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Handbook of Rocks,

FOR USE

WITHOUT THE MICROSCOPE.

JAMES FURMAN KEMP, A.B., E.M.,

Professor of Geology in the School of Mines, Columbia University

New York.

glossary of the names of rocks _^^

AND OF other ^^ ^^

LITHOLOGICAL TERMS. ]v3£-n

GeoU Lib,

SECOND EDITION, REVISED.

NEW YORK:

D. VAN NOSTRAND COMPANY

23 MURRAY ANn 27 WARREN STREETS
1900

Press op

The New Era Printing Company,

Lancaster, Pa.

PREFACE.

The clear presentation of the subject of rocks to bej^inners is
not an especially simple undertaking. The series of objects is ex-
tremely diverse, and many unrelated processes are involved in
their production. In order not to confuse and bewilder students,
the teacher must emphasize the intelligible points and the recog-
nizable characters, avoiding alike distinctions that have their chief
foundations in past misconceptions, such as the time element in
the classification of igneous rocks, or that require microscopic
study to substantiate them. In the following pages the attempt
has been made to avoid these difficulties, and only to mention and
emphasize the characters that a beginner, properly equipped with
the necessary preliminary training in mineralogy, can observe and
grasp.

Some years of annually going over this ground have convinced
the writer that for this purpose we are not likely to reach a more
serviceable, fundamental classification than the time-honored one
of Igneous, Aqueous (or Sedimentary) and Metamorphic rocks.
They furnish not alone convenient central groups, but also prepare
the student for subsequent geological reading. With the Aqueous
have been placed the Eolian as a similar, although ver>^ minor di-
vision, so that fire, water and air, the ancient elementary agents,
are emphasized in their work upon the earth, and the fundamental
classification is based, as it should be, on method of origin. The
only illogical step involved is the placing of the breccias together
with the sediments, but breccias are so subordinate and go so con-
veniently with conglomerates, that it has been done.

The igneous rocks are the ones that present the greatest diffi-
culties to the learner. In the following pages, after a preliminary
exposition of principles, the very minor group of the volcanic
glasses is first taken up, because it is the simplest and because it
illustrates cooling from fusion most forcibly. Passing then through
the felsitic and porphyritic to the granitoid textures, rocks of in-
creasing complexity are one after another attacked. Analyses
have been freely used to illustrate the chemical differences of mag-

PREFACE.

mas, because in no other way can the varieties be fundamentally
described. Within fairly narrow limits the chemical composition
of the magma establishes the mineralogy of the rock.

The Aqueous and Eolian rocks arc not difficult to understand.
The Metamorj^hic are in many respects the most obscure of all,
but it is hoped that enough varieties have been selected and em-
phasized to serve for field use and for the reasonably close deter-
mination of the great majority of those that will be met in Nature.

Many names will be encountered in geological reading that are
not mentioned in the book proper. To explain them and to avoid
confusing the main text with unessential matter, they have been
compiled in a Glossary. Practically all the names for rocks will
be found there, and some related, geological terms. The chief
guide in its preparation has been the index of Zirkel's great Lehr-
biich der Petrograpliie, but not a few American terms are intro-
duced, which are not in it nor in Loewinson-Lessing's Pctrograph-
isches Lexikon, to which the writer is also greatly indebted. Other
works, English, French and American, have likewise been at
hand. One only needs to compile a glossary to appreciate what
numbers of unnecessary and ill-advised names for rocks burden
this unfortunate branch of science, and to convince one that the
philological petrographer comes near to being the enemy of his
kind.

So far as possible, technical words of classical derivation have
been avoided in the main work in favor of simple English, and for
the rocks described, American types have been especially sought
with which to illustrate the different species, because they are more
significant and accessible to readers on this side of the ocean. The
text except the glossary, appeared as a series of papers in the
School of Mines Quarterly during 1895-96.

J. F. K.

A^•<;I^sT, 1896.

NOTE TO THE SECOND EDITION.

In the preparation of the .second edition, but little change has
been made in the main text. The Glossary has, however, been
rewritten and brought up to date,

J. F. K.

Dkckmher, 1899.

TABLE OF CONTENTS.

Preface iii

Abbreviations vi

Chaiter I. — Introduction. Rock-forming Minerals. Princi-
ples of Classification . . i

Chapter II. — General Introduction to the Igneous Rocks i 2

Chatter III. — The Igneous Rocks, continued. The Glasses.
The Rocks whose chief feldspar is orthoclase. The Pho-
nolites and Nephclinc-Syenites 20

Chapter IV. — The Igneous Rocks, continued. The Dacites,

the Andesites and the Rocks of the Basalt Group. 39

Chapter V. — The Igneous Rocks, continued. The Diorites,
Gabbros, Pyro.xcnites and Peridotitcs. Ultra-basic Igne-
ous Rocks 47

Chapter VI. — Remarks in Review of the Igneous Rocks . 54

Chaffer VII. — The Aqueous and Eolian Rocks. Introduc-
tion. The Breccias and Mechanical Sediments not Lime-
stones 58

Chapter VIII. — Limestones. Organic Remains not Lime-
stones. Rocks Precipitated from Solution. Determina-
tion of the Aqueous and Eolian Rocks 70

Chapter IX. — The Metamorphic Rocks. Introduction. The

Rocks Produced by Contact Metamorphism .... 84

Chapter X. — The Metamorphic Rocks continued. The Rocks
Produced by Regional Metamorphism. Introduction.
The Gneisses and Crystalline Schists 93

Chapter XI. — The Metamorphic Rocks, continued. The
Rocks Produced by Regional Metamorphism. The
Quartzites and Slates. The Crystalline Limestones and
Dolomites. The Ophicalcites, Serpentines and Soapstones 106

Chapter XII. — The Metamorphic Rocks, concluded. The
Rocks Produced by Atmospheric Weathering. The De-
termination of the Metamoqihic Rocks 116

Glossary 121

Index 180

ABBRFA'IATIONS.

A. A. A. S., or Proc. Amer. Assoc. Adv. Sci. — Proceedings of the
American Association for the Advancement of Science.

Amer. Geol., or A. G. — American Geologist.

Amer. Jour, of Sci., or A. J. S. — American Journal of Science, some-
times called Silliman's Journal.

Bull. Geol. Soc. Amer. — Bulletin of the Geological Society of
America.

Bull. Mus. Comp. Zool. — Bulletin of the Museum of Comparative
Zoology, Harvard University, Cambridge, Mass.

Jahrb. d. k. k. g. Reichs. — Jahrbuch der kaiserlichen, koniglichen,
geologischen Reichsansalt, Vienna, Austria.

Jour, of Geol. — Journal of Geology, published at the University of
Chicago.

Neues Jahrb., or N. J. — Neues Jahrbuch fiir Mineralogie, Geologic
und PalKontologie, Stuttgart, Germany.

Quar. Jour. Geol. Soc, or Q. J. G. S. — Quarterly Journal of the
Geological Society of Ix)ndon.

Tsch. Mitth. — Tscherraak's Mineralogische und Petrographische
Mittheilungen, Vienna, Austria.

U. S. Geol. Surv. — United States Geological Survey, Washington.
The i)ublications are Bulletins, Monographs and Annual Reports.

Zeits. d. d. g. Ges. — Zeitschrift der deutschen geologischen Gesell-
schaft, Berlin, Germany.

Zeits. f. Krys.— Zeitschrift fiir Krystallographie, Munich, Germany.

A IIAXI) HOOK OI KOCKS

For Use Without the Microscope.

CHAPTER I.

Introduction. Rock-forming Miner.als. PkiNcirLEs
OF Classification.

A rock may be best defined a.s any mineral or aggregate of min-
erals that forms an essential part of the earth. The word mineral
is used because this is our most general term for all inanimate na-
ture, and while the lifeless remains of organisms often contribute in
no small degree to rocks, no rock is made up of those which are
still alive. In instances a single mineral forms a rock, but among
minerals this is the exception. B}' far the greater number are in
such small amount that they cannot properly be considered rocks.
Rock-salt, ice, calcite, serpentine, cemented fragments of quartz,
kaolin and a few others arc in suflficient quantity, but the vast ma-
jority of rocks consist of two or more. The condition that a rock
should form an essential part of the earth is introduced to bar out
those minerals or aggregates which, though important in them-
selves, are none the less insignificant as entering into the mass of
the globe. Thus the sulphide ores, while locally often in con-
siderable quantity, when broadly \ iewed are practically neglectable.
Yet this is somewhat arbitrary and there are single minerals and
aggregates that may properly give ri.se to differences of opinion.
The following pages err, if at all, on the side of demanding that
the amount should be large. A rock must also have an individual
character, sufficient to establish its identity with satisfactory sharp-
ness. The species cannot be marked off with the same definition
as in plants, animals or minerals, and there is here again reason-
able opportunit)' for differences of opinion as to the limits which
should be set, some admitting of finer distinctions and greater multi-
I

2 A HAND HOOK OF ROCKS.

plicity of species than others ; but, after all has been said, there
should be a well marked individuality to each rock species such that
any careful and qualified obser\cr may readily see. Too great re-
finements and too minute subdivisions ought to be avoided. The
determining conditions of species will betaken up at greater lengtli,
when the preliminaries of classification have been set forth, but it
must be appreciated that the jjoint of view is also a most important
factor. Thus if one is studying the geology of a district with close
accuracy, and is tracing out the history and development of its
rocks with microscopic determinations and descriptions of min-
erals and structures which may be minute, finer distinctions will
naturally be drawn than those that suggest themselves to one who
is engaged in ordinary' field work or in mining or engineering
enterprises. It is for the latter class that these pages arc prepared
and throughout the descriptions and classification here given, the
necessary limitations and the practical needs of such observers are
always kept in mind. Textural and mineralogical distinctions are
alone emphasized where easily visible on a specimen, although
never made contradictory of principles of origin and classification
that could be carried to greater length and subdivision.

Rocks embrace matter in a great variety of structures and con-
ditions. While in general we picture them to ourselves as solid,
yet under the terms of our definition, we have no logical right to
bar out liquids or even gases. The physical condition may vary
with ordinary temperatures. Thus we cannot reject ice as an ex-
tremely abundant and important rock, and yet its solid condition
results from water with a moderate loss of heat, and at ordinary
temperatures the same molecules may be in a liquid or gaseous
state. All that we know of volcanoes indicates that liquid, molten
magmas exist for long periods deep in the earth, yet they are
none the less rocks becau.se of their liquidity. In general, how-
ever, rocks are solid, and gases or liquids (except water) de-
serve no further attention. In texture rocks may be loose and in-
coherent as in sand, gravel, volcanic dust and the like, or they
may be extremely dense, hard and solid, as in countless familiar
examples. This solidity or massiveness has its limitations, for all
observation and experience show that what are apparently solid
masses are really broken up by multitudes of cracks into pieces
of varying size. All quarries and mines have these, and they may
aid or annoy the operators according to the purpo.ses of excava-

INTRODrCTORY. 3

tion. Tlic\- will as^Min he rcfciicd to at length. Unless too deep
within the earth, rocks are also in all cases permeated with minute
pores and spaces that admit of the penetration of water and other
lic|uids, especially if under pressure.. These are important factors
in terrestrial circulations.

Tnr. ClIKMK.M lu.EMENTS I .Ml'OKTANT IN RoCKS.

The chemical elements reall)' important in rocks arc compara-
tivel}' few. antl arc those which are most witlespread in nature.
The best estimate that has been made is that of F. W. Clarke, in
Bulletin -^S, of the U. S. Geological Survey, pp. 34-43. The crust
to ten miles below sea level and the air and the ocean are embraced.
The composition of the solid crust is reached by averaging up
analyses of igneous and crystalline rocks, 880 in all; 321 from
the United States, 75 from Europe, 486 from all quarters. Ig-
neous rocks being tiic source of all the others, furnish the best
data for the general cliemistry of the globe. The composition of
the ocean is then averaged in with that of the rocks, on the basis of
7% for the former and 93*% for the latter, witii a further addition
of 0.02% for the nitrogen of the atmosphere. Other ingredients,
as the oxygen of the air, arc less than o.ox^/o and are neglected.

49.98

2.28

0.09

.Si

25 30

2.23

0.07

7.26

0.94

0.04

Ft-

5.08

I'i

0.30

0.03

35»

0.21

0.02

2.50

CI, Br

0.15

O.OI

The remaining elements may be omitted in this connection,
although, as a moment's reflection will show, the\' include all the
common metals except iron and manganese.

There is good ground for believing that toward the centre of the
earth the metallic elements become much more abundant, and
that near the centre some of the heaviest known are in excess, but
these inferences, however well-based, concern materials far beyond
actual experience, and of no great moment in this connection. As
regards rocks we have to deal with the outer portions of the globe,
to which we are accustomed to refer as a crust. This term is not
meant to indicate anything as to the condition of the interior, but
merely its exterior as contrasted with tlie inner parts.

The chemical elements above cited arc combined, except per-
haps in volcanic glasses, in the definite compounds that form

4 A IfAXDIiOOK OF ROCKS.

mineral species. These compounds change, more or less, in the
course of time, under the action of xarious natural agents, chief of
which are water, carbonic acid and oxygen, but at any particular
stage, however complex the rock may be, it is made up of definite
chemical compounds, though we may not be able to recognize
them all. The most important compounds are not numerous and
are practically limited to the following : silicates, oxides, carbo-
nates, sulphates, chlorides, and of far inferior moment phosphates,
sulphides, and one native element graphite.

As a broad conception in speaking of these compounds it is in
many respects advantageous to have the igneous rocks primarily
before our minds, because as stated above they are the sources of
the others. In taking up the minerals the purpose here is to em-
phasize their chemical composition and relative importance, not to
describe them as would be done in a text-book on mineralogy so
as to enable a student to recognize them, for such preliminary
knowledge is here assumed. Our purpose is to make prominent
the chief chemical compounds entering into the earth, and to pre-
pare the way for a true conception of the range and relations of
its constituent rocks.

The Silicates.

The Silicates are grouped as follows : the feldspars and feld-
spathoids ; the pyroxenes ; the amphiboles ; the micas ; olivine.
*rhe last four groups are often collectively called the ferro-magne-
sian silicates. Zircon and titanite conclude the list of those impor-
tant in igneous rocks. In addition there are a number of others
that are especially characteristic of altered or metamorphosed
rocks, viz : epidote, scapolite, garnet, tourmaline, topaz, andalusite,.
cyanite, fibrolite or sillimanite, and staurolitc. Finally a few hy-
drated silicates complete the list.

The Feldspars and their related minerals are all double sili-
cates of alumina and an alkali or an alkaline earth or both. We
.speak of them as alkali-feldspar, potash-feldspar, soda-feldspar,
lime-soda feldspar, etc., based on this fact. They are generally
grouped as orthoclase, representing monoclinic feldspar with its
two cleavages at right angles ( hence the name ), and as plagioclase
or triclinic feldspar, with oblique cleavages^ and one striated cleav-
age plane. Orthoclase is chiefly KAlSijO^, but Na replaces
more or less of the K, without affecting the crystal system. Suffi-
cient amounts of soda are however capable of changing the system

IXTKOnrC/OKV. 5

to triclinic ami the fcklspar is callcil anortIu)clasc. Microclinc is
also a triclinic variety of potash feldspar, with a cleavage angle
slightly less than a right angle, but with peculiar and character-
istic optical properties, which are chietly of moment in micro-
scopic work. The clear, unclouded orthoclase of the later volcanic
rocks is often called sanidine. It does not differ essentially from
the orthoclase of the oUler rocks, and the distinction based on
geological age is obsolete, but as the terms are still used in the
literature of the subject it is well to understand them.

The plagioclase feldspars embrace a practically unbroken
series from pure soda-alumina silicate in albite, NaAlSi^O^, to
pure lime-alumina silicate, anorthite, CaAl^Si^O^. Various mix-
tures of these two molecules give the intermediate species, but the
two on which special stress is ordinarily placed are oligoclase,
with soda in excess and hence called soda-lime feldspar, and labra-
dorite with lime in excess and hence called lime-soda feldspar. If
we represent the orthoclase molecule, KAlSi.,0^ by Or ; the
albite molecule, NaAlSi.,0^ by Ab, and the anorthite, CaAl^Si20g
by An, all the intermediate feldspars can be algebraically ex-
pressed. Thus anorthoclase lies between Ab^Or,, and Ab^ j,(3r, ; al-
bite embraces those from Ab through Ab^An,; oligoclase, Ab^-Anj,
through Ab.,An, (the intermediate mi.Ktures Ab^An,^ through
Ab^Ang are called andesine); labradorite includes Ab^An, through
AbjAn, ; bytownite Ab,An., — Ab,An,. ; anorthite AbjAn^, to An.
This conception of feldspars as isomorphous mixtures of molecules
is a very valuable one and by determining specific gravity, optical
properties and chemical composition, one or all, the different
members can be identified. Practically, however, in the ordinary
determination of rocks, aside from microscopic work we are forced
by the difficulty of distinguishing the intermediate varieties, into
the general use of orthoclase and plagioclase, and rely on the
presence or absence of the striations peculiar to the basal cleavage
of the latter in distinguishing between the two, but of course ex-
perience and familiarity with the general characters and associa-
tions of minerals in rocks often enables one to determine very
closely the minor varieties. We would naturally look for ortho-
clase, albite and oligoclase in acidic rocks or those high in silica,
while in basic rocks we would expect tho.se near the anorthite end.

All the feldspars have ver>' similar crystal forms when these are
developed, as they occasionally are in rocks. When the\' are

6 J J/AXDBOOK OF ROCKS.

small and irrci^ailarly bounded, cleavage faces should be sought
out and examined with a pocket lense. It is interesting to note
that only in igneous rocks do we obtain crystals uniformly de-
veloped on all sides, for only in a fused magma do they swim and
grow without a fixed support.

The word feldspar is spelled by English writers " felspar," but
among Americans the more correct form, based on the etymology,
is employed, following the German original " Feldspath."

h'KLDsi'ATHOins. With the feldspars are placed two other im-
portant and closely related minerals, nepheline and leucite, to
which may also be added one that is quite rare, melilite. Ncphc-
linc is an hexagonal soda-alumina silicate 4Na20,4Al.,0.,,9SiO.„ in
which some of the Na^O is replaced by K.,0 and CaO. It ap-
pears in a subordinate series of igneous rocks that are rich in
soda. Leucite is an isometric potash silicate, K.,0,Al.,0.,,4SiO^,
with a little Na^O replacing part of the K^jO. It appears as an
important rock-making mineral in the igneous rocks of ten or
fifteen localities the world over, and is therefore of very limited
distribution. Melilite is an extremely basic lime-alumina silicate,
i2CaO,2Al,03,9Si02, and appears in a few rare basalts.

Reference may also be made to sodalite, no.sean and haiiyne
which are occasionally met, but which are chiefly of microscopic
interest.

The feldspars, together with the fcldspathoids nepheline and
leucite, are the most important of the rock-making minerals in
their relations to the classification of rocks.

In order to have a standard series of analysis with which to
compare those of rocks later given, the following table is inserted
of theoretical feldspars and fcldspathoids. The relative amounts
of the several oxides will suggest the extent to which the mole-
cules are present in any rock whose analysis is known :

f)K

All

OKTlIOCI.ASi;

AI.IIITK

ANOKTHITK

NEPIIP.I.IN'i:

i.Rr< iTP.

MBLILITR

KAISijO,

NaAlSijO,

CaAljSijO,

NagAl,Si„0,^

KAlSijO,

Ca,,Al«Si,0„

SiO,

64.7

68.6

43- 1

45.0

55.0

38.1

Al./)3

18.4

19.6

36.8

34-3

235

M-S

K,0

1 6. 9

. 21.5

Na,0

. 1 1.8

20.7

CaO

20.1

47-4

Recalling w hat has been said about the replacement of the al-
kalies by one another, and that we never meet any of these min-
erals chemically pure, according to the formulas above given, and

fXTRODCi TORY

makirif; suitable allowance for this replacement, we may still ap-
preciate that orthoclasc and albitc, bcin^ high in silica, favor
acidic rocks, and the others beinj; low in silica, basic ones ; that
ncpheline implies a magma rich in alumina and soda, leucite one
rich in potash, ami mclilite one low in silica and alumina, but high
in lime.

The Pykoxknks and the Ami'UIHoi.Ivs are best described to-
gether. Kach embraces a series of conijiounds of the same chemical
composition, differing only in physical and optical properties. As
the table shows, they vary from magnesia silicate through a .series
of lime and lime-alumina silicates, with an iron silicate generally
present. All the p)TOxenes have a prismatic cleavage of nearly
90° (87° 10' or thereabouts), while the amphiboles cleave along a
prism of nearly i 20° ( i 24° 11').

COMPOSITION

I'YRO.KENE.

.XMI'IIUIOIK.

SYSTEM.

r MgOSiOj-)
\l-eOSR).j )■

Knstatite
Hronzite
Hypersthene

Anthophyllite

- ( )rthorhomhic

CaMgSi,0„

'Diopside

Tremolile

CaMg.Si./), 1

Malacolite
(Diallage)

Actinolite

CaFeSi/),,

CaMgSi.,U, 1

CaFeSi.,()«

- Monoclinic

Mg.Al.>iO,

Augite

Hornblende

MgFe.,SiO,

Fe.AljSiO. .

NaFeSi.,Og

-Acmite
.I'.girine

.\rfvedsonile

Under the orthohombic pyroxenes cnstatite has least of the
molecule FeOSiO... /. c, FeO less than 5'/;, bronzite has FeO
less than 14^^' and hypersthene the higher values. The increase
brings about a darker color and changed optical properties. The
orthorhombic pyroxenes are much less frequent than the mono-
clinic, but are of wide distribution, especially h\persthene. The
orthorhombic amphiboles are of minor importance and are but
seldom met.

The light-colored monoclinic pyroxenes are almo.st pure lime
magnesia silicates, and are called diopside. They are chiefly found
in crystalline limestones. As iron increases, they pass into malaco-
lite, which may also contain small amounts of the aluminous mole-
cules. Neither of these pyroxenes is of sjx;cial abundance as a rock
maker. When pinacoidal cleavages around the vertical axis appear
in addition to the prismatic ones in pyroxenes of the general compo-

8 J /lAXDnOOK OF KOCA'S.

sition of mahicolitc thc)' arc called diallage and arc imi)ortant in
some igneous rocks. But the chief rock -making pyroxenes arc
the dark aluminous, ferruginous ones, which are called augite, and
these are among the most important of all minerals in this con-
nection. The igneous rocks rich in soda, in which nepheline is
common, are thc ones that contain acmite and icgirine, thc soda-
pyroxenes.

Thc monoclinic amphiboles are closely parallel in their occur-
rence and relations to the pyroxenes. Tremolite is met in crystal-
line limestones. Actinolite may form schistose rocks by itself, but
much the most important variety is hornblende, the aluminous
variety corresponding to augite. The soda amphibole, arfvedso-
nite is rare.

The pyro.xenes and amphiboles are often collectively referred to
as thc bisilicatcs, thc oxygen of the base being to the oxygen of
the silicon, as shown in the first two formulas, in the ratio of 1:2.
It is also interesting to note that man)' blast furnace slags are cal-
culated on thc basis of thc formulas for pyroxene.

The Micas. The commonest of these is biotite and its distribu-
tion is very wide. It is a complex silicate involving magnesia in
large amounts and is often called magnesia mica for this reason.
The other bases are hydrogen, potassium, iron and aluminum, and
the general formula is (H,K)^ (Mg, Fe )^ Al,Si.,0,2. It is impor-
tant in its bearings on the classification of rocks. Phlogopite is
of related composition but is almost entirely limited to crystalline
limestones. Muscovite, from its richness in potash, is often called
potash mica. It is widespread in granites and schists and as an
alteration product. Its general formula is ( H, K ) AlSiO^.

Olivine, the unisilicate of magnesium and iron, 2(Mg,Fe)0
SiO^, completes the list of silicates which arc of thc first order of
importance in igneous rocks. The above name is usually em-
ployed in preference to chrysolite. Olivine is practically limited
to basic igneous rocks.

Zircon and titanite are interesting microscopic accessories, but as
rock-making minerals they are seldom visible to thc naked eye.

Along the contacts of intrusions of heated igneous rocks, and in
regions where thc original sediments have undergone strong dy-
namic disturl)anccs, with oftentimes attendant circulations of waters
more or less heated, a series of characteristic silicates is in each
case developed. Garnet, tourmaline, topaz, andalusite, scapolite

/XTROPi'Cn^RY. 9

and bidtitc arc cspcciall}' characteristic of the former ; ^'arnet,
cwuiilc, siUinianite, staurolite, biotite, and muscovitc of the latter.
I'^pidote results when feldspars and the ferro-ma^ncsian silicates
uniler^o decay and alteration in proximity, so that the solutions
afforded may react on one another.

The hydrated silicates of chief importance include a magncsian
series, embracing talc and .serpentine, which result from the ferro-
mai^nesian minerals; a ferrui^inous aluminous series, with much iron
oxide, usuall)' coUectivcl)' called " chlorite," and derived from the
iron-alumina silicates ; and fiiiall>- kaolin, the hvilrated silicate of
alumina that is chiefly \'iekled by feldspar. Zcolitic minerals are
also often met, but rather as vein fillings and in ain\'gdaloidal
cavities than as important rock makers.

The oxides include quartz and its related minerals chalcedon)'
and opal, and the oxides of iron — magnetite and hematite and the
hydrated oxide, limonite. With these should be mentioned chro-
mite and ilmenite (mcnaccanitc), which arc of minor importance.
Quartz is found in all rocks high in silica. Magnetite and hema-
tite are at times almo.st abundant enough to constitute rocks
themselves. They favor igneous and metamorphic varieties when
present in a subordinate capacit)'. Magnetite is the most wide-
spread of all the rock-making minerals. Limonite is an altera-
tion product. Chromite is practically limited to the basic igneous
rocks and their serpcntinous derivatives. Ilmenite is a common
accessory in many igneous rocks.

The carbonates are calcite, dolomite and siderite, all three bcmg
really members of an unbroken series from pure carbonate of cal-
cium, through admixtures of magnesium carbonate to pure magne-
site on the one hand, or with increasing carbonate of iron to pure
siderite on the other. The sulphates of moment are anhydrite and
gypsum, the latter the hydrous, the former the anh\drous salt of
lime. The one chloride is the sodium chloride, rock salt or halite,
and the one phosphate is apatite, which is a phosphate and chloride
of lime. The two sulphides of iron, pyrite and p)'rrhotite are the
only ones sufficiently widespread to deserve mention, and graphite
is the chief representative of the elementary substances, although
native sulphur might perhaps with propriety be also mentioned.

We speak of minerals as essential and accessor}', meaning b)' the
former term those that constitute a large part of the rock, and that
must be mentioned in the definition ; bv the latter those that are

lo A HAXDBOOK OF ROCKS.

present in small amounts or that are more or less fortuitous,
Primar)' minerals are those that date back to the origin of the rock,
as for instance the ones that crystallize out from a molten magma
as it solidifies ; secondary minerals are formed by the alteration of
the primary. Feldspars, pyroxene and hornblende arc good illus-
trations of the former ; hydrated silicates of the latter.

Tiiii Prlncii'LE-s Underlying the Cl.assification of Rock.s.

Rocks must of- necessity be classified in order to place them in
their natural relations so far as possible and to allow of their syste-
matic study. At the same time they arc so diverse in their nature
and origin that the subject is not an easy one. They must however
be grouped on the basis of their structures and textures ; or of their
mineralogical composition ; or of their chemical composition ;
or of their geological age ; or of their method of genesis. One or
several of these principles enter into all schemes. On the basis of
the first, rocks have been classified as massive and stratified ; as
crystalline and fragmental or clastic, each with subdivisions on one
or more of the other principles. On the basis of the second we
have had those with only one mineral (simple rocks) and those with
several (complex rocks). The chemical composition as shown by
a total analysis (bausch-analysis) without regard to special min-
eral components is of almost universal application in a subordinate
capacity. It must be regarded in the group of igneous rocks and
in those that are deposited from solution, chiefly highly calcareous
or highly siliceous rocks. The principle of geological age was
formerly much valued in connection with the igneous rocks, but it is
a thoroughly exploded one. The principle of origin or genesis is
the most philosophical of all as a fundamental basis, but while in
the greater number of cases it may be readily applied there are
some puzzling members whose entire geological history is not well
understood. Very early in the development of the subject it
was appreciated that there were two great, sharply contrasted
groups, according as they had consolidated and crystallized from a
molten condition or had been deposited in water either as mechan-
ical fragments or as chemical precipitates. Widened observation,
especially in arid and sandy regions, has added to these a less
important group of those whose particles have been heaped to-
gether by the wind. They are called the eolian rocks and will be
taken up together with the aqueous, with which they have many

/.\ /A( '/'/ ( /{'A')'. II

points in common. Two grand divisions have therefore been es-
tablished, the igneous, on the one hand, and the ac|iieous and
cohan on the other.

Even a hmited fiekl experience soon convinces the observer that
many rocks are encounteretl which cannot be reatlily placed with
either of the two great classes whose origin is comparatively
simple. Rocks for instance are met having the minerals common
to the igneous, but with structures that resemble those of sediments
in water.

Great geological disturbances, esJ3ecially if of the nature of a
shearing stress, may so crush the minerals of any igneous rock and
stretch them out in bands and layers as to closely imitate a re-
crystallized .sediment. The b^^king action of igneous intrusions on
fine sediments, such as clays and muds, makes it difficult for an
observer, without the aid of thin sections and a microscope to say
where the former sediment ends and the chilled magma begins.
Sediments buried at great depths and subjected to heat and hot
water become recrystallized with their chemical elements in new
combinations. These excessively altered rocks have been often
grouped into a separate, so-called " metamorphic " division, which
was a sort of " omnibus " of unsolved geological problems. This
metamorphic group is useful, and the term is a common one in the
science, but wherever possible it is well to appreciate the true
affinities of its members which though altered are still referable to
their originals.

In the following pages these three divisions will be adopted, but
the metamorphic group will be reduced to a minimum by remark-
ing, in connection with descriptions of the unaltered, changes that
igneous and aqueous undergo.

We take up, therefore, in this order :

A. The Igneous Rocks.

B. The Aqueous and Eolian Rocks.

C. The Metamorphic Rocks.

CHAPTKR II.

General Introduction to the Igneous Rocks. Classification.

The Igneous rocks are first treated because they have been the
originals, according to our best light, from which all the others
have been directly or indirectly derived, for either from the frag-
■ ments, as afforded by their decay, or from the mineral solutions,
yielded by their alteration, possibly in the primitive history of the
globe, all the others have been produced.

The igneous rocks occur in dikes, sheets, laccolites, bosses
and vast irregular bodies, for which we have no single term.
Dikes (sj)elled also dykes) have penetrated fissures in other rocks,
and have solidified in them. They therefore constitute elongated
and relatively narrow bodies, of all sizes, from a fraction of an
inch in thickness and a few feet in length, to others a thousand or
more feet across and miles in length. Sheets are bodies of rela-
tively great lateral or horizontal extent, compared with their thick-
ness. They are either surface flows, which may be afterwards hur-
ried or else are intruded between other strata. In the last case
they are often called laccolites, especially if lenticular in shape.
Roughly cylindrical masses, such as might chill in the conduit of a
volcano are called necks. Irregular, projecting, rounded bodies
are called bosses. The enormous masses of crystalline rocks like
granite that often cover hundreds of square miles, and that fre-
quentl}' appear to have fused their way upward by melting into
their substance, overlying rocks, are called batholites. They have
in most if not all instances, only been uncovered by erosion, for
the name means a rock belonging to the depths of the earth. It
will be later brought out that the character of the occurrence,
whether as dike, surface flow, intruded sheet, or batholitc, has an
important influence on the texture.

IGNEOUS ROCKS. CLASSIFICATION. i;

Igneous rocks arc characteristically massive, as contrasted with
the stratified structure of the sedimentary, and the term massive
is sometimes employed as a synonym of igneous. Other synony-
mous terms are eruptive and anogene, both meaning that the rocks
have come up from below. Many years ago the distinction was
made between those that have crystallized dcc\i within the earth,
the plutonic, and those that have been poured out on the surface,^
the volcanic. The words intrusive and effusive or extrusive have
been emplo\'ed in much the .same way. Ik'tween surface flows
and deep-seated masses (batholites) and their characteristic tex-
tures, every gradation is to be expected and is met, and an inter-
mediate group has even been established by some writers for rocks
that have cooled as intruded sheets and dikes. This three-fold
distinction is not carried out here, the two extremes being believed
to illustrate the varieties .satisfactorily when accompanied by aux-
iliary remarks on the intermediate types.

\Vc are tending more and more to employ the word structure
for the larger features of a rock, as for instance a massive
structure as against a stratified, while the smaller features are de-
scribed as textures, as for instance a glassy texture, a porpii}Titic
or a granitoid, terms that refer to characters which may be seen
even" on a small fragment. Glassy texture, as the name implies, is
that of glass or slag and has no definite minerals. It results when
a molten magma is so quickly chilled that the minerals have no
opportunity to form. Porphyritic implies larger crystals, well
formed or corroded and rounded, embedded in a more finely crys-
talline, or even in a glassy " groundmass." There may be several
sizes and kinds of these crystals, and because of their prominence
in the rock they are called phenocrysts, /. c, apparent crystals,
but phanerocryst is better etymologically. If a magma crystal-
lizes as a mass of very fine or microscopic crystals without pheno-
crysts, its texture is described as felsitic. A granitoid or granular
texture has the component cr\'stals all about the same size, and
very seldom pos.sessing their own crystal boundaries. Strictly
speaking, there is no groundmass in granitoid rocks. Sometimes
from a local abundance of mineralizers (as later explained), grani-
toid rocks have small cavities into which the component minerals
project with well bounded crystals. Such are called uiiarolitic.

Textures in igneous rock are due to several factors that have in-
fluenced the development of the magma during its consolidation.

14 A HANDBOOK OF ROCKs.

The most important are chemical composition, tenijjcrature, rate
of coohn^, pressure and tlie original presence of dissolved vapors
called mineralizers. The fusibility varies with the chemical com-
position. The most acid or siliceous magmas, /. c, those with 65-
75^ SiOo are least fusible. When molten they are viscid and ropy.
The fusibility increases with the decrease of silica down to the
basic rocks with 40 to ^o'jo SiOj. The ultra-basic rocks which
graduate into practically pure bases, as in some rare, igneous iron
ores are less fusible. This statement that acid rocks are least fusi-
ble often puzzles a student who is familiar with blast furnace practice
and the composition of slags, in which the most siliceous are re-
garded as most fusible, but slags themselves, as a comparison of
analyses will readily show, are to be paralleled with basic rocks.
The importance of the fusibility as regards textures lies in the
fact that the highly siliceous quickly chill, become ropy and
freeze. They therefore especially yield glasses. The easily fusi-
ble remain fluid at lower temperatures, crystallize out as min-
erals to a greater degree and seldom 'yield glasses. They flow
farther from the vent and tend to develop the porphyritic or
even a variety of granular texture. The influence of tcmpcraturt
has been partly outlined in speaking of composition, but it will
readily ajipear that in its progress to the surface a basic magma
might stand for a considerable period at a temperature of flu-
idit\', whereas an acid magma in the same situation would con-
solidate. The I'atc of cooling \s important. Cooling magmas tend
to break up into minerals. As a general thing it requires a very
quick chill to prevent their formation. Hence it is that even vol-
canic glasses which appear to be perfect glass to the eye are shown
to be full of dusty, microscopic minerals under the microscope.
Volcanic glasses are chiefly found on the outer portions of flows or
dikes, but instances arc known where sheets of them are very thick,
as at Obsidian Cliff in the Yellowstone Park. The common experi-
ence with lavas is that certain crystals develop to notable size, it
may be an inch or more in diameter, while the magma stands be-
neath the surface, in circumstances favorable to their formation.
These are then caught up in the moving stream and brought to the
surface or near it where the final consolidation takes place and fixes
them in the so-called groundmass. A quick chill makes a fine-
grained groundmass when not a glassy one, and slow cooling yields
one more coarsely crystalline, but in the final cooling or consoli-

/GW'JiOl S A'( )( /\S. Cf. ASS/I- It . / 1 /OX. \ 5

ilatioii at 01 near the surface, crystals arc seldom if ever developed
of a si/c C(MiiiiKMisiiral)lo with those formed in the depths. I^y this
process of partial crwstalli/atioii below and final consolidation on
the surface, the |)orph\Titic texture is almost always developed,
but in strict accuracy it should be stated that cases are known
where phenocrysts apjjcar to have formed in lavas after coming to
rest. Magmas also flow to the surface with no phenocrysts (or
" intratelluric " crystallizations) and then consolitlate not as glass,
but as finely crystalline aggregates, practically all groundmass.
The resulting texture is called felsitic:

Pn'ssmr, such as is developed upon a magma deep within the
earth or during its passage to the surface is thought to exert an
inlluence upon the formation of many phenocrysts and to be ncces-
sar\' for their development. Dissolved vapors, such as steam,
lu'drofluoric and boracic acids arc also important factors. Acidic
magmas are more general 1)' pro\idcd with them than basic, and
where locally abundant they lead to variations both in the mineral
composition and texture at different places in the consolidated
rock. They may prevent the development of glass, and cause a
sheet such as Obsidian Cliff, in the Yellowstone Park, to present
alternations of glassy and stony layers, the latter being formed of
microscopic crystals.

A word should be added about the chemical composition of
rocks and about the interpretation of analyses before the rocks
themselves are taken up. The analyses are reported in percent-
ages of oxides, for the most part, and these arc arranged in the fol-
lowing series, SiO,, Al^O,, Fe^Oj. FeO, CaO, MgO, Na,0, Kfi,
H.,0. In order to have anhydrous materials, it is customar\' to
ignite and determine loss on ignition. This loss includes both
H2O and CO.^ and where large throws uncertainty over the relations
of the elements left behind, because of the evident advance of deca)-.
Small percentages of other oxides are quite invariably present and
in refined work are determined. These are TiO,, MnO, NiO,
BaO, SrO, S, CI, ^.^0^, I-i^^O, and even rarer ones. They are how-
ever always in very small quantity. We often recast an anal-
ysis, by dividing, as in the determination of a mineralogical
formula, each percentage by the molecular weight. W'e thus get
numerical molecular ratios which indicate the relative numbers of
individual molecules and enable us to draw conclusions as to the
way they are combined with one another in the component

1 6 J HANDBOOK OF ROCKS.

minerals. Variations in chemical composition entail variations in
resulting minerals, but it is also true that the same ma<jma, if
consolidating under different physical conditions of heat, pressure,
etc., at different times may yield somewhat different minerals, for
instance, hornblende instead of augite, or vice versa. A study of
analyses soon makes one more or less familiar with the minerals
that would necessarily result. The more important points are the
amounts of silica, of the alkalies and alkaline earths, of iron oxides
and of alumina. For instance, as a rule, only magmas high in
SiO.^ yield quartz, for otherwise it would combine with the bases.
Much K^O is necessary for an orthoclase or leucite rock, but much
Na^O for one with nepheline. MgO in relatively large amount is
required to yield olivine or an orthorhombic pyroxene, and when
feldspars drop away and rocks become very basic we expect high
CaO, MgO, FeO, Fe^O^, and low SiO,. In rocks tested for pur-
poses of building, the percentage of sulphur is important and very
little should be present. It occurs in some form of pyrites, which
by its decay generates sulphuric acid and destroys the stone or
stains it with limonite. It should never reach i '/.

The specific gravity or density of a rock is an important feature
in its practical bearings. While it may in ice be less than i, and
in coals and certain carbonaceous deposits drop as low as 1.25, and
in very porous sandstones reach 2.25, yet in the common rocks it
is seldom below 2.50, and ranges from this to over 3.00. Granites
are usually about 2.65, but basic rocks, rich in iron, attain to the
higher limits, even above 3.0. Determinations are important in
those rocks used for building purposes, and are expressed in
pounds per cubic foot.

Of recent years we have come to regard molten magmas as es-
sentially solutions of some compounds in others, and to appreciate
that solutions do not cease to be such, even when the temperature
is very high. It results from this that the crystallization of the
minerals of an igneous rock takes place from the magma as this
in its cooling successively reaches a point of .saturation for the salt
in question. The least .soluble thus .separate the earliest of all,
and then the others in order ; but as the pressure under which
they rest is also a factor, and this is subject to variation, as indeed
is the temperature during movement to the surface, one mineral's
period of formation may overlap another's more or less. The
f)rclLr of formation will l)e determined by the laws of thermo-

IGNEOUS ROCKS. CLASS/FICAT/Oy. i;

dynamics and necessarily the ininnal that develops the most
heat in crystaMizing will be the first to crystallize. As a general
rule, the relations of the minerals in rocks show that the earliest
to form are apatite ; the metallic oxides (ma^Mietite, ihnenite,
hematite) ; the sulphides (pyrite, pyrrhotite) ; zircon and titanite.
These are often called the j^rouj) of the ores. Next come the
ferromagnesian silicates, olivine, biotite, the pyroxenes and horn-
blende. Next follow the feldspars and feld.spathoids, ncpheline
and leucite, but their period often laps well back into that of the
ferromagnesian group. Last of all, if any excess of SiO.^ re-
mains, it yields quartz. In the variation of the conditions of
pressure and temperature just referred to, it may and does often
happen that crystals are again redissolvcd in the magma, or are re-
sorbed, as it is called ; and it may also hajipen that after one series
of minerals, usually of large size and of intratcUuric origin, has
formed, the series is again repeated on a small scale as far back as
the ferromagnesian silicates. Minerals of a so-called second gen-
eration thus result, but the)- are always much smaller than the
phenocr}'sts, and are characteristic of the groundmass.

It results from what has been stated that the residual magma is
increasingly siliceous up to the final consolidation, for the earliest
crystallizations are largely pure oxides. It is also a striking fact
that the least fusible minerals, the feldspars and quartz, are the
last to cr}'stallize.

In the matter of the study and determination of a rock species,
especially of an igneous rock, it is desirable to procure materials as
fresh and unaltered as possible. If feldspars have all changed to
kaolin and clay, and if ferromagnesian silicates are merely chlorite
or serpentine, and if secondary quartz, calcite and the like have
formed, it is \ery difficult if not impossible to draw correct or even
well-grounded inferences. Rocks near ore bodies are very often
of this character.

Bearing in mind these differences of texture and the causes of
them, it is possible to group igneous rocks in such arrangement
that they can be intelligently studied, and identified with a reason-
ably close approximation to the truth. It should be appreciated,
however, that with finely cr>'stalline rocks, whose components are
too small for the unassisted eye, the microscope is the only re-
source, and with this as an aid much greater subdivision can be

Jo'SC.

.; IfAXDPyOOk' OF ROCKS.

IGXEOUS ROCKS. CLASSIFICATION. 19

attained. The object licre in view is to limit the discussion purely
to the study without the microscope.

The scheme of classification of the igneous rocks has three
principles underlying it, viz : te.xturc, mineralo^ical composition and
chemical composition. The textures are five: glas.sy, felsitic, por-
phyritic, fragmental and granitoid, and the table is arranged from
top to bottom so that they come in this order. The arran^^ement
is adopted because it brings the glassy which are the simplest of
all rocks at the outset, where they can be best taken up by the
beginner. The rocks arc arranged from left to right on a mincral-
ogical principle, and chiefly on the basis of the predominant feld-
spar present, as is the usual custom. This also makes possible a
general succession from those most acidic on the left to those most
basic on the right, but while this is true for the extremes it is not
strictly so for intermediate points because dacites and quartz-dio-
rites are far higher in silica than are phonolites and nepheline-
syenites, and even than trachytes and .syenites. The general range
of silica is indicated on the lowest line. At the same time the im-
portance of the bases is not to be overlooked and subsequent tables
of analyses are given so as to show the range.

The general and larger truths of igneous rocks are fairly well
brought out in condensed tables of this character, although excep-
tional cases are known that would require its modification. But no
attempt has been made to confuse the larger truths by mention of
the rarer occurrences, for, as before stated, only ordinary' examina-
tion is assumed in connection with this text. When rare and ex-
ceptional varieties are met they should be placed in the hands of a
microscopical worker. It should also be appreciated in connection
with the table that groups of rocks shade into one another by im-
perceptible gradations and that they are not marked off with the
sharpness of ruled spaces. In general the glassy, felsitic and por-
phyritic constitute the la-'as or surface flows, the dykes and the
laccolites, while the granitoid rocks are the deep-seated, or abys-
sal ones, but there are cases where the latter show porphyritic
tendencies and others wherein the former shade into granitoid
textures.

CHAPTER III.

The Igneous Rocks, Continued. The Glasses. The Rocks
Whose Chief Feldspar is Orthoclase. The Phono-

LITES AND NePHELINE-SyENITES.

The Glasses.

SiOj AljOj FePa FeO. CaO. MgO. Kfi. Na^O. Loss. Sp. Gr.

1. 79.49 11.60 0.33 0.49 1.64 0.09 1.52 4.04 0.68

2. 76.20 13.17 0.34 0.73 0.42 0.19 4.46 4.31 0.33 2.352

3. 75.52 14.11 1.74 0.08 0.78 o.io 3.63 3.92 0.39 2.342

4. 74.70 13.72 1. 01 0.62 0.78 0.14 4.02 3.90 0.62 2.345

5. 74.05 13.85 tr. . . 0.90 0.07 4.31 4.60 2.20

6. 74.05 12.97 2.73 . . 0.12 0.28 5. II 3.88 0.22 2.37

7. 74.01 12.95 . . 1.42 0.99 0.48 4.65 5.34 0,29 2.391

8. 72.87 12.05 1.75 . . 1.30 1. 10 tr. 6.13 3.00

9. 71.6 12.0 i.o . . I-I 0.2 4.3 2.5 7.4

10. 71.56 13.10 0.66 0.28 0.74 0.14 4.06 3.77 5.52

11. 65.13 15.73 2.24 1.86 3.62 1.42 3.96 2.93 2.43

12. 60.5 19. 1 4.2 0.3 0.6 0.2 3.5 10.6 . . 2.48

13. 54.28 14.83 14.73 • • 7-02 3.65 1.27 4.22 . . 2.704

14. 50.82 9.14 7.33 7.03 11.63 7.22 1.02 3.06 1.74 2.66
15- 45-73 20.15 12.46 . . 8.67 3.59 4. II 5.74 0..12

I. Pumice, Cinder Cone, Calif., J. S. Diller, Bull. 79, U. S. G. S., p. 29. 2. Black
Obsidian, Tewan Mtns., N. M., J. P. Iddings, 7th Ann. Rep. U. S. G. S., 219. 3. Red
Obsidian, Yellowstone Park, J. P. Iddings, 7th Ann. Rep. U. S. G. S., 219, also
FeSj 0.1 1. 4. Black Obsidian, Yellowstone Park, J. P. Iddings, 7th Ann. Rep. U. S.
G. S. , 219, also FeS.^ 0.40. 5. Scoriaceous Obsidian, Mono Lake, Cal., I. C. Russell
8th Ann. Rep. U. S. G. S., 380. 6. Obsidian, Lipari Is., Abich. Vulk. Ersch., 62.

7. Obsidian from Andesite, Clear Lake, Cal., G. F. Becker, Mon. XIII., U. S. G. S! 104.

8. Perlite, Hungary, Kalkowsky, Elemente der Lith. , p. 75. 9. Pitchstone, Meissen, Lem-
berg, Z. d. d. g. G., XXIX., 508. 10. Pitchstone, Silver Cliff, Colo., W.Cross, Phil. Soc.
Wash., XL, 420. II. Andesitic perlite, Eureka, Nev. Hague, Mono. XX., U. S. G. S.,
264. 12. Phonolite obsidian, Teneriffe, Abich. Vulk. Ersch. , 62. 13. Hyalomelane,
Ostheim, Germany, Lemberg Z. d. d. g. G., XXXV., 570. 14. Pele's Hair, Hawaii,
Cohen, N. J., 1880, II., 41. 15. Tachylyte, Gethurms, Germany, Lemberg, See No. 13.

Coinmoits on the analyses. — An examination of the table of analy-
ses indicates that the magmas are high in SiO,„ and relatively low in
all other bases except the alkalies. The high Na,0 of Number 1 2

THE VOLCANIC GLASSES. 21

is worthy of remark, because this is the rule with a nephehne rock.
The percentages under the column headed loss, which practically
indicate the HgO present are characteristic for different varieties.
They are low in the case of obsidians, Nos, 2, 3, 4, 6, 7 ; un-
usually high in No. 5, described by Russell as scoriaceous obsidian ;
still higher in the perlites Nos. 8, 1 1 ; and reach a maximum in
the pitchstones Nos. 9 and 10.

Basic glasses are seldom sufficiently free from included crystals
as really to be separable from the porphyritic rocks. Frothy and
cellular crusts do, however, appear on lava streams, and are known
as scorias, and rare, homogeneous glasses have been called tachy-
lyte and hyalomelane.

Varieties. — The chief glasses are obsidian, pumice, perlite and
pitchstone. The name obsidian is applied to homogeneous
glasses with low percentages of water. The word is of classic and
ancient origin and is now used with a perfixed name for all glasses,
such as rhyolite-obsidian, basalt-obsidian, etc. Pumice is an ex-
cessively cellular glass, caused by expanding steam bubbles.
Perlite is a glass broken into small onion -like, individual masses,
by concentric cracks, from contractions in cooling. The concentric,
shelly masses lie in between intersecting series of larger, straight
cracks ; the perlites have considerable water, usually 2-4 ^. The
word is also written pearlstone, and was suggested by the fancied
resemblance of the concentric shells to the familiar gem. Pitch-
stone is a homogeneous glass, like obsidian, but contains 5-10^
of water. Pitchstones have often a more resinous appearance than
obsidians, but there is no very essential difference apparent to the
eye. The name was formerly used for glasses of earlier geological
age than the obsidians. Obsidians are usually black or red, with
translucent edges ; pitchstones are mostly reds and greens, but thin
slivers are practically colorless ; all the glasses contain dusty,
embryonic crystals, gas pores, and sometimns skeleton crystals of
larger growth and even a few phenociysts which are often ar-
ranged in flow lines and swirling eddies. Almost all large de-
velopments of the glasses show dense, stony or lithoidal layers,
and streaks, that are due to the development of minute crystals of
feldspar and quartz, which may be arranged in radiating rosettes,
called spherulites. The individual crystals are not often large
enough to be seen with the unassisted eye. Expanded, bubble-
like cavities are also met, with perhaps several concentric walls,

2 2 A HANDBOOK OF ROCKS.

on which at times are perched Httle well-formed crystals. These
cavities are called lithophysje, i. e., stone bubbles. Topaz, quartz,
tridymite, feldspars, fayalite and garnet have been found in beauti-
ful crystals in them. The lithophysae are due to the influence
and escape of mineralizers, and may reach a diameter of over an
inch.

Relatio)isJiips. — The glasses are all mere varieties of volcanic
rocks, which a quick chill has prevented crystallizing. At the
same time, it is only possible by field associations or by chemical
analysis to refer them to their corresponding porphyritic types,
although in the great majority of cases they are formed from
rhyolitic magmas.

Geological Occurrence. — The glasses sometimes appear as inde-
pendent sheets and dikes ; more often they form the surface of
well crystallized lava-sheets or the outer portions of dikes.

Alteration. — Glasses resist alteration notably well, but in the
long run are subject to decay along cracks and exposed surfaces.
They yield quartz, kaolin and fine, scaly muscovite. In instances
they devitrify, as it is called, or break up into aggregates of quartz,
and feldspar in excessively minute crystals, so that we can only
trace them back to the original glass, by the flow lines, spheru-
lites, etc., that still remain. Such devitrified forms have been
called by F. Bascom, apobsidian. Petrosilex is an older term ap-
plied to these and other similar rocks, and felsite has been also
used.

Distribution. — The glasses are widespread in the West. Obsi-
dian Cliff, in the Yellowstone Park, yields black, red and stony
varieties, and has been made a type locality by the studies of J. P.
Iddings. Silver Cliff, Colorado, has furnished some remarkable
pitchstones, described by Whitman Cross. The extinct volcanoes
of New Mexico, Utah, Montana and around Mono Lake, Califor-
nia, are well-known localities. Alaska has supplied much
from near Fort Wrangel, and in Mexico and Iceland are other pro-
lific sources. Along the Atlantic Coast there are only the devitri-
fied glasses of ancient (pre-Cambrian) volcanoes. These are well
developed in New Brunswick, Maine, Massachusetts and Pennsyl-
vania. Abroad the obsidian of the Lipari Islands is a famous one,
and the perlites of Hungary supply the usual type specimens in
our collections. The best known of all pitchstones are found at
Meissen, near Dresden, in Saxony, and on the island of Arran,.
off the west coast of Scotland.

IHE RHYOLITES. 23

The Rhvolites.

SiO,

AIA

Fe/)3

FeO

CaO

MgO

K,0

Na,0

Loss.

Sp. Gr.

83-59

5-42

tr.

3-44

tr.

I 37

5-33

0.76

2-54

78.95

10.22

3-23

1. 84

0 14

1.76

4 18

77-5

9-7

6.1

5-8

0.4

75.20

12.96

0.37

0.27

0.29

0.12

8.38

2.02

0.58

73-91

15.29

0.89

0.77

4-79

3-62

1. 19

73-07

11.78

2.30

2.02

0.39

6.S4

1. 19

2.24

71.12

14.58

1.69

1.50

0.15

6.01

326

0.95

70.92

13-24

3 54

0.66

1.42

0.23

4.25

4.28

0-57

7074

14.68

0.69

0.58

4.12

0.28

2.59

2.29

2.09

2.68

10.

68.84

15.73

3-II

3-58

0.90

3-59

2.89

1.50

2.4

II.

68.10

M97

2.78

1. 10

3<H

1. 10

2-93

346

1.28

2.636

12.

67.20

1495

5- 19

0.30

2.39

0.89

4.00

2 13

13-

66.91

14 13

5.00

235

0.95

5.40

3.86

1.42

14-

66.60

16.69

2.06

0.93

1.40

I.15

5 23

2.46

1.70

243

15- 63.63 17.42 0.15 5.76 2.86 . . 5.54 4.52 0.15

I. .Soda-rhyolite, Berkeley, Cal. Palache. Bull. Geol. Dept. Univ. Calif., I.. 6l. 2.
Rhyolite, Iceland, BSckstrom, Contrih. to Icelandic I.iparites. 3. khyolite, Wales,
A. Marker, Bala Vole. Ser., 13. 4. Rhyolite, Silver Cliff, Colo., Cross, Colo. Sci. Sec.,
Dec. 5, 1887, 229. 5. Rhyolite. I'into Peak, Kiireka, Nev., A. Hague, Mono. \X.,
264. 6. Rhyolite, McClelland I'cak. Washoe, Dist., Nev., F. A. C.oixh, Bull. 17, U.
S. G. S., '^'^. 7. Rhyolite, Island of Ponza, near Naples, <|uote«l by Kalkowsky,
Elem. d. I.ith., p. 75. 8. Rhyolite, \'clIowstone I'ark, Iddinu\ Origin Igneous
Rocks, Tal). l. 9. White I'orphyr)', I.eadville, Colo., Cross, Mono. XII., I'. S. G. S.,
326. 10. Rhyolite, I,as.sen's Peak. Cal., Fortieth Paral. Sur\-ey, I., 65*. II. Gray
Porphyni', I-cadvilc, Colo., Mono. \II., U. S. G. S., 332. 12. <Juart/ Porphyry, Flag-
stafi' Hill, Colo , Palmer vS: Fulton, Colo. Sci. See, III., 356. 13. Rhyolite, Hungary,
V. Hauer, Verh. d. k. k. R., 1867, I18. 14. (Quartz Porphyry, I'pper <^>uinnesec Falls,
Mich., G. H. Williams, Bull., 62, U. S, G. .'^., I20. 15. (^>unrl/ P..r]>hvrv, Water-
ville, N. H., (;. W. Hawes. N. H. Geol. Sur%-., III., 178.

