reached the trunk channels. But in some places, as for instance
where the pyrite occurs in a carbonaceous argillite, but not in quartz,’ the
gold may have been precipitated directly by the carbonaceous material.
But since the gold in the Sierra Nevada is mainly deposited in open
fissures,“ the suggestion already made of direct reduction of the major
portion of the gold by ous salts, and especially ferrous sulphate, is thought
to be the more plausible, although if the formation of the ferrous sulphate
be due to carbonaceous material in the country rock, the precipitation is
indirectly due to organic matter.
The sulphides of the base metals also precipitate gold from its
solutions. Below the level of ground water the rocks most commonly
associated with gold are the sulphides of the base metals. Thus gold
occurs on a great scale associated with pyrite. Very often it is found also
with sulphides of the other metals, especially copper. In such occurrences,
where the sulphides are abundant, the gold is likely to be plentiful; where
the sulphides are present in small quantity, the gold also is likely to be
deficient. This relation is illustrated in both California and Australasia,’
# Rickard, T. A., The Enterprise mine, Rico, Colo.: Trans. Am. Inst. Min. Eng., vol. 26, 1897,
pp. 978-979.
>Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California:
Seventeenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1896, p. 81.
¢ Lindgren, op. cit., p. 140, Pl. VIII.
“Lindgren, op. cit., p. 259.
¢Lindgren, Waldemar, The gold-quartz veins of Neyada City and Grass Valley, California:
Seventeenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1896, pp. 124-126. Don, J. R., The genesis of certain
auriferous lodes: Trans. Am. Inst. Min. Eng., vol. 27, 1898, p. 567.
1096 A TREATISE ON METAMORPHISM.
and suggests that the original solution and deposition of native gold and
the sulphides are frequently connected; therefore it is reasonable to infer
that the conditions which produced sulphides may also have resulted in
the precipitation of gold.”
Liversidge has shown experimentally that pyrite, galena, arsenopyrite,
and nearly all other sulphide minerals will precipitate gold completely from
solutions of auric chloride.’ The frequency with which gold occurs associ-
ated with or inclosed by pyrite suggests that this is a very important
reaction. This relation is well illustrated by the gold-quartz veins of the
Sierra Nevada described by Lindgren, where in the deeper parts of the lodes
beyond the belt of surface oxidation the pyrite and the gold are mtimately
associated. Lindgren says that the intimate connection of the gold with
the sulphides was very likely caused by the tendency of gold solutions to
be precipitated by particles of sulphides.’
In certain cases gold occurs associated with both carbonaceous material
and the base sulphides. This is well illustrated by the argillite of the
gold-quartz veins of the Banner mine described by Lindgren,“ and by the
Bendigo reefs of Australia.’ In such cases the question may arise as to
which of the two is the precipitating agent; but the more intimate
association of the gold with the pyrite suggests that the pyrite is the
precipitating agent for the gold (see Pl. XI, 4), and this agrees with the
experimental work of Liversidge, which shows that the metallic sulphides
precipitate gold from solutions more readily than organic matter’
Recently Lenher and Hall’ have shown that the natural tellurides of
gold, silver, and mercury are capable of rapidly and completely precipitat-
ing metallic gold from its solutions. This they have accomplished with
calaverite (AuAgTe,; Au 39.5, Ag 3.1, Te 57.4) (Au 41.76, Ag .80, Te 56.64,
«Lindgren, cit., p. 184.
> Lindgren, Waldemar, Gold-quartz veins of Nevada City and Grass Valley: Seventeenth Ann.
Rept. U. 8. Geol. Survey, pt 2, 1896, p. 182. Liversidge, A., On the origin of gold nuggets: Proc.
Roy. Soc. New South Wales, vol., 27, 1893, p. 303.
¢ Lindgren, cit., p. 184, Pl. VII, p. 138, and Pl. VIII, p. 140.
@ Lindgren, cit., Pl. VIII, fig. c, p. 140, and p. 156.
€Rickard, T. A., Origin of gold-bearing quartz of Bendigo reefs, Australia: Trans. Am. Inst.
Min. Eng., vol. 22, 1894, pp. 314-317.
f Liversidge, A., On the origin of moss gold: Proc. Roy. Soc. New South Wales, vol. 27, 1893,
p. 287.
yg Lenher, Victor, Naturally occurring telluride of gold: Jour. Am. Chem. Soc., vol. 24, 1902, pp.
355-360. Lenher, Victor, and Hall, R. D., Action of tellurium and selenium on gold and silver salts:
Thid., pp. 918-927.
PRECIPITATION OF GOLD. 1097
from Kalgoorlie),” krennerite (AuAgTe,; Au 35.5, Ag 19.4, Te 45.1, uncer-
tain), sylvanite (AuAgTe,; Au 24.5, Ag 13.4, Te 62.1), hessite (Ag, Te),
kalgoorlite (1gAu,Ag,Te,), nagyagite (Au,Pb,,Sb,Te8,,), and coloradoite
(HgTe). It thus appears that whether these minerals are largely tellurides
of gold, as calaverite; of gold and silver, as krennerite, sylvanite, ete; of
silver, as hessite, or of mercury, as coloradoite, when brought into contact
with chloride of gold solutions metallic gold is precipitated. This experi-
mental work of Lenher and Hall is of very great importance, probably
largely explaining the intimate association of free gold with the tellurides
at various places.
Tt will be explained (pp. 1170-1171) that in many places the associa-
tion of free gold with tellurides is partly due to the reconcentrating action
of descending water, the tellurides being the precipitating agent. Doubtless
im many cases this process of reconcentration is also combined with that of
direct oxidation of the tellurium of the tellurides, leaving gold behind. The
spongy gold pseudomorphous after telluride, such as that which occurs at
Cripple Creek, is almost certain evidence of one or both of the above
processes. But in many cases the free gold associated with tellurides
may be due to a first precipitation from ascending waters, the precipi-
tating agent being the telluride. his possibility is illustrated by the
Kalgoorlie district of Australia, described by Bancroft, who says: ‘The ore
is very slightly altered country rock containing iron pyrites and tellurides
of gold. . . . One of the interesting phenomena of the ore is the occurrence,
once ina while, of small particles of crystalline gold surrounded by crystals
of tellurides low in gold.”’
The conclusion that the Kalgoorlie gold may be precipitated by the
tellurides is confirmed by observations made by Rickard. He states that
‘free gold at Cripple Creek has invariably that appearance which charac-
terizes the metal when it has originated from the disintegration of tellurides;
but at Kalgoorlie ordinary gold, ina bright and crystalline condition, also
Ne
occurs.
“Rickard, T. A., The telluride ores of Cripple Creek and Kalgoorlie: Trans. Am. Inst. Min. En
vol. 30, 1901, p. 711.
+ Bancroft, Geo. J., Kalgoorlie, Western Australia, and its surroundings: Trans. Am. Inst. Min.
Eng., vol. 28, 1899, pp. 93-94.
¢Rickard, T. A., Telluride ores of Cripple Creek and Kalgoorlie: Trans. Am. Inst. Min. Eng.,
vol. 30, 1901, p. 714.
oy
6
1098 A TREATISE ON METAMORPHISM.
Bearing in the same direction are facts given by Kemp: ‘From Mr.
Baneroft the writer learns that the actual ore at Kalgoorlie consists of
calaverite and native gold. In a specimen kindly given the writer there is
a yellow telluride and a silvery one. In the midst of the latter a bit of wire
gold is embedded. There is no sign of alteration, and the native metal and
the telluride must have crystallized together.”*
The very general occurrence of tellurides in gold deposits leads me to
the belief that the precipitation of gold by tellurides is probably an impor-
tant reaction in the first concentration by ascending waters. The work of
Hall and Lenher shows conclusively that where gold-bearing solutions
enter deposits containing tellurides, whatever their origin, the gold will be
rapidly precipitated by the tellurides. In this connection the manner in
which the tellurium travels and is precipitated is important, and is discussed
with the tellurides. (See pp. 1119-1125.)
Hall and Lenher’ have shown that silver selenide also reduces gold to
the metallic form almost as rapidly as selenium does. The reaction takes
place only slightly in the cold, but readily on warming. Whether or not
this has any importance with reference to ore deposits is uncertain.
In many cases the precipitation of gold is not produced by a single
one of the causes given, but by some combination of them. For instance, it
has already been pointed out that the abundance of ows solutions and dimin-
ishing temperature work together. Again, the reducing action of organic
material and that of base sulphides or tellurides, or both, may work together.
In still other cases all of the above favorable conditions may occur simulta-
neously. The combination of organic matter with sulphides or tellurides is
believed to be very common.
Such a combination is probably illustrated by the deposits of New
South Wales, in which the gold is associated both with organic matter and
with pyrite. In this case the organic matter was probably the chief agent
in precipitating the base sulphides from the sulphates. In consequence of
this reaction there are two compounds present capable of precipitating the
gold, both the pyrite and the carbonaceous material. Since the work of
Liversidge shows that the sulphides precipitate gold more readily than
«Kemp, J. F., Geological occurrence and associates of the telluride gold ores: Mineral Industry,
1898, p. 319.
> Hall and Lenher, Action of tellurium and selenium on gold and silver salts: Jour. Am. Chem.
Soc., vol. 24, 1902, p. 927.
SOLUTION OF SILVER. L099
organic material, and since the gold is more intimately associated with the
sulphides, it seems probable that, in proportion to its mass, pyrite is a more
important precipitating agent than the organic matter; but undoubtedly
both are effective.
In many eases it is probable that the precipitation is due fundamentally
to carbonaceous material, but that the influence of the organic matter is one
step removed. That is to say, the organic matter first precipitated the
baser sulphides by reducing them from sulphates, as explained under the
succeeding section, and then the base sulphides precipitated the gold. This
would seem to be the natural explanation which accounts for the close
connection of the gold of some deposits with both carbonaceous materials
and pyrite and other base sulphides. *
SILVER.
Metallic silver as a product of the first concentration by ascending
waters is not an abundant source of the metal. One of the most notable
occurrences of metallic silver of this origin is that of the Keweenawan
of Lake Superior, where the silver, in subordinate amounts, is closely
associated with copper.
Solution—'I"he form in which silver most extensively occurs is sulphide.
It is well known that oxidizing waters change silver sulphide to a sulphate
which is a readily soluble compound. It is therefore probable that the silver
is very largely transported as a sulphate. It is also certain that silver is
transported as a chloride, which may be produced either by the direct action
of the chlorides or of hydrochloric acid on the sulphides of the metal, or by
the interaction of the chlorides and sulphates. Silver carbonate is also
somewhat readily soluble in carbonated solutions. Silver also occurs as a
metal. It is well known that metallic silver is soluble in ferric sulphate,
the reaction being:
2Ag+Fe, (SO,),=Ag,SO,+2FeSO,
This reaction takes place in proportion as the ferric sulphate is abundant,
and it has already been pointed out that such sulphates are likely to
be abundant in the early part of the journey of the underground waters
where they are descending. Stokes has shown that after this reaction has
«Don, J. R., The genesis of certain auriferous lodes: Trans. Am. Inst. Min. Eng., vol. 27, 1898,
pp- 569, 612.
1100 A TREATISE ON METAMORPHISM.
extended to equilibrium it will continue farther where the temperature is
rising, and thus descending solutions with increasing temperature are
favorable to solution of metallic silver. Doubtless silver is soluble to some
extent in all the strong acids which occur underground and also in their
salts. When, however, the silver becomes a salt, the form in which it would
subsequently travel depends on the amount of acids in the solutions and
their strength. It is well known that silver sulphide is soluble im alkaline
carbonates and in hydrosulphuric acid, but probably silver thus transported
is not an important source of metallic silver of the first concentration by
ascending waters. From such compounds the silver is likely to be thrown
down as a sulphide.
Precipitation — The precipitation of silver in the metallic form from its
solutions follows to a certain extent the same lines as that of gold; but silver
is not nearly so readily reducible as gold, and therefore is not thrown down
in the metallic form by so many compounds Silver is precipitated by
metallic iron, by metallic copper, by cuprous compounds, readily by ferrous
sulphate, and probably slowly by all other ferrous compounds. Of the ous
salts, ferrous sulphate is doubtless the most important. The precipitation
ef the silver by ferrous sulphate is commonly written—
Ag,SO,-+ 2FeSO, = 2Ag + Fe.(SO,)3
The reduction is a function of the abundance of the reducing agent. Where
the ferrous sulphate is abundant, and the temperature moderate it is believed
that the more probable reaction for the precipitation of silver from its
solutions is represented by the following equation:
Ag,SO,-+3FeSO, + 4H,0 =2Ag + Fe,0,+4H,S0,
Since ferric sulphate will dissolve silver, and ferrous sulphate will
reduce silver, the relations of these two compounds are, as Stokes notes,
reversible according to the following expression:
2Ag-+ Fe,(SO,),a2 Ag,SO, + 2FeSO
5 2 4 52 4 4
Stokes says that the reaction moves in the direction of solution when the
temperature is rising, and in the direction of precipitation when the tempera-
ture is falling. Thus, in ascending solutions, where the temperature is
falling, silver is especially likely to be thrown down by the ferrous salts.
Silver may be also precipitated by ferrous compounds in a solid form,
as in magnetite and the silicates. That this is certain follows from the fact
PRECIPITATION OF COPPER. 1101
that such compounds are capable of reducing copper to a metallic condition
from its solutions, and this element is more difficult to reduce than
silver. That such reduction has actually taken place by the ferrous iron of
silicates has been maintained by Vogt for the native silver at Kongsberg,
Norway.”
COPPER.
Solution —Copper in underground solutions is doubtless carried in many
forms, probably more largely as copper sulphate, but as a chloride to an
important extent. It may be transported also as copper carbonate in
carbonate solutions carrying excess of carbonic acid. Copper as a sul-
phide is soluble in the alkaline sulphides, especially acid sodium sulphide
(NaHS), and is soluble in considerable quantity in alkali sulpharsenates
and sulphantimonates.
Precipitation. —Copper as a metallic compound is precipitated by metallic
iron and by ferrous compounds. The precipitation by metallic iron is
probably unimportant, but that by ferrous compounds is of great conse-
quence. Copper occurs most extensively in the metallic form as precipitate
of a first concentration in the Lake Superior region. Many years ago
Pumpelly’ noted the fact that in this region there is an intimate connection
between the native copper and the iron-bearing minerals carrying ferrous
iron. On this subject Pumpelly says:
Throughout its deposits the copper exhibits a decidedly intimate connection
with delessite, epidote, and green-earth silicates, containing a considerable percentage
of peroxide of iron as a more or less essential constituent; while among the other
silicates, viz, analcite, laumontite, datolite, prehnite, only the last named, which
alone seems subject to a considerable replacement of its alumina by ferric oxide, is
especially favored by copper. This association is so invariable * * * that there
exists a close genetic relation between the metallic state of the copper and the ferric
condition of the iron oxide in the associated silicates; that the higher oxidation of
the iron was effected through the reduction of the oxide of copper and at the expense
of the oxygen of the latter. ¢
* % * x * * *
Now, may we not consider the presence of iron in prehnite generally to be due
to a beginning change, and the deposition of native copper in the Lake Superior
g sg ge, ne : :
prehnites to be partially or wholly correlated with the higher oxidation of the iron?¢
«Vogt, J. H. L., Ueber die Bildung des gediegenen Silbers, etc.: Zeitschr. f. prak. Geol., April,
1899, p. 118.
> Pumpelly, Raphael, The paragenesis and derivation of copper and its associates on Lake Superior:
Am. Jour. Sci., 3d series, vol. 2, 1871, pp. 353-354.
¢Punpelly, cit., p. ¢
d@Pumpelly, cit., p. 354.
2
IDO.
OZ A TREATISE ON METAMORPHISM.
These observations of Pumpelly convinced me some years ago that
the copper was reduced by the ferrous compounds. These conclusions
have been later confirmed by experimental work. Biddle, in a thick-walled
flask, reduced both cupric and cuprous chlorides by ferrous chloride in a
saturated solution of potassium bicarbonate." More recently Stokes, by
heating an acidified solution of cupric sulphate and ferrous sulphate in a
closed tube, has produced metallic copper at the cold end of the tube and
pure hematite at the hot end of the tube. The reactions involved are as
follows: °
2CuSO,+-2FeSO,2Cu,S8O,+ Fe,(SO,);
The Cu,SO, is decomposed on cooling to Cu and CuSO, while Fe,
(SO,),(decomposes on heating, thus:
Fe, (SO,);+3H,0=Fe,0,+-3H,S0,
Stokes further was able to reduce cupric sulphate to metallic copper
by heating with hornblende in a closed tube to a temperature of 200° C.
The reduction of the copper he attributes to the ferrous iron silicate of the
hornblende. Thus experimental work completely confirms the conclusion
that under natural conditions copper may be reduced from its salts by
ferrous compounds, and that the metallic copper of the Lake Superior
region was probably reduced both by ferrous solutions and by the ferrous
iron of the solid compounds.
In the reactions written above it is supposed that where the copper
salts are reduced by ferrous salts, ferric sulphate or hematite is produced.
However, this is not necessary, nor where the ferrous salts are in excess is
it believed to be probable that these are the common reactions. The
natural reaction under such circumstances would be to produce magnetite,
a compound not oxidized to the extent of ferric salts. The reaction may
be represented as follows:
CuSO,+3FeSO,+4H,0=Cu- Fe,0,+4H,80,
By this reaction three molecules of the ferrous salts are required to
produce one molecule of metallic copper, whereas if ferric salts be pro-
duced only two molecules of ferrous compound are needed. It is needless
« Biddle, H. C., The deposition of copper by solutions of ferrous salts: Jour. Geol., vol. 9, 1901,
pp. 480-436.
> Stokes, manuscript.
PRECIPITATION OF COPPER. 1103
to say that where the ferrous salts are very abundant well-known chemical
principles lead to the conclusion that a partial oxidation of the ferrous
compound is far more probable, since by the reduction of the copper an
amount of oxygen is liberated sufficient to transform only a small part of
the iron to the ferric state. In this connection it is to be remarked that in
the Lake Superior native copper deposits ferrous compounds are very
abundant, and magnetite is a very common mineral, both in the amygdaloid
and conglomerate deposits. For example, Irving illustrates the intimate
association of the native copper and the magnetite in the cupriferous sand-
stones of the Nonesuch mine, the copper frequently surrounding grains of
magnetite.” My first interpretation of this relation was that the magnetite
was probably the reducing agent which threw down the copper, but if this
were so it should be transformed to hematite. My present view is that
the native copper and the magnetite were both precipitated as a result
of the reaction of ferrous salts upon copper salts. According to Stokes’s
work these would not be simultaneously precipitated at the same place.
The magnetite would form where the temperature is higher and the copper
where the temperature is lower. To explain the existence of both at the
same place one is obliged to suppose that the magnetite formed first,
and that later when the temperature was lower the copper was thrown
down. It is notable that in the illustrative case mentioned, the copper
does surround the grains of magnetite, and thus corresponds with Stokes’s
experimental work.
It is hardly necessary to say that in one district the chief reaction
precipitating the copper may produce ferric salt and in another district may
produce magnetite, while in still other districts the precipitation of the
copper may be due to the combination of both reactions.
The Lake Superior copper deposits are believed to be an ideal case of
ores deposited by ascending waters, the sources of which are the igneous
rocks of the Keweenawan. In this region the only locality at which the
ore has been found in paying quantities is at Keweenaw Point, and the
productive district is at present confined to a very small area about Calu-
met and Houghton. Notwithstanding this fact, there is scarcely a locality
in the Lake Superior region where the Keweenawan basic lavas occur in
«Irving, R. D., The copper-bearing rocks of Lake Superior: Mon. U. 8. Geol. Survey, vol. 5, 1883
fig. 1, Pl. XVI, p. 127. (Description of same pp. 131-132.)
?
1104 A TREATISE ON METAMORPHISM.
which small amounts of copper are not found. Almost every porous amyg-
daloid shows flakes of it. In many localities it is so abundant that exten-
sive exploration has been undertaken, with the hope of finding large ore
bodies, as, for instance, in Douglas County, Wis., Isle Royale, and Mamainse.
But all of these explorations have resulted in failure. To me the almost
universal association of small quantities of copper with the Keweenawan
lavas is the most conclusive evidence that these lavas are the source of
the metal.
While copper commonly occurs in many of the igneous rocks in its
native state, especially in those which are porous, it is found in the less
porous and therefore less altered rocks in minute quantities in the form of
a sulphide, and this is thought to be the original form of the metal. That
is to say, at the time the lavas crystallized the copper separated as copper
sulphide or copper-iron sulphide. When the lavas were upturned by the
formation of the Lake Superior syncline, and denudation began truncat-
ing them, the segregation of the copper deposits was inaugurated. The
descending oxidizing waters transformed the copper sulphides into copper
sulphates and took them into solution. The underground water was finally
converged into trunk channels, and there met solutions bearing ferrous salts
or came into contact with ferrous compounds. At such places reduction
and precipitation took place. Where there were good trunk channels, as
in the upper surfaces of many of the amygdaloids, there was sufficient segre-
gration to encourage widespread exploration, as has already been noted.
But only a few of the amygdaloids were open and scoriaceous enough to
become the centering points of a sufficiently extended circulation to pro-
duce workable ore deposits. The greatest trunk channels were in the con-
glomerates. Where these conglomerates were interstratified with abundant
lavas bearing a sufficient amount of copper, and there existed other neces-
sary favorable conditions, the rising circulation has fortunately segregated
the metal in great quantity. The most notable of these deposits is in the
conglomerate upon which the Calumet and Hecla and the Tamarack mines
are located
SULPHIDES.
It has been stated that metals which occur as sulphides comprise iron,
copper, lead, zinc, nickel, arsenic, antimony, mercury, and silver. Of
these sulphides iron is the most abundant. Indeed, the sulphides of iron—
THE SULPHIDE ORES. TL INO}S)
pyrite, mareasite, pyrrhotite, and other forms—exceed in quantity many
fold all other sulphides. Standing next in amount is the group of zine,
lead, and copper sulphides, the order of abundance probably being that
named. Nickel, arsenic, and antimony sulphides, also occur in important
amounts. Mercury and silver sulphides, whileimportant ores of these metals,
are insignificant as compared with those previously mentioned so far as
absolute quantity is concerned. As to the original source of the sulphides,
it is well known that sulphide of iron occurs as an original constituent of
igneous rocks. Probably the same is true of the other sulphides, but as their
quantity is very much less they have been little noticed. Even if sulphides
had not been observed to be original constituents of the igneous rocks, the
large amount of sulphur compounds issuing from the interior of the earth,
in connection with volcanism, would lead to the conclusion that sulphides
must exist in the igneous rocks. This makes it highly probable that sulphur
as sulphide is or was present in sufficient quantity in the original rocks to
fully account for all of the sulphur compounds of the ore deposits.
While it is doubtless true that the original sources of all the sulphide
ores are the sulphides of the igneous rocks, it-does not follow that the
igneous rocks are the immediate source of existing sulphide deposits.
Indeed, in many instances this is not the case. For instance, it will be
seen (p. 1146) that the sulphides of the lead and zine deposits of the Missis-
sippi Valley are segregated from sulphides in limestones. These sulphides
were derived from the sulphides of earlier rocks which were probably trans-
formed to sulphates, transported to the sea, and in the sea, as shown by
Chamberlin, precipitated as sulphides at the same time the limestone was
formed. In other cases, the sulphides of a particular ore deposit may be
derived from a metamorphic rock. In many instances the sulphides of an
ore deposit have their immediate source very largely in sulphides which
have undergone one or more cycles of segregation by the processes which
are given below. In many cases the sulphides of a given ore deposit are
not derived from any single source or rock, but from the various sulphides
in all the rocks through which the particular system of underground
circulation producing the ore deposit passed.
MON XLV1I—04——70
1106 A TREATISE ON METAMORPHISM.
SOLUTION OF SULPHIDES.
It is well known that the sulphides of copper, mercury, iron, nickel,
lead, zine, arsenic, and antimony are soluble m alkaline sulphides. Illus-
trating this general statement, Becker* has shown experimentally that the
natural sulphide of mercury, cinnabar, the natural sulphide of iron, pyrite,
the sulphide of copper, and the sulphide of zinc are soluble in sodie sul-
phide. Along the same line as Becker's work Doelter has shown that
pyrite, stibnite, sphalerite, arsenopyrite, chalcopyryite, bournonite, and
galena are all soluble in sodic sulphide.’
Becker has further shown that the sulphides of iron, copper, zine, ,
arsenic, and antimony are also soluble in alkaline carbonates containing,
but not saturated with, hydrosulphuric acid.’ This is partly equivalent to
saying that they are soluble in sodium sulphide; for if hydrogen sulphide be
introduced in sodium-carbonate solution, the following reaction immediately
takes place:
Na,CO,+H,S=Na,S+H,0+C0,
But the observation by Becker is important, since it gives a method by
which sodium sulphide, a solvent for the sulphides of the heavy metals,
may be produced. In this connection it should be recalled that the
carbonates of the alkalies are among the most abundant compounds
carried by underground solutions, and hydrogen sulphide is known to be
very common in such solutions. Therefore it is certain that considerable
quantities of sodium sulphide will be produced where these two classes of
solutions come together, as they are often sure to do. The formation of
sodium sulphide in consequence of the mixture of sodium carbonate and
hydrogen sulphide is probably of great importance in the genesis of ores.
Confirming the conclusions of Becker and Doelter, observation has
shown that at Steamboat Springs, Nevada, and Sulphur Bank, California,
mercuric sulphide and iron sulphide are transported in solutions containing
sodic sulphide, hydrogen sulphide, sodium carbonate, and carbon dioxide.
“Becker, G. F., Quicksilver deposits of the Pacific coast: Mon. U. 8. Geol. Survey, vol. 13,
1888, pp. 428-435.
>Doelter, C., Einige Versuche tuber die Léslichkeit der Mineralien: Tschermaks Mineral.
Mittheil., Bd. XI, 1890, pp. 323-324.
¢ Becker, cit., pp. 482-435.
SOLUTION OF SULPHIDES. 1107
At Sulphur Bank boric acid is also present.” Hydrogen sulphide has also
been observed by Lindgren in the ascending waters of the Federal Loan
mine of the Sierra Nevada. He says that at the Federal Loan mine “an
unmistakable odor of sulphureted hydrogen was noted in the vicinity of
the spring.” ’
Finally, Doelter has shown tnaat the sulphides are soluble in pure
water, to some extent. He has thus dissolved measureable quantities of »
pyrite, galena, stibnite, sphalerite, chalcopyrite, arsenopyrite, and bour-
nonite.°
Since sodic carbonate, sodic sulphide, and hydrogen sulphide are prob-
ably so important in the transportation of the sulphides of the metals, it is
advisable to consider the possible sources of these compounds. This sub-
ject is of further importance, because it will be seen that hydrogen sulphide
is also an important precipitating agent.
It has been fully explained in other places (pp. 176-177, 609-610,
677-679) that carbon dioxide is liberated on a great scale in the zone of
anamorphism, and that most of this carbon dioxide joins the deep circulation.
It has been seen also that the process of carbonation of the silicates is
perhaps the most fundamental reaction of the zone of katamorphism—that
in which aqueous ore deposits occur. Of the elements which the silicates
contain, the most easily carbonated is sodium. It should follow that sodium
carbonate is an abundant salt in the ground water. This theoretical
conclusion is fully confirmed by observation.
We shall now see that abundant sodium carbonate results in the for-
mation of the other compound most frequently present where sulphides of
the valuable metals are in solution.
Stokes has shown recently that by the reaction of sodic carbonate
alone upon pyrite or marcasite sodic sulphide is produced. His reaction is
as follows:
8FeS,+15Na,CO,—4Fe,0,+-14Na,S-++Na,$,0,-+15C0,
«Le Conte, Joseph, and Rising, W. B.,.The phenomena of metalliferous vein formation now in
progress at Sulphur Bank, California: Am. Jour. Sci., 3d ser., vol. 24, 1882, pp. 23-33. Le Conte,
Joseph, On the mineral vein formation now in progress at Steamboat Springs, compared with the
same at Sulphur Bank: Am. Jour. Sci., 3d ser., vol. 25, 1883, pp. 424-428.
+ Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley districts, California:
Seventeenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1896, pp. 121-122.
¢ Doelter, cit., pp. 321-323.
1108 A TREATISE ON METAMORPHISM.
It is therefore evident that, since iron sulphide is the most common
of the sulphides in nature and sodium carbonate an abundant carbonate,
the reaction given is likely to take place on a considerable scale, and there-
fore that sodic sulphide, which is available to dissolve the sulphides of
the other metals, is probably produced under natural conditions in large
quantities.
Since experiment has shown the solubility of the sulphides in sodium
sulphide and sodium carbonate, and observation has shown the frequent
presence of these compounds, especially sodium carbonate, in underground
water, there can be little doubt that the sulphides as such are transported
oh an extensive scale in the ground waters.
While the sulphides are somewhat readily soluble by various com-
pounds, it is not believed that the material of the sulphide ores is carried
to the openings in the rocks to be deposited by the ascending waters in the
form of sulphide only. It has been fully explamed that the sulphides in
the belt of weathering are largely oxidized to sulphites and to sulphates,
mainly the latter, and taken into solution by the descending waters. In
this connection it is to be noted that the sulphates of iron, copper, zinc,
nickel, mereury, and silver are all readily soluble; and even the sulphate
of lead is dissolved to the extent of one part in 31,500 parts of water at
15° ©.,“ which is probably entirely adequate for the purposes of under-
eround transportation. While the sulphur compounds of these metals are
very largely transported as sulphates, they may also to some extent be
transported as sulphites. Of these metals the sulphites of iron, copper,
zinc, and silver are rather readily soluble.
PRECIPITATION OF SULPHIDES.
We have now seen that the metals which may be deposited as sul-
phides in the ore bodies may be transported as sulphides in solution or as
sulphates. The question now arises as to the conditions which will result
in their precipitation. It will be necessary to consider separately the pre-
cipitation of compounds transported as sulphides and as sulphates.
The more important metals which are known to be precipitated as
sulphides in sufficient quantities to constitute ore are those of iron, lead,
zinc, nickel, cobalt, copper, silver, mercury, arsenic, and antimony. As to
«Comey, A. M., Dictionary of Chemical Solubilities: London, 1896.
PRECIPITATION OF SULPHIDES. 1109
whether or not gold is also precipitated as a sulphide no definite statement
can be made. The majority of those who have most closely studied gold
deposits hold that the gold occurs either as free gold or as telluride. In
evidence of this they cite the fact that where gold occurs with pyrite, as it
so generally does, the microscopical study of the sulphides shows flakes of
free gold within the sulphide. Admitting these facts, it does not at all follow
that another portion of the gold is not present in the pyrite as a sulphide.
Indeed, since gold sulphide is known to be a definite compound, which can
easily be produced in the laboratory, and is produced by a metallurgical
process, it seems to me highly probable that some of the gold is precipitated
as a sulphide and in this form is associated with the sulphides of the other
metals.
Precipitation of sulphides transported as such— he precipitation of transported sul-
phides in the form in which they are found in the ore bodies may be
accomplished either by simple dilution of the compounds, as pointed out
by Becker* for mercury sulphide, by decreasing pressure and tempera-
ture, or by both. These are the conditions in ascending solutions, hence
the transported sulphides are likely to be precipitated where the water is
rising in trunk channels. The precipitation of the sulphides may result
also from mingling of solutions. If, for instance, an acid, such as boric
or sulphuric acid, be added to the solutions of the sulphides carried in
alkaline carbonates and alkaline sulphides, the simple neutralization will
result in the precipitation of many of the sulphides. By such neutraliza-
tion the alkaline sulphide, the solvent, is destroyed. For instance, if sul-
phuric acid be added to a sodium sulphide solution the following reaction
takes place:
Na,S+H,SO,=Na,.S0,+ H,.S
This equation shows that not only is the solvent destroyed, but a precipitat-
ing agent for the sulphides is produced. Under such conditions, there
ran be no doubt that the sulphides will be rapidly precipitated. Also,
where the sulphides are transported in solutions of sodium sulphide and
sodium carbonate, the addition of hydrogen sulphide in excess will result
in the precipitation of many of the metals.
«Becker, cit., pp. 429-431.
1110 A TREATISE ON METAMORPHISM.
Very frequently the mingling of alkaline solutions of sulphides with
acid solutions is at the points where descending and ascending waters meet.
This is very well illustrated by the conditions at Steamboat Springs and
Sulphur Bank, described by Le Conte,“ where the surface descending
waters are strongly acid, largely resulting from the oxidation of sulphides
to sulphates, whereas the waters rising from the deep sources are strongly
alkaline and bear sulp..ides in solution.
Precipitation of sulphides .‘ansported as oxidized salts—WW herever below the level of
ground water in the belt of cementation the sulphates and sulphites come
in contact with buried organic material, or with solutions carrying organic
compounds, the sulphates and sulphites may be reduced to sulphides. By
such reduction ores may be directly produced. Where the salts of the
metals are transported as sulphates the organic matter has merely to take
away the oxygen of the sulphate in order to transform it to a sulphide.
For instance, if the compound be lead sulphate (PbSO,) the abstraction
of the four atoms of oxygen produces galena (PbS). But in order that
such compound be formed it is not necessary to suppose that the metals
precipitated are transported as sulphates. It is only necessary to suppose
that oxidized compounds of the metals and sulphates, with a reducing
compound, are present. ‘To illustrate, if silver be transported as a chloride
with sodium sulphate in the solutions and oxygen be taken away from
the sodium sulphate, this would be transformed to sodium sulphide,
which would immediately precipitate the silver as silver sulphide. Again,
supposing copper to be traveling mainly as carbonate, sodium sulphate
to be present, and the reducing agent to be carbon, the end result is repre-
sented by the following reaction :
CuCO,+Na,S0,+2C=CuS-+Na,CO,+2C0,
Of course the actual process would be as in the previous case. The
compound containing the sulphur would be reduced to sulphide. This
would react upon any of the compounds present which could produce
insoluble sulphides.
Many ore deposits give evidence that the reducing action has been
caused by organic material. Such cases are illustrated by disseminated
sulphides occurring through carbonaceous material, as the graphite schist
“Le Conte, Joseph, Genesis of metalliferous veins: Am. Jour. Sci., 3d ser., vol. 26, 1883, p. 9.
PRECIPITATION OF SULPHIDES. ill ateal
of the Vermont copper mine described by Cazin.* The direct effect
of the carbonaceous material in the production of the sulphides is also
illustrated by the indicator veins of the Bendigo reef of Australia’ and
by the pyrite in the argillite described by Lindgren.’ Another clear
case of the direct reduction of oxidized salts by organic matter is that of
the lead and zine deposits of the Mississippi Valley. In Wisconsin, as
shown by Blake,* lead and zine are precipitated by the organic matter of
the oil rock, and in Missouri, as shown by Bain,’ by the organic matter of
the Devonian shales. More frequently, however, the direct action of
organic material upon the sulphates and sulphites is to form sparsely
disseminated sulphides, which must be further worked over in order to
produce ore deposits.
Recently Jenney has brought together a large number of instances
in which there is a close association of ores with organic compounds, and
these suggest that the organic material has directly or indirectly reduced
sulphates to sulphides, and thus resulted in their precipitation’
Probably sulphites and sulphates also may be reduced to sulphide by
ferrous iron in the rocks. In the case of silver sulphate the reaction
may be supposed to be as follows:
Ag,SO,+-12FeSO,+12H,0=Ag,S+4Fe,0,+12H,S80,
The reaction for the production of chalcocite from cuprous sulphate is:
Cu,SO, + 12FeSO, + 12H,0=Cu,S+4Fe,0,+12H,80,
and for cupric sulphate is:
2CuSO,+15FeSO,+16H,0=Cu,S+5Fe,0,+16H,80,
It is to be noted that in these reactions magnetite is formed simultane-
ously with the production of the sulphides, and in order that the reactions
“Cazin, F. M. F., Discussion of genesis of ores: Trans. Am. Inst. Min. Eng., vol. 24, 1894, pp-
604-608.
>Rickard, T. A., The origin of the gold-bearing quartz of Bendigo reefs, Australia: Trans. Am.
Inst. Min. Eng., vol. 22, 1894, p. 314.
¢Lindgren, Waldemar, Gold-quartz veins of Nevada City and Grass Valley: Seventeenth Ann.
Rept. U. 8. Geol. Survey, pt. 2, 1896, Pl. VIII, C, pp. 140, 156.
@ Blake, Wm. P., Lead and zinc deposits of the Mississippi Valley : Trans. Am. Inst. Min. Eng.,
vol. 22, 1894, pp. 630-631.
eBain, H. F., Van Hise, C. R., and Adams, Geo. I., Preliminary report on the lead and zine
deposits of the Ozark region : Twenty-second Ann. Rept. U. S. Geol. Survey, pt. 2, 1901, pp. 128-129.
JJenney, W. P., The chemistry of ore deposition: Trans. Am. Inst. Min. Eng., vol. 33, 1903,
pp. 445-48.
iste 2, A TREATISE ON METAMORPHISM.
shall take place a very large amount of ferrous salt is necessary. Similar
reactions may be written with the ferrous reducing salt in other forms than
sulphate.
The class of reactions represented by the above equations has not been
confirmed by experimental work in the laboratory. It was suggested by
geological facts. In the basic voleanic rocks which have, been profoundly
altered by metasomatic changes in the belt of cementation secondary sul-
phides and magnetite are very generally present in close association. Such
oceurrences are very well illustrated by the ancient volcanics of the Lake
Superior region——as, for instance, the Hemlock volcanic formation of the
Crystal Falls district of Michigan.“ For these rocks it can hardly be as-
sumed that organic material or reducing agents other than the ferrous salts
are present. But ferrous silicates, and, therefore, ferrous compounds, are
very abundant; hence the suggestion that they are the agents which have
resulted in the reprecipitation as sulphide of the sulphate compounds which
have come down from the belt of weathering. The widely disseminated
association of pyrite and magnetite in these rocks suggests that even the
following reaction may possibly take place:
22FeSO,+20H,0=FeS,-+7Fe,0,+20H,S0,
If heat be liberated by this change, there is good chemical reason for
its occurrence. But it must be admitted that this reaction is highly specu-
lative and needs confirmation by experimental work.
At those places where the sulphates enter trunk channels, mingling of
the solutions may cause precipitation when one of the solutions bears reduc-
ing agents. It is believed, however, that the precipitation of sulphates
as sulphides in the trunk channels of circulation is far more frequently
accomplished by hydrogen sulphide.
An important source of hydrogen sulphide is the reaction of organic
matter upon the sulphides. Of such sulphides, that of iron is, of course, of
the greatest consequence. Confirming the conclusion that the reaction of
organic matter upon the sulphides may produce hydrogen sulphide is the
fact that artesian waters held in sediments containing organic material
are frequently marked by the presence of hydrosulphuric acid. This is
«Clements, J. Morgan, and Smyth, H. L., The Crystal Falls iron-bearing district of Michigan:
Mon. U. §. Geol. Survey, vol. 36, 1899, pp. 73-154.
PRECIPITATION OF SULPHIDES. WLS}
illustrated by the artesian waters of Wisconsin,* which are contained in a
series comprising limestones and shales that are rich in organic matter. The
influence of organic matter in producing hydrogen sulphide is further
strongly suggested by the conditions at Sulphur Bank, where hydrogen sul-
phide is especially abundant in the solutions and the circulation of the ground
water is through sedimentary rocks bearing abundant organic material.
Another source of hydrogen sulphide is the action of the dilute strong
acids upon the sulphides. For instance, the reaction of sulphuric acid
upon many of the base sulphides liberates hydrosulphurie acid.
Even the weak acid, carbonic, where abundant, produces hydrogen
sulphide by reaction upon the alkaline sulphides, such as sodium sulphide.
As already noted, Stokes has shown that the carbonates of the alkalies, by
reaction upon the sulphides of the base metals, such as pyrite, form alka-
line sulphides. If the sodium sulphide thus produced comes into contact
with streams bearing carbonic acid, hydrogen sulphide will be produced.
When it is remembered that in the solutions sodium carbonate is an abun-
dant salt, that pyrite is the most plentiful sulphide, and that carbonic acid
is the most abundant acid, it will be realized that the conditions must
frequently arise which result in the production of hydrogen sulphide on a
considerable scale.
Another way in which hydrogen sulphide can be formed is by the
partial oxidation of the sulphides, especially the abundant iron bisulphide
and pyrrhotite. On page 214 it was seen that the following reactions may
take place:
3FeS, + 4H,0440=Fe,0,+4H,8+280,
3Fe,,8,.+36H,0+80=11Fe,0,+36H,S
The frequent association of magnetite with iron sulphide where the con-
ditions have been favorable for partial oxidation suggests that this reaction
may have taken place upon a considerable scale.
The hydrogen sulphide from any of the above sources may join the
circulating waters bearing oxidized salts in solution and precipitate copper,
zine, lead, silver, and other metals as sulphides in the veins.
In this connection it is very interesting to note that Doelter, by treating
metallic salts of the metals by hydrogen sulphide in somewhat dilute solu-
4, 1882, p. 547.
1114 A TREATISE ON METAMORPHISM.
tions at temperatures of 100° or somewhat higher, has actually succeeded
in producing artificially the following sulphide minerals: Pyrite, chaleopy-
rite, bornite, chalcocite, covellite, galena, bournonite, miargyrite, jamesonite,
and pyrrhotite.*
Doubtless in many instances the precipitation of the oxidized salts is
accomplished by an alkaline sulphide instead of by hydrogen sulphide.
It has already been pointed out that the action of alkalie carbonates upon
salts such as iron sulphide produces alkaline sulphides. For instance,
sodium carbonate reacts upon iron sulphide, producing sodium sulphide.
Where this sulphide comes into contact with any of the oxidized salts, or
other soluble sulphates, the metals may be thrown down as sulphides.
A particularly good case of precipitation due to mingling of solutions
is that of the lead and zine deposits of the Missouri-Kansas district, described
by Bain,’ where waters rise through the Carboniferous limestone and come
into contact with precipitating solutions derived from it. It is uncertain
whether the precipitating solutions are alkaline sulphides, hydrogen sul-
phide, or reducing agents derived from organic matter, but probably
hydrogen sulphide produced by reactions upon base sulphides is the more
important precipitating agent.
Of the greatest importance in the precipitation of sulphides from
ascending waters, the metals of which have been transported in the form of
oxidized salts, is the reaction upon the solutions of previously precipitated
sulphides. The sulphides of the less valuable but more abundant metals
are important agents in the precipitation of the sulphides of the more valu-
able metals from the solutions of their oxidized salts. "That such precipitation
occurs was shown by Anthon in 1837." In 1888 the same subject was inves-
tigated by Schiirmann, whose results are but slightly different from those of
Anthon. Schiirmann’ gives the following series of metals, the order being
increasing strength of affimity for sulphur: Manganese, thallium, arsenic,
iron, cobalt, nickel, zine, lead, tin, antimony, cadmium, bismuth, copper,
silver, mercury, palladium. He shows that the sulphide of the first mem-
@Tschermaks Min. u. Petrog. Mitth. vol. 7, 1886, p. 535 et seq.
> Bain, H. F., Van Hise, C. R., and Adams, G. I., Preliminary report on the lead and zinc deposits
of the Ozark region: Twenty-second Ann. Rept. U. 8. Geol. Survey, pt. 2, 1901, pp. 212-214.
¢Anthon, E. F., Ueber die Anwendung der auf nassem wege dargestellten Schwefelmetalle bei der
chemischen Analyse: Journ. f. Prak. Chemie, 1st series, vol. 10, 1837, p. 353.
4Schiirmann, Liebig’s Annalen der Chemie, vol. 249, 1888, p. 342.
PRECIPITATION OF SULPHIDES. 1115
ber of the series, manganese, reacts upon the oxidized salts of the
following metals and precipitates them as sulphides, the precipitating
agent at the same time going into solution as an oxidized salt. In a simi-
lar manner the sulphide of any other member reacts upon the oxidized
salts of all the following members, precipitating the sulphides of them
and going into solution as an oxidized salt. Thus the sulphide of iron
is a precipitating agent for the oxidized solutions of zinc, lead, copper, and
silver, throwing them down from their solutions as sulphides. Similarly
the sulphides of copper throw down silver as sulphide from oxidized silver
solutions. It is to be noted that the base metals, manganese, iron, and
arsenic, as sulphides, are capable of precipitating practically all of the
higher-priced metals; and that these compounds and zine, lead, and copper
sulphides are capable of precipitating silver. It is thus shown by experi-
ment that the abundant sulphides of the baser metals are capable of pre-
cipitating, as sulphides, the more valuable metals from their oxidized salts.
In this connection the widespread occurrence of rhodochrosite and
rhodonite in connection with ore deposits is very interesting. One or both
of these minerals are found in almost every Western camp. They have
been especially observed at the Butte and Little Belt Mountains districts
of Montana, and the San Juan, Cripple Creek, Rico, and Aspen districts of
Colorado. If manganese should be present in the veins as a sulphide, for
instance, as alabandite (MnS), and oxidized solutions of any of the other
metals come in, the sulphides of these other metals would be formed and
the manganese would pass to the oxidized condition, and it is found as a
carbonate or silicate in the deposits of the first concentration and frequently
as a hydrated oxide in the deposits which have been affected by secondary
alterations by descending waters. The oxidized manganese is precipitated
as a carbonate or silicate because carbonic and silicic acids are the two
abundant rock-making acids, and because these acids, with manganese,
make insoluble compounds. When later the rhodochrosite and rhodonite
in the belt of weathering are affected by secondary alterations, hydrated
oxide of manganese, so prevalent in the upper parts of deposits, is pro-
duced. It at, least is a suggestive coincidence that the rich gold streaks
in the Camp Bird mine, Colorado,” are closely associated with impure
« Purington, C. W., Woods, T. H., and Doyveton, G. D., The Camp Bird mine, Ouray, Colo.: Trans.
Am. Inst. Min. Eng., vol. 33, 1903, p. 570.
1116 A TREATISE ON METAMORPHISM.
rhodonite, and that in the Golden Fleece vein rhodochrosite ‘‘forms with
quartz the gangue of the rich free gold ore.”* At Rico also, according to
Ransome, rhodochrosite “is important in these mines as a rough indication
of good ore.”’
Since manganese is not a very abundant metal, and since as a sulphide
it has precedence over all other sulphides in its reactions upon oxidized
salts of the other metals, being at the same time transformed to an oxidized
product, it is natural to expect that manganese as rhodochrosite and rhodonite
would be the dominating forms of manganese minerals in ore veins, and
that manganese sulphide would be very rare, and such are the facts.
While manganese sulphide in many veins has doubtless been an
important agent in the precipitation of ores of the first concentration by
ascending water, it is probable that the much more abundant iron sulphide
is of even greater importance in this connection. Just as oxidized salts of
manganese are produced by the precipitation of the valuable metals by
manganese sulphide, so the precipitation of such metals by iron sulphide
produces oxidized salts of iron. Siderite is an even more important gancue
in ores of the first concentration than rhodochrosite or rhodonite, thus
indicating that the reaction suggested has taken place. Since, however,
iron is so much more common than manganese, ordinarily not all of the
iron sulphide has thus been oxidized, and hence in the veins pyrite is a
very common gangue.
The reactions which caused the precipitation of the sulphides of the
various metals from their oxidized solutions by the sulphides of other metals
were not written out by Anthon and Schurmann, but recently Stokes has
investigated the reaction in the case of iron sulphide with some of the solu-
tions. Thus he has shown that pyrite and marcasite precipitate copper
sulphide from cupric sulphate according to the following reaction:
5FeS,-+14CuS0,+12H,0=7Cu,8-+-5Fe80,+12H,S0,.
This reaction took place at 100° C. and above. Stokes has shown also that
where sodium carbonate or potassium bicarbonate is present with the
sulphide of iron and carbonates of various metals at 180° C., sulphides are
“Ransome, F. L., A report on the economic geology of the Silverton quadrangle, Colorado: Bull.
U.S. Geol. Survey, No. 182, 1901, p. 73.
>bRansome, F, L., The ore deposit of the Rico Mountains, Colorado: Twenty-second Ann. Rept.
U.S. Geol. Survey, part 2, 1901, p. 252.
PRECIPITATION OF SULPHIDES. TULA
precipitated in a different manner. For instance, in the case of lead
carbonate with pyrite and marcasite, the reaction is as follows:
8FeS,-+14PbCO,+Na,CO,=14PbS+4Fe,0,+ Na,8,0,+15C0,
An interesting point in connection with this reaction is the production
of hematite and sodium thiosulphate. Stokes shows also that zine, copper,
and silver are precipitated as sulphides by iron as sulphide in an analogous
manner, the reactions being:
8FeS,-+15ZnC0,+Na,CO,=15ZnS-+4Fe,0,--Na,S0,-+16C0,
and
8FeS, +15Ag,CO,+Na,CO,;=15Ag,S-+4Fe,0,+Na,SO,+16C0,.
While Stokes in the laboratory used temperatures of 100° C. and 180°
C., it does not follow that the above reactions do not take place at lower
temperatures but more slowly.
The above reactions in the presence of an alkaline carbonate are very
suggestive, since they possibly explain in large part the association of lead,
zinc, copper, and silver ores with limestone; for if calcium carbonate be
substituted for sodium carbonate, similar reactions take place. It is not
necessary to rewrite the equations, but the character of the reactions may
be seen by substituting for Na,CO, on the left-hand side the equations
CaCoO,, and for Na,S,O; and Na,SO, on the right-hand side the equation
CaS,O, and CaSO, Since these reactions are confirmed by experiment,
we probably have the clue to the world-wide preference of many ores of
lead, zine, copper, and silver for the limestones. (See pp. 1086-1087.)
GENERAL STATEMENTS.
From the foregoing it appears that at the trunk channels of circulation
the sulphur may enter in sulphides or in oxidized compounds in the forms
of sulphites, sulphates, or other oxidized salts. If present as sulphides in
alkalime solutions, the sulphides may be precipitated in consequence of the
neutralization of the solution by an acid. They may be thrown down also
in the ascending solutions in consequence of diminishing temperature and
pressure. However, where the oxidized sulphur-bearing solutions reach
trunk channels, as they frequently do, in order to precipitate the metals
as sulphides they must become mingled with solutions bearing reducing
agents, hydrogen sulphide, or alkaline sulphides, or must come into contact
ILLS) A TREATISE ON METAMORPHISM.
with other sulphides previously precipitated or with other reducing agents.
Any of these factors, or any combination of them, may cause the precipita-
tion of the sulphides.
A ease due to the combination of these factors is the precipitation of
sulphides which later may react to precipitate the metal of the oxidized salts.
For instance, in the early stage of the development of a deposit, the sul-
phides of iron and other base metals may be precipitated by hydrogen
sulphide or some other compound, and later these previously precipitated
sulphides take part in the precipitation of the sulphides of copper, silver, and
mercury from the oxidized solutions of these compounds, such as sulphates.
While it is believed that sulphides are generally segregated in the first
concentration by upward-moving waters, this is not supposed to be univer-
sal. Nature’s processes are always too complex to be coverd by a single
general statement. As a result of mingling solutions at various places, and
of reactions between solutions and walls, many lateral-moving and downward-
moving streams doubtless deposit rather than dissolve sulphides. Indeed,
in the frequent case where in downward-moving waters sulphites or sul-
phates are reduced by organic matter to sulphides, precipitation of a portion
of the sulphide is usual. Still the statement would hold true that upon the
average more sulphides are dissolved than deposited by descending waters,
and more sulphides are deposited than dissolved by ascending waters.
I conclude, therefore, that sulphide ores, whatever their source, as first
concentrates are generally deposited by ascending waters in the trunk channels.
It appears that whether sulphur-bearing compounds reach trunk chan-
nels as sulphides or as oxidized salts, they are likely to be precipitated in
such channels because solutions from various sources mingle there and
come into contact with previously precipitated sulphides. The sulphides
thus produced in the trunk channels may by denudation again reach the
belt of weathering when the cycle is complete. These sulphides may be
again oxidized to sulphate, and so on. It is therefore clear that sulphur, as
sulphide and sulphate, may again and again take part in the deposition of
ores,“ but the first chief source of the sulphur is probably the sulphides of
the original crystallized rocks. and of magmas.
4Le Conte, Joseph, Genesis of metalliferous veins: Am. Jour. Sci., 3d ser., vol. 26, 1883, p. 13.
THE TELLURIDE ORES. TEI)
TELLURIDES.
The only important telluride ores are those of gold and silver. These
compounds, until comparatively recently, were regarded as somewhat
exceptional, and for the most part rather as mineralogical rarities than as
ores. The earliest gold and silver tellurium ores to be mined were those of
Nagyag, in Transylvania.* Gold mining in Colorado has added other and
much more important localities where the tellurides are produced in impor-
tant amounts. About 1872 the tellurides of Boulder County became of
consequence. But of more importance than all of the previously known
tellurides are those of Cripple Creek, Colorado, which began to be exploited
in 1891.2 Here the various tellurides are found, but the greater part of the
gold is believed to occur in calaverite.’
Close mineralogical studies of ore deposits in recent years have shown
that subordinate and usually unimportant amounts of tellurides, from an
economic point of view, occur very generally associated with gold. Indeed,
it appears that the presence of the tellurides with the gold ores of the first
concentration is scarcely less prevalent than the association of pyrite with
gold ores. The widespread occurrence of tellurides in connection with gold
ores is illustrated by the Kalgoorlie district of West Australia, described
by Baneroft;* by the Sierra Nevada gold deposits, described by Lindgren; °
by the Judith Mountains and Little Rocky Mountains, Montana, described
by Weed and Pirsson;’ by the Potsdam gold ores of the Black Hills,
described by Smith and Irving;’ by the mines of Custer County, Colo.,
and by the Appalachian gold deposits, described by S. F. Emmons.’
«Kemp, J. F., Geological occurrence and associates of the telluride gold ores: Mineral Industry,
vol. 6, 1898, p. 295.
> Penrose, R. A. F., jr., Mining geology of the Cripple Creek district: Sixteenth Ann. Rept.
U.S. Geol. Suvey, pt. 2, p. 118.
¢ Penrose, cit., p. 121.
d Bancroft, Geo. J., Kalgoorlie, Western Australia, and its surroundings: Trans. Am. Inst. Min.
Eng., vol. 28, 1899, pp. 88-100.
e Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California: Seven-
teenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1896, p. 117.
f Weed, W. H., and Pirsson, L. V., Geology and mineral resources of the Judith Mountains, Mon-
tana: Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 3, 1898, pp. 589-592. Weed, W. H., and Pirsson,
L. V., Geology of the Little Rocky Mountains, Montana: Jour. Geol., vol. 4, 1896, pp. 426-428.
gSmith, F. C., The Potsdam gold ores of the Black Hills of South Dakota: Trans. Am. Inst. Min.
Eng., vol. 27, 1898, pp. 414-420. Irving, J. D., A contribution to the geology of the northern Black
Hills: Annals New York Acad. Sci., vol. 11, 1899, pp. 297-311.
hk Emmons, S. F., The mines of Custer County, Colorado: Seventeenth Ann. Rept. U. S. Geol.
Survey, pt. 2, 1896, p. 433. Emmons, §. F., Notes on the gold deposits of Montgomery County, Mary-
land: Trans. Am. Inst. Min. Eng., vol. 18, 1890, p. 407.
1120 A TREATISE ON METAMORPHISM.
Since the tellurides are very generally associated with gold ores and
the free gold deposited by the deep circulation is almost universally
associated with pyrite and other sulphides, it follows that the tellurides
deposited by the deep circulation are usually associated with sulphides.
This is not only true of those localities in which the tellurides are unim-
portant as ores, but is equally true of the great telluride camps. Of
Cripple Creek Penrose says: “The close association of tellurides with
sulphides, especiaily iron pyrites, which themselves often carry gold, is
also a point worthy of note. Not only are auriferous tellurium and sulphur
compounds often thus mechanically associated im different minerals, but
sometimes they replace each other in the same minerals, such as tetrady-
mite, nagyagite, and tellur-sulphur.” “
The tellurides are also very generally associated with fluorite, and
Penrose notes that where the gold is mainly in sulphides fluorite has not
often been observed.
