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105 12) ,56 2' 



55th Gokobbss, ) HOUSE OF EBPEESBNTATIVES. (Dooument 
2d Session. J \ l^o. 667. 



DEPAETMBNT OP THE Il^TEEIOB 



BULLETIN 



OF TBM 



UNITED STATES 



GEOLOGICAL SURVEY 



N"o. 150 



TBK BDUCAT70KAL SERIES OF ROCK SPECIMENS COLLEGTEB 
AND DISTRIBUTED BY THE UNITED STATES 
GEOLOGICAL SURVEY.^Diujeb 



* 



WASHINGTOir 

GOTEENMENT PRINTING OFFIOB 

1898 



VBmy OP mt 

iiUHfO BTAMFORD JR. UmERSITY. 






DEPABTMENT OF THE IHTEBIOH 



BULLETIN 



UNITED STATES 



GEOL0GICi\;^ ^pRVET 



N"©. 150 




WASHINGTON 

OOVEBNHENT PRINTING OFPIOE 

1898 



(JNITET> STATES GEOLOGICAL STTBVEY 

CHABLBS I). VAtAXyiT. UIBEOTOB 



EDUCiTlONAL SERIES OF ROCK SPECIMENS 



COLLECTED AND DISTRIBUTED 



JOSEPH SILAS IJILLEB 




WASHINGTON 

OOVEBNHENT PBINTnfG OPPICB 

1898 



CONTENTS. 



Letter of tranemittal 9 

Preface 11 

The study of rocks 13 

Introduction 13 

Strnotnral featares of rocks 13 

Methods of physical analysis 18 

Specific- grayity method 19 

Electro-magnetic method •. 22 

Microscopic method : 23 

The principal rock-making minerals 27 

Minerals illustrated hy this series of rock specimeus 29 

Minerals of the isometric system 30 

Minerals of the tetragonal system 33 

Minerals of the hexagonal system 34 

Minerals of the orthorhomhic system 37 

Minerals of the monoclinic system 40 

Minerals of the triclinic system 45 

Homogeneous aggregates 46 

Classification of rocks 48 

Tahular view and classification of the rocks of this series 52 

DeHcriptions of specimens 56 

Unaltered sedimentary rocks of mechanical origin (Nos. 1-22) 56 

Unaltered sedimentary rocks of chemical origin (Noh. 23-38) 91 

Unaltered sedimentary rocks of organic origin (Nos. 39-56) 1 15 

Unaltered igneous rocks (Nos. 57-114) 145 

Granite-rhyolite family (Nos. 58-69) 146 

Syenite-trachyte family (Nos. 70-71) 181 

Nephelite-leucitc rooks (Nos. 72-78) 186 

Diorite-andeeite family (Nos. 79-94) 211 

Oahbro-basalt family (Nos. 95-109) 245 

Peridotite family (Nos. 110-114) 286 

Metamorphio sedimentary rocks (Nos. 115-135) 298 

Metamorphic igneous rocks (Nos. 136-146) 343 

Residual rocks (Nos. 147-150) 376 

ninstrations of surface modifications (Nos. 151-156) 385 

Index 393 

5 



ILLUSTRATIONS. 



Page. 
Puts T. Bedded stmctnre, bedding, and cross bedding, Oak Canyon , Arizona . 14 

II. Massive structure in dioritey Shasta County, California 16 

IIL Rhonibohedral jointing in gneiss, Potomac River near Washington, 

District of Columbia 18 

IV. Columnar jointing, Watchung Mountain, Orange, New Jersey 20 

V. Petrographic microscope 24 

VI. High part of South Manitou Island, Lake Michigan, showing forest 
formerly buried beneath drifting sand and now exposed by eolian 

erosion 62 

VII. A typical exposure of southern loess, Muscatine, Iowa 66 

VIII. Clay bank at Brick Haven, Virginia, opposite Washington, Dis- 
trict of Columbia 68 

IX. Thin sections of sandstone: A, pebbly sandstone ftom Barron, Ore- 
gon, as seen under a microscope in ordinary light; B, brown 
sandstone from Hnnmielstown, Pennsylvania, as seen under a 

microscope in ordinary light 76 

X. Thin section of Potsdam sandstone fh>m Ablemans, Wisconsin, as 
seen under a microscope : J, in ordinary light; B, between crossed 

nicols 80 

XI. Ripple marks on sand dnnes near Golden Gate Park, San Francisco, 

California 82 

XII. Ripple marks cut by mud cracks filled with sand, from the Grand 

Canyon of the Colorado, Arizona 84 

Xni. Thin section of graywacke from Hurley, Wisconsin, as seen under 

a microscope in ordinary light 86 

Xiy. Old Faithful geyser in action, Yellowstone National Park 92 

XV. Vein of quartz in gneiss near Chain Bridge, District of Columbia. . 94 
XVI. Thin sections of siliceous oolite from Center County, Pennsylvania, 
as seen under a microscope: A^ in ordinary light; B^ between 

crossed nicols 96 

XVII. Deposits in Marengo Cave, Indiana 98 

XVIIL Deposits in Luray Cave, Virginia 100 

XIX. Terraces of Mammoth Hot Springs, Yellowstone National Park.... 102 

XX. Concretions of marcasite from Thatcher, Colorado 110 

XXI. Fossil trees, Yellowstone National Park 114 

XXII. Section of a flint nodule from Austin, Texas 120 

XXIII. Beds of fossiliferons iron ore between beds of limestone, Rochester, 

New York 138 

XXrV. Dike of andesite cutting andesitic breccia, Absaroka Range, W^yo- 

ming 146 

XXV. Lithophysa) in lithoidite of Obsidian Cliflf, Yellowstone National 

Park; natural size 154 

XXVI. Sections of spherulites, from Yellowstone National Park; figs. 1-7. 156 
XXVn. Branching crystals of orthoclase, Manebacher twins, from Yellow- 
stone National Park 158 

XXVUI. Sections of metarhyolite from Utley, Wisconsin: /i, secondary 
growth of mi croperthite about plagioclase in polarized light; 

Bf phonocryst of microperthite in polarized light 168 

7 



8 ILLUSTRATIONS. 



Pl. XXIX. Thin section of nephelite-syenite from Litchfield, Maine, as seen 

under a microscope between crossed nicols 204 

XXX. Thin section of dacite f^om Bear Creek, California, as seen under 

a microscope 214 

XXXI. Nodules in dacite, Lassen Peak, California 218 

XXXII. Thin section of hypersthene-andesite from Mount Shasta. Cali- 
fornia, as seen under a microscope 228 

XXXIII. Vesuvius in eruption, April 26, 1872 246 

XXXIV. Cinder Cone, 10 miles northeast of Lassen Peak, California 248 

XXXV. Volcanic bombs at base of Cinder Cone, 10 miles northeast of 

Lassen Peak, California 250 

XXXVI Surface of lava field 10 miles northeast of Lassen Peak ; latest 

lava, not covered by sand 252 

XXX VI I. Sections of quartz grains and surrounding shells of glass and 

augite, from same locality 254 

XXXVIII. Thin sections as seen under a microscope : J, oli vine-diabase from 
Pigeon Point, Minnesota ; B, gametiferous gabbro from Granite 

Falls, Minnesota 278 

XXXIX. Thin sections of kimberlite fVom Elliott County, Kentucky, as 

seen under a microscope 292 

XL. Thiu section of marble f^om Cockeysville, Maryland, as seen under 

a microscope : A, in ordinary light ; By between crossed nicols. . SOO 

XLI. Syncline and anticline , 316 

XLII. Thin section of tourmaline-biotite schist from the Black Hills, 
South Dakota, as it appears under a microscope in polarized 

light 328 

XLIII. Thiu section of aporhyolite from South Mountain, Pennsylvania, 

showing altered spherulites, as seen under a microscope 346 

XLI V. Thiu sectious of aporhyolite : A, from Lake Superior, Minnesota, 
showing pcrlitic structure; J^, from South Mountain, Pennsyl- 
vania, showing perlitic, spherulitic, and lithophysal struct- 
ures 348 

XLV. Thin sections : A , quartz-nori te-gneiss from Od essa, Minnesota ; B, 

hornblende -gabbro-gneiss from Minnesota Falls, Minnesota 360 

XLVI. Spheroidal weathering in granite near Woodstock, Maryland 386 

XLV II. Irregular weathered surface of Cambrian limestone at York, 

Pennsylvania.. 388 

Fig. 1. Pegmatitic structure 17 

2. Smeeth separating apparatus 20 

3. Westphal balance 22 

4. Balsam bottle 26 

5. Wire saw. United States Geological Survey 26 

6. Grinding apparatus, United States Geological Survey 27 

7. Section of coast, showing beach and sediments 57 

8. Sliella in marine sand at Carter's Grove, on James River, Virginia 65 

9. Brecciated Devonian limestone, Fayette, Iowa 73 

10. Section of vein 94 

11. Travertine fan, Mammoth Hot Springs, Yellowstone National Park. .. 100 

12. Cells of silicified wood, seen under a microscope 114 

13. Globigerina and Textularia from the chalk of White Cliff, Texas... 116 

14. Globigerina ooze, from Indian Ocean, at a depth of 1,800 fathoms 117 

15. Fragments of volcanic ghiss in tuff, as seen under a microscope 212 

16. Pyrope, showing border of biotite and magnetite 293 

17. Cross section of staurolite from staurolitic-mica-sohist 334 

18. Glaciated rock surface showing frontal and lat'eral grooves caused by 

a projecting nodule of harder rock 389 



i 



LETTER OF TRANSMITTAL. 



Dbpabtment of thb Interior, 
United States Geological Sueyby, 

Wdshington^ D, 6\, November P, 1897, 
Sir: I have the honor to sabmit herewith the manuscript and illus- 
trations for a bulletin to accompany the Educational Series of Eock 
Specimens. 

Very respectfully, 

J. S. Diller, 

Geologist, 
Hon. Charles D. Walcott, 

Director United States Geological Survey. 



PREFACE. 



It was early perceived that the field parties of the Geological Survey 
oocasioDally liad favorable opportunities for the collection of important 
rock types not^ readily accessible to others, and that, by systematic 
oooperatioii, tliese parties, without much additional expense, could in 
the course of years gather a large collection of duplicate type speci- 
mens of Tocks ^wliich would be valuable to educational institutions for 
the purposes of teaching. 
This collection was started in 1882, under the directorship of Maj. 
J.W.Powell, and at first contemplated 200 suites containing only 100 
speoimeBB eacli of typical rocks belonging about equally to the two 
gieat gronpa, sedimentary and igneous. 

To Messrs. Becker, Hague, and Emmons was assigned the collecting 
of the crystalline rocks, while the others were to be obtained by Messrs. 
Gilbert, Powell, Ohamberlin, Hague, and Kerr. The work progressed 
steadily for several years, but, in order that important types not met 
by any of the field parties in their regular work might be obtained, it 
was found necessary to assign more definitely the task of completing 
tbe series and preparing it for distribution. 

Although the responsibility for the selection, numbering, and arrange- 
ment of the series rests almost wholly with the present writer, these 
were determined after much correspondence and conference with his 
colleagues and were controlled largely by circumstances. Although 
most of the specimens in this collection are intended to illustrate rock 
types, there are a number — such as concretions (33-36), dike(57), jointing 
(103, 123), and others — intended to illustrate special ieatures, and it has 
been thought best to treat each with its kind among the classified rocks 
rather than under a separate heading. 

Acknowledgments are due to nearly all the geologists of the Survey 
and to many persons outside who have generously aided in collecting 
specimens within their reach. Special acknowledgment should be 
made to Prof. 0. H. Hitchcock, who collected all the specimens from 
New Hampshire. 

In the preparation of this bulletin, 29 specimens have been described 
by Prof. J. P. Iddings, of the University of Chicago; 16 by Prof. W. 
8. Bayley, of Oolby University, Waterville, Maine; 9 by Mr. Whitman 
Cross, of the United States Geological Survey; 5 by Mr. George P. 

Herrill,of the United States National Museum ; 5 by Prof. J. E. Wolff, of 

11 



12 PREFACE. 

Harvard University, Gambridge, Maasachnsetts; 4 by Mr. George Otis 
Smith, of the United States Geological Survey; 3 by Mr. G. K. Gilbert, 
of the United States Geological Survey; 3 by Mr. Waldemar Lindgren, 
of the United States Geological Survey; 3 by Prof. C. R. Van Hise, of 
the State University, Madison, Wisconsin ; 2 by Mr. Bailey Willis, of 
the United States Geological Survey; 2 by Mr. H. W. Turner, of the 
United States Geological Survey; 2 by Mr. Walter Harvey Weed, of 
the United States Geological Survey; 1 by Prof. E. B. Matthews, of 
the Johns Hopkins University, Baltimore, Maryland; 1 by Prof. Flor- 
ence Bascom, of Bryn Mawr College, Bryn Mawr, Pennsylvania; 1 by 
Prof. L. Y. Pirsson, of the the Sheffield Scientific School in Yale Uni- 
versity, New Haven, Oonnecticut; and 1 by Mr. S. Weidman, of the 
University of Chicago; in all of which cases the descriptions are duly 
accredited in the text. All of the descriptions not thus accredited and 
the other portions of the bulletin have been prepared by the present 
writer, who desires to acknowledge the aid he has received from 
Microscopical Physiography of Bock-making Minerals, by J. P. Iddings 
(translation of Rosenbusch) ; the excellent Handbook of Rocks, by J. F. 
Kemp; Petrology for Students, by Alfred Harker; Stones for Building 
and Decoration, by G. P. Merrill ; Rocks, Rock- weathering, and Soils, 
by the same author, as well as to the more complete works on petrog- 
raphy, especially those of Rosenbusch and Zirkel. 

J. S. DiLIiEB. 

JxmE 9, 1897. 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS. 



By J. B. DiLLEB. 



THE STUDY OF ROCKS. 

INTRODUCTION. 

A rock is a large mass of mineral matter forming an essential part of 
the earth. Granite and sandstone are rocks made np of a number of 
minerals, but in other rocks, as, for example, limestone and serpentine 
▼hen pare, the mass is composed wholly of one mineral. In still other 
exceptional cases, like obsidian, the mineral matter may be in a wholly 
amorphous condition, like glass — not made np of any definite min- 
eral or minerals. The material of which the rock is composed may 
be loose, as sand and gravel, or cemented (lithified), as sandstone and 
conglomerate. 

Rocks may be studied (1) in the field, as large masses, where their 
greater structural features and relations to other rocks are discovered; 
and (2) in the laboratory, where only hand specimens (stones) are 
available for investigation. The science of rocks is petrology. It is the 
branch of geology that treats of the origin, mode of occurrence, struc- 
tore, composition, and alteration of rocks. The branch of petrology 
that embraces a knowledge of the structural (megascopic and micro- 
scopic), mineralogic, and chemical characters of rocks is petrography^ 
The knowledge of rocks especially as masses of minerals, acquired 
largely by microscopic and other laboratory methods, is called by some 
li^logy. 

It is hoped that the lithology and petrography to be learned in 
studying this educational series of rock specimens may arouse an 
interest in the study of petrology, and also in that of geology, of which 
the former is a part. 

STRUCTURAL FEATURES OP ROCKS. 

Among the first features of rocks noted by an observer are those 
arising from structure. They may be megascopic — that is, large enough 
to be seen by the unaided eye— or microscopiCj visible only with the aid 
of a microscope. 

FragmefUal or elasticy brecciatedj agglomerated^ conglomerated^ pebbly ^ 
granular^ cryptoelastic or compa^t^ horny. — Specimens 10, 11, and 12 are 
all made of fragments and illustrate a kind of structure characteristic 

13 



14 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [bull. 150. 

of Buch rocks, viz, clastic. Glastic rocks composed of fragments of 
igneous rocks are said to be pyroclastic. Specimens 79 and 99 are 
pyroclastic. Glastic orfragmental strnctore, as it is sometimes called, 
may be coarse or fine^ according to the size of the component parts, 
and other designations may be applied to it on account of variation in 
the shape of the fragments. When the fragments are angular the 
structure is hrecciated (11). If they are coarse, with occasional large 
blocks of many shapes and sizes irregularly intermingled with finer 
material, the structure is agglomerated. When the fragments are dis- 
tinctly rounded pebbles the structure is conglomerated (10). Such 
pebbles intermingled with sand produce pebbly structure (12). Bock 
composed of sand, either angular or well rounded, has psammitie or 
sandstone-like structure (12-19). It is also called granular (fragmental 
granular) ; but this term has a wider application than the others, for 
granites (67), diorites (94), gabbros (109), and similar igneous rocks, 
whose mineral constituents form distinct, angular, often interlocking 
grains, are also granular (crystalline granular), but their fundamental 
structure is crystalline and not fragmental^ as is the case with sandstone. 

In specimens 21 and 22, whose fragments, although visible in thin 
sections under the microscope, are so small that they can not be seen 
by the unaided eye, the structure is cryptoclastic or computet. The 
structure of specimen 134 is more than compact; it is esi)ecially fine 
and close, with the great hardness and conchoidal fracture of flint, due 
to the induration it has experienced in contact with an igneous rock. 
It« structure is horny or flinty. 

Stratified, banded, cross bedded^ laminated. — Most fragmental rocks 
are formed in water, and the material of which they are composed is 
stratified, i. e., arranged in layers and beds (strata), as illustrated in the 
accompanying PI. I, which represents a series of parallel horizontal 
strata consisting chiefly of sandstones. A stratum may be thick or 
thin, and may contain one or more layers. In specimen 16 an indistinct 
banded structure is due to the presence of differently colored layers in a 
larger stratum of sandstone. A similar structure is produced in igne- 
ous rocks by flowing (see below). The layers are usually parallel to 
the stratification, but in some cases, as in the lower beds of PI. I, the 
layers run diagonally across the stratum and produce cross bedding. 
The bedding may be thick and without layers or it may be thin, as in 
PI. I, and also in specimen 17, where the fine material is arranged in 
sheets so thin that it is called laminated. This structure is often well 
illustrated in deposits of sedimentary clay. 

Unstratified or massive. — Successive flows of lava, where they spread 
over a fiat country, arrange themselves in sheets so as to show a bedded 
structure similar to that of stratifled rocks, but igneous rocks gener- 
ally, such as gabbros, diorites (Pi. II), granites, etc., show no such 
arrangement. They are unstratified — that is, massive. 

Flow structure, streaked. — Ehyolites and other acid lavas are usually 




aCOOEil STBUCrURE. 



awa.] STRUCTURAL FEATURES OF ROCKS. 15 

visoons at the lime of their emption, and occasionally preserve lines 
produced by flowing. Specimen 80 shows a streaked arrangement of its 
black glass, due to this cause. The elongated parts may be drawn out 
80 as to form bands and produce banded structure. All structures of 
this kind are included under the general synonyms fltixion structure, 
Jluidal structure, ekuAflow structure. When not visible to the unaided 
eye it may frequently be seen under the microscope in the stream-like 
arrangement of the small crystals and other material variously colored. 
Parous^ cavemouSy cellular^ pumiceouSy vesicular — Specimens 23 and 29 
are fall of small irregular cavities, and their structure is porous. Both 
are hot-spring deposits, and the porous structure originated at the time 
the rock was formed. In si)ecimen 153 the cavities are larger. It is 
eaf emoiw. Similar structures occur in acid volcanic rocks. The si)ecial 
form illustrated by specimen 100 occurs most frequently in basic lavas. 
The cavities are nearly round cells or vesicles, produced by expanding 
Tapor contained in the molteii rjock material at the time of its eruption. 
This structure is cellular. Sometimes the lava is so inflated by expanding 
vapors as to be froth-like, pumiceausj as in specimen 59. Pumice is 
uaoally a product of highly explosive volcanic eruptions. Vast quan- 
tities of it were thrown into the air by the great eruption of Krakatoa 
in 1883. When there are but few cells, so that tbey appear as separate 
vesicles, the rock is vesicular. The cells and vesicles of lava are formed 
while it is yet viscous, and if the lava afterward moves, the cells will 
be elongated in the direction of motion. 

AmygdaloidaL — Aiter vesicular lavas solidify, percolating waters 
freqaently deposit mineral matter of various kinds in their cavities. 
Quartz, calcite, zeolite, etc., are deposited in the cells, producing, as in 
specimen 139, an amygdaloidal structure. The almond-shaped kernels 
filling the cells are known as amygdules. 

Vitreous or glassy yperliticj devitrified. — Specimen 60 illustrates vitreous 
OF glassy structure, so named on account of similarity to artificial glass. 
It is limited to igneous rocks, and results from the chilling of molten 
rock material {magma) so suddenly as not to permit an appreciable 
amoant of crystallization before it becomes solid. Specimen Gl is almost 
wholly glass, and is divided by sets of more or less concentric fissures, so 
as to give it the globular {perlitic) structure characteristic of perlite. 
Some very ancient acid volcanic rocks which were originally glassy have 
by slow crystallization gradually lost their glassy character and become 
demtrified. Specimen 136 illustrates this feature. The minerals devel- 
oped are chiefly quartz and feldspar. 

CrystaUineyparphyritiCj holo'crystalline, hypocrystallinej avwrphous^ lith- 
oidaly even-crystalline granular, — In specimen 90 the prominent white 
portions are feldspar crystals, the dark blade-shaped crystals are horn- 
blende, and the hexagonal ones are mica. These forms result from crys- 
tallization. The rock structure produced by crystallization is crystalline, 
a feature which is common to many igneous and metamorphic rocks. 



16 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [BULL.m 

Specimen 90 is a good example of one type of crystalline rock. The feld- 
spar, hornblende, and mica each has its own i>eculiar form, i. e., is idio- 
morphic. The gray portion of the rock appears uniform to the unaided 
eye. It is the groundmms^ in which the prominent crystals {phenacrysU) 
are embedded, producing the porpkyritie structure. In a thin section of 
90, it may be seen under the microscope that all the gray matter of the 
groundmass is crystallized. There is no amorphous matter {b€ise) pres- 
ent; the rock is completely crystalline, i. e., holocrystalline. Granites, 
syenites, diorites, and similar igneous rocks, as well as gneiss, mioa- 
schist, and others among the metamorphics, are holocrystalline. The 
other extreme is represented by obsidian (60), in which there may be 
no visible crystallization, its structure being amorphous. Many lavas, 
such as 62, 63, 82, 86, 101, etc., contain various proportions of both crys- 
talline and amorphous material, and their structure is said to be hypo- 
crystalline. Si>ecimeu 62 is an acid lava having a low degree of crys- 
tallization. On account of its compact, stony character the structure is 
called lith^oidal. In granite the minerals crystallized under circum- 
stances that made them interfere with one another and prevented the 
development of perfect crystals, such as produce porphyritic structure. 
On the other hand, the mineral grains are angular, interlocking, and of 
approximately equal dimensions, so that the rock is even-crystalline 
granular. Even-crystalline granular structure is so well illustrated by 
granite (66-69) that it is often called granitic or granitoid. The various 
minerals, instead of being bounded each by its own peculiar form (idio. 
morpbic), as are the phenocrysts in porphyritic rocks, have irregular 
angular forms determined by interfering crystallization ; each takes the 
form imposed on it by its growing neighbors, and is allotriomorphic. In 
specimen 90, among the phenocrysts of feldspar, hornblende, and mica 
there are occasional prominent round grains of qaartz embedded in the 
gray groundmass. These grains of quartz differ from the other pheno- 
crysts in not being idiomorphic. They lack the crystalline form peculiar 
to quartz and are said to be anhedral. 

Phanerocrystalliney cryptocrystalline^ compa^^t, aphanitic. — The series 
of terms already noted designate structures arising from varying 
amounts of crystalline and amorphous material. There is still another 
series (besides coarse and fine), to designate the size of the crystallized 
grains. When the particles are sufficiently large to be visible to the 
naked eye the structure is phanerocrystalline. When the particles are 
so fine as to be visible only with the aid of the microscope the structure 
is microcrystalline or cryptocrystalline. Very fine-grained igneous rocks, 
like fragmental ones, are said to be compact. In diabases and diorites 
compact structure has been called aphanitic, 

PegmatitiCj granophyricj graphic. — ^In granite, quartz-porphyry, and 
rhyolite, quartz aud feldspar sometimes intercrystallize in parallel posi- 
tions so as to produce, in certain sections, a graphic appearance. The 
structure is pegmatiticj granophyricy or graphic^ and is well illustrated 



STRUCTURAL FEATURES OF R0CE8. 



17 



in paphic gratiito (&g. 1), of vbich there is no Hpecimen in the collec- 
tion. Wheo the stmctare in so fine as to be visible only ander the 
mienHcope, it is micropegmatttic, mierogranophtfric, or micrographM, 

Spliervlites and lithophyaa. — In some acid lavae there is a low grade 
of crjstalhzation, illaatrated by specimen 62, in which there is a more 
or leas distinct radial arrangement of the crystalline particles and 
derebpment of spheroidal bodies {spherulitea). These are more dis- 
tinct ander the microscope between crossed nicols, when each is marked 
bya black cross. The hollow sphenile8(I*th<>pAy«(e)cont^ned in speci- 
men 63 form its most oonspimons fBatnre. The lithophysffi often show 
conceu trie sheUs, and arelined by a multitude of minate crystals. Like 
tpheralites, they are found chiefly in acid lavas, especially in the 
bthoidal form of rhyolite. 

MieroUtet or cryatalliteSf and globuUtea. — Orystale vary in size, and may 
beBogmallas to be visible only nnder the microscope. Extremely miunte 
eryUatgare called microUtetor cri/atailitea. Feldspar and aagiteoccnr as 
micrDlitee. Generally 

nierolitea are acicn- |lP^^^Biy^^i!l|!Wi^ W'*4 ^^^^ 
lar, ot rod-shaped, gTja^ t j/fTWP**! ^ '' ■ ■ V ^^ 
md, nIthOQgh crys- 
talliiol and dimbly 
rBfracting, are not 
determlQable miner- 
abgically. Besides 
tbese there are other 
"tremely minnte 
Mes which do not 
possess crystallo- 
Erapbic form and do 
Dot seosibly affect 
polarized light. The simplest of these are ronnd and are termed globu- 
iilt». They are common in hypocrystalline lavas, especially basalt and 
rhyolite. Branchiug hair-like forms are called triekttes and straight 
ones belonites. 

Concretionary, oolitio. — ^The spheroidal forms lately noted are found 
ID igneous rocks. Forms somewhat similnr, but of different origin, ture 
fonndin stnitifled rocks. Specimen 3i shows a piece of fern embedded 
in Band, which has been indurated in nodular form about the fern by 
tie deposition of carbonate of iron collected from the surrounding rock. 
Sncb -A structure, formed about a center by additions to the outside, is 
fiKretionarg. Specimens 33 and 35 also are concretions; so is each 
oftbe many little round grains in specimen 30, as well as in 26 and 31. 
^ben, as in 26, they are abundant and so small that the mass resembles 
tbe roe of fish, the stroctore is oolUic. 

Seeretionary, geode. — A nodule that is hollow and lined with crystals, 
*B specimen 36, in a geode. It is a secretwn, and is formed In a cavity 
Ball. 160 2 




no. 1 FegmftUtlg alractaTe of gnphlc gnnlM. The dark anM 

■re qaftrts; tb« light, feldspar- The fl^rewu prepArad tntia % 
■pHlmen iDHiied b; Ur. O. P. Merrllt, of ths If itloDBl Mnwiani. 



18 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [buix.1W. 

by coating its walls and then gradually making additions on the inside. 
Amygdules, as illustrated in specimen 139, are secretions and are of the 
same general character as the geode. 

Shaly structure^ slaty cleavage^ foliated or sehistosej schiatj gneiss. — 
Some fine-grained fragmental rocks, whether laminated (17) or not 
(21 and 22), may be much more easily split into irregular chips or plates 
parallel to the stratification than perpendicular to it, and the structure 
that renders them thus fissile, being characteristic of shales, is called 
shaly structure. Specimen 122 may be split readily into thin sheets, 
like roofing slate (122). The character that renders such splitting 
possible is called slaty cleavage^ since it is a common feature of slate. 
Shaly structure is parallel to the stratification and is determined in the 
deposition of the material, but slaty cleavage may be parallel or inclined 
to the plane of stratification, and is induced by movements in the mass 
due to pressure in the process of mountain building. (See description 
of specimen 122.) Specimen 129 splits readily in one direction and'yet 
it possesses neither shaly structure nor slaty cleavage, but has what 
is usually denominated foliated or schistose structure. This fissility is 
due to the parallel, more or less lenticular arrangement of foliated and 
other minerals. The plane of easiest splitting is through the sheets or 
lenses of the foliated mineral. Foliated structure is a common feature 
of metamorphic rocks, both sedimentary and igneous. When the 
foliated structure is coarse the rock is gneissy and when fine the rock is 
schist Formerly the term gneiss had a mineralogic significance, but 
here, following Gordon ^ and Kemp,^ the writer uses it in a structural 
sense only. Gneiss is well illustrated in the collection by specimen 
128, which may be called conglomerate-gneiss as well as metamorphic 
conglomerate. Specimens 137 and 138 also illustrate gneiss, and 129, 
131, and 133 are good examples of schist. 

Jointingy rJiomhohedraly columnar, — Bocks are frequently intersected 
by divisional planes called jointSy which divide them into more or less 
regular bodies. PI. Ill represents jointing in metamorphic rocks, in 
which the several systems of fissures divide the rock into rhombohedral 
bodies, illustrated by specimen 123. In igneous rocks the jointing gives 
rise to columns (PI. lY ), like those of the Giant's Causeway and Fingal's 
Gave. Specimen 103 is a small column. 

METHODS OF PHYSICAL ANALYSIS. 

Within the last thirty years great advances have been made in the 
study of rocks, especially in the appliances for their detailed chemical 
and physical investigation in the laboratory. Chemical analysis gives 
the sum total of the chemical elements they contain, but in most cases 
this does not disclose the mineral composition of the rocks analyzed, 

1 C. H. Gordon, Bull. Geol. Soo. America, Vol. VII, p. 122. 
* J. F. Kemp, Huidbook of Kooks, p. 96. 




BHOMQOHEDRAL JOI 



i ■■ • 



iiiiiiB.1 METHODS OF PHYSICAL ANALY8I& 19 

nor does it throw any light on their stractnre, which is oloeely related 
to their origin and history. For these we must depend on physical 
analysis. 

There are three important methods of x)artial or complete physical 
analysis of rocks: (1) By means of heavy solutions, (2) by means of an 
electro-magnet, the rock in both cases being nsi^d in the form of small 
particles, and (3) by means of a polarizing microscox>e, thin sections of 
rock being used. 

The first method depends for its efficiency upon a difference in the 
specific gravity of the minerals composing the rocks, and is quantita- 
tive as well as qualitative; the second depends upon the relative mag- 
netic susceptibility of the associated minerals; and the third depends 
npon their optical properties. The last is by far the most important and 
complete method, and when practicable should be employed first, as it 
will certainly furnish information of value in the application of the 
other methods. It is chiefly qualitative, determining the minerals and 
stractural elements of rocks, but affords also a means of estimating 
approximately the quantitative relations of the minerals. 

SPBCIPIC-GhBAVITY METHOD. 

If a mixture of dry sawdust and iron filings be thrown into water 
the sawdust will float and the iron filings will sink, the two being sepa- 
rated by means of a liquid whose specific gravity is between those of the 
mixed substances. Minerals of which rocks are comiK)sed differ in 
specific gravity, and by employing heavy liquids of intermediate spe- 
cific gravity the minerals may be separated. The best liquids for this 
purpose readily mix with water, so that their specific gravity can be 
conveniently changed by the addition or evaporation of wat«er. 

Preparation of the material. — ^The rock to be investigated should be 
pulverized by stamping (not grinding) in a mortar. To separate the 
grains and dust a series of sieves or bolting cloths may be used. Those 
found most generally useful are bolting cloths Nos. 10 and 16. The 
grains which go through No. 10 and not through No. 16 are usually of 
the best size for separation in the heavy liquid, and, having been rubbed, 
are clean and free from dust. 

Solutions most commonly used. — ^Thoulet brought into general use a 
solution of potassium iodide and mercuric iodide mixed in the propor- 
tions of KI to Hgia as 1 to 1.24. With this solution a specific gravity 
of 3.i96 may be obtained. It is readily miscible with water, and as 
easily evajKirated, so that its specific gravity may be conveniently 
changed as desired. 

Oadraium borotungstate, barium mercuric iodide, and methyl iodide 
have been used, but the separation of minerals having high specific 
gravity is most easily attained by means of a solution prepared by 
mixing nitrates of silver and thallium in the proportion of 1 to 1. 
For the methods of preparing the solutions and descriptions of the 



20 



THE EDUCATIONAL SERIES OF" ROCK SPECIMENe. 



apparatus used, as well aa its manipalatioo Id tnakiog the separation, 
reference may be made to tlie following works: 

Alikroskopiscbe Pbysiograpfate der Uineralien and Oesteine, I, Band 
{die petrograpfaisch wichtigen Ifineralien), by H, Boseiibascb, third edi- 
tion, pp. 226-260. 

Translation of tbe above, Vol. I, by J. P. Mdings (third edition, 1893), 
pp. 08-107. 

Lehrbnofa der Petrographie, by F. Zirkel, second edition. Vol. I, pp. 
M^IOS. 

American Joomal of Science, December, 1S95, third series, Vol. L, 
p. 446. 

To convey a more definite idea of this method of analysis, a descrip- 
tion may be here inHert«d of the Smeeth separating apparatus, which 
is illustrated in fig, 2. 

This consists of a wineglass-shaped base (a), with a hollow standard, 
a tube (b) to contain the heavy liquid in which tbe separation takes 
place, and stoppers {c and d) to close the upper and 
lower ends of the tube. 

All of these separate purts have ground fittings, 
80 as to be water-tight. To make a separation, 
place the tube fr in a, as in the ligare, but with 
the stoppers o and d rwnoved. Fill the tube b 
about three quarters fall of tbe heavy solntion; 
add the rock iwwder, then the stopper c, and after 
tbe tnbe has been shaken so as to thoroughly wet 
the powder allow it to stand for separation. Tbe 
particles heavier than the liquid will sink to tbe 
bottom and those lighter will rise to the sorfiwe. 
On account of adhesion among the particles some 
of the heavier ones will be lifted to the top, while 
lighter ones will be pulled down to tbe bottom. 
These may be liberated by stirring both portions 
at the same time with a small glass rod inserted 
from above. By repeating the stirring several times 
it is i)08sible to secure a complete separation. 
It will be readily seen that by inserting the stopper d the tnbe fr, 
with its contents of heavy liquid and light material floating on top, 
may be removed and the heavy material washed out of a for stady. 

The tube b may be again inserted in a and the stoppers removed, 
allowing the heavy liquid to fill the standard of a, Distilleil water 
may be added to the liquid to lower its si>ecific gravity, so that the 
next heavier particles will sink and separate as before. Thus by suc- 
cessive additions of small quantities of water a powdered rock com- 
posed of many minerals differing in specific gravity may be separated 
into its constituents. 
In making mineral separations it is esi)eciany desirable to discover 





COLUMN*" JOINTING, WATCHUNG MOUNTAIN, ORANGE. NEW 



muM,] METHODS OP PHYSICAL ANALYSIS. 21 

as closely as xiossible the specific gravity of the miDcral separated, and 
this may be accomplished approximately by determining the specific 
gravity of the solution at the time of separation. There are several 
Trays of doing this: (1) By means of indicators of known specific 
gravity; (2) by using the Westphal balance; or (3) by using Solomon's 
apparatus for determining the specific gravity of liquids. Only the 
first two of these need be noticed more ftilly here. 

With a series of indicators, the specific gravity of which has been 
accarately determined, the specific gravity of the liquid may be found 
by placing the indicators in the liquid. These indicators may be suit- 
able fragments of minerals, or other specially prepared material. Those 
heavier than the liquid will sink; the others will rise; if one has the 
same specific gravity as the liquid it will remain suspended and thus 
indicate the specific gravity of the liquid. If several such indicators 
are placed in the tube with the powder to be separated, the specific 
gravity of the material may be approximately determined by compari- 
son at the time of separation. 

A simple but efficient series of indicators has been devised in the 
petrographic laboratory of Harvard University, under the direction of 
Prof. J. E. Wolff. To prepare them, seal up by melting at one end a num- 
ber of short lengths of small glass tubes; put in each a certain amount 
of mercury or lead, so that they will form a regular series increasing in 
weight from the lightest to the heaviest; seal up the other end of each, 
leaving a short piece of platinum wire projecting to form a loop or hook 
by which the indicator can easily be removed from the heavy liquid. It 
is best to make a large number of these at once, and then by successive 
dilutions of the heavy solution to determine the specific gravity of the 
serie^by using the Westphal balance. Thus solutions are obtained in 
which the minerals remain exactly suspended, and by measuring with 
the balance the specific gravity of the solution, that of the minerals is 
determined. 

The Westphal balance, made by O. Westphal, of Oella, province of 
Hanover, Oermany, is illustrated in fig. 3. It consists of a base with 
an upright hollow cylinder a, in which slides the rod &, controlled by 
the screw c. On b is supported the firame d, which at e forms the ful- 
cram of the beam /. The base may be leveled by the screw g, and when 
a is vertical and the two points at h are together the beam /is horizon- 
tal. From the hook k hangs the sinker I. The liquid whose specific 
gravity is to be determined is put in a glass vessel m^ and the specific 
gravity is weighed directly by a decimal series of weights and riders hun g 
atH* or upon the dividing beam /between e and Jc. The specific gravity 
indicated by the riders in the figure, that at k having the value 2, is 
2.916. 

As the particles of different minerals in the same rock vary greatly 
in size, structure, and hardness, it is not to be expected that all of them 
will be present in the powder prepared for separation in the same pro- 



22 THE EDUCATIONAL SERIES OF ROCK BPECIHENB. [bitu-Uo. 

portion as in the original rock. In fact, some of them may diaappcar 
entirely in the dust while the powder is preparing for separation. 
Upon examination of the powder it ia generally foand also that many 
of the grains, instead of being homogeneons, are composed of more 
than one mineral. Furthermore, different minerals may have the same 
specific gravity. For these reasons it iB not possible by using heavy 
liquids to make accurate mineral analysea of rocks, but only a rough 
approximation of their mineral compositiou can be attained. This 
method, however, affords a most convenient means of obtaining sep- 
arately considerable quantities of particular rook-forming minerals for 
study. 
Thin laminated minerals may be separated from others by allowing 



B* 



a ' ti ' 



1 



the rock powder to glide gently over paper, or to fall a|>on the moistened 
sides of a glass funnel. On acconnt of their shape they sink slowly in 
water, and may be separated to some extent by vigorously starring the 
powder in water and, after allowing it to stand a few moments, decant- 
ing quickly. 

ELBCTRO-MAGNKTIC METHOD. 

Miignetite may rea<lily be removed from rock powder prepun^d as 
already iudicatetl, by passing an ordinary magnet closely over the 
powder spread out upon paper. With a much stronger magnet — an 
electro- magnet — a nomber of other minerals, such as amphibole, pyr- 
oxene, oliviue, epidote, garnet, etc., having couHiderable iron in their 



MU*^] METHODS OP PHYSICAL ANALYSIS. 23 

composition, may be lifted from the paper and thus separated from 
feldspar, qnartz, and other nonmagnetic minerals. By varying the 
maguetic moment of the instrument the iron-bearing minerals may be 
more or less completely separated from one another. 

MIOBOSGOPIO METHOD. 

As minerals differ among themselves in the forms of their crystals, 
80 also they differ in their effect on light which passes through them. 
The optical pTox)erties of minerals are directly dependent on crystallo- 
graphic relations, and afford especially valuable data for their deter- 
mination in rocks. 

Petrograpbic microscopes are arranged with reference to studying 
rocks by transmitted ordinary and polarized light so that the optical 
properties of their component minerals may be discovered. 

In the accompanying cut (PI. V) is shown such a microscope,^ a 
description of which is here given. 

The screw supporting the arm between the piUars allows the instrnment to be 
inclined at any angle. The main tnbe is provided with a cloth lining, into which 
the drawtnbe carrying the ocular is fitted. There is a coarse adjustment by rack 
•Dd pinion, and a fine adjustment by a micrometer screw. The mirror is both flat 
ind concave and the mirror-bar is adjustable. 

Coming now to the peculiarly petrographical features, we have the lower nicol- 
prism, or polarizer, inclosed in a cylindrical metal box, both ends of which are pro- 
tected by glass. This box is capable of a complete revolution, aud is provided with 
ft grsdoated silvered circle and index. It is held by a cylindrical frame, in which 
it may be raised or depressed at will by a rack-and-pinion movement. This frame 
is attached to the under side of the stage by a swinging arm, so that the whole polar- 
izing apparatus may be thrown to one side if desired. A strong compound lena may 
be screwed upon the upper end of the polarizer whenever strong illumination or 
converged polarized light is needed. 

The circular stage (9.5 cm. in diameter) is provided with a beveled silvered edge, 
graduated to degrees. Upon this is mounted, for smooth aud concentric revolution, 
the admirable mechanical stage known in the manufacturers' catalogue as No. 1052. 
This carries an index for reading the graduated circle, and is also provided wiih 
silvered graduations for it-s two rectangular movements, whereby any point within 
a section can be readily located. The upper sliding bar, which carries the object, 
has been shortened, so as to be flush with the revolving stage only when pushed to 
its extreme limit on either side. With this, square or short rectangular glasses must 
he used for mountingi which will avoid any interference with the revolution of the 
stage. 

Into the nose piece, Just above the objective, is an opening, intended to receive 
the four following accessories, each mounted in a separate brass frame: (1) A Ber- 
trand lens for magnifying the interference figures ; (2) a quarter undulation mica 
plate; (3) a quartz wedge; (4) a Klein quartz plate or a gypsum plate, with red of 
the first order. 

The centering of the various objectives is secured by two screws, having motions 
at right angles to each other. 

The upper nicol-prism, or analyzer, is inserted in the tube, in order to avoid the 
diminishing of the size of the field, which is unavoidable when the prism is placed 
OTer the ocular as a cap. To accomplish this, and at the same time to keep the 



1 iCaniifactiired by the Baaach &. Lomb Optical Company, Rocheeter, New York. 



24 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [buu^MO. 

tobe dust-tight, the nicol is inclosed in one side of a doable-chambered box. The 
other side is left vacant, and the box may be slid to and fro as ordinary or polarized 
light is deHired. A metal sheath protects this box from above. > 

For the microscopic study of rocks the material is best prepared in 
the form of thin sections. 

Preparation of thin sections. — For the preparation of thin sections of 
rock, a thin but strong chip about as large as a 25-ceut piece is selected. 
The chip is held in the hand and ground smooth on one side. The 
grinding may be done on a fine sandstone, emery stone, whetstone, or 
iron plate on which emery, first coarse (No. 70) and finally fine (flour), 
is used to make the surface perfectly smooth. It is much more con- 
venient, however, and less laborious to grind on a revolving iron plate 
mounted on the top of a vertical axis to which power is applied by the 
foot or from some other source. 

After making a smooth plane surface as large as the chip will permit, 
cement it with Canada balsam to a piece of heavy glass about 1 inch 
square in the following manner. Place the square glasses and ground 
chips on a small, gently heated brass table. When the chips are 
thoroughly dry, place on the glasses some balsam, hardened by evap- 
orating part of the turpentine, and press the fiat side of the chips on 
the glasses, so as to remove all bubbles of air. When cooled, the chips 
should adhere firmly to the glass. 

A cement which is preferred by some to Canada balsam is obtained 
by slowly melting together a mixture of 16 parts by weight of viscous 
Canada balsam and 50 parts of shellac and keeping them heated for 
some time. The ma«s, as it completely cools, may be drawn out in 
strings and rolled between the hands into convenient rods about 1 cm. 
thick and 20 to 30 cm. long. 

Holding the chip by means of the thick glass, after mounting as 
above described, grind down its other side carefully until the rock shows 
signs of breaking away. The glass should be thick, so as not to bend, 
and should be pressed evenly, so that the section may be ground 
uniformly throughout. Finer emery (No. 120) should be used toward 
the last, on a perfectly fiat iron plate, and for the final grinding the 
finest washed emery (No. FFFF), on a smooth glass plate, until the 
section is completely transparent. 

The extra balsam around the section should be removed with a warm 
knife blade and the section thoroughly cleaned by Washing it first 
in alcohol with a soft camel's hair brush, and finally with clean water. 
It is then put, section upward, in a shallow bath of turi)entine for some 
hours, until the balsam is dissolved and the thin section completely 
loosened from the thick glass. After removal from the bath it is care- 
fully washed again, to prepare it for transfer to a thinner glass. The 
thoroughly cleaned object-glass on which the section is to be finally 



1 Copied from a paper on a new petrographio microscope of American mannfactnre, by the late 
Prof. G. H. WiUiamt : Am. Jour. Set., Feb., 1888. 3d aeries. voL XXXV, pp. 114-117. 




PETROGRAPHic Microscope. 



MUJkl IfETBODS OP PHYSICAL ANALYSIS. 25 

nxHiiiteil is placed on a Bmall table as before, and a drop of balsam is 
placed on it from tbe bottle (flg. 4) by preasiug the rnbber bulb. The 
BectioDS are then carefully transferred by slidiug them with mounted 
needles from the thick glaea to the drop of balsam. The balaam, heat«d 
by a lamp beneath tbe table, becomes more liquid, and the section in 
manipulated by tbe mounted needles ttntil all parts of it above and 
below are covered with balsam and all bubbles are removed. Bubbles 
are easily removed by pricking them with a heated needle. When the 
balsam has been evaporated sufficiently, a bit of it removed by a mount- 
ing needle will not adhere to the thumb-nail if applied while yet fluent. 
It most be heat«d gently and not too long, else it will become too 
brittle. The thin cover glasses, having been thoroughly cleaned with 

alcohol and water, are then put on the section and pressed down gently, 

w as to squeeze oat tbe 

Bnperfluons balsam. 

vbicli, after cooling, is re 

moved and the cover sec 

tioD cleaned as before. 
Oriented and other sec- 

ttoDs are often desired of 

rocks which may not be 
. chipped. In such cases, 

to obtain a slice for grind- 

ing. it is necessary to have 

> net saw. An appara- 

toB combining both saw 

■nd grinding disk with 

motive power was devised 

by tbe late Prof. George H. Williams. It contains a storage battery 

Md an electric motor, a grinding disk, an emery wheel, and a diamond 

saw,' Diamond saws may be made by taking a flat circular sheet-tin 

(tinned iron) disk 6 or 8 inches in diameter and placing it in a vise 

betweea flat wooden disks, allowing tbe tin to project one- fourth of an 

inch all around. Place a knife-edge across that of the tin disk at an 
, angle of about 15 to 20 degrees and strike it a blow, so as to cut a 

nick an eighth of an inch deep into the edge of the disk. Repeat this 

operation until the entire periphery of the tin disk has been notched 

or nicked, the nicks to be abont one-teuth of an inch apart, cut in 
directions which may vary in different quadrants of tbe disk. While 
nicks in the same quadrant of the disk's edge should be parallel, those 
of adjacent quadrants should be nearly perpendicular to eat^h other. 
Mix diamond dust or bort with a little oil to make it adhere, and flII 
the nicks, using a small pointed stick. Lay the edge of the tin disk on 
a flat iron and hammer gently on the sides along the edge, so as to 

icu. He Am. Jour. Scl.. Febnury. 