Comments on tJic Analyses. — The analyses illustrate the range.s
of the various molecules. No. i is a very e.xceptional rock, alike
in its high SiO., and CaO, and low AUO,. The gradual increase of
AU03 in all the others, with decrease of SiO,, and in general the
same relation as regards CaO are worthy of remark, as is the pre-
vailingly low MgO. Sometimes K.O, sometimes Xa.O, is in ex-
cess, and this brings out the reason why we spoke of orthoclase as
the chief feldspar, not as the only one in the table, p. 19. The
specific gravit)' is in general low.

[ ariitics. — Rhyolites proper are porphyritic rocks with pheno-
crysts of quartz, alkali-feldspar, usually orthoclase.of biotite, and less
commonly hornblende and augite, in a groundmass that is cither
glassy, or a finely ci\'sta!line aggregate of cjiiartz and feldspar, or

24 A HANDBOOK OF ROCKS.

both. The name rhyoHte was coined from the Greek verb to flow, on
account of the frequent flow structure. It is the name mostly used
in America and England, whereas liparite (from the Lipari Islands)
and quartz-trachyte are employed in Europe. A variety with very
little groundmass and an approximation to a granitoid texture is
called nevadite, from the State of Nevada. In former years a dis-
tinction was made between the volcanic rocks of pre-Tertiary age
and those of later date, and as against the later rhyolites the older
were called quartz-porphyries, but the distinction has no serious
foundations. At present some authors define quartz-porphyries as
rocks corresponding to the rhyolites, but which have crystallized as
intruded sheets, laccolites, sills and dikes. Nevertheless, while this
distinction has some force, in any extensive collection of specimens
no very noticeable difference can be detected in the hand specimens
even by a very practised observer. Rhyolites from the inner parts
of thick surface flows, and from their branching dikes, can be
found, especially when alteration has advanced somewhat, to match
all quartz-porphyries, but as a general rule quartz-porphyries are
denser and less cellular than rhyolites. The meaning of both
words should be well understood on account of their presence in
the literature. Certain microscopic structures due to the inter-
penetration of quartz and feldspar are also seen in the quartz-por-
phyries (z. €., intruded sheets), that are seldom met in surface
flows, but these have no value in ordinary study. An old and
very useful term is felsite, which has been applied especially to
acidic lavas of ancient geological date, that lack phenocrysts,
wholly or largely. Whether they correspond to rhyolites or to
less acidic magmas, such as trachytes, is not always apparent with-
out chemical analyses. Felsites are dense, usually green, red or
gray rocks, which only indicate to the eye that they are very finely
crystalline. They really consist almost entirely of minute quartz
and feldspar crystals, practically a groundmass without pheno-
crysts, but it is not always apparent whether they represent origi-
nal crystallizations from fusion, or devitrified obsidians (apob-
sidians) or recrystallized tuffs, all of which have been demonstrated
in one place or another. Those certainly derived from rhyolites
have been called aporhyolites.

Rhyolites high in soda are called soda-rhyolites, or pantellerites,
(see analyses i and 2). Ancient rhyolites (quartz-porphyries, fel-
sites) rich in soda have been called quartz-keratophry (see analysis

THE RHYOLITES. 25

1 2), the distinction being made because the feldspar present must
be anorthoclase as against true orthoclase. As the true porphy-
ritic texture graduates into the granitoid, we have intermediate
rocks called granite porphyries, of which mention is made under
granites.

Mineralogical Composition. — The principal minerals present of re-
cognizable size are quartz, in rounded or doubly terminated (/. c, di-
hcxagonal) pyramids with practically no prism faces ; and feldspar,
including orthoclase, less often anorthoclase, and a soda-lime plagio-
clase belonging in the series from albite to oligoclase. Biotite is
much the commonest dark silicate, although hornblende, and less
often augitc, arc occasional. It is important to appreciate that the
dark silicates arc vastly inferior in quantity to the light ones. As
regards the groundmass, b\' the unaided eye, we can only deter-
mine that it is glassy (called also hyaline), or finely crystalline,
that is, felsitic, or coarsely cr>'stalline. Vesicular groundmasses are
met in surface flows, not in intruded masses. Lithophysa; also
occur in rhyolites as well as in volcanic glasses.

Relationships. — Rhyolites pass by insensible gradations into
glasses on one side, trachytes on another, granites on a third and
dacites on a fourth. Without the micro.scope rhyolites can only
be identified with certaint)- by recognizing the (juartz, and may
then be confu.sed with dacites. The striated feldspar of the latter
is our chief means of distinction between the two.

Altcratioti. — ( )rdinar\' decay leatls to the formation of clays and
kaolin. In metamorphic alterations the rhyolites pass into very
finely cr\stal!ine aggregates of fjuartz and feldspar, and then it is
<lifficult to decitie what minerals are original and what secondary,
and whether the original rock was a massive one or a tuff Shear-
ing stresses develoji schistose structures, and when tiecay is further
sui)cratlded, sericite schists may result that are extremel}' difficult
geological problems.

Distributioit. — Rhyolites are common in the Western States,
being well known in the Hlack Hills : the Yellowstone Park ; in
Colorado, where Chalk Mountain, near Leadville, is a type locality
for nevadite ; in Nevada, both near Eureka and near the Com-
stock lode, and in California. The so-called quartz-porphyries have
been also met in many Western districts, but are of especial im-
portance at Leadville, where they are intimately associated with
the ores. The ancient rhyolites (ciuartz-por[)h\-ries) have also an

26 A HANDBOOK OF ROCKS.

important development on Lake Superior. The greater part of the
boulders in the Calumet, copper-bearing conglomerate consists of
them, and Lighthouse Point, near Marquette, furnishes an outcrop.
Along the Atlantic Coast the pre-Cambrian rhyolites (felsites) are
present in the same localities as those cited for volcanic glasses.
Recentr hyolites are in vast quantity in Iceland. Many are known
in Europe, but the enormous development in Hungary is especially
worthy of note. The sheets of rhyolite on the Lipari Islands be-
tween Naples and Sicily, suggested the name liparite. In almost
all volcanic districts they are liable to occur. In the Tyrolese
Alps quartz-porphyries are of great extent, and in Scandinavia and
in Cornwall, they form important dikes, familiar to all students of
the subject.

Rhyolite Tuffs. — These are the fragmental ejectamenta from
explosive eruptions that often afford very extensive strata of rock.
Although loose at the time of falling, they may become consoli-
dated in the course of time, or before this occurs they may be sorted
and redeposited in water so as to share the nature of a true sediment.
Fragments of volcanic glass and of all the component minerals of
rhyolite make them up, while larger fragments of rock and vol-
canic bombs are at times intermingled. Tuffs of ancient geolog-
ical date become metamorphosed and recrystallized, so as to afford
products not to be easily distinguished from compact felsites.

Rhyolite tuffs are abundant along the eastern foothills of the
Front Range of Colorado, and are extensively quarried for a rather
soft, building stone.

The Trachytes.

SiOj

Al,03

Fe,03

FeO

CaO

MgO

K,0

Na.,0

Loss.

Sp. Gr

66.03

18.49

2.18

0.22

0.96

0-39

5.86

5.22

0.85

2.59

65.07

16.13

517

2.74

0.67

4-44

4-77

0.70

62.28

19.17

3-39

1.44

5-93

5-37

2-33

2.65

62.17

1858

2.15

1.05

1-57

0.73

3.88

7.56

1.70

58.70

19.26

3-37

0.58

1. 41

0.76

4-53

8.55

2.64

57-7

17.9

4-4

3-9

3-7

1.8

7-7

3-8

0.1

2.61

I. Trachyte, Game Ridge, Custer Co., Col., Cross, Proc. Col. Sci. Soc, 1887, 237,
2. Oligoclase-tracliyte, Drachenfels on Rhine, Rammelsberg, Z. d. d. g. G. , XL, 440.
1859. 3. So-called Bostonite dike, Lake Champlain, J. F. Kemp, Bull. 107, U. S. G.
S., 20. 4-5. Acmite-trachyte, Crazy Mountains, Mont., Wolff and Tarr, Bull. Mus.
Comp. Zool., XVI , 232. 6. Trachyte, Arso Flow, Ischia near Naples. Abich, Isola
d'Ischia, 38. Silica determinations on eleven trachytes from the Black Hills afforded
J. H. Caswell values from 65.46 to 52.02.

THE TRACHYTES. 27

Comvients on the Analyses. — The decrease in silica and the in-
crease in alumina and the alkahes as against the rhyolites are note-
worthy. The alkalies in particular are high, with sometimes pot-
ash, sometimes soda, in excess. The latter marks the passage to
the phonolites.

Varieties. — Trachyte is a name derived from the Greek word for
rough, and refers to the rough surfaces of those first studied.
The name was of much wider application in earlier years than to-
day and was used for both rhyolites and trachytes. If soda is
high, the .soda pyroxene, acmite, may form and give name to the
rock as in analyses 4 and 5. Pantellcritc is another name for those
that arc rich in soda and that have anorthoclasc feldspar. Por-
phyry, the name so widely and loosely used is applied to pre-Ter-
tiary trachytes, and to intruded sheets and dikes. Hostonite has been
employed for such dikes and to both dikes and sheets when rich
in soda, the name keratophyrc has been given. Many felsites also
belong here, but for all these the remarks made under rhyolites ap-
pl)'. The distinctions are onl)' of importance in close micro-
scopical work and when the idea of the geological relations is in-
volved in the definition, but both porphyry and fclsite are useful
current terms. Porphyry, it should be remarked, is employed in
mining circles in the West, for almo.st ever)' rock that occurs in
dikes or sheets.

Mineraloi^ical composition. — The principal minerals of recogniza-
ble si/.e arc alkali-feldspar, chiefly orthoclasc of the variety called
sanidine, a little acidic plagiocla.se ; more or less biotitc, hornblende
and pyroxene in this general order. The light-colored silicates arc
in notable excess over the darker ones. The groundmass is glassy
or finely, or (rarely) coarsely crystalline.

Alteration. — The alteration is practically the same as that de-
scribed under rhyolites.

I\elationsliif>s. — With increase of soda, trachytes pass into phono-
lites, to which iiulceil the\' are closely related. With the develop-
ment of granitoid texture they (including porph)ries) pass into
syenites. They are easily confused with some andesites unless the
eyx can detect the striated feldspar of the latter, but, as noted in
the next paragraph, they are comparatively rare rocks.

Distribution. — True volcanic trachytes are extremely rare in this
country, for many of the otlitr cited localities, as, for instance,
some of those in the reports of the Fortieth Parallel Survc}', have

28 A HANDBOOK OF ROCKS.

been shown to be andesites. Beautiful examples do, however, oc-
cur in the Black Hills, with superbly developed orthoclases. Others
are known in Custer county. Col. (see Analysis i), and in Montana
(Analyses 4 and 5). The porphyries, strictly so-called, are not
identified with certainty in very wide distribution, although, doubt-
less, many dikes in the West are properly described as such. In
southeast Missouri, at Iron Mountain and Pilot Knob, they are
very abundant. Many curious dikes occur around Lake Cham-
plain, and among the pre-Cambrian volcanics of the Atlantic Coast
they are not lacking. Abroad trachytes are more common, and
along the Rhine, — where the peak of the Drachenfels is situated,
which furnishes the commonest specimens for collection, — in the
Auvergne, in Italy and in the Azores they are well known.

TracJiytc Tuffs are not common in America, and offer only micro-
scopic points of difference from those formed of rhyolitic material.

The Phonolites.

SiOj

Al,03

Fe.p3

FeO

CaO

MgO

K,0

Na,0

Loss.

Sp. Gr.

61.08

18.71

1. 91

0.63

r.58

0.08

4-63

8.68

2.21

2.582

60.02

20.98

2.21

0.51

1. 18

tr.

5-72

8.83

0.70

2.576

59-46

23.00

352

1. 00

0.50

4.90

7-13

0.71

59-17

19-74

3-39

0.92

015

6.45

8.88

1. 18

2.566

56-43

22.25

2.66

0.97

1.41

tr.

2.77

II. 12

2.05

2.54

49.18

20.65

5-97

2.43

0.29

6.88

9.72

1.60

2-553

45-18

23-31

6.11

4.62

1-45

5-94

II. 17

1. 14

44-50

22.96

6.84

8.65

1.65

4-83

6.70

2.06

I. Devil's Tower, near Black Hills, Wye, Pirsson, A. J. S., May, 1894, 344. 2. El
Paso Co., Colo., Cross, Proc. Col. Sci. Soc, 1887, 169. 3. Island of Fernando de No-
ronha, Brazil, Giimbel Tscher, Mitt., 1880, II., 188. 4. Near Zittau, Saxony, v.
Rath. Z. d. d. g. G., VIII., 297. 5. Wolf Rock, Cornwall, Eng., Phillips, Geol. Mag.,
VIII., 249. 6. Leucite-phonolite, near Rieden, Germany, Zirkel, lyehrbuch II., 465.
7. Eleolite-porphyry, Beemerville, N. J., J. F. Kemp, N. Y. Acad. Sci., XL, 69. 8.
Eleolite-porphyry, Magnet Cove, Ark., J. F. Williams, Igneous Rocks of Ark., 261.

Comments on the Analyses. — It is at once apparent from the
analyses that the range in silica, except in the last two, is much
like that of the trachytes, but that the alumina goes higher, .and
that the alkalies are in extremely large amounts. No other rocks,
except the corresponding granitoid types, reach these amounts in
alkalies. The soda which is necessary for the formation of the
nepheline is naturally in excess. The rare leucite-phonolites, as a
general thing, are more basic and show comparatively high potash.

THE PHOXOLJTES. 29-

The last two analyses of intrusive or dike members are abnormally
basic for phonolitic rocks.

Varieties. — The phonolites were named because in the thin plates
in which they often break up the\- xm'g under the hammer. Xeph-
eline or eleolite-porphyries have been described by a few obser-
sers, and certain ones in dikes have been named tin^uaite from a
Brazilian locality, but in their mineralogical composition they are
practically phonolite, although at times exceptionally basic. Leu-
citophyr is applied to phonolites with both ncphcline and leucite,
while leucite phonolite is used for those that have no nepheline.

Mineralogical Composition. — The principal minerals are ortho-
clasc, variety sanidine, and ncphcline or leucite, or both. The
nepheline is seldom visible to the eye, and indeed it is practically
necessary, in order to insure oneself of the rock to warm up a
little of it powdered, in dilute acid, filter, and evaporate for gelatin-
ous silica. Nepheline gelatinizes so readily that it is easily de-
tected. No.scan and hauyne arc frequent in phonolites. The
commonest of the dark silicates are the soda-pyroxenes, acmitc and
scgirine. Hornblende is known, but biotite is .seldom seen. The
rocks have usually a compact gray or green groundmass in which
are visible, shining sanidines, seldom ncphcline or leucite, and rarely
dark rotls of jiyro.vcnc, because the pyroxene, though alwa\s pres-
ent, is usuall}' microscopic. In connection with pre-Tcrtiar)- rock.s
the name elcolite is sometimes used for nepheline.

Relationships. — The jihonolites are closely related to the tra-
chytes as already stated. Ihey have also intimate connections
with certain rare, basaltic rocks to be referred to later that contain
nepheline and leucite.

Alterations. — The nci)helinc changes quite readily to natrolite
and perhaps analcite, while leucite yields analcite. Metamorphic
processes arc yet to be studied.

Distribntion. — The true volcanic phonolites are onl)* known in
a few localities in this country, such as the Hlack Hills, where they
form dikes, sheets and isolated buttes ( Devil's Tower ), and the
Cripple Creek mining district of Colorado, where the comparatively
few dikes known haxc proved of great importance as associates of
the ores. Nepheline- or eleolite-porphyries (tinguaites) as recorded
as exceedingly rare rocks near Magnet Cove, Ark., and Ik-emer-
ville, N. J., associated with nepheline-syenite. Phonolites are
much more abundant abroad, being well known in many parts of

so A HANDBOOK OF ROCKS.

Germany. The varieties with leucite are especially familiar from
the vicinity of Rieden, in the extinct volcanic district of the Eifel. A
peculiar leucite rock, with abundant scales of biotite, gives the
name to the Leucite Hills, two or three miles north of Point of
Rocks, Wyo. Leucite tinguaites occur near Magnet Cove, Ark.,
in the Highwood Mountains, Mont., and near Rio Janeiro, Brazil.
Tuffs are known abroad but not in this country, and exhibit few
features calling for special mention.

The Granites.

SiO,

AiPa

Fe,03

FeO

CaO

MgO

K,0

Na^O

Loss.

Sp. Gr

73-76

13-43

1.42

0.75

5-22

4.01

0.42

2.63

73- 70

14.44

0.43

1.49

1.08

tr.

4.43

4.20

0.40

2.69

73-05

14-53

2.96

2.06

tr.

5-39

1-73

0.29

72.73

1.05

tr.

8.15

0.90

0.22

72.26

15-59

1. 16

2.18

I-I3

0.06

5-58

3-85

0.47

2.65

71.78

14-75

1-94

2.36

0.71

4.89

3.12

0.52

71.64

15.66.

2.34

2.70

tr.

5-6

1.58

0.48

69.46

17-50

2.30

2-57 .

0.30

4.07

2.93

0.82

2.68

69.28

17.44

2.30

0.27

2.76

3-64

lO.

68.68

16.28

0.66

2.55

2.24

0.81

4.07

2.88

0.85

II.

66.84

18.32

2.27

0.20

3-31

o.8x

2.80

5-14

0.46

12.

66.68

14-93

1-58

3-23

4.89

2.19

2.05

2.65

1-25

13-

66.40

17-13

3-77

4-05

0.94

2.08

4-49

1.03

I. Biotite granite, Green's Landing, Me., E. F. Hicks. Privately conimunicated.
2. Granitite, Peterhead, Scotland, Phillips, Q.J. G. S., XXXVL, 1880, 13. 3. Red
Granitite, Westerly, R. L, F. W. Love, for J. F. K., unpublished. 4. Red Granite,
Stony Point, Conn., L. P. Kinnicut, Anal., unpublished. 5. Albany granite, N. H.
Hornblende granite, G. W. Hawes, A. J. S., iii., XXL, 25. 6. Hornblende granite
with biotite, Cottonwood Canon, Utah, T. M. Drown, 40th Parallel Surv., I., no. 7.
Gray granitite. Westerly, R. L, see No. 3. 8. Typical granite, Chester, Mass., L. M.
Dennis, for J. F. K., N. Y. Acad. Sci., XL, 129. 9. Biotite granite, Raleigh, N. C,
G. P. Merrill, Stones for building and decoration, 418. 10. Biotite granite with horn-
blende. Wood Cone, Eureka Dist., Nev., Arnold Hague, Mono., XX., U. S. G. S., 228.
II. Augite-soda granite, Kekequabic Lake, Minn., U. S. Grant, Araer. Geol., June,
1893, 385. 12. Granitite, Rowlandville, Md., Jour. Cin. Soc. Nat. Hist., 1894, p. 32.
13. Biotite granite with hornblende. El Capitan, Yosemite, see No. 6.

Comments on the Analyses. — These analyses illustrate the general
range of SiOg, but granites are known outside of both limits. As
SiOj decreases the bases increase, and soda tends to exceed potash,
marking the passage to the diorites. Those high in NagO, like
No. 1 1, are often called soda-granites. They are analogous to the
keratophyres, soda-rhyolites and pantellerites, earlier referred to.

THE GRANITES. 31

The whole table is a close parallel to that of the rhyolites. The
analyses are selected, so far as possible, to represent prominent
building stones.

Mincralogical Composition and Varieties. — Granites are, par excel-
lence, granitoid rocks consisting of orthoclase, sometimes micro-
clinc, some acid plagioclase, quartz, and in the typical variety both
biotite and muscovitc. Magnetite, apatite and zircon are always
present, though small, and garnet is not at all unusual. Biotite is
much the commoner of the micas, and when it is present alone the
rock is sometimes called granititc. Granites with muscovite alone
are especially found in the form of dikes. They are called aplite-
Hornblende is also frequently met, either with biotite or by itself,
giving then hornblende-granite. In former years this aggregate
was called syenite, but the modern usage is different. Augitc in
granites is uncommon, and marks a passage to the gabbros. All
forms of dark silicates and mica may fail, and then we have the
so-called binary granites. Some Missouri granites are of this
character.

Kspccialh" in regions ot gi.iiiitc intrusions aiui ot c.\tcnsi\c meta-
morphism, veins or dikes — it is an open question which is the more
correct term — are met, which arc formed of very coarsely crystalline
aggregates of the same minerals that constitute granite. These
are called pegmatite and in them is the home of graphic granite,
the curious intcrgrowth of quartz and feldspar, such that a cross
fracture of the blades of quartz suggests cuneiform characters.
Garnet, tourmaline, beryl and minerals involving the rare earths,
are often found in pegmatites, and they supply the feldspar and
mica of commerce. The outcrops may be two hundred feet
broad or more, and again the same aggregates arc found as small
lenses or " Augen " in metamorphic rocks. In regard to the larger
veins or dikes it .seems improbable that true igneous fusion could
have afforded such coarsely crystalline aggregates, and so we are
forced to assume such abundance of steam and other vapors, /. e.,
mineralizers, as to almost, if not quite, imply solution.

•Undoubted dikes of the composition of granite are also known,
that have no such unusual size of minerals, but that tend to de-
velop a porphyritic texture from the presence of feldspars larger
than the general run of the component minerals. These are called
granite-porphyries, and they pa.ss by insensible gradations, through
finer and finer groundmasses, into typical quartz-porphyries. The

32 A HANDBOOK OF ROCKS.

phenocrysts of granite-porphyries may also fail and the ground-
mass may become finer and finer, passing through a stage called
micro-granite into the felsites.

The outer portions of granite masses are often subjected to the
action of escaping vapors, containing boracic and hydrofluoric acids
(fumarole action). These develop tourmaline in quantity and often
fluorite, and in rare instances cassiterite. In a famous case near
Luxullian, in Cornwall, the feldspar has become changed to an ag-
gregate of tourmaline needles and quartz, and the rock is called
luxullianite. Tourmaline granite is, however, also known in which
tourmaline plays the role of mica or hornblende, as at Predazzo, in
the Tyrol. Fumarole action may change the borders of granites
to a mass of quartz and a lithia mica, affording the rock that is
called greisen and that is a familiar gangue for tin ores.

Granites are commonly gray, bluish or reddish in color. The
feldspar is mainly responsible for this, as quartz is colorless and
transparent and biotite and hornblende are not specially abundant ;
but unusual richness in the last named silicates tends to darken the
shade. These latter are very frequently segregated into the black
bunches that are noticeable in many building stones. They may be
spheroidal in their alignment, affording so-called orbicular granite.

Relationships. — The passage of granites, through granite-por-
phyries and micro -granites, into quartz-porphyries and felsites, has
been remarked. Sometimes along the border of an intrusion, this
can be traced inch by inch to a place where the porphyritic tex-
ture is due to a quick chill. Mt. Willard, in the Crawford Notch
of the White Mountians is a classic locality of this phenomenon.
It was described in i88i by Geo. W. Hawes (see analysis 6), and
will be referred to again under the rocks of contact metamorphism.
The close relationship of the granitoid rhyolites or nevadites with
granite need only to be referred to. As quartz decreases, syenites
result by insensible gradations, and as hornblende or biotite and
plagioclase increase, the same passage is made to diorites. Inter-
mediate varieties, which are very common, are often called granite-
diorites or grano -diorites. Transitional passages to gabbro, from
increase of augite and plagioclase, are also well recognized.

Geological Occurrence. — Granites in their most typical develop-
ment constitute great irregular masses that have solidified at
depths ; such are called batholites, and it is generally believed that
before consolidating they have often fused their way upward by

////: CRAX/TES. ' y:,

melting into themselves overlying rock. Granites also appear as
irregular or rounded outcrops in the midst of other rocks (bosses
or knobs) and as dikes. There is no reason why granites should
not form at all geological ages, but those open to our observation
are most!}' Archean and Paleozoic because, being deep-seated
rocks, only the older ones have been exposed by erosion. The
relations of pegmatites to veins have been earlier set forth. Granites
tend to break apart along jointing planes into rectangular blocks,
a property that much facilitates their quarrying. They also have
lines oi weakness admitting of their further division into smaller
ma.sses. The development of these is more or less characteristic of
each particular locality.

Uses. — Granites are much more extensively employed for struc-
tural purposes than any other igneous rock, and indeed in the
trade any cry.stalline rock consisting of silicates is called granite,
riicy are in general the strongest of the common building stones.
Crushing resistances range from io,000 to 25.OOO pounds per
square inch in a 2-inch cube. The important points arc homo-
geneity of te.vture. good, rectangular cleavages in the {juarry,
adaptabilit)' to tf)ol treatment, durability and pleasing color.

Alteration MctnmorMtism. — In ordinary decay granites suffer
first by the oxidation of the protoxide of iron in the ferromagnesian
silicates (biotite, hornblende), and the formation of chlorite and
other secondary minerals. The feldspars also kaolinize, and the
rock thus becomes h)'drated. Pyrite, if present, is an active agent
in deca)'. \'et the chemical changes involved, except hydration,
seem to be comparatively slight even in the change from granite
to soil. (i. P. Merrill gives the following analyses of unaltered
and altered biotite granite from the vicinity of Washington, D. C.
(Bull. Geol. Soc. Amer.. VL, 323).

.Si( ),

Al./.,

Kc,( )

I- c( )

CaO

MnO

KjO

Na./ )

Ijrnition.

6933

'4-3.?

3.«'x)

3-21

2-44

2.67

2.70

I 22

66.82

15.62

1.88

1.69

I'^l,

2.76

2.04

2.58

3 27

65.69

15 23

2.63

2.64

2.00

2.12

4.70

No. I is fresh and undecomposed rock ; No. 2, decomposed but
still moderately firm rock ; No. 3, soil. It is evident at once that
there has been considerable hydration, and that a notable decrease
in the alkalies has occurred, each being affected about equally in
the end, althougli K.^O yields first ; MgO has relatively increased ;
CaO has suffered loss ; the FeO is all oxidized, the A1,0, has rcla-
3

A HANDBOOK OF ROCKS.

tively increased and the SiO^ decreased. While appreciating these
chemical changes, Dr. Merrill still emphasizes the much greater
importance of the physical alteration and attributes this to swelling
from hydration. Other interesting data are given in the citation.
Similar sets of parallel analyses have been made abroad with analo-
gous results in the case of the chemical rearrangements.

Under dynamic stress granites are more or less crushed and
have their minerals drawn out into laminations from shearing
strains so that they readily assume gneissoid structures. Beyond
question many gneisses have resulted in this way, and in the
geology of some districts, as, for instance, the Front Range of
Colorado, we employ the term granite-gneiss. The structures
were, doubtless, induced while the granite was deeply buried and
subjected to pressure when closely confined, so that the yielding
came in a gradual flow.

Distribution. — Granites are abundant along the Atlantic coast,
and are near tidewater from Canada to Virginia. Further south
they lie back of the Coastal Plain. They are chiefly biotite granite
and are extensively quarried. A famous hornblende granite is
obtained at Quincy, Mass., that was formerly called syenite. In
the old crystalline areas of Michigan, Wisconsin and Minnesota
they are common. Missouri has many in the region of the por-
phyries, already cited. In the West, the Black Hills, the Rocky
Mountains, the Wasatch and the Sierras are abundantly supplied.
They are equally common in Europe and elsewhere the world
over.

The Syenites.

SiO^

AI.P3

Fe,03

FeO

CaO

MgO

Kf>

Nap

Loss

Sp. Gr.

60.03

20.76

4.01

0.75

2.62

0.80

5.48

S-96

0.59

59-83

16.85

7.01

4-43

2.61

6.57

2.44

1.29

2.73

59-78

16.86

3.08

3-72

2.96

0.69

5.01

5-39

1.58

2.689

59-37

17.92

6.77

2.02

4.16

1.83

6.68

1.24

0.38

2.71

56.45

20.08

1.31

4.39

2.14

0.63

7-13

5-61

1-77

46.11

14.7s

2.20

4-51

7.82

5-73

3.84

1.29

1-59

2.904

46.73

10.05

3-53

8.20

13.22

9.68

3.76

1. 81

1.24

I. Fourclie Mtn. near Little Rock, Ark., J. F. Williams ; Igneous rocks of Ark.,
88. 2. Plauen, near Dresden, Y. Zirkel, Pogg. Ann., CXXIL, 622. 3 Custer Co.,
Colo. Cross, Proc. Colo. Sci. Soc. , 1887, 240. 4. Biella, Piedmont, Cossa., Turin Acad,
ii., XVIIL, 28. 5. Sodalite-syenite, Highwood Mtns., Mont., W. Lindgren, A. J. S.,
Apr., 1893, 296. 6. Minette, Rhode Island, badly decomposed, contained CO, 7.32,

THE SYENITES. 35

Pirsson, A. J. S., Nov., 93, 375. 7. Shonkinite, Highwood Mtns , Mont., Weed and
Pirsson, Bull. Geol. Soc. Amer., VI., 414.

Comments on the Analyses. — The syenites mark a decrease in
SiOg from the granites and a general increase in all the bases. The
high percentage of alkalies is especially worthy of remark, and the
notably large amounts of soda, showing the passage to the nephe-
line syenites. The parallelism with the trachytes is close. The
last two analyses exhibit excessively basic extremes, whose theo-
retical significance is commented on in the next paragraph.

Mincralogieal Composition, I'arieties. — The name sjenitc was
suggested by Syene, now Assuan, an Egyptian locality, where a
hornblende granite was formerly obtained for obelisks, and if its
local significance were perpetuated, syenite as formerly should be
applied to this rock. But Werner used it in the last century for
the well-known typical rock from the Plauenschcn Grund (see
Analysis 2), near Dresden, that contains almost no quartz, and of
recent years this has been its correct use. Topical .syenites have
orthoclasc and hornblende ; tho.se with biotite are called mica-
.syenites. .Some plagioclase is always present ami magnetite,
apatite and zircon are invariable. Mica .syenites in dikes, basic
and of dark Cf)lor have been called minette. (Orthoclasc and augite
afford augite-syenite. An excessivel)' basic one (Analysis 7), from
the Highwood mountains, Mont., has recently been described by
Weed and Pirsson under the name Shonkinite. It is of great
theoretical importance, as it shows that orthoclase is not limited to
acidic rocks, but may be the prevailing feldspar in very basic ones
Still more recently J. V. Iddings has noted others of similar char-
acter from the region of the Yellowstone Park. (Jour, of Geology,
December, 1895, 935.) Basic nepheline-syenites have been earlier
known. Still the table on page 55 expresses the general truth,
the exceptions being excessively rare rocks so far as yet known.
Syenites arc themselves rare rocks. With high soda, the mineral
sodalite develops and yields sodalite syenites which are passage
forms to nepheline .syenites.

Ri/iitions/tips. — Syenites are most closel)- allied with nepheline-
.syenites, into which with increase of soda they readily pass. They
also with increasing plagioclase shade into diorites and the augite-
syenites are closely akin to gabbros.

Geological Occnrrcnce. — Sj'enites form irregular masses antl
dikes, precisely as do granites.

36 A HANDBOOK OF ROCKS.

Altcrat'uvi. — There is little to be said that was not covered un-
der granite. The rarity of syenite makes it a much less serious
factor. In metamorphism they pass into gneisses.

Distribution. — Syenites occur in the great igneous complex of
the White Mountains. They form large knobs and dikes near
Little Rock, Ark., and a dike is known in Custer county, Colo.
The only American minette yet discovered, is a dike on Conanicut
Island, R. I., described by Pirsson (see Analysis 7). Abroad,
syenites are better known. The Plauenscher Grund, near
Dresden, Biella in the Piedmont, and the vicinity of Christiania,
Norway, are the best known. Minettes are especially famous in
connection with the mining district about Freiberg, Saxony, and
in the Vosges mountains.

The Nefheline Syenites.

SiO,

Alp,

Fe.,0,

FeO

CaC)

MgO

K.p

Na.p

Loss.

60.39

22.51

0.42

2.26

0.32

0.13

4-77

8.44

0.57

59-70

18.85

4.85

1-34

0.68

5-97

6.29

1.88

59-OI

18.18

1.63

3-65

2.40

1.05

5-34

7-03

0.50

5630

24.'4

1.99

0.69

0.13

6.79

9. 28

1.58

54.20

21.74

0.46

2.36

1.95

0.52

6-97

8.69

52.75

22.55

3-65

1-85

C.15

7-oS

8.10

3.60

51.90

22.54

403

315

3-II

1-97

4-72

8.18

0.22

5096

19.67

7.76

4-38

0.36

6.77

7.67

1.38

50-36

19-34

6.94

3-43

7-17

7.64

3-51

10.

41-37

16.25

16.93

12-35

4-57

3-98

4.18

0-45

Sp. Gr.

I. So-called Nepheline-syenite, or Litchfieldite, Litchfield, Me., W. S. Bayley, G.
S. A., IIL, 241. 2. Nepheline-syenite, Fourche Mountains, Ark., J. F. Williams,
Igneous Rocks of Ark., 88. 3. Nepheline syenite, Red mountains, N. H., W. S.
Bayley, G. S. A., IIL, 250. 4. Ditroite, Hungary, Fellner, Neues Jahrb., 1868,
83. 5. Foyaite, Portugal, Jannasch, Neues Jahrb., II., II. 6. Nepheline syenite,
Sao Paulo, Brazil, Machado, Tsch. Mitt., IX., iS88, 334. 7. Laurdalite, variety of
Nepheline-.syenite. Lund, Norway, Brogger, Syenit-pegmatit-gange, t,!,- 8. Leucite-
syenite, Arkansas, J. F. Williams. Igneous Rocks of Ark., 276. 9. Nepheline-
syenite, Beemerville, N. J., F. W. Love for J. F. K., N. Y. Acad. Sci., XL, 66.
10. Basic Nepheline-syenite, Beemerville, N. J., J. F. Kemp, N. Y. Acad. Sci., XL, 86.

Covuiiciits on the Analyses. — A considerable range is shown in the
SiO^, some analyses going below the usual percentages for syenites
and the last analysis being abnormal. In general the amounts of
alkalies are extremely high, with Na.,0 in excess, in which respect
the phonolites are paralleled.

Mincralogical Composition mid Varieties. — The minerals of neph-

THE NEPHn.LINE SYEX/TES. 37

eline syenite are in general the same as those of syenite proper,
with the addition of nepheline, often sodalite, and several charac-
teristic ones into which the rare earths enter as bases. Zircon
is widespread and is often large enough to afford fine crystals.
For this reason the rocks were named zircon-syenite many years
ago. The nepheline is often called cleolite (or ela;olite), from the
former custom of speaking of this mineral in pre-Tertiar>' rocks as
el?colite and in later ones as nepheline. just as we have hatl ortho-
clase and sanidine, but the custom is gradually falling into disuse.
Attempts have been made to give different names according to the
tlark silicate ; for instance, those with hornblende were called fo\aite .
from Foya, a Portuguese locality ; those with biotite. miascite from
Miask, in the Urals. But both these minerals so often appear to-
gether or with pyroxene that the practice is not generally observed.
Ditroite is a variety rich in blue sodalite. The Litchfield, Me.,
rocK has been shown by Hayley to have as its feldspar almost ex-
clusively albite, and he. therefore, has called it litchfieldite. The
texture of nepheline-.syenites varies very much. At times it is
very coarsely granitoid, and again it is what is called tr.ichytic,
/. e., with little rods of feldspar, more or less in flow lines, like a
trachyte and marking a pas.sage to the phonolites. Types have
been based on these characters. Where at all finely cr\'stalline.
the determination of nepheline-syenites, as against true syenites,
is a matter for the microscope. Xepheline-s\cnites are compara-
tively rare rocks. Corresponding rocks with Icucitc arc as ycl
only known from Arkan.sas and Montana.

Relationships. — As already remarked, the nepheline-syenites are
closely related to the true syenites, and to the phonolites. With
certain basic plagioclase rocks with nepheline. called theralites,
they are also of near kinship.

Giological Occurrence, Alteration. — Ihe nepheline-syenites are
special U' prone to appear as dikes, often on a very large scale.
Their alteration affords no special features, as distinguished from
the syenites or granites, e.xcept as regards the secondary minerals
from tile nepheline. Xatrolite, muscovite and kaolin are all known
in this relation and the last two have been called liebenerite and
gieseckite. Cancrinite also results from the alteration of nepheline.
The rarity of the nepheline-.s\'enites has prevented their playing
an important role among metamorphosed rocks.

Distribution. — Nepheline-.syenites are known in this c<>iintr\- at

38 A HANDBOOK OF ROCKS.

Montreal and Dungannon, Ont.; Litchfield, Me.; Red Hill, N. H.;
Salem, Mass.; Beemerville, N. J., where a superb dike is exposed ;
and near Little Rock, Ark., where the area is extensive. Very in-
teresting ones occur near Rio Janeiro, and in the State of Sao
Paulo, Brazil. Abroad the Portuguese locality, in the Monchique
Mountains ; the one at Ditro, in Hungary, and the wonderful dikes
near Christiania, in Norway, so prolific in rare minerals, are of
especial interest.

CHAPTER IV.

The Igneous Rocks, Continued. The Dacites, the Andesites
AND the Rocks of the Basalt Group.

The Dacites and Andesites.

The Dacites.

SiO, A1,0, FcjO, FeO CaO MgO N\0 K,0 I^ss. Sp. Gr.

1. 69.96 15.79 2.50 1.73 0.64 3.80 4.12 1.53

2. 69.36 16.23 088 1.53 3.17 1.34 406 3.02 0.45

3. 67.49 16.18 1.30 1.22 2.68 1.34 4.37 2.40 2.69

4. 67.2 17.0 3.5 1.2 4.5 1.5 3.7 1.6 0.9

5. 67.03 16.27 . 3.97 3.42 1.19 2.71 3.50 1.56

6. 66.03 14.57 257 1.19 3.38 1.89 3.71 2.70 2.07

7. 63.36 16.35 2.12 3.05 4.79 3.28 3.58 2.92 0.99

Thk Andesitfis.

8. 67.83 15.02 . . 5.16 3.07 0.29 2.40 3.20 1.11

9. 65.50 14.94 1 72 2.27 2.33 2.97 5.46 2.76 1.37
to. 63.49 18.40 2.44 1.09 2.30 0.66 5.70 4.62 1.04

11. 62.94 18.14 • ' 382 6.28 3.06 3.83 1.22 0.60

12. 61.62 i6.8(> 6.61 6.57 2.07 3.93 1.66

13. 61.58 16.34 6.42 5.13 2.85 2.69 3.65 0.64
14 59.48 16.37 3.21 3.17 4.88 3.29 3.30 2.81 2.02

15. 56.19 16.12 4.92 4.43 6.99 4.60 2.96 2.37 1.03

16. 56.91 18. iS 4.65 3.61 7.11 3.49 4.02 1.61 0.36

1. McClelland Teak, near Com.stock Ixxlc, Ncv.. F. A. (looch, Hull. 17, U. S. (j
S. , 33. 2. Lassen's Peak, California, Hague and Iddings, A. J. S., Sept., 1883, 232

3. Sepulchre Mountain. Yellowstone Park. J. P. hidings, Iliil. Soc. Wash., XI., 210

4. Nagy-Sebes, Hungary, Doelter, Tscher. Min. Petr. Mitt., 1873, 93. 5. Fureka
Dist., Nev. A. Hague Mono., XX., U. S. G. S.,264. 6-7. Colombia, S. America
From Kuch's Petrographie of Colombian Volcanoes, quoted in Jour. Geol., I., 171. 8
Hb.-mica-andesite, Fureka Dist., Nev., Mono. XX., U. S. G. S., 264. 9. Mb. -mica
andesite, Sepulchre Mountain, Yellowstone Park, J. P. Iddings, Phil. Soc. Wash., XI.
210. Compare No. 3. 10. Mica-andesitc, Kosita Hills, Colo., \V. Cross, Colo. Sci. Soc.
1887, 250. II. Ijisscn's Peak, Calif., Hague and Iddings, A. J. S., Sept., 1S83, 225. I2
Mt. Rainier. Sec last reference. 13. Pyroxcne-andesite, Eureka Dist., Nev., Mono. XX.

40 A HANDBOOK OF ROCKS.

U. S. G. S. , 264. Compare No. 5. 14. Hyperstheneandesite, near Red Bluff,
Mont., G. P. Merrill, Proc. U. S. Nat'l Museum, XVII., 651. 15. Hypersthene-an-
desite, Buffalo Peaks, Colo., W. Cross, Bull. I., U. S. G. S. 26. 16 Colombia, S.
America. See Nos. 6 and 7.

Comvioits oil the Ajialyses. — It appears at once from the analyses,
that the dacites are high in silica, in which they equal the lower
ranges of rhyolites. As compared with the latter, soda is prevail-
ingly in excess of potash, and as a rule the other bases run higher.
The andesites lap over the lower limits of the dacites and have
much the same range in silica as the trachytes. All the bases reach
notable percentages, but the alkalies recede as the others increase.

Mineralogical Composition, Varieties. — The name dacite is derived
from the old Roman province of Dacia, now a part of modern
Hungary. The name andesite was suggested by the abundance of
these lavas in the Andes Mtns. Quartz occurs usually in abundant
crystals in the dacites, causing them to closely resemble the
rhyolites. The distinction, when it can be made, and this is not
always without the microscope, depends on the prevalence of
striated feldspars. The prevailing dark silicate is biotite, as is
usually the case with an acidic rock. Hornblende and augite are
rarer. Magnetite and the small accessory minerals, apatite, zircon,
etc., are generally present. In the acidic andesites, biotite is also
commonest ; hornblende and then pyroxene favor those of decreas-
ing silica. Andesites with biotite are usually called mica-andesites.
Andesite when used alone implies hornblende-andesite. Prevail-
ing augite is indicated by the name augite-andesite. If magne-
sia is in considerable amount hypersthene may result and af-
ford hypersthene- andesite, a frequent rock in the West, but
for all one can usually say from ordinary examination, the
rocks may be augite-andesite, or even hornblende-andesite. An-
desites strongly resemble trachytes, but it is to be appreciated
that trachytes are comparatively rare rocks, while andesites are
among our commonest lavas, and along the west coast of North
and South America are the prevailing volcanic rock. The dark
silicates are also increasingly abundant in the andesites. • They
pass insensibly into diorites, by the development of granitoid tex-
ture. The older andesitic lavas have been called porphyrites^ on
the analogy of porphyry, and then mica, or hornblende or augite
is prefixed. Others use porphyrite for intruded sheets and dikes ;
and others still for varieties, with a eroundmass of medium coarse-

THE DACITES AXD AXDESITES. 41

ness, restricting thus andesite to the finely crystalhne or glassy
varieties of groundmass and diorite to the coarse, granitoid rocks.
These distinctions belong, however, to the refinements of the sub-
ject.

On analogy with the name trachyte, which was formerly ap-
plied in the field to all these more or less rough and cellular lavas,
but which is now, by universal consent, restricted to the orthoclase
rocks, G. F. Becker has suggested that " asperite " be used for
those with plagioclase, basing it on the Latin word for rough.
With general acceptance it ought to prove a verj' useful term, be-
cause the observer is often in doubt whether a rock is dacite or
andesite, and if the latter, to which group it belongs. Propylitc
is a name still more or less current in the West. It was created
for a series of rather coarsely crystalline or granitoid andesites, that
are of early Tertiary age, and that often have the dark silicates altered
to .secondary minerals. The name means •' before the gates." and
the significance was that coming just before the geological time of the
true volcanics, yet resembling them, they de.servcti this distinction.
It is now obsolete and reasons for its special existence were long
ago exploded, but having been employed on the Comstock lode, it
has pa.ssed into western usage. There is no special and neces-
sary geological age for any igneous rock.

Alteration, Mctamorpliism. — The andesites in decay afford kaoli-
nized material and mixtures of this with chloritic products that are
very difficult to identify. Thus the now famous andesitic breccia
at Cripple Creek, Colo., can rarely be shown to the eye to be other
than a white, kaolinized mass, and decomposed outcrops of mas-
.sive flows are no less unsatisfactory. Where metamorphic proc-
esses affect older flows, felsitic and silicified forms result similar
to those mentioned under rhyolites. The tracing of the history
of the rock is then a matter for the microscope and chemical
analjsis when indeed it can be done.

Tuffs. — Andesitic tuffs and breccias (/. <•.. aggregates of angular,
volcanic ejectments coarser than tuffs) are rather common in the
western volcanic districts. With ordinary observation they can
only be identified by finding fragments large and fresh enough to
indicate the original. Such have proved of great economic im-
portance at Cripple Creek, Colorado.

Distribution. — Andesites are' very wide-spread in the West. The
vast laccolites that form many of the peaks in Colorado are in-

42 A HANDBOOK OF ROCKS.

truded andesites (porphyrites) of a rather acidic type, frequently
with some orthoclase. In the Yellowstone Park they are impor-
tant. In Nevada, as at Eureka and the Comstock lode, they have
proved of great geological interest, and especially near the latter,
with its many miles of drifts, shafts and tunnels, very important data
for the study of rock masses have been afforded. The old cones
along the Pacific, Mt. Hood, Mt. Shasta, Mt. Rainier and others
are chiefly andesite. The products of Mexican and South Amer-
ican volcanoes are also of this type, and indeed along the whole
Pacific border the recent lavas have many features in common.
Abroad andesites are seldom lacking in great volcanic districts.

The Basalts, Including Diabase.

Basalts.

SiOj

MP,

Fe,03

FeO

CaO

MgO

Nap

Loss

Sp. Gr.

57-25

16.45

1.67

1.77

7-65

6.74

3.00

1-57

0.45

53-81

1348

3.02

7.39

10.34

6.46

3-23

0.64

0.57

2.75

53-62

22.09

4.21

6 02

6.24

3.16

0.57

503

52.27

17.68

2.51

5.00

8-39

6.05

4.19

1-58

0.82

5>.58

11.92

2.96

13.05

8.52

4.09

0-95

0-34

1-52

2.989

50.38

19.83

6.05

2.00

10.03

5-36

2-15

1.76

1.37

49-45

17.58

3-41

7.20

4.05

5-83

1-57

4.34

49.04

18.11

2.71

7-70

7. II

4.72

4.22

2. II

1.29

2.738

48.40

17-95

2.28

8.85

10.05

6.99

2.86

1.03

0.34

2.8

lO.

47-54

19-52

4.24

6.95

11.70

6.66

3-09

0.16

2.981

II.

46.43

17.10

II. 16

10.38

9.78

2.50

2.65

Diabases

12.

54-52

19.10

2.83

5.89

7.25

3-92

3-73

2.30

0.59

2.7

13-

53-13

13.74

1.08

9.10

9-47

8.58

2.30

1.03

0.90

2.96

14.

49.28

15.92

1. 91

10.20

7-44

5-99

3-40

0.72

3- 90

2.86

48.75

17.17

0.41

13.62

8.82

3-37

1.63

2.40

2.985

i6.

46.28

12.96

4.67

6.06

10.12

8.71

3-75

3-34

2.921

17-

45-46

19.94

15.36

8.32

2,95

2.12

3.21

2.30

2.945

Limburgi

te.

i8.

46.90

10.17

1.22

5-17

6.20

20.98

1. 16

2.04

5.42

2.86

19.

40.22

14.41

17.42

2.36

11-53

7.29

3-94

1.90

1. 10

2.89

Nepheline-basalt.

20.

38.35

9.18

20.32

11.76

13.78

2-77

2.02

1.20

3-223

I. Basalt with quartz. Cinder Cone, Calif., J. S. Diller, A. J. S., Jan., 1887, p. 49,
Anal. Hillebrand. 2. Kilauea, Sandwich Is.; Cohen., NeuesJahrb.l88o, IL, 41. 3. Ice-
land, Schirlitz, Tsch., Mitt., 1882, 440. 4. Rio Grande Canon, N.M., J. P. Iddings, A. J.
S., Sept., 1888, 220, Anal. Eakins. 5. Dalles, Oregon, Lemberg, Z. d. d. g. G., XXXV.,
116. 6. Richmond Mtn., Eureka Dist., Nev., A. Hague, Mono. XX., U. S. G. S.
164, Analyst Whitfield. 7. Point Bonita, Calif., F. L. Ransome, Bull. Geol. Dept. ,
Univ. Calif., I., 106. 8. Buffalo Peaks, North Park, Colo., Woodward, 40th Parallel
Surv., II., 126. 9. Shoshone Mesa, Nev., Woodward, 40th Par. Surv., II., 617. lO.

THE BASALTS. 43

Cascade Mts., Oregon, Jan nasch, Tsch. Mitth., l88l, 102. 11. tilassy basalt, Edge-
combe Island, near Sitka, Alaska, Lemberg, Z. d. d. g. G., XXXV., 570. 12. Dia-
base Hills, Nev., Woodward, 40th Parallel Su^^•., I., Table opposite p. 676. 13. Penn.
R.R. cut, Jersey City, N. J., G. \V. Hawes, A. J. S., iii., IX., 186. 14. Lake Salton-
stall,Conn., Ibid. 15. Dike near Boston, Mass.,\V. H. Hobbs, Bull. Mus. Comp. Zool.,
XVI., I. 16. Point Bonita, Calif., F. L. Ransome, Bull. Geol. Dept, Univ. Calif.,
I., 106. 17. Dike at Palmer Hill, Ausable Forks, X. V., J. F. Kemp, Bull., 107, U.
S. G. S., 26. 18. Limburgite, Bozeman. Mont., G. P. Merrill, Proc., U. S. Natl.
Mus., XVII., 640, Anal. Chaiard. 19. Limburgite, Palma., L. Van Werveke, Neucs
Jahrb. , 1879, 485. 20. Xephelinc-basalt, Pilot Knob, near .\ustin, Texas, J. F.
Kemp, Amer. Gcol., Nov., 1890, 293.

Coinincnts. — Tlic first analysi.s is very like the more basic ande-
sites, except in its high percentage of MgO. It is of a curious and
exceptional basalt with quartz phenocrysts. regarding which men-
tion is made later. In general, the othersare notably high in the
oxides of iron, in CaO and MgO. The specific gravity is also
high. The analyses of diabases differ in no es.sential from those
of true basalts. No. 13 is of especial interest, as it is the one
usually (j noted as the type of our Triassic diabases. The last
three are representatives of the unusual varieties, later mentioned,
that are of rare occurrence in the United States, but that represent
the limiting percentages of SiO^ in rocks mostly composed of
ferromagnesian silicates.

Mincr-.xlogical Composition. I iirii/iis. — The name basalt is a ver)'
ancient term and has been explained in .several ways. Many re-
gard it as a corruption of basanites which was used by Pliny, but
for what rock is uncertain. The Greek word for the black touch-
stone or Lydian stone u.sed by the ancient jewellers is. similar to
this last form. Others refer it to Basan or Bashan, the kingdom
of Og, as mentioned in the Old Testament, Deuteronomy III., i.
Again an ICthiopian word " basal," used by lMin\' for an iron-bear-
ing rock, has been suggested. Agricola in the sixteenth centurj'
gave it its present signification.

It stands for a very large and important group, which has man)-
mineralogical varieties, but which can seldom be subdivided with-
out exact microscopical stutl\-. The name basalt, therefore, em-
braces them all when megascopically considered. They are all
heavy, black, gray or brown rocks, usually porphyritic, but at
times lacking all phenocrj'sts and merely a closely crystalline, dark
mass. Plagioclase, augite, olivine and magnetite are the chief
minerals present, and the groundmass is usually a finely crystalline
aggregate of these and of some dark glass. .\t the acidic extreme

44 A HANDBOOK OF ROCKS.

of basalts are certain dark rocks, with abundant augite, that yet
lack olivine. Though closely related to the augite-andesites, they
are sometimes called olivinc-free basalts. The typical basalt has,
however, olivine, and often exhibits this mineral in large rounded
masses. Coarsely crystalline basalts are called dolantes, a very
common and useful field term. Typical dolerites are porphyritic
but shade into granitoid varieties. An old group of rocks and one
whose name often puzzles beginners as regards its special signifi-
cance is called diabase. The diabases are generally entirely crys-
talline and apparently of a granitoid texture. The feldspars are,
however, in long and relatively narrow crystals, as contrasted with
the broader ones of typical granitoid rocks. In the interstices of
these lath-shaped feldspars are found the dark silicates and mag-
netite. The texture is called opJiitic and is more especially of
microscopic importance. (Ophitic is more fully explained in
the Glossary.) There is, therefore, ground for difference of
opinion as to whether the diabases should be placed with the
granitoid or with the porphyritic rocks, but as they are always in
sheets or dikes, which shade into porphyritic forms on the contacts,
and which are really volcanic in their nature, they are put here as
essentially basalts at the extreme of the group toward the granitoid
division. In former times the name was only used for pre -Tertiary
rocks ; but as often stated, the time distinction has been long since
exploded. There are both diabases and olivine-diabases, but really
except in connection with microscopic work the term diabase is
superfluous, while we have and use basalt, dolerite and gabbro. It
is, however, so intimately involved with the literature of many im-
portant mining regions and others of great geological interest that
the student should be familiar with its employment and its signifi-
cance. The name is derived from the Greek verb meaning to
penetrate or pass through, and was suggested by the dikes in which
the early occurrences were met. Greenstone and trap are also old
names chiefly applied to diabase.

Into many basalts nepheline or leucite enter, and if in notable
amount, with little or no olivine, the rock is called tephrite, or
leucite-tephrite ; if with much olivine, basanite or leucite-basanite ;
if with little or no olivine and no plagioclase, nephelinite or leuci-
tite ; and if there is much olivine, nepheline-basalt and leucite-
basalt. The distinctions are, however, microscopic and in the field
basalt is sufficient. Again the rocks may lack both the feldspar

THE BASALTS. 45

and the feldspathoids and consist merely of augite in a glassy
groundmass, /. c, augitite, or of augite and olivine in a glassy ground-
mass, giving limburgite, but these are also only of microscopic
moment, although the significance of the names should be under-
stood. Basalts with melilite are the rarest of the group.