It seems to me certain that the widespread association of the tellurides
with gold and silver ores, especially with gold ores, has a genetic signifi-
cance probably less important only than that of the base sulphides, especi-
ally iron sulphide. As to the source of the tellurium, nothing is positively
known. In all of the above-mentioned districts in which tellurium is
especially abundant, with the possible exception of the Kalgoorlie district,
which has not yet been thoroughly studied and in which the age of the
mineralizing dikes has not been established, are found comparatively recent
igneous rocks, in which doubtless the tellurium largely had its origin. At
first thought one might conclude that the gold, silver, and tellurium were
transported as tellurides and deposited as such without chemical change,
but the recent work of Lenher and Hall’ is decidedly against this view.
They have found no solvent whatever for tellurides of gold without
breaking up these compounds and producing salts of tellurium, the gold
usually being left in the metallic form. Moreover, as has already been
noted, they have shown that metallic tellurium and seven of the more
common mineral tellurides of gold and silver rapidly reduce gold from
“Penrose, R. A. F., jr., Mining geology of the Cripple Creek district, Colorado: Sixteenth Ann.
Rept. U. 8. Geol. Survey, pt. 2, 1895, p. 157.
>Lenher, Victor, Naturally occurring telluride of gold: Jour. Am. Chem. Soe., vol. 24, 1902, pp.
355-360.
Hall, R. D., and Lenher, Victor, Action of tellurium and selenium on gold and silver salts: Ibid.,
pp. 918-927.
THE TELLURIDE ORES. Z|
its solutions, forming metallic gold, the tellurium at the same time going
into solution. Further, they have proved that the presence of any of the
soluble tellurides, including hydrogen telluride, is sufficient not only to throw
the gold out of solution, but to prevent it from getting into solution. Other
soluble salts, where tellurium acts as an acid, are the tellurites and tellurates.
Tellurous oxide or acid is also sparingly soluble. All of these compounds,
like tellurides, precipitate gold from its solutions in a metallic form, not as
telluride. From the foregoing it appears that if tellurium compounds in
which tellurium is a part of the acid are essential for the formation of the
tellurides of gold, these tellurium salts and the gold have come into the
trunk channel from separate sources. They could not have traveled
together; else the gold would have been thrown from the solutions before
reaching the trunk channels.
It is to be remembered that tellurium, besides acting in an acid, also
acts as a metal to the halogens (chlorine, bromine, and iodine). The
telluric compounds of this class, such as telluric chloride (TeCl,), being
completely saturated, so far as tellurium is concerned, have no reducing
power, and they may therefore travel with gold solutions. But the tellurous —
combinations with the halogens, such as tellurous chloride (TeCl,), can not
travel with gold, for Lenher has shown that such tellurous salts in the
presence of water and its compounds react upon the water, producing
metallic tellurium and telluric chloride according to the following equation:
2TeCl, + H,O—Te+TeCl,-H,0
The metallic tellurium would then reduce the gold, which would therefore
be precipitated before reaching the trunk channels.
The almost invariable association of tellurides with sulphides, already
noted, naturally leads to the suggestion that the same conditions were
favorable to the deposition of the sulphides and the tellurides. In this
connection the occurrence at Cripple Creek is especially interesting. Mr.
Moore, consulting engineer of the Portland mine, informs me that in this
district near the surface the values are found in the oxidized zone as native
gold; that deeper, the values are mainly in the tellurides, the associated
sulphides being very lean; and that still deeper the values are found both
in the tellurides and in the sulphides of iron. The depth at which the
sulphides at the Portland mine begin to have value is about 200 meters.
fal
MON XLVII—O04
1122 A TREATISE ON METAMORPHISM.
These facts strongly favor the precipitation of the gold as a first concentrate
simultaneously in the sulphides and the tellurides. It is believed (see pp.
1172-1174) that the abundant native gold at the surface and in the tellu-
rides at intermediate zones to the exclusion of the sulphides is due to
secondary reaction by descending water.
It may be that the key to the problem of the deposition of the
tellurides lies in their association with sulphides. We have already seen
that telluric salts of the type of TeCl, may travel with gold in solutions. —
It has already been pointed out that gold, in most cases, probably also
travels as a chloride, and thus solutions of auric chloride and _ telluric
chloride may together enter trank channels which contain sulphides. In
such trunk channels the reaction of the sulphides might reduce both the
gold and the tellurium simultaneously and thus produce tellurides of gold;
or, by the reaction of the sulphides upon the telluric salts, these may be
reduced to tellurous salts, which, as already explained, would decompose
into metallic tellurium and telluric salts, and the tellurium would precipitate
the gold. Under these or some other conditions, the gold and tellurium go
down together, with a definite composition, and thus form the tellurides.
The invariable association of silver with the tellurides of gold
suggests the possibility that the presence of silver salts is essential for the
formation of the tellurides of gold. But in some cases the silver is very
low in the tellurides, although rarely as low as 0.8, as in the case of one
specimen of calaverite from Kalgoorlie,* which makes it improbable that the
presence of silver salts is essential for the precipitation of the tellurides.
The association of the tellurides with fluorite has already been men-
tioned, and this association suggests a genetic relation between the two;
but bearing in the opposite direction are the observations made by Penrose
at Cripple Creek, where the abundance of fluorite has no definite relation
to the abundance of tellurides.’ Possibly pointing in the same direc-
tion is the experimental work of Lenher,’ who has shown that gold and
gold oxide are absolutely insoluble in hydrofluoric acid. Not only is this
the case, but when any gold salt is brought into contact with a soluble
fluoride the gold is immediately precipitated as a hydrate. This is true
“Rickard, T. A., Telluride ores of Cripple Creek and Kalgoorlie: Trans. Am. Inst. Min. Eng.,
vol. 30, 1901, p. 711.
>Penrose, cit., pp. 158-159.
¢Lenher, Victor, Fluoride of gold: Jour, Am. Chem. Soc., vol. 25, 1903, pp. 1136-1138.
bo
THE TELLURIDE ORES. 1123
even for silver fluoride, which, according to Lenher, acts upon gold chloride
as follows:
AnCl,4-3A¢F+3H,0=3A¢Cl+ Au(OH),+3HE
Lenher’s work suggests the advisability of experimenting as to the effect
of the soluble fluorides and of hydrofluoric acid upon solutions bearing chlo-
rides of both gold and tellurium. In this or some other combination of the
three elements gold, tellurium, and fluorine with perhaps other compounds
may lie the key to the problem of the precipitation of the gold tellurides.
The abundant fluorite associated with the tellurides has frequently
been cited as evidence of pneumatolitic action in connection with such
deposits. But Penrose’s description of the Cripple Creek ores gives no
support to this hypothesis. All his descriptions show the phenomena to
be those which, in many other districts, are well known to be due to water
solutions. For instance, the most abundant gangue mineral of the district
is silica in quartzose, chalcedonic, and amorphous forms. It is well known
that these compounds are the most abundant ones deposited by under-
ground water. Others of the more abundant gangue minerals are oxides
of manganese, calcite, gypsum, barite and kaolin."
Furthermore, in some of the mines, as the Gold Coin mine, the ore
of which is in granite, there has been solution of biotite, quartz, and
feldspar, and this can not be attributed to pneumatolitic action.’
I therefore find no evidence of pneumatolitie action in connection with
the genesis of the telluride ores. It is uncertain what conditions produce a
somewhat homogeneous compound consisting of definite proportions of
silver, gold, and tellurium, as in the telluride minerals. Doctor Lenher has
heen at work on this problem for some time, but as yet without determin-
ative results. He has treated gold solutions with hydrogen telluride and
has worked on the various tellurium compounds, such as the chloride,
bromide, fluoride, and alkaline tellurites, all experiments being made in the
presence of a reducing agent, such as sulphurous acid; vet in no case has he
been able to get homogeneous compounds of gold and tellurium, but, on the
contrary, has obtained metallic gold and tellurium salts, or mixtures of
gold and teilurium of irrational composition. While the problems of the
:
“Penrose, cit., pp. 123-129.
>Penrose, R. A. F., jr., Mining geology of the Cripple Creek district: Sixteenth Ann. Rept. U. S.
Geol. Survey, pt. 2, 1895, pp. 159-161.
1124 A TREATISE ON METAMORPHISM.
transportation of the compounds resulting in the tellurides and of the pre-
cipitation of the tellurides have not yet been solved, there is little doubt
that careful chemical work will solve them.
In view of all the foregoing facts, the most plausible suggestion which
IT am able to make as to the development of the tellurides is as follows:
The gold is taken into solution by descending waters in the early part of the
journey in the form of a chloride, as explained on pages 1089-1091. The
tellurium is widely dispersed, possibly to some extent as metallic tellurium,
but more largely as a telluride, and is oxidized in the belt of weathering.
It there unites with an acid and travels as a soluble salt, perhaps, also, in
large part, as telluric chloride. That reactions of this kind take place with
reference to the tellurides in the belt of weathering are certain, as shown
by the abundant spongy gold at. Cripple Creek, pseudomorphous after the
various tellurides, showing that such reactions have taken place there on
an extensive scale. The auric chlorides and telluric salts, such as telluric
chloride, may travel together without reaction, or they may come together
in the trunk channels. There they become mingled with reducing com-
pounds in solution, or, as has been suggested, come into contact with
reducing solids, such as the sulphides. It is believed the tellurium and
gold are simultaneously reduced under such conditions that they unite in
definite compositions, and thus produce the telluride minerals.
Hall and Lenher were more successful in the production of telluride of
silver. An ammoniacal solution of silver chloride in contact with tellurium
at room temperatures resulted in the production of silver telluride according
to the following reaction:
4Ag¢C1+3Te=2Ae,Te+TeCl,
When the nitrate of silver was used ‘‘a greater range of temperature
was possible, and the solution was either boiled continuously or kept at a
temperature of about 80° C. for a long time.”* ‘The reaction in this case as
given by them is:
4AgNO,+3Te—2Ae,Te+Te(NOs),
Since, however, it is difficult to understand how metallic tellurium can
be carried in solution, there is a possibility that the silver tellurides are
«Hall, R. D., and Lenher, Victor, Action of tellurium and selenium on gold and silver salts: Jour.
Am. Chem. Soc., vol. 24, 1902, p. 922.
THE TELLURIDE ORES. UNDS
produced in nature not by the reaction of metallic tellurium but by tellu-
rous salts which, according to Lenher, precipitate silver by the following
reactions:
(a) 2TeCl,+H,0O=Te+TeCl,+H,O
(b) 4AgNO,-+3Te=2Ag,Te-+Te(NO,),
While the reactions given may possibly explain the formation of a
part of the silver telluride in the rare mineral hessite, it is to be remem-
bered that pure silver telluride is very exceptional.
Commonly, so far as silver occurs with tellurium, it is in minerals in
which there is also telluride of gold, and frequently telluride of lead, mer-
cury, and antimony. It therefore follows that the conditions under which
the telluride of silver is precipitated must be the same in most cases as those
which cause the formation of the telluride of gold. Hence the solution of
the problem of the precipitation of tellurides of gold will probably also be
the solution of the problem of the precipitation of the telluride of silver.
OXIDES.
The oxide ores that are deposited by ascending water below the belt in
which the effect of descending waters is evident are dominantly hematite
(Fe,O;) and magnetite (FeO . Fe,O;), and possibly zincite (ZnO), franklinite
(feZnMn)O .(FeMn),O;), and cassiterite (SnO,).
The association of hematite and magnetite with the sulphides of the
first concentration far below the belt where oxidizing waters can act is so
well known that it is not necessary to amplify by giving illustrations.
So far as I know the chief occurrences of zincite and franklinite in
deep-seated deposits with no evidence of superficial oxidation are in New
Jersey, especially at the famous localities of Franklin Furnace and Sterling “
It is notable that here magnetite also occurs associated with the zincite and
franklinite. Magnetite associated with pyrite and marcasite occurs very
extensively in the Lake Superior region, especially in Canada, although
these minerals are found together on the south shore of Lake Superior, as
at the Republic trough. The question at once arises whether the metals
were transported to their present positions in solution and deposited by
ascending waters, or, on the contrary, were produced by alterations of solid
«Kemp, J. F., The ore deposits of Franklin Furnace and Ogdensburg, New Jersey: Trans. New
York Acad. Sci., vol. 13, 1893-94, pp. 76-96.
1126 A TREATISE ON METAMORPHISM.
compounds in the zone of anamorphism by the processes of dehydration,
decarbonation and deoxidation.
Magnetite—he formation of magnetite in the zone of anamorphism is
fully discussed on pages 845-846, and will not again be described here.
The reactions producing magnetite are believed to be as follows:
3FeCO,-+-O=—Fe,0,+3C0,
2FeCO,-+FeS,-| 2H,O=Fe,0,+ 2H,S-+200,
3(Fe,0, . nH,O)—O=2Fe,0,+3nH,0
22(Fe,O, . nH,0) +FeS, + 2H,0=15Fe,0,+ 2H,S0,+22nH,0
zincite—In order to produce zincite in the deep-seated zone from smith-
sonite, it is only necessary that the water and the carbon dioxide be driven
off, thus:
ZnCO, . H,O=Zn0 +H,0.+-C0,
If smithsonite were formed in the belt of weathering, as explained on
pages 1144-1145, 1147-1148, and later by burial transferred to the zone
of anamorphism, the above reaction would almost certainly take place.
Franklinite—In1 the case of franklinite it is necessary only to suppose
that with the smithsonite there was a certain amount of hydrous oxide of
iron and manganese. It is well known that the smithsonite produced in
the belt of weathering almost universally contains hydrous oxide of iron.
Not infrequently it is also associated with hydrous manganese oxide. If
smithsonite, hydrous oxide of iron, and hydrous oxide of manganese be
taken in the proper proportions, and subjected to dehydration, decarbona-
tion, and deoxidation, franklinite may be produced. For simplicity suppose
that the iron, zinc, and manganese in the resultant franklinite are equally
abundant as ferrous oxide, and iron and manganese equally abundant as
ferric oxide. The reaction may then be written as follows:
2(ZnCO, . H,O)+4(Fe,0, . 1}H,0)+4(Mn,0, . H,O)—40=
6[(FeZnMn)O. (FeMn),0,]-+-12H,0+2CO0,
Hematite —]t is well known that hematite occurs as a gangue mineral in
connection with the sulphides in veins, although so far as I know there is
no case in which such hematite occurs in sufficient amount to be called an
ore. In this connection the recent work of Stokes is interesting.“ He has
shown that FeS, as pyrite and marcasite is a ferric iron; and further, that
«Stokes, H. N., On pyrite and marcasite: Bull. U. 8. Geol. Survey No. 186, 1901, pp. 50.
THE OXIDE ORES. WUD
from this ferric iron hematite can be produced by the action of certain
carbonates and alkaline carbonates without oxidation. By reactions of
zinc, lead,.and silver carbonates with an excess of alkaline bicarbonate, he
has produced hematite according to the following reactions:
8FeS,-+147nC0,-+Na,CO,=4Fe,0,+-14Zn8 +Na,$,0;+15C0,
8FeS, +-14PbCO,-+Na,CO,=4Fe,0,+14PbS-+Na,8,0,+ 1500,
8FeS,+15Ag,CO,+-Na,CO,;=4Fe,0,+15A¢,8-+Na,SO,+16C0,
He has also produced hematite from the sulphide through the reaction of
the alkaline carbonate alone at temperatures of 100° C. and 190° C., thus:
8FeS, -+15Na,CO,24Fe,0,-+-14Na,S-+Na,8,0;+15C0,
This last reaction is reversible, but Stokes remarks it will proceed to an
end if the sulphide and thiosulphate be carried away, as by a circulating
solution, or if there be present a metallic salt capable of precipitating
the alkaline sulphide as fast as it is formed, as in the first three reactions
above given.
Cassiterite —In reference to cassiterite (SnO,) Lindgren calls attention to
the work of Winkler, Sandberger, and Verbeek, which shows that tin
‘may be held in solution and deposited at ordinary pressure by thermal
waters.”” However, in this connection it is to be remembered that both
Vogt and Beck strongly insist that cassiterite is formed, to use the words
of Vogt, “by eruptive after-action.” Vogt says of the cassiterite veins that
their “material contents were extracted from the not yet fully congealed
”» Further, Vogt and Beck, if I understand them correctly, follow-
granite.
ing Elie de Beaumont and Daubrée, regard the extraction from the “not
yet fully congealed granite” as accomplished by pneumatolytic processes.
Upon the point that tin ore may be thus formed I express no opinion.
I do not doubt that tin, like other metals, has been transported by
underground waters, and from such solutions precipitated as cassiterite.
As is well known, tin may be readily carried by water as the sulphate,
nitrate, or chloride, and doubtless is also transported in other forms. From
stannic solutions at ordinary temperatures hydrated stannic oxide is thrown
down by the alkaline carbonates. It may be that at the relatively high
@Vindgren, Waldemar, Metasomatic processes in fissure veins: Trans. Am. Inst. Min. Eng., vol.
30, 1901, p. 625.
>Vogt, J. H. L., Problems in- the geology of ore deposits: Trans. Am. Inst. Min. Eng., vol. 31,
1902, p. 135.
1128 A TREATISE ON METAMORPHISM.
temperatures and pressures which exist in the deeper parts of fissure veins,
this reagent would result in anhydrous stannic oxide, or cassiterite; but
this is a mere suggestion. The conditions under which anhydrous
stannic oxide is precipitated from its solutions should be experimentally
investigated.
In conclusion it may be said that one oxide which very probably is
deposited by ascending waters as an ore is cassiterite.
CARBONATES.
Aside from hydrosulphuric acid, the acid of great importance in the
deposition of ores is carbonic acid. This, as is well known, is indeed
the dominant acid contained in issuing underground waters. ‘This point
both Le Conte* and Posepny” strongly emphasize. I have already pointed
out sources for the excess of carbon dioxide held in the underground
waters. (See p. 678.)
A very interesting confirmation of the theory given on the pages
referred to regarding the liberation of silica by the process of carbonation
near the surface and the liberation of carbon dioxide probably by the
process of silication at depth is furnished by the Geyser mine of Custer
County, Colo., described by Emmons.’ Here waters were analyzed from
the 154-meter and the 615-meter levels. The surficial waters contain ten
times as much silica as the deep-seated waters, and the deep waters hold a
greater quantity of carbonic acid than the vadose circulation.
The carbonates which are most exteusively deposited in the veins of.
the first concentration are those of calcium and magnesium as calcite
(CaCO) and dolomite (CaCO,.MgCO,). While calcite and dolomite are
the dominant carbonates in the gangue, the carbonates of the other alka-
line earths, strontianite (SrCO;) and witherite (BaCQ,), are also well known.
The other important carbonates are those of iron—siderite (FeCO,)—and
of manganese—rhodochrosite (MnCO,). Of course, it is well known that
carbonate of zinc (smithsonite) and carbonates of copper (azurite and mala-
chite) occur abundantly as ores; but these are mainly secondary products
of descending waters and do not come under the subject now being consid-
«Le Conte, Joseph, Genesis of metalliferous veins: Am. Jour. Sci., 3d ser., vol. 26, 1883, p. 11.
b Posepny, F., The genesis of ore deposits: Trans. Am. Inst. Min. Eng., vol. 23, 1894, p. 287.
¢Emmons, S. F., The mines of Custer County, Colo.: Seventeenth Ann. Rept. U. 8. Geol. Survey,
pt. 2, 1896, pp. 460-464.
THE GANGUE MINERALS. 129
ered, the deposits of a first concentration by ascending waters. It has been
explained in various parts of this treatise that the two great rock-making
acids are carbonic acid and silicic acid, and further it has just been explained
that in the deep-seated zone carbonic acid is extensively produced. Thus
it is entirely natural that so far as the metals are somewhat soluble and can
travel as carbonates, they will do so. This applies especially to the alkalies
and the alkaline earths, and to iron and manganese. When carbonate solu-
tions containing these various metals rise in the trunk channels of circula-
tion with diminishing temperature and pressure, it naturally follows that the
less soluble of the carbonates are thrown down in large amounts. These
less soluble carbonates are calcite, dolomite, siderite, and rhodochrosite.
The carbonates of sodium and potassium are not thrown down, because
these compounds are so readily soluble.
In conclusion it may be said that the carbonates, as original deposits
by ascending waters, are essentially gangue minerals, not ores. So far as
I know none of them are mined as ores within the United States, although
in some places siderite and rhodochrosite, as deposits of the first concentra-
tion by ascending waters, may be an unimportant source of iron and man-
ganese ores.
Excellent illustrations of rhodochrosite as a gangue mineral are fur-
nished by the San Juan district, Colorado. This carbonate is generally
found in that area, but is an especially important and abundant gangue
mineral at the Smuggler Union mine.
SILICATES.
The silicates are well known to be important gangue minerals deposited
by the deep circulation. If one were to give a list of the silicates which
have been found as gangue in veins in various parts of the world, it would
comprise a large proportion of the entire list of silicates. This is an inevi-
table consequence of the facts that, first, the dominant acid of nature is
silicic acid, and second, the silicates for the most part are rather insoluble,
many of them extremely so. It has been explained at various places that
silica as colloidal silicic acid travels in great quantities through the rocks,
being produced on an enormous scale in the belt of weathering by the
process of carbonation. (See pp. 173-177, 480.) Silicic acid formed
in this manner and released by other reactions thus enters the belt
of cementation and finds its way to the trunk channels of circulation, to
1150 A TREATISE ON METAMORPHISM.
which come also metals in various combination. Where silicic acid and
the salts of the metals thus meet it is certain that the silicates form exten-
sively, according to the law of mass action that the most abundant com-
pounds are most likely to be precipitated, and in consequence of the law
that the most insoluble compounds are likely to be thrown down.
Of the silicates in fissure veins Lindgren regards the following of
sufficient importance to be cited: Muscovite, biotite, chlorite, pyroxene,
amphibole, garnet, epidote, orthoclase, albite, tourmaline, topaz, kaolinite,
and zeolites.”
Linderen’s description of the occurrence of these minerals shows that
the hydrous silicates are dominant. While a considerable number of the
anhydrous silicates form, they are very subordinate in amount. The signifi-
cance of these facts is discussed on pages 1055-1060. The abundance
and prevalence of the hydrous silicates are beautifully illustrated by the
copper deposits of the Lake Superior region. Here there has been scarcely
any discoverable effect of descending waters, as recent glacial erosion has
entirely removed the belt of weathermg of pre-Glacial time. Pumpelly
gives the following list of hydrous silicates as occurring in this region:
Analcite, apophyllite, chlorite, datolite, delessite, epidote, laumontite,
prehnite.’ This list has since been supplemented by Hubbard, the following
being added: Chabazite, chlorastrolite, chrysocolla, harmotome, heulandite,
lennhardite, mesotype, natrolite, pectolite, saponite, stilbite, thomsonite.°
While in the Lake Superior copper deposits the-dominance of the hydrous
silicates is clear, the gangue minerals are not restricted to them, but com-
prise anhydrous minerals—orthoclase and wollastonite being mentioned.
It is to be noted that the silicate gangue minerals retain even some
of the alkalies, i. e., sodium and potassium. Sodium is found in analcite,
chabazite, chlorastrolite, mesotype, natrolite, pectolite, stilbite, and thom-
sonite; and potassium is found in apophyllite, chabazite, harmotome, and
orthoclase. By the union of the alkalies with other bases and with silica it is
possible to produce compounds insoluble in the liquids present, and where
this could take place as demanded by the laws of chemistry, it has occurred,
«Lindgren, Waldemar, Metasomatic processes in fissure veins: Trans. Am. Inst. Min. Eng., vol.
30, 1901, pp. 607-615.
>Pumpelly, Raphael, The paragenesis and derivation of copper and its associates on Lake Supe-
rior: Am. Jour. Sci., 3d series, vol. 2, 1871, pp. 254-255.
¢ Hubbard, L. L., Macroscopic minerals of Michigan: Rept. State Board Geol. Survey Michigan,
1891-92, p. 176.
THE GANGUE MINERALS. 113
and thus even the most soluble of the abundant elements have been fixed
in the veins to some extent.
While the silicates are dominantly gangue minerals, silicates of zine,
manganese, and nickel occur as ores. Silicate of zine occurs both as
willemite (2ZnO.Si0,) and as calamine (H,O.2ZnO.SiO,). The most
notable occurrence of willemite is in the New Jersey zine deposits.“ In
reference to this mineral the question arises whether it was formed as an
original precipitate from the deep circulation or was produced by dehydra-
tion of calamine in the zone of anamorphism, the calamine having been
formed originally in the belt of weathering in the same manner as in the
Mississippi Valley. Whether calamine occurs as zinc ore as an original
deposit by ascending waters [do not know. The abundant development
of calamine in consequence of the reactions of the belt of weathering is
very well known, as explained on pages 1144, 1147-1148.
Manganese silicate occurs rather abundantly as rhodonite (MnO.SiO,).
For the most part, however, this is a gangue mineral rather than an ore, its
occurrence being well illustrated at Butte, Mont., and in the Smuggler _
Union mine and other places in the San Juan district, Colorado. Although
sometimes found in considerable quantities, it is not now used in the United
States as a source of manganese on account of its high percentage of silica.’
The most notable occurrence of nickel as a silicate is in New Caledonia.
Packard says: “The nickel ore of New Caledonia is a hydrated silicate of
nickel and magnesium (garnierite) which occurs intimately associated with
serpentine. ... The field observations showed that the nickel silicate is
clearly an alteration product of the serpentine. ... lot springs have
played a conspicuous part in furthering the decomposition of the nickel-
bearing serpentine into garnierite and the other products.”* Packard also
notes the occurrence of nickel silicate at Riddles, Oreg., in North Carolina,
in the Urals, and in Silesia. In all cases it is a secondary alteration
product usually derived from a serpentine which itself was derived from a
nickeliferous olivme. From Packard’s descriptions it appears probable that
the nickel silicate mined is an oxidized product formed by the reaction of
«Kemp, J. F., The ore deposits at Franklin Furnace and Ogdensburg, New Jersey: Trans. New
York Acad. Sci., vol. 13, 1893-94, p. 90.
> Penrose, R. A. F., jr., Manganese, its uses, ores, and deposits: Ann. Rept. Geol. Survey Arkansas
for 1890, vol. 1, 1891, p. 85.
¢ Packard, R. L., Genesis of nickel ores: Mineral Resources, 1893, pp. 170-171.
1132 A TREATISE ON METAMORPHISM.
descending waters upon a sulphide of nickel, the latter being deposited by
the deep circulation.
In conclusion, it appears that the silicates deposited by ascending
waters, like the carbonates, are essentially gangue minerals. The only sili-
cates deposited by ascending waters which may be ores are those of zine,
manganese, and nickel. It yet remains to be shown that any of these
compounds are sufficiently abundant as first concentrates by ascending
waters to serve as important ores.
CRITERIA FOR DISCRIMINATING DEPOSITS OF THE DEEP CIRCULATION.
The ascending waters depositing ores are necessarily those of the deep
circulation. The criteria by which ores may be determined to have been
deposited by ascending waters from a deep circulation have not yet been
fully worked out, but certain suggestions may be made.
The first and most conclusive criterion discriminating ores produced by
deep waters is that at the present time the waters are observed to be ascend-
ing. Locally, as in the Comstock lode, great volumes of hot water are rising,
and doubtless ores are still being deposited. In such cases the evidence in
favor of deposition from ascending waters is complete. A characteristic of
ascending streams of the deep circulation is their independence of rainfall
and other local conditions. The volume and the temperature of the water
remains nearly constant from one year’s end to the other. The best illustra-
tion of such an occurrence known to me is that furnished by the Comstock
lode, already mentioned. An almost equally good case is that in the Smug-
gler Union mine at Telluride, Colo., where, about 600 meters below the sur-
face, in the Pennsylvania shaft, a stream of water at a definite temperature
rises with uniform flow entirely unaffected by the seasons. The regularity
of the temperature and volume of this water have led to the use of the stream
for mine purposes. Equally good instances of an extended rising circulation
independent of local precipitation are found in the Joplin district of Mis-
souri. Here in practically all of the sulphide mines below a very moderate
depth there is unanimous agreement among the miners that the volume of
water which rises is not influenced in the slightest degree by precipitation.
Whether the season be dry or wet the steady flow of rising water must be
handled. Where for some reason the process of concentration of the ores
has ceased, as is doubtless the case in many regions, we do not find
DEPOSITS OF THE DEEP CIRCULATION. NBS
evidence of the ascending waters. One of the common causes for the
cessation of a vigorous underground circulation is the nearly complete
cementation of the openings by the circulation. When the openings become
subeapillary the circulation necessarily ceases.
Where there exist underground streams with steady flow the channels
which they occupy are usually clean and fresh. ‘They are lined with beau-
tiful clear crystals without a stain. Along the channels there may be open-
ings or even caves lined with crystals of various sizes, some of them reaching
a foot or more in diameter. Such vugs, openings, and caves are superbly
illustrated in the Joplin district of Missouri, where, in consequence of the
lowering of the level of ground water by mining, such openings may be
visited. When first freed from water the crystals lining the cavities are as
untarnished as polished plate glass. The same thing may be seen at the
Enterprise mine at Platteville, Wis., in the Upper Mississippi Valley. At
this locality, as well as many in Missouri, the ores are clearly associated
with the trunk channels of circulation. The miners in their operations
follow the channels as guides, working against the streams. At places
along a channel the water is concentrated into a single stream, and at such
places there is likely to be comparatively narrow belts of very rich material
along the walls. In other places the stream is subdivided among many
small openings, and at such places the ore is dispersed.
Probably this criterion of ascending waters and freshness of the depos-
ited material furnishes the best single line of evidence in favor of a deep
circulation. But one must not conclude that the deposits of the ascending
waters extend, unaffected by descending waters, to the very surface.
For instance, while the present mining in the Joplin lead and zine district
of Missouri is in the belt in which descending waters have produced com-
paratively little effect, i. e., from 30 to 60 meters deep, at the surface and
for a few meters below the surface descending and ascending waters mingled.
Together they moved off laterally, and in consequence of their joint action
the oxidized products and the galena deposits mentioned on pages 1444—
1446 were produced. In this particular area the distance to which the
descending water produced an effect is very smal, but such is not univer-
sally the case. Where the vertical elements are greater and the relief is
steeper, the descending waters working against the ascending currents may
produce an effect to a very considerable depth.
iLike A TREATISE ON METAMORPHISM.
The very marked difference between deposits made by deep waters
and those made by shallow waters is beautifully illustrated by the Missouri
district. Thus in Joplin, where pumping has lowered the level of ground
water by 30 to 45 meters, the once clean openings, formerly below the
ground-water level, are now in the belt where waters are descendmg., The
caves which when first entered showed crystals fresh and untarnished are
now coated with oxide of iron and other products carried by the descend-
ing waters. Still more striking is the contrast between the clean mines in
which waters are ascending and the dirty ones in other areas in which
descending waters have produced an important effect to a considerable
depth. The latter mines are on smithsonite and calamine, and are muddy
and dirty to the last degree. hick, sticky, iron-stained clay is everywhere,
adhering thickly to one’s boots. The amazing contrast between the con-
ditions of the two classes of deposits is illustrated by the sulphide mines
of the Joplin:eamp and the oxidized product mines of the Aurora camp
About 2 kilometers distant from Aurora is Sand Ridge, where the capping
Carboniferous impervious shale still remains. Here the descending waters
have not had a chance to enter, and the deep-seated waters are still rising,
and in this mine, which is only a short distance from the dirty oxidized
mines, are found the same clean sulphides and beautiful crystal-lined
cavities so characteristic of the mines of Joplin.
In many cases, where currents are still rising, the ores are still being
deposited. This is believed to be true in the lead and zine district of
southwestern Missouri, in the Comstock lode, and is known to be the
case at Steamboat Springs and Sulphur Bank. Where, in mines, streams
of clear water are found, with a uniform flow, unaffected by the seasons,
and deficient in oxygen, the evidence seems conclusive that the ore deposits
associated with such a circulation are the products of the deep circulation.
But in many distriets the conditions of circulation which resulted in the first
concentration have ceased, the openings having been perhaps fully cemented
at the time of concentration, thus causing the circulation to die out. In
such cases the present circulation may be of an entirely different character,
even if the first main concentration was that of deep waters.
A second criterion indicating that ores are deposited from the deep
circulation is the phenomenon of crustification, so strongly emphasized by
Posepny. (Fig. 28.) Posepny argues that the regular laminar arrangement
DEPOSITS OF THE DEEP CIRCULATION. NS)
of different minerals or different combinations of minerals along the walls
of a vein or along one wall is evidence that the waters have risen from
a deep-seated source, the mineral solutions changing from time to time and
so resulting in the banded or comb structure.* I follow Posepny in the
belief that, in most cases, such regular crustification is especially charac-
teristic of ores deposited by ascending waters. But undoubtedly, in many
cases there are superimposed upon the primary crustification later similar
minerals deposited by descending waters, as fully explained on subsequent
pages. (See pp. 1146, 1151, 1152, 1154-1157.) Such secondary sulphides
have been described at many localities, but so far as I know ne one has
claimed that such descending waters have added definite continuous sec-
ondary crusts, one above the other. Certainly it has never been shown that
an ore deposit produced solely by descending waters shows a definite
b
IIR
a
ey
a)
Y
Fig. 28.—Cross section of banded vein near the London shait, Mineral Point, Colo. a, country rock; ), quartz and
.chalcopyrite; c, tetrahedrite; dd’, quartz; e, galena. After Ransome.
crustification. It must be remembered that the existence of crustification
is no evidence that the alternating minerals contained in the veins have not
been modified and enriched by descending waters. Indeed, this is very
frequently the case. Therefore crustification is an evidence of a first con-
centration by ascending waters, but is not evidence that descending waters
have not since greatly modified the character of the deposit.
A third criterion which is of great importance in showing that ores are
deposited from the deep circulation is the somewhat even tenor of values
vertically. It is explained (pp. 1139-1143) that a special characteristic of
descending waters is to produce a rich upper belt which diminishes in value
“with depth. It is shown, also, that one of the most characteristic effects of
«Posepny, F., The genesis of ore deposits: Trans. Am. Inst. Min. Eng., vol. 23, 1894, pp. 254-262.
1136 A TREATISE ON METAMORPHISM.
descending waters is to produce a differentiation of the products, in a verti-
cal sense, one mineral or one combination of minerals being at one horizon
and another mineral or combination of minerals being at another horizon.
Commonly these horizons have a depth varying from a few meters to many
or even hundreds of meters. This is very different from the phenomena
of crustification, in which the change of minerals is lateral within very
short distances at right angles to the walls.
It is certain that for various reasons, as fully explained under “Ore-
shoots,” there are great variations vertically in the richness of deposits pro-
duced by deep waters. A case of such extreme variation is that of the
Comstock lode of Nevada, where there were numerous great bonanzas
separated from one another by comparatively lean material; and yet in this
yein the evidence of water ascending under pressure is most conclusive,
with no evidence of descending water; but the variations in tenor in
deposits of the deep circulation are in both directions. The bonanzas may —
be found lower than the poorer material, and vice versa. In contrast with
this, descending waters produce an upper rich belt which is not again dupli-
cated below. While the deposits of the deep circulation may vary greatly
in values, in many cases there is remarkably even tenor for great vertical
depths. The best illustration of this known to me is the copper deposits
of the Lake Superior region which are known to extend from the surface
to a vertical depth of 1,600 meters, equivalent to 2,066 meters along the
dip, 40°, with values sufficient to warrant working throughout the distance.
As already explained, the copper was in all probability reduced and
precipitated directly as metallic copper from upward-moving cupriferous
solutions. The reducing agents were ferrous solutions derived from the
iron-bearing silicates, and ferrous compounds in the solid form, magnetite
and silicate. It is well known that metallic copper once formed is but
slowly affected by the oxidizing action. Oxidation has m fact occurred in
the Lake Superior region, but not to an important extent. An oxidized
belt may have formed in pre-Glacial time, but if so it was swept away
by glacial erosion, and a sufficient interval has not elapsed to form another.
The ore deposits now worked have apparently remained practically un-
changed since their first concentration. In this fact we have the explana-
tion of the extraordinary depths to which these deposits extend.
DEPOSITS OF THE DEEP CIRCULATION. Use
Lindgren cites the gold veins of Nevada City and Grass Valley, Cali-
fornia, as another case of constant tenor. According to him,” ‘It can be
confidently stated that there is no gradual diminution of the tenor of the
ore in the pay shoots below the zone of surface decomposition,” although
within the same shoot there are many and great variations in richness.
This statement is applicable to deposits which reach a vertical depth of 500
or 600 meters. If Lindgren is correct in thinking that the gold-quartz
veins of the Sierra Nevada do not diminish in depth below an extremely
superficial upper zone, this would be a case in which sulphuret ores were
sufficiently concentrated by ascending waters alone to afford workable
ore deposits.
In conclusion, it may be said that even tenors, in a vertical direction
for any considerable depth, is very strong evidence of a concentration by
ascending waters; but the reverse can not be said, that variability in a
vertical direction is evidence that the depesits were not deposited by
ascending waters.
Fourth. The absence of oxidized products in deposits, especially of
limonite, favors the view that the ores were deposited by the deep circula-
tion and were unaffected by secondary concentration; but this criterion is
severely limited by the fact that enrichment of the sulphides takes place
below the level of ground water in consequence of secondary descending
solutions. An excess of base sulphides prevents the formiation of the more
palpable evidence of oxidizing waters.
Another difficulty in reference to using oxides as a criterion is that, in
consequence of denudation, the ores deposited by the deep circulations rise
into the belt of oxidation and are modified by the oxidizing waters to a
variable extent. Very early in the process such oxidized products as hema-
tite and limonite form. The ores are still essentially those of the deep
circulation. From this stage to that in which the secondary action of
oxidizing waters is of such consequence as to produce abundant hydrated
hematite and limonite there are all gradations; but in many cases the
secondary effect of descending, oxidizing waters can be allowed for and a
judgment reached as to the nature of the deposit before it was modified by
descending waters.
“Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California: Seven-
teenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1896, p. 168.
MON XLVII—04——72
1138 A TREATISE ON METAMORPHISM.
In the case of a given ore deposit, by wise combination of the criteria
given, it is in general possible to make a somewhat reliable judgment as to
whether or not the ore was produced solely by ascending waters.
GENERAL STATEMENTS.
The foregoing statements explain to some extent the source, nature, and
manner of deposition of the compounds deposited by ascending waters. But
it is not the intention here to discuss their application to the known dis-
tricts. This I do not attempt, because I lack the necessary accurate obser-
vations upon which such a discussion should be based. To tellin what man-
ner the deposits of an individual district are formed requires very detailed
investigation of that district.
While it is not the purpose here to take up the solution and deposition
of the compounds which occur in individual ore deposits, it is well again to
recall the law of mass action. Other things being equal, those compounds
which are abundant will be dissolved in larger degree during the downward
course of the waters, and will be most abundantly precipitated in the trunk
channels. It is well known that, with the exception of aluminum, which
does not form a sulphide, iron is the most plentiful of all the metallic com-
pounds in the crust of the earth, and therefore iron sulphide occurs in greater
abundance than the sulphide of any other metal. It is well known that in
many cases the deeper a mine goes below the level of ground water the
greater becomes the proportion of iron sulphide and the less that of the
other metals. As a result of this, combined with increased cost of working,
it frequently does not pay to mine a deposit beyond a certain depth. The
law of mass action explains the abundance of the iron sulphide; it does not
explain the frequent relative increase of the iron sulphide and the decrease
of more valuable sulphides as one passes from the level of ground water into
deeper workings. To explain this we must take into account the effect of
the downward-moving waters, discussed under the succeeding heading.
We have now seen that the zone of fracture is traversed by the perco-
lating waters; that metalliferous materials taken into solution by the down-
ward and lateral moving waters are carried to the trunk channels of
underground circulation, and that in these trunk channels the metalliferous
materials are precipitated in various ways. Thus a first concentration by
ascending waters, giving sulphurets, tellurides, and metals of some of the
elements, is fully accounted for.
SECOND CONCENTRATION BY DESCENDING WATERS. 1139
In some cases the deposits thus produced are rich enough to be of
economic importance. In these cases, which undoubtedly exist, but which
perhaps are less numerous than.one might at first think, a first concentra-
tion by deep waters has been sufficient.
SUBCLASS 2. ORES PRECIPITATED FROM ASCENDING AND DESCENDING AQUEOUS
SOLUTIONS.
Thus far ores precipitated by ascending waters alone have been consid-
ered. However, many of the ores thus produced have been profoundly
modified by the action of descending waters.
Where the point of exit of the ascending waters of the trunk channels
is in a valley or near the level of surface drainage the waters may reach
the surface. However, where the openings are below slopes, ascending
waters ordinarily do not continue to the surface, but make their way
laterally from the trunk channels at and below the level of ground water.
(See fig. 26.) Above the level of ground water, and frequently for a
certain distance below it, the movement is downward in the openings. The
water thus moving downward includes not only that which directly passes
into the trunk openings at the surface, but a much larger quantity which”
converges into them from the smaller openings on all sides.
In regions in which mining is going on denudation has ordinarily
truncated the veins for considerable depths, in many cases to hundreds or
even thousands of meters. It is therefore clear that, in a majority of cases,
the upper portions of the fissures in which the waters are now descending
were in all probability much deeper below the surface, and therefore the
waters in many of the larger fissures were once ascending. During the
time the water was ascending the first concentration of sulphurets and other
products took place. But as a result of the downward migration of the belt
of weathering and the downward movement of water in that belt alteration
and secondary concentration of ore deposits have taken place. This second
concentration of ore deposits is of the greatest consequence, and I believe
largely explains the frequent greater richness of the upper 50, or 100, or
500 meters, and in some cases 1,000 meters, as compared with lower levels.
As soon as denudation results in the transfer of the sulphides,
tellurides, and metals of the first concentration into the belt of weathering
they are subjected to the action of the descending waters bearing oxygen
1140 A TREATISE ON METAMORPHISM.
and carbon dioxide. As denudation continues the belt of weathering
continues to migrate downward. As a consequence of the reactions of the
belt of weathering, combined with denudation, the character of the upper
part of the lode is greatly changed, there being produced in the upper part
of it a second concentration. If this process continues long enough under
favorable conditions, the deposits formed may be divided into three belts:
(1) Above the level of ground water, and in some instances extending
somewhat below it, is a belt largely composed of oxides of manganese, iron,
and copper; carbonates of lead, zinc, and copper; hydrated silicates of
nickel and zine; chloride of silver; metallic gold, silver, and copper; and
residual enriched sulphides and tellurides; all with the associated gangue
minerals. (2) At and below the level of ground water, and in some cases
extending somewhat above it, is a transition belt composed of rich
sulphides of silver, mercury, copper, lead, zinc, nickel, ete.; tellurides
of gold and silver; and free gold, silver, and copper; with these are
subordinate amounts of oxidized products and of course the associated
gangue minerals. (3) Deeper down is a belt in which iron sulphides,
pyrite, and pyrrhotite preponderate; but in which are considerable amounts
of the more valuable sulphurets, the tellurides of gold and silver, and free
gold, silver, and’ copper. Between the three classes of material there are
gradations. The oxidized belt passes gradually into the rich sulphide and
telluride belt; the rich sulphide and telluride belt passes gradually into
the lean sulphide belt. It is not to be supposed that all of the above
products are to be found in a single lode. The development of the belts
is due to a complicated set of reactions caused by descending waters.
These reactions will be more fully appreciated when individual combina-
tions of metals are considered. Here only a brief general statement will
be made.
In the belt of weathering the reactions which transform the original
products—sulphides, tellurides, and metals—are oxidation, hydration, and
carbonation, the fundamental reactions of the belt. In this belt the forma-
tion of the materials mentioned is but the result of the application of these
general processes to the exceptional geological products, ore deposits.
Before hydration and carbonation’can take place oxidation must occur. If
the sulphides be equally abundant, the sulphide which is most easily oxidized
is the first to disappear. The order of disappearance for the metals in
SECOND CONCENTRATION BY DESCENDING WATERS. 1141
this hypothetical case, as shown by the work of Anthon* and Schiirmann,’
omitting the less important elements, is therefore manganese, arsenic, iron,
cobalt, nickel, zinc, lead, tin, antimony, copper, silver, mercury. It is,
however, understood that the oxidation of an easily destroyed sulphide is
not complete before the oxidation of a more refractory sulphide has begun.
All of the sulphides are being oxidized all the time, but the more readily a
sulphide is oxidized the more rapidly it is destroyed.
The sulpbur of a sulphide may be oxidized without the oxidation of
the metal, in which case a metal is produced by the process of oxidation.
At the same time the sulphur is oxidized, or subsequently, the metal may
also be oxidized, and thus oxides be formed. As soon as the oxides are
produced they may unite with carbon dioxide and form carbonates, or with
silicic acid and form silicates. If water also unites with the compounds the
carbonates and silicates are hydrated. But the carbonates and silicates
may also be produced in a different manner. The sulphides as originally
oxidized may be tranformed to sulphates. If thus changed, the sulphates
may immediately react upon carbonates or silicic acid, producing carbonates
or silicates, at the same time forming sulphates of other elements, such as
barium, strontium, or calcium. The evidence that reactions of this kind
have taken place upon an extensive scale in veins is perfectly clear. In
mine waters sulphates of copper, zinc, and iron occur very frequently. In
some cases the amount of sulphate of copper in such water is so great that
it is worth while to run it over iron and thus precipitate the copper.
Further evidence of the formation of the sulphates is shown by the frequent
precipitation of basic ferric sulphates in the veins, as, for instance, at
Cripple Creek.’ That the sulphates produced by oxidation of the sulphides
react upon the other elements is shown by the frequent development in the
upper part of the veins of such gangue minerals as barite, celestite, gypsum,
and oceasionally even magnesium sulphate.
An admirable case illustrating this principle is described by James
Douglas. In speaking of the Arizona mines he says that gypsum is abun-
« Anthon, E. F., Ueber die Anwendung der auf nassem wege dargestellten Schwefelmetalle bei der
chemischer Analyse: Jour. f. Prak. Chem., vol. 10, 1837, pp. 853-356.
>Schtirmann, E., Ueber die Verwandtschaft der Schwermetalle zum Schwefel: Liebig’s Ann. d.
Chem., vol. 249, 1888, pp. 326-350. f
¢Penrose, R. A. F., jr., Mining geology of the Cripple Creek district: Sixteenth Ann. Rept. U.S.
Geol. Survey, pt. 2, 1895, p. 130.
1142 A TREATISE ON METAMORPHISM.
dantly present in the driest mines of the Southwest, but at Bisbee, a more
humid area, it is absent.” Probably the reaction producing the sulphate
has taken place extensively in both areas, but in the driest area there was
not sufficient amount of water to dissolve the calcium sulphate formed and
hence it was precipitated as gypsum. The sulphates may even react upon
the aluminum salts and form hydrous sulphate of aluminum. This occurs
in the gold veins of California.’
The oxidized belt in some cases may be altogether above the level of
ground water, but in other cases where the descending oxidizing solutions
are strong may extend somewhat into the belt of cementation. An excel-
lent case illustrating the almost perfect coincidence of the oxidized belt
with the belt of weathering is furnished by the Monte Cristo district,
described by Spurr. He says, ‘‘the zone between the surface of the ground
and the lowest level of the ground-water surface is practically coincident
with the zone of oxidation.” °
It has been shown how the upper oxidized belt is produced, and next
the manner of formation of the rich sulphide and telluride belt will be consid-
ered. The oxidized products, oxides, carbonates, and sulphates, may either
in situ react upon the unaltered sulphides, or the carbonates, sulphates, and
other oxidized compounds may be transported downward in solution and
there react upon the sulphurets. In either case the result is to produce a
richer sulphuret. The reactions may be between an oxide or a salt of a
metal and its sulphide, for instance between the oxide or sulphate of cop-
per and the sulphide of copper, as shown by the following equations:
6CuS-++20u,0=5Cu,8+80,
and
6CuS-+2Cu,S80,+3H,0=5Cu,S+2H,S0,+H,S0,
The reaction, however, may be between the oxide or salt of one metal and
the sulphide of a second, as for instance, between zine oxide, sulphate, or
carbonate and iron sulphide. In this case a sulphide of the first metal is
« Douglas, James, The Copper Queen mine, Arizona: Trans. Am. Inst. Min. Eng., vol. 29, 1900,
p. 535.
b Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California: Seven-
tenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1896, p. 120.
¢Spurr, J. E., The ore deposits of Monte Cristo, Washington: Twenty-second Ann. Rept. U. 8.
Geol. Survey, pt. 2, 1901, p. 859.
SECOND CONCENTRATION BY DESCENDING WATERS. 1143
produced. Thus Stokes has shown that zine carbonate and sodium
carbonate together react upon iron sulphide as follows:
8FeS,+14ZnCO,+-Na,CO,=14Zn8+4Fe,0,+Na,8,05-+15C0,
The particular reactions in an individual ease depend upon the relative
proportions and solubilities of the compounds present, upon the existing
conditions of pressure and temperature, the amount of oxygen, ete. This
will more clearly appear upon subsequent pages.
The deep belt in which the sulphides of iron are preponderant is that
of the first concentration already considered on previous pages.
The concentrations by ascending and descending waters have been
spoken of as if they were successive. In some instances this may be the
case; but frequently it is much more probable that ascending and descending
waters are at work upon the same fissure at the same time, and that their
products are, to a certain extent, simultaneously deposited. For instance,
under the conditions represented by fig. 26 there is taking place in the
lower part of the fissure a first concentration by ascending waters and, in
the upper part, a second concentration by descending waters. Between
the two there is a belt in which both ascending and descending waters are
at work. In an individual case the rich upper part of an ore deposit which
is worked may now be in the place where ascending waters alone were first
acting, where later, as a consequence of denudation, both ascending and
descending waters were at work, and still later, where descending waters
alone were at work. The more accurate statement for this class of ore
deposits, therefore, is that deep or ascending waters are likely to be potent
in an early stage of the process, that both ascending and descending waters
may work together at an intermediate stage, and that descending waters
are likely to be important in the closing stage.
The above general statement may perhaps be better understood if
supplemented by a consideration of specific associations of the metals. The
associations which are chosen for illustrative purposes are as follows:
Associations (1) of lead, zine, and iron; (2) of copper and iron; (8) of silver
and gold with the base metals.
1144 A TREATISE ON METAMORPHISM.
ASSOCIATION OF LEAD, ZINC, AND IRON COMPOUNDS.
Tn order to understand the relations of the lead, zinc, and iron com-
pounds where they occur together in ore deposits, it seems advisable to
consider an individual region rather than tomake a general statement.
An excellent illustration of the association of these metals is furnished
by the deposits of the Mississippi Valley, and this region will therefore be
considered.
FACTS OF OCCURRENCE.
In the Mississippi Valley, as is well known, in openings in limestones,
lead and zine minerals are associated with marcasite, pyrite, and some
chalcopyrite.“ Calcite and dolomite are abundant gangue minerals, as
would be expected. Barite is also a common gangue mineral and locally
is very plentiful. There are other gangue minerals which will not be
taken into account. Since chalcopyrite is very subordinate, it will not again
be alluded to.
The vertical order of occurrence in the region is commonly as follows:
Above the level of ground water in the belt of weathering the dominant
valuable minerals are galena and the oxidized ores—smithsonite, calamine,
cerussite, and anglesite. Other oxidized ores occur in subordinate amount,
but they will be ignored. The cerussite and anglesite frequently incrust
the galena or are in crystals upon it. With the smithsonite and calamine
there is some sphalerite. The smithsonite and calamine may extend 5 or
10 meters below the level of ground water, but deeper the oxidized products
almost wholly disappear. The presence of oxidized products below the
level of ground water is due in some cases to the occurrence of the material
along a main channel of downward-percolating waters or to the well-known
general downward movement of oxidizing water somewhat below the level
of ground water; in other cases probably is due to a present higher level of
ground water than in pre-Glacial time, as a result of depression and valley
filling at the close of the Glacial epoch; and in still other cases is probably
due to reactions between oxidized lead salts and sphalerite. (See p. 1150.)
Below the galena and the oxidized ores are sphalerite and galena together,
the former being dominant except in the Southeastern district of Missouri.’
«Chamberlin, T. C., The ore deposits of southwestern Wisconsin: Geol. of Wisconsin, vol. 4,
1882, pp. 380-393.
> Chamberlin emphasizes the inferior position of the zinc as compared with the lead and the asso-
ciation of the zine and iron, but he does not consider the positions of these compounds with feference
to the level of ground water. Loe. cit., pp. 488-491.
LEAD AND ZINC ORES. 1145
~ Associated with the sphalerite and galena are large amounts of marcasite
and pyrite. For much of the region the workings have not extended far
below the level of ground water, but in certain districts they have reached
a considerable depth. In some places, as at Granby, Mo.,* deep exploration
has shown pyrite and marcasite to dominate at the lowest level reached.
Under the present heading a full consideration of the concentration by
ascending waters is excluded; but in order to consider the secondary work
of descending waters it is necessary to outline the work of the first concen-
tration by ascending waters.
In the Upper Mississippi Valley the order of occurrence of the minerals
at many openings from the wall to the center of the veins or to the druses
is marcasite, ferriferous sphalerite, galena in cubic crystals; or is sphalerite,
galena. In some cases the order is marcasite, blende, these two being
repeated perhaps several times. Not infrequently the galena of the first
succession is followed by mareasite and (very subordinate in quantity)
galena in octahedral crystals. Occasionally between the galena and the
second marcasite is ferriferous sphalerite in small amount. If the druses
are not fully closed by the sulphides there usually follows calcite, and
occasionally barite on the calcite. Thus if there were a full succession at
any one place, it would be (1) marcasite, (2) ferriferous sphalerite, (3) galena
in cubic crystals, (4) ferriferous sphalerite, (5) marcasite, (6) galena in
octahedral crystals, (7) calcite, (8) barite.
In the Lower Mississippi Valley sequences similar to those in the
Upper Mississippi Valley occur in certain places, but in the best known and
most productive district, that of southwestern Missouri, there seems to be
no regular sequence for the sulphides. According to Bain, the general
order of deposition for this district, within the openings, was dolomite,
metallic sulphides, and chert. ‘‘To some extent these processes were con-
temporaneous or recurrent, but as a whole they occur successively in the
order named.” As to the relations of the sulphides to one another, Bain
says: ‘“Blende rests on galena and galena on blende indiscriminately, and
both cover and in turn are covered by iron sulphide.”’ Also blende and
galena are in many cases intimately intermingled.
a Bain, H. F., with Van Hise, C. R., and Adams, G. I., Preliminary report on the lead and zine
deposits of the Ozark region: Twenty-second Ann. Rept. U. 8. Geol. Survey, pt. 2, 1901, p. 162.
> Bain, H. F., cit., pp. 151-152.
1146 A TREATISE ON METAMORPHISM.
I know of no clearer illustration of the phenomenon of crustification
described by Posepny, and indeed of the general principles of ore deposition
by underground waters, than that furnished by the Upper Mississippi Valley
district. The larger parts of the crustified sulphide products are believed to
be the work of ascending waters. In many cases it appears that there were
two or more cycles of precipitation, so far at least as the sphalerite and galena
are concerned, but the first cycle was by far the more important. As
explained below, it is thought to be probable that the galena in octahedral
crystals (No. 6 of the general succession), and in many cases the sphalerite
(No. 4) and the marecasite (No. 5), are due to a second concentration.
As to the manner of transportation of the metals which in the Missis-
sippi Valley were precipitated as sulphides at the first concentration, it is
believed that the lead and zine travelled mainly as oxidized salts, and
probably largely as sulphates. All the evidence goes to show that in this
region the concentration took place under physical conditions substantially
the same as those which exist at the present time. There is no proof
that the solutions went to any great depth, or that unusual temperatures
prevailed. Nor is there any evidence that abundant alkaline sulphides and
carbonates of the alkalies were present. Therefore, it can not be supposed
that the sulphides were transported as such. Probably they were originally
disseminated through the limestones as sulphides, were oxidized to sulphates
and transported in that form, or as carbonates.