26 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. Ibdu-IM. 

close the nicks and hold the diamond dost. If the dost is deeply 
baried in the nick the saw will not cnt at first. 

A very eerviceable saw may be made of annealed iron wire, as illns- 
trated in fig. 5. c is an endless wire (gange 16) rnnning over two 
pulleys, a and b, 18 inches in diuneter. To the lower pulley, b, the 
power is applied by a shaft. These pulleys at work shonld make aboot 
250 revolutions a minate. The apper one is pulled up od a sliding 
frame by the weight and cord t;, to keep the wire stretched. The ends of 
the wire mast be carefally soldered together and filed smooth. Water 
sfaonld be dropped by the tube t 
where the specimen to be cat is 
pressed against the wire, which may 
be supplied with emery by the fin- 
gers of the opOTator. The wire 
may be used as a scroll saw and a 
slice may be thus oat out of the 
middle of the specimen. This saw 
has been succesafally used in the 
petrographic laboratory of the 
United States Geological Survey 
for the last seven years. 

In the same laboratory the grind- 
ing apparatus represented in fig. 6 
bas been found equally snccessfuL 
The cast-iron firame a carries the 
steel spindle b, to which is screwed 
on top the cast-iron plate c, 12J 
inches in diameter. The plate re- 
volves when at work about 1,200 
times a minnt«, in a box, d d, and is 
underlain by a block of same size, 
e, to prevent emery from getting to 
the spindle. Separate plates may 
FM. 6.-wire »w. United BUM G«iogi«i be kept forcoaTse and fine grinding, 
Snrrey. a.a,puUiiyBir<ui vgruoven.sDdieu and ft ^Qfif wheel maybe fitted to 

IroDwlM; d. fnune: t, ^nide: /, bolla. three- .1 -^ ji ^ i> i_ - -n 

to»Hh,in2:g.u^l«,.h.ai^t:i,y.»i^ the Same spiudle for polishing. For 
tnba. grinding and polishing large speci- 

mens for exhibition a larger plate may be used. 

iSoft, xwrous material may be hardened by heating it in Canada 
balsam, and when cooled a section may be made as already indicated. 
Bock powder may in many cases be most conveniently studied by 
mounting it between a glass slide and cover in a drop of water. To 
study the internal structure and optical properties, one must avoid the 
total reflection from the surface of the particles, and it is better to 
niouDt the powder, which should be of nearly uoiform size, in a sub- 
stance whose index of refraction is nearly the same as Chat of the 
j%>iFdered mineral. For this puriwse glycerin (n = 1.46), Canada 




niuB.] 



METHODS OF PHYSICAL ANALYSIS. 



27 



t 



mmn 



ITTM 






W/////WM////M 



1^1 



balsam (n = 1.54), and a concentrated solution of iodide of potassium 
and mercury (i» = 1.73) have been used, but for preservation Canada 
balsam is preferred. To secure a uniform distribution of the powder 
in the balsam, pat a thin film of solid balsam on the central portion of 
the slide and spread tlie powder properly on it, then upon heating it 
slightly the powder will adhere. It can then be covered with balsam 
dissolved in etlier or chloroform and furnished with a glass cover. 

Three of the methods of physical analysis by means of which rocks 
maybe separated more or less completely into their mineral conipo- 
Dents have been given in the preceding pages. In applying the first 
aod second of these methods, the observer discovers some of the 
fa&portant properties of the minerals by which they are distinguished 
from one another, but their final 
determination generally depends 
largely upon the microscopic and 
optical study of the separate 
grains. 

The third method, the micro- 
scopic method of rock analysis, is 
by far the most comprehensive, and 
affords the most valuable means of 
discovering not only the mineral 
comiH>8itiou but also the structure 
of rocks, and throws much light 
upon their genesis and history. To 
apply it successfully it is neces- 
sary to know something of minerals, 

especially of their optical proper 

ties. In the succeeding chapter are 

given brief descriptions of the minerals which play a part, more or less 

important, in any of the rocks of the series* Only brief mention is 

made of their optical properties. 
Full and systematic accounts of rock-making minerals may be found 

in the works of Dana,^ Bosenbusch,' Zirkel,^ and Fouqu(^ and Michel 

Levy.^ 
For the convenience of English and American students an excellent 

translation of Bosenbusch has been made by Prof. J. P. Iddings.'^ 




te*^ 



jaUieHes 






FlQ. 6.- 



-Grinding appara«.UB, United Staten Geo- 
logical Survey . 



THE PRINCIPAL ROCK-MAKING MINERALS. 

Although there are over 800 species of minerals, there are only about 
100 which enter, to any considerable extent, into the composition of 



*8ystflni of Mineralogy of J. D. Dana, 6th ed., by Sdward S. Dana; New York, 1892. 

'Mikroakopiache Pbyniographie der Ifineralien nnd Gesteine, von H. Roaenbusch; Band I: Die 
petrof^phiaeb wichtigen Minemlien ; 1892. 

*Lehrbach der Petrographie, von Dr. Ferdinand Zirkel; Vol. I; Leipsig, 1893. 

^Ifio^ralogle mlcrographiqne, Rochea 6rnptives frauvalses, par Foiiqu6 et A. Michel L6vy ; 1879. 

•Mieraacopical Phyalography of the Rock-making Minerals; An Aid in the 8tndy of Rookn, by 
H. IbMenlmach ; tranalated and abridged for Use in Schools and Colleges by Joseph P . IddVnga -, 1\Aid 
ttitiaB.l«W. 



28 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. Ibull.150. 

rocks, and scarcely a score of them are of prime importauce as rock 
constituents, liock-making minerals Lave been variously classified 
according to their conditions and the part they play in the constitution 
and history of rocks. 

Those minerals whose presence is necessarily implied in the defini- 
tion of the rock are essential minerals, while the others present are 
nonessential or a^^cessory. This is well illustrated by granite, of which 
the essential minerals are quartz, orthoclase, and a ferromagnesian sili- 
cate. They are always present, otherwise the rock would not be 
granite. Apatite, magnetite, titanite, and a number of other minerals 
are accessory in granite, because they may be present or absent with- 
out affecting the kind of rock. 

Minerals present in the rock at the time of its origin are original 
or primary minerals. The others now present are secondary minerals, 
in that they originated in the rock after it was formed. Secondary 
minerals may originate within a rock either from the alteration of 
primary minerals or from the accession of new material from without. 
Orthoclase, which is an original constituent of granite, is frequently 
altered to muscovite, quartz, and kaolin, all of which, when so derived, 
are secondary minerals. A granite may contain at the same time not 
only the essential and original quartz and muscovite, but also some 
secondary quartz and muscovite which have resulted from the altera- 
tion of the feldspar. 

The mineral matter of rocks is in one of two conditions, either amor- 
phous or crystalline. The amorphous condition arises in the solidifica- 
tion of matter from a molten or gelatinous state under such circum- 
stances as prevent its crystallization. In this way glasses and opals 
are produced. 

The degree of crystallization in solidification from a molten condition 
dei)end8 chiefly on the rate of cooling. If very sudden, practically all 
the material becomes glass, but in other cases, where the cooling is 
slower, some of the matter crystallizes, and the amount that becomes 
glass gro^s less and less as the rate of cooling decreases. It follows 
that, if allowed to cool slowly enough, all the mineral matter will crys- 
tallize and the rock will become holocrystalline. While many lavaa, 
like pumice (59), obsidian (60), and perlite (61), are glassy, and others, 
like nevadite (64) and dolerite (105), are holocrystalline, a large num- 
ber of volcanic rocks, like many basalts, andesites, and rhyolites, con- 
tain more or less glassy material. The chemical composition of the 
glassy material found in rocks varies greatly, not only in its original 
constitution, but in proportion to the degree of crystallization in the 
solidification of the lava. Under the microscope between crossed nicols 
glass is seen to be isotropic and generally to contain minute but more or 
less distinct crystals. 

Opal is hydrated silica; composition, Si02+aq. Like glass, it is 
wholly isotropic, unless affected by internal strains which give it anom- 



Dnm.] 



THE PRINCIPAL ROCK-MAKING MINERALS. 



29 



aloos doable refraction. Hardness, 5.5 to 6 ; specific gravity, 2.2 ; infusi- 
ble, and so distin^aishable from glass, which is fusible. It forms irreg- 
ular patches, strings, and veins, as well as pseudomorphs after various 
minerals, in altered acid eruptive rocks. Its most important occurrence 
is in siliceous sinter deposited by many hot springs and geysers (23), and 
insilicifled wood (37). 

In the following list are given the minerals commonly found in rocks, 
bat it mast not be supposed that all are equally important and abun- 
dant The most important mineral of unaltered sedimentary rocks of 
mechanical origin is quartz, on account of its ability to withstand attri- 
tion, as well as its resistance to solvents, both chemical and organic. 
Galcite and dolomite, although of no importance among sedimentary 
rocks of mechanical origin, are of greater importance than quartz in the 
unaltered sedimentary rocks of chemical and organic origin. Among 
the igneous and metamorphic rocks, quartz, the feldspars, nephelite, 
lencite, the amphiboles, the micas, the pyroxenes, and olivine are the 
most important rock-making minerals, and are generally regarded as 
of prime significance in their classification. Magnetite, zircon, and 
titanite are of somewhat less importance, while the scapolites, garnet, 
toormaline, topaz, epidote, andalusite, cyanite, sillimanite, staurolite, 
chlorite, talc, and serpentine are of nearly equal importance. 

MIHEBAL.S rL.r.nSTBA.TED BY THIS SSBIES OF BOGK SPECIMENS. 

The minerals of the succeeding list are arranged according to their 
systems of crystallization, which bring together generally those having 
similar optical and other properties. 





iBometrie syBtem, 




Pyrite. 


Fluorite. 


Hailynite. 


Magnetite. 


Garnet. 


Noselite. 


Chromite. 


Lencite. 


Analcite. 


8pmeL 


Sodalite. 

Teirtigonal Byatem, 


Perofakite. 


Bntfle. 


Zircon. 

Hextigonal syBtem. 




Graphite. 


Quartz. 


Apatite. 


Pyrrhotite. 


Tridymite. 


Nephelite. 


Hematite. 


Calcite. 


Cancrinite. 


Ihnenite-leacoxene. 


Dolomite. 

Orihtfrhambio system. 


Tourmaline. 



Uarcaaite. Pyroxenes (orthorhombic). Phrenite. 

Aodalosite and CliiMtolite. Oliyine. Talc. 

Topaz. Fayalite. 

Staaiolite. Zoiaite. 



30 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS. (bcu.UO; 





Monoolinic Bystem, 




Gypsum. 

Pyroxenes (monoolinic). 

Amphiboles. 

Mioaa. 


Chlorite. 
Epidote. 
AUanite. 
Titanite. 

Triolinio system. 


Zeolites. 

Feldspars (monoclinic). 


Feldspars (triclinic). 


MicTocline. 

Homogeneous aggregates. 


Plagioclase. 


SerpeDtine. 
Kaolin. 


Glanoonite. 


Limonite. 



MINERALS OF THB ISOMKTRIC SYSTEM. 



Pyrite. — Disnlphide of iron. Oomposition, FeSg. Isometric. When 
well crystallized, usually appears in the form of cubes or pentagonal 
dodecahedrons. Fine-granular, sometimes radially fibrous, forming 
nodules. Brittle; hardness, 6 to 6.5.^ Specific gravity, 4.8 to 5.2. 
Luster, metallic; color, pale brass-yellow. Streak, grayish black. 
Opaque. Before the blowpipe on charcoal or in a closed tube gives 
off sulphurous odors and becomes magnetic. Often occurs as crys- 
tals and grains, rarely in large masses. In many rocks, both sedimen- 
tary and igneous, but is always an accessory constituent. It is read- 
ily distinguished by its color, hardness, and chemical reaction. It 
occurs as small grains, generally microscopic, in minette (91), campton- 
ite (92), diorite (93), garnetiferous gabbro (199), and hornblende-schist 
(131). Marcasite (page 37) is essentially like pyrite in all respects 
excepting crystallization and its easier alterability. Instead of being 
isometric, it is orthorhombic. 

Magnetite, — Magnetic oxide of iron. Composition, FcsOf. Isometric, 
octahedrons common, dodecahedrons less common, and generally occurs 
in irregular grains. When massive, often granular. Hardness, 5.5 to 
6,5. Specific gravity, 4.9 to 5.2. Color, iron black. Streak, black. 
Opaque. Strongly magnetic. Magnetite occurs occasionally in large 
deposits, and is mined as a valuable ore of iron. It is one of the most 
widely distributed accessory constituents of igneous and metamorphic 
rocks, in which it occurs as microscopic crystals and grains. Being 
strongly magnetic it is readily recognized. It forms the whole of 
specimen 146, but in most of the other metamorphic and igneous rocks 
it can not be seen without the aid of the microscope unless the rock is 
powdered and brought under the influence of a magnet, when the little 



■The Bcale of hardness by Moha sad that of fusibility by Von Kobell, used in these descriptiona, an 
as followa : 

Hordnets.— 1. Talo. 2. Gypsum. 3. Calolto. 4. Flnorite. 5. Apatite. 6. Feldspar. 7. Quarts. 
8. Topaz. 9. Corondnm. 10. Diamond. 

FtinbilUy. — \. Stibnite: fusible in the flame of a candle in large fragments; 2. Natrolite: fusible 
in the flame of a candle in small Augments; 8. Almandite garnet: infkisible in the flame of a candle, 
but easily fusible before a blowpipe, even in somewhat large pieces ; 4. Oreen actinolite : fusible before 
the blowpipe in rather flno splinters ; 5. Orthoolase: fusible before the blowpipe in finer splinters; 
6. Bronsite : before the blowpipe becomes rounded only on the finest points and thinnest edges. 



THE PRINCIPAL ROCK-MAKING MINP:RAL8. 



31 



graiDS of magnetite will leap forth. Orains of magnetite resemble 
chromite and ilinenite, but may be distiuguisbed from them by the 
strong magnetism and the lack of definite blowpipe reaction showing 
the presence of chromium or titanium. 

Chromite. — Isometric, generally in regular octahedrons. Oomposi- 
tioD, FeGr204. Color, black, but in thin sections by transmitted light, 
brown. Hardness, 5.5. Specific gravity, 4.3 to 4.5. It commonly 
occurs in serpentine and olivine and is closely related to picotite, from 
which it differs chiefly in the amount of chromium it contains. It 
OGcnni in the olivine of saxonite (114) and in serpentine (145). 

^meh. — The spinels are aluminous minerals whose crystals are 
i^ar octahedrons. The common varieties are given in the table: 

The common rarieties of spinel. 



Hcroynite 
Pieoiite... 



Formula. 



MgAI,04 

(MgFe)(AlF«),04. 
FeAls04 

(FeMg)0(AlCr)^4 



Color. 



Specific 
gravity. 



Red 

Dark green 

Dark green 

Yellowiah brown 



3.5-4.1 
3.6 
3.9 
4.0 



H. 



8 

8 

7.6-8 

8 



The first two occur most commonly in gneiss, while the last, occurring 
in eruptive rocks, is generally enveloped by olivine. All the spinels 
are accessory minerals, usually small, but unchangeable, for they 
remain x>erfectly fresh in rocks whose other minerals are decomposed. 
Although the spinels occur in many of the rocks of the collection, they 
are most common in the olivine of the basalts and peridotites, but are 
visible only in thin sections with the aid of the microscrope. Picotite 
occurs in the olivine of quartz-basalt (101) and olivine nodules (104), 
and pleonaste has been observed in cortlandite (113). 

Fluorite, — Calcium fluoride. Composition, CaFj. Isometric crystals, 
usually cubical. Hardness, 4. Speciflc gravity, 3.1 to 3.25. Luster, 
yitreous. Color, various, usually bright, making pretty cabinet speci- 
mens. Occurs generally in veins, and rarely in small particles as an 
accessory constituent of rocks, as in the biotite-granite (67) of Platte 
Canyon, Colorado. 

GarneU, — Silicates of aluminum, iron, manganese, chromium, calcium, 
and magnesium. The principal varieties are given in the table: 

The prinoiptU varietiee of garnet. 





Formula. 


Color. 


Speciflc 
gravity. 


GnMinilaritJi ....... 


Ca.Ai.Si.0i 


Pale green or yellow. . . 
Red 


3. 4-3. 75 

3.5-4.3 

3.^-4.3 


1 Almandit^ 


^^•^ 1 1 ig^j'ivri] .......................... 

7^ALSi«0.« 


tf Mlantt^k , . . . 


Ca.Fe.Si.0i. 


Black 


Common garnet 

; Spetaartite 


*'■•■ "l*"!^^!! ••-•••••••--••••-••••-••-■ 

Isomorphona mixture of three above. . 
Mn.A)«Sl«Ot« 


Reddish brown, yel- 
lowish red. 
Red or vellow .......... 


3.7-4.3 
8.7-3.8 


MffaAUSiU).. 


Blood-red 


' J»V|W .........••• 


'" b9" ■•'■'•■"ll • • • •■•••••••••••••••••-•-• 





32 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bull. 150. 

Isometric crystals, usually rhombic dodecahedrons. Hardness, 6.5 to 
7.5. Luster, vitreous-resinous. Transparent. Sometimes massive, form- 
ing garnet rock. As an accessory constituent it occurs usually in crys- 
tals in altered rocks, both eruptive and sedimentary. In the collection, 
crystals of common garnet appear in garnetiferous gabbro (109), mica- 
schist (130), and garnetiferous hornblende-schist (141). Pyrox)e appears 
in kimberlite (112). 

Leuoite, — Silicate of aluminum and potassium. Composition, KAl- 
Si206. Probably isometric, with the characteristic roundish crystals 
icositetrahedral, generally in grains, rarely massive, granular. Hard- 
ness, 5.5 to 6. Specific gravity, 2.4 to 2.5. Luster, vitreous. Color, white 
or gray. Translucent to opaque. Infusible, but decomposed without 
gelatinization by hydrochloric acid. Resembles analcite, but distin- 
guished by infusibility and greater hardness. 

In thin sections under the microscope leucite is polyhedral or round. 
Small crystals are practically isotropic, but larger ones show compli- 
cated twinning lamellae between crossed nicols, and frequently contain 
numerous inclusions arranged in zones. Leucite is known only in 
igneous rocks, chiefly in volcanics. It occurs in orendite (72), where, 
although abundant, it can be seen only in the thin section with the 
microscope. 

Sodalite, — Chlorosilicate of aluminum and sodium. Composition, 
NaiAlsClSisOis. Isometric. Crystals dodecahedral or octahedral. 
Hardness, 5.5 to 6. Specific gravity, 2.1 to 2.3. Luster, vitreous. Color, 
blue, greenish, or gray. Easily and completely soluble in hydrochloric 
acid. On standing, gelatinous silica separates out, and when the solu- 
tion is evaporated crystals of common salt appear. Heated in a closed 
tube, blue varieties become white. Before the blowpipe, at 3.5 they 
fuse, with intumescence, to a colorless glass. 

Under the microscope, cross sections of its crystals are quadratic or 
hexagonal, and appear isotropic, with a low index of refraction. Soda- 
lite occurs chiefly in granular-crystalline Igneous rocks of the nephelite- 
leucite series. It occurs in some parts of pulaskite (74), and also in the 
nephelite-syenite (77) of Litchfield, Maine, where its small blue grains 
may be seen occasionally in a hand specimen. 

Raiiynite and noselite, — Salphureto-silicate of aluminum, sodium, cal- 
cium. Composition, an isomorphous mixture of Na5Al3(S04)Si30i2 
and Na3CaAl3(S04)Si30i2. The members rich in sodium are called 
no8€lit€j while those rich in calcium are called haiiynite. Isometric 
crystals as in sodalite. Hardness, 5.5 to 6. Specific gravity, 2.3 to 
2.5. Color, blue, gray, red, or various. Heated in a closed tube, unlike 
sodalite it retains its color, but before the blowpipe at 4.5 it fuses to a 
white glass. 

Both haUynite and noselite gelatinize easily in hydrochloric acid, and 
on evaporation abundant needles of gypsum are found in the case of 
haUynite, but few or none in the case of noselite. Noselite ^in some 



THE PRIT^CIPAL ROCK-MAKING MINERALS. 33 

kH a characteristic dark border. Uaiiyuite aud uoHelite are fre- 
full of inclusions. Haiiynite occurs in the .theralite (76) of 
\ Butte, Montana. 

ite, — Hydrous silicate of alaminam and sodium. Coini>osition, 
O^HiO. It is one of the zeolites, and, although sometimes a 
constituent, in this series of rocks it is always a secondary min- 
iquently resulting from the alteration of nephelite or leucite. 
ibles leucite, and may be distinguished by fusing at 2.5 and 
Ein^ in hydrochloric acid. It occurs in the groundmass of por- 
theralite (76), and possibly also in camptonite (92). 
tkite. — Calcium titanate. Composition, CaTiOa. Isometric. 
I generally very small octahedrons or rounded grains. Hard- 
5. Specific gravity, 4.1 to 4.4. Luster, metallic. Grayish 
ir brown. Has high index of refraction and anomalous double 
on. It frequently occurs in melilite, nephelite, and leucite 
nd also in some peridotites. It occurs abundantly in kimberlite 
ut can be seen only with the aid of a microscope. 

MINERALS OF THE TETRAGONAL SYSTEM. 

le, — Titanic oxide. Composition, Ti02. Tetragonal crystals, 
lly i)i*i^iii^^i<^ but sometimes in grains, and also extremely 

microscopic crystals in certain schists and slates. Hardness, 
K Specific gravity, 4.19 to 4.25. Luster, metallic, adamantine. 
yellow, brownish, or red. High index of refraction. Not afl'ectrd 
is. It is often an original accessory mineral, but in other cases 
iiirs to be secondary from titanite or ilmenite. Rutile is common 
isses, mica-schist, and rocks rich in hornblende and augite. Tlie 
;e crystals often seen in quartz are supposed to be rutile. It 

in the quartzite of Eureka, Nevada (118), the phyllite of Ladies- 
Maryland (126), and the schistose biotitc gneiss of Manhattan 

(132). 

on. — Silicate of zirconium. Composition, ZrSi04. Tetragonal. 
kls usually small, square prisms, ending in pyramids. Hardness, 
Specific gravity, 4.05 to 4,75. Colorless to yellow, pink, violet, or 
ish. Luster, adamantine, translucent. Double refraction strong, 
ligh index of refraction. Infusible, not affected by acids exc(»[)t- 
'' concentrated sulphuric acid when the mineral is in i)owdered 

on is one of the most widely distributed original accessory miu- 
in many igneous rocks as well as metamorphosed sediments, 
ipinel, it does not decompose readily, but remains unaltered in 
ddual material of the crystalline rocks in which it was contained. 
y sometimes be obtained by washing such material. It is con- 
in hornblende-biotite-granite (68), nephelite syenite (77), schis- 
iotite-gneiss (132), and many other specimens of this series. 
Bull. 150 3 



34 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [bull.UO. 

MINERALS OF THE HEXAGONAL SY8TSM. 

Graphite. — ^Neairly pure carbon. Hexagoual crystals, flat six-sided 
tables, and foliated grains. Hardness, I to 2. Specific gravity, 2.09 
to 2.23. Lnster, metallic. Oolor, iron-black. Opaqne. Tbin lamellae, 
flexible, but not elastic. Feel, greasy. Infusible and not affected by 
acids. It is widely disseminated as a pigment in the older metamor- 
pbic rocks and occasionally occurs in beds. It occurs in metamon^liic 
conglomerate (128T). 

Pyrrhotite, — Sulphide of iron, ofben containing nickel. GompositioD, 
chiefly FejSs to FcuSij. Irregular opaque grains of a bronze-yellow to 
copper-red color and metallic luster. Magnetic, and thus readily dis- 
tinguished from pyrite. Occurs in cortlandite (113), but is usually 
microscopic; occasionally it may be seen among the scales of talc in 
steatite (142). 

Hematite. — Ferric oxide. Composition, FcjOs. Hexagonal. Crystals 
usually thin hexagonal plates. In transmitted light, blood-red. Lnster 
of crystals splendent, hence called specular iron. Sometimes irregular 
scales. Hardness, 5.5 to 6.5. Specific gravity, 4.5 to 5.3. Color, iron- 
black to red. Streak, red. Usually opaque, but when thin enough to 
be transparent is blood-red in transmitted light. Is infusible, but 
becomes magnetic when heated. 

Micaceous liematite and red hematite of themselves form rock masses, 
often of large size, occurring in beds among the metamorphic rocks, 
chiefly Archean. Specular iron is widely distributed as an accessory 
constituent of acid eruptive rocks. It is one of the first minerals to 
crystallize, and in the form of minute red scales is usually included by 
the minerals of later crystallization. Occurs forming almost the whole 
of specimen 121, and also in 52 and 120, as well as in many others. 

Ilmenite. — Mainly ferrous titanate, but variable in composition. 
Formula commonly FeTiOs. Hexagonal. Crystals rhombohedral, but 
generally in irregular grains. Hardness, 5 to 6. Specific gravity, 4.5 
to 5.2. Oolor, black and opaque. Luster, submetallic. Streak, nearly 
black. Not magnetic, or feebly magnetic; less so than magnetite. It 
frequently alters to a white or yellowish-brown, strongly refracting 
substance, called leucoxene, which, although it contains titanium, is of 
variable chemical composition. Ilmeuite is a widely distributed ingre- 
dient of eruptive rocks, especially diorite, diabase, gabbro, and perido- 
tite, and is found less fr<*quently in metamorphic sedimentary rocks. 
It occurs in kimberlite (112). Leucoxene occurs in aporhyolite (136) 
and in the residual sand of diabase (148). 

Quartz. — Pure or nearly pure silica. Composition, SiOj. Hexagonal. 
Six-8ided prisms terminated by six-sided pyramids. In eruptive rocks 
the prismatic faces are very short or altogether absent, so that the faces 
of the two terminal pyramids come together, forming bi-])yramidal 
crystals. Hardness, 7. Specific gravity, 2.6. Generally colorless in 
rocks, but sometimes smoky or bluish. 



THE PRINCIPAL ROCK-MAKING MINERALS. 35 

the microscope it is clear and transparent. It is nnlaxially 
and these features nsaally distinguish it from other minerals 
ich it is associated. It often contains numerous extremely 
avities filled with water or liquid carbon dioxide in which there 
)le in continuous rapid motion. 

'. is one of the most important and widely distributed of all 
aiug minerals, in both altered and unaltered sedimentary and 
rocks. It is an essential constituent of granite^ quartz-diorite, 
and dacite, as well as of quartzite^ many gneisses, schists, and 
les. 

making qaartz occurs in three phases : granitic phase, porphy- 
ise, and clastic phase. 

tic quartz is such as occurs in granite. It is the youngest origi- 
itituent of the rock, and tills the irregular spaces left between 
3r minerals. Its form is therefore determined by its surround- 
liat is, it is allotriofnorphic. It is usually filled with fluid inclu- 
The quartz in gneiss, mica-schist, and other similar crystalline 
J irregular, granular, and closely related to granitic quartz. It 
eius and sometimes contains gold. In California the auriferous 
\ are derived from rocks containing such veins. Granitic quartz 
illustrated by that in the granites (66 to 69); and that in many 
gneisses (132 and 137), schists (119, 129 to 131), and veins (25) 
&ly related to it. Jaspilite (120) is composed largely of quartz 
. character. 

ihyritic quartz usually has well-developed crystal form — that is, 
iomarphic. Fluid inclusions are common, but not so abundant as 
uitic quartz, and, furthermore, porphyritic quartz often contains 
inclusions. In many cases crystals of porphyritic quartz have 
corroded by the magma in which they originated and reduced to 
grains; that is, they have become anhedral (90) or are completely 
yed. Porphyritic quartz is illustrated by nevadite (64) and dacite- 
yry (90). It belongs wholly to igneous rocks. 
itic quartz is such as is found in sandstones and conglomerates 
liiuentary origin. It is illustrated in sx)ecimens 3 to 5, 10, 12 
Some of the quartz found in rocks is occasionally in the form of 
)dony, which is optically negative and usually fibrous. Of such 
ial agates are composed. Silica in the amorphous state usually 
ins water and is called opal. 

iymiie. — Pure silica. Composition, Si02, like quartz. Hexagonal 
Js, usually minute, thin tabular forms. Imbricated, like tiles 
oof. Within rocks tridymite is always of microscopic dimensions. 
>und chiefly, almost exclusively, in acid volcanic rocks. It occurs 
oidite (62) and aporhyolite (136). 

nte. — Calcium carbonate. Composition, CaCOs. Hexagonal. 
Us rhombobedral, but massive in limestone. Hardness, 2.5 to 3.5. 
ic gravity, 2.5 to 2.7, Bhombohedral cleavage perfect, so that it 



36 THE EDUCATIONAL SERIES OP BOCK SPECIMENS. [bull. 15a 

may generally be split with ease into rhombobedrons. Sometimes (*>ol* 
orless and transparent (Iceland spar) and used to illustrate doable 
refraction. By placing it on a dot on white paper two dots api)ear. 
Often yellowish gray or white. Infusible, but gl(»ws and becomes lime. 
Effervesces readily in cold hydrochloric acid. When powdered, will 
effervesce in strong vinegar. 

Calcifce is one of the most important and widely distributed of min- 
erals in rocks. It forms essentially the whole of many limestones (28 
to 31, 39, 40, 42, etc.), is the cementing material in calcareous frag- 
mental rocks (12), and is generally abundant in the altered basic 
eruptives. 

Under the microscope it is usually clear and may be distinguished 
by its cleavage — two sets of lines crossing at an acute angle — and its 
peculiar pinkish-yellow gray, often irridescent, colors between crossed 
nicols. 

Dolomite. — Carbonate of calcium and magnesium. Composition, 
(CaMg)0O3, in which varying amounts of magnesia may be replaced by 
iron or manganese. Hexagonal. Crystals, cleavage, color, and optical 
and other properties very like those of calcite, but it may usually be 
distinguished from the latter by means of the characteristic that, unless 
powdered, it does not eflervesce readily in acetic or cold dilute hydro- 
chloric acid. Hardness, 3.6 to 4. Si)eciftc gravity, 2.8 to 2.9. 

Dolomite, like calcite, is widely distributed, often in crystals whose 
cross sections under the microscope appear triangular, six sided, or 
rhombic. It is an essential constituent of limestone and massive dolo- 
mites. Much of the limestone in the United States is dolomite. It 
occurs in specimens IIG and 117 of the collection, also as a secondsu'y 
product in the kimberlite (112) of Kentucky. 

Apatite, — Calcium phosphate + calcium chloride or fluoride. Coni])0- 
position, 3(Ca3P208)+Ca(ClFl)2. Hexagonal, crystals generally micro- 
scopic, six-sided prisms. Granular in crystalline schists. Hardness, 5. 
Sjieciflc gravity, 3.16 to 3.22. Usually colorless or whitish under the 
microsco])e. Rather high index of refraction, so as to appear to stand up 
in surrounding minerals of later crystallization by which it is included. 
Soluble in nitric acid without gehitinization, and the solution gives, 
with ammonium molybdate, a yellow precipitate. Moistened with sul- 
phuric acid and heated, it colors the flame pale bluish green. This dis- 
tinguishes it from nephelite, with which it is often associated. Widely 
distributed as an original accessory constituent of granular igneous 
rocks; common, but less abundant, in basic lavas. Eeddish-brown 
grains of apatite may be seen in the magnetite (146) from Port Henry, 
New York. It is a microscopic constituent of granite (68), pulaskite 
(74), nephelite-syenite (78), dacite-porphyry (90), and many other rocks 
of this series. 

Nephelite, — Silicate of potassium, sodium, and aluminum. Composi- 
tion probably i^aAlSi04, in which one-fourth of the Na is generally 



wuBl THE PRINCIPAL ROCK-MAKING MTNEKAL8. 37 

replaced by K. Hexagonal. Crystals six-sided prisms, or massive. 
Hardness, 5.5 to 6. Specific gravity, 2.6. Colorless. Luster vitreous 
in crystalR, but greasy when massive. Fuses at 3.5 into colorless glass. 
Gelatinizes in acid. The glassy colorless variety in younger volcanic 
rwks is nephellte; the massive form, usually with greasy luster and 
often colored, in iutrusive rocks is sometimes called eleolite. They are 
identical in chemical comi)osition. Nephelite readily alters to zeolites. 
Tliey are essential constituents of a whole series of extrusive and intru- 
sive igneoas rocks. Nephelite occurs in phonolite (73), pulaskite (74), 
theralite (75 and 76), and, in the phase commonly called eleolite, in 
nephelite- syenite (77 and 78). 

Cancriniie. — Silicate and carbonate of aluminum, sodium, and hydro- 
gen. Composition, Na4HAl3(C03)Si30i2. Hexagonal. Crystals columnar 
and massive. Hardness, 5 to 6. Specific gravity, 2.42 to 2.5. Colorless 
and transparent when fresh, but frequently yellow and of other colors. 
Fuses easily, effervesces in hydrochloric acid, and when heated gelati- 
nizes. Like nephelite, cancrinite alters to zeolites, and a« yet has been 
found only in nephelitesyenite. It occurs in that of Litchfield, Maine 
(77), usually as conspicuous yellowish grains. 

Tourmaline. — Silicate and borate of aluminum, magnesium, and iron; 
of complicated composition. Hexagonal. Crystals usually three-sided. 
Hardness, 7 to 7.5. Specific gravity, 2.94 to 3.24. Color, usually black ; 
Inster, vitreous. Readily distinguished under the microscope by its 
strong absorption when the longer axis of the section is perpendicular 
to the short diagonal of the polarizer. Tourmaline occurs in granitic 
igneous rocks, especially near the borders of regions of contact meta- 
morphism and in fissure veins. It occurs commonly also in the members 
of the crystalline schists; for example, mica schist (130) and quartz- 
schist (119). It occurs also in phyllite (126) and other altered rocks. 

MINEKALS OF THE ORTIJOKHOMBIC SYSTEM. 

Marcasite. — Iron disulphide. Composition, FeS2. Orthorhombic. 
Before the blowpipe, on charcoal or in a closed tube, gives off sulphurous 
fumes and becomes magnetic. Color, pale bronze-yellow. Like pyrite 
iu all essential respects, excepting form of crystallization, by which 
means alone are the two distinguishable. Marcasite alters much more 
esisily than pyrite. Marcasite is illustrated by specimen 35. The gen- 
eral form of its crystals is suggested by the points projecting from 
the surface of the nodule. Hoth pyrite and marcasite readily change 
to limonite. In specimen 35 the marcasite is almost wholly changed to 

liinonite. 

Andalusiie and chiastolite, — ^Aluminum silicate. Composition, AlaSiOs. 
Orthorhombic. Crystals generally in square thickset prisms, rarely 
in rounded grains. Hardness, 7 to 7.5. Specific gravity, 3.1 to 3.2. 
Color, various, often reddish brown or dark; under the microscope 
usually colorless or reddish; when colored, is pleochroic. Transparent. 



38 THE EDOCATIONAL SERIES OF ROCK SPECIMENS. [bull. 150. 

replaced Cleavage quite perfect, parallel to prismatic faces. Infusible. 
Saturated with cobalt solution after ignition and then again ignited, 
becomes blue. Occurs in argillaceous rocks affected by contact meta- 
morphism, less commonly in mica-schists and gneisses of the Archean, 
and rarely in granites. 

(Jhiastolite is andalusite containing carbonaceous matter so arranged 
as to form light and dark squares or a cross on a cross section of a 
crystal. It is especially abundant in some zones of contact metamor- 
phism, and is well illustrated by the chiastolite-schist (135) near 
Mariposa, California. 

Topaz, — A Huosilicate of aluminum. Composition, Al2Si04F2. Ortho- 
rhombic. Crystals usually short, flattish prisms, rarely in grains. 
Hardness, 8. Specific gravity, 3.52 to 3.50. Color, light yellow, blue, 
or colorless. Transparent, infusible, and only partially attacked by 
sulphuric acid. Often contains inclusions of hematite and ilmenite 
and minute cavities filled with water or carbonic acid in a liquid state. 

Crystals of topaz occasionally occur in cavities of the rhyolite 
(nevadite, 04) of Chalk Mountain, Colorado. 

StauroUte. — Silicate of iron and aluminum. Composition, HAloFe- 
Si2< )i3. Orthorhombic. Generally crystallized in short prisms, often 
twinned, forming a cross. Hardness, 7 to 7.5. Specific gravity, 
3.3 to 3.8. Color, usually brownish ; under the microscope pleochroic. 
High index of refraction, and yields brilliant colors between crossed 
nicols. 

Staurolite does not occur in igneous rocks, but appears in crystalline 
rocks, esi>ecially of the Archean, in gneiss, mica-schist, and others poor 
in amphibole. It is found also in rocks highly altered by contact meta- 
morphism, and is well illustrated by prominent crystals in the stauro- 
lite-mica schist (133) of Charlestown, New Hampshire. 

Pyroxenes. — The pyroxenes are silicates chiefly of iron, magnesium, 
and calcium, and form a series with considerable range of chemical 
composition. With the amphiboles, micas, and olivine they are the 
most important rock-forming ferroniagnesian silicates in igneous masses. 
Pyroxenes may be divided according to their crystallization into three 
groups — rhombic, monoclinic, and triclinic. At this place only the 
rhombic pyroxenes, enstatite^ hronzite^ and hypersthencj will be consid- 
ered. Their crystals are prismatic. Cleavage parallel to the pinacoidal 
planes is sometimes better developed than that of the prismatic planes. 
Enstatite contains less than 5 per cent of iron, brouzite from 5 to It 
per cent, and hypersthene over 14 per cent While enstatite in thin 
sections is colorless and not pleochroic, hypersthene is usually greenish, 
light red, or brownish, and strongly pleochroic. The most important 
distinguishing feature under the microscope is the parallel extinction 
in prismatic sections and distinct rectangular cleavage in cross sec- 
tions, which are usually octagonal, with four of the sides larger than 
the others. 



MU».] THE PRINCIPAL ROCK-MAKING MINERALS. 39 

The orthorliombic pyroxenes, esi>ecially hypersthene, are important 
rockfonnlug minerals^ occnrring in many andesites and basalts, as well 
as gabbros aud x>^Tidotites, bat are of little consequence in rocks of 
sedimentary origin, either unaltered or metamorphic. 

Enstatite (MgSiOa) occurs as an essential constituent in saxonite 
(114), also in some of the olivine nodules (104), and occasionally in 
basalt It occurs also in the pyroxenite (110) from near Baltimore. 
Bronzite occurs in the feldspathic peridotite (111) of the same region. 
Hypersthene occurs in hyperstheneandesites {SG and 87), and forms an 
important part of the gabbro (108) of Mount Hope. 

A fibrous substance having the composition of serpentine, but usually 
forming pseudomorphs, resulting from the alteration of orthorhombic 
pyroxene, is htistite. It occurs in peridotite firom near Baltimore (HI). 

Olivine, — Silicate of magnesium and iron. Composition, (MgFe)^- 
SiOf. Orthorhombic. Crystals short, prismatic. Sections usually four, 
six, or eight sided. Often granular. Hardness, 6.5 to 7. Specific grav- 
ity, 3.3 to 3.45. Colorless or yellowish. Decomposed, with separation of 
gelatinous silica, by heated hydrochloric acid. Under the microscope, 
in thin section, olivine is seen to be without distinct cleavage and to 
have a high index of refraction. Its rough surface apparently rises 
above the adjacent minerals, and becomes brilliantly colored between 
crossed nicols. 

Three phases of olivine have been distinguished: (1) That of gran- 
alar igneous rocks, such as olivine-gabbros and peridotites, where it is 
granular; (2) that of porpbyritic igneous rocks, where it forms promi- 
nent crystals, as in many basalts; (3) that of crystalline schists, where 
it occurs as an accessory constituent. In the first two cases it is one 
of the minerals which crystallize early in the solidifying molten mat 
ter, and includes only such minerals as magnetite, ilmenite, picotite, 
etc., of earlier crystallization. In igneous rocks it is always an original, 
and generally an essential, constituent. 

Olivine alters to calcium carbonate, iron oxide, silica, and serpentine, 
and a large proportion of the masses of serpentine exposed among the 
metamorphic rocks have resulted from the alteration of rocks rich in 
olivine. The process will be described under rocks of the peridotite 
series (110 to 114) and serpentine (145). Specimens 104 and 114 illus- 
trate granular olivine. The porphyritic form may be seen in porpby- 
ritic theralite (76); also in kimberlite, as illustrated in PI. XXXIX. 

Fayalite. — Silicate of iron. Composition, Fe2Si04. Orthorhombic, 
usually in minute tabular crystals. Hardness, 6.5. Specific gravity, 
about 4. Color, light yellow, but upon exposure generally becomes 
opaque, dark brown, or black. Fuses readily to magnetic globule and 
gelatinizes in acids. The minute nonmagnetic crystals in the lithophysoB 
of specimen (62) are fayalite. 

ZaiMte. — Silicate of calcium and aluminum. Composition, HCa2Al3< 
SijOu. Orthorhombic. TJsually in columnar aggregates. Hardness, 6. 



40 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bullISj. 

Specific gravity^ 3.25 to 3.36* Has two cleavages, at right angles to 
each other, one perfect and the other imperfect. Usually colorless or 
gray, index of refraction high, but doable refraction low, and between 
crossed nicols its color is a feeble blue. Epidote, which it resembles, 
gives brilliant colors between crossed nicols. 

It occurs in some altered gabbros (143), and more commonly in horn- 
blende schists (131?), but is much less common than epidote. 

Prehnite, — A silicate of lime and aluminum. Composition, HsCa^Alr 
Si30i2. Orthorhombic, but usually in irregular plates. Hardness, (i toC.5. 
Specific gravity, 2.8 to 2.95. Odorless or greenish. Fuses at 2 with 
intumescence, and then readily gelatinizes in acid. Occurs only as a 
isecondary mineral in basic eruptive rocks (102), but occasionally may 
be an original constituent of calcareous rocks and crystalline schists. 

Talc, — Hydrous silicate of magnesium. Composition, Mg3H2Si40i3. 
Orthorhombic. Usually occurs in plates or rods. Basal cleavage 
perfect, giving it a foliation like mica, but soft (hardness, 1), and not 
elastic. Specific gravity, 2,8. Colorless in thin scales, but in larger 
bodies usually greenish or gray, with pearly luster and greasy feel. 
Like muscovite, gives brilliant colors between crossed nicols, but may 
be distinguished by the fact that after ignition, if it is moistened with 
a solution of nitrate of cobalt and heated again, it becomes when 
cooled pale red. 

Talc is an essential constituent of talc-schist and steatite (142), and is 
not uncommon among metaniorphic rocks. In basic eruptive rocKs it 
sometimes appears as a secondary mineral, resulting from the altera- 
tion of hornblende. 

MINERALS OF THE MOKOCLINIC SYSTEM. 

Gypsum, — Hydrous calcium sulphate. Composition, CaS04+2aq. 
Monoclinic, crystals tabular, but in rocks generally irregular, granular, 
lamellar, or fibrous aggregates. Hardness, 2. Specific gravity, 2.2 to 
2,4. Colorless to gray and yellowish. Yields water in a closed tnbe, 
is slowly soluble in hydrochloric acid without gelatinization or effer- 
vescence, and fuses rather easily. Occurs in beds as gypsum rock. 
It is of chemical origin and is interstratified with other sedimentary 
rocks. 

Pyroxenes (monoclinic), — The pyroxenes are silicates, chiefly of iron, 
magnesium, and calcium, with sufficient variation in chemical comiK>- 
sition and physical properties to give rise to a number of species. The 
orthorhombic pyroxenes are treated on page 38. The only monoclinic 
forms which appear in this series of rocks are augitCj diallagcy diopside, 
and ccgirite. Crystals, short prismatic. Also in irregular grains. 
Hardness, 5 to 6.- Specific gravity, 3.23 to 3,6. Color, grayish white, 
ranging through green and black. By transmitted light it is usually 
greenish, yellowish, or pale brownish. Sections parallel to the prism 
show one cleavage, and between crossed nicQls the extinction angle 



DiLLEi-l THE PRINCIPAL BOCK-MAKING MINEKAL8. 41 

may be as large as 45°. Corresponding sections of orthorbombic pyrox- 
enes bave parallel extin(;tion, and tbose of ampbibole only a small 
extinction angle. Cross sections are often nearly square, witb sligbtly 
truncated corners, or octagonal, witb sides of nearly equal lengtb. 
They sbow two directions of cleavage nearly at right angles. Diallage 
differs from augite in having, besides prismatic cleavage, a distinct 
parting, parallel to the orthopinacoid, giving diallage a more or less 
lamellar structure. Like bronzite, diallage often contains minute 
inclusions whicb give it a metallic sheen. 

Augite is one of the most common essential primary minerals in igne- 
ous rocks. The pbenocrysts of dolerite (105) and tlieralite (76) are 
augite. In granular form augite constitutes a large part of diabase 
(106), basalt (05 to 102), and quartz-norite-gneiss (140). It occurs as an 
accessory constituent in many eruptive rocks. Among metamorphic 
rocks it is not common, for in tbe process of dynamometamorphism 
augite is usually changed to hornblende. 

Diallage lias rather limited distribution, occurring with olivine form- 
ing nodules in liasalt (104), in olivine-diabase (107), and as an essential 
constituent in gabbros ( 108 and 109). Diopside (malacolite) is a mono- 
clinic pyroxene, i>oc)r in alumina and without lamination parallel to the 
orthopinacoid. It occurs in ])ulaskite (74), basalts (102), and pyroxenite 
(110), as well as in some olivine nodules (104) and some of the Archean 
granites and limestones. Acmite and icgirite are forms rich in sodium 
and iron, ^^^girite occurs in the theralite (75 and 76) of Crazy Moun- 
tains, Montana. 

Amphibole. — The amphiboles are silicates, mainly of magnesium and 
calcium, in which the former predominates over the latter. The ortho- 
rhombic forms do not occur in any of the rocks of this series. Only 
the raonoclinic forms will be considered. Crystals jirismatic, with 
cleavage angle 120^, well marked and (jharacteristic. Hardness, 5 to 6. 
Specific gravity, 2.9 to 3.55. Not affected by acids. Color, various. 
The colorless or wbite variety is IreniolHc, The green is actinolite, arf- 
redsonite^ or uralite. The black and brown are hornblende or gr'dnerite^ 
and tbe blue is generally glaucophane. 

Cross sections of ampbibole prisms are four or six sided, with promi- 
nent cleavage lines parallel to the principal sides and crossing ciich 
other at an angle of 124^. In prismatic sections the extinction angle 
ift small, wbicb distinguishes it from augite. Pleochroism usually 
Rtrong in green, brown, and blue amphiboles. The cleavage, angle of 
extinction, and pleochroism distinguish the amphiboles from thi*. 
pyroxenes, which they so closely resemble chemically. 

Tremolite (CaMg3Si40i2) occurs as lamellar masses or crystals in 
Archean limestones, silicate hornfels, and some serpentines. Actino- 
lite occurs in many green schists of the Archean. It occurs also as 
tufts of radiating needles in the cortlandit<» (113) of Stony Point, New 
York. Common black hornblende is an essential constituent of born- 



42 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [bulliso. 

blende-granite (69), many dacites (81 and 82), bornblendivandesites 
(83 to 85), camptonite (92), and cortlandite (113) among igneous rocks. 
In bomblende-scbist (131) and garnetitbrous hornblendescbist (141) the 
hornblende is green. Arfvedsonite occurs in lencite and nephelite rocks 
(pulaskite 74), griinerite in rocks ricb in iron (hematite 121), and glaa- 
cophane in glaucopbaneschists, not re]>resented in this series. Ui^alite 
and fibrous green hornblende occur in metamorphic rocks, and are 
UHually pararaorpliic after pyroxene. As such, urab'te occurs in horn- 
Meiide-gabbro-gneiss (143 and 144). 