Basalts rather rarely have hornblende, but when this is present
it is a deep brown variety known as basaltic hornblende. I^iotite
is also uncommon, except in the varieties with ncphcline or leucite.
A most extraordinary basalt has been met by J. S. Oilier and others
in our western volcanic regions that contains quartz in moderately
large phenocrysts. The presence of this mineral in such basic
rocks is most peculiar, but it is explained by assuming exceptional
conditions of crystallization in the early histor>' of the magma, lead-
ing to the separation of quartz which was never re-absorbed.

Alteration, Metamorpliistn. — The olivine of basaltic rocks is the
first mineral to alter, and it soon becomes a network of serf)cntinc
vcinlets enclosing unchanged nuclei. The augite also pa.sses
rcadil)- into chlorite and finally the feldspar kaolinizes. The pre-
valence of green, chloritic j)roducts suggested the name green-
stone for the old diabases. The basaltic rocks arc extremely im-
portant in connection with metamorphism, and the iron-mining
regions around Lake Superior |)rcsent su|x:rb illustrations of the
process. The augite has the greatest tentlenc)- to pass into green
hornblentle, by what is called a " paramoqihic " change, /. <•., a
change in the mineral without change in the chemical conijiosition
and without, as in pseudomorphs, preserving the original lorm.
Under shearing stresses and movements, accompanied by this
paramorphic change, diabases, so-called, pass into hornblende-
schists, and even chlorite-schists or green-schists, losing their mas-
sive structure entirely and becoming a very different rock, and one
that can be traced to its original with great difficulty. Such horn-
blendic rocks arc also called amphibolites.

Tuffs. — Basaltic tuffs, agglomerates, breccias, etc., are well known
aiul often accompany the massive flows. The\- mark an explosive
stage of eruption before the actual outi)ouring of lava.

Distributio)i. — Basaltic rocks are enormously developed in this
couiUr)-. The oldest strata are penetrateil b\- numerous black,
igneous dikes, in practical!}- all their exposures. The New I''ng-
land seacoast is especially seamed by them, and hundreds may be
met in a short distance. The Adirondacks and the White Moun-

46 A HANDBOOK OF ROCKS.

tains, the Highlands of New York and New Jersey, have many.
In the East are the intruded sheets of Triassic diabase, up to 500
feet in thickness, forming many of the most prominent landmarks,
such as Cape Blomidon, N. S.; Mts. Tom and Holyoke, Mass.;
East and West Rock, near New Haven, Conn., the Palisades on
the Hudson, and many dikes in the Richmond, Va., and Deep River,
N. C, coal fields. Around Lake Superior, both in the iron and in
the copper regions, are still greater sheets, for many thousands of
feet of basalt (diabase) are present on Keweenaw Point. On the
north shore near Port Arthur, the head-lands of Thunder Bay ex-
hibit superb examples. The iron-bearing strata are penetrated by
innumerable dikes. The greatest of all the American basaltic areas
is, however, met in the Snake River region of southern Idaho and
extends into eastern Oregon and Washington. Many thousands
of square miles are covered with the dark lava and are locally
called the " Lava Beds." In Colorado, as at the Table Mtns., near
Golden and Fisher's Peak, near Trinidad, there are prominent
sheets, and the same is true of many other points in this State.
In New Mexico, Arizona and Texas they are also met. The vol-
canoes of the Sandwich . Islands are basaltic. Basaltic rocks with
nepheline are scarcely known in the United States. Some minor
dikes in the East, a volcanic neck at Pilot Knob, near Austin,
Texas, dikes and sheets in Uvalde Co., Texas, and a few dikes at
Cripple Creek, Colorado, are practically the only localities yet
identified. Leucitic rocks, more phonolitic than basaltic, are known
in the Leucite Hills, Wyo., and in Arkansas. Of basaltic affinities
they occur in New Jersey, but these and the nepheline rocks are
of small practical moment, although of great scientific interest.

Basalts have quite as great development abroad as here. The
islands off the north coast of Scotland are famous localities, and
many of the volcanic regions of the continent are no less well
provided. The lavas of Etna are chiefly basaltic, and those of
Vesuvius are remarkable for their richness in leucite. In India
are the great basalt fields of the Deccan, which are comparable in
extent with those of the Snake River ree^ion of the West.

CHAPTER V.

The Igneous Rocks, Continued. The Diokites, Gahhkos,

Pyroxenites and Pekidotites. Ultra-Basil

' Igneous Rocks.

Thk Diorites.

SiO,

AlA

Kep,

FeO

CaO

MgO

Na,0

K,0

Ix)ss Sp. Gr.

67-54

17.02

2.97

034

2 94

I 51

4.62

2.28

0-55

65.27

15.76

'•36

3-44

2.14

4.57

3 97

0.42

61.75

18.88

0.52

3-52

3-54

1.90

367

1.24

4.46 2.79

.S8.05

18.00

2.49

4.56

6.17

.155

3-M

2.18

0.86

56.71

1 8. 36

6.45

6.11

392

352

2.38

. . 2.86

52.35

15.72

2.90

7 32

8.98

7-36

2.81

» 32

« 35

5047

18-73

4.19

4.92

8.82

348

4.62

356

0.58 2.87

48.98

17.76

2.14

6.52

8.36

2.09

6.77

2.0S

4.50

'»•

48.19 •

16.79

18.37

^' '

6.85

»-32

5 59

111

2.31

I. (Quartz mica-dioritc, Electric Peak, Yellowstone Park, J. P. Iddinf^s, Anal, hy
Whitfield, Hull. ITiil. Soc. of Washington, II., 206. 2. Quartzaugite-diorilc, Watah,
Minn., A. Strcng, Neucs Jnhrbuch, 1877, 232. 3. Diorilc. Pcn-macn-mawr, Wales,
J. A. Phillips, Q. J. G. S., XX.XIII.. 424, 1877. 4. iJiorite. see under No. 1. 5.
Dioritc (granitoid andcsite?), Conistock Lode, Nev., R. W. Wootl ward, 40th Par.
Survey, I., op|). p. 676. 6. Augitc-diorite, Little Falls, Minn., A. Strcng, Neues Jahrb.,
1877, 129. 7. Augitc-diorite Mt. Fnirvicw, Custer Co., Colo., W. Cross, Anal, by
Kakins, Col. Sci. Soc, 1887, 247. S. Poqihyritic-di<>rite, St. John, N. B., W. 1).
Matthew, Trans. N. Y. Acad. Sci., XIV., 213. 9. Diorite dike rich in magnetite,
Forest of 1 )LaM Mine, N. Y., J. F. Kemp, A. J. S., Apr., 1888, 331.

Cotmmiits. — The analyses represents a scries that is closely paral-
lel with the dacites and andesitcs in the first five analyses, but that
recalls the basalts and diabases in the la.st four. M[^0 rules lower
than in the latter. It is evident that the prevailing feldspar would
be a lime-soda variety, but that orthoclase might readily be pro-
duced, for even when we allow some for biotite the percentage of
K2O is notable. The parallelism with gabbros will be shown by
the ne.xt table to be close.

Mincralogical Couiposition and Varieties. — The name diorite is

48 A HANDBOOK OF ROCKS.

derived from the Greek verb, meaning to distinguish in allusion to
the fact that the hornblende and feldspar could be distinguished one
from another in the coarsely crystalline ones first studied. Diorites
are granitoid rocks consisting of hornblende, biotite and plagio-
clase. Those with hornblende are diorites proper, while those with
biotite are called mica-diorites. Some augite is occasionally present,
marking passages to the gabbros and giving the name augite-
diorite. An intermediate type occurring in dikes, and containing
both biotite and augite is kersantite. Acidic diorites often have con-
siderable quartz, and are called quartz-diorites. Tonalite is a quartz-
hornblende-mica-diorite. Quartz-diorites shade insensibly into
granites, and the importance of the intermediate forms or grano-
diorites was emphasized under granite. These acidic diorites are
prevailingly light in color, but the more basic ones become de-
cidedly dark. Certain dikes with the minerals of diorite are called
camptonites. The great tendency of augite to change to green
hornblende causes a doubt to hang over the true character of
many diorites. They may often be metamorphic products from
gabbros or diabases.

Alteration, MetamorpJiisni. — In ordinary alteration the feldspar of
diorites kaolinizes and the hornblende changes to chlorite, affording
one of the varieties of the so-called greenstones. Under shearing
stresses in metamorphism the diorites pass into gneisses, and into
hornblende schists or amphibolites. In many mining regions even
decidedly schistose varieties are still called diorite. A final stage
is chlorite-schist, wherein the hornblende has altered to chlorite.

Distribution.. — True, original diorites are not very common rocks
in America. A well-known quartz-mica-diorite is extensively de-
veloped in a series of igneous rocks, called the Cortlandt Series,
on both sides of the Hudson, below Peekskill. In the Sudbury
nickel district, north of Lake Huron, dense, dark diorites are the
chief rock containing the ore, but there is always the possibility
that the hornblende is altered augite. Mt. Davidson, above the
Comstock Lode, is either a true diorite or a granitoid phase of
andesite. Authorities differ as to its interpretation. Grano-
diorites, the intermediate rocks between granite and diorite. are
well recognized both in the East and the West.

Diorites are well known abroad and have been described from
various places in Great Britain, Germany, France and Austria.
The typical tonalite is obtained near Meran, in the Tyrol.

GAB BROS, PYROXEXITES AXD PERIDOTITES. 49

The Gabbros, Pvroxexites and Peridotites.

(labbro.

SiO,

AlA

Fe/'.i

FeO

CaO

MgO

Na,0

K,0

Loss.

Sp. Gr.

59-55

25.62

0-75

7 73

ir.

5.09

0.96

0.45

2.66

55-34

16.37

0.77

7-54

7 51

5-05

4.06

2.03

0.58

5472

17-79

2.08

6.03

6.84

5-85

3.02

3.01

2.928

54-47

26.45

1.30

0.67

10.80

0.69

4-37

0.92

053

2.72

53-43

28.01

0-75

11.24

0.63

4.85

0.96

tr.

2.67

52.14

29.17

3-26

10.81

0.76

3.02

0.98

0.58

49 «S

21.90

6.60

4-54

8.22

303

3-83

1.61

1.92

48.02

17.50

1.80

7-83

13.16

10.21

1.48

Ir.

0.79

46.85

19.72

322

7-99

13.10

7-75

1.56

0.09

0 56

10.

46.85

18.00

6.16

8.76

10.17

8.43

2.19

0.09

0.30

3097

II.

45.66

16.44

0.66

1390

7-«3

11.57

2.13

0.41

0.07

I'yroxcnitc.

12.

55 M

U.J 5

348

4-73

•Viv

26.66

0.30

'3-

5398

1 32

1. 41

390

»547

22.59

0.83

330»

44.01

11.76

15.01

4.06

2525

I'cridfitit

15.

47 4'

6.^9

7.06

4.80

14.32

>5-34

0.69

1.40

2.10

3 30

K..

46. OJ

9.27

2.72

9-94

353

25.04

1.48

0.87

0.64

3.228

17-

41.00

7.58

5-99

10.08

2359

0.52

4-73

2989

18.

36.80

4.16

. .

8.33

8.63

2598

0.17

2.48

0.51

3384

5.88

7-<^4

5-16

9.46

22 96

0.33

2.04

7.50

20.

29.81

2.01

5.16

435

7.69

32 41

0.1 1

0.20

8.92

2.78

1. Anorllidsitc, Chateau Richrr, Canada, T. S. Hunt, Ocolofty of Canada, 1863. 2.
Norite, Cortland .Series, Montmsc Point, Hudson River, Anals. by Munn, for J. 1).
Dana, A. J. S. , Aug., 1881, p. 104. 3. Gabbro, near Cornell Dam, Croton River,
H. T. Vult6, for J. !•". Kcaip, unpul>li.she;l. 4. Anorthosite, Summit of Mt. Marcy,
.Adirondacks, A. R. I.fc<ls, 30th Ann. Rep. N. V. State Museum, reprint, p. 14, 1876.
5. Anorthosite, Nain, Labrador, A. Wichmann, '/.. d. d. r. Gcs., 1884. 6. Gabbro,
Iron Mtn., Wyo. , 40th Parallel .Sur>'.,IL, 14. 7. Ciabbro, near Duluth, Minn.,
.Streng, Neucs Jahrb., 1S76, 117. 8. Ciabl)n)dioritc, Baltimore, Md., average of seven-
teen samples, Mackay forG. IL Williams, L*. S. G. .S., Hull XXVIIL, 39. 9. Ciab-
bro, Baltimore average of twenty-three samples, ibid. 10. ( labbro, Southwest Adiron-
dacks, C. M. Smyth, Jr , \. J. S., July, 1894, 61. 11. Gabbro. Northwest Minn.. \V. S.
Hayley, Anals. by .Stokes, Jour, lieol.,1., 712. 12. rvTo.xenite. var. NVebsterite, Webster,
N. C, E. A. Schneider for Geo. IL Williams, Amer. Geol., July, 1890, p. 41. 13.
Pyroxenite, Baltimore, Md., T. M. Chatard for G. IL Williams, ibid. 14. Pyroxe-
nite, Meadow Creek, Mont., Geo. P. Merrill, PriK. U. S., Nafl. Mus., XVIL, 658.
15. Pcridotite. Cortland .Series, Montrose l*t., N. V.. Emerson for (i. IL Williams, A.
J. S., Jan., 18S6, 40. 16. Peridolite, Custer Co. Colo., L. G. Eakins for W. Cross.
Proc. Colo. .Sci. Soc, 1887, 245. 17. Peridotite, Baltimore, Md., L. Mackay for G,
H. Williams, Amer. Geol., July, 1890,39. 18. Peridotite, Dewitt, N. V., IL S.
Stokes for 1 )arton and Kemp, .Vmer. Jour. Sci., June, 1895, 456. 19. Mica Perido-
tite, Crittenden Co., Ky., W. F. Hillebrand for J. S. Diller, A. J. S., Oct., 1892, 288.
20. Peridotite, Elliott Co., Ky., J. S. Diller, Bull. 38, U. S. G. S.. p. 24.

50 A HANDBOOK OF ROCKS.

Comments. — The range in composition presented by the gabbros
is in many respects the same as that of the basalts. As a general
rule the most feldspathic members ( the anorthosites) are the
highest in silica, Nos. i, 4 and 5. No. 6, although described as
gabbro, is doubtless of the same character, for the low FeO and
MgO indicate few dark silicates. These rocks are also highest in
AI2O3 of all the rock analyses yet quoted. As the CaO and MgO
increase in amount, the [pyroxenes and olivine grow notably more
abundant. In the gabbros of the Cortland series (Nos. 2 and 3) and
in those near Duluth (No. 7) there is often considerable orthoclase
as is indicated by the K2O. The pyroxenites are distinguished by
the falling off in Al^Og, due to the disappearance of feldspar, and
by the increase in CaO and MgO for the .pyroxenes. The perido-
tites reach a lower percentage of silica than any other igneous
rocks so far cited, but if this is accompanied by high H2O, allow-
ance must be made for the relative decrease of the original SiO^ in
the change to serpentine. The great percentages of MgO are very
notable, and are due to the presence of much olivine, magnesian
pyroxene and, in instances, biotite. Chromic oxide is also always
present in small amounts, and oxides of nickel and cobalt are
usually in perceptible quantity.

Miiicralogical Cojiiposition, Varieties. — The name gabbro is of
Italian origin, and has been applied of recent years, and with
growing favor to the great group of granitoid rocks consisting in
the typical cases of plagioclase and pyroxene. The diabases, as
explained under basalt, are now generally classed with the vol-
canic rocks, although texturally and mineralogically they really
lap over true gabbros. The so-called gabbro group is a very large
and characteristically variable one. Originally the name gabbro
was only applied to a mixture of plagioclase and the variety of
monoclinic pyroxene called diallage, that has pinacoidal as well
as prismatic cleavages, but of late years all granitoid, plutonic
pyroxene-plagioclase rocks are collectively spoken of as the gab-
bro group. At the acidic extreme we have in Canada and the
Adirondacks enormous masses of rock that are practically pure,
coarsely crystalline labradorite. Pyroxene is the dark silicate
when any is present, but often it is insignificant. These pure feld-
spar rocks are best called anorthosites, from the French word for tri-
clinic feldspar, but the word is not to be confused with anorthite,
the lime-feldspar, with which it has no special connection. As

GABBROS, PYROXENITES AND PERIDOTITES. 51

monoclinic pyroxene increases they pass into gabbros proper.
More or less biotite and hornblende may also be present. If the
pyroxene is orthorhombic we call the rock norite. Varieties with
olivine are frequent, giving olivine-gabbro and olivine-norite.
Gabbros and norites are not readily distinguished without the
microscope, unless the bronzy appearance of hypersthene can be
recognized. In the former case, gabbro is a good collective
term. An old and obsolete synonym of anorthosite is labradorite-
rock, of interest becau.se widely used in the early reports on the Adi-
rondacks. Norites were called hypersthene rock, or hyjxrrsthene-
fels, both of which arc undesirable rock names. Gabbro intrusions
of not too great extent or irrcgularit\- for careful stud)' have been
observed to grow more basic toward the outer margins.

The gabbros pass insensibly, by the decrease of plagiocla.se. into
the pyro.xenites and peridotites, and in any great gabbro area all
these are usually present, but they may occur also as indcjxrndcnt
masses. The pyroxcnites are practically pjroxcnc, with little if
any other minerals. There is some variety, according as the rock
contains one or .<-cveral of the following : cn.statite, bronzite, hyper-
sthene, diallage <raugite ; but with the unassisted eye, it is sekloni
that one can be sure of these tlistinctions, c.vcept as the ortho-
rhombic |)yr()xcncs exhibit a bronze luster. Hornblende, magnetite
antl pyrrhotite may also be present. With the accession of olivine,
peridotite results, so named from the I'Vench word peridot for
olivine, and a number of varieties ha\e been made accortling as the
olivine is associated with one or more of the minerals cited for
pyroxenites. The list is given under Peridotite in the Glo.ssary. The
distinctions are however hardl\- po.ssible without microscopic aid.
As the extreme of peridotites we have a nearly pure olivine rock,
called d unite, important in North Carolina. Much magnetite may
be associated with peridotite; indeed at Cumberland Hill, R. I., there
is enough to almost make the rock an ore. Chromite, too, is a fre-
quent associate. As peridotites shade into a porphyritic te.xture,
especially in dikes, they have been called picrites, and even further
varieties, such as kimberlitc, have been made. Black hornblende,
which is brown in thin sections, is frequent in both pyroxenites and
peridotites, and may even form a rock itself, hornblendite. Dark
brown biotite is also often jire.scnt in considerable amount.

Some writers have regarded the pyroxenites and peridotites as
of doubtful igneous origin and have placed them with metamorphic

52 A HANDBOOK OF ROCKS.

rocks, but from their frequent association with gabbro, and from'
their independent occurrence in dikes, there is no good reason to
doubt their true, igneous nature.

A very rare granitoid rock, consisting of plagioclase, nepheh'ne
and ferro-magnesian siHcates has been called theralite from the
Greek verb to seek eagerly, because its discovery was anticipated
before it was actually found by J. E. Wolff in the Crazy Mountains,
Montana. It is an extremely rare combination of minerals, but of
special scientific interest because it corresponds among the gran-
itoid rocks to the tephrites and basanites of the porphyritic.

Alteration, MctaniorpJiisin. — The gabbros alter chiefly by the
formation of serpentine and chlorite from the dark silicates. The
pyroxenites and peridotites change readily into serpentine, often
with an intermediate stage as hornblende-schist. Under dynamic
stresses, especially shearing, anorthosites and gabbros pass into
gneissoid types, and in the process much garnet may be developed.
This is especially true in the Adirondacks. The larger feldspars
may be left in the gneisses as " eyes," or to adopt the German
term, as " Augen," affording Augen-gneiss, /. e., gneisses with
comparatively large lenticular feldspars. Much hornblende, espe-
cially in true gabbros, is often developed in the process. The
basic members, the pyroxenites and peridotites develop amphibo-
lites or hornblende-schists, which latter often furnish very puzzling
geological problems.

Distribution . — The anorthosites are, so far as we know, limited to
several Canadian areas, as at the headwaters of the Saguenay river,
and again north of Montreal ; and to the higher peaks of the
Adirondacks and some of their outliers. Mt. Marcy and its
neighbors consist of them. Gabbros are also present in vast quan-
tity, and are likewise well known in the White Mountains, in the
famous Cortlandt series, near Peekskill, on the Hudson, and in the
vicinity of Baltimore. Around Lake Superior gabbros are of
great importance, as the basal members of the Keweenawan system
and other older intrusions are largely formed of them. Fine
specimens can be had at Duluth. They are a characteristic wall
rock of titaniferous magnetite. Pyroxenites occur as subordinate
members of the gabbro areas, especially near Baltimore. Peridotites
are in the same relations in the Cortlandt series, in the Baltimore
area and in North Carolina. They are also known on Little Deer
Island, Me.; at Cumberland Hill, R. I.; in dikes near Syracuse,

GABBROS, PYROXEXITES AXD PERIDOTirES. 53

N. Y.; at Presqu' Isle, near Marquette, Mich.; in Kentucky ; in
•California and elsewhere in the West. When outlying dikes are
met, far from any visible, parent mass of igneous rocks and in sedi-
mentar)' walls, they are very frequently peridotite.

Abroad, anorthosites and gabbros are abundant in the Scandi-
navian peninsula, whose geology is in many respects like that of
Canada and the Adirondacks. In the north of Scotland gabbros
are of especial interest because they have been shown by Judd to
be the deep-seated representives of the surface basalts. On the
continent they are important rocks in many localities. The same
is true of Australia and such other parts of the world as have been
studied. Of especial interest are the peridotite dikes in South
Africa that have proved to be the matrix of the diamond.

Ultra-B.asic I(;neous Rocks. Meteorites.

A few ultra-basic igneous rocks arc known in which the silica
decrea.ses almost to nil, and in which the ba.scs, especially iron, are
correspondingly high. They are in general rather to be con-
sidered as basic .scgregrations in a cooling and crystallizing magma
than as individual intrusions. Tlie Cumberland Hill, R. I., so-
called peridotite, cited above, has ver)' little silica. Titanifcrous
ores have almost none, but they arc often exceptionally rich in
alumina. In a few cases metallic iron has been detectetl in basic
igneous rocks, suggesting analogies with meteorites.

Meteorites are rare and onl\' of scientific interest, but it is
extremely suggestive that such silicates as are met in them arc
chiefly olivine and cnstatitc. minerals rather characteristic of very
basic rocks. The commoner meteorites are an alloy of metallic
iron and nickel, but some rare sulphides are occasionally present.

As filling out the theoretical series we cannot bar out water and
ice. There is no reason why they .«hould not be considered igneous
rocks of extremely low fusing point, but they are so familiar that a
a simple reference to them is sufficient.

CHAPTER VI.
Remarks in Review of the Igneous Rocks.

Chemical Cotnposition. — Igneous magmas vary from about 8o^
silica as a maximum to practically none as a minimum, but impor-
tant rocks rarely drop below 40/0. Alumina is highest in the an-
orthosites or feldspathic gabbros, where it may exceed 25^. It is
lowfest in the pyroxenites and may be less than I'jo. The oxides
of iron are almost lacking in the highly siliceous, but may reach
beyond 20^ in the basaltic rocks, and with TiO., may be nearly
100% in some igneous iron ores. Lime attains its maximum of
12-15% i" ^^^ gabbros and pyroxenites, while magnesia in the
pyroxenites and peridotites may even surpass 25^. Potash is most
abundant in the orthoclase and leucite rocks ; soda in those with
nepheline. Combined alkalies may reach 15^ in the phonolites
and nepheline syenites. In general they are, however, about 4-10,
and may practically fail. Water in quantities over i % as a rule
is an indication of decay, but in the pitchstones this is not posi-
tively true, for the water reaches close to 10% and the rocks are
apparently unaltered.

Texture. — All of the typical textures are easily recognized in
characteristic development, but the glassy shade insensibly into the
felsitic, the felsitic into the porphyritic, and the porphyritic into the
granitoid. There are, therefore, intermediate forms that are diffi-
cult to classify. Yet, on the whole, the four textures are the
most satisfactory basis for classification, and as a guide, in accor-
dance with which to study. Chemical composition being the same,.
texture is a result of the physical conditions surrounding the magma
at the time of crystallisation and of the presence of niineralizers.

RFJ JEW OF IGMEOUS ROCKS.

TRACHYTES-SYEMTE-S RARE BASIC

RIIYOI ITES AND r.RANITES rHONOMTES NEPH. SYEN't'S ORTHOCLASE

ROCKS

iJAi iit>i A^u^--.IT^..^ li a •. a it . h d v r

q'tz diorites uiobites oabbros pyroxenites peridotites ores

Diagram intended to illustrate graphically the mineralogical composition of the
igneous rocks. The numbers indicate jxTCcntagcs in silica. The upper diagram in-
cludes the orthodase rocks, the lower the plagioclase and non-fcldspathic ones, Q is
(juartz ; C>, orlhocl.ise ; NT., nejiheline and leucite ; P, plagioclase; M, luuscovitc ; B,
hiotite ; II, hornhlcnile ; A, augitc ; Ol, olivine ; the black area, magnetite, pvrrho-
tite, and other metallic minerals.

Mineralos^y. — The above tliarjram.s, with a reasonable approxi-
mation to the truth, illu.strate the (juantitative niineralo^)- of the
igneous rocks. A section cut through the charts at any one point
expresses the relative amounts as well as kinds of the several min-
erals in the rocks whose names are along the top lines, and whose
percentages in silica are approximately shown. No mention is
made of texture. In the orthoclase rocks quart/ di.sappears at
about 65^^ SiO^, while orthoclase continues to the end ; plagio-
clase in small amount is quite constantly present throughout the
series. Xcpheline and leucite come in as indicated. Muscovite
appears only in the more acidic granites. Hiotite and hornblende
vary in relative quantity, but toward the basic end both yield to
augite. The rocks at the basic end are chiefly those recently dis-
covered by Weed and Pirsson in Montana, by Iddings in the
\''ellowstone Park and by Lawson in the Rainy Lake region. In
the plagioclase and non-feldspathic rocks quartz and orthoclase soon
run out, so far as any notable or regular amount is concerned.
Plagioclase holds along to about 45'/ SiO^, and at about 55^ SiO.,
may in the anorthosites be the onl)- mineral present, liiotite and

56 A HANDBOOK OF ROCKS.

hornblende are present all the way through, but toward the basic
end they tend to yield in importance to augite, which latter in some
pyroxenites, at about 49/r) SiO^, may be the only silicate present.
Olivine begins to appear at 55^0 and steadily increases with occa-
sional lapses almost to the end, where it may be the chief mineral.
The ores, as the extreme case, and without regard to silica, increase
so as to be the only minerals in the rock, forming thus the theo-
retical, basic limit. The diagrams also emphasize the fact that
igneous rocks shade into one another by imperceptible gradations,
and this is true of the orthoclase and plagioclase groups them-
selves, although not suggested by the separation of the two in the
drawings. The continuation of the orthoclase series to a basic
extreme is a fact that we have only appreciated in very recent
years.

A careful scrutiny of analyses and mineralogical composition
leads to the conclusion that practically the same magma may,
under different physical conditions of crystallization, afford miner-
alogical aggregates that vary considerably in the proportions of the
several minerals — now yielding more hornblende, again more
augite, and even affording quartz in a basalt. Hence, analyses in
different groups overlap more or less, and the difficulty of drawing
sharp lines of distinction is increased. Yet, allowing for this vari-
ation, chemical composition determines the resulting mineralogical
aggregate and is fairly characteristic.

Dcicnniiiatioii of Igneous Rocks. — In determining an igneous
rock, the texture should first be regarded, next the feldspars. If
orthoclase prevails, the presence or absence of quartz establishes
the rock. If plagioclase prevails, we look for biotite, hornblende,
pyroxene and olivine. If no feldspar is present, we look for the
presence or absence of olivine. On this basis the table on page 1 8
is to be used — there are, however, many finely crystalline rocks
which elude the power of the unassisted eye. If of light shades,
they can generally be referred with reasonable correctness to the
rhyolites, trachytes, felsites or andesites. If dark, they are prob-
ably basaltic in their nature, and the name " trap " is a very useful
and sufficiently non-committal term. Care should be exercised
not to confuse the porphyritic rocks having angular phenocrysts
with the amygdaloids, or the rocks whose almond-shaped cavities,
produced by expanding steam, have been filled by later introduced
calcite, or quartz, or other secondary minerals. While books are

REVIEW OF IGXEOUS ROCKS. 57

of great assistance, really the only way to become properly familiar
with rocks is to use the books in connection with correctly labeled
and sufficiently complete study collections.

Field Observations. — A rock is not to be considered by any intel-
ligent observer as a dead, inert mass in nature, but as an important
participant in the ceaseless round of changes that confront us on
every side. Familiarity with specimens and varieties in collections
ought always to be followed by observation in the field. We have
all grown to believe that in limited areas igneous rocks, however
varied they may be, are yet intimately related in their origin, or
are bound together by ties of kinship, " consanguinity " as Iddings
has called it. Some regions like eastern Montana and the Black
Hills have especial richness of high soda or potash magmas, giv-
ing rise to nepheline and leucite rocks, and sodalite syenites ; Colo-
rado, Utah and New Mexico have wonderful and enormous lac-
colites of andesites (porphyrites). The Pacific coast in South
America has andesites in vast extent from active volcanoes, and in
North America from extinct cones. Idaho, Oregon and Washing-
ton arc marked by basalts. The Atlantic coast region has a long
series of very ancient volcanoes, that preceded the early fossilifcrous
.strata from Newfoundland to North Carolina and that yielded nearly
the entire .series of the volcanic rocks. In the Adirondacks, on the
Hudson near Peekskill, near Haltimorc, and around I^ikc Superior
we find the members of the gabbro family ; while near tidewater
along the Atlantic seaboard we have granites, almost all with biotite.
Such facts as these suggested the creation of the term " pctrographic
provinces," to J. W. Judd. in the endeavor to suggest these kin-
ships of magmas in certain limited districts. There arc many
others even in North America that could be cited, but the above
will suffice to remind the reader that these broader relationships
should be always before him while extending his acquaintance
with rocks as they occur in the natural world about him.

CHAPTER VII.

The Aqueous and Eolian Rocks. Introduction. The
Breccias and Mechanical Sediments Not Limestones.

The members of this, the second grand division, are much
simpler, and, as a general thing, much easier to identify and to
understand than are the igneous. No single term is comprehensive
enough to include them all, and even the double one selected
above, in the endeavor to embrace as many as possible, and to avoid
the multiplication of grand divisions, still falls short of including sev-
eral. Nevertheless, those not embraced (the breccias) are of limited
distribution, and, for many reasons, go best with the other fragmental
rocks, even if, strictly speaking, they are neither aqueous nor eolian
in origin. Sedimentary is the most useful term, and is universally
applied as a partial synonym of the above, for it fairly includes the
most important members of the series, but the rocks deposited from
solution and the eolian rocks can hardly be understood by it.

The rocks will be taken up under the following groups :

.1. Breccias and Mechanical Sediments, not Limestones.

II. Limestones.

III. Organic Remains, not Limestones.

IV. Precipitates from Solution.

The limestones are reserved for a special group, although they
belong in instances to each of the other three. They form, how-
ever, such an important series in their scientific and practical rela-
tions, that it is in many respects advantageous to take them all up
together.

I. Breccias and Mechanical Sediments, Not Limestones.

Group I. is described in order from coarse to fine in the following
series, minor varieties not cited in the table being mentioned in the
text under their nearest relatives.

BRECCIAS. 59

1 ' 1

Coarse to Fine.

7. <

< a:

. ;j ARfiii LACEous Silt AND r, .v
y. ^ Sandstone. Sjiai.k. ^'"^^■

"^ H

j5 < Calcareois Calcareous ..
■^ Sandstonfu Shale. marl.

BRECCIAS.

The word breccia is of Italian origin and is used to describe ag-
gregates of anaiilar fragments cemented together into a coherent
ma.ss. The brcccia.s cannot all be properly considered to be cither
aqueous or eolian, and some have already been referred to under
the fragmental igneous rocks. Oftentimes they resemble con-
glomerates, but, unless formed of fragments of some soluble rock,
whose edges have become rounded by solution, there is no diffi-
culty in distinguishing them, lireccias, as regards their angular
fragments and interstitial filling, may be of the same materials or
of different ones. We may distinguish Friction breccias (Fault
breccias), rt///v hrccci'X^, and, for the sake of completeness, may
also mention here liruptii'c breccias.

Friction breccias are cau.sed during earth-movements by the
rubbing of the walls of a fault on each other, and by the con.sequent
crushing of the rock. The crushed material of finest grade fills in
the interstices between the coarser angular fragments, antl all
the aggregate is soon cemented together by circulating, mineral
waters. Such breccias occur in all rocks and are a fretjuent source
of ores, which are introduced into the interstices by infiltrating
solutions. Quartz and calcite are the commonest cements.

Talus breccias are formed b\- the angular debris that falls at the
foot of cliffs and that becomes cemented together b\- circulating
waters, chiefly those charged with lime.

Fniptive breccias may be producetl either by the consolidation of
coarse and fine, fragmental ejectments like tuffs, or by an erupting
sheet or dike that gathers in from the wall rock sufficient fragments
as inclusions to make up the greater part of its substance. These
are finally cemented together by the igneous rock itself and afford
curious and interesting aggregates, oftentimes representing all the
rocks through which the dike has forced its way to the surface.

6o A HANDBOOK OF ROCKS.

A crust may also chill on a lava stream, and when an added im-
pulse starts anew the flowing, the crust may be shattered into an
eruptive breccia of a still different type.

We often speak of breccias as "brecciated limestone," "brecciated
gneiss," or some other rock, making thus the character of the original
prominent. When the fragments and the cement are contrasted
in color, very beautiful ornamental stones result, which may be sus-
ceptible of a high polish.

A moment's consideration of the above methods of origin will
convince the reader that breccias, except as formed of loose, vol-
canic ejections are of very limited occurrence. Although
deeply buried rocks that share in profound earth rnovements often
suffer crushing and brecciation on a large scale, the effects are
chiefly detected by microscopical study.

Generalities Regarding Sedimentation.
In the production of the rocks next taken up, moving water
plays so prominent a part that its general laws are described
by way of necessary introduction. All streams or currents
charged with suspended materials exercise a sorting action dur-
ing the deposition of their loads. With materials of the same
density the sorting will grade the deposit according to the sizes of
the particles. With materials of different densities, smaller par-
ticles of heavier substances will be mixed with larger particles of
lighter ones. Assuming a swift current, it readily appears that,
when it slows up, the large and heavy fragments drop first of all ;
then the smaller fragments of the heavier materials and the larger
ones of those successively lighter, until at last the smallest particles
of the lightest rock alone remain in suspension. It is also impor-
tant to bear in mind that, the density being the same, the diameter
of the transportable particle increases with the sixth power of the
velocity. Thus, if we have a current of the proper velocity it will
be able to lift a grain of quartz a sixteenth of an inch in diameter ;
but if the velocity is doubled, the transportable particle will be four
inches in diameter. An appreciation of this law makes the size of
boulders moved by many streams, in times of flood, less surprising.
On the other hand, when the suspended material becomes exces-
sively fine, the ratio of its surface to its volume is so extremely
high that adhesion, or chemical action akin to hydration, or some
other influence not well understood, operates in pure, fresh waters,

ox SEDIMEXTATION. 6r

so as to practically render sedimentation impossible, even if the
medium be perfectly quiet. \V. M. Brewer has shown by a series
of experiments with all sorts of clays, lasting over many years, that
if we introduce into such an emulsion a mineral acid or a solution
of salt or of some alkali, the turbidity clears with remarkable
quickness. When, therefore, sediment-laden streams flow into the
ocean, or into salt lakes, even the finest jjart of their load speedily
settles out.

While we ma)' state thus simjily the laws of sedimentation, we
must not expect in Nature such well-sorted and differentiated
results as would at first thought apjjear to be the rule. Of rivers
and shore current.s — the two great transporting agents — the former
are subject to floods and freshets, giving enormousl)' increased
efficiency for limited periods, and again to droughts, with the same
at a minimum. Hence varying sediments overlap and are involved
together. ICddies and cjuiet portions in the streams themselves con-
tribute further confusion, and an intermingling of coarse and fine
materials. Shore currents have parallel increases of violence in
times of high wind and storms, and sink again in times of calm.

Kolian deposits are subject to even greater fluctuation, and their
irregularities are more pronounced than tho.se of the true Aqueous.
Both these cla.sses of rocks are marked by a more or less perfect
arrangement of their materials in la)ers. The layers give rise to
regular beds in deposits from quiet and uniform currents, and, al-
though in tho.se from swift ones they are ver\' irregular, as ex-
plained above, nevertheless, biddnii^, or stratification, is in the high-
est degree characteristic of the Aqueous and Kolian grand division.

When in the presence o{ these sedimentary rocks in the field,
the obser\er should alwa)s appreciate that they reproduce past
conditions, and that they indicate the former presence of water,
either in a state of agitation and with high transporting j)ower for
the coarse varieties, or as quiet reaches in which were laid down
the finer deposits. Rightly approaching and interpreting them,
we may .see that the ocean has advanced across the land in times
of submergence, leaving behind its widening trail of shore gravels,
now conglomerates ; that the.se have been followed up and buried
first by fine offshc^re sediments, and later by the remains of organ-
isms now appearing as limestones, until succeeding elevation
cau.ses the waters again to retreat ant! prepare the way for another
" cycle of tleposition."

62 A HANDBOOK OF ROCKS.

Gravels and Conglomerates.

Loose aggregates of rounded and water-worn pebbles and
boulders are called gravels, and when they become cemented to-
gether into coherent rocks they form conglomerates. Sand almost
always fills the interstices. Silica, calcite and limonite are the
commonest cements. The component pebbles are of all sorts of
rock depending on the ledges that have supplied them, hard rocks
of course predominating. Rounded fragments of vein quartz are
especially frequent. Gravels and conglomerates, if of limited ex-
tent, indicate the former presence of swift streams ; if of wide area
they suggest the former existence of sea beaches and the advance
of the sea over the land. Component pebbles are of course older
than the conglomerate itself, and if igneous, they may establish
the age of the intrusion as older than the conglomerate. Fossil-
iferous boulders prove the age of the conglomerate as later than,
their parent strata. Under favorable circumstances gravels may be
cemented to conglomerates in a comparatively few years. Con-
glomerates are exclusively aqueous. Gravels and conglomerates
graduate by imperceptible stages into pebbly sands and sandstones,
and these into typical sands and sandstones. Notably unsorted
aggregates of relatively large and more or less angular boulders in
fine sands or clay indicate glacial action.

Metanwrphisni. — Under dynamic stresses, especially in the nature
of pressure and shearing, the pebbles of a conglomerate may be
flattened and rolled out into lenses, and such are often observed.
If the pebbles are feldspathic as in the case in those from granite
ledges, and if the inter^^titial filling is aluminous and not purely
quartzose as in the commonest cases, conglomerates, when re-
crystallized, may pass into augen-gneisses with their characteristic
'' augen," or " eyes " of feldspar and quartz that but faintly sug-
gest their original character. Excessive metamorphism may
further develop types closely simulating granite, forming thus the
so-called " recomposed granite " of the Lake Superior regions.

Occurrence. — Gravels are too familiar to require further reference.
Conglomerates are met in all extended sedimentary series. Our
greatest one lies at the base of the productive Coal Measures of
Pennsylvania and adjacent States. It is properly called the " Great
Conglomerate." Remarkable ones with squeezed pebbles are met
in the Marquette iron region of Michigan. In Central Massachu-

SANDS AND SANDSTONES. 63

setts there is an augen-gneiss, that has been derived from a Cam-
brian conglomerate. It is quarried at Munsen, and sold as granite,
and is a widely known building stone. Around Xarragansett Bay,
R. I., are conglomerates, in part at least of Carboniferous age, in
all stages in the progress to gneiss.

Sands

AND

Sandstones.

•

FeO

.SiO,

A1.A

!•>,(•:,

, MnO

CaO

MgO

K,0

Nap

Ix)SS.

Sp. Gr.

99.78

0.22

98.84

0.17

0-34

tr.

0-23

99.62

0-I3

0.7

99-47

0,17

0.12

0.90

0.50

0.07

0.15

0.12

2.648

.«;•

95.85

.64

M Si

0.08

0.45

(2-24.S)

94-73

0.36

2.64

.38

0.36

0.83

91.67

6.92

tr.

0.28

0.34

I 17

2.240

82.52

7.07

3-55

1.42

1.83

tr.

3.61

69.94

13 '5

2.48

0.70

3-09

tr.

3.30

5 43

1. 01

(2-36)

I. Sand from Cambrian Quartzite, Chcsirc, Ma.'w , S. Dana Hayes, Mineral Re-
sources, i883-'84, p. 962. a. Oriskany Sandstone, Juniata Valley. Pcnna. A. S.
McCrealli, /i/rw. 3. Siluro-Camhrian saccharoidal sand.stonc, Crystal City, Mo. Ana
lyzed by Gln.ss Co. 0.22 not delcnnincd, /ifem. 4. Novaculite.Kockiiort, Ark. R.
N. Hrarkctt, for L. S. Griswold, (ict>l. Ark., 1890, III., 161. 5. Salmon rc<l Triassic
Sandstone, ClenoMj, Colo. Quoted by G. V. Merrill, " Stones for building; and Deco-
ration," p. 420. 'n>c Sp. (Jr. is of one from Ralston, near by. 6. Cambrian re<l
.Sandstone, I'ortage I>akc, Mich., Idem. 7. Light-gray sub-carboniferous sandstone,
near Cleveland,. Ohio, IJem. 8. Olive-green carboniferous sandstone, Dorchester,
N. H., /Jem. 9. Red tria.ssic sandstone (brownstone, arko5c), Portland, Conn., /Jem.

Co)nmeuts on the Analyses, — The first three illustrate the purity
of the sand in exceptional cases. We may projxrrly infer that the
sediments were derived either from prcc.xisting saiulstoncs, that
had already been once sorted and separated from their aluminous
ingredients, or from excessively weathered and kaolinized quartzose
rocks, such that the feld.spar had entirely passed into clay, and had
been eliminated in deposition. No. 4 is a novaculitc, and is an ex-
cessively fine, fragmental deposit. Nos. 5, 6 and 9 are red sand-
stones, and indicate the comparatively small percentage of iron
oxides that may cause a deep coloration. No. 7 is free from iron,
but has some aluminous material, evidently a ver>' pure clay, from
the lack of iron. No. 8 has its iron as protoxide, for the rock is a
green variet)'. Its manganese oxide is worthy of remark. No.
9 is a feldspathic sandstone, or arkose, whose analysis, except that
the AL^Og is low and the CaO rather high, might answer for a
granite.

64 A HANDBOOK OF ROCKS.

The specific gravity of sandstones varies widely. Quartz itself
is 2.6—2.66, and specially dense sandstones reach 2.5, but, being
characteristically porous rocks, the usual range is 2.2-2.4. They
often go lower and many even reach 1.8.

Mineralogical Composition, Varieties. — The mechanical sediments
whose predominant particles are finer than pebbles, and yet, in
most cases, of notable size, are grouped under this head. They
are found in all stages of coherence, from loose sands to exces-
sively hard, metamorphic rocks called quartzites. Quartz is much
the commonest mineral that contributes the grains, as it is the most
resistant of the common rock-making minerals. In river sands the
grains are angular, but in those continually washed together on a
sea beach, they become more or less rounded. Garnets, magnetite,
zircon and other hard and resistant minerals are widely distributed
in small quantities. Feldspathic sands also occur, and when they
are compacted to firm rock they are called arkose. As in the con-
glomerates, the cementing materials of sandstones are silica, calcite
and limonite, but in many the character or cause of the bond is
rather obscure. Those with siliceous cement yield the most
durable stone for structural purposes ; those with ferruginous afford
the greatest range of colors, such as olive-green, yellow, brown, and
red. Calcareous cements may be detected by their feeble efferves-
cence. Sandstones entirely formed of calcareous fragments are
known, but are described under limestone.

A curious and exceptional rock is novaculite, that is extensively
developed in Arkansas. It was long thought to be allied to the
cherts, which it much resembles, but microscopic investigation has
led Griswold to determine it to be a finely fragmental deposit of
quartz grains, practically a siliceous ooze. In fineness it is parallel
with the clays, but it contains little else than silica.

Aqueous sandstones generally exhibit well-marked bedding
planes, although cases are familiar in which the bedding is exces-
sively coarse and the layers are extremely thick. Swirling eddies
in the original stream or currents give rise to cross-bedding and
various irregularities. In fact, all the phenomena of beaches and
stream-bottoms, such as ripple-marks, worm-borings, shells, etc.,.
are preserved in sandstones.

Eolian sands are usually of aqueous deposition in their original
condition, but they are afterwards taken up by the wind and driven
along as dunes and dust into more or less remote districts. When

SANDS AXD SAXDSTOXES. 65

they finally reach a state of rest and consolidate, they have ver\''
irregular stratification, cross-bedding, swirling curves, pinching and
swelling layers and other characteristic phenomena. Finer varieties
afford a surface deposit that is generally called " loess," and that
may lack all stratification. More or less water-transported mate-
rial is also intermingled, making the term one of not particularly
sharp definition. This mixed character has made the loess of
many localities a rather puzzling geological j)roblem. It is always
loosely textured and is important in its relations to agriculture.

Sands and sandstones pass by insensible gradations into the
varieties in the upper line of the series shown in the tabulation on
p. 59 by the increasing admixture of clayey or argillaceous ma-
terials. The base is kaolin, Al^O,, 2SiO.„ 2H^,0, a mineral that
forms microscopic, scaly crystals and that has the property of plas-
ticity, and this property it lends to the last members of the series,
which in exceptional cases may contain little else. The lower series
passes gradually into the fragmental limestones, by the increasing
admixture of calcitc.

Mttaniorpliisin. — The purer .sandstones in metamorphism yield
quartzites which are denser and harder than their originals, because
by deposition of cementing quartz, the fragmental grains are
very firmly bound together. The later deposited quartz often con-
forms to the optical and cr>'stallographic properties of the grain
around which it crystallizes. No sharj) line divides sandstones and
quartzites ; the\' shade imperceptibly into one another. Less pure
sand.stones, if crushed and sheared in the metamorphic process,
yield siliceous or (juartz schists from the develojjment of micaceous
scales between the grains. I'Mexible sandstone or itacolumite has
been thought to owe to them its property of bending, but it is due
to interlocking grains.

OccNrrciiif. — Sandstones are so common in all e.xtended geolog-
ical sections as to deserve slight special mention. Next to lime-
stones they arc the most widely used of building stones in quantity,
although the annual value of granite is greater. The Potsdam
sandstone of the Cambrian in New York and on the south shore
of Lake Superior is extensively quarried, and other prominent ones
are the Medina of New York, the Berea grit of the Subcarbonifer-
ous of Ohio ; and the red and brown Triassic sandstones both of
the Atlantic seaboartl and the Rocky Mountains.

A HANDBOOK OF ROCKS.

Argillaceous Sandstone, Shale, Clay.

FeO

(a)Si02 (b)Si02

AiPa

Fe,0,

CaO

MgO

KjO Na,0

(a)H20 (b)Hp

Shale.

I. 69.92

23.46

0.20

0.48

0.40

1-43

3-84

2. 67.29

15-85

6.16

0.95

0.19

8.71

3- 64.37

19-73

9.07

0.82

2.32

3-78

4. 62.86

20.65

9.21

0.48

0.34

6.26

5- 58-45

21.96

8.43

1.05

1-57

4.00

6.51

6. 43.13

40.87

3-44

8.90

5-32

2.42

0.20

Brick Clay.

7. 81.71

9.81

3.80

0.48

0.26

3-91

8. 75.88

11.22

5-04

0.48

0-35

6.76

9. 65.14

13-38

7-65

2.18

2.36

8.51

10. 62.00

18.10

9.11

5-66

II. 57.80

22.

1.85

2.07

12.68

12. 53-77

20.49

923

2.04

4.22

9.60

13- 45-73

29.69

6.86

0.44

1. 01

3-42

12.86

Potter's Clay.

14. 27.68 36.58

22.95

1.28

0.45

0.37

1.96

6.74 2.05

15. 42.28 18.02

24.12

1.46

0-59

0.68

2.42

7.77 0.86

Fire Clay.

16. 61.60

28.38

0.52

0.46

0.36

5.08

17. 38.10 12.70

31-53

0.92

0.40

11.30 2.50

18. 45.29

40.07

1.07

0.26

0.08

0.48

13-18

Residual Clay.

19- 55-42

22.17

8.30

0.15

1-45

2.49

9.86

20. 33-55

30.18

1.98

3-89

0.26

1-57

10.72

Kaolin.

21. 46.50

39-57

13-93

Note. Where two values of SiO.^ are given, the first is the combined silica, i. e.,
chiefly in kaolin, and the second the free silica, which is practically comminuted quartz.
Under HjO, where two values are given, the first is combined water, likewise chiefly
in kaolin, the second is the free water, which has simply soaked in.

I. Haydensville, Hocking Co., O. Quoted by H. Ries, XVI. Ann. Rep. Direc-
tor U. S. Geo/. Survey, Fart IV., p. 572. 2. Hornellsville, Steuben Co., N. Y.,
Ibid., 572. 3. Kansas City, Mo., Ibid., 570. 4. Red Shale, Sharon, Mercer Co.,
Pa., //i?V/., 572. 5. Leavenworth, Kan., /^/(/., 570. 6. Clinton, Vermilion Co., Ind.,
Ibid., 570. 7. "Washington, Davies Co., Ind., Idem., 566. 8. Salem, Washington
Co., Ind., Idem., 566. 9. Red Clay, Plattsburg, Clinton Co., N. Y., Ibid., 568.
10. Red Clay, Lasalle, 111., Ibid., 564. il. Rondout, N. Y., Ibid., 568. 12. Brown
Clay, Fisher's Is., N. Y., Ibid., 568. 13. Hooversville, Somerset Co., Pa., Ibid.,
568. 14. Akron, O., Ibid., 562. 15. East Liverpool, O., Ibid., 562, alsoTiO^, 1.20.
16. Woodbridge, N. J., Ibi'd., 556. 17. Cheltenham, Mo., Ibid., 556. 18. Wood-
land, Pa., Ibid., 556. 19. Morrisville, Calhoun Co., Ala., Ibid., 574. 20. Near
Batesville, Ark., Ibid., 574, also Pp^, 2.53. 21. Pure Kaolin— Al^Oj 2SrO.^, 2H.p.

Coinniciits on the A)ialyses. — The analyses are significant when
compared with those of the sandstones on p. 63. It appears at
once that there is a great decrease in silica, and a great increase in
alumina, and, as a rule, in all the other bases and water. Among

ARGILLACEOUS SANDSTONES, SHALES, CLAY. 67

themselves there is wide variation, but by using No. 22, as indi-
cating pure kaolin, it is possible to infer how much quartz sand is
mingled with the clay, due allowance being made for the fragments
of unaltered feldspar, as shown by the alkalies, for silicates, hydrous
and anhydrous, involving iron, lime and magnesia, and for carbo-
nates of lime, magnesia and iron. Shales and brick clays are shown
to be comparatively impure admixtures of kaolin and quartz ;
potter's clay is much less so, and fire-clay is little else than these
two. No. 19 is practically pure kaolin.

Miiieralogical Composition. Wirictics. — The argillaceous sand-
stones have a finer grain than the sandstones proper, and tend to form
thin but tough beds. They find their best examples in the flag-
stones of our eastern cities. Shales lack this coherence and break
readily into irregular slabs and wedge-shaped fragments of no no-
table size. As sands give rise to sandstones, so on hardening and
drying, muds and silt yield shales. Shales show all gradesfrom gritty
and coarse varieties to fine and even ones apprc^ximating clays. The
finer shales when ground have the .same plasticity as clay, and arc
often moulded and baked into brick, especially of the vitrified
kinds for paving. Shales may be black from bituminous matter
in them, and are then described as " bituminous." They grade
into cannel coals, but great areas of them such as the Genesee Shale
of New York and the Huron Shale of Ohio, have as much as 8 to
20% hydroairbons and yield quite copious products on distillation.

As the particles of quartz become finer and finer and not too
abundant, the plasticity of the kaolin presently asserts itself so
that the shales pass into clays. In the most even and homo-
geneous grades, they show but slight grit to the teeth, but in
coarser varieties they are decidedly gritty even to the fingers.
The)' are often sci)aratcd into thin Ix-ds b\' la\ers of sand that
mark the times of freshets during their formation and the attendant
deposition of coarse material. Clays of earlier geological date are
hard and dense rocks and must be ground before use. Such are the
fire-clays immediately beneath Carboniferous coal-seams. Clays
are blue, red and brown according to the state of the iron oxide,
whether ferrous or ferric, or they may be nearly white when it
fails. The less pure brick clays as shown by the analyses con-
tain oxides of iron, calcium, magnesium and of the alkalies in quan-
tity, but fire-clays practically lack these.

As contrasted with the transported or sedimentary clays just
mentioned, there are residual clays that result from the decay of

68 A HANDBOOK OF ROCKS.

impure limestones and that are found on their weathered outcrops.
They are very impure and variable in composition, but they are
markedly plastic.

Metamorpliism. — In metamorphic processes shales become com-
pacted and oftentimes silicified. Their lack of homogeneity causes
them to yield irregularly breaking and very tough rocks called
graywackes, which differ only in greater hardness from their
unaltered originals. Excessively silicified shales are called phtha-
nites and are important in the Coast Range of California. Shales
also under shearing stresses and attendant mineralogical reorgani-
zation pass into schists of various kinds, such as quartz-schist,
mica-schist and possibly hornblende-schist. G. F. Becker even
mentions rocks derived from them that are mineralogically like
diabases and diorites, but their recognition is a matter for micro-
scopic study. Clays under shearing stresses develop new cleavages
without regard to their original bedding and from the homo-
geneous character of the original and the perfection of the cleav-
age, slates result, which are of great practical importance.

Ocawrcncc. — Shales and clays are such common members of
extended geological sections as to deserve no special mention.
They are often a thousand feet or more in thickness and cover
great areas.

Calcareous Sandstones, Marls.

HP or
Na^O CO2 I^oss.
. . 326

0.29

Calc.

FeO

Sandst. SiO^

k\.f

\ Fe,03

Ca(3

,MgO

K2O

I. 79.19

3-75

7.76

3.20

2. 38.41

5-77

1.79

20.08

8.82

0.12 (

Calc. Shales.

3- 3970

26.83

19.28

2.43

511

4- 28.35

12.37

21.47

8.24

S-73

Marls.

5- 43-70

25.00

8.85

2.33

6. 38.70

10.20

18.63

9.07

1.50

3-65

7. 28.78

11.63

2.96

24.50

2.91

2.12

I. Calcareous sandstone, Flat

^staff, Ari

iz. Que

.ted by G.