The first precipitation of the ascending salts is believed to have
been accomplished through the agency, direct or indirect, of organic matter.
There is abundant organic material in the shales associated with the ores,
and a less amount in the limestones. While the precipitation of the
sulphides disseminated through the shales may have been largely accom-
plished by the direct reaction of the solid carbonaceous material, the more
common precipitation within the openings was probably accomplished
through the reducing solutions which were produced by the contact of the
water with the organic compounds. Waters which have been in contact
with such compounds, which also bear sulphides, are very likely to contain
hydrogen sulphide. This substance is a direct precipitant of the sulphates
as sulphides, but it is probable that dissolved organic compounds also act
as reducing agents at the point of precipitation in the crevices.
LEAD AND ZINC ORES. 1147
SECOND CONCENTRATION.
It is highly probable that the regular vertical distribution of the
minerals is due to a second concentration mainly controlled by downward-
moving waters combined with denudation.
OXIDIZED ORES.
The oxidized ores include those above or near the ground-water level.
As already noted, the important ones comprise smithsonite, calamine,
cerussite, aud anglesite. These ore bodies have been formed from the
sulphides by oxidation, carbonation, and hydration. The beginning of
these processes is coincident with the appearance of the sulphides above
the level of ground water in consequence of progressive denudation. Lead
sulphide (PbS) and zine sulphide (ZnS), by simple oxidation are changed
to lead sulphate (PbSO,) and zine sulphate (ZnSO,). The lead sulphate
constitutes the mineral anglesite. If this sulphate in solution reacts upon
calcium carbonate, lead carbonate and hydrated caleium sulphate are
produced according to the following equation:
PbSO,-+ CaCO, +2H,0=PbCO,+CaS0,.2H,0.
If zinc sulphate reacts on calcium carbonate, smithsonite is produced
according to the following equation:
ZnSO,+CaCO,+2H,0=ZnCO,+ CaSO,.2H,0.
In this connection it is interesting to note that in Wisconsin the smith-
sonite often occurs in beautiful pseudomorphs after the various forms of
calcite characteristic of the district. In many cases when these pseudo-
morphs are broken they are found to be partly hollow, showing not only
that the calcite was replaced, but that solution of the calcium carbonate
went on faster than the deposition of the smithsonite. In many other
instances the smithsonite is in plates of greater or less thickness over calcite
or limestone, the latter showing solution by unequal etching.
Where the zine sulphate finds abundant amorphous silica in chert the
reaction may be between this compound and the zine salt in the presence
of water, the equation being as follows:
SiO, +27n80,+3H,O=(ZnOH),Si0,+2H,S0,.
1148 A TREATISE ON METAMORPHISM.
In the Upper Mississippi Valley chert is not abundant and in that district
the chief oxidized zinc ore is smithsonite. In the southwestern district of
Missouri chert is very abundant, and the dominant oxidized ore is calamine,
although smithsonite is also there present in important amounts.
The oxidized ores are often very rich and frequently they are concen-
trated in large irregular masses. These features, exceptional as compared
with the sulphides of the first concentration, are due to two processes: First,
through downward transportation by the solutions there may be segregated
in a small vertical distance a large part of the materials which had a wider
vertical distribution as sulphides; and second, residual concentration takes
place—that is, the carbonates and sulphates of lead and zine are much
more insoluble and heavier than the country rock with which they are
associatated—the limestone. As the limestone is eroded the oxidized deposits
sink down and thus accumulate.
SULPHIDE ORES.
Gatena—If it be premised that the ascending waters evenly distributed
the sulphides, at least so far as the vertical element is concerned, although
across the vein these sulphides may or may not be arranged in a definite
order, it is certain that downward-moving waters, combined with denuda-
tion, may concentrate the galena at high levels and the sphalerite at lower
levels. (See pp. 1144-1145.)
Galena is the most difficultly oxidizible of the sulphides of lead, zine,
and iron. (See p.1140-1141.) Moreover, this compound is immeasureably
more insoluble than the limestone. By the solution and mechanical erosion
of the limestone and by the oxidation and solution of the sphalerite and
iron sulphide above the level of ground water, the galena is concentrated.
Whitney estimates that to make one-third meter of residual clay in the lead
and zine district of Wisconsin requires 10 to 12 meters of limestone and
shale.” Since in this district the residual material is often several meters
thick, it follows that at and near the surface there may be concentrated as
a residual product an amount of galena which was originally distributed
through several or many meters of limestone. That these processes of
erosion of the limestone and the removal of the sphalerite and marcasite
by oxidation have taken place upon an extensive scale is shown by the
@ Hall, James, and Whitney, J. D., Geology of Wisconsin; vol. 1, 1862, pp. 121-125.
LEAD AND ZINC ORES. 1149
occurrence of many detached fallen crystals and masses of galena in the
openings above the level of ground water, and also at the bottoms of the
wider openings and caves a short distance below it. Indeed, a considerable
portion of the lead which has been mined was taken above or within a short
distance below the level of ground water. This strongly corroborates the
idea that concentration resulted from the solution of the other sulphides
which held the galena to the walls and from the erosion of the limestone,
thus permitting the material to drop to lower positions in the crevices.
While the concentration of the galena was partly as above stated, it
may have been caused in part by chemical reactions between the various
compounds. In the belt of weathering part of the galena, as already noted,
is being oxidized, as is shown by the incrustations and superimposed erystals
of cerussite and anglesite. During the formation of the carbonates and
sulphates a certain amount of these salts is taken into solution and carried
downward. ‘These carbonates and sulphates react upon the other sulphides
present and reprecipitate the lead as galena. ‘These reactions may take
place to some extent above the level of ground water, but are especially
likely to occur below it. As a result of the downward migration of the
belt of weathering, there is in the downward-moving waters a continual
supply of the sulphates and carbonates of lead. The chief reaction is that
between the lead salts and the dominant iron sulphide. Supposing the iron
were in the form of FeS, the reactions may be written as follows:
PbSO,+ FeS=PbS-+FeSO,.
PbCO,+Fes=PbS-+ FeCO .
If the iron be supposed to be in the form of FeS,, as is most likely, and
oxygen were present, the reactions may be—
PbSO,+-FeS,-+0,—PbS-+ FeS0,-+ 80).
PbCO,+FeS, + 0,=PbS-+ FeCO,-+S0,.
Stokes has also shown that the precipitation of lead as a sulphide from
its oxidized solutions may be accomplished by the bisulphide of iron
without the presence of oxygen. ‘The reaction by which this takes place is
as follows, with lead sulphate:
7PbSO,+4FeS, + 4H,0 =7PbS+4FeSO,+4H,80,.
1150 A TREATISE ON METAMORPHISM.
A similar reaction takes place with lead carbonate and bisulphide of iron
where sodium carbonate is present. Stokes writes the reaction as follows:
14PbCO, + 8FeS, + Na,CO,=14PbS+4Fe,0,--Na,S,0,+15C0,.
Subsequent experiment has shown that calcium carbonate may be substituted
for the sodium carbonate. The last two reactions were performed at tem-
peratures of 100° and in the absence of oxygen. Since in the upper part
of ore deposits such temperatures are seldom found, and there is no reason
whatever to suppose that they existed during the development of the lead
and zine ores of the Mississippi Valley, and as oxygen is likely to be present,
it is probable that the precipitation of the sulphide of lead by the bisulphide
of iron in the upper part of veins was by the reactions written with oxygen
rather than by the reactions without oxygen.
However, it has been stated that zine sulphide is present with the
original sulphides, and this may also react upon the lead salts, according to
the following equations:
PbCO,-++ ZnS=PbS-+-ZnC0,.
PbSO,+ZnS=PbS+ ZnSO,.
In the case of the former reaction, smithsonite would be formed. In
this connection it is notable that frequently smithsonite is associated with
the galena for some distance below the level of ground water. While a
part of the smithsonite below ground water is of this origin, doubtless the
larger portion of it is differently explained. (See p. 1147.)
To the foregoing reactions, partly explaining the concentration of
galena, objection may be made on account of the slight solubility of lead
carbonate and lead sulphate. It is true that these substances are very
sparingly soluble in pure water; however, they are sufficiently soluble in
waters bearing carbon dioxide to account for the phenomenon. But some
of the lead may have been carried downward as a chloride. Independently
of chemical theory, we know that much of the galena has been changed to
some soluble form upon*an extensive scale. As evidence for this inference,
galena crystals above the level of ground water are much corroded, and the
amount of cerussite and anglesite associated with them is so small as not to
account for the corrosion, therefore the lead has been transformed to a
soluble salt which has been transported below in important amounts. In
the deposits of southwestern Missouri, and in those of Wisconsin adjacent to
LEAD AND ZINC ORES. 1151
the oil rock, which have by denudation passed into the belt of weathering,
numerous partially dissolved erystals of galena in openines and many
) ] g \
e
cubical openings once evidently occupied by galena but now wholly vacant
show.abundant evidence of oxidation and solution of lead sulphide.
Whether or not the reactions written above express the exact chemical
changes, it is certain that oxidized lead salts are precipitated by the sul-
phide of iron and by the sulphide of zine as lead sulphide, as was shown
by the experimental work of Anthon and Schiirman already referred to.
(See pp. 1114-1115.)
So far as my argument is concerned, it is of no consequence whether
the lead be transported as a sulphate, carbonate, chloride, or other salt.
However, it is believed that these are the forms in which the lead was trans-
ferred on the most extensive scale. I regard the cerussite and anglesite as
evidence of the partial transfer of the lead as sulphate and carbonate. A
large amount of sulphate and carbonate probably formed, but the com-
pounds are so insoluble that a part of the salts produced was not carried
downward, but precipitated near the places of formation. ;
In the upper Mississippi Valley for a short distance above and below
the level of ground water there are cubic crystals of galena of very large size
as compared with those disseminated through the sphalerite at lower levels.
The crystals at this upper horizon are commonly from 5 to 8 centimeters
in diameter, and a considerable proportion have diameters of 10 centimeters
and some of 15 to 20 centimeters. In contrast with this, the galena
intimately associated with the sphalerite at the lower levels is very rarely
in crystals larger than 5 centimeters in diameter, while a large part of it
is in smaller particles. It is thought probable that the large size of the
crystals at the upper horizons is the result of additions made by descending
water to the smaller crystals of the first concentration, Below the level of
ground water octahedral crystals of galena are frequently superimposed
upon the cubic crystals of this compound.
Sphalerite—Hvidence of the oxidation of sphalerite and the transporta-
tion of the material elsewhere is as clear as in the case of galena, both in
the upper and lower Mississippi Valley. At many places in southwestern
Missouri sphalerite crystals may be seen in the chert in various stages of
oxidation, and the porous rocks contain numerous casts of the sphalerite
crystals, now vacant or-occupied by a film of iron oxide. Precisely the
1152 A TREATISE ON METAMORPHISM.
same thing may be seen in Wisconsin, only that the matrix holding the
partly dissolved sphalerite crystals or casts is limestone instead of chert.
The sphalerite oxidized to sulphate is carried downward. Zine holds
sulphur less strongly than lead, but much more strongly than iron. 'There-
fore, the sulphate is reduced to sulphide below the galena, the reactions
being similar to those producing the galena. They may be written as
follows:
ZnSO,+FeS=ZnS-+ FeSO,
ZnCO,-+FeS=ZnS+FeCO,,
or,
ZnSO,+-FeS, +-O0,=Zn8+ FeSO, +80,,
ZnCO,+ FeS, +O,=ZnS+ FeCO,-+-S80,.
While these, and especially the last two, are regarded as probable
reactions, Stokes has shown that at temperatures from 100° to 180° C.
precipitation may take place without the presence of oxygen, thus:
14 ZnCO,+8 FeS,+Na,CO,=14 ZnS+4 Fe,0,+Na,8,0,+15 CO,
Under the conditions accompanying this reaction, calcium carbonate
may be substituted for sodium carbonate, but the reaction is slower.
While it has not been shown by experiment, it is probable that, in analogy
with the reaction of copper sulphate and lead sulphate, zine sulphate
may at these higher temperatures react upon FeS, as follows:
7 ZnSO,+4 FeS,-+-4 H,O=7 ZnS+-4 FeS0,+-4 H,S0,
As confirmatory of the deposition of secondary sphalerite by the reac-
tions above written or by other reactions which produce the same result,
Bain has found in the Joplin district * of Missouri at various places ruby-
colored crystals of sphalerite superimposed upon the main mass of sphalerite
attributed to the first concentration.
Marcasite and pyrit.— At a certain depth in the openings below the level of
eround water nearly all ot the salts of lead and zine descending from the
belt of weathermg would be precipitated by reactions between them and
the iron sulphide, as above explained. In veins in which the first concen-
tration extends to a depth greater than that to which downward-moving
waters are effective, only the sulphurets of the first concentration would be
«Bain, H. F., Van Hise, C. R., and Adams, Geo. I., Preliminary report on the lead and zine
deposits of the Ozark region: Twenty-second Ann. Rept. U. 8. Geol. Survey, pt. 2, 1901, p. 161.
LEAD AND ZINC ORES. 1155
found below this level. These sulphurets might consist mainly of marcasite
and pyrite, with subordinate amounts of sphalerite and galena. However,
even in this deep belt, concentration of galena and sphalerite may occur to
some extent, although it receives no contribution from the lead and zine
salts from above; for even after the salts of lead and zinc traveling down-
ward from the belt of weathering are all precipitated, the waters may still
hold oxygen. This oxygen would, to the largest extent, act on the marca-
site, producing to some extent soluble salts which would be abstracted, and
thus reduce the quantity of this material, and relatively enrich the deposits
in lead and zine, although not increasing the absolute amount of lead and
zinc present in a given vertical distance. So far as the zine and lead salts
were oxidized by the oxygen-bearing water, these would react on the iron
sulphide again, and they would be precipitated according to the reactions
given above.
The above paragraph can not be said to apply generally to the deposits
of the Mississippi Valley. These deposits are usually of very limited
vertical extent. Many of them are apparently cut off by impervious strata
within short distances of the surface, but drilling at the Granby area, Mis-
souri, has developed a large amount of pyrite at depth, and thus, so far
as information goes, this area appears to conform to the general rule. In
the upper Mississippi Valley area, while information is not very full,
apparently below the galena horizon the iron sulphide is rather more abun-
dant at high than at low horizons. As a consequence the zinc sulphides
become less impure with depth. This has been noted at Shullsburg, at
Platteville, at Benton, at Dubuque, and at Mineral Point. But this is in the
belt in which marcasite and sphalerite alternate in the crustified ores, and
both are apparently for the most part the products of ascending water. If
this vertical distribution be regarded as chiefly due to the primary concen-
tration by ascending water, the order is as it should be, for at first the zinc
sulphide should be thrown down to a greater extent than the iron sulphide.
GENERAL STATEMENTS.
It is believed that concentration by ascending waters largely explains
the erustified sulphide deposits of zine and lead of the upper Mississippi
Valley, at least so far as their main masses are concerned, although, as has
been explained, superimposed upon the sulphides of the first concentration
MON XLVII—04——73
TLL A TREATISE ON METAMORPHISM.
are subordinate amounts of sulphides of the second concentration. It is
believed also that the intimately intermingled sulphides at the lower hori-
zons in southwestern Missouri and the great disseminated deposits in the
massive limestone of southeastern Missouri are results of the first concen-
tration. It is thought that a second concentration by descending waters
explains through the reactions given the orderly distribution of the ores in
a vertical direction, i. e., the oxidized products and the enriched galena
above and ‘near the ground water and the sphalerite deposits below the
level of ground water. The intermingled sphalerite and galena below the
level of ground water, while largely of the first concentration, are modified
to a variable extent by the second concentration. Further, it is believed
that the second concentration was a determinative factor in the production
of many of the rich deposits, especially their upper portions. The process
of concentration by descending waters is primarily chemical, but is also to
some extent mechanical. The latter is especially true of the galena loosened
by solution from the walls and of the oxidized products mixed with galena
which have accumulated as residual material in consequence of erosion of
the limestone.
The above conclusions were reached from a study of the facts in the
field, without any knowledge of the experimental work in the laboratory
showing that iron sulphide precipitates zinc and lead as sulphides from
their oxidized salts, and that zinc sulphide precipitates lead as a sulphide
from its oxidized salts. It is thus seen that the facts in the field and the
experimental work of the laboratory supplement each other and strongly
confirm the correctness of the conclusions as to the manner in which the
vertical distribution of the ores is explained.
Thus the above theory fully explains and harmonizes the following:
(1) ‘The galena and sphalerite crystals are frequently corroded and
partly dissolved, and very numerous casts of them are found in the chert
and limestone. ‘These facts show conclusively that the sulphides have been
dissolved above the level of ground water.
(2) Upon the calcite, and frequently pseudomorphous after its crystals,
smithsonite is found, the calcite at these places showing corrosion and solu-
tion. This furnishes strong evidence that the oxidized salt of zinc, probably
zinc sulphate, has reacted upon the calcite.
4 Ba
y
PLA xe
A. Vein quartz, Banner mine, California. Showing shattering of black argillite by quartz seams;
pyrite developing in the argillite but not in the quartz; further, brownish (weathered) carbonate
deposited next to the argillite in the seams. Beginning of metasomatic alteration apparent by
bleaching of the black argillite. After Lindgren.
B. Secondary galena and blende in ores from Missouri. After Bain.
U.S. GEOLOGICAL SURVEY MONOGRAPH XLVII PL.XI!
JULIUS BIEN & CO.LITH.NAY.
/A/ NEIN QUARTZ, BANNER MINE, CALIFORNIA
(B/ SECONDARY GALENA AND BLENDEIN ORES FROM MISSOURI.
LEAD AND ZINC ORES. 1157
(3) With the oxidized products of lead and zine are associated barite
and gypsum. These are the by-products which should be formed by the
reactions of the sulphates of lead and zine upon the alkaline earth carbonates.
(4) At and near the level of ground water are very large crystals of
galena as compared with those deeper in the deposits. These crystals are
fully explained by secondary growths of galena deposited by descending :
waters.
(5) The products have a vertical order; the oxides, carbonates, and
silicates ie mainly above the level of ground water, the galena largely
above but extending below it, and the sulphide of zine with the iron sul-
phides mainly below it. This vertical order is that which should be pro-
duced by the reactions of the descending oxidized salts of lead and zine
on the sulphides of zine and iron.
(6) Superimposed upon the crustified sulphides of the first concentra-
tion are later crystals of galena and sphalerite, often different in character
from the earlier sulphides. For instance, in Wisconsin the later galena is
usually in octahedral crystals, and in Missouri the sphalerite is pure and
ruby colored. (See Pl. XII, B.) The exceptional appearances of these
products are fully explained by the reactions between the oxidized products
of lead and zine and the sulphides deposited by the first concentration.
While the precipitation at the second concentration of downward-moving
sulphates is partly accomplished, as has been explained, by the reaction
upon them of the low-grade sulphurets, thus producing rich sulphides, the
reduction and precipitation of these oxidized products is partly accomplished
by the direct and indirect action of the organic matter, precisely as in the
case of the original precipitation. The power which organic matter has for
the reduction and precipitation of the oxidized products has been recognized
by several others who have written upon the lead and zine of the Missis-
sippi Valley. Among these are Chamberlin,* Blake,’ Bain,’ and Grant.
«Chamberlin, T. C., The ore deposits of southwestern Wisconsin: Geol. of Wisconsin, vol. 4, 1882,
pp. 544, 546.
> Blake, Wm. P., Lead and zine deposits of the Mississippi Valley: Trans. Am. Inst. Min. Eng.,
vol. 22, 1894, pp. 630-631. Also, Wisconsin lead and zine deposits: Bull. Geol. Soe. America, vol. 5,
1894, pp. 28-29. :
¢ Bain, H. F., with C. R. Van Hise and G. I. Adams, Preliminary report on the lead and zine
deposits of the Ozark region: Twenty-second Ann. Rept. U. 8. Geol. Survey, pt. 2, 1901, p. 213.
a Grant, U.S., Preliminary report on the lead and zine deposits of southwestern Wisconsin: Bull.
Wis. Geol. and Nat. Hist. Survey, No. 9, 19038, p..83.
1158 A TREATISE ON METAMORPHISM.
Chamberlin states that organic matter from the surface may have made its
way down through the openings of the limestone, and thus assisted in the
reduction of the descending oxidized products.* As pointed out by Blake,
the effect of the organic matter is especially clear in the case of the large
sphalerite deposits of Wisconsin which rest upon the oil rock at the base of
the Galena.
While the above statement concerning concentrations of the lead and
zinc ores by ascending and descending waters combined is made with
reference to the Mississippi Valley, it is believed that many of the lead and
zine districts of other parts of the world have had a substantially similar
history.
oi ASSOCIATION OF COPPER AND IRON COMPOUNDS.
Another very general association of metals is that of copper and iron.
Where this association occurs it is well known that above the level of
ground water metallic copper, cuprite (Cu,O), tenorite (CuO), malachite
(CuCO,;.Cu(OH),), and azurite (2CuCO,.Cu(OH),) are very frequently
found.
These compounds may be produced from any of the sulphides. It is
not worth while to attempt to write out the reactions showing their forma-
tion from all of the sulphides, but to make the principles clearer it may be
well to consider one illustrative case. For this purpose chalcocite is taken,
since this is the richest sulphide and the one which, as shown below, is
likely to be developed in the upper part of the belt of cementation and pass
in large quantity into the belt of weathering. By simple oxidation of the
sulphur of chalcocite, metallic copper is produced, according to the follow-
ing reaction:
Cu,S+20=2Cu+S0,
Simultaneously, or later, the metallic copper thus formed may be oxidized
to cuprite, according to the equation:
2Cu+O0=Cu,O
By further oxidation the cuprite may pass into tenorite, according to
the reaction:
Cu,O+O=2Cu0
«Chamberlin, cit., pp. 544-545.
COPPER ORES. 1159
When the oxidation has gone far enough to produce tenorite, carbona-
tion and hydration may take place, and malachite be produced, according to
the following reaction:
2Cu0-+C0,+-H,O=CuCO0,. Cu(OH),
Or if the carbonation goes further as compared with hydration, azurite may
be produced, according to the following reaction:
6Cu0-++4CO,+2H,O0=2[2CuCO,. Cu(OH),]
The product represented by any stage in this process may be regarded
as formed directly by a combination of two or more of the preceding
reactions. But in the southwestern portion of the United States it is
certain that the reactions frequently if not usually occur step by step, as is
shown by numerous specimens In a single specimen I have seen a core
of chalcocite surrounded by metallic copper, outside of which is cuprite;
beyond this lies tenorite, and beyond the tenorite hydrated carbonate.
While it is rather rare to find this full succession, it is easy to get specimens
of metallic copper interlaced or surrounded by cuprite, the latter being
surrounded by at least a film of tenorite, beyond which occur the ecar-
bonates. In this connection it may be recalled that cuprite is a rather
abundant oxide and that tenorite is rare. The reason which may be sug-
gested for this is that as soon as tenorite is formed this oxide may unite
with carbon dioxide and water and produce malachite or azurite. This
process goes on almost as fast as the tenorite develops, and thus very little
of this oxide is found in the mines. The oxidized copper products, instead
of being formed from chalcocite, may be produced from any of the leaner
sulphides of copper; but in such cases the associated iron forms iron com-
pounds which may largely remain in situ as oxides or be carried away as
sulphates.
The oxidized ores of copper occur extensively in arid regions. The
precipitation is small, but there is enough moisture underground to carry
on the work with the assistance of oxygen and carbon dioxide. In
such a region the level of ground water is far below the surface and erosion
is exceedingly slow, so that there is ample room and sufficient time for a large
amount of material to accumulate above the level of ground water. In
humid regions, while water is more abundant and the conditions to that
1160 A TREATISE ON METAMORPHISM.
extent are more favorable to the development of the oxidized products, denu-
dation is frequently so rapid that time is not given for the transformation of
the sulphides to such products, for a given part of the belt of weathering
is above the level of ground water only a short time before the material
of that portion is removed by erosion. Also the belt above the level of
ground water is narrow, and thus there is comparatively little room for the
oxidized deposits.
While under favorable conditions oxidized products are very common
above the level of ground water, it is well known that below it the oxidized
and carbonated products occur in greatly diminished quantity, and there
are frequently present rich sulphurets, such as chalcocite (Cu,$), bornite
(Cu,FeS;), chalcopyrite (CuFeS,), and sometimes covellite (CuS). Some-
what deeper below the level of ground water the oxides and carbonates are
not found. Furthermore, the chalcocite, covellite, and bornite are very
generally restricted to the upper part of the belt of cementation; deeper,
the places of these minerals are largely occupied by chalcopyrite. Not
only is this true, but still deeper in many instances the chalcopyrite is less
prominent, and the iron sulphides more so. In the lower workings of many
of the deeper mines the only metalliferous product found is cupriferous iron
sulphide, the chalcopyrite having wholly disappeared. But in some places
where the dominant sulphide of the deeper levels is pyrite or pyrrhotite
fracture openings within these sulphides are filled with the rich copper
sulphides.
Whether or not this general statement is correct for a particular area,
each mining engineer can judge from his own knowledge. There may be
exceptions to it, due to various causes, one of which has been alluded to in
explaining bonanzas. Thus below cupriferous pyrites there may again be
found richer copper sulphides. Indeed, as before stated, ore deposits vary
greatly in their richness both horizontally and vertically, and the above
statement can only be considered as a general average.”
The above order is believed to be explained by the work of downward-
moving waters. The combinations of lead, zinc, and iron were foilowed from
above downward. The reactions which occur in the case of the copper-
iron deposits may perhaps be followed, to vary the treatment, from the base
«See Raymond, R. W., Discussion of ‘‘Genesis of ore deposits,’ by F. Posepny: Trans. Am. Inst.
Min. Eng., vol. 24, 1895, pp. 991-992.
COPPER ORES. 1161
upward. At greater or less depths below the level of ground water the ores
are often dominantly cupriferous pyrites or pyrrhotite, the direct deposit
of the ascending waters. At a little higher level oxygen from above may
have oxidized a portion of the iron and transported it elsewhere, relatively
enriching the deposit in copper; at a still higher level there is a contribution
of soluble copper salts from above. Since copper sulphate is certainly the
most common salt, for convenience all the soluble salts will be regarded as
sulphates. Reactions similar to those given below may easily be written for
other salts.
At the level where salts of copper appear from above, the action of the
copper salt on iron sulphide may produce chalcopyrite, the reactions result-
ing from the greater affinity of sulphur for copper as compared with iron,
and from the law of mass action. With cupric sulphate the reaction may
be written, for ferrous sulphide:
(1) :2FeS+CuSO,=CuFe$,+ FeSO,
For bisulphide of iron, in the presence of oxygen, the reaction may be:
(2) 2FeS,-+CuSO,+0,=CuFeS,+FeS0,+280,
or, if oxygen be absent,
(3) 7CuSO,+8FeS,+8H,0=7CuFeS,-++FeSO,+8H,80,
Where the iron sulphide is pyrrhotite, intermediate between FeS and
FeS,, the reactions may be written by combining the equations (1) and (8)
in proper proportions. ;
The chalcopyrite may also be produced by the reaction of cuprous
sulphate on pyrite or marcasite. In the presence of oxygen the reaction
is as follows:
3F eS, +Cu,80,+40=2CuFeS,+FeS0,+280,
and in the absence of oxygen:
15FeS,+7Cu,SO,+8H,0=14CuFeS,+ FeSO,+8H.S0,
In passing upward from the lowest level at which the chalcopyrite
appears, this mineral may steadily increase in quantity until it becomes an
important constituent, and finally the iron sulphide may become subordinate.
Under these circumstances bornite is likely to appear also. The produc-
1162 A TREATISE ON METAMORPHISM.
tion of bornite by the direct reactions of the cupric salts may be supposed
to be as follows:
For ferrous sulphide in the presence of oxygen the reaction is:
(1) 4FeS+3CuS0,+20=Cu,FeS,-+3FeS0,+80,
For iron bisulphide the reaction is:
(2) 3FeS,+3CuSO,+40=Cu,FeS,+2FeS0,+ 480,
or, if oxygen be supposed to be absent, the reaction for cupric salt may be:
(3) 13FeS,-+210uSO,+20H,0=7Cu,FeS, + 6FeS0,+20H,S0,
The production of bornite by the direct reaction of cuprous sulphate
on the bisulphide of iron in the presence of oxygen may be written:
(4) 5FeS,+3Cu,SO,+80=2Cu,FeS,+-3FeSO,+480,
or, if no oxygen be present, the reaction is:
(5) 23FeS,+21Cu,SO,+16H,O=14Cu,FeS,+9FeSO,+-16H.S0,
However, the bornite may also be produced by the reaction of the
cupric salt on the chalcopyrite itself. For instance, in the presence of
oxygen, for cupric sulphate, the reaction might be:
(6) 2CuFeS,+CuSO,+20=Cu,FeS,+FeSO,+80,
If oxygen be absent the reaction for cupric sulphate would be:
ye I
(7) 18CuFeS,+11CuSO,+8H,0 =8Cu,FeS,+-5FeSO,+8H,SO,
The reaction for cuprous sulphate in the presence of oxygen may be
written:
(8) 4CuFeS,+Cu,SO,+60=2Cu,FeS,+2FeSO,+S80,
and in the presence of oxygen:
(9) 23CuFeS,+-11Cu,80, +4H,0=15Cu,FeS,-+-8FeS0,+4H,S0,
Further reaciions might be written by combining corresponding
equations for cupric and cuprous salts, as a result of which the bornite is
produced by the reaction of both cuprous and cupric compounds. Reac-
tions might also be written by which the bornite is partly produced from
bisulphide of iron and partly from chalcopyrite. These reactions would be
merely the combination of corresponding equations, and it seems hardly
necessary to write them out, since no new principle is illustrated.
COPPER ORES. 1165
Still higher up, chalcocite may appear with the chalcopyrite and born-
ite. For a short distance below the level of ground water this mineral in
some mines is the dominant sulphide and may be produced by the reaction
of cupric sulphate on the iron sulphide. For ferrous sulphide the reaction
in the presence of oxygen would be:
(1) 2FeS+2CuSO,+20=Cu,S+2Fe80,+80,
For iron bisulphide in the presence of oxygen, the reaction would be:
(2) 2FeS, +2CuS0,+60=Cu,S+2Fe80,+380,
or, if oxygen be absent, the reaction is:
(3) 5¥FeS,+14CuS0, +12H,0=7Cu,S+-5FeS0,+12H,S0,
Chalcocite may be produced also by the reaction of cuprous salts on
iron sulphide. For ferrous sulphide in the presence of oxygen the reaction is:
(4) FeS+Cu,SO,=Cu,S+ FeSO,
for iron bisulphide in presence of oxygen:
(5) FeS,+Cu,80,+-20=Cu,8+ FeSO,+S0,
or, if oxygen be absent,
(6) 4FeS,+7Cu,S0,44H,0=7Cu,S+4FeS0,+ 4,80,
Furthermore, the chalcocite may be produced by reactions of the sul-
phates either on the chalcopyrite or on the bornite. In the case of cupric
sulphate upon chalcopyrite in the presence of oxygen, the reaction may be
written as follows:
(7) CuFeS,+CuSO,+20=Cu,S+FeSO,+S80,
or, if oxygen be not present, the reaction may be as follows:
(8) 5QuFeS,-+11CuS0,+8H,0=8Cu,8 +5FeSO,+8H.S0,
For cuprous sulphate upon chalcopyrite, in the presence of oxygen,
the reaction is:
(9) 2CuFeS,+Cu,SO,+60=2Cu,S+2FeSO,-+SO,
and in the absence of oxygen is:
(10) SCuFeS,-+11Cu,80,+4H,0=15Cu,8+8FeS0,+4H,80,
1164 A TREATISE ON METAMORPHISM.
In the production of the chaleocite from the bornite the reaction for
cupric salts in the presence of oxygen may be as follows:
(11) Cu,FeS,+CuS0O,+20=2Cu,S-+ FeSO,--SO,
or, if oxygen be not present, the reaction may be:
(12) 5Cu,FeS;+-11Cu80,+8H,0=13Cu,8+5Fes0,+8H,S0,
For cuprous salt in the presence of oxygen the reaction is:
(13) 2Cu,FeS,+-2Cu,SO,+20=5Cu,S-+-2FeSO,+S80,
or, in the absence of oxygen:
(14) 8Cu,FeS,+-11Cu,S80,-+4H,0=23Cu,S8-+8FeS0,+4H,80,
In all of the foregoing reactions the fundamental principle, so far as
iron and copper are concerned, is the same. In every case the acid radical
of the oxidized copper salt passes to the iron, and the sulphur in the iron
unites with the copper, forming copper sulphide, thus producing a greater
proportion of copper sulphide in the ore than before the reaction.
Too much stress must not be laid on the particular chemical reactions
written. They are designed to show the nature of the reactions which may
occur rather than to assert that the particular reactions written do occur
exactly. It is believed that the reactions written are possible, but much
experimental work must be done in the laboratory in order to ascertain
which of the reactions is most common in an individual case. It is believed
that the reactions for the production of rich sulphides of copper by
descending waters are often those in which oxygen is present, but doubt-
less reactions without oxygen also take place frequently.
In all cases where SO, is regarded as produced by the reactions it may
be that instead of this compound sulphuric acid is formed. Indeed, it is
well known that SO, in the presence of ferric sulphate is oxidized to sul-
phuric oxide. In order to modify the equations in which SO, appears so as
to produce sulphuric acid it is merely necessary to substitute H.SO, for each
SO, on the right side of the equation, and add H,O-+0 on the left side.
Probably the reactions in which oxygen is absent also occur in the first
concentration of copper in consequence of the reactions of ascending copper
solutions upon previously precipitated lower-grade materials.
COPPER ORES. . 1165
While in some cases the sulphides were precipitated by reactions in
which oxygen is present and in other cases by those in which it was absent,
in still other cases I have no doubt that the reactions utilized some oxygen,
but not as much as indicated by the equations as written. In such instances
the real changes when precipitation took place are represented by equations
which combine in various proportions those in which oxygen is represented
as present and those in which it is absent. Moreover, for a certain part of
a deposit the conditions may for a time have been those in which the reac-
tions were without oxygen, later those in which it was insufficient, and
finally those in which it was sufficient. Thus, all combinations of the reac-
tions with and without oxygen may have taken place in the production of a
single deposit. i
The vertical relations of the richer sulphides which appear in passing
from depth to the surface are very different in various regions. In some
districts there may be somewhat regular gradations from the poor
sulphurets at depth to the very rich sulphurets at or near the level
of ground water. In other cases this change may be very abrupt.
For instance, at Ducktown, Tenn., the lean cupriferous pyrrhotite changes
rapidly into, a zone of very rich sulphuret at and near the level
of ground water. At Butte, Mont., the rich sulphurets are found at a
much greater depth, and in the deeper workings of some of the mines
secondary fracture openings in the lean sulphurets now contain small veins
of rich sulphides of various kinds, even bornite or chalcocite, which have
evidently been reduced by the lean sulphides adjacent. These illustrations
show that the various sulphides overlap one another. In passing upward
from the poor material, bornite may appear before the iron sulphide has
been replaced largely by chalcopyrite, and at the place where bornite has
become reasonably abundant chalcocite may be found. If the dominating
material be iron sulphide, the copper mineral which is present is likely to be
chalcopyrite rather than the richer sulphurets. Chalcopyrite is likely to
be associated on the one hand with pyrite or pyrrhotite and on the other
hand with bornite, or even chaleocite. Bornite and chalcocite are likely to
be associated with each other and with chalcopyrite.
At still higher levels in a mine, a moderate distance below the
level of ground water, oxidized and carbonated products may appear
with the sulphurets. These mixed products, sometimes called oxysul-
ILS A TREATISE ON METAMORPHISM.
phurets, are well illustrated in the Appalachian, Arizona, and Montana
deposits.” Still higher, and especially above the level of ground water,
the oxidized and hydrated products may become dominant, for there
the rich sulphurets which have emerged from the ground water have been
directly acted upon by the oxygen, carbon dioxide, and water. Spurr, J. E., The ore deposits of Monte Cristo, Washington: Twenty-second Ann. Rept. U. S.
Geol. Survey, pt. 2, 1901, p. 851.
¢ Penrose, R. A. F., jr., Mining geology of the Cripple Creek district, Colo.: Sixteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1895, pp. 131-132.
MON XLVII—04———74
1170 A TREATISE ON METAMORPHISM.
oxidized in order that the free gold may be produced. This may take place
first by the oxidation of tellurium to an oxide, and then the solution of the
oxide by acids. Or the sum total of all the changes may be represented in
a given case as follows:
Au,Te,+12NaCl+6C0,+60=4Au+3TeCl,+6Na,CO,
That this or some similar process of the oxidation of the tellurium,
leaving gold behind, has taken place upon an extensive scale at Cripple
Creek, is shown by the very frequent pseudomorphs of spongy gold after
the various tellurides.
The free-gold ores thus formed in the belt of weathering are commonly
very much richer than the downward extensions of the deposits, in which
the gold is associated with sulphides or tellurides, or occurs as a telluride.
This exceptional richness of the upper part of the gold deposits is so well
known that it is unnecessary to give many cases illustrative of it. However,
one or two of the more important districts may be mentioned. In Australia,
Don says, ‘‘for ounces per ton above the ground-water level, only penny-
weights per ton have been found below it.”“ In the Sierra Nevada deposits,
according to Lindgren, near the surface the values are from $80 to $300
per ton, whereas deeper they are somewhat uniform for the given vein,
and run from $20 to $30 per ton.’ In many instances the decrease in values
is so great, in passing from the belt of weathering to the deeper workings,
that while the belt of weathering and the upper part of the belt of cementa-
tion may be very profitable, the deeper portions of the deposits are so
lean as not to warrant working.
These facts seem to me to be conclusive evidence that, in some way,
the downward-moving waters have concentrated in a comparatively narrow
belt an amount of gold which originally had a much wider vertical extent.
That is to say, in some way, as denudation continued downward, the gold
deposited in one of the original forms has been taken into solution by the
descending waters and has been reprecipitated, thus producing the enriched
upper part of the veins. As this process continues the belt of weathering
becomes richer and richer, there being segregated within a comparatively
@Don, J. R., The genesis of certain auriferous lodes: Trans. Am. Inst. Min. Eng., vol. 27, 1898,
p. o96.
> Lindgren, Waldemar: The gold-quartz veins of Nevada City and Grass Valley, California:
Seventeenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1896, p. 128.
GOLD ORES. 1171
small vertical distance (from a few meters to 100 meters or more) a large
part of the gold which, as a first concentration, may have had a vertical
extent of 1,000 to several thousand meters.
It has been seen that gold is soluble in ferric chloride, cupric chloride,
sodic carbonate, alkaline sulphides, and in other compounds. There
are likely to be present in descending waters ferric chloride and ferric
sulphate, and, in case the lode is a copper-bearing one, also cupric
chloride. It is therefore believed that in the upper part of the belt of
weathering these reagents or others take the gold in solution. As the
solutions pass below the belt of weathering and into the belt of cemen-
tation they come into contact with sulphides of the base metals, tellurides,
or organic materials, or are mingled with reducing solutions. It has
already been seen that any of these conditions results in the precipitation
of gold from its solutions, and therefore the gold is thrown down by
these compounds in the upper part of the belt of cementation. Probably
in the majority of instances the sulphides are the chief reducing com-
pounds, although the others are not unimportant. The reduction of gold
by sulphides is somewhat different from the reduction of any of the metals
previously considered. Zinc, lead, copper, and silver are thrown down
from their salts as sulphides by the baser sulphides. The gold is thrown
down from its salts by those sulphides not as a sulphide but as metallic
gold, because gold and sulphur have such weak affinity and gold is so
easily reduced to the metallic form.
As denudation goes on the enriched upper portion rises into the belt
of weathering. The sulphides and tellurides are there again oxidized,
the gold is again partly dissolved and transported downward to be again
precipitated, and thus an horizon of steadily increasing richness and breadth
is formed below the belt of weathering in the belt of cementation, the gold
commonly being largely free, but associated with sulphides.
During the process not all the gold which rises into the belt of weath-
ering is dissolved, and the richer the sulphide zone which rises into the
belt of weathering the greater the amount of gold left behind. This
process of chemical concentration in the belt of weathering is supplemented
to an important degree in many districts by residual concentration. As
denudation continues the rocks are disintegrated, dissolved, and transported
to the streams to a greater extent than is the gold, and thus the gold in the
1172 A TREATISE ON METAMORPHISM.
upper parts of the veins is segregated. Segregation by this method is
especially important at and near the surface. It thus appears that the
production of the free gold of the upper parts of lodes is largely a chemical
process, but is partly a process of residual concentration The two together
continued for a sufficient length of time are believed to adequately explain
the rich free gold ores of the upper parts of lodes.
Cripple Creek furnishes an excellent illustration of rich free gold at and
near the surface. Near the surface numerous deposits in this district were
exceptionally rich, containing a large amount of free gold above and near the
level of ground water. Usually much of this free gold was in spongy
pseudomorphs after tellurides, showing its formation in large measure by
the direct oxidation of the tellurium of the tellurides, the gold being left
behind. For some years it has been well known that below the upper belt
of rich material in many of the veins the deposits were very much poorer.
Indeed, many of the deposits were found to be so poor below the upper
narrow belt of rich ores as to lead to their abandonment.
In reference to this poor ground Mr. Moore says:
“Tt is undoubtedly true that some mines have entered poor ground in
the veins at depths of 250 feet [75 meters] to 400 feet [120 meters] below
the surface, and have sunk down to depths of 400 feet [120 meters] to 500
feet [150 meters] deeper without finding payable ore bodies again.” But
this was by no means true of all the deposits. Mr. Moore further says, ‘It
is a fact that in a greater number of mines the values have been carried
down to more than 1,200 feet [360 meters] below the surface continuously,
and in at least two mines the values have greatly increased from 1,100 feet
[3830 meters] downward.”“ A most interesting feature in reference to the
Cripple Creek district has been developed recently. Deep in some of the
mines which showed a depleted horizon there has been discovered a lower
horizon of very rich ores, partly sulphides and tellurides, but also in large
part free gold.
According to Mr. Hills: ‘The original ground-water level at Cripple
Creek is about altitude 9,500 feet [2,850 meters] for the west side of the
district and considerably higher for the Bull Cliff region.”’
The upper part
“The Daily News, Denver, Colo., Jan. 1, 1903, p. 3.
bNinth Ann. Rept., Portland Gold Mining Co., Victor, Colo.; Report of Victor G. Hills, Consult-
ing Engineer, 1903, p. 87.
GOLD ORES. 1173
of these rich deposits appeared upon an average at least 120 meters below
ground-water level. Mr. Moore“ states that the depths of the deepest shafts
in the Cripple Creek district vary from 240 to 450 meters. On Battle
Mountain are the Portland, Granite, Burns, Ajax, Stratton’s Independence,
Strong, Gold Coin, and Modoc. At Bull Hill are the Last Dollar, Blue Bird,
Logan, American Eagles, Wild Horse, Isabella. At East Spur Bull Hill
are the Independence T. & M. Co., Vindicator, Lillie, and Golden Cyele.
He says in reference to these deposits: “All but one of these shafts have
good bodies of ore and excellent indications for the future at the lowest
levels to which they have thus far attained. Certain of these shafts, to wit,
the Last Dollar and Blue Bird, show some of the richest ore ever mined in
the district at their lowest levels.” Mr. Moore regards these deep, rich
deposits as due to the secondary action of descending waters. It has
already been noted that in the Cripple Creek districts the values in the
upper part of the veins near the ground-water level are very largely in
native gold; deeper they are largely in the tellurides, the sulphides being
comparatively poor; while still deeper they are to a large extent in the
sulphides.
Tf these are facts they seem to be evidence that the ores of the Cripple
Creek district have undergone two concentrations. The first concentra-
tion was by ascending waters, and the ores were originally deposited in
some measure as tellurides, but perhaps more largely as free gold associ-
ated with sulphides. In consequence of later denudation, with action by
descending waters, secondary segregation may have taken place and _pro-
duced two rich upper horizons, that of free gold largely above the level of
ground water and that of the rich tellurides and associated free gold, mostly
below it.
Exceptionally the Cripple Creek mines have a deep horizon of rich
tellurides. In most districts where rich ores occur at some depth below
the level of ground water the reduction of the gold to its free state is
believed to be largely the work of the base sulphides. As already noted,
in the deeper workings of the Cripple Creek district the values are largely
in the sulphides, and the tellurides are subordinate. The natural explana-
tion of the rich tellurides is that as the original sulphides and _ tellurides
rose above the level of ground water the gold and tellurium were both
@ The Cripple Creek Times, December, 1902.
1174 A TREATISE ON METAMORPHISM.
oxidized and traveled down, the first as gold chloride or a similar salt, and
the tellurium as telluric chloride or a similar salt. Below the level of
ground water these two compounds would both be reduced where they
came into contact with the base sulphides. At the moment of reduction
they may have united, and thus the rich tellurides of gold may have been
formed. It is thought possible that precipitation was caused in this manner
rather than by previously existing lean tellurides of the first concentration,
because in the deeper workings of some of the mines the tellurides seem to
be very subordinate to the sulphides, and the values are mainly in the
sulphides. If the above suggestion proves correct, in the Cripple Creek
district the deep, rich tellurides represent a concentration of tellurium as
well as of gold. If the amount of tellurium for the upper 500 meters or
more of the lodes was not originally greater than in the deeper workings,
the tellurium now found in the deep rich horizon must have had a much
wider original vertical distribution."
The formation of the rich free gold near and above the level of ground
water in the Cripple Creek district is not materially different from the
formation of such products elsewhere. As denudation continued the
enriched sulphides and tellurides rose above the level of ground water,
the sulphur and tellurium were there oxidized, and some of the free gold
was left behind. This chemical process was accompanied by residual
concentration.
CONCENTRATION BY REACTION UPON SULPHIDES COMPARED WITH METALLURGICAL CONCENTRATION.
One of the more common processes of metallurgy for the separation
of gold, silver, copper, and lead from iron is based upon the principle
explaining the second concentration given on preceding pages, viz, that
iron holds sulphur less strongly than the other elements named. The
sulphureted ores are imperfectly roasted, thus partly oxidizing them to
oxides and sulphates. The ores are then smelted in a furnace with a flux.
The oxides of the valuable metals and the sulphates react on the remaining
sulphides of all the metals, producing a matte containing the sulphides of
the valuable metals. The iron gets all or nearly all of the oxygen; the
iron oxide unites with the fluxes and passes into the slag.
“Since the above was in page proof I have been informed by Dr. Lindgren, who has closely
studied the district, that he doubts the existence of enriched deep tellurides at Cripple Creek. If
these tellurides do not exist the two preceding paragraphs need radical modification.
EFFECT OF DESCENDING SOLUTIONS. 1175
OTHER REACTIONS OF DESCENDING SOLUTIONS.
In the foregoing pages the second concentration of metals by solution,
downward transportation, and precipitation by reactions on the sulphides of
the first concentration has been emphasized. However, it is not supposed
that this is the only process which may result in enrichment by descending
waters of the upper parts of vein deposits. The enrichment of this belt
may be caused by reactions between the downward-moving waters carrying
metallic compounds and the rocks with which they come in contact, and by
reactions due to the meeting and mingling of ascending and descending
waters.
The descending waters carrying metals dissolved in the upper part of
the veins may be precipitated by material contained in the rocks below.
This material may be organic matter, ferrous salts, etc. So far as precipi-
tating materials are reducing agents, they are likely to change the sulphates
of most of the metals to sulphides, and precipitate the metals in that form.
While sulphides may thus be precipitated to some extent above the level of
ground water, because of the deficiency of oxygen they are thrown down
much more largely below it. The reducing solutions ordinarily precipitate
gold from its solutions in the metallic form.
In a trunk channel, where ascending and descending waters meet,
there is a considerable horizon in which the circulation is slow and
irregular, the currents now moving slowly upward and now moving slowly
downward, and at all times being disturbed by convectional movements.
Doubtless this belt of slow general movement and convectional circulation
reaches lower levels at times and places of abundant rainfall than at other
times and places, for under such circumstances the descending currents are
strong. The ascending currents, being controlled by the meteoric waters
falling over wider areas, and subject to longer journeys than the descending
currents, do not so quickly feel the effect of abundant rainfall. Later, the
ascending currents may feel the effect of the increased rainfall and carry
the belt of upward movement to a higher level than normal. However,
where the circulation is a broad one, little variations in ascending currents
result from irregularities of rainfall.
In the belt where ascending and descending waters meet (see fig. 26),
convectional mixing of the solutions due to difference in temperature is an
important phenomenon. ‘The waters from above are cool and dense, and
1176 A TREATISE ON METAMORPHISM.
tend to sink downward, while those from below are warm and less dense
and tend to rise; thus the waters are mingled. But even if the water were
supposed to be stagnant at the neutral belt, it is probable that by diffusion
the materials contributed by the descending and ascending waters would
be mingled.
Ascending and descending solutions are sure to have widely different
compositions, and an accelerated precipitation of metalliferous ores is a
certain result of their mixture. As an illustrative case in which precipita-
tion is likely to occur, we may recall that ascending waters contain prac-
tically no free oxygen, frequently contain hydrogen sulphide, and are
often somewhat alkaline, while descending waters are usually rich in
oxygen and frequently contain acids, as at Sulphur Bank, described by
Le Conte.” The mingling of such waters as these is almost sure to result
in precipitation of some kind. As illustrating the effect of the mingling of
descending and ascending solutions we may suppose that the sulphides
of any of the metals are rising in solutions of sodium sulphide, and that
the descending waters are carrying sulphates of the metals and sulphuric
acid. The sulphuric acid would destroy the sodium sulphide according to
the following reaction:
Na,S+H,80,=Na,S0,+H,S
The sodium sulphide being destroyed, the sulphides traveling as such in
the ascending solutions would be thrown down. ‘This reaction also pro-
duces hydrogen sulphide, which, formed in this way or originally present,
would throw down metals from the descending oxidized salts. For
instance, sulphate of copper would be thrown down as a sulphide by
hydrogen sulphide. Another illustrative case is the mingling of descending
waters bearing oxygen with ascending waters bearing iron carbonate. The
result is to throw down the iron as ferric oxide, according to the following
reaction:
2 FeCO,+0-+nH,0=Fe,0,. nH,0+2C0,
Le Conte also suggests that by the mingling of the waters from
a
to)
below with those from above the temperature of the ascending column is
rapidly lessened, and this also may result in precipitation. The dilution
may work in the same or in the reverse direction.
«Compare Le Conte, Am. Jour. Sci., 3d series, vol. 24, 1882, p. 33, and vol. 26, 1883, p. 9.
EFFECTS OF DESCENDING WATERS. IIe
The metals precipitated by the mingling of waters may be contributed
by the descending waters, by the ascending waters, or partly by each. In
so far as more than an average amount of metallic material is precipitated
from the ascending waters, this results in the relatively greater richness
of the upper part of veins independently of the material carried down from
above.
The avove methods of precipitation and enrichment of the upper parts
of deposits follow from the reactions of downward-moving waters. Their
effect is to precipitate the metals of the ascending water to some extent, and
thus assist in the first concentration. But the results of these processes can
not be discriminated from the second concentration, which is caused by an
actual downward transportation of the material of the first concentration.
It is believed that the peculiar character of the upper portions of lodes is
mainly owing to downward transportation of metals previously precipitated
(see pp. 1182-1189); but whether this be so or not, it is certainly due to
descending waters.
SECOND CONCENTRATION FAVORED BY LARGE OPENINGS NEAR THE SURFACE.
The concentration of large ore bodies in the belt of weathering and in
the upper part of the belt of cementation is greatly favored by the abun-
dance and size of the openings as compared with the openings at greater
depths.
The openness of the rocks above the level of ground water and the
rapid lessening of the volume of the openings below it have already been
alluded to as general phenomena, and an explanation has been offered that
in the belt of weathering solution is the law, and in the belt of cementation
cementation is the law. (See pp. 484-487, 562-565, 612-617.) Of course,
it is understood that there is usually not a sudden change in the amount
of pore space at the level of ground water, but at and below it the
extremely open upper ground grades into the much less open lower ground.
In some instances the gradation requires some distance. Thus, so far as
the openings are concerned, the conditions for the formation of large ore
deposits are more favorable above the level of ground water and as far
below it as openings are numerous than at deeper levels.
This openness of the belt of weathering and the comparative closeness
of the belt of cementation are well illustrated by many limestone regions;
1178 A TREATISE ON METAMORPHISM.
for instance, the lead and zine district of southwestern Wisconsin, already
described. (See pp. 1144 et seq.) Another excellent illustration of very
loose and open ground above the level of ground water and tight ground
below it is furnished by the Monte Cristo district of Washington.” Here
near the surface all the minor joints are open, circulation has been free,
and the larger ore deposits are found. At depth the joints are mostly tight,
only a few being sufticiently open to allow of much water circulation.
But numerous and large openings may exist below the level of ground
water. In various kinds of rocks—such as sandstones, conglomerates,
amyedaloids, and tuffs—the openings are original, and may not have been
closed by cementation. Of course, the more recent the earth movements
the more numerous and larger are the openings. In some places the
descending waters are not saturated when they reach the level of ground
water, and solution continues for some distance below it. Furthermore,
the level of ground water varies under different circumstances. Where a
region is being uplifted the level of ground water, other things being equal,
is descending, and where a region is subsiding it is rising. As a result of
physiographic changes there may be alternate valley fillmg and valley
erosion. These changes affect the level of ground water. In Pleistocene
time there was an extensive period of valley filling instead of erosion.
Consequent on this the level of drainage, and therefore the level of ground
water, rose. Also there may be very considerable variations in the level
of ground water as a consequence of long-continued climatic changes, such,
for instance, as the alternating periods of humidity and aridity in the Cor-
dilleras of the West in connection with the Pleistocene.’ To illustrate, at
the present time in the mining districts of New Mexico and Arizona the
level of ground water is far below the surface, but it can not be doubted
that during the humid epoch evidenced by the existence of Lake Bonne-
ville and Lake Lahontan the level of ground water was much higher.
Emmons gives a specific case in which the ground water was apparently
once nearer the surface than at present. According to him, at the Delamar
aSpurr, J. E., The ore deposits of Monte Cristo, Washington: Twenty-second Ann. Rept. U.S.
Geol. Survey, pt. 2, 1901, p. 847.
> Gilbert, G. K., Lake Bonneville: Mon. U. 8. Geol. Survey, vol. 1, 1890. Russell, I. C.,
Geological history of Lake Lahontan, a Quaternary lake of northwestern Nevada: Mon. U. S.
Geol. Survey, vol. 11, 1885.
LARGE OPENINGS NEAR THE SURFACE. 1179
mine the lower levels are as dry as the upper levels. The region is arid
oc
and the mine is never wet, but the mine shows ‘‘universal evidence of a
secondary enrichment that must have proceeded from the surface down-
wards.” ”
This fact Emmons regards as evidence that at the comparatively
recent Bonneville epoch the water level was comparatively near the surface.
Aside from secular changes in the level of ground water, due to varia-
tions in rainfall extending through geological periods, the shorter periods
of varying rainfall produce some effect upon the level of ground water.
Thus the annual and several-year period variations in rainfall cause slight
changes in it.
All these changes favor alternate solution and deposition—solution
when the level of ground water falls, precipitation when it rises. Where
the ground water has been at a low level large openings to some depth
are likely to be produced. Where later for some reason the level of
ground water rises these openings are in a very favorable position to be
filled with ore, as a result of precipitation from ascending solutions, of the
reactions of descending solutions, and of the mingling of ascending and
descending waters.
Tt might be argued that the existence of ore deposits in the large open-
ings of the belt of weathering is evidence that the ores were not first
deposited by ascending waters. However, as has been already explained,
in such openings there may be concentrated mineral material originally
distributed by ascending waters through a much greater vertical distance.
Thus a very rich ore deposit in large openings, formed by the reaction of
descending waters upon a first concentration produced by ascending waters,
may be bounded below by a horizon in which the ground is very close, the
comparatively small openings which once existed having been greatly
enlarged by solution during reconcentration by descending water.