The micas. — The micas are silicates, chiefly of aluminum, magnesium, 
iron, and the alkaline metals. In composition and other properties 
there is considerable variation, and the group is divided into a number 
of species. The most common, for the sake of brevity, may be included 
under the names of biotite and muscovite. The former, containing con- 
siderable iron, is dark colored, strongly pleochroic, and approximately 
uniaxial, while the latter, rich in potassium, is colorless or gray and 
biaxial. The micas are all characterized by crystallizing in thin hex- 
agonal plates with eminent basal cleavage, producing foliated structure 
and easily affording very thin elastic laminae which show a black cross 
or bisectrix apparently at right angles to the cleavage surface. They 
are generally regarded as belonging to the monoclinic system, although 
their habit is hexagonal. 

In porphyritic igneous rocks the mica is often in well-developed hex- 
agonal ])lates, but in granitic rocks and schists it is in irregular scaler. 
Under the microscope sections parallel to the cleavage are frequently 
isotropic, yield an optic-axial figure, and show no cleavage lines, but 
sections perpendicular to the cleavage show conspicuous cleavage lines, 
brilliant polarization colors, and parallel extinction. 

The colorless transparent micas, included for petrographic conven- 
ience under muscovite, are lepidoliU, zinnwaldite, sericite, damourite^ 
paragoniie, and margarite. The micas generally black by reflected 
light but by transmitted light colorless, deep brown, yellowish, reddish, 
or green, with strong pleochroism, included under biotite, are anomite^ 
rubellan, and phlogopite, 

Muscovite (KH2Al3Si30i2) is not a volcanic mineral. As an original 
constituent in igneous rocks it is limited to the granitic series (GO), 
but is a widely distributed and common constituent of metamorphic 
sedimentary rocks, gneisses, and schists (119, 129, and 130). In the 
form of sericite it is often an early product of metamorphism in pliyllite 
(126), staurolite-mica schist (133), and aporhyolite (136;. 

Biotite has a wide distribution in igneous rocks. In hornblende- 
niica-andesite (83), dacite (81 and 82), and dacite-porphyry (90) it 
a(!curs in rej;:ular crystals among phenocrysts. It is an eLseiitial con- 
stituent also of biotite-granite (67), minette (91), and cortlandite (113), 
and an accessory constituent in many others. It is widely distributed 
as an essential constituent of many gneisses and schists, especially in 



Diu«.] THE PRINCIPAL ROCK-MAKING MINERALS. 43 

specimens 66, 129, 1^, and 132. Lepidoinelane occurs in nephelite- 
syenite (77). 

Chlorites. — ^Tbe chlorites are hydrated silicates of magnesium, iron, 
and aluminam. On acconut of diiferences in chemical composition 
and also to a limited extent in physical properties, the chlorite group 
has been divided iuto a number of species, but for ordinary petro 
graphic purposes these need not be distinguished. Monoclinic. Crys 
tals often hexagonal, but occurring generally, like mica, in flat or 
bent plates of irregular outline. Hardness, 1 to 3. Sper*ific gravity, 
2.(» to 2.96. Color, green. Basal cleavage perfect, producing foliated 
structore. Laminae flexible. Basal sections show a distinct black cross 
or bisectrix. Sections perpendicular to the cleavage are lath-shaped. 
Pleocliroic, yielding greenish or bluish to yellowish or red colors. 
Interference colors between crossed nicols much lower than those of 
mica. 

Chlorites are widely distributed among igneous and metamorphic 
rocks, but are always secondary, resulting irom the alteration of sili- 
cates rich in magnesium and iron, such as mica, pyroxene, and ampbi- 
bole. They occur in hornblende-pyroxene- andesite (85) and many other 
i^eons rocks, but can be seen only in the thin section under the 
microscope. They occur also in chlorite-phyllite (127), whirh is com- 
posed chiefly of chlorites, and in steatite (142), of which chlorite some- 
times forms a considerable portion, although steatite is usually talc. 

Epidote. — A hydrous silicate of aluminum, iron, and calcium. Com- 
position, H(CaFe)2(AlFe)3Si30i3. Monoclinic. Crystals prismatic and, 
in rocks, generally granular. Hardness, G.5. Specific gravity, 3.3 to 3.5. 
Basal cleavage perfect. Color, generally yellowish green, sometimes 
brownish. Under the microscope, in thin sections, usually colorless, 
bat yields between crossed nicols especially brilliant colors, which 
distinguish it from zoisite. 

Epidote is never a primary or essential constituent of igneous rocks. 
It frequently occurs as a secondary mineral resulting from the altera- 
tion of igneous and sedimentary rocks. It occurs in some gneisses (138), 
schists, and phyllites (127), and in aporhyolite (136), and is one of the 
commonest of all silicates resulting from weathering. 

Allanite. — Silicate of aluminum, iron, cerium, and other rare ele- 
ments. Occurs in crystals and irregular grains, like epidote. Hard- 
oess, 5.5 to 6. Specific gravity, 3 to 4.2. Color, generally pitch black, 
brown, or gray; in thin sections reddish brown or greenish, usually 
with strong pleochroism. Cleavage indistinct or absent. Optical 
properties vary; many allanites are isotropic, others highly double- 
refracting. 

Allanite occurs in gneisses (138) as well as in diorite (93), diorite- 
porphyry {S9)^ and other igneous rocks. 

Titanite, — Silico-titanate of calcium. Composition, CaSiTios. Mono- 
clinic, Crystals frequently double-wedge shaped. Occurs also in 



44 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [Brixisa 

irregular grains. Hardness, 5 to 5.6. Specific gravity, 3.3 to 3.7. 
Color, brown to black and reddish yellow. Feebly transparent. Index 
of refraction very high and surface appears rough, but double-refrac- 
tion low, giving between crossed nicols a brownish-gray color. Not 
affected by acids. Light-yellow titanite usually in wedge-shaped crys- 
tals is commonly called sphene. It is one of the primary accressory 
minerals in many granites, syenites, diorites, and other igneous rocks. 
Granular titanite is of secondary origin, frequently resulting from the 
alteration of titaniferous magnetite, ilmenite, or rutile in igneous and 
metamorphic rocks. Titanite occurs in pulaskite (74), diorite (93), and 
in the form of sphene in minette (91), and in many other igneous and 
motnmorphic rocks, but rarely in particles sufficiently large to be seen 
without the aid of a lens. 

Zeolites {monoclinic). — Zeolites are hydrous silicates of aluminum, 
calcium, potassium, and sodium, with other bases, and, excepting rare 
cases of analcite, all are secondary minerals. They have low index of 
refraction and low double refraction. They are transparent, usually 
colorless or white, readily attacked by hydrochloric acid, and easily 
fusible. The most common forms besides analcite, already mentioned 
(page .'^), are heulandite^ Htilhite^ scolecite, epistilbite^ and laumoniitr^ 
which may be distinguished chiefly on optical grounds. Laumontite 
is well illustrated in the amygdules of the diabase amygdaloid (139); 
scolecite, in the feldspathic peridotite (111) near Baltimore. 

FeldHpars (monoclinic). — The feldspars are silicates, chiefl}' of alumi- 
num, with more or less potassium, sodium, or calcium. According to 
crystallization they may be divided into two groups, monoclinic and 
triclinic, but all are very closely related in chemical and physical 
properties. 

Their principal common feature is the possession of two cleavages, 
one parallel to the base, perfect, and the other parallel to a lateral pin- 
acoid, less perfect and variable. The angle between the two cleavage 
planes in monoclinic feldspar is 90 degrees, and the basal cleavage 
shows no twinning stria\ In the triclinic feldspars, however, the two 
cleavage planes are slightly inclined to each other, and the basal plane 
is striated. These strije are well illustrated by the large feldspar crys- 
tals in specimen 90. The striation is due to oft-repeated lamellar 
twinning, which under the microscope and between crossed nicols pro 
duces numerous distinct parallel alternating color bands, and affords an 
easy means of distinguishing the tri<'linic from the monoclinic feldspars. 

Monoclinic feldsi)ar, usually (tailed ortlioclase or potash feldspar, is a 
silicate of alumina and potash. Composition, KAlSi^OH. Crystals 
short tables and square prisms, but in rocks it usually occurs in irreg 
ular grains. Crystals may be simple, but are often twinned parallel to 
the orthopinacoid (Carlsbad twins). Color generally white or gray, but 
often reddish. Frequently more or less turbid and semitransparent 
Not acted on by ordinary acids, and before the blowpipe fuses at 5. 



wiiXE.1 THE PRINCIPAL ROCK-MAKING MINERAlA 45 

Swtioiis of simple crystals of orthoelase between crossed nicols are 
seen to be optically the same throughout, but Carlsbad twins show two 
bftiulsof interference colors, less bright than those of quartz. 

Orthoelase is an essential constituent of granites (66-69), syenite (71), 
melarhyolite (quartz-i)ori>hyry, 65)^ phonolite (73), and pula^kite (74), 
and traces of it have been found in dolerite (105). 

In the recent acid volcanic rocks orthoelase is clear and glassy, and 
has been designated sanidine. This form of orthoelase is an essential 
constituent of nevadite (64) and trachyte (70). It occurs also in the 
cavities of lithoidite (62). 

MINERALS OK TUB TRICLINIC SY8TKM. 

Feldspars (triclinic), — As already stated, feldspars are silicates, chiefly 

ol alaminuir, with more or less potash, soda, or lime. There are three 

e.s8entia11y distinct ehemiciil compounds whieh exist as separate varieties 

of feldspar, often designated, respectively, potassium feldspar (ortho- 

da^e)j sodium feldspar {albite)^ and calcium feldspar (anorthite). The 

second of these compounds combines in definite proportions with the 

first and t hinl, forming the intermediate members of several scries of 

feldspars, designated, respectively, the potash-soda series (anorthoclase) 

and the soda-lime series (plngioclase). Orthoelase is monocliiiic, but 

anorthoclase and plagioclase are triclinic. Although the cleavage 

angle of plagioclase is clearly oblique (86^ to 87°), that of anorthoclase 

varies scarcely any from a right angle. 

The characteristic feature of triclinic feldspar is the lamellar or poly- 
synthetie twinning, producing striatious on the basal cleavage plane. 
The twinning parallel to the brachypinacoid (least ])erfect cleavage 
plane) is the albite type, while that at right angles to it is the pericline 
type. The tine parallel banding such twinning produces is evident 
under the microscope between crossed nicols, and is highly characteristic 
of the triclinic feldspars. 

The potassium-feldspar compound crystallizesin the monoclinic system 
asortlicM'lase, and in the triclinic system it is ealled microcline. It occurs 
in irregular grains, and in the thin section under the microscope between 
crossed nicols may be readily distinguished by its colored rectangular 
grating, produced by the two sets of fine parallel lamellar twins, cross- 
ing each other at right angles. Basal cleavage plates between crossed 
oieols extinguish at an angle of lo^ 30' to the line of the piuacoidal 
cleavage, and a piuacoidal cleavage x>late extinguishes at an angle of 
5^ to the line of basal cleavage. Reddish microcline is abundant in 
the biotite granite of Platte Canyon, Colora<lo (67). It is present also 
in the granite of Fox Island, Elaine (68), granitoid gneiss (137), and 
epidote-mica-gneiss (138). Microcline often contains parallel inter- 
growths of orthoelase and albite. 

Plagioclase feldspars are those of the lime-soda series, including 
albite and anorthite and their isomorphic mixtures, oligoclase, andesine. 



46 THE EbUCATIONAL SERIES OP ROCK SPECIMENS. [euu-isa 

labradorite, and bytownite. The composition of albite is KaAlSisOg; 
of anorthite, CaAl38i208. The average intermediate mixtures are: 

Intermediate mixture* of albite and anorihite, 

Albite Ab 

Oligoclasc AbgAO' 

Andesine AbrAoA 

Labradorite Ab^Aos 

Bytownite Ab^Acy 

Anorthite An 

These feldspars are much alike, and are most readily distinguished by 
their specific gravity and extinction angles, which are given in the Id- 
lowing table and in the diagram prepared by Michel Levy: ^ 

Extinction anyles and specific gravities of plagioclase feldspars. 



Albite 

Oligoclase . 
Andeaine . . . 
Labradorite. 



Extinction 

angle on base, 

OP. 



Extinction , o^^^xfL^ 
pinacoids, a. I g™^»*y- 



o o 

+ 4 to I- 3 

+ 3 to -}- I 

— 2 to - 3 

— 5 to —12 



o o 

-I 19 to +13 2.«2 

+13 to — 2 i 2.64 

— 2 to —10 2.65 

—16 to —26 2.60 



Bytownite 1—17 to —27 —29 to —33 2.71 

Anorthite 1—28 to -39 -35 to —36 2.75 



Plagioclase is an important and essential constituent of diorite (1)3 
and 94), basalt (100 to 102), dolerite (105), diabase (IOC and 107), and 
gabbro (108 and 109). It occurs as an accessory mineral in many 
metamorphic rocks; for example, gneiss (132) and schists (129 and 131). 
It is especially prominent, showing fine striations, in specimen 90. 

Albite occurs in albite-schist (129), oligoclase in trachyte (70), andes- 
ine in dacite (81 and 82) and hornblendemica-andesite (83), labradorite 
in most of the andesites (83 and 88), basalt (102), and diabase (lOG and 
107); bytownite in gabbro (108), peridotite (111), and gneiss (144); and 
anorthite in the hornblende-gabbro-gneiss near Baltimore (143). 

Anorthoclase, as far as is yet definitely known, is very rare in rocks 
as compared with orthoclase and plagioclase. It often shows inter- 
secting systems of extremely fine, twinning lamelhe, which pass into 
areas free from them without there being any visible boundary between. 
Like orthoclase, it occurs in the acid series (granitic and syenitic) of 
rocks. It possibly occurs in theralite (76). 

HOMOGENEOUS AGGREGATES. 

Serpentine. — Serpentine is a hydrous silicate of magnesium. Chemi- 
cal composition, H4Mg3Si209. Serpentine is always a secondary mineral, 
and when in the form of crystals is generally pseudomorphous. It is 
usually massive and sometimes fibrous (chrysotile). Hardness, 2.5 to 



»:fitnde 8ur la determination des feldepaths, 1«94, p. 32. See, also, Becker's PI. XI in Eighteenth 
Ann. Bept., Part III, 1898, p. 36. 



DiLual THE PRINCIPAL ROCK-MAKING MINERALS. 47 

4. Specific gravity, 2.5 to 2.6. Color, generally green, sometimes red 
or mottled. Feel, ofteii greasy. Structure, splintery. Chrysotile, a 
silky, fibrous form of serpentine, belongs to the orthorbombic system 
and bas parallel extinction. Massive serpentine has low double refrac- 
tion with aggregate polarization, as if composed of minnte scales, fibers, 
or grains. 

Seri>entine results chiefly from the alteration of olivine. The change 
begins along the cracks in the o]i\ ine, and often gives rise to a pecul- 
iar reticulated structure, in which the net is serpentiue. If the altera- 
tion is not complete, remnants of olivine occur in the meshes. 

Serpentine results also from the alteration of augite and hornblende, 
and in the latter case it is generally characterized by a grating structure 
with rhombic interspaces, due to the arrangement of the fibers normal 
to the cleavage of the hornblende. In that derived from augite the 
meshes are rectangular. 

Serpehtine has a wide distribution among metaniorphic rocks, and 
has been derived from intrusive rocks rich in olivine, such as x>eridotite ; 
also sometimes from gabbro-diabase and pyroxeuite. 

Specimen 145 illustrates serpentiue derived from the alteration of 
pyroxenite. 

Kaolin. — Kaolin forms earthy aggregates which under the microscope 
are seen to be comprised of minute scales resembling muscovite, but 
tLey have a weak double refraction. It is usually white and results 
from the alteration of feldspar, nephelite, scapolite, and similar minerals 
JD the process of weathering. It occurs in specimen 149, also in the 
white portions of 147 and 148. 

Glauconite. — ^Glaucoiiite is essentially a hydrous silicate of potassium 
and iron, and is amorphous. It occurs in small grains, which are gen- 
erally spherical, but also elliptical or Irregular. Hardness, 2. Specific 
gravity, 2.2 to 2.4. Refraction and double refraction weak. It consti- 
tutes the essential part of greensand (5). 

Limonite. — Limonite is a hydrous ferric oxide having the following 
composition: 2Fej033H20. It is not crystallized, but often fibrous, 
stalactitic, botryoidal, concretionary, or earthy. Its color is various 
shades of yellow or brown, whence its other name, brown hematite. 
When heated before a blowpipe it becomes magnetic, and in a closed 
tube yields water. Its streak is yellowish brown and difi'ers from the 
eberry-red streak of red hematite. Hardness, 5 to 5.5. Specific gravity, 
3.Gto4. Iron rust and most of the yellowish-brown stains resulting 
from the alteration of iron-bearing minerals are limonite. It accompa- 
nies siderite in specimen 32, and results from the alteration of marca- 
Mte in specimen 35. The yellow tint of phyllite (126) is due to the 
presence of limonite. 



48 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. IbuilUO. 



CLASSIFICATION OF ROCKS. 

Eocks may be classified according to any of their many features, bat 
the most fundamental one upon which the primary divisions are based 
is genesis. From this point of view rocks are of two great groups, 
igneous and sedimentary. Igneoas rocks are those which, like basalt 
and granite, have solidified from a molten state. The material of whicb 
they are formed, like that erupted from volcanoes, has come up from tbe 
interior of the earth in a highly heated igneous condition. Sedimentary 
rocks, on the other hand, are formed by the accumulation of sediments 
brought together by some transporting agent, generally water, and are 
well illustrated by sandstones, conglomerates, shales, and limestones. 
The material of which they are made up is derived from the degradation 
of the land of the earth's surface. Sedimentary rocks are sometimes 
called aqueousj fragmentalj or stratified^ and igneous rocks are corre- 
spondingly designated eruptive^ massive^ or tinstratifiedj but the various 
terms in the two series are not strictly synonymous. 

Before proceeding further with the classification, attention should be 
given to the various stages through which a rock may pass in the cycle 
of its existence. 

Under the influence of the weather, rocks exposed upon the earth's 
surface gradually decompose and disintegrate, and the loose material 
thus formed is carried away by the rills, brooks, and rivers as sediment 
to the sea, where it is deposited to form new rocks. By the long-con- 
tinued accumulation of sediments and outpouring of igneous material, 
sedimentary and igneous rocks once upon the surface may become 
deeply buried and alfected by the internal heat of the earth. Further- 
more, many of them become involved in mountain-building processes 
and subjected to such high degrees of heat and pressure that the mate- 
rial of which they are composed is rearranged more or less completely, 
in accordance with crystallographic and other forces. By this process, 
which is largely dynamic, the unaltered sedimentary and igneous rocks 
become altered or metamorphosed into slates, schists, gneisses, and 
other forms of metamorphic rocks. In specimen 128, metamorphic 
conglomerate, the original fragmental structure may yet be seen. In 
Hoosac Mountain this conglomerate passes into gneiss, in which the 
original structure has entirely disai)peared. It should be borne in mind 
that this change, called metamorphisiUj does not produce new rocks; it 
simply modifies old ones. 

The earth movements by which rocks are metamorphosed may raise 
them in mountain masses above the sea. After erosion has removed the 
surface material the once deeply buried metamorphic rocks become 
exposed, weathering takes hold of them, and they disintegrate, forming 
residual sands and clays, which are ultimately washed awayanddepos- 
ited in the sea. Old rocks are thus destroyed in providing material for 
new rocks. The history of a rock embraces three stages: (1) the stage 



WLO.) CLASSIFICATION OP BOCKS. 49 

ofits origin, including the time it remained uiialtered; (2) the stage of 
its alteration ; and (3) the 8tage of its disintegration, which is usually 
Tery local and brief. 

In accordance with these stages rocks may be arranged under three 
heads: (1) unaltered rocks; (2) altered or metamorphic rocks; (3) 
residaal rocks. 

It is evident that the classification of rocks should refer primarily to 
those which are unaltered. Metamorphic rocks should, as far as possible, 
be traced back to their original condition, and be arranged accordingly. 
As already indicated, the two fundamental groux)s of unaltered rocks, 
based on origin, are sedimentary and igneous, and to the subdivision of 
these two great groups attention will now be given, beginning with the 
sedimentary. 

Sandstone (13), vein quartz (25), and diatom earth (51) are all sedi- 
mentary rocks, and being composed chiefly of silica have essentially 
the same chemical eom]>osition. In each case water was the agent 
wliich bore the material to thex>oint of accuninlation, but the modes of 
deposition were nnlike. To make the sandstone, the sediment was pre- 
cipitated from mechanical suspension in the water, while for both vein 
quartz and diatom earth the material was precipitated from solution, 
tbe former by a chemical agent and the latter by an organism. On 
account of these differences sedimentary rock mny be divided into 
mechanical J chemical ^ and organic j as indicated in the table, page 53. 

Tlie ultimate classification of igneous rocks is a matter concerning 
wliich there is much difference of opinion. The. chief divisions are often 
Uu»ed in large measure upon mode of geologic occurrence and chemical 
composition, for these two factors are the principal ones in a deter- 
miuation of the structural and mineralogic features of igneous rocks. 

Igneous material (^^ magma") rising from the earth's interior may 
break entirely through the overlying rocks and reach the surface, where, 
exposed to the atmosphere, it cools rapidly; or it may stop beneath the 
surface on the way up and cool slowly to solidification under circum- 
'stances very unlike those which obtain upon the surface. Igneous 
rocks have thus been divided by some geologists into two groups, vol- 
canic and plutoniCj sometimes called extrusive and intrusive rocks. 
Although the igneous rocks of this collection are not divided into 
these two groups, some of the structural features of these, as usually 
given, will now be considered, for convenience in other subdivisions. 
Volcanic or extrusive rocks are delivered upon or near the surface 
by volcanic action and spread out in streams or layers, either as con- 
tinuous flows or as accumulated fragments of lava. On account of their 
sadden cooling many volcanic rocks are either wholly glassy or only 
partially crystalline. Others are holocrystalline, but the mineral par- 
ticles forming the principal mass in such cases are usually, but not 
always, very minute. 
Platonic rocks, on the other hand, cooling at some distance beneath 
Bull. 150 i 



50 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bull. 150. 

the surface, solidify more slowly than volcanic rocks and attain a rather 
coarsely crystalline granular structure, excepting along their borders, 
where they come in contact with cooler rocks. They occur in masses 
of various shapes and sizes, ranging from dikes to bosses and irregular 
batholiths miles in extent. 

Eising through stratified rocks, the molten material often spreads 
out between the strata, forming low dome-shaped masses (laccoliths) or 
sheets of nearly uniform thickness. Intrusive sheets lying conformably 
between strata resemble contemporaneous surface tiows which were 
buried in the sediments as they were deposited. Generally the two 
cases may be easily distinguished by studying the contact. Intrusive 
rocks highly heat the rocks with which they come in contact, and fre- 
quently metamorphose them, and are themselves finer grained near the 
contact, owing to the cooling influence of neighboring masses. 

Although plutonic rocks generally do not extend to the surface, it is 
evident that volcanic rocks always connect, at the orifice from which 
they issue, with plutonic masses extending into the earth. Plutonic 
and volcanic rocks grade into each other, forming more or less continu- 
ous series. All these, coming from essentially the same magma, may be 
regarded as belonging to the same series. 

As to chemical composition, igneous rocks have a wide range. For 
example, their content of silica may range from 30 per cent to 80 
per cent, and on this basis alone igneous rocks may be conveniently 
spoken of as adrf, that is, containing over 05 per cent of sili(;a; inter- 
mediate J 55 per cent to 65 per cent; and basic j below 55 per cent. 

The chemical nature of the magma determines almost wholly the 
mineral composition of the resulting igneous rocks. For this reason 
the classification of igneous rocks on a chemical basis, which is really 
fundamental, may be expressed in large measure also in terms of mineral 
association, and its application may be thus rendered much more con- 
venient. 

On a chemical-mineralogic basis igneous rocks may be divided into a. 
number of families, Ciich embracing all rocks, both volcani(5 and plu- 
tonic, which have essentially the same composition, and each series rang- 
ing in structure from the glassy and porphyritic forms of lava to the 
coarsely crystalline, even-granular forms, like granite and gabbro. 

Silica is one of the most abundant and widely distributed components 
of rocks. For this veason the ])rincipal rock-forming silicates become 
the most important basis of classification, especially the silicates of the 
alkalies — feldspar, nephelite, and leucite — and the ferromagnesian sili- 
cates — amphibole, mica, pyroxene, and olivine. Only those families that 
are represented by specimens in the collection will be here considered, 
and some of tliese families are not limited as they might be if the collec- 
tion were larger and of greater variety. The following classification 
corresponds closely to that reported May 27, 1897, by a committee con- 
sisting of 0. R. Yan Hise, W. H, Weed, H, W, Turner, Whitman Cross, 



M"*^l CLASSIFICATION OF ROCKS. 51 

aud J. S. Diller, appointed by the Director of the United States Geo- 
logical Survey to consider the nomenclatare of the igneous rocks and to 
adopt a system for use iu the geologic folios pablished by the Survey. 

Qranit^-rhyolite /dfnily. — Granite among plutonic rocks and rhyolite 
among volcanic rocks are the types of this family. They are rich in 
silica, usually containing about 70 per cent {(io to 80), with approxi- 
mately 8 per cent of the alkalies, K2O generally but not always being 
a little in excess of the FaiO. When holocrystalline, their essential 
mineralB are quartz aiid alkali feldspars. Oligoclase is present, some- 
times in considerable but subordinate quantities; also one or more of 
the ferromagnesian silicates hornblende, mica, or augite, with smaller 
amounts of accessory minerals. 

8iienite'tr€iehyte family. — Syenite among plutonic rocks and trachyte 
among volcanic rocks are the types of this family. They are not so 
rich in silica as the members of the granite-rhyolite family, usually 
averaging about 59 per cent (55 to 68), with approximately 9 per cent 
of the alkalies, K2O being in general slightly in excess of ^aiO. Min- 
eralogically they are characterized by the general absence of quartz 
and the predominance of alkali feldspars. As iu the granite-rhyolite 
fiunily, there is often a large but always subordinate amount of oligo- 
clase present. Hornblende, mica, or augite may be rather abundant, 
bnt the whole amount of the ferromagnesian silicates present is less 
than that of the feldspar. The rocks of this series are rare. 

yephelit€'leucite rocks, — Nephelite-syenite and phoiiolite are respec- 
tively the abyssal and surface igneous rocks of this family. They con- 
tain so low a percentage of silica (about 54 per cent) and so high a 
percentage of the alkalies (about 14 per cent) that nephelite and leucite 
are formed and, although generally associated with feldspar, are con- 
sidered the characterizing minerals. In the phonolite and nei)he]ite- 
syenites the associated feldspar is largely orthoclase, but in the theral- 
ites it is chiefly plagioclase, and on account of such difierences the 
nephelite-leucite rocks, although rare, may be divided into several 
faDiilies. The hornblende and augite which these rocks contain are 
usually rich in alkalies. 

Diarite-andesite family. — Diorite and andesite are the plutonic and 
volcanic tyi)e rocks of this family. They average about 60 per cent 
(48 to 70) of silica and 7 per cent of the alkalies, Na^O being in excess, 
with about 6 per cent of the alkaline earths, of which the greater part 
is generally lime. Consequently the characterizing minerals are soda- 
hme feldspars. Quartz is often present, and also hornblende or mica, 
bat Dot in predominating quantities. Augite is common in andesites, 
but less so in diorites, where its relative abundance indicates a degree 
of approach to the gabbros. 

Odbbro-basalt family. — Gabbro and basalt are respectively the plu- 
tonic and volcanic types of this family. They average about 50 per 
cent of silica^ 9 per cent of iron oxides, 9 per cent of CaO, 6 per cent 



52 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bdll.15o. 

of MgOj and 4 per ceut of the alkalies. Feldspar is usually less 
abundant in this family than in the preceding ones, and on account of 
increase in the GaO and decrease in the alkalies, it is prevailingly 
of the lime-soda variety, as distinguished from the soda-lime variety 
which prevails in the diorite family, although in some cases the alkali 
feldspars become imx)ortant. As the CaO and MgO increase, pyroxene 
becomes more abundant, so that the holocrystalliue members of this 
family in general are essentially labradorite-pyroxene rocks. The mem- 
bers of this family and of the diorite-andesite family are the most 
abundant of igneous rocks. 

Peridotite family. — Pyroxenite and peridotite are the plutonic types 
of this family and are composed, essentially, the first of pyroxene aud 
the last of olivine, by which they are characterized. In chemical com- 
position they are especially rich in MgO, containing an average of about 
29 per cent, with rather low silica (about 44 per cent) and a very small 
percentage of the alkalies and alumina, so that they are essentially 
feldspar-free rocks. Volcanic rocks of this series are rare, and as yet, 
perhaps, wholly unknown in the United States. 

TABULAR VIEW AND CLASSIFICATION OF THE ROCKS OP THIS SERIES. 

STAGES OF A ROCK CYCLE. 

Originatiou. Alteration. Disiotegration. 

CLASSIFICATION OF ROCKS. 

Unaltered. Meiamorpkic,^ BeeiduaL 

Sedimentary : 

Mechanical. Mioa-scliist. 

Chemical. 

Organic. Crystalline limestone. Clay of limestone. 

Igneous : 

6ranite-rhyolit« family. Granite-gneiss. Sand of granite. 

Syenite-trachyte family. 

Nepholine-leiu'ite rocks. 

Diorite-andesite family. 

Gabbro-basalt family. Gabbro-gneiss. Sand of diabase. 

Peridotite family. Serpentine. 



1 Only a few examples of m4tamorphie and residual rocks are given, for the i)urpo8eof showing their 
relations to the unaltered forms. 



DILLBm.] 



CLASSIFICATION OF UOCKS. 



53 



Unaltered teditnemiary rockt. 



Number of apecimen in the series. 



M«chaiiical : 

Grarel 1,2. 

Sand ' 3,4,5,6. 

Loess 7. 

Clsy ' 8,9. 

CongloiDenite 10. 



11. 



BreeciA 

Sendstone 

Gray waeke 

Shsle 

Chemical: 

Silioeoos sinter 

Vein— vein quartz 

Siliceoas oolite 

Gypsum '27. 

Limestone — 



12, 13. It. 15, 16. 17. 18. 10. 

20. 

21,22. 

23. 

24.25. 

26. 



Stalactiie 

Trarertine 

f)oliUcsan(l 

Oolite 

Limonite 

Concretions — 

Clay stone 

Containing fern . 

Marcasite 



28. 
20. 
20. 
31. 
32. 

33. 
34. 

35. 



Geode 30. 

SUicifiedwood 37. 

Sllieifled shell 38. 

Orsanie: 

Limestone ' 30, 40, (41 ),42, 43. 44, (45),4«, 47,48,49,50. 

Diatom earth , 51. 



Fossib'feitnM iron ore 

Peat 

Coals 



52. 
53. 
54,55,50. 



( 



54 



THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [boll. 15 





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DILLES.1 



CLASSIFICATION OP ROCK& 



55 



Metamorphic sedimentarif rocks. 



Metamarphio igneoun rockt. 



CrystalliBe limestone 

Marble 

Marble (dolomite) 

Qoartxite 

Qoartc-acbiBt 

Jaspilite 

Magnetic speealar hematite 

Slate (clay slate, roofing alate) 

Indurated jointed shaltf 

Crumpled shale 

Faolted pebble 

Pbjllite (nericite-achiat) 

Phyliite (chlorite- phyllite) 

MetamorpUc conglomerate (conglonierato gneinn) 

Albitescbiat 

Mica-achiat 

Homblende-achist 

Schiatone biotite-gnciwi 

.Staorolite-niica-Hchiiit 

Horn fels 

Cliiastolite-schiat 



Ko.of 
•peoimen. 

115 
116 
117 
118 
119 
120 
121 
122 
123 
124 
125 
126 
127 
128 
120 
130 
131 
132 
133 
lU 
135 



i No. of 
specimen. 



Aporhyolite 

Granitoid gueias 

Epidote- mica-gneiss 

Diabase^amygdaloid 

Quarts-nonte-gneiss 

Garaetiferons liomblende-schint 

Steatite '... 

flomblendic gabbro-gneiss (gabbro-diorlte) 

Do 

Serpentine 

Magnetite 



136 
137 
138 
139 
140 
141 
142 
143 
144 
14:> 
14G 



Residual rocks. 



KeMdual sand of gneiss 

Ilesidual sand of diabase 

Residual clay of feldspathic rock (kaolin) 
Reniihisl fla v of limestone 



No. of 
specimen. 

147 
148 
149 

l.'iO 



56 



THE EDUCATIONAL SERIES OP BOCK SPECIMENS, [bulliso. 



IUustratian$ of aur/ace modificaiioM, 



Spheroidal weathering in igneous rocka 

Spheroidal weathering in shale 

Differential weathering on impure limestone 
Differential weathering, tinted limestone — 

Glaciated rock 

Desert varnish 



No. of 
specimen. 




BESCRIPTIONS OF SPECIMENS. 

UNALTERED SEDIMENTARY ROCKS OF MECHANICAL ORIGIN. 

No. I. Beach Gravel. 

(From Nahant, Massachusbtts. Described bt J. S. Dillbr.) 

Specimen Ko. 1 consists of smooth pebbles of varioas sizes. These 
illustrate the character of the pebbles accumulated to form the hea<ih 
gravel of Nahant. They range in size from that of a pea to that of an 
^gg* When the fragments are as large as a man's head, the accumula- 
tion of pebbles and cobblestones is called shingle. The round form and 
smooth surface of the pebbles in specimen No. 1 show that they have 
been subject to much attrition on the beach. The breaking waves 
(breakers) are continually dashing against the shore, often with great 
violence, knocking the pebbles together, breaking off their corners, and 
grinding all the material within their grip to finer gravel, sand, and 
clay. The water of the broken wave returning to the sea flows down 
the beach, carrying with it some of the gravel, sand, and clay. As the ' 
water at the surface of the ocean is carried landward by the wind 
waves,^ that on the beach flows seaward as an undercurrent (undertow) 
and distributes the fragniental material, according to size, over the 
bottom. This process is illustrated in the accompanying flg. 7. 

The beach is composed largely of pebbles, with some sand. The 
undertow is generally not strong enough to carry pebbles far from 
shore, but off shallow and stormy coasts sand may be carried to a con- 
siderable distance (nearly 100 miles off the Atlantic coast), and the silt 
and clay will be deposited still farther beyond, where the undertow 
meets the deeper water. In this manner the waste of the coast land is 
spread over the bottom of the sea to form new rocks. As the trans- 
porting power of the undertow varies with the force of the waves 
in changing tide and winds, so its deposits will vary and become 
arranged in layers — i. e., stratified. 

The cliff cut by the sea in forming the beach is a sea cliff. It is the 
source of the beach gravel. Pebbles may already exist in the loose 



1 G. K. Gilbert, Lake Bonneville: Mon. U. S. Geol. Survey, Vol. X, 1890, pp. 29 and 30. 



MLua.1 DESCRIPTIONS: NO. 1, BEACH GRAVEL. 57 

material of the cliff, as along the New England coast at many points, 
bat this is not generally the case. The gravel of the beach is usually 
made from the solid rocks which form the cliff. The force of the waves 
IB so great on stormy coasts that fragments, small and large, sometimes 
weighing tons, are broken off from the fissared cliff and tumbled about 
by the dashing waves, pounding one another until ground to small peb- 
bles and clay. The rocks of the sea cliff are weakened for the attack 
of the waves by weathering, but a large part of the destructive effect 
of the waves is due to the fact that they are armed with fragments of 
rock, which are hurled with great force against the cliff, as in a bom- 
bardment. The noise of such rock-pounding by the waves on a stormy 
coast may sometimes be heard a number of miles. 

Pebbles of soft material are readily ground to pieces and disappear. 
It is only those, like specimen No. 1, of hard, relatively tough, homo- 
geneous material that stand the continuous battering and become well 
rounded. Of all minerals, quartz is the most common one which is 
both physically and chemically hard and can well resist the wear of 
wave action. For this reason many of the beach pebbles are com[)osed 



Jjan€f 




Fio. 7.>-Seciion of coast, showing beach and sediments. 

largely of quartz. When the waves are not normal to the beach the 
pebbles are thrown up the. beach in the direction of the moving wave 
and made to travel along the beach. Such pebbles are well rounded, 
and their size is usually, but not always, proportional to the distance 
they have traveled. The original shape and structure of the fragments 
are the chief factors in determining the ultimate form of the pebbles 
subject to the attrition brought, about by the waves on a beach. If the 
original fragments are flattish, and esi>ecially if their material splits 
readily in one direction, the pebbles will be flat with rounded edges, 
but if of nearly cubical blocks of practically homogeneous material, 
such as granite, syenite, and many other igneous rocks, they will be 
spheroidal, as specimen No. 1. 

There is another natural process of gravel formation which is of 
much importance and should be mentioned in this place, although 
not represented by si>ecimens. Streams of water flowing over the 
surface of the land carry with them rock fragments which have been 
detached in various ways, rolling them along the bottom of the chan- 
nel and sometimes picking them up and dropping them again. The 



58 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. Ibull.i50. 

stones thus carried give and receive many blows, not only striking 
the bed rock of the channel, but striking one another, and by the^ 
blows they are rounded, just as are the stones rolled upon the beach. 
The resulting forms are the same, and if a typical sample of stream 
gravel were added to this collection it would merely duplicate the 
sample of beach gravel. 

No. 2. Glacial Gravel (Striated Pebbles). 

(From Elkuorn, Walworth County, Wisconsin. Described ur G. K. Gilbkbt.) 

A glacier is a large mass of ice moving slowly under the influence of 
gravity. Glaciers on mountiiin slopes descend valleys and have the 
form of broad, deep rivers. They are fed at the upper end by annual 
accumulations of snow, and waste away by melting at the lower end. 
Glaciers on plains are called ice sheets. They are built up by acca* 
mulatious of snow until their own weight forces them to spread 
outward, and they waste away by melting at their margins. A moun- 
tain glacier carries a load of rock. Some stones fall upon its back and 
are borne along without modification; others are picked up from 
beneath and embedded in the ice. As one part of the ice moves faster 
than another, the embedded pebbles are rubbed against one another 
and are thereby scratched. Some of them are also rubbed against the 
rocky bed over wliich the glacier moves and are still more vigorously 
abraded. Often tliey are rubbed until quite flat on one side, the flat 
surface being polished and marked by parallel scratches. Sometimes 
after su(;h flattening they are turned and similarly rubbed upon another 
side, and in this way a large number of facets may be ground upon the 
same x>6bble. Ice sheets, similarly, have stones embedded in their 
lower portions, but usually carry none upon their backs. The parts of 
glaciers and ice sheets which waste by melting are traversed by 
streams, which may flow across the upper surface, beneath the under 
surface, or in tunnels midway. These streams wash along the x>ebbles 
that come in their way, rolling them as they go and giving them all 
the ordinary characters of stream gravel or beach gravel (see specimen 
No. 1). The stones carried by glaciers are thus of three types: (1) The 
unmodified material of the back load, which usually consists of fragments 
fallen from cliffs and is angular; (2) the pebbles and bowlders embedded 
in the ice, which are usually subangular in general form, and are char- 
acterized by scratches, or by scratches and ground facets ; (3) the x>ebbles 
finally acted upon by glacial streams, which are well rounded and hfcve 
smooth surfaces. Those of the first type resemble the stones of talus 
slopes; those of the third are not distinguishable from other stream- 
worn pebbles, nor from beach- worn pebbles (specimen No. 1); those of 
the second type are peculiar to glaciers and are distinctively said to 
bo fjlaciated. They are illustrated by the specimen No. 2. 

The corners of glaciated pebbles are usually rounded, so that there 



Mii«.l • l>E8CBIPnONS: NO. 2, GLACIAL GRAVEL. 59 

are bo sharp angles, but the rounding does not approach in perfection 
that illustrated by stream pebbles and beach pebbles. On each flat facet 
the strisB are parallel, but the direction is often different on contiguous 
&oet8. Usually the facets show also irregular scratches, and such 
scratcbes, with little parallelism, characterize the uufaceted portions 
of the surface. 

A glacier moves continuously, but slowly, from the region of accumu- 
lating snow to the region of ablation or melting, so that the stones it 
receives are all carried in the same direction, and where the ice stream 
ends by melting, the stones stop and are accumulated. Where pebbles 
are deposited without other material they constitute glacial gravel, 
hut more frequently the glaciated x>6bbles are deposited in association 
▼ith clay and sand, constituting a material called bowlder cUiy or till, 
(See specimen I^o. 9.) 

In the Pleistocene period there were many glaciers in the high 
mountains of Colorado, Utah, Fevada, California, and the area lying 
north of these States, and there were ice sheets in the northern part of 
the continent. One of the ice sheets, having its center of accumulation 
ill Canada, spread over New England, covered most of the area bor- 
dering the Great Lakes, and occupied the northern part of the Great 
Plains. During its existence it moved an immense quantity of clay, 
sand, and stcmes southward, producing a deposit which in many 
districts deeply buries the bed rock. The pebbles constituting 
siiecimen No. 2 are from a deposit^at Blkhorn, Walworth County, Wis- 
consin, where they were embedded in clay. 

The reader will find further information in the descriptions of speci- 
mens Nos. 9 and 155. He is also referred to the second chapter of The 
Great Ice Age, by James Geikie; to The Drift: Its Characteristics 
and Relationships, by R. D. Salisbury, in Vol. II of the Journal of 
Geology, and to The Rock Scorings of the Great Ice Invasion, l)y 
T. C. Chamberlin, in the Seventh Annual Report of the United States 
Geological Survey. 

No. 3. Beach Sand. 

(Pbom Suluvans Island, near Charleston, Soutu Carolina. Described by 

J. S. Diller.) 

The mineral fragments next smaller than pebbles are grains, and an 
accumulation of them is sand. Fragmental material grades in size from 
gravel through sandy gravel and pebbly sand to pure sand. Sand is 
variously designated, according to its mode of occurrence and origin, 
as well as with reference to its color, shape, and mineralogic compo- 
sition. Beach sand, river sand, residual sand, black sand, sharp sand, 
quartz sand, etc., are among the names thus given it. 

8x>ecimen No. 3, from the ocean beach on Sullivans Island, at the en- 
trance of Charleston Harbor, is a uniformly fine-grained sand , composed 
chiefly of angular grains of colorless transparent quartz. Some of the 



60 THE EDUCATIONAL SERIES OP ROCK SPECIMENS.- [bullim. 

grains of quartz are roanded and a few are stained yellowish by oxide 
of iron. If the sand is stirred with a small horseshoe magnet, only a few 
small grains of magnetite may be found adhering to the poles. Some 
of the black grains are relatively earthy material, such as results from 
the decomposition of ferromagnesian silicates like augite. Other black 
grains are yellowish green by transmitted light, and slightly pleochroic, 
with a large cleavage angle, but a small angle of extinction, and are 
believecl to be hornblende. There are a few thin, light-gray, pearly 
scales of a cloudy, transparent, biaxial mineral like somewhat-altered 
muscovite. Clear, angular, glassy grains of microcline and plagioclase 
were seen, without traces of kaolinization. Numerous small white frag- 
ments of molluscan shells are present. 

The sand is so largely composed of quartz that it affords a good 
example of quartz sand. Many of the grains are sharp-angular, show- 
ing shallow-curved pittings, resulting from conchoidal fracture. A 
larger proportion of the grains show more or less rounded corners with 
dull surfaces like ground glass, due to the attrition of the grains among 
themselves when moved. Grains of sand are so light in water that the 
feeble knocks they give one another when hustled about by the waves 
and currents make little impression as compared with that made and 
received by pebbles. For this reason grains of beach sand will remain 
angular where pebbles become well rounded. Sand of well-rounded 
grains worn only by water indicates long-continued exposure to the 
action of the waves. Waves and currents inclined to the coast cause 
the sand to travel along the coast. The farther the sand travels the 
larger the proportion of its rounded grains. 

The character of the sand in specimen No. 3 is such as to indicate that 
it is derived from two sources: (1) From the decomposed and disinte- 
grated crystalline rocks, like gneiss and granite, rich in quartz; and 
(2) from the breaking up of quartz- feldspar rocks without decomposi- 
tion. The iron-stained grains of quartz and the complete absence 
of decomposed feldsjiar indicate that it was derived from residual 
materials; the feldspar, having completely decomposed, forming kaolin 
and muscovite, was entirely separated from the quartz by the sorting 
action of the waves. The glassy quartz with conchoidal-fracture sur- 
faces and the clear fresh feldspar indicate rock breaking, for such 
unaltered material could not be of residual origin. The feldspar would 
show clouding, due to alteration. 

The beach sand of the Atlantic coast is part of the formation now 
developing in that region from the waste of the land. Other minerals 
besides feldspar are reduced to finer particles by decomx)osition, and 
removed in the process of sorting, thus concentrating the quartz to 
form sand. The bulk of nearly all sand is quartz, a condition which 
naturally results from the chemical and physical durability of that 
mineral. A considerable portion of it has been derived from the erosion 
of the Cretaceous, Tertiary, and lUeistocene sandstones of the Coastal 



mica. Feldgpatkic tand is ricb in feldspar. Oold sand or aurif- 
sand contaiQS particles of gold, aLd at some places aloug the 
of Oregon and elsewhere has been washed for gold. Auriferoas 
is usually black and is sometimes catted black sand, because of 
agnetite and other heavy black minerals it contains. 

No. 4. DuTTB Sand (Eolian Sard). 

*Fbom San Kiiancisco, California. DKhcitiiiKD by J. S. Diller.) 

» sand was collected from the dunes near Goldeu Gate Park, 
«D tSau Francisco and the Cliff House. PI. XI illustrates a imr- 
if the wind rippled surface of one of these dunes. The twigs in 
ureground show the direction of the prevailing wind from the 
The sand from the adjaceut seabeach, like that on many other 
s, is blown inland by the storms Irom the sea to form the knolls 
idges of the dunes. Tbey are well developed on-'asioually in arid 
us, and also along the shores of lakes; for example, Lake Michifriin 
?\. VI), where tbe evidence of strong winds is seen in the incliua- 
»f the trees. 