5-40

9.21

6.14

10.00

!2.66

4.18

p. Merrill, Stones for
Building and Decoration, ^20. 2. Calcareous sandstone, Jordan, Minn., Idem. 3.
Genesee Shale, Mt. Morris, N. Y. , supplied by H. Ries. 4. Niagara Shale, Rochester,
N. Y., H. T. Vult^, analyst. Supplied by H. Ries. 5. Cretaceous Marl, Hop Brook, N.
J., Geol. of N. y., 1868, 419; also PjO^, 2. 18. 6. Cretaceous Marl, Red Bank, N.
J., Idem, 418; also ^-f)-^, 1.14, SO.j o. I4. 7. Subcarboniferous marl. Bowling Green,
Ky. , Ky. Geol. Siirv., Chem. Analyses A, Part 3, 90 ; also, PjOj, 0.25.

Conunents on the Analyses. — The analyses illustrate in a very
suggestive way the passage of these mechanical sediments into im-
pure limestones. The gradual intermingling of more and more of

CALCAREOUS SAXDSTOXES, MARLS. 69

shells and other remains of organisms brings it about. The high
PjO., of the marls, as cited under the references, is worthy of re-
mark. It is to be appreciated that the lime and magnesia and
some of the iron of the analyses are to be combined with COj, even
though the CO^, is not mentioned.

Mincralogical Composition. I \iritties. — Calcareous sandstones are
practically sandstones with rich calcareous cement, or with a large
amount of organic fragments intermingled with the prevailing quartz
sand. They arc passage forms to the fragmcntal limestones. Cal-
careous shales derive their lime partly from the fine organic sedi-
ment that is deposited with the siliceous and aluminous particles
and partly from contained fossils. Beds of these rocks arc partic-
ularly favorable layers for the discover)' of the latter, and often
break the monotonous barrenness of a geological section composed
of ordinary shales. Marls, strictly speaking, are calcareous clays,
and originate in typical cases by the deposit of limy slimes along
with the aluminous. The lime destroys the placticit>- of the clay
and yields a crumbling rock, often richly provided with fossils and
of value a.s a fertilizer. Grains of glauconite. the green silicate of
potash and iron, are at times present, and characterize the so-called
"green sands " that are valuable as fertilizers. The term marl is
somewhat loosely used in its applications, and moderately coarse
calcareous sands, and even beds that show but small percentages
of lime on analysis are designated by it in the States along the At-
lantic seaboard from New York south. It is clear that marls are
intermediate rocks between clays and impure earthy limestones.

Mctamorphisin. — The rocks of this group are altered in meta-
morphic processes to schisto.sc forms, not so essentially different
from tho.sc resulting from the common aluminous shales and clays,
except that the richness in lime facilitates the production of min-
erals requiring it. The marls, when high in lime, behave like im-
pure limestones, and arc prolific sources of silicates. Marls are,
however, much more common in later and unmetamorphosed for-
mations than in older ones, although it may be that in the latter
they have yielded some schistose derivatives not readily trace-
able back to them.

Occurrence. — Calcareous .sandstones and shales arc met as occa-
sional beds in series of the more abundant, distinctively aluminous
varieties. Marls are chiefly developed in the Cretaceous and Tertiar)-
strata ofthc Atlantic seaboard and around the Gulf of Mexico. Fresh-
water ones are not lacking in the Tertiary lake basins of the West.

CHAPTER VIII.

Limestones ; Organic Remains, not Limestones ; Rocks

Precipitated from Solution. Determination of

the Aqueous and Eolian Rocks.

. Limestones.

FeO

SiO^ Al.Oj

Fefi,

CaO

MgO

CO., H2O

Insol.

CaCOg MgCOj

Living Organisms.

I. (Coral)

54.57

2.54

97.46

2. (Reef-rock)

53-82

1. 01

96.11 2.13

3. (Lagoon Sed.)

54.58

0.85

97.47 1-79

4. (Coral)

44.96

3-87

80.29 8.14

5. (Oyster Shells)

44-4

1-3

35-4 14.5

(79.28)(2.73>

Calcite.

6. Pure Mineral.

56.

44.

100.

Dolomite.

7. Pure Mineral.

30.43

21.72

54-35 45-65

Marine Limestones.

8. 0.63

0.55

55.6

0.23

99.30 0.49

1.06

53-78

0.34

0.90

I-I3

96.04 0.72

10.

1-25

53-89

0. 10

96.24 0.21

II. 1.84 0.64

1.82

51.40

2.23

41.19 0.27

91.80 4.68-

12. 12.34

7.00

44.41

0.44

79.30 0.92:

13. 3.77 0.08

6.80

33-79

15-32

42.21

60.35 32-61

14.

0.55

29.54

21.08

1.82

0.60

52.75 44.28

Waterlime.

15- 18.34

7-49

37.60

1.48

3-94

67.14 2.90

16. 15-37

11.38

25.70

12.44

1.20

45.91 26.14

Siliceous.

17.

1.20

17.69

10.59

1.24

43-72

31.60 22.24

Freshwater Limestone.

18.

0.37

54.16

0.15

43-68

1-49

96.71 0.31

19. 1.83

0.22

34.20

0.1 1

26.79 4.64

31.28

61.07 0.23

Travertine.

20. 0.08

0.15

53-83

0.90

41.79 1.43

94-97 0.43

1. Stag's horn coral i^Millepora aicicorms'), S. P. Sharpless, Afiier. Jotir. Sci.y
Feb., 1871, 168. 2. Bermuda coral reef rock. A. Q.Mo^ova., Neues Jahrb., 1894,
I., 269. 3. Bermuda coarse lagoon sediment, /i:/^;«. 4. Average of 14 analyses of the
coral Lithotkamniufii from localities the world over, Iciefn, 272. 5. Oyster shells, Geol^

LIMESTONES. 71

of Xno Jersey, 1868, 405. 6. Calculated from CaCO^. Calculated from CaCOj,
MgCOj. 8. Crystalline Siluro-Camb. limestone, Adams, Mass., E. E. Olcoit for Marble
Co. 9. Limestone, Bedford limestone, Ind. Quoted by T. C. Hopkins, Mineral In-
dustry, 1894,505. 10. Solenhofen lithc^raphic stone. Quoted by G. P. Merrill, Stones
for Buildinj^ ami Decoration, 415. II. Limestone, Hudson, N. V., Th. Egleston,
12. Trenton Limestone, Point Plea.sant, Ohio, vide No. 10. 13. Surface Rock, Bonne
Terre, Mo., J. T. Monell, unpublished. 14. Limestone, Chicago, T. C. Hopkins,
Mineral Industry, 1895, 508. 15. Hydraulic limestone, Coplay, Penn. (>uot«d by
W. A. Smith, Mineral industry, 1893, 49. 16. Hydraulic limestone, Rosendale, N.
Y., Idem. 17. Siliceous limestone, Chicago, 111., vide No. I4. 18. Miocene lime-
stone, Chalk Bluffs, Wyo., K. W. Woodward, 40th Parallel Sur\'., L, 542. 19. Eocene
limestone, Henry's Forks, Wyo. , B. E. Brewster, Idem. 20. Travertine, below Hotel
Terrace, Yellowstone Park, J. E. Whitfield, for W, H. Weed, gth Ann. Kef. Dir.
U. S. Geol. Sun., 646.

Comments on the Analyses. — The first three and the fifth analyses
indicate that the calcareous parts of living organisms arc quite pure
calcium carbonate. The fourth analysis is of that species of coral
which, so far as we know, is highest in magnesia. Small amounts
of calcium jjhosphate are often present as well, some shells being
richer than others. Xos. 6 and 7 are introduced so as to give a
basis for estimating the purity of the following limestones : Nos.
8, 9 and lo are extremely pure varieties, and from these, as a start-
ing point, the other components increa.se in one analysis and
another. No. 14 is a nearly typical dolomite. Nos. i 2 and 17 arc
highly siliceous, and Nos. i 5 and 16 are both strongly argillaceous.
The last two are closely parallel in composition with marine varie-
ties. An analysis of a travertine is given in No. 20.

It at once appears that Nos. 13, 14, 16 and 17 arc far higher in
magnesia than an\' known living organism, and it is evident that
an original organic ileposit must have undergone an etirichment in
magnesium carbonate to bring them about. Dana suggested many
years ago that coral or other organic sanil, while agitated in
sea-water, jirobably exchanges a part of its calcium for magnesium,
and there is much reason to think that it does. Otherwise, the
change must ha\e been brought about by magnesian solutions per-
colating through the rock and altering it by the replacement pro-
cess called dolomitization, or dolomization. Much of the silica, no
doubt, results from radiolarians and sponge spicules, but much
also, together with the alumina, from fine fragmental sediments.

Origin. — Much the greater number of the important limestones
are of marine origin, but in certain geological formations fresh-
water ones are well developed. The calcareous remains of organ-

72 A HANDBOOK OF ROCKS.

isms have been their principal source, and of these the forami-
nifera, the corals, and the molluscs the chief contributors. Their
shells have often become thoroughly comminuted to a calcareous
slime before final deposition, so that the resulting rock affords no
trace of organic structure. The solubility of the carbonate of lime
aids in the cementation of the slime to rock and tends to efface the
organic characters. Limestones pass by insensible gradations
through more and more impure varieties into calcareous shales and
marls, but, as a rule, they are deposited in deeper water than the
true shales and sandstones. This conception must n6t be applied
too strictly, because, beyond question, a depth of a few feet has
often sufficed, and too much emphasis has often been placed upon
the depth regarded as necessary for limestones. Coral sands ac-
cumulate on or near the immediate shore, and may even be heaped
up by the wind.

In confined estuaries of sea water subjected to evaporation,
enough carbonate of lime is precipitated directly from solution, to
yield important strata, and such are often met in a series of beds
associated with rock salt and other precipitated rocks as later set
forth. Calcareous deposits from limy springs may also almost
reach the dignity of rocks, and when abundant are called travertine
or calcareous tufa. If particles of dust, etc., are suspended in limy
springs or in concentrated estuarine waters, they gather concentric
shells of the carbonate and may yield oolitic deposits from the co-
alescence of the concretions. Some algae likewise secrete oolitic
calcite and contribute extensively to rocks.

Mineral Composition. Varieties. — Calcite is the chief mineral of
limestones, and when thin sections are magnified it exhibits its
characteristic cleavages. Dolomite and siderite accompany it fre-
quently, and their molecules also replace the calcium carbonate, in a
greater or less degree, so as to form double carbonates. An unbro-
ken series can readily be traced from pure calcium carbonate, through
more and more magnesian forms, to true dolomite. Those with
over 5<}^ MgO are usually described as magnesian limestone, and
when the MgO mounts well toward the 21.72% in the mineral
dolomite, we use the latter name. In the same way, a series of
ferruginous varieties may be established toward the clay ironstone
and black-band ores, and a siliceous series toward the flints and
cherts. Cherty limestones are a very common variety, and are
referred to again in connection with chert. When the argilla-

LIMESTONES. 73

ceous or clayey intermixtures enter, argillaceous or hydraulic
varieties result that are generally drab and close-grained, and are
useful in the manufacture of cement. Bituminous matter may be
present, making the limestones black, and this, in the form of
asphalt, may yield asphaltic varieties.

Besides these varieties established on the basis of chemical com-
position, special names may be given because of structure. Thus
earthy limestones tend to crumble to dirt ; oolitic limestones re-
semble the roe of a fish ; pisolitic varieties consist of concretions
of size comparable with peas ; and other terms are employed, that
are sclf-cxplanator)'. Prominent fossils suggest names, such as
crinoidal, from fossil crinoids ; coraline, foraminiferal and many
more of local or stratigraphic significance. Practical applications
play a part in nomenclature, supplying " waterlime," "cement-
rock," " lithographic limestones," etc.

MetamorpJiism. — Limestones feel the effect of metamorphism
with exceptional readiness and under deforming stresses, probably
accompanied by elevation of temperature, and in the presence of
water, or along the contacts with intruded dikes and sheets of
igneous rocks, they lose their sedimentary characteristics, such as
bcddiiig-plancs and fossils, and change into crj'stallinc marbles.
The contained bituminous matter becomes graphite ; the alumina
and silica unite with the lime, magnesia and iron to give various
silicates. Other oxides together with the bituminous ingredients
contribute to the varioiis colorations. Mechanical effects are
manifested in flow lines, brecciation and other familiar features of
many that are cut and polished for ornamental stones. Impure
limestones that undergo these metamorphic changes are the most
prolific of all rocks in variety and beauty of minerals. Arendal,
Norway, and the crystalline limestone belt from Sparta, N. J.,
north through I'ranklin Furnace arc good illustrations. The crys-
talline limestones will be again mentioned under the metamorphic
rocks.

Occnnence. — Limestones are too common to deserve special
mention as regards occurrence. They are frequently met in all
parts of the country, but the Trenton limestone of the Ordovician,
the Niagara of the Silurian and the Sub-carboniferous limestones
of the Mississippi Valley are specially worthy of note.

74 A HANDBOOK OF ROCKS.

III. Remains of Organisms not Limestones.

Calcareous remains are much the most important of the contri-
butions made by organisms to rocks, but there are others, respec-
tively siliceous, ferruginous and carbonaceous, that deserve mention.

Siliceous Organic Rocks.

The principal members of this group are infusorial or diatoma-
ceous earths ; siliceous sinters ; and cherts, hornstones or flints, the
three last names being practically synonymous. Infusorial earths
consist of the abandoned frustules of diatoms, which are micro-
scopic organisms belonging to the vegetable kingdom. Though
not a common rock, they yet are met in series of sedimentary
strata, both freshwater and marine, with sufficient frequency to
justify their mention. Some foreign earthy materials are unavoid-
ably deposited with them. The siliceous sinters are extracted from
hot springs by algae that, as shown by W. H. Weed, are capable of
living and secreting silica in waters up to i85°F. They are far
less important geologically than the infusorial earths. Chert is a
rock consisting of chalcedonic and opaline silica, one or both. It
possesses homogeneous texture and is usually associated with
limestones, either as entire 'beds, or as isolated, included masses.
It often has druses of quartz crystals in cavities, and in thin sections
under the microscope it sometimes exhibits sponge spicules.
Cherts not provided with these organic remains may be regarded
with great reason as chemical precipitates, and as American varie-
ties in the great majority of cases lack them the cherts receive
more extended mention under the chemical precipitates.

Infus. Earths. SiO^ Al^Og Fe^Oj

1. 91.43 2.89

2. 86.90 4.09

3. 75-86 9.88 2.92
Silic. Sinter.

4. 89.54 2.12
Chert. > . '

5. 34.0 0.80

I. Miocene, Little Truckee River, Nev., R. W. Woodward, 40th Parallel Survey,.
I., opposite p. 542. 2. Fossil Hill, Nev., Idem. 3. Richmond, Va., M. J. Cabell, Min-
eral Resources, 1883-84, p. 721. 4. Deposit from Old Faithful, Yellowstone Park,
J. E. Whitfield, for W. H. Weed, 9th Ann. Rep. Dir. U. S. Geol. Sur., 670. 5. Cre.a-
Ceous chert, England, Jukes- Brown and Hill, Quar. Jour. Geol. Soc, Aug., 1 889.

FeO

CaO

MgO

Na^O

K.fi

H2O

0.66

0.36

0.25

0.63

0.32

3-8

1.26

0.14

0.51

0.77

0.41

5-99

0.29

0.69

0.08

0.02

8.37

tr.

1. 71

tr.

1. 12

0.30

513

CaCOg.
63-4

MgCOg.

1-5

0.3

SILICEOUS ORGANIC ROCKS. 75

Comments on the Analyses. — The infusorial earths are fairly high
in water, and this is the main cause of low silica, but, as stated
above, their growth and accumulation in water make it unavoid-
able that more or less clay and other sediments should mingle
with them. In these and the other members of the series, it is
important to understand that much of the silica is opaline, or amor-
phous, hydrated silica, and not quartz or chalcedon)-. Tests of
the amounts soluble and insoluble in caustic alkali are usually
made to determine the proportions of the two, for, while it is not
an accurate separation — quartz and chalcedony being themselves
somewhat soluble — it gives an approximate idea. No. 4 is a de-
posit separated from the gey.sers by alg.e and evaporation. No.
5 is largely due to sponge spicules, mixed in with chalk, and there-
fore is high in calcic carbonate.

Mincraloi^ical Co>fif>osition. I 'tiriitiis. — The mineralogy of the
infusorial earths can be stated less definitely than the chemical com-
position. The individual diatoms are very minute, but the analyses
indicate both opaline and chalcedonic silica as lx:ing present. In
the sinters and cherts, when the latter can be shown to be organic,
the same two varieties arc recognizable, and with them are varying
amounts of calcite. The infusorial earths are fine, powder)' de-
posits, resembling white or gray, dried clays, but they lack plas-
ticity and are best recognized with the microscope. Siliceous sin-
ters, often called geyscrite, are cellular crusts and fancifully shaped
masses that closely resemble calcareous tufas, but that are reatlily
distinguished by their lack of effervescence. Chert is dense, hard
and homogeneous, and of white, gray or black color. It readily
strikes fire with steel, ami when it breaks has a splintery or con-
choidal fracture. It is often decomposed to powdery silica on the
outside, and in extreme ca.ses may yield rather large deposits of
this powder, that are called " tripoli," and are u.sed for various
practical purposes. Mention may again be made of the cherts
that seem best explained by chemical precipitation.

Mctatnorf^ltisui. — The cherts alone of these rocks are of suffi-
cient importance to attract attention in this connection, and their
metamorphism is briefly referred to on page 81.

Occurrence. — Infusorial earths are abundant near Richmond, \'a.,
and on Chesapeake Hay, at Dunkirk, and Pope's Mills, Md. Beds
deposited in evanescent ponds or lakes are also well known in
States farther north. In the West, the Tertiar)' strata have yielded

76 A HANDBOOK OF ROCKS.

them in Nevada. In California and Oregon great areas are re-
ported by Diller. Siliceous sinters produced by algae are quite
extensive in the Yellowstone Park, and similar deposits, perhaps
caused by the same agent, are found in many, hot spring regions.
Sinters chemically precipitated also occur. The most important
•occurrences of chert are all mentioned together on page 8i.

Ferruginous Organic Rocks.
It is a question whether these deserve the dignity of rocks, for
they may with great propriety be classed with the minerals, dis-
tinctively so called. It will therefore only be mentioned that
many have attributed the formation of beds of limonite to the
separation of iron hydroxide by low forms of organisms.
Even granting this, it is still true that such limonites are insignifi-
cant when compared with those that result by purely inorganic
reactions in the decay of rocks. Important strata of cherty car-
bonates of iron are present in the iron mining districts around
Lake Superior and have been, no doubt, the principal source of
the hematites. Van Hise regards them as probably of organic
origin, but the evidence is not decisive and they may be chemical
precipitates. Clay-ironstone and black-band ores, — that is, argil-
laceous and bituminous ferrous carbonate, — sometimes form con-
tinuous beds instead of the usual isolated lenses, but when they do,
they are not organic in origin, although decaying organic matter
may be instrumental in preserving the reducing conditions that are
necessary to the formation of the ferrous salt.

Carbonaceous Organic Rocks.

When plant tissue accumulates in damp places and under a pro-
tecting layer of water that prevents too rapid oxidation, new ac-
cessions may more than compensate for loss by decay so that ex-
tensive deposits may result. These become progressively rich in
carbon by the loss of their other elements and yield beds of con-
siderable geological, but much greater practical importance. The
course of the changes and the several stages are indicated in the
following table.

C. H. O. N. Total.

Woody Tissue 50 6. 43. I, loo.

Peat 59 6. 2,^. 2. 100.

Lignite 69 5.5 25. 0.8 100.3

Bituminous Coal 82 5. 13. 0.8 100.8

Anthracite 95. 2.5 2.5 trace. 100.

CARBONACEOUS ROCKS. PRECIPITATES. 77-

The changes are in' the nature of loss of oxygen and hydrogen,,
and also of carbon, but the decrease of the first two is relatively so
much greater, that the carbon actually is enriched. The table is
theoretical in that no account is taken of the more or less fortuitous
mineral matter that forms the ash together with a small percent-
age of incombustibles in the vegetable tissue itself Peat is a
more or less incoherent mass of twigs and stems, decidedly car-
bonized and darkened, but with the original structures, as a general
rule still well preserved and recognizable. By gradual stages it
passes into lignite, which is still further compacted, and which ex-
hibits the original structures more faintly. In bituminous coal. tiic\'
are seldom recognizable, and the aggregate is compact and black.
In anthracite the coal is dense, amorphous and lustrous. The
oxidation necessary to the later varieties may have been largch'
performed before actual burial in otlur ri)cks, hut the changes .ire
continuous and progressive in all.

Other organic dervatives, such as asphalt, petri)lcum, etc., arc
not considered of sufficient abundance to rate as rocks.

Mitamorphisnt. — Anthracite is locally produced from bituminous
coal near igneous intrusions, and by regional mctamorphism, as
later explained. The chemical changes arc the same as those
progressive ones above outlined, but are doubtless more rapidK*
brought about. Anthracites become graphitic, and, as a theoret-
ical extreme, pass into graphite. Natural cokes are also produced
along intrudetl dikes.

Occurroicc. — I'cat favors cool ami moist latitudes in all parts of
the world, and is chiefl\' of fresh w.iter origin. Lignites and coals
are best developed in the Carboniferous and Cretaceous strata, and
where the former occur in the East ami the latter in the West,
they often contain coal scams.

IV. pKixiprrATius FROM Solution.
The name of this group indicates the character of the rocks
that comprise it. Bearing in mind the condition established at
the outset, p. i, that a rock should form an essential part of the
earth, it is evident that water is the only natural solvent abundant
enough to j'ield such rocks, and that on!}' the most widespread
compounds that are notabl)' soluble in it, or in its common solu-
tions of other more soluble salts, can meet this requirement. The
rocks may be conveniently taken up under the following heads.

78 A HANDBOOK OF ROCKS.

I. Precipitates involving the alkaline earths and alkalies. 2.
Siliceous precipitates. 3. Ferruginous precipitates.

Precipitates Involving the Alkaline Earths and Alkalies.

The carbonate of lime in stalactites, stalagmites and crusts on
the walls and floors of caves in limestone or in the surface deposits
from limy springs, affords a rock of this character. It is a form
of limestone, from pure varieties of which it does not differ in com-
position, although its banded structure and rings of growth, which
we may describe by Posepny's useful word " crustification," in a
measure distinguish it. Naturally such deposits are often beauti-
fully crystalline, free from admixture except of associated dissolved
materials and as a rule purer than sedimentary limestones. They
yield our well-known onyx marbles. Some regularly stratified de-
posits of limestones that are associated with the precipitated rocks
next discussed have doubtless originated together with the latter.

Gypsum and rock salt are the chief members of this sub-group.
They occur quite invariably in association, and have resulted alike
from the evaporation of sea-water and from the drying up of
lakes, originally fresh. Both are mixed more or less with dust and
other mechanical sediments washed or blown into the evaporating
reservoir, or are interbedded with other salts that were present in
a minor capacity in the mother liquor, but instances of thick beds,
especially of rock salt of surprising purity, are well known. When
these attain several hundred or even a thousand feet, it is evident
that more than twenty-five times this depth of salt water, on the
basis of the known composition of the sea, would have to be evap-
orated, and this is a practical absurdity even for any conceivable
confined body, with occasional renewals from breaches of the bar-
rier. It would be necessary to assume wide stretches of shallows
that were practically evaporated to dryness, while at the same time
subsidence of the coast was progressing at just about the necessary
rate to keep pace with the growth of the salt. The recent ex-
planation, however, advanced as the " Bar theory," by Ochsenius,*
clears it up. We need only to assume a relatively deep and nearly
land-locked estuary, with a shallow bar between it and the sea.
Evaporation continually concentrates the confined salt water and

* Zeitschrift f. Praktische Geologic. May and June, 1893. An excellent abstract by
L. L. Hubbard appears in the Geol. of Michigan, V., Part II., p. ix.

PRECIPITATES FROM SOLUTION. 79

especially the portion on the shallow bar. This, becoming rich
in mineral matter and of high specific gravity, flows inward and
down the slope of the bar to the bottom of the estuar}'. In the
course of time, and allowing for the influence of pressure in the
depths and of temperature, conditions favorable to precipitation,
first, of the insoluble gypsum, later of the more soluble common
salt will be reached, and in varying and alternating layers they will
be built up indefinitely, or until some upheaval or subsidence alters
the relations of the estuar)' to the sea. More or less anhydrite is
also deposited, and is later found in extended cross-sections of
salt-bearing strata. The most soluble ingredients, such as KCl,
MgCl.,, MgSO,, etc., become continually richer in the mother
liquor, and unless this is also finally evaporateii, they escape and
are not found in the series. So far as we know, the Strassfurt dis-
trict, in Germany, is almost the only place where this escape has
been prevented on a large scale, although rock salt is of world-
wide distribution.

Gypsum forms at times gray or black earthy beds, that look very
much like limestone, but of course do not effervesce. Again, it is
in white, cream-colored or more deeply tinted layers, yielding ala-
baster. Minor portions afford sclenite, the clear, transparent variety,
and thin coats of native sulphur arc seldom lacking. R«)ck salt forms
crystalline beds, often stained red or brown, by iron oxide, lioth
gypsum and .salt may impregnate associated .sediments more or less,
yielding gyp.seous or saline shales and marls. In many localities
gypsum deposits have undergone a complex scries of chemical
changes in the general nature of deoxidation from carbonaceous
matter present, so as to yield native sulphur in large amounts.

Mitaiiiorp/tisjH. — None of the above rocks are worthy of mention
as regards metamorphism.

Occurnncc. — In America, gj'psuni is found especially in the Up-
per Silurian of New \'ork ; the Lower carboniferous of Michigan
and Nova Scotia ; the Tria.ssic in the Kastern prairie states, Kansas
and Texas ; in undetermined Mesozoic in Iowa; ami in the Jura-
Trias or in undetermined strata in Colorado, Utah and the West.
Rock salt occurs in the Upper Silurian of Southern New York ; in
the Triassic of Kansas ; in the Quaternar}' (?) of Petite An.se, I^a.,
and at many places of recent geological age in the West.

A HANDBOOK OF ROCKS.

Siliceous Precipitates.

H^Oor

Geyserite. (

a)Si02(b)Si02

AI2O3 FcjOg CaO

MgO

K2O

Na^O

Loss

Sp.Gr.

81.95

6.49 tr.

0.56

0.15

0.65

2.56

7-50

Cherts.

99.46

0.29

0.4

tr.

0.34

3-35 95-78

0.16

tr.

O.OI

0.20

4-52 93-65

0.83

0.05

O.OI

0.78

98.10

0.24 0.27

0.18

0.23

1. 16

94.91

2.85

0.42

tr.

Sil. Oolite.

95-83

2.03

1-93
CaCOg

tr.
MgCOj

2.63

56.50

1.50

16.84

2.60

12.54

2.688

3-70

i.42

88.71

8.09

2.654

Cherty iron

carbonates.

CaO

MgO

FeO

MnO

CO2

lO.

58-23

0.06 5.01

0.38

9-59

18.41

0.25

2.08

5-22

II.

46.46

0.24 0.64

1.87

3.10

26.28

0.21

1.22

19.96

12.

28.86

1.29 1. 01

0.74

3-64

37-37

0.97

0.68

25.21

Note. (a)Si02 means silica soluble in caustic alkali ; (b)Si02 silica insoluble in
the same.

I. Geyserite, Splendid Geyser, Yellowstone Park, J. E. Whitfield for W. H. Weed,
gth Ann. Rep. U. S. Geo/. Stir., 670. 2. Gray unaltered chert, Joplin, Mo. An-
alysis made by U. S. Geol. Surv. Quoted in Ann. Hep. Geol. Sur. Ark., 1896, III.,
161. 3. White altered chert, Galena, Kan. , /(/ifw. 4. Unaltered chert, Bellville, Mo,.
Idem. 5. Decomposed chert, or Tripoli, Seneca, Mo., W. H. Seamen. Quoted by E. O.
Hovey, Amer. Jotir. Sci., Nov., 1894, 406. 6. Chert, Roaring Springs, Newton Co.,
Mo., J. D. Robertson, for E. O. Hovty, Idetn. 7. Siliceous oolite, Center Co., Penn.,
Barbour and Torrey, Amer. Jour. Sci., Sept., 1890, 249. 8. Silica-lime o5lite. Idem.
9. Lime-Silica oolite on same specimen as No. 8, Idem. 10, 11. Cherty iron carbo-
nates, N. E. Minn., T. M. Chatard, for C. R. Van Hise, Monograph, XIX., U. S. Geo!
Survey, 192. 12. Cherty iron carbonate, Sunday Lake, Gogebic Range, Mich., W.
F. Hillebrand, Idem.

Connncnts on the Analyses. — The first seven are high in silica,
some approximating chemical purity. No. i has admixtures of
mud thrown out by the geyser from its walls. The five cherts
2-6 inclusive have but slight amounts of alumina, iron and lime,
and low percentages of water. Nos. 3 and 4, by the determina-
tions of soluble silica give us some idea of the amount of the
opaline form that is present. The three analyses 7, 8 and 9 are
a most instructive series, passing as they do from nearly pure silica
into a moderately siliceous, magnesian limestone, from which the
first two are thought to have been derived by replacement. Nos.
10, 1 1 and 1 2 are the curious cherty carbonates of iron from which
the Lake Superior iron ores have been formed by subaerial decay.
Their richness in magnesia as compared with lime is noteworthy.

SILICEOUS PRECIPITATES. 8i

Mincralogical Composition. J ^ariffirs. — Cherts are so exceedingly
fine grained that they give no indication of their constituent
minerals to the unaided eye. The microscope shows, however,
that they are chiefly chalcedony in excessively minute crystals,
with which are associated varying amounts of opaline silica, quartz
crystals, calcite or dolomite rhombs and dusty particles of iron
oxide. In foreign cherts as stated above on p. 74, sponge spic-
ules have been met, but not in the important American varieties.
Cherts often have an outer powdery crust, due to alteration, and
while as shown by analysis 5, this may not mean any notable
chemical change, it may penetrate whole beds and leave only a
white, incoherent mass called "tripoli," that is used for a polishing
powder and for various other purposes. Cherts have spherulites
occasionally and are still more often oolitic. The chcrty or sili-
ceous rocks of the formations containing the I^ke Superior iron
ores are mixtures of chalcedonic silica and carbonate of iron in
varying proportions, and in their alteration they afford more or
less sharply differentiated ja.spers and hematites. Three analyses
of varj'ing composition are given above*, Nos. 10, li and 12.

As stated earlier, cherts are intermingled in all proportions with
limestones. They are very puzzling problems as regards origin.
Where devoid of organisms, the majorit)' of observers regard them
as in some way precipitated chemically from sea water, pcssibl)- as
gelatinous silica and by replacement of limestone. Their structure
and relations give us few definite clues on which to base a firm
conclusion. As earlier stated, others regard them as derived from
siliceous remains of organisms, such as sponges, radiolarians and
the like, which may have been redissolved and worked over into
clialcedony, making them practically jirccipitates. Cherts are also
called hornstonc and Hint.

Mctauiorpliis))t. — Purely siliceous cherts are unpromising sub-
jects for metamorphism, except as they }ield silica for the pro-
duction of silicates from cherty limestones. The ferruginous cherts
of I^ke Superior pass into actinolitic and magnetitic slates, a most
interesting change, especially in the former case. The lime, mag-
nesia and iron are combined with silica under the metamorphosing
influences, so as to yield the actinolite.

Occurrence. — The abundance of cherts or related rocks in the re-
gion of Lake Superior, either associated with limestone or in the
cherty carbonates described above is remarkable. In their eco-
6

82 A HANDBOOK OF ROCKS.

nomic products, they are the most important strata present. The
Siluro-Cambrian Hmestones are often cherty both east and west,
and in the New York and Ohio Devonian, the so-called " Cornifer-
ous " limestone was named from its richness in "hornstone." In
the Mississippi Valley the lower Carboniferous strata are particu-
larly prolific in cherts.

Ferruginous Precipitates.

Some iron ores doubtless originate in this way, and the processes
by which the soluble proto-salts are oxidized and precipitated as
the insoluble ferric hydroxide are well understood. But they may
be considered rather as minerals than as rocks. The cherty iron
carbonates of the preceding section have already been cited, and
the clay ironstones and black-band iron ores are omitted from
further mention for the same reasons as are the limonites.

The Determination of the Aqueous and Eolian Rocks.

The members of this series are much easier to recognize than
are the igneous. Breccias, conglomerates and sandstones are at
once apparent from their fragmental character. Breccias differ
from conglomerates in the angular shape of their component frag-
ments. As the sandstones become finer, the argillaceous varieties
may be distinguished by the peculiar odors emitted by all clays and
clayey rocks when breathed upon. The calcareous sandstones
and marls betray themselves by effervescence with acid. All lime-
stones, unless too rich in magnesia, effervesce in cold acid, and the
more readily if first scraped up into a little heap of powder with a
knife. Dolomites effervesce much less readily, and warm acid may
be necessary. Infusorial earth may need the microscope for its cer-
tain identification, and then the abundance of the little organisms
is very apparent. The cherts are so characteristic in appearance
as to admit of little uncertainty, except as compared with the silici-
fied tuffs and excessively fine felsites, called petrosilex, in which case
geological surroundings or the microscope are the only resources.
The ferruginous rocks, if such be allowed, are self-evident, as are the
carbonaceous. Gypsum is easily recognized when in the crystal-
line form, but when black and earthy, the observer may be forced
to determine its lack of effervescence, and to make a sulphur test
with the blowpipe. Nevertheless with these rocks as with the
igneous, although to a less degree, it is very advisable to gain ex-

REMARKS ON DETERMIXATIOX. 83

perience with correctly labeled study collections or with the syste-
matic exhibits of a museum, so that the observer may have a fund
of personal observation back of him from which to draw, and on
which to depend when a rock comes up for determination.

For field work and travel, it is well to appreciate that a few dr>^
crystals of citric acid, that can be dissolved in a little water as
needed, serve very well for tests of effervescence. They are more
safely carried than are liquid mineral acids.

CHAPTER IX.

The Metamorphic Rocks. Introduction. The Rocks Pro-
duced BY Contact Metamorphism.

The word metamorphism was first introduced into geological
Hterature by Lyell in 1832, and was used to describe the pro-
cesses by which rocks undergo alteration. It was particularly ap-
plied by him to those stratified rocks that, fi-om deep burial in the
earth, and fi-om the consequent heat and pressure to which they
have been subjected, have assumed structures and textures resem-
bling those of the unstratified primary or plutonic. In this sense
it has been generally employed since, and it implies an increase in
crystallization, hardness and those attributes, which are especially
associated with the crystalline schists, as contrasted with the un-
altered sediments.

The literal meaning of the phase " the processes by which rocks
undergo alteration " may, nevertheless, be somewhat more com-
prehensive than this, and may be made to include the changes
produced by atmospheric agents, which we ordinarily describe by
the term weathering, and in the following pages the products of
this latter form of alteration will be briefly considered as a third
and concluding group.

The metamorphic rocks will therefore be taken up under the
following three classes :

I. Rocks reduced by Contact Metamorphism.

II. Rocks produced by Regional Metamorphism.

III. Rocks produced by Atmospheric Weathering.

THE METAMORPHIC ROCKS. IXTRODCCT/OX. 85

By contact metamorphism is meant the series of changes that
are effected by an igneous intrusion, such as a dike or a laccoHte
upon the rocks through which it is intruded. These changes are
often profound, and are brought about by the heat of the intrusion
as well as by vapors and hot solutions which it may likewise give
forth. The wall-rock may be itself igneous or sedimentar>-, or
even metamorphic. This form of metamorphism is sometimes
called " local " as contrasted with " regional."

By regional metamorphism we describe the series of changes
that are produced in the rocks of wide areas or " regions " by deep
burial, mountain-making upheavals, and by heat and pressure.
Although LycU had stratified rocks before him as the chief ma-
terials on which these agents acted, yet it is well recognized to-day
that igneous rocks are no less profoundly affected, and indeed that
the results of their alteration may be almost or quite indistinguish-
able, from those derived from .sediments. But there is great un-
certainty as to the original condition of many regionally meta-
morphosed rocks, and although the endeavor has been made in
previous pages to throw as much light on them as possible, by
systematically referring to the alteration and metamoqjhism of
simple types, nevertheless, many are obscure, and in their history
are involved some of the profoundest problems of geology.

By atmospheric weathering is meant the .series of changes
wrought in rocks at or near the surface of the earth, by the ordi-
nary atmospheric agents, water, oxygen, carbonic acid and the like.
The changes are chiefly in the nature of disintegration, lo.ss of
soluble ingredients and decomposition, and in general they pro-
duce a marked shrinkage of bulk.

It is important to appreciate that under whatever form the met-
amorphic rocks are met, they are of necessity alteration products
of the two grand divisions over which we have already passed.

Generalitie-s Regarding Contact Metamorphism.

Widening observation has shown that contact metamorphism is
produced by all varieties of igneous rocks and that it may be
broadly stated to be independent of the kind of rock forming the
intrusion. Granites, sj'cnites, nepheline-.s\enites, dioritcs, gabbros
and even peridotites have in one place and another proved to be
efficient agents. Yet the following statements may be said to
hold good :

86 A HANDBOOK OF ROCKS.

1. Plutonic rocks are more favorable to it than volcanic. This
follows because plutonic rocks cool slowly at considerable depths
and stand therefore at high temperatures for long periods next
their walls.

2. Magmas rich in. mineralizers are much more favorable than
are those poor in them. This naturally follows from the powerful
influence exerted by escaping vapors. It is tantamount to saying
that acidic rocks are in general more efficient than basic ones,
because experiment shows, and field observation indicates, that
abundant absorbed vapors accompany and facilitate the fusion of
the rocks high in silica, whereas basic rocks are much more largely
the results of dry fusion. Granites, for instance, are the common-
est and most effective agents of contact metamorphism.

3. As regards the walls, sedimentary rocks posses varying sus-
ceptibilities. Highly siliceous sandstones and conglomerates, for
example, are stubborn subjects, and manifest but slight altera-
tion ; but highly aluminous or calcareous beds are favorable to
recrystallization, because they contain the alumina, iron, lime,
magnesia and the alkalies that will combine with silica, under
metamorphosing influences, to yield copious contact minerals. Of
all rocks, impure limestones yield the most varied and interesting
results.

4. With a favorable intrusion, the apparent distance to which
the metamorphosing influence penetrates, depends on the angle of
emergence of the intrusion. If it comes up at a low angle it may
lie but a short distance below the surface for a considerable stretch
on one side of the outcrop, so that the metamorphosed area may
apparently extend to a great distance, although at no point far
from the source of heat. Around a vertical dike the distance
would naturally be less. Again, the alterations progress much
less readily across the bedding of stratified rocks than along it.
Hence, an intrusion that cuts across the bedding produces more
widespread effects than does one parallel with it.

5. It is believed by many, especially among English and Ger-
man observers, that there is very slight migration of material dur-
ing metamorphism, and therefore that the contact minerals have
resulted from the silica and the bases that were practically in the
same places before the intrusion as after it. It follows that there
has been no chemical introduction or substitution, but only re-
arrangement of molecules during the process. An analysis, there-

'I HE METAMORFHIC ROCKS. LXJRODrCT/OX. ^7

fore, of a reasonably large-sized sample would indicate the compo-
sition of the original rock, except so far as water, carbonic acid
and other volatile ingredients have been driven off. From obser-
vations upon an intrusion of granite in Westmoreland, England,
that cuts a decomposed, basic, amygdaloidal lava, Alfred Harker
concluded that the migration had not exceeded one-twentieth of
an inch. But among the French much greater power of chemically
affecting the walls is attributed to intrusions, and in instances it
certainly .seems as if, in addition to the fluorine and boron that
we all know penetrate into wall rocks during the escape of min-
eralizcrs, hydro-fluosilicic acid might impart silica and that some
of the bases, and especially the alkalies, niit^ht mii^ratc in heated
solutions, to a moderate distance.

6. Notwithstanding the truth of the foregoing generalities, it is
a curious fact that contact effects are sometimes strangely lacking
where we would naturally expect them, and they are often of
varying intensity and irregular distribution, where they do occur.
The.sc anomalies can in part be explained by the general principles
already cited, of which no doubt the presence or absence of min-
eralizers, the superheated or relatively cold condition of the intru-
sion are chief. Hut ever)' ob.ser\'er of wide experience is some-
times much puzzled by what he meets in Nature.

I. Till-: Rocks Pkodlckd iiv Cosr.vcr Met.vmoki'mi.s.m.

Although the principal results of contact metamorphism are
manifested in the walls of the intrusion, the igneous rock is itself
influenced. It is therefore necessary to note both the />//<r//rt/ and
the externa/ effects, or those upon the intrusion and tho.se upon the
walls. The area over which the latter are manifested is often
called the aureole, and the concentric rings of decreasing alteration
as one pas.ses outward from the intrusion are called zones.

Interna/ Effects. — The igneous rock suffers a relatively rapid loss
of heat in its marginal portions as compared with its interior, and
as a result it very commonly assumes a porphyritic, felsitic or even,
just as the contact, a glas.sy texture, although it may be granitoid
within. W^here these textures are well developed the passage
from one to the other is extremely gradual, and if the wall rock
has been originally a shale or a clay that has been baked to a

88 A HANDBOOK OF ROCKS.

dense mass, one may need microscopic examination to determine
where the intrusion ends and the wall rock begins. The changes
in texture in the intrusion are accompanied more or less by
changes in chemical composition and in not a few cases progres -
sive analyses have shown the margins to be much more basic than
the interior of the intrusion. The chilling of the former has thus
produced chemical rearrangements in the magma previous to con-
solidation.

External Effects. — Recalling the statement earlier made that
within the limits already set forth the character of the intrusion is
immaterial, the most convenient and intelligible method of treat-
ment will be to briefly outline several typical cases wherein the
commoner sedimentary rocks are known to have been affected,
and then to refer to one or two instances wherein igneous or re-
gionally metamorphic ones have suffered alteration. The same
order will be preserved for the sediments as appears under Chap-
ters VII. and VIII.

Breccias are too limited in distribution to be a serious factor.
Conglomerates and sandstones so generally consist of silica, that
they supply but little raw materials of a favorable kind. The small
amounts of alumina present may combine with the silica to afford
sillimanite (AlgOg, Si02) and stimulated circulations of hot water
may cause added deposition of quartz around the grains so as to
develop increased hardness.

With shales and clay rocks, even if in the form of slate (see
later, p. 107), the effects are more pronounced; and around intru-
sions in them well-marked and well -identified zones have been de-
scribed.

At the contact of the igneous rock with the sediment a breccia
or "mixed zone" of intrusive and fragments of wall rock is some-
times, although not always, met. More commonly the shales,
slates, clay or their kindred rocks are baked and altered to a dense,
flinty product known as a hornfels or hornstone, which latter name
in this sense is, however, not to be confused with its use for flints
and cherts. It breaks in irregular, angular masses and has a
very close resemblance to dense trap. Its mineralogy is, as a
general thing, a subject for microscopic study, but it may be said
that biotite in small scales is rather the most widespread min-
eral present, and that andalusite, garnet, cyanite, staurolite, tour-
maline, ottrelite, rutile, hornblende, feldspars and other min-

THE METAMORPHIC ROCKS. INTRODL'CTIOX. 89

erals more or less characteristic of such surroundings frequenth"
appear. They may be of considerable size and the prisms of
andalusite of the variety chiastolite with the light and dark mal-
tese crosses showing in their cross-sections, are especially frequent.
As the contact is left the hornfels often passes into mica schist.
Farther out the mineralogical changes become less marked ; the
andalusite and other crystals are less and less well developed and
finally shade into mere dark spots or aggregates of biotitc, mag-
netite and bituminous matter. When even these fade out the un-
changed sediment is met. In some localities it has therefore been
possible to establish three zones, which arc, in the reverse order of
the above succession, the knotty or spotted slates, the knotty
mica schists, and the hornfels, usually with andalusite. By knotty
is meant the aspect given by the larger contact minerals in the
midst of finer aggregates.

These are the names adopted for a well-known contact studied
by the eminent German petrographcr, Roscnbusch, in the V^osges
Mountains. At a famous American locality in the Crawford Notch
of the White Mountains, on the slopes of Mt. Willard and not far
from the Crawford House, the granite has penetrated an argillitic
mica schist or micaceous slate, and the zones are somewhat differ-
ent, (i. W. Ilawes in 1881 established the following seven:
I. The argillitic mica schist (chloritic) ; 2. Mica schist (biotitic) ;
3. Tourmaline hornstone ; 4. Tourmaline veinstone (a small con-
tact band, rich in tourmaline) ; 5. Mixed .schists and granite ;
6. Granite porphyry (biotitic) ; 7. Granite (hornblendic). This is
one of the most complete and best-exposed contacts known, and
illustrates both external and internal effects.* The succession
illustrates the alteration of chlorite to biotite by the granite, and
then near the contact the develojiment of tourmaline from the
boracic and fluoric emanations which were afforded by it. On
the southeast corner of Conanicut Island, in Xarragansett Bay,
granite has penetrated Carboniferous shales, as described by L.
V. Pir.sson,! and has baked them to compact hornfels near
the contact. Spotted slates are likewise met resembling those de-
scribed above. Immediately beneath the diabase of the Palisade
ridge at Hoboken, N. J., the Triassic shales arc baked to a compact

* Hawes' paper is in the Ametuan Journal of Science, January, l88l, p. 21.
t L. V. Pirsson. On the Getilogy and Petrography of Conanicut Island, R. I.
Ainrrican Journal of Sdenn-, Nov., 1 893, |i. 363.

90 A HANDBOOK OF ROCKS.

hornfels with abundant tourmalines, and near Beemerville, N. J.,*
a great dike of nepheline-syenite has come up through Ordo-
vician shales and has altered them in places to remarkably dense,
black hornfels. Near Crugers, on the Hudson River, mica-
diorites have penetrated mica schists and have developed in them
a considerable number of characteristic, contact minerals, but the
changes in the schists are not specially apparent to the eye.f As
western and other eastern areas are further studied, no doubt addi-
tional cases will be fully described. Many are known and await
careful field work.

The contact effects on limestones furnish extremely interesting
phenomena and a series of minerals somewhat different from those
just described. On account of the general lack of migration of
material the elements of the minerals must be present in the unal-
tered rock. Pure limestones therefore merely crystallize into equally
pure marbles. Siliceous and argillaceous ones become thickly
charged with biotite, garnet, vesuvianite, scapolite, pyroxenes and
amphiboles, tourmaline, spinel, and not a few more. Garnet and
vesuvianite are especially characteristic. Good contacts have been
met at several American localities. Near St. John, N. B.,J granite
has penetrated Laurentian limestone and has developed a garnet
zone, with more or less pyroxene. Diorites cutting or including
limestone in the Cortland series§ have caused the formation of
pyroxene, scapolite, hornblende and other minerals.

In the valley extending from Warwick, N. Y., southwest to
Sparta, N. J., are most instructive exhibitions, and rich mineral
localities are based on them. Granite is the principal intrusive. ||
The western Adirondack region of New York contains many more
where gabbro and limestone come together, and where the well-
known mineral localities occur. C. H. Smyth, Jr., has lately
identified their contact nature and will in time describe them.
Abroad, the region about Christiania in Norway has proved to be
classic ground for these phenomena, and a great contact of diorite
on Triassic limestone at Predazzo in the Tyrolese Alps has pro-
duced the characteristic zones on a grand scale.

* J. F. Kemp, Trans. Nerv York Acad. Set., XL, p. 60.

fG. H. Williams, Amer. Jour. Set , Oct., 1888, p. 265.

:t:W. D. Matthew, Trans. N. V. Acad. Sci., XIII., 194.

§ G. H. Williams, Amer. Jour. Sci., Oct., 18S8, 267.

II J. F. Kemp and Arthur Hollick, Annals N. V. Acad. Sci. VII., 644.

THE METAMORPHIC ROCKS. IXTRODUCTIOX. 91

Increasing experience, in the West and in Mexico, has shown that
copper ores are often deposited along the contacts of eruptives and
limestone. Thus in the Seven Devils district, in western Idaho,
bornite occurs between diorite and white marble, and is mixed
with epidote and garnet as a gangue, both being minerals char-
acteristically developed in these surroundings.

The inclusions of wall rock caught up by an advancing intru-
sion on its way to the surface are instructive examples, and often
are afterwards found entombed in the igneous rock and more or
less altered. The lava flows of Vesuvius and the ejected bombs
have been of extraordinar)' interest in this respect. Limestones
are frequent among them and they exhibit the same zones as the
larger occurrences. Vesuvianite, in fact, received its name from
this association.

Of the remaining members of the grand division of the Aqueous
rocks, the Carbonaceous arc the principal ones deserving mention.
Coal seams of the normal bituminous variety have been cut in not a
few places by igneous dikes, and display in a marked degree the
metamorphosing effect. The volatile hydrocarbons have been
driven off and the coal has become an impure coke. The Triassic
coal basins of \'irginia and North Carolina exhibit man\' instances
where diabase dikes have wrought the change, antl in the region of
Puget Sound basalt intrusions have effected similar results. In
Colorado and New Mexico, the near approach of an igneous sheet
has brought about the formation of anthracite, and in fact all
grades of coal can be detected from rich bituminous to hard anthra-
cite, according to the nearness of the dike or laccolite.

Reference may also be made to the hills of soft magnetite, near
Cornwall, I'enn., where a great dike of diabase has altered limonite
to this more crystalline and thoroughly anhydrous mineral.

Where intrusions cut other igneous or metamorphic rocks the
effects arc much less apparent, because the walls are resistant to
change, being themselves already crystalline. Around granites,
however, even in these conditions, great pegmatite dikes and veins
are copiously produced, which seems to be in large part brought
about by escaping heated vapors and solutions.

Remarkable cases of contact mctamorphism are, however, cer-
tainly caused by these last named agents. As rocks they are not
specially abundant, although of great scientific interest. Some in-
trusions have emitted copious emanations of hydrofluoric and bor-

92 A HANDBOOK OF ROCKS.

acic acid in conjunction with superheated steam. These vigorous
reagents have attacked the wall rocks, when originally formed of
crystalline silicates, making them porous and cellular from the de-
struction of feldspars, and have often caused the crystallization of
quartz, tourmaline, topaz, fluoric micas, fluorite, apatite and other
characteristic minerals of which cassiterite is of much economic
importance. Such metamorphic products when essentially consist-
ing of quartz and mica are called greisen. Tourmaline granites
hkewise result. It is not to be overlooked, however, that mineral-
izers have also played a large part in the cases earlier cited, nor
should the remark be omitted in conclusion that they and similar
agents have been of vast practical importance in the formation of
ores.

CHAPTER X.

The Metamorphic Rocks, Continued. The Rocks Produced

BY Regional Metamokphism. Introduction. The

Gneissf_s and Crystalline Schists.

Introduction.

This subcli\isi()n embraces rocks that differ widely among them-
selves, but that have nevertheless important features in common.
The following generalities are applicable in a large way ant! will
serve to emphasi/.e some of the most important i)oints.

1. Rcgionall}' metamorphosed rocks are all more or less per-
fectly crystalline. This is least developed in the slates.

2. They are all more or less decidedly laminated or foliated, al-
though .some amphibolitcs, marbles and .seqientines are quite mas-
sive. The laminations are due to the arrangement of the con-
stituent minerals, and especially the dark-colored ones, in parallel
alignment, so that light and dark layers stand out con.spicuously.
The terms bedded and stratified should never be applied to them
because the banding is largely due to dynamical processes, and
has no necessary connection with original sedimentation.

3. They are of ancient geological age or else are in greatly dis-
turbed districts.

It is important in connection with the.se rocks to distinguish be-
tween the effects produced by heat or thermal metamorphism and
the effects produced by mechanical forces or dynamic meta-
morphism. Hy thermal metamorphism we understand the alter-
ations caused by heat not necessarily accompanied by the mechan-
ical effects such as shearing, crushing and the like, that are
comprehended under dynamic metamorphism. Contact metamor-
phism is of course a variety of the former which, however, is also
brought into play alike when rocks are so deeply buried that they

94 A HANDBOOK OF ROCKS.

come within the sphere of influence of the earth's interior heat, and
when from dynamic stresses, they are crushed so that their particles
move or sHde under -great pressure on one another and develop
heat by friction. If we imagine for a moment great bodies of
rocks which have definite crushing resistances, buried under a load
of overlying strata, so deep within the earth that their limits of re-
sistance are exceeded, yet so confined that they cannot fly apart,
we perceive that they must yield by internal crushing, and if the
upheaval of a mountain range eases the strain, that they must flow
in a mass. It is to this flow, accompanied by shearing, that the
lamination of metamorphic rocks is largely due. Prominent or
conspicuous minerals are strung out in parallel alignment, often-
times with v/avy folds and curves, and in the end a foliated or
laminated structure is superinduced that suggested the bedding of
sediments to the early geologists. It is not to be denied, however,
that the laminations do at times correspond to original bedding,
because where the contrasts in chemical and mineralogical compo-
sition, among the layers are pronounced, they doubtless mark such
correspondence, but cases are well known of old conglomerate
beds passing directly across the prevailing schistosity of a gneissic
district.

During these shearing and flow movements large crystals, such
as the feldspars of porphyries, and the larger uncrushed nuclei of
minerals in a general pulp are squeezed and stretched into lenses,
and remain like eyes between eyebrows, so that they are called
" Augen " from the German word for eyes. Swirling curves and
eddies in the laminations are also familiar phenomena and cannot
be explained in any other way.

These changes may take place without mineralogical alteration,
as when granitoid rocks pass into gneisses that contain simply the
crushed fragments of the originals, but as a general thing new com-
binations are formed in the metamorphosed rock. Pyroxene passes
into hornblende ; soda-lime feldspars become scapolite or saussurite,
and other changes ensue that are best detected with the microscope.
Sedimentary rocks suffer entire recrystallization, and sometimes so
thoroughly lose their original characters that no clue is afforded as
to their history. In regional metamorphism precisely as in the
case of the contact metamorphic rocks, it is generally believed that
there is no change in composition, except perhaps by the loss of
volatizable ingredients, but only rearrangement of elements. A

THE METAMORPHIC ROCKS. INTRODUCTIOX. 95

gross analysis of a reasonably large sample will therefore give a
clue to the composition of the original. Heated waters, generally
charged with mineral matter and steam have no doubt contributed
largely in bringing about the final results.

The Regionally Metamorphosed rocks will be described under
the following heads :

1. The Gneisses and Crystalline Schists.

2. The Quart/.ites and Slates.

3. The Crj'stailine Limestones and Dolomites : The Ophical-
cites, Serpentines and Soapstones.

The Gneisses.