DEPTH OF EFFECT OF DESCENDING WATERS.
There can be no doubt of the importance of downward-percolating
waters in the production of ore deposits in the zone in which they are
active. The only question which remains open is the depth to which they
are effective. This varies greatly in different districts and in the mines of
«Emmons, S. F., The Delamar and the Horn-Silver mines; two types of ore deposits in the deserts
of Nevada and Utah: Trans. Am. Inst. Min. Eng., vol. 31, 1902, pp. 672-673.
11380 A TREATISE ON METAMORPHISM.
the same district. In general, the effect is likely to be deep seated in pro-
portion as the lode worked is on high ground. (See p. 1181.)
In arid regions the level of ground water is far below the surface and
the process of denudation is slow, so that the downward-moving waters
have both a wide belt in which to work above the level of ground water
and a long time in which to work upon a given horizon. In such regions
the oxides and carbonates are likely to occupy a considerable horizon.
This is very well illustrated by the copper mines of Arizona and New
Mexico and by the ‘‘colorados” of the silver-gold deposits in various arid
regions.
In humid regions, on the other hand, the level of ground water is
likely to be near the surface. If this be combined with marked relief so
that denudation is rapid, the processes of oxidation and carbonation may
be very incomplete above the level of ground water. Indeed, m many cases
erosion may be so rapid that the sulphurets do not have time for more than
very partial oxidation, and they may extend nearly or quite to the surface.
If the rainfall is abundant, descending waters are likely to be effective below
the level of ground water; and if there are large openings and strong relief,
the waters are effective to.a considerable depth. Consequently even where
there are no oxidized products above the level of ground water, the ores are
likely to have been enriched by descending waters in the belt of cementa-
tion. It is very seldom that a deposit is found in which no effect of
descending waters can be discovered.
It has already been seen that the level of ground water may vary from
the surface to a depth of 300 meters or more. Hence it is certain that
from the surface to a depth of 300 meters the ground waters may be a
potent factor in the production of the rich ore deposits. The deposits in
this belt are particularly profitable, not only because of the accessibility of
the material, but because of the fact that there is no expense for pumping;
furthermore, the products are commonly easily reducible. This may be
illustrated by the gold and silver deposits. In the former, the native gold
is free from its entanglement of sulphurets and tellurides; in the latter the
silver is largely in the form of the readily extracted chloride, or in some
instances as native silver. Aspen Mountain furnishes an excellent illustration
of a broad belt of weathering. Here, at a depth of 250 meters, at least one
great body of lead carbonate has been developed. This carbonate contains
a few small crystals of galena, showing its derivation from the sulphide, and
DEPTH OF EFFECT OF DESCENDING WATERS. 1181
therefore showing that the oxidizing, carbonating waters above the level of
ground water have been extremely effective.
Up to this point there will be no disagreement on the part of anyone.
The question now arises as to the depths below the level of ground water to
which descending waters produce their effects. This is a question to be
answered, not by deduction, but by observation. Even Posepny, who
emphasizes the effect of ascending waters, agrees that oxidized products are
the evidence of the work of vadose circulation, or the circulation of lateral
and downward-moving waters. Furthermore, Posepny agrees that the
iron ores of the Lake Superior region,” which are oxidized products, have
been produced by downward-moving waters. A number of the mines
have been worked to a depth of 500 meters or more below the level of
ground water; and in these cases it is perfectly clear that to this depth the
downward-percolating waters have produced an oxidizing effect. This is
true in a region in which the level of ground water is relatively near the
land surface, and which is not mountamous. In various other regions
oxidized products are found also to a very considerable depth below the
level of the ground water.
In the San Juan district of Colorado, in the Revenue tunnel of the
Virginius mine, the effect of oxidizing waters has been observed to the
depth of nearly 1,000 meters, as shown by iron-oxide stains through the
sulphides. How much deeper oxidation may be effective is not known,
and below this depth there may be a belt in which the sulphides may
have been enriched by descending solutions from above without the actual
production of oxidized products. In the Cripple Creek district of Colorado
oxidized products are not found to a great depth, but if the suggestion is
correct that the high-grade tellurides are due to the effect of descending
waters, such waters must have been effective to a depth of 400 meters.
As yet the lowest limits of the enriched belt has not been reached, and
how much deeper it extends is unknown. Both these districts are humid
and of very accentuated topography, and thus furnish exceptionally favor-
able conditions for descending waters to produce deep effects.
The illustrations given furnish positive evidence that the belt in which
descending waters are effective in producing rich secondary concentrates
under favorable conditions extends to a very considerable depth.
«@Posepny, F., Genesis of ore deposits (discussion): Trans. Am. Inst. Min. Eng., vol. 24, 1895,
pp. 966-967.
1182 A TREATISE ON METAMORPHISM.
ILLUSTRATIONS OF SECONDARY ENRICHMENT AND DIMINUTION OF RICHNESS WITH DEPTH.
The processes have now been explained by means of which a rich upper
belt may be produced. If the argument be correct, it is an inference from
this that ore deposits which have undergone a second concentration are
likely to diminish in richness with depth, provided a considerable belt be
considered. It remains to give the facts which confirm the above hypoth-
esis, and illustrate diminution of richness with depth.
At Ducktown, Tenn., at the level of ground water, appeared a belt
of rich black copper ore (copper glance), which varied from less than 1 to
about 25 meters in thickness. Above this belt was gossan, very poor in
copper; below it a very low grade cupriferous pyrrhotite.“ In this instance
it can hardly be doubted that originally the lean cupriferous pyrrhotite
extended not only to the present surface but probably much higher. The
downward-moving waters transported copper to its locus near the level of
eround water. Here the copper salts reacted on the iron sulphide and
produced rich sulphurets.
A case which has been, perhaps, more closely studied than any other
in the United States is that of the deposits of Butte, Mont. Douglas
states that here rich oxysulphurets are found near the surface. On the
summit of the hill “it seems as if the copper, leached out of the 400 feet
(120 meters) of depleted vein, had been concentrated in the underlying ore,
and had thus produced a zone of secondary ore about 200 feet (60 meters)
deep, which contains, as might be expected, about thrice its normal copper
0b
content. Of the Butte deposits Emmons says:
Secondary deposition, or transposition of already deposited minerals, has played
an unusually important rdle. In the case of the copper veins it has not been confined
to the oxidizing action of surface waters, which has resulted in an impoverishment of
the ore bodies, but below the zone of oxidation it has resulted in the formation of the
richer copper minerals bornite, chalcocite, and coyellite, in part, at least, by the
breaking up of original chalcopyrite. Unusual enrichment of the middle depths
of the lodes has thus been caused. Whether the two processes of impoverishment
and enrichment have been differing phases of the action of descending waters, or
whether the latter may have been a later result of the rhyolite intrusion, has not yet
«Blake, W. P., The persistence of ores in lodes in depth: Eng. and Min. Jour., vol. 55, 1893,
p. 3. Henrich, C., The Ducktown ore deposits and the treatment of the Ducktown copper ores: Trans.
Am. Inst. Min. Eng., vol. 25, 1896, pp. 206-209.
> Douglas, James, The copper resources of the United States: Trans. Am. Inst. Min. Eng., vol. 19,
1891, p. 693.
ILLUSTRATIONS OF SECONDARY ENRICHMENT. 1183
been definitely decided. It is, however, fairly well determined that the enrichment
of the copper deposits is so closely associated with the secondary faulting that it may
be considered to be a genetic result of it.¢
While the larger and richer sulphide bodies were found at from 60 to
125 meters below the water level, rich sulphide bodies of smaller size are
here and there found at a depth of 450 meters below the water level,
“though in apparently decreasing amount as compared with the immense
»’ Brown states that in the same area oxidized
thickness of pyritous ore.
products extend to the level of ground water. These oxidized products,
according to Brown, promptly change at water level into normal sulphurets.
‘““There follows below a region of varying height, of valuable rock,. which
again slowly deteriorates in depth, this deterioration, however, being so
Nec
retarded finally as to be scarcely appreciable.”’ He further says that
above the level of ground water is gossan “‘ carrying high values in silver,
and particularly in gold.”* Thus at Butte we have enrichment in silver and
gold and depletion in copper in the belt above the level of ground water as
compared with the material below it; and at and below the level of ground
water we have rich sulphides of copper which grade into leaner sulphurets.
In the case of the Butte deposits it can hardly be doubted that the compar-
atively lean sulphides in the deeper workings represent the product of a
first concentration, and that the modifications of this material found above
and below the level of ground water represent the work of downward-
moving waters. ‘To account for the high values of gold and silver above
the level of ground water, one must suppose that this belt has received con-
tributions of these metals from the upward extension of the veins which has
now been removed by erosion. The great richness of the copper below
the level of ground water Douglas clearly attributes to the downward trans-
portation of the material from the depleted copper veins. However, a part
of this material was doubtless derived from an upward extension of these
veins, precisely as in the case of the gold and silver. For my own part, I
have little doubt that the precipitation of the rich sulphides was produced
by reactions upon the lean sulphurets, as given in the equations on
pages 1161-1164.
«Emmons, 8. F., Economic geology of the Butte special district, Montana: Geologic Atlas U. &.,
folio 38, 1897.
6Emmons, 8. F., The secondary enrichment of ore deposits: Genesis of ore deposits, Am. Inst.
Min. Eng., 2d ed., 1902, p. 444.
¢ Brown, R. C., The ore deposits of Butte City: Trans. Am. Inst. Min. Eng., vol. 24, 1895, p. 556.
@ Brown, cit., p. 555.
1184 A TREATISE ON METAMORPHISM.
Penrose cites the Arizona copper deposits as instances of secondary
concentration. These deposits he regards as produced by leaching of the
copper from a lean copper-bearmg pyrite, and its segregation at the places
where the rich ores occur. In this process Penrose, however, says that the
volume of the deposit must be decreased; but he makes the point that
the smaller amount of the rich product is more valuable than a larger
lean deposit, because more easily mined and more readily reduced.“ This
process of concentration is further described by Douglas, who notes, also,
that the changes have resulted in the production of enriched sulphides from
very lean sulphides in the Copper Queen mine. Here, according to Doug-
las, a large very low-grade copper-bearing pyrite deposit running from the
60- to the 120-meter level contains rich oxysulphides and black sulphides
on the outside and mainly lean pyrite in the interior.’ The original mate-
rial in the Arizona locality is as plainly a lean cupriferous pyrite as in
Tennessee. Here, however, on account of the peculiar climatic conditions,
the alterations have been of a different kind and have not extended to a
uniform depth. Instead of the rich belt being a horizontal sheet, it oecurs
in a zone about a large cupriferous pyrite mass; but the principles of
concentration are identical, and the rich products are unquestionably in
part due to reactions between the oxidized salts and the lean sulphides.
The rich oxidized products of the Southwest doubtless were produced
directly from the enriched sulphurets. Therefore, in the formation of the
rich oxidized products there were two stages of alteration; first, the pro-
duction of rich sulphurets by the reaction of oxidized products upon the
lean pyritiferous material, and after that oxidation of the rich sulphurets.
These oxidized ores formed partly in situ, but there has been also more or
less of transfer of material from one place to another.
An excellent illustration of an enriched upper belt in the case of gold
is furnished by the gold-quartz veins of Grass Valley, California, where,
according to Lindgren, the decomposed belt of weathering, about 50 meters
in depth, contains “from $80 to $300 per ton, while the average tenor in
depth is from $20 to $30.”° Furthermore, the rich 50 meters, which con-
«Penrose, R. A. F., jr., The superficial alteration of ore deposits: Jour. Geol., vol. 2, 1894, pp.
306-308.
> Douglas, James, The Copper Queen mine, Arizona: Trans. Am. Inst. Min. Eng., vol. 29, 1900,
pp. 532-533.
¢ Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California: Seven-
teenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1896, p. 128.
ILLUSTRATIONS OF SECONDARY ENRICHMENT. 1185
tain from four to ten times as much gold as the sulphurets below the level of
ground water, are depleted in silver. In some veins the sulphurets extend
almost to the surface. Lindgren further states that the sulphurets below
the level of ground water continue with undiminished richness to a depth
of 500 or more meters.* He adds that the California region is one in which
denudation has extended to a depth of 500 to 1,500 or more meters.’ From
these facts it is highly probable, as suggested by Lindgren, that sulphurets
similar to those below the level of ground water were deposited above the
present surface. If this were the case the only possible explanation of the
belt of weathering rich in gold and depleted in silver is that descending
waters have abstracted a large part of the gold from the material removed
by erosion, and have deposited it in the belt of weathering. Its precipita-
tion there was, doubtless, mainly due to reduction by the sulphides,
producing sulphurets richer in gold. Later, the sulphides have been
oxidized, leaving the enriched belt of free gold. The silver apparently has
been transported downward to a greater extent. One would expect that
correlative with the belt above the level of ground water poor in silver,
there would be a belt at and below the level of ground water richer in
silver. Upon this point Lindgren does not give us information.
Another very interesting case of richness of the belt of weathering
in gold, as compared with the unaltered sulphides below, is furnished by
the Australian gold fields, where the belt above the level of ground water
is several times as rich as the unaltered tellurides and sulphides below;
some mining men say ounces above to pennyweights below.’ This rich belt
is from 15 to 120 meters wide. Ina portion of the mines of some districts—
when the bottom of the oxidized zone
for example, the Kalgoorlie district
is reached, the ores are so lean as to be valueless, so that mines which were
profitable in the weathered zone did not pay below it. Many of the mines
of that district, however, are profitable below the weathered horizon. If it
had not been for the secondary enrichment by denudation and downward
transportation of material, many of the mines would not have been exploited,
@ Lindgren, cit., pp. 161-163.
> Lindgren, cit., pp. 182-183.
¢ Don, J. R., The genesis of certain auriferous lodes: Trans. Am. Inst. Min. Eng., vol. 27, 1898,
p. 596.
@ Hoover, H. C., The superficial alteration of western Australian ore deposits: Trans. Am. Inst.
Min. Eng., vol. 28, 1899, pp. 762-764.
5°)
MON XLVII—04——75
1186 A TREATISE ON METAMORPHISM.
although Hoover thinks that in this strange country the downward concen-
tration is more mechanical than chemical. Thus the secondary concentra-
tion by descending waters is no less an important part of the genesis cf the
gold ores of Australia than the first concentration by ascending waters.
It has already been fully explained (p. 1144 et seq.) that the lead and
zine deposits of the Mississippi Valley furnish clear cases of the importance
of the action of descending waters in enriching ores first concentrated by
ascending waters.
The Leadville deposits furnish an instance of the decrease of the rich-
ness in silver with depth. Emmons says: ‘There is a fair foundation for
the generalization that in the deposits, as developed at the time of this
investigation, the ores were growing poorer in silver as exploration extended
farther from the surface.”*
The ores early mined contained scarcely any zinc. Emmons states of
the ores of the deeper levels later mined that the ‘‘sulphide ores consisted
of mixtures of pyrite, galena, and zinc blende, the latter in fairly equal
amounts.” The sulphide bodies immediately below the belt of oxidation
are richer in zine blende than the ore at greater depth. The material rich
in zinc below correlates with the ores low in zine near the surface, the zine
sulphide of that horizon having been oxidized, and the zinc carried down
and redeposited.”
Another case of the diminution of the richness of sulphurets with
depth is furnished by the nickel mines of Lancaster Gap, Pennsylvania,
which were not worked beyond a depth of about 75 meters, presumably
because ‘‘the ore decreased in richness as depth was attained.” °
The San Juan district of Colorado gives excellent illustrations of
decrease of values with depth. The upper parts of the lodes were usually
very rich in silver, not infrequently ores running 300 ounces or more per
ton. In the deeper workings of the veins the values in silver usually ran
down, in many cases very rapidly, so that, at a depth of a few hundred
meters the amount of silver became so low that work upon many of the
vol. 12, 1886, pp. 554-555.
> Emmons, 8. F., The secondary enrichment of ore deposits: Genesis of ore deposits, Am. Inst.
Min. Eng., 2d ed., 1902, pp. 439-440.
¢Kemp, J. F., The nickel mine at Lancaster Gap, Pennsylvania: Trans. Am. Inst. Min. Eng.,
vol. 24, 1895, p. 626. Discussion by E. E. Olcott, p. 884.
ILLUSTRATIONS OF SECONDARY ENRICHMENT. 1187
lodes ceased comparatively early in the history of the district. Regarding
these ores Ransome says:
In spite of the diversity shown by the different ore bodies, there is after all
remarkable uniformity to be found in the change at very moderate depths—usually
less than 300 feet [91 meters]—from an ore consisting chiefly of argentiferous galena
to highly argentiferous silver-copper ores, and then a gradual diminution of value
downward through the increasing proportion of low-grade pyrite in the ore bodies.
These changes are best recorded in the Yankee Girl, Guston, and Silver Bell mines.“
In some of the deeper lodes of the district the values have become so
low that they do not warrant working. While in some lodes there has also
been a decrease in the amount of gold with depth, the gold values in gen-
eral have held up very much better than the silver, and in some of the
mines at a depth of 500 or more meters below the surface, as at the Camp
Bird, the values are still very high in gold. Possibly the explanation of
the difference between the two metals is that from the descending solutions
the silver was thrown down more rapidly than the gold. Since silver is
precipitated as a sulphide, it is very quickly thrown down from its soluble
salts in consequence of the reaction of the sulphides of the base metals,
such as galena, sphalerite, and pyrite; whereas by these compounds the
gold is thrown down from its solutions in metallic forms, and is therefore
probably more slowly precipitated, and consequently secondary enrichment
extends much deeper.
The Monte Cristo district of Washington, described by Spurr, gives a
most excellent illustration of the principle of decreasing richness with
depth. In that district there is a definite arrangement of the ores verti-
cally, which arrangement has a very marked relation to the topography.
The order downward is, (1) oxidized ores; (2) upper sulphide belt, char-
acterized by galena, blende, and chalcopyrite, carrying the highest values
in gold and silver and extending not more than 100 meters below the
surface; (3) intermediate sulphide zone, having reduced amounts of galena,
blende, and chalcopyrite, and with these realgar, arsenopyrite, pyrite,
and pyrrhotite; and (4) lower sulphide zone, low in gold and silver, and
consisting of pyrite, pyrrhotite, and arsenopyrite, and in which galena and
blende are subordinate or absent.’ In the upper sulphide belt the ‘“galena
«Ransome, F. L., A report on the economic geology of the Silverton quadrangle, Colorado: Bull.
U.S. Geol. Survey No. 182, 1901, pp. 111-112.
oSpurr, J. E., The ore deposits of Monte Cristo, Washington: Twenty-second Ann. Rept. U.S.
Geol. Survey, pt. 2, 1901, pp. 841-857.
1188 A TREATISE ON METAMORPHISM.
zone is shallowest and is overlapped by the blende zone, and this by the
chalcopyrite zone.”* Thus in this upper belt there is a definite order of
prominence of these three minerals—galena, blende, and chalcopyrite.
In addition to these specific instances of the production of a rich
upper belt, some general statements have been made which need to be
referred to. One of these is made by Douglas in reference. to sulphuret
mines as a whole. Says he, in the conclusion of his discussion as to the |
copper resources of the United States, with reference to the various Appala-
chian deposits, ‘Like all sulphuret mines, they became poorer as depth
was attained.” ’
With the exception of the San Juan and Monte Cristo districts, the
above illustrations of secondary enrichment and diminution of richness with
depth are the same as originally published by me.’ Recent articles by other
‘authors have given many other instances of secondary enrichment which
fall in line with the above illustrations, but which in this treatment of
principles need not be repeated. Some of the more notable contributions
upon this subject are those by Emmons,” Weed,’ Ransome,’ and Bain.’
Upon the hypothesis that the rich ores of the upper parts of deposits
usually result from secondary action of descending solutions upon material
no richer than the deeper parts of the deposits, the extent and richness of
such deposits may give an approximate idea of the minimum denudation
which a district has suffered. For example, at Ducktown, Tenn., to
produce the rich oxidized products of copper above the level of ground
water, and the very rich sulphides at and near it, from the lean, cuprifer-
ous pyrrhotite below, which bears about 2 per cent of metallic copper,
«Spurr, cit., p. 841.
> Douglas, James, The copper resources of the United States: Trans. Am. Inst. Min. Eng., vol. 19,
1891, p. 694.
¢ Van Hise, C. R., Some principles controlling the deposition of ores: Trans. Am. Inst. Min. Eng.,
vol. 30, 1901, pp. 128-134.
d@ Emmons, 8. F., Secondary enrichment of ore deposits: Trans. Am. Inst. Min. Eng., vol. 30, 1901,
pp. 177-217.
é Weed, W.H., Enrichment of mineral veins by later metallic sulphides: Bull. Geol. Soc. Am., vol.
11, 1900, pp. 179-206.
f Ransome, F. L., Ore deposits of the Rico Mountains, Colorado: Twenty-second Ann. Rept. U. 8.
Geol. Survey, pt. 2, 1901, pp. 229-397. Report on the economic geology of the Silverton quadrangle,
Colorado; Bull. U.S. Geol. Survey No. 182, 1901, pp. 265.
g Bain, H. F., with Van Hise, C. R., and Adams, Geo. I., Preliminary report on the lead and zinc
deposits of the Ozark region: Twenty-second Ann. Rept. U.S. Geol. Survey, pt. 2, 1901, pp. 23-227.
ILLUSTRATIONS OF SECONDARY ENRICHMENT. 1189
there must have concentrated in the shallow deposits at the surface an
amount of copper which was originally distributed through a vertical
distance of hundreds of meters. At Butte, Mont., the production of the
rich sulphide deposits characteristic of that district must also have required
enormous denudation. In this case where the belt of weathering is depleted
in copper, it may be supposed that during the secondary concentration the
major part of the copper originally distributed through a great vertical
distance was carried downward and reprecipitated; and that the losses in
consequence of denudation were not great. But in other cases, where
geological studies show that great denudation must have taken place, the
amount of enriched material is comparatively small. For example, in the
Sierra Nevada gold belt, the enriched product has a thickness of not more
than 30 meters and is not more than five times as rich upon the average as
the sulphides of the first concentration below. It would therefore follow
that the gold distributed through 150 meters of the first concentration
would be sufficient to account for all of the enriched product, but Lindgren
places the extent of the denudation in this district as about 3,000 meters.
Hence in this case but a small part of the gold has been taken into solution
by descending waters and transported below and reprecipitated. It
therefore follows that the larger part of the gold produced by the original
concentration has been carried away from the fissures by the processes of
denudation, and correlative with these great losses to the veins are the
placer deposits of California. While the material was mainly carried away
from the veins, it was in great measure segregated in the beds of streams,
past and present, from which deposits it is being extracted.
These instances of the relation of depth of denudation and amount of
enriched deposits, where the evidence seems to be particularly clear, are
mentioned with a view of suggesting an interesting line of study with
reference to ore deposits rather than with an idea of giving exact or
comprehensive Gata upon the subject.
GENERAL STATEMENTS.
It is believed to be a very general rule, if a long enough seale be used,
that ore deposits diminish in richness with depth. But it is well known
that above the level of ground water the valuable materials may be almost
wholly dissolved and deposited at or below the level of ground water
1190 A TREATISE ON METAMORPHISM.
by the reactions above stated, as at Ducktown, Tenn., or partly dissolved
and transported below, as at Butte, Mont. Thus for a certain depth the
ores may increase in richness. This exception, however, does not affect
the common rule as to diminution of richness with increasing depth.
Penrose,“ in 1894, in discussing the superficial alteration of ore
deposits, says :
Asaresult of these various changes, certain materials are sometimes leached
from the upper parts of ore deposits, which have become porous by alteration, and
carried down to the less pervious unaltered parts. Here they are precipitated by
meeting other solutions, or in other ways, and hence the richest bodies of ore in a
deposit often occur between the overlying altered part and the underlying unaltered
part. This is not always the case, but it is true of some copper, silver, iron, and
other deposits.”
De Launay,’ in 1897, emphasizes the frequent occurrence near the
surface of rich deposits, which in some cases are oxidized products and in
others are sulphides. He explains the richness of the deposits by the
abstraction of more soluble material. This frequently results, he thinks, in
transforming a low-grade product into a rich ore. By this process a poor
sulphide may be changed to a rich sulphide, as, for instance, cupriferous
pyrites or chalcopyrite may be transformed to covellite or chalcocite by
abstraction of iron sulphide. It is a natural deduction from de Launay’s
explanation” that the volume of the material is decreased, although he
does not make this point.
De Launay further emphasized the point that the metallic material of
veins may have been repeatedly transferred from one place to another, and
suggests that a part of the material now found in veins may have been
transferred from vein material which was once above the present surface of
denudation. :
Le Conte’ suggested that the rich belt may be explained by supposing
that precipitation by ascending waters does not occur at great depth,
because the solutions do not get saturated until comparatively near the
level of underground water. But it is to be remembered that the upper
“Penrose, R. A. F., jr., The superficial alteration of ore deposits: Jour. Geol., vol. 2, 1894, pp.
288-317.
>Penrose, cit. p. 294.
¢Launay, M. L. de, Contributions 4 l’étude des gites métalliféres: Annales des Mines, 9th ser.,
vol. 12, 1897, pp. 151-152.
@ Launay, de cit. p. 194.
e Le Conte, Jos., On the genesis of metalliferous veins: Am. Jour. Sci., 3d series, vol. 26, 1883, p. 12.
RICH DEPOSITS NEAR SURFACE. IL
part of a fissure receives abundant lateral waters which have been trans-
ported a comparatively short distance under conditions of low pressure and
temperature; whereas the solutions lower down have taken a longer journey
under conditions of high pressure and temperature. If Le Conte’s explana-
tion be satisfactory, one would expect the most insoluble constituent to be
precipitated at the greatest depth. In the case of the lead-zinc-iron deposits
this would make the galena abundant at depth, the sphalerite abundant
at a higher level, and the iron sulphide the dominating constituent at the
highest levels. In the case of the copper-iron deposits the rich sulphides
of copper would be in the lower levels and the cupriferous pyrites at the
higher levels.
From the foregoing it is apparent that there has been a general under-
standing that a rich upper belt has been produced in many ore deposits.
My explanation of a rich upper belt is, mainly, that oxidized soluble com-
pounds are produced in the belt of weathering, and that these in situ or
lower down react upon the lean sulphides and tellurides. In this way arich
belt is formed. Later,in consequence of denudation, these rich sulphides and
tellurides pass into the belt of weathering. Here they are again exposed
to the oxidizing forces, are largely transformed in situ to oxides, carbonates,
ete., and a belt of rich oxidized products is formed above the ground water.
In part, when oxidized, the materials are taken into solution, again trans-
ported downward, and again react upon the sulphides and tellurides. In
arid regions, where the amount of downward-moving water is small, the
oxidized products formed from the rich sulphides and tellurides are likely
to remain in large part in situ; but in humid regions, where water is abundant,
the metals, after oxidization, are in large measure carried downward and
again react upon the sulphides and tellurides below and further broaden and
enrich this belt. Thus, under different climatic conditions, we may have
a rich oxidized zone, a rich sulphide and telluride zone, or both, in varying
proportion.
It is notable that the vertical distribution of the sulphides in the rich
zone is precisely that which should be expected from the affinity of the
metals for sulphur In the lead-zinc-iron districts the galena is found at
the highest horizon, lower down is the sphalerite, and still lower the iron
sulphide. In the copper-iron lodes the high-grade copper sulphides,
chalcocite and bornite, are at higher levels; lower these are intermixed
192 A TREATISE ON METAMORPHISM.
with chalcopyrite; still lower the chalcopyrite is mixed with pyrite; and in
the deepest levels the pyrite is dominant. Apparently this vertical arrange-
ment of the sulphides can not possibly be explained by precipitation by
ascending waters. From such waters we would expect exactly the reverse
distribution.
While the reaction between the oxidized products and the sulphides
has been strongly emphasized, because it is believed to be the most funda-
mental of the causes producing a rich upper belt, it is understood that
other factors may also help in this process. As already pointed out,
reduction and precipitation of the metals of descending solutions may take
place through the agency of organic matter or other reducing materials
contained in the rocks, or by meeting ascending solutions carrying precipi-
tating agents. Near the surface more than an average amount of original
precipitates from ascending solutions is a possibility in some cases. (See
p- 1077.)
Concluding, it appears to me that the existence of a rich upper belt
in many deposits, and the frequent diminution of the ores in richness in
passing from the surface to some distance below the level of ground water,
can not be the work of ascending or descending waters alone, but is due to
ascending and descending waters combined. Ascending waters produce a
first concentration. A second concentration by descending waters produces
the rich deposits. These rich products are found in the few meters or few
hundred meters of the outer crust of the earth. When it is remembered
that the greater part of the ores which have yet to be abstracted from the
earth comes from the first 500 or 700 meters, and when it is further con-
sidered that the effect of descending waters may be felt to these depths, it
becomes evident that the process of second concentration by descending
waters is a very important one indeed, so far as the economic value of ore
deposits is concerned. Indeed, as a result of it there is concentrated in
the extreme outer shell of the crust of the earth a large portion of the
products which during the first concentration may in many cases have been
distributed over 1,500 or 3,000 meters or more, but which have now been
largely removed by erosion. We therefore conclude that for a large class
of ore deposits a second concentration by descending waters can not be
said to be one whit less important in the genesis of ores than a first concen-
tration by ascending waters.
COMPLEXITY OF CONCENTRATION. 1193
It follows from the foregoing that one of the most important classes of
ore deposits is that produced by the joint action of ascending and descending
waters. First comes the action of the downward-moving, lateral-moving
waters, mainly of meteoric origin, which take into solution metalliferous
material. In regions of recent volcanism these waters may be joined by
subordinate amounts of water exuded during the crystallization of magmas;
and such waters may be rich in metallic material. The waters, after
collecting metals from any or all of the rocks with which they come into
contact, are converged into trunk channels and there, while ascending,
may deposit ore of the first concentration. During and after this first
concentration many of the ore deposits which are worked by man have
undergone a second concentration not less important than the first as a
result of descending lateral-moving waters. In some cases a concentration
by descending lateral-moving waters alone is sufficient to explain ore
deposits. It, therefore, appears more clearly than heretofore that an
adequate view of ore deposits must not be a descending-water theory, a
lateral secreting water theory, or an ascending-water theory alone. The
descending, lateral-moving, and ascending waters alike are driven mainly
by gravity. Each performs its own work.
SUBCLASS 3. ORES PRECIPITATED FROM DESCENDING AQUEOUS SOLUTIONS.
For the sake of simplicity and continuity of exposition, the effects
produced by descending waters have been applied to deposits which have
been first concentrated by ascending waters. However, it is perfectly clear
that a concentration by descending waters alone may be adequate to
produce ore deposits. The more important ores which are produced by
descending waters alone are those of iron and manganese. The ores of
these metals thus formed are chiefly oxides, and so far as iron and
manganese are concerned are largely hydrated oxides.
IRON ORES.
The ores of iron, unlike those of the other metals, occur in such large
masses that they properly belong with the rocks, hence their general
development has been already considerered in Chapters IX and XI. It
remains here only to summarize the special factors which have been of
consequence in their segregation. Of the ore deposits which it has been
1194 A TREATISE ON METAMORPHISM.
shown are produced by descending waters alone, those of the Lake
Superior region are unquestionably of the greatest importance.
In another place I have fully discussed the segregation of these
iron-ore deposits.” Without exception the great iron-ore deposits of the
Lake Superior region are found resting upon impervious formations. In
the great majority of cases these impervious formations are in pitching
troughs. (See Pl. XIII.) Thus the trunk channels of circulation are
formed. For the most part these troughs are somewhat close and have
steep pitches, but in the Mesabi district the troughs are less definite in form
and have a very shallow pitch. The impervious basement for an iron ore
deposit may be an impervious sediment such as a slate, may be an igneous
rock, ora combination of the two. The essential thing is that the basement
shall be relatively impervious to water as compared with the iron-bearing
formation in which the ores occur. The source from which the iron oxide
is obtained is the original rock of the iron-bearing formation, either siderite
or hydrous ferrous silicate, greenalite. As the water starts on its down-
ward journey the oxygen it contains is exhausted in the oxidation of the
iron carbonate or ferrous silicate, thus producing the ferruginous slates and
cherts, at the same time liberating the carbon dioxide of the carbonates.
The waters from which the oxygen is exhausted and which are enriched in
carbon dioxide are now able to take the ferrous carbonate and hydrated
ferrous silicate into solution. Where impervious basements exist in pitch-
ing troughs, the downward-moving waters are deflected toward these, and
thus are converged into such troughs. Other waters more directly from
the surface which have not come into contact with iron carbonate and iron
silicate, and therefore bear oxygen, are also converged into the troughs.
The mingling of the two classes of waters in the troughs results in the
precipitation of the hydrated hematite and limonite, the reaction in the
case of the iron carbonate being:
2FeCO,+nH,0+0=Fe,0;. nH,O+ 2C0,
At the same time the abundant waters converged in the trunk channels
dissolve the silica, and thus the ores are depleted in that compound. So
far as there was iron carbonate or iron silicate in the pitching troughs the
ferrous oxide is largely oxidized in situ. Thus, the ore deposits are the
aVan Hise, C. R., The iron-ore deposits of the Lake Superior region: Twenty-first Ann. Rept.
U.S. Geol. Survey, pt. 3, 1901, pp. 305-434.
PLATE XIII.
IRON ORE DEPOSITS IN PITCHING TROUGHS.
(Both ore exploited and ore now in mine are represented as ore, since the purpose of this plate
is to show the manner of the development of the ore rather than the present stage of exploitation. )
Fig. 1. Vertical north-south cross section of Chandler mine, showing relations of the ore deposit
to the soap rock, Ely greenstone, and ore-bearing formation. The iron ore is in a broad
U-shaped trough, bottomed by soapstone or paint rock which grades down into greenstone.
It is capped by the ore-formation material. At the place where the cross section is made
the ore does not extend to the surface along either limb. Therefore, at the particular place
where this cross section exists, although there is a very large ore deposit below the surface, at
the surface the only rocks which are found are the greenstone, soapstone and iron-bearing
formation.
Scale: 1 inch equals 250 feet.
Fig. 2. Vertical east-west longitudinal section of Chandler mine, showing the same relations as fig. 1.
The figure very well illustrates how the ore body increases in size from the surface. Where
the ore reaches the drift its area is small; and this great ore deposit, which extends eastward,
where it constitutes the Pioneer mine, is below a heavy capping of the ore-bearing for-
mation.
Scale: 1 inch equals 250 feet.
1196
U. S, GEOLOGICAL SURVEY MONOGRAPH XLVII PL. XilI
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IRON-ORE DEPOSITS IN PITCHING TROUGHS.
CONCENTRATION OF IRON ORES. LID?
result largely of the transportation of iron carbonate to the pitching troughs,
the iron being there precipitated, and partly of oxidation of ferrous carbonate
or silicate in place. Simultaneously with these processes the silica is dis-
solved and abstracted.
It is to be noted that the precipitation of iron ores by the above process
is due essentially to the mingling of solutions from two different sources.
The solutions from one source have taken a longer journey, or else have
passed through rocks more capable of abstracting oxygen, so that they
early acquired the properties of deep solutions—an excess of carbonic acid
-and an absence of oxygen. Under such cireumstances they carry iron ear-
bonate precisely as do the deep solutions of the first concentration in ores
produced by ascending waters. The solutions from the other source pass
through material in which the iron has already been oxidized to the ferric
form, and therefore do not have the oxygen extracted. The course of this
water is not necessarily shorter than that of the other waters, but upon the
average is likely to be so. When the two solutions come together the iron
oxide is precipitated, in consequence of the mingling of solutions. While
for the Lake Superior iron ores apparently the solutions from both sources
were descending to the point where they became mingled, in iron-ore
deposits in other regions it may be found that the precipitation of the iron
oxide was due to the mingling of ascending solutions bearing iron carbonate
with descending solutions bearing oxygen.
For instance, at certain localities the iron ores are magnetite instead of
hydrated hematite. In Chapter IX the various ways in which magnetite
may be produced are discussed, but it may be suggested that magnetites
may be due to the direct precipitation of ascending solutions bearing iron
carbonate and descending solutions bearing an insufficient amount of
oxygen for transformation to the ferric oxide, the reaction being as follows:
3FeCO;+ 0=Fe,0,+3C0,.
Perhaps the most notable magnetite deposit in the United States is that
of the Cornwall mine in Pennsylvania. Here the ore is a replacement of
Silurian limestone, and rests upon a trap dike as an impervious basement.
The Mesozoic rocks are faulted against the limestone and ore. A plausible
suggestion as to the origin of this deposit is that ascending waters bearing
iron carbonate, moving up along the fault and through the limestone, met
1198 A TREATISE ON METAMORPHISM.
descending waters bearing oxygen and following the impervious basement,
and as the oxygen was not in sufficient quantity to oxidize the iron to the
ferric oxide it was thrown down as magnetite.
It has been pointed out (pp. 845-846), that magnetite may be pro-
duced in two other ways—by the reaction of siderite and pyrite upon
each other, or by the partial oxidation of pyrite or pyrrhotite. In the
Cornwall deposits* sulphur is a constant associate in small percentage
except at the surface, and this fact suggests the possibility that the reactions
between iron carbonate and iron sulphide or between iron sulphide and
oxygen may have been also instrumental in the production of the magnetite;
but the amount of residual sulphur is so small that the theory of the
derivation of the deposit in large measure from pyrite has scanty support.
‘To what extent the principles worked out in reference to the Lake
Superior ores are applicable to the iron-ore deposits of other parts of the
world it is yet too early to say, but, as just suggested, the descriptions of
the Cornwall deposit show that it has many features in common with the
ores of the Lake Superior region. Many of the Silurian deposits of the
Appalachian region are at contacts of limestones with shale or slate. In
some places these latter are certainly in such positions as to furnish imper-_
vious basements, and therefore the ores present many features analogous to
those of the Lake Superior region. Buta much more extended study of the
iron-ore deposits of other regions must be made before any generalization
can be made with reference to them.
MANGANESE ORES.
The manganese deposits, like the iron-ore deposits, are very largely
secondary segregations by descending waters. Indeed, iron and manganese
are very closely associated. Many of the iron-ore deposits are manganifer-
ous, and some of them sufficiently so locally that they also become ores of
manganese. For instance, in the Gogebic district of Michigan a small
amount of the iron ore was sufficiently manganiferous to be sold as a man-
ganese ore rather than an iron ore. Manganese ores are always associated
with iron oxide. The amount of iron is variable, running from a small
percentage to an amount considerably in excess of the manganese.
«Lesley, J. P., and d’Inyilliers, E. V., Report on the Cornwall iron-ore mines, Lebanon County:
Ann. Rept. Geol. Survey Pennsylvania for 1885, pp. 533-534.
CONCENTRATION OF MANGANESE ORES. 1199
Manganese is present in the original igneous rocks in very much
smaller quantity than iron, and therefore the manganese deposits are much
smallcr than those of iron; but so far as can be made out, the segregation of
manganese and iron are strictly parallel processes. Thus in the secondary
rocks manganese is found only as traces in the sandstones and shales, but
in the limestones—carbonate rocks—it is more than half as abundant as in
the original rocks. Since the limestones are so small in mass, only a small
fraction of the manganese of the original rocks is in the limestones.
Apparently the great mass of the manganese of the original rocks has been
segregated in the ores of manganese and iron and in the iron formations.
: In view of all the foregoing facts it is thought to be extremely probable
that the manganese is largely transported as a carbonate from the original
rocks to lagoons, and is there precipitated as an oxide. By this process
bog ores of manganese are produced just as are the bog limonites. But to
a large extent the manganese oxide is reduced to the manganous form by
the organic matter, reunites with carbon dioxide, forming carbonate, and in
this form is built up into carbonate formations. From such carbonate
formations it is segregated by underground solutions which have different
sources and mingle in the trunk channels. Some solutions bear the manga-
nese carbonate and others the oxygen. When these two meet the manga-
nese is precipitated as hydrated oxide according to the following reaction:
MnC0,+0+nH,0=Mn0,. nH,0-+CO,.
It is yet too early to state to what extent the manganese ores have been
concentrated on impervious basements in pitching troughs.
IV. SPECIAL FACTORS AFFECTING THE CONCENTRATION OF ORES.
Thus far the discussion of ores deposited from aqueous solutions has
not taken into account a number of the special factors which affect the
concentration of ores. The general discussion may need great modification
to adapt it to a particular district. To illustrate my meaning, it may be
well to consider some of the additional factors affecting the deposition of
ores, and to point out the more obvious possible modifications of the general
theory which may result from them. There will be briefly considered the
effect of (1) variations in porosity and structure, (2) the character of the
topography, and (3) physical revolutions.
1200 A TREATISE ON METAMORPHISM.
VARIATIONS IN POROSITY AND-STRUCTURE.
It has been fully explained that a vigorous circulation is essential for
the production of aqueous ore deposits. It has been pointed out that in
order to produce an ore deposit the amount of water which must flow
through the openings is many thousands of times the volume of the ores
deposited. Also it has been explained that waters do not produce channels.
In order to have a vigorous circulation original and continuous channels
must exist. Thus initial porosity in rocks is an essential factor in the devel-
opment of ore deposits. In the foregoing discussion the existence of trunk
channels has been recognized, but otherwise it has been assumed _ that
the rocks were equally porous in all directions. As a matter of fact, the
rocks range between the widest extremes in porosity; from those in which
the openings are subcapillary and which are substantially impervious to
those which are as open as a sieve in all directions.
The different masses of sedimentary rocks, igneous rocks (especially
the lavas), and metamorphosed equivalents of either sedimentary or igneous
rocks may differ in porosity.” The contacts of rocks of different kinds fre-
quently furnish trunk channels for circulating water. Bedding partings
produced by shearing stresses during deformation furnish sheet channels
parallel to the strata, or openings on the anticlines or synclines. Some
strata when deformed may yield by fracture, furnishing channels for water
circulation, while interlaminated strata may yield by flowage, thus remain-
ing relatively impervious. These various irregularities may combine in
different ways.
All irregularities in porosity and structure require modification of the
simple general statements of the present paper (see pp. 129-156, 571-589)
concerning the character of underground circulation and the concentration
of ore deposits. At some time it may be possible to divide the moditica-
tions of the general circulation due to variation of porosity and structure
into classes, but at present this can not be done. The modifications of the
general circulation which occur in many districts must first be studied and
deseribed individually, after which generalizations may possibly be made.
At present some statements may be made in reference to certain modifica-
tions of the underground circulation.
«Compare Emmons, Structural relations of ore deposits: Trans. Am. Inst. Min. Eng., vol. 16,
1888, pp. 804-839.
VARIATION OF POROSITY AND STRUCTURE. WADI
DISTRIBUTION AND SIZE OF OPENINGS.
From the foregoing general statement it is clear that one of the essen-
tial conditions for the production of large ore deposits is that openings should
be of adequate size. As will be seen, these openings may be original or
they may be largely secondary. A close analysis of the manner in which
the waters work shows that it is not of advantage in ore deposition to have
equal porosity in all directions. Were the rocks equally porous in every
direction there would be no reason for the concentration of the water in
trunk channels, and therefore the conditions which have been discussed as
essential for the mingling of solutions of different kinds would not exist.
Oftentimes the essential condition is to have considerable masses of rocks
nonporous or moderately porous adjacent to or associated with other areas
which are very porous. Oftentimes where the porous belts are too numer-
ous there is a segregation of a small amount of ore in them all, and not an
adequate segregation in any one.
The most frequent combination of porous and relatively nonporous
rocks favorable to the production of ore deposits is to have certain belts
with openings of adequate size associated with more extensive areas in which
the openings are much smaller, although they may be equally continuous.
As illustrations of such combinations we may have layers of sandstene or
conglomerate between igneous rocks or shale. The large openings may be
furnished by fault fissures.
For many reasons trunk channels may vary greatly in size and con-
tinuity. For instance, fault fissures, in consequence of the irregularity of
the fractures and the displacements, may range in width from great rooms
to mere seams. The effect of the dip upon fissures is of great importance.
Where the dips are vertical or nearly so, the veins may be wide; whereas,
where the dips are flat, the veins may be relatively narrow. This is a nat-
ural consequence of the effect of gravity. The more inclined the veins the
more forcibly does gravity pull down the overhanging wall, and thus tend
to close the openings. This association of wide veins with steep dips, and
narrow veins with low dips, is beautifully illustrated at the Smuggler Union
mine, of southwestern Colorado.
MON XLVII—O+ 76
1202 A TREATISE ON METAMORPHISM.
COMPLEXITY OF OPENINGS.
In the general discussion an ore deposit has been spoken of as if it were
a single continuous mass formed in a large opening. An ore deposit in a
single large opening is exceptional. Ore deposits show all gradations of
openings from large single ones to those of an extraordinarily complex
character. A trunk channel may be a set of distributive faults; a group of
parallel or intersecting sets of joint openings; the minute parallel openings
of fissility; a group of openings along bedding planes; the shrinkage open-
ings formed within or along the borders of cooling magma; the openings in
an autoclastic rock or reibungs breccia; the multitude of openings of a
sandstone or a conglomerate; or the very minute openings between the
mineral particles of a limestone, schist, gneiss, or other dense rocks.
Trunk channels may vary from vertical to nearly horizontal attitudes.
Ore deposits ordinarily have important vertical components, although they
may be found in nearly horizontal positions. In the latter case the trunk
channels forming the deposits probably have vertical components some-
where else.
It is hardly necessary to give illustrations of ore deposits for each of
these complex conditions, but, as very excellent instances of veins of a very
composite character may be mentioned the Cripple Creek deposits” and the
gold-quartz veins of Nevada City and Grass Valley, Cal.’ The essen-
tial point, so far as the discussion of the foregoing pages is concerned,
is that ore deposits commonly occur at places where there are trunk chan-
nels for ascending or descending waters, or both. In order that metallifer-
ous material shall be brought to a place and deposited in large quantity,
there must be long-continued circulation. It matters not whether a trunk
channel is a single passage or is composed of an indefinite number of minor
passages, the principles given on the previous pages are applicable to the
deposition of ores in such trunk channels.
In various regions the conditions are so exceedingly complex that ore
deposits close together may differ greatly in their mineral content.
This is the best evidence that, notwithstanding their continuity, the under-
«Penrose, R. A. F., jr., Mining geology of the Cripple Creek district, Colo.: Sixteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1895, pp. 187-153.
> Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California: Seven-
teenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1896, pp. 158-160, 259.
COMPLEXITY OF OPENINGS. 1203
ground systems of water circulation have been, if not independent, at least
partly so. This is well illustrated by the ore deposits of Butte, Mont.
Apparently the metallic contents of the individual feeding streams and even
the trunk channels were very different within short distances. There are
two main zones of mineralization. The more important product of one of
these mineral zones is silver sulphide, which is associated with sulphides of
lead, zine, and iron, and with silicate of manganese. The chief product
of the other mineral zone is copper, which carries silver in important
amounts.®
PREEXISTING CHANNELS AND REPLACEMENTS.
When it is understood that ore deposits from aqueous solutions ordi-
narily form in trunk channels the question as to whether ores are deposited
in preexisting openings or are replacements is easily answered, as a general
proposition. It has been shown that such channels are not originally
formed by aqueous solutions. (See pp. 1007-1008.) Original channels for
underground circulation are primarily due to the structures of rocks or to the
effects of deformation. It therefore follows that ores are, to some extent at
least, deposited in preexisting openings. However, the conditions favoring
vigorous circulation are also those causing reactions on the wall rocks. It
has been fully explained that solution and deposition are commonly simul-
taneous processes. Wherever there is a trunk channel it is certain that the
walls of the openings are dissolved to some extent, and at the same time, or
subsequently, metalliferous minerals are precipitated. Indeed, either enlarge-
ment by solution and subsequent precipitation of ore, or synchronous solu-
tion and precipitation by which the wall rocks are replaced in various
degrees, molecule by molecule, by the ore, or both together, are almost
universal phenomena.
I therefore believe that the large majority of ore deposits, if not all,
are partly deposited in preexisting openings and are partly replacements of
the wall rocks. However, in some cases the filling of the preexisting cavi-
ties is more important, and in other cases substitution for the wall rocks is
the dominant process.
Other things being equal, ore deposits which are largely in preexisting
cavities are more likely to occur in strong refractory rocks, such as quartzite,
«Emmons, §. F., Notes on the geology of Butte, Montana: Trans. Am. Inst. Min. Eng., vol. 16,
1888, p. 54.
1204 A TREATISE ON METAMORPHISM.
granite, and porphyry; and ore deposits which are largely replacements
are more likely to be found in easily soluble rocks, such as limestone. The
gold-quartz veins of California give an excellent illustration of the deposi-
tion of ores in preexisting cavities in refractory rocks, such as siliceous
areillite, diabase, diorite, and granodiorite.“ This case is all the more
interesting since the wall rock itself is greatly modified, and has lost and
gained various elements. The San Juan district of southwestern Colorado
furnishes another excellent illustration of veins deposited in openings of
hard, refractory rocks. Here great continuous lodes in preexisting open-
ings are known to extend for a vertical distance of 1,000 meters or more;
but even here there is very frequently complex fracturing or even breccia-
tion, as a result of which the veins form either a series of sheets separated
by layers of wall rock or breccia cemented by vein material. Ore deposits
which are largely replacements in easily soluble rocks are well illustrated by
the silver-lead deposits of Eureka, Nev.,” and Leadville* and Aspen,’ Colo.
Replacements are likely to be important also in proportion as the
trunk channels are complex rather than simple. This follows from the law
of mass action. In proportion as a trunk channel is complex, the surface
of action upon the wall rock for a given quantity of solution is large.
As conspicuous examples where there are large surfaces of action may be
mentioned sandstones and conglomerates and the reibungs breccias, or
crushed rocks along fault zones. Where the trunk channels are very
complex, the rocks, even if refractory, may be replaced to a considerable
extent by the metalliferous ores. A conspicuous instance of this in a
sedimentary rock is that of the copper conglomerate deposits of Lake
Superior, where many grains, pebbles, and bowlders of porphyry are partly
or wholly replaced by metallic copper. On some bowlders the metallic
copper occurs as partial or complete skulls surrounding the fragments of
porphyry; in others these skulls are thicker, and in still others the entire
masses of the bowlders, as described by Pumpelly,* are fully replaced by
« Lindgren, cit., pp. 172-257, 259, 261; also pp. 146-157.
> Curtis, J. 8., Silver-lead deposits of Eureka, Ney.: Mon. U. 8. Geol. Survey, vol. 7, 1884, pp.
93-106.
¢Emmons, 8. F., Geology and mining industry of Leadville: Mon..U. 8. Geol. Survey, vol. 12,
1886, pp. 565-569.
@Spurr, J. E., Geology of the Aspen mining district, Colorado: Mon. U. 8. Geol. Survey, vol. 31,
1898, pp. 231-234.
¢Pumpelly, R., Copper district: Geol. of Michigan, vol. 1, pt. 2, 1875, pp. 37-88. Pumpelly, R.,
The paragenesis and derivation of copper and its associates on Lake Superior: Am. Jour. Sci., 3d series,
vol. 2, 1871, pp. 348-355.
PREEXISTING CHANNELS AND REPLACEMENTS. 1205
the metallic copper. While the conglomerate deposits of Lake Superior
are in part replacements, they also are in large part fillings of preexisting
cavities between the clastic particles. An excellent example of replacement
in igneous rocks where there is wonderfully complex distributive faulting
and folding, and thus a large surface of contact for substitution, is furnished
by the Cripple Creek district, in which, according to Penrose,“ ore mainly
occurs replacing and blending into various igneous rocks.
By substitution a rock may be completely replaced by ore.. This is
particularly likely to occur where the rock is uniform in structure and com-
position, as limestone or dolomite. Where, on the contrary, the rock is of
complex composition, such as granite or porphyry, or where there are
different kinds of rock present, as, for instance, diorite and granite, the
replacement is usually largely selective. This selective replacement may
apply to the mass of the wall rock, to the mdividual fragments of it, to
clastic fragments of sandstones or conglomerate, and to the different con-
stituent minerals in a single fragment. The particular minerals or masses
which are most soluble in the solutions present are most rapidly dissolved.
Where the wall rock varies greatly in the solubility of its minerals the
selective replacement of the country rock may extend for some distance
from the central deposits. The readily soluble minerals are dissolved, and
in place of them are precipitated the metalliferous minerals. This process
is ordinarily called impregnation. Selective replacement of this kind is
well illustrated by the Butte, Mont., granite, in which “the basic constitu-
ents of the granite are naturally attacked first, then the feldspars, and
finally the quartz itself may be removed, so that in some parts there are
found large masses composed entirely of metallic minerals.”’
In regions of heterogeneous rocks the frequent occurrence of main
masses of the ore deposits in the more soluble rocks is due in part to its
greater solubility. For instance, where limestone occurs in intimate asso-
ciation with sandstone, with quartzite, with diorite, with trachyte, with
porphyry, with granite, or with almost any other rock, and ore deposits are
found, the ore is likely to be largely in the limestone, partly because it is
«Penrose, R. A. F., jr., The mining geology of Cripple Creek, Colorado: Sixteenth Ann. Rept.
U. S. Geol. Survey, pt. 2, 1895, pp. 140-141, 144-146, 161-162.
b> Emmons, S. F., Notes on the geology of Butte, Montana: Trans. Am. Inst. Min. Eng., vol. 16,
1888, p. 57.
1206 : A TREATISE ON METAMORPHISM.
more soluble than the other rocks.* But other factors enter into the
matter. It has already been explained that country rock, especially lime-
stone, may furnish solutions which react on the mineral-bearing solu-
tions and thus cause precipitation or furnish the metallic material. (See
pp. 1086-1087.) Furthermore, where limestone and stronger rocks are
deformed together, the limestone, having less strength, is more likely to be
crushed and broken in a complex manner and thus furnish trunk channels
for circulation.
Change in the level of ground water may be an important factor in
opening up rooms in which ores may be deposited later. It is explained
(pp. 409-411, 565-566) that for various reasons the level of ground
water may vary greatly, and that above it large openings, such as caves,
are formed. (See pp. 484-487.) If, later, the level of ground water for
any reason rises, these openings pass into the belt of cementation and
in them large ore bodies, including sulphides, may be deposited. This
may be the history of the great cave-like openings, lined with gigantic
geodal crystals, which are found in the lead and. zine districts of Missouri,
and which were below the level of ground water before pumping was begun.
While the larger caves apparently have not been especially favorable places
for ore deposition, in some of the smaller ones ores have been deposited.
Probably this history applies to ‘he Butte district. Emmons’ says
that for this district the level of ground water was once very much lower
than now, possibly as much as 300 to 600 meters. Doubtless much of the
solution occurred when the water was at a lower level. Later, when the
ground-water level was raised, the openings formed by solution were utilized
by the ore precipitated from descending waters transporting material from
higher levels. This process may have been important in the development
of the large sulphide bodies now below the level of ground water.
in conclusion, I hold that ores deposited from aqueous solutions form
where there existed original trunk channels of circulation. Ore deposits
«Wendt, Arthur F., The copper ores of the Southwest: Trans. Am. Inst. Min. Eng., vol. 15, 1887,
pp. 25-77. Curtis, J. S., Silver-lead deposits of Eureka, Nevada: Mon. U. 8. Geol. Survey, vol. 7,
1884, pp. 64-79. Emmons, S. F., Geology and mining industry of Leadville: Mon. U.S. Geol. Survey,
vol. 12, p. 540-543.
> Emmons, 8. F., The secondary enrichment of ore deposits: Genesis of ore deposits, Am. Inst.
Min. Eng., 2d ed., 1902, pp. 444-445. See also Weed, W. H., The enrichment of gold and silver veins:
loc. cit., p. 497.
INFLUENCE OF IMPERVIOUS STRATA. 1207
formed along trunk channels occur commonly, if not universally, to some
extent in preexisting openings and to some extent as a substitution for the
wall rock. Where the trunk channels are simple and the rocks refractory,
the ore deposits may be largely in preexisting openings. Where the trunk
channels are complex and the rocks soluble, the ore deposits to a large
extent are likely to be replacements.