[lance at specimen No. 3 with a lens discloses the fact that the grains 
II well rounded. In this re8i)ect dune aand differs fr<im ordinary 
1 and river saud, but the difference is only one of gradation. Sand 
Atively much heavier in air than in wuter, so that when hurled by 
the grains strike much harder blows than when borne by water, 
bis fact fully accounts for the greater roundness of dune sand, 
miiieralogic composition dune sand is generally as variable as 



62 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. Ibull.150. 

extinction and cleavage angle of hornblende. Other clear-green vit- 
reous grains without pleochroism and with a large angle of extinction 
are probably pyroxene. The grains with a dull or waxy Inster, ranging 
in color between red, light yellow, gray, green, and brown, when 
crushed and exiK>sed under a lens appear to be fine, powdery, siliceons 
material like that obtained by crushing small pieces of variously colored 
siliceous slates and jasper, such as are abundant in the Coast Hange. 
Sand dunes border nearly all lacustral and marine coasts that are not 
rocky, and are usually in the form of irregular rounded ridges or 
mounds parallel to the beach and with the steeper slope to the land- 
ward. They occur not only along the coasts, where the sand is derived 
from the beaches, but also in arid regions of the interior. They are 
abundant in the great desert region of North Afirica and Arabia, as 
well iis at a numl>er of j>oints in the western x)art of our own country. 
They sometimes cover many square miles, and the accumulated sand 
may be hundreds of feet in thickness. San Francisco is largely built 
on sand dunes, which cover most of the northern end of the peninsula* 
The prevailing winds from the southwest blow the sand inland in a 
rhythmical way and iH'autifhlly ripple-mark the surHsMe of the dunes. 
l>wing to the liK*al etldies and fre<|uent shifting of the winds, the struc- 
ture of the dunes is very irreguhir. It may be compared with the 
cn^ss l>eddinir ot*o;Ksionally seen in water deposits. (See PL I.) 

With v;)riation in the direction and force of the winds and iu the 
supply of Siind, the dunes change their form and sometimes travel 
slowly inland. l>n the western bonier of Enroi^e, where blowing sand 
oivupies nearly half the *xxicjJt tri>m the Pyrenees to the Baltic^ their 
advance v;u*ies fn>m place to plat*e, and ranges between ;> and 24 feet 
I¥*r annum. Fertile lioltis and houses of ont*e iiopnious districts have 
lH»en luirieil by advanoinij dunes. Streams have been turned from 
their i^nirse^. and the whole rviiion has l>een con verted into sand wastes. 
Such dova^trtt ion can l>e avert e*l to a great extent by planting grass to 
hold the S3vnd, and tives to break the wind. In Gascon v, on the south- 
west c*»ast of France, where dunes are very large, extentiing along the 
sea for ir»(> niilt-s, with a bivadth of Wit at times as Buch as ^ mUes^ 
and rising fiv<jnt»ntly to the heii^ht of ^^^0 feet, the devastation by the 
advancing dunes was chei'ktxl and avert^ni to a considerable ej^teiit by 
phuUiniT and srr^^wing pino^ 

Saiuls shifttHl by the wiuils are ni*t oi>n lined to the steaooast, but 
tvt^ur t>n the K>:>iers of many lakes, as Trell as in the aiid repons of 
the imenor. Tliev niav W :^vn on the western shore of Lake Miohi- 
irsin ^see Tl. VI , whore the dunes reakh a height of l<«t» to iW* feet 
FoTVsis onoo entonilvii Wneaih them are being a^run exposmd by eolian 
ei\\s^>n. 

Ill f'.e^iaert re^rions the al>seT»<>i* of veg>e^tation exposess the iwks direirtJy 
t*> The woaiher. and iho dryni«s is es^^ecially favorable to transportaTion 
of sami by the winds, which are oiYen vigorous. The lik>wn sand in 




1 L 



Diiin.) DESCRIPTIONS: NO. 5, GREEN8AND. 63 

snch regions carves tbe exx)osed stones and ledges in a peculiar manner. 
The discovery of this geologic phenomenon led to the invention of the 
sand blast, which is now a mechanical process of considerable import- 
ance in the arts. 

Tbe eolian sands of the arid regions of North America have been 
described by Prof. I. C. Russell: Geological Magazine, July 1889, p. 289. 
Like those of the coast, the dunes of the interior regions are composed 
chiefly of quartz, but exceptions to this rule occur occasionally. Pro- 
fessor Kussell reports small dunes in Carson Desert com^iosed of casts 
of small crustaceans {Cypris), and others near Fillmore, Utah, com- 
posed of crystals of gypsum. On parts of the coast of Cornwall, 
England, the dune sand consists mostly of fragments of land and sea 
shells. The material has been used as a fertilizer. In places it is so 
firmly lithifled by calcareous cement that the rock can be used for 
building. References to the most important pai)ers on dune sand are 
given by Geikie in his Text-Book of Geology, third edition, pp. 335, 336. 

No. 5. Grkensand. 

(Fkom Fakmingdalk, Monmouth County, Nkw 'Ikrsky. Dkscrihkd hy 

J. S. DiLI.ER. ) 

Greensand is a sand characterized by the presence of the greenish 
mineral glauconite, which is essentially a hydrous silicate of aluminum, 
iron, and potash. It is abundant in the Cretaceous formation along 
the Atlantic coast, especially in New Jersey, where the series which it 
characterizes has a thickness of nearly 5f)0 feet. The greensand series 
18 well stratified, and contains in x>laces an abundance of marine fossils. 

Typical greensand, of which specimen Xo. 5 is an example, is composed, 
besides the glauconite, of some clay, and generally also some calcareous 
matter, with minerals like those derived from the disintegration of 
crystalline rocks in the waste of the land. Quartz is most abundant, 
bat feldspar, hornblende, magnetite, augite, zircon, epidote, tourmaline, 
garnet, and other minerals are present in small amounts. It contains 
also a few small fragments of gneiss and other rocks. 

The principal component, glauconite, is usually in more or less 
loanded, sometimes mammillated grains about 1""" in diameter. By 
means of a pliosphatic cement the smaller grains are occasionally bound 
together, forming nodules. The color of the glauconite grains is black 
or dark green when fresh, and brownish when altered. The mineral is 
soft and easily crushed, yielding in thin section light-green colors by 
transmitted light and fine aggregate polarization between crossed 
nii-ols. Fine punctures may occasionally be seen upon the surface of 
the grains; at other times they are smooth and shining; but generally 
they are dull and the surface is rather irregular. Some of the grains are 
distinct internal casts of foraminiferal calcareous shells, but generally 
the form of the shell chambers is not sharply preserved. Dr. C. E. Lord, 
who examined in the laboratory the upper marl of Farmingdale, reports 



64 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [mvLm 

that, besides containiiig glaacoiiite, the marl is characterized niiiieralog* 
ically by the presence of niicrocline, biotite, and moFcovite, a great 
scarcity of chlorite and amphibole, and an increase in the size of the 
quartz grains, which are round and often attain 3*""* in diameter, 
although generally in the greensands the average diameter is about l*"*". 
Gyx>8um and phosphorite are present. These minerals are rarely sepa- 
rated, and occur in large quantities in the argillaceous gray marl near 
Farmingdale, The phosphorite occurs in the form of an earthy, amor- 
phous, colorless substance^ frequently inclosing grains of glauconite or 
fragments of other minerals, and occasionally the spines and teeth of 
fishes. It is easily separated and subjected to qualitative analysis to 
prove that it is phosphate of lime. 

Much light has been thrown upon the formation of greensand by the 
work of the Ckalienffrr expedition. Approximately 1,000,000 s(}uare 
uiUes of the sea floor is now covered by glauconitie deposits, and they 
are limiti'd in their development to certain areas adjacent to the land 
where Foraminifeni are ]>re«ent and the amonnt of sediment is small. 
Tlie depth generally is between 100 and 200 fathoms, although glauco- 
nite is sometimes found at much greater depths. 

In the glauconitie material dredged up from the present sea floor by 
the CkailrHijer and other expeditions, the groins of glauconite are com- 
monly found in the cavities of Fon^minifera and other organisms, and 
in many other cases the grains show clearly the interior casts of such 
shells^ the shells having disapi>eared. This association is so general 
that all the pn>lKibiIitie^ apinnir to favor the opinion that the glauconite 
was formed orisrinallv in the cavities of organisms. Greensand has been 
found in grtniter or less amounts in nearly every geologic horizon from 
the Cambrian down to the present time« but is es^iecially abundant, as 
already indicateil, in iKtrtions of the Cretaceous. Greensand is exteii- 
sivelv used in Xew Jersev as a fertilizer. 

The student will find much additional information concerning the 
formation of the glauituutie deix^sits of the pre^^nt oceans in the Chal- 
lenger Re|H>rt of Det»p Sea DeiH>sitSs esi>eoially on pp. 378 to 391. Con- 
cerning the givens;inds of the Atlantic coast, reference may be made 
to A l^Tvliniinary KeiH>rt on the Cretaceous and Tertiary formations 
of New Jersey, by W, 1*. Clark, in the Annual ReiMirt of the State 
Gei^loirist of New Jcrst^y for ISiVJ. pp. lt»5^ to 245; also to the Journal of 
Get>Iogy « February and March, lSiM\ VoL 11, p. 161. 

Xix ti. Fossils fko>i Marixs Sakd. 

^FR'^m Gr^^vik Wharf, .Iamks City CorxTY, Vikoixia. Dk$4.^kibei> by 

J, S. lnil-FR,» 

In the beach sand of specimen Niv 3 there are fragments of shells 
bn»ken u]> by the w;ives. At m:uiy p*;u*x\^ alon^ the «:oast the shells are 
buried in the sand before they aiv brx^ken. This takes place es|)ecially 



DE8CEIPTIOMS: NO. 7, LOESS. 



65 



beneath tbe water a little offshore, or in a bay, where the force of the 
vsves ia not so violeDt as ou an exposed beach aud animal life is 
abuudant to supply the shells. When baried so as to become a part of 
the earth, and thus to form a record of tbe kind of animals that lived 
wbile the sand was being deposited, tbe shells are fossilii. The two 
shells of specimen No. 6 are fossils whose specific names are Venus 
mrreenaria and Pecten jeffergonivs. They were found bnried in the sand 
a& illustrated in fig. 8. The clams and scoUopii which bore these shells 
livvd and died in the sea during the geologic epoch known as tbe Mio- 
cene. The bodies disapiMtared, bnt the shells remained unaltered, 
buried in tbe sand, to tell of tbe life of that ancient time. 

The presence of the shells iu tbe blufi's on tbe land far from the sea 
allows that during tbe Miocene epoch the ocean covered much of what 
'» DOW dry land, and that since then tbe sea bottom at that pavticular 
plat:e. and in fact all 
^oiig the Atlantic coast 
oftbe United States from 
Vas-sachusetts to Florida, 
has been raised and the 
coast line moved farther 
eastward to its present 
pMition. 
Specimen No 6 fur 

nishes an example of the 

simplest type of fossiliz v- 

tioii. The organism is 

bnried and a large part 

of it. the shell remains 

Doaltered. Specimen No 

38 illustrates a type of 

fossilization in which tbe 1- 




Bhell is completely ?«» «— aiwUi in m.m.e «iil «l Crlers Grme on J»uw« 

changed, aud only the 

outer form is preserved, while in specimen No. 37, which is a third type, 
although tbe organic matter is completely replaced, the outer form and 
debcate iutemal structure of the wood are completely preserved. 



No. 7 



Loess. 



I. S. DiLLBR.) 

The pale- yellowish, homogeneous, fine-granular, somewhat calcare- 
ous earthy material forming the bhiiTs at Muscatine, Iowa, and else- 
where along the Mississippi is called loess. The name was first given 
to material of the same sort occuitying the valley of the Rhine. 

Tbe loess of Muscatine is intermediate in fineness of gniiii between 
sand and cluy, and, although it appears remarkably uniform, there is 
Goosiderable range in the size of the particles — from about 1 mm. to 
Boll. 150 5 



66 THE EDUCATIONAL BEEIES OF BOCK SPECIMENS. [bcll.150. 

0.0001 min. in diameter — the finer material forming by far the greater 
X>ortion of the mass.^ Under the microscope it is seen that most of the 
grains are angular or subangnlar, but many are distinctly roauded, 
showing that at some stage of its existence they have been sabjected 
to considerable attrition. Quartz is the most abundant mineral iu 
both rounded and angular grains. Clear, colorless grains of orthoclase 
and plagioclase feldspar and green pleochroic fragments of hornblende 
are common but not abundant. Yellowish, strongly pleochroic, foliated 
scales of biotite occur occasionally, and rarely fragments of a pale- 
reddish, strongly absorbing mineral, supposed to be tourmaline. There 
are a small number of black, opaque grains, some of which are prob- 
ably ores of iron, although none were found to be magnetic, even after 
heating. The flocculent yellowish or rusty-gray particles are clay, col- 
ored chiefly by oxide of iron. They become decidedly reddish by igni- 
tion, just as in the manufacture of red bricks the yellowish hydrous 
oxide of iron (limouite), by heating and driving off the water, is 
changed to red hematite. 

In dilute hydrochloric acid the loess effervesces vigorously, but only 
for a few moments, showing the presence of small amounts of carbon- 
ate of lime. It generally occurs in the form of small nodules or tubes, 
and by studying the loess in the field it has been found that the 
minute tubes represent the rootlets of vegetation penetrating from the 
surface. They ramify the deposit, but as their general direction is ver- 
tical they give to the loess a vertical structure, which tends to make 
it form cliffs where sufficiently thick and properly exposed. This fea- 
ture can be seen in PL VII, which shows a typical exposure of the 
loess at Muscatine. At this exposure the loess appears to be massive, 
i.e., without evident stratification. Such is the general character of 
the loess, but at other exposures in the same region there are distinct 
traces of stratification due to aqueous deposition. 

Besides the mineral constituents already noted, the loess from Mus- 
catine contains a few minute coiled shells of land snails. According 
to W J McGee,'^ the loess of thcj southern part of the State contains 
also the fragments of other land animals, a few water snails, and still 
fewer mussels, the latter of subarctic and arctic facies. The land ani- 
mals are most closely related to arctic and subarctic forms, indicating 
that the loess originated during a cold epoch. 

The distribution of the loess in the Mississippi Valley, as recently 
pointed out by Chamberlin, may be summed up in two great features: 
It is distributed (1) along the leading valleys, and (2) along the border 
of the former ice sheet at the stage now known as the lowan. The 
loess occurs principally in the valleys of the Missouri, Mississippi, 
Illinois, and Wabash rivers. Along the Missouri River it is found 
from southern Dakota to its mouth, and along the Mississippi Kiver 



' Report of the Illinois World's Fair Coinmissiou, by Milt'on Whitney, pp. 93-114. 
^ISloventh Ann. Bept. U. S. G«ol. Survey, Part I, 1891, pp. 800 and 471. 



wu«Ll DESCRIPTIONS: NO. 7, LOEBS. 67 

from Minnesota to southern Mississippi. Along the Dlinois and the 
Wabash it occurs from the points of their emergence from the territory 
of the later glacial sheet to their months. In all these valleys the 
loess is thickest, coarsest, and most typical in the bluffs bordering the 
rivers, and becomes finer, thinner, and nontypical as the distance from 
the rivers increases. 

The distribution of the loess along the frt>nt of the ice sheet in glacial 
times was elaborately worked out for Iowa by Mr. McGee, who showed 
that the loess material was the fine stuft' ground up by the glacier 
which, during the loess epoch, terminated in that region. Many others 
have so greatly extended the evidence of this relation that it is now 
generally accepted as one of prime importance in considering the origin 
of the loess. While it appears to be clear that the material of the 
loess in tbe Mississippi Valley was furnished by the ice sheet, and that 
in some way it was distributed chiefly by water, there are many puz- 
zling features of its distribution that are not easily accounted for; as, 
for example, the wide range in altitude of the loess deposits. The 
extent of the vertical range, as pointed out by Ghamberlin, is about 
1,000 feet, and frequently the range within a score of miles is from 
500 to 700 feet. The fluctuating floods of the loess epoch must have 
ex])osed portions of the loess-covered flats to the action of the winds, 
and the fact that evidences of wind action in the original deposition of 
the loess have been observed has led Ghamberlin to suggest that eolian 
as well as aqueous agencies may have played an important part in the 
deposition of the loess of the Mississippi Valley. 

Loess similar to that of the Mississippi Valley has been recognized 
in many parts of the earth, especially in the valley of the Ehiue, along 
the Danube, and in various portions of southeastern Asia. Notwith- 
standing the similarity of its general features in many regions, its 
origin may be very diverse. In China, for example, its large masses 
are supposed by Kichthofen to be chiefly wind-blown material derived 
from the disintegration of the rocks in the adjacent hills. Prof. I. O. 
Bassell^ has shown that the ^^ adobe ^^ in the arid regions of the West 
is practically identical with the loess of China, and that its subaerial 
accumulation is to be mainly ascribed to the action of ephemeral 
streams. Wherever it occurs the loess or adobe is a fertile soil and, 
excepting in arid regions, sustains a large agricultural population. 

The student will find further information on this subject in the fol- 
lowing papers: The Driftless Area of the Upper Mississippi Valley, by 
T. C. Chamberlin and E. D. Salisbury, Sixth Annual Report of the 
United States Geological Survey (1884^'85), pp. 278-307; The Pleisto- 
cene History of ^Northeastern Iowa, by W J McGee, Eleventh Annual 
Report of the United States Geological Survey (1889-'90), Part I, pp. 
291-303; Supplementary Hyi>othesis Respecting the Origin of the Loess 
of the Mississippi Valley, by T. C. Chamberlin, Journal of Geology, 
Vol. V (Xovember-December, 1897), pp. 795-802. 

> Geological Magaslne, 1880, p. 349. 



68 THE EDUCATIONAL SERIES OF BOCK SPECIMENS, [bull. 150. 

Ko. 8. Brick Clay. 

(From Brick Havbn, Alexandria County, Virginia. Described by J. 8. 

DiLLER.) 

Fine earthy material like specimen No. 8, which is somewhat firm and 
brittle when dry. but plastic and tenacious when wet, is clay. When 
pure, it is composed essentially of silicate of aluminum. Generally, 
however, it is impure from the presence of fine m'ains of quartz and 
oxide of iron, as well as other minerals. 

Specimen No. 8 is clay from a bank, represented in PI. VIII, on the 
Virginia side of the Potomac, nearly opposite Washington. Its pale- 
yellowish color is due to the presence of hydrous oxide of iron (limonite), 
which when the brick is burned loses water and becomes hematite, 
coloring the brick red. 

When stirred in water the clay readily goes to pieces; the coarse 
particles subside rapidly, the finer ones slowly. By decanting, the 
material can be separated into portions differing in size of the particles. 
Under the microscope the coarser portion is seen to contain numerous 
grains of quartz, with a few of clear banded feldspar. There are occa- 
sional minute round nodules of limonite; and nearly all the grains of 
sand, as well as of clay, are stained yellowish by oxide of iron. The 
coarser portion is made up chiefly of quartz grains and oxide of iron, 
and many of the grains of quartz are well rounded. In the finer por- 
tion clay particles are more abundant, but they are much smaller than 
the quartz and other mineral fragments. The clay is in very fine scales 
and is stained reddish yellow by oxide of iron. The scales look like 
minute flakes of mica, but may be distinguished by their very weak 
double refraction. The basis of clay is silicate of aluminum, and when 
pure in minute scales it is colorless. The aggregation of such scales 
looks white, like snow, on account of its porous structure. 

Clay is the insoluble residue left from the decomposition and disinte- 
gration of aluminous silicates, especially feldspar, but it is derived also 
from nephelite, sea polite, and other minerals containing much silica and 
alumina. The wliite clay (kaolin) derived from feldspar is illustrated in 
specimens Nos. 147, 148, and 149, which are the residual material of feld- 
spathic rocks. Specimen No. 150 is clay derived from the decomposition 
of an argillaceous limestone. The clay was originally deposited with 
the carbonate of lime in the limestone as it formed. Later, when the 
limestone was raised above the sea and exposed to the weather, the 
rain dissolved the soluble carbonate of lime and left the insoluble 
residunl clay. 

The clay represented by specimen No. 8 forms a de])osit of consider- 
able extent along the Potomac near Washington, and differs from tbe 
clays mentioned above in that it has been transported by water from tbe 
place whore it originated to its present position. The deposit rises less 
than 150 feet above the river. It is extensively used in the manufacture 



wuM.] DESCRIPTIONS: NO. 9, BOWLDER CLAY. 69 

of brick, and a view in one of the clay pits is ^iveii in PL YIIL Ac- 
cording to W J McGee,* the clay belongs to the Golnmbia formation, 
and was dei)08ited in an estuary on the borders of a river delta of the 
ancient Potomac. At the time this clay was deposited (i. e., daring 
tbe later Columbia epoch) the Atlantic slope in the vicinity of Wash- 
ington stood about 150 feet below its present level. The Potomac en- 
tered the estuary at Washington and dropped its load of sediment, of 
which the clays were the finer portion. They were derived, at least 
in largo part, from the loose mantle of residual material resulting 
from the decomposition and disintegration of the rocks in the Potomac 
basin. 

From a study of the relation of the Columbia formation to bowlder- 
bearing strata and to the great terminal moraine which stretclies 
across the country from Cape Cod in Massachusetts to Dakota, Mr. 
McGee concludes that the clays of the Potomac river near Washington 
were deposited during the first and second glacial periods, in the early 
part of the Pleistocene. 

No. 9. Bowlder Clay. 

(From Rociiestrr, Xew York. Described by G. K. Gilbert.) 

Certain clays produced by glaciers contain pebbles and bowlders, 
and are therefore called bowlder clays. In the description of specimen 
No. 2 some account is given of glaciers and ice sheets, and mention is 
made of the ice sheet which in Pleistocene time covered a large portion 
of northeastern North America. As the stones held in an ice sheet 
^b one on another and are gradually reduced in size, the particles 
ground off remain embedded in the ice. Where the stones nib against 
the bed rock, not only are they ground away, but the bed rock itself is 
^orn, and the product of all this abrasion is received by the ice and 
carried forward. Where the ice melts, its load comes to rest, forming a 
<l^ix)8it, iind in this deposit the coarser and finer fragments may be 
indiscriminately mixed. Such a mixed deposit is called iilU Some- 
times its finer part is sand, but usually there is enough clay to make it 
coherent, and the name bowlder-clay is then commonly used. Bowlder- 
days are heterogeneous not only in mechanical structure but in compo- 
sition. The ice in its journey abrades many rocks, and the particles 
gromid from all these are mingled together in the resulting till. The 
ratio ill which the different rocks traversed are represented in the till 
tlepends partly on the extent of their outcrops and partly on tlie readi- 
uess with which they are ground away; and it is also found that the 
nearest rocks are relatively better represented than the more remote, 
^n the northern States the motion of the ice sheet was soutliward, and 
the till at each locality is largely composed of pebbles and rock flour 
from the formation lying immediately north of it. Specimen No. 9 is 



'Piroc.Am.AMOc. Adv.Sci.. Vol. XXXVI, p 221. Alt»o Am.Jour.Sci., 3d aeries. Vol. XXX V, p. 331. 



70 THE EDUCATIONAL SERIES OP BOCK SPECIMENS, [bull. 150. 

from a till deposit underlain by the Niagara limestone^ bat a short dis- 
tance to the north begins a broad outcrop of Medina shale^ and this 
formation furnished the chief material of tbe clay, giving its reddish 
color. 

The specimen appears, on first examination, to contain a few fine 
pebbles and very little sand ; but when the coarser portion is separated 
from the finer by successive washings, sand is found to form a consid- 
erable proportion of the whole mass. The sand is chiefly quartz, with a 
small mnountof feldspar and green hornblende and much oxide of iron. 
Some of the larger grains are angular, but the smaller ones are often 
rounded. The very fine material is not a normal clay, such as results 
from the decomposition of aluminous silicates, but is a rock flour, due 
to the trituration of a variety of materials, including various unaltered 
as well as decomposed silicates and the carbonates of lime and mag- 
nesia. The sand grains were probably derived chiefly from tbe sand- 
stone of the Medina formation, and had been rounded before they were 
received by the ice. 

Further information will be found in the descriptions of specimens 
Nos. 2 and 1 55. The reader is also referred to The Terminal Moraine of 
the Second Glacial Epoch, by T. G. Chamberlin, in the Third Annual 
Report of the United States Geological Survey; and to The Surface 
Geology of New Jersey, by R. D. Salisbury, in the Annual Report of 
the State Geologist of New Jersey for 1891. 

No. 10. GONGLOMEEATE. 
(From Utica, Oneida County, Nkw York. Described by J. S. Diller. ) 

When gravel is cemented so that the pebbles and sand of which it is 
composed are bound together, the rock formed is conglomerate. It dif- 
fers from gravel only in containing a cementing substance which con- 
verts the loose material into a solid mass. 

In specimen No. 10 the pebbles are quartz. They are small and 
generally well rounded. Some, indeed, are subangular, but most of 
them have the corners completely reduced. The sand, and also the 
cement which holds the fragments together, are chiefly quartz. 
Although there are a few brass-yellowish grains of pjrrite, and some 
black ones of other minerals, almost the whole mass of the rock is 
silica. It is a deposit in which quartz is so abundant as to indicate 
that at the time the conglomerate was formed the conditions were 
especially favorable for the accumulation of quartz. 

The bed of conglomerate from which specimen No. 10 was taken is 
well exposed in Oneida County, New York, and on this account is called 
Oneida conglomerate. It is associated with a thick sandstone — the 
Medina — which varies in color from red to white, and which may be 
traced almost continuously from western New York eastward to near 
the Hudson, where its upturned edge swings to the southwest, stretching 



wLLttO DESCRIPTIONS: NO. 10, CONGLOMERATE. 71 

away thioagb New Jersey, Pennsylvania, Maryland, Vir^nia, and Ten- 
nessee into Alabama. From this long line the outcrops of the Medina 
sandstone and conglomerato extend westward to the Mississippi Valley, 
but in that direction the material gradually becomes finer. 

The shells found in the sandstone, as illustrated in specimen No. 19, 
show that it was deposited in the ocean. The reduction in the size of 
the {^articles of the sediment toward the Mississippi Valley, in accord- 
ance with the general relations of littoral deposits illustrated in fig. 7 
(p. 57), indicates that the shore of the ocean lay to the eastward, in the 
Appalachian region, and that at that time the sea occupied the Missis- 
sippi Valley. 

Most conglomerates contain a considerable proportion of quartz peb- 
bles, or pebbles of siliceous rock, but besides these they usually contain 
pebbles of many other kinds. The most abundant pebbles are those of 
tbe most durable rocks, such as vein quartz, quartzite, graywacke, 
granite, and various kinds of volcanic and plutonic rocks, especially 
those which contain a high percentage of silica. When pebbles of any 
one kind predominate, special names, such as quartz conglomerate, lime- 
itone conglomerate^ and volcanic conglomerate, may be given to the rock, 
according to the prevailing constituent. 

Conglomerates may be coarse or fine, according to the size of the 
pebbles of which they are composed. Fragments larger than pebbles 
are often called bowlders^ and conglomerates containing them have 
been designated bowlder conglomerates. The size of the fragments and 
their sarface features are indexes to the character and strength of the 
transporting i)ower by which they were deposited. 

As indicated in the descriptions of specimens Nos. 1 and 2, gravel 
originates wholly in the process of erosion, either by glaciers or streams 
of water on the land, or by waves of the sea beating on the coast. On 
stormy coasts, where much gravel is produced and the undertow is 
strong enough to carry it seaward, it is spread over the bottom near 
shore to form a bed of conglomerate, as illustrated in fig. 7. Being 
beneath the sea, the deposit is below the level of erosion and is pre- 
served so long as it remains in that position. On the land, however, 
the conditions are diflferent. The gravel deposits of glaciers and water- 
eonrses are ever exposed to erosion. By shifting fioods they are gradu- 
% washed down to lower levels toward the sea, and finally the material 
is carried into the sea, where it finds a resting place. For this reason 
ravel deposits of the laud are ephemeral. In the process of erosion 
they are carried into the sea, where, like those formed by the waves, 
they are added to the more permanent deposits which contribute to the 
upbuilding of new formations. The coarsest material is deposited 
i^earest shore and marks approximately the direction of the shore line, 
80 that by tracing out the coarse conglomerate among the ancient sedi- 
Dientary rocks we can to a considerable extent make out the geography 
of the land and sea. The distribution of the sandstone and conglomerate 



72 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bull.150. 

of which specimen No. 10 is a sample shows clearly that the shoreline 
of the Medina epoch was in the Appalachian region and that the land 
fhrnishing the sediment lay to the eastward. 

The cement which binds the loose material together and converts 
gravel into conglomerate varies from place to place, and sometimes holds 
a more or less definite relation to the composition of the rock. In con- 
glomerates where the sand and pebbles are chiefly qaartz the cement 
is often siliceous; in those containing fragments rich in iron it is 
generally ferruginoas. Silica and oxide of iron are the most common 
cementing sabstances, bat carbonate of lime also occurs in some 
regions. These three cements are illustrated in sandstones by speci- 
mens No8. 12, 14, and 15. In some cases there is no visible cement, the 
pebbles and matrix being so closely pressed together that they adhere. 

No. 11. Breccia* 

(From Virgixia, opposite Point of Rocks, Marylaxd. Dbscbibed by 

J. S. DlIXER.) 

Breccia differs from conglomerate in the shape of the fragments of 
which it is composed. In conglomerate most of the pebbles are rounded, 
but in breccia the fragments are angular. Intermediate stages between 
the two rocks have been called brecciated conglomera'te. 

Breccias are much less common than conglomerates, and are pro- 
duced in various ways. Those of sedimentary origin are of little 
imiK)rtance and grade into conglomerate. Specimen No. 11 is of this 
type. It was selected on account of its distinctly fragmental structure, 
its availability, and its architectural application. Although many of 
the fragments are angular, others are well rounded; in fact, at most 
places where this rock crops out the round pebbles predominate, so 
that generally the rock is a conglomerate. The fragments are nearly 
all limestone, and at the time the breccia originated the fragments 
were transported only a short distance from their source. This fact is 
readily determined by studying the rock in the field, where it occurs 
near the limestone from which the fragments were derived. 

The color of the limestone fragments in specimen No. 11 varies accord- 
ing to that of the parent rock, but the interstitial material of sand, car- 
bonate of lime, and oxide of iron in which the pebbles are embedded is 
uniformly red, like much of the Triassic sandstone belonging to the 
same formation. 

As in conglomerate, there are three substances which act as cement 
in the breccia — carbonate of lime, oxide of iron, and silica — ^aud all are 
of nearly equal imi)ortance. If a piece of the red material between the 
I>ebble8 is placed in hot hydrochloric acid until the carbonate of lime 
and oxide of iron are dissolved away, the fragment usually retains its 
form, owing to the siliceous cement present. 

The rock of which s}>ecimen No. 11 is a sample has been used for orna- 
mental building pur[)oses. A series of large columns of this material 



DE8CBIPTION8: NO. 11, BRECCIA. 



73 



adorn tbe old Hall of Bepreaeutatives, now calleil Statuary Hall, in 
the Capitol at Wasbiugton. But it i^ so difficult to drestt and poliali 
evenly that it is not oztensively used tiir hucIi puriwses. 

For further information concerning thia rock, reference slionld be 
made to a p&|>er bj- Arthur Keith on The Geology of the Oatoctiu Belt, 
in the Foart«eoth Annual lieport of tbe United States Geological 




Fio. B.-.-BnccUtal DaTimiui 



FayetM, low*, 



Snrvey, Part II, page 340, and to Stoneb for Building and Det'oration, 

by G. 1'. Merrill, page 93. 
Besides the breccias of sedimentaiy origin, illustrated by specimen 

Ho 11, there are talm breccias^ fault ot friction /»rccciVij(, and vohanicai 
trwptire breccias. Under the Influence of the weather, on steep slopes 
lorktt break up into angular fragments, and beneath cliffs such frag- 
ments generally accumulate and form a talas, which in some places 
^iDeBccmente«l so as to form lalus breccia. In breccias of this sort 



r 
■J 



74 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BrLL.U«. 

none of the fragments are rounded. They occur perhaps most fre- 
quently in regions of extensive limestones, especially such as are 
cavernous, limestones furnishing both the firagments and the cement 

Along lines of fracture, where rocks have been faulted and crushed, 
the angular fragments thus formed may be cemented by substances hdd 
in solution by water circulating in the fissures, and thus form breccia. 
Such breccia is called fault breccia or friction breccia. Breccias of a 
smilar sort, whose origin is in some cases at least not yet clearly under 
stood, are illustrated by fig. 9, which rex)resents a limestone breccia 
described by W J McGee in the Eleventh Annual Report of the 
United States Geological Survey, Part I, pages 319-321. 

Fragments ejected from volcanoes are often angular, and a consoli- 
dated accumulation of them forms volcanic breccia. Acid lavas are 
usually viscous, and during their eruption are sometimes so crushed 
and broken as to become a mass of angular fragments. When cemented 
together by material of the same kind as the fragments, as is often the 
case, such lavas are breccias, and to distinguish them from others are 
called lava breccias or brecciated lavas. Lava flowing over angular frag 
mehts on the surface may pick them up, and the mass may thus become 
brecciated. 

In the various kinds of breccias there may be a wide range not only 
in the size of the fragments but also in their chemical comx)ositioD. 
Usually, however, there is less variety in composition than among the 
pebbles of a conglomerate, but a greater range in size. 

No. 12. Pebbly sandstone. 

(From Barkon, nkar Ashland, Jackson County, Oregon. Describkd by J. S. 

DlLLER. ) 

Conglomerates are composed of pebbles, and sandstones are composed 
of sand. They are often found intermingled as alternating layers of the 
same mass and pass into each other gradually or abruptly, recording the 
gradual or sudden change in the currents by which the material was 
deposited. Between conglomerates and sandstone there are many 
intermediate grades. These may be represented by specimen No. 12. 

Pebbly sandstone is composed chiefly of sand, but contains so large 
a proportion of conspicuous pebbles that these deserve mention in the 
name. This specimen was collected from a pebbly bed in a mass of 
Cretaceous sandstone resting unconformably on the older rocks of the 
Klamath Mountains, from which the material to make the sandstone 
was derived. The dark-colored i)ebbles are chiefly slates, while the 
lighter-colore<l ones are from masses of serpentine and other eruptive 
rocks. The fragments are all metamorphic rocks, and some are full of 
small veins. 

Gray sand, which constitutes the greater portion of the rock, is com* 
pose<l chiefly of quartz and feldspar, with some mica and other minerals 
derived from the diorities, granites, and similar eruptive rocks, as well 
as from the slates with which they are associated in the Klamath 



Diu«.l DESCRIPTIONS: NO. 13, GRAY SANDSTONE. 75 

Moantains. Some of the small, dark grains of sand are, like the })eb- 
Ues, intersected by microscopic veins. Although the x)ebbles are well 
rounded, the grains of quartz and feldspar are angular. This feature 
is well illustrated in PI. IX, A, Some of the grains are of plagioclase 
feldspar, but their banding can not be seen in ordinary light. The 
other grains are of quartz, with sharp, angular outlines, strongly 
ooDtrasting with the rounded forms of the pebbles with which they are 
associated. This association shows clearly that the pebbles are rounded 
iBore easily than the grains of sand, and the reason for this is to be 
foand in the fieust that the grains of sand, being so light in the buoyant 
water, strike such tiny blows when they collide with one another 
daring transportation that but little effect is produced. On the other 
band, the pebbles, on account of their greater weight, strike much 
nore effective blows and soon get their corners knocked off. 

A drop of acid on specimen No. 12 causes brisk effervescence, show- 
ing the presence of carbonate of lime as a cementing substance between 
the grains of sand and pebbles. 

At the time the pebbly sandstone was formed the Klamath Mountains 
were an island in the Cretaceous seas and received the beat of the 
waves, which, to a large extent at least, produced the fragments and 
deposited them to make the pebbly sandstone. Although all the 
fragments in the pebbly sandstone are of metamorphic rocks, the 
pebbly sandstone itself is entirely unaltered. So it is evident that the 
rocks of the Klamath Mountains were metamorphosed before the 
Cretaceous sandstone was deposited* 

No. 13. Geay Sandstone. 

(FltOM BSREA, CUTAHOOA COUNTT, OhIO. DESCRIBED BY J. S. DiLLER.) 

Sandstone is consolidated sand. As long as the material is loose and 
iQcoberent it is sand, but whenever by any process the particles are 
insde to cohere so as to form a solid rock, they become sandstone. All 
saudstones were originally sand and show similar variations in com- 
position and texture. The range of variation, however, is greater 
tbjui in sands, on account of the differences in the composition, color, 
And other properties of the cement. 

The grains of some porous sandstones are angular and so loosely 
cemented that when broken the surface of the rock is rough and gritty 
to the feel. Such sandstones are commonly called grits, and of these 
Bpeciiuen No. 13 is a good example. It is well exposed at Berea, Ohio, 
where it is extensively quarried for building stones and for grindstones 
ftod is generally known as the Berea grit. 

The Berea grit is a fine-grained, homogeneous sandstone, composed 
^ost wholly of quartz and orthoclase feldspar. The latter is greatly 
^tered and in most cases completely changed to kaolin, which is readily 
distinguished by its whiteness. With the gray quartz, it gives color to 
tberock. Some grains of kaolin contain fresh cores of feldspar, and a few 
comparatively fresh graina of microcline and plagioclase may \)e b^w. 



76 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. Ibuu«1M. 

Muscovite and pjrrite are rather rare. The cementing substance is 
argillaceous and is impregnated by oxide of iron, but there is not a 
sufficient quantity present to modify the color of the rock. The cement 
is weak and does not fill all the interstices between the grains of sand. 
On this account the rock is soft and porous, so that it can be easily 
carved and readily absorbs a large amount of water. 

Kaolin occurs in distinct grains of essentially the same shape as th< 
quartz, although somewhat more rounded. These grains, as well » 
the thick coating of soft kaolin which envelops some of the roundec 
grains of feldspar, had not yet been formed at the time the sandstoni 
was deposited, else it would have been removed by the attrition th< 
grains have experienced during their transportation. 

The original freshness of the material and the presence of such { 
considerable portion of feldspar suggest that the surface of the lan< 
at the time the sandstone was formed must have been one of consid 
erable relief. It is only when streams have considerable fiall tha 
the currents are swift and strong enough to carry grains of sand au< 
pebbles. In a rapid stream pebbles and bowlders are rolled along oi 
its bed. They frequently knock together, break to pieces, and b; 
long-continued attrition are reduced to sand and finer sediment, bu 
the sand formed in such cases is composed of fresh minerals. Th 
feldspar, at least when the sand originates, is chiefiy unaltered, bu 
on subsequent exposure to weathering it may readily become changei 
to kaolin, as in the Berea grit. Had the land been one of gentle relief 
worn down to almost a plain (peneplain), the streams would have beei 
sluggish and able to remove only the material resulting from th 
decomposition and disintegration of the rocks. The fine particles o 
kaolin derived from the alteration of the feldspar are readily 8eparate( 
from the quartz during transportation, leaving the sand composei 
almost exclusively of quartz. Thus it appears that the original pres 
ence of a considerable proportion of unaltered feldspar indicates tha 
the land from which it was derived at the time the Berea grit wa 
formed was one of considerable relief. Chemical analysis of the rod 
according to Mr. G. P. Merrill,^ shows that the rock contains abou 
95 per cent of silica, with a small amount of lime, magnesia, oxid 
of iron, alumina, and alkalies. When freshly quarried it contain 
from 5.83 to 7.75 per cent of water, but when dry only 3.39 to 4.28 pc 
cent. 

The Berea grit has a wide distribution in Ohio, having an extent ( 
about 15,000 square miles above and below ground. This wide exteii 
is remarkable considering its thickness, as it seldom reaches 50 fee 
In the northern x)art of the State it is medium grained and contain 
some pebbles, but in the middle and southern portion of the State i 
is fine grained. Its surface is often ripple-marked, and worm burrow 
abound as on the sands of modern beaches, indicating that the Berc 
grit was formed along an ancneut shore line. 



' StoueB for Building aud Decoratiou, p. 77. 



DllLBB.] 



descriptions: no. i4, brown sandstone. 



77 



On account of its agreeable color, its dorability, and the ea8e with 
which it is worked, it is a valaable building stone. Its grit makes it 
Taluable for grindstones, and its i)oro8ity makes it a reservoir for 
petroleum and gas. According to Prof. Edward Orton,^ it is '^ the most 
important single stratum in the entire geological column of Ohio. Its 
economic value above ground is great, but it is greater below. In its 
outcrops it is a source of the finest building stone and the best grind- 
stone grit of the country, and when it dips beneath the surface it 
becomes the repository of valuable supplies of petroleum, gas, and 
salt water.^ 

ISo. 14. Brown Sandstone. 

(From Hummelstown, Dauphix County, Pennsylvania. Described by 

J. 8. DlIXER.) 

The sandstone of Hummelstown, Pennsylvania, is a typical sandstone 
with ferruginous cement. In color it is usually purplish brown with 
minute white specks. The uniformity in the size of its rather small grains 
gives the rock an even texture. Its feel is decidedly gritty, owing to 
the angular form of the graihs. It is composed chiefly of angular grains 
of quartz with some clear, fresh microcline and plagioclase, showing dis- 
tinct twinning. Occasionally fragments of a mineral with very strong 
absorption perpendicular to the prismatic axis, and parallel extinction 
like that of tourmaline, may be found. The kaolin present is sometimes 
in distinct grains of about the same size as those of quartz. It was 
deposited chiefly as finer silt between the grains of sand, and is much 
less abundant than the quartz. The largest grain shown in the lower 
left-hand quarter of PI IX, B^ is kaolin. The others are nearly all 
quartz. The brownish cement which is the chief interstitial substance 
and coats many of the grains of quartz is ferric oxide. On account of 
its abundance it gives color to the whole mass. As its color varies 
through shades and tints of brown and red, so also the rock varies in 
color. 

The following analysis, by E. A. Schneider, shows the chemical com- 
position of the rock : 

AnalyH$ of brown aandttone from HummeUtown, Pennsylrania, 



SiO, , 

Al^ 

FejOj 

PeO 

c»o 

MgO 

KjO 

N»,0 

H,O(i05O) 

H,0 (IgDition) 

Total .... 



Per cent. 

88.13 

5.81 

1.77 

.31 

.20 

.S3 

2. 03 

.06 

.23 

.26 



90.03 



1 Geological Survey of Ohio, Economic Geology, vol. 6, p. 28. 



78 THE EDyCATIONAL SERIES OF ROCK SPECIMENS. [bull.i». 

At the quarry from whicli specimen No. 14 was obtained the rock near 
the surface is reddish brown, and the greater body of the rock deeper 
in the earth is purplish brown. The bedded arrangement of the rock 
is a prominent feature of the quarries. The even layers are usually less 
than 10 feet in thickness and are cut by joints, which greatly facilitate 
quarrying. 

The formation represented by the brownstone of Hummelstown, 
Pennsylvania, has a wide and irregular distribution along a belt 
stretching from New England to South Carolina. Throughout the 
whole belt, although it varies in texture considerably, ranging all tbe 
way from a coarse conglomerate and angular breccia to shale, it is 
everywhere deeply colored by oxide of iron and frequently associated 
with compact, dark, heavy, igneous rock, such as the basalt of Orange, 
New Jersey, illustrated by specimen No. 102. Specimen No. 11 is 
breccia from the border of the same formation near the Potomac. 

Much has been written on this formation. Its bibliography is given 
by Prof. I. C. Kussell,^ who illustrates its distribution by maps and 
fully describes its character and the hypotjieses concerning its history. 
He calls it the Newark system. Fossil plants and fishes, and also the 
footprints and bones of huge reptiles and batrachians, have been found 
at a number of places, and their evidence fixes the age of the Newark 
as Juratrias. Whether it was deposited in a series of local basins, 
corresponding to the present disconnected distribution of the rocks, or 
as a broad terrane in one irregular and continuous arm of the sea 
stretching from New England to South Carolina, is as yet a matter of 
discussion. 

The uniformly red and brown color of the formation throughout its 
whole extent indicates uniform conditions over the whole area. As 
shown by Mr. Bussell,'^ it suggests a mild, moist climate. Gneiss, 
schists, and similar rocks containing much pyroxene, hornblende, and 
mica or other ferromagnesian silicates are not usually red when unde- 
composed. Under the influence of the weather, however, these iron- 
bearing minerals may be altered and much ferric oxide developed, coat- 
ing the grains of quartz and other unchanged minerals red or brown. 
If these products of subaerial decay are washed away and deposited to 
form sandstones and shales without wearing off the ferric oxide coating 
the grains, the new rock will be red or brown; and thus, it is thought, 
the red color of the Newark system may be explained. 

The great length of the formation, taken in connection with its small 
breadth and great thickness, and its intimate association with basaltic 
igneous rocks having the same lineal arrangement and being, at least 
in part, of contemporaneous origin, are among its most important 
features. The rock is extensively used as a building stone in cities of 
the Atlantic States, and at several points coal beds of importance have 
been discovered. 



« Bull. U. S. Gail. Survey No. 85, 1802. 

« Ball. U. S. Geol. Survey No. 52, 1889, p. 5«. 



ma] DE8CRIPTIOK8: NO. 15, P0T8DAM SANDSTONE. 79 

No, 15. Potsdam Sandstone. 

^Fkom Ablsmans, Sauk County, Wisconsin. Described by J. S. Duxer.) 

The Potsdam sandstone is so named from its occurrence at Potsdam, 
JD uorthem New York. It has been traced through a wide stretch of 
ooanfry southwest and west of New York; and at many points, as in 
Wisconsin, where it has been positively identified, the same name is 
applied. 

It is a typical quartz sandstone in which the sand is largely siliceims. 
It is light, almost colorless, on account of the transparency of the quartz 
of which it is composed, although there is here and there a suggestion 
of pale rusty yellow, due to the trace of ferric oxide present in the 
cement. 

Its stnicture is decidedly granular and somewhat porous. The inter- 
stices between the grains are in many cases not completely filled. The 
feel of specimen No. 15 is less gritty than that of specimen No. 14, and 
ilthe siufaces of the two specimens be examined with a lens it will be 
observed that when specimen No. 15 is fractured many of the grains 
break, bat in specimen No. 14 the cement breaks and the grains pull 
apart, leaving the surface with more angular projections. 

The grains are nearly all quartz, colorless and transparent, excepting 
the faint gray clouding due to the occasional presence of the large num- 
ber of liquid inclusions. The dark material forming part of the cement 
between the grains in Fig. A, PI. X, is ferric oxide. The outlines of 
the original well-rounded grains are generally indicated by a clouded 
border which marks off the siliceous cement between them. In Fig. 
B, PI. X, it may be seen that the cement is sometimes oriented, so as to 
be optically continuous with the adjoining grain and extinguisheil at 
the same time. In the lower right-hand portion of Fig. B, PI. X, is a 
banded grain of feldspar. At the upper left hand of the feldspar, as 
seen in Fig. A, PI. X, is an elongated triangular grain of ferric oxide, 
and at its right a well-defined area of interstitial quartz which belongs to 
the adjoining grain . The fragment of quartz crystal represented by the 
grain grew by additions to the outside until the intervening space was 
completely occupied. In some places complete crystal faces have been 
developed. The growth of quartz grains and of other minerals in this 
manner, and the consequent induration of the rocks containing them, 
is a metamorphic process. It has been illustrated and discussed by a 
nomber of authors, especially by Irving and Van Hise.^ 

This rock is much used for building purposes, and on account of its 
siliceous cement is especially durable. In some localities, however, 
where the formation crops out, the cement is not siliceous, or the rock 
nay contain small accumulations of clay. Both of these fe<atures, wher- 
ever they occur, lessen the value of the stone for building purposes. 

The »and of which the Potsdam sandstone was formed is much more 



'Bull. r. S. Geol. Survey No. 8, and Am. Jour. Sci., Vol. XXX, p. 231, wid Vol, XXXIII, p. 285. 