Introductory. — Gneiss is an old word that originated among the
early German miners in the Saxon districts. It was es[x:cially ap-
plied by them to laminated rocks of the mineralogical composition
of granite, and in this sense it is quite widely employed to-day.
But there are main' important gnei.s.ses that correspond in min-
eralogy to the other plutonic rocks, and that are quite as properly-
designated by this name, .so that gneiss has come to be a term that
is of loo.se geological significance ver\' much as is trap, but that
is none the less useful for this reason. We may therefore define
gneiss as a laminated, metamorphic rock that usually corresponds
in mineralogy to some one of the plutonic types, (ineisses differ
from schists in the coarseness of the laminations, but as the.sc
become finer they pass into schi.sts by insensible gradations.
Varieties are sometimes indicated by prefixing the name of the
most prominent silicate, usually one of the ferro-magnesian group,
thus hornl)lciulc-gnciss, biotite-gneiss, p\roxene-gnciss, but we
also often speak of quartz-gneiss, orthoclase-gnciss, plagioclase-
gneiss, garnet-gneiss and the like.

It is evident at once that the above names are incomplete.
Hornblende-gneiss, for instance, does not indicate whether the
rock contains orthocla.se or plagioclase, quartz or no quartz, and
the other ones cited are open to the same or similar objections, and
if in the endeavor to embody fuller descriptions we string together
the names of all the minerals in the rock, we employ an objection-
able and awkward method of coining words. A .system has, how-
ever, been suggested by C. H. Gordon,* in a recent paper that
obviates many of these objections and that is adopted below with

* Hullelin of the Geological Society of America, V'lT., 122.

A HANDBOOK OF ROCKS.

some abbreviations suitable for an elementary book. It is based
on the parallelism that exists between the mineralogy of gneisses
and that of the massive plutonic rocks, and it avails itself of the
short names of the latter, that indicate in each case, a definite
combination of minerals to describe the aggregates present in the
former. Two sedimentary terms are also added.

Massive Type.

Granite

Syenite

Diorite

Gabbro

Pyroxenite

Peridotite

Gneiss of Correspond-
ing Mineralogy.

Granitic Gneiss
Syenitic Gneiss
Dioritic Gneiss
Gabbroic Gneiss
Pyroxenitic Gneiss
Peridotitic Gneiss

Sedimentary
Type.

Conglomerate
Sandstone

Derived Gneiss.

Conglomerate Gneiss
Quartzite Gneiss

Dr. Gordon also suggests that when gneisses are evidently dy-
namic derivatives from a massive rock, that this relationship be
indicated by using the terms granite-gneiss, syenite-gneiss and so
on. If, however, differentiations in the magma before crystallizing
have given rise to laminations, that such be distinguished by the
adjective gneissoid, as gneissoid gabbros.

Gneisses are occasionally met which do not exactly correspond
to any of the above names. Chlorite, for example, is a not un-
common mineral, and while it is evidently an alteration product
from pyroxene, hornblende or biotite, the original mineral is not at
once apparent, and some such name as chlorite-gneiss must be
used. In the same way cordierite-gneiss describes those rare
varieties containing cordierite (iolite and dichroite are synonyms
of cordierite); sillimanite-gneiss, garnet-gneiss, epidote-gneiss and
others convey in their names their characteristic features.

Analyses of Gneisses.

Chemical analyses often enable us to trace back gneisses to their
original rocks, whether igneous or sedimentary, but it requires
careful study of correct type analyses and some familiarity with
their ranges in composition to do it. So far as their number
admits the analyses quoted on earlier pages will be found sug-
gestive :

THE GNEISSES. 97

SiO,

AiA

Fe,03

FeO

CaO

MgO

K,0

Xa,0

Ix>ss or UjO

76.61

12.45

^■11

0.84

5-42

3 12

0.53

74.95

9.42

7.47

1.65

0.13

2.02

4-05

1.02

73-47

15.07

1 15

4.4S

0.12

0.38

5-59

71.46

15.06

2-43

1.40

0.42

5-17

l-^l

0.S3

69-35

18.83

2.00

5.94

.78

69.94

14.85

7.62

2.10

0.97

4-33

4 30

0.70

61.96

19-73

4.60

0-35

1. 81

2.50

0.79

1.82

57.66

22.83

7-74

1.16

3-56

5-72

0.60

1.50

57-20

«9.57

9.52

0.59

5-73

4.40

0.28

^-n

0.88

10.

54.89

13.67

1-35

563

4.70

8.34

1.95

2.76

I. Granite gneiss, west side of JUack Hills 40th Par. Surfn\ I., p llo. R. \V.
Woodward, .Anal. 2. Called a diorilic gneiss in reference, contains hornblende,
<|iiart/, plagioclasc. orthocla.se, /</<•/« , R. W. liunsen, .Anal. 3. Conglomerate gneiss,
so called granite ; Miinson, Ma.ss. Quoted by G. I'. Merrill, StonfS for BuilJitii^ and
Decoration, p. 418. 4. (iranitc gneiss. Iron Mountain, Wyo. , R. \V. Woodward,
Anal. .See under No. I. 5. Dark variety of No. 3. 6. Granite gneiss, tlcrived
from a hornblende granite. Trembling Mountain, Quebec, Fumlamental I.aurcntian
of Logan. F. D. Adams, Amer. Jour. S<i., July, 1895, p. 67. W. C. Adams, Anal.
7. <^uart/.ilic gneiss, with garnet, sillimanite, graphite and pyrite ; St. Jean de Mntha,
<,)uebcc, IJeiii^ N. N. Evans, Anal. 8. Ciranitc gneiss, probably a metamorphosed
clay or slate. Trembling Lake, (^)uet)€C. Contains garnets and sillimanitc, F. D.
Atlams, Anirr. Jour. Set., July, 1895, p. 67, W. C. Adams, Anal. 9. Dioritic
gnci.ss. New York City, P. Schwcit/er, Amer. Chtmist, VI., 457, 1876. 10. Gneiss
containing malacolitc, sca[Kilit(', orthoclasc, graphite, pyritc. Rawtlon, Quebec.
See under NO. S.

CoitmnHfs OH till Analysis. — Nos. i, 4 ami 6 arc clearly derived
from granites, presumabl)* by dynamic metamorphism. The aii-
aly.scs correspond closely in their general features with tho.se
given on p. 30 except that the Al.O., of No. i is a trifle low, and
the re.,©., of No. 6 a trifle high. Nos. 3 and 5 are now known
to be metamorphosed Cambrian conglomerate, although so thor-
oughly recrystallized as to be a well-known, commercial granite.
The conglomerate must have come from granitic or dioritic orig-
inal rocks. Nos. 7 and 8 correspond to the analyses of slates as
noted b>' F. D. Adams in the original reference (see also under
slates, p. 107). No. 10 as noted by Adams is of doubtful interpreta-
tion. The high alkalies, lime, magnesia and the moilerate silica
suggest a basic syenite or trachyte, but the alumina is exception-
ally low for these. It may be a tuff or a slightly altered sediment
from these originals. No. 2 is a very anomalous rock, and it is
difficult to refer it to an original diorite, it is so high in silica and
so low in alumina. The iron is very large for so acidic a rock.
No. 9 is undoubtedly an altered sediment as indicated by the local
geology. Notwithstanding the anomalies of composition, chem-
7

98 A HANDBOOK OF ROCKS.

ical analyses supply one of the surest clues to the geological his-
tory of gneisses and it is to be hoped that they will be multiplied
in America. At present but few are available, far fewer than of
igneous rocks.

Alteration. — The alteration of gneisses is similar in all respects to
that of their corresponding massive types. The feldspars alter to
kaolin, the micas and hornblende to chlorite and the rock softens
down to loose aggregates that contribute heavily to the sedimen-
tary rocks.

Distribution. — Gneisses are abundant in ancient, geological form-
ations. The early Archean is their especial home, and they form
the largest part of its vast areas in Canada, around the Great Lakes,
along the Appalachians and in the Cordilleran region. But no
singl; division of geological time monopolizes them any more than
such an one does plutonic rocks. There are Cambrian and even
Carboniferous gneisses in New England, and dynamic meta-
morphism may produce them from massive rocks of almost any
age. The later geological formations are, however, seldom buried
sufficiently deep to be in favorable situations. Much the same
holds true of Europe and the rest of the world. The gray and red
gneisses of the mining districts about Freiberg, in Saxony, those
of the Highlands of Scotland, and in Scandinavia, and the wonder-
ful exhibitions of dynamic metamorphism in the Alps are to be
cited as of unusual historic and scientific interest.

Gramditc. — Granulite is a word that has possessed somewhat
contrasted meanings according as it has been used in Germany,
F" ranee or England. In Germany as first employed it was applied
to a finely gneissoid rock that consists chiefly of feldspar, quartz and
garnets. These original granulites have other minerals more or
less prominently developed, of which cyanite, augite, biotite and
hornblende are chief The texture of the rock is extremely dense,
and except for the garnets, cyanite or augite the individual min-
erals are hardly discernible. Among French and English speaking
peoples granulite has been applied to granite rocks that appear to
the eye to be chiefly quartz and feldspar, although the microscope
may show muscovite. They are practically binary granites, or
rich quartz and feldspar gneisses. The name has also been used
for coarse plutonic rocks that have been crushed down by dyna-
mic metamorphism into a finely granular and homogeneous aggre-
gate. But so far as metamorphic rocks have been met in America,

THE MICA-SCHISTS. 99

cases are very rare which cannot be satisfactorily described without
the use of this word, that has been so perverted from its original
application as to be practically valueless without an accompanying
explanation.

The Crystalline Schists.

The crystalline schists have finer laminations than the gneisses,
but in other respects the mineralogy is often much the same, and
as already stated no very sharp line can be drawn between them.
It is important to note that the words " schistc " of the French and
" Schiefer " of the Germans are applied to shales, slates and meta-
morphic schists indiscriminately, but in English schist is only used
for mctamorphic rocks. The more important schists arc broadly
classified, according to the principal ferro-magncsian silicate that is
present, into the following three groups under which they will be
taken up.

{a) Mica schi.sts.

(tt) Hornblende-schists or Amphibolitcs.

(f) Various Minor Schists.

The Mica-Schist.s.

SiO,

Al,< ),

I- . ,< ),

KcO

CaO

Mg<)

K,(J

Na,0

H,U

82.38

11.84

2.28

1.00

0.83

0.38

0.77

79.50

«3-36

2.87

0.71

0.95

4.69

0.36

0.78

6945

14.24

^'.54

2.66

» 35

252

4.02

0.52

66.21

1S.60

534

0.44

1.24

380

2.16

2.04

62.98

16. 88

2.48

5.00

tr.

1.58

7-45

302

61.57

»9 53

.S-44

2.61

tr.

1.90

2 14

348

60.49

>9-35

0.48

5.9S

1.08

2.89

3-44

2-55

3.66

5767

17.92

9.00

319

329

3.86

1.09

3 «9

55.12

24.32

6.13

4-99

ir.

tr.

283

2.71

10.

49. CO

2365

8.07

0.63

0.94

9.II

»-75

341

Micasch

ist, rich ii

11 <|unr(/,

Monte 1

Rosa, Swi

tzcrland,

, /.iilk..

wskv. Sill.

Wiener

Akad., XXXIV., 41, 1895. 2. .Mica-schist, with i|uartz and green mica, Zermatt,
Swit/crlaml. Bunscii in Rolh' i TnhelUn, 1862. 3. Clamelifcrous mica-schist with
feldspar, Hrixen. Tyrol. Schonfcld and Roscoe, Ann. tier. Chem. u. Phar., XCI.,
1854, 305. 4. Mica-schist near Meissen, Saxony, Ililgcr (iuote<l in /^ofA's I'lihelUn,
1879. 5. Mica-schist, Crugers, N. V., contains (juartz, orthocla.->c, biolite, muscovite,
little oligocIa5c, etc. F. L. Nason for G H. Williams, Amer. Jour. Sci., Oct., 1888.
259. 6. Crumpled gametiferoiis mica-schists, Idem. 7. Argillitic mica-schist, («. W.
W'XviQS,, Geolos;y of N^e-,i< Hampshire, Part III., 219. 8. Mica-schist near Messina,
Sicily, Ricciardi, quoted in Roth' s 7'a/>ellen, 1884, p. ix. 9. Slaurolite mica-schi.st,
with biotite, muscovite, quartz, siliimanite, garnet. See under No. 6. 10. Sericite
schist, Wisconsin, IVis. Geol. Sun., I., 304.

loo A HANDBOOK OF ROCKS.

Comments on the Analyses. — Like the majority of gneisses the
mica-schists are more or less closely parallel with the granites in
chemical composition because the constituent minerals are so
largely the same in both. But where they have been formed from
metamorphosed sediments such as shales, clays, and the like, the
alkalies are often lower than in the case of siliceous igneous
rocks, and, what is still more characteristic of sediments as con-
trasted with highly siliceous igneous rocks, the magnesia is in
excess of the lime. A comparison of the above analyses with
those of the rhyolites, trachytes, granites and syenites earlier
given will forcibly bring this out. The local geology as well as
the analyses, indicate that there is little doubt that Nos. 5, 6, 7
and 9 are altered sediments, and the presumption is strong that
most of the others are also.

Mineralogical Composition. Varieties. — The most prominent and
abundant minerals in the mica-schists are quartz, muscovite and
biotite. While they are more or less interleaved together, yet
close examination of the coarser varieties shows that they are in
layers irregularly parallel and to a large extent distinct. The
minerals are in all degrees of relative abundance, quartz sometimes
largely predominating and marking a passage to the quartzites,
while again the micas may be in great excess. Both muscovite
and biotite are met, the former being, perhaps, rather the more
abundant. With these chief minerals are almost always asso-
ciated very considerable amounts of feldspar, both orthoclase and
plagioclase, and variable proportions of garnet, staurolite, cyanite,
sillimanite, tourmaline, apatite, pyrite and magnetite.

The garnet and staurolite may exhibit surprisingly well devel-
oped crystals and illustrate the extraordinary power of certain
compounds to crystallize under circumstances apparently ill-adapted
to their perfect development.

Mica-schists embrace a series from rather coarsely crystalline
varieties to others that are excessively fine-grained and that are
near relatives of the slates. The minerals of the latter may be of
microscopic dimensions, and only the aggregate of shining scales
reveals them as mica. Such aggregates, of a silvery white color
but of composition essentially the same as normal muscovite, are
called sericite, and the corresponding schists, sericite-schists. A
related soda-mica ( muscovite and its relatives are potash micas)
is called paragonite. Hydromica is a name applied many years

THE AMPHIBOLITES. THE MIX OR SCHISTS. loi

ago by Dana to sericite, paragonite, and perhaps others resembling
them, so that for these finely micaceous schists, especially in our
eastern states, hj'dromica schist is a field name that is largely used
in practice and in geological reports. These fine-grained mica-
schists that approximate slates are also made a special group by
many, under the name phyllite, a very useful term and one to be
strongly commended. Mica-schists are also met that are high
in lime and that mark transitions to the cr>'stalline limestones.
The abundance of calcite or dolomite betrays them, and to such
the names calcareous schist or calc-schist are applied.

Mica-schists result from the thorough metamorphism or recrj's-
tallization of sandstones, shales and clays, and also from the crush-
ing and excessive shearing of igneous rocks, granitoid and por-
phyritic alike. A possible origin from ancient volcanic tuffs is
always to be considered in the study of a district, but the ques-
tions of origin arc obscure and are subjects for thorough chemical
and microscopical investigation.

A/tcnitioN. — The mica-schists are rather resistant to alteration
and often appear on mountain tops. When alteration docs prevail,
they soften to masses of quart/ .sand, chlorite .scales and kaolin.

Distribution. — The mica-schists form the countr>' rock over \ast
areas in New l*!ngland and to the south along the eastern Appa-
lachians. Altlunigh long regartied as of uncertain or ob.scure geo-
logical relations they are now recognized as being in large part at
least metamorpho.sed Cambrian and Ordovician shales or related
.sediments. Around Lake Suj)erior and in the regionally meta-
morphosed areas of the West they are not lacking.

TiiK Hdrnhlkndi-: Schists ok AMi'imtoi.iTRs.

lutroductory. — Under dynamic metamorphism the basic igneous
rocks whose chief bisilicate is pyroxene, pass very readily into
liornblcndic rocks, with a greater or less development of schistosity.
On account of the prevailing parallel arrangement of the prismatic
crystals of hornblende, schistosity is seldom entirely lacking, but
where less distinct the name amphibolitc has proved to be a u.se-
ful alternative, and indeed is of wide general application. Sedi-
mentary rocks are also known in rarer instances to yield similar
results.

I02 A HANDBOOK OF ROCKS.

SiO,

M,i\

Fe.,03

FeO

CaO

MgO

Na,p

H.,0

52-39

16.13

1.64

1.44

8.76

4.70

1.42

2-59

0.17

50-44

8.18

1.06

6.28

11-55

17.63

0.50

2.98

0.98

49.19

18.71

5-03

4.04

592

7.98

0.77

1.44

5-05

46.31

II. 14

21.69

9.68

tr.

6.91

4.44

44-49

16.37

5-07

5-50

7-94

7-50

0.56

2.59

4-99

I. Hornblende-schist, Grand Rapids, Wis., Geology of Wis., IV., 629. Also, Fe in
pyrite, 0.34; 8,0.39; P.^j, 0.28; Ca in apatite 0.815. 2. Pseudo-diorite of Becker,
Knoxville, Calif., Monograph XIII. U. S. Geo/. Siirv. loi, W. H. Melville, Anal.
Also, MnO 0.213, Cr^Og 0.480. 3. Hornblende-schist derived from gabbro, Lower
Quinnesec Basin, Wis. R. B. Riggs for G. H. Williams, Bull. 62 U. S. Geol. Surv.,
p. 89. Also, CO.2 1.82. 4. Hornblende-schist near Cleveland Mine, Mich., Foster and
Whitney, Hept. on the Iron Lands of Lake Superior, p. 92. 5. Hornblende-schist,
Lower Quinnesec Falls, Wis., R. B. Riggs for G. H. Williams, Bull. 62 U. S. Geol.
Surv., p. 91. Also COj 5.38.

Comments on the Aiialyses. — The analyses indicate basic rocks, of
decidedly variable composition. Nos. 3 and 5 are certainly sheared
igneous rocks. No. 2 is regarded by Becker as a metamorphosed
sediment. It is quite different from the others in its low alumina,
and its great excess of magnesia over lime. No. i appears to be
an altered igneous rock and No. 4 is probably the same. Aside
from exhibiting the composition of these rocks, the analyses are
interesting when compared with those of the basic diorites (p. 47)
and the gabbros and pyroxenites (p. 49).

Mineralogical Composition. Varieties. — The most abundant min-
eral in these rocks is naturally hornblende. With it are associated
oftentimes biotite, augite, plagioclase, garnet, magnetite, pyrite and
pyrrhotite ; but quartz, except as forming veinlets, is not often met
nor is it to be expected in such basic rocks. The commonest va-
riety of hornblende is black to the eye but is green in thin section.
It forms prismatic crystals from moderately coarse to microscopi-
cally fine. The prisms are interlaced so as to make a very tough
aggregate and one that breaks with difficulty under the hammer.
Light green actinolite may also form schists. Black scales of bio-
tite appear interlaminated with the hornblende. The augite is not
readily distinguished from the hornblende with the eye alone. It
is in large degree the remnants of original pyroxenes that have
partially passed into hornblende during the metamorphic process.
The plagioclase also represents to a great extent the feldspar
that was in the original gabbro or other igneous rock from which
the amphibolite has been derived. The plagioclase is often re-
placed by secondary products, such as epidote, calcite, scapolite

THE AMPHIBOLITES. THE MLXOR SCHISTS. 103

and others, which together make up the aggregate formerly called
saussurite, and regarded as an individual mineral. The minor
accessories, magnetite, pyrite, pyrrhotite and garnet deser\e no
special mention. Except magnetite, which never fails, they are of
more or less irregular occurrence. ,

Alteration. — The hornblende passes readily into chlorite and
softens to a scaly mass with the separation of much limonite
that yields a characteristic, rusty outcrop. If any pyrite or pyr-
rhotite is present it greatly expedites the alteration by its contri-
bution of .sulphuric acid. The feldspars yield calcite and kaolin
and the whole mass becomes a rusty clay or soil.

Occurrence. — The hornblende-schists constitute individual belts in
schistose regions in the midst of other metamorphic rocks and also
great areas by themselves. Dikes and sheets of diabase and plu-
tonic ma.sses of gabbro in districts that have been subjected to
violent dynamic upheavals readily pass into them. The same
areas in the Eastern States that were cited under gabbro contain
them, and they are minor members in the schistose districts of
New England. Around Lake Superior they form a most impor-
tant part of the geology of the iron ore regions, and in the lilack
Hills, the Rock)' Mountains and the ranges of California tliey are
often met.

\'.\Ki()is Mini IK Schists.

Under this collecti\e term are assembled a .series of minor rocks,
no one of which compares in importance with the .schists already
mentioned, but all of which may be met as subordinate members
of metamorphic districts. There are also others in considerable
variety which are esteemed too unimportant for an elemcntar)'
book.

sio, wp^

IViO,

Fc( )

Cat)

M«<>

K/.)

Na,0

H,0

Chlorite schist.

I. 49.18 15.09

12.90

10.59

5.22

1. 51

3 <n

1.S7

2. 47.10 2.14

44-33

0.36

0.13

5.19

Talc schist.

3. 58.66 9.26

4.42

0.94

22.78

4.09

4. 50.81 4-53

3- 52

4.26

3«-55

4.42

Kpidote schist.

5. 41.28 18.48

9-44

8.20

7.04

7.48.

2. 21

352

2-74

ICcloyitc.

6. 48.89 14.46

2.00

7-15

i3-7t'

12.21

0.17

1-75

0.40

(ilaucophane schist.

7. 47.84 16,88

4.99

556

11.15

7.89

0.46

J. 20

1.98

I04 A HANDBOOK OF ROCKS.

I. Chlorite schist, Klippe, Sweden, Cronqvist for Tornebohn. Quoted by Roth,
Gesteinsanalysen, 1884, p. VIII. 2. Chlorite schist, Foster Mine, Mich., C. F.
Chandler, Geol. of Mich., I., 91. 3. Talc schist, Fahlun, Sweden, Uhde quoted by
Roth, Gesteinsanalysen, 1861, 56. 4. Talc schist, Gastein, Austria, R. Richter,
Idem. 5. Epidote schist from diabase. South Mountain, Pa., C. H. Henderson,
Tf-atis. .fimer. Inst. Min. Eng., XII., 82. 6. Eclogite, Altenburg, Austria, Schuster
Tscher. Mitt., 1878, 368. 7. Glaucophane schist, Monte Diablo, Calif., W. H. Mel-
ville, Bull. Geol. Amer., II., 413.

Comments on the Analyses. — These analyses are too variable to
admit of much in the way of comparative remarks, for the rocks
are so totally unlike. No. i suggests an original diabase or some
such rock. No. 2 is abnormally rich in iron, doubtless in large
part from magnetite or hematite. The high magnesia in Nos. 3
and 4 is characteristic and indicates their close relations with ser-
pentines. No. 5 is an altered diabase. No. 6 is of a rock variable
in its mineralogy and obscure in its history. No. 7 is practically
a hornblende-schist with glaucophane, an amphibole that is high
in soda, instead of common hornblende.

Mineralogical Composition. Varieties. — The chlorite schists are
marked by the presence of this green micaceous mineral in large
amount. More or less quartz is also generally present, and not in-
frequently plagioclase, talc, epidote and magnetite. The schistose
texture is pronounced. The chlorite-schists are manifestly altera-
tion products from some rock, with abundant, anhydrous, iron-
alumina silicates. Hornblende-schists, presumably from basic
igneous rocks are the general source. Certain chlorite-schists are
often called " green schists."

Talc-schists are characterized by sufficient talc to make this min-
eral prominent and in addition they have quartz as the next most
abundant constituent. Feldspar may at times be noted, and some
micaceous mineral is not rare. Care is necessary not to confuse
fine scales of the last named with talc itself Various accessory
minerals likewise occur, and the magnesian carbonates, dolomite
and magnesite are often present. Obviously the talc-schists have
resulted from the alteration of some rock with one or more anhy-
drous, magnesian silicates that lacked iron. Tremolite and ensta-
tite are the most available, but the original sources of these are
often obscure. Siliceous dolomites or intrusive pyroxenites at
once suggest themselves, but the iron must of necessity have been
low, so as not to yield serpentines.

Epidote-schists result when the ferro- magnesian silicates and the

THE MIXOR SCHISTS. 105

plagioclases are so favorably situated with reference to each other
as to establish mutual reactions. They especially arise as phases
in the metamorphism of pyroxenic or hornblendic rocks, such as
diabase, hornblende-schists and the like. Eclogite is a rock scarcely
known in America, having, as yet. only been noted near the Wash-
ington Mine, Marquette District, Mich. (Geol. of Wis., III., 649).
It is a well recognized variety, however, in Europe. It consists of
bright green amphiboles and pyro.xenc, of garnet and of a variety
of minor minerals. In ordinary determination it would not be
distinguished from a garnetiferous, actinolitc schist. Glaucophane
is a blue soda amphibolc that is rare in America, except in the
coast range of California, where it characterizes certain important
schists. The rocks have a pronounced blue shade, and contain in
addition quartz and feldspar. In California they certainly are al-
tered shales. Graphite appears cjuite commonly as a characteristic
mineral of certain schists, and may justify the use of the name
graphite schist. More or less mica, and always quartz and feld-
spar are associated.

Distribution. — Chlorite -schist and talc-schist are not uncommon
members of our larger metamorphic series, especially along the
Appalachians, in New I'jigland antl around I^ike Superior.
ICpidotc-schist is less common in the same relations. The occur-
rence of eclogite and glaucophane-.schist has already been cited.
Graphite-schist is not iiifre(|uent in the metamorphosed Paleozoic
.strata of the l-'.ast.

CHAPTER XL

The Metamorphic Rocks, Continued. The Rocks Produced
BY Regional Metamorphism. The Quartzites and
Slates. The Crystalline Limestones and
Dolomites, Ophicalcites, Serpen-
tines AND SoAPSTONES.

The Quartzites.

SiOj AI0O3 Fe.Og FeO CaO MgO K/J Na,0 H.p Sp. Gr.

1. 97.1 1.39 1.25 0.18 0.13

2. 96.44 1.74 0.33 017 022 0.13 0.19 0.90

3. 84.52 12.33 2.12 0.31 tr. o. II 0.34 2.31 2.74

I. Quartzite, Chickies Station, Penn., Penn. Geol. Sw'v. Rep. M., p. 91. 2.
Sandstone partly altered to Quartzite, Quarry Mtn., Ark., R. N. Brackett for L. S.
Griswold, Geol. of Ark , 1890, III., 140, 161. 3. Quartzite, Pipestone, Minn., W. A.
Noyes in Mhin. Geol. Surv., I., 1 98.

Comments on the Analyses. — There is no essential difference in
the analyses of quartzites and sandstones, as the few quoted above
will show, but doubtless the resulting quartzite is somewhat richer
in silica than the original sandstone. Comparatively few analyses
of quartzites have been made in America.

Mineralogical Compositio)i, Varieties. — The quartzites are meta-
morphosed sandstones, and differ from the latter principally in their
greater hardness, and to a certain extent in their fairly pronounced
crystalline character. These qualities are due to the presence of
an abundant siliceous cement that is crystalline quartz, and that is
often deposited around the grains of quartz of the original sand-
stone, so as to continue their physical and optical properties. The
original grains have, therefore, had the power of controlling the

106

THE QUARTZ ITES. THE SLATES. 107

orientation of the molecules of the new silica as it cr>'stallized.
When the original sandstone has been argillaceous the resulting
quartzite contains mica and especially muscovite, and with increase
of the mica, such quartzites pass through the intermediate varieties of
quartz-schist into mica-schists. A ver>' curious and more or less mi-
caceous variety is the so-called flexible sandstone or itacolumite,
whose grains have the power of slight movement on one another
from their loosely interlocked arrangement, so that thin slabs may
be bent through a considerable arc. Quartzites also result from
pebbly sandstones and conglomerates, and the pebbles of these
latter are often flattened by the dynamic movements with which
the metamorphism is at times associated. There is no sharp line
of demarcation between quartzites and sandstones, and while the
extremes of soft sandstones and hard quartzites are entirely differ-
ent, the determination of intermediate \.iriit!is is more or Uss ar-
bitrary.

Alteration. — Quartzites sometimes soften to siind on their out-
crops, and in the process, almost the last vestiges of alumina or
lime may be removed. In this way the .sands in analysis No. i.
p. 63, were formed. In general, however, they arc excessively
resistant rocks, and tend to form prominent ledges.

Distribution. — Quartzites occur in almost all series of metamor-
phosed sediments, and as these are best developed in the later
Archean (Iluronian, Alg<Mikian) .strata, they especially char-
acterize them. In the metamorphic belt in New l^ngland and
along the Appalachians, they are frequent, as well as in the Huro-
nian, around Lake Superior and Lake Huron and in the similar
areas of the West.

The Si

.ATES.

SiO.,

A1,0,

l'>/».

Yk( )

CaO

Mk< )

K.j()

Naji>

11,0

66.45
66.00

n-i^

1.71

I 4«

2.86.
tr.

6.28

tr.

0.05
367

0.90
2.22

403

24.60

3.00

65.85

16.65

5 31

0.59

2.95

3-74

'•31

3 »o

(^4-57

17 30

7.46

1. 16

2.60

1.99

4.62

^ii^

16.16

3-79

0.15

4 44

7.56

J 54

265

60.50

19.70

7-83

1. 12

2.20

318

2.20

iZO

60.32

23.10

.05

0.87

y'^i

0.49

4.08

5700

20.10

10. 98

I 23

3 39

» 73

»-30

4.40

55.88

21.85

903

0.16

1.49

3- 64

0.46

3 39

10.

54.80

23 15

9.58

1.06

2.16

3-37

2.22

390

io8 A HANDBOOK OF ROCKS.

I. Slate, Llanberis, Wales. Quoted by G. VMenWl, Stones forBuilding and Deco-
raiion, p. 421, also MnO, 0.91, COj, 1.30. 2. Slate, Etchemin Riv., N. B., T. S.
Huni, Fhil. Mag:, (4) VII., 237, 1854. 3. Roofing slate, Westbury, Can., /t/^w.
4. Roofing slate, Lehesten, Germany. Frick, quoted by Roth, Gesteinsanalysen, 1861,
p. 57. 5. Damourite slate, Hensingerville, Pa., Geol. of Penn. Rep. M., 91. 6.
Roofing slate, Wales, T. S. Hunt as under No. 2. 7. Slate, Lancaster Co., Penn.,
also FeSj, 0.09. See under No. i. 8, Roofing slate. Angers, France, T. S. Hunt, as
under No. 2. 9. Blueblack carbonaceous slate. Peach Bottom slate, York Co., Penn.,
also MnO, 0.586, CoO tr. C, 1.974, FeS.^, 0.51, 80^,0.022. See under No. I. 10.
Roofing slate, Kingsey, Quebec, T. S. Hunt, as under No. 2.

Coiiwients on the Analyses. — The analyses are especially signifi-
cant when compared with those of the shales and clays, p. 66,
and with those of the mica-schists, p. 99, with which latter they
are closely parallel. Two features at once impress the observer,
the excess of magnesia over lime, and the excess of potash over
soda. The former stamps their origin as from sediments rather than
from igneous rocks of these percentages in silica, because this rela-
tive excess of magnesia as noted under the mica-schists is rather
characteristic of sediments.

Mlnerulogical Composition. Varieties. — As the sandstones during
metamorphism pass into quartzites, so the shales and clays become
slates, when not so thoroughly recrystallized as to yield mica-
schists or phyllites. The more sandy shales afford varieties that
break irregularly and that lack homogeneity, but tough and even
slates result from homogeneous clays and are among the most
remarkable of rocks. The distinctive feature of slates as against
shales is the possession of a new cleavage that may lie at any
angle with the original bedding of the rock, and that has no defi-
nite relation to it. The cleavage has been developed by dynamic
strains that have, beyond question, involved a shearing stress and,
some differental movement among the layers, though it may have
been microscopic. As a matter of observation the component
grains of slates have become flattened and lie parallel with the new
cleavage, and any mica flakes or hornblende needles that may be
present lie along it.

Various explanations have been advanced for slaty cleavage, and
its artificial production in different substances has occupied several
investigators. Based principally upon experiments performed by
Professor John Tyndall, over forty years ago, it has been usually
referred to a compressive force at right angles to its plane. Tyndall
subjected blocks of wax to pressure, using wet glass plates as his

THE SLATES. 109

buttress of resistance. The blocks were of course greatly reduced
in thickness and were forced to spread or bulge laterally. Shortly
afterward H. C. Sorb}-, partly on the basis of the flattening of the
component grains, and the alignment of mica scales, explained the
cleavage as due to planes of weakness caused by this new arrange-
ment. Recently, G. V. Becker of the U. S. Geological Survc)' has
repeated the experiments of Tyndall with modifications. So long
as the resisting glass plates were wet with water the slaty cleavage
was developed, but when they were smeared with a heavy lubricat-
ing oil, although there was lateral expansion during compression,
no bulging took place and no cleavage was developed. Manifestly
therefore the frictional drag of the plates enters into the problem,
and although the resolution of the forces involved is somewhat
complex, a shearing stress results that is a strong factor in pro-
ducing the cleavage.* In the case of the large beds or strata
which are metamorphosed into slate in Nature, the case is even
less simple, and the contrasts in rigidity, between the beds that
yield slates, and their enclosing strata, are less pron(iuncctl than in
the experiment, but there is little iloubt that the cc^mpression aiul
slight lateral flow which occasion a flattening of the grains and an
alignment of the seal)' minerals across the direction of application
of the force in this wa)- produce the cleavage. All slates have cross-
cleavages, or, it may be, joints, more or less well develojied. anti one
of these may even be perfect enough in connection with the regu-
lar cleavage, tocau.se the slate to break into small prisms available
for slate pencils, for which in earlier years they were employed.
All slate quarries also show curly slates, where cjuart/.-veins or .sandy
and harder streaks in the original sediment have caused imperfec-
tions in the cleavage. It has been noted that in some (juarries the
available plates appear to become thicker in depth, as if the surface
weathering had been a factor in developing the cleavages. Though
commonl)' drab to black, they may be red. green or purple.

Slates pass by all intermediate gradations into phyllites and
mica-schists. The word slate is also loosel)' used for shales that
have never had any secondary cleavage induced in them, and this
is especially true of the black, bituminous shales that occur with
coal seams, but in strict, geological u.se. the new cleavage and
metamorphism should be an essential of a true slate.

*G. F. Becker, Finite homogeneous Strain, Flow and Rupture in Roci<s, Bu/l. Gtol.
Sor. Anirr., IV., 82, 1893.

no A HANDBOOK OF ROCKS.

Alteration. — Slates are exceedingly resistant as is shown by their
use in thin slabs for roofs, and they often constitute prominent
ledges or even peaks. They soften down to a clay in the last stages
of alteration, but always on the outcrop are more tender than in
depth, so that much dead work is unavoidable in opening quarries.

Distribution. — Our most prominent slates are Cambrian or Or-
dovician in age. Along the Green Mountains and especially in
northern Vermont they are strongly developed. Again in east-
ern Pennsylvania, in Virginia and in Georgia they are met in great
areas. On the south shore of Lake Superior merchantable grades
have been somewhat developed. Along the western slopes of the
Sierra Nevada Mountains they are very important rocks.

The Crystalline Limestones and Dolomites.

SiO., Al.,0,

Loss.

CaCOg

MgCOg

99-51

99.24

0.28

98.43

0.30

98.21

2-35

98.00

96.82

1.89

92.42

6.47

70.1

25.40

54.62

45 -04

lO.

54-25

44-45

FeO

InsoL

0.29

0.20

0.31

0.38

0.15

0-35

0-57

1.63

O.IO

2.12

0-35

0-9S
2.40

O.IO 0.7

0.60

Statuary Marble, Brandon, Vt. Quoted by G. P. Merrill, Stones for Building and
Decoj-ation, 417. 2. Marble, Carrara, Italy, Idem. 3. Marble, Knoxville, Tenn. ,
Idem, also S, 0.014, Organic Matter, 0.068. 4. Cross-grained black and white mottled
Marble, Pickens Co., Ga., locally called Creole ; Geol. Surv.' Ga., Bulletin I., 87.
5. White Marble, Rutland, Vt., see under No. I. 6. Coarsely crystalline while Mar-
ble, Cherokee Quarry, Pickens Co., Ga. , see under No. 4. 7. White Crystalline
Limestone, Franklin Furnace, N. J., Geo. C. Stone, unpublished. 8. Crystalline Mag-
nesian Limestone, Tukahoe, N. Y., H. L. Bowker for Lime Co. 9. Crystalline Dolo-
mite, so-called " Snowflake Marble," Pleasaniville, N. Y.,XVI., Arm Rep. Dir. U. S.
Geol. Sui'vey, Part IV., p. 468. 10. Crystalline Dolomite, white marble, Inyo Co.,
Calif., Ann. Rep. Calif. State Mineralogist, 12S.

Comments on the Analyses. — The analyses do not differ essen-
tially from those of unaltered limestones except so far as the ones
in the table are purer carbonates of lime and magnesia. The avail-
able analyses are of merchantable marbles, and in the nature of
the case these are derived from very pure sedimentary limestones.
They are interesting as illustrating the series from a rock that is
almost chemically pure carbonate of lime to one in which the
carbonate of magnesia reaches the values of typical dolomite.

CRYSTALLINE LLMESTONES AXD DOLOMITES, in

Comparison with the analyses of limestones earlier given, on p. 70.
is recommended. It will be seen that in this case there is apparently
no change in gross composition from metamorphism, but of course
the relations of the silica and the bases are different. In the sedi-
mentary limestones the silica is largely in the form of quartz and
in combination with alumina forming hydrated silicates, such as
kaolin. In the crystalline limestones it is largely in silicates of
lime, magnesia and alumina, such as tremolite, pyroxene, phlogo-
pite, etc., minerals whose formation has been one of the results of
metamorphism. The percentages in the insoluble column do not
therefore indicate pure silica. There may be even microscopic,
barite crystals present.

Mincraloi^ical Composition. I 'nritties. — The cr\'stalline limestones
and dolomites arc metamorphosed forms of the sedimcntar\' varie-
ties earlier described. The change involved is, as the name im-
plies, one of crystallization. I'ossils, and to a large degree bedding
planes, are destroyed and a more massive aggregate of calcite or
dolomite crystals results. Such carbonaceous material as was origi-
nally jiresent usually affords streaks of graj)hite which occasion
dark veinings. They bring out the brccciation or flow-lines in-
duced by the pressure from the mountain making upheavals
usually attendant on the metamorphism. Other bituminous or
ferruginous matter may \ield pronounced colors of many hues.

If the original limestone has been an impure variety and has
contained silica, alumina and iron oxides, as illustrated by the
analyses on page 70, these components have furnished the neces-
sary materials for the various silicates that the metamorphism
has caused to form. Tremolite is a common result, light-colored
pyroxenes are not infrequent, and phlogopite and other micaceous
minerals are the most abundant of all. I^rge quarries always
show borders or streaks that are characterized by these minerals,
and where the original limestone pa.s.seil into shales or .sandstones
at its upper and under surfaces, these micaceous varieties are
almost always met. I'^or ornamental purposes, the included sili-
cates serve to mar the stone, being, except in the case of micas, of
greater hardness than the calcite.

Crystalline limestones form more or less extensive strata in the
midst of other mctamorphic rocks. Slate, phyllitcs, mica-schists
and quartzites are their most common associates. The dolomites
may lia\c formed in man\' cases from pure calcareous limestones b)'
the infiltration of magiu'sian solutions, and by an exchange of a por-

112 A HANDBOOK OF ROCKS.

tion of the magnesia for a portion of the lime, as earher referred to
on page 7 1 , but so many unaltered limestones are high in magnesia,
that the change is not a necessary attendant of metamorphism.

Alteration. — Crystalline limestones are soluble rocks and weather
with comparative facility. Where they occur in metamorphic
belts they are invariably in the valleys, and are potent factors in
determining the direction of the drainage lines. Where exposed
for long periods they afford a coarse, crumbling sand or gravel,
that is much used for roads in the borders of the Adirondacks and
in western New England. The final stage is a mantle of residual
clay from which the calcareous material has been largely leached.

Occurrence. — The crystalline limestones are frequent in our me-
tamorphic districts. In the Appalachian belt they are of great
areal and economic importance, and are largely quarried in Ver-
mont, Massachusetts, New York, Pennsylvania and Georgia. In
western Colorado they are strongly developed, and in the Sierras
of California the same is true, Inyo County being a rather large
producer of marble. The foreign mountainous and metamorphic
districts exhibit enormous exposures. The great series of ranges
that begin in the Pyrenees and extend through the Alps and the
Carpathians to the Himalayas, have many famous quarries and
ledges. The region of the " Dolomites " in the Tyrolese Alps is
a district of especial richness. The Carrara marble of the Ap-
penines, the Pentelic of Greece and the colored varieties from
Northern Africa, indicate their presence in those regions.

The Ophicalcites, Serpentines and Soapstones.

Ophicalc.

CaCO

. Mg

;C03

FeC03

SiO,

MgO

H2O

FeO

AlA

57-37

0.74

13-18

10.29

4.06

3.57

0.85

23-85

.28

1.97

22 42

18.74

6.43

4.30

7.65

.98

1.78

3653

28.08

8.63

6.49

Serp.

SiOj

MgO

AlA

Cr,0,

Fe,03

FeO

NiO

CaO

44.14

42.97

12.89

43-87

38.62

9-55

0.31

7.17

0.27

0.02

42.52

42.16

14.22

1.96

41-54

40.42

14.17

2.48

1-37

0.04

40.67

32.61

12.77

5-13

8.12

40.06

39.02

12.10

1.37

0.20

3 43

0.71

10.

36.95

3307

10.40

16.50

II.

34-84

30.74

17-39

0.42

0.68

6.08

1.85

tr.

7.02

Soapst.

MnO

12.

64.44

33-19

0.34

0.48

1.39

0.23

13-

62.10

32.40

2.05

1.30

2.15

U-

62.00

33 I

4-9

OPHJCAL CITES, SERPEXTINES, SOAPSTO,\ES. 113

I. Ophicalcite, Oxford, Quebec, T. S. Hunt, Amer. Jour. Set., March, 1858, 220.
The analysis as cited is assembled from several partial analyses 2. Ophicalcite.
Brompton Lake, Quebec, Idem, p. 221. Original results recast as in No. I. 3. Ophi-
calcite, Brompton Lake, Quebec, Idem, p. 222. Recast as before. 4. Theoretical ser-
pentine, HjMgjSiOg. 5. Massive serpentine, Webster, N. C, F. A. (lenth, Amer.
Jour. Sei., IL, xxxiii, 20I. 6. Massive serpentine, Montville, N. J., E. A. Manice,
Dana's Mineralogy, 1877, 467. 7. Serpentine, a metamorphosed sandstone, New
Idria, Calif., \V. IL Melville for G. H. Becker, in Monograph XIII., U. S. Geol.
Surv., 110. 8. Serpentine, decomposed peridotile, Syracuse, N. Y., T. S. Hunt,
Amer. four. Set., Sept., 1 858, 237. 9. .Serpentine, Dublin, Harford Co., Md. Quoted
by G. P. Merrill, Stones for liuilding and Decoration , ^IJ^. lO. Serpentine from peri-
dotite, Presq' Isle, Mich., J. D. Whitney, Amer. Jour. Sei., II. , xxviii, 18, also
Na,0, 0.97. II. Serpentine from peridotite, Monte Diablo, Calif., W. H. Melville,
Bull. Geol Sot. Amer., IL, 408, also Na,0, 0.42, K,0, 0.07. Soapstone, Webster,
Jack.son Co., N. C, F. A. Genth, Minerals of North Carolina, p. 61. 13. Talc,
Gouverneur, N. Y., Analysis quoted by C. H. Smyth, Jr., School of Mines Quarterly,
July, 1896, p. 340. 14. Theoretical talc, 6MgO, sSiO,, 211,0.

Cottn)icnts on the Analyses. — The ophicalcites mark a passage
from the dolomites to the .serpentines. They are practically crys-
talline magncsian limestones or dolomites, that are mottled with
inclusions of serpentine in var)'in^ amounts. The analyses begin
with one that is over half calcite and over two-thirds calcite and
dolomite. The ratios of the remaining oxides are just about those
recjuired by serpentine. In the .second the amount of .ser|x;ntine
has much increa.sed, and in the third the carbonates have notably
retreated. Under the serjientines, as compared with the theoret-
ical mineral, No. 4, the succeeding analyses are all notably rich in
iron, b^.xccpt in the cases of Nos. 10 and 1 1, they are remarkably
uniform considering their diverse origin. In No. 10 the SiO,
drops, probably from the presence of magnetite, while in the last
the p\ro.\ene of the original peridotite has contributed consider-
able lime. In all these rocks Al.O., is notabl)- low. It is most
abundant in No. .S, a seqientine that is derived from a rock with
much augitc. Chromium is a rather characteristic element in ser-
pentines that result from basic igneous rocks, and nickel can be
very generally detected on analysis. Lime practically fails except
in No. II. It should be apjireciated that as a mineral, serjK-ntine
is a unisilicate, whereas talc is a bisilicate, and this explains the much
larger percentage of silica in the latter. The soapstones are fairly
pure, aggregates of talc, as a comparison of Nos. 12 and 13 with
No. 14 will indicate.

Mincralogical Composition. I 'ariitics. — The ophicalcites are mot-
tled rocks consistingof irregular or rounded masses of green serpen-

114 A HANDBOOK OF ROCKS.

tine embedded in white calcite and dolomite. The proportions of
the constituent minerals are variable. The serpentine may be in
small nodules a fraction of an inch in diameter or in large stringers
and masses several feet across. This irregularity renders it dif-
ficult in quarrying to preserve a uniform grade. The stone is
mottled green and white, and when uniform is a very beautiful one.
The serpentine varies from dark green or almost black, to light
clear shades, and has been derived in a number of cases, as has
been shown by G. P. Merrill,* from original pyroxene crystals.

The ophicalcites are therefore in many cases alteration products
from a crystalline limestone, that has been surcharged with pyrox-
enes, and this itself may probably be referred in most cases to an
original siliceous, magnesian sediment, recrystallized by regional
metamorphism.

Ophicalcites are also called ophiolites, serpentinous marbles and
verd antique. The syllable "ophi," in all these words is derived
from the Greek for serpent and ophicalcite means therefore a ser-
pentinous limestone.

The serpentines are green or red aggregates of scales, fibers or
massive individuals of the mineral serpentine. They display con-
siderable variety of texture according to the characters of these
components. Other minerals are not especially prominent. Grains
of chromite or magnetite may be detected and garnets of the
variety pyrope are sometimes well developed. Veinlets of calcite or
of magnesian carbonates ramify through the rock in many expo-
sures. Remains of the original olivine, pyroxene, or hornblende
from which the serpentine has been derived may often be detected
and biotite or some hydrated magnesian mica is not infrequent.
The varieties of the mineral serpentine are numerous, but many of
them are too rare to be serious rock-makers. Almost all serpen-
tines have been formed by the alteration of basic igneous rocks,
among which the pyroxenites and peridotites are the chief con-
tributors. Hornblende schists also yield them and G. F. Becker
has recorded the remarkable case of sandstones that pass into
them in the Coast Ranges of California.

Soapstones, called also steatites, are chiefly talc as the analyses
show. Quartz veinlets often run through the rock and scattered
grains of quartz are not infrequent. Magnesian carbonates

*G. P. Merrill, Amer. Jcur. Sd., March, 1889; Proc. U. S. NatU Museum, XII.,
595, 1890.

OPHICALCITES, SHRPIiXT/XIiS, SOAPSTOXES. 115

are likewise evident in many exposures. In the case of the
Gouverneur beds of talc (see Anal. 13), C. H. Smyth has shown
that the original minerals have been tremolite and enstatite,
and that the beds occur in crystalline limestone, but it is a hard
problem to determine from what tremolite and enstatite have been
derived. Two reasonable sources suggest themselves, either
a siliceous dolomite, or a non-ferruginous, basic intrusive. The
soapstones are not particularly abundant rocks but are of eco-
nomic value where met. They arc close relatives to the talc
schists earlier cited.

Altiratioti. — The scr|KMitinous rocks themselves are thoroughly
altered derivatives from fresher anhjdrous ones and in their further
decomposition simply soften to incoherent dirt and clay. The
more resistant, included minerals arc thus set free, and as in the
case of platinum and garnets they may be concentrated in gravel.

Distribution. — Ophicalcites arc most abundant in Quebec, the
northern Green Mountains and the foothills of the Adirondacks.
The serpentines are especially notable on Staten Island, in south-
eastern Penn.sylvania and the neighboring parts of Maryland, where
the gabbros, as stated on p. 52, and their related rocks are abun-
dant. They share in an important belt ofthe.se basic intrusives in
North Carolina and Georgia. In the basic igneous rocks around
Lake Superior they are occa.sionally met as alteration products.
In the Coa.st ranges the serpentines are of very great importance,
and in part are altered sediments. ThcN' are likewise common
abroad, and in a minor capacity appear in man\' metamorphic dis-
tricts. Soapstonc is much less common, but is met in this country
as a subordinate member in much the same regions as tin sir
pentines and crystalline dolomites.

CHAPTER XII.

The Metamorphic Rocks, Concluded. The Rocks Produced

BY Atmospheric Weathering. The Determination

OF THE Metamorphic Rocks.

Introduction. — It is a matter of common observation that out-
crops of rocks and loose boulders are always more or less decom-
posed and broken down or " weathered" for a greater or less dis-
tance below their surfaces. This may not be Serious enough to
prevent the accurate recognition of the rock, and usually within
the area once covered by the great ice sheet of the Glacial Period it
is not, because the moving ice has ploughed away all loose and de-
composed materials, but south of the terminal moraine, and above
all in the tropics, the decomposition is excessive and may produce
to a depth of a hundred feet or more a mass of alteration prod-
ucts that give of themselves slight, if any, clue to their originals.
This is a common experience in the Southern States, where, as
well as in Central and South America, the indefinite character of
the surface rock throws great difficulties in the way of accurate
geological mapping. So difficult at times is the determination of
the country rock, that, for example, during field work in Brazil, O.
A. Derby has felt compelled to resort to the panning out of the
surface materials with a gold-seeker's pan in order, by concentrat-
ing the heavy but small and undecomposed, accessory minerals,
such as zircon, titanite, monazite, xenotime, apatite and others, to
get from their characteristic associations some clue to the original
rock. Many travelers have noted the brilliant colors of the soils
of latitudes toward the equator and the comparatively somber
tones of those toward the poles.

These products of weathering are so widespread, therefore, and
5o individual that a few pages have been reserved for their particu-
lar mention. Special names for them have been suggested at various
times. The oldest one and the one most current is laterite. The

ii6

ROCKS FROM A TMO SPHERIC \ \ 'EA THERIXG. 1 1 7

word means brick earth and was originally applied to the red or
brown ironstained surface soils occurring in the tropical lands, and
derived by direct decomposition from the country rock in place.
It has been applied in later years, however, to all sorts of these sur-
face soils from whatever rocks derived, and whether colored red or
not. G. F. Becker, of the U. S. Geological Survey, has recently
(1895) proposed saprolite,* a word meaning literally rotten rock,
as " a general name for thoroughly decomposed, earthy, but un-
transported rock." This is practically the modern use of laterite,
although it is broader than the latter's original application. The
U. S. Geological Survey in the invaluable .series of atlas sheets now
being issued employs the term "surficial," /. <•., surface rocks, as a
general designation for these untransported products of decom-
position. We also often speak of residual clay as was done on
pp. 66 and 67 for the less soluble, aluminous residues left bLhind
in the removal of the more soluble portions of limestones.

The general scope and application of these names having been
set forth, a brief consideration will be given to the mineralogical
processes of change that have produced them from several of the
commoner groups of rocks.

The chief causes of this sujicrficial breaking down or " degenera-
tion," as it has been aptly called by G. P. Merrill, t are, the chem-
ical action of rain and ground-waters, especially when charged with
carbonic acid or other dissolved matter ; organic life, both vege-
table and animal, operating through the agency of the organic acids
produced by their living processes or b\- their tiecomposing re-
mains ; and the mechanical disintegration produced by changes
of temperature, by the freezing of water and by swelling from hy-
dration or from some of the chemical or mineralogical changes
among those referred to above. Altlunigh having no connection
with the.se atmospheric processes, yet hot springs and allied exhala-
tions from dying volcanic energy bring about closely similar re-
sults and are able to change great sheets of volcanic rock to bril-
liantly variegated mas.ses of clay and kaolin. At the Falls of the
\'ellowstone River, in the National Park, the.se are wonderfully and
impressively displayed, more than a thousand feet of rhyolite hav-
ing been changed practically to kaolin.

Under the action of the chemical agents the more easily soluble

* (rtildfields of the Southern Appalachians, p. 43, XVI., Ann. Rep. Dir. U. S. Geo!.

\ Bulletin of the Geological Society of America, VII., 378.

ii8 A HANDBOOK OF ROCKS.

elements are removed or put in such relations to one another
as to facilitate their rearrangement in new and secondary com-
binations. In the rocks composed of silicates the most vulnerable
oxides are lime, magnesia, potash and soda. Iron oxides also
suffer .extensively, but the ferric form is sometimes very resistant.
Silica yields more or less, especially to the alkaline solutions from
the potash and soda referred to above. Alumina, on the whole,
is least readily attacked of all, and is usually the one that furnishes
the best basis of comparison between analyses of altered and un-
altered materials.

Among the igneous and metamorphic rocks, open or porous
varieties naturally suffer more than compact and finely crystalline
ones. Rocks high in the bases that are most readily attacked
chemically, are easier victims than those especially rich in the re-
sistant ones. Basic rocks, therefore, with their high percentages
of lime and magnesia and their relatively low silica, suffer espe-
cially, whereas, granites and related gneisses are much more stub-
born subjects, the large amount of quartz in them furnishing a very
resistant component.

Granites, syenites, acid diorites and their corresponding porphy-
ritic types alter especially through the feldspathic member present.
The constituent quartz is but slightly affected, and the dark silicates
are not present in sufficiently large amounts to be very serious fac-
tors. The resulting product is a kaolinized or clayey mass through
which are distributed quartz grains, and which is more or less
stained by the hydrated oxide of iron that is yielded to some ex-
tent by the dark silicates. The characteristic products of the latter
are also present in small amounts, but are more extensively men-
tioned subsequently. The exposed ledges furnish loose pieces
that often weather in concentric shells and simulate rounded, water-
worn boulders. The next result is a large contribution of clay and
sand to sedimentary or eolian deposits, it may be at a great distance.

In the basic igneous or metamorphic rocks the dark ferro-mag-
nesian and aluminous silicates are in excess, and in decomposition
their peculiar products predominate. The distinctively magnesian
ones yield serpentine, the aluminous change to chlorite. Both
these minerals are prevailingly green, and dark green, surficial
rocks result. The abundance of iron in them leads to the forma-
tion of very rusty outcrops.