IMPERVIOUS STRATA AT VARIOUS DEPTHS.
Slichter’s theoretical investigations on the motions of ground waters
show that, m order to discuss the flowage under any given set of condi-
tions, it must be assumed that it is limited only by an impervious stratum.”
It is, of course, understood that there are no strata which are
absolutely impervious, but there are many which are practically so.
Wherever there is an impervious stratum the effective underground circula-
tion is there limited or divided, whether the stratum be at the depth of 100
or 1,000 or more meters. The impervious stratum may be a plastic shale
which yields to deformation without fracture, or it may be a rock intruded
after deformation has occurred, thus making a barrier. Of course there
are all gradations, from practically impervious strata to strata which merely
check the circulation. It is believed that in the average case the limit of
effective circulation is probably much less than the theoretical limit of
10,000 meters given by the depth of the zone of fracture.
If an impervious stratum be but 100 meters from the surface and fissures
be limited to that depth or interrupted, the laws given (pp. 1021-1028)
apply to the circulation above the stratum. Therefore, such a fissure may
be occupied by ascending water in the lower part and by descending water
in its upper part. Hence an ore deposit contained in such a shallow fissure
may be the result of a single concentration by ascending or descending
waters, or of two concentrations, the first by ascending and the second by
descending waters.
The foregoing statement in reference to the practical limits of under-
ground circulation for the ore deposits of a given district may be true even
if below the impervious stratum there are other strata, fed from a distance,
in which circulation is occurring. Such lower pervious strata may have
“Slichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann.
Rept. U. S. Geol. Survey, pt. 2, 1899, 329-357. .
1208 A TREATISE ON METAMORPHISM:
circulations of their own, independent of the higher circulations, and this
circulation may produce ore bodies. This is beautifully illustrated by the
Enterprise mine, of Rico, Colorado (see fig. 29), described by Rickard*
and Ransome,’ in which the ore is confined to fissured and broken lime-
stones, gypsum, and sandstone below a black shale, which, when bent, did
not fracture, and therefore afforded no channels for water circulation.
In this connection it may be well to mention the Mercur district of
i Utah (see pp. 1213-1214), where a
_| Sandstone silver ledge and a gold ledge, about
= Sandy shate 30 meters apart, occur in limestone,
each below a shale-like stratum of
“| Sandstone
altered porphyry. Spurr regards
Black shale 0
the silver ledge as produced during
S|‘ Blanket ei i ale %
a an earlier mineralizing period, and
: Blanket limestone i
a iach shale the gold ledge during a later one.’
Wp ftp sce rnsn| Sandstone
Receayperere It may be suggested that the true
: explanation of the existence of two
eM mineral ledges so near together and
of such different mmeral character
Sandy shale a : i) ees 3
een is that in this district there were two
SeTdy hae independent circulations separated
Sandstone 5s t
by impervious strata—the upper
Sandy shale
= one, producing the gold ledge,
“| Sandstone
lying between the two impervious
Rae = Sandy shale
‘ BOER porphyry belts; while the lower
ee)
Fic. 29.—Diagrammatic section of Enterprise mine, Colo- one, forming the silver deposit, Was
rado, and its blanket pay shoot. After Ransome.
below the lower impervious layer.
The lead and zine ores of the Mississippi Valley? furnish another illus-
«Rickard, T. A., The Enterprise mine, Rico, Colo.: Trans. Am. Inst. Min. Eng., vol. 26, 1897,
pp. 918, 960-973, 976-977.
> Ransome, F. L., The ore deposits of the Rico Mountains, Colorado: Twenty-second Ann. Rept.
U.S. Geol. Survey, pt. 2, 1901, pp. 299-397.
cSpurr, J. E., Economic geology of the Mercur mining district, Utah: Sixteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1895, pp. 367-369.
d@ Bain, H. F., with C. R. Van Hise and G. I. Adams, Preliminary report on the lead and zine
deposits of the Ozark region: Twenty-second Ann. Rept. U. S. Geol. Survey, pt. 2, 1901, pp. 33-227.
Van Hise, 0. R., Some principles controlling the deposition of ores: Trans. Am. Inst. Min. Eng., vol. 30,
1900, pp. 102-109. Van Hise, C. R., and Bain, H. F., The lead and zine deposits of the Mississippi
Valley: Trans. Federated Inst. Min. Eng., England, 1902, pp. 1-60.
INFLUENCE OF IMPERVIOUS STRATA. 1209
tration in which impervious layers are of the greatest importance. Taking
this region as a whole, and considering only the lithology, we have the
following succession in descending order: (1) Thick shale formation; (2)
limestone, bearing ore bodies; (3) shale; (4) limestone, bearing ore bodies;
(5) shale, often interstratified with limestone; (6) sandstone; and (7) pre-
Cambrian. As illustrating the influence of the impervious strata in the
formation of the ores, we can consider more particularly two districts, the
Missouri-Kansas district and the upper Mississippi Valley district.
In the Missouri-Kansas district the descending succession of strata is:
(1) Undifferentiated Carboniferous sandstones and shales; (2) Lower Car-
boniferous (Mississippian) limestones; (3) Devono-Carboniferous shales;
(4) Cambro-Silurian limestones; (5) Cambrian sandstones; (6) pre-
Cambrian. In this area there was an artesian circulation in the Cambro-
Silurian limestones and sandstones under the Devono-Carboniferous shales,
and in the Lower Carboniferous limestones under the Carboniferous
IMPERVIOUS
PERvVIOUS ' eR ODS
i
Fie. 30.—Illustrating mingling of circulations of two limestones separated by a shale.
sandstones and shales. (See fig. 30.) Probably the deposition of the ores
began in consequence of a connection of the circulations of the Cambro-
Silurian and the Carboniferous limestones by faults cutting the impervious
Devono-Carboniferous shales. The ores of the district are deposited in the
Carboniferous limestone, which is not dolomitic. The Cambro-Silurian
limestone is strongly dolomitic. A very important gangue mineral deposited
with the ores in the Carboniferous is dolomite. It is therefore believed
that the metals for the ores and the dolomite alike were derived from the
Cambro-Silurian limestone and were precipitated in the Lower Carbon-
iferous limestone in consequence of the reducing action of the solutions
furnished by it and by the direct action of the organic matter in the shales
and limestones. At an early stage of erosion, when the Carboniferous
shales and sandstones were removed, the artesian water under pressure
arose along joints, faults, and other trunk channels to the surface, and thus
the first concentration of the sulphides in the Carboniferous limestone took
1210 A TREATISE ON METAMORPHISM.
place. In some places in the district where the capping shale still remains,
apparently little or no secondary concentration has taken place; but at
many places the capping shale has been largely removed, and in such areas
descending waters have produced the oxidized ores above the level of ground
water, have enriched the sulphides below it, and have formed the vertical
arrangement of the lead and zinc ores described on pages 1144-1145.
In the upper Mississippi Valley district it is believed that the lead and
zinc have undergone two concentrations, as in the Missouri-Kansas district,
although the evidence is not quite conclusive. Here Galena limestone is
overlain by the Cincinnati shale, in Iowa called the Maquoketa shale,
Fie. 31.—Ideal vertical section of flow of underground water in the Galena limestone of the upper Mississippi Valley.
When the surface was at A A’ A” the first concentration by ascending water took place; when erosion had
reduced the surface to B B/ the second concentration by descending water occurred.
and is underlain by the Trenton limestone, with an oil-bearing shale
between. When the impervious overlying shale was partly removed an
artesian circulation was inaugurated which resulted in the first concentra-
tion. Later, when the overlying shale was largely removed, a second
concentration by descending waters took place, which has continued to the
present time. (See fig. 31.)
The main difference between the upper Mississippi Valley and Missouri-
Kansas districts is that in the upper Mississippi Valley district the important
metals are believed to have their chief source in the limestones in which
PITCHING TROUGHS AND ARCHES. 1211
the deposits are now found; whereas, as has been pointed out, in the
Missouri-Kansas district the source of the ore is believed to be the Silurian
dolomite and the deposits are in the overlying Carboniferous limestone.
The lead and zine deposits of central and southeastern Missouri are more
nearly analogous in their history to the Upper Mississippi Valley district
than to the Missouri-Kansas district.
PITCHING TROUGHS AND ARCHES.
Another interesting special case of influence of porosity and structure
is that where alternately pervious and impervious layers are in a set of
pitching folds. The varying porosity may follow from original difference
in the porosity of the layers, or it may result from the deformation itself.
The more rigid strata may be deformed by fracture, and the less rigid by
flowage. The convex sides of the brittle layers are likely to be more
fractured, and therefore more porous than the concave sides. For synclines
this would place the more porous parts of a stratum at the bottom and for
anticlines at the top.
Where strata are deformed so as to produce a set of similar or nearly
similar folds, if no openings are produced at the synclines and anticlines,
the layers on the limbs of the folds must be thinned and those on the arches
and troughs thickened, or both. But more frequently, in the zone of
fracture, instead of the layers on the limbs being thinned and the arches and
troughs thickened sufficiently to occupy all the space, openings form on the
anticlines and synclines, thus furnishing trunk channels. Where the folds
are large, on account of their being less weight above an arch than above
the corresponding syncline, the openings of the anticlines tend to be larger;
but this does not apply to anticlines and synclines which are at the same
level.
Any combination of porous layers with impervious layers in folds is
likely to give trunk channels for underground water in the troughs above
impervious strata, and at the crests below impervious strata. When
descending waters come into contact with an impervious stratum, they are
deflected toward the synelines, and there finding trunk channels, they follow
the troughs downward along the pitch. When ascending waters come into
contact with an impervious stratum they are deflected toward the anticlines,
and there finding trunk channels they follow the arches upward along the
pitch. Therefore, ore deposits produced by descending waters are often
1212 A TREATISE ON METAMORPHISM.
found in pitching troughs underlain by relatively impervious strata; and
ore deposits produced by ascending waters are often found in pitching
arches overlain by impervious strata.
If this statement be reversed we have the suggestion that where ore
deposits oecur in connection with pitching anticlines and synclines, their
positions furnish a criterion by which it may be decided whether their first
concentration was accomplished by ascending or descending waters. Where
the ores occur in pitching arches bounded above by impervious strata, the
presumption is that they were concentrated by ascending waters; where the
ore deposits occur in pitching troughs bottomed by impervious strata, the
inference is that they were concentrated by descending waters, for it is dif-
ficult to see how waters can be converged at such positions by moving in
the reverse directions. Of course this criterion can not be too rigidly applied,
for independently of the impervious strata, openings which so frequently
oceur on anticlines and synclines might furnish trunk channels which could
be taken advantage of by ascending or descending waters.
The Lake Superior iron ores furnish an admirable illustration of the
concentration of ores by descending waters in pitching troughs which are
on impervious basements. Since these ore deposits, which fully illustrate
the principles of concentration of ores by descending water in pitching
impervious troughs, have already been discussed, ores of this class will not
be here further considered. (See Pl. XIII.)
A case in which ore is probably deposited by ascending waters in
arches, because there concentrated by impervious roofs, is furnished by the
Bendigo gold district of Australia. The typical position for the gold in
the district, according to Rickard, is immediately below a slate, on top of
a sandstone. The slate is the impervious stratum and the sandstone the
pervious stratum. In this district there are a large number of alternations
of pervious and impervious strata, as a result of which concentrations have
occurred at a number of horizons, one above the other. While Rickard does
not specifically speak of the pitch of the anticlines, the longitudinal sections
show that they do have a marked pitch.
It is not supposed that the location of ore deposits in pitching arches
below impervious strata is wholly controlled by the existence of the impervi-
ous roofs, for, as already explained, when a heterogeneous mass of strata is
@ Rickard, T. A., The Bendigo gold field: Trans. Am. Inst. Min. Eng., vol. 20, 1892, pp. 463-545.
PITCHING TROUGHS AND ARCHES. Ws)
folded, openings are likely to be formed at the anticlines and synclines.
Rickard’s explanation of the location of the ores is that the apices of the
anticlines furnish more open passages than the synclines. As already
explained, if it be assumed that there be less load above the anticlines
than above the synclines, this would be favorable to the production of larger
openings at the anticlines. This explanation may possibly be to some
extent applicable, but the pitching arches concentrating the ascending
solutions below impervious strata are believed to be an important factor in
the localization of the ores. Various other factors—for instance, intersecting
fractures—may be very important in the process. The Bendigo gold fields
are very interesting in a scientific way in that such a large number of
bonanzas have been found at a depth of 600 to 700 meters.* At the present
time information is not available which adequately explains these bonanzas.
A pervious layer or other opening furnishing a trunk channel for cir-
culating waters may be bounded on both sides by impervious strata. An
excellent illustration of ore deposits at the openings of anticlines between
relatively impervious Strata, presumably formed by ascending waters, are
the gold-bearing quartz ores in the slates and quartzites of Nova Scotia,
described by Faribault.” Here are a great many parallel deposits
directly at the anticlines or on some parts of the anticlinal folds, the de-
posits bemg separated by layers of relatively impervious slate. Further-
more, the largest deposits are located on the great pitching anticlines rather
than on the subordinate ones.
Another excellent illustration of ascending ore solutions concentrated
by an impervious roof is furnished by the Mercur district, Utah, described
by Spurr,’ where two ore-bearing beds, one called the silver ledge and the
other the gold ledge, about 30 meters apart, occur in a limestone below
seams or beds of very much altered porphyry resembling shale. The ores
are especially localized where fissures reach these beds, and thus displace
them, and in some cases form local arches. Moreover, the entire ore dis-
trict is located upon a general anticline, furnishing a gently pitching arch.
“Rickard,.T. A., The Bendigo gold field: Trans. Am. Inst. Min. Eng., vol. 20, 1892, pp. 538-539.
> Faribault, E. R., The gold measures of Nova Scotia and deep mining: Paper read before the
Canadian Mining Institute, March, 1899; published by the Mining Society of Nova Scotia, 1899.
eSpurr, J. E., Economic geology of the Mercur mining district, Utah: Sixteenth Ann. Rept.
U.S. Geol. Survey, pt. 2, 1895, pp. 365-367, 395, 399-401, 449, 454; see also Pls. XXV and XXXIV,
and figs. 44 and 45.
1214 A TREATISE ON METAMORPHISM.
No better illustration could be furnished of an impervious pitching arch
than that of the Elkhorn mine described by Weed. (See fig. 32.) Here the
ore is at the contact of limestone and an overlying argillaceous rock which
Weed calls hornstone. The deposit is altogether below the hornstone, and
extends down an irregular distance into the limestone which it replaces.”
It can hardly be doubted that this deposit received its first concentration
by ascending waters. :
If the criterion that ores in troughs are deposited by descending
waters and ores in arches are deposited by ascending waters be applied to
the Leadville ore deposits, the conclusion is that the sulphides of Leadville
were originally deposited by ascending waters. These ores occur below a
relatively impervious porphyry in a much-broken limestone, mainly the
Blue limestone.’ Later, when the second concentration by downward-
Depth from
surface.
910
ON
SRR AN
SSS 970
=a, =—
SSS SSM oo
= 990
° 30 60 SOFEET.
Fia. 32.—Ore deposit in limestone beneath impervious shale, Elkhorn mine, Montana. After Weed.
moving waters occurred, the material which in many places was on denuded
anticlines was in part carried down the limbs of the folds under the
porphyry into the limestone. At this time, doubtless, also, the limestone
was largely dissolved, the ores were carried down, not only along the dip
but across the beds, thus producing the very great irregularities which are
characteristic of the deposits. If the above explanation be correct, the
Leadville ores present another case in which both ascending and descending
waters were concerned.’
«Weed, W. H., Geology and ore deposits of the Elkhorn mining district, Jefferson County, Mon-
tana: Twenty-second Ann. Rept. U. 8. Geol. Survey, pt. 2, 1901, pp. 472-483.
>’ Emmons, §. F., Geology and mining industry of Leadville, Colo.: Mon. U.S. Geol. Survey, vol. 12,
1886, pp. 539-584.
¢ Blow, A. A., Geology and ore deposits of Iron Hill, Leadville, Colo.: Trans. Am. Inst. Min. Eng.,
vol. 18, 1890, pp. 173-181.
PITCHING TROUGHS AND ARCHES. - 1215
In this connection it may be suggested that the positions of the ores
in reference to the limestone and porphyry in the Leadville district are
remarkably similar to those of the ores in the Mercur district in reference
to almost identical formations. The forms of the deposits, their irregular
under surfaces in the limestone, and the regular surfaces at the porphyry are
all identical. Both Emmons and Spurr agree that the ore in the Mercur
district was deposited as sulphides by ascending waters. If this be true,
the same explanation is probably applicable to the Leadville district.
Another exceedingly interesting illustration of the deposition of ores
below an impervious stratum in pitching arches is that furnished by the
Enterprise mine of Rico, Colorado, described by Rickard’ and Ransome.’
In this district above the ore bodies is an impervious shale, which is very
rarely broken by the fissures. The ore occurs in two places: First, ore in
nearly vertical fissures extends indefinitely downward below the shale, but
not upward into it. The verticals are cut by cross fissures, and where the
intersections occur the fissures are likely to be unusually rich. (See p. 1084.)
Second, the larger masses of ore are in crushed or fractured limestone
below the black shale, but the rich blanket of ore replaces gypsum.’
These ore bodies are narrow laterally, some being parallel to the strike of
the verticals and others parallel to the cross veins. Fig. 29 shows that
they occur below anticlinal flexures of the shale made by the deformation
resulting in the fracturing and faulting in the more brittle rocks. Rickard
regards the deposits as the result of ascending waters, since the fissures
continue downward hut do not extend upward into the shale. The anticlinal
arches have a pitch. Probably the waters issuing from the verticals and the
cross fissures followed these arches upward until the pitch somewhere
brought them to the surface, at which places the waters escaped as springs,
for the waters of the ascending circulation must have somewhere escaped,
and that they could not do through the impervious shale.
Porous pitching troughs below, above, or between impervious strata
may have different origins from those mentioned. Very frequently such
troughs are produced in part or in whole by intrusive igneous rocks. For
instance, if an impervious sedimentary stratum has a monoclinal dip and a
“Rickard, T. A., The Enterprise mine, Rico, Colo.: Trans. Am. Inst. Min. Eng., vol. 26,
pp. 918-973.
>Ransome, F. L., The ore deposits of the Rico Mountains, Colorado: Twenty-second Ann. Rept.
U.S. Geol. Survey, pt. 2, 1901, pp. 254-302.
1216 A TREATISE ON METAMORPHISM.
dike cuts across the stratum, a pitching trough may be produced, as, for
instance, in the Penokee-Gogebic district of Michigan and Wisconsin.” An
intruded igneous rock may follow the contact between folded strata and
thus furnish a trough or arch bounded by an impervious formation. Various
other ways will occur to one in which pitching troughs or arches with
impervious basements, or roofs, or both, may be produced. A trough or
arch, no matter how formed, is favorable to the concentration of ores, pro-
vided a porous stratum or an opening between the layers furnishes a trunk
channel. Of course, other favorable conditions must cooperate with these
in order to produce an ore deposit.
Combinations of pervious and impervious strata, united with joints,
faults, and other structures which affect some impervious strata and not
others, furnish extraordinarily complex sets of conditions which will
undoubtedly yield interesting results when studied in special cases.
GENERAL STATEMENTS.
The importance of variation in porosity and structure is sufficiently
illustrated in the foregoing pages to show that one of the first steps in study-
ing the genesis of ore deposits formed by aqueous solutions is to investigate
the underground circulation. If a geologist wishes to know whether or
not adequate supplies of underground water can be obtained for a city, he
should at once begin to study the strata, in order to ascertain their varying
porosity, their accessibility, the amount of water they carry, their import-
ance as water bearers and water restrainers, and the possible sources
of supply; in short, he should make a complete investigation of all of the
factors which influence the underground cireulation for that district. Fre-
quently this would involve the study of the stratigraphy for extensive areas,
often for entire geological provinces. From my own point of view it is
equally important, in order to ascertain successfully the genesis of the ore
deposits of a given district, to begin by studying the stratigraphy in all its
bearings, especially with reference to underground systems of circulation.
The problems of stratigraphy and underground circulation being solved,
including both the ascending and descending currents, the geologist will
have gone far toward the solution of the problem of the genesis of the ores
aTrving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wiscon™
sin: Mon. U. S. Geol. Survey, vol. 19, 1892, pp. 268-275.
{INFLUENCE OF TOPOGRAPHY. 1217
for that district. But for some districts not only is it necessary to solve the
problem of the present circulation of the underground water, but it is essen-
tial that possible past circulations should be studied. Thus, where there are
impervious strata which have been largely removed by erosion, the condi-
tions for circulation are very different before and after the removal of the
impervious stratum. To illustrate, it has already been pointed out that in
various districts in the lead and zine region of the Mississippi Valley the
first concentration took place under conditions of artesian circulation, the
upper confining stratum being an impervious shale. Subsequent to that
time this shale has been removed from most of the region and a second
concentration has taken place by descending water under very different
conditions of circulation. }
CHARACTER OF TOPOGRAPHY.
EFFECT OF VERTICAL ELEMENT.
Where the relief is marked, the underground circulation is likely to
penetrate much deeper than in regions where the variations in relief
are slight.
In mountainous and elevated plateau regions the lithosphere is likely
to have more numerous, larger, and deeper openings than low-lying areas.
Elevated areas are usually those of comparatively recent orogenic or
epeirogenic movement. Therefore, they are regions in which the rocks
have recently been fractured, and hence the processes of cementation
are less likely to’ have closed the openings. In regions of very steep
topography the tendency for the material to glide down the slope under
the stress of gravity also tends to widen openings which have been once
formed. Such movements are known to be effective to the depth of
hundreds of meters. It is hence clear that elevated and rough regions
are those in which the underground circulation is likely to find large,
numerous, and deep openings. Further, elevated and mountainous regions
are those in which the underground water has the greatest difference in
head, and this is favorable to deep circulation.
Thus, in mountainous regions, like the Cordilleras, ‘it is to be expected
that the underground circulation, both ascending and descending, is
effective to greater depths upon the average than in regions of gentle
topography. ‘The Cordilleran region is well illustrated Dy the San Juan
MON XLVuI—04——17
1218 A TREATISE ON METAMORPHISM.
district, where the topography is very rugged. It has been pointed out
that oxidizing effects are here produced to a depth of a thousand meters,
and it can hardly be doubted that the breadth of the belt of effective
ascending circulation is great. In the lead and zine district of south-
western Wisconsin the topography is gentle, and here the effective
circulation, ascending and descending, was confined to a vertical distance
of not more than 150 meters; but this is partly due to impervious strata.
EFFECT OF HORIZONTAL ELEMENT.
The horizontal position of an ore deposit with reference to topography
often has an important influence upon its richness and magnitude. If the
correct theory of circulation of underground waters and the deposition of
ores has been given, certain corollaries follow with reference to this point:
(1) Commonly ores deposited by ascending waters should be formed
below the valleys, or at least below the lower parts of the slopes; for
these are the places where waters are ascending in the trunk channels.
(2) Commonly ores deposited by descending waters should be formed
below the crests or below the upper slopes of elevations; for these are the
places where water would be descending. Probably the upper slopes are
more favorable places than the crests; for at an annular belt on the
upper slope of an elevation the quantity of descending waters would be
greater than at the crests. (3) Commonly ores which receive a first
concentration by ascending waters and a second concentration by
descending waters should be on the slopes, probably in many instances
nearer the valleys than the crests. At such places the meteoric waters
falling at the higher elevations have sufficient head to search deeply the
zone of fracture for ores. Therefore, the ascending circulation in trunk
channels is strong. Further, at such places the level of ground water is a
considerable distance below the surface, and abundant descending waters
are concentrated in the upper parts of the openings. (See fig. 26, p. 1076.)
The downward migration of the belt of weathermg furnishes the final
favorable condition for the accumulation of a large amount of second
concentrates by descending waters.
Admirable illustrations of ore deposits corresponding to the second of
the corollaries are furnished by the iron-ore deposits of the Lake Superior
region. ‘These are the products of descending waters, and the great major-
ity of the ore deposits are found near the tops of hills or upon the slopes.
INFLUENCE OF TOPOGRAPHY. 1219
For instance, in the Gogebic and Menominee districts of Michigan,
where the drainage is across the range, the great ore deposits reach the sur-
face on the slopes or crests, and comparatively little ore has been discovered
below the strong cross drainage lines where the waters are probably ascend-
ing. All of the great iron-ore deposits reach the surface, but many of them
have a pitch which carries them below the rocks. An ore deposit which at
the surface starts on a slope or crest may extend below a subordinate cross
valley.. In such cases the water had head sufficient to carry it down and
make it effective below the valley; but, so far as I know, it has yet to be
proved that any great deposit continues in force across the valley of an
important stream.”
The topography in the Mesabi range of Minnesota exerts a somewhat
different influence. The productive portion of this range extends from a point
west of Hibbig to Embarrass Lake. Here the Giants range to the north is
strongly emphasized, and there are good drainage lines to the south. West
of Hibbing the Giants range is much less marked; indeed, for much of the
distance to the Mississippi River is almost imperceptible.’ While ore depos-
its have been discovered in this part of the range, they are not nearly so
numerous and extensive as in the area to the east, and the ores which have
been found are sandy Gndicating an imperfect removal of the silica).
There is no known stratigraphic reason why the ore deposits should not
have developed as extensively between Hibbing and the Mississippi River
as to the east. The only suggestion which has occurred to me in reference
to the matter is that because of the dying out of the Giants range and the
lack of sufficiently marked elevation and good slopes to produce a vigorous
circulation, the groundwater circulation has not been sufficiently vigorous
to concentrate the iron oxide and to remove the silica to the same extent as
farther east.
An excellent illustration of the third corollary is furnished by the
San Juan district of Colorado. In that district the richer parts of the lodes
are generally below the middle slopes of the mountains. Nearly all of
the great lodes have been opened on veins which outcropped in the so-called
‘‘basins” at altitudes of 3,000 to 3,500 meters. Above these basins are the
«Van Hise, C. R., The iron-ore deposits of the Lake Superior region: Twenty-first Ann. Rept. U.S.
Geol. Survey, pt. 3, 1901, pp. 329-330.
> Van Hise, cit., Section on the Mesabi district, by C. K. Leith: pp. 352-353.
1220 A TREATISE ON METAMORPHISM.
high peaks, rising to 4,000 and 4,300 meters. Below the mines are the
towns at altitudes from 2,300 to 2,700 meters—that is, the outcrops of
the veins are 700 to 1,000 meters above the main valleys.
Another illustration of deposits which are on slopes and are the result
of two concentrations is furnished by the lead and zine district of the upper
Mississippi Valley. Chamberlin‘ notes that in the valleys of the Wisconsin
part of the district the waters generally ascend to the surface; therefore, at
such places only a first concentration would be expected. It is the general
impression among miners in this district that a lode makes better on the
slope of a hill “than at the summit or at the foot of a hill.”’ Also, it is
held by the miners that the lodes which run parallel to a contour of a hill
“Vike an eave trough”’
are more likely to be rich than those which run
toward the summit of the hill. When considered in connection with the
topography both of these practical conclusions of the miners are fully
explained by the theory of a first concentration by ascending waters and a
second concentration by descending waters. This case of the Wisconsin
lead and zine district must not be too strongly insisted upon, for at many
places the streams have cut through the lead-bearing limestone into the
St. Peters sandstone below, which is a barren formation.
In regions in which there have been recent important changes in the
positions of the drainage lines and elevations, the generalizations concern-
ing the relations of ore deposits and topography are only partly applicable.
Tt is well known that in consequence of the varying hardness of rocks,
of structure, of the unequal strength of streams and of unequal declivity,
drainage lines are almost constantly shifting, in many regions somewhat
rapidly. - Another way in which the position of the drainage lines with
reference to ore deposits may be shifted is by base leveling and subsequent
uplift. After a region has been cut down to the level of the sea and is
again uplifted it is well known that some of the streams are likely to be
along old drainage lines, but that others will be in new positions. It fol-
lows that when the second cycle of erosion is well inaugurated an ore deposit
which during the first cycle of erosion was in a valley may be on a crest,
and vice versa. When it is remembered that there have been various
«Chamberlin, T. C., The ore deposits of southwestern Wisconsin: Geology of Wisconsin, vol. 4,
1882, p. 565.
> Chamberlin, cit. p. 563. _
INFLUENCE OF TOPOGRAPHY. 1221
epochs of extensive base leveling from pre-Cambrian to Tertiary time, it is
seen that shifting of drainage lines in consequence of different cycles of
topographic erosion may be of great consequence,
Doubtless in consequence of changes in drainage many ore deposits
which, when below valleys, received a first concentration by ascending
waters, are now well up on slopes or even at crests. A change of this
kind is especially favorable to the development of ore deposits which are
formed by two concentrations, the first by ascending and the second by
descending waters. In an early stage of the history of a deposit it-may be
in a favorable place to receive a first contribution of ore. Later, if in con-
sequence of a topographic change, it be on higher ground, it is then in a
favorable place for the work of descending waters. Although it is difficult
to prove, I have littie doubt that many ore deposits have had this history.
Where a region has been base leveled it is very difficult to reconstruct
the topographic conditions which led to the concentration of the ores. To
illustrate, the Lake Superior copper deposits are believed to have been
deposited by ascending waters. The waters may have made their way
down along fault fractures and through the minor pores to the sloping con-
elomerate beds and have risen along the beds which had their surface out-
crop at a lower elevation than the feeding areas. But all this is purely
hypothetical, for the region has been base leveled and the copper deposits
unquestionably developed before the end of the base-leveling period. Since
that time the region has been uplifted and the present topography incised
in the base-leveled plateau. Se far as one can see the present topography
has no recognizable relation to the ore deposits.
From the foregoing it is apparent that the relation of topography to
ore deposits is an important one, and also that in many districts its deci-
pherment is a difficult problem.
PHYSICAL REVOLUTIONS.
The genesis of many ore deposits is undoubtedly further complicated
by physical revolutions of various kinds. After an ore deposit has partly
formed, either by ascending or descending waters, or both, the region may
go through a physical revolution, and after the revolution the concentration
of the ores may again be taken up by nature’s processes.
1222 A TREATISE ON METAMORPHISM.
After an ore deposit has been formed the country may pass below the
level of the sea by denudation and subsidence, may be deeply buried
under sedimentary rocks, and may be again uplifted and undergo a second
cycle of reactions which affect the nature of the ore deposits. An ore
deposit partly formed may be buried deep under volcanic rocks. This
undoubtedly has occurred on a great scale in the great region of Tertiary
voleanism in the Cordilleras of the West. The ore deposits there buried
are placed in a new environment and are undergoing a second cycle of
concentration or depletion. In that district it is entirely possible, indeed
probable, that ore deposits formed in the pre-Tertiary volcanic rocks, and
with a very complex history involving the work of both ascending and
descending waters, are now receiving additions of ores from ascending
waters, or, on the contrary, are being depleted, and in consequence are
contributing metals to waters which are rising. Such waters may deposit
the metals in openings within the volcanic rocks, thus making new ore
deposits. When in the future denudation shall have stripped off these
voleanics, or the upper part of them, these -ore deposits will be at the
surface. In this connection it is to be remembered that in the Yellowstone
Park and other areas there are extensive Tertiary volcanic tuffs in almost
every respect like formations which in the San Juan district of Colorado
bear important ore deposits, but the San Juan region was so elevated that
denudation has thoroughly dissected the volcanic material. Indeed, the
major streams have cut down through the voleanics to the sediments below.
It may be suggested that when denudation of the voleanic series shall have
progressed as far in various other i Tertiary volcanic regions as in the San
Juan region ore deposits will be éxposed, but this may not occur while man
occupies the earth.
It is well known that fissures are places of weakness, and that move-
ment has again and again recurred along the old planes. Thus favorable
conditions for ore concentration may recur in the same places through
various revolutions. Physical changes of various other kinds may take
place. Each of the complex changes in physical history produces its
effect upon an ore deposit.
COMPLEXITY OF ORE DEPOSITS. 1223
V. GENERAL STATEMENTS.
It is clear from the foregoing that an ore deposit may have a wonder-
fully complex history. It may not represent the work of a single period of
ascending or of descending waters, or one cycle of ascending and one cycle
of descending waters, but may be the result of a number of cycles of depo-
sition by ascending or descending waters, or both.
Thus irregularities in certain of the ore deposits in very ancient rocks
may be explained. But in many cases it appears probable that the main
work in an existing ore deposit has been done by continuous concentration,
first by ascending waters, second by descending waters, although in such
cases part or even all of the metals may have been derived from earlier
concentrations.
Any of the special and local factors above discussed and others not
considered may in an individual case be so conspicuous as to appear to be a
controlling factor in the formation of an ore deposit. One might say that
the existence of a given trough was the cause of the production of an ore
deposit. The truer statement would be that the factor under consideration is
one essential among many. The porosity of a formation, the existence of a
pitching trough, favorable topography, the presence of igneous rocks fur-
nishing heat to make the waters active, and many other special factors may,
in a given case, all be essential, and without the help of any one of them an
ore deposit would not have been produeed, but no combination of factors
will form an ore body if a source of the metal is not available upon which
the underground waters may act. In short, each case of the development
of an ore deposit requires the fortunate combination of many favorable
factors, working harmoniously together, the absence of any one of which
may prevent the concentration of the ore deposit.
VI. ORE SHOOTS.4
No fact is better known concerning ore deposits than that they vary
in the most remarkable fashion, both in size and richness. Moreover, these
variations are both vertical and horizontal. Frequently rich deposits decrease
in size or are wholly cut off with extraordinary abruptness. Other equally
rich deposits may appear somewhere else on the same level or on another
“For a general discussion of ore shoots in fissures, see Penrose, R. A. F., jr., The mining geology
of the Cripple Creek district, Colorado: Sixteenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1895, pp.
162-166.
1224 A TREATISE ON METAMORPHISM.
level in an equally strange and apparently inexplicable manner. The ore
masses of exceptional richness are generally called ore shoots. Sometimes
. they are spoken of as pay streaks, at other times as bonanzas, at other times
as chimneys. In this paper ore shoot’ is used as a general term to include
all deposits of exceptional richness or size, of whatever origin. At various
places in this paper factors have been mentioned which produce ore shoots.
However, because of the very great economic importance of ore shoots, it
seems to me advisable to consider under one heading some of the more
prominent features of ore shoots deposited from aqueous solutions, even at
the risk of repetition. :
Ore shoots may be grouped into those which are largely due to
structural features, to the influence of the wall rocks, and to secondary
concentration by descending waters.” ” Ey
One large class of ore shoots may be explained principally by structural
features. These structural features may be the varying size of openings,
varying complexity of fractures, flexures, intersections of fractures, and
later orogenic movements. ~~
A fracture through a. mass of rocks is necessarily uneven. Where
there are movements, it follows that the walls are not adjusted to each
other. Where convex surfaces are opposite each other, a fissure may be
represented by a mere seam. Where, on the other hand, two concave sur-
faces are opposite each other, a widening may occur which in some cases
is sufficient to produce a great room. This is beautifully illustrated by
the Comstock lode. This lode showed remarkable variations in its width,
here being represented by a narrow seam of clay, there by a great
bonanza. The story that tells how Fair and Mackay, with persistent
courage, drove along a little seam of clay, at places almost unnoticeable,
for hundreds of meters, from the 360-meter level of the Gould and Curry
mine to the great bonanza, is one of the romances of mining.“ The clay
seam marked a place where two pretuberances of the walls had been
jammed together so that there was no opening through which circulating
waters could pass, whereas the great bonanza represented a place where
both walls bowed outward and thus gave ample space for the deposition
of the great deposit. Rooms may be produced also partly or largely by
solution, and may be connected by comparatively large channels.
aLord, Eliot, Comstock mining and miners: Mon. U. 8. Geol. Survey, vol. 4, 1883, pp. 809-314.
—
ORE SHOOTS. 1225
It has been shown, other things being equal, that the underground cir-
culation follows the larger openings. Thus the abundant circulation is
converged wherever there are rooms, especially where they are connected
with passages of considerable width. Moreover, the solutions of such a
circulation are likely to be derived from various sources. Hence, upon the
average, in the large openings the ore is in greater quantity and very
frequently richer than in the narrower ones, where the solutions are both less
abundant and less complex.
It is evident from the above that there may be every variation in the
width of an ore deposit due to uneven walls, from zero to many feet.
Ore shoots are frequent where the fractures, instead of being simple, are
complex; that is, where there is a crushed zone, or zone of brecciation and
mashing. It has been pointed out (pp. 1086-1088) that some ore deposits
are largely due to reactions between the solutions and the rocks through
which they pass. Such an ore deposit is most likely to be rich at a crushed
zone, where the interaction between the solutions and the rocks through
which they circulate is much greater than at a point where there is a single
fracture, even if the latter furnishes a larger space than the multitude of
smaller openings. (See pp. 1213-1215.) x
The Cripple Creek district is an excellent illustration of a region in
which the fracturing is of an extremely complex character. The ores and
gangue are largely impregnation and replacement products of the rocks in
which they occur, including granite, andesite, phonolite, basalt, ete. The
solutions in this district evidently followed the innumerable minute
fractures of all kinds and had a very large surface of contact.* A condition
for the deposition of the ore and gangue was the previous enlargement of
the openings by solution.
| Very frequently the rich shoots of ore are located by flexures, the ore
being either at the crests of anticlines or at the bottoms of synclines. As
pointed out (pp. 1206-1212), this is especially likely to be the case where,
in connection with the folds, there are impervious strata. Under such
conditions, as has already been fully explained, ore is likely to be
converged from ascending solutions in the arches of perviotis strata below
impervious strata, and from descending waters in troughs of pervious
strata above impervious strata. In the cases cited, such as those of
« Rickard, T. A., The lodes of Cripple Creek: Trans. Am. Inst. Min. Eng., vol. 33, 1903, pp. 613-618.
1226 A TREATISE ON METAMORPHISM.
Australasian and Nova Scotian gold ores and the Lake Superior iron ores,
these relations are perfectly clear. Doubtless in many mines there are
minor flexures which have been overlooked, but which may be sufficient
to control the movement of the circulation, and thus produce the
chimneys of ore. These minor flexures may be parallel with the dip of
a deposit, or they may pitch to the right or to the left as one looks
‘down the dip.
The intersections of fractures furnish one of the most frequent explana-
tions of ore shoots. The intersections may be those of faulted fissures,
those of fissures and joints, or the intersections of joints. In many instances
one set of fractures carries the larger ore deposits, and the intersecting set
or sets of fractures are known as side fractures. In some instances deposits
may occur in two or more sets of intersecting fractures, and in others one
of the intersecting sets may be barren of deposits.
In all cases where intersecting fractures occur solutions are contrib-
uted from two or more sources. The solutions invariably have different
compositions, and therefore precipitation is likely to occur at or near the
junctions. In some cases metalliferous material may be furnished by more
than one set of fractures, while in other cases it may be contributed by one
set of fractures and the precipitating agents by the others. In those
instances where the intersecting veins all carry ore it is easy to see why the
deposits at the intersections are unusually large and rich. Where the side
veins are small or are wholly filled with gangue material their importance
in the genesis of ore deposits has been very generally overlooked. In
many instances there is little doubt that the metallic material was precip-
itated in a main fissure at or near the point where the side veins join,
through the influence of the solutions contributed by the latter veins. A
very clear case of the influence of barren side veins is that already cited of
the Enterprise mine, of Rico, Colo., where the pay shoots are especially rich
in the main fissures at the places where barren side veins intersect them.
Not infrequently, where a side fracture intersects a vein carrying the
main deposit, the high values are not exactly at the intersection, but a
little way from.it. This is a natural consequence of the laws of circulating
waters. Where two streams come together from different sources they are
sure to have different heads. If the side fracture which furnishes the pre-
cipitating solution has the greater head its water will occupy the main
ORE SHOOTS. 1227
fissure for some distance on either side of it, the solutions furnished by the
main fissure being excluded; but at a greater or less distance from the
intersection the solutions from the two sources become mingled and precipi-
tation takes place, thus producing rich ore shoots. This relation is very
well illustrated at the Silver Age mine, at Idaho Springs, Colo. Here the
ore shoots are connected with intersecting fractures. They are in the main
vein at places near, but not directly at intersections of the subordinate
fissures. Again, at the Last Dollar mine in the Cripple Creek district the
main vein is cut by numerous cross veins, which upon the whole have not
been very profitable; but at numerous places the very rich values are found
in the main veins comparatively close to the cross veins; that is, at distances
from them varying from 15 to 60 meters. Where ore shoots are found to
be connected structurally with barren side veins a consideration of the
minerals of the main vein and of the side veins ought to lead to more exact
knowledge concerning the manner of the precipitation of the metal; for
presumably the precipitation of the metals was connected with some of the
compounds which occur as gangue, and in the side veins these may be
different from those of the main vein.
Side fractures may be at right angles to the main fractures or incline
to them. They may extend directly down the dip or pitch to the right or
left along the dip. Therefore almost any curious distribution of the rich
shoots may occur. All of these relations of fractures with lodes are beau-
tifully illustrated in the San Juan district. Here some fractures strike
parallel with the lodes, but dip at considerable angles with them. There
are also cross fractures which differ from the lodes both in strike and dip.”
Usually side streams bearing either metals or precipitating agents or
both do not issue equally all along the faults or joints, but may be largely
converged into large channels, and under such circumstances strong springs
enter the fissures. Where such springs empty into a room produced by
the structural features, discussed on pp. 1224-1225, bonanzas may be
formed, such as those of the Comstock lode.
While the relative influence of the different sets of intersecting frac-
tures is very complex, in an individual mine a close study of the number,
order, and relations of the fractures and joints, many of which are, perhaps,
«Ransome, F. L., A report on the economic geology of the Silverton quadrangle, Colorado: Bull.
U.S. Geol. Survey No. 182, 1901, p. 59.
1228 A TREATISE ON METAMORPHISM.
almost imperceptible, may furnish rules which will enable one to more
intelligently search for ore. :
Between the two cases of trunk channels produced by flexures
(described on pp. 1225-1226) and by cross fractures (described on pp.
1226-1228) there are complete gradations.
Late orogenic movements explain certain ore shoots. After openings
have received a first contribution of ore, and are, perhaps, fully cemented
by ore and gangue materials, orogenic movements frequently recur, which
again fracture the ground and produce openings. Some parts of a deposit
may escape fracture, while other parts may be broken. The fracturing of
the broken parts may be simple or complex. The latter may produce
zones of parallel fractures, zones of intersecting fractures, brecciated zones,
or even zones in which the material is finely mashed. Between the parts
of a deposit where fracturing is absent and those where it is most complex
there may be all gradations. The fractures may be confined to a narrow
belt of a deposit or to one side of it, and to varying limits laterally or
vertically. Entirely new sets of openings may be produced in the wall
rock. All of the above statements in reference to a main deposit apply
equally well to intersecting deposits. Therefore an ore deposit which has
received a first contribution and is subjected again to orogenic movements
is in such a condition that it may again receive a contribution of ore
material under the same complex laws as at first. The late fractures may
be filled with new contributions of metals from the original source; they
may be fed by the solution of the deposits betore formed; or they may
derive metal both from the original source and from the earlier deposit.
The new ore may be distributed very irregularly through the older
ore, may be superimposed upon the old material where there are openings,
may be deposited as relatively small secondary veins, or any combination
of these may take place. The original irregularities in the tenors of the
veins, combined with the irregular deposition of the secondary material,
may give extraordinary and apparently inexplicable variations in richness.
Excellent examples showing how secondary fracture produces very rich
subordinate veins are furnished by the San Juan district of Colorado. At
the Smuggler Union, deep in the mine, in a secondary fracture adjacent
to the main vein, are found extraordinarily rich deposits of free gold in
quartz, having an exceedingly irregular and “ pockety” distribution of
values. It seems probable that in this fracture the gold has been precipi-
ORE SHOOTS. 1229
tated as a result of reducing solutions furnished by the adjacent sulphides
of the main vein. At the famous Camp Bird mine there are thin black
seams deep below the surface, which are extraordinarily rich in gold.” The
main part of the vein has its gold and silver values in copper sulphides,
galena, blende, and other base sulphides and in tellurides, but the fine
black material in the small veins apparently contains free gold, as is shown
by the fact that 75 per cent of the value of the ores is collected on the
plates. It seems perfectly clear that these minute seams of rich gold-
bearing material are the results of precipitation in secondary fractures.
Ore shoots in many cases are explained by the influence of the wall
rocks. It is well known that where ore deposits intersect a complex set of
rocks the pay shoots are likely to have a decided preference for one rock.
For instance, if a fissure passes from granite to diorite, or from either of
these to limestone, or from any of these to sandstone, the character and rich-
ness of the deposit may vary greatly as the rock changes. For this vari-
ability, due to the character of the wall rocks, different explanations apply
in different cases. In some instances the restriction of the ore shoots to one
rock is largely explained by its more ready solubility. This is particularly
applicable to the substitution deposits, the wall rock being dissolved in
advance of or pari-passu with the deposition of the ore. By the solution of
the soluble reck sufficient room is furnished for a large ore deposit. The
above is undoubtedly the partial explanation in many cases of the occurrence
of the ores in limestone rather than in the adjacent more insoluble rocks.
Gypsum is even more readily soluble than limestone. Of the Enterprise
mine of the Rico district, Ransome’ says the rich blanket of ore is “due
essentially to the removal by solution of a massive bed of gypsum which
may have been from 15 to 30 feet [5 to 10 meters] im thickness.” All
stages of the process of replacement are seen.
In other instances the occurrence of the rich and large bodies with one
wall rock rather than with another is due to the fact that the wall rock itself,
by reaction upon the solutions, precipitates the ore material. This also
partly explains the preference of certain ore deposits for limestone, for, as
already explained (pp. 1116-1117), calcium carbonate probably accelerates
«Ransome, F. L., A report on the economic geology of the Silverton quadrangle, Colorado: Bull.
U.S. Geol. Survey, No. 182, 1901, pp. 89-90.
+ Ransome, F. L., The ore deposits of the Rico Mountains, Colorado: Twenty-second Ann. Rept.
U.S. Geol. Survey, pt. 2, 1901, pp. 278-295.
1230 A TREATISE ON METAMORPHISM.
the precipitation of metals as sulphides from their oxidized salts by iron
sulphide.
In still other instances the wall rock itself furnishes solutions contain-
ing metalliferous material which is precipitated in the trunk channels or
furnishes solutions capable of precipitating metalliferous material in the
trunk channel. Either of these may be true for igneous rocks. Also
igneous wall rocks may furnish heat and thus in a very important measure
promote segregation, as explained on pages 1015-1016.
An excellent illustration of the influence of the wall rock is exhibited
by the Lake vein of the San Juan district, near Telluride. This vein,
profitable in the volcanic tuff, extends through the tuff into the limestones
below, and through them into the sandstones. In passing from the tuff
into the limestones the vein changes from a quartz vein to a calcite vein,
and as it goes into the sandstone it gradually changed again to a quartz
vein. There can be no better illustration than this of the influence of the
wall rock and of the domination of the law of mass action. The vein
material deposited in the limestone and sandstone has essentially the same
composition as the rock through which it passes, showing that the minerals
deposited in this vein were dependent upon the solutions derived from the
adjacent formations. Each formation contributed as its chief material
compounds which could be derived from it. Solutions entering the vein
from other sources were subordinate to those derived from the wall rock.
The influence of the wall rock is further illustrated by the fact that so long
as the vein remains in the tuffs it is of economic importance, and as soon
as it passes into the sedimentary rocks the amounts of gold and silver
become so small that it is valueless.
A third class of ore shoots are those produced by the process which has
been so fully explained in this paper, viz, the secondary enrichment of
deposits by descending waters, the first concentration of which was pro-
duced by ascending waters. This process produces rich ore bodies, either
oxidized or sulphureted, or partly each, which are limited in depth by the
distance to which the descending waters are effective.
GENERAL STATEMENTS.
Of necessity, in this analysis, the various factors which may produce
ore shoots have been separately treated. In a given case it is rare, indeed,
to find that the entire explanation lies in the application of a single one of
ORE SHOOTS. Zoi
them. To explain an ore shoot of an individual mine, ordinarily a number
of the above causes need to be combined, and in some cases doubtless,
other causes which have not been treated. No study is more important
economically, more fascinating, or more difficult than to ascertain the par-
ticular combination of factors which produced the ore shoots in a given
district or mine.
From the foregoing it is plain that no general statement can be made
in explanation of ore shoots. In each district, in each mine, in each part of
a mine, all the phenomena must be studied closely in the light of correct
theories of ore deposition in order to reach an explanation applicable to
the particular case.
It is well known that in the districts which are mineralized the work-
able ore deposits are ordinarily confined to relatively small areas, although,
so far as one can see, the amount of metalliferous material furnishing ore
deposits may have been the same throughout the districts. The absence of
workable ore deposits for the larger parts of the districts ordinarily is due
to lack of favorable combination of the various special factors mentioned,
and doubtless many others which have not been considered. As better
illustrating my meaning, I may again mention the iron and copper ores of
the Lake Superior region. The iron-bearing formation has an extensive
occurrence through the Lake Superior region, but the workable iron ores
are confined to small areas which have been subject to ancient and recent
metamorphism, and in which there are favorable structural features. The
Lake Superior copper deposits equally well illustrate the principle. All of
the mines now being exploited are confined to an exceedingly narrow area
on Keweenaw Point, while the copper-bearing rocks occupy an extensive
belt about nearly the entire Lake Superior basin. Moreover, these
copper-bearing rocks are mineralized in many places, as is shown by the
widely disseminated copper. But, unfortunately, in many areas a little
copper is concentrated in many amygdaloid or sandstone belts, rather than
in a single amygdaloid or sandstone. For instance, in certain districts
scores of amygdaloidal flows lie upon one another. In each of these beds
the scoriaceous upper surface bears metallic copper, but in none is it in
sufficient amount to make the copper a workable deposit. Had the copper
deposited in a number of these amygdaloid formations been concentrated
in one of them, a workable ore deposit would have been produced.
‘
123 A TREATISE ON METAMORPHISM.
From the foregoing it is clear that an investigation of the local factors
in a district should include both those which are favorable and unfavorable
to the concentration of ores, for a study of the latter in many districts may
prevent the expenditure of large sums in exploration where the mineraliza-
tion is general, but the conditions are not such as to have concentrated the
valuable material in sufficient quantity at any one position to warrant
exploitation.
A treatise on ore deposits, including descriptions of individual districts,
necessarily deals with the special factors which are important in each
district. These special factors may be considered so conspicuous that. the
entire attention is given to them. But it is to be remembered that each of
these is subordinate to the general principles controlling the deposition of
ore deposits in all districts.
SUMMARY AND CONCLUSION.
I shall now make a summary statement of the genetic classification of
ore deposits proposed, and consider briefly the relations of these different
classes of deposits. It is proposed that ore deposits be divided into three
divisions: (A) Sedimentary ores, (B) igneous ores, and (C) metamorphic
ores. The sedimentary ores may be divided into two classes: (1) Chem-
ical precipitates, and (2) mechanical concentrates. The mechanical con-
centrates may be divided into (a) residuary deposits, (b) stream deposits,
and (c) beach deposits. As yet criteria have not been formulated by which
igneous ores may be subdivided, and therefore there is only one class (1)
magmatic segregations. Metamorphic ores are divided into two classes:
(1) Gaseous solution ores, and (2) aqueous solution ores. The aqueous
solution ores are subdivided into (a) ores deposited by ascending waters,
(b) ores deposited by descending waters, and (c) ores deposited by ascend-
ing and descending waters combined. The scheme is therefore as follows:
Classification of metallic ore deposits.
(1) Chemical precipitates. ;
(A) Sedimentary -- (a) Residual deposits.
(2) Mechanical concentrates.{ (b) Stream deposits.
(c) Beach deposits.
(B) Igneous .-.---- (1) Magmatic segregations. ; :
(1) Deposits from gaseous
solution.
a) By ascending waters.
b) By descending waters.
c) By ascending and descending water..
(C) Metamorphic - . (
2) Deposits from aqueous
solution. (
SUMMARY AND CONCLUSIONS. 1233
Of the sedimentary ores the important chemical precipitates are those
of iron, manganese, and aluminum. Of the mechanical concentrates the
residuary deposits include ores of iron and tin; the stream deposits include
ores of gold, of tin, and of platinum; the beach deposits include ores of
gold and magnetite.
In igneous ores—magmatic segregations—we can only certainly include
titaniferous iron ores and the aluminum ores which occur in corundum
syenite.
In the metamorphic ores deposited from gaseous solutions are probably
to be placed those impregnation deposits associated with which there have
contemporaneously developed the heavy anhydrous minerals, such as gar-
net, wollastonite, amphibole, pyroxene, biotite, and tourmaline. Doubtless
many if not all of the ores here belonging developed under the conditions
of the zone of flowage or anamorphism. ‘This class is a subordinate one,
but includes a number of mines in various districts. Copper seems to be
the most important metal of this class, but zine, lead, gold, silver, ete., occur.
The class of aqueous solution ores is believed to be more important
than all others combined. The criteria by which these ores are to be
recognized are as follows:
(1) The major portion of the material of the ore deposits, and especially
the gangue minerals, is the same as the general cementation materials which |
fill the openings of rocks of all kinds, from fissures to subcapillary openings.
General cementation is universally recognized as the work of aqueous
solutions.
(2) The gangue minerals of the ores are dominantly hydrous silicates,
carbonates, and oxides, such as are now being produced by aqueous solu-
tions in innumerable tocalities throughout the world, as, for instance, in the
Yellowstone National Park, the Mississippi Valley, Iceland, ete.
(3) At some of these localities in which the gangue minerals are being
formed on an extensive scale, metallic ores are also being deposited in small
quantities by the aqueous solutions, as at Steamboat Springs, Sulphur
Bank, and Boulder Hot Springs.
All of the three subclasses of ores deposited by aqueous solutions are
important. Ores of the first subclass, those which at the point of precipi-
tation are deposited by ascending waters alone, comprise ores of gold, silver,
and copper, the sulphides of all of the valuable metals, and the tellurides of
MON XLViI—04 78
1254 A TREATISE ON METAMORPHISM.
gold and silver. This subclass is of very great importance. It is illustrated
by the copper ores of the Lake Superior region, by all but the superficial
portion of the pyritiferous gold veins of the Sierra Nevada, and by the
deposits of many other regions.
The ores which are deposited by descending waters alone are the most
important subclass, both as to volume and as to importance to mankind.
These ores are chiefly oxides, anhydrous and hydrous. The dominant ores
here placed are those of iron. While subordinate amounts of iron ores
belong in other divisions, certainly more than 90 per cent and probably
more than 95 per cent of the iron ores yet exploited are in this subclass.
The ores of manganese also are mainly oxides produced by descending
waters.
The ores deposited by ascending and descending waters combined con-
stitute a very important subclass. The materials of this subclass include
oxides, carbonates, anhydrous and hydrous silicates, and chlorides, chiefly
above the level of ground water; sulphurets and tellurides chiefly below,
but often also above it, and native metals both above and below it.- There
is frequently a transition belt of considerable breadth between the various
products. At or near the level of ground water all of these products are
often intermingled. Important ores of copper and zine are included in the
oxides; of lead, zinc, and copper in the carbonates; of zinc, copper, and
nickel in the hydrous silicates; of silver in the chlorides; of iron, zinc, lead,
copper, nickel, mercury, and silver in the sulphides; of gold and_ silver in
the tellurides. The metals include important ores of gold and silver, and
subordinate amounts of copper.
It is believed that the ores denouned by ascending and descending
waters combined are more numerous than those of any other subclass, even
if they do not occur in such great volume and are not of the same impor-
tance to man as the ores deposited by descending water alone. I suspect
that a close study of the origin of ore deposits will show that of ores formed
by underground water the most numerous subclass is not made up of ores
deposited by ascending waters alone, but is composed of ores which have
undergone a first concentration by this process and a second concentration
by descending waters. As a result of this there is found in the upper 50 to
500, or possibly even 1,000, meters of an ore deposit a large portion of
the metalliferous material which originally had, as a result of the first
concentration, a much wider vertical distribution.