80 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bulliso. 

distinctly rounded than most of the beach sand along the Atlantic coast 
from New England to Florida. It has been so much worn that the feld- 
spar and other minerals softer than the quartz have been almost com' 
pletely ground to silt and removed. It suggests, also, that the material 
from which the sand was derived may have been in a greatly decoin- 
posed and disintegrated condition, and the landscape one in which 
gentle slopes prevailed ; for under such circumstances the altered min- 
erals are unusually soft, so that they are easily ground fi.ne during trans- 
portation, and separated from the quartz. 

The following chemical analysis, by E. A. Schneider, shows the 
highly siliceous character of this sandstone: 

Analysis of Potsdam sandstone from AhlemanSf Wisconsin. 




SiO^ 

A1,0, 

HjO (ignition) 

Total . . . 



No. 16. Banded Sandstone. 

(From Pkoa, Summit County, Utah. Dbsckibed by J. 8. Dillkh.) 

This sandstone is arranged in distinct layers, beds, or bands, whic 
in the field, where large exposures may be seen, are clearly expresse 
chiefly in differences of color. The banding may be seen in the han-«: 
specimen, but is not conspicuous. It is parellel to the stratilicatiorm 
and was determined when the material was dex)osited, although tlm< 
peculiarities of color may not all have been developed at that time. 

The sand of which this stone is composed is almost wholly qnnrtz^ 
Here and there are traces of unaltered feldspar. Grains of kaolin are 
more abundant than those of fresh feldspar, and they may in some 
cases be seen as minute white specks in the hand specimen. 

Under the microscope the granular structure is much more distinct 
The grains of quartz are well rounded. Both silica and ferric oxide 
appear in the cement. The former is perhaps the more abundant, and 
is in places optically continuous with the adjoining grain, showing tfaat 
the crystallograpliic force in the grain controlled its deposition, and 
the matter was so arranged as to form a growth in the crystal like that 
in the Potsdam and many other sandstones. The ferric oxide is suffi- 
ciently abundant to give a decidedly reddish color to the rock, and its 
arrangement has given rise to the banding of the rock. Some bands 
contain much oxide of iron and others but little. The grains of sand 
in the various bands do not differ among themselves essentially in size 
or material. This is not generally the case in beds of stratified rocks, 
for in such rocks the sediments are ordinarily arranged according to the 



DESCRIPTIONS: NO. 17, LAMINATED SANDSTONE. 



81 



size and weight of the fragments, thus producing stratification, as 
illustrated in the laminated sandstone, specimen No. 17. 

Following is a chemical analysis of the sandstone, by E. A. Schneider, 
which shows the very siliceous character of the rock : 

Analysis of handed sandstone from Peoa^ Utah, 



Per cent. 



2.02 



I SiO, 96.60 

I Fe,0,i 

FeO 

CaO 

MgO 



.04 
.08 



K,0 

X«aO 

H,O(105o).... 
H3O (ignilion) 

Total... 



11 
29 



99.14 



No. 17. Laminated Sandstone. 

(From Holyokk, Hampdex County, Massachusetts. Dks( ribed my .1. S. 

DiLLER.) 

The sand of which this stone is composed is very line and is arranged 
in such thin sheets as to produce laminated structure. The material is 
much finer than that of other sandstones in the series, and approaches 
mud or clay in character. Upon the broad surfaces of the hand speci- 
men may be seen many glistening scales of mica, which lie parallel to 
the stratification. On account of their extreme thinness the scales of 
mica readily float and are carried away to be dei)osited with finer sand 
and mud. 

Tlie composition varies greatly perpendicular to the stratification or 
lamination, but parallel to it within the same layer the composition 
is comparatively uniform. The lighter-colored layers are composed 
chiefly of quartz grains with mica, some grains of feldspar, tourmaline, 
and other minerals. The quartz is often well rounded and coated with 
oxide of iron. The darker-colored, red layers contain finer material and 
more angular particles. Mica, oxide of iron, and argillaceous material 
are much more abundant, and represent a quieter stage of the water 
than the coarser films. Systematic variation of these layers indicates 
a corresponding variation in the conditions of deposition. Where 
tbe coarser and thicker portions were laid down the water was more 
vigorously in motion than where the fine sediment was deposited. 
The coarser may represent times of heavy rains, melting snow, or flood- 
tide, and the finer, periods of low, quiet water, carrying a much smaller 
amoant of sediment. 
Bull. 160 6 



82 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [buu.150. 

Specimen No. 17 illastrates the laminated sandstone of the Triassic 
rocks in the Connecticut Valley, and belongs to the same formation as 
the brownstone of Hammelstown, Pennsylvania, the distribution of 
which is indicated under specimen No. 14. Throughout this large area 
the conditions vary greatly, and while the laminated sandstone was 
forming in some places, coarser sandstone and conglomerate were form 
ing in other places, so that the same stratum may show lateral transitions 
from the finer to the coarser sediments. 

No. 18. ElPPLE-MARKED SANDSTONE. 
(From HolVokk, Hampden County, Massachusetts. Described by J. S. 

DiLLER.) 

The wavelike marks upon the upper surface of specimen No. 18 are 
ripple marks, and were produced by corresponding movements of tbe 
water at the time the sediments were deposited. In the water they are 
formed only where it is shallow, and they do not extend beyond the 
depths to which the water is agitated by the wind. At low water they 
are well exposed along the sandy shores of the ocean, but they gener 
ally attain their most regular development upon the land in regions of 
windblown sand. 

PI. XI illustrates the ripple-marked surface of one of the sand dunes 
near Golden Gate Park, San Francisco, California. The bent twigs in 
the foreground show the direction of the prevailing strong winds to bo 
from the left; that is, from the ocean. The axes of the ripple marks 
are perpendicular to the course of the wind, and the slopes of the 
small ridges are not equal. Upon the windward side the slope is long 
and gentle; to the leeward it is short and steep. The sand blown by 
the wind moves up the long slope and falls over the shorter one,- caus- 
ing the ridge to move forward with the wind, but at a much slower 
rate. Under the influence of strong winds from the Pacific, the ripple 
marks illustrated in PI. XI gradually advance from left to right. The 
whole surface is in motion and the dunes travel landward. 

The development of ripple marks under water is not so simple a mat- 
ter as their subaerial development, where they are due wholly to tbe 
influence of the wind. The ripple marks formed by water are rarely 
so regular as those illustrated in PI. XI. In specimen No. 18 the upper 
surface is of finer material than that of which the ripple marks are 
chiefly composed, and in the deposition of this sediment the irregulari- 
ties of the rippled surface were rendered less conspicuous. Originally 
the material now exposed upon the ripi)led surface was mud. It was 
soft and easily impressed. At low water the surface was uncovered 
and exposed to tbe weather. Insects, birds, and other animals crossing 
the mud flat afoot left tracks. The footprints were covered up and pre 
served by later deposits so as to remain in the rocks, and to-day afford 
evidence of the character of tbe animals that lived when the rock was 



Mixni] DESCRIPTIONS: NO. 19, F088ILIFEROUS SANDSTONE. 83 

fonned. The sandstone of tlie Connecticut Valley, from which speci- 
mens Xos. 17 and 18 were collected, have long been celebrated for the 
large fossil footprints it contains in certain localities. During the early 
portion of the Juratrias period amphibians and reptiles of large size 
traversed the muddy flats of the Connecticut Valley and left tracks in 
some cases nearly 2 feet in length. In places where the mud was 
exi)os€d long enough to dry, reticulated cracks were developed. The 
succeeding flood filled the mud cracks with sand, and when the rock is 
split apart the filled mud cracks intersecting the ripple marks appear 
as in PI. XII. 

No. 19. FOSSILIFEBOUS SANDSTONE. 

(From Medina, Orleans Cointy, New York. Described by J. S. Diller.) 

Fossiliferous sandstone differs from other sandstone only in that it 
contains fossils. In this specimen from Medina, New York, commonly 
called the Medina sandstone, the sand, like that of the Potsdam sand- 
stone, is almost exclusively quartz. The fossils it contains are either 
casts or shells shaped somewhat like a tongue, on which account the 
little mollusk was named LinguJa, Lingula is one of the most ancient 
genera, ranging from the lower Cambrian to the present day. In the 
course of evolution it has been remarkably persistent, and, unlike most 
forms, has suffered comparatively little moclificatiou under the influence 
of a long series of geologic changes. 

The fossils found in the sandstone are generally of animals which 
lived upon, or directly above, the sandy bottom of the seii. Rarely the 
fc^sils are of land animals or plants which have been brought to the 
sea by rivers and buried in the sand. 

The fossiliferous sandstone at Medina is composed chiefly of quartz, 
the grains of which are rather angular. The preservation of the deli- 
cate shells indicates that the sand, at the time of its deposition, was 
not subjected to great attrition, else the fragile shells would have been 
ground to pieces. The cementing substance between the grains is 
chiefly carbonate of lime, probably derived in large part from the 
fossils. 

This sandstone has a wide distribution through the eastern part of 
the United States, especially along the Appalachian Mountains, where 
it is evidently a shore deposit, passing into conglomerate, and contain- 
ing ripple marks, rill marks, and other evidences of littoral origin. 
The Medina sandstone is associated with the Oneida conglomerate, 
and its distribution is more fully given on pages 70 and 71. Being a 
hard, siliceous rock, interstratified with shales, limestones, and other 
softer rocks, it more eff*ectually resists the general degradation of the 
land than its associates, and in the course of h>ng continued expo- 
sure has come to be the principal mountain forming rock in the imme- 
diate vicinity of its outcrop. The North Mountain, of Pennsylvania, 
the Massaimtteu, of Virginia, and the Clinch Mountain, of Tennessee, 



84 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [buli^ik 

are good examples of mountains formed by the Medina sandstone 
They are remarkable for their smooth, even crests, and give evidenc 
of an earlier topographic cycle, when the land of that region stood at 
much lower level with reference to the sea, having been, in fact, wor 
down by the streams daring the long-continued period of degradatio 
almost to a plain (peneplain). The even-crested mountains of har 
rocks are the only remnants of this ancient peneplain. 

No. 20. Graywacke. 

(From Hurley, Iron County, Wisconsin. Describbd by W. S. Baylky.) 

The graywackes difl'er from the sandstones in composition. Wherea 
the latter consist essentially of quartz grains (or of quartz and fek 
spar, as in the case of the arkoses) cemented by quartzitic, calcareous 
or other cement simple in composition, the graywackes contain grair 
of many different minerals and small fragments of rocks, united by 
cement of the composition of many slates. In the formation of tfa 
sandstones the rocks from which the sands were derived were broke 
down into their constituent mineral components, and these were sorte 
by the waters in which they were deposited. On the other hand, tl 
rocks from who^e detritus the graywackes were made were not so con 
pletely disintegrated. Tlie sands contained not only quartz and othc 
mineral grains, but also little particles of rock, all so intermingled ths 
we can not believe that much sorting took place. When rock partich 
are not to be found in the graywackes, the distinction between the^ 
rocks and the sandstones must rest upon the cementing material, whic 
in the former is dark in color and contains much chlorite and some micsi 

The specimen in the collection was taken from a low ledge on tl 
south side of the Milwaukee, Lake Shore and Western Railroad, aboi 
three miles west of Hurley, Wisconsin. According to Irving and Va 
Hise, the rock belongs among the upper beds of the Penokee ire 
formation,^ which is Huronian. It is not foliated — that is, it is not 
schist — but it is heavily bedded, the different beds appearing as fim 
or coarser grained bands, in the latter of which rock fragments ai 
quite conspicuous, while in the former the grains are so fine that tl 
rocks seem to grade upward into black slates. All the beds have a lo 
dip, a little east of south, and a strike north of east. The low dip < 
the rock over large areas indicates that it has not been subiected 1 
severe orographic forces — a fact explaining its lack of foliation, an 
thus serving to distinguish it, together with all the other Penoke 
Gogebic rocks, from the much squeezed, highly foliated rocks of pr 
Huronian age in this region. 

The specimen is an excellent representative of the typical gra 
wacke, although but few rock fragments can be tliscbvered in it. 



^Cf. Geikie: Text-Book of Goology, 2(1 ed., 1885, p. 162; and J. Roth: Allgemeine nod Chemlw 
G«ologie, B. 11, p. 622. 
* Tenth Ann. Kept. V. S. Geol. Survey, Part I, pp. 426 ot hwi- 




'1I'H>LE MIRKS CUT 6V IW 



WLLm] DESCRIPTIONS: NO. 20, GRAYWACKE. 85 

IS a fine-grained gray rock of nearly uniform texture. In it may be 
detected a few grains of quartz that often appear black, small dull 
white grains of feldspar, and occasionally tiny black streaks tiiat look 
like slate fragments. It splits quite easily parallel to its bedding, and 
breaks with a more or less concboidal fracture in other directions. Its 
density is 2.687. 

Tnder the microscope its coarser components are easily distinguished 
from the matrix or cement, which is not in very large quantity. The 
most numerous grains are clear fragments of quartz, some of which 
present the rounded outlines of waterworn grains, while others are sub- 
angular. They often contain rows of tiny vacuoles filled with liquid, 
and sometimes a few little specks of dust. Many of them have a wavy 
extinction, which is usually ascribed to deformation in the internal 
structure of the mineral showing it, as a consequence of pressure. A 
few grains are seen to be composed of several portions of different crys- 
tals, since their various parts extinguish in difierent positions. All 
these features are those belonging to the quartz of crystalline scliists. 
Uence, we may conclude that some of the material of the gray wacke 
was derived from rocks belonging to the crystalline-schist series. 

Next to quartz in quantity are feldspar grains. Some of these are 
quite fresh, and consequently clear, grains of an unstriated variety 
that is probably orthoclase. Others show the bars of plagioclase, while 
a few are marked by the gridiron crossbarring of microcline. A few 
are as well rounded as those of the quartz, but most are more or less 
angular. The greater portion of the feldspar is highly altered. In 
natural light such grains appear cloudy, and olten they possess a red- 
dish tinge. Under crossed nicols they break up into a sort of mosaic 
of tiny x>articlesof quartz and small needles and shreds of a bi'ightly 
polarizing micaceous mineral that may be kaolin. In the most highly 
colored grains the pigment is discovered to be a red or reddish-brown 
iron compound intermingled with the other components of the mosaic. 
The shapes of the altered feldspar grains no longer resemble those of 
waterworn fragments. Decomposition has destroyed their original 
OQtlineSy for now the kaolin needles extend into the interstitial cement, 
80 that the margin formed by the replacing mosaic is rough and ragged. 
The only other substances that are clearly seen to be grains are a 
few homblendic pieces and fragments of dark rock. The pieces of 
hornblende are very few in number. They are irregular in their out- 
line, as if broken, unworn pieces, and are strongly pleochroic, being 
yellowish green in a direction perpendicular to their cleavage, and 
dark green parallel to this. The rock fragments are dark, and some- 
times nearly black. Under high powers they are found to consist of 
fine grains of quartz and of a dark substance, probably some mineral 
eolored black by magnetite. 

As a nile^ all the fragments lie with their longer axes in one direc- 
tion, which a little examination of the hand specimen shows to be in 
the plane ot bedding. 



86 THE EDUCATIONAL SERIES OP ROCK SPECIMENS, [bull. 150. 

The grains make up the larger part of the rock mass. Its most char- 
acteristic portion, however, is the material cementing these. This i* 
present iu but small quantity. It consists largely of chlorite, whicU 
gives it its greenish hue and the rock its dark color. The minerals that 
can be detected in it are chlorite, quartz, biotite, magnetite, muscovite 
or kaolin, and a very little hematite. The first two are the most com- 
mon. Under high powers much of the quartz is found to be iu small 
grains, while a good deal of the same mineral is intermingled witk 
chlorite, etc., as a very fine aggregate of secondary origin. Professoi 
Van Hise, who has studied the graywackes of the Penokee district^ 
very carefully, believes that this quartz lias come principally frona 
feldspar, whoso other product of decomposition is chlorite. In a few 
instances the origin of the quartz and chlorite may be told from the 
general shape possessed by their aggregates, which is that of feldspar 
grains, but iu most cases the aggregates possess such indefinite out- 
lines that nothing can be learned from them. 

The true cement is the interstitial substance between the smallest 
recognizable grains. This represents what was originally clay. At 
present it is composed largely of tiny flakes of green chlorite — a little 
secondary quartz between the chlorite — and, embedded in this matrix, 
little particles of magnetite and pyrite, both of which are opaque, 
small round aggregates of a dark-brown translucent rutile, and occa- 
sionally a little flake of green biotite and tiny shreds of kaolin or mus- 
covite. Since the same muscovite or kaolin shreds are observed sur- 
rounding some of the larger grains of feldspar, it is probable that this 
mineral in the cement is derived, like the larger pieces, from substances 
of the composition of feldspar in the original clay. 

The entire cement is thus found to be crystalline. None of the origi- 
nal clay remains. The alteration is regarded by Van Hise, in the article 
alluded to above, as due entirely to the influence of circulating waters 
holding certain substances in solution, the most important being some 
magnesium salt. By the action of this salt on the material of the 
plagioclase and of the iron-bearing minerals in the original deposit an 
abundance of chlorite was developed, and at the same time the excess 
of silica in the feldspar was separated as quartz in the interstices 
between the chlorite flakes iu the matrix.^ 

Such a change as this, produced by the partial solution of substances 
in a rock and their chemical reaction upon one another, is known as 
a metasomatic change. By it a clastic rock sometimes loses nearly 
all evidences of its original fragmental nature and becomes crystal 
line. If the metasomatic change is attended by pressure, the rock may 
have developed in it a foliation, and may thus give rise to a new rock, 
which, if its origin were not known, would undoubtedly be called a 
crystalline schist. Bocks of this nature have been described by Van 



>Am. Joar. Sci., 3d scries, Vol. XXX [. 1886, p. 453. 

'The percentage of SiO^ in oUgoclaae is about 62 per cent, whUe the proportion in ohlorit« ia only 
abont 28 per cent. 




""« SECTION Of GRAVWACKE FBOM M 



N ORDIN*RV LIGHT, > 



J 



\ 



DUIB.] 



DESCRIPTIONS: NO. 21, SHALfi. 



87 



Hise,' aD(l the cliemical processes that have changed them from gray- 
wackes to mica-schistB have been very carefully worked out. 

PI. XIII shows the fragmental character of the rock quite plainly. 
The lighter grains, both the rounded ones and the sharply angular 
oues, are qaartz. The large clouded grains are altered feldspar. In 
the lower i)ortiou of the figure are two grains that have retained their 
waterworn outlines, while to the right of the center and a little above 
it is one whose original outlines have nearly disappeared through 
decoini)osition. The muscovite, biotite, and the fine components of the 
groQDdmass are not visible in the photograph. 

The chemical composition of the graywacke, as reported by H. N, 
Stokes, 18 as follows : 

Analysis of graywacke from Hurley, Wiacansin, 



SiO, 

AlA 

Fe,0, 

FeO 

MnO 

CaO 

MgO 

K,0 

Na,0 

LoM (ignition) . 



Per cent. 

76.84 

11.76 

.55 

2.88 

trace 

.70 

1.39 

1.62 

2.57 

1.87 



Total 100.18 



This is not very diflferent from the composition of a feldspathic quartz- 
ite. Nor should we expect it to be different, for the original rock of 
which the graywacke is an altered phase was a plagioclase-quartz-clay 
^k, as we learn from its microscopic study.^ 

No. 21. Shale. 

(Prom Cashaqua Crekk, Livingston County, New York. Described by J. S. 

DiLLER. ) 

Shale is comi)08ed of sediment finer than sand. It usually splits or 
breaks most easily parallel to its stratification, showing that this line 
<>f weakness originated at the time of deposition. The deposit is chiefly 
^^ay with very fine sand, and the rock became shale by induration. 

The rock from which specimen No. 21 was collected is best exposed on 
Cashaqua Creek, Livingston County, New York. On this account it 
^as locally called the Cashaqua shale. It is light greenish gray, and 
^ther solid for shale. It was obtained at a fresh exposure, where the 



'Cf- C. R. Vftn Hise, upon the Origin of tlie mica-BchisU and black mica-slateH of the Penokee- 
^<>gebic iron-bearing series : Am. Jour. Sci., 3d aeries, Vol. XXXI, 1880, p. 453. 

'For other descriptions of in'ay^ackes see F. D. Adams : Appendix to Ann. Rept. Canadian Geol. 
^rv*yfor 1880-81-^, Montreal, 1883, pp. 20-23; and Irving and Van Hise: Tenth Ann. Kept. V. S. 
^i. 8anrey, pp. 426 et seq. 



88 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [bull.1S0. 

corrasion by the stream is rapid, so that the weathered portion of the 
shaie has been removed and brought the solid rock to the surface. 
When exposed for a considerable time to the weatlier, shale becomes 
fissile, crumbles to small pieces, and is ultimately reduced to a tenacious 
clay, sometimes more or less sandy, ready to be carried away by the 
rains, rills, and rivers into the sea, and again deposited to initiate a new 
rock cycle. In the field the Cashaqua shale contains a few fossil shells 
and some flattened calcareous concretions, with traces here and there, 
but no continuous beds, of sandstone. On the Genesee Kiver this shale 
is 110 feet thick. It gradually thins toward the west, appearing on Lake 
Erie with a thickness of 33 feet. To the east of Casha<]ua Creek sandy 
layers become more and more abundant and the shale is gradually 
rei)laced by sandstone, indicating that the source from which the sedi- 
ment of the shale was derived lay in that direction. 

Under the microscope the specimens of shale from Cashaqua Creek 
are seen to vary considerably in the size of the particles of which they 
are composed. The component material may be conveniently divided 
into argillaceous and sandy. Both occur in the same specimen, and 
may be present in nearly equal amounts, or either may predominate, 
forming almost the whole of the mass. 

The sharp, angular sand grains are chiefly quartz. It is generally 
clear and colorless, but occasionally contains liquid and other inclu- 
sions. Greenish grains of hornblende and chlorite are rather common. 
'No fresh feldspar appears among the grains of sand; it has all dis- 
appeared in the process of decomposition and disintegration of the rock 
from which the sediment was derived. The space between the grains 
is occupied chiefly by argillaceous matter. It incloses scales of musco- 
vite, numerous crystals of ferruginous carbonate of lime, and a multi- 
tude of minute prismatic crystals, the exact nature of which is not 
well understood. The muscovite is usually in very small scales and 
stripes intermingled with argillaceous material such as one sees ordi- 
narily resulting from the alteration of feldspar. The mica in large 
part, and perhaps wholly, and also the clay, originated in the decom- 
posing feldspar, although it is possible that the former may have been 
derived in part from a rock containing i)rimary muscovite. 

The small colorless or reddish and yellowish brown crystals are some- 
times perfect rhombohedra, but more commonly they are round grains 
or groups of crystals. When suflBciently transparent the colors between 
crossed nicols are high. In acetic acid they effervesce like calcite, 
and when dissolved in hydrochloric acid the solution, upon the addition 
of ammonia, yields a precipitate of ferric oxide, showing that the carbon- 
ate contains iron. Its easy solubility suggests that it is not dolomitic 
The mineral is apparently ferrocalcito. In weathered portions of the 
shale the ferrocalcite is reddish and yellowish brown, owing to the 
oxidation of the iron, and occasionally the iron oxide is in sufficient 
abundance to give a rusty tinge to the rock. 

The sharx), angular cystals and grains of ferrocalcite are quite uui 



MLLD.) descriptions: no. 21, SHALE. 89 

formly distribated througli tbe whole mass of the rock, and were 
apparently deposited with tbe other sediment. It is evident, however, 
tbat they differ widely in their origin from the fragments of quartz 
and tbe greater portions of tbe sediments in which they occar. The 
grains of qnartz are minate fragments of the rocks from which they 
were derived. They are not crystals, but their boundaries are not crys- 
tallographic lines. The fact that many of the ferrocalcite grains are 
perfect crystals indicates tbat they were deposited directly from solu- 
tion in water. Whether their precipitation took place with tbe deposi- 
tion of tbe sediment from suspension, or at a later date, maybe learned 
from their relation to the associated material in the shale. In altered 
rocks carbonat'es frequently originate in the decomposition of other 
minerals, but in such cases there are usually more or less distinct traces 
of tbe decomposed mineral. In the Casha<iua shale there are no traces 
of dec^omposed minerals to indicate that the ferrocalcite originated that 
way. Tbe complete absence of veins and other irregular accumulations 
of tbe material suggests that it was not deposited by waters circulating 
through the mass after the rock was formed. The regularity of its 
distribution and its idiomorphic character indicate tbat it was precipi- 
tated from solution in the ocean waters at the same time tbat tbe sus- 
pended sediments were, so tbat tbe chemical and mechanical sediments 
intermingled upon the bottom. It is known that mechanical precipita- 
tion tends to promote chemical precipitation, and it is possible that the 
association of the two sediments may be in some measure accounted 
for in this way. 

The occurrence of ferrocalcite in this shale is interesting on account 
of its bearing upon the origin of extensive deposits of iron ore among 
the crystalline schists.^ 

The minute, brownish, prismatic crystals occurring in irregular 
abundance, frequently in small swarms, throughout the argillaceous 
material range in length from 0.01"»™ to 0.005™'". Generally they are 
80 small that their effect ni)on transmitted light can not be observed, 
bat tbe larger ones have a high index of refraction, giving strong color 
between crossed nicols, and have parallel extinction. They sometimes 
join in such a way as to suggest twinning, and occasionally are bent. 
These minute crystals, to which the Germans give the name "Thon- 
schiefemadelcben," occur not only in shale, but more frequently in clay 
date. They are referred to in the description of specimen No. 122, and 
are less abundant than in the Gashaqua shale. The exact nature of 
these minute crystals has been the subject of much investigation. On 
accoaut of their small size it is difficult to isolate them for chemical 
determination, but Saner, Gathrein, and Werveke have shown conclu- 
sively that at least in many cases the minute crystals are rutile. 

The Gashaqua shale in New York is a member of tbe Portage group 
of the Devonian system of rocks, which has a wide distribution over 
the whole country between the Appalachian and Kocky mountains. 

1 B. D. Irving: Am. Jour. Sol., third series, Vol. XXXII, 1886, pp. 366-272. 



90 



THE EDUCATIONAL 8EBIE8 OF BOCK SPECIMENS, [bull. 190. 



No. 22. Gabbonaceous Shale. 

(From Duo Gap, Walker County, Georoia. Dkscribbd by J. S. Dilucr.) 

This shale was collected at Dag Gap, near Lafayette, in tbe nortb- 
western corner of (Teorgia. On account of its color it is sometimes 
called the black shale, or Devonian black shale, to indicate also its 
geologic age. 

The upper portion of the shale, 3 or 4 feet in thickness, is usually 
dark gray in color, and often carries a layer of round concretions about 
an inch in diameter. The remainder of the formation, the portion from 
which the specimens were taken, is jet black from an abundance of car- 
bonaceous matter, and when freshly broken it emits a strong odor like 
petroleum.^ 

To the unaided eye it is homogeneous and compact, but under the 
microscope it is minutely granular. The grains are chiefly angular 
particles of quartz, with minute globules and crystals of pyrite embedded 
in dark-colored material which is largely carbonaceous, whence the 
name carbonaceous shale. That this material is for the most part car- 
bonaceous is indicated by the fact that when heated red-hot for a short 
time the dark color disappears, owing to the combustion of the carbon- 
aceous matter. When highly heated for a longer time before the blow- 
pipe it turns black again and becomes magnetic, due to the presence 
of iron. 

The highly carbonaceous character of the shale is most clearly indi- 
cated by the following chemical analysis, made by L. G. Eakins in the 
chemical laboratory of the United States Geological Survey: 

Analysis of thalefrom Dug Gap, Georgia. 



Per cent. 



I Sil>, I 51.03 

, A1,0. 13.47 

Fe,Oi 8 « 

! CaO j .78 

MgO j 1.15 

1 Ktt) 3.16 

Na,0 ^ .41 

P,<>. ; -31 

S 7.29 

Fixed carbon 13.11 

Volat ile hydrocarbon 3. 32 

H,0 81 

102. 80 

O.for S 2.74 

Total 100.16 



■ I n »oute States! tbU black 8bale ha^t b«'en di»tiUed for oil, yieldiug 30 to 40 ipJlon« per ton. 



DOUB.] DESCRIPTIONS: NO. 23, SILICEOUS SINTER. 91 

ITie iron ijyrites in the shale oxidizes and, according to Hayes,^ fre- 
qnently stains the weathered surface of the rock with iron oxide and 
sulphate and mineralizes many springs. The shaly structure is not 
prominent on fresh fractures, but is brought out somewhat by weath- 
ering. 

Associated with the black shale locally in Tennessee are small nod- 
ules and beds of phosphate of lime, which is of economic importance 
as a fertilizer. Although only a few feet in thickness, this shale is 
perbaps the most x)ersistent and uniform of all the Paleozoic formations 
of the South. It extends over the whole of middle Tennessee from the 
Tennessee River to the eastern edge of the Cumberland Plateau and 
southward across northwestern Georgia and northern Alabama, its 
outcrops indicating an original extent over at least 38,000 to 40,000 
square miles. 

This shale, on aeconnt of its distinctive and striking appearance, has 
attracted much attention from miners and has been prospected in many 
places for coal and various ores. Such exploitation, however, has 
always been attended by failure, as the shale contains nothing of present 
economic importance excepting the local deposits of phosphate of lime. 
Although it carries a large proportion of carbonaceous matter which 
hums when it is placed in a hot fire, the amount is not sufficient to 
coDstitute it a fuel, and no true coal is ever found associated with it. 
Besides the fertilizer it affords, this formation is of economic importance 
chiefly as a starting point in prospecting for the red fossil ore, like No. 
52 of this series, which belongs below it at a uniform depth over con- 
siderable areas. 

UNALTERED SEDIMENTARY ROCKS OF CHEMICAL ORIGIN. 

No. 23. Siliceous Sinter.^ 

(From Yellowstone National Park. Described by Walter Harvkv Weei>.) 

The hot springs and geysers of the Yellowstone are surrounded by 
large areas of siliceous sinter that often entirely cover the floor of the 
geyser basins. About the spouting vents this material has been built 
Q]) into mounds and cones of unique forms and great beauty. The 
more quiet pools have built up more or less regular mounds of white 
sinter, which are in places as much as 20 feet in height above the sur- 
roanding level. Besides these deposits, the alkaline waters of the 
geyser regions have left deposits of silica wherever they have flowed, 
and many square miles within the park are covered by white and 
glistening deposits of this material. 

Until the Yellowstone deposits were studied it was the generally 
accepted theory that the geyser waters reached the surface heavily 

^ 'Geologic AUas U. S., folio 2, Ringgold, Georgia^Tenneseee. Abo, Sixteenth Ann. Kept. U. S. GeoL 
Sorrey. Part IV, pp. 61 1-023. 

'See Formation of hot-spring depoeiu, by W. H. Weed : Ninth Ann. Kept. U. S. Geol. Survey, 1890, 
P1»-6MC3. 



92 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [buu^iM. 

charged with silica, which by relief of pressure, by cooling, and by 
evaporation was precipitated out and deposited by the waters. Obser- 
vation of the natural conditions under which the Yellowstone deposits 
are forming, together with experiments and a study of the chemical 
analyses of the geyser waters, showed that the silica brought to the 
surface by the geyser waters was rarely separated out and deposited 
by the first two causes, but that deposits are formed about the geysers 
and the margins of springs by evaporation, producing a true geygeriie, 
A new mode of deposition was then recognized, namely, the separation 
of silica by plant life, by the alg^e that are abundant in the hot waters 
of the region. It is by this agency that much the largest part of the 
sinter dei>osits of the region have been formed. 

This algous vegetation is sure to be observed by every visitor to 
the region. Its varied tones of pink, yellow, orange, red, brown, and 
green adorn the slopes of geyser cones, flush the white silica of the 
little basins with their tints, and mark the waterways with their bril- 
liant colors. It is ever present where the temperature does not exceed 
185^ F., often lining the great bowls of the cooler springs and ikx>1s 
with leathery sheets of brown or green. Where a constant overflow 
prevails, the channel is often fllled by a vigorous growth in which an 
alga^ mat is formed having the consistency of a firm jelly, and most 
beautifully colored. In whatever form it is found, and no matter how 
brilliantly tinted, this algous material, if removed from the water and 
dried in the hot sun of the region, rapidly loses its color, shrinks in 
size, and becomes an opaque white mass of silica, whose weight is not 
one per cent of its former state. Chemical analysis, made in the Sur- 
vey laboratory by J. E. Whitfield, shows this dried material to be silica 
and water, viz : 

Anahjais of dried algous regetation from geyser margin, Tellowetone National Park. 




Experiments showed the writer that the growing algae form a jelly 
of hydrous silica. It is of this material that the algie filaments are 
formed, and the algic slime of other waters is here a hydrous silica 
binding the threads together. The nature of this separation may be 
seen under the microscope, though the fresh hydrous silica is difiUcult 
to study, and the dried material becomes opaque. In most cases the 
glassy rods can be readily distinguished, and the inclosing past^ 
usually shows globules and pellets of the dehydrated silica. 

The specimen No. 23 is a sinter formed by algae. It is light anc 
porous, showing in part a fibrous structure, is soft, and resembles chalk 
in appearance because it soils the fingers with a white impalpable 



DBJJE^l DEgCBIPTIONS: NO. 24, VEIN. 93 

povdet. TUe specimeiis represent the lighter, truly algous form of 
giUceona smter, »nch as is commonly formed by the gelatinous mats 
about the hot springes. 

In thiu sectioii, when under low powers, the siliceous sinter usually 
appears as au indistinct aggregate of glassy silica. If, however, a 
power of 200 dianieters be used, and the section is thin enbngh, the 
sinter is seen to be composed of a delicate network of minute fibers or 
threads which pass in and out, and are often composed of extremely 
minute globnles of silica glass. In most cases the algous origin of the 
sinter is not apparent except under high powers. Generally the 
or^nic nature of the sinter can not be satisfactorily determined in thin 
section, owing to the fact that the plant rods are composes! of hyaline 
silica. It is only in the minutest details that the structure may be 
recognized. In some specimens of the sinter we see extremely minute, 
thread-like particles of the silica held in a mat of small spheres of 
silica closely crowded together. In such cases it is not reasonable to 
assume that the former organic jelly made by the living i)lant has, 
upon drying, separated into such globular particles. . It is rather more 
probable that the silica was itself in the globular form during the life 
of the plant. 

The specimen represents the most common form of freshly formed 
sinter about the the geyser basins of the Yellowstone, The illustra- 
tion, PL XIV, represents Old Faithful Geyser in action. The erup- 
tions are of four or five minutes' duration, and take place every hour 
from an opening a couple of feet across in the summit of a mound built 
up of white siliceous sinter. The sinter forms a series of shallow 
terraced basins, in the cooler of which the alga) colors are noticed. 

No. 24. Vein. 

(From Paskesta, Tehama County, California. Described by J. S. Diller.) 

Much of the rain that falls upon the earth sinks into it and circulates 
through the pores and fissures of the rocks beneath the surface. Sol- 
uble portions of the rocks are dissolved, ai!d the water becomes charged 
with mineral matter. When this water again reaches the surface in 
springs it deiwsits the mineral matter held in solution, and thus siliceous 
sinter (No. 23), travertine (No. 29), etc., are formed. It may deposit its 
dissolved material beneath the surface, filling fissures in rocks, or form 
icicle shaped masses, as stalactites (No. 28), in caverns. 

The fissures in rocks filled in this way with mineral matter are veins. 
They are illustrated by specimen No. 24, which was collected at Paskenta, 
in Tehama County, California, where they occur abundantly ns stream 
pebbles brought down from the Coast Eange. The vein in this case is 
qnite uniformly white, and is comx>osed of quartz and calcite. When 
placed in dilate hydrochloric acid it effervesces briskly, dissolving the 
calcite and leaving the quartz. The calcite is most abundant in the 
middle portion of the vein, while the quartz usually predominates upon 



94 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bcu.150. 

the sides, showing that much of the quartz was deposited before the 
calcite. This relation is shown in fig. 10, illustrating a vein from which 
the calcite has been removed by acid. The quartz is often tooth- 
shaped, projecting into the vein. 

Some veins are composed almost wholly of calcite, with but a trace 
of quartz here and there along the edges, while others are chiefly 
quartz. Generally, however, the two minerals are present in nearly 
equal amounts. 

The rock fissures or openings which, when filled with mineral mat- 
ter, become veins are produced in various ways. Some originate in 
the contraction of the rock while drying or cooling, others are due to 
solution, but the greater number that become veins are produced in 
connection with movements of the land in making mountains and con- 
tinents. It is for this reason that veins are most abundant in mountain 
regions among rocks which have been upturned and metamorphosed. 
Veins of quartz are perhaps more abundant than those of any other 
mineral, and those of calcite are next in number. Feldspar, barite, 

fiuorite, hornblende, epidote, and pyroxene are other 
vein minerals, besides precious metals, such as gold and 
silver, and a large number of ores, which are sometimes 
of great economic value. Where native gold occurs it is 
usually found in quartz veins. 

PI. XV illustrates a vein of quartz 2 feet in thickness 
cutting jointed gneissoid rocks on the Potomac River 
near Washington, District of Columbia. 
„. ,^ „ ,. A vein containing ore is usually called a ioc?€ by miners, 

Fig. 10.— Section . ^ . , . , . 

of vein composed and the miucral matter assocmted with the ore in the vem 
ofqiiartMi)and is the //ani/ue, whilo thc rock containing the vein is the 
wiiichthecaicj"© country rock. Veins when first formed are usually not 
has been r»v far from Vertical, but they may be displaced by subse- 
by acid. q^|^y|^ movemeuts and intersected by several series of 
later veins. They vary in breadth from a mere film to over a hundred 
feet, and occasionally have banded structure produced by the deposition 
of corresponding layers on opposite sides of the fissure until it is com- 
pletely filled. When the filling is not complete the cavities are fre- 
quently lined with beautiful crystals similar to those found in geodes. 
Fibrous minerals are usually found in veins, and the fibers are peri)en- 
dicular to the walls. 

1^0. 25. Vein Quartz. 

(From Castleton, Harfokd County, Maryland. Described bt J. S. Dillgr.) 

Of the various kinds of vein-forming minerals, quartz is the most 
common and important. Veins of quartz in altered crystalline rocks 
are sometimes 100 feet or more in width and may be traced across the 
country for miles. It occurs in sufficiently large masses to be consid- 
ered as a rock. Although usually associated with metamorpliic rocks, 
it originated, like siliceous sinter, by deposition from aqueous solution. 





< 

OQ 



o 



O 



UJ 

O 
9 

CO 



< 
I 
o 

< 

UJ 



(/> 

UJ 

z 
o 

z 

t- 

< 
C 

u. 
O 

UJ 



iLLEBl DESCRIPTIONS: NO. 26, SILICEOUS OOLITE. 95 

Specimen No. 25 is from a large vein at Castleton, Maryland. The 
3ck is light colored, vitreous lostered, hard enough to readily scratch 
lass, infusible, insoluble in hydrochloric acid, and in fact has all the 
tiaracteristic properties of the mineral quartz. It^ structure is granu- 
ir, but the grains being angular and interlocked, as in granite, they 
ccupy all the space, and as all the particles are of the same material it 
oes not appear granular to the naked eye. This structure is, however, 
ell ilisplayed under the microscope between crossed nicols, owing to 
iie unlike optical orientation of the grains. The grains are transpar- 
Dt, although somewhat clouded by a multitude of minute liquid incln- 
ions, usually less than 0.002 """ in diameter. These minute inclusions 
an be seen only with a high magnifying power, but are known to be 
Iquid on account of the moving bubble which each one contains. If 
ho section be gently heated, the liquid expands so as to occupy the 
irhole cavity and the bubble disappears. The cause of the rapid motion 
of the bubble in these inclusions is not well understood. 

Similar inclusions occur in the quartz associated with the calcite in 
tbe vein of specimen No. 24, as well as in the quartz of granite, gneiss, 
and schistose rocks generally. 

The included liquid is usually water, but in some cases has been 
shown * to be carbon dioxide in a liquid state, and the two may occur 
together in the same cavity. Vein quartz is a water deposit from solu- 
tion, and the included water was usually imprisoned in the cavities at 
the time the quartz was formed. Occasionally minute cubes of salt are 
found in these liquid inclusions. 

The quartz at Castleton is mined and finely pulverized for use in the 
manufacture of pottery. In order to render it more pulverable it is 
bighly heated in a kiln. It comes out snow-white and brittle, ready for 
grinding. 

No. 26. Siliceous Oolite. 

(From Crkter County, Pknnsyjaania. Described bt J. S. Dillek.) 

The occurrence of silica in the form of sinter deposited by geysers is 
illustrated by specimen No. 23 ; that deposited in fissures as veins is rep- 
resented by specimens Nos. 24 and 25. We now come to a form in which 
tbe silica appears in small spherules like fish roe, whence the name 
oolite. In this case the chief component of the oolite is silica, and it is 
designated siliceous oolite to distinguish it from the more common form 
of oolite, such as specimen No. 31, which is composed of carbonate of 
lirae. The material was collected near State College, in Center County, 
(Pennsylvania, where it occurs as bowlders associated with flint over an 
irea of several square miles. 

This oolite is remarkable for the uniformity in shape, size, and distri- 
)ation of the spherules. On a fresh fracture their centers are usually 
lark, while their rims and the matrix which fills the interspaces and 



> G. W. Hawea : Am. Jour. Sci., 3d series, Vol. XXI, p. 203. 



96 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [bcllU 

incloses the spherules are light colored. Both spheniles and matr 
are firmly coherent, so that a fracture passes directly through tl 
spherules instead of around them. In some places the hand si)ecim& 
show cavities which are lined with minute crystals of quartz. It 
evident that if conditions had continued favorable for the developmte 
of quartz these drusy cavities would all have been filled. 

The structure of the spherules may be rearlily seen in the hand 8[>e 
men witli a iK>cket lens. Their centers are usually dark, granular, ai 
vitreous, and surrounded by a dense waxy envelope, which is in man 
cases beautifully banded, like agate. This relation is illustrated by tli 
spherule in the center of the figures in PI. XVI. The concentric stra* 
ture shows that the spherules result from growth and not from attritioi 
At the center of many of them rounded grains of sand are found as 
nucleus, about which the silica was deposited in successive layer 
Some of the layers are granular quartz; others are fine, granular, c 
fibrous chalcedony. The black bands and clouding in the figures ai 
brown oxide of iron. 

The two large spherules cut through the middle by the left edge 
the figures in PI. XVI have each a round grain of quartz at the cente 
These grains are full of minute needles and liquid inclusions, like the; 
of granitic quartz. The minute needles observed in these nucle 
quartzes do not occur in the granular quartz of the concentric layei 
As pointed out by O. K. Hovey, this nucleal quartz is occasional 
surrounded by secondary growths of quartz optically continuoas wi 
the original grain.^ In each of the two spherules alluded to the quai 
is enciivled by a rather coarsely granular band in which many of t 
longer diameters of the grains radiate from the center of the sphera 
Some of the spherules are composed wholly of such grains, and ti 
radial structure is phun. Others appear to be made chiefly of chi 
cedony. All the sections showing a nucleal grain are correspondii 
set'tions, and supiK>sed to be through the middle of the spheral 
When compared it is evident that the composition and 8tnictare< 
the spherules vary considerably. Occasionally the nucleal quartz i 
enveloiHHl in a thick, dense layer of oxide of iron, and more rarel 
there are finely fibwus layers ne;irly midway between the center an 
circumference of the spherule. The outermost layers of the spherule 
whii'h form the borders of the interstitial spaces, are generally fibroa 
This structure is well illustrated about the clear, triangular area towai 
the lower lelt hand portion of Fig. B of PI. XVI. 

Oi>lite conipi>sed of ci^rlmnate of lime is of common occurrence, som 
times in large masses containing marine shells, and must l>e of marii 
origin. Its formation hac? l>een observe*! in lakes and springs, al;s 
On the other hand, silire^uis oolite is rare, and from the fact that, ! 
in si>eciinen Xo. i*S, silica is known to replace carbonate of 1ioK\ it li 
been suggested that the silii-eons iH^Iite was onginally cak*areoi 



' Bull. «.;«*>!. Sx-. America. V^.!. V, p. «2n 




COU»TV, ««NSYt.«««. 



MLLB.] 



DESCRIPTIONS: NO. 27, GYPSUM. 



97 



Pertinent to this view are the following chemical determinations taken 
from complete analyses published by Edwin H. Barbour and Joseph 

Torreyjr.:^ 

Ompoiitian of oolite from near State Collegey Pennsylvania, 





Lime-Ailica 
colite. 


Silica-limo 
oolite. 


Siliceous 
oolite. 


Silica 


3.7 


filLSO 


95.83 
].93 


Calciam carbonate or oxide 


88.71 IB- 81 









The lime-silica oolite and silica-lime oolite occur in the same hand 
specimen, but are said to be separated by a sharp line. Siliceous 
oolite and flint occur at the same locality. The same observers report 
organic remains in the siliceous oolite they describe, but none were 
observed in the specimens of this series nor in the material described 
by Hovey. 

It does not admit of doubt that the nuclear quartzes filled with 
iDiDute needles were originally grains of sand about which the con- 
centric layers of silica were formed, but whether or not the concentric 
byers when first formed were calcareous or siliceous may yet be ques- 
tioned. Dr. W. Bergt,* who has studied the material most carefully in 
the laboratory, and G. B. Wieland,^ who has investigated it in the field, 
have advocated its direct deposition from hot siliceous springs and have 
described associated phenomena indicating such origin. Furthermore, 
similar deposits have been observed by W. H. Weed now forming about 
the siliceons hot springs of the Yellowstone IN'ational Park. They 
^ill be described by him in Monograph XXXII of the Geological 
Survey series, under the head of "Hot-spring deposits." 



Ko. 27. Gypsum. 

(From Gypsum, Ottawa County, Ohio. Described by J. 8. Diller.) 

Gypsam, like quartz and calcite, occurs at times in such large masses 
*« to be properly considered a rock. At Gypsum, Ohio, it is found 
^^r the middle of a great sheet of limestone of upper Silurian age. 
■I^fi specimens for this series were collected by Messrs. Marsh & Co. 

Gypsam is generally fine-granular, or compact, and so soft as to be 
•l^ite easily scratched with the finger nail, but, unlike limestone, does 
•Jot effervesce in acid. Its normal color is white, though by admixture 
^^ clay, carbonaceous matter, and iron oxide, it becomes gray, brown, 
<)' reddish. The clear, transparent form of gypsum is selenite, and the 
^•Dpact, white form, frequently used for statuary and ornaments, is 
"^abaster. Its composition is 0aSO42H2O. When heated, the water 

'Am. Jftor. Se!., 3d serle*. Vol. XL, pp. 248, 249. Se« also Petrology for Students, by Alfred Barker, 

^232. 

'(^iMhaft laifl, in Dresden. 1892. p. 115. 

'Am. Joar. Scl., Oct., 1897, 4th Mries, Vol. IV, pp. 262-264. 

Bull. 150 7 



98 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [ButLiM. 

is readily driven off, and the resalting powder is the common plaster of 
psrris of commerce. 