In the case of limestone, the lime and magnesia are dissolved

DETERMIXATIOX OF METAMORPHIC ROCKS. 119

away, while the alumina, silica and iron oxides remain behind in
the mantle of impure residual clay alread)' referred to. The other
sedimentary rocks suffer especially from mechanical processes,
although chemical changes are not lacking among them, for, as re-
marked on page 107 regarding analysis No. i, of page 63, during
the breaking up considerable leaching may result that leads' to the
production of nearly chemically pure quartz sand.

The mechanical and associated, chemical breaking down of rocks
tends to place them in more favorable conditions for further chem-
ical alterations, and for erosion and removal.

All the changes in the weathering of rocks have been well-de-
scribed by M. \\. Wads worth as " resulting from the general dissi-
pation and degradation of the potential energy of the constituents
of the earth's crust in the universal pas.sagc of matter from an ac-
tive state towards a passive and inert condition."*

TiiK Dktkk.min.mion or the Met.vmoki'Mk Rocks.

The rocks resulting from contact mctamorphism arc rather of
local interest, than of wide, areal distribution. The spotted schists
and slates, and the hornstones are readil\- recogni/.ed by a practiced
observer. Ilu- crystalline limestones even when chargeil with
silicates may closely resemble the products of regional mctamor-
phism. In dealing with the latter, famiiiarit)' with well character-
ized types is the safest guide. The gnci.sses are at once apparent
from their laminated character and granitoid texture. Transition
members between them and the mica-schists on the one hand,
and the hornblende-schi.sts on the other, may cause hesitation as
to which group they belong to. The finel\- laminated ones are
certainly members of the schists, those with prevailing mica be-
longing with the mica-schists, those with prevailing hornblende,
with the hornblende-schists. Again as the fineness of the lamina-
tion or foliation increases, the schists pass into the phyllites and
slates, that are easily recognized. The quartzites likewise present
little difficulty as they are practically liard .sandstone. The crys-
talline limestones and dolomites are only to be distinguished by
the ease or difficult)' of obtaining effervescence. The ophicalcites
look like no other rocks, and the seipentines and soapstones are

* The Theories of i trc Deposits, Proc. Bost. Soc. Nat. Hist., Vol. XXIII., p.
202 1884.

I20 A HANDBOOK OF ROCKS.

also at once apparent. The soapy feel of all these magnesian
rocks aids in their recognition. There are, of course, rare and
obscure, metamorphic rocks that cause trouble, but, just as in the
case of the finely crystalline igneous rocks, they are best referred
to someone familiar with the use of the microscope.

GLOSSARY.

Note. — In the following definitions, when fuller explanations are to be found in
preceding pages, references are given to them and they should be consulted. No at-
tempt has been made to unnecessarily repeat previous statements.

Aa, a Hawaian word si)e(;ially introduced into American usage, by
.\Iaj. CI'.. Dutton, and employed to descril)e jagged, scoriaceoiis, lava
flows. It is contrasted with i)ahoehoe. 4th Ann. Rep. U. S. (ieol.
Survey, 95.

Ablation, a name applietl to the process whereby residual dej>osits
are formed by the washing away of loose or soluble materials.

Absarokite, a general name given by Iddings to a group of igneous
rocks in the .Vhsaroka range, in the eastern portion of the Yellowstone
Park. They have porphyritic texture with i)hcnocrysts of olivine and
augite in a grounilma.ss, either gla.ssy or containing leucite, orthoclase or
plagioclase, one or several. They range chemically, SiO,, 46-52 ;
AljOj, 9-12 ; MgO. 8-13 ; alkalies, 3-6.3, with j)otash in excess. The
name is of greatest significance when taken in ( onnection with shoshonitc
and banakite. Jour, of (ieol., III., 936.

Abyssal-rocks, a synonym of plutonic rocks as used in prec eding pages.
The word has been suggestetl and especially used by W. C Hriigger.

Accessory ( omponents or minerals in rocks are those of minor im-
|)ortance or of rare occurrence, whose presence is not called for by the
definition of the species.

Acidic, a des(ri|)tive term applied to those igneous rocks that con-
tain more than d'^'/o ^iO.^, as contrasted with the medium of 65%-55%
and the basic at less than 55%; still the limits are somewhat elastic.

Acmite-trachyte, a trachyte whose pyroxene is acmite or a-girine
and whose feldspar is anorthoclase. It therefore differs from normal
trachyte in its prevailing soda instead of potash, as is shown by the ac-
mite, a soda-pyroxene, and the anorthoclase, a soda -feldspar. The ac-
mite-trachytes are intermediate between the true trachytes and the
phonolites. They were first described from the Azores fMiiggc, Neues
Jahrbuch, 1883, II., 189) and have also been found in the Crazy
Mountains, Mont.; see p. 26, .Vnals. 4 and 5.

Adamellite, a name proposed by Cathrein as a substitute for tonalite,

121

122 A HANDBOOK OF ROCKS.

on the ground that tonalite means a hornblende-biotite granite, rich in
plagioclase, whereas adamellite, which better describes the rocks at the
Tyrolese locality, means a quartz-hornblende-mica-diorite with granitic
affinities. Adamellite emphasizes the dioritic characters ; tonalite, the
granitic. The name is derived from Monte Adamello, near Meran, Tyrol,
the locality of tonalite. Neuesjahrb., 1890, I., 75. Brogger usesit for
acidic quartz-monzonite. Eruptions-folge bei Predazzo, 61.

Adinole, a name for dense felsitic rocks, composed chiefly of an aggre-
gate of excessively fine quartz and albite crystals, such that on analysis
the percentage of soda may reach 10. Actinolite and other minerals are
subordinate. Adinoles occur as contact rocks, associated with diabase
intrusions and are produced by them from schists (compare spilosite and
desmite). They also constitute individual beds in metamorphic series.
(Compare porphyroid, halleflinta. ) The name was first given by Beu-
dant, but has been especially revived by Lessen. Zeits. d. d. Geol.
Ges., XrX., 572, 1867.

Aerolite, a synonym of meteorite.

Agglomerate, a special name for volcanic breccias as distinguished
from other breccias and from conglomerates.

Akerite, a special name coined by Brogger for a variety of syenite at
Aker, Norway, that is a granitoid rock consisting of orthoclase, consider-
able plagioclase, biotite, augite and some quartz. (W. C. Brogger,
Zeitsch. f. Krys., 1890, 43.)

Algovite, a name proposed by Winkler, for a group of rocks, practi-
cally diabases, or porphyritic phases of the same, in the Algauer Alps.
They also embrace gabbros according to Roth, and are doubtless various
textural varieties of an augite-plagioclase magma. Neues Jahrbuch,
1895, 641.

Allotriomorphic, an adjective coined by Rosenbusch in 1887 to
describe those minerals in an igneous rock which do not possess their
own crystal faces or boundaries but which have their outlines impressed
on them by their neighbors. They result when a number of minerals
crystallize at once so as to interfere with one another. They are espe-
cially characteristic of granitoid textures. The word was unnecessary,
as xenomorphic had been earlier suggested for the same thing, but it is
in more general use than xenomorphic. See also anhedron.

Alluvium, Lyells' name for the deposit of loose gravel, sand and mud
that usually intervenes in every district between the superficial covering
of vegetable mould and the subjacent rock. The name is derived from
the Latin word for an inundation (Elements of Geol., 6th Ed., N. Y.,
1859, p. 79). It was employed by Naumann as a general term for sedi-
ments in water as contrasted with eolian rocks. It is generally used
to-day for " the earthy deposit made by running streams or lakes, espe-

GLOSSARY. 123

cially during times of flood." (Dana's Manual, 1895, p. 81.) In a
stratigraphical sense it was formerly employed for the more recent water-
sorted sediments, as contrasted with "diluvium," or the stratified and
unstratified deposits from the melting of the continental glacier of the
Glacial Period. This use, with fuller study of the Glacial deposits, is
practically obsolete.

Alnoite, a very rare rock with the composition of a melilite basalt,
that was first discovered in dikes on the island of Alno, off the coast of
eastern Sweden. The special name was given it by Rosenbusch to em-
phasize its occurrence in dikes and its a.ssociation as a very basic rock,
with nepheline syenite. Alnoite has been discovered near Montreal
by F. D. Adams. ( Amer. Jour. Sci., April. 1S92, p. 269) and at
Manheim Hridge, N. V., by C. H. Smyth, Jr. ( .\mer. Jour. Sci. .\ug.,
1893, 104).

Alsbachite, a name given by Chelius to a variety of granite por-
phyry, forming dykes in Mt. Melibocus, and containing large mica
crystals and rose -red garnets. Noti/bl. Ver. Krdk. zu Darmstadt, 1S9;.
Heft. i;,. I.

Alum-shales, shales charged with alum, which in favorable localities
may be commercially leac hed out and crystallized. The alum results
from the decomposition of pyrites, l)€cause the sulphuric acid, thus pro-
duced, reacts on the alumina present, yielding the double sulphate that
is alum.

Ampelite, a name, specially current among the French, for shales,
charged with pyrite and carbonaceous matter, which may yield alum-
shalcs.

Amphibole, the generic name for the group of bisilicate minerals
whose chief rock-making member is hornblende. It is often prefixed to
those rocks which have hornblende as a prominent constituent, as am-
])hil>olc-andcsitc, amphibole-gabbro, amphibole-granite, etc.

Amphibolite, a metamorphic rock consisting chiefly of hornblende,
or of some member of the amphibole group. It is as a rule a synonym
of hornblende-schists, but is preferable to the latter, when the schistos-
ity is not marked. See j). 10 1.

Amygdaloids are cellular lavas, whose cavities, caused by expand-
ing steam -hubbies, resemble an almond in size and shape. Basaltic
ro< ks arc most |)rone to develop them. The term is used in the form of
the adjective, amygdaloidal, and properly should be limited to this.
As a noun it is also employed for secondary fillings of the cavities, which
are usually calcite, cpiartz or some member of the zeolite group. .Amyg-
daloidal rocks are of chief interest in America, because certain basal-
tic lava sheets on Keweenaw Point, Lake Superior, have their amygdules
filled with native copper and are imj)ortant sources of the metal. .Amyg-

124 A HANDBOOK OF ROCKS.

daloidal cavities are limited to the upper and lower portions of lava
sheets. The name is derived from the Greek word for almond.

Analcite-basalt, a variety of basalt whose feldspar is more or less
replaced by analcite. The analcite is at times in such relations as to
give reason for thinking it an original mineral and not an alteration
product from feldspar. Analcite basalts occur in the Highwood Moun-
tains, Mont, (see W. Lindgren, loth Census, XV., 727, Proc. Calif.
Acad. Sci., Ser. II., Vol. III., p. 51. Comptes Rendus, Fifth Inter-
nat. Geol. Cong., 364). Analcite-diabase has been met in California.
(H. W. Fairbanks, Bull. Dept. Geol. Univ. of Calif., I., 173.) See
also in this connection teschenite.

Anamesite, an old name suggested by von Leonhard in 1832, for
those finely crystalline basalts, which texturally stand between the dense
typical basalt, and the coarser dolerites. The name is from the Greek
for "in the middle."

Andalusite-hornstone, a compact contact rock containing andalusite.
It is usually produced from shales or slates by intrusions of granite.

Andesite, volcanic rocks of porphyritic or felsitic texture, whose
crystallized minerals are plagioclase and one or more of the following :
biotite, hornblende and augite. The name was suggested by L. von
Buch in 1836, for certain rocks from the Andes, resembling trachytes,
but whose feldspar was at first thought to be albite, and later oligoclase.
See p. 40.

Anhedron, a name proposed by L. V. Pirsson for the individual, min-
eral components of an igneous rock, that lack crystal boundaries, and
that cannot therefore be properly called crystals according to the older
and most generally accepted conception of a crystal. Xenomorphic
and allotriomorphic are adjectives implying the same conception. The
name means without planes. Bulletin Geol. Society of America, Vol.
VII., p. 492, 1895.

Anogene, a general name for rocks that have come up from below ;
/. ("., eruptive rocks. See p. 13.

Anorthite-rock, a name given by R. D. Irving to a coarsely crystal-
line, granitoid rock, from the Minnesota shore of Lake Superior, that
consists almost entirety of anorthite (Monograph V., U. S. Geol. Sur-
vey, p. 59). The rock is a feldspathic extreme of the gabbro group,
practically an anorthosite formed of anorthite.

Anorthosite, a name applied by T. Sterry Hunt (Geol. Survey,
Canada, 1863, 22), to granitoid rocks that consist of little else than
labradorite and that are of great extent in eastern Canada and the Adi-
rondacks. The- name is derived from auorthose, the French word for
plagioclase, and is not to be confused with anorthite, with which it has
no necessary connection, although anorthosite is used as a general name
for rocks composed of plagioclase. Mt. Marcy and the neighboring

GLOSS ARV. 125

high peaks of the Adirondacks are formed of it. The rocks are extremes
of the gabbro group, into whose typical members they shade by insen-
sible gradations. See p. 50.

Apachite, a name suggested by Osann, from the .\pache, or Davis
Mountains of western 'I'e.xas, for a variety of phonolite, that varies from
typical phonolites in two particulars. It has almost as much of amphi-
boles and of itnigmatite as of pyroxene, whereas in normal phonolites the
former are rare. The feldspars of the groundmass arc generally micro-
jjerihitic. Tscher. Mitth.. X\'., 454.

Aphanite, an old name, now ])ractically obsolete, for dense, dark
ro( ks, whose components are too small to be distinguished with the eye.
It was chiefly applied to finely crystalline diabases. An adjective
aphanitic is still more or less current.

Aplite is now chiefly applied to the muscovite-granite that occurs in
dikes, and that is, as a rule, finely crystalline. Its original application
was to granites poor or lacking in mii a See |). 31. The name is from
the Greek for simple.

Apo, theCreek preposition lor " Irum,"' suggested by F. liascom as a
prefix to the names of various volcanic rocks to describe the devitrified
or silicified varieties, mostly of ancient date, that result from them, and
that indicate their originals only by the i)reservation of characteristic
textures. Thus apobsidian, aporhyolite, apandesite, apobasalt, etc.,
have been used. (See \k 22.) Many rocks called by the old indefinite
name petrosilex are of this character. Journal of (Ecology, I., 828,
Dec. 1893.

Arenaceous, an adjec tive apjdied to ro( ks that have l)een derived
from saiul, or that contain sand.

Argillite, a synonym of slate.

Arkose, a special name for a santlstone ru h in feldspar fragments, as
distinguished from the more common, richly cpiartzose varieties. See
p. 64.

Aschafiite, a name suggested by Ciiimbel for a dike rock occurring
near .\s( liaHenburg, 15avaria. (Havana, Vol. IV., Heft 11, p. 23.) It
is defined by Rosenbusch as a dioritic, dike rock, containing quartz,
plagio( lasc and biotite as the chief dark silicate.

Ashbed diabase, a local name used on Keweenaw Point, Lake Su-
perior, for a rock resembling a conglomerate, but which is interpreted by
^^'adsworth as a very scoriaceous, amygdaloidal sheet into which much
sand was washed in its early history. See Monograph V., U. S. CJeol.
Surv. , p. 1 3S.

Asiderite. Daubree's name for stony meteorites that lack metallic
iron.

Asperite, a collective name suggested by G. F. Becker for the rough
cellular lavas whose chief feldspar is plagioclase, but which it is im-

126 A HANDBOOK OF ROCKS.

possible to speak more closely without microscopic determination. The
name is intended for general field use much as trachyte was employed in
former yeare. It is coined from the Latin word for rough. See p. 41.
Also Monograph XIII., U. S. Geol. Surv., p. 151.

Ataxite. , See under Taxite.

Augen, the German word for eyes ; used as a prefix before various rock
names, but more especially gneiss, to describe larger minerals or aggre-
gates of minerals, which are in contrast with the rest of the rock. In the
gneisses, feldspars commonly form the augen and are lenticular with the
laminations forking around them, in a way strongly suggesting an eye.
The term is seldom used in any other connection than with gneiss in
America.

Augite, the commonest, rock-making pyroxene. As distinguished
from other pyroxenes augite refers to the dark varieties with consider-
able alumina and iron. The name is used as a descriptive prefix to
many rocks that contain the mineral, as for instance augite-andesite,
augite-diorite, augite-gneiss, augite-granite, augite-syenite, etc.

Augitite, non-feldspathic, porphyritic rocks consisting essentially of
a glassy groundmass, with disseminated augite and magnetite. Various
minor accessories also occur. The name was first applied by Doelter to
lavas from the Cape Verde Islands. (Verhandl. d. k. k. Geol. Reichs-
anst., 1882, 143.) See above, pp. 44, 45.

Aureole, the area that is affected by contact nietamorphism around an
igneous intrusion. See p. 87.

Anthigeneous, an adjective coined by Kalkowsky to describe those
minerals which form in sediments after their deposition, as for instance
during nietamorphism. The name emphasizes in its etymology the
local origin of the minerals as contrasted with that of the other com-
ponents, the latter having been brought from a distance.

Autochthonous, an adjective derived from two Greek words, meaning
indigenous. It is applied to those rocks that have originated in situ,
such as rock salt, stalagmitic limestones, peat, etc., but it is of rare use.

Autoclastic, an adjective applied to fragmental rocks, which owe their
fragmental character to crushing or dynamic nietamorphism, and not to
sedimentation.

Automorphic is the contrasted term with xenomorphic or allotrio-
morphic, and is used to describe those minerals in rocks, which have
their own crystal boundaries. The later suggested word, idiomorphic,
means the same thing and is somewhat more widely used.

Axiolite, a term coined by Zirkel in his report on Microscopical Pet-
rography, for the U. S. Geol. Survey along the Fortieth Parallel, 1876,
to describe those spherulitic aggregates that are grouped around an axis
rather than around a point. The application comes in microscopic work
rather than in ordinary determination.

GLOSSARY.

Banakite, a general name given by Iddings to a group of igneous
rocks in the eastern portion of the Yellowstone Park and chiefly in
dykes. They are porphyritic and richly feldspathic. The phenocrysts
are labradorite and the groundmass consists of alkali-feldspars. A
little biotite and subordinate augite may be present. Chemically they
range SiO,, 51-61; A1,0„ 16. 7-19.6; CaO, 3.5-6; MgO, 1-4;
Na/), 3.8-4.5; K/^, 4.4-5.7. The group should be considered in
connection with absarokite and shoshonite. Jour, of Geol., III., 937.

Banatite, a name coined by B. v. Cotta in 1865 to describe the dioritic
rocks that are connected with a series of ore deposits in the Austrian
province of the Banat. Accurate microscopical study has shown them to
be of such varying mineralogy that the name has now slight definite sig-
nificance. The rocks are largely (piart/.-diorites. Krzlagerstatten im
Banat und in Scrbien, 1865.

Barolite, Wmlsworth's name for rocks composed of harite or celes-
titc. Kept, of State Cieol. Mich., 1891-92, p. 93.

Barysphere, a term for the deep interior portions of the earth, pre-
sumably ( omposed of heavy metals or minerals. It is contrasted with
lithosphere, the outer stony shell.

Basalt, a word of ancient but uncertain etymology as stated on p. 43.
It is eni])loyed as a rock name in its restric ted sense for porphyritic and
felsitic rocks consisting of augite. olivine and plagioclase with varying
amounts of a glassy base which may entirely disap|>ear. In a broader
sense the ba.salt or basaltic group is used to include all the dark, l)asic
volcanic rocks, such as the true l)asalts ; the ncpheline-, Icucite- and
mclilite-basalts ; the augitites and limburgites ; the diabases, and mela-
phyres. The word basalt is an extreniely useful field name, as in many
instances the finer discriminations can only be made with the micro-
scope.

Basanite, a very old term, first used as a synonym of basalt ; also
formerly ai)plied to the black, finely < rystalline quart/ite, used by old-
time workers in the prec ious metals as a touch-stone or test-stone to dis-
tinguish gold from brass by the streak. This variety was often called
Lydian stone or lydite. Hasanite is now universally employed for those
volcanic rocks, that ])Ossess a porphyritic or felsitic texture and that con-
tain plagioclase, augite, olivine and nepheline or leucite, one or both,
each variety being distinguished by the prefix of one or the other, or of
both of the last named minerals. See p. 44.

Basanitoid, a term suggested by Hiicking for basaltic rocks, without
definite nepheline, but with a gelatinizing, glassy base (H. Biicking,
jahrb. d. k. k. ])reus. Landesanst., 1S82).

Base or Basis is employed to describe that i)art of a fiiscil roi k

128 A HANDBOOK OF ROCKS.

magma that in cooling fails to crystallize as recognizable minerals, but
chills as a glass or related, amorphous aggregate. It differs thus from
groundmass, which is the relatively fine portion of a porphyritic rock
as distinguished from the phenocrysts.

Basic, a general descriptive term for those igneous rocks that are
comparatively low in silica. 55 or 50 per cent, is the superior limit.
See also Acidic and Medium.

Batholite, a name suggested by Suess for the vast irregular masses of
plutonic rocks that have crystallized in depth and that have only been
exposed by erosion. Seep. 12. The word is also spelled bathylite,
and batholith.

Bed, the smallest division of a stratified series, and marked by a more
or less well-defined divisional plane from its neighbors above and below.

Beerbachite, a name given by Chelius to certain small dikes, asso-
ciated with and penetrating large, gabbro masses, and having themselves
the composition and texture of gabbro. The name was coined in the at-
tempt to carry out the questionable separation of the dike rocks from
large, plutonic or volcanic masses of the same mineralogy and structures.
Notizbl. Ver. Erdkunde Darmstadt, 1892, Heft 13, p. i.

Belonite, rod or club-shaped microscopic minerals, which usually
occur as embryonic crystals in a glassy rock.

Benches, a name applied to ledges of all kinds of rock that are
shaped like steps or terraces. They may be developed either naturally
in the ordinary processes of land-degradation, faulting, and the like ; or
by artificial excavation in mines and quarries.

Beresite, a name coined by Rose many years ago for a muscovite-
granite that forms dikes in the gold district of Beresovsk in the Urals.
It is, therefore, practically a synonym of aplite, as earlier defined, but
some of the beresites have since been shown to be practically without
feldspar, and to form a very exceptional aggregate of quartz and musco-
vJte. (Arzruni, Zeitsch. d. d. g., Gesellsch., 1885, 865.)

Binary-granite, a term more or less used in older geological writings
for those varieties of granite that are chiefly quartz and feldspar. See
p. 31. It has recently been applied to granites with two micas. C. R.
Keyes, XV., Ann. Rep. Div. U. S. Geol. Survey, 714.

Biotite is used as a prefix to many names of rocks that contain this
mica ; such as biotite-andesite, biotite-gneiss, biotite-granite, etc.

Bituminous, an adjective applied to rocks with much organic, or at
least carbonaceous matter, mostly in the form of the tarry hydrocarbons
which are usually described as bitumen.

Blue-ground, local miners' name for the decomposed peridotite or
kimberlite that carries the diamonds in the South African mines.

Bombs, masses of lava expelled from a volcano by explosions of

GLOSSARY. 129

steam. They fall as rounded masses and lie on the slopes of the cone,
or become buried in tuffs.

Boninite. Petersen's name for a glassy phase of andesite with bron-
zite, augite and a little olivine, from the Bonin Islands, Japan. Jahrb.
Hamburg Wissensch. Anst., VIII., 1S91. Compare sanukite.

Borolanite, a rare rock related to the nepheline-syenites and de-
scribed by Home and Teall from Borolan, Sutherlandshire, Scotland.
It has granitoid texture, and consists principally of orthoclase and the
variety of garnet called melanite. As accessory minerals, biotite, py-
roxene, alteration products of nepheline, sodalite, titanite, apatite and
magnetite are met. (Trans. Roy. Soc. of Kdinburgh, 1892, p. 163.)

Bostonite, a name proposed by Himter and Rosenbusch for certain
dikes, having practically the mineralogical and chemical composition of
trachytes or porphyries, except that anorthoclase (and therefore soda) is
abnormally abundant and dark silicates are few or lacking. They arc
much the same as dike-keratophyres and were named in carrying out the
questionable separation of the dike-rocks, as a grand distinct division
from the i)lutonic and volcanic rocks. The name was suggested by
their supposed presence near Boston, Mass., but Marblehead, 20 miles
or more distant is their nearest locality. They have l)een since met in
largest amount around I^ke Champlain and in the neighboring parts of
Canada. Tscher. Min. u. Petrog. Mitth., 1890, 447. See also Bull.
107, V. S. (icol. Survey.

Bouteillenstein, /. <'. , bottlcstone, a peculiar green antl very pure
glass, found as rolled pebbles near Moldau, Bohemia. It is also called
moldauite and pscudoc hrysolite, the latter from its resemblance to oliv-
ine. It is not certainly a rock, as it may be a j)rehistoric slag or glass.

Boulder-clay, unsorted glacial deposits, consisting of boulders, clay
and nunl ; till, hardpan.

Breccia, a fragmcntal rock whose components are angular and there-
fore, as distinguished from conglomerates, are not water-worn. There
are friction or fault breccias, talus-breccias and eruptive breccias. The
word is of Italian origin. See p. 59.

Broccatello, an Italian word for a brecciated and variegated marble.

Bronzite is often used as a prefix to the names of rocks containing
the mineral. Rocks of the gabbro family are the commonest ones that
have the prefix.

Buchnerite, a name proposed by Wadsworth for those peridotites,
terrestrial and meteoric, which consist of olivine, enstatite (bronzite)
and augite. The name was given in honor of Dr. Otto Buchner, an
authority on meteorites. I.ithological Studies, 1884, p. 85.

Buchonite, a special name given by Sandberger to a ncpheline-teph-
9

I30 A HANDBOOK OF ROCKS.

rite that contains hornblende. Sitzungsberichte d. Berl. Akad. Wis.,
July, 1872, 203; 1873, vi.

Buhrstone, a silicified fossiliferous limestone, with abundant cavities
which were formerly occupied by fossil shells. Its cellular character
and toughness occasioned its extensive use as a millstone in former years.

Bysmalith, a name suggested by J- P- Iddings for an igneous intru-
sion that forms a huge cylindrical mass or plug, with length and width
approximately the same, but of relatively great height. Journal of
Geology, VI., 704.

Calc-schist, schistose rocks, rich in calcite or dolomite and forming
intermediate or transitional rocks between the mica-schists and crystal-
line limestones. See p. 10 1.

Camptonite, a name given by Rosenbusch to certain dike rocks,
having in typical cases the mineralogical composition of diorites, /. e.,
with dark brown hornblende, plagioclase, magnetite, and more or less
augite. They are often prophyritic in texture, and may even have a
glassy groundmass. Without the microscope camptonites usually ap-
pear as dark basaltic rocks with a i&w shining crystals of hornblende or
augite ; their determination is essentially microscopic. Intimately as-
sociated with the camptonites of typical composition have been found
others corresponding to all varieties of basaltic rocks. Such with pre-
vailing augite have been called augite-camptonite. The name campto-
nite is derived from the township of Campton, in the Pemigewasset
Valley, N. H. The original camptonites were discovered near Liver-
more Falls, on the Pemigewasset river, many years ago, by O. P. Hub-
bard. They were microscopically described by G. W. Hawes in 1878,
and on this determination Rosenbusch based the name. They, or their
near relatives, have ofcen intimate associations with nepheline syenites.
(See also, monchiquite, fourchite, ouachitite.) Camptonites are es-
pecially abundant throughout the Green Mountains and near Montreal.
G. W. Hawes, Amer. Jour. Sci., 1879, XVII., 147. H. Rosenbusch,
Massige Gesteine, 1887, 333. Bulletin 107, U. S. Geol. Surv.

Carbonolite, Wadsworth's name for carbonaceous rocks. Rept. State
Geol. Mich., 1891-92, p. 93.

Carmeloite, a name given by A. C. Lawson to a group of erup-
tive rocks at Carmelo Bay, Calif., which are intermediate between the
basalts and andesites. They range in silica from 52 to 60 per cent.,
have augite and plagioclase for phenocrysts ; and a peculiar, orthorhom-
bic, hydrated silicate of iron, lime, magnesia and soda, which is a
secondary mineral after some original, probably olivine. The sec-

GLOSSARY. 131

ondary mineral has been called Iddingsite. Bull. Geol. Dept. Univ.
of Calif., I., 29. 1893.

Cataclastic, a structural term applied to those rocks that have suf-
fered mechanical crushing in dynamic metamorphism. Compare auto-
clastic.

Catawberite, a name given by O. Lieber to a rock in South Caro-
lina that is an intimate mixture of talc and magnetite. Gangstudien,
III., 353. 359-

Catlinite, a local name in Minnesota for a red argillaceous sandstone,
presumably of Cambrian age, that was used by the Indians for pipe
bowls. C. T. Jackson, Amer. Jour. Sci., 1839, 388.

Catogene, /. e., sedimentary rocks, whose particles have sunk from
abo\c downward.

Cement, the material that binds together the jiarticles of a fragmen-
tal rock. It is usually calcareous, siliceous or ferruginous. See p. 64.
The word is also used in gold-mining regions to describe various con-
solidated, fragmental aggregates, such as breccia, conglomerate and the
like, that are auriferous.

Chalk, a marine, calcareous and excessively fine, organic sediment
usually ( onsolidatcd.

Chert, a compact, siliceous rock formed of chalcedonic or ojjaline
silica, one or both, and of organic or precipitated origin. See pj). 74,
80, 81. Cherts often occur distributed through limestone, affording
cherty limestones. Flint is a variety of chert. Cherts are especially
( onmion in the subcarboniferous rocks of southwest Missouri.

Chlorite, a general name for the green, secondary, hydratcd silicates,
which contain alumina and iron, and which are especially derived from
augite, hornblende and biotitc. Chlorite is used as a prefix for various
names of rocks that contain the mineral, such as chlorite schist. The
name is coined from the Creek word for green.

Chlorophyr, a name given by .\. Dumont to certain, porphyritic
quartz diorites near (^)ucna.sf. I'llu'iuin. See Dclcssc, lUill. Soc. Ceol.
de France, 1850, 315.

Ciminite, a name derived iroiu the Monti Ciinini in Italy, and given
by H. S. Washington to a group of lavas, intermediate between trachytes
and basalts. They are ])orphyritic in texture and are characterized by
the presence of alkali feldspar and basic plagioclasc, augite and olivine,
with accessory magnetite and apatite. Kiotiteand hornblende are either
absent or are insignificant. They range from 54 to 57 SiO,, 5—9 CaO,
and 3-6 MgO. Journal of Geology, V., 351. 1897. Compare I^tite.

Clastic, descriptive term applied to rocks formed from the fragments
of other rocks ; fragmental.

Clay, general name for the fine, aluminous sediments that are plastic.

132 A HANDBOOK OF ROCKS.

Though usually soft, they may be so hard as to need grinding before the
plasticity manifests itself, as in numerous fire clays. See p. 67.

Clay slate, metamorphosed clay, with new cleavages developed by
pressure and shearing. The term is used in distinction to mica-slate,
and other slaty rocks. See p. 108.

Claystone-porphyry, an old and somewhat indefinite name for those
porphyries whose naturally fine groundmass is more or less kaolinized,
so as to be soft and earthy, suggesting hardened clay.

Clinkstone. See phonolite.

Comendite, a variety of rhyolite, containing phenocrysts of sanidine,
quartz and regirine, in a granophyric and spherulitic groundmass. Horn-
blende and some blue, soda-amphibole, together with zircon, magnetite,
titanite, tridymite, and plagioclase as accessories. The name was given
by Bertolis, an Italian geologist, from a locality on the island of San
Pietro, Sardinia. Rend. Roy. Acad. Lincei, IV., 48. 1895. Com-
pare Paisonite.

Complementary Rocks, a term suggested by W. C. Brogger for the
basic rocks, which, usually in the form of dikes, accompany larger in-
trusions of more acidic types, and "complement " them in a chemical
sense. Quar. Jour. Geol. Soc. L., 15. 1894. Compare Lamprophyre,
Oxyphyre and Radial Dikes.

Composite dike, a dike formed by two intrusions of different ages
into the same fissure (W. Judd, Quar. Jour. Geol. Soc, 1893, 536).

Concretions, spheroidal or discoid aggregates formed by the segre-
gation and precipitation of some soluble mineral like quartz or calcite
around a nucleus, which is often a fossil.

Cone-in-cone, a curious structure, occasionally met in clay rocks,
whereby two opposing and interlocking sets of cones or pyramids are
developed, with their axes parallel and their bases is approximately
parallel surfaces.

Conglomerate, consolidated gravel. See p. 62.

Consanguinity, a team used by Iddings to describe the genetic rela-
tionship of those igneous rocks which are presumably derived from a
common, parent magma. See p. 57, and Bull. Phil. Soc. Washington
XL, 89.

Contact, the place or surface where two different kinds of rocks come
together. Although used for sedimentary rocks, as the contact between
a limestone and sandstone, it is yet more especially employed as be-
tween igneous intrusions and their walls. The word is of wide use in
western mining regions on account of the frequent occurrence of ore-
bodies along contacts. On contact-metamorphism, see pp. 85-92.

Cordierite, a synonym of iolite or dichroite, employed as a prefix to
those rocks that contain the mineral, as cordierite -gneiss.

GLOSSARY. 133

Comubianite, a name coined by Boase from the classic name for
Cornwall, Kngland, to describe a contact hornfels, consisting of anda-
lusite, mica and quartz. It was proposed as a substitute for an earlier
but indefinite term proteolite. Bonney suggests restricting cornubianite
to tourmaline hornfels. Quar. Jour. Geol. Soc, 18S6, 104.

Corrasion, geological term for the wearing away of rocks by grit sus-
pended in moving water or air ; to be distinguished from erosion.

Corroded Crystals, phenocrysts that after crystallization are more or
less realjsorbed or fused again into the magma.

Corsite, a name applied by Zirkel to the orbicular or spheroidal
diorite from Corsica ; synonym of na|)oleonite. Lehrb. d. Petrographie,
1866, 11., I -53, 320.

Cortlandite, a special name given by G. H. Williams to a peridotite
that consists chiefly of hornblende and olivine and that occurs in the so-
called (Portland series of igneous rocks in the township of Cortland, just
south of I'cekskill, on the Hudson River. This rock had been previously
called hud.sonite by K. (*ohen, a name rejected by Williams because
already used for a variety of pyroxene. Amer. Jour. Sci., Jan., 1SS6, 30.

Corundolite, Wadsworlh's name for rocks composed of corundum or
enuTv. Kept. .State (leol. Mich., 1891-92, p. 92.

Crenitic, a word derived from the Greek for spring, and especially
used by T. S. Hunt for those rocks, which were thought by him to have
come to the surface in solution and to have l>een precipitated. He used
the so-called "crenitic hypothesis" to explain certain schists whose feld-
spars were supposed to have been originally zeolites, but his views have
received slight, if any, support. Troc. Roy. Soc. Canada, Vol. II., Sec.
HI., 1884. Reprint, ]). 25. Crenitic is also used by W. (). Crosby to
describe those mineral veins which have been deposited by uprising
springs. Tei hnology (Quarterly, .\|)ril, 1894, p. 39.

Cross-bedding, or Cross-stratification, descri|)tive terms applied
to those minor or subordinate layers in sediments that are limited to
single beds, but that are inclined to the general stratification. They are
caused by swift, local currents, deltas, or swirling wind gusts, and are
esi)e( ially characteristic of sandstones, both aijueous and eolian. See pp.
64, 65.

Crustification, the English etpiivalent of a term suggested by Posepny
for those dc|)osits of minerals and ores that are in layers or crusts and
that, therefore, have been distinctively deposited from solution. Trans.
Amer. Inst. Min. ling., XXI 1 1., 207, 1S93.

Crypto-crystalline, formed of crystals of unresolvable fineness, but
not glassy. A submicroscopical crystalline aggregate.

Crystallites. The term is most properly applied only to small, rudi-
mentary or embryonic crystals, not referable to a definite species, but it
is also used as a general term for microscopic crystals.

134 A HANDBOOK OF ROCKS.

Cumberlandite, a name derived from Cumberland Hill, R. I., pro-
posed by Wadsworth for the ultra-basic, igneous rocks, forming the hill.
It is an aggregate of titaniferous magnetite, plagioclase, olivine and sec-
ondary minerals, but contains from 40-45 per cent, iron oxides and
about 10 per cent. TiOj. Lithological Studies, 1884.

Cumulites, Vogelsang's name for spherulitic aggregates of globulites.
Die Krystalliten, 1875.

Cuselite, Rosenbusch's name for a peculiar variety of augite-porphy-
rite from Cusel, in the Saar basin. Massige Gest. , 503, 1887.

Dacite, quartz-bearing andesites. The name was suggested by the
ancient Roman province of Dacia, now in modern Hungary. See p. 40.

Damourite-schist, a micaceous schist whose micaceous mineral is
damourite. Much the same as hydro-mica schist. Seep. loi.

Dellenite, a name proposed by Broegger for an intermediate group of
effusive rocks, between the dacites and the liparites (rhyolites). The
name is derived from Dellen, Helsingland, Sweden. Die Triadische
Eruptionsfolge bei Predazzo, p. 60 and footnote to p. 59. Compare
Toscanite.

Desmosite, a banded contact rock developed from shales and slates
by intrusions of diabase. Compare spilosite and adinole. See Zincken,
Karsten und V. Dechen's Archiv, XIX., 584, 1845.

Detritus, a general name for incoherent sediments, produced by the
wear and tear of rocks through the various geological agencies. The
name is from the Latin for " worn."

Devitrification, the process by which glassy rocks break up into defi-
nite minerals. The latter are usually excessively minute, but are chiefly
quartz and feldspars.

Diabase, igneous rocks, in sheets or dikes, consisting essentially of
plagioclase, augite and magnetite, with or without olivine, and possess-
ing a texture often called ophitic, but which may, perhaps, be better
described as diabasic. The feldspars are lath-shaped and automorphic
while the augite is xenomorphic and packed in their interstices. See
p. 44, and also Ophitic. Diabase has had a somewhat variable sig-
nificance during its history, but with the final exit of the time-element
in the classification of igneous rocks its present meaning is generally
accepted as above given.

Diabase-porphyrite, a porphyrite whose ground -mass is a finely crys-
talline diabase, and whose phenocrysts are prevailingly plagioclase. It
is contrasted with augite-porphyrite, whose phenocrysts are prevailingly
aueite.

GLOSSARY. 135

Diallage, the variety of monoclinic pyroxene that in addition to the
prismatic cleavages, has others parallel to the vertical pinacoids. Used
also as a prefix to many rocks containing the mineral.

Diatomaceous earth, rocks essentially formed of the abandoned frus-
tulcs of the microscopic organisms called diatoms.

Dichroite, see under cordierite.

Dikes, spelled also dykes, intrusions of igneous rocks in fissures ; not
to to be confused with veins which are precipitated from solution.

Diluvium, a name formerly applied to the unsorted and sorted de-
posits of the Glacial period, as contrasted with the later water sorted
alluvium. Compare Alluvium.

Diorite, a granitoid rock consisting essentially of plagioclase and
hornblende. More or less biotite is usually present, which may even
replace the hornblende, yielding mica-diorite ; augite also often ap-
pears. Acidic varieties with (piartz are called (juart/.-diorites. See pp.
47, 48. Diorite is often used as a prefix for porjjhyritic or other rocks
related to diorite. The name is from the Greek to distinguish and was
given l)y Hauy in 1S22. Traite de .Mineralogie, IV., 541.

Diorite-porphyrite, a porphyrite whose ground-mass is a finely crys-
talline diorite, and whose phenocrysts arc prevailingly plagioclase. It
is contrasted with hornblende-porphyrli.-, wh.ise phenocrysts are pre-
vailingly hornblende.

Dipyr, a variety of sca|)olite, often used as a prefix to the names of
ro( ks that contain the mineral.

Disthene, synonym of cyanite, sometimes used as a prefix in rock
nanus.

Ditroite, a nepheline syenite from Ditro in Himgary, especially rich
in blue sodalite. See p. 37.

Dolerite, coarsely crystalline basalts. The word has had a somewhat
variable meaning during its history and among different peoples. The
English use it interchangeably with dial)a.se ; indeed the definitions
given here justify this usage, except that the characteristic texture of
diabase is not essential to this definition of dolerite. Hut the diabasic
texture is more of a microscopic feature than a mega.scopic. Holerite
is from the Greek for deceptive, and was given by Hauy in allusion to
its application to later rocks, that could not be distinguished from older
greenstones. The name is a long standing indictment of the time ele-
ment in the classification of igneous rocks.

Dolomite is applied to those rocks that ap|)roximate the mineral do-
lomite in com|iosition. Named by Saussure, after Dolomieu, an early
French geologist. See p. 72.

Dolomitization or Dolomization, the process whereby limestone be-
comes dolomite by the substitution of magnesian carbonate for a portion

136 A HANDBOOK OF ROCKS.

of the original calcium carbonate. If the MgCOg approximates the
45,65 per cent, of the mineral dolomite, there is great shrinkage in
bulk, leading to the development of porosity and cavities, up to 11 per
cent, of the original rock.

Domite, a more or less decomposed trachyte from the Puy de Dome
in the French volcanic district of the Auvergne. The typical domite
contains oligoclase and is impregnated with hematite.

Drift, a general name for the unsorted deposits of the glacial period.
Where subsequently worked over by water they are called modified drift.

Dunite, a member of the peridotite group that consists essentially of
olivine and chromite. It was named from the Dun Mountains in New
Zealand, the original locality, but it also occurs in North Carolina.
The name was given by v. Hochstetter in 1859. Geol. v. NeuSeeland,
2i8, 1864.

Durbachite, a name given to a basic development at the outer border
of a granite intrusion in Baden. It has the general composition of
mica syenite. The name was given by Sauer, Mitth. d. grossh. bad.
Geol. Landesanstaldt, II., 233.

Dykes, see dikes.

Dynamometamorphism, a general term for those metamorphic
changes in rocks that are produced by mechanical as distinguished from
chemical processes, but the former are seldom unattended by the latter.
See p. 93.

Dysyntribite, a name given by C. U. Shepard, Amer. Assoc. Adv.
Sci., 1851, 311, to a mineral or rock in St. Lawrence Co., N. Y., which
is a hydrated silicate of aluminium and potassium, and is related to
pinite ; the name means hard to crush. Compare parophite. See also.
Smith and Brush, Amer. Jour. Sci., ii., XVI., 50, and C. H. Smyth,
Jour, of Geol., II., 678, 1894.

Eclogite, a more or less schistose metamorphic rock, consisting of a
light-green pyroxene (omphacite), actinolite (var. smaragdite) and
garnet. Scarcely known in America. See p. 105 and anal. 6, p. 103.
The name is from the Greek to select, in reference to its attractive ap-
pearance.

Effusive, a name applied to those rocks that have poured out in a
molten state on the surface and have there crystallized, /. e., volcanic
rocks. See p. 13.

Elaeolite or Eleolite, a name formerly current for the nepheline of
pre-tertiary rocks. It is best known in the rock-name eleolite-syenite,
a synonym of nepheline-syenite, which latter is preferable. See nephe-
line-syenite.

GLOSSARY. 137

Elvan, Cornish name for dikes of quartz -porphyry or of granite-por-
phyry.

Endomorphic, used as a descriptive adjective for those phases of
contact-nietamorphism that are developed in the intrusion itself. It is
synonymous with internal as used on p. 87.

Euktolite, see Venanzite, which has priority.

Enstatite, the variety of orthorhombic pyroxene with less than 5 per
cent. FeO. It is largely used as a prefix to the names of rocks that con-
tain the mineral.

Eorhyolite, eobasalt, etc., a series of names proposed by O. Norden-
skjoeld for the older ecjuivalents of the rhyolites, Ijasalts, etc. The terms
are practically e<|iiivalent to aporhyolite, apobasalt, etc., but the latter
have priority. Hull. Cieol. Inst. Univ. of Upsala, I., 292. 1893.

Epidiorite, a name applied to dikes of diabase, whose augite is in part
altered to green hornblende. The name was coined before it was un-
derstood that the hornblende was secondary in this way. It was first
applied by Oiimbel in 1879 to a .series of narrow dikes that cut Cam-
brian and Ordovician strata in the Kichtelgebirge. The name empha-
sizes their later age than the typical pre-Cambrian diorites, but its sig-
nifican* e has been expanded in later years.

Epidosite, rocks largely formed of epidote. 'i'he epidotc seems gen-
erally to be produced by the reactions of feldspars and bisilicates upon
eachotiur during alteration.

Epidote, the name of the mineral is often |)refixcd to the names of
rocks containing it. As a rule, the presence of epidote indicates the
advance of alteration.

Erlan or Erlanfels, a name proposed by Breithaupt for metamorphic
rocks, which consist essentially of augite, /. r., augite schists. The name
is derived from the iron-furnace at Krla, near Crandorf, Saxony.

Erosion, geological term for the process of the removal of loose ma-
terials in sus|)ension in running water or in wind.

Eruptive, the name ought properly to be only applied to effusive or
volcanic rocks, but it is often used as a synonym of igneous.

Essexite, a name derived from Es.sex County, Mass., and applied by
J. 11. Sears to a peculiar rock, occurring with the nepheline-syenite 01
Salem Neck. It is an intermediate rock between the nepheline-syenites,
the diorites, and the gabbros, and contains labradorite, orthoclase,
and more or less nepheline or sodalite, together with augite, biotite
barkevicite, olivine, and apatite. Hulletin Es^ex Institute, XXIII.,
146. 1S91, H. S. Washington, Journal of Geology, VII., 53, 1899.

Eucrite, a name given by G. Rose to rocks and meteorites that con-
sist essentially of anorthite and augite. . The term is practically obsolete.
Pogg. Annalen, 1835, I., i.

138 A HANDBOOK OF ROCKS.

Eudyalite, the name of the mineral is sometimes prefixed to the
rare nepheline-syenites that contain it.

Eulysite, a name given by Erdmann to rocks interlaminated with the
gneisses of Sweden, and consisting of olivine, green pyroxene and gar-
net. Neues Jahrb., 1849, ^37-

Euphotide, the name chiefly used among the French for gabbro. It
was given by Hauy, and is derived from the Greek words for well and
light, in allusion to its pleasing combination of white and green.

Eurite, used among the French as a synonym of felsite, but also ap-
plied to compact rocks chiefly feldspar and quartz, such as some granu-
lites. The name was first given by Daubisson to the groundmass of
porphyries, because of their easy fusibility as compared with hornstone
or flint.

Eutaxitic, a general name for banded volcanic rocks. The banding
is due to the parallel arrangement of portions of the rock that are con-
trasted either in mineralogy or texture.

Exomorphic, a descriptive term for those changes produced by con-
tact-metamorphism in the wall rock of the intrusion ; the antithesis of
endomorphic. It is synonymous with external as used in p. 88.

Extrusive, synonym of effusive, much used in America.

Farrisite, a name derived from Lake Farris in Norway, and applied
by Broegger to a very peculiar rock, which is as yet known only in one
small dike. The rock is finely granular in texture and consists of some
soda-bearing, but not sharply identified, tetragonal mineral related to
melilite, together with barkevicite ', colorless pyroxene ; biotite ; ser-
pentinous pseudomorphs after olivine; magnetite and apatite. Das Gang-
gefolge des Laurdalits, 66. 1898.

Feldspar, the name of the mineral is often prefixed to the names of
those rocks that contain it, as feldspar-porphyry, feldspar-basalt, etc.

Feldspathoids, silicates of alumina and an alkali or alkaline earth,
that are practically equivalent to feldspars in their relations in rocks.
The principal ones are nepheline, leucite and melilite, but sodalite,
nosean and hauyne could perhaps be also considered such, although their
composition varies from the above.

Felsite, the word was first applied in 1814 by Gerhard, an early ge-
ologist, to the fine groundmasses of porphyries. These were recognized
to be fusible as distinguished from hornstone, which they resembled (com-
pare eurite). Felsite is now especially used for those finely crystalline
varieties of quartz-porphyries, porphyries or porphyrites that have few or
no phenocrysts, and that therefore give but slight indications to the un-

GLOSSARY. 139

aided eye of their actual mineralogical composition. The microscope
has shown them to be made up of microscopic feldspars, quartzes and
glass. Petrosilex has been used as a synonym. See p. 24.

Felsitic has been employed as a megascopic term in the preceding
pages to describe those textures which are characteristic of felsites, /. e.,
micro-crystalline, but without phenocrysts. Seep. 15. It is often used
also to describe the groundmasses of truly porphyritic rocks, that are
rnicro-crystalline, but clearly not glassy. In this sense we have felsite-
porphyry, felso-liparite, felso-dacite, etc.

Felsophyre, a contraction for felsite-prophyry.

Felspar, the current spelling of feldspar among the I'.nglish. It is
based on an old typographical error in Kirwan's Mineralogy, I., 317,
1794, now, however, firmly established in general usage.

Ferrite, microscopic crystals of iron oxide.

Ferrolite, Wadsworth's name for rocks composed of iron ores. Kept.
State (ieol. Mich., 1891-93, p. 92.

Fibrolite, synonym of sillimanite, and sometimes used as a prefix to
ro( k names.

Fiorite, siliceous sinter, named from Mt. Santa Fiora, in Tuscany.

Firn, Swiss name for the granular, loose or consolidated ice of the
high altitudes licfore it forms glacial ice below.

Flaser-structure, a structure developed in granitoid rocks and espe-
cially in gabbros by dynamic metamorphi.sm. Small lenses of granular
texture arc set in a scaly aggregate that fills the interstices between
them. It appears to have been caused by shearing that has crushed
some portions more than others, and that has developed a kind of rude
flow-structure.

Flint, a compact and crypto- crystal line aggregate of chalcedonic and
opaline silica. Chert and hornslone are synonyms. See i)p. 74. 80.

Float, a term much u.sed among Western miners for loose, surface de-
posits, which are usually somewhere near their parent ledges.

Flow-Structure, a structure due to the alignment of the minerals or
inclusions of an igneous rock so as to suggest the swirling curves, eddies
and wavy motions of a flowing stream. It is caused by the chilling of
a flowing, lava current, l-'luxion-structure is synonymous.

Foliation, the banding or lamination of metamorphic rocks as dis-
tinguished from the stratification of sediments.

Forellenstein, a variety of olivine-gabbro, consisting of plagioclase,
olivine and more or less pyroxene. The dark silicates are so arranged
in the lighter feldspar as to suggest the markings of a trout. (German,
Forelle. )

Formation, as defined and used by the U. S. Geological Survey, is a
large and persistent stratum of some one kind of rock. It is also loosely

140 A HANDBOOK OF ROCKS.

employed for any local and more or less related group of rocks. In
Dana's Geology it is applied to the groups of related strata that were
formed in a geological period.

Fourchite, a name proposed by J. Francis Williams for those basic
dike rocks that consist essentially of augite in a glassy groundmass, /. <?.,
dike-augitites. The name was suggested by Fourche Mountain, Ark.,
where they are abundant. Ann. Rep. Geol. Sur., Ark., 1890, II., 107.

Foyaite, a name originally applied to the nepheline syenite, with
supposed hornblende, of Mt. Foya in the Monchique range of Portugal.
Although the hornblende has since proved to be augite and aegirine, the
name foyaite is still employed for nepheline syenite with hornblende.
See p. 37.

Fragmental, descriptive term for the rocks formed from fragments of
preexisting rocks, such as sandstones arid breccias. Clastic is synonymous.

Fraidronite, a name used by early French geologists for a variety of
minette.

Freestone, a quarryman's name for those sandstones that submit
readily to tool treatment.

Fruchtschiefer, German name for a variety of spotted, contact schists
in the outer zone of the aureole. See p. 89.

Fuller's earth, a fine earth, resembling clay, but lacking plasticity.
It is much the same chemically as clay, but has a decidedly higher per-
centage of water.

Fulgurite, little tubes of glassy rock that have been fused from all
sorts of other rocks by lightning strokes. They are especially frequent
in exposed crags on mountain tops. The name is derived /rom the
Latin for thunderbolt.

Gabbro, an Italian word formerly used for a rock composed of serpen-
tine and diallage. It was later applied to igneous rocks, of granitoid
texture, consisting of plagioclase and diallage, but as now employed,
any monoclinic pyroxene may be present, with or without diallage.
As the name of a group, it includes in addition to the rocks with mono-
clinic pyroxene, those with plagioclase and orthorhombic pyroxene as
well ; and even the peridotites and pyroxenites, from their close geo-
logical connection with the gabbros may conveniently be embraced.
Although of the same mineral composition with gabbro, yet the peculiar
and contrasted texture of diabase may be remarked. Intermediate
types have even been called gabbro-diabase. See p. 50. A full review
of the meaning and history of gabbro, by W. S. Bayley, will be found
in Jour, of Geology I., 435, Aug., 1893.

GLOSSARY. 141

Gabbro-diorite, gabbro with hornblende which may, in fact, be
secondary after augite. Intermediate rocks between true gabbros and
diorites.

Ganggesteine, (ierman for dike rocks.

Garganite, a name suggested by Viola and de Stefani for a dike rock
in the Italian province of Foggia, which in the middle, with prevailing
alkali-feldspar contains both augite and amphibole, /'. f., is a vogesite ;
on the edges it contains biotite, hornblende, olivine and resembles ker-
santite. Holl. Roy. Com. Geol. d' Italia, 1893, 129.

Garnet-rock, a rock composed essentially of garnets.

Generations of minerals in an igneous rock refer to the groups of
individuals that crystallize out at a definite period and in a more or less
definite succession during cooling. The same species may have one,
two, or very rarely three generations. See p. 17.

Geodes, hollow, rounded boulders lined with crystals projecting in-
ward from the walls.

Geest, a name proposed by J. A. DeLuc in 1S16 for " the immediate
j)roducts of rock decay in situ." It is a provincial word for earth in
Holland and northern (lermany. -Xbrcge (Icologi(pic, I'ari-;, 1816,
112, as quoted l)y W J Mcdee, XI., Ann. Rep. I'. S. (leol. Survey,
Part I., 279, Compare I^terite, Saprolite.

Geyserite, siliceous deposits from a geyser. See p. 80.

Gieseckite-porphyry, a nepheline |)orphyry from (irccnland, whose
nephclinc phcnoi ry.sts are altered to the aggregate of muscovite scales,
which was called gieseckite under the impression that it was a new
mineral. Liebenerite i)orphyry is the same thing from Preda/.zo, in the
Tyrol.

Glass, the amorphous result of the quick chill of a fiised lava. See
p|). 20, 21, 22, and for glassy texture, p. 14.

Glauconite, the green silicate of iron and potassium that is impor-
tant in many green sands. See p. 69.

Glaucophane, a blue sodaamphibolc found especially in certain, rare
schists. Sec pp. 103, 105.

Glomeroporphyritic, a tcxtural term proposed by Tate for those por-
phyritic rocks whose feldspar phenocrysts are made up of an aggregate
of individuals instead of one, large crystal. British Assoc. Adv. Sci.,
1890, S14. Com|)are Occllar.

Gneiss, a laminateil or foliated granitoid rock that corresponds in
mineralogical composition to some one of the plutonic rocks. The name
originated among the Saxon miners. See p. 105.