SUMMARY AND CONCLUSIONS. 1235
It is to be noted that the ores assigned above to the work of descending
waters alone do not include any sulphides. It is by no means asserted that
sulphide ores belonging to this subclass do not exist, but I know of no
sulphide deposit which can be certainly placed here. If more careful
investigation shall show that no important sulphide ores are deposited by
descending waters alone it will follow that all aqueous solution sulphide
ores belong either to the class deposited by ascending waters or to the class
produced by ascending and descending waters combined. It would
therefore follow that in the production of all sulphide ores ascending waters
are concerned. ‘To my knowledge the only sulphide ores which are held
to be the result of descending waters alone are those of the Monte Cristo
district of Washington, described by Spurr.“ But Spurr’s statement does
not seem to me to prove the conclusion. The mining has extended to only
a moderate depth in this district. So far as the facts are known, one might
explain the ores as being first concentrated by ascending waters and recon-
centrated by descending waters, or as produced by ascending waters
alone. It seems to me more probable that the interpretation should ‘be
along the line applicable to so many other districts—a first concentration
of the sulphides by ascending waters and a later concentration by
descending waters.
If the conclusion be established that ascending waters are concerned in
the production of all sulphide ores, to this extent the contention of
Posepny, who strongly insisted that all sulphide ores are the work of
ascending waters, is confirmed. His error would be that he overlooked
the importance of the concentrating effect of descending waters. Since
the tellurides are so closely associated with the sulphides it is highly
probable that they also will be found to be produced not by descending
waters alone, but by either ascending waters alone or by ascending and
descending waters combined.
To the foregoing classification objections will at once occur. It may
be said that there are no sharp dividing lines between the divisions, classes,
and subclasses. But transitions are everywhere the law of nature. In a
previous chapter (see pp. 786-787, 904-905) I have explained that there
are gradations between different classes of rocks, and this statement
applies equally well to ore deposits.
«Spurr, J. E., The ore deposits of Monte Cristo, Washington: Twenty-second Ann. Rept. U. S.
Geol. Survey, pt. 2, 1901, p. 857.
1236 A TREATISE ON METAMORPHISM.
\
There may be complete gradations between ores deposited by processes
of sedimentation and those produced by processes of metamorphism, and
between ores apparently most widely separated. Even ores of igneous
origin and those deposited by aqueous solutions may grade into each other.
It has been fully explained that ore deposits produced by processes of
sedimentation may grade into those produced by processes of metamor-
phism. Illustrating this, it may be recalled that a placer deposit formed
by mechanical concentration may be profoundly modified by the circulating
waters of the belt of cementation. Such circulating waters may add greatly
to or subtract much from the amount of gold in a mechanical concentration
deposit. It is highly probable (as explaimed on pp. 1042-1043), that the
gold conglomerates of the Rand and the deep placers of California have
this composite origin. It is also conceivable that ore deposits primarily
produced by sedimentation may also be modified by the actions of gaseous
solutions, but this is probably much less common than the modification by
aqueous solutions.
If it be agreed that ore deposits are produced by magmatic segre-
gation, by gaseous solution, and by aqueous solution, it is certain that a
deposit may be partly segregated by one of these processes, and this work
supplemented by one or both of the other processes. This may be true
of a deposit which is the result of continuous segregation. It is hard
to see how an ore deposit produced by magmatic segregation can be formed
wholly independent of the action of gaseous and aqueous solutions. If at
the time a magma solidifies, magmatic segregation of some valuable metal
takes place to a certain extent, after solidification is complete the tempera-
ture is still above 365° C. for a considerable time, and the conditions are
almost certainly those favorable to the action of gaseous solutions. These
may continue the process of segregation for a long time, but finally the
temperature falls below 365° C., and then when the temperature is still high
and hot water potent, the aqueous solutions begin their work. This work
of aqueous solutions may continue to the time the ores are exploited.
Thus an ore deposit partly formed by magmatic segregation is sure to
be be subsequently modified by the action of gaseous and aqueous solu-
tions. The modification may be by addition or subtraction of the valuable
material; that is, in the direction of further enrichment or in the direction of
depletion. For the titanic magnetites and the corundum-syenite ores it is not
SUMMARY AND CONCLUSIONS. 1237
supposed that the action of the gaseous and aqueous solutions is usually of
great consequence; but for the sulphide ores and the gold ores which have
been held to be produced by magmatic segregation alone, I hold the work
of the solutions to be a necessary and in most instances the dominant part
of the process.
Whether ore deposits produced by magmatic segregation are likely to
be altered mainly by gaseous solutions or by aqueous solutions will depend
largely on the zone in which the igneous rocks are intruded. If the rocks
are batholiths in the zone of anamorphism the magmatic segregations which
there form are likely to be modified by gaseous solutions, and so long as
they remain in that zone aqueous solutions are likely to be unimportant.
Where the igneous rocks are intruded into the zone of fracture the con-
ditions are not favorable for action of gaseous solutions for any length
of time, but are especially favorable for the long-continued action of
aqueous solutions. Thus we would expect that magmatic-segregation ores
would be especially likely to be modified by gaseous solutions in the zone
of anamorphism, and by aqueous solutions in the zone of fracture.
Metamorphic ore deposits formed mainly by gaseous solutions or by
aqueous solutions, or by the two combined, may form without any implica-
tion that they are partly due to magmatic processes. Often after igneous
rocks are intruded ores do not begin to form until gaseous solutions are
active, or much later, when aqueous solutions are at work.
Not only are there transitions between ore deposits produced by
sedimentary, igneous, and metamorphic processes, but there are transitions
between the classes and subclasses. To illustrate, there are transitions
between chemical precipitates and mechanical concentrates; between
residual, stream, and beach deposits; between the deposits produced by
gaseous and aqueous solutions; between deposits produced by ascending
waters, descending waters, and ascending and descending waters.
The chemical precipitates and mechanical concentrates are connected
by the residual deposits. Very frequently chemical solution of a rock in
which the ore was disseminated is of equal or greater importance than the
mechanical concentration. The residual iron ore mantling Iron Mountain,
Missouri, is quite as much due to the solution of the porphyry as to its
mechanical disintegration. The tin ore in the alluvium of the Malay
Peninsula is probably concentrated more extensively by chemical than by
1238 A TREATISE ON METAMORPHISM.
mechanical processes. Thus one might consider that the residual deposits
which are classified as mechanical concentrates are partly chemical concen-
trates, and on this basis chemical deposits might be divided into chemical
precipitates and residual concentrates. However, residual deposits are
placed with the mechanical concentrates mainly because of their close
alliance with stream and beach deposits, which are plainly mechanical
concentrates.
This leads to the transition between residual, stream, and beach
deposits. It is well known that a residual deposit on a hill or slope may
pass by imperceptible stages into the concentrates at the bottom of a stream.
It is known also that stream deposits may extend along the stream until
the ocean is reached, where a beach deposit may exist. It is therefore clear
that there is every possible gradation between residual, stream, and beach
deposits.
An ore deposit which is formed mainly by gaseous solutions may be
modified by aqueous solutions, for when the temperature has fallen so that
the water is below its critical temperature, it is little short of certain that
the processes of segregation may and usually will be continued by aqueous
solutions, just as in the case of magmatic ores. Thus ore deposits are
probably seldom formed solely by gaseous solutions without any subse-
quent modification by aqueous solutions. Probably very often ore
deposits, perhaps inaugurated by precipitation from gaseous solutions, have
been profoundly modified by aqueous solutions. Like the ore deposits
first segregated by magmatic segregation, the later water action may be in
the direction of addition or subtraction. As to the importance of subse-
quent action by aqueous solutions upon ore deposits partly formed by
gaseous solutions, there may be difference of opinion. It is possible that
rocks which do not freely permit the circulation of underground water may
be more pervious to gaseous solutions, and that in very dense rocks deep
within the earth gaseous solutions may segregate ore deposits—such as those
associated with heavy anhydrous silicates or gangue minerals—which while
in that position are not materially modified by the subsequent action of
aqueous solutions. But ore deposits which are formed in the zone of
anamorphism must pass well up into the zone of fracture, if not to the
surface, before they are accessible to man. They are now in the zone in
SUMMARY AND CONCLUSIONS. 1239
which aqueous solutions are active, and while migrating they may be
profoundly modified. The amount of subsequent modification, while
migrating from the deep zone to the surface, depends upon the conditions
of fracture, water circulation, and all the other factors which have been
discussed in connection with the deposition of ore deposits by aqueous
solutions. The modifications in the zone of fracture of an ore deposit the
concentration of which was begun in the zone of flowage, may be, therefore,
inappreciable or of dominating importance.
Ore deposits formed by aqueous solutions are not nearly so likely to
be modified by gaseous solutions as are the latter by aqueous solutions.
Under normal conditions an aqueous ore deposit would not be modified by
gaseous solutions, but if there were sufficiently powerful orogenic move-
ments or great igneous intrusions adjacent to an ore deposit partly formed
or primarily produced by aqueous solutions, it is conceivable thatsthe water
might be raised above its critical temperature and thus a further concentra-
tion take place as a result of the work of gaseous solutions. It might be
supposed also that an ore deposit partly formed by aqueous solutions might
become so deeply buried as to pass into the zone of anamorphism and there
be modified by gaseous solutions. Thus, while it is admitted that modifi-
cation of aqueous solution deposits by gaseous solutions is possible, it is
believed that it is exceptional.
Ore deposits which are precipitated almost solely by ascending waters
often grade into those in which descending waters have produced an
important effect. Thus there is transition between the ores deposited by
ascending waters and those deposited by ascending and descending waters.
Similarly there is every gradation between ore deposits formed by ascend-
ing waters and those produced by descending waters, and between ore
deposits formed by descending waters and those precipitated from ascending
and descending waters. It may not infrequently happen that a single fissure
may fall partly in one subclass and partly in another. Thus a single ore
deposit may belong partly in the subclass of ores formed by ascending
waters, and partly in the subclass of ores produced by ascending and
descending waters. However, most aqueous deposits belong chiefly to one
of the three subclasses.
Finally, not only are there gradations between different varieties of the
ore deposits among themselves, but there are gradations between the ore
1240 A TREATISE ON METAMORPHISM.
deposits and the rocks, for the ore deposits in many cases are not sharply
separated from the country rocks, but grade into them in various ways.
Therefore it appears that not only are there no hard and fast lines
between ore deposits of different modes of origin, but that a single ‘ore
deposit may be produced by various combinations of most widely diverse
processes. Indeed, it is certain that many ore deposits are not produced
by a single process, but by several processes, one following another, or
even by more than one cycle of processes. Thus, ore deposits which
receive the first concentration by magmatic segregation may be concen-
trated further by gaseous solutions, still further by aqueous solutions, still
further by sedimentary processes, and again by aqueous solution. It is
clear that there may be every gradation between ore deposits produced
solely by magmatic segregation and solely by aqueous solutions.
For the production of many ore deposits in their present condition
there have been many stages of concentration by various processes. One
cycle of concentration may have followed another through a large part of
geological time in accordance with the general law for the segregation of each
of the metals in certain formations. (See pp. 947-948.) Many iron-ore
deposits have certainly, and many deposits of other metals have probably,
been produced as the result of the work of several cycles.
While a part of the metals for many and perhaps most of the ore
deposits have an exceedingly composite history, in the majority of cases a
final process is dominant, and ore deposits may therefore be fairly classified
on the basis of the last permanent process. or instance, there is no ques-
tion that the Lake Superior iron ores were deposited by descending aqueous
solutions; the copper deposits of the same region by ascending aqueous
solutions, and the lead and zine deposits of the Mississippi Valley by
ascending and descending aqueous solutions.
Looked at broadly from the above point of view, the total mass of
existing ore deposits of all kinds taken together represents the accumulation
of the valuable metals of all previous processes of concentration. The
processes of segregation are continuous. We catch the series of cycles
at a certain stage, and upon the basis of the existing facts we classify
them. In coming geological periods existing ore deposits will be sources
from which, by various processes of segregation, the ore deposits of that
distant time will derive a portion of their material. Another source of
SUMMARY AND CONCLUSIONS. 1241
material for such ores will be the igneous rocks which in the future enter
the upper part of the lithosphere from the centrosphere.
In conclusion the merits of the classification proposed may be sum-
marized as follows:
First, the classification is strictly genetic, and is based upon a
consistent plan.
Second, the primary divisions of metallic ore deposits are parallel with
those of rocks. Just as there are sedimentary, igneous, and metamorphic
rocks, so there are sedimentary, igneous, and metamorphic ores. If this
division of rocks be logical the division of ore deposits is equally logical.
Third, the classification recognizes the gradation and connection
between different classes of deposits, one of the most fundamental laws of
geology. Recognizing that many ore deposits have a long complex history
in which many processes have been concerned, it makes the basis of the
classification the last process which resulted in the deposition of the ores
where they are now found.
Fourth, it is believed that the classification offered, being strictly
genetic and recognizing gradation, is enlightening from a scientific point of
view and gives a better idea of the relations of various ore deposits to one
another than classifications heretofore proposed.
Fifth, it is believed that the classification will assist mining engineers
and geologists in accurately describing deposits, and will give rules to
guide them in the exploration and exploitation of ore deposits.
As an illustration of the practical usefulness of the classification is
the connection between genesis and depth for those ore deposits in which
descending waters were an important factor in concentration. Ores depos-
ited by ascending waters alone are likely to continue to great depth.
Therefore, where a given ore deposit has been shown to belong to this
class, the expenditure of money for deep exploration may be warranted.
Where a deposit is produced by descending waters alone the extent in
depth is probably much more limited. In such cases, when the bottom of
the oxidized product is reached, it would be folly to expend money in
deep exploration with the expectation of finding other rich oxidized masses.
Where the ore deposit belongs to the third class, that produced by ascend-
ing and descending waters combined, there is a rich upper belt which we
can not hope will be duplicated at depth. However, this class of deposit
1242 A TREATISE ON METAMORPHISM.
may grade into the first class, and after the transition the deposit may be
rich enough to warrant exploitation at depth. If such work be undertaken,
it must be understood that the rich upper products peculiar to the belt of
weathering and to the belt of sulphide and telluride enrichment will not be
duplicated at depth. It therefore appears to me that the determination
to which of the classes of ore deposits produced by underground waters a
given deposit belongs has a direct practical bearing upon its exploration
and exploitation.
Again it has been seen that the ores produced by magmatic segrega-
tion and those which are preduced by gaseous solutions form a relatively
small class, and not only so, but that the ore deposits themselves are
usually rather circumscribed. When, therefore, deposits have either of these
origins, experience does not justify the belief that they will be of great
importance. For instance, if a copper deposit is associated with heavy
anhydrous gangue minerals, and has irregular boundaries, it can not be
expected to prove comparable in richness and extent to the great copper
deposits produced by aqueous solutions.
It is my hope that mining engineers and geologists will study ore
deposits in various regions with reference to the principles discussed in this
treatise. It appears to me that he who does this will be capable of inter-
preting better than before the phenomena which he finds in the ore body
or bodies with which he is particularly concerned.
In addition to the points specially emphasized in this volume, accurate
descriptions should be made of the relations of the different minerals of ore
deposits, of the occurrence of each ore mineral with reference to the wall
rocks, and of the variations of the ores in composition and richness at
various depths, reckoning both above and below the level of ground water.
Moreover, such a study should include close observation of the gangue
minerals in their relations to one another and to the valuable minerals, for
in many instances they give important testimony as to the origin of the ore
deposit. A study should be made of the stratigraphy, including the changes
of country rocks and wall rocks, the pervious and impervious formations,
the phenomena of deformation, such as folding, faulting, jointing, breccia-
tion, secondary structures, ete. The volcanism of the district should be
fully investigated. Finally, the general metamorphism should be worked
out. In short, my point of view is that the principles controlling the
SUMMARY AND CONCLUSIONS. 1243
deposition of ores must be interpreted in the light of the general principles
of geology and especially of metamorphism. It is believed that when a
comprehensive study of various ore-bearing districts has been made from
this point of view a more satisfactory treatise upon ore deposits may be
written than has yet appeared.
Such a study of ore deposits must be a difficult one, involving as it
does a working knowledge of stratigraphy, of physical geology in its
broadest and most intricate aspects, of petrology, including igneous, sedi-
mentary, and metamorphic rocks, of mineralogy, and of modern physical
chemistry. Undoubtedly many ore deposits will be found to be exceedingly
complex and not to come fully within the scope of the principles discussed.
So far as any ore deposit fails to do this, it will give data upon which to
state a more nearly complete theory of ore depasits than that here proposed.
EAN 1D) 18) 2
A. Page.
Absorption, capacity of rocks for, an index to
power of transmission --------...---- 155
Acid rocks, resistance of, to weathering --___-____- 532-533
Acid|saltsyins tance jo fos eases eee eee ene 84
PAGIdS 4 definitions \o Laeee eae ee ee 84
solution of, by rock weathering 536, 537
strength of, dependence of, on degree of disso-
Cla GON! 2 2s See ee ee noe 94
strong and weak, discussion of _- = 93
work of, in ground-water solutions -- aspje AS)
Acmite, chemical and physical constants of- = 1955271
OCCULTONCE) Of. a= 22 aeons ae eee eee 273
source and mode of origin of -_ 273,280,286, 289, 369,378 |
Actinolite, alterations and alteration productsof__ 285-
290, 872, 375, 398, 399
chemical and physical constants of --..---.---- 195, 283
formation of, from ankerite, chemical reaction
inv Olv.e Chinese ae see
occurrence of
sources and mode of formation of -_--___
270, 271, 274, 277, 279, 283-284, 286-287, 309,
310, 354, 369, 376, 379, 382, 388, 391, 407, 408
Actinolite-magnetite-quartz rock, origin of 834-841
photomicrograph of---------.-.---------- 836
Actinolitic marbles, origin of: -2------2--=--.-.---.-- 833
Adams, F. D., figures cited from 674, 814
on deformation of quartz grains in rocks 695
on granulation and recrystallization as affected
-- 243, 268
by water content of rocks -_________. 742
on mechanical granulation of anorthosite______- 675
on rock flowage by granulation -___--.-._______. 759
Adams, F. D.,and Barlow, A. E., onschist conglom-
Orates assets ee a Se eee ae ea 859
Adams, F. D., and Nicolson, J. T., on deformation
ofcalcitelcrystalseae= =e ae Vis)
on deformation of marble under pressure ____ 49,670,
681, 696, 747, 772, 809, 811, 812, 814, 815, 816, 1012
|
ontmobilityofical cites =a sana ee 754
Adams, G.I., on chert beds of Missouri-----..______ 851
Adaptation of minerals to environment, illustra-
tions of ----- -- 33-34
Adhesion, influence of, in ground-water circula- |
TO) ees See er Reece ocpdosherceeccs 150-152
Adsorption, phenomena of____--.------..._------ 6465, 121
Egirite, chemical and physical constants of ___--._ 195 |
Aggregation, state of, in rocks, effect of, on rate of
weathering 222i .2.5 225 sso eee 533-534 |
Agriculture, eftect of, on ground-water level.__-... 427
efrectiof ony cun-O ft se maaan eee eens eer 415
ON SOlIS Somer aes See es are eee sa eae 451
Aikman, C. M., on nitrogen compounds, decompo-
SItIONIOLS =.= Reee ane e eee noes aneee e465 p|
Page.
Aikman, C. M.,on nitrogen, fixation of, by bacteria. 453
on nitrogen precipitated from the air, amount
QE GS Sos eae SIS Re eee re ee 453
on oxygen in soils, amount of _________ 470
on reduction of nitrates to nitrites 472
on temperature at which bacteria show maxi-
AM CLV t yee ee ae 466
Air, chemical composition of _____- Soo eae Odd
See also Atmosphere.
Albite, alteration of, to analciteand quartz, volume
changes produced by-_-_---...___.._- 632-633
alterations and alteration products of________ 260-265,
366, 372, 375, 398, 400, 404
chemical and physical constants of____________ 195, 259
sources and mode of formation of-_ 260,261,265, 274,281,
294, 333, 334, 335, 366, 369, 375, 376, 377, 384, 386,
388, 389, 392, 393, 396, 397, 400, 404, 405, 406, 407
Albite and anorthite, alterations and alteration
products of 375, 376, 377, 397, 398
Albiteand gibbsite, alterations and alteration prod-
ucts (of, 222 ee Sone eee 375, 400
Albite and halite, alterations and alteration prod
cts (0 bose eee ee ne eee 375, 400
Albite, anorthite, and leucite, alterations and alter-
ation products of 375, 376, 386, 397
Albite, anorthite, and microcline, alterations and
alteration products of ________________
Albite, anorthite, and orthoclase, alterations and
alteration products of______ 375, 377, 389, 404
Albite-gneiss, photomicrograph of___ 902
Albite-schist, thin section of _______________ 704
Alfalfa, disintegration of rocks by -...______....-___ 445
Alkalies, decomposition of, in’ rock weathering,
OTder Of S222 Seka ee ae 536
in arid regions, accumulation of_______________ 545-546,
in soils, amount of, necessary to prevent agri-
Culture Ses - sess ae eee 543
migration of, to surface, process of __ _ 546
prevention of deposits of________- 546
susceptibility of, to weathering. _______ nee GBP
Alkaline rocks, susceptibility of, to weathering _ 532-533
Allanite, alterations and alteration products of____ 328
chemical and physical constants of - 195,320
Occurrencelo ne sssse=s =e nee een nae soe || GP
Allophane, chemical and physical constants of ___ 195,254
sources and mode of formation of 369, 389
Almandite, alterationsand alteration products of_ 372,375
chemical and physical constants of -_._________ 195, 299
occurrence of --- 300-301
Almandite and pyrope, alterations and alteration
productsofs-==se==sses= 375, 383, 390, 401, 404
Almandite, melanite, and pyrope, alterations and
alteration products of -_ 375,383, 387,390, 396
1245
1246
Page.
Alteration of minerals, chemical reactions and yol-
ume changes inyolyed in, table show-
in geese ee eee ee eee enews 375-394
classification of, with volume changes- --- 395-408
general nature of ._--.-.------- ---- 202-206
products of, tables showing-------------------- 372-374
with addition or subtraction of material, cate-
gories and examples of ------------- 203-206
with change in chemical composition, character
‘andconditions|of*-==222=-----=-2=<— 202-206
without addition or subtraction of material,
categories and examples of -.-------- 203
without change in chemical composition, char-
acter and conditions of --.-.-.-------- 202
Alterations of rocks, general nature of ___....------ 31-38
geological factors affecting --....---------------- 40-44
See also Metamorphism.
Aluminates, deposition of -_.......-..---------------- 541
Aluminum, in earth’s crust, amount and percent-
AeelOL a ehean = ees 934, 936, 937, 938, 983-986
in rocks of different kinds, proportions of --_---- 984
mineralsicontainingy-=--22 22) nena ease eae 628
decomposition of, in rock weathering, order
OL eRe aoe eee ae eros aee 536
Aluminum ores, formation of__-..--.------------- 541, 1037
Aluminum-silicate group of minerals, character,
occurrence, formation, and altera-
tions of 316-319
Amesite, alterations and alteration products of__ 347-348
chemical and physical constants of -_....------ 196, 345
OCCULTENCOO fae teen eee een e eee e eee eee 346-347
source and mode of formation of -__...-.-------- 302,
304, 328, 369, 390, 393, 399, 403
Ammonia, production of, from urea, formula for
MEACLIO Nate suet tee eas ae ee ersEnE Nan 465
Amorphous compounds, crystallization and conden.
SBELOTILO Lee ee eee ea pene Smee eras 103
Amphiboles,alterations and alteration products of 282, 354
chemical and physical constants of -...---- 195,197,281
formation of, from limestones----_--- 822
fromypynoxenes=----2-- sone — eee 276-278
occurrence of _._..---------- E Secures 281-282
percentage of, in igneous rocks 937
sources and mode of formation of ______________. 274,
276-278, 354, 380, 383, 394, 822
See also names of minerals of the group, Actino-
lite, ete.
Amphibole-gabbro, mineral composition of, and
volume changes possible in 632-633
Analcite, alterations and alteration products of ._ 333-335,
366, 372, 376, 401
chemical and physical constants of ------------ 195, 330
occurrence 0 fase eseenenn mena sees --- 331-333
sources and mode of formation of -_- - 260,266,
267, 292, 293, 295, 296, 334, 335, 366, 369,
375, 386, 388, 392, 397, 398, 400, 402, 404
Analcite and quartz, alterations and alteration
PLOGUCtS Olsen ease ete ones
production of, from albite, volume change inci-
Gen tito penser mertae naan eens 632-633
Anamorphism, propriety of use of term -____.__-_.. 169
zone of, chemical changes and reactions in__--__ 93-94,
168, 171, 180, 181, 186, 590, 675-707, 764-766
chemical reactions in, heat effects of _____- 110-111
common to zone of katamorphismand_ 181-182
opposed to those taking place in zone of
katamorphism 181, 366-369
thermal and volume conditions and ef-
fectsiol ese seateeee eee 167, 170, 186
INDEX.
Page.
Anamorphism, zone of, cirevlation of gaseous solu-
PIONS Hn CS ee setae ela IN 1020-1021
zone of, condensation of yolume of rocks in____- 169
conditions and materials in ________________ 668-670
definition of=—- = 2-2 = --- 48,167,170, 657-659
deformation}inseeesse ese ee eee 1011-1012
energyarequired Loree: sees eee 769-774
dehy.drationtine sess sees seen ene 178-179, 180
Geoxidation in esse eae ee 169, 955
factors dominant in _ Soi seus NOY
gaseous solutions in, circulation of ______ 1020-1021
gradation between, and zone of katamor-
HIST Soe ena eae A a 191
igneous work in ___ 707-748
materials and conditions in _____- 668-670,
mechanical work in __...._____._- --. 670-675
minerals formed in 169, 363-364
oxidation and deoxidation in, thermal and
volume changes inyolved in____.____- 172
relation of, to zones of fracture and flow-
UE Oye AWE sae ad bd aha Wp A 187,190
to zone of katamorphism _- 170-186, 766-768
TOCK OWA geil n Ste NL EE ana eeeee 748-764
rocks formed in, character of_- ees Key)
silication in ____._= oe eke Ne AOS eee 176-177
specific gravities of minerals in____ 169,363-364, 924
temperature conditions in ___________-____. 168, 170
thermal effects of chemical reactions in 170
yolume changes in___..___.-_--2--__-.-. 167, 182-183
water in, circulation of 128, 129, 661-668
CONGITION OL Memes ea nee ee ee ape 659-661
quantity of__ ._._-_.
worksinyChemicallae sas mess nee ae a eee
IsNeous esate oe ay Neen ieee ena
MeChamical ses Neh Wei eee Leva Nee tlie 670-675
Anatase, alterations and alteration products of _... 280-
231, 874, 388
chemical and physical constants of______..__-- 199, 230
OCCUTTEN COLO Ly amare ees ae pe a a eto ae ON 230
sources and modes of formation of __.._________- 227,
228, 230-231, 355, 371, 385, 386, 393, 396, 405
Andalusite, alterations and alteration products
C0} a gece pean a ar 317-319, 372, 399, 402
chemical and physical constants of ___________- 195, 316
occurrence and mineral associates of __________-- 316
Andesine, mineral composition and physical con-
stantslofeses. see eee eee eer 259
occurrence, alterations, and alteration products
Co ea ea ey 2 ey ee Ce ae ee 259-260
Anglesite, reaction of, on calcium carbonate, equa-
tiontsho wan eae e ee eee en 1147
Anhydrite, alterations and alteration products of__ 357,
i 372, 402
chemical and physical constants of __- - 195, 357, 788
source and mode of formation of_______________- 357,
358, 369, 384, 400, 788-789
Anthophyllite, source and mode of formation of... 310
Anhydrous minerals, order of decomposition of_... 533
Animals, abundance of, favorable to decomposition
of rocks 503-504, 505-506
phosphates and nitrates produced by ----..---- 542-543
scarcity of, fayorable to rock disintegration
without decomposition 500
work of, in soils 447-451, 456-457, 504-506
Ankerite, alterations and alteration products of_ 242-243,
372, 405, 406
chemical and physical constants of 195, 237
formation of actinolite from, chemical reaction
involved yi niesessse ase eae eee
837
Se
INDEX. 1247
Page.
Ankerite, formation of sahlite from, chemical reac-
TION NVOlvie dei nasa = ea eee 837
sources and modes of formation of_ 242-243, 625, 823-829
Ankeriteand parankerite, alterationsandalteration
LOGUCLS|O eee = a ee 376
Ankerite and quartz, alterations and alteration
(OLOCUCTSO fee see eens 376, 407
Ankerite, parankerite, and quartz, alterations and
alteration products of _--.------------ 376
Anorthite, alteration of, to gismondite, volume
changeyincident)}tos---. =e eee 633
alteration of, to thomsonite, volume change in-
CId ent LO see eee eee ene eeu 633
alterations and alteration products of -- - 260,
261. 264, 367, 372, 376. 402, 404
chemical and physical constants of___._._----- 195, 259
OCCUTTED COO Leen taee wae ae eee aetna 259-260,
sources and modes of formation of _______- 260, 264, 367
Anorthite, trisilicic, alterations and alteration prod-
chemical and physical constants of____.....----- 201
Anorthite, albite, and leucite, alterations and alter-
ation products of ____._____- 375, 376, 386, 397
Anorthite, albite, and microcline, alterations and
alteration products of__._.....-.----- 389
Anorthite, albite, and orthoclase, alterations and
alteration products of_____- 375, 377, 389, 404
Anorthite and albite, alterations and alteration
products of ---_... .__... 375,376, 377.397, 398
Anorthite and calcite, alterations and alteration
PROGUCLSO Leena 377, 400
Anorthiteand hematite, alterations and alteration
PLOGUCTS 0 aaa aa eae 77, 402, 404
Anorthite and olivine, alterations and alteration
DROCUCTSIO Lee eee nen ee ae 388, 408
Anorthoclase, alterationsand alteration products of 258-
259, 369, 370, 371, 372, 377, 398
chemical and physical constants of___________- 195, 253
OCCULTON COLO fame ee ee ee ae eee 257-258
sources and modes of formation of ____._________ 258
Anorthoclase and gibbsite, alterations and altera-
tion products of __ 377,400, 401
Anorthoclase, hematite, and calcite, alterations and
alteration products of -______._.___..- 3i7
Anthon, E. F., on order of disappearance of the
metals onoxidation===--2----- 221 t 1141
on precipitation of sulphides_________________ 1114, 1116
Anthophyllite, alterations and alteration products
Of MSS ee eect trees 282, 372, 377, 378, 404
chemical and physical constants of _________... 195, 281
OCCULTENCE 0 hisses eae aan ean e ne ney oe ene 281-282
sources and modes of formation of _______..____. 268,
270, 271, 282, 309, 310, 354, 369, 379, 382, 388, 399, 406
PAM TS ay O Ls Of ONtSOS pee eee ae ete 448, 449
Apatite, alterations and alteration products of_____ 356,
372,378
OCCULTEN CO OLs sana awe eee ee ee ae ae 356
chemical and physical constants of___.________ 195,356 |
Aphrosiderite, chemicaland physical constants of 195,327
sources and modes of formation of _______.______ 303,
304-305, 369, 376, 383, 390, 404
Apo, defini tionjof term: —2 see eee eee T6777
Apophyllite, alterations and alteration product of_ 333,
334, 372, 378, 401
chemical and physical constants of____._________ 330
(OC CUTTEN COO Leese ae ee ee ee 331-333
Aqueous solutions, ascending, ores precipitated
LT OM 2 saose Soe ee see ee ae 1072-1139
Page.
Aqueous solutions, ascending and descending, ores
precipitated from -_---___....---- 1139-1193
Cizculabionio feeses a= ee ee ee 1021-1029
descending, ores precipitated from___.____.. 1193-1199
LOLM ALON O Leena eee eee 59
metamorphic work of 63-158
minerals now in course of deposition by ---- 1059-1060
loresye enosite dsb yaaa res a een ee eee 1058-1234
metalsioL;SOURCES|OL Meese te ee ee 1069-1072
modification of, by gaseous solutions. _____-. 1239
precipitation of minerals by reactions between 116-119
precipitation of minerals by reactions between
gasesiandieee aa eee eae 119
SOULCEIOLAW a LOI O hee een eee 1065-1069
work of, in segregating ores________________- 1072-1199
Aragonite, alterations and alteration products of__ 245-
246, 372, 378, 399
chemical and physical constants of ___________. 195, 245
OCCUTTENCE (Of es Sere ee ae eerie nee eu 245, 793
sources and modes of formation of _______.- y---- 240
Arches in strata, influence of, on ore deposition. 1211-1216
Arctic regions, disintegration of rocks in_________ 498-499
Arfyedsonite, alterations and alteration products
OPC mean Ser ae cata att eR lbs 289, 372, 378
chemical and physical constants of ---- 195,283
OCCULT ENC EO Leesa era aaa Rene On ee 285
Argentite, alteration of, to silver sulphite, equation
BHOWIN ges whee kewl ed eT EN ee 1167
formation of, equation showing -___...___.--____ 1168
Argillite, association of gold with 1095, 1096
losses in, by weathering, analyses showing. 510, 515, 522
Arid regions, accumulation of alkalies in_________ 545-546
disintegration of rocks im___._.----._________.___ 497
ground-water level in, as affecting depth of ore
deposits mem eesm mesa: ene nane eerie 1180
497-498
Arizona, copper deposits of, secondary enrichment
CO) Oe kee mea ai epee 1184
Arkansas, limestone from, losses in, by weather-
ingyess pee eS he ene 513,523
syenite from, losses in, by weathering, analyses
Showin geo Cee hs eee te 508, 522
Arkansas River, ground water of, rate of flow of... 584
Arrhenius, Svante, on climatie effect of increased
amount of carbon dioxide in atmos-
phere Bees ee eee arena enae 464
Arkose, deposits of, origin and character of ______ 874-875
Arkoses, metamorphosed, discrimination of______ 909-910
Arkose-gneiss, deposits of, originand character of_ 875-876
Arkose-schist, deposits of, origin and character of_ 875-876
Artesian water, flow of, loss of head in_____________. 142
Artesian wells, section showing requisite conditions
80) PE eee aa cma uae Lea Yi
Ascending aqueous solutions, ores deposited by__.__ 1072-
1139, 1235
See also Water, ground, ascending.
Ascending and descending aqueous solutions, ores
deposited from _________ 1139-1193, 1284, 1235
Aspen district, Colorado, silver-lead deposits of,
theory of formation of __________--___ 1084
Atmosphere, chemical composition of_______________ 944
percentage of known matter of globe formed
Dracaena ee RECS cen eee QO
Augé, on bauxite deposits_- 985
Augite, alterations and alteration products of _____ 273
277-280, 372, 378, 397. 399, 408
chemical and physical constants of_____.______ 195, 271
occurrence of_ 272
1248
Augite, source and mode of formation of ----------- 272,
286, 289, 369, 385, 408
Augite, siderite, and magnesite, alteration products
(os ee aE Ne i eee eee 378, 408
Austin, L. W., and Thwing, C. B., on volume of
water at different temperatures- ----
Australia, gold fields of, secondary enrichment in
1185-1186
ore deposits of, influence of organic matter of
wall rock on deposition of -_----- 1087-1088
Axinite, chemical and physical constants of ------ 195, 323
occurrence of
Azurite, formation of, from malachite ------------- 1159
B.
Bacteria, work of, in disintegrating plant and ani-
TALIS RSME VE) 9 aececo eee eeSe 455-406, 457
work of, in plant growth-----------------------
Bain, H. F., figure cited from_--.--------
on rcs beds of Missouri --.-..------------
on dolomitization of limestone by necendine
waters---
on lead and zine ores of the Mississippi Val-
Wei Sco Soho nak ecacen eco aEeE Eas 1145, 1208
on precipitation of lead and zinc ores by min-
gling of solutions. --_----.-.------------ 1114
on precipitation of lead and zinc ores by organic
Mattern sees ee 1111, 1114
on reduction and precipitation of oxidized prod-
ucts by organic matter--_-_-...._--.----
on secondary enrichment of ore deposits
on secondary sphalerite in the Joplin district... 1152
Bain, H. F., and Calvin, Samuel, on dolomitization
Oflimestoner-s-sereess sea ate eee 800
Ball, S. H., aid rendered by, in measurement of
grains of sandstone --.....------------ 861
on density of saturated solution of sodium chlo-
47
on granulation of anorthosite- 675
on size of grains of feldspar in original and gran-
ula tedinocks pase saesana sesso nea ee 739
on size of particles of sand composing sand-
SEONG peso nee eee ai amen eee ee 892
Bancroft, G. J., on precipitation of gold by tellu-
Mid eg Ue sanawenmee enn ste a Se ees 1097
on tellurides in the Kalgoorlie district of West
INSITE 3 5355 be he ee ee 1119
Banner mine, California, vein quartz from, figure
Oc tecensesdccce SasedeaSeSe cores See 1156
Barium, percentage of, in earth’s crust --_____.__ 936, 1002
Barium oxide, percentage of, in earth’s crust--__-_- 988
Barker, G. F., on capillary and molecular attrac-
tion - 150, 151
on strength of surface tension of water _._..____ 150
Barlow, A. E., on fusion of sedimentary rocks _____ 734
Barlow, A. E.,and Adams, F. D., on schist-conglom-
TM CTALCS Sas eeepeme meta ierene ene es 859
Barometric pressure, influence of, on ground-water
leven coos sree aren Scere SN Se 428
Barus, Carl, on decrease of volume of glass and hot
Wwaterlonsolubionaeeseneeee eases a= 78
on miscibility of water with rock magma__ 723
on nonaction of steam on diabase 493
on potentialization of energy in strained min-
OTS IS 28 So EE eee eee sna 96,691
on solubility of glass in water._....-----.------- 79, 80,
112, 630, 637, 692, 723, 740, 749, 750
INDEX.
Page.
Barus, Carl, on temperature increase with increase
Of depth Se se see eae Nee
on temperatures required for aqueo-igneous
fusions isi 2o bocce sb eae eee eeace 729
Basalt, losses in, by weathering, analyses showing--
strong and weak, discussion of ______-_- 92
substitution of, alterations produced by 408
iBasicisalts;instanceloljess sss eens 84
Bastite, chemical and physical constants of -__-_---- 195
sources and mode of formation of________-_-. 268, 270,
275, 278, 282, 285, 286-287, 288, 290, 369,
375, 378, 379, 381, 382, 391, 398, 399, 404
Bauxite deposits, formation of__..----.---..-..------- 1037
Bayley, W. S., on arkose-gneiss deposits of Lake
SUperlomnesion eee. sae eeaeee eee 876
on biotite and chlorite, mutual replacement of 179-180
on fusion of rocks on Pigeon Point, Minn 732, 733
on schist-conglomerates 860
on schists of Lake Superior region __-..._.__---. 904
on water content of rocks as affecting granula-
tion and recrystallization ____-_.__..- 743
Beaumont, Elie de, on ore production by gaseous
solutions) 32225 tel See eee ea ae 1053
on origin of pegmatites 72
term ‘‘agents minéralisateurs”’ first used by --- 59
Beck, R., on formation of cassiterite _____._-.-_--__- 1127
on igneous origin of certain ore deposits --.. 1049-1050
on ores deposited from gaseous solutions-_-_____- 1054
Becke, F., chemical reactions given by -..--.----.- 261, 264
on alteration of olivine to anthophyllite ________ 310
on alteration of orthoclase and plagioclase to
albite and other minerals -_-...---... 264
on enlargement of mineral particles. ___-_______- 644
Becker, G. F., figures cited from __.-_.___________. 643, 882
on bowlder-like masses produced by weather-
Inge teeeee 527-528
on consolidation of rocks- = BES
on contact metamorphism 715
on precipitation of mercury sulphide_--.-..____- 1109
oniserpentini zations sss. sees sere ee ean eee g
on solubility of gold
on solubility of sulphides
on strain, flow, and rupture of rocks ____.____- 600, 753
on Witwatersrand banket -_--..- -- - 1041
Bedding partings, size of -_ emo ss
Bedding planes, importance of, in ground-water
Circulation ee eee eee eee 130
Bell, Robert, on volume changes in rocks by weath-
Ting ses ee eo ele eae 524
Belt of cementation, weathering. See Cementation;
Weathering.
Belt, Thomas, on rock decomposition in tropical
TE LIONS sae a 476, 530-531
Bendigo gold district of Australia, ores formed be-
neath impervious arches in _____- 1212-1213
Berea sandstone, strain in, shown by quarrying.... 598
Berlanite, source and mode of formation of __ 347,369,380
Berthelot, Marcellin, on production of heat in
chemicalichanges*sseeeesseescesie eee 106-107
INDEX. 1249
Page. Page.
Beta-spodumene,alterations and alteration products Boussingault, J. B., and Lewy, B., on carbon dioxide
Ofeeee 392, £97, 405 in soils 474
chemical and physical constants of -__________- 200,280 | Bowlder, gravel,and pebble deposits, discussion of_ 853-860
sources and mode of formation of - 280-281, 369, 392,408 | Bowlders of disintegration, production of ________ 527-528
Biddle, H. C., on precipitation of copper -_---_------ 1102 | Branner, J. C., on ants and termites, burrowing
Bingley, ©. W., on devitrification.____.___..._______- 248 WOLD )cO Loamet eis Ret de D Ai nel dss 448, 449, 456
Biotite, alterations and alteration products of ___ 339-343, on bowlders and mountains of disintegration in
854, © 72, 378, 379, 396, (97, 399, 402 Brazil Sasa a Aes ee a eae 528
chemical and physical constants of -_--._--_--- 195, 336 onearbon dioxide brought down by rain, amount
OCCUTRETICOlO fee eee sap een aaa es eee Ee BOO) ORAS se uae eal as fe eI I 3 TAD
sources and modes of formation of__-. 255,257,278, 280, on disintegration of rocks by changes in tem-
286, 312, 354, 369, 377, 578, 385, 387, peratures 0228 ke eS wd 39
389, 393, 394, 897, - 99, 400, 401, 405 on diurnal change of temperature in Brazil_____ 436
Biotite and hematite, alterations and alteration Brauns, R., on formation of webskyite----__-- Cine 350
products of _.. 379,396, 398, 401
Biotite-chlorite, chemical and physicalconstants of. 195
SOURCEOL cere ten eer oat OU eaiheren cen Ate 36), 379, 697, 401
Biotite-gray wacke, photomicrographs of _______--.- 888
Biotite-slate, photomicrograph of -____-.______-__--. 902
Birkinbine, John, on magnetite beach sands of New
Derlands ts ssce cee eves a ee 1039
Bischof, Gustay, on carbonation of silicates______- 175-176
on carbon dioxide in earth’s crust_----_---.-..-. 474
on carbonic acid in ground waters __-____..___-- 610
on carbono-silicic cycle of metamorphism. 176-177,181
on decomposition of silicates by alkaline car-
bona bes ese eee ee a eee 476477
on interchange of carbon dioxide and silica in
chemical reactions at different tem-
Peratures ys eae eee 108
on magnesium content of corals and marine
Shells)sit She. case ae ee eee 798
on oxygen contained in ground waters, amount
(Be SS EE Se aS ee Lar re SN 05
on penetration of basalt by water___-__________- 123
on precipitation of calcium and magnesium car-
bonates; order/of ===) 225 -e ee 799
on replacement of carbonic acid by silicic acid_ 94
on sea water, density of_...-_---_-.--..__--______ 147
Biake, R. F., on effect of work of burrowing ani-
Ama S{ONUSOIS panera ee eee or 450
Blake, R. F., and Letts, E. A., on amount of oxygen
INSOUS uss a ee Nt ee ea en eis 470
on bicarbonates in zone of weathering -_________ 536
on carbon dioxide in the atmosphere___- 962
on carbon dioxide in soils --___- +... 22-2 _ 2... 475
Blake, W. P., on ore deposits at Ducktown, Tenn __ 1182
onorganic matter,action of, inreducing and pre-
cipitating oxidized products--___ 1157, 1158
on precipitation of lead and zine by organic
HOY TALIS) Cl eset a see ae En 111
Blende, secondary, figure showing ---_.---_________- 1156
Blow, A. A., on ore deposits of Leadville, Colo__ 1214
Blue Mountains, Oreg., copper deposits of _____- 1055-1056
Bog iron ore, formation of deposits of 550)
Bohemia, basalt from, losses in, by weathering___ 509,522
phonolite from, losses in, by weathering--_______ 512
Boration, definitionof= 222022.) sean eae 206
Bornite, production of, reactions showing __ 1162
zone of deposition of 1160
Bornstein, Hans, and Landolt, Richard, on viscosity
of water at different temperatures.. 141
Boulder Hot Springs, Mont., mineral yein forma-
LOM bee ase eaa ee 1059-1060, 1064, 1066, 1068
SOUTCeS\OLgwa tel; OL neon sane ne eee eee eae 1068
Boussingault, J. B., on amount of carbon dioxide
emitted by volcanoes-_------....--.... 969
MON XLVII—04——79
IBLECCIAS POT eISPACe nyeaaes aes ane a ean aa 127
Rrecciation. See Fracture.
Breunnerite, sources and mode of formation of __ 309,369
Brewster, David, on devitrification of glass_________ 248
Briggs, L. J., on absorbent power of soil for carbon
Gioxide 2 ee ee ees 475
Brégger, W. C., on alteration of arfvedsonite to
BGCINI Teas Se IE AED Net Nee 286
on pegmatization--_--...----..-.-- afeeea ee home 721, 725
Bromide of magnesium, See Magnesium bromide.
Bromine, in earth’s crust, amount and percentage
OF S22 FE eNO Rea ee ule Ee aed 978, 1002
in ocean, amount and percentage of___-.._ 936,944, 978
in sea salt, percentage of__-...._...__._..___... 942,943,
Bronzite, alterations and alteration products of_ 268-271,
372, 379, 399, 404, 405
196, 267
268
chemical and physical constants of
(OGCURTO21 COO Lae ee eae ee
Bronzite, calcite, and quartz, alteration products
(0) faye ene pases Sale eee Mp 379, 407
Brookite, alterations and alteration products of____ 230-
231, 372, 379
chemical and physical constants of ____-______- 196, 230
occurrence of 230
sources and mode of formation of _________ 231
Brown, R. C.,on ore deposits of Butte, Mont________ 1183
Brucite, alterations and alteration products of_____ 235,
372,379, 396
chemical and physical constants of -__________. 196, 234
OCCULTENCelOL eae Nees a seees Bo a ne 235
sources and mode of formation of_____.________- 325,
349, 369, 380, 385, 391, 396, 403, 404
Buckley, E. R., figure cited from -__....______-._... 814
on crushing strength of Berlin rhyolite-gneiss
and Niagara limestone. -_-__..._______- 534
on effect of freezing on rocks, as influenced by
pore space -- 441
on pore space in sandstones________-... 124, 126,569, 863
Burrowing animals, work of, on soils_________ 447-451, 456
Butte, Mont., copper ores at, secondary enrichment
OP scien reese 1182-1183
mineralized zones at, copper and silver ores
separately borne by_---..-..--.__---. 1081
ore aeposits of, contiguous mineral zones in,
bearing different ores________________ 1203
Bytownite, character, occurrence, f-rmation, and
alterations Oeste see eee se ae 259-265
Cc.
Cacti, disintegrating effects of, on rocks__ 445,
Calamine formation) obsess sae eee 828
Calcite, alterations and alteration products of “__ 98-240,
373, 379, 380, 407, 408
1250
Page.
Calcite, change of, to dolomite, volume effects of -_- 801
chemical and physical constants of 196, 237
MO Dili tysO Leese eee ee eee 54-155
OCCUITENCO)O Leese ee een nea 237-238
sources and modes of formation of__.. 216, 237-238, 241,
245-246, 273, 274, 282, 285, 286, 298, 312,
322, 358, 369, 375, 376, 377, 378, 379, 381,
882, 383, 384, 385, 887, 390, 391, 393, 394,
396, 397, 398, 399, 402, 407, 408, 624-625
Calcite and anorthite, alterations and alteration
DLOGUCES|O Las=ee ease eee eae aaa = 377,400
Calcite and quartz, alterations and alteration prod-
TIC tS (OL saa eee eee eeninenaanae 380, 407
Calcite, anorthoclase, and hematite, alterations and
alteration products of __..-.---------- 377
Calcite, bronzite, and quartz, alterations and altera-
tion products of__......------------- 379,407
Calcite, diaspore,and quartz,alterations and altera-
tion products of __-.-.-----.--------- 382,407
Calcite, forsterite, and quartz, alterationsand altera-
tion products of___..-.-------------- 383, 407
Calcite, gibbsite, and quartz, alterations and altera-
tion products oteesseeetecsess eee 883, 407
Calcite, hematite, and microcline, alterations and
alteration products of -----.-------- 389, 401
Caleite, hematite, and orthoclase, alterations and
alteration products of _-- 389,401
Calcite, hypersthene, and quartz, alterations and
alteration products of __.....----..-- 379, 407
Calcite, ilmenite, and quartz, alterations and altera-
ition\productsiofsssese-e seen ran 385, 408
Calcite, olivine, and quartz, alterations and altera-
tioniproducts\ofe==------se=sen-—=2-= 388, 407
Calcite, quartz, and corundum, alterations and al-
teration products of ___.------------ 381, 408
Calcite, quartz, and rutile, alterations and altera-
iON PrOdUC Ss Obeeeree essen ans aaa 391, 407
Calcium, agricultural benefits of, in soils -___-..---- 92
in earth’s crust, amount and percentage of __._- 934,
936, 990,
in ocean, amount of -...-------------------------- 944
in sea salt, percentage of......----.-------------- 943,
oceurrence and combinations of -_----.-------- 990-992
Calcium-bearing minerals, deposition of -_.--------- 627
Calcium carbonate, amount of, annually remoyed
Lr OMSOL eee eee eee ene eee
deposition of
deposits of, metamorphism of --.-------------- 795-797
in ocean 943
in salts of sea water - 943
solution and precipitation of __-...----- 119, 539, 557-
Calcium-magnesium-carbonate family of rocks, de-
posits composing =.. 791-823
Calcium oxide, percentage of, inearth’scrust__ 934, 937, 938
See also Lime.
Calcium sulphate, amount of, annually removed
IBROAY OW) pene anscoocncoN Se esEC SCONES 486
AMOUNG OLN OCEAN See ae naan eee mee ine asian 943
inisaltiof/seanwatenessseensseee=s es 943
production of, equation showing reaction for_._ 1147
solubility of, in salt water --..------------------- 119
California, depth of disintegration of rocks in_.--.. 531
ground water in, rate of flow of -:--------------- 585
Calvin, Samuel, and Bain, H. F., on dolomitization
Oflimestone 2 2asse eee eee een ens 800
Cameron, F. K.,on absorption of carbon dioxide by
SOUS ea a ee ae eae nee 475
INDEX.
‘ Page.
Cameron, F. K., on adsorption.._..__....------------ 64
on alkaline reaction of sodium silicate in solu-
tionisio 2 seco Se ee eae 86
on hydrolysisin case of aluminates and ferrates_ 87
ON OSMOtICHPNESSUTC ana ene eee 7.
on precipitation of alumina and ferric oxides... 541
on solubility of gypsum and calcium carbonate
inisaltiwaterieer sso ese es ae eee ee 119
OTSOLIHONS eee See ere reece earn een NDS
Camp Bird mine, Colorado, ore seams in secondary
fracturesiat S20 2 1229
Cancrinite, alterations and alteration products of.. 294-
295 373, 379, 402
chemical and physical constants of 196, 292
occurrence of 294
Capillarity, definition of 152
effect of, on level of ground water--_- 412
influence of, in ground-water circulation -__-- 151-152
upward transfer of materjal by------------------ 549
Capillary openings in rocks, flow of water in -_-_- 138-143
Capillary, subcapillary, and supercapillary openings
in rocks, sizes of 135-186, 188, 145-146
Carbon, chemical relations of, to silicon, discussion
173-177
962, 968, 970, 971-974
-- 936, 938, 962-974
im atmosphere _-----.--.-----
in earth’s crust --
in igneous rocks 968, 969
imi meteorites ass eae eee eee eae 964
in ocean -_--- 944, 962, 965, 967-968
iniseaSalt Sah occ.ssasenaewaanse se enieeee eee eaeteee Oxo
oxidation of, in belt of weathering 461-465
replacement of, by silicon, discussion of __-.-- 175-177
segregation of, modes and extent of____.-__--- 964-967
Carbon-bearing minerals, deposition of -__-.-...-.-- 628
Carbon compounds, specific volumes of, compared
with volumes of similar silicon com-
POUND GS ie SU aha see eve eae ee 175
Carbon dioxide, action of, in metamorphism_--_-._- 60,
61, 62, 71, 165
action of solutions of, on minerals-_--.-------- 475476
appropriation of, by plants, effects of -_----._--- 452
emitted by fumaroles and solfataras, sourcesof_ 492
gravitational gathering of, from interstellar
SDS.CC eee ane are a eee anetey nee 973
in atmosphere, amount of_____-..------ 72,474, 962-963
increase of, by human agencies-------- 457, 463-465
now and formerly, amount and proportion
@kees 949-950, 956, 964-965
in earth’s crust, percentage of___....-.---------- 938
in rain water, amount of_-_.-.-.-.---------------- 474
imsrocks.asiliguidsa2 ese een e een enaenaenn 746-747
in salts of sea water, percentage of. 942
in sedimentary rocks-_--...----------------- 965, 966, 973
Talons oS oe ee oe i SSeS SSS 474
increasing amount of, thrown into atmosphere,
TESUltslOLssrs teen eo eee nee 464-465
influence of pressure on action of__-.-.----.----- 71
liberation of, by erosion---.-..----------------- 969-970
pressure of, at earth’s surface-------------------- 71
production of, conditions favoring -------------- 609
by combustion of coal __-...----.------- 464, 972-978
replacement of, by silicon dioxide- ------------ 173-177
replacement of silicates by--------------------- 175-176
segregation of, by gravity from interstellar,
space 973
sources of, in metamorphism -- 164,609, 610, 950, 967-974
work of, in metamorphism 60, 61, 62, 71, 165
INDEX.
Page.
Carbonaceous rocks, occurrence of gold at intersec-
tion of quartz veins with----._..- 1094-1095
Carbonate of lime. See Calcium carbonate.
Carbonate order of rocks, deposits of -.--..-..-.-- 791-842
Carbonates. cement-forming, in belt of cementa-
tion, enumeration of-_-..-.--..-------- 621
character, occurrence, formation, and altera-
tions|ofjeses=e == --- 236-245
chemical relations of, to silicates - --- 1738-177
decomposition of, by silica ---..---..--.------.- 176-177
deposition of, conditions of ____-.-.-.--.------- 624-625
formation of, extent of _____ --- 971-972
in belt of cementation -_- eae aeeweLGo
with decomposition of silicates ....-.....-.-. 98
gangue-forming, depcsition of-_____--___---- 1128-1129
in waters of mineral springs, content of -_-_--_- 611
solubility of, increase of, by pressure -_.-------- 78
Carbonation, alteration produced by ---.-.----------- 396
Gefinitioni of eee ee eee ee noe 204-205
depth and pressure at which silication re-
= places 180-181
discussion of -_..-.------ - 473-480, 608-612
Ox ten bio fiei ae sti ee ele ee a adn eee 964-966,
as compared with extent of silication -_-_- 971-972
in belt of weathering -__.-.-...--.._..-_._ 163,479
in belt of cementation-- : Borne kays
MOtH OCS Of sete toee ey eee teh k ey MEN gS 475,
method and extent of, inzoneof katamorphism_ 161,
163, 164-165
of: Silicates: discussion of -—- 22. ------22-n se see- 175-176
relations of, to vegetation AUT
source of carbon dioxide for, in zone of kata-
MOTD NSM sees wee ee eee leh ea ae 161
volume changes incident to ---.-_-._--.-..---_. 478479
Carbonation and defluoridation, alteration pro-
ducedib yt are eens eee one na aon 396
Carbonation and dehydration, alterations pro-
AUC AB ye eee cee SUS ye ON ene Meee 396
Carbonation and deoxidation, alterations produced
Neh mee oceans Race cH BEL Se RSC eee eae InEsle
Carbonation and desilication, alterations produced
AD Ye Rare en AG a Se Sh eb ok ees Ae 396
Carbonation and hydration, alterations produced
PD Yas snes ae ee 397
Carbonation, deoxidation, and dehydration, altera-
tions produced by -----------.-._-..-. 396
Carbonation, dehydration, and desilication, altera-
tion s}producedibysrseseeees eae eee 396
Carbonation, dehydration, and desulphation, altera-
LIONS PrOducedibyaees se essen een 396
Carbonation, hydration, and dechloridation, altera-
tions produced by ---- 398
Carbonation, hydration, and desilication, altera-
tions\producedibyjeeseeseet enon ee 898-399
Carbonation, hydration, oxidation, and desilica-
tion, alterations produced by -...___- 399
Carbonation, hydration, and desulphation, altera-
tions}produced byes = eee seen tee S98)
Carbonation, hydration, and silication, alterations
PDPOGUCECI bya ese set ee SOD
Carbonation, oxidation, and hydration, discussion
Of perreee sacs te nae Fate cereale apd BOR k Od
Carbonation, oxidation, dehydration, and desilica-
tion, alterations produced by _-.._--- 399
Carbonic acid, acids replaced byzeeeees 205
dissociation of, equation showing_- 85
importance of, in metamorphism---...__-.---.-- 85
Page
Carbonic acid, in ground water, presence of_______- 93
in ground-water solutions, work of_____________. 93
in salts of sea water, percentage of____._________ 942
replacement of, by silicic acid -_-___-_.----_____- 94
replacement of, by silicic acid, volume changes
Mesum biN Lee OM asses eee ee eee 93.