G3rpsum is usually found in beds or lenticular masses associated with 
clay and rock salt, or anhydrite, and it has been found locally in rocks 
of all ages later than the Silurian. This intimate association with rock 
salt clearly indicates its marine origin in many cases at least; for it is 
well known that in the manufacture of salt by evaporation from sea 
water gypsum is precipitated before the more soluble salt. Gypsum 
occurs also in irregular masses in limestone and is not associated with 
salt. It then appears to have resulted directly from the action of sul- 
phuric acid upon the limestone, converting the carbonate into the 
hydrous sulphate of lime. Gypsum is rarely found forming incrusta- 
tions in caves.* 

Gypsum is used chiefly in the manufacture of plaster of paris and 
land plaster. The former has many applications in the arts, and the 
latter is a valuable fertilizer. Thin plates of selenite are used to secure 
certain optical effects in microscopic investigations, and from alabaster 
statuary, vases, and other ornaments are carved. 

No. 28. Stalactites. 

(From Luray Cave, Page County, Virginia. Described by J. S. Diller.) 

Limestone is one of the most soluble among the common rocks, and in 
certain regions, as for example Virginia and Kentucky, where circum- 
stances are favorable, large caves are made in it by solution. The under- 
ground water in such regions contains much dissolved limestone, and 
when it seeps through the roof of a cave and evaporates the dissolved 
limestone is deposited in various shapes, depending chiefly upon the rate 
of supply. The most common and characteristic feature is the stalactite 
illustrated by specimen No. 28. A number of stalactites in position are 
shown in the accompanying PI. XVII, a view of Marengo Cave, Indiana, 
taken by Mr. Ben Haines. 

A drop of water with carbonate of lime in solution, upon reaching the 
roof of the cave where it is exposed, begins to evaporate and lose its 
carbonic acid by virtue of which it holds the limestone in solution. As 
a result the calcareous matter is deposited around the edge of the drop, 
and if the proper supply continues a tubular pendant will develop. 
While some of the small stalactites are hollow, they do not long remain 
so, for the hollow soon fills and the growth of the stalactite continues 
by the addition of successive layers upon the outside. A cross section 
of a stalactite shows it to be composed of one or more concentric bands, 
which are usually radial fibrous. Their color is generally yellowish, 
owing to the presence of iron oxide, but occasionally they are snow- 
white and translucent or beautifully colored by copi)er or other sub- 
stances. Their chemical composition is CaOOa, which generally crys- 



' See paper by G. P. Merrill, Prooi U. S. Nat. Mu8., Vol. XVII, pp. 80-^1. 




< 
o 



tli 

> 
< 
u 

o 

z 
tli 
a 

< 



c/) 
O 

a. 
tli 
o 



DDUBl DESCRIPTIONS: NO. 29, TRAVERTINE. 99 

tallizes in the hexagonal system as calcite^ but occasionally in the 
ortborhombic system as aragonite. 

In PI. XVII stalactites are seen hanging like icicles from the roof of 
a cave. When the supply of water is greater than can be evaporated 
from the stalactites, some of it (lroi)s to the floor and forms a stalagmite 
which grows upward as the stalactite grows downward, and if the 
process be continaed long enough the two will meet and form a column, 
as sbown in the plate. Variations in the rate and distribution of the 
supply give rise to many beautiful as well as grotesque forms. Luray 
Cave, of Virginia, from wbich specimen N'o. 28 was collected, is remark- 
able for tbe abundance and varied character of its deposits. Part of 
these are illustrated in PL XVIIf, from a photograph taken by Mr. C. H. 
James, of Philadelphia. All caverns are not so beautifully ornamented. 
Mammoth Cave, of Kentucky, although remarkable for its size, contains 
a very small amount of cave deposits such as are shown by the two 
illnstrations. 

The formation of stalactites has been described and illustrated by 
G. P. Merrill in Proc. U. S. Nat. Mus., Vol. XVII, pp. 77-«0. 

No. 29. Tbavertine. 

(From Mammoth Hot Springs, Yellowstone National Park. Described by 

Walter Harvey Weed.) 

Among the many interesting natural phenomena that claim the 
attention of the vi8itx)r to the Yellowstone National Park, the geysers 
auil hot springs rank first in general interest. Their novelty and 
beauty are sare to attract universal admiration, while the vast quanti- 
ties of hot water that flow from the ground are convincing evidence 
of the nearness of internal heat. These steaming fountains and boiling 
pools are usually surrounded by snowy white borders of mineral matter 
<leposite(l by the hot waters. 

At the Mammoth Hot Springs the deposit consists of carbonate of 
lime; this forms the unique marble terraces and pulpit basins of those 
springs. The total area covered by travertine at this place is over a 
tkoasand acres, and tbe greatest thickness is about 250 feet. When 
first seen the main mass of the deposit is striking from its whiteness, 
resembhng an immense snow bank, filling a narrow valley environed by 
dark-green, pine-clad slopes. This deposit of travertine forms benches 
or terraces, the largest one, on which the hotel stands, being 83 acres 
in extent. While the deposit is white,^ or if old and weathered a light 
gray, the hot-spring bowls and overflow basins, and especially the slopes 
^^i by the streams of hot water, are brilliantly tinted with red, yellow, 
orange, green, and brown. These colors are due to a low form of plant 
life— alpfsjc — whose presence is not easily recognizable, owing to the 
covering of travertine. The illustration shows the general character 
^ the terraced basins that are built up on the overflow slopes. 



100 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [»ru.i». 

Tlie illuBtratiou, Fl. XIX, sbows tbe hot-water basiDS that cover the 
slopes, like irregular steps. They are formed by the more ra])id deposi- 
tion about the edges of pools of hot water; and the crennlated border 
produced by the rippling of the water, together with the presence of 
alg:i', causes an overflow that builds up stalactitic masses that hang 
like a fringe down the sides of tbe basinn. 

The occurrence of plant life in these hot waters has a special signifi- 
cance, as the travertine deposits are built up chiefly by the activity of 
these little plants, which abstract carbon dioxide from the hot waters, 
thus caasing the precipitatiuu of the carbonate of lime held in soliilion. 
The specimen shows the fibrous variety of travertine formed by the 
encrusting of tbe yellow, silky skeins of algic filaments growing in the 
hottest waters near tbe orifices of the springs. This deposit forms 
fan-shaped masses (tig. 11), which eventually choke up tbe spring. 

The specimen shows a beautiful fibrous structure, and is apjtarently 
compact, but tbe transverse section shown by the ends of tbe specimen 




TnTerUne fan, Uuumoth Hot Springi, TeUowstoue NaUanal Park, 



prove the rock to be cellular. The freshly formed rock is soft and 
friable, but becomes indurated and compact with age, the change 
being unaccompanied by any variation in chemical composition. While 
freshly formed and wet, tbe travertine retains tbe brilliant colors of 
the alga;, but these gradually fade upon drying and the death of the 
plant life, and the deposit becomes chalky white. The older deiwsita 
have a gray surface, due to the organic matter &om tbe alga; present 
ill the rock. 

Though the specimen of travertine described is a hot-spring deposit, 
this rock is also formed by cold-spring waters which are highly charged 
with carbonate of lime. The beautiful cave onyx and other cave 
deposits are similar in composition, but they are formed more slowly 
and are therefore of different physical character. 

In thin section under the microscope the organic origin is not dis- 
closed, but fresh specimens may be dissolved in dilnte acid, which will 
reveal the algie sheaths and threads. The following analysis, made in 




DEPOSITS IN LURAV CAVE. VlRGI 



DQilB.] 



DESCRIPTIONS: NO. 29, TRAVERTINE. 



101 



the laboratory of the United States Geological Survey by J. E. Whit- 
field, shows the material to be a nearly pure carbonate of lime: 

Andytii of irarertine from rent near Blue Springs, main ierracCf Mammoth Hot'Springa, 

Tellowaione National Park, 



SiO^sUIca 

Al,0, + Fe,0, 

SOs, snlphnric acid 

CaO.lime 

CaCOg, lime carbonate 

MgO, magnesia 

KaCl, sodium chloride 

KsO, potash 

CO|, carbonic acid 

HaO, water 

C, carbon 

Other constituents 



Per cent. 



Total 



0.06 

O.U 

0.70 

55.02 

a (86. 02) 

0.06 

0.20 

&0.08 

42.25 

1.06 

0.24 



99.81 



a If all the COs be supposed to l.e eonibimd with lime. 
b PotasMc chloride. 

The striking feature of the analysis is the remarkably low percentage 
of carbon when it is remembered that the travertine is formed of lime- 
incnisted algie threads. 

The waters of the Mammoth Hot Springs are so heavily charged 
with carbonate of lime in solution that this substance is deposited by 
other agencies than plant life, but they are insignificant in comparison 
with it. This is taken advantage of in the preparation of coated 
articles of metal and glass — often sold to tourists — the articles being 
prepared by exposing them to hot-water spray. 

In thin section under the microscope the travertine shows tangled 
fibers, which are surrounded by a furry growth of calcite spicules, which 
are in general at right angles to the fibers. Under high powers with 
crossed nicols these rods appear brownish gray in color, and the spaces 
between the fibers are filled with a finely cryptocrystalline aggregate 
of calcite. In many of the sections, where the rods are thickly crowded, 
the lace-like arrangement is not so apparent and the calcite aggregate is 
wanting. The calcite spicules which surround the rods are but rudely 
riidial in appearance, and sometimes appear felted, but generally 
fumble delicate frostwork. This microscopic structure quite accords 
wilh what we know of the origin of the travertine. The rods are the 
Ratings of the algse threads, which in the section are probably merely 
^bells and contain no recognizable organic matter. 



i 



102 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [buu.150. 

No. 30. Oolitic Sand. 

(From Pyramid Laj(b, Washoe County, Nevada. Described by J. S. Dillbr.) 

Among tlie Needles on the shore of Pyramid Lake in Nevada, accord- 
ing to Prof. I. 0. Eussell, the short beaches are made up of oolitic 
sand, represented by specimen No. 30. Although most of the grains are 
well rounded, there are many subangular ones. In size they are very 
irregular, ranging from 1 ™™ to 6 "»" in diameter. The smaller, when 
cemented, give rise to oolite, but the larger ones produce pisolite. 
Among the well-rounded grains are occasional shells of a small snail. 

The middle portion of each spherule is minutely granular and envel- 
oped by series of concentric ftbrous granular layers. Each spherule is 
a concretion. Some have grains of sand or minute shells at the center 
as a nucleus, about which the carbonate of ]ime accumulated. Occa- 
sionally the nucleus is large and complex, including one or more 
concretionary grains, each having its own concentric banding. Most 
of the spherules, however, are without such foreign nucleus. They 
appear to have started by the segregation of the more or less floccu- 
lent particles of the precipitate itelf, forming globular accumulations 
upon which the further precipitation continued with variations, giving 
rise to successive rings of growth. This subject will be considered 
more at length under " Concretions" (Nos. 33 to 30). 

The spherical form of the grains and the uniform thickness of the 
concentric layers indicate that the grains were in motion while the suc- 
cessive coats were formed. Professor RusselP remarks that "this 
material is evidently now forming, and in places has been cemented 
into a compact oolite by the deposition of a paste of calcium carbonate 
between the grains, and forms irregular layers several inches in thick- 
ness that slope lakeward at a low angle." The formation of oolitic 
sand on the shore of Pyramid Lake is limited to the immediate vicinity 
of the Needles, where warm calcareous springs rise near the shallow 
margin of the lake.^ 

Similar sand, but much finer, is now forming locally upon the shores 
of Great Salt Lake. The waters of Pyramid Lake and Great Salt Lake 
differ widely in chemical composition, and it is evident that the con- 
ditions under which oolitic and pisolitic sand may be formed vary 
considerably. 

Dr. A. Rothpletz has shown ^ that the oolitic sand of Great Salt Lake 
is of vegetal origin. It is due to certain algae having the power to 
secrete carbonate of lime. When the grains of sand are dissolved in 
weak acid, there remain the dead and shriveled bodies of such alga), 
and in the lake these granules are usually covered, at least in part, with 



•Mon. U. S. Geol. Survey, Vol. XI, p. 168. 
«Ibid.,p.61. 

'Botacisclies Centralblatt, Kr. 35, 1892; translated by F. W. Cragin iu Aiu. Geologist, Nov. 1892, 
Vol. X, p. 279. 



) 



DiLLKB.] DESCRIPTIONS: NO. 31, OOLITIC LIMESTONE. 103 

a blaish-green algse mass. In Pyramid Lake, however, no sach masses 
have been reported in connection with the oolitic sand, and the evi- 
dence does not appear to indicate that it is of vegetal origin. To settle 
this matter, if possible, specimens of this oolitic sand from Pyramid 
Lake were referred to Prof. W. G. Farlow, of Harvard College, who 
reports that they contain " nothing like Gloeocapsa and Glceothece.'^ 
He adds: "I find no trace of vegetable structure except the rare occur- 
rence of Leptothrix-like threads broken up into fragments. These 
are so rare and so imperfectly shown that I can only regard them as 
accidental impurities, coming, perhaps, from the outside. There is 
nothing hke the mass of algse found in the Salt Lake material." 

No. 31. Oolitic Limestone. 

(From Bedford, Lawrsncs County, Indiana. Described by J. S. Diller.) 

The oolitic limestone, or oolite, from Bedford, Indiana, represented by 
Rpecimen No. 31, is composed chiefly of oolitic grains, among which there 
is great variety in form, but a smaller range in actual size. They are 
generally less than 1°"™ in diameter, and can not be studied to advan- 
tage without a microscope. The}* do not have the usual concentric 
banding, like the grains of siliceous oolite (No. 26) and oolitic sand (No. 
30), and are much more irregular. Discoidal and elongated forms pre- 
dominate, possibly for the reason that under their conditions of growth 
the environment was not symmetrical in all sections, and development 
along difierent radii proceeded unequally. Much of the variation in 
shape, however, is due to the fact that the grains are fragments of coral, 
mollnscan shells, worm tubes, and numerous other more or less rounded 
forms* 

Each rounded oolitic grain is enveloped by a coat stained more or 
less deeply by oxide of iron. Many of these grains are completely 
crystalline, and, all parts being in parallel position, they extinguish at 
the same time when rotated between crossed nicols. Some of these 
grains, as shown by the distribution of their iron oxide, were evidently 
once organic, and, besides being optically continuous within, they are 
in some cases continuous upon the outside with the clear granular 
calcite of the matrix. It is evident that such grains have grown by 
additions of calcite upon the outside, in just the same manner as the 
quartz has grown in many slightly altered sedimentary rocks.^ The 
matrix is usually more crystalline and clearer than the grains which 
it incloses, and is often optically continuous about several grains. 

Mr. George H. Girty has examined this limestone microscopically, 
and famishes the following remarks concerning the fossils it contains: 

Protozoa are Tery aLaDdantly represented by the foraminiferal gen as Endothyra. 
Most of the forms observed can probably be referred to Endothyra haileyi Hall, a com- 
mou fossil in the St. Loals Hmestone of the Mississippiau series. A trace of sponges 



^ See de«oriptionB of Xo«. IS and 118. 



104 THE EDUCATIONAL SERIES OP ROCK SPECIMENS, [buu.150. 

haB been noticed, and corals, belonging chiefly to the genus Zaphrentis, occur spar- 
ingly. Fragments of echinoderm plates and spines constifcnte, perhaps, the greater 
portion of the organic remains of this rock. Brachiopods and bryozoa are not well 
represented, and the articulates are represented by ostraeods and trilobites. 

Ill the oolitic sand (Bpecimen No. 30) of Pyramid Lake there are 
usually a few shells of a small fresh-water gastropod; but in the oolite 
from Bedford, Indiana, the organisms of various kinds are marine and 
so abundant as to indicate a rich fauna. This commingling of fossils 
and oolitic grains is illustrated in the formation of oolites upon the 
seashore to-day. Oolitic sand and oolite, as shown by Dana,^ are of 
common occurrence on the beaches of coral islands. Coral rock is 
broken off and ground up to calcareous sand by the force of the waves 
that break against the coast. These sands, worm shells, etc., are 
thrown by the tides and waves upon the beach. The water of such 
calcareous shores gets much carbonate of lime in solution, and as the 
sand dries, after the tide recedes, the dissolved material is dei>osited, 
thus forming successive coats about the grains, filling the spaces 
between them, and, if the grains are not frequently moved, ultimately 
cementing them together to form oolite or pisolite, according to the size 
of the grains. Dana observed that such << beach deposits consist of 
regular layers, commonly from an inch to a foot in thickness, and are 
generally consolidated up to a line a little above high-tide mark. In 
all instances observed the layers dip at an angle of from 6^ to 8^ down 
the beach. The dip is nothing but the slope of the beach itself, and 
arises from the circumstance that the sands are deposited by the 
incoming waves or tides on such a sloping surface." 

Such deposits are forming on the Florida Keys. Much of the drift 
sand of that region is consolidated into true oolite, containing uumer- 
ons traces of organic remains, wholly unlike those of the oolite at Bed- 
ford, Indiana. 

As shown by Eothpletz* and Wethered,^ it is probable that algie 
and other low forms of life which secrete carbonate of lime may have 
contributed much in the formation of oolite. The fragments of coral 
and other grains of calcareous sand in water containing much carbonate 
of lime in solution would, under especially favorable conditions, afford 
nuclei for the growth of algse and girvanellae, forms which are known 
to have played an important rdle in the formation of certain oolitic 
granules. The more we learn of ^sedimentary rocks precipitated from 
solution the more clearly we perceive the importance of the work of 
such organisms in the process. 

The Bedford oolitic limestone is extensively quarried at many points 
in a belt extending southeast from Gosport by Bloomington, Bedford, 
and Salem to near the Ohio Eiver. The belt has a length of about 90 
miles and an irregular breadth averaging nearly 5 miles. The rock 



'Coral and Coral Islands. 3d edition, pp. 153, 156, 206, and 392. 

* BotaniBches Centrolblatt, Nr. 35, 1892 ; translated by F. W. Cragin in American G^ologiat, l^or., 
1892, Vol. X, p. 280. 
' Quart. Jour. Geol. Soc. London, May, 1895, vol. 5L, p. 196. 



wu».l DESCRIPTIONS: NO. 32, LIMONITE. 105 

varies in thickness from 25 to 100 feet, and is massive, showing only 
traces here and there of stratification. Upon a weathered surface these 
lines are more distinct and cross bedding is occasionally visible. The 
dip is gentle toward the Mississippi Valley. Joints are common, but 
not abundant, and the water thus gaining access to the rock mass has 
in places given rise to great caverns. Where weathered, the limestone 
is buft" colored, but in the deeper parts of the quarries it is blue. Tbe 
difference in color is thought to be due to a change in the oxidation of 
the iron and the loss of carbonaceous matter upon exposure. When 
first taken from the quarry the rock is soft and easily carved, but 
luffdens nx>on exposure. 
Oolitic limestones of great extent are rare, although a local develop- 
inentof oolitic structure in extensive limestones is not uncommon. Its 
formation depends, apparently, upon the large local supply of carbo- 
nate of lime along the shore, but this must be combined with mobility 
of deposit in order to produce spherical deposition. 
On account of the abundance of oolite at a certain horizon among 
the rocks of the Jurassic system in Europe, this portion of the system 
is called the Oolite. It must be understood, however, that as a name 
for a group of rocks in the Jurassic system it includes many other 
rocks besides oolite properly so called. 

Oolite is generally carbonate of lime, but occasionally it is siliceous, 
fts in specimen No. 26, or ferraginous, as in specimen No. 52. In tbe 
second case, however, the oolitic structure was originally calcareous; 
uid it is possible, but not probable, that the same was true in the first 
<^^. Barely oolite is made of pyrite. Mr. G. P. Merrill has an excellent 
example from the Black Hills. The student will find an excellent paper 
on tbe Bedford oolitic limestone in the Twenty-first Annual Report of 
the Department of Geology and Natural History of the State of Indiana, 
for 1896, pp. 291-427. The paper includes a bibliography of oolites. 

No. 32. LiMONITB. 
(Pboji Lowmoor, Alleohaky County, Virginia. Described by J. S. Djller.) 
Of the various substances accumulated by deposition from solution 

m 

^ ^ater, limonite is among the most common. When heated before 
a blowpipe in the reducing flame it becomes magnetic on account of the 
^n present. In composition it is a l^drated oxide of iron, expressed 
by the formula 2Pe2033H20. Its streak is usually yellowish brown, 
Ul^eiron rust, which is practically the same substance. Another name 
for it is brown hematite, to distinguish it from red hematite (No. 121), 
^Mch has a red streak and contains no water. 

The simplest form of limonite is represented by specimen No. 32, in 
^bich it occurs as a stalactite. Carbonate of lime frequently occurs in 
^e form of well-developed stalactites, but such forms of limonite are 
'^ although it is frequently seen in small shapes which are somewhat 
^actitic or botryoidal. The mode of occurrence, origin, and struc- 
^ of the stalactite of which specimen No. 32 is a fragment are in a 



106 THE EDUCATIONAL SERIES OF BOCK SPECIMENS, [bull ISO. 

general way exactly analogous to those of specimen "So, 28, which is 
composed of carbonate of lime. 

The stalactites of liraonite were collected by Mr.G. L, Welch from the 
roof of a limestone cavern within a mile of Lowmoor. That there were 
some stalactites of carbonate of lime in the same cavern is indicated 
by the fact that in a few of the specimens the limonite is completely 
inclosed by calcite^ and the limonite core is discovered only when a 
cross section of the stalactite is examined. 

It is not quite correct to say that this stalactite is limonite, for it is 
in some cases largely carbonate of iron (siderite), and a few of the 
stalactites in the series are coated by carbonate of lime, like specimen 
No. 28, although the middle portions of such are limonite or siderite. 

The structure of specimen No. 32 is well shown at the ends. The brown 
concentric layers differ in texture; some are very compact and more or 
less wiEtxy or dull, while others, upon a fresh fracture showing cleavage 
faces, have a somewhat vitreous luster when seen in the proi)er posi- 
tion; still other bands are spongy or pulverulent. Upon applying a 
drop of hydrochloric acid it is found that the lustrous layers effervesce 
vigorously, while the others are not affected. The lustrous layers are 
siderite, the others are limonite. By a study of the soft ocherons 
layers it is found that the limonite, in many cases, grades into the 
siderite in the same layer, the oxide being the exposed portion, while 
the carbonate is within, where not so exposed to the influence of 
atmospheric agents. This relative position at least suggests, if in fact 
it does not demonstrate, that the limonite is derived from the car- 
bonate by oxidation. As shown by T. Sterry Hunt' this change is 
accompanied by a considerable loss of volume, and we are enabled to 
understand why limonite derived from the carbonate is so porous. 

The compact layers of limonite which usually form the outer coating 
of the stalactite, and occur also within, sometimes alternating with the 
layers of siderite, do not appear to be derived from the carbonate. 
Their structure is not porous, and precludes the idea that they were 
derived directly from the carbonate, but indicates rather that the 
change took place before the deposition. It is possible that in all cases 
the iron may have been in solution as a carbonate, but was oxidized 
before actual deposition, so as to show no loss of volume. 

At Lowmoor there are large deposits of limonite, and it occurs in the 
form of an extensive bed, averaging 15 feet in thickness. It is of 
Devonian age. This is but one of many similar examples throughout 
the Appalachian region. 

The conditions necessary to the accumulation of iron upon the earth-s 
surface are well illustrated in many places to-day, and deserve special 
consideration. By the decomposition of rocks containing ferriferous 
minerals the iron is usually liberated in the form of ferric oxide, which, 
being insoluble, remains behind, coloring the residual material red- 



MiDeral Physiology and Pfayiiograpby, p. 202. 



DiuBt.] descriptions: no. 33, claystone. 107 

di8b. To render it soluble it must be reduced to a ferrous state, and 
this is accomplished by acids, chiefly carbonic acid, resulting from the 
decomposition of vegetal matter. Ferrous oxide readily combines 
with carbonic acid to form carbonate of iron, which is soluble in water, 
containing an excess of carbonic acid, and it is thus that the accumu- 
lation of large bodies of iron is rendered possible. 

Perrons carbonate when exposed to pure air loses carbonic acid and 
takes oxygen, thus returning to the insoluble ferric oxide. This reac- 
tion forms a precipitate and deposits limonite. The process is illus- 
trated about many chalybeate springs where the reddish ferric oxide 
accumulates. 

The precipitation and deposition of carbonate of iron, such as that 
which is associated with the limonite in specimen No. 32, could not take 
place with full exposure to pure air. There must be present some 
reducing agent — as, for example, carbonic acid — to prevent its oxida- 
tion. As shown by Le Oonte,^ the condition most favorable for its 
deposition is the presence of a large amount of decomposing organic 
matter — for example, a peat bog, where there is an excess of carbonic 
acid. This subject will be reverted to in considering fossiliferous iron 
ore, specimen No. 52. 

Limonite is a valuable ore of iron, and when pulverized is used also as 
a polish and for paint. 

No. 33. Concretion (calcareous). (Claystone.) 

(From Hartford, Connecticut. Described by J. S. Diller.) 

In studying the oolitic sand (p. 102) it was noted that each grain is 
essentially a concretion, i. e., it is nodular in shape and composed of 
material which accumulated by systematic additions upon the outside. 
It l)egan to form at the center, and gradually grew outward. 

Specimen Ko. 33 is a concretion, and, like the grains of oolitic sand, 
is composed of carbonate of lime, but it was formed under very different 
conditions. On account of its composition it is sometimes called a 
caleareouit concretion^ and in consideration of the fact that it occurs in 
clays, it is frequently designated claystone. 

It was obtained from the bank of the river at Hartford^ Connecticut, 
where many of them have been washed out of the adjoining clay bank. 
Their mode of occurrence at other localities was described and their 
great variety of form illustrated as early as 1841, by President Edward 
Hitchcock.* 

In form there is great variety, including spherical, discoidal, anuu- 
lated, and cylindrical. Most of the forms represented in this collection 
are discoidal, and show traces of the planes of stratification in the clay 
where they were found. 

lElementfl of Geology, p. 137. 

'Final report on the Geology of MMSMhnsetts, Vol. II, pp. 407-417, pU. 15, 16, 17, and 18. 



108 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [buu^iso. 

The stratification planes are marked directly throngli the claystones, 
and they have their greatest extension parallel to the stratificatioD, 
which is horizontal. That the concretions did not exist at the time the 
clay was deposited, but originated in place later, is clearly indicated by 
their mode of occurrence and structure. The lines of stratification in 
both clay and claystone were marked out by differences of size, color, 
or composition of the particles at the time the clay was deposited. At 
a later period the carbonate of lime, of which the greater portion of the 
concretion is composed, was deposited by waters circulating through 
the porous layers of the sandy clays. 

Why the circulating waters bearing calcic carbonate should deposit 
it in such shapes is not always clear, but it appears from the general 
form that the precipitating infiuence must have emanated from the cen- 
ter. Many calcareous concretions contain fossils, and the decomposing 
organic matter furnished the surrounding atmosphere with the pre 
cipitatit that developed the concretion from the center outward. The 
form of growth, however, is determined largely by the relative porosity 
of the various layers. The greatest development takes place along the 
most porous layers, where precipitation is most free. As the concretion 
grows, it loses organic matter at the center, and this process may be 
carried on until the latter has entirely disappeared in such a way as 
not to preserve its original form, ^o record of the fossil will remain. 
Many concretions have no visible traces of organic matter, and its 
absence may be explained in this manner. 

Excepting the conditions which mark the planes of stratification, 
claystones are often homogeneous throughout, but in other cases they 
possess concentric banding like that so well shown in the siliceous oolite, 
specimen l^o. 26. 

For further information on the subject of claystones, the student is 
referred to Monograph XXIX, United States Geological Survey, by 
Prof. B. K. Emerson, where he will find the matter discussed under the 
heading "Concretions.'^ 

FO. 34. CONCBETION (FERRUGINOUS, CONTAINING A FOSSIL). 

(From Mazon Creek, Grundy CouyxY, Illinois. Described by J. S. Dillkr.) 

The rocks along Mazon Creek, Grundy County, Illinois, from which 
specimen No. 34 was collected, according to Mr. Frank H. Bradley,' con- 
sist of very sandy clay shales and sandstones, in some places becoming 
nearly pure clay shales with many fossiliferous nodules of carbonate of 
iron. These nodules contain many species of ferns, as well as numeroas 
fossil insects, occasional crustaeeans, fish scales, and shells of moUusks. 
It appears that in this case most, if not all, the concretions have 
organic nuclei, and in this respect they are strongly contrasted with 
the claystones of Hartford. It should be remembered, however, that 

* Geol. Survey of Ulmols, VoL IV, p. 190. 



wtLtt] BE8CRIPTI0N8: NO. 34, CONCRETION. 109 

in the shales of Mazon Creek there are numerous fossils, but in the 

clays of Hartfoid fossils are rare or entirely absent. 
The ferru^nous sandy shale, which contains the concretions (speci- 

mea ^o. 34) irregalarly scattered through it, is 25 feet in thickness and 

oyerlies the main coal bed of that region. 
In form and size the concretions vary according to the inclosed 

or{i;anism. A small fern leaflet may be contained in a concretion less 

tban an inch in diameter, while another concretiou, inclosing a piece of 
calamite, may be a foot in diameter. Generally, however, the concre- 
tions are flattened in one direction and elongated oval in another. 

As with the calcareous concretions, the plane of greatest extension 
is the plane of stratification. Traces of stratification may be seen in 
the banding or shaly structure of the concretion. 
The fossil vegetal fragments contained in these concretions have been 
studied, described, and figured by Leo Lesquereux,^ who remarks:^ 

Peenliar species of plants and animals or their fragments seem to have been 
selected as the nuclei of the nodules. They contain, for example, an abundance of 
leaflets of Tarions species of Neuropteris . . . whioh are either rare or have not yet 
Wq found in the shale at Morris, while these shales are rich in [other] remains 
Kircely or not at all preserved in the concretions. 

lie states further : ^ 

Th« pinme or leaflets of ferns are always found in them in a flaMened position, 
tlieiraxis or rachis extending through the center of the elongated nodule, with the 
(imsioQB on lK)th sides; the surface of the pinnules, slightly swollen, as when in 
their living state, is marked by recognizable hairs or fruit dots, with distinct veins 
lod yeinlets, antl their appendages, like the scales, are seen in the various modifica- 
tions which they present in living specimens; for example, long, straight, flat, 
diTerging, or prmiary rachis, and becoming shorter, ruffled and curled on their 
Dpper divisions. The small organs of plants appear, therefore, in a better state of 
ptcaerration than in the shales. With small animals like crustaceans, scorpions, 
iniectBof a fleshy and very delicate texture, the preservation q^T form is still more 
reinarkftble. They are found entombed in the middle of the nodules just as if they 
vere in life, or as if they had been transformed into stone while still living. The 
froitaor nutlets are not flattened. By the section of the nodules, which generally 
break into two equal halves by hard strokes on their edges, the middle and internal 
ptft of the frnit is exposed to view, while the outside surface is immersed in the 
stone. The nnmeroas cones also of Lepidodendron found in these concretions are 
eqnally well preserved, either in whole or in part, by horizontal cross sections. Some 
speeimens not only show distinctly the pedicels of the sporanges and the blades in 
tbeir natural position, but even sporanges with their seeds have been found in 
them, without perceptible alteration. In the cross section of these Lepidostrobi 
the Bporange cells form a central row, which is surrounded by the blades in the 
form of a star. 

Aside from the inclosed organism, part of whose carbon usually 
fBinains in the fossil, the concretion is composed chiefly of carbonate of 
iron. When dissolved in dilute hydrochloric acid, upon the addition of 
ammonia the solation becomes at first greenish, then reddish, precipi- 
^ng an abundance of ferric oxide. There is but little calcium and 



' Geo]. Snrvey ot Illinois, Vol. IV, pp. 376-508, and numerous plates. 
< op. cit., pp. 48^-483. 
"Ibid., p. 482. 



110 THE EDUCATIONAL SERIES OP ROCK SPECIMENS, [bull.150. 

magnesium iu the solntion, and it is evident that the iron, being in the 
ferrous state, as shown by the greenish color of the solution, is pres- 
ent in the nodule in the form of a carbonate. The presence of the iron 
is most simply shown by the fact that a small fragment of the concre- 
tion heated in the reducing flame becomes magnetic. It is interesting to 
note this occurrence of carbonate of iron in connection with organisms 
where carbonic acid was probably present to prevent the oxidation of 
the iron at the time of deposition.^ 

Kg, 35. Concretion (Marcasite). 

(From Thatcher, Las Animas County, Colorado. Described by J. S. Diller.) 

These concretions of marcasite were obtained, according to Mr. G. K. 
Gilbert,^ from the lower layers of the limestone at the base of the Timpas 
formation.^ The limestone has a light-gray color, which becomes 
creamy white on weathered surfaces. It is compact and fine grained, 
and in the eastern portion of the district its texture becomes somewhat 
coarser, its color paler, and the fractured surface resembles chalk. 

Ab the limestone is broken up and removed by the action of the weather, the more 
resistant nodules are freed from the matrix, so as to lie loose on the Hurface. They 
are of a dark-brown color, and of oval or cylindrical form, with a diameter of about 
half an inch. Their surfaces are not even, but are set with angular projections, tLe 
ends of crystals.^ 

In PI. XX, taken from Gilbert's report, the variation in form and 
size of the nodules is illustrated upon the natural scale. 

That the crystals referred to above are marcasite is known by their 
tbrm in connection with the fact that, when pulverized and heated in a 
closed tube, they yield a sulphurous odor and become magnetic, thus 
indicating that they are composed of sulphur and iron. From the shape 
of the projecting crystals it appears that their axes, although at right 
angles, are of unequal length, thus showing that the mineral is mar- 
casite and not pyrite, which has nearly the same color and composition 
but diii'ers in form of crystallization. 

The structure of the concretion is radial and connected directly with 
the arrangement of the pointed crystals upon its surface. Although 
all the concretions are rusty brown outside, when broken open some of 
them are found to be brass-yellow within, and the radial structure dis- 
tinct. In most of the nodules, however, the marcasite has completely 
changed to limonite, and the structure is not well marked. No nuclei 
were observed. 

The chemical composition of marcasite is sulphur 53.4, iron 46.6, 
represented by the formula FeS2. When exposed to the air, it usually 
decomposes, forming limonite, which at first only coats the^nodules but 



I See also deacription of Limonite, specimen No. 32, and Fossiliferons Iron Ore, specimen No. 62. 
'The nnderground water of the Arkansas Valley in eastern Colorado: Seventeenth Ann. Rept 
U. S. Geol. Survey, Part II, 1896, p. 56«. 
■ The Timpas formation is the upper portion of the Niobrara (Cretaceous) groap of that region. 
* Gilbert, op. cit., p. 506. 



^ ^ 




1^ 








CONCRETIONS OT MiRCASITE FROM THATCHER. COLORADO 



Lix».l DESCRIPTIONS: NO. 36, GEODE. Ill 

I time completely replaces the marcaslte. When the replacement is 
)mplete the crystals are psendomorphs of limonite after marcaslte. 
[) the process of decomposition snlphuric acid is set tree and may 
ttack the surrounding minerals, thus contributing to the effects of 
reathering. Sulphate of iron is frequently formed, and the nodules 
all to pieces. This is especially likely to.be the case if the air is damp, 
the nodules found loose upon the weathered surface of the limestone 
ire chiefly limonite, but according to Mr. 6. W. Stose, who collected 
lome of the specimens, those found flrmly embedded beneath the sur- 
face of the limestone are marcaslte, and when broken exhibit the radial 
itrncture much better than those changed to limonite. 

In the case of the concretions Nos. 33 and 34 of this series, it is evi- 
leiit that they were formed after the deposition of the argillaceous 
strata in which they are contained; but the concretions of marcaslte 
from the Timpas limestone do not indicate so clearly their origin. In 
some other cases, however, fossils which were originally calcareous are 
found completely replaced by marcaslte or pyrite. Such replacements 
were not found in the strata ^m which specimen No. 35 was collected. 

Xodales of marcaslte or pyrite are perhaps most frequently found 
in limestone, but they are of common occurrence also in shales and 
sandstones.* 

No. 36. Oeobe. 

(From Warsaw, Hancock County, Illinois. Described by J. S. Dillbr.) 

Geodes, like concretions, are nodules, but differ widely in their mode 
of growth. A concretion begins at the center and increases in size by 
additions upon the outside. On the other hand, a geode begins upon 
theoQtside as the lining of a cavity, and grows by successive additions 
upon the inside of the cavity. As in specimen No. 36, the cavity is 
asually well studded with crystals. 

The geodes at Warsaw are contained in a bed of the Keokuk group, 
)f lower Carboniferous age, and it has been said that there is no other 
ocality known in the West where a few hours of labor of a good col- 
ector will be rewarded by so large a variety of finely crystallized speci- 
Qens. The occurrence of these geodes in the field has been studied by 
•Ir.A. H. Worthen,^ and their structure and mineralogic composition 
lave been described by Prof. George H. Brush.^ 

Mr, Worthen says: 

The geodes occar disseminated throngb the shale and shaly limestone, sometimes 
) thickly dispersed through it that the individuals press against each other as they 
e embedded in the matrix; and again, are so sparsely disseminated that several 
ibic feet of the shale will afford not more than a single specimen. They are most 
kndant at Warsaw, in the lower part of the bed, which also affords nearly all the 
rge-sized individuals. The general form of those tilled with siliceouH minerals is 



> The fomiAtioii of saDdatone concretions, by G. P. Kerrill: Proc Kat. Mua., Vol. XVII, p. 87. 

>Geol. Sorvey of Illinois, ToL I, pp. 89-98. 

*Op.cit.,pp.00-06. 



I 



112 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [boti^UO. 

globular, and many of them are solid spheres of quartz, the interior of which is 
generally crystalline, with a thin crust of chalcedony coating the exterior surface. 
Through the middle of the geode beds there is a band of shale, which, at Warsaw, 
is from 8 to 10 feet thick, in which nearly all the geodes are lined with calcareous 
minerals, and these present less regularity of form than those lined with quartz. 
Many of them are flat disks, nearly or quite solid, but always containing calcite, 
and frequently fine crystals of blende. 

Mr. Worthen states also that in Hancock Goanty a siliceous geode 
was found filled with liquid bitumen, and at St. Francisville, Missouri, 
others were observed to be partially filled with clear water. 

The collection of thirty-three geodes studied by Professor Brush was 
specially selected, and his report describes the variations. The geodes 
of this series of rock specimens, although generally quite uniform, are 
somewhat varied, as were those described by Professor Brush, from 
whose report the following is quoted: 

You will observe that, in every case I have examined, the outer layer of the geode 
is siliceous and is of that form of silica which is called chalcedony, althongh some- 
times this outer siliceous rim is extremely thin. The next in the order of snperpoei- 
tion is crystalline' quartz. In every geode which contains crystalline quartz this rests 
directly on the chalcedony. In some instances a second layer of chalcedony rests 
on the quartz crystals (Nos. 11, 14, and 22), and in one instance a second series of 
quartz crystals rests on the second layer of chalcedony. Calcite occurs in great 
beanty and variety of form, sometimes resting directly on the chalcedonic crust, 
and sometimes resting on the lining of quartz. In no instance where calcite and 
quartz occur in the same geode have I found the quartz resting on calcite; they all 
indicate that the calcite is subsequent in formation to the quartz. The calcite 
crystals are worthy of special crystal lographic study. 

The occurrence of pyrites shows that in some cases its formation was simultaneous 
with that of calcite, while in other instances it was apparently subsequent to it. 
The elongated crystals of tarnished pyrites are quite remarkable and might easily 
be confounded with rutile ; but they show a yellow color and a cross fracture, and 
a blowpipe examination reveals their real character. 

Blende seems to have been simultaneous in formation with the calcareous layer of 
the geodes in which it occurs, for in two instances I have observed it embedded in 
the calcareous layer without resting on the chalcedonic base. 

GypeurHf observed in minute crystals in only two instances, is subsequent in for- 
mation to the second layer of chalcedony in the geodes in which it occurs. PearU 
spar, dolomite, or hrotcn'Spar, as it might very appropriately be called, occurs in 
several geodes, and is almost always of subsequent formation to the calcite. In a 
few instances, however, calcite crystals appear resting on a. dolomite base, and this 
leads me to call attention to the occurrence of calcite of at least two distinct periods 
of formation, as shown by the form- and color of the crystals (Nos. 20 and 21). The 
dolomite in the geodes seems to be peculiarly liable to decomposition by the 
oxidation of the iron. An analysis of it shows it to contain a large percentage of 
carbonate of iron with the carbonates of lime and magnesia. 

Aragonite was found in but one instance, and then resting on dolomite. Geode No. 
32 contained a considerable amount of a loose white powder, which, on chemical 
examination, proved to be a hydrous silicate of alumina; and it is exceedingly 
curious that the crystals of calcite in what must have been the lower part of the 
cavity, contain, disseminated through them, this same silicate, as, upon solution in 
acid, they leave behind an insoluble white powder, similar in character to that 
found loose in the geode. Moreover, the crystals differ in form from thoae lining the 
upper portion of the cavity. 



Diu^l DESCRIPTIONS: NO. 37, 8ILICIFIED WOOD. 113 

Geode No. 4 is one of the most interesting of the snite, being almost filled with 
uphaltnm, and having isolated quartz crystals embedded in the asphaltum. 

The geodes in this series are composed of a thin crast of chalcedony, 
here and there showing indistinct banding parallel to the surface of the 
geode. Inside of the chalcedony isa thick layer of crystalline quartz 
with granular or radial structure about the inclosed cavity, which is 
bristling with the pyramidal points of quartz crystals. Superimposed 
apon the faces of the quartz crystals, here and there in some of the 
geodes, are a few crystals of dolomite, calcite, pyrite, or other minerals, 
but they form a very subordinate portion of the whole mass. 

That geodes originate in cavities there can be no question, but the 
way in which the cavity was produced is often a matter of doubt. The 
form of the geode is in some cases conclusive evidence that the cavity 
was produced by dissolving and removing a shell or other fossil. The 
shape of the geodes in this series does not clearly indicate any organism, 
bat it has been suggested that the geodes of the Keokuk limestone of 
Iowa and Illinois (including those of Warsaw) *' occupy the centers of 
sponges that were at some time hollowed out by siliceous solutions, like 
the hollowed corals of Florida, and then lined with crystals by deposi- 
tions from the same or some other mineral solution.'' ^ 

Among the most interesting geodes are those occasionally found in 
fossiliferons shales, as at Yaquina Bay, on the coast of Oregon, where, 
after the calcareous shells are removed, leaving a cavity, the mold is 
tilled with translucent quartz by successive additions upon the walls 
of the cavity. In most cases the cavities are completely filled, but in 
others the cavity is only partially filled with quartz, the remaining 
portion containing water with a movable bubble, whose motion may be 
observed through the translucent quartz. 

Similar small geodes are occasionally found in amygdaloids, where 
the original vesicles in the lava are not completely filled. The same 
term is also applied to crystal-lined cavities in veins, without reference 
to their form. 

No. 37. SiLiciFiED Wood. 

(From Gallatin Basin, Gallatin County, Montana. Described by J. S. 

DiLLER.) 

The occurrence of silicitied wood is illustrated in PL XXI, prepared 
from a photograph taken by Prof. J. P. Iddings upon the slopes of 
the Lamar Biver. The trunks of the two trees are shown standing 
erect where they grew. They were buried by the accumulation of sand 
aud gravel of an ancient geologic flood, which gave birth to the Tertiary 
conglomerate. At the time of their burial they were not silicified, but, 
while under the ground, the circulating siliceous waters effected a com- 
plete change in their composition. The wood was removed particle by 
particle, and in the position of each was placed a particle of silica, so 

— m ■ ■ - ■ — ■ _^ - ■ I - — 

I Haxraal of Geology, 4th edition, by J. D. Dana, pp. 97, 98. 

Bull. 150 8 



114 THE EDUCATIONAL 8EKIE8 OP EOCK SPECIMENS. |Bru.ia 

that ultimately the wood was all removed and the whole trunk was 
completely changed to silica; yet the replacement occurred in aoch a 
way as to preserve with wonderful detail, not only the remarkable cellu- 
lar structure of the original wood, but also the peculiar markings upon 
the walls of the cells. TLiu sections have been made of specimen Ko. 37, 
and fig. 12 shows its cellular structure, as seen under a microscope. The 
cells of the wood are filled with silica, slightly different in apx>earatic« 
from that which replaces the vegetal matter, and it is this differeace 
that marks the cellular structure so distinctly. It has been studied by 
Mr. V, H, Knowlton, who describes it as follows: 

FUj/orylon peaM. — Annual ring very proiiouncod, 2.10'"'" broad; chIIh of Bununer 
wood large, tbiD-wallcd; oplU of full wood thick, niach comprMSOd ; cells of iiim- 
mer wood witL a aingle aeriaa of Iftrge, Bcaltered punctations; medullar; mjB ia * 
Hingle writs of two to almut twenty long cells, market! radially with one tn tlires 
small -bordered pits in the width of each wood cell; resin tuljes rather Dumeroos, of 
largr size. 

The fossil wood contains in places much amorphous silica, but at 
other pla<%s is generally cryptocrystalline, with here and there small 
radial fibrous groups with negative optical properties, 
indicating that it is chalcedony. 

The special lesson to be learned from this specimen 
IS the excellent example it a&brdsof the replacement 
by substitution of one substance for another, in con- 
nection with fossilizatiou by casting. 

The doable character of the process may be illus- 
trated by an experiment. If wood be repeatedly 
soaked iu a solution of sulphate of iron until its cells 
are filled, and then burned, its structure will be pre- 
neraiukHiBcope served in the remaining ferric oxide in the form of a 
cast The plate formerly occupied by the wood will be vacant. Id 
sihiified wood, however, there is the cast of silica as before, and also 
the repl.itmg silica which occupies the position of the organic matter. 
Such perfect replacement by silica may occur also in the animal king- 
dom. The internal structure of a spirifer, as well aa the delicate parts 
of other mollusks, may be replaced and preserved in the greatest detail 
by silica. 

Silicified wood is of common occurrence in many of the sandstones 
and conglomerates of all ages, especially in those later than the Juras- 
sic These rocks usually contain much quartz, and, being porous, 
afford special facilities for the circulation of underground waters, 
through whose agency the silicification of the wood is effected. 

No. 38. SiLiciFEED Shell. 

(From Ciiarlestown, Clark County, Indiana. Dbscribed bt J. 8. DiLUtR.) 

This shell, Spiri/er oireni, is one of the Devonian forms found in the 

hydraulic limestone, or cement stone, which extends northeastward from 

Louisville, Kentucky, to Gbarlestowu, Indiana. The shell waa origi- 





TREES. YELLOWSTONE NATIONAL PARK, 



I 



pOJ^l DESCRIPTIONS: NO. 39, CHALK. 115 

iially carbonate of lime, but is now qaartz, the carbonate of lime baving 

been removed and the silica deposited in its place. This has been done 

in such a way as completely to preserve the exteinal form of the shell, 

but the internal structure is lost. It appears that a perfect impression 

of this shell was n'kade in the material by which it was inclosed at the 

time the rock was formed. The shell was then removed by solution in 

tbe water circulating in the rocks, thus leaving a mold which was 

afterwards filled with silica, producing a cast. The method of fossiliza- 

tion by molds and casts is common and of much importance in the 

geological record. Molds of shells are perhaps more frequently seen in 

tbe Oriskany sandstone than in any other formation. 