Granite, in restricted signification is a granitoid, igneous rock consist-
ing of cjuart/, orthoclase, more or less oligoclase, biotite and muscovite,
but is widely used in a more general sense. The first three may also be

142 A HANDBOOK OF ROCKS.

combined with either of the micas alone, with hornblende or with au-
gite. In its technical applications as a name of a building stone it is
used for almost any crystalline rock composed of silicates, as contrasted
with sandstones, slates, limestones and marbles. It is a very old term.
See p. 31-

Granitelle, a granite with comparatively little mica, so that it con-
sists almost entirely of quartz and feldspar ; binary granite. It has
been also used by R. D. Irving for augite-granite. U. S. Geol. Surv.,
Monograph v., p. 115.

Granite-Porphyry, practically a quartz -porphyry with a coarsely crys-
talline groundmass ; an intermediate rock between granites and typ-
ical quartz-porphyries, having the same minerals as the former, but be-
ing porphyritic like the latter. The chief phenocrysts are, however,
feldspars. See pp. 25, 32.

Granitite, a special name for biotite-granite. It is much the com-
monest of the granites.

Granitoid, used in preceding pages as a textural terra to describe
those igneous rocks which are entirely composed of recognizable min-
erals of approximately the same size. It was suggested by granite, the
most familiar of the rocks which show this characteristic. See p. 13.
In the granitoid texture each kind of mineral appears in but one gener-
ation, and the individuals seldom have crystal boundaries.

Granodiorite, a term which has been given special currency by the
usage of the U. S. Geological Survey, and which is employed for the
intermediate rocks between granites and quartz-diorites. It is a con-
traction for granite-diorite and is a very useful, rock name. Compare
Adamellite.

Granoph3rre, a descriptive term used in connection with microscopic
study, to describe those groundmasses in quartz-porphyries in which
the quartz and feldspar crystals have simultaneously crystallized so as to
mutually penetrate each another. The several parts of one individual,
though separated from one another, extinguish together between crossed
nicols. Micropegmatitic is synonymous. See also micro -perthitic,
micro-poicilitic, and micro-granitic.

Granulite, properly speaking a finely crystalline, laminated, meta-
morphic rock consisting essentially of orthoclase, quartz and garnet,
but having also at times cyanite, hornblende, biotite or augite. The
name means garnet rock (/. c, German for garnet, granat). The
granulites are best developed in the mountains of Saxony. Sometimes
the name is less correctly used for muscovite granite, or for granites
containning little else than quartz and feldspar. See p. 98.

Graphite, the name of the mineral is often prefixed to the names of
rocks containing it, as graphite-gneiss, graphite-schist, etc.

GLOSSARY. 143

Graywacke, an old name of loose signification, but chiefly a|)i)lied to
metamorphosed, shaly sandstones that yield a tough, irregularly break-
ing rock, different from slate on the one hand and from quartzite on the
other. The components of graywacke may be largely bits of rocks,
rather than fragments of minerals.

Greenschists, chlorite schists, which may, however, be of quite
diverse ori^'in. See p. 104.

Greenstone, an old field name for those compact, igneous rocks that
have developed enough chlorite in alteration to give them a green cast.
They are mostly diaba.ses and diorites. Greenstone is partially synony-
mous with trap. It is often used as a prefix to other rock names.

Greisen, a granitoid but often somewhat cellular rock, composed of
quartz and muscovite or some related mica, rich in fluorine. It is the
characteristic mother rock of the ore of tin, cassiterite, and is in most
cases a result of the contact action of granite and its evolved mineral-
izers. See p- 92.

Grit, I oarse sandstone.

Grorudite, liroegger's name for a porphyritic, dike rock from Grorud,
near Chrisliania, Norway. The j)henocrysts are microcline and aegirine ;
the groundmass consists of rectangular orthoclase, cpiart/, and aegirine.
It is a variety of granite porphyry. Zeilsch. f. Krys. , X\'I., 65.

Groundmass, the relatively finely crystalline or glassy portion of a
jjorphyritic rock as contrasted with its phenocrysts. Not to l>e con-
fused with basis, as will be seen by referring to the latter. On ground-
mass, see p. 14.

Gumbo, a name current in Western and Southern States for those
soils that yield a sticky mud when wet.

Halleflinta, a Swedish name for dense, compact metamorphic rocks,
consisting of microscopic (piart/. and feldspar crystals, with occasional
phenocrysts and sometimes hornblende, chlorite, magnetite and hema-
tite, 'riicy arc associated with gneisses, but are of obscure origin.

Haloidite, Wadsworth's name for rock-salt. Rojjt. State (Jeol. Mich.,
1891-92, p. 92.

Haplite, a name proposed by L. Kictcher for that variety of granite
which ( onsists of <iuartz and j)Otash feldspar. The name is derived
from the Greek for simple. \n Introduction to the Study of Rocks.
British Museum Guide-books, 1S95, 58, 63, 102. Compare Binary
granite.

Hard-pan, a name specially developed in the digging of auriferous
gravels, and applied to the layers of gravel which are usually jjrescnt a

144 A HANDBOOK OF ROCKS.

few feet below the surface and which are cemented by limonite or some
similar bond. They therefore are resistant. It is also used to describe
boulder-clay, which is likewise difficult to excavate.

Harzburgite, a name proposed by Rosenbusch for those peridotites
that consist essentially of olivine and enstatite or bronzite. Mass.
Gest., 1887, 269. Saxonite was earlier proposed by Wadsworth (1884)
for the same rock, and has priority.

Hauyne, the name of the mineral is often prefixed to the names of
those rocks that contain it, as hauyne-basalt, hauyne-trachyte, etc.

Hedrumite, a name proposed by Broegger for certain, syenitic rocks
that are poor or lacking in nepheline, but that have a trachytic texture.
Zeitschr. Krys., XVI., 40. Das Ganggefolge des Laurdalits, 183.

Hemithrene, Brogniart's name, current among the French, for cer-
tain, dioritic rocks that have a large amount of calcite, presumably an
alteration product.

Heronite, a name proposed by A. P. Coleman, for a dike rock, con-
sisting essentially of analcite, orthoclase, plagioclase, and a;girine, the
analcite having the character of a base, in which the other minerals
form radiating groups of crystals. The name is derived from the lo-
cality, Heron Bay, on the north shore of Lake Superior. Journal of
Geology, VII., 1899, 431.

Heumite, a name proposed by W. C. Broegger for a dike rock, com-
posed of minerals, too small to be recognized with the eye alone, but
which under the microscope prove to be natronorthoclase, natronmicro-
cline, barkevicite, biotite, and in small amount, nepheline, sodalite and
diopside. The accessories are apatite, magnetite, pyrite and titanite.
The silica in two dikes was found to be respectively 47.10 and 48.46.
The name was derived from Heum, a small town on Lake Farris. Das
Ganggefolge des Laurdalits, 90, 1898.

Holocrystalline, a textural term applied to those rocks that consist
entirely of crystallized minerals as distinguished from those with more
or less glass.

Hornblende, the name of the mineral is prefixed to many rock names.

Hornblendite, a granitoid, igneous rock consisting essentially of horn-
blende and analogous to pyroxenite. See p. 51.

Hornfels, dense, compact rocks produced from slates by the contact
action of some igneous intrusion, especially granite. Various micro-
scopic minerals are developed in them. See p. 88.

Hornstone, synonym of flint and chert.

Horses, a miner's term for fragments of wall rock included in a vein.

Hudsonite. Seecortlandite.

Hyaline, a synonym of glassy, which is often prefixed to the name of
volcanic rocks to signify a glassy development, as hyalo-rhyolites.

GLOSSARY. 145

Hyalomelane, Hausmann's name for basaltic glass. The word is de-
rived from the (Ireek for black glass.

Hydato, a syllable prefixed to lithological terms to indicate an origin
through aqueous processes.

Hydatopneumatolithic, a term used in the discussion of certain ore de-
posits to describe their origin through the agency of water and vapors.

Hyperite, used in Sweden loosely for the rocks of the gabbro family,
and in a restricted sense for olivine-norite.

Hypersthenite, a somewhat obsolete name for norite.

Hysterogenite, Posepny's term for mineral deposits derived from the
debris of other rocks. The word means of secondary or later formation.
Trans. Amer. Inst. Min. Kng., XXIII., 211. Com[)are idiogenite,
xenogenite.

Hysteromorphous, a term suggested by Posepny for those ore de-
posits that have been formed by the chemical and mechanical influences
of the surface region from some other original deposits. Trans. Amer.
Inst. Min. V.ng., XXIII., 331, 1.S93.

Idiogenites, a term suggested by Posepny to describe those ore de-
posits which are contemporaneous in origin with the wall rock. 'I'he
word means of the same origin. Trans. Amer. Inst. Min. Kng.,
XXlll., 211, 1893. Comj)are xenogenite, hysterogenite.

Idiomorphic, a descri[)iive term for those component minerals of a
ro( k that have their own crystal faces. Rarely all are of this character,
and then the rock is called j)ani(liomorphic. .Again, some are, and
others are not, giving the hypidiomorphic texture. The phenocrysts of
porj)hyritic rocks are most prone to be idiomorphic. When no minerals
have their own crystal faces, as in most granites, the rock is allotrio-
morphic, as earlier explained. All these terms were suggested by
Rosenbusch. Mass. (Jest., 1S87, but Rohrbach's automorphic and
xenomorphic, as is stated under the former, have a year's priority and
mean the same thing. The words arc of chief importance in micro-
scopic work.

Ijolite, a granitoid, nepheline rock, occurring in Finland and corre-
sponding in mineralogy to the nephelinites. It contains chiefly nephe-
line and pyroxene. The name is ilerived from the lijoki River, Fin-
land, and was given by Ramsay and Herghell. Stockholm geol. foeren.
focrb., 1S91, 300.

Ilmenite is sometimes prefixed to those rocks, which contain enough of
the mineral to receive attention as ores ; thus ilmenite-gabbro, ilmenite-
norite, etc. See J. H. L.,Vogt, Zeitschrif. prakt. Geologic, I., 11, 1893.
10

146 A HANDBOOK OF ROCKS.

Inclusions, the term is applied both to crystals or anhedra of one
mineral involved in another, and to fragments of one rock inclosed in
another, as when a volcanic flow picks up portions of its conduit.

Infusorial earth. • See diatomaceous earth.

Intratelluric, a term applied to those processes that take place deep
within the earth. For example, the large phenocrysts of a porphyry
are usually of intratelluric crystallization. Seep. 14.

Intrusive, the contrasted term with effusive, and applied to those
rocks that have crystallized without reaching the surface. They there-
fore form dikes, laccolites and batholites. Plutonic is, to a certain ex-
tent, synonymous. See p. 13.

Itabirite, a metamorphic rock, first described from Brazil, of schis-
tose structure and composed essentially of quartz grains and scales of
specular hematite. Some muscovite is also present. It is a close rela-
tive of itacolumite. It was named from Itabira, a place in Brazil.
When it crumbles to powder it is called jacotinga. Heusser and Claraz,
Zeit. d. d. geol. Gesellsch., XL, 448, 1859.

Itacolumite, or flexible sandstone, is a peculiar quartz schist first de-
scribed from Brazil, but since found in North Carolina and elsewhere.
It is composed of interlocking quartz grains, to which it owes its flexi-
bility, of muscovite, talc and a few other minerals, and has been re-
garded as the mother rock of the Brazilian diamonds. See. p. 107.

Jacupirangite, a name derived from Jacupiranga, Prov. Sao Paulo,
Brazil, and applied by O. A. Derby to a group of igneous rocks, consisting
sometimes of pure magnetite; again of magnetite with accessory pyroxene;
or of pyroxene with accessory magnetite ; or of pyroxene and nepheline
with biotite and olivine in greater or less quantity. Amer. Jour. Sci.,
April, 1891, 3 [4.

Jasper, red chalcedony, abundant enough on Lake Superior and else-
where to be a rock.

Jaspilite, a name originally proposed by Wadsworth for all the acid
eruptive rocks, whose chemical and physical condition carries them above
the rhyolites, but now used more or less loosely around Lake Superior
for the jasper associated with the local iron ores.

Kaolin, the hydrated silicate of alumina, Al^O^, 2Si02, 211^0, that is
the base of clays, and that gives them plasticity. See p. 67.

Kelyphite-rim, a name applied by Schrauf to rims of pyroxene, horn-

GLOSSARY

blende and spinel that sometimes surround the garnets of peridotites. It
is of microscopic apjjlication.

Keratophyre, a rock intermediate between porphyries and porphyrites,
and differing from either in having as the principal feldspar, anortho-
clase instead of either orthoclase or the soda-lime feldspars. Keratophvre
ap])lies to pre-tertiary rocks, whereas pantellerite is used for the same
aggregate of more recent geological date. The name was given in 1874
by (iiimbel to certain Bavarian felsitic and porphyritic rocks, that re-
sembled hornfcls, hence the name from the CIreek for horn. Its sig-
nificance has since been restricted.

Kersantite, a very old name of somewhat varying application, but
formerly used for rocks, that are intermediate between diorites or their
corresponding porphyrites and gabbros or diabases. Mica-diabase was
used as a synonym. Rosenbusch, in carrying out the separation of the
dike rocks from the effusive and intrusive grand divisions has sought to
restrict the name to those dike roc ks with plagioclase'that have prevail-
ing dark silicates, of which the chief is biotite. Kcrsanton is practically
a synonym. Hoth names are derived from a town in Brittany.

Kies, a general term for the sulphide ores, now adopted into Knglish
from tlu- original (>erman.

Kieselguhr, Cierman name for diatomaceous earth, .lud more or less
current in Knglish.

Killas, Cornish miner's term for the slates or schists that form the
country rock of the Cornish tin veins.

Kimberlite, a name given by H. Car\ille Lewis to the |)eridotite, that
forms the diainantiferous dike at the Kimberly mines, of South Africa.
(Ceol. Maga/inc, 1887, 22.) The rock is more porphyritic than
typii al peridotite.

Kinzigite, a metamorphic rock consisting of biotite, garnet and oli-
godase. It was named in i860 by Fischer, from the Kinzig Valley,
in the V<\mV l-'orest. Neues Jahrb., i860, 796.

Knotty, a descriptive term for those slates or schists, which are so
altered by contact metamorphism as to have new minerals developed in
th«'ni. K'^i'iK them a spotted or knotty appearance. See p. 89.

Krablite, ejected blocks from the volcano of Krafla, in Iceland,
which were regarded many years ago by Korchhammer, under the name
baulite, as a felds|)ar, of percentage in silica far beyond that of albite.
(Jour. f. prakt. Chemie, 1843, 390; Jahresber. iiber die Fortschrit.
Chemie u. Mineralogie, 1844, 262. ) It was soon shown by the micro-
scope to be an aggregate.

Krassyk, a local name for a decomposed ferruginous schist — in the
Beresov gold-mining district ofthel'rals. Archiv fiir jiractische Geo-
logie, II., 537.

148 A HANDBOOK OF ROCKS.

Kugel, the German word for ball or sphere, and often prefixed to
those igneous rocks that show a spheroidal development, such as cor-
site, orbicular granite, etc.

Kulaite, a name derived from the Kula basin in Lydia, Asia Minor,
proposed by H. S. Washington, for those rare basalts (there abundant)
in which hornblende surpasses augite in amount. "The Volcanoes of
the Kula Basin." Privately printed. New York, 1894, Amer. Jour.
Sci., Feb., 1894, p. 115.

Labradorite, the name of the feldspar is prefixed to many rock
names. Labradorite rock was formerly much used for anorthosite,
which see.

Laccolite, a name based on the Greek word for cistern and suggested
by G. K. Gilbert for those intrusions of igneous rock that spread out
laterally between sedimentary beds like a huge lens, and that never reach
the surface unless exposed by erosion. See p. 12 ; also Geology of the
Henry Mountains, Utah, p. ig.

Lamprophyr, a general term, now used in a somewhat wider sense
than as originally proposed by Giimbel, who suggested it. Rosenbusch,
in the Massigen Gesteine, gave it its present significance. Lamprophyrs
are dike rocks of porphyritic texture, whose predominant phenocrysts
are the dark silicates, augite, hornblende or biotite. They are practi
cally basic dikes. The word means a shining rock, and was first ap-
plied in 1874 to small dikes in the Fichtelgebirge that were rich in
biotite. In a somewhat modified sense it has recently been employed
by L. V. Pirsson, as single term for the basic " complementary rocks,"
(see Complementary Rocks), and as the antithesis of oxyphyre, which
applies to the acidic complementary rocks of an eruptive area.

Lapilli, volcanic dust and small ejectments, the results of explosive
eruptions.

Lassenite, Wadsworth's name for unaltered, glassy trachytes. Rept.
State Geol. Mich., 1891-92, p. 97. The name is derived from Las-
sen's Peak, Cal.

Laterite, a name derived from the Latin word for brick earth, and
applied many years ago to the red, residual soils or surface products, that
have originated in situ from the atmospheric weathering of rocks.
They are especially characteristic of the tropics. Though first applied
to altered, basaltic rocks in India, laterite has had in later years a general
application without regard to the character of the original rock. Com-
pare saprolite. See pp. 116, 117.

Latite, a name suggested by F. L. Ransome, for the rocks that are

GLOSSARY. 149

intermediate between the trachytes and andesites. Latite is meant to
1)6 a broad family name and to include the effusive representatives of
the plutonic monzonites. Piagioclase and orthoclase are both present,
augite, hornblende, biotite and olivine vary in relative amounts. The
textures may be glassy, felsitic or porphyritic. The name is derived
from the Italian province of Latium but was suggested by studies on
Table Mtn., Tuolumne Co., Calif. Bull. 89, U. S. Geol. Survey.
Compare trachydolerite, ciminite, vulsinite, monzonite.

Laurdalite, a name given by Broegger to a coarsely crystalline variety
of nei)hcline-syenite, that is abnormal in having for its feldspar natron-
orthoclase, rarely natron-microcline, instead of the normal potash ortho-
clase. The dark silicates are biotite, diallage and olivine. Zeitsch.
f. Kryst., XVT., 28, 1890.

Laurvikite, a name applied by Broegger to a Norwegian variety of
augite-sycnite that contains natron-orthoclase as its chief feldspar and
most abundant mineral. The other components are rare piagioclase,
pyroxene, biotite, barkevicite or arfvedsonite, olivine and magnetite.
Besides microscopic acces-sories, nepheline is occasionally met. Zeitsch.
f. Kryst., XVI., 29, 1890. Compare pulaskite.

Lava, a general name for the molten outpourings of volcanoes.

Laxite, Wadsworth's name for the fragmental or mechanical rocks,
especially when unconsolidated. Kept, of State Geol. of Mich., 1891-
92, p. 98.

Lentils, a short name for lenticular beds in a stratified series.

Leopardite, a siliceous rock from North Carolina, spotted with stains
of man_L;aiK>.e oxide. It is usually considered to be a (|uartz porphyry.

Leopard rock, a local name in Canada, a|)plicd to pegmatitic
rocks which are assoc iatoil with the apatite veins of Ontario and Quebec.
See C. H. Cordon, Bulletin (ieolog. Society of America, VIl., 122.

Leptinite, or Leptynite, the French synonym of granulite as used
among the CicTiuans. See granulites.

Leptomorphic, a term suggested by Giimbel for crystallized substances
which lack definite crystalline borders, as the nepheline in many groun-
masses. l-'ichtclgebirge, 1879, 240.

Lestiwarite, a name proposed by Rosenbusch for the aplitic dike-
rocks that accompany nepheline-syenites in Norway and Finland. They
are chielly or almost entirely alkali feldspar, with very subordinate
pyroxene or amphibole. They had been previously called syenite-
aplites by W. C. Broegger. Lestiwarite is derived from the Finnish
locality Lestiware. Massige Gesteinc, II., 464. Das Ganggefolgc des
Laurdalits, 207.

Leucite-basalt, basaltic rocks with olivine, in which leucite replaces
piagioclase. See p. 44.

I50 A HANDBOOK OF ROCKS.

Leucite-basanite, basaltic rocks that contain both leucite and pla-
gioclase. As contrasted with leucite-tephrites, they contain olivine.
See p. 44.

Leucitite, basaltic rocks without olivine in which leucite replaces
plagioclase. Compare leucite-basalt.

Leucitophyre, a name formerly used as a general one for the leucite
rocks, but now by common consent restricted to those phonolites that
contain both leucite and nepheline.

Leucite -tephrite, basaltic rocks without olivine, that contain both
plagioclase and leucite. Compare leucite-basanite.

Leucophyre, originally applied by Glimbel in 1874 to light-colored
diabases whose feldspar was altered to saussurite and whose augite had
largely changed to chlorite. Rosenbusch restricts it to diabases poor in
plagioclase. The name means a light-colored or white porphyritic rock,
and has little claim to consideration either in etymology or application.

Lherzolite, a variety of peridotite, first discovered in the Pyrenees,
and containing olivine, diopside and an orthorhombic pyroxene. Much
picotite is also present. It was named from Lake Lherz, by de la Metherie,
Theorie de la Terre, II., 281.

Liebenerite-porphyry, nepheline-porphyry whose nepheline pheno-
crysts are altered to muscovite. Its original locality is near Predazzo,
in the Tyrol. Compare gieseckite-porphyry.

Limburgite, porphyritic basaltic rocks consisting of olivine and augite
in a glassy groundmass. They lack feldspars. See p. 45. The name is
derived from Limburg, a locality on the Kaiserstuhl, a basaltic mountain
in Baden. It was suggested by Rosenbusch in 1872, and at the same
time Boricky described similar rocks from Bohemia as magma-basalt.

Limestone, the general name for rocks composed essentially of cal-
cium carbonate. See p. 71.

Lindoite, Brogger's name for certain dike rocks, in the region of
Kristiania. They have trachytic texture ; are seldom and then but
slightly porphyritic ; are medium to coarsely crystalline in the larger
dikes; possess light colors and often lack dark colored minerals. When
such are recognizable they are pyrite and chlorite. Ferriferous carbo-
nates are present. Traces of aegirineand of a dark, alkaline hornblende
may be occasionally detected. (Die Eruptivgesteine des Kristianiage-
bietes, I., 131, 1894. )

Liparite, a synonym of rhyolite, and largely used for the latter
among Europeans, though rhyolite is chiefly current in America and
England. The name is derived from the Lipari Islands, off the coast of
Italy, where the rocks are abundant. It was proposed by Justus Roth
in i86r. Gesteins-analysen, p. xxxiv.

Listvanite, a local name for a rock in the gold-mining district of

GLOSSARY. 151

Beresov, in the Urals. It is regarded as a contact zone produced from
dolomite, and is a coarsely crystalline aggregate of magnesite, talc,
quartz andlimonite, pseudomorphic after pyrite. Archiv liir practische
Geologie, II., 537.

Litchfieldite, a name proposed by W. S. Bayley for the variety of
nepheline-syenite, occurring in loose boulders near Litchfield, Me.,
whose chief feldspar is albite and which differs therein from normal
nepheline-syenite. Bull. Cieol. Soc. Amer., III., 243.

Lithical, a term proposed. by L. Fletcher for the finer, textural char-
acters of rocks, i. e., those for which, texture as distinguished from struc-
ture is employed above. See p. 13. Lithical from theOreek for stone
is contrasted with petrical from the Clreek for rock. Introduction to
the Study of Rocks; British Museum Handbooks, 1895.

Lithionite-granite, a name proposed by Rosenbusch for granites with
lithia ini( a or litliioiiitc.

Lithographic limestone, an exceptionally homogeneous and fine-
grained limestone, suitable for lithography.

Lithoidal, a descriptive term applied to those groundmasses, espe-
cially of rhyolitcs, that are excessively finely crystalline, like porcelain,
as distinguished from glassy varieties. The English etjuivalent, stony,
is also used.

Lithophysae, literally "stone bubbles," a name applied to those
cellular cavities in acidic lavas, obsidian, rhyolitc, etc., that have con-
centric walls, and that are caused by a special development of mineral-
izers at that particular |)oint. They are usually hemispherical in shape
and oil the walls may have various well crystalli/ed minerals. See p. 22.

Lithosphere, the outer stony shell of the earth. See barysphere.

Local metamorphism, /. e., contact metamorphism. See p. 85.

Loess, fine surfai e soils chiefly formctl of wind-blown dust. See p.
65. I lie name is a (lerman word, akin to loose, and appears to have
been first applied geologically in the Rhine valley.

Luciite, Chelius' name from the Luciberg in Hesse, for finely crystal-
line, diorite dikes, whose minerals are xenomorphic. Notizblatt V'erein.
f. F.nlkunde, Darmstadt, i8y2, i.

Luijaurite, a name projjosed by Broegger for a nepheline-syenite, rich
in a;girine and eudialyte. Zeit.sch. f. Kryst , XVI., 204. The name
is from a Lapland locality, where the rock was tliscovercd by Ramsay.

Lustre-mottlings, a name aj)plied by I'umpelly to certain augitic
rocks, which have a shimmering lustre because the shining cleavage faces
of the augite crystals are mottled by small inclusions Proc. Amer.
Acad., XIII., 260, 1878. Compare Foicilitic and Schiller.

Luxullianite, a tourmaline granite from Luxullian, in Cornwall,
that is a ])roduct of contact metamorphism. See p. 32.

Lydite. See Basanite.

152 A HANDBOOK OF ROCKS.

ISA

Macroscopic, a word formerly current as a synonym of megascopic,
/. e. , recognizable by the naked eye. It is etymologically less correct
as an antithesis of microscopic than is megascopic, for "macro" is
from the Greek for broad, whereas " mega ' ' means large. Nevertheless,
it preceded megascopic in general use and is still current.

Madupite, a name given by Whitman Cross to a peculiar group of
rocks which are illustrated by one forming Pilot Knob, a butte about 6
miles northwest of Rock Springs, Wyo. Cross defines Madupite "as
consisting essentially of diopside and a magnesia-potash mica with
leucite in decidedly subordinate amount. Its magma was low in
silica, alumina and iron, rich in potash, and contained so much lime
and magnesia that silicates of these bases are the principal constituents,
yet controlled in their development by the strong potash element."-
The Pilot Knob case is a vitropliyric representative of the type, so de-
fined. Amer. Jour. Sci., Aug , 1897, i39*

Magma is now generally employed for the molten masses of igneous
rock before they have crystallized. An original, parent magma may
break up into several derived ones. See pp. 2, 13, 57. The word is also
used in the sense of basis as earlier defined, but this use is unfortunate.

Magma-basalt, a synonym of limburgite, which was proposed by
Boricky, in 1872, at about the same time that Rosenbusch suggested
limburgite. Some authorities give the former the preference.

Magnetite. The name of the mineral is prefixed to the names of
many rocks in which it is prominent. It almost furnishes a rock itself,
at times.

Magnetite-olivinite, a name coined by A. Sjogren in 1876 for the
igneous iron -ore at Taberg, in Sweden. The work is an aggregate of
magnetite and olivine, with a few shreds of biotite. The rock is prac-
tically a peridotite, greatly enriched with titaniferous magnetite. On
the borders of the intrusion it shades into gabbro. Geol. Foren. in
Stockholm, Forhandl., III., 42. Compare Cumberlandite.

Malchite, a variety of diorite dikes which have, in a groundmass of
quartz, feldspar and hornblende, phenocrysts of plagioclase, hornblende
and biotite. The name was given by A. Osann, and is derived from
Malchen, another name for Mt. Melibocus, in Hesse.

Malignite, a name proposed by Lawson for a group of rocks on the
Maligne river, Rainy Lake district, province of Ontario. They are de-
scribed as "basic, holocrystalline, plutonic rocks, rich in alkalies and
lime. ' ' Iron is present in moderate amounts, almost entirely combined
in the silicates. Iron and magnesia are more abundant than is usual in
the alkali-rich plutonic rocks. The chief minerals are orthoclase, often

GLOSSARY. 153

microscopically intergrown with an acid plagioclase ; segirine-augite,
which may predominate with but a moderate admixture of biotite, or
may be subordinate and intergrown with preponderant soda amjjhibole,
biotite being present as before. There are three tyjjes of malignites,
one of which has much melanite and another much nepheline. Bull.
Dept. Geol. Univ. Calif., I., 340, 1S96

Manganolite, Wadsworth's name for rocks composed of manganese
minerals, such as wad, psilomelane, etc. Rept. State Geol. Mich.,
1891-92, p. 93.

Marble, in lithology, a metamorphosed and recrystalli/ed limestone.
In the trade the name is applied to any limestone that will take a |)olish.

Marekanite, a rhyolitic perlite from the banks of the Mcrekana
river, near Ochotsk, Siberia. At times a very clear glass, it is found in
balls and cores of large perlitic ma.sses and may even be under strain
like Prince Ru|)ert's drops. See Zirkcl's Petrographie, II., 299.

Marl, a calcareous clay, or intimate mixture of clay ami particles of
calcite or dolomite, usually fragments of shells. Marl in .\nicrica is
chiefly ap|)lied to incoherent sands, but abroad compact, impure lime-
stones arc also called marls.

Marmarosis, the general name for the process of crystallization of
limestonch to marble, whether by contact or regional metamorphism. It
was coined by (Jeikie from the I^tin for marble.

Massive, the antitlicsis of stratified, and therefore, often used as a
synonym of igneous or eru|)tivc rocks as contrasts. I with tin- l.i dil..]
sedimentary and laminated melamorphic varieties

Megascopic, a descriptive term meaning large enough to be dihtiu
guishcd with the naked eye ; the antithesis of microscopic. See ma-
croscopic. U.sed also to describe methods of observation without the
mi<'ros( ope or with the eye alone.

Melaphyre, a rock name first introduced by Brogniart in 1.S13, practi-
cally for porphyritic rocks with a dark groundmass and with feldspar
phenocrysts. After having had various meanings for many years, by
common consent, it is now generally used as suggested by Rosenbusch for
pre-tertiary olivine-basalts. that is, for porphyritic etpiivalents of olivine-
di abase.

Melilite-basalt, a rare basaltic rock whose feldspathoid is melilite.
It was first identified by Stel/ner in 1.SS2. The rock is excessively
basic. Sec p. 45. .Alnoite is the same rock in dikes.

Mesostasis, a synonym of basis suggested by (iiimbel.

Metabolite, Wadsworth's name for altered, glassy trachytes, of which
lassenitc is llie unaltered form. Rept. State (leol. Mich., 1891-92, p.

97-

Metachemical metamorphism, Dana's term to describe that variety

154 A HANDBOOK OF ROCKS.

of metamorphism that involves a chemical change in the rocks affected.
Amer. Jour. Sci., July, 1886, p. 69.

Metadiabase, a shortened form of metamorphic diabase, suggested
by Dana for certain rocks simulating diabase, but supposed to have
been produced by the metamorphism of sediments. Amer. Jour. Sci.,
Feb., 1876, 121. Compare Pseudo-diabase.

Metadiorite, dioritic rocks produced as just described under meta-
diabase. Compare Pseudo-diorite.

Metarhyolite, a name applied to rocks which were originally rhyo-
lite, but which are now altered in mineralogy, by recrystallization,
so as to develop microperthite, or products of devitrification, or some
other change from their original condition. Bull. U. S. Geol. Survey,
No. 150, p. 164. Aporhyolite being generally accepted, metarhyolite
would appear to be superfluous.

Metamorphism, a collective term for the processes by which rocks
undergo alteration of all sorts. It is more fully set forth on page 84.

Metasomatic, /. e., a change of substance ; it is used to describe the
replacement of one or more of the minerals of a rock by others. The
form of the originals is not at all preserved as in pseudomorphs, nor
does the chemical composition remain the same while the form alters as
in paramorphs, but both customarily change. The term is especially
used in connection with the origin of ore deposits. The corresponding
noun is metasomatosis, but replacement is a good English equivalent.

Metaxite, a name of Hauy's for micaceous sandstone.

Mezo or Meso is sometimes prefixed to the names of igneous rocks of
Mesozoic age.

Miarolitic, a descriptive term applied to those granites that have
small cavities, into which well-terminated crystals project. See p. 13.

Miascite, a name coined from Miask, a locality in the Urals where a
nepheline-syenite occurs whose dark silicate is biotite. Used also as a
general name for biotitic nepheline-syenites. See p. 37.

Mica-schist, finely laminated, metamorphic rocks, consisting of
quartz, mica, feldspar and several minor minerals. See p. 99.

Mica-peridotite, a name applied by J. S. Diller to a peculiar perido-
tite, occurring as a dike in Crittenden County, Ky. , and consisting
chiefly of altered olivine and biotite. Amer. Jour. Sci., Oct., 1892,
288. See Analysis 19, p. 49.

Mica-trap, an English field name for dark, dike rocks rich in mica.

Micro-felsite, a name used in microscopic work for those varieties of
groundmass that do not affect polarized light, but that are not true
glasses because they have a fibrous, a granular or some such texture.
The textures are no doubt in many cases the results of devitrification
of a glassy base.

GLOSSARY. 155

Micro-granite, a name used in microscopic work for those ground-
masses of porphyritic rocks, that consist of small quartz and feldspar
cystals with granitoid texture on a small scale, /. <r. , with components of
about the same size and usually without crystallographic boundaries.
See granophyric.

Micro-granulite, the French equivalent of granophyric, as earlier ex-
plained.

Micro-crystalline, granular rocks, whose components are recogni-
zable, but are so small as to require the microscoi^e for their identification.

Microlites, generally used for microscopic, but still identifiable
minerals.

Micropegmatite, /. e., microscopic pegmatite, a term applied to those
groundmasses of jjorphyritic rocks whose microscopic cpiartz and feld-
spars mutually penetrate each other. The several parts of the same
crystal, though isolated, extinguish together. Sec granophyric.

Microperthite, /. <•., microscopic perthite, a term applied to that
variety of orthocla.se which is thickly set with flat spindles of albite. It
is very < onmion in gneisses. Comi)are granophyric.

Micropoikilitic, a textural term suggested by (I. H. Williams to
describe those minerals that are sj)eckled with microscopic inclusions of
other minerals, having no definite relations to each other or to their
host. Jour, of (Ecology, I., 176, 1893. I'oikilitic is often spelled
poi( ililic or poe< ilitir.

Millstone-grit, an old I'.nglish name for the conglomeratic sandstone
at the base of the Carboniferous Coal Measures. It is more or less cur-
rent in this country as a synonym of the Great, I'ottsville or Serai con-
glomerate.

Mimesite, an obsolete synomym of dolerite.

Mimophyre, a name suggesteil by l-.lie de Beaumont in 1S41 for
metamorphosed, argillaceous rocks in which feldspars had developed, so
that they resembled porj)hyries. Volcanic tuffs are a fretjuent original,
but gray\va( kcs and arkoses have also yielded them. Compare Porphyroid.

Mineralizers, the dissolved vapors in an igneous magma, such as
steam, hydrofluoric acid, boracic acid and others, that exert a powerful
influence in the development of .some minerals and textures. See p.
15. The word is also technically used in some definitions of ore.
Thus it is said that an ore is a comj^ound of a metal and a mineralizer,
such as co|)per and sulphur, iron and oxygen, etc.

Minette, a variety of mica-syenite, usually dark and fine-grained, oc-
curring in dikes. See p. 35, Anal. 6.

Missourite, a granitoid rock consisting of leucite, biotite, augite,
olivine, iron ores and apatite, and corresponding to the efi"usive leucite-
basalts. It was discovered in the Highwood Mountains, Mont., by

156 A HANDBOOK OF ROCKS.

Weed and Pirsson, and named by them from the Missouri river, "the
most prominent and best known geographical object in the region."

Moldauite, a very pure glass, from the valley of the Moldau river,
Bohemia. See Bouteillenstein.

Monchiquite, a name suggested by Hunter and Rosenbusch from the
Monchique Mountains of Portugal, for basaltic dikes corresponding in
mineralogy and texture to limburgite. They often accompany nephe-
line-syenite. Tsch. Mitt., XL, 445, 1890.

Monzonite has usually been considered as a variety of augite-syenite
that displayed, however, considerable mineralogical variety. Broegger
has recently used the name for a transitional and intermediate group of
granitoid rocks between the granite -syenite series (/. e., the alkali-feld-
spar series) and the diorites (/. e., the lime-soda feldspar series). The
monzonites have both alkali -feldspar (or orthoclase) and lime-soda
feldspar (or plagioclase) in approximately equal amounts, or at least
both richly. (Die Eruptivgesteine des Kristianiagebietes, II. , 21, 1895).

Mortar-structure, a term suggested by Toernebohm to describe those
granites, gneisses or other rocks that have been dynamically crushed, so
that larger nuclei of their original minerals are set in crushed and com-
minuted borders of the same, like stones in a wall.

Mulatto, a local name in Ireland for a Cretaceous green sand.

Muscovado, the Spanish word for brown sugar, used by Minnesota
geologists for a rusty, brown, outcropping rock that resembles brown
sugar. It has been applied to both gabbros and quartzites. XVI.,
Ann. Rept., Minn. Geol. Surv.

Mylonite, a name suggested by the English geologist Lapworth for
schists produced by dynamic metamorphism. Rept. of Brit. Assoc,
1885-86, p. 1025.

Nadel-diorite, /. c, needle-diorite, a German term for diorites with
acicular hornblende.

Napoleonite, a synonym of corsite.

Natron-granite, granites abnormally high in soda, presumably from
the presence of an orthoclase rich in soda, or of anorthoclase. They
are also called soda-granites. Natron is likewise used as a prefix to
minerals and rocks that are rich in soda, as natron-orthoclase, natron-
syenite, etc.

Navite, Rosenbusch' s name for pre -tertiary, porphyritic rocks, con-
sisting of plagioclase, augite and olivine as phenocrysts, with a second
generation of the same forming the holocrystalline groundmass. The
name is from Nava, a locality in the Nahe Valley, Mass. Gest., 1887.

GLOSSARY. 157

Necks. Lava-filled conduits of extinct volcanoes, exposed by erosion.

Nepheline-basalt, an old, general name for basaltic rocks with nephe-
line, but now restricted to those that practically lack plagioclase, and
that have nepheline. augite, olivine and basis. See p. 44.

Nepheline-basanite, basaltic rocks with plagioclase, nepheline, au-
gite, olivine and basis. Compare nepheline-tephrite. See p. 44.

Nephelinite, basaltic rocks consisting of nepheline, augite and basis,
but without olivine. See p. 44.

Nephelinitoid, Horicky's term, now used in microscoi)ic work for
nepheline-glass, or the glassy basis in nepheline rocks, whose easy gela-
itnization indicates its close relations with this mineral ; unindividu-
alix.cd nciilu'linc.

Nepheline syenite, /. f., eleolite-syenite, a name to be preferred to
the latter as there is no real need of the wordeleolite. Granitoid rocks
consisting of orthoclase, nepheline, and one or more of the following:
hornblende, augite and I)iotite. The rocks result from magmas espec-
ially rich in alkalies, and possess great scientific interest on account of
their richness in rare, associated minerals. See p. 36.

Nephelite, a later method of spelling ncj)hcline and one consistent
with approved, mineralogical orthography.

Nevadite, a name coined by von Richthofen from Nevada, for those
rhyolites that approximate a granitoid texture, /. r., with little ground-
mass. Mem. Calif .\( ad. Sci., I., p. 54, 1867. See |). 24 and Hague
and hidings, .\nicr. Jour. .Sci., June, 18S4, 461.

Neve, a French synonym of firn.

Nonesite, |)or|)hyrites with orthorhouibic pyroxene. The name was
given i)y I.cpsius. Das wcstliche Siid-Tyrol, Herlin, 1878.

Nordmarkite, Hroegger's name for a variety of granitic rocks consist-
ing of ortho( lase, some oligoclase, more or less microperthite, <|uartz
and somewhat subordinate biotite, pyroxene, hornblende and .tgirine.
It is chemically high in silica and the alkalies. Zeitsch. f Kryst., XVI.,

54. I'^^OO-

Norite, a roi k of the gabbro fiimily that consists of plagioclase and or-
thorhouibic j)yroxene, usually hyj)ersthene. The name has had a variable
history and was originally proposed in 1832 by Msmark for aggregates of
feldspar and hornblende which were lacking or notably poor in diallage
and hypersthene. Hut as many localitcs were cited in later years which
on microscopic examination were found rich in these minerals, Rosen-
busch finally gave the name its above definition anil this is its generally
accepted signification.

Normal metamorphism, /. c, regional metamorphism. See p. 93.

Normal pyroxenic, Bunsen's name for his assumed, typical, basic, ig-
neous magma with 48 per cent. SiO^ as contrasted with the correspond-

158 A HANDBOOK OF ROCKS.

ing Jiormal-trachytic one with 76 per cent. SiO.^. He sought to explain
all intermediate rocks by the intermingling of these two. Although ap-
parently applicable at times and serviceable in their day, the conceptions
have long since been exploded. See J. Roth's Gesteinsanalysen, 1861.

Nosean. The name of the mineral is often prefixed to the names of
rocks containing it.

Novaculite, excessively fine grained, quartzose rocks supposed to be
consolidated, siliceous slimes and of sedimentary origin. They are
especially developed in Arkansas, and are much used as whetstones.
See p. 64.

Obsidian, a general name for volcanic glass. When used alone it
implies a rhyolite-glass, but it is now much employed with a prefix as
andesite-obsidian, basalt-obsidian. See p. 21.

Ocellar- structure, a microscopic term used by Rosenbusch for pe-
culiar aggregates of small pyroxenes, that resemble eyes, buds and the
like, and that are especially common in nepheline and leucite rocks.
Mass. Gest., 625, 1887.

Odinite, a name given by Chelius to certain porphyritic dikes in Mt.
Melibocus, which have a groundmass of plagioclase and hornblende rods,
with phenocrysts of plagioclase and augite. Notizbl. Ver. Erdkunde,
Darmstadt, 1892, Heft 13, p. i.

Olivine. The name of the mineral is prefixed to the names of many
rocks that contain it. It is of especial importance in this respect, as
its presence marks a more basic development in many rocks, as con-
trasted with their varieties that lack it.

Oolitic, a textural term for those rocks which consist of small concre-
tions, analogous to the roe of fish. Oolites are calcareous, siliceous and
ferruginous.

Opacite, a noncommittal microscopic term, less current than formerly,
for minute, opaque grains observed in thin sections of rocks. They are
generally regarded to-day as chiefly magnetic dust.

Ophicalcite, Brogniart's name for crystalline limestones, spotted
serpentine. Seep. 113.

Ophiolite, Brogniart's name for the serpentines. See p. 113. It is
also employed in America in the sense of ophicalcite as above given.

Ophite, a name given in 1798 by the Abbe Palassou to a green rock
of the Pvrenees. It was later recognized to be composed of feldspar and
hornblende, and still later was determined by Zirkel to be a uralitized
diabase. The name has chief significance to-day because used to de-
scribe the textural peculiarity of some diabases. Strictly speaking an
ophitic texture is one in which rod-like or lath-shaped, automorphic.

GLOSSAR): 159

plagioclase feldspars are involved in augite, as it were, in a paste, so as
to form a variety of poicilitic texture, but the term was used in the first
edition of this book, and is employed by many in the sense in which
the diabasic texture is defined on a preceding page. The difference
between the two meanings lies in the fact that in the former the augite
is in excess and the feldspar is involved in it. In the latter, the feld-
spar is in excess, and the augite fills the interstices between its lath-
shaped crystals. The peculiar significance of these textures is that the
feldsi)ars crystallized before the augite, contrar}' to the usual succession.
See p. 44 and p. 17.

Orbicular, a textural term for those rare rocks whose minerals have a
spheroidal grouping, such as corsite and orbicular granite. See Kugel
and Spheroidal.

Orbite, a name proposed by Chelius for certain diorite dikes near
Orbeshohe, Hesse, of porphyritic texture and having large phenocrysts
of hornblende, biotite and plagioclase. Notizbl. Ver. Krd. Darmstadt,
1892, I.

Orendite, a name proposed l)y Whitman Cross, for the peculiar leu-
citic rocks at Orenda Butte in the Leucite Hills, Wyo. They contain
leucite and sanidine, in about eipial amounts, magnesia, potash, mica,
and diopside as essentials. A peculiar aniphibole is also present. The
rock is a leucite-phonolite as the latter term is used by older writers,
but the objection to calling any rock a phonolite which lacks ncphelinc,
led to the name. Amer. Jour. Sci., Aug., 1897, j). 123. Compare
Madujiiti' and Wyomingite, etc.

Orthoclase. The name of the mineral is often pn iImiI to tlu- names
of rocks that contain it.

Orthophyre, /. e., orthocla.se porphyry or porphyry proper.

Ortlerite, a name given by the .Austrian geologists, Stac he and von
John, to ( crtain porphyrites of the eastern Alps that resemble the old
greenstones and that have plagioclase, hornblende, generally augite,
and more or less basis. They range from 4S-54 SiO^. Jahrb. k. k.
g. Rei< hsanst, 1879, 342.

Ossipyte, a name suggested by C. H. Hitchcock for a rock from
Watcrville, N. H., which on examination in 1871 by K. S. Dana (be-
fore the use of thin sections in .America) was thought to consist of oli-
vine and labradorite, with a little magnetite. Ossipyte is derived from
the "Ossipees," a tribe of Indians, who fomierly lived in the region.
Amer. Jour. Sci., Jan., 1872, p. 49. Hy means of thin sections the
rock was later shown to contain diallage, by G. W. Hawes, and to be
a gabbro. (ieol. of New Hampshire, Vol. III., Part IV., p. 166. Os-
sipyte was a forerunner of troctolite over which it has priority.

Ottrelite schists, schistose rocks with the peculiar micaceous mineral

i6o A HANDBOOK OF ROCKS.

ottrelite. They are best known from the Ardennes, Belgium, but are
found in New England.

Ouachitite (pronounced wavv-shee-tite), a name coined by Kemp
from the Ouachita River, Arkansas, for certain, basic dikes containing,
in a glassy groundmass, prevailing and often phenomenally large pheno-
crysts of biotite, very subordinate augite and magnetite. They also
occur at Beemerville, N. J., associated with nepheline syenite. Ann.
Rep. Geol. Surv. of Ark., 1890, II., 393.

Oxyphyre, Pirsson's general name for the acidic rocks, as contrasted
with Lamprophyre, for the basic rocks. The two are complementary,
see Lamprophyre, and Complementary Rocks.

Pahoehoe, the Hawaian word for a lava sheet, whose surface consists
of smooth or fluted hummocks. It is contrasted with aa, which refers
to jagged and cindery crusts. See Aa. It has been specially intro-
duced into English speech by Capt. (now Major) C. E. Button. 4th
Ann. Rept. Dir. U. S. Geol. Surv., 95, 18S3.

Paisanite, a name, proposed by Osann from the Paisano Pass, on the
Southern Pacific R. R., in western Texas, for a variety of quartz-por-
phyry, consisting of phenocrysts of microperthitic orthoclase and rarer
quartz, in a groundmass of quartz and feldspar. Occasional groups of
small hornblendes (riebeckite) are met. Tscherm. Min. u. Petr,
Mitth., XV., 435. Compare Comendite.

Palaeophyre, Giimbel's name given in 1874 to certain porphyritic
dike rocks corresponding to quartz-mica-diorites in mineralogy. They
cut the Silurian strata of the Fichtelgebirge.

Palaeophyrite, a name proposed by Stache and von John (compare
ortlerite) for certain porphyrites in whose strongly prevailing ground-
mass are phenocrysts of plagioclase, hornblende and augite. Jahrb. d.
k. k. g. Reichsanstalt, 1879, 342.

Palaeopicrite, a name proposed by Giimbel in 1874, in his paper,
"Die palaeolithischen Eruptiv-gesteine des Fichtelgebirges," a contri-
bution to which we are indebted for a great number of useless and un-
necessary rock names, for picrites which were considered by him to be
similar to the rocks from the Cretaceous formation, originally named
picrite by Tschermak. Giimbel called his specimens palseopicrites be-
cause they occurred in Palaeozoic strata. He regarded them as aggre-
gates of olivine, enstatite, chrome-diopside and magnetite, but they
are now known to be chiefly olivine and augite. More or less brown
hornblende and biotite also occur.

Palagonite-tuff, an altered basaltic tuff from Palagonia, in Sicily.

GLOSSARY. i6i

The name palagonite was originally applied to problematical, brown in-
clusions in the tuff, which were thought at first to be a definite mineral.
They are now known to be a devitrified, basaltic glass. The name was
given by v. Waltershausen in 1846. See Vulk. Gesteine in Sicilien und
Island, 1853, 179-

Palatinite, a name ])roposed by I>aspeyres for certain rocks in the
German jjrovince of Pfalz, supposed by him to be gabbros with diallage
and to be of Carboniferous age ; but they have since been shown to be
essentially diabases. Neues. Jahrb., 1869, 516. The word is derived
from the classic name of the district.

Pallasite, originally proposed by Gustav Rose for a meteorite that
fell near Pallas, in Russia, has been used by Wadsworth in a wider
sense for both meteoric and terrestial, ultra-basic rocks, which in the
former average about bo^/c iron, and in the latter have at least more
iron oxides than silica. Cumberlandite (which >^<-'i i- the chief ex-
ample. Lithological Studies, 18S4, 68.

Panidiomorphic, Rosenbusch's term for those rocks, all of whose
components possess their own crystal boundaries.

Pantellerite, a group of rocks intermediate between the rhyolitesand
trachytes on the one hand, and the dacites on the other. They differ
from all these in having anorthocla.se as the princi|)al feldspar. Cossyrite,
a rare and probably titaniferousamphibole, occurs at the original locality
on the island of I'antclleria, in the Mediterranean. See p. 27. The
name was given by Koerstner. /.eitschr. f Kryst., 1881, 348.

Paragenesis, a general term for the order of formation of a.ssociated
minerals in time succession, one after the other. To study the paragene-
sis is to trace out in a rock or vein the succession in which the minerals
have (levelo]>e(l.

Paramorphism, the passage of one mineral into another without
change of composition, as augite into hornblende in uralitization. It is
also used in connection with metamorphism to describe such thorough
changes in a ro< k. that its old comjjoncnts are destroyed and new ones
are built up.

Parophite, a name given by 1". Sterry Hunt, (ieol. Surv. Can., 1852,
95, to a rock or mineral similar to dysyntribite. The name means like
ser])entine.

Pearl-diabase, see \'ariolite.

Pearlite or Pearlstone, vokanit gla.ss with concentric, shelly tex-
ture and usually with a notable percentage of water. See p. 21.

Pegmatite, originally applied to graphic granite, but of later years

used as a general name for very coarse, dike or vein granites, such as have

large quartz, feldspar, muscovite, biotite, tourmaline, beryl and other

characteristic minerals, and are often called giant -granite. See p. 31.

1 62 A HANDBOOK OF ROCKS.

Pele's Hair, a fibrous, basaltic glass from the Sandwich Islands,
named after a local, heathen goddess.

Pelite, a general name for mud rocks, /. e. , shales, clays and the like.

Pencatite, see predazzite.

Peridotite, granitoid rocks consisting of olivine and pyroxene with
little or no feldspar. See p. 51. Many varieties have been made de-
pending on the kind of pyroxene present, or on its absence in favor of
related minerals, viz :

Olivine, augite — Picrite.

Olivine, diopside (diallage), enstatite^Lherzolite.

Olivine, enstatite — Saxonite, harzburgite.

Olivine, enstatite, augite — Buchnerite.

Olivine, augite, garnet — Eulysite (metamoiphic?).

Olivine, diallage, hornblende — Wehrlite.

Olivine, hornblende — Cortlandite. >

Olivine, biotite — Mica-peridotite.

Olivine, hornblende (secondary?), biotite — Scyelite.

Olivine, alone or with chromite — dunite.

Further particulars about each of these will be found under the indi-
vidual names. Compare also kimberlite.

Perthite, a name given by Thomson (F/iiV. Mag., 1843, 1S8) to
parallel intergrowths of orthoclase and albite, originally described from
Perth, Ontario.

Petrical. L. Fletcher's name for the coarser structural features of
rocks. See lithical.

Petrography, properly the descriptive part of the science of rocks
for which the more general name is petrology or lithology, but petrog-
raphy is widely used as a synonym of the latter.

Petrosilex, an old name for extremely fine, crystalline porphyries
and quartz -porphyries and for those finely crystalline aggregates we now
know to be devitrified glasses ; also for theground masses of the former,
which though not glassy are yet not resolvable by the microscope into
definite minerals. See felsite, micro-felsite. It was practically a confes-
sion by the older petrographers, that they did not know what the rock
consisted of.

Phenocrysts, a name suggested by J. P. Iddings (Bull. Phil. Soc.
Wash., XI., 73, 1889), for porphyritic crystals in rocks. It has proved
an extremely convenient one, although its etymology has been criticized.
It may be best to change to phanerocryst, just as in botany, phenogam has
yielded to phanerogam ; but one form or the other is a necessity.

Phonolite, volcanic rocks, of porphyritic or felsitic texture, consist-
ing of orthoclase, nepheline, pyroxene and more rarely amphibole.
Leucite may replace the nepheline and yield leucite-phonolites. See p.

GLOSSARY. 163

28. The name is Klaproth's rendering into Greek of the old name
Clinkstone. Abhandl. Berlin, Akad. , 1801.

Phosphorite, massive calcic phosphate, of the composition of apatite
but usually lacking crystal form.

Phosphorolite, Wadsworth's name for phosphatic rocks, guano -phos-
phorite, apatite, etc. Rept. State Geol. Mich., 1891-92,' p. 93.

Phthanite, Hauy's name for silicious schists. Its use has recently
been revived in America by G. F. Becker, who applies it to certain
silicified shales in California. Quicksilver Deposits of the Pacific Coast,
Mono., XIII., 105, U. S. Geol. Survey.

Phyllite, a name for intermediate rocks between the mica schists
and slates, usually finely crystalline; mica-slates. See p. loi.

Phyre. The last syllable of porphyry, often used with prefixes, as
vitrophyrc, orthoi)hyre, granoi)hyre, etc.

Picrite, a name originally given by Tschermak to certain, porphyritic
rocks from the Carpathians, that have abundant and large phenocrysts
of olivine, with less pyroxene, hornblende and biotite, in a glassy
groundma.ss, more or less devitrified. The rocks are practically preter-
tiary iimburgites. Picrite is now also applied to those peridotitcs that
consist of olivine and augite. It is derived from the Greek for bitter, in
allusion to the high percentage of magnesia, Biltererde in German.

Pistazite, a synonym of e])idote, more current in ICurope than Amer-
ica, and uscmI in rock names for epidote.

Pitchstone, a glassy rock, usually corresponding to the rhyolites or
trachytes, but with a considerable percentage of water, 5-8 % for cx-
ami)le. It was formerly specially used for pretertiary gla.sses, /. r., the
glasses of quartz-porphyries and porphyries, but time distinctions are
obsolete. Pitchstones have a marked resinous luster as the name im-
])lies. See p. 21.

Plutonic, a general name for those rocks that have crystallized in
the depths of the earth, and have therefore assunied as a rule, the grani -
toid tecture. See p. 13.