See also Carbon dioxide.
Carrara marble, deformation of, experiments in_ 811-816
814
deformation of, photographs showing effects of -
Casehardening of rocks, an illustration of chemical
action between solids
ISCUSSION Of pes Sere eee eae eee
Cassiterite, deposition of, from gaseous solutions... 1054
formation) ofmeenns ee ean ese ee seer eeee 1127-1128
Caves in lead and zinc district of Missouri, features
OBR So ea SE ey Se ee OW BENE DBS.
Cavities containing liquid, cut showing __-- 746
Cazin, F. M. F., on ore deposition 1
Cellulose, composition and geological effects of ____. 452
oxidation of, acids produced by _____--...__---- 461-462
Cementation, belt of, aqueous solutions in, circula-
ELOMYOL | BIN eR es LIE 1024-1028
ibeltriof boundaries ofp ese essees seen ne ee 565-566
belt of weathering contrasted with_-..___- 166-167
Cal boOna tl OMe sess ee eee ee 164-165
chemical changes in, variation of amount of,
with varying porosity-...-.---.___. 655-656.
chemical reactions in_-_-_---_---.-. 164-167, 603-604
thermal effects of=-2-22- 2222)--2--. 2=-.-. 690
cireulation in, aqueous solutions. 571-593, 1024-1028
gaseous solutions -_----_---.--___ - 1019-1020
WEEP Sees eneS ses bu Meeco-susoeces cuecHo 571-593
conditions and materialsin, variability of _. 594
definition of 162, 164, 562-565.
eoxidatlonpin nae ae een anne ane 164-165
gaseous solutions in, circulation of _______ 1019-1020.
hydra cionkin yess eee seer eee 164-165, 612
THE OVSOWTISS iO ea 646-652
limitsioi teen ne sence oem a maenmne ape Ste hae 43
materials and conditions in, variability of. 594.
metamorphic agents in___..-._-....__-..--__- 64
name of, reasons for choosing___._._-...--_-- 563:
Oxi da blonpinyes sana eee eee
processes at work in__ ------ 562-563, 594-652
silicification in, dominance of_____...__.__. 176, 630:
transfer of material from belt of weather-
ing toe nec reese ae ae Meee 538
transition from belt of weathering to_____ 560-561
variable materials and conditions of _._..... 594
water in;|jamount of 22-222 2 es 569-571
CincUla TION Of asset een ee eae ee en 571503)
COndiblonyofeeeces teen ae nee eee 566-569
work in, chemical __
TETCOUS Ret esse tes see eae mer lle deed
mechanical
Cases|Ofvasesesee === ee
discussiontofeeee see :
in zone of anamorphism, discussion of -
IME EHO SiO freee rape eee na eo A RR Pe EERE 619-621
methods and materialsof _.______- oS SES AESS 563-564
operation of, by igneous injections___..._.______ 634
in conjunction with metasomatism _________ 654
TAO OL ss stuece eset ones Seaton ee nese 618-619
relations of diffusion to 636-639
time necessary for 618-619
Cementing materials in belt of cementation, discus-
SOMO Lae eee ae ee ne een G2 IBY
1252
Page.
Cementing materials in belt of cementation, distri-
bution of 628
distribution of elements in
| precipitation of, principles of -_.-..-.------------ 629
similarity of, to rocks cemented __-.--..--------- 628
Centrosphere, definition of term -~--..-.--.-------- 31, 658
Cephalonia, sea mills of, cause of water power of-_. 149
Cerargyrite, production of, equation showing_____- 1167
Chabazite, alterations and alteration products of .. 334,
373, 380, 401
chemical and physical constants of -_.-._------ 196, 331
OCCURED COLO Laeeee meee ene ena 331-333
sources and mode of formation of 262,
298, 299, 369, 375, 377, 384, 398, 899
Chalcedony, alterations and alteration products
OL ee ae es a ee as 222
sources and mode of formation of —----.--------- 222,
273, 369, 389, 400
Chalcedony and chert, alterations and alteration
WLOGUCES{O fypeeec eee eee ennai 380
Chalcocite, alteration of, to metallic copper, equa-
LLOTMESH O Wall eens eer ape eae 1158
production of copper from, equation showing -_ 1158
production of, from cuprous and cupric sul-
phates, equations showing reactions
OT aa ne ce ee tga ae OG) AE Us 11
reactions involving - --- 1162, 1163, 1164
ZONC\OL CEPOSItON! Of a=. eee oe ae eee een eee 1160
Chaleopyrite, reactions involving - 1161, 1162, 1163
ZONE OL GePOSivlON Of sees sean soe ae ee ne 1160
Chalk, pore space in, amount of -_,__.-..------------ 125
Challenger expedition, citations of scientific results
Ofeaee ee 794, 942, 943, 944, 964, 967, 974, 991, 997
Chamberlin, T. C., on carbon dioxide drawn from
interstellar spaces by gravitation... 973
on carbon dioxide in air and in the ocean, bal-
ANCObOt WICC c= e- nae aa eee 968
onclimatic effect of increased amount of carbon
dioxide in atmosphere caused by use
Ofc all eee see senate seen onan 464
on ground water, systems of__...---.----.---..-. 578
on hydrogen in atmosphere-_-_-----__-.----_...-. 954
on hydrogen sulphide in artesian waters issuing
near Lake Michigan---_--.-....------.- 607
on hydrosulphuric acid in artesian waters__ 1112-1113
on loess of Mississippi Valley ----.--------------- 501
on minerals associated with lead and zinc ores.. 1144
on ore deposits of southwestern Wisconsin -____- 1220
on organic matter as an agent in reducing and
precipitating oxidized products - 1157,1158
section cited from, showing requisite conditions
foriarbesiatnw.ellsteees— seen sss. sscce 5I7
Chamberlin, T. C., and Salisbury, R. D., on size of
particles\ofloesst= a2 2222 e 2. eee
on thickness of residual clay in upper Missis-
Sippi Valley --=—=--=-=--2 =< 7-52 530
Chandler iron mine, Lake Superior region, figures
showing occurrence of ores at -_-.-.-
Chapman, R. M., aid by, in mathematical computa-
29, 208, 210
1196
Chemical action, importance of heat in -___...--_- 105-110
metamorphic effects of. ___.---------------------- 45-46
production of heat by ahh eee tery y
relation of, to mechanical action and heat.-._ 110-113
through solutions, definition of ------------------ 66
Chemical changes in earth’s crust, temperature ef-
fects lO Leena eee nee nee aes 1013-1014
INDEX.
Page.
Chemical composition of rocks, changesin. 507-518, 764-766
changes in, variation of, with porosity 655-656
effect of, on rate of weathering__.._______.____ 532-533
importance of, in metamorphism ______________- 359
use of, as criterion for discriminating metamor-
phosed sedimentary and igneous
= TOC IES ees SEE oe eee eee ne ne 914-915
Chemical composition of earth’s crust, discussion
OES 0 OS RL Tee pe eee 192-201
Chemical compounds, heat of formation of__.___-_. 106
Chemical effects of heat, discussion of _____________- 56
Chemical elements, distribution of, as affected by
metamorphism) 222-2522 see 932-1003
in earth’s crust, percentages of, tables showing. 934,
936, 938
TO GIS EI OU GLO TAO Lessee ea 947-1003
relative abundance of-.---.---_--=--_---.-- 192-193
Chemical energy, metamorphic effects of __ ---- 45-46
Chemical precipitation, ore formation by____.....-. 1037
Chemical reactions, categories of, in zone of kata-
AMOI; hi Sn eee ee eee 161-162
complementary, examples of __-__-_-___....____ 204,206
effectionpressune ONS. seee--ee ane was eee ene 100-104, 168
heatanvolvediimsseess sere eee ee ee 106
reversibility of _- 87-90, 110-111
equations illusurating -_-....-.-._--.------------ 88, 110
speed of increase in, with increase of tempera-
ture _- 107-108
stoppage of, by pressure and heat generated by
the reactions themselyes-___..._..--- 89
thermal aspects of, inzonesof metamorphism __ 182
volume changes consequent on, energy factors
iny.olved@:ints sos atte eee 170-171
principle applicable to_ 209
table showing, as applied to minerals_____ 375-394
Chemical solutions, increase of solubility of, with
increaseof heat asset seen 116
Chemical work, combination of, with mechanical
work, in metamorphism -__.____--.. 495-496
in belt of cementation, discussion of ___-_----- 602-646
in belt of weathering, agents and their work
DTM eS ev I Sto Lape oe 451-488
in metamorphism, combinations and relations
of, to mechanical and igneous work. 653-
‘ 655
in zone of anamorphism, discussion of __-____-
Chemical-physical factors on which nature of alter-
tions depends -_--_-_-_-------- urs 359-365,
Chemical-physical principles applicable to action of
@LOUN dewiateresess ee eee 65-123
Chert, alterations and alteration products of __.-__- 222
concretionary, photomicrograph of -_.._--..---- 836
definitioniofssssce Hates Nt eee a eee eres weal 816
deposits of, origin and features of -. 847-853
ferruginous, origin of -- $29, 830-831
sources of 222,389, 400
| Chert and chalcedony, alterations and alteration
WLOGUCLSO Leese ates ene cree eet eC
Cherty limestones, weathered forms of ______--___.- 529
Cherty limestones, dolomites, and marbles, fea-
tures and origin of --.------..----.- 816-820
Chicago, artesian wells at, conejusions drawn from
flow of, asto friction of water flow in
Potsdam sandstone. -..-------------:- 587
loss of head of artesian waters at... ---.-------- 142
Chicago, University of, aid by students of______-.-- 29
| Chile, nitrate deposits in -.....-..-------------------- 543
INDEX.
Page
Chloridation, alterations produced by--------------- 400
definition of 206
Chloride of magnesium. See Magnesium chloride.
Chloride of sodium. See Sodium chloride.
Chloride order of rocks, deposits composing---_-- 789-790
Chlorine, emission of, during volcanic eruptions, ob-
servations on_-.---..------ Regen 789-790
in earth’s crust and in the ocean, amount of-. 978-979
in earth’s crust, percentage of____...----.. 936,978-979
INOCeaM AMOUNT 0 fesen sen ee eee eee eee 944
in sea salt, percentage of___-____.-_--_--_------ 942, 943
Chlorophyllite, chemical and physical constants
196, 291
source and mode of formation of ___._. 291,370, 386, 402
Chlorites, alterations and alteration products of __. 347-
348, 373, 380
chemical and physical constants of -______- 196, 345-346
OC CUTTON COO Lp ees ae ee ee ee 346-347
sources and modes of formation of________ 278, 275-276,
278, 285, 287, 290, 302, 303, 304, 322, 324,
326, 327, 328, 339, 340, 341, 342, 343, 347,
853, 354, 369, 375, 378, 381, 382, 384, 386,
389, 390, 391, 393, 398, 402, 403, 404, 625
See also names of minerals of the group, Ames-
ite, etc.
Chloritoid, alterations and alteration products of__ 345
chemical and physical constants of__---_------ 196, 344
occurrence and source of________.---------------- 345
porphyzritics thinisectionof=--— = ----- 8=)- == W04
sources and modes of formation of _.....-..-_--- 345
Chondrodite, alterationsand alteration productsof_ 325,
73, 880, 403
chemical and physical constants of -_---_------ 196, 325
OCCUTTEN COO lyase ae ela ena eel ere 325
Chromite, occurrence of - 228, 229-230
chemical and physical constants of__----..___- 196, 228
source and mode of formation of ___..._-_____- 309, 370
Chromium in earth’s crust, percentage of .__-.__ 936, 1002
Chrustschoff, K. von, on laboratory production of
certain minerals----_--__- Baers 685
on minerals obtainable from water solutions at
high temperature and pressure-______ 1057
Chrysolites, alterations and alteration products
Co Biel ap a a a a Ag 308-311
chemical and physical constants of ___-___ - 199,308
OC CUUTONGC EO fees eee ae ore eee eee ee 308
Cimolite, chemical and physical constants of -___- 196, 254
SOURCES! OL fees rar eI ae al tad 254, 370,389
Circulation of ground water, causes of, summary.- 152
faclOImOD POSING ease e eee ee ee eae 153-154
forces and movements involved in _.__ 146-152, 416-423
influence of sizeand continuity of rock openings
onjspeediofes en an Ss Sree es 154-155
rapidity of factors determining--__.-__...---.-- 154-155
See also Ground water; Water.
Clarke, F. W.,on alterations of albiteand anorthite. 264
on atmosphere, hydrosphere, and litrosphere,
chemical composition of.___..---.---- 933
on carbon in earth?sicrust.--.--------ssee ee eee 963
OniGalboOusin: Shales fre seeeee asa eae eee eee 897
on chemical action in solutions, throry of - 68
on chlorophyllite, formation of, from iolite_____ Pashl
on clays and soils, ayerage chemical composi-
tIOMKO PSs =e ce ee aes ea Usa ew eee 891
on ‘‘crust of the earth,” definition of term _____ 192
on depth limit of observationofearth’sstructure 31
1253
Page.
Clarke, F. W., on earth’s crust, chemical composi-
tion of ._ 192-193, 934-938, 959, 974, 975, 981, 983,
984, 986, 987, 989, 990, 992, 993, 999, 1000, 1002
on feldspar minerals, abundance of __- 252
relations of, to zeolites ----_.--..-.-.------:-- 261
on ferric oxide in clays and shales, amount of-. 898
on igneous and crystalline rocks, average chem-
ical composition of -__- 889, 890
on iron (metallic) in original igneous rocks,
UIMOUN GO Leesan eee eae eran 1045
on lithosphere, limits of..._......---------------- 932
on mesolite 262
on ocean, chemical composition of.____-.-.------ 944
on oxygen in earth’s crust, percentage of__._--- 948
on penninite, composition of _______._____-------- 344
oniquartz, abundanceofmess: sss een ena naene 217
on shales, average chemical composition of___ 890,891
on silicates, alkaline reaction of -___..._.______-- 86, 87
on tourmaline, constitution of________.__.___-_. 326
on water combined in rocks, amount of________- 162
on‘zeolites; alterations'of -__-_.--2-2--2 2222 1 333
composition of 329
Clarke, F. W., and Hillebrand, W. F., on chemical
action in solutions, theory of ________ 68
onrockjanalySes foes sean oa ne eee 693
onrocksof Pigeon Point, Minn., composition of. 733
on slate, chemical composition of_______________- 895
on water content of rocks, relation of, to recrys-
tallization 743, 744
Clay, chemical composition of.__.____.....-.-..---.-- 891
DOLE |SPACONNaeme sas e eee ee ene 126
volume changes requisite to produce_ ---- 523-524
Clay soils, effect of freezing on ____...______=.-----.- 443
Cleavage, display of, by weathering -.._____..__.._- 525
MOAN GeO LAE ESE ee dae eam ony neh eames 760
monoclinal, existence of, over extensiveareas_ 928-931
Clements, J. M., on cementing material and rock
cemented, similarity of__..-.-___.__<-_ 628
onchert rocks of Lake Superior region _________ 851
on contact metamorphism
on'(@rystalHalls\voleanicss2snes2 22) ose
on{devyatrifica tion messsee seen ne eens
on schists of Lake Superior region __-
on secondary sulphides and magnetite, associa-
RKO} 0 ho) Capea, ee eee ens Saher lab 1112
Cleopatra’s Needle, Central Park, New York, effect
of freezing and thawing on_________ 441-442
Climate, effect of, on metamorphism__-________.____ 41
Climatic changes, influence of, on level of ground
WSUS TS eras mae eet tee yO 424
Clinochlore, alterations and alteration products
O Ba etn Sete rao CM oe aera 347-348
chemical and physical constants of - -- 196,346
OGCULTEN COOL eee ene ee ae an a -- 846-347
sources and modes of formation of ________ 344, 370, 390
Clinohumite, alterations and alteration products
(0) aN ae ea Ree i ae 325, 373, 380, 403
chemical and physical constants of _ -- 196,325
OCCULTEN COLO LE seas aa ane ees neem 825
Clintonites, alteration and alteration productsof___ 345
chemical and physical constants of _<______-_____ 344
occurrence, sources, and modes of formation
Of eae eee ce Sees ee Seeniaetanne Fee 344-345
Clintonite group of minerals, character, occurrence,
formation, and alterations of__.... 344-345
See also Chloritoid; Margarite; Ottrelite.
Coal¥icarbonsintamountlofeseesacee sess tee eee aoe
carbon @ioxide produced by combustion of,
RMOUNTO Lest eaees eee eee 464, 972-973
Coalibedsformation\of&essesesese- ence enna sence 472
Coarse-grained rocks, rate of weathering of, as
compared with that of fine-grained
TOCKS pies skate a ekS Ae ee wen 533-534
Cohesion, influence of, in ground-water circula-
Teo a ey eS ase 150-152
Colloidal silica, silicification by .-- 540,547
Colorado, Aspen district of, silver-lead deposits of. 1084
San Juan district of. See San Juan district.
Comey, A. M.,on solubility of the sulphates_---_ 1075, 1108
Comminution, effect of, on amount of soluble ma-
teria lhinisollsssssa= sae see see eeee 495
Complementary reactions, examples of__----_---- 204, 206
Composition, chemical, importance of, in metamor-
Phismi eee Re ae Se 359
Comstock lode, history of exploitation of-_..---.--- 1224
precipitation of metalsat,hypothesisconcerning 1083
SW EUG EL Seopa eee aoa ania eee Serene oe 1132
width of, variations in 2 1224
Concentration, second, conditions favoring- -___- 1177-1179
See also Second concentration; Secondary en-
richment.
Concentration of soluble material by ground water,
iscussion{o fees eee ees seen ee 543-551
Concretionary chert, photomicrograph of_-.--_-..- 836
Condensation with recrystallization and mineral
changes, features of__._..-.______-- 102-103
Conductivity of rocks as regards heat, degree of--- 53
Consolidation of rocks, methods of ---___-_-------- 595-597
Contact metamorphism, direct contact effect of. 489-490
exomorphic and endomorphic effects of - 488
extent of, conditions governing -----_-___.____- 649-652
PEALUTESIO heen cree eee neste een ae eee 648
indirect contact, effect of 490-494
Conglomerate, original structure in, preservation
of, after weathering __...___._____.__ 526
Conglomerates, origin and character of ~ 855-857
Conglomerates and pseudo-conglomerates, discrimi-
nation between, criteria for_____-___. 910
Conglomerate-schist, origin and character of____- 857-860
Conn, H. W., on nitrogen compounds, decomposi-
tlonio fess es ce ese Uy Se ee 465
on nitrogen in soils, loss of, by manufacture of
explosive powders-.-...-_-....-.------.. 466
Copper, solution and precipitation of _______._.- 1101-1104
Copper and iron compounds, association of -___- 1158-1165
Copper ores, formation of_____-__--...-..--. 1062, 1158-1166
of Lake Superior region, features of___._____ 1204-1205
origin of 1136
of Oregon, types of 1055-1056
secondary enrichment of_.---_..--...-------.22.. 1184
Copper Queen mine, Arizona, secondary enrichment
Gis isens kbaoe AONE Rare RON OS CEES ESS ease 1184
Coquand, Henri, cited on bauxite deposits __.-_.___- 985
Coral limestones, dolomitization of
Corals, magnesium content of __.-.
Cordierite, alterations and alteration products of_. 291,
373, 386, 402
chemical and physical constants of --.-..----.. 198,291
occurrence of 291
Cornwall, Pa., iron ore deposits at -_--
Corundophilite, alterations and alteration products
Of eae ea See eee ee a ene ane 347-348
chemical and physical constants of
INDEX.
Page
Corundophilite, occurrence of ___....-_-.-.-------- 346-347
sources and mode of formation of-__._..___... 318,347
Corundum, alterations and alteration products
Ofsiees ae ease 223-225, 373, 380, 381, 402, 408
chemical formula of, physical constants of, and
other data concerning_-_--.-------- 196, 223
OCCULTENCOE!O flees Mana ern SEES oy ee 223
sources and mode of formation of___._______.._- 232,
235-236, 318, 328, 367, 370, 381, 383, 400, 624
Corundum and magnesite, alterations and altera-
tlonsproducts (0 feasseee ses aaa 381, 400
Corundum and quartz, alterations and alteration
TVR ON? a 381, 406
Corundum, quartz, and calcite, alterations and al-
teration products of__-____.-._----- 381, 408
Corundum, quartz, and potassium carbonate, alter-
ations and alteration products of-._ 381,408
Corundum-syenite, deposits of._..._...._..-.-------- 1045
Cotta, B. yon, on contact metamorphic origin of
Covellite, zone of deposition of __....__--..--..__--.-
Cramer, Frank, on strain in rocks-_.-
Credner, H., on origin of pegmatites
Cripple Creek mining district, Colorado, fracturing
in, complex character of _-____-------
225
woldrones sofas sees Sas See So ee es ees 1172-1174
ore depositsjatw origin ofjs sc ase esee eee eee eee 1085
ore shoots near, intersecting fractures in ______-
oxidationtinydep th) 0 fees ease eae eee eee 1181
replacement of igneous rocks by ores in -_-____- 1205
Crosby, W.F., and Crosby, W. O.,on cause of water
power of sea mills of Cephalonia --.. 149
Crosby, W. O., on induration of Pikes Peak granite
alongsjoin ts sess see ee eee 549
on red color of soils, cause of __ 482
Crosby, W. O.,and Fuller, M. L., on origin of peg-
Ma ties: i ees as se eae 122,123
Cross, Whitman, on fluviate origin of psephites of
Cordilleras eos ee soe eee 854
Cross, Whitman, Iddings, J. P., Pirsson, L. V., and .
Washington, H.S.,on mineral com-
position of amphibole-gabbro-_-___-_-- 632
Crustification; examplesiof 222222 ss -e eee eee ene ee 1146
Crystalline rocks, average chemical composition
OPN SOREN AS RLS Sebo lek NAD Soles peace se 890
Crystalline and igneous rocks, analyses of -_-__._- 934-935,
Crystalline form, symmetry of, influence of, on sta-
bilityzofmineralss = es eee 360-3861
Crystallization, alteration of rocks by--------------- 208
effect of pressure on 103
effect of temperature on - _- 78
importance of, in metamorphism-----_--.-_-_---- 76
methodsiofeesses sass se seems Bese ice See 74-16
Culver, G. E., and Hobbs, W. H., on decomposition
of olivine-diabase in South Dakota. 560
Cummingtonite, alterations and alteration prod-
ucts|ofa== eerie 285, 287, 373, 381, 404
chemical and physical constants of --.....----- 196, 283
(OCCULT EN COLO fee sees an eae te ae a 284
Cuprie salts, reactions involving.--.... 1161, 1162, 1163, 1164
Cupric sulphate, production of chalcocite from,
equation showing reaction for -__--_- lll
yeactionsrnvoOlyin piss staan one ee 1161, 1162
reversible reaction of, with ferrous sulphate... 1102
Cuprite, production of, equation showing----.._--.- 1158
Cuprous chloride, precipitation of gold by, equation
showing : 192, 1082
INDEX.
Page.
Cuprous sulphate, production of chalcocite from,
equation showing reaction for-__--_- 1111
reactions involving-__------..------ 1161, 1162, 1163, 1164
Curtis, J. S., on genesis of ore deposits_--...--..---- 1206
on ore deposits of Eureka, Nev ------------------ 1204
on Witwatersrand banket---_-.----..------------ 1041
Cushing, H. P., on effects of freezing and thawing
on rocks in Alaska---.-.--.---+.------ 443
Cyanite, alterations and alteration products of-. 318-819,
373, 381, 402
chemical and physical constants of ----..------ 196, 316
occurrence and associations of--
sourcesand modes of formation of_ 223, 224, 225, 232, 236,
317, 318, 319, 370, 376, 381, 382, 383, 399, 406, 407
Cymatolite sourcelofeseeseesseeesenaeeeen ee eeee eens 370
ID).
Dale, T. N., on cleavable marbles -_--.....---------- 810
Dale, T. N., Pumpelly, Raphael, and Wolff, J. E.,on
crystallographic orientation of por-
phyritic feldspars|=-------------=2---- 698
Dall, W. H., and Harris, G. D., on phosphates -_---- 976
Dalmar, K., on zonal arrangement of metamorphic
minerals peripheral to intrusive
Damourite, sources and modes of formation of-.___ 223,
224, 225, 255, 318, 319, 327, 328, 353, 370,
376, 381, 389, 393, 400, 402, 403, 406, 408
See also Muscovite.
Dana, E. §., on chlorine in sodalite ___.--_--...-.---- 978
Dana, J. D.,on composition of dolomitic coral lime-
Stone Pesan on see nas aeee ence aeecce 799
on magnesian deposits of coral reefs - 803
on red color of soils, cause of ___._.-.------------ 482
on sedimentary rocks, thickness of-----...------ 939
on water contained in rocks ----.-_-_------------ 128
use made of System of Mineralogy of ____----__- 211
Dana, J. D.and E.S., on Hautefeuille’s experiments
producing rutile group of minerals. 231
on garnet, products of fusion of --.-.--..-------- 300
on scapolites 312
on wernerite 265
Oonizeolitessessie een ene 329
Dana, Mount, disintegration of rock at, by freezing
and thawing 443
Daniell, Alfred, on capillary flow -- 40)
on dissipation of energy------.------------------- 57
on friction between ground water and contain-
ing srocks= <2 SS ioe Se 153
On} gaseous! SoOlUbiONS eens ae eee eee eae 59
on heat production in all transformations of
CONNOLLY ase ae an eee ee aes 111
on size of rock openings availablefor water flow. 134
on surface tension of water ---.-.------.--------- 150
on water flow in rock openings ---_---.-----.---- 137
Darton, N. H., on flow of artesian water-- 142
on level of ground water. --2222 2-22-5222 522-: 410
Darwin, Charles, on effect of work of earthworms
onisoils 2.220 252222 cS 448
Darwin, G. H., on ‘‘stress difference” 672
on tidal forces of the past 931
Daubree, A., on combined effect of chemical and
mechanical action on minerals. --..-- 495
on penetration of rock by water gas under pres-
sure _ 144, 665, 666
Daubree, A., on tin ores, formation of
on trituration of rocks, limit of -__...__-
on zeolites, artificial formation of
Dayis, W. M., on fluviate origin of psephites of Cor-
Gilleras snes eee areas 854
Dayison, Charles, on effect of work of lobworms on
soils 448
Dawson, G. M., on peripheral structure_- 716
Debsration, definition of -2---5- 2s ee obese se eee 206
Deboration and decarbonation, alterations pro-
Gucediby sss 22s oe ea eee 400
Decarbonation, alterations produced by--- - 400
definition {ofc 2e sek eee Ee Sets)
in zone of katamorphism-- 162
Decarbonation and deboration, alterations pro-
duced by:see2t ce ee ee 400
Decarbonation and hydration, alterations pro-
Guced! byjeets see aa see ee soee eee 401-402
Decarbonation and titanation, alterations produced
SD Yea ee eee ee eee ee oe ae eee 400
Dechloridation, definition of._.-...-..._...-...-_--_-. 206
Dechloridationand hydration, alterations produced
TD Ypres he ee 403
Dechloridation, carbonation, and hydration, altera-
tions produced! by ------ ------------.- 398
Dechloridation, decarbonation, and hydration,
alterations produced by--_---.-------- 403
Decomposition of minerals in rocks, order of____- 518-521
of rocks, regions favorable to__-.-.__-.-----__- 501-506
Decomposition and solution, relations of disintegra-
tion) to =-256e 22s oe 494-507
Defiuoridation, definition of --
Defluoridation and carbonation, alterations pro-
Guced ibyj-s aie ee 396
Defiuoridation and hydration, alterations produced
DY ssoees ee a eee eee 403
Deformation of minerals, relation of pressure to
TADIGI by {OL ese es eee ees 741
of rocks, diagram illustrating. --.-.--....-------- 769
energy required for, in zones of katamorphism
andianamorphism\==ssss2522eeso- ee 769-77:
lag of recrystallization behind-_----..---...__-- 696-698
mechanical energy a cause of_---..__-...----.--- 95-98
DTessure] a CAUse|O fesse eee ee ene 657-659
speed\ofi2-2222 22sec sere ee ee
temperature effects of
time relation of recrystallization to --.____-__- 696-698
Dehydration, alterations produced by -------------- 400
conditionsjandjeftects!ofsesssese seen eee 89,169
definition of____...--.... 204
depths necessary for -_- 178
IISCUSSIONIO fe seem eens see ee ee 482-483
examples\Of i222. 222 2o 22 ae cc hese oe see nsec 179-180
in\beltiof weatherin yea so = ass aae sate ee 163, 164
Dehydration, deoxidation, and carbonation, altera-
itlons;producedibyjessesse sae eee 396
Dehydration, desilication, and carbonation, altera-
tions producedybyj-ssseseeeee eee 396
Dehydration, desulphation, and carbonation, alter-
ations produced by ..-.......-..-..... 396
Page.
Dehydration, carbonation, oxidation, and desilica-
tion, alterations produced by --------
Dehydration and decarbonation, alterations pro-
Guced pb yess Soe eee 401
Delamar mine, Nevada, secondary enrichment in,
showing former presence of circulat-
ingiwatensecesseee ce ese aeons 1178-1179
Delesse, A., (7) on decrease of density by vitrifica-
tion See le cer ee a eee 103
on nonaction of dry steam on minerals --_------ 493
Demorphism, belt of, name proposed for belt of
weathering 163
Density of minerals composing rocks, increase of
with increase in depth ___...------- 182-185
Denudation, belts of ores deposited at different
levels by reason of__-.-.--.------- 1189-1140
influence of, on ground-water level --..--------- 426
Deoxidation, alterations produced by---------------
Gefinitionol, fees ee ee basen cae ea eee 204
inibeltjohcementationesses assesses =e ee eee a a= 165
in belt of weathering---- 163
in zone of anamorphism --_--_------------------ 172, 676
in'zone of katamorphism.-..----...-------------- 162
of iron compounds in soils, examples of -____-- 471-472
Deoxidation and carbonation, alterations produced
Daye See ie te Se 396
Deoxidation, carbonation, and dehydration, altera-
tions.\produced by, 222223212) =2--==-- 396
Dephosphation, definition of 206
Deposition, discussion of 487
in belt of cementation, discussion of -_ 165-166, 612-617
Depth from earth’s surface, density of minerals as
determined tbysecs sone ns eee 182-185
effect of, in metamorphism --- 43-44
increase of temperature with increase of ___-._- 578
limits of, to which rocks haye been observed_-- 31
Depth of no annual yariation in temperature, prob-
able coincidence of, with level of
PLOUN daw ale rseeseeee onan eens 590
Derby, O. A., on hydration of rocks ____.--.-------- 481
Desert varnish, reference to. ---.--.-------------=--- 547
Descending waters, depth of effect of.....__.._- 1179-1181
ores precipitated from --.-.._--.-.------
Desilication, agents and processes of______-.--
1193-1199, 1234
75-476, 480
alterations produced by.-.-..-------------------- 401
Gefinition{of ses sve reese ee ae es See ee 205
Desilication and carbonation, alterations produced
LO pam eee ae ae a ig 396
Desilication and hydration, alterations produced
LO Rete aeRO SERA HEB AN SERRA B Seem 404
Desilication, carbonation, and dehydration, altera-
tions produced by: == 22 22--=2 2 ===. 396
Desilication, carbonation, and hydration, altera-
tions produced bys==25 22. -2-- === 398-399
Desilication, carbonation, hydration, and oxidation,
alterations produced by-_----------.-- 399
Desilication, carbonation, oxidation, and dehydra-
tion, alterations produced by --..---- 399
Desmine. See Stilbite.
Desulphation, definition of-__....-.---.-------------- 205
Desulphation, dehydration, and carbonation, alter-
ations produced by--.--.------------- 396
Desulphation, hydration, and carbonation, altera-
tions;producedibyjesessssseeen eee = 399
Desulphidation, definition of
Detitanation, definition of
Deville, H. St. Claire, ‘‘agents minéralisateurs”’ de-
fined tb years ot ae eases 59
INDEX.
Page.
Devitrification of glass, conditions governing -.-_ 248-249
heat and volume, relations of ._....__.....------- 251
minerals produced by
scale, rate, and zonal place of _________._.__._.- 247-252
Diabase, losses in by weathering, analysis showing.
510-511, 515,522
Diallage, alterations and alteration products of-.._ 274,
378, 381
Diaspore, alterations and alteration products of ___ 282,
373, 381, 400
chemical and physical constants of___.---_---- 196, 231
occurrence of -- 282
sources and mode of formation of________- 228, 235-236,
292, 296, 298, 299, 312, 318, 337, 343, 353, 370, 379,
380, 383, 388, 390, 392, 397, 398, 400, 401, 402, 624
Diaspore and magnesite, alterations and alteration
HOY ROOHUCORES) CPEY se a a s 381,401
Diaspore and quartz, alterations and alteration
products\ofyessees se ae ees 381, 382, 407
Diaspore group of minerals, character, occurrence,
formation, and alteration of _______ 231-234
Diaspore, quartz, and calcite, alterations and alter-
ation products of ____.-_..-_-__--__- 382, 407
Diaspore, quartz, and potassium carbonate, altera-
itionsproducts\ofesssses sees ee eee 382, 407
Dichroite. See Tolite; Cordierite.
Dickson, C. W.,on ore deposits of Sudbury, Canada_ 1047
Diffusion, influence of, in cementation of rocks ._ 636-639
Diffusion by solution, discussion of ---__---_-_- ... 82-83
rates of, table showing ------------- u 83
Diller, J. S., figure cited from Se AB 14:
Diopside, alteration of, to tale, quartz, magnetite,
and calcite, volume change incident
tO e Es eee Se eee DE 633
alterations and alteration products of- nes AB)
274-275, 277, 278, 373, 382, 398, 408
chemical and physical constants of -.---------- 196, 271
formation of, from dolomite, chemical reaction
involved Sales snes s 2 siee anes eae 822
occurrence of 272
source and mode of formation of __ 241,272,370, 382, 407
Diopside and magnesite, alterations and alteration
DYOCUCtSO hese een eee 382, 408
Diopside-augite series of minerals, character, oc-
currence, formation, and alterations
OLE OS Te SU ON tthe er on inayat ee ae ae 272-280
Diorite, losses in, by weathering, analyses showing- 510,
515,522
Dip, steepness of, effect of, on width of fissures ---. 1201
Disintegration of rocks, regions favorable to -_--- 496-501
relations of decomposition and solution to -_._ 494-507
temperature changes producing, examples of. 434-439
Dissociation.-Cheomyao Le wssee eee eee eae 84-85
Dissociation of salts in water, phenomena of --_--_- 73
Dissolved materials resulting from weathering, dis-
tribution of --- 536-554
Disthene, alterations and alteration products of-. 318-319
chemical and physical constants of __._-_------ 196, 316
See also Cyanite.
District of Columbia, granite from, losses of con-
stituents in, by weathering --__--_- 507, 522
Dittmar, William, on calcium in the ocean, amount
OE SEB IESE ke nfo Aer Sees 9 Te
on carbon in atmosphere, amount of --.-..--..-. 964
onecarbon dioxide in the ocean and the air,
AMOUNtO Leese we see uae te Rie ne mn OO Gp
on chlorine in the sea, amount of ___..._----.---- 979
on ocean, chemical composition of-__-_.-..---.---- 944
INDEX. Pay
Dittmar, William, on ocean, volume of______-.__---- 942
on sea salt, amount and composition of ___.... 942,943
on sea water, composition of _________- esx 104
on sodium in the ocean, amount of - == Coy
Doelter, C., on gold, solubility of ____........--_- 1089-1090
on sulphide minerals, artificial production of__. 1114
on sulphides, solubility of _.._....._.....-_-. 1106, 1107
on zeolites, solubility of, in water ___....._.-____ 112
Dolomite, alterations and alteration products of_ 241-242,
378, 382, 407
chemical and physical constants of___________- 197, 237
cherty, features and origin of ____.__________-. 816-820
formation of, from limestone, volume change
involved in eee 209
formation of diopside from, chemical reaction
‘inwOliye dimer eee een es eek aire mill Fatt 822
formation of tremolite from, chemical reaction
nr Oliv 6d binwetesss) sensed vere ew ee 822
sources and modes of formation of _______.__ 238-239
>
240-241, 370, 379, 380, 408, 624-625, 798-808
Dolomite and quartz, alterations and alteration
DLOGUCTS 10 flee ee eee eee es 382, 407
Dolomitization, conditions of _ .. 802-808
magnesium) for;|source Of — 2222222 ee 802
occurrence of, at sea bottom_-_-_...__._...__-_-- 802-804
subaorialit se 220 yee - 804-808
Don, J. R., on gold deposits of Australia ________ 1170, 1185
on gold ores and sulphides, association of __ 1095-1096,
1099
on occurrence of gold at intersection of quartz
veins and carbonaceous shales___ 1094-1095
Douglas, James, on copper deposits of Arizona ____- 1086,
1183, 1184, 1188
on gypsum in Arizona mines ___.-___....____ 1141-1142
on oxysulphurets ______.._._._...-.-.-... 1165-1166, 1182
Doyeton, G. D., Purington, C. W.,and Woods, T.H.,
on occurrence of rhodonite at the
Camp Bird mine, Colo___________- 1115-1116
Dry mines, mistaken inferences concerning_____ 1063-1065
Ducktown, Tenn., copper deposits at, secondary en-
richment of 1182, 1188-1189
Dudleyitey formation Ofsessss es -se5 tosses eee 345
SOULCE OL on none ee eee et an eM 370, 387
Dynamic action, metamorphic effects of____________ 42-43
Dynamic metamorphism, definition of-____________- 47
Mse.of term) discussion Of=sssss-e ae eete ene ennan 763
EK.
Earth, specific gravity of, discussion of facts bear-
nbeY= Kohat oe ina ee cer rae eh tina ey aU 364-365
Earth movements, effect of, on temperature of
PTOUN iw aite rays eee aeons 590
Earth’s crust, chemical composition of____ 192-193, 933-948
CeformationloLssswen eee se een Ne EE NnEe 1005-1014
density of 933
elements in, order of abundance of, compared
with order of abundance of elements
IMME tECOLIteS sae = eee =e ene 947
relative abundance of ___.___-_...-_._.___.. 192-193
movements of, in mountain making, discussion
ORS E ee ee raat eos as 924-931
percentage of known matter of globeformedby_ 933
Earthworms, effect of work of, on soils 448, 456
Eldridge, G. H., on origin of phosphates 976, 977
Elements (chemical) in earth’s crust, percentages
of, tables showing 193, 934, 936, 938
in earth’s crust, relative abundance of________ 192-193
Page.
Elements (native), occurrence of, as minerals______ 212
Elkhorn mine, Mont., ores formed beneath imper-
VAR) CIRO NES TN a 1214
Emerson, B. K., on fusion of schist by granite in
Massa chusetts)ssss-9) same ene ae 735-736
on minerals formed by igneous intrusion _____ 717,718
on\peripherallistructure 9-224 95... 716
on porphyritic gneisses 705
Emmons, §. F., on carbonic acid in underground
WELCOTS ae hos Nie Ree SAT ay | 1128
on mineral veins at Butte, Mont_________________ 1081
on ore deposits, genesis of _______________ 1183, 1200, 1206
on ore deposits of Butte district, Mont______ 1182-1183,
1203, 1205
on ore deposits of Delamar mine, Nev_______ 1178-1179
on ore deposits of Leadville district, Colo __ 1168-1169,
1186, 1204, 1214
on ore production from igneous rocks__________- 1043
on psephites of Cordilleras, fluviate origin of _.. 854
on secondary enrichment of ore deposits 1188
on tellurides associated with gold ores ______ oo abot)
Endomorphic effect of igneous intrusion, definitio
OP eS ROU ea SO Seed Bara ae ss Coa 648
Endomorphism, definition of 648
Energy factors of zones of metamorphism, consid-
er ablomio fess sees noes ee ee ane 170-171
Enlargement of minerals in rocks by deposition of
new matter, discussion of __________ 619-621
Enophite, source of 347, 370, 380
Enstatite, alterations and alteration products of ___ 268,
269, 373, 382, 397, 404
chemical and physical constants of __- 197, 267
sources and modes of formation of ____ 268,
302, 304-305, 307, 367, 370, 386, 390
Enterprise mine, Rico, Colo., ore deposits at 1084,
1208, 1215, 1229
Environment, adaptation of minerals to____________ 33-35
importance of, in metamorphism ________________ 42
Epeirogenic movement, influence of, on ground-
water lovel 426
Epidote, alterations and alteration products of _._ 322-323,
373, 382, 383, 398
chemical and physical constants of _____._____ 197.320
occurrence and associations of____________._____. 321
sources and modes of formation of _____________. 255,
260, 263, 264, 265, 273, 274, 275-276, 285, 288, 303,
305, 312, 318, 321, 339, 340, 841, 342, 353, 354, 370,
377, 378, 379, 381, 382, 383, 384, 385, 387, 389, 390,
391, 394, 396, 398, 399, 401, 402, 404, 405, 406, 625
Epidote group of minerals, character, occurrence,
formation, and alterations of 320-823
hydration of, degree of = Gs
Epistilbite, chemical and physical constants of___ 197,330
occurrence of
Equilibrium, chemical, notes on
Erosion, belts of ores deposited at different levels
in consequence of_________._______ 1139-1140
Eucryptite, chemical and physical constants of ____ 197
source and mode of formation of ______ 281, 370, 392, 405
Evaporation, influence of, on ground-water
leVveltsaa seas sae ee aera as 425-426
Exomorphie effect of igneous intrusion, definition
(6) Ie eee pa etd eee I 648
Exomorphism, definition of-_--..-___________- - 648
Expansion reactions, discussion of_____-___________ 631-643
. Page.
Faribault, E. R., on gold ores of Nova Scotia ------- 1218
Farrington, O. C., cited on meteorites---_- 945, 946, 947, 964
Fassaite, source and mode of formation of-_-._ 314,370,383
Fault OPENING SHSiZO.O Leeann ernie se ee 138
Fault planes, importance of, in ground-water circu-
TE SVoy th A ee ee eee ae ee 1380
Faults, importance of, in ground-water circula-
biome ies aa Mle eae A RI Eas 130-131
pore space added to rocks by -------------------- 127
Fayalite, alterations and alteration products of _... 308
chemical and physical constants of -_-.-------- 197,308
occurrence of .---..------- 3 MSN RL NOEL 308
Federal Loan mine, sulphureted hydrogen at ------ 1107
Feldspar, alteration of, to quartz and mica, thin
section showing. ---
fragment of, cut showing enlargement of ------ 626
in igneous rocks, percentage of ___.__..---------- 937
Feldspar group of minerals, character, occurrence,
formation, and alterations of__--- 252-265,
294, 353, 380, 381, 625-627
Feldspar-porphyry, weathered, original structure
preserved ins ases sas eee een ane
Feldspathoid class of minerals, alterations and
alteration products of-_._-------.-- 352-393
Werrates, deposition) of=-<-22=22--=----=-------------- 541
Ferric chloride, hydrolysis of, equation showing... 108
Ferric oxide in earth’s crust, percentage of_._ 934,937,938
in sedimentary and igneous rocks, ratio of, to
526
ferrousioxidejeesse- sesso esa 950-951
red or yellow soils produced by ------------------ 469
sources and modes of formation of-_-. 172, 467, 540-541
Ferric sulphate, formation of, chemical reaction in-
VOlvied Hine seme eee sete c eens oseane 826, 828
reversible reaction of, with ferric sulphate, in
presence! of/silerici-. -2.)------.--- == 1100
solution of silver by, equation showing ------.-- 1099
Ferrodolomites, formation of-_-------.------ 242-248, 625
Ferromagnesian silicates, alterations of --___------- 853
Ferromagnesian sands, deposits of, character and
ORI FINO fever ee eee 877-879
Ferrous carbonate, formation of, chemical reaction
imvolvedtinie sete eee n eee 828
origin of, from basic ferric sulphate, chemical
reaction involved in -----.---.-----.-- 828
Ferrous chloride, precipitation of gold by, equation
Showin gate eer an ene anes yee Soe
Ferrous compounds, oxidation of -
precipitation of copper by ----.-
work of, in metamorphism ----------------------
Ferrous oxide, decomposition of, inrock weathering 536
in earth’s crust, percentage of. __-________- 934, 937, 938
in sedimentary and igneous rocks, ratio of, to
ferri cCloxid Ojsesse se sae enone ne eee ere 950-951
OCCULLCN COO hee tee eee eee ee eee mene n eam ae ee 467
OXIGAtIONYO fre eee a eee aaa 467
DTeCipita tony fee nee ae ee ae eee eee ee ae 540
Ferrous silicate, formation of, chemical reaction in-
ViOlv.e Gaim yee eee eee ner sree 5 828
Ferrous sulphate, precipitation of gold by, equation
SVOVO ADOYS oe etn cbeaSosoaacheRosssS 1093
precipitation of silver by, equation showing- 1082, 1100
reversible reaction of, with cupric sulphate.... 1102
reversible reaction of, with ferric sulphate, in
presence of silver: -2-:2_---2-------—-- 1100
Ferruginous cherts, origin of. _- 829, 830-831
Ferruginous shales, origin of.._......-.------------ 829-830
INDEX.
Page.
Fine-grained rocks, rate of weathering of, as com-
pared with that of coarse-grained
TOCKS EES Gee ice Se eR Se 533-534
Fischer, Alfred, on bacterial action in decomposi-
tion of carbohydrates _______-___--._- 463
on bacterial action in plant growth ______.____ 452,453
on carbon dioxide in rain water, amountof_... 474
ONFITONB DAC LOT Aenea etn 826
on nitrogen compounds, decomposition of -- 465
Fissility, display of, by weathering 525
See also \'racture.
Fissility openings, importance of, in ground-water
Ginculationieseeeese sean ae nese eee 131
Size! OF 2 eos SE a Oe EE BAL LN Ua RUN) aS 138
Fissures, width of, effect of dip on .___-...___._-.... 1201
Flexibility of rocks, conditions affecting ________- 188-189
Flexures, occurrence of rich ore shoots near ... 1225-1226
Flow of ground water, rate of___........----____-- 582-588
See also Ground water; Water.
Flow of rocks, effect of,on texturesand structures. 760-762
Flowage, zone of, circulation of water in _______-_-- 1029
zoneiof, deformation\in <---.---=--- 1011-1012
relation of zones of anamorphism and kata-
morphism\to sess ese eee eee 187,190
Np PETA Of eee eee ee eee 187-191
Fluid inclusions in rocks, figure showing 746
sizes of cavities containing-___-__.______.__ 658
Mluoridation; definition\o fewsesse se eee 206
Fluorides, character,occurrence, andalterationsof. 216
Fluorine, percentage of, in earth’s crust______._. 936, 1002
Fluorite,alterations andalteration product of_ 373,383, 396
chemical and physical constants of__-._--._.-. 197, 216
occurrence and alterations of____-__-_...--..---- 216
Foerste, A. F., Shaler, N.S., and Woodworth, J.B.,
on occurrence of graphitic coals--__- 212
Forces of metamorphism, consideration of_ 39-40, 44, 45-57
Forchhammer, Georg, on magnesium content of
corals and marine shells forming
limestone deposits-- 798, 804
on potassium silicate, dissolution of, in water. 112
Forsterite, chemical and physical constants of --_ 197,308
occurrence and alterations of__.._.-__-----_--- 308, 311
Forsterite, calcite, and quartz, alterations and alter-
ationsproductsiofesseseiss see 383, 407
Friction, internal, of ground water, character and
imfluenceiOhmeecessseee seen =e 153-154
Fracture, secondary, veins produced by ------------ 1228
zone of, circulation of aqueous solutions in__- 1022-1028
features of, and chemical reactions in_-. 1005-1009
opening sine sesss eet e eee eee 1006-1008
relation of zones of anamorphism and kata-
TOTP MIS MNLO eae a nner 187,190
Fracture and flowage, zone of, aqueous circulation
Tee a ee eS ase nee 1028-1029
zone of, phenomena in-_--..--.---------------- 1009-1011
Fractures of rocks, causes and character of___.... 599-602
GinectioniOlys=seseene saute nae sae et acai oer ae 601
intersections of, ore shoots at or near -- 1226-1227
pore space added to rocks by --.----.------ Sie 127
SDPACIN G1OL seas nana ee ee eee cnn Sa en eee 600
Fragmental rocks, classification of - . 784-787, 853-904
deposits|\com posing Sapo ane eee eee eee 853-904
France, basalt from, losses in, by weathering --_--- 509, 522
Franklinite, formation oles ssa eee 1125
production of, from Smithsonite, equation show-
ipg oe. ene ae se ee eee 1126
Freezing, likeness of, to precipitation -_-__-_.__-__--
INDEX.
Freezing and thawing, effect of, on rocks-----_---
Fuller, M. L., and Crosby, W. O.; on origin of peg-
Matites.--..-+---
Fumaroles, alums produced by
chloridesiproduced\by=eescs eee ase eae
gases emitted by, sources of ___-__---_--------- 491-492
metamorphic work of 490-494
DLOGUCtS\O eee eee 493-494
Furlonge, W.H.,on depth of decomposition of rocks
injthevGransyvaalesssse esse nana eee 531
Fusion and absorption of intruded rocks, discussion
OP Sean SSS ek Sa eet 728-736
Futterer, Karl, figure cited from________________---- 704
on new minerals formed in rocks_____----------- 753
on quartz grains (flat) in rocks -_-.-__---..-.---- 692
onrecrystalization of quartzinquartz-porphyry 738
G.
Gabbro, mineral composition of,and volumechanges
possibleinas sew Is ee
Galena, occurrence and genesis of _
secondary, figure showing -.---.----.___.-._._---
Galena limestone, strain in, shown by quarrying-._ 598
Gangue minerals of aqueous solution ores, enumer-
ationolos ses ee See 1233
Gangue minerals of ore deposits, citations concern-
AT eae ee SAME Es 1055-1057
Gannett, Henry, onrangeoftemperatureinAlaska. 437
Gardner, F. D., and Means, T. H., on effect of alka-
ieSnVSOil seesaw eas ney eae 478,543
Gardner, F. D.,and Stewart, John, on calcium chlo-
ride and sodium carbonate in soil at
Great Salt)Lake -----22----------_- 542, 543
Garnet, absorption of other mineralsin rocks by -. 701
alterations and alteration products of. 302-307, 373, 383
chemical and physical constants of ._-_-_-____-
OCCULTENCO OL see eee eee eee we (senee esse 300-302, 1057
sources and mode of formation of ___- 300-301,
353, 370, 380, 393, 903
Gas, water, in zone of anamorphism, action of____ 660-661
Gas content of rocks, effect of, on metamorphism__ 41
Gaseous solutions, circulation of._-.-..---..-____ 1018-1021
formation of gengse 59
metamorphism by ---------------- 59-63, 490-494, 648-649
Ores produced ib ype see eee eee 1052-1058
work of, in belt of cementation___---..--._____ 648-649
Gaseous and aqueous solutions, precipitation by re-
actions! between) ==---55---2--2-- 222-22 119
Gases, chemical and physical principles applicable
tolaction\orsaeiesen see eaten oe 60-62
important in rock alteration, list of _------____- 59
inclusions of, in rocks, figure showing--~----___- 620
metamorphic work of -._.._.-.---.----- 59, 61, 62-63
mixtures of, properties of - 60
precipitation between aqueous solutionsand_.. 119
pressure of, importance of, in metamorphism__ 61-70
quantity of, dissolved by liquid, proportional to
PVESSUL yee seen eUees ue ee een 7
solubility of, as affected by temperature_______-
solution of, in ground waters-----.--------------- 68-72
temperature of, importance of, in metamor-
[DHS Ie ee ee ee eee 61-62
Gedrite, chemical and physical constants of _- 197, 281
(OCCUTTON COLOR See ean eee ner ane anes 281-282
sources and mode of formation of -_-..----_----- 282
Gehlenite, alterations and alteration products of... 314,
373, 383, 406
1259
Page.
Gehlenite, chemical and physical constants of__._ 197,314
OCCUTTONCEO Laas ae eee oe eats eae ea 314
Gehlenite and quartz, alterations and alteration
TDROGHOWIC oe ae poe ocoeee enue 383, 406
Geikie, Archibald, on dolomitezation of Carbonifer-
ous limestones in England _____._____ 800
on graywackes of Great Britain_______._.._____- 882
on hydrogen of volcanoes, source of__......-.... 492
on lavas, temperatures of - 652
term schist defined by.--...-.----_.-..--...---- 779, 780
Geographic conditions, effect of, on metamorphism - 41
Geological factors affecting alteration of rocks,
enumeration of -_-
Geological work of ground water, factors influenc-
Ing Se aan SALE a NON BOE Wes 156-158
Geology, data composing great volume of
Georgia, depth of decomposition of rocks in__-_
granites from, losses in, by weathering _________
Geyserite, deposits of, reference tu
Gibbsite, alterations and alteration products of__.. 235-
236, 373, 383, 400
chemical and physical constants of____-_-- 197,198, 234
OCCULTEN COO Lee sree se eae ae ee ee ee eee 235
sources and modes of formation of ___...________ 223,
224-225, 235-236, 254, 255, 256, 258, 260, 261, 263, 264,
265, 292, 293, 294, 296, 298, 299, 304, 312, 313, 318, 319,
322, 323, 328, 337, 338, 339, 341, 343, 353, 370, 375, 376,
377, 378, 379, 380, 381, 382, 384, 386, 387, 388, 389, 390,
392, 393, 394, 397, 398, 399, 400, 402, 403, 404, 406, 624
Gibbsite and albite, alterations and alteration prod-
UCTS OL se Dee SNE SAO 375, 400
Gibbsite and anorthoclase, alterations and altera-
tion products of 77, 400, 401
Gibbsite and magnesite, alterations and alteration
PLOGUCLS OLepaae eee ae 383, 401
Gibbsite and microcline, alterations and alteration
PLOCUC TSO Laaan eee Rene 389, 400
Gibbsite and orthoclase, alterations and alteration
DLOGUCES Of sates se eeset es yeaa uE SB O84 ()0)
Gibbsite and quartz, alterations and alteration
products of _--..--.__.- 383, 407
Gibbsite, magnesite, microcline, and siderite, alter-
ations and alteration products of__ 389,401
Gibbsite, magnesite, orthoclase, and siderite, alter-
ations and alteration products of... 389
Gibbsite, quartz, and calcite, alterationsand altera-
LIOMsPROCUCTS{O fee eee eee 383, 407
Gibbsite, quartz, and potassium carbonate, altera-
tions and alteration products of ___ 383, 407
Gibbsite. See also Hydrargillite
Gismondite, alterations of. ----.~----.-------_-.___. 335, 367
chemical and physical constants of -___________ 197, 231
YOR ARET NCE) One a oe Se eee 331-333
sources and modes of formation of _____._______. 262,
332, 367, 370, 376, 402, 633
Gilbert, G. K., on climatic changes in the Far West
in Pleistocene time ----2-----_--_____- 1178
on contact metamorphism _________ 651
on deposits of Lake Lahontan 554
on exfoliation of rocks in Sierra Nevada (note). 438
on magnesium in salts of Great Salt Lake ______ 994
on post Glacial anticlinal arches in shale and
limestone Sates seeweeeeee ee ne 598
on potassium in the Dead Sea 1000
ODSHOLE|GePOSIbS ee eee ee eee ae 795
on warm or hot springs of Cordilleran region. 591-592
Glaciers, disintegrating work of-__.._........-..22-. 498
1260
Page.