Oasts may be of other substances than silica, and when of a compar- 
atively stable substance, like oxide of iron, the fossil may be preserved 
althoagh the rock inclosing it may undergo great change. One of the 
most striking examples of this kind is in a mica-schist of Scandinavia, 
where the original sediment was so metamorphosed as to have been 
completely crystallized, and yet the forms of large trilobites it contains 
are perfectly preserved in the casts of oxide of iron. 

There are three principal types of fossilization illustrated by speci- 
mens in this series. In the first type, represented by specimen No. 6, 
tbe organism is simply buried and remains wholly or partly unchanged; 
in the second type, represented by specimen No. 38, the organism has 
been completely removed and its mold or cast preserves its external form 
only; in the third type, represented by specimen No. 37, the organism 
is completely removed and replaced by mineral matter in such a way as 
to preserve not only the external form but also its delicate internal 
stmcture. 

UNALTERED SEDIMENTARY ROCKS OF ORGANIC ORIGIN. 

No. 39. Chalk. 

(FttOM White Cliffs of Little River, Sevier County, Arkansas. Described 

BY J. S. DiLLER.) 

Of the various rocks deposited by water, limestone is among the 
most important. It Is composed essentially of either carbonate of lime 
or carbonate of magnesia, or both. Its various forms are quite fully 
illustrated in this series by the following specimens: Chalk (No. 39), 
^atdliua limestone (No. 40), coquina (No. 42), shell limestone (No. 43), 
cberty limestone (No. 44), compact limestone (No. 46), lithographic lime- 
stone (No. 47), hydraulic limestone or cement rock (No. 48), amorphous 
iAdrl(No. 49), and shell marl (No. 50) among the unaltered rocks, and 
wystalliiie limestone (No. 115), marble (No. 116), and dolomite (No. 117) 
^Dioug the metamorphic rocks. 

Chalk is a white earthy limestone which is so soft as to be easily 
^rked by the finger nail, and is composed of fine calcareous sediment 
derived chiefly from the shells of foraminifera. When pure, its chemi- 
cal composition is almost wholly carbonate of lime. 




116 THE EDUCATIONAL SERIES OP BOCK SPECIMENS. [buu^MO. 

Although well known for centuries in England, its occurrence among 
the rocks of the United States was not appreciated fully until 1887,^ 
when it was described by Mr. E. T. Hill, who has shown ^ that there 
are several distinct beds of this material, having wide vertical and 
areal distribution, in the Cretaceous rocks of Texas and neighboring 
States. The specimens for this series, which are a little harder and 
less porous than typical chalk, were collected from the uppermost of 
the various chalk beds of the Texan region at White Cliffs, on Little 
Biver, in Arkansas, where, according to Mr. Hill, a great bed of the 
purest whit« chalk occurs in section 35, T. 11 S., B. 29 W. He says: 

These cliffs have long been a laudniark of the region, are aboat 150 feet in heigh t, 
perpendicular, and as white and almost as pure as the celebrated chalk cliffs of 
Dover, England. Their remoteness from the lines of travel is the probable explana- 
tion of their -having so long been overlooked by American geologists. 

The chalk of these cliffs scales off rapidly in great oon- 
choidal flakes, and owing to the irregularity of this 
process, its face, instead of being a continuous plane, is 
composed of many acute and reentrant angles, resem- 
bling the bastions of a fortress. The summit of the cliff 
is covered with gravel, but, measuring from the top of the 
hill a short distance from the margin, the present thick- 

-n. <» rii ut ^ / wiAA. i^ess of this chalk is found to be about 135 feet from the 
FlG.13.— Globigerin»(a,XlOO) x 4. xt, ^ ^ a ^ * -4. 0,1.. v ii v 1 

andTextalariiKfr. xi40)froin Bummit to the bed underlying it. This chalk has a low 

the chalk of White Cliffs, southeastern dip. 

Texas. The regularity of this bed throughout its exposure— 

about one-fourth of a mile — and its reappearance a few 
miles to the east and across the Saline watershed shows that it is not a local bed, 
but the remnant of a great and extensive horizon, worn away by the denudation 
through Tertiary and Quaternary times of the deposits of the Red River embayment. 

In the large fragments from which specimen No. 39 was prepared 
fossils were rather common and conspicuous, although but few traces of 
them can be seen in the hand specimen. Gamptonectes, Inoceramus, 
Baculites, and Ananchytes ovalis are the fossils rei)orted by Hill from 
this bed, but it appears that generally <^the chalk is almost free from 
fossils." This statement, however, refers only to fossils which can be 
seen by the unaided eye, for if properly prepared and examined under 
the microscope the chalk is found to be comx)Osed almost wholly of 
material derived from the shells of minute organisms. Some of the 
shells are complete, more are broken, but most are reduced to a fine 
powder. In fig. 13 are represented two of the most common forms 
which occur in specimen ^o. 39. These have been studied by Mr. 
George H. Girty, who reports as follows: 

The chalk sections which 1 have examined seem to be scantily supplied with 
recognizable organic fragments. The latter consist of foraminifera, with an occa- 
sional coccolith. The foraminifera can be referred to the genera Globigerina and 
Textularia, of which the former seems much more common. 



» Am. Jour. Sci., 3d series, Vol. XXXIV, 1887, p. 308. 

> Ann. Rept. QeoL Survey Arkansas for 1888, vol. 2, pp. 87 and 88. 



iwnl DESCEIPTI0N8: NO. 39, CHALK. 117 

rtie chemical aaalysla * of the chalk £rom White Clififs is as follows 

Analtftit of chalk/rom White Cliffi, Saner County, JrkanioM. 
lawilable nutUr and silJw 



Ferrit . 



lidel 



Cu-boDAie arilnie ' »■ 

Carbon>l«ariiiaKDwii> | 1 

LOHOD iipilUOD and water I 

Total ; 101.00 j 



It contains some iron, which here and there tinges it yellowiah. The 
iisoloble matter aud silica are readily accounted for by the presence of 
qiicnla; of spoDges and other siUceons orgauiams, which may be seen 
nnder the microscope. Mr, Hill reports that in the field it does not 

contain nodnles of fliut. In this 

respect it is like the " chalk with- 
ont iiints" of Earope, and differs 
from the "nodnlar chalk" or 
"chalk with dints," so well ex- 
posed in the cliffs at Dover, 
England. Althoagh flints do 
uot occnr in the chalk at White 
Cliffs, they do occur in the 
Caprioa limestone near Austin, 
Texas, from which specimen Ko. 
41 was collected. 

Much light has been thrown 
npoD the origin of chalk by the 
deep-sea soundings of the Chal- 
lenger expedition.' From the 
sea floor, at depths of between ^^ H-Gi"biB"ii." ou^,. fnm, ludu,., i>r,^i,, at » 
2,500 and 17,000 feet, where not *"'"' "' ' ^ '"^•'""■ 

too cold, the dredge brought op a white ooze, consisting largely of the 
Bbells of foraminifera and other organisms having calcareons tests, in- 
termingled with a small amountof radiolarian and other siliceous shells. 

In tig. 14 is shown the appearance of globigerina ooze as seen 
under the microscope. ' It is composed chiefly of the calcareous shells of 
members of the family Qlobigeriiiida;, but with these are a few lozenge- 
Bbaped and other siliceous shells.^ This is so strikingly similar to 
chalk in its structure and composition that its deposition practically 
illnstrates the origin of chalk. 

' Ann. Repit. Oeol, Snm; ArkHuu for IMS, raL 3, p. 230. 

■RiTonofibsKlentiacrniiltgDf thaeiploriDgToyM'af H. U.S. OAoRfnpn-, IKTBtu UTO^ Deap- 
ta nipoalta, 2\ i-V3 . 
■niid.,nppv(ne-tliiid orBg.*, PI. XII. 




118 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [buli.150. 

The organisms whose remaius now form the chalk lived in the sea 
ander various circumstances. The larger animals livpd upon the bottom, 
where they died and their shells contributed to the accumulating 
chalk. 

The minute foraminifera, of whose tests the chalk is chiefly composed, 
lived not upon the bottom, but far above it, near the surface of the ocean. 
The species of foraminifera caught in the surface nets of the Challenger^^ 
are the ones whose dead shells have sunk to the bottom to make up a 
large part of the globigerina ooze. 

It is evident that these minute organisms must have drawn from the 
ocean water the carbonate of lime of which their shells are com]K>sed. 
The same is true of corals, mollusks, and all other marine organisms 
whose skeletons or shells are calcareous and whose remains play sucb 
an important part in the formation of limestones. 

Notwithstanding the fact that chalk is one of the rarest of sedimentary 
rocks, globigerina ooze is one of the most widely distributed of the 
marine deposits forming at the present day. 

Chalk is very porous, so that it will absorb in some cases an amount 
of water equal to about one- third its own bulk. On this account beds 
of chalk are great reservoirs of underground water, and in some places, 
as in the vicinity of London, such beds yield a large supply of water by 
means of artesian wells. 

Chalk has wide application in the arts, industries, and agriculture. 
It is calcined to make lime, producing a superior quality of that mate- 
rial for chemical and structural uses. In the semihumid portion of its 
extent, where the material indurates through a process of interstitial 
hydraulic setting, it is sawn and extensively used as a building material. 
By saturating chalk with siliceous solutions to give it hardnoss, and 
mineral stains to give it color, ornamental marble of great variety and 
beauty is manufactured in Europe. Its most remunerative application, 
however, is its use in the process of making hydraulic and Portland 
cements. For this purpose it is mixed in definite proportions with clay 
and silica. It is the use of chalk that has enabled Europe to control 
the supply of su])erior Portland cement in the United States, and Hill 
has pointed out the fact that the neglect to utilize the extensive deposits 
of chalk in our own country amounts to a serious commercial loss. When 
pulverized, washed, and elutriated it is known as creta preparataj and 
is extensively used for toilet and fine abrasive purposes, as well as for 
medicinal purposes. It is also used in the manufacture of carbonate of 
soda and carbonic acid. On wet clay soil it is a valuable fertilizer, and 
for such purpose it is extensively used in parts of England, and could 
be most profitably so employed upon the noncalcareous lands of the 
southern coastal region of the United States. 

The white crayons used for marking purposes were formerly made of 
chalk, but now they are composed chiefly of artificially produced sul- 



1 Challenger KeportH ; Deep Sea Deposits, p. 218. 



mMR.] DESCRIPTIONS: NO. 41, FLINT. 119 

phate of lime, with a small admixture of lime carbonate. The lump 
chalk used by carx>enter8 and other tradesmen is the natural chalk as 
romoved from the ground. 

No. 40. Patbllina Limestone. 

(From Austin, Texas. Described by J. S. Diller.) 

The specimens of this series were collected on Bull Creek, 5 miles 
west of Austin, Texas. It is a light-colored, earthy limestone, which 
to a considerable degree resembles the chalk of Arkansas, although it 
belongs to a much older horizon. According to Hill,^ it forms a stratum 
10 feet or more in thickness near the middle of the Glen Hose beds, in 
the basal portion of the lower Cretaceous. 

The typical Patellina limestone dififers from the chalk (!N'o. 39) chiefly 
in hardness and in the macroscopic fossils present; but the material 
also occurs in nature as beds of pulverulent chalk, or marl. As in the 
ease of chalk (No. 39), there are a number of mollusks present in this 
limestone, but few, if any, of them appear in the hand si)ecimens. 

It is composed largely of a small, flat-conical foramiuiferal shell, 
Patellina texana Boemer,^ and on this account, at the suggestion of 
Mr. Hill, has been called the Patellina limestone. While Patellina tex- 
ana is the only foraminifer in the limestone visible to the naked eye, 
in a thin section of the rock under the microscope the finer material is 
seen to be comiK)sed chiefly of foraminifera similar to those of chalk, 
illustrated in figs. 13 and 14, and might well be called chalky IhneHtone, 
Its conditions of formation must have been in general quite like those 

of chalk. 

No. 41. Flint. 

(From Austin, Texas. Described by J. S. Diller.) 

Flint nodules are of common occurrence in the upper chalk beds of 
England, but, as noted under the description of chalk (No. 39), they do 
not occur in the equivalent of that horizon in Texas. The specimens of 
the series were collected from the lower- lying Caprina limestone (Shu- 
mard) of Hill's section,^ 2 miles west of Austin, Texas. 

In this chalky limestone are well-defined layers of exquisite flint nodules, occa- 
pyini;, apparently y persiHt^nt horizonH iti localities. These flint nodules are oval and 
kidney shape^l, ranging in size from that of a walnut to about 2 feet in diameter. 
Ext4'norly they are chalky white, resembling in general character the flint nodules 
of the English chalk cllfls. Interiorly they are of various shiules of color, from 
light opalescent to black, somethnes showing a banded structure. These flint 
DMlnlee are heantifully displayed tn situ in the Deep Kddy Canyon of the Colorado, 
above Austin, where they can be seen occupying three distinct belts in the white 
chalky limestones. . . . 

The fact that these are the only flint horizons, so far at least as is known to the 
writer, in the whole of the immense Cretaceous deposits of the United States is very 



■Pmleontology of the CreUceoun formation of Texas: Proc. Biol. Soc. WaHhiugtuu, Vol. VIII, 1893, 
pp. 14, 20, and 21. 
'lUoBtratMl in Dana's Manual of Geology, 4th ed., 1895, p. 834. 
*GeoL Sur\-ey Texas, Bull. No. 4, p. xix. 



120 THE EDUCATIONAL SERIES OP BOCK SPECIMENS. [bulcUO. 

intereatinjir, and espeoially since they ocour abont the middle of the Lower Cieta- 
ceouH series instead of at the top of the upper series, as in England. It was firom 
them that the Indians made their flint implements, and the ease of their lithologic 
identity will be of value to the anthropologist in tracing the extent of the inter- 
conrse and depredations of former Indian tribes inhabiting this region.' 

These flints have been distribnted in later geologic epochs over a 
wide area coastward of the present oatcrops of the Gaprina limestone. 

The specimens illastrate the light-colored exterior of the flints as 
well as the extremely compact textare and the perfect conchoidal 
splintery fracture of the darker interior. Pi. XXII, taken from a pho- 
tograph, shows a section of one of these nodules in which the concentric 
banding, as well as the planes of original stratification, are distinctly 
marked. When highly heated in a flame, the dark-colored flint becomes 
white, indicating that the color is dne to the presence of organic matter. 
Mr. Hill reports a nodule containing a small cavity filled with liquid ; 
others are found with fossils (Bequienia and Monopleura) as nnclei, but 
these occnrrences are exceptions. The nodules are generally without 
nuclei. Nevertheless, they contain a large number of fossil fragments, 
which are visible only with the aid of a microscope. 

Mr. J. A. MerrilP has made a sx>ecial study of the fossils in the flint 
nodules of the Lower Cretaceous of Texas, and not only described the 
forms, but considered the conditions of their preservation and the origin 
of the nodules. He examined a number of slides of specimens in this 
series, and reports three species of monactiuellid and three tetracti- 
nellid forms of sponge spicules, besides the remains of foraminifera, 
echiuoderms, and shell fragments. 

The large number of the siliceous organisms found in the flint, both 
of this country and of Europe, leave scarcely any doubt as to the 
source of the silica of which they are formed. It was originally taken, 
at least in large part, directly from the sea water by siliceous organ- 
isms, especially sponges, for their skeletons and shells, in much the 
same way as the carbonate of lime is secured from the same water by 
organisms having calcareous parts. In globigerina ooze calcareous 
and siliceous organisms are found intermingled, and the Texas material 
contains a similar association of forms. The organisms found by Mr. 
MeiTill in the flint of Texas are foraminifera, sponges, mollusks, and 
fish scales. The foraminifera were principally globigerina whose 
shells are well known to have been originally calcareous. In the flints, 
however, the calcareous matter is completely replaced by silica, and it 
is evident that the flint is not made wholly by an accumulation of sili- 
ceous organisms, but in part, at least, by the replacement of calcareous 
organisms by silica brought in solution. Spicules of sponges and other 
organisms are found in all stages of preservation. A few are well pre- 
served; more are partially destroyed, while the greater number have 
either almost or entirely disappeared under the attack of mechanical 
attrition and solvents. 

1 Hill, Geol. Survey Teras, Bull. No. 4, p. xix. 

'Bull. MuB. Comp. Zool. Harvard College, Vol. XX VIII, No. 1, pp. 1-26. See alao » Beview of 
the general work by Mr. Wayland Vaugban, in Jour. Geol., Vol. IV, p. 112. 



wu^l DESCRIPTIONS: NO. 42, COQUINA. 121 

Although the forms of some of the sponge spicules are well preserved, 
the sUiceous material of which they are now composed is not amorphous, 
as it was originally, but so arranged, or perhaps we may say rearranged, 
as to be crystalline. The silica of the flint is in two forms, crystalline 
and amorphous. The first is practically insoluble, but the second is 
soluble in caustic potash. By far the larger xK>rtion of the silica in the 
f int is in a crystalline condition. Mr. George Steiger, by treating sx>eci- 
men No. 41 with a 10 per cent solution of caustic i)otash for one hour 
over a water bath, found that 15.39 per cent of the flint was dissolved. 
It is generally very fine microgranular, but occasionally it is radial 
fibrous like chalcedony. However, it is optically positive, while chal- 
cedony is negative. Even that of the sponge spicules and other fossils, 
which was originally amorphous, is now crystalline. Perhaps this 
structural change may have resulted directly from the removal of the 
spiculin originally associated with the silica in the sponge spicules. 
However this may be, it is certain, as already noted, that there is 
much actual replacement by silica in the fossils of flint nodules. 

It has generally been supposed that originally the silica was rather 
uniformly distributed throughout the bed, and that the flint nodules 
were formed in much the same manner as claystones by concretional 
action. A somewhat dififereut view is suggested by SoUas,^ advocated 
by Merrill,' and commented upon by Vaughan^ — that each nodule rep- 
resents a separate sponge bed, in which many generations of si)onges 
have lived and died in all stages of development. 

In the local accumulations thus produced Sollas and Merrill see the 
origin of the nodules, but in support of a somewhat different view 
Yaughan refers to a fact noted by Murray — that sponge spicules col- 
lect around shells. That flint nodules, sometimes at least, have nuclei 
has been shown by Hill, and the weight of present opinion appears to 
favor the view that flint nodules are largely concretionary. 

Flint nodules are extensively imported from England into the United 
States, where they are ground and mixed with kaolin for the purpose 
of making potter's clay, such as is used in the manufacture of porcelain 
and other finer grades of china. The material is also valuable, especially 
in Texas, for road making, and is extensively used for track ballast. 

No. 42. OOQUINA. 
(Fbom St. Augustine, Florida. Described by J. S. Diller.) 

Goquina is a very porous limestone, composed almost exclusively of 
shell fragments cemented together by carbonate of lime. It is a tthell 
Imestonej but on account of its peculiarities is generally known through- 
out this country by the local Spanish name of coquina. 

According to Mr. R. Dietz,* it forms a considerable portion of Anas- 

1 AnnaU Mag. N»t. Hist., 6th seriea, Vol. VI, pp. 441-443. 
>BalL Mna. Comp. Zool. Harvard College, Vol. XX VIII, p. 22. 
* Joar. GeoL, Vol. IV, p. 114. 
« Jour. Acud. Kat. Sol. Philadelphia, Vol. IV, 1824, p. 73. 



122 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bull.150. 

tasia Island, and occurs in horizontal layers, which easily separate, 
forming slabs. The layers are from 1 inch to 18 inches in thickness. 
The fragments of shells composing them vary in size, and occasionally 
entire shells are found. In general, the material is finest near the snr- 
face. When first removed from the ground the rock is soft and may 
easily be cut into any desired shape, but upon exposure to the air it 
becomes indurated. On this account it is a good building material, 
and has been extensively used in the construction of the fort, the quays, 
and other structures at St. Augustine. 

The shells are chiefly, if not wholly, of species now living along the 
adjoining coast. In the coquina studied by Dietz they belong princi- 
pally to the genus Area, but Dr. W. H. Dall says that they vary greatly 
from place to place, according to the locally dominant species. 

The shell fragments are all arranged with their largest surfaces par- 
allel to the plane of stratification. The space between them is partially 
filled with clear quartz sand, and the whole is cemented by calcite, and 
in such a way as to give the rock, when examined under a small lens, a 
crystalline appearance. The quartz is easily loosened and isolated by 
dissolving a small piece of the coquina in acid. Some of theTgraius 
thus liberated are well rounded, but generally they are sharp, angular, 
and clear, as if near their original source. Numerous minute rutile 
needles occur in some of them, as in the quartz of granitic rocks. The 
sand drifts southward along our Atlantic coast, and it is probable that 
the sand in the coquina has been carried from far northward, for the 
beach of Florida exposes no rocks from which it could have been 
derived. 

Dr. Dall tells me that cof]uina is now forming at many points along 
the coast of Florida. The shallow-water shells washed up by the 
waves to the beach, when placed about high-tide level, are alternately 
wet and dry. The water laving the shell beach gets a large amount of 
carbonate of lime in solution, and as it dries, after the waves recede, 
the lime carbonate is deposited upon the fragments, gradually binding 
them together and forming a more or less solid shell rock — coquina. 

The loose shell-beach material is mixed with cement to make an arti- 
ficial building stone, quite extensively used in St. Augustine. The 
trimmings of buildings made of this material are of coquina. 

No. 43. Shell Limestone. 

(Fpom Rochester, New York. Described by J. S. Diller, ) 

This limestone, like coquina, is composed of shells, and is theretbre a 
shell limestone. It differs from coquina, however, in being compact 
and containing almost exclusively the shell of one species, formerly 
atUed Atrypa hemispherica^^ but now known as Anoplotheca heinispherica? 
As the limestone is composed almost entirely of Anoplotheca, it is 
sometimes called Anoplotheca limestone by paleontologists. 



» Geology of New York, by Jaiue» Hall, 1843, pp. 6-1 and 73. 
"Paleontology of New York, Vol. VIII, by James Hall, 1894, p. 136. 



Dnjji.1 DESCRIPTIONS: NO. 44, CHERTY LIMESTONE. 123 

This bed of limestone is only 3 or 4 inches in thickness along the 
Genesee Elver near Rochester, and lies within the upper green shale 
of the Clinton group. The sediments associated with it are all fine, 
and, although of littoral origin, evidently do not belong to beach depos- 
its. The shells are so well preserved as to retain their pearly appear- 
ance, and but few of them are broken. They accumulated upon the 
8ea floor at a favorable spot, where not disturbed by the influx of ordi- 
nary sediments or the beat of waves. 

The relation of the strata in that region is illustrated in PL XXIII. 
At the base of the exposure shown in the figure is a Tnass of shale, 
which is overlain by a thin-bedded limestone, near the bottom of which 
is a bed of iron ore (No. 52). The limestone above the ore, having a 
thickness of 14 feet, forms the middle falls of the Genesee. As its most 
abundant shell is a Pentamems, it is referred to as the Pentamerus 
limestone. Immediately above the Pentamerus limestone, which is well 
shown in the figure, is the mass of green shale containing the thin bed 
of Aooplotheca limestone from which specimen No. 43 was obtained. 

No. 44. Cherty Limbstone. 

(From Buffalo. New York. Described by J. S. Diller.) 

The bed of limestone from which specimen No. 44 was taken extends 
from near the Hudson westward through the State of New York into 
Canada, Ohio, Indiana, and other States of that region. At many 
places in New York it contains a large amount of siliceous material 
called chert or hornstone. The limestone containing it is cherty lime- 
stone. It was formed during the Corniferous* period of the Devonian 
era. Fossils, especially corals, are so abundant in some places that 
the limestone looks like the reef- rock of modern coral reefs. The fossils 
are often silicifie<l, forming chert. 

The chert occurs irregularly distributed throughout the mass of lime- 
stone, as in specimen No. 44, or it may be arranged in layers, nodular 
sheets, or series of separate nodules in the same plane, alternating with 
layers of limestone. In such cases the limestone layers are generally 
thicker than those of chert, although the relative proportions of chert 
and limestone vary greatly from place to place. In general, the occur- 
rence of chert in limestone is analogous to that of fiint and chalk. 

The limestone, being soluble under conditions of weathering, is gradu- 
aOy carried away, leaving the exposed surface of the limestone rough 
^ith chert. As weathering progresses the surface in places becomes 
paved with angular fragments of chert. 

In band specimen No. 44 the bluish-gray compact chert and the 
dark limestone are distinctly separable. In some specimens the chert 
predominates, in others the limestone. Their boundaries are almost 
always sharply denned, although in some places there is gradation 



1 From the Latin "oomu," horn. 



124 THE EDUCATIONAL SERIES OP BOCK SPECIMENS, [bull. 150. 

I 

from one to the other. Both effervesce in acid, but the soft, dark por- 
tion, which will hereafter be referred to as the limestone, effervesces 
much more freely than the hard, flinty chert. 

That the limestone is crystalline, at least in part, is indicated by the 
minute glistening grains, visible under a small hand lens. When 
viewed in a thin section under a microscope it is found to be irregu- 
larly granular, containing numerous perfect crystals of calcite, rang- 
ing from 0.005°"°^ to 0.03"^™ in diameter. Most of the calcareous mate- 
rial is dark or brownish, owing to the presence of carbonaceous matter, 
which disappears upon ignition, leaving the limestone white. Clear, 
colorless calcite occurs in veins, but more commonly in single crystals 
or variously shaped patches, irregularly intermingled with the clouded 
matenal. 

Embedded in the limestone are occasional angular grains of clear 
quartz. They occur in chert also, but are much less common. 

The chert effervesces rather feebly in acid, owing to the presence in 
it of some carbonate of lime, but when that is dissolved away nothing 
but the hard, horny siliceous material is lefb. Under the microscope 
it appears cryptocrystalline, with here and there more coarsely crystal- 
line, clear areas, having confused radial fibrous structure, some of which 
appears to be optically negative, as chalcedony. 

Scattered throughout the chert is much carbonate of lime, often in 
sharp rhombohedral crystals. These crystals are completely enveloped 
by the chert, as if they were formed before the chert was deposited, or 
at least before it had hardened so as to prevent the development of 
crystals of calcite. This matter will be more fully considered under 
"Chert," as illustrated by specimen No. 46, which was taken from the 
same bed of cherty limestone as that which occurs at Buffalo, but at a 
different locality. 

No. 45. Chert. 

(From Leroy, Genesbb County, New York. Described by J. S. Dilleb.) 

The chert at Leroy, New York, according to Hall,^ occurs in the Cor- 
niferous limestone of the Devonian group, where it is arranged in irreg- 
ular bands between layers of compact gray or blue limestone. In a 
thickness of about 50 feet of the Corniferous limestone there are at 
least twelve horizons of chert, ranging from a few inches to several feet 
in thickness. Westward the proportion of chert in the limestone 
diminishes, and in some places the chert nearly disappears. 

Chert, illustrated by specimen No. 45, is a highly siliceous material of 
light-gray color. When freshly broken some of it effervesces for a little 
while, showing the presence of a small amount of carbonate or lime; 
but a fragment long exposed to the weather does not effervesce, the 
carbonate of lime having been removed in solution. Upon exposure it 



* Geology of Kew York, part 4, 1843, p. 167. 



Witt] DESCRIPTIONS: NO. 45, CHERT. 125 

bleaks up into small angular pieces, which accumulate in the soil at 
the Burface. 

The chert contains shells, corals, and other fossils which were orig- 
inally composed of calcareous matter, but are now completely silicified. 
Id addition to these fossils, it contains numerous organisms which were 
once BihceouB. Mr. George H. Girty, who examined the thin sections of 
this chert, states that <^ sponge spicules and fragments of the spicular 
skeleton of sponges of both the hexactinellid and lithistid orders are 
not uncommon, although they frequently are much broken. The hexac- 
tinellid elements are chiefly flesh spicules ornamented with numerous 
sharp nodes." 

The structure, as revealed by the microscope, is cryptocrystalline, 
and 00 the whole considerably flner than that of the chert in specimen 
No. 44. The minute grains are rarely greater than 0.001 """^ in diameter, 
and there is comparatively little variation in size. Radial flbrous chal- 
cedony, such as occurs in specimen No. 44, was not seen in the body of 
the chert, but does occur in a few remarkably weU-developed veins. 
These veins are made up of several bands of fibrous chalcedony, with a 
final filling of granular quartz in the middle. Here and there through- 
oat the chert are small areas of amorphous silica, and there may be 
much in the cryptocrystalline mass, where it can not be so readily dis- 
tingaished. 

The chert, excepting that which replaces calcareous organisms, is 
remarkable for the abundance of sharp rhombohedral crystals, like 
thoaeofcalcite, it contains. There is some variation in the forms of 
the crystals, but in general they are rhombohedral and average about 
0.02"'" in diameter. A few minute crystals of other substances besides 
carbonates, as well as irregular grains of quartz, are present. 

The siliceous material of the Corniferous limestoue, illustrated by 
the specimens I^os. 44 and 45, is sometimes called hornstone. There 
appears to be no distinct line of division between flint, hornstone, and 
chert The term << flint," although used in a comprehensive sense to 
include chert, so that chert may be defined as impure flint, is applied 
chiefly to the more purely siliceous rock which occurs in chalk. Flint 
18 often, but not always, rather dark colored. A special characteristic 
of flint, according to Griswold, seems to be that a considerable part of 
the silica is in the amorphous soluble form of opal. 

In New York the siliceous material of the Corniferous limestone 
was formerly called ^^ hornstone,"' and the name is still used to some 

'Oeikie applies the term *' hornstone" (Text-Book of Greology, 3d ed., p. 154) to "au exceedingly 
^^IMet uliceoas rock, asnally of some dall tint, occurring in nodular masses or irregular bands or 
^Bt. The name has sometimes been applied to more flinty forms of felsite. " In the United States, cer- 
^ owre or less flinty rocks which result ttom the alteration of sediments in contact with igneous 
""cb have been caUed hometone. It is what the Germans call "homfels." Hawen described such 
ia K«w Hampahire, Am. Jonr. Sci., 3d series, 1881, Vol. XXI, p. 27; Emerson in Now Jersey, Am. 
'wu-. Sd., 8d series, 1882, Vol. XXIII, p. 303; and Kemp in Trans. New York Acad. Sci., Vol. XI, pp- 
^' 128. It thus appears that the term ** hornstone *' has been used to designate rocks of widely dif* 
fertQt origin, and the needs of science would be better subserved by dropping it altogether, and using 
the tenia /in( and chert for rocks like specimens Nos. 41 and 45, and horn/els for those like specimen 
Ho. 130. 



126 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bcu^iso. 

extent,^ but with growing infrequency,* while the term ^' chert" is com- 
ing into more general use to designate impure flint, especially when 
it is calcareous. 

Much has been written concerning the origin of flint and chert, and 
it is evident that all have not been produced in the same way. Some 
observers consider that the silica of chert is derived from sponges and 
other siliceous organisms, while others consider that by some chemical 
reaction the silica was precipitated directly from sea water to make the 
chert. 

The presence of siliceous organisms^ in many cherts leaves no doubt 
that their silica, at least in large part, was derived from siliceous 
organisms. 

Oolitic structure (specimen No. 26) occurs in some cherts, and has 
been regarded as indicating the replacement of carbonate of lime by 
silica. The silicified corals and moUusks (specimen No. 3d), so common 
in the chert of the Gorniferous limestone, afiford positive evidence of this 
replacement, and since we often find in the same specimen of Gorniferous 
chert both sponge spicules and replaced calcareous fossils, the traces of 
its history are essentially the same as those noted under flint (specimen 
No. 41). In fact, the chert (specimen No. 45) difl'ers from flint chiefly in 
containing numerous crystals of carbonate of lime. These crystals can 
not be regarded as remnants of the calcareous organisms. They crys- 
tallized in place before they were enveloped in hard chert to interfere 
with their development. Irving and Van Hise,^ after an extensive 
study of the cherty limestone and the cherty carbonates of the Penokee 
iron-bearing series of Michigan and Wisconsin, conclude: ^^ First, that 
the chert was mainly deposited simultaneously with the iron carbonate 
with which it was so closely associated ; and, second, that it is probable 
that the chert is of organic origin, although we have no i>08itive proof 
that it is not an original chemical sediment, while it may in part be 
from both sources." 

The acids resulting from the decomposition of organisms afifect the 
solubility of silica, and, as suggested by Julien,^ may cause its precipi* 
tation. This would account for *the siliciflcation of organisms, both 
vegetal and animal, so common in the various formations. 

The evidence of the formation of chert by direct precipitation firom 
sea water without the intervention of life is negative. Although there 
are cherts in which no trace of life has been found, it is possible that such 
traces have been obliterated by more pronounced activity of the same 
agents which in other cases only partially destroy them. 

I See Dana, Manual of Geology, 4th ed., 1895, p. 583. 
'See " Homstone" in Century Dictionary. 

•W. J. SollaH, Annals Mag. Nat. Hist., 5th series, Vol. VII, 1881, p. 141; Cr. J. Hinde, Geol. Mag^ 
1887. p. 435; J. A. Merrill, Bull. Mus. Comp. Zool. Harvard CoUego, Vol. XXVIII, pp. l-a«. 
« Tenth Ann. Kept. IT. S. Geol. Survey, Part I, p. 397. 
sproc. Am. Assoc. Adv. Sci., 1879, p. 396. 



DESCRIPTIONS: UNALTERED SEDIMENTAEIES. 127 

No. 46. Compact Limestone. 

Grkason, Cumberland County, Pknnsylvania. Described by J. S. 

DiLLER.) 

limestone is one of the most extensive and most ancient, as well 
most iiiix>ortant, economically, in the United States. It occurs 
Teat limestone belt extending from western New England through 
rner of New York, New Jersey, Pennsylvania, Maryland, and 
la iuto Tennessee, and represents widespread, long-continued, 
ratively uniform conditions of the sea in Cambro- Silurian time. 
20unt of its softness and solubility by long exposure to atmo- 
15 agents it has wasted away more than the harder rocks adjacent 
The latter rocks form mouutaius, while the limestone api)ears 
intervening valley. Lebanon and Cumberland valleys of Penn- 
lia and the Great Valley of Virginia have this limestone as their 
Diental ix>ck. The soil in these valleys is rich, and furnishes the 
df one of the greatest agricultural regions of the country. 

limestone is compact, with numerous minute glistening particles, 
3 with a hand lens. Its dark color is due to impurities, probably 
aaceous, at least in part, for when highly heated the dark color 
[lears. In dilute hydrochloric acid it effervesces freely, but not so 
>asly as pure calcite, and ^fter the carbonate of lime is completely 
ved there remains a very fine, dark sediment. Under the micro- 

the structure of this limestone is found to be what would be 
d mieroporphyritic. It contains a multitude of minute rhombohe- 
crystals of calcite, about 0.05 to 0.06'"*" in diameter, embedded in a 
fine-granular matrix, which is chiefiy carbonate of lime, but con- 
in addition nearly all the various impurities found in the limestone, 
ny of the crystals are very sharply defined and contain traces of 
npurities in the matrix. Occasionally the material is arranged in 
s, of which the darker and more carbonaceous usually contain the 
8t number of well-developed crystals. In places over very small 
. they have so grown as to mutually interfere and interlock in a 
suggesting the crystalline structure of marble. In the lighter- 
ed bands the microphenocrysts are less abundant, and occasion- 
;he tine-granular groundmass prevails. 

ter the carbonate of lime is removed by acid, much of the fine 
ual material is doubly refracting, but in general it is too fine for 
finite mineralogic determination. The chemical analysis of the 
e rock, however, shows that the residual material must be com- 
i chiefly of quartz, with silicates of alumina, magnesia, and iron« 



I 



128 



THE EDUCATIONAL SERIES OF BOCK SPECIMENS, [bull 150. 



The chemical analysis, made by E. A. Schneider, is as follows; 

Analy$i8 of limestone from Greason, Penneylvania. 



SiO, 

Al,0,i 

Fe^S 

loBolable residue, undetermined . 

CaO 

MgO 

CO, * 

Organic matter 

HjO (105°) 

Total 



Peroent. 



7.00 

3.82 

.45 

30.20 

0.00 

38.82 

.75 

.18 



00.08 



Specimen No. 46 contains no fossils, although there are beds of the 
same belt in that region containing an abundance of marine shells, and 
there can be no doubt in such cases that a considerable part of the 
limestone is of organic origin. For this reason, in the classification of 
the limestones of this series it was placed among those of organic 
origin. There is reason to believe, however, that it may be of chemical 
origin, and occasion will be taken at this i>oint to consider the evidence 
concerning such a view. 

The principal evidence furnished by the limestone itself is to be 
found in its porphyritic structure. The relative age of the ground- 
mass and the minute crystals so conspicuous under the microscope 
(microphenocrysts, which produce the porphyritic structure) may be 
best understood by considering a porphyritic igneous rock, such as 
dacite-porphyry (specimen No. 90), where the phenocrysts of quartz 
and feldspar are clearly older than the groundmass by which they are 
enveloped; that is^ these crystals were formed before the groundmass 
solidified. This is shown especially by the corrosive action of the 
magma upon the quartz crystals. At the time the large feldspar crys- 
tals developed the inclosing material was soft, so as not to interfere 
with their symmetrical growth and structure. The same must have 
been true of the ininute crystals of calcite in the limestone. They 
must be the oldest solid portion of the mass. Although it may have 
accumulated about the same time, it was not lithified until the crystals 
of calcite were fully developed. 

This microporphyritic structure of the limestone is not a local modi- 
fication in the rock; it belongs to the whole mass, and may not be 
attributed to metamorphism, either local or regional, for both are absent 
The associated layers of shale and fossiliferous limestone are unaltered. 
It appears as if the crystals of calcite developed directly in the solution 
from which they drew their carbonate of lime, and that they are the 
fundamental portion of the original mass. 

The conditions under which large masses of limestone originate by 



wun] DESCRIPTIONS: NO. 46, COMPACT LIMESTONE. 129 

chemical depositaon are not well understood, and there is much differ- 
ence of opinion concerning the early history of such rocks. This sub- 
ject has been recently discussed, by Mr. Bailey Willis,^ and from his 
paper the foUo^rin^ quotations are taken. The conditions favorable to 
chemical deposition are — 

(a) Evaporation firom an inclosed sea. 

(6) Precipitation of lime and magnesia from ocean waters, charged by solution 
from the land, throngli evaporatioD^ through reaction of salt water on fresh, and 
tiiToagh varying atmoepheric conditions at the surface of the sea. 

(a) Evaparatian from an inclosed sea, — When a limited body of water, such as a lake, 
u sabjected. to a change of climate, so that evaporation exceeds precipitation of rain, 
the volume yr\\\ shrink, ontflow will cease, and the solution of salt will be concen- 
tnted. If the process is sufficiently continued, the solution will become saturated, 
first for one salt, then another, and they will be deposited in the order of their in solu- 
bility. This process is important as an indication of climatic variation in the past. 
U has been fully described by Gilbert, Russell, and Chatard for Pleistocene Likes 
lod the chemical relations, and these studies suggest the conditions to which appeal 
mast be made to explain the less exact facts known in ancient formations of the kind. 
{b) Frecipiiation from WaekUh waters. — ^The chemical precipitation of lime and mag- 
nesia from sea water is a much mooted (question. There are two lines of evidence 
relating to it which are apparently opposed. On the one hand, the scientists who 
have decMribed material obtained by soundings ou modern limestone deposits have 
recognized only organic remains. The Challenger in the open oceans, remote from 
great rivers; the Coast Survey vessels in the Caribbean, the Gulf of Mexico, and otf 
the Atlantic coast; the Norwegian expedition in the North Atlantic, and English 
vessels in the Indian Ocean have found calcareous oozes of various kinds and rocky 
limestone formations, bnt in every ciiMO the calcareous matter is described as com- 
posed wholly of the tests of pelagic organisms, many of them of microscopic size. 
It is known that carbonates of lime and magnesia are to a greater or leQ^ extent solu- 
ble in waters containing carbonic acid, and that the proportion of these carbonates 
diflsolved in ocean waters is small. According to Dittmar, the salts in solution in 
ocean waters contain 0.345 per cent of carbonate of lime and 3.600 per cent of sul- 
phate of lime,^ and the ocean is capable of dissolving all the lime poured into it by 
rivers.^ This view being accepted, it follows that pelagic organisms, which possess 
the power of secreting solid carbonate of lime from solution, alone (;an cause lime 
deposits. Chemical precipitation is, according to this vimv, inipOHsible, or, if it 
(iccars, is followed by speedy re-solution, and all limestones deposited under condi- 
tioQK of the existing oceans are of organic origin. 

On the other hand, there are many limestones, deposited at difierent periods of 
geologic time, from Algonkiau to the present, including some now forming, which 
consist of more or leas clearly crystalline calcite, devoid of organic structure. If 
this caloite was originally built into orgauic forms they have been entirely oblit- 
erated. Snch limestones do indeed contain fossils which sometimes exhibit more 
or less crystalline texture, but the occurrence of these organic forms in the holn- 
erystslline matrix only raises the question, If the mass was originally all organic 
aodhas andergone secondary crystallization after lithifaction, why was the process 
10 complete in the matrix and relatively so ineffective in structures whose delicate 
anatomy can still be traced even to microscopic details? Thin sections of lime 
stone which show a mass of interferant crystals suggest that this was the primary 



' Jonr. Gool., Jaly-Aagust, 1883, Vol. I, Na 5, pp. 500-517. 

* Report on the Scientific Besalts of the Voyage ot V..ii.ii. ChalUsnyer ,- PliyHUui ami CheiuiHtry, 
V(iLI,p.204. • 

*Op.oit..p.221. 

Bull. 150 9 



130 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [buli.150. 

structure of the rock, and organic remains appear to bo foreign bodies which are 
accidentally of the same substance as the matrix. If this view be correct, then only 
the alteration of the organic carbonate is the measure of the alteration of the rock- 
mass, and it is a fair inference from the original crystalline structure that the lime- 
stone may have been produced by chemical precipitation. 

In explauation of this coutradiction it may be suggested that broad 
shallow seas resemble inclosed bodies of water rather than the open 
ocean, so far as concentration of their dissolved salts is concerned; atid 
that observations on organic deposits have been made in the ocean, 
whereas it is not improbable that many limestones were deposited in 
relatively shallow seas. If oceanic waters enter a broad basin which 
is so nearly inclosed as to impede their outflow, they may circulate 
until more or less concentrated by evai)oration, much as they would be 
in a completely inclosed water body. Under such conditions lime- 
stones may have a chemical origin. 

It is not proposed here to argue that limestones are prevailingly of one origin or 
the other, but only to show that the assumption of organic origin for all the cal- 
careous deposits of the stratified series is too sweei)ing. To this end it is desirable 
to consider the chemical and mechanical conditions which affect the precipitation 
of carbonate of lime, to estimate the solubility of the carbonate in salt water, to 
review the conditions under which lime is contributed to, and distributed in, the sea, 
and to describe several cases of modern limestone formation by precipitation. . . . 

As to the chemical and mechanical conditions which affect the precipitation of 
carbonate of lime, chemists describe two under which bicarbonate of lime held in 
solution may be decomposed, liberating carbonic acid and precipitating tbe neutral 
carbonate: First, by diminution of the tension of the carbonic acid in the atmos- 
phere; second, by agitation of the solution. 

Theoretically, either one of three things may occur to the neutral carbonate of 
lime if it be thrown out of solution by either one of these processes, which we may 
admit are active on some portions of the salt-water surface. The carbonate may be 
redissolved, or deposited as a calcareous mud, or built into organic structures. We 
may discuss these alternatives in turn. 

The solvent action of sea water has been the subject of direct observation in the 
ocean and of experimental determination. . . . 

The pelagic pteropods and foraminifera, living at the surface, sink on dying and 
are slowly dissolved. If the water be too deep, the carbonate of lime never reaches 
the bottom; only the insoluble residue gets there. The limits below which the cal- 
careous remnants are not found are about 1,500 fathoms for pteropods, thin shells 
exposing large surfaces to solution, and 2,800 for globigerina, smaller shells, rela- 
tively more massive. . . . 

The solvent power of sea water is very moderate and may be satisfied, so far as 
carbonate of lime is concerned, by two sources — by organic tests in suspension, and 
by chemical precipitate. The lime used by organisms is derived from the solution 
to which it is partly returned by re-solution, but another part is deposited, and the 
sea thus suffers constant loss. This loss is supplied by the streams from the land. 
If this terrigenous supply is less than the amount of organic deposit, the sea will 
become less alkaline and will more efficiently dissolve calcareous tests, until tbe 
solvent is satisfied. If the land contribution is continuously equal to the amount 
organically subtracted, there will be equilibrium. If the land yields more carbonate 
of lime than that which is being locked up in organic limestones, the alkalinity of 
the sea will gradually increusi* until there is chemical precipitation. This condition 
is favored by the (uitraiicc of lime-boariug fresh water into a sea free from active 
ourrents and exposed to evaporation which balances the inflow. 



Diutt.1 DESCRFETIONS: NO. 46, COMPACT LIMESTONE. 131 

After Teviewing the conditions under which lime is carried from the 
laDd and distributed in the sea, Willis finds that — 

The hme brought down by rivers, though measarable by hundreds of thousands 
of tons per aannm, is so widely diffused in the vast volume of the ocean that it 
escapes recognition. 

There are several instances of modern limestone formation which, 
thongh local, illustrate the processes of chemical deposition on a hirge 
scale. A reference to this may close the suggestion concerning lime- 
stone deposition by other than organic means. 

Chemically deposited limestone is forming in the southern part of 
Florida, i>robably over extensive areas. It occurs in the Everglades, 
and the precipitation is in two forms: 

First, from the mass of the water as a flocculent mud; second, from the lower 
layers of the water in contact with limestone as crystals forming an integral jiart 
of the solid rock. 

The limestones formed upon the shores of the Pleistocene lakes 
Bonneville^ and Lahontan,^ of Utah and Kevada, as well as the one now 
developing at the mouth of the Ehone, are referred to as examples of 
limestones formed under the conditions considered in the preceding 
discussion. 

These conditions are favored at the mouth of the Ehone by the salinity of the 
Uediterranean and the absence of strong currents. 

The examination of a few thin sections of limestone of different ages, from Cam- 
brian to the present, shows that they have three principal types of structure. There 
are tho:»e which resemble the Everglades limestone in that they consist of more or 
less coarsely crystalline calcite, yet include unaltered organic remains. Of these 
the Trenton limestone and the marbles of corresponding age in Tennessee, which 
occur interstratified with unaltered calcareous shales, are the most striking exam- 
ples examined. Cambrian limestones and the Knox dolomite show similar crystalline 
atractnre. The second type, the precipitated sediment which forms the muds of the 
Kverglades and which was deposited in Lake Bonneville, is represented by speci- 
nens composed of exceedingly fine grained, apparently pulverulent, material; the 
best of these are from tho Knox dolomite and the Solenhofen lithographic stone. 
The third variety of limestone consists of the thoroughly crystalline marbles, which 
contain no unaltered material, and which occur in such field relations that they are 
known to be completely metamorphosed. Extended study is required to determine 
the nature of deposition of the first and second types. They may have been organic 
and have suffered moderate alteration only, but there is a reasonable presumption 
that they did to some extent crystallize in place from sea water, and were, to a still 
grater extent, precipitated from the outspread fans of fresh water radiating from 
rivers' months, whence they spread as fine silt over the bottom of the sea. . . . 