Pneumatolitic, a general name for those minerals which have been
l)rodu( cd in ( onnection with igneous rocks through the agency of the
gases or vapors called mineralizers. They may be in the igneous mass
itself or in cracks in the wall rock. Compare the ca.ses cited on pp.
91, 92. The term is much used in discussions of ore de|)Osits.

Poicilitic, /. <r., speckled, a term i)roposed by G. H. Williams for
those rocks which have a mottled luster, because on the shining cleavage
faces of some of their minerals, small inclusions of others occur, pro-
ducing the effect. The same thing was earlier called " luster mottling "
by Pumpelly, but poicilitic has proved a u.seful term both in megascopic
and microscopic work. (Journal of Geology, I., 176, 1893.) It is
also spelled poikilitic and poecilitic.

1 64 A HANDBOOK OF ROCKS.

Porcellanite, fused shales and clay, that occur in the roof and floor
of burned coal seams. The rock is quite common in the lignite dis-
tricts of the West, where apparently spontaneous combustion has fired
the seams in the past.

Porodine, Breithaupt's name for amorphous rocks, such as are derived
from gelatinous silica.

Porodite, Wadsworth's name proposed in 1879, for all the altered,
fragmental forms of eruptive rocks, commonly called diabase tuff, schal-
stein, etc. Bull. Mus. Comp. Zool., 1897, V., 280.

Porphyrite, a porphyritic rock, belonging to the plagioclase series
and corresponding in mineralogy to the diorites. To distinguish it
from andesite, it is necessary to draw a contrast between surface flows
(andesites) and intruded dikes or sheets (porphyrites); or between ter-
tiary and later lavas (andesites) and pre-tertiary ones (porphyrites); or
between those with glassy or very finely crystalline groundmasses (ande-
sites) and those with groundmasses of moderate coarseness (porphyrites).

Porphyritic, a textural term for those rocks which have larger crys-
tals (phenocrysts) set in a finer groundmass, which may be crystalline
or glassy, or both. See p. 13. Rosenbusch has sought to define it as
the texture due to the recurrence of the period of crystallization of the
same or similar minerals (Neues Jahrb., 1882, II., 3). While, except
for porphyritic rocks with a glassy groundmass, this practically amounts
to the same thing as the textural definition just given, it is idle for any
writer to try to change so old, well-established and indispensable a
conception.

Porph3n:y, a word derived from the classic name of the shell fish, a
species of Murex, that yielded the Tyrian purple of the ancients. It
was later applied to the red, porphyritic rock of the Egyptian quarries,
" porfido rosso antico," whose red color is due to piedmontite, a man-
ganese epidote. In course of time it was applied to all porphyritic
rocks as we now understand the term. In its restricted sense it implies
orthoclase-porphyry, the porphyritic rock corresponding to syenite, but
to give it any essential significance as contrasted with trachyte, one of
the three distinctions must be drawn, which are cited above under por-
phyrites, and of which the second is of no real value. See p. 24.
Porphyry is colloquially used for almost every igneous rock in the West,
that occurs in sheets or dikes in connection with ore bodies.

Porphyroid, metamorphic rocks with porphyritic texture, /. e., with
phenocrysts of feldspar or other minerals in a finer groundmass, yet
shown by geological relations to be altered sediments, or tuffs. Fossil
remains have even been detected in some. They are close relatives of
halleflintas.

Pozzuolane, a leucitic tuff, found near Naples and used for hydraulic
cement.

GLOSSARY. 165

Predazzite, a contact rock at Predazzo in the Tyrol, produced by an
intrusion of syenite in crystalline dolomite. It is partly calcite and
partly brucite or hydromagnesite. Pencatite is the same aggregate,
darkened by grains of pyrrhotite.

Primary, an old synonym of Archean. Also used for those rocks
which have crystallized directly from fusion or solution, as contrasted
with transported or secondary sediments.

Propylite, a name given by von Richthofen in 1867 to certain ande-
sites, formed at the beginning of Tertiary time, that were thought to re-
semble the old diorites, and diorite-porphyrites. They had been previ-
ously called by him greenstone-trachytes in Hungary, but were not
named propylite until he met them again in Nevada and California (Me-
moirs of the California Academy of Sciences, I., 60, 1867). The west-
ern propylites have been since conclusively shown by several American
petrographers to be only more or less altered andesites. The literature
of the name furnishes an interesting and amusing e.xhibition of the elTorts
of those petrographers, who are influenced by the time-myth in the
classification of igneous rocks, to draw distinctions, where there are no
differences. The name means before the gates, alluding to their position
at the beginning or entrance to the Tertiar)', which was supposed to
usher in the true, volcanic eruptions of geological time. See p. 41.

Proteolite, an old name for certain ( ontact roiks pro<luced by gran-
ite intrusions from slates in Cornwall. It has been lately revived by
Bonney for andalusite-hornfcls. ((^. J. (i. S., 1886, 104.) Compare
Cornubianite.

Proterobase, originally ajiplied by (iiimbel, 1874, to Silurian or ear-
lier diabahos with hornblende. The freipiency of the paramorphism of
augite to hornblende has led others to apply it to diabases with uralitized
augitc. Rosenbusch restricts it to diabases with original hornblende.

Protogine, an old name for a granite or gneiss in the AIjjs, consist-
ing of quartz, orthoclase and chlorite or sericite, which was formerly
erroneously taken for talc. The laminated structure from dynamic met-
amorphism is often ])ronounced.

Psammite, a general name for sandstones, from the Greek word for
a grain of sand.

Psephites, a general name for conglomerates and breccias, /. e.,
coarse, fragmental rocks as lontrasted with psammitesand pelites. The
name is derived from the Greek for pebble.

Pseudo-diabase, a name proposed by G. K. Becker for certain met-
amorphic rocks in the coast ranges of California that are supposed to
have been derived from sediments, yet that have the minerals and tex-
ture of diabase. Monograph, XIII., U. S. Geol. Surv., p. 94. Com-
pare Mctadiabase, which means the same thing and has precedence.

i66 . A HANDBOOK OF ROCKS.

Pseudo-diorite, dioritic rocks produced as described under pseudodia-
base above. See the same reference.

Pseudo-chrysolite, synonym of moldauite, bouteillenstein.

Pseudomorph, the replacement of one mineral by another, such that
the form of the first is preserved by the second, despite the difference in
composition.

Puddingstone, conglomerate.

Pulaskite, a special name given by J. Francis Williams to certain
syenitic rocks from Pulaski County, Arkansas, that have trachytic tex-
ture and that consist of orthoclase (kryptoperthite), hornblende (arfved-
sonite), biotite and a little augite (diopside), eleolite, sodalite and ac-
cessory minerals. Ann. Rep. Geol. Surv. Ark., 1890, II., 56. Com-
pare laurvikite.

Pumice, excessively cellular, glassy lava, generally of the composi-
tion of rhyolite. See p. 21.

Pyromeride, a name given by the Abbe Haiiy to the orbicular diorite
or corsite of Corsica. The word means " partly fusible," and refers to
the properties of the two constituent minerals, of which the one, quartz,
was infusible, and the other, the feldspar, could be melted.

Pyroschists, a name suggested by T. Sterry Hunt for those sediments
that are impregnated with combustible, bituminous matter. Amer,
Jour., March, 1863, 159.

Pyroxene. The name of the mineral is often prefixed to the name
of the rocks that contain it.

Pyroxenite, a name first proposed by T. Sterry Hunt for the masses
of pyroxene occurring with the apatite deposits of Canada. It is now
generally employed in the sense advocated by G. H. Williams, for grani-
toid, non-feldspathic rocks, whose chief mineral is pyroxene, and which
lack olivine. See p. 51. (Amer. Geologist, July, 1890, p. 47.)
Williams proposed the name websterite, from Webster, N. C, for a
variety consisting of diopside and bronzite, with the latter porphyritic-
ally developed. Idem, 35.

Quartz, the name of the mineral is prefixed to the names of many
rocks that contain it, as quartz-porphyry, p. 24; quartz -trachyte, p.
24, etc.

Quartzite, metamorphosed sandstone. See p. 106. Not to be used
for vein quartz.

Radial Dikes, a descriptive term specially used by L. V. Pirsson
for those dikes which radiate outward from an eruptive center. Amer.
Jour. Sci., Aug. , 1895, p. 116.

GLOSSARY. 167

Reaction-rims, a term mostly used in microscopic work, for the
curious rims of hypersthene, garnet, hornblende, biotite, magnetite and
perhaps other minerals, that surround grains of magnetite or of ferro-
magnesian silicates, wherever, in many gabbros, they come next to feld-
spar. They are supposed to be produced by the reaction of the miner-
als on each other, probably in the crystallization of the rock. (See J.
F. Kemp, Bull. Cieol. Soc. of Amer. , V., 221, 1S94.)

Regional-metamorphism, Daubree's name for that extended meta-
morphism that, as contrasted with contact effects, is manifested over
large areas. See pj). 85-93.

Regolith, a name coined by G. 1'. Merrill from two Greek words
meaning "blanket of stone" for the layer of loose materials that
mantles the land areas of the globe and rests on the solid rock. These
materials are derived from the decay of rocks, accumulations of vege-
tation, talus, debris, sediments, wind-blown sands, and glacial deposits.
Rocks, Ro<k-weathering and Soils, 299, 1897.

Rensselaerite, V.. Kmmons' name for a talcose rock from St. Law-
rence Co., N. V. .Vnnual report of the N. V. Gcol. Survey, 1837, p.

'52-

Resorbed, a term used in microscopic work to describe those pheno-
crysts \vhi( h after crystallization are partly fused again into the magma.
See p. 17.

Retinite, tin* ( nrrcnt name for i)iti hstone among the Krcni h.

Rhomben-porphyries, a name api)licd to certain Norwegian por
phyries, whose phcnocrystsof orthoclase are bounded by 00 P and 2P 00,
so that they rescml)le a rhombohedron. The orthoclase is rich in sotla.

Rhyolite, volcanic rocks, of porphyritic or felsitic texture, whose
phenocrysts are prevailingly orthoclase and cjuartz, less abundantly bio-
tite, hornblende or pyroxene, and whose groundmass is crystalline,
glassy, or both. The name is from the Circck to flow, and refers to the
frerjuent flow structure. Rhyolite is current in .\merica, whereas
liparite and cpiartz-trachyte are more used abroad. The name was given
in i860 by v. Richthofen. ( Jahrb. d. k. k. Reichsanst, XI., 153,
i860. )

Rill-marks, small depressions in sandstones, produced by the eddying
of a retreating wave on a seabeach under the lee of some small obstruc.
tion, such as a shell or pebble.

Ripple-marks, corrugations in sandstones produced by the agitation
of waves or winds when the rock was being deposited.

Rockallite, a name proposed by J. W. Judd for a rock from Rockall
Island, a small reef in the North Atlantic, 240 miles west of Ireland.
Rockallite is a granitoid rock, consisting of cjuartz, albite and aigirite,
in proportions respectively of 38 : 23 : 39, in the specimen investigated.

168 A HANDBOOK OF ROCKS.

Trans. Royal Irish Academy, XXX [., Part III., 39 ; Amer. Jour. Sci.,
March, 1899, 241.

Rock-flour, a general name for very finely pulverized rocks or min-
erals which lack kaolin and, therefore, the plasticity of clay, and which
are much finer than sand. Rock-flour which is largely pulverized
quartz may be separated from most clays.

Saccharoidal, a term applied to sandstones whose texture resembles
that of old-fashioned loaves of sugar.

Sagvandite, a curious rock from near Lake Sagvand, Norway, that
is mainly bronzite and magnesite. A little colorless mica, and more
or less chromite and pyrite are also present. The name was given by
Petterson. Neues Jahrb., 1883, II., 247.

Sahlite, a variety of pyroxene, sometimes prefixed to rock names.

Salband, a term current among miners for the parts of a vein or dike
next to the country rock.

Sand, incoherent fragment of minerals or rocks of moderate size, say
one-quarter of an inch (6 mm.) and less in diameter. Quartz is much
the commonest mineral present. See p. 6t,.

Sandstone, consolidated sands. See p. 63.

Sanidinite, a name applied especially to certain trachytic bombs that
occur in tuffs in the extinct volcanic district of the Laacher See, Ger-
many. Recently it has been suggested by Weed and Pirsson for those
syenitic rocks which are all orthoclase. Amer. Jour. Sci., Dec, 1895,
p. 479.

Sanidinite, a name proposed by Weed and Pirsson for the extreme
case of feldspathic syenites, in which all other minerals except ortho-
clase practically fail. They establish a series as follows :

All orthoclase, no augite - Sanidinite.

Orthoclase exceeds augite — Augite-syenite.

Orthoclase equals augite — Yogoite.

Augite exceeds orthoclase — Shonkinite.

All augite, no orthoclase — Pyroxene and peridotite rocks of various types.

Amer. Jour. Sci., Dec, 1895, p. 479. Subsequently yogoite was
withdrawn in favor of monzonite, which has priority. Idem, May,
1896, 357, 358.

Santorinite, a name proposed by H. S. Washington for those excep-
tional andesitic or basaltic rocks, which, with a high percentage of
silica (65-69), yet have basic plagioclases, of the labradorite-anor-
thite series. The name was suggested by the volcano Santorini. (Journal
of Geology, V., May-June, 1897, 368.) See also Fouque, Santorini et

GLOSSARV. 169

ses Eruptions, Paris, 1879, and Etude des Feldspaths, 317-320. The
prevailing bisilicate at Santorini is pyroxene.

Sanukite, Weinschenk's name for a glassy phase of andesite that con-
tains bronzite, augite, magnetite, and a few large plagioclases and gar-
nets. The rock is related to the andesites as are the limburgites to the
basalts. Neues Jahrb. Beilageband, VII., 148, 1891.

Saussurite-gabbro, gabbro whose feldspar is altered to saussurite.
See p. T03.

Sazonite, Wadsworth's name for peridotites consisting of enstatite
or bronzite and olivine. Synonym of harzburgite, but saxonite has
priority. Lithological Studies, 1884, p. 85.

Schalstein, an old name for a more or less metamorphosed diabase -
tuff.

Schiller-fels, enstatite or bronzite peridotite with poicilitic pyrox-
enes. Orthorhombic pyroxenes possess the poicilitic texture to a pecu-
liar degree, and especially when more or less altered to bastite, and the
term schiller, which expresses this, is especially a|)plied to them.

Schillerisation, Judd's name for the process of producing poicilitic
texture by the (lcveloi)mcnt of inclusions and cavities along particular
crystal planes. The cavities are largely produced by solution, some-
what as arc etch figures, and arc afterwards filleil by infiltration. Quart.
Jour. Geol. Soc, 1885, 383; 1886, 82.

Schist, thinly laminated, metamorphic rocks which split more or less
readily along certain planes approximately jjarallel. See p. 99.

Schlieren, a useful (lerman term, largely adopted into English, for
those smaller portions of many igneous rocks, which are strongly con-
trasted with the general mass, but which shade insensibly into it. Thus
portions of granite are met, much richer in biotite and hornblende than
the normal rock, or much more coarsely crystalline. Pegmatite streaks
occur and other differentiations of the original magma. Several dif-
ferent varieties may be made, for a discussion of which see Zirkel's
Lehrbuch der Petrographie, I., 787, 1893.

Schorl, an old name for tourmaline, still sometimes used in names of
rocks.

Scoria, coarse, cellular lava, usually of basic varieties.

Scyelite, Judd's name for a rock, related to the peridotites, that oc-
curs near Loch Skye, in Scotlanil. Its principal mineral is green horn-
blende, i)rcsuniably seconilary after augite ; with it are bleached biotites
and serpentine, supposed to be derived from olivine. See (^. J. G. S.,
1885, 401.

Secondary, a term used both for rocks and minerals, that are derived
from other rocks and minerals, such as sandstone, clay, or other sedi-
ments ; chlorite from augite, etc.

I70 A HANDBOOK OF ROCKS.

Sedimentary, rocks whose components have been deposited from
suspension in water. See p. 60.

Selagite, a name of Hauy's for a rock consisting of mica, dissemi-
nated through an intimate mixture of amphibole and feldspar, but it has
been since applied to so many different rocks as to be valueless.

Selenolite, Wadsworth's name for rocks composed of gypsum or an-
hydrite. Rept. State Geol. of Mich., 1891-92, p. 93.

Septaria, literally little walls, a name applied to concretions, largely
of argillaceous material, which are traversed by cracks. The cracks are
filled as a rule with calcite or quartz, affording an intersecting network
from which weathering may have removed the original, included, argil-
laceous matter.

Sericite-schist, mica-schist whose mica is sericite. See p. 100.
Sericite is also used as a prefix to many names of metamorphic rocks
containing the mineral.

Serpentine, a metamorphic rock consisting chiefly of the mineral ser-
pentine. See p. 113.

Shastalite, Wadsworth's name for unaltered, glassy forms of andesite.
Rept. of Mich. State Geol., 1891-92, p. 97.

Shonkinite, a name given by Weed and Pirsson to a rock from the
Highwood Mountains, Mont., which they define as "a granular, plutonic
rock consisting of essential augite and orthoclase, and thereby related to-
the syenite family. It may be with or without olivine, and accessory
nepheline, sodalite, etc., may be present in small quantities." Bull.
Geol. Soc. Amer., VI., 415, 1895. See Anal. 7, p. 34. Later they
state that augite should exceed orthoclase. Amer. Jour. Sci., Dec,
1895, p. 479.

Shoshonite, a general name proposed by Iddings for a group of igne-
ous rocks in the eastern portion of the Yellowstone Park. They are
porphyritic in texture, with phenocrysts of labradorite, augite and oli-
vine, in a groundmass that is glassy or crystalline ; in the latter case or-
thoclase and leucite, alone or together, are developed. Chemically they
range: SiO.,, 50-56; Al^Og, 17-19-7; CaO, 8-4.3; MgO, 4-4-2.5;
Na^O, 3-3.9; K.jO, 3.4-4.4. The rocks are to be considered in con-
nection with absarokite and banakite. Journ. of Geol., III., 937.

Siderolite, as used by Fletcher and generally in English, is a name
for meteorites that are partly metallic iron and partly silicates. As used
by others it is applied to more purely metallic ones.

Sideromelane, von Waltershausen's name for a basaltic glass from the
palagonite tuffs of Sicily. Vulk. Gest. v. Sicilien und Island, 202, 1853.

Silicalite, Wadsworth's name for rocks composed of silica, such as
diatomaceous earth, tripoli, quartz, lydite, jasper, etc. Rept. State Geol.
Mich., 1891-92, p. 92.

GLOSSARY. 171

Silicification, the entire or partial replacement of rocks and fossils
with silica, either as quartz, chalcedony or opal.

Sillite, Gumbel's name for a rock from Sillberg, in the Bavarian Alps,
variously referred by others to gabbro, diabase, mica-syenite and mica-
diorite. Beschr. der bay. Alpen, 184, 1861.

Sills, an English name for an intruded sheet of igneous rock.

Silt, a general name for the muddy deposit of fine sediment in bays
or harbors, and one much employed in connection with engineering
enterprises.

Sinaite, an alliterative substitute for syenite proposed by Rozieres be-
cause on Mt. Sinai true, quartzless syenites occur, whereas at Syene the
ro( k is a hornblende-granite. See p. 35.

Slickensides, polished surfaces along faults, or fractures produced by
the rubbing of the walls upon each other during movement.

Soapstone, metamorphic rocks, consistingchiefly of talc. Seep. 114.

Soda-granite, granites especially rich in soda, or whose soda exceeds
the potash. Compare analyses, p. 30. See natron-granite.

Sodalite-syenites, syenites rich in sodalite ; close relatives of nephe-
linc syenites. See anal. 5, p. 34. Sodalite-trachytes also occur.

Soggendalite, a name proposed by C. V. Kolderup for a variety of
dial)asc that is especially rich in pyroxene, and that is intermediate be-
tween true diabases and pyroxenites. The type rock forms a dike near
Soggendal, Norway. Bergens Museums .\arbog, 1896, 159.

Soil, surface earth mixed with the results of the demy of veuetable or
animal matter, so as usually to have a dark color.

Solvsbergite, Bnigger's name for quartzless or <|uartz-poor grurudites;
that is, medium to finely crystalline, dike rocks, with prevailing alkali-
feldspar (mostly albite and microcline) with legirine, or in the basic
varieties with hornblende (kataforite), sometimes also with a peculiar
mica. In the most basic members (|uartz entirely fails anti nepheline
appears. (Die Kruptivgesteine des Kristianiagebietes, I., 67.)

Sondalite, a name proposed by Stache and von John for a meta-
morphic rock consisting of cordierite, (piartz, garnet, tourmaline and
cyanite. Jahrb. d. k. k. g. Reichsanst, 1877, 194.

Sordawalite, an old name for the glassy salbands of small diaba.se
dikes that were regarded as a mineral. It is derived from Sordawalar,
a localitv in Finland. Compare wichtisite.

Spessartite, a name proposed by Rosenbusch, for those dike rocks,
which, whether porphyritic or granitoid in texture, consist of prevailing
plagioclase, hornblende and diopside. Orthoclase and olivine oc-
casionally appear. Massige Clesteine, 532, 1896. The name from
Spessart, a group of mountains in the extreme northwest of Bavaria, but
as it has already been used for a variety of garnet, it is a very unfortu-
nate selection.

172 A HANDBOOK OF ROCKS.

Spheroidal, a descriptive term applied to igneous rocks that break up
on cooling into spheroidal masses, analogous to basaltic columns ; also
used as a synonym of orbicular as applied to certain granites.

Spherulites, rounded aggregates or rosettes, large or small, of acicu-
lar crystals that radiate from a center. They are chiefly met in the
microscopic study of acidic, volcanic rocks and commonly consist of
feldspars and quartz. When of one mineral they are called by Rosen-
busch sphero-crystals. They may reach large size, though mostly
microscopic. See p. 21.

Spilite, an early French name for dense, amygdaloidal varieties of
diabase.

Spilosite, a spotted, contact rock produced from shales and slates by
intrusions of diabase. It corresponds to the hornfels of granite contacts.
Zincken in Karsten und v. Dechen's Archiv., 1854, 584.

Stalactite, depending, columnar deposits, generally of calcite, formed
on the roof of a cavity by the drip of mineral solutions. Compare sta-
lagmite.

Stalagmite, uprising, columnar deposits, generally of calcite, formed
on the floor of a cavity by the drip of mineral solutions from the roof.
Compare stalactite.

Steatite, soapstone, talc rocks.

Structure, used generally in America for the larger physical features
of rocks, as against texture, which is applied to the smaller ones. See
p. 13. Many, however, employ them interchangeably. Compare also
petrical and lithical.

Stylolite, small, columnar developments in limestones or other cal-
careous rocks that run across the stratification. They appear to have
been caused by some unequal distribution of pressure in consolidation,
or by a capping fossil, as against the surrounding rock.

Subsoil, the layer of more or less decomposed and loose fragments of
country rock that lies between the soil and the bed rock in regions not
covered by transported soils.

Suldenite, a name given by Stache and von John to gray, acidic, an-
desitic porphyrites in the eastern Alps. They range from 54-62 SiO,
and have, in the prevailing gray groundmass, phenocrysts of hornblende,
plagioclase, a little orthoclase and accessory augite, biotite and quartz.
Compare ortlerite.

Surficial, a general name, lately introduced by the U. S. Geological
Survey, for the untransported surface, alteration products of igneous
rocks.

Sussexite, a special name suggested by Brogger for the eleolite por-
phyry, originally described by Kemp, from Beemerville, Sussex Co., N.
J. Die Eruptivgesteine des Kristianiagebietes, 1895. The name was,

GLOSSARY. 175

however, applied years ago to a hydrated borate of manganese and
magnesia, from Franklin Furnace, N. J.

Syenite, granitoid rocks consisting in typical instances of orthoclase
and hornblende. In mica-syenites, biotite replaces hornblende. In
augite-syenites augite does the same. For etymology and history see
p. 36. Compare also laurvikite, mon/.onite, nordmarkite, pulaskite,
sanidinite, shonkinite, yogoite.

Syssiderite, Daubree's name for those meteorites which consist of
silicates cemented together by metallic iron.

Tachylyte, Breithaupt's name for a basaltic glass. It was originally
regarded as a mineral and was named from two Greek words suggested
by its quick and easy fusibility. Sec analysis 15, p. 20, and description,
p. 21. Kastner's Archiv fiir die gcsammte Naturlehrc, \'\\.. 112.
1826. Compare hyalomelane.

Taconyte, a name proposed by H. W Winchcll for the chcriy or |;is-
pery, but at times ( alcareous or more or less quartzitic rock, that encloses
the soft hematites of the Mesabi Range, Minn. Taconytcs are regarded
as in large part altered greensands by J. K. Spurr. The term is current
in the Mesabi iron range. XX. Ann. Rept. Minn. (leol. Survey, 124.
The name is derived from Taconic, \\. Knimons' rejected geological
system.

Talc-schist, schistose rocks consisting chiefly of talc and quartz.
*See p. 104. Talc is also prefixed to several other roi k nanies.

Tawite, a name given by W. Ramsay to a very peculiar rock of both
granitoid and porphyritic texture and consisting of pyroxene and
sodalite. It occurs in the ncphelinesyenite area of Kola in I-'inland
and is derived from Tawajok, a local geographical term. Fcnnia.
XI., 2, 1894.

Taxite, Loewinson-l.cssing's name for lavas, that, on crystallizing,
have broken up into contrasted aggregates of minerals so as to present
an apparent, clastic tci turc — either banded, /. c, eutaxitic, or brec-
ciated, /. <'.. ataxitic. Hull. Soc. Belg. (leol., \'., 104, 1891.

Tephrite, basaltic rocks containing lime-soda feldspar, nepheline,
augite and basis. I.eucite-tephrites have leucite in place of nepheline,
and some tephrites have both. Tephrites tliffcr from basanites in lack-
ing olivine. The name is from the Greek for " ashen," alluding to the
color. Ft is an adajjtation of an old form, tephrine. Neuesjahrb.,
1865, 663.

Teschenite, a name given in 1861 by Hohenegger to a group of
intriisi\e ro( ks in the Cretaceous strata near Tcschen. .Austrian Silosia.

174 A HANDBOOK OF ROCKS.

They have, however, been since shown to embrace such a variety of type,
that the name has little value, but as analcite occurs quite constantly in
most of them, many still use the term for diabasic rocks with this mineral.

Texture. See structure and also p. 13.

Theralite, granitoid rocks consisting essentially of plagioclase, nephe-
line and augite, with the common accessories. They were first discovered
by J. E. Wolff in the Crazy Mountains, Montana. They were previously
and prophetically named by Rosenbusch from the Greek to seek eagerly,
because this mineralogical and textural aggregate was believed to exist
before it was actually discovered. A spelling therolite is also advocated.

Tholeite, Rosenbusch' s name for augite-porphyrites, which, aside from
the usual phenocrysts, have a groundmass, with but one generation of
crystals and with a little glassy basis between them, affording a texture
called intersertal. Massige Gest., 504, 1887.

Till, unsorted glacial deposits, consisting of boulders, clay and sand.

Timazite, a name given by Breithaupt to certain porphyritic rocks in
the Tiniok Valley of Servia, that have since proved to be varieties of
andesite and dacite. Berg, und Hiittm. Zeit, 1861, 51.

Tinguaite, a name given by Rosenbusch to rocks consisting of alkali
feldspar, nepheline and abundant segirine, which form dikes in or near
areas of nepheline-syenite. It was first applied to specimens from the
vicinity of Rio Janeiro, where in the Serra de Tingua the rocks were
first discovered and described by O. A. Derby as phonolites. They
have since proved of very wide distribution and not always to accom-
pany nepheline-syenites (Black Hills, S. D.). By many the name tin-
guaite is regarded as an unnecessary and undesirable synonym of pho-
nolite. It first appears in Hunter and Rosenbusch, Tschermaks Min.
and Petrog. Mitth., XL, 447, 1890.

Toadstone, an old English name for certain, intruded sheets of amyg-
daloidal basaltic rocks in the lead district of Cumberland, England.
Also locally applied near Boston to a mottled felsite, apparently spheru-
litic.

Toellite, a biotite, hornblende, porphyrite, with garnets, that forms
dikes in mica-schist and gneiss near Meran, in the Tyrol. Pichler,
Neues Jahrb., 1873, 940.

Tonalite, a quartz-mica-hornblende diorite from near Meran in the
Tyrol. It was named by vom Rath, from Tonale, a place on Mt. Ada-
mello. Zeit. d. d. g. 'Gesellsch., XVI., 249, 1864. Compare adamel-
lite.

Topazfels, a brecciated, contact rock, near granite contacts, and
formed of topaz, tourmaline, quartz and some rarer accessory minerals,

Toscanite, a name proposed by H. S. Washington for a group of
acid, effusive rocks in Tuscany (Italian, Toscana) and elsewhere, which

GLOSSARY. 175

are characterized mineralogically by the presence of basic plagioclase, as
well as orthoclase, and by occasional quartz ; and chemically by high
silica and alkalies, and (for the acidity) high lime, and low alumina.
They range from 63-73 silica and are intermediate between rhyolites
and dacites. Journal of Geology, V., 37, 1897. Compare dellenite.

Touchstone. See basalt.

Tourmaline-granite, a variety of granite with tourmaline as the dark
silicate. It is usually due to fumarole action, and is developed on the
borders of intrusion of normal granites.

Trachorheite, a name proposed by F. M. Endlich as a collective des-
ignation for the four rocks, propylite, andesite, trachyte and rhyolite,
as used by von Richthofen. Hayden's reports, 1873, p. 319.

Trachy-andesite, effusive rocks, intermediate between trachytes and
andcsites. Used by H. S. Washington for trachytes which have also
much acidic plagioclase (andesine to oligoclase). Jour. Geol., \., 351.

Trachy-dolerite, a name suggested by Abich for a group of rocks in-
termediate between the trachytes and basalts. Natur u. Zusammenset-
zung der V'ulkanisch liildungen, loi, 1841. Compare I^titc. 'I'rachy-
dolerite as used by H. S. Washington means a trachyte with consider-
able basic plagioclase (labradorite to anorthitc). Jour. Geology, V.,

351-

Trachyte, igneous rocks of porphyritic or felsitic texture consisting
cs.scntially of orthoclase and biotite or hornblende or augite, one or
more. Seep. 27. It was formerly u.sed for both rhyolites and trachytes
proper, or practically as a field name for light -colored lavas and jior-
jjhyries. As such in older reports it is to be understood. Compare
also acmite-trac hytes and pantellerites.

Trachytic lecture, a special microscopic name for those ground-
masses that are made up of rods of feldspar, usually in flowlines, but
without basis.

Trap, a useful field name for any dark, finely crystalline, igneous rock.
It is a Swedish name from the occurrence of such rocks in sheets that
resemble stei>s, "trappar." Seep. 56.

Trass, a trachytic tuff from the I^aacher Sec, used along the Rhine
for hydraulic cement.

, Travertine, calcareous tufa. The name was given by Naiunann and
is of Italian origin.

Trichite, a microscopic term for hair-like crystallites ; so named from
the Greek for hair.

Tripoli, a name applied to diatomaceous earth and to pulverulent
silica derived by the breaking down of cherts from some change not
well understood. See p. 81.

Troctolite, Bonney's name for a variety of gabbro consisting of

1/6 A HANDBOOK OF ROCKS.

plagioclase and olivine with very subordinate diallage. The olivine
may be serpentinized. Geol. Magazine, 1885,439. Compare Ossipyte.

Trowlesworthite, a variety of granite which has been so altered by
fmiiarole action that it consists of fluorite, orthoclase, tourmaline and
some quartz, the last named having been largely replaced by the first.
The name is derived from an English locality, and was given by Worth,
Trans. Roy. Geol. Soc. of Cornwall, 1884, 180. Mineralog. Mag.,
1884, 48.

Tufa, the cellular deposits of mineral springs, usually calcareous or
siliceous. See p. 72. Not to be confused with tuff.

Tuff, the finer, fragmental ejectments from the explosive eruptions of
volcanoes. They may afterwards be water-sorted or cemented to firm
rock. Coarser ones are called volcanic breccias, but in neither is it fre-
quent to see much sorting unless by subsequent erosion. Tufa is also
used in this sense, but the custom should be discouraged.

Typhonic rocks, Brogniart's name for rocks that have come from the
depths of the earth, /. e. , plutonic and eruptive rocks. Typhon is used
as a synonym of boss or stock.

Umptekite, a name proposed by Ramsay for the border facies of the
nepheline-syenite mass at Umptek, Finland. It lacks nepheline almost
entirely, and contains perthitic intergrowths of the alkali-feldspars.
Arfvedsonite is the chief, dark silicate, but segirine is also present. The
accessory minerals are numerous. Fennia, XL, 2, 1894.

Uralite, a special name for that variety of hornblende, that is derived
by paramorphism from augite. The word is often used as a prefix be-
fore the names of those rocks that contain this variety. It has also
suggested various rock names, such as proterobase, scyelite, etc. The
name is derived from the original occurrence in the Urals. (G. Rose,
Reise nach dem Ural, II., 1842, 371.)

Variolite, a special name for a curious, border development of dia-
base intrusions, which is a very dense, finely crystalline mass of rounded
spheroids, largely spherulitic in texture. They give the rock a pock-
marked aspect and hence the name, which is a very old one. Pearl
diabase is synonymous.

Venanzite, a name proposed by Sabatini, an Italian petrographer,
for an effusive rock from a small volcanic cone at San Venanzo, Um-
bria, Italy. Venanzite contains phenocrysts of olivine in a ground-
mass of melilite, leucite and black mica, together with a little pyroxene,

GLOSSARY. 177

nephelite and magnetite. Bolletino Reale Comitato Geologico, Sept.,
1898. Rosenbusch subsequently described the same rock under the
name Euktolite, but Venanzite has priority. Sitzungsber. k. pr. Akad.
Wissensch., Berlin, VII., no, 1899; Amer. Jour. Sci., May. 1899, 399.

Vintlite, a (juartz-porphyrite occurring in dikes near Unter-Vintl, in
the Tyrol. Compare toellite from the same region. Pichler, Neues
Jahrb., 1871, 262.

Viridite, a microscopic name suggested by Vogelsang and formerly
used for the small, green, chloritic scales often met in thin sections.
As their true nature has now been determined, they are generally called
chlorite.

Vitro, a prefix meaning glassy and used before many rock names, as
vitrophyre, in order to indicate a glassy texture.

Vitrophyre, \'ogelsang's name for (|uartz-porphyries and porphyries
with gla.ssy groundmass.

Vogesite, Rosenbusch's name for syenitic dikes, in which the dark
hornlilendcs or augites are in excess over the light colored feldspars.
Ma.ss. Gest., 1887, 319. The name is derived from Vogesen, the Ger-
man form of Vosges.

Volcanic, surface flows of lava as distinguished from plutonic rocks.
See p. 13.

Volcanite, a name proposed by U'. M. Hobbs, for an anorthoclase-
augitc lava with the chemical composition of dacite. Bull. Geol. Soc.
Amer., V., 598. 'i'he name was suggested by the original occurrence
on the island of Volcano, one of the I.ipari group, where the rock oc-
curs as a cellular bomb.

Volhynite, a pori)hyritc containing j)lagioclase, hornblende and bio-
tite pheno( rysts in a holocrystalline groundmass of feldspar and chlorite.
The name was given by Ossovsky, and it is based on the original occur-
rence in Volhynia. See Chrustschoff, Bull. Soc. Min. France, 1885,
441.

Vulsinite, a name suggested by H. S. Washington for a group of
rocks intermediate between trachytes and andesites. They contain
much labradorite in addition to the usual minerals of trachyte. The
name is derived from Vulsinii, an ancient Etruscan tribe inhabiting the
region where the type specimens were obtained. Journal of Geology,
IV., 547. Compare latite and trachydolerite.

Wacke, an old name for the surficial, clayey products of the alter-
ation of ba-salt. The syllables.are still current in graywacke.

Wash, a miner's term in the West for loose, surface deposits of sand,
gravel, boulders, etc.
12

178 'A HANDBOOK OF ROCKS.

Websterite, a name proposed by G. H. Williams for the pyroxenites
near Webster, N. C, that consist of diopside and bronzite, with the
latter porphyritically developed. Amer. Geol., VI., 35, 1890. The
name websterite had been previously used by A. Brogniart in 1822 for
aluminite. Hauy's Mineralogie, II., 125.

Wehrlite, a name originally suggested by von Kobell for what was
supposed to be a simple mineral, but which proved to be a peridotite
consisting of olivine and diallage.

Weiselbergite, Rosenbusch's name for those augite-porphyrites
whose groundmass consists of a second and sometimes third generation
of plagioclase rods and augites, arranged in flow lines in a glassy basis.
Mass. Gest., 501, 1887. Wordsworth uses the name for an altered
andesite glass. Rept. of State Geol. of Mich., 1891—92, p. 97.

Whinstone, a Scot<:h name for basaltic rocks.

Wichtisite, a glassy phase of diabase, named from a Finland locality,
Wichtis. Compare sordavalite.

Wyomingite, a name suggested by Whitman Cross, for the variety of
rock from the Leucite Hills, Wyoming, which consists almost entirely
of leucite and phlogopite, small, acicular crystals of diopside are
very subordinate, and apatite is also present. Amer. JourSci., Aug.,
1897, 120. This is the rcok described by Zirkel in 1876 and was the
first known occurrence of leucite in America. Fortieth Parallel Survey,
VI., 259.

Xenogenites, Posepny's term for mineral deposits of later origin than
the wall rock. The name means foreigners, and refers to their later in-
troduction. Compare idiogenites. Trans. Amer. Inst. Min. Eng. ,
XXIII.,_2o5, 1839.

Xenolith, a term proposed by W. J. Sollas, for included masses of
rock, caught up in an igneous intrusion. The term means foreign rock.

Xenomorphic, Rohrbach's textural name for those minerals in an
igneous rock, whose boundaries are determined by their neighbors. Its
antithesis is automorphic, which see. Xenomorphic is synonymous
with allotriomorphic, over which it has priority. Tsch. Mitt., 1886, 18.

Yogoite, a name suggested by Weed and Pirsson from Yogo peak, one
of the Little Belt Mountains, Mont., for a syenitic rock composed of
orthoclase and augite in about equal amounts. See also sanidinite and
shonkinite. Amer. Jour. Sci., Dec, 1895, 473-479.

GLOSSARY. 179

Zircon-syenite, a name originally given by Hausmann to certain Nor-
wegian nepheline syenites which were rich in zircons. I^ter it was
practically used as a synonym of nepheline syenite, but is now obsolete.

Zirkelite, a name proposed by Wadsworth in 1887 to designate al-
tered, basaltic glasses, in distinction from their unaltered or tachylitic
state. Geol. Surv. Minn., Bull., 2, 1887, p. 30.

Zobtenite, Roth's name for metamorphic rocks with the composition
of gabbros, /. c, rocks not certainly igneous. The name is derived
from the Zobtenberg, aSilesian mountain. Sitz. Berl. Akad., 1887, 611.

Zonal-structure, a term especially used in microscopic work to de-
scribe those minerals whose cross-sections show their successive, con-
centric layers of growth.

Zwitter, a Saxon miner's term for a variety of greisen. Only of sig-
nificance in connection with tin ores.

INDEX.

Note. — The index only concerns the main portion of the book and not the Glossary
Attention may be called to the latter as embracing many rocks not otherwise mentioned

Nev.; Cascade Mts., Oregon ; Edge-
combe Island, Alaska, 42

Basalt, defined, 43

Alterations, Metamorphism, . , , 45

Distribution, 45

Mineralogical Composition, ... 43

Tuffs, 45

Varieties, 43

Basanite, 44

Batholite defined, 12

Becker, G. F., cited on Asperite, . . 41

Metamorphism of Sandstone, . , 68

Pseudodiorite, Analysis, .... 102

Slate, 109

California serpentine, 114

Saprolite, 117

Beemerville, N. J.,hornfels, .... 90

Nepheline syenite, 36

Binary granite, 31

Biotite, 8

Bituminous Coal, ']6

Bosses, defined, 12

Bostonite, defined, 27

Brazil, Weathered rocks of, .... 1 16

Breccias, . 59

Bronzite, 57

Bytownite,

Accessory Minerals, 9

Acmite, 7

Actinolite, 7

^girine, 7

Alabaster, 79

Amphibole, ... 7

Amphibolites, see hornblende-schists, loi

Andalusite, 8

Andesine, 5

Andesites, analyses, Eureka, Nev.;
Yellowstone Park ; Rosita Hills,
Colo.; Lassen's Peak, Calif.; Mt.
Rainier, Wash.; Red Bluff, Mont.;
Buffalo; Peaks, Colo., Colombia,

S. A , 39

Defined, 4°

Anhydrite, 9

Anogene, 13

Anorthite, 5

Anorthoclase, 5

Anorthosites, analyses ...... 49

Defined, 50

Anthophyllite, 7

Anthracite, 76

Apatite, 9

Aplite, 31

Apobsidian, 24

Aporhyolites, 24

Aqueous Rocks, 10, 58

Arfvedsonite, 7

Asperite 41

Atmospheric weathering defined, . . 85

Rocks produced by I16-119

Augen, 31. 52

Augen-gneiss, 94

Augite, 7

Defined, 8

Augite-andesite, 40

Augitite, 45

Bar-theory of Rock-salt, 78

Basalt, Analyses, Cinder Cone, Calif.,
Kilauea, S. I.; Iceland ; Rio Grande
Canon, N. M.; Dalles, Oregon ; Eu-
reka, Nev.; Point Bonita, Calif.;
North Park, Colo.; Shoshone Mesa,

Calcareous sandstone, analyses. Flag-
staff, Ariz.; Jordan, Minn., ... 68
Calcareous shale, analyses, Mt. Morris,
N. Y.; Rochester, N. Y., . . . . 68

Calcite, 9

Calcite anals. , ^o

Calc-schist, loi

Camptonite, 48

Cancrinite, 37

Carbonaceous Organic Pvocks, ... 7^

Metamorphism, 77

Occurrence, ... 77

Chemical Elements in Rocks, ... 3

Chert, analyses, England, 74

Joplin, Mo.; Galena, Kansas ; Bell-
ville. Mo.; Seneca, Mo.; Newton

Co., Mo., 80

Cherty iron carbonates, analyses,

Minn.; Gogebic Range, Mich., . 80

Chlorite, 9

180

INDEX.

I8i

Chlorite-schist, analyses, from Klippe,
Sweden ; Foster Mine, Mich., . . 103

Defined 104

Chromite 9

Qarke, F. W., cited, 3,

Gay,

Brick Clay, analyses, Washington,
Ind.; Salem, Ind.; Plattsburg,
N. Y.; La Salle, 111.; Rondout,
N. Y.; Fisher's Island, N. Y.;

HooversN'ille, Pa., 66

Potter's Clay, Akron, Ohio; East

Liverpool, Ohio, . . .66

Fire C/<y,\Voodbridge, N. J.; Chel-
tenham, Mo.; Woodland, Fa., . 66
Residual Clay, Morrisville, Ala.;

IJatesville, Ark., 66

Conanicut Island, K. I.; contact met-

amorphism, 89

Gjnsanguinity, 57

Contact Metamorphism, internal effects 87

External effects 88

On sedimentary rocks, .... 88, 9 1

Zones, 88

Cornwall, Penn., magnetite ... 91
Crawford Notch, N. H., contact met-
amorphism, 32. 89

Cross, W., cited, 22

Crugers on Hudson, contacts, ... 90

Crystalline Limestones, analyses of,

from Hrandon, Vt.; Carrara, Italy;

KnoxvillcTcnn.; Pickens Co., (ia. ,

Rutland, \'t.; Franklin Furnace, N.

J.; Tuckahoe, N. Y., HO

Mineralogy, HI

Varieties, Ill

Alteration, 112

Occurrence 112

Crystalline Schists, 99

Cyanite, . . 9

Cycle of deposition 61

Dacites, analyses : Comstock Lode,
Nev.; I^ssen's Peak, Cal.; Yellow-
stone Park; Hungary; Eureka,

Nev.; Colombia, S. /\., 39

Defined 40

Dana, J. D., cited, loo

Degeneration II7

Derby, O. A., cited on alteration of |

rocks, 116

Diabases, analyses. Diabase Hills,
Nev.; Jersey City, N. J.; Lake Sal-
tonstall, Conn.; Hoston, Ma.ss.; |

Point Bonita, Cal.; Ausable Forks,

N. Y., 42

Defined, 44 ,

Diallage, 7

Defined, 7i 8

Diatomaceous earth. See Infusorial
Earth, 74

Dikes, defined, 12

Diller, J. S., cited, 45

Diopside, 7

Diorites, analyses, Yellowstone Park ;
Watab, Minn.; Wales; Comstock
Lode ; Little Falls, Minn. ; Custer
Co., Colo.; St. John, N. B.; Forest

of Dean, N. Y., 47

Defined 48

Alteration, ... 48

Distribution 48

Dissolved vapors 15

Ditroite, Anals., 36

Defined, 37

E>olerite, 44

Dolomite, 9

As a Mineral, 9

As a Rock, 70, 95

Crystalline, analyses from Tucka-
hoe, N. Y.; Inyo Co., Cal., . . Iio
Dykes, see dikes.

Eclogite, analysis, Altenburg, Austria, 103

Defined, 105

Effusive, 13

Eleolite porphyr)', analyses, Becmer-

ville, N. J.; Magnet Cone, Ark., . 28

Eleolite-syenite.see Nepheline-syenite, 36

Enstatite. 7

Eolian Rocks, lo, 58

F.olian sandstone 64

Fpidote, 9

Kpidote schist, analysis from South

Mountain, Pa., 103

Defined 104

Essential Mineral's, 9

Extrxisive 13

Feldspars, analyses, 6

Defined, 4

Feldspathoids, 4

Defined, 9

Felsite . . 24

Felsitic texture, 15

Ferromagnesian silicates, defined, . . 6

Ferruginous organic rocks 74

Ferruginous precipitates, 82

Foyaite anals., 3^

Defined 37

Freshwater Limestone, analyses. Chalk

Bluffs, Wyo.; Henry's_Forks,Wyo., 70

Friction breccia 59

Gabbro, analyses. Chateau Richer,
Can.; Montrose Pt. , N. Y.; Croton
River, N. Y.; .Mt. Marcy, N. Y.;
Xain, Labrador; Iron Mt. , Wyo.;
Duluth, Minn.; Baltimore, Md.;

Adirondacks, N. Y.; N. W. Minn., 49

Defined, 50

I«2

INDEX.

Gabbio — Mineralogy, 50

Varieties, , 5°

Alteration, 52

Metamorphism, 52

Distribution, 52

Garnet, 8

Generations of Minerals, 17

Geyserite, analysis, Yellowstone Park, 80

Glasses, 20

Varieties, 21

Relationships, 22

Geological occurrence, 22

Alteration, 22

Distribution, 22

Glauconite, 69

Glaucophane-schist, analysis from

Monte Diablo, Cal., 103

Defined, 105

Gneisses, 95

Defined and classified, .... 95, 96
Analyses from Black Hills ; Mun-
son, Mass.; Iron Mtn.; Wyo. ;
Trembling Mtn., Quebec; St. Jean
de Matha, Quebec; New York City ;

Randon, Quebec, 97

Alteration, ..." 98

Distribution, 98

Gordon, C. H., cited 95> 96

Granite, analyses, Green's Landing,
Me.; Peterhead, Scotland ; West-
erly, R. I.; Stony Point, Conn.;
Crawford Notch, N. H.; .Cotton-
wood Canon, Utah ; Chester, Mass ;
Raleigh, N. C; Eureka Dist., Nev.;
Kekequabic Lake, Minn., Row-

landville, Md. ; Yosemite, .... 30

Defined, 31

Alteration, Metamorphism, . . 33

Distribution, 34

Geological Occurrence, 32

Mineralogical Composition, ... 31

Relationships, 32

Uses, 33

Varieties, . 31

Granite porphyry, 24, 31, 32

Granitoid texture, 13

Grano-diorites, 32

Granulite, 98

Graphic granite, . ' , . 31

Graphite schist, 105

Gravels, 62

Greensand, 69

Greenstone, 44

Greisen, 32

Groundmass, 13

Gypsum, 9> 78

Halite, 9

Hawes, G. W., cited, 32, 89

Hematite, 9

Hoboken, N. J., contacts .... 89

Hornblende, 7, 8

Hornblende-granite, 31

Hornblende-schist, loi

Analyses from Grand Rapids, Mich. ;
Knoxville, Cal.; Lower Quinnesec
Basin and Falls, Wis.; Cleveland

Mine, Mich., . 102

Mineralogy and varieties, .... 102

Alteration, 103

Occurrence, 103

Hornblendite, 51

Homfels, 88

Hyalomelane, analysis, 20

Hydroniica schist, lOo

Hypersthene, 7

Hypersthene-andesite, 40

Hypersthene fels, . , 5^

Hypersthene rock, 5 1

Iddings, J. P., cited, ...... 22, 36

Ilmenite, 9

Infusorial earth, analysis, Little Truc-
kee River, Nev.; Fossil Hill, Nev.;

Richmond, Va., 74

Intratelluric, 15

Itacolumite (See especially p. 107), . 65

Kaolin, 9» 66

Kaolin, anals., 66

Keratophyre, 27

Kimberlite (see errata), •. 5^

Knotty schists, ......... 89

Knotty slates, 89

Labradorite, 5

Labradorite-rock, 5 1

Laccolite, defined, 12

Laterite, 1 16, 117

Laurdalite, anals., 36

Lavas, 19

Leucite, 6

Leucite-basalt, 44

Leucite-basanite, 44

Leucite-phonolite, anals., Rieden, Ger-
many, 28

Defined, 29

Leucite-syenite, anals., 36

Leucite tephrite, .... ... 44

Leucitophyre 29

Lignite, 76, 77

Limburgite, analyses, Bozeman, Mont. ;

Palma, Italy, 42

Defined, 45

Limestone, analyses, Adams, Mass.;
Bedford, Ind.; Solenhofen, Ger-
many; Hudson, N. Y.; Point Pleas-
ant, Ohio; Bonne Terre, Mo.;

Chicago, 111., 70

INDEX.

183

Limestone — Metamorphism, .... "3

Mineralogy', 72

Occurrence, 72

Origin 7*

Varieties, 72

Limestones, Crystalline, no

Limonile, 9

Liparite, defined, 24

Litchfieldite, anals. , 36

Defined, 37

Lithophysae, 21,25

Living organisms, analyses of calcare-
ous parts, 70

Loess, 65

Luxulianite 32

Lyell, Sir Charles, cited on Metamor-
phism, 84, 85

Magnesian Limestone, 72

Magnetite 9

Malacolite, 7

Marble, 5ce Crystalline Limestone, . ill
Marls, analyses. Hop Hrook, N. J.;
Red Hank, N J. ; Bowling Green,

Ky. 68

Melilitc . 6

Melilite basalt, 45

Merrill, C«. 1'., cited on,

Alteration of (iranite, 33

(^phicalcite, 114

Degeneration 1 17

Mctain<>q)hic Kocks, 1 1. 84

Determination of 1 19

Metamorphism, defined 84

Contact metamorphism defined, . 85

Described, S5-87

Rocks protluced by, . . . 87-92

Regional metamorphism, .... 93

Meteorites, • 53

Microlitic, 13

Mica, 8

Mica-andesite, ... ... 40

Mica schists, analyses from Monte
Rosa, .Switzerland; Zemiatt, Swit-
zerland ; Urixen, Tyrol ; Meissen,
Saxony; Crugcrs, N. Y.; New-
Hampshire; Messina, Sicily; NVis

consin , 99

Mineralogical composition, . . . 100

Varieties lOO

Alteration, loi

Distribution, • loi

Microcline, 5

Minerali/.ers .... 15

Minette, analysis, from Rhode Island, 34

Mount Willard, N. IL, 32, 89

Muscovite 8

Necks, defined, 12

Nepheline, 6

Nepheline-basalt, analysis, Pilot

Knob, Texas, 42

Defined, 43

Nepheline- syenite, analyses, Litch-
field, Me. ; Fourche Mtn., Ark. ;
RedMtn., N. H.; Diuo, Hungary;
Foya, Portugal ; Sao Paulo, Brazil ;

Lund, Norway ; Beemerville,N. J., 36

Mineralogy, 36

Relationships 37

Alteration, 37

Distribution, 37

Nevadite. 24

! Norite, analysis, 49

Defined, 51

Novaculite, analysis, 63

\ Defined, 64

I o

I Obsidian, analyses, Tewan Mtns. ;
i Yellowstone Park ; Mono I^ake ;

Lipari Island ; Clear Lake. Cal., . 20

i Defined 12

Obsidian Cliff, cited, 14

{ )chsenius, cited, 78

I Oligoclase, 5

I Olivine 8

Olivine-dialmse 44

Ophicalciles, 112

1 Analyses from Oxford, Quebec ;

I Brompton I^ke, (Quebec. . . . 112

Mineralogy, 113, 1 14

.Mtcration II5

Distribution, . . . • . . II5

Orbicular granite, ],2

j Organic Remains not Limestone, . . 74

Orthoclasc 4

Pantellerites 24, 27

Paramorphism 45

I Pearlite or Perlite, analyses, Hungary ;

' Kureka, Nev. 20

Defined 21

Peat, . . 76, 77

Pegmatite 31

Pele's Hair, anals .20

Peridotite, analyses, Montrose Point,
N. Y.; Custer Co., Colo.; Baltimore,
Md., Dewitt, N. Y.; Crittenden Co.,

I Ky.; Klliott Co., Ky., 49

Defined, . . 51

( For varieties see Glossary. )

Petrographic Provinces 57

I Phanerocryst, 13

Phenocryst, 13

I Phlogopite, 8

j Phonolites, analyses. Devil's Tower,
\Vyo. ; Fl Paso Co., Col.; Fernando
de Noronha, Brazil ; Zittau, Saxony;

Wolf Rock, England, 28

J Defined 29

1 84

INDEX.

Phonolites — Mineralogy, 29

Relationships, 29

Alteration, 29

Tuffs, 30

Phonolite-obsidian, analysis, , . . , 20

Phthanites, 68

Phyllite, loi, 109

Pirsson, L. V., cited, 89

Pitchstone, analyses, Meissen ; Silver

Cliff, Colo., 20

Defined, 21

Plagioclases, 4

Defined, , . . , 5

List of, 5

Plutonic, 13

Porphyrites, 40

Porphyritic texture, .13

Porphyry defined, , . . . 27

Precipitates from Solution, 77

Pressure as influencing texture, ... 15

Primary Minerals, 10

Principles underlying the Classifica-
tion of Rocks, 10

Propylite, 41

Pumice, analyses, 20

Pyrite, 9

Pyroxenes, 7

Tabulated, . 7

Pyroxenite, analyses, Webster, N. C. ;
Baltimore, Md.; Meadow Creek,

Mont.; Montrose Point, N. Y., . . 49

Defined, 51

Pyrrhotite, 9

Quartzites, analyses from : Chickies
Station, Pa.; Quarry Mtn., Ark.;

Pipestone, Minn.; 106

Mineralogy, 106

Varieties, . . 106

Alteration, 107

Distribution, 107

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