Glass, alteration of, minerals produced by ---------- 251
devitrification of, phenomena of_---....-.------ 247-252
heat and volume relations of-_--...---.---- 251-252
natural, occurrence, and alterations of __ ~ 246-352
solubility of, in water -- soca SB
80, 112, 630, 637, 692, 723, 740, 749, 750
StralnvinNweXaIN Pleo Leora eee enone ana 691
Glauconite, character, occurrence, and alterations
351
chemical and physical constants of -.....------ 197, 351
Glaucophane, chemical and physical constants of. 197,283
occurrence, sources, and modes of formation
283, 285
Gmelinite, character and occurrence of-_
Gneiss, definition of 782-783
features Of sesso me ese ee een acim = 782-783
lossesin, by weathering, analysis showing... 508,515
metamorphic, igneous, and sedimentary rocks
often combined in -----.---.-------- 911-912
minerals of, development of -_.-_--.------- Seca 899
photomicrograph of ----------_--- = ---------22--- 902
Gneiss-arkose, deposits of, originand character of_ 875-876 |
Gneiss-gray wacke, deposits of, character and origin
OLAS meee tannin ae eee SSS 883-886
Gneiss-pelite, deposits of, origin and character of_ 894-904
Gneiss psephite, origin and character of 857-860
Gneissic tuffs, discrimination between sedimen-
tary gneissic rocks and___-.-----.-- 909, 913
Gold, association of, with argillite____..._____._- 1095, 1096
association of, with base metals 1169-1174
deposition of, by ascending solutions_______- 1089-1099
forms\of occurrence of 2.2 settee a
precipitation\of—--<22 --- 228 1091-1099
solubility of 1075, 1089-1091, 1171
Gold-bearing quartz veins, origin of, by magmatic
segregation, theory concerning-- 1048-1049
Gold deposits, formation of, possible modes of. 1041-1042,
1048-1049, 1050
1084
Gold ore of Sierra Nevada, origin of
Gold placer deposits, formation of
Gogebic districts of Michigan, iron ore deposits of,
as affected by topography __-________- 1219
Gordon, H. A., on gold deposits of New Zealand ___-
Grain, silica potash, soda, etc., iIn-..-....---..------- 454
Grain of rocks, effect of, on rate of weathering -_ 533-534
See also Recrystallization.
Grains and grasses, disintegration of rocks by
Granite, induration of, along joints
injected, exampleiofss22 ses .ee es ase s eee
losses of constituents of, by weathering
strain in, shown by quarrying. ----..-...---._..-
Grant, U. S., on action of organic matter in reduc-
ing and precipitating oxidized prod-
1157
on agency of igneous intrusions in producing
iron-silicite rocks. -.-2.-.-----------.- 840
on zonalarrangement of metamorphic minerals
peripheral to iz trusive masses_...... 719
Granulation of minerals, energy factors involved
FLT ees Sen eet Soe et ce anata 7710-771
relation of recrystallization to- - 737,748
temperatures favorable to__..___- eee h40
water content of rocks as affecting -_--_-_____- 741-748
Graphite, character, occurrence, and alterations of. 212
chemical and physical constants of -__..--.----.- 197
Grasses and grains, disintegrating effects of, on
TOCK Si ee Us AUB eae atte tala 445
Grass Valley, Cal., gold veins of, constant tenor of. 1137
secondary enrichment of gold ores at. -_----- 1184-1185
331-333,
1169 |
INDEX.
Page.
Gravel, pebble, and bowlder deposits, discussion
OPE Ge See SU Se SE NSLS 853-860
Gravity, metamorphic effects of __..._....-_-___...- 46-50
ground-water motions as affected by_- 146-147, 148-149
Gravity, specific. See Specific gravity.
Grays Peak, Colo., disintegration of rock at, by
freezing and thawing 443
Gray wacke, deposits of, character and origin of._ 880-883
metamorphosed, character and origin of _____- 883-886,
discriminationlofseseesese eae ae ener 909-910,
photomicrograpks of--.._-...-......-..---------- 888
serpentinized, formation of__-..____...___..--_ 881-882
Gray wacke-gneiss, deposits of, character and origin
C0) ie ere oe hee at aC an 883-886,
Gray wacke-schist, deposits of, character and origin
OP Se ae Ue ee WG RON 883-886,
Gray wacke-slate, deposits of, character and origin
of _. 883-886
Great Basin, deposits in, reference to_______________ 559
ground water of, rate of flow of___- 585
streams and lakes of, soluble material trans-
ported and deposited in_________--. 551-554
Great Salt Lake, calcium chloride deposits of-__.__ 542
precipitation of salts in ---.-_-.-..--------.---- 552,553
Greenalite, character and occurrence of -____._____- 284
Photomicrographyoh esse sa as eee H mie ealaae 836
Griswold, L.S.,on whetstones of Arkansas -_-_---_- 853
Grits, deposits of, character and origin of ___.____ 879-880
Groddeck, A. von, on contact metamorphic origin
Of OPES HAE SOSA ae een ee eee mens eet tase LODO
Grossularite, alterations and alteration products
of ____...----.---- 302,303, 305, 307, 373, 384, 398
chemical and physical constants of____________ 197, 299
occurrence and associations of -___-_..-_-_------
sources and modes of formation of- 301,314, 370, 383, 406
Grossularite and melanite, alterations and altera-
tion products of 383, 384, 387, 398
Grossularite, pyrope, and melanite, alterations and
alteration products of -_ 383, 384, 387, 390, 398
Groth, P. H., on composition of zeolites_____--_.---- 329
Ground water, absence of, in ore-bearing districts,
mistaken inferences from_______- 1063-1065
ACIdS presen tiin sass aaa eee eee eye nee 93
channels traversed by, size of _-._._--..-_- 580-582, 1201
chemical and physical principles controlling ac-
tion of --- 65-123
circulation of - 123-158, 416-429, 571-593, 661-668, 1021-1029
CausesoL SuUmMMaryOL pease eee ee eee 152
channels of, variations in size and continuity
1201
129,
187, 189-146, 572-579
concentration of soluble material by _----- 543-551
conditions governing---_-_-_._--- 129-158
downward movements of, factors influ-
ETC LIE Se CA ana aE inte ie mee 417-419
factors or forces influencing-____.. 146-156, 416-417
adhesion se=sssesesee meena ------- 150-152
frictions. ss eee --. 153-154
gravity. 2S ie ee ee - 146-147, 578
gravityjandhea ticassonss 0s sesame 148-149
We ate U cers NS eS AES 147-148
limiting formations _- --- 576-578
mechanical action -----.-------.---------- 149
molecular attraction --..-------.------- 150-152
openings in rocks____- --- 129-146
temperature sete ee eee 157
vegetation ___--- .-- 152, 422-423
Tarte OL 22 sbi eS et Se ee ear ave 154-156, 583-588
INDEX.
Page.
Ground water, circulation of, sections (diagrams)
SHOWIN See se eee eee ae 570,571
tree-likeisystemofi2 sais sate oe eee 583
upward movements of,factorsinfluencing- 419-422
WAP OTOL sate Behahe ee er ee eee ee Loe 571-572
compounds; presen tin = sae seeee eee ease ae 76-77
deposition of minerals from ___.__-._----------- 613-617
dissolution of material by_--....-.-.----------- 613-617
effect of, in cases of igneous intrusion -_- eeeeGO0)
evaporation of, effect of cultivation on_--_---- 421, 422
gases)presemtiin 22%. We ee es kee ee eee 68-69
geological work of_____-____-_- .. 156-158
as 92
580-582
ions present in _-
large channels preferred by~----
level of, change in,influenceof,onore deposition. 1206
C52) 0) lc (ay Ge berere appa ey seth aes aa ey ea er ee 409-411
factors affecting or determining-- -. 423-429
BETICU] ture ee es ee ee ree NEO ee re 427
barometric pressure_____----------------- 428
capillanity #2 see eo Saaremaa 412
LST at ree eee ane 426
evaporation 425-426
AMATI 2 LSU Oa ae A Sed 427-428
Tainfall soe oe aos ee 423-426
Seepag es esa: Ja ees Se 425,
LEMP sla bUNO woe ee ee eee eae aera 428
uplift and subsidence 426
form of -.-- 411-413
in arid and humid regions, as affecting ore
epOsita yesh te ee ee eee Aah 1180
ore deposition as affected by change in ______ 1206
relations of, to topography --------___- 410, 411-413
Varia tio nye sae eee ae ween 423-429 1180
importance of, in lead and zine regions
of Mississippi Valley _.-...-...-.-...-- 429
materials added to, in belt of weathering_____ 617-618
metamorphism by ... 65-123
MOVEMENtSiO fae ss aes ee sa De ee ELE 123-158,
416-429, 571-593, 661-668, 1021-1029
in transporting metals -__...____.-..____. 1075-1081
Speecdioheeessese eee Mie ee oe bere e Becat ed DO B=DO8
vertical and horizontal, relative length of_ 579-580
See also Ground water, circulation of.
pressures, temperatures, and depths of, table
showing relations of____--..-._______. 567
principles of chemical reactions applicable to__ 84-123
principles of solutions applicable to ____________- 65-83
Tate of ow: OL eA awa ES ek SEES IL 583-588
silica in, large amount of, where vegetable mat-
telehis |e) DUN dam tees ee eens 76
SOlUtIONLOME ASCs} Ia seal nnnn ee ee aan 68-72
SOLUbIONEO LES OLIGS meee ee ean ee 72-81
SOUP COOL sae ME as DN ae a sae 128-129
temperature of, asaffected by earth movements. 590
importancelo fess ses sexe er aneke ee ane 615
increase of, with depth__ 578-579
on entering and issuing from the earth ___ 589-593
thermalleffects of. --2- 2-2 22222 -25. 589-693
upwardmovements Of l= ses ssee eee 419-422
forces influencing, vegetation -___________. 422-423
molecular attraction__-----. ----__-._._. 419-422,
UNL versalipresencelo fees sesas = seen eee mene 123-124
See also Water; Ascending waters; Descending
waters.
Groye, Sir William, on segregation of carbon diox-
ide from interstellar spaces by gravi-
1261
Page.
Griinerite, alterations and alteration products of_. 284,
373, 384
chemical and physical constants of _______- 197, 283, 284
occurrence and associations of__________________- 284
origin of, from iron carbonate, chemical reaction
FLO live Clie eae ae a 834
source and mode of formation of____.._._.-.-____- 244,
245, 284, 370, 392, 407
Griinerite-magnetite-quartz rocks, origin of ______ 834-841
Grtineritic marbles, origin of__._-_____-...-.--_-____ 833
Giimbel, K. W. von, on use of term ‘“‘epi?? _____.__.- V7
Gypsum, alterations and alteration products of... 358,
373, 384, 396, 400, 788-789
chemical and physical constants of_______- 197, 357, 788
occurrence and associations of_________...._____- B57
solubilityioty inisalt waters sess seas aan: 119
source and mode of formation of______________ 357-358,
370, 376, 402, 788-789
HH.
Hague, Arnold, on calcareous deposits in Yellow-
stone National Park__-_......_..___.- 5d
Hahn, Julius, on annual range of temperature _____ 437
Halite, chemical and physical constants of _.______- 197
Halite and albite, alterations and alteration prod-
RIGES1O LEN ete ey i out ent TS Neate 375, 400
Halite and nephelite, alterations and alteration
DLOCUICTS OL sass ee anes ee 388, 400
Hall, R. D., and Lenher, Victor, on precipitation of
gold 1091-1092, 1096-1097, 1098
on telluride of silver, artificial production of ___ 1124
on tellurides, formation of___________._- 1120-1121, 1124
Hallock, William, on variations in rigidity of bodies
accordant with degree of pressure ___ 672
Halloysite, chemical and physical constants of ._____ 197
sources and mode of formation of _____ 253, 254, 370, 389
Hambuechen, Carl, on potentialization of energy in
strained metals and minerals______- 97, 691
Hammond, J. H., on Witwatersrand banket________ 1041
Hann, Julius, on temperatures at surface of the
earth svwe nah sen Wear Ns ead 52
Harmotome, character, occurrence, formation, and
alterationslofypessees ose sa eweaees 330-835
Harris, G. D., and Dall, W. H., on phosphates ______ 976
Hautefeille, Paul, experiments of, producing rutile
group of minerals ____._______________ 231
Haiiy, R. J., on rock nomenclature Sen 35)
Haiiynite, alterations and alteration products of_ 298-299,
373, 384, 399
chemical and physical constants of._______ 197, 295, 297
(OCCUNTONCOXO fees te teen ne en 297-298
Hayes, C. W., on bauxite deposits --.._-.-__.__.._ 985, 1037
on colors of soils of Nicaragua asindicating pres-
ence of ferric or ferrous oxide_____ 470, 471
on Nicaraguan streams, lack of sedimentin__.. 504
onjorigin’ of phosphates) 252222. e eases eee ae 976, 977
on quartz crystals, corrosion of__-......------._. 218
onisolution(of silica sass. se see ee ee 76, 848
Heat, absorption and liberation of, by solution ____ 81
chemical effects of----------.---..- 56, 89, 105, 113, 208, 362
conductivity, ofrocks|foree =: sees eee eee tees 53
derived from the sun, metamorphic effects of __ 51-53
derived from within the earth, metamorphic
effec tsloh es ae were eee eee 53-54
increase of, effect of, on chemical reactions______ 89
liberation of, dominance of reaction causing ___ 362
mechanical effects produced by, in belt of ce-
mentation 595
1262 INDEX.
Page Page.
Heat, metamorphic effect of _......_...--.----------- 51-56 | Hillebrand, W. F., and Clarke, F. W., on chemical
phenomena of, in chemical action 54, 105-110, 182, 208, 362 action in solutions, theory of _______- 68
production of, by mechanical action in rocks, on chemical composition of slate --.__-....---.-- 895
importance of, in metamorphism. -54, 99-100
relation of, to chemicaland mechanicalaction_ 110-113
work of, in ground-water circulation -__.----- 147-149
Heat toning of chemical reactions, mathematical
StatementiOle essen eae enon ee 106
Heat and gravity, work of, inground-water circula-
SOS Ag Ss Ses a sec ar ae 148-149
Heat and light, metamorphic effect of .-....-.------ 51-56
Hedenbergite, occurrence of--------.---------------- 272
chemical and physical constants of --..---..--. 197,271
Hematite, alterations and alteration products of... 226-
227, 378, 877, 379, 384, 401, 402, 408
chemical and physical constants of --.----- 197, 223, 842
formation of, chemical equations showing--- 830, 1126
from cupric sulphate and ferrous sulphate
solution, chemical equations show-
aS SOAS COC ROSSER ARES CHB A DESEO 1102
from iron carbonate, chemical equations
SHOWIN Geen ene eae eae ee 844
from siliceous siderite, etc., chemical equa-
bLONS|SNO Wil Pees ear eee enn 830
formation of magnetite from, chemical reac-
tiombimyOlye dian ssss-ss teense ene OSG
residual deposits of --..-..------------ 1039
sources and modes of formation of --..--- 214, 225-226,
227, 229, 233, 242, 244, 245, 268, 269, 282, 303,
309, 313, 828, 342, 370, 375, 376, 377,378, 379,
384, 385, 386, 387, 390, 391, 398, 399, 400, 404,
405, 467, 468, 623, 830, 843-845, 846, 1102, 1126
Specular Origin Ofjes-ssee eee ete enna n ans 844-845,
Hematite and anorthite, alterations and alteration
PLOGUCtS OL eee eae eee eee en 377, 402, 404
Hematite and biotite, alterations and alteration
DROGUCLS Olea seas e ceases == 379, 396, 398, 401
Hematite and meiorite, alterations and alteration
TOON EKO O}e A a 387, 402
Hematite, anorthoclase, and calcite, alterations and
alteration products of -..------.----.. 377
Hematite, calcite, and microcline, alterations and
alteration products of -_-...-.-.---- 389, 401
Hematite, calcite, and orthoclase, alterations and
alteration products of -.-----.------ 389, 401
Hematite-schistworlgimiofsesssss sos ee eee eee ae 845
Henrich, C., on ore deposits of Ducktown, Tenn_.-_ 1182
Hercynite, chemical and physical constants of -.... 198
source of 370
Hess, W. H., on origin of nitrates in cavernearths. 543
Heterogeneous and homogeneous chemical systems,
MO tes Ons eestene senna eeeeeae naa 90-91
Heulandite, alterations and alteration products of. 333,
334, 373, 384, 400
chemical and physical constants of --.....-...- 198, 330
sources and modes of formation of --_-.-- 262,
381-333, 370, 375, 376, 397
High lands, disintegration of rocks in_--..-------- 499-500
Hilgard, E. W., on amount of soluble salts con-
tained in soils in California --..--.... 545
on decomposition of rocks in arid regions. _-.... 501
on decomposition of silicates in calcareous and
noncaleareous soils -_...-------------- 47
on endurance of plants to alkalies. -_-_....-.------ 478
on soils of arid regions --..----_----.--.-_=-.----- 497
Hill, R. T., on systems of circulating underground
NUE Ses accaLHodeonas Sonces EeeHS ae eanS 578
onmockjanalysessaesssss=s ease ea aaanaee 693
on rocks of Pigeon Point, Minn___--__- 733
on water content of rocks, relation of, 56) crys-
tallization=23-3sssvess eee 743, 744
Hills, R. C., on ground-water level in Cripple Creek
istrict Sr eS hos as a 1172
Hinde, G. J., on cherts of organic origin___--____ _- 847
on silicification at Spitzbergen ___..___..._---.-- 646
Hitchcock, Edward, on schist conglomerates___-__- 859 -
Hobbs, W. H., on enlargement of mineral particles. 644
on regeneration of mineral particles 705
Hobbs, W. H., and Culver, G. E., on decomposition
of olivine-diabase in South Dakota-. 560
Hoff, J. H. van’t, on heat of chemical reactions-_-___- 107
ONOSMOLICIPRESSUNE Sean eee eee eee TA:
On reactions 82) Sere ae Se ee eae 90
on solutions 74
on thermal effects of chemical reactions at high
and low temperatures_.....-.----.--- 168
Homogeneous and heterogeneous chemical sys-
tems notes!ontesses= se eee ee eene 90-91
Hoover, H.C.,on ore deposits of western Australia. 1185-
1186
Horizontal movement of ground water, limits of. 579-580
Hornblende, alterations and alteration products
of .._. 285-286, 287, 288, 289, 290, 373, 384, 385, 398
chemical and physical constants of___-.--.---- 198, 283
fragment of, cut showing enlargement of_ 626
OC CUTTENCC)O fae ee eee eee ere 284-285
sources and modes of formation of___ 274,279, 285,303,
305-306, 370, 376, 378, 383, 387, 390, 396, 408, 625-627
Hornblende and quartz, alterations and alteration
Productsiofpe ssa ee eee enon 385, 399, 405,
Hoskins, L. M.,on flow and fracture of rocks ____--- 671
Hoskins, L.S., on shearing of rocks -_....--.--.----- 763
Howell, E. E.,on warm or hot springs of Cordilleran
TO GLOD Sas Parcabee Stel ale pA Rd taut iar 591-592,
Hubbard, H. L., on hydrous silicates in the Lake
Sietoe TOPIOTN soe sehU eee tel so Ne 1130
Hudson schists, igneous and sedimentary rocks
combined ins ee aaa eee 912
Human influences on ground-water level, charac-
torof se ee ee 427-428
Humic acid, formation and action of, in soils_-_-.- 462-463
Humid regions, decomposition of rocks in -_-__--- 501-502
level of ground water in, as affecting depth’ of
ore deposits 225-5 5-2 ses eaam UN 1180
Humidity, importance of, in decomposition of
TOCKS 42s oss ce Sects eee ee 501-502
Humites, alterations and alteration products of-. 325,
878, 385, 403
chemical and physical constants of _--..----.-- 198, 325
OCCUTTON COO fps ss ee ecet ee sen sae ae aero nea 325
Hunt, T. Sterry, on carbon dioxide formerly in at-
mosphere!/s: 22252 sees ee 964-965
on carbon dioxide of mineralsprings, source of. 678
on carbon dioxide segregated from interstellar
spaces by gravitation.-.....--.-..--.- 973
on origin of pegmatites--.....--...--------------- 722
on silica in underground water ---...---.-.------ 476
Huttonian principles of geology, application of, to
metamorphism sess nee eee - 35-36
reversal of, as applied to metamorphism-.------- 36-37
Hyalosiderite, chemical and physical constants of-. 199
INDEX.
Page.
Hydrargillite, chemical and physical constants of_ 198, 234
occurrence, sources, and alterations of --_----- 235-236,
See also Gibbsite.
Hydration, alterations produced by-----.------------ 401
conditions favorable to__-.-..---.--.----------.-- 89
definitionyofe ie oP eee ea eae 204
depth and pressure required for reversal of -. 180-181
Gisctssionioleecs tena sees e sie aa tos oaseee 481-483
examplesiofac: Mi Saeed eee acsee 179-180
extent of, in belt of cementation _-.... 164,165, 166,612
limitation of, by expansion of volume-- So 481!
occurrence of, in zone of katamorphism cos alts}
water fixed by, amount of_-_..._---.------------ 161-162
Hydration and carbonation, alterations produced
Ayer SN eae oe a 396
Hydration and decarbonation, alterations produced
Dy gree ee 8 eps a Serene ALOR alata 401-402
Hydration and dechloridation, alterations produced
Bye Bea ee nO A A RI 403
Hydration and defluoridation, alterations produced
ND Yio toe nee ee ee ae aerate eae 403
Hydration and desilication, alterations produced
[DY eae Sena ee eA 404
Hydration and oxidation, alterations produced by.. 404
Hydration and silication, alterations produced by.. 405
Hydration, carbonation, and dechloridation, altera-
tions|producedibygens--seeeeee eee 398
Hydration, carbonation, and desilication, altera-
tions produced by 398-399
Hydration, carbonation, and desulphation, altera-
tions\produced! byji---o ses ee nee tee 399
Hydration, carbonation, and oxidation, discussion
OPE Sac R EE Ne DES ee aie male Mia 483-484
Hydration, carbonation, and silication, alterations
MLoOduced ib ygeassee eee eee 399
Hydration, decarbonation, and dechloridation, al-
terations produced by --..-.----.-_..- 403
Hydration, dechloridation, carbonation, and desili-
eation, alterations produced by--.._- 403
Hydration, desilication, and decarbonation, altera-
tions|producedibyp2s-s-s sss eee eee 401
Hydration, oxidation, and desilication, alterations
PLOCUCSCND yee eae ae eee eee, 405
Hydration, oxidation, carbonation, and desilica-
tion, alterations produced by ________ 390
Hydrobiotite, chemical! and physical constants
OL eee Sates eae ee eee 198
source and mode of formation of --.-..-----..... 339,
340, 342, 370, 378, 397
Hydrogen in earth’s crust, amount of____-..---._- 981-982
in earth’s crust, percentage of 934, 936
in sea water, percentage of-__-__-..--------_--_.- 942
occurrence and combinations of __-.-.---._-__- 981-983
of voleanoes and fumaroles, sources of ___.._-___ 492
oxidation of, in belt of weathering----_________ 461-465
Hydrogen sulphide, sources and importance of_ 1112-1114
Hydrolysis, increase of amount of, with increase of
TOMperavurre| asa sees eee eee
importance of, possible, in metamorphism -.-
of sodium silicate, observations on
phenomena ofss: 22 $2.2. ta es ae ee
Hydromagnesite, chemical and physical constants
Ol seascoacecarcesacensneSeceseeess sens 198
sources and modes of formation of --._ 235, 370,379,396
Hydromuscovite, sources and modes of formation
Ooo Hepoeeaanecodo =eoete 255, 293, 312, 853, 370
See also Pinite.
1263
Page.
Hydronephelite, chemical and physical constants
198, 329
occurrence and associations of -- 331-333
sources and modes of formation of --...---..---- 292,
295, 331-333, 370, 388, 392, 397, 398
Hydrophlogopite, chemical and physical constants
(Op CS eet Beh Ry oe rere ey eo 198
source and mode of formation of ______ 343, 370, 389, 397
Hydrosulphuric acid, oxidation of, by bacteria, for-
mula for reaction--------------------- 468
production of _____..--------
solubility of sulphides in -_-.._--.---_------------
Hydrosphere, percentage of known matter of globe
formed! bye =: een eee
See also Ocean.
Hydrotalcite, source and mode of formation of--. 309-371
Hydrous silicates, alterations and alteration prod-
UCts\OL = Jee eee ee CE ee 519
Hygrometric water in rocks, definition of -_-..._--- 124
Hypersthene, alteration of, to talc, magnetite, and
quartz, volume change incident to... 633
alterations and alteration products of __------ 268-271,
378, 379, 385, 399, 404, 405
chemical and physical constants of __-__.____-- 198, 267
occurrence and associations of 268
sources and modes of formation of _.._ ..------- 268,
302, 339, 342, 370, 375, 376, 379, 383, 390, 396, 401
Hypersthene, calcite, and quartz, alterations and
alteration products of --....-..----. 379, 407
I.
Ice, change of water to. See Freezing.
Idaho, South Mountain district of, ore deposits and
gangue minerals of _____...-_--._.....
Idaho Springs, Colo., ore shoots near, intersecting
fracturesiat jos essen. je pee 1227,
Iddings, J. P., Cross, Whitman, Pirsson, L. V., and
Washington, H.S., on mineral com-
position of amphioble-gabbro ________ 632
Igneous and crystalline rocks, analyses of ________ 934-935
Igneous intrusion, features of______ 488-494, 646-652, 707-736
ground-water temperature as affected by-___- 590-591
metamorphic effects of. _.--__.- 42,54, 716-736, 1014-1017
factors influencing, composition of intrusive
and intruded rocks -.--...-....--2..-. 650
depth of intrusion -...-._- = OY
eTOUNCIWaLeTy He swessoe ee ae cee Wh)
length of time of intrusion_____-_..___. 651-652
POLosityzohTOCKS Sse ses eso eee 649-650
DICSSULCE = aa aaa enna - 652
size of intrusive masses --..____ 5 Gal
temperature of intrusive -- 652
minerals}fOrm 6dybypessassessa ema ae 717-720
water released by, in zone of anamorphism_____ 662
work of, in belt of cementation__-.______._____ 646-652
Igneous processes, ores produced by ----.-. 1043-1052, 1233
Igneous rocks, chemical composition of -_.-.-___..__ 890
classification and nomenclature of _. 904-906
effects of, in metamorphism -.--.--__....__..___- 42
importance of, as sources of ore deposits--. 1030-1034
metamorphosed, and metamorphosed sedimen-
tary rocks, discrimination between,
‘criteria;fonia est eens ene 908-917
minerals in, percentages of _.....---_-_.... 35) M987.
ore deposition as affected by.....-----.------ 1014-1017
oreidepositsiinys : soe eee eee 115-116, 1081-1082
rock disintegration as affected by --------- 434-439
range of, at which work of water solutions is
done
relations of, to solubility
zone of constant, in earth, depth of_____-_._____. 590
Temperature and solution, relations of ___-_____--_- 79-81
Temperature of earth at surface and below, ayer-
(ee) eee ae oes Sane eee 51-52
Tenorite, alteration of, to malachite, equation show-
Tn oS a ae RE aE ON ae 1159
formation of, equation showing -------.--------- 1158
Tension. See Strain.
Termites, effect of work of, on soils -___-_--__- 448,449, 456
Textures of rocks, category of, produced by meta-
TONY AO) MIS per Hee 37-38
coarse, favorable to permanency --..------------- 40
rock flow as affected by ---------.----=--------- 760-762
weathering as affecting = --- = _-_ 22225 - 524-527
preservation of, after metasomatic alteration. 644-645
Thawing and freezing, effect of, on rocks -______- 440-444
Thermal factors involved in chemical changes, dis-
cussion of ----:-.--------..<.-.-- 182; 208, 362
Thomson, Sir William. See Kelvin.
Thomson, W., and Tait, P. G., on effects of ‘‘stress
Gifferen ce? he 52s) Sia eee ee aeses 672
Thomsonite, chemical and physical constants of - 201,329
formation of, from anorthite, volume change
INCICeN GLO seer eee 633
OCCUTLEN COO hasan ae Seen ene ee 331-333
sources and modes of formation of ___---__---.-- 261,
292, 293, 295, 296, 372, 376, 388, 392, 402, 403
Thwing, C. B., and Austin, L. W., on volume of
water at different temperatures -... 147
Tidal forces, effect of, on movements of earth's
erust 930
Tilden, W. A., on carbon gases inclosed in rocks - 968, 969
Time as a factor in metamorphism, importance of. 41-42
Tin deposits; character of =-2-__ 22-222 5222-22-2- 1038-1039
Tin ores, formation of, modes of -__-------- 1054-1055, 1058
Titanates, character, occurrence, formation, and
alterablOon Of = seen s sea eee O04 BOD)
Mitanation: definitioniofiss-ss.ses aa aeee oe eee 205
Titanation and decarbonation, alterations pro-
duce db ye nets ee eee eae eed 400
Titaniferous iron ores, deposits of_----.--------- 1044-1045
Titanite, alterations and alteration products of -_.. 355,
874, 393, 396, 401
chemical and physical constants of _---- 201, 354
(OCCULNONGCC Ole esan sere een ene - 854
sources and modes of formation of ---_-_-------- 227,
228, 231, 854, 372, 385, 391, 407, 408
Titanium, in earth’s crust and in original and sedi-
mentary rocks)_--.--2.=2.--- 938, 936, 974-975
Titanium dioxide, percentage of, in earth’s crust. 937,938
Tolman, C. F., on balance between carbon dioxide
in air and that in the ocean --_--_---- 968
on pressure, effect of, on crystallization -------- 103
on Spring’s experiments in compaction of clay. 100
INDEX.
Page.
Topaz, alterations and alteration products of. 318,374, 393
chemical and physical constants of
occurrence of
Topography, character of, effect of, on ore de-
201,316 |
316 |
[DOSTt eee ne 1217-1221
Tourmaline, alterations and alteration products
Ofer ae ee 326-827, 374, 393, 400
chemical and physical constants of -
occurrence of
sources and modes of formation of
Transmission of water, capacity of rocks for, de-
terminable by capacity of imbibi-
Glom see oe Saas ete Oe nee 155
Transportation of material, agency of water solu-
LOST EA ee 64
Transyaal, depth of disintegration of rocks in__-.-- 531
Travertine, deposits of, reference to ----..---------- 550
Treadwell, F. P., and Reuter, M., on solubility of
calcium carbonate in salt water ---_- 119
Tree-like system, ground-water circulation in____ 583,588 |
Trees, disintegrating effects of, on rocks_______--- 445-447 |
Tremolite, alterations and alteration products of __ 285,
286, 290, 374, 393, 397
chemical and physical constants of -__--------- 201, 283
formation of, from dolomite, chemical reaction
involve dein sae. sehen eae ae 822
occurrence of - 283-284 ,
sources and modes of formation of --_..-_..-. 241,242,
274, 277, 279, 283-284, 309, 354, |
372, 382, 383, 386, 407, 408, 822
Tridymite, chemical and physical constants of_.- 201,220 |
occurrence and modifications of-.-__------__--
Trisilicic anorthites, chemical and physical con-
stants of
Tropical lands, decomposition in 502
Troughs, pitching,influence of,on ore deposition_ 1211-1216
Trunk channels of ground-water circulation, atti-
tudes of
size and continuity of, variations in_____-_____--
Tschermak, Gustav, on enorphite and berlanite-
on olivine
MirfastOrmablon Olea ss) senses ees ahaa = =n OOU, DOS TIS
Mutts cementation! Of ss = ae oe ee 1025
Character ots == sana daaeere eee a eee 596-597
COUSIN a WOM Ohya tpeeee nae een en ee 597
gneissic, discrimination of ____.--.._--.-_-._-...- 909
schistose, discrimination of_-_-.__.--__-.________ 909
re
Unaka Mountains, variable metamorphism in______ 920
Underground circulation, variation in size and con-
tinuity of channels for -___-_-._-_--_- 1201
See also Ground water.
Uplift and subsidence, influence of, on ground-water
Urea, transformation of, to ammonia, formula for
TEACLIONsproducin pass sees 465
Uvaroyite, chemical composition of, and other data
CONCOLUIN GS tess eee ees 201, 299
OCCOETENGE| Oferta sets EN eae ane eee 302
Vv. .
Van Hise, C. R., on alteration of clastic rocks _____- 184
on belt of cementation, depth of___.._____--.____ 657
on cementing material, similarity of, to rock
cementeds = so eae Lance ees 628
220-221
1283
Page.
Van Hise, C. R., on chlorine, emission of, during
> voleanic eruptions---.--22-.---------- 789
on cleavage tS,
on closure of rock openings at depths -_-________ 566
‘onserustalishortenin pease sas er enee 774
on deformation of rocks placed in unusual posi-
LIONS Stee een keane Wea ee ee 188
on earth movements _____--___- 111, 146, 774, 924, 926, 927
on enlargement of mineral fragments____-_ 625, 626, 644
on granulation and recrystallization, influence
of rock character on_---...----.._.._. 740
on igneous intrusion, periods of, coincidence of,
with periods of rapid orogenic move-
NON ES es oes eee ee Se ee eee 709
on igneous intrusions, agéney of, in producing
InON-sili cate mocks) asses eee 840
on gravity, domination of, in earth moyements_ 46
on iron-ore deposits of the Lake Superior re-
gion 831, 1194, 1216, 1219
on ore deposition ___ - 1031, 1208
on ore deposits. secondary enrichment of _______ 1188
on liquid inclusions, secondary character of ____ 746
on magnetite deposits of Michigan
onimashingyoturockstsseeea. se see
on metals, source of ____
on metamorphism, variations in, within short
Gistances |r te. es aan ern ee EE 919
on miscibility of rock and water____._________ 723, 1030
on movements of earth’s crust - 924, 926, 927
on pepmatization =. sas scceeene so se aes 725
on peripheral structure =-252--- 2222-2 716,717
on porphyritic crystals in schists_________.___... 702
on quartz grains (flat) in rocks_______.___.____.__ 692
on recrystallization of quartz and development
fofimicayin\ rocks eseeee eee oe 694
on residual cores of recrystallized quartz grains
OF TOCKS: 22 2h ee es ees 752
on replacement of quartz by iron oxide_________ 220
on rock alteration in intermediate belt between
zone of katamorphism and zone of
anamorphism 768
on rock cleavage _____ 760
on rock flowage 7.
on schists of Lake Superior region 904
on schist-conglomerates________..-_._____________ 860
on secondary enrichment of ore deposits __.____ 1188
on strain and fracture of rocks 600
on strain in minerals, effect of-_________.________ 97
on zones of fracture and flowage, subdivision of
outer part of earth into ______________ 187
Van Hise, C. R., and Irving, R. D., on actinolite,
growth or penetration of, into
QUAL tZ ees OeN Sees ee PN 219
on alteration of clastic yocks_____-____.._________ 184
on diabase from Michigan, losses in, due to
SW.Ga CHO Wr ri rye Heyes open eee ea 511
on enlargement of mineral particles 644, 874
on igneous intrusions, agency of, in producing
iron-silicate rocks -____________._.____ 840
on iron-ore deposits, origin of __ 831
on recementation of rocks -_--:_______2__-______ 564
on crystallization of quartz and development of
MICe AN TOCKS ae eee ee en te 694
onrock textures (original), preservation of,after
metasomatic alteartion __-___________ 644
on schists of the Lake Superior region_.________ 904
1284 INDEX.
Page.
Van Hise, C. R., and Leith, C. K., on igneous intru-
sions as an agency in producing iron-
(SHC NE® TROON ass ee eae SND 840
Valley filling, influence of, on ground-water level_. 426
Variations in metamorphism within a single dis-
trict, discussion of ___--..-._ 2) 5 917-921
Varnish, desert, reference to _.........----.-.------. B47
Vegetation, carbonation of soils promoted by -__---- ATT
ground-water motion as affected by___ 152,417, 422-423
rock decomposition and solution by_-_ 444-447, 505-506
upward transfer of material by .__..__.._________ 550
Vein quartz, figure showing 1156
Venezuela, diabase from, losses in, by weathering. 508:
509, 522
Vermiculite, alterations and alteration products
(0) Heap aM De opm ees i 353
source and mode of formation of ______ 337, 353, 372, 388
Vertical movement of ground water, limits of ___ 579-580
Vesuyianite, alterations and alteration products
Of S205. Se LTD ES Sere eos 315, 374, 393
chemical and physical constants of ___ 201,315
OCCUTTENCOOL = eee a0 ens yee =a) dik}
Virginia, diorite from, losses in, by weathering ____ 510,
515,522
gneiss from, losses in, by weathering, analyses
SHO Win oy so SS ve Se URE ae Were 508
pyroxenite from, losses in, by weathering ______ 510
Viscosity of solutions, importance of degree of, in
ground-water circulation ____________ 153
Viscosity of water, decrease of, with increase of
depth from surface _____.._____.__._ 578-579
decrease of, with increase of temperature__ 140-141, 153
table showing, for various temperatures________ 141
Vogt, J. H. L., on cassiterite, deposition of .________ 1127
on ore deposits, production of, from igneous
TOCKS (42-03 52s aes 1044, 1049-1050, 1051
on ores deposited from gaseous solutions_-_______ 1054
on precipitation of silver by ferrous iron of sili-
CAbeS2 22 RSS lis See ee elas 1101
Volcanism, effects of, on ore deposition --- 1014-1017
Volcanoes, carbon dioxide emitted by____-__________ 969
vapors from, sources of ___......--5-- 1 492
Volume, changes in, chemical reactions as affecting 93,
170-171, 182-182, 208-211, 375-408, 632
changes in, metasomatism as affecting_______.__ 632
weathering as affecting ______.__.__________ 528-524 |
diminution of, in rocks. ______ 101-104
mressurewertects ones: se2 tte eee 363-365
Ww.
Wadsworth, M. E., on casehardened sandstones____ 547
Walcott, C. D., on source of silica for marine or-
BSUS S)ae trans ota 2 ach 848
Walker, T. L., on nickel ores of Canada_____________ 1046 |
Wall rocks, ore shoot formation as affected by_ 1086-1088,
1229
Wallich, G. C., on cherts of organic origin.___..____ .847
Walther, Johannes, on deposits of ferromagnesian
Sands peemeeeeee eet ec ee 877
on disintegration of rocks by changes in tem-
OTA TUNC eee eee Lee 437,439
Washington, H. S., Cross, Whitman, Iddings, J. P.,
and Pirsson, L. V.,on mineral compo-
sition of amphibole-gabbro -__-_-______ + 632
Washington, Monte Cristo, district of. See Monte
Cristo district.
Page.
Water, combined, in rocks, proportion of ______.... 123
deep, deposits made in, compared with deposits
made in shallow water _._____________ 1134
dissociation and dissociative power of__.____- 66-67, 86
dissociation of salts by ......).-.. 2) 73
expansion of, with increase of temperature __._ 149
flow of, through rock openings 137-146
gravitative action of, metamorphism ibyeeaae 50
ground, ascending, carbon dioxide exhaled
DY S aot SeN Ee Bie eae Sanit Oe ie 970-971
ascending, deposits by, criteria for discrimi-
mating. Pelee are alain Joe ees 1132-1138
first concentration of ores by -_--..___- 1079
descending, deposition of ores by_______- 1079, 1080
circulation of, forces producing ___._______ 146-152
See also Ground water.
hot, work of, in solution and recrystallization._ 692
| in minerals, granulation and recrystallization
as/atfected by 22-2) = ee Aqeas
in belt of cementation, condition of ___________ 566-569
animines tf ea buresio fas a aee eae eee, 1182-1134
| in rocks, amount of_____________ 123, 934, 937, 938, 981-983
metamorphism as affected by______..._______ 41
| in zone of anamorphism, condition, quantity,
| and surfaces of -_____._____..___.___ 659-662
| miscibility of, with rock magma __________ 114-115, 723
movement of, below zone of anamorphism _____ 668
| CHUNG) Cea Kone) Oye SPE 953
| pressure necessary at different temperatures to
maintain liquidity of _______________ 440,568
| shallow, deposits made by, compared with de-
posits made in deep water ___________ 1134
surface, concentration of soluble materials by 551-554
work of, in metamorphism __________-_______ 432
viscosity of, at different depths from earth’s
SUTLaC ek Sassen e terns n eee eee an 578-579
at different temperatures_______ eeel4ate53}
Water bubbles in rocks, figure showing ___ -. 746
Water gas, in zone of anamorphism, action of____ 660-661
penetration of rocks by __-.--________._________ 144-145
Water solutions, metamorphism by__._.__________ 602-656
pressure range at which work is done by ______- 603
temperature range at which work is done by __ 03
| Watson, T. L., on losses of granites by weathering. 523
on weathering of granitic rocks of Georgia_____ 520
Wavellite, chemical and physical constants of ______ 198
| Weathering, acids going into solution during - 536, 537
additions made to rocks in ______________._.___.__ 523 5
belt of, alterations in, contrasted with those of
belt of cementation___________:_____ 166-167
belt of oxidation coincident with____________ 1142
carbon dioxide in, amount of ____.___________ 474
chemical reactions in_______
circulation in
definition{of=2--2 =
- 416-423, 1018, 1023
--- 162,163, 409-411
mineral concentration in________..._.._____ 544-550
openings in, promotion of second concentra-
tom Of Ones jb yseeeae eres ne ee li7-1179
gases in: circulation of22---2 (=) ee 1018
Limits ohio ee sates Man Re als aay oat ue eae 43,529
material abstracted from, by run-off______ 538-539
material dissolved and reprecipitated in__ 539-554
materials and conditions in ____________.___ 429,431 ©
mechanical work in ________ ---- 164, 431-451
metamorphismyjine soe ose sea Oke e ears 429-527
agencies of -=---2-=-2 -. 64, 480-431
1286
Zeblites, sourcesand modes of formation of__________ 260,
261-263, 265, 292, 294, 295, 298, 353, 625
Zine, silicate of, formation of.__._....-._____........ 480
Zine and lead regions of Mississippi Valley, caves
iNiee.= Se Sessscocess a ateresesece A
depositions in, conditions of ___..____________ 1083, 1084
ground-water level in, effect of changes of ____- 429
Zinc blende, secondary, figure showing_____________ 1156
Zine carbonate and sodium carbonate, reaction of,
on iron sulphide, equation showing.. 1143
Zine, lead, and iron compounds, association of__ 1144-1158
Zinc ores, associated minerals of 1144-1145
Zinc salts, reactions involying 1151-1152
Zine sulphate, reaction of, on amorphous Silica,
equation showing. ---__-._.______..___ 1147
reaction of, on calcium carbonate, equation
showings see eae nn ee eee 1147,
Zine sulphide, precipitation of, conditions govern-
din DESO Sore ie Wee Newent 1154-1158
RE CCACtIONSHin vo hy ino eemeeenenee sence ines epee 1150
ZINE OCCULT Ces|O fp eeen ee eases mean us enna 1125
production of, from smithsonite, equation show-
BM amen se ee bee et cia ee ae 1126
O
INDEX.
R Page.
Zircon, alteration and alteration product of_______. 315,
3874, 394, 402
chemical and physical constants of_____ - 201,315
hydrous, formation of __.._.._______ =o | GG
SOUT CES! Ofer ee sueetrse 27 we mae ee a lee 394, 402
OGOUBRING OS he aes ee Se ea a 315
See also Malacon.
Zirkel, Ferdinand, and Naumann, C. F., on forma- —
tion of smaragdite___._.-_.___________
Zoisite, alterations and alteration products of__- 322-323,
374, 394, 399
chemical and physical constants of____________ 201, 320
OCCULTON COOL Sasa t oes ne weet Hl = ores ee 320, 625
sources and modes of formation of ____.______ 223,225,
232, 236, 263, 264, 265, 302, 303, 320,
322, 353, 372, 375, 377, 381, 382, 383,
384, 389, 398, 402, 404, 407, 408, 625
Zonal arrangement of minerals consequent upon
igneous intrusion 717-720
Zone. See Anamorphism; Flowage; Fracture; Kata-
morphism.
Zones and belts of metamorphism, definitions of ___ 159,
160, 163, 164, 167, 189-190
INDEX.
Page.
Weathering, belt of, phenomena of..-- ------------ 409, 561
TREO NOING Ot 25 ee eesti 1140-1141
relations of, to sedimentary rocks--------- 555-560
residual material in, distribution of-_-_-.- 554-555
rocks produced from material of, without
transportation to the sea--.-------- 559-560
temperatures in= == 22-22 enn oe eee na 431
thickness of
transfer of material from, to belt of cementa-
tion 25 es ee see ee peg OSS.
transition from belt of cementation to_-.. 560-561
water in, amount and source of------------ 413-416
depthiand degree\of/ss2 asta ea ee eee 529-532
dissolved materials resulting from, distribution
OPC SRS SOS OR ane wae eee 536-554
effect of, on structures and textures ---_---.-. 524-526
forces\and/ agents Of. snes pare ae en ee 532
gains and: losses}in) 222-9 =o sense 522-524
minimum and maximum, regions of-_- 531-532
Mate otis. te ete nee ea eee 532-536
rock constituentslost Dyce ss oases eee 516
stages of, discussion of -----_-.-----.--------:--- 535-536
surfaces produced by - ee ae 527-528
ultimatesproducts of2=-2—— se= = eeeee = 2 be1
volume changes in rocks by -.--------- 522-524
Webskyite, formation and sources of ._ 349, 350, 372,391, 402
Weed, W.'H:figure cited! from 2-2 -22-22. 2222-2 1214
on copper deposits in Mexico - 1056
on ore deposits of the Elkhorn mining district. 1214
on ores, genetic classification of _-...._--.-------- 1053
on present mineral vein formation -____ 1060, 1066, 1068
on secondary enrichment of ore deposits_-.____- 1188
on travertine depositsof Mammoth HotSprings_. 1032
Weed, W. H., and Pirsson, L. V., on tellurides asso-
ciated with gold ores of Judith and
~ Little Rocky mountains, Mont_____-- 1119
Weidman, Samuel, on agriculture as affecting
ground-water leveli-_-2--.- 2-2.) 2-22. 427
on deformation of rocks by combined flowage
EbaalibgKeibiRs) ass Sak eS 1009
on idevitrificationss= sess seers 247
on regeneration of mineral particles -- 705
Weight. See Specific gravity.
Welding in zone of anamorphism, extent of ______ 670-671
Wells, influence of, on ground-water level________ 427-428
Wells, artesian, section showing requisite conditions
30) ae per Pa ee engi a a 517
Wendt, Arthur F., on genesis of ore deposits _______ 1206
Wernerite, character, occurrence, formation, alter-
ations, andalteration productsof_ 311-314,
Whitney, J. D., on residual clay in upper Missis- oj
SippliVallley see eae tee ee 520, 1148
on volume changes in rocks by weathering_____ 524
Whitney, Milton, on rainfall and run-off, relations
414
on sand grains and clay grains in residual lime-
stone subsoil, number and surface
area Of ft ee ee ee RG ne 496
on soils, size of particles of__- --- 136,892
oni water injaridisoils: ss.) ssa mie eames 413, 418, 419
Whitney, Milton, and Means, T. H., on endurance
of plants to alkalies. -__-._____.__2__- 478
Whittle, C.L.,on enlargement of mineral particles. 644, 705
onischisticonglomerates=s__-- 2. ee eee 859
Wiechmann, F. G., on atomic weights <....-_-____- 936
Willemite, deposits of _--
Williams, G. H., dedication of monograph t
on deyitrifica tions = 22s 4-6 s- ee
on enlargement of mineral particles___-
on metasomatism
on origin of pegmatites ---.---_--------:+-
OnspegMatization: ssa se ase ee
Willis, Bailey, on Hamilton shale, origin of.
on limestone depositsin the ocean, forms
Winchell, A. N., on Duluth gabbro, content
Winslow, Arthur, on flexibility of rocks___.
on lead and zine in limestone ---_-------
Wisconsin, ground water in Potsdam sand:
ratorof How Of es =e ee a
lead and zine district of, conditions in, 1)
second concentration of ores
Wisconsin, University of, aid by students 0
Wolff, H. C.,on capillary flow of water,
tion of, by meniscus------ aes
on water in dry soil, motion of ------.--.-
Wolff, J. E., on albite crystals in Hoosac -
orientation of .._.._....._...!
on albite-pelite-gneiss of Hoosac Mount='
on porphyritic texture in schist _.-_-..__
on regeneration of mineral particles_.!.
on schist conglomerates. _..-...-.----.- -
photomicrograph cited from __-___--- id
Wolff, J. E., Pumpelly, Raphael, and D j4
on crystallographic orie 11
porphyritic feldspars -_---
Wollastonite, chemical and physical consis: =
formation of, from calcite and quari. \
changes involved in____-_._._-
formation of, from limestone, chem. a’
tion involved in-_...-.....-4
occurrence of ont
sources and modes of formation of... | 4
241, 242, 272-278, 280, 318, 372.9
Woods, T. H., Purington, C. W.,and Donethh
on occurrence of rhodonits
aD
Camp Bird mine, Colo-_--_-- Vis 5
Woodward, R.S., on mass of the atmosphe™e
on volume of lithosphere --_-_-.--------- b
Woodworth, J. B., Shaler, N. S.,and Foerste
on occurrence of graphitic cle)
Worms, effect of work of, on soils. -_----__---.
Wright, A. W., on occurrence of oxygen in met
ItGS ie Saas Se ee
Wyatt, Francis, on precipitation of phosph:
Yellowstone Park, deposits of travertine ar
Seriteinisssss sae eee
springs of, sources of water of___ 2
Yung, M. B., and McCaffery, R. S., on copy:
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Zeolites, alterations and alteration products
character, occurrence, formation, and
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chemical and physical constants of __
occurrence of
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XXVITT.
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XXXII.
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XXXIV.
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XLVII.
PUBLICATIONS OF UNITED STATES GEOLOGICAL SURVEY.
Mollusca and Crustacea of the Miocene formations of New Jersey, by R. P. Whitfield.
1894. 4°. 193 pp. 24 pl. Price 90 cents. .
The Glacial Lake Agassiz, by Warren Upham. 1895. 4°. xxiv, 658 pp. 38 pl. Price
$1.70.
Flora of the Amboy clays, by J. 8. Newberry; a posthumous work, edited by Arthur
Hollick. 1895. 4°. 260 pp. 58 pl. Price $1. i
Geology of the Denyer Basin in Colorado, by S. F. Emmons, Whitman Cross, and G. H.
Eldridge. 1896. 4°. 556 pp. 31 pl. Price $1.50.
The Marquette iron-bearing district of Michigan, with atlas, by C. R. Van Hise and W. S.
Bayley, including a chapter on the Republic trough, by H. L. Smyth. 1897. 4°.
608 pp. 35 pl. and atlas of 39 sheets folio. Price $5.75. 2
Geology of old Hampshire County, Massachusetts, comprising Franklin, Hampshire, and
Hampden counties, by B. K. Emerson. 1898. 4°. xxi, 790 pp. 35 pl. Price $L.90.
Fossil Medusze, by C. D. Walcott. 1898. 4°. ix, 201 pp. 47 pl. Price $1.50.
Geology of the Aspen mining district, Colorado, with atlas, by J. E. Spurr. 1898. 4°.
xxxv, 260 pp. 43 pl. and atlas of 30 sheets folio. Price $3.60.
Geology of the Yellowstone National Park, Part II, descriptive geology, petrography, and
paleontology, by Arnold Hague, J. P. Iddings, W. H. Weed, C. D. Walcott, G. H. Girty,
T. W. Stanton, and F. H. Knowlton. 1899. 4°. xvii, 893 pp. 121 pl. Price $2.45.
Geology of the Narragansett Basin, by N. 8. Shaler, J. B. Woodworth, and A. F. Foerste.
1899. 4°. xx, 402 pp. 31pl. Price $1. = ;
The glacial gravels of Maine and their associated deposits, by G. H. Stone. 1899. 4°.
xiii, 499 pp. 52 pl. Price $1.30.
The later extinct floras of North America, by J. S. Newberry; edited by Arthur Hollick.
1898. 4°. xviii, 295 pp. 68 pl. Price $1.25.
The Crystal Falls iron-bearing district of Michigan, by J. M. Clements and H. L.
Smyth; with a chapter on the Sturgeon River tongue, by W. 8. Bayley, and an intro-
duction by C. R. Van Hise. 1899.. 4°. xxxvi, 512 pp. 53 pl. Price $2.
Fossil flora of the Lower Coal Measures of Missouri, by David White. 1899. 4°. xi, 467
pp. 73 pl. Price $1.25.
The Illinois glacial lobe, by Frank Leverett. 1899. 4°. xxi, 817 pp. 24 pl. Price $1.60.
The Eocene and Lower Oligocene coral faunas of the United States, with descriptions of
a few doubtfully Cretaceous species, by T. W. Vaughan. 1900. 4°. 263 pp. 24 pl.
Price $1.10.
Adephagous and clavicorn Coleoptera from the Tertiary deposits at Florissant, Colorado,
with descriptions of a few other forms and a systematic list of the nonrhyncophorous
Tertiary Coleoptera of North America, by 8. H. Scudder. 1900. 4°. 148 pp. 11 pls.
Price 80 cents.
Glacial formations and drainage features of the Erie and Ohio basins, by Frank Leverett.
1902. 4°. 802 pp. 26 pls. Price $1.75.
Carboniferous ammonoids of America, by J. P. Smith. 1903. 4°. 211 pp. 29 pls.
Price 85 cents.
:-. The Mesabi iron-bearing district of Minnesota, by C. K. Leith. 1903. 4°. 316 pp. 33
pls. Price $1.50.
. Pseudoceratites of the Cretaceous, by Alpheus Hyatt, edited by T. W. Stanton. 1903.
4°. 351 pp. 47 pls. Price $1.
The Vermilion iron-bearing district of Minnesota, with atlas, by J. M. Clements. 1903.
4°. 463 pp. 13 pls. Price $3.50.
The Menominee iron-bearing district of Michigan, by W. S. Bayley. 1904. 4°. 513 pp.
43 pls. Price $1.75. > :
A treatise on metamorphism, by C. R. Van Hise. 1904. 4°. 1286 pp. 13 pls. Price
$1.50.
Ail remittances must be by MONEY ORDER, made payable to the Director of the
United States Geological Survey, or in cURRENCY—the exact amount. Checks, drafts,
and, tage stamps can-not be accepted. Correspondence should be addressed to
The Director, _
UNITED STATES GEOLOGICAL SURVEY,
Wasuineton, D. C.
October, 1904.
PUBLICATIONS OF UNITED STATES GEOLOGICAL SURVEY.
[Monograph XLVII.]
The serial publications of the United States Geological Survey consist of (1)
Annual Reports, (2) Monographs, (3) Professional Papers, (4) Bulletins, (5) Mineral
Resources, (6) Water-Supply and Irrigation Papers, (7) Topographic Atlas of the
United States—folios and separate sheets thereof, (8) Geologic Atlas of the United
States—folios thereof. The classes numbered 2 oi and 8 are sold at cost of publica-
tion; the others are distributed free. =
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