In discussing the solubility of shells in sea water it has been pointed out that the 
l>yer of organic matter which accumulates at the sea bottom contains a solvent 
ftwTiied by the evolution of carbonic acid in the process of decay. Through this 
layer all substances must pass before they can become part of a lithified stratum. If 
^Ware plant tissue or flesh they will become more or less oxidized ; if they are cal- 
^reoua testa they will be more or less completely dissolved, and if there be any 
chemically precipitated lime arriving on the sea bottom it, too, would be dissolved 
'" this menstruum. The earlier forms of dredge which scooped into the sea bottom 

' Men. U. S. G«M>1. Survey, Vol. I, by G. K. Gilbert. 
• Mon. U. S. Geol. Sarvey, VoL XI, by I. C. RuaseU. 



132 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bull. 150. 

brought np a mass of ooze, formed of fine particles, burying organic forms. The 
later forms of dredge, arranged to skim the surface of the bottom, bring np sbellB 
and organisms remarkably free from mud. Now, it may be conceived that the layer 
of mud on which the creatures live, die, and with sunken organic remains decay, 
grades from the fresh surface of recent accumulations downward into a much more 
completely decayed and dissolved mass, and that this rests upon a surface of lime- 
stone. In the upper part of this unconsolidated stratum carbonic acid may most 
abundantly be evolved; in its lowest part the more concentrated solution of lime 
may accumulate. Then it is conceivable that lithification by crystallization of the 
carbonate of lime from the more concentrated solution is constantly proceeding on 
the limestone surface. If this conception be correct, the formation of limestooe by 
organic means involves the re-solution and crystallization of more or less of the cal- 
cite in the primary formation, and only those organic forms can remain unchanged 
which resist the solvent action. If they are delicate, as the trilobites' branchia 
from the Trenton limestones, described by Walcott, they give evidence that they 
were rapidly buried and protected. 

It is thought by some that limestones are evidences of organic life at whatever 
period of sedimentary history they were deposited, but it has here been shown 
that the source of all lime in the sea is the land, and that under conditions exist- 
ing in certain localities both crystalline limestone and ' calcareous mud are now 
forming chemically. It has also been shown that lime converted into organic forms 
is subtracted from that which would otherwise go to saturate the sea water. If, 
then, in any early age of the earth's history, lime-using organisms were not present 
to subtract and deposit lime from sea water, and if the atmospheric ageneies 
worked then ae now, the contributions, from the land must have continually added 
to the alkalinity of the sea until chemical precipitation occurred. Such a process 
must have been limited to seas rather than extended to oceans, because the condi- 
tions of delivery of lime from the laud were then, as now, localized. With tlie 
development of marine life aud the increased demand for lime for organic use, and 
with the corresponding deposition of organic limestone, the sea water must have 
become less alkaline, aud tho conditions of chemical precipitation must have l»eeD 
still more restricted. In time it might occur that pelagic organisms should deoiaud 
so much lime for circulation from the water to calcareous algie, to herbivorous, and 
then to carnivorous formn, and so back into solution, that lime could escape from 
solution by precipitation only under exceptional conditions. If it be true that I he 
oceanic oozes, the muds of the Caribbean, the mud flats of Flurida, aud similar 
calcareous deposits in different seas the world over, be wholly organic, then marine 
life has locked up more lime than the continents could concurrently supply, and the 
balance is now turned against chemical precipitation. But it has not always been so. 

No. 47. LITHOGRAPHIC STONE. 

(From Flint Ridge, Greenwood County. Kansas. Described by J. S. Diller.) 

Lithographic stone is a limestone characterized by its very fine, uni- 
form texture, structure, and composition — so fine and compact, indeed, 
that it will receive very delicatid markings by the engraver's tools, as 
well as by etching with acids in lithography. 

The best lithographic stone comes from the neighborhood of Solen- 
hofen, near Munich, in Germany, where the rock is extensively quarried. 
Besides the uniformity of texture aud composition which makes it 
equally resistant throughout to the engraver's tool, it is soft enough to 
be easily engraved aud possesses a degree of porosity which renders it 
properly absorbent, so that it will receive and retain the greasy i>rep- 
arations used by the lithographer in transferring and printing. 



iwu«] DESCRIPTIONS: XO. 48, HYDRAULIC CEMENT ROCK. 133 

Specimeii No. 47 is a poor example of litbographic stone, although it 
is one of the best that could be readily obtained in this country. It 
lacks nniformity of texture and composition, and for this reason is not 
good for lithograpliic purposes. Under the microscope, in a thin section, 
it is seen to contain not only very fine-granular carbonate of lime, but 
also DomeroaB angular particles of quartz, which, although usually less 
than 0.02™"* in diameter, render it worthless for lithography. The 
minute particles ^would turn the engraver's tools aside, and in etching 
wonld not be affected by acid, like the surrounding carbonate of lime. 
It contains also numerous small patches of clear calcite, which modify 
the absorption of the stone. 

^hen a bit of this stone is dissolved in acid, a large amount of 
residual material is obtained that is composed chiefly of quartz. 
When similarly dissolved, the Solenhofen stone, used in printing the 
United States Geological Survey maps, leaves a considerable insoluble 
residue of dark argillaceous matter with some minute grains of quartz, 
although chemical analysis has shown that in many cases it is composed 
almost wholly of carbonate of lime. All the other substances it con- 
tains put together rarely make as much as 4 per cent of the whole 
mass.^ A simple means of testing the lithographic qualities of a stone 
wbich to the naked eye appears so fine grained and homogeneous as to 
promise to be of use for lithographic purposes, is to examine a thin 
section under the microscope. If the stone is of value, it will appear 
homogeneous in comx)osition and have a very fine texture. 

Good lithographic stones have been much sought for in this country, 

bat thus far with but little success. Stones have been found in several 

of the States within the Mississippi Valley region, bat so far as known 

none of them have proved very satisfactory. Some rocks have been 

discovered yielding small stones, but none of these have come into 

extensive use. It is possible, however, that good lithographic stone 

may yet be found in this country. A paper by Mr. G. P. MerrilP on 

lithographic limestone will be found useful to students and others 

interested in this subject. 

No. 48. Htdbaxtlio Cement Rock. 

(From Akron, Erie Couivtt, Nrw York. Described by J. S. Diller.) 

Hydraulic cement rock is a limestone containing nearly half as much 
^% as carbonate of lime. It affords a quicklime, the cement from 
^bich, when properly prepared, will harden under water to a stone- 
like mass. On this account the rock is often called by geologists^ 
h^raulic limestone. It occurs interstratified with other limestones of 

'Paper ud Press (Philsdelphia, January, 1896), Vol. XXU, p. 90. 

*th« Mineral Indnatry, by R. R. Rothwell, 1893. vol. 2, p. 453. 

*Hr. U. Cnmmlnga, general manager of the Standard Cement Company, who kindly obtained the 
"PKlmena of faydraalic cement rock for thin series, informed me that the term hydraulic limetdone is 
^ the trade appUed to a limestone that contains only abont half as much clay as the ermtnt rock, and 
^t the lime deriTed from it will not make a cement that will harden under water. 



134 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS. Ibuu.150. 



various compositions, aud, in places, contains fossils of marine origio. 
The cement made from it is of great importance for building purposes. 

Hydraulic cement rock is usually of a gray color^ and has a more or 
less fine-granular crystalline structure. Under the miscroscope it \f^ 
seen to contain a large number of angular grains of quartz, and here 
and there a grain of fresh feldspar. Some of the feldspar is microcline, 
and so firesh as to be clear and show distinctly the characteristic crossed 
striations. The microscope reveals numerous circular spots or pellets, 
which are fine-granular, and contain much of the argillaceous material. 

When dissolved in acid the rock leaves a large amount of gray 
residual material, which, under the microscope, is found to be chiefly 
argillaceous, with much quartz, some feldspar, and a trace of a few 
other minerals. These represent the sediment deposited in the lime- 
stone while forming. 

The chemical analysis given below, by George Steiger, shows the 
large amount of impurity present; and that the greater portion of it 
is quartz, with much clay, is evident. It seems hardly proper to call 
these materials impurities, for the value of the rock, as a source of 
cement, depends upon their presence in the limestone. 

Analy8i9 of eemeni rock from Akron, New York, 



SiO, 

TiO, 

AlaOg 

FejOg 

FeO 

MuO 

CaO 

MgO 

K,0 

Na,0 

Waterloo^ — 
Water 100° + 

PaO, 

CO, 

Total . . . . 



Per cent. 



9.03 

.18 

2.25 

.85 

.52 

none 

26.84 

18.37 

.85 

none 

.21 

.77 

.03 

40.33 



100.21 



The property of hardening under water, possessed by hydraulic 
cement, is attributed to a chemical union of the clay (silica and alumina) 
with the lime and water. Mortar made of ordinary quicklime will 
harden only upon evaporating to dryness, and is therefore of no value 
for many of the most important structures. 

The hydraulic cement rock represented by specimen No. 48 belongs 
to what is called the Water lime group in the upper Silurian system of 
New York, where it is extensively used for making cement, especially 
in Ulster County. Large quantities of cement rock are quarried also 
in Indiana and Kentucky, and to a less extent at various points ia 



wuxRl DK8CR1PTT0NR: NO. 49, AMORPHOUS MARL. 135 

Georgia, Illinois, Kansas, Maryland, Missonri, New Mexico, Ohio, 
Pennsylvania, Texas, Utah, Virginia, West Virginia, and Wisconsin. 
The total product of hydraulic cement in the United States in 1895 
was nearly eight million barrels, and the output is rapidly increasing. 

Portland cement, which is of the same character as the hydraulic 
cement already referred to, is made in England by mixing 70 per cent 
of chalk with 30 per cent of line mud from the Thames. It is now 
being made quite extensively in this country from a nonmagnesiau 
argillaceous limestone. 

Information concerning the production of cement in this country is 
published annually by the United States Geological Survey in the 
report on the Mineral Kesources of the United States, and special 
mention may be made to the report for 1891, pp. 529 to 538, the report 
for 1894 (Part III of the Sixteenth Annual Eeport), pp. 576 to 585, and 
the report for 1896 (Part V of the Eighteenth Annual Eeport), pp. 
1179-1182. 

No. 49. Amorphous Marl. 

(From Cortland, Cortland County, New York. Described by J. S. Diller.) 

A belt of limestones and highly calcareous rocks extends across the 
State of New York from the Niagara to near the Hudson. During the 
Glacial period much material from this belt was carried in the drift 
soQthward, so that there is a broad belt in which the springs and other 
waters rising in the drift carry much carbonate of lime in solution. 

Upon the loss of the carbonic acid by means of which the carbonate 
is held in solution, as well as by means of plants and animals, much of 
the carbonate is precipitated, forming calcareous tufa or marl. The 
calcareous tufa generally contains traces of the vegetation which grows 
upon the bottoms of the lakes and streams or upon the adjacent slopes. 
On the other hand, the marl generally contains shells. Marl is an 
earthy calcareous rock in which the carbonate of lime is intermingled 
with much clay, sand, or other earthy material. The proportions may 
rary from a small percentage to over one-half of the whole mass. 
James HalP says that <' in nearly all situations the muck swamps are 
underlain by a deposit of calcareous marl. This is usually very finely 
palverulent, and, though cohering when wet, is very friable when dry." 

Specimen No. 49 is of this character. A few small shells of both 
univalve and bivalve moUusks occur in it where the specimens were 
collected, but as they are not in large numbers the marl appears 
amorphous, although in other portions of the mass shells are locally 
abundant. It contains 94.32 per cent of carbonate of lime, and is 
almost completely soluble in dilute hydrochloric acid, leaving a whitish 
residne composed chiefly of more or less rounded grains of quartz. 

The calcareous material is very fine-granular, the particles averaging 
apparently less than 0.01"*™ in diameter, while the associated quartz 



» Geology of New York, Part IV, 1843, p. 360. 



13n THE KDUCATIONAL SERIES OF ROCK SPECIMENS. fBox-lSOL 

grains are generally about six times as large. Both are crystalline, 
altboagh neither shows crystallographic outlines. The carbonate of 
lime closely resembles that obtained by pulverizing the contained shells, 
and suggests that it may have originated by their disintegration. Pro- 
fessor Hall remarks : ' 

In the greater number of the marl beds the remains of fluviatile testacea are very 
abandant, though it is only in a few situations where they have formed any large 
proportion of the deposit. The shells appear to have flourished in immense nnm- 
bers; probably from the facility with which they obtained calcareous matter, and 
other favorable circumstances; but still it is plain that the fbrmations of this kind 
are generally due to calcareous springs or to the peroolation of rain water throngh 
the snrrounding rocks, which, from its excess of carbonic acid, dissolves the calca- 
reons particles in the soil or the harder strata. 

No. 50. Shell Mabl. 

(From near Rochester, New York. Described by J. S. Diller.) 

A few miles east of Eochester, New York, is a small place which until 
recently was a swamp, bat is now dry land and cultivated. Originally 
it was a small lake, which was gradually filled by sediments washed 
from the adjacent slopes, and became first a swamp and then arable 
land. In the lake lived numerous moUusks, whose remains were 
buried in the mud of the lake, converting it into marl. As the shells 
may still be plainly seen, the material is shell marl. In some cases the 
shells are so abundant as to form the greater portion of the mass, but 
in specimen Ko. 50, although numerous, they form but a small portion 
of the whole. When the marl is placed in dilute acid it effervesces 
vigorously for a short time only, and the greater x>oi*tion of the material, 
which under the microscope is seen to be composed of sand and argil- 
laceous matter, with traces of fibrous vegetal remains, is left as a 
residue. 

Ko. 51. Diatom Eabth. (Infusorial Eabth, or Tripolite.) 

(From White Plains, Churchill County, Nevada. Described by J. S. Diller.) 

Diatom earth, sometimes called tripolite (or tripolyte^), and more 
frequently infusorial earth, is a soft earthy material like chalk (No. 39), 
volcanic dust (No. 5S), or kaolin (No. 149), but differs from all of these 
in being composed chiefly of extremely minute siliceous plants, or dia- 
toms. The diatoms were once included under the general term "infu- 
soria," hence the name infusorial earth. Tripolite, or tripoli, takes its 
name from the country in Africa, where similar material occurs. In 
that case, however, instead of being composed of diatoms, it appears to 
be derived from the leaching of a siliceous limestone. 

The shells of organisms in lacustrine waters of limestone regions, as 
we have already seen, are calcareous, for there large masses of calca- 
reous rocks give carbonate of lime to the waters of the lakes. In 



» Geology of New York, Part TV. 1843, p. 361. 
*Daiia'H Manual of Geology, 4tli edition, p. 81. 



Diuwl T>KSCKIPTIONS: NO. 51, DIATOM EARTH. 137 

legions ^wbere the prevailing rocks are rich in silica the shells of 
organisms which tloarish in the waters are siliceous. The most com- 
iDonof sach siliceous organisms are diatoms, a very minute species of 
plant, of which there are many forms. A few of these are illustrated 
in Dana's Manual of Geology, fourth edition, pages 164 and 894. Occa- 
sionally they are so abundant that their dead shells, falling to the 
bottom of the water in which they lived, accumulate and form large 
deposits. Such dex>osits are common in the volcanic regions of the 
I^orthwest, where the streams carry much silica from decomposing 
lavas and are occasionally interrupted and ponded by outflows of new 
coulees. Several excellent examples of such ponding, produced by the 
recent outflows of lava forming dams in the bed of the stream, occur 
along Pit Biver and Klamath Eiver, in northern Galifornia. In such 
cases there was developed above the dam a temporary lake ia which 
diatoms flourished and gave rise to small local deposits, now exposed 
on the banks of the river, which by long-continued corrasion has cut a 
canyon through the lava and drained the lake. The diatom earth of 
White Plains, Nevada, forms a bed of larger extent. It is of Miocene 
age, and has been tilted with the associated volcanic rocks.' Its mode 
of aecnmolation is illustrated by Shaler.^ Although diatom earth is 
often of lacnstral origin, it is produced also in the warm waters of the 
siliceons springs of the Yellowstone National Park, where, as Weed 
has shown, beds 3 to 6 feet in thickness cover many square miles.^ 

Diatoms flourish in the surface water of parts of the ocean, especially 
in the South Atlantic, where they are so abundant as to becloud it and 
where they serve as food for whales. Their remains sink to the bottom 
and form great accumulations of diatom ooze.^ Their tests, unlike the 
calcareons ones of foraminifera, are insoluble and may sink to the 
bottom of the deepest ocean. 

Diatom earth is found in many parts of the world, and is extensively 
^^ for polishing. It has been used also as an absorbent in the manu- 
fftctore of explosives, and as a packing about steam boilers. The 
"silver white ^ of commerce is diatom earth. 

In the United States it occurs at many localities, of which two may 
fce mentioned. Near Eichmond, Virginia, it forms a bed 30 feet thick 
^d 100 miles in extent; and near Monterey, Galifornia, there is a bed 
^it 50 feet in thickness, but of unknown extent. There are many 
other localities. The output for 1896 in this country was 2,796 tons, 
haloed at $16,042. 

'I?. S. Ge<J. ExpL Fortieth Par., Vol. I, Systematic Geology, p. 421 ; Vol. II, DcHcriptive Geology, 

'Th« origin and nature of soils: Twelfth Ann. Kept. TT. S. GeoL Sarrey, Part 1, 1891, p. 316, fig. 22. 

>Botnicai Gazette, Vol XIV. No. 6, p. 117, 1889. 

*fieports of the ChaUenger Expedition, Deep Sea Deposite, p. 208. 



138 THE EDUCATIONAL SERIES OF EOCK SPECIMENS. [BULulsa 

No. 62. FossiLiFEBOus Iron Orb. 

(From Rochester, Nkw York. Described by J. S. Diller.) 

The rocks of the Clinton series, exposed along the Genesee Biver, 
near Eochester, are illastrated in PI. XXIII. The main mass is a thin- 
bedded limestone of the Clinton series in the upper Silurian system, but 
near its base, plainly visible in the figure, is a bed of iron ore^red 
hematite — illustrated by specimen No. 52. At this point the ore bed 
has a thickness of 14 inches. Twenty miles to the east it attains its 
greatest thickness, 24 inches. Westward froui Eochester the ore bed 
extends only a short distance, for it does not reach the Niagara Eiver. 
To the south, however, in the Appalachian region, it has a remarkable 
distribution. Sometimes there is only one bed, as at Eochester; then 
again there may be three beds, ranging from 1 foot to 10 feet in thick- 
ness. They can be traced from New York through Pennsylvania, 
Virginia, Kentucky, and Tennessee into Alabama. They occur also in 
Wisconsin. 

The ore is usually fossiliferous, as is specimen No. 52, and is sometimes 
called '' red fossil ore." At other times it is oolitic, and is referred to 
as the oolitic iron ore; also as the Clinton ore, on account of its age 
and place of best exposure. The fossils are chiefly broken crinoids and 
bryozoa. 

The rock is made up of flattish or elongated grains, many of which 
are fragments of shells, but when seen in the hand specimen all appear 
to be oxide of iron. Under the microscope, however, these fossil frag- 
ments are, in most cases, found to be only partially made up of iron 
ore. Ill some cases, there is a fine coating of the oxide, such as may be 
seen about the grains of beach sand, as well as about the grains of 
many sandstones and quartzites, but generally it is thicker than a mere 
coating, and in many cases it completely replaces the carbonate of lime 
of the original fossil. Some of the grains look oolitic, but in a thin 
section no concentric or radial arrangement, suggesting concretionary 
structure, was observed. At many other places, however, as shown by 
Smyth,^ the oolitic structure is well developed. In specimen No. 52 the 
cement binding the ferruginous grains together is caleite and silica. 
They may be intermingled or may occur separately, but in either case 
the cement contains but little oxide of iron. The silica is occasionally 
radial-fibrous and optically negative, like chalcedony. 

By dissolving a fragment of this ore in hydrochloric acid, after the 
carbonate of lime has completely disappeared it is found that the ore 
is intimately associated with much silica, which is not easily recognized 
before the carbonates are removed. Upon close examination of a thin 
section a considerable part of the carbonate of lime in the fossil frag- 
ments is seen to be replaced by silica. 



1 Am. Jour. Scl., June, 1801, 8<l Buriea, Vol. XLIII, p. 487. 




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MLLBE-l DESCKIPTION8: NO. 52, F0SSILIFER0U8 IRON ORE. 139 

When this rock is long exposed, at or iienr the surface, to atmos- 
pheric infiaences and drainage, it is softened by the removal of mach 
of the carbonate of lime cement, and thus the proportion of oxide 
of iron is greatly increased, making the rock a valuable ore of iron. 
Below drainage the carbonate of lime is not removed, and the rock is 
harder and of less value for the iron it contains; but as it affords the 
necessary flux for more siliceous ores, it may still be used for its iron. 
It is important, however, to note that the ore in both cases is the anhy- 
Irous oxide of iron — that is, red hematite — and not the carbonate, or 
limonite, as might be expected from its association with carbonate of 
lime. 

This ore has been extensively used of late years in the Southern 
States, especially at Birmingham, Alabama, and at various points in 
Tennessee. Originally it was considerably used in Tennessee, not only 
as a source of iron, but also in dyeing, and it has been known in that 
region for many years as the "dyestone ore." 

The fossiliferous and bedded character of this ore, and its extensive 
distribution, are altogether exceptional, so that its origin is a matter 
of much interest. James HalP regarded it as derived from pyrite, in 
part at least, through the action of thermal waters, but this view has 
long since given place to other hypotheses. 

The facts that the oxide of iron in the bed replaces bryozoa and other 
calcareous fossils, and that below the drainage level the rock is largely 
carbonate of lime, have led to the view that the ore originated entirely 
^y the replacement of limestone after the strata were tilted up into 
their present jwsition. This view was advocated by A. F. Foerste,^ 
^ad e8X)ecially by James P. Kimball.^ 

The type locality of the rocks of the series to which the iron ore 
^longs is at Clinton, New York, where, instead of one bed, there are 
•hree beds of ore, and, as pointed out by O. H. Smyth, they are 
^^sociated with shales, sandstones, and conglomerates, whose ripple 
c^arks and mud cracks clearly indicate the littoral condition under 
^hich they were deposited along the borders of an interior sea that 
^nce filled the Mississippi Yalley, and at an earlier stage of its history 
leposited the great mass of limestone noted in the description of 
kl)ecimen No. 46 (p. 127). The abundance of marine life furnished the 
naterial for the coquina-like accumulation of shells, and the streams 
>f the adjacent lands brought the oxide of iron to tide* washed, salt- 
Bratcr flats, where, in the association of the two, the deposit of iron 
originated. 

According to Dana,* "The beds were evidently made over tide- 
washed, salt-water flats, where trituration is gentle. They indicate a 
wonderful degree of uniformity in continental level over a wide area.'' 



' Oeology of New York, part 4, p. 61. 
> Am. Jour. Sci., Jan., 1891, 3il series, Vol. XLI, p. 28. 
•Am. Geologist, Dcp., 1891. Vol. VIIT, pp. 35ft-357. 
^Dana's Manual of Geology, 4tli ed., p. 539. 



I 



140 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BrLL.150. 

The oolitic stiructui e of the ore, as showu by Kewberry,^ favors the 
yiew which regards the ore as an original deposit instead of a sabse- 
quent replacement of the limestone. 

C. Willard Hayes has recently examined many of the Clinton iron- 
ore mines of Tennessee,'^ Georgia, and Alabama with special reference 
to data concerning the origin of the ore, and he reports^ that he has 
never foand It passing into a nonferrnginous limestone. ^^ While it is 
true that the proportion of iron to lime in the unweathered ore varies 
in some cases rather rapidly, it is quite as apt to vary along the strike 
as upon the dip, showing that the variation is original and not con- 
nected with depth below the surface." His observations lead him to 
conclude, as did dewberry and Smyth, that the iron ore is an original 
constituent of the bed, and is not due to later replacement. 

No. 53. Peat. 

(From North Cambridgk, Middlesex County, Massachusetts. Described by 

George Otis Smith.) 

Peat is of recent origin, and the processes of its formation can be 
observed by anyone who has opportunity to visit a swamp. It is gen- 
erally formed by the accumulation of moss-like plant matter that has 
been preserved from the usual decay by moisture which prevented the 
free access of air. Classified by origin, peat may be termed phytog- 
enous and sedimentary; with respect to composition, it is essentially 
carbonaceous. The specimen of this collection well exhibits the char- 
acter of peat in its dry, compact form, in which it is used as a fuel. In 
color, it is dark brown or black; in texture, somewhat earthy, but 
with almost all of the characteristics of matted plant fibers. In its 
natural condition peat varies much in degree of compactness and the 
amount of water it contains. 

Peat is a sedimentary deposit in stagnant or almost stagnant water. 
Thus the plant remains are usually deposited in close proximity to the 
place of growth, and often the growing plants are in contact with the 
underlying peat, which simply represents past generations of the same 
species. Peat-forming conditions obtain in countries of humid tem- 
perate or subarctic climate. The places of peat accumulation are small 
lakes, marshes, or bogs, where the moisture favors both the growth of 
mosses and water plants and the preservation of their remains.^ The 
drainage modifications which have resulted from the glacial invasion 
have favored the existence of such swamps and bog3 in the northern 
parts of this country; but in Ireland peat bogs attain the greatest 
extent, one having an area of over 100 square miles, with the deposit 
of peat almost 50 feet thick. 

I The geneaiB of our iron ores, by J. S. Xewberry : School of Mines Quart., Nov., 18A0, Vol. II, 
pp. 1»-14. 

'Creologio Atlas U. S., folio 8, Sewanee, Tenn. 

■Letter, June 10, 1806. 

* Professor Sbaler gives the best <lesoription of these mnrsh conditions, in his paper on Fn>«h-wster 
morasses of the Unitwl States: Tenth Ann. Krpt. IT. S. Geol. Survey, 1800, pp.25&-339. 



wua.1 DESCRIPTIONS: NO. 54, CANNEL COAL. 141 

Specimea No. 53 was collected in North Oambridgef Massachusetts, 

wbere it occurs in a small basin eroded in brick clay similar to Ko. 8 of 

this collection, and of inter-Glacial age.^ The shallow pond has been 

fiUed during post-Glacial time by this deposit of plant remains, which 

is from 3 to 4 feet thick. In the lower part of this deposit the peat is 

compact and composed of finely comminuted material, which grades 

upward into a yegetal mat, on the surface of which the peat-making 

plants are growing in some places. The origin of the compact peat is 

tbns clearly shown. 

Many varieties of x>eat have been distinguished, classification being 
based upon the kind of peat-forming plants, or upon the physical and 
chemical characters of the peat itself. The amount of carbon present 
varies from 50 to 62 i)er cent, but usually there is little increase over the 
amoant contained in peat moss, Sphagnum. The principal use of peat 
is as a fuel, but it is far inferior to other fuels; and it is also used as an 
absorbent and as a fertilizer. 

No. 54. Cannel Coal. 

(Fkom Wauliks Cbxkk, Harlan Countt, Kentucky. Described by George 

Otis Smith.) 

Coal, like peat, is composed essentially of carbon derived from vege- 
tal matter accumulated under conditions in which water played an 
important part. It may therefore be regarded as a phytogenous sedi- 
mentary rock, but it is unlike peat in all its physical and chemical 
characters, and far superior to it in economic imi)ortance. Goal is not 
believed to have been deposited as coal, hence its origin needs to be 
discussed under two heads — the conditions of accumulation and the 
character of subsequent change. Peat has been defined as simply pre- 
served plant debris, and is essentially an unaltered rock ; but in the case 
of coal the changes subsequent to deposition have been considerable. 
This alteration has consisted of changes in structure or texture and in 
chemical composition. Thus, the coal specimens might be described 
later as altered rocks were it not for their natural relation to the peat 
just described. 

That coal represents old deposits of vegetal matter is well proved by 
both its megascopic and its microscopic structure. Distinct plant im- 
pressions are often seen on coal. Logs are found changed to coal, and 
its inner structure in many cases is fibrous or cellular. The microscope 
feveals even delicate spores as constituents of coal. The fossil leaves 
and other plant remains, which are so abundant in the associated shales 
And sandstones, strengthen the proof of its organic origin. Goncerning 
the conditions of accumulation of this mass of plant debris, two rival 
hypotheses may be mentioned, bj^t a complete discussion of them would 
involve the consideration of the most complex of geologic phenomena. 



'The gUcial brick clays of Rhode Inland »nd soatheaAt«ru Masaachii setts; Chap. Ill, Tlie clays 
•bout BMt4ni. by C. F. Marbut and J. B. Woodworth : SeveutovotU Ann. Kept. IT. S. Geol. Survey, 



futI,18M^pp.M»-996. 



I 



142 THE EDUCATIONAL SEKIES OF ROCK SPECIMENS, [bull. 150. 

The one hypothesis gives to coal a growth-in-place origin^ the other a 
drift or transport origin. 

The growth-in-place origin involves a comparison of Paleozoic coal 
beds and recent peat deposits. The hypothesis extends i>eat-foriniDg 
conditions over vast areas, and pictures a laxuriance of vegetation 
hardly to be comprehended from our knowledge of present conditions. 
Sach a view is supported by the presence of the underclay, supposed 
to represent the soil which supported this growth, and also by the 
frequent occurrence of roots (Stigmaria) in this clay and of tree truuks 
in the sandstone above the coal. An accumulation as great as that 
represented in the coal beds seems to imply tropical vegetation ; yet the 
process was one of preservation as well as of production of the organic 
matter. The climate that would promote the latter might prevent the 
former, since, as has been stated in the description of i)eat, accumula- 
tion of vegetal matter takes place to-day only in humid temperate and 
subarctic climates. 

The drift origin of coal deposits involves sedimentation processes 
differing somewhat from those which have produced other sedimentary 
rocks. Sedimentation of this chanicter would take place in the quiet 
waters of marginal lagoons, into which abundant vegetal matter drifts. 
Along the margins of these lagoons marsh vegetation would flourish, 
and here subaerial decay and subaqueous preservation might proceed 
side by side. The invasion of currents bearing mud or sand into portions 
of the lagoons would occasion the deposition of shale or sandstone ; and 
thus the splitting of a coal bed may be explained as a simple phenome- 
non of sedimentation. 

The two hypotheses have much in common. Both involve base-level 
conditions, luxuriance of vegetation, and slow subsidence of the area 
of accumulation. They differ in that growth-in-place necessitates 
subsidence of an oscillatory character to explain the alternation of 
coal beds and subaqueous sediments, while the sedimentation of 
drift material needs only slow subsidence, possibly somewhat intermit- 
tent. Both hypotheses are ably supported, the latter receiving more 
attention from later workers on the subject. It is reasonable to believe, 
moreover, that the coal deposits, with so great a distribution both geo- 
logically and geographically, have not all had exactly the same origin, 
and that even in the same coal field both hypotheses may be necessary 
to explain all the phenomena. 

The subsequent alteration of these deposits of carbonaceous debris 
has resulted in the destruction of much of the organic structure and 
in quite marked chemical changes. Peaty fermentation doubtless 
initiated such changes, while later geological processes contributed to 
the metamorphism. It must be noted that the metamorphism has been 
selective, the avssociated shales and sandstones remaining unaltered. 
This is explicable from the readiness with which hydrocarbon com- 
pounds respond to changes in their physical environment. Increivse of 



wLtn-l DESCRIPTIONS: NO. 55, BITUMINOUS COAL. 143 

temperature and of pressure attending the folding and faulting of the 
rocks of the coal formation has caused the chemical changes which 
constitute the difference between woody fiber and coal. The alteration, 
in short, consists of a decrease in volatile or gaseous constituents and 
a corresponding relative increase in carbon. The principal physical 
change has been a corresponding loss of volume, and from compression 
there have resulted various structures more or less characteristic of the 
different varieties of coal. These changes have been slow, but the 
degree of alteration attained in some of the Tertiary coals shows that 
time is not the most important factor. Coal beds are found in forma- 
tions of the Paleozoic, Mesozoic, and Tertiary ages, but the Carbonifer- 
oas coals are the most important, and the coal specimens described 
below are all of this age. 

Cannel coal is black in color, with a rather dull luster, compact and 
homogeneous in texture, and with a flat conchoidal fracture, although 
often with a fissility approaching that of shale. Its specific gravity 
is lower than that of most bituminous coal,^ but higher than that of 
lignite or brown coal. Chemically, cannel coal is characterized by its 
richness in volatile hydrocarbons — a quality which makes it preemi- 
nently a gas coal. Its name, cannel, or candle, was given because of 
the readiness with which it burns with a bright flame, so that pieces 
were used as candles by the poor people in England. 

Cannel coal is more clastic in character than other coals, and is com- 
posed of carbonaceous material thoroughly macerated. Fossil fishes 
and remains of other aquatic animals are often found associated with 
cannel coal, showing its sedimentary origin. The proportion of impuri- 
ties or ash is larger than in other coals, and cannel coal thus grades 
into carbonaceous shale by increase in amount of clayey material. 

The specimens of this collection came from the Carboniferous area of 
southeastern Kentucky. Here the measures lie in a broad syncline, 
with a gentle fold as its southeastern limit, and an overthrust fault 
bounding it on the northwest. Analyses of cannel coal from the same 
part of the basin show a rather low percentage of carbon, with ash 
amounting to over 26 per cent. 

No. 56. Bituminous Coal. 

(From Geobgbs Crkek, Allegany County, Mauyland. Described by Geoiuje 

Otis Smith.) 

This coal is intensely black, with a velvet or pitch luster and a 
cnbicalor uneven fracture. A block of bituminous coal usually shows 
* banded structure, with some of the bands dull, others bright and glis- 
^ning. The hardness of this coal is less than that of anthracite, hence 
it« common name, soft coal; and it soils the hand, thus differing from 
<5annel coal. Its specific gravity varies from 1.2 to 1.5. Chemically, it 
varies considerably in the proportion of carbon and volatile constitu- 
ent bat is intermediate in comx)osition between anthracite and lignite. 



144 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [buu.15«. 

Bitaminoas coal ignites easily, bams with a yellow flame, and gives off 
mach smoke. The rapidity of its combastion fits it for general use in 
the industries, while certain varieties are especially adapted for the 
production of gas and coke. 

The Georges Creek Basin, from which locality this specimen was col- 
lected, is situated in western Maryland. It is a shallow basin of Coal 
Measures, containing several valuable beds of coal. The " Big Vein," 
which occurs near the top of the 1,200 feet of Carboniferous rocks here 
exposed, is correlated with the Pittsburg coal bed, the most valuable 
of the Appalachian field. It forms the lowest member of the upper 
Coal Mcfasures, and its structure persists over large areas, showing n ide 
prevalence of nearly uniform conditions at the time of its deposition.^ 
The coal represented by specimen Ko. 55 is a high-grade coal, standing 
well in the market. 

No. 50. Anthracite Coal. 

(From Gilbertox Mine, Schuylkill County, Pennsylvania. Described by 

George Otis Smith.) 

As will be observed from an examination of this specimen, anthra- 
cite coal is black, but with a bluish or brownish tinge. Its luster is 
adamantine to submetallic and its fracture is plainly conchoidal. It is 
dense and brittle, and no megascopic structure can be distinguished, 
yet the microscope sometimes reveals traces of plant remains. Anthra- 
cite is the hardest and heaviest of all the coals, with a hardness of 2 
to 2.5 and a sx>ecific gravity of 1.4 to 1.7. Its chemical characteristic 
is the high proportion of carbon, which ranges from 85 to 92 per cent 
in the Pennsylvania anthracite and is known to reach even 95 per 
cent in the anthracite of Wales. Because of the decrease in volatile 
constituents, anthracite ignites with more difficulty than does bitumin- 
ous coal, and burns with a small bluish flame and no smoke, but affords 
more heat, pound for pound, than any other coal. 

The locality from which this anthracite was collected is in the Western 
Middle Anthracite Field of Pennsylvania.'^ The eastern basin of this 
field is a long and narrow spoon-shaped syncline, subdivided by anti- 
clines into many smaller basins. Overturned strata are quite common, 
and the coal beds of this area are steeply inclined, the average dip 
being not less than 35^ to 40^. Such close folding has tended to crush 
the coal, thus increasing the proportion of waste. 

The chemical and physical characteristics of anthracite are such as 
seem due to a greater degree of alteration than that attained in the 
other coals. It has been suggested that there were differences in 
the original vegetal deposits, since beds of anthracite have been found 
between or even above beds of bituminous coal. However, these rela- 



*Tbe stratigraphy of the bituminoua-coal fields in deHcribed by I. C. White in BiiU. U. S. G«ol. 
Survej' Xo. 65. 
> Pvscribed iu llie Fiual Keport of the reuunylvaaia Geol. Survey, voL 3, Part I, Chap. CXXL 



w^^] DESCRIPTIONS: NO. 57, DIKE. 145 

tioDs are exceptional, and it is well known that in the same bed 
bitominons coal may grade into anthracite, and in many eases this 
change is seen to be the result of contact metamorphism. Perhaps 
the bei$t instances of alteration are foand in the Cretaceoas coals of the 
Anthracite-Crested Batte area in Colorado.^ The noncoking bitumi- 
nous coals are found in regions of least metamorphism, the coking coals 
in the localities of more advanced alteration, and the anthracite only 
in areas of ^eat regional metamorphism or in the neighborhood of 
large bodies of porphyrite.**^ 

If the metamorphism of coal beds is more profound, the coal becomes 
graphitic, as is the case in the anthracite of Ehode Island. 

In short, there is a complete gradation in chemical composition from 
wood which contains about 50 per cent of carbon to graphite, which is 
pure carbon. Intermediate members of this series are the peat and the 
eoals which liave already been described. The group is a natural one, 
the members having an origin essentially the same, but with present 
characters dependent ux>on the degree of subsequent alteration. 

UNALTERED IGNEOUS ROCKS. ' 

No. 57. Dike. 

(From Wiluamsons Point, Lancaster County, Pknnsylvania. Desciubed 

BY J. S. DiLLER.) 

By the activity of forces not fully understood, fissures (great and 
small) are produced in the rocks at and near the earth's surface and 
filled from below with molten rock material. Such rock-filled fissures 
are dikes. They are common in volcanic regions, and illustrate one of 
the forma of connection between the volcanic effusions upon the sur- 
face and the highly heated interior of the earth in which the lavas 
originate. They are generally nearly vertical, as in PI. XXIV, break- 
ing through sedimentary as well as igneous rocks, and occasionally 
&bow a marked columnar structure perpendicular to the walls of the 
dike, like that of specimen No. 103. Specimen No. 57, which includes 
tbe whole thickness of the dike, shows the cross fractures distinctly. 
This jointing was determined by the cooling influence of the adjacent 
rocks. 

Dikes range in size from a few inches to several hundred feet in 
thickness and from a few rods to miles in length. Igneous rocks such 
as form dikes are generally harder than the adjoining rocks, so that 
they are able longer to withstand the destructive influence of weather- 
'"g» and give rise to ridges which may be tracked for miles. The same 
dike may not continue for many miles, but the group to which it belongs 
Diaybe more or less continuous for long distances. In connection with 



1 Geologic Atlac U. S., folio 9. Aiitliracite-Creetcd Butte. 
> Eldridge, descriptive text of folio 0. 

Bull. 150 10 



146 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bui^wo. 

the Triassic formation of the Atlantic coast, they extend with Inter- 
raptions from Massachusetts to Xorth Carolina. 

The material of large dikes cools more slowly than that of small dikes, 
and becomes more coarsely crystalline. Small dikes, like specimen No. 
57, are usually fine grained. Large dikes are in general more coarsely 
crystalline in the middle than upon the borders, where they come in 
contact with the country rock and are sometimes glassy. A number 
of volcanoes may be in line on the same fissure, affording escape at 
certain points for the rising magma. In other places the outflow upon 
the snrfacb appears to have taken place all along the fissures in sheets, 
as observed by Eussell,^ Le Conte,^ and others. 

Dikes may occur in radial groups about a volcano. Volcanoes some- 
times become plugged up, and the great pressure of the molten material 
within bursts the mountain asunder. The fissures afford an escape 
for the magma, and dikes are produced. This arrangement is well 
marked in the Highwood and Crazy mountains, as well as other moun- 
tains in Montana^ and about the Spanish Peaks of Colorado/ Mount 
Etna affords an illustration of such a group in connection with an active 
volcano. Upon its slopes are numerous minor vents arranged in lines 
radiating from the main crater. 

Like joints, dikes may occur in parallel or intersecting systems. 
Where they intersect, their relative age can be determined; the newer 
cut across the older. 

Many kinds of igneous rocks, both plutonic and volcanic, occur in 
dikes. Some forms are known only in dikes, and for this reason have 
been placed in a separate group by some authors.^ 

The most common of all dike-forming rocks is probably diabase, the 
group to which the small dike illustrated by specimen No. 57 belongs. 

GRAJaTE-RHYOLITE FAMILY. 

No. 58. Volcanic Dust (rhyolitic !). 

(From Gallatin Valley, Gallatin County, Montana. Dkscribkd by J. P. 

Iddings.) 

This fine dust forms a deposit about 20 feet thick within Neocene 
lake beds of the Gallatin Valley, Montana,® where it has been studied 
by A. C. Peale. The major part of these lake beds consist of volcanic 
dust, similar but less pure, and presumably brought into the lake basins 
by waters from the neighboring slopes, where it has been deposited by 
the wind. The purer material occurring in these beds is considered to 



' Bull. U. S. Geol. Survey No. 108, pp. 11 and 22. 
« Am. Jour. Sci., 3d neries. Vol. VII, p. 167. 

■Highwood MoQQtainH of Montana, by Walter H. Weed and Loula V. Pirssoii: Bull. Geol. Soc- 
America, vol. 6, p. 392. See, also, Geologic Atlas U. S., Little Belt folio, by W. H. Weed. 

* Geologic Atlas U. S., Spanish Peaks folio, by ft. C. Hills. 

* " Ganggesteine " of Rosenbuach : Mikroakopische Phyaiographie der Ma«sigen <re«iteiiie, 18K7,p.6. 
c Pcale, A. C. : Geologic Atlas V. S., folio 24, Three Forks, Montana, 1896. 



descriptions: no. 58, VOLCANIC DUST. 



147 



Deen deposited directly from the air. It occars in beds 2 to 5 feet 
separated by thiu calcareoas layers, the thickness of the whole 
20 feet. 

en examined with a microscope, it is seen to be made up of minute 
ents of colorless glass, whose angular shax>es in some instances, 
iread-like form in others, together with the presence of air pores, 
are spherical, elliptical, and tubular, indicate plainly that the 
ents are broken pumice. The size of the fragments is very small, 
rgest being about 0.4"°» in diameter, the average diameter being 



Oniiii 



O.L> 

ery small i>ercentage of the fragments are pieces of crystals, aiid 
appear to be feldspar, hornblende, and pyroxene, and possibly 
quartz. But quartz and unstriated feldspar may be easily con- 
when in angular fragments, unless cleavage is pronounced. This 
percentage of crystals as compared with glass may be due to the 
al paucity of crystals in the magma exploded into dust, or it may 
) result of a partial separation of the material during its trans- 
ion through the air, by which means the denser and more compiact 
les settled nearer the vent from which the eruption took place 
the lighter and more attenuated ones. Hence it can not be 
led that the material found in this deposit necessarily represents 
>ini)osition of the lava before explosion. The glass itself is abso- 
free from microlites and is i)erfectly colorless in the thin bits 
Qg the dust. 

J chemical composition of the dust is shown by the following 
ses: 

Analyses of volcanic du«i and of rhyoUie. 




klysis II shows the percentage when the loss upon ignition is left 

This loss is unusually large and indicates a highly hydrated glass. 

compared with the rhyolitic rocks of the Yellowstone Park 

, which is 50 miles to the southeast, the volcanic dust of the 



148 THE EDUCATION Al, SERIES OF ROCK SPECIMENS, [bull. 150. 

Gallatin Valley is found to be much richer in K2O and poorer in Na^O, 
and to contain more MgO and a little more GaO and iron oxide. Analy- 
sis III, of a rhyolite from the Xorth Madison Plateau, Yellowstone 
Park, is pliwjcd by the side of that of the volcanic dust for comparison. 
It is a fair representative of the composition of most of the rhyolite 
of the region. The relatively high percentage of MgO and CaO, and 
the presence of fragments of hornblende and pyroxene and feldspar, 
suggests that the volcanic dust may be the glassy portion of a lava 
having the composition of a dacite, in which potash was concentrated 
in the groundmass by the crystallization of the soda in lime-soda feld- 
spars. Since this is purely speculative and the actual history of this 
particular deposit of dust is unknown, it will be better to refer it to the 
rhyolites on the basis of its high content of silica, remembering that 
its chemical composition is not normal when compared with the great 
bodies of rhyolite that were erupted in the region to the southeast, and 
noting the fact that it may be related to the explosive eruptions of 
andesite which also took place in earlier Neocene times in the same 
region. 

For accounts of the trans[)ortation of volcanic dust, sometimes to 
distances of «500 and 700 miles, the student is referred to the text-books 
of Dana, Geikie, and others. 

No. 59. Rhyolitic Pumice. 

(From Mono Lake, Mono County, California. Described by Wali>kmar 

LiNDGREN.) 

At most volcanic eruptions the effused or ejected masses are partly 
of a porous or scoriaceous structure, due to the expansion of gases and 
vapors contained in the magma. The extreme result of such a dilation 
of the magma by expanding gases is a light, spongy, froth-like sub- 
stance, usually more or less drawn out in threads and sometimes, 
indeed, forming fibrous masses with silky luster. It is called pumice. 

Pumice may be formed during the eruption of any magma, and we 
may thus speak of rhyolitic pumice, of andesitic pumice, etc. No doubt, 
on account of the more viscous character of the magma the kind first 
mentioned is most common, and such is the rock here described. 

The late Tertiary or Pleistocene volcanoes south of Mono Lake, at 
the eastern base of the Sierra Nevada, when active poured out heavy 
masses of rhyolitic glass or obsidian. 

Violent explosions accompanied the eruptions, and fine volcanic dust, 
together with "volcanic bombs" of pumice, were scattered all over the 
surrounding country for miles beyond the craters. 

The pumice is light gray, usually somewhat fibrous or silky on 
account of the long drawn-out pores which it contains. As a rule the 
l)ieces will float on water. Under the microscope it is found to consist 
ol a colorless glass, i)erfectly isotropic, containing a few large por- 



Dai£S.1 



DESCRIPTIONS: NO. 59, RHYOLTTIC PUMICE. 



149 



phyritic crystals or plienocrysts of plagioclase with narrow striations; 
one or two larger greenish pyroxeoe grains were also noted. More 
abandant are foils of a brown biotite, sometimes 1"""' long. The glass 
is much purer than that of the obsidian from the same locality. It 
contains no or very few trichites, and only a moderate quantity of 
microlites. ^mong these, biotite foils, sometimes hexagonal and occa- 
sionally bent and twisted, together with minute feldspar microlites, are 
the most abundant. Local accumulation of opacite grains and obscure 
microlites are frequent. Very characteristic is the stringy or drawn- 
oat structure of the glass, caused by long, winding, thread-like gas 
inclusions. 

The macroscopic i)ores of the pumice of course appear prominently in 
the sections. They are usually drawn out in an elongated shape par- 
allel to the thin, microscopic gas inclusions in the glass, and are some- 
times so abundant that only a slender network of glass remains. 

The appended analysis by W. H. Melville shows by the high per- 
centage of alkali and small amount of lime that the rock belongs in the 
gnmp characterized by the prevalence of alkali feldspars, albite, and 
orthoclase, though probably a little oligoclase is also present. The 
silica is rather low for a rhyolitic pumice, and in this respect the rock 
approaches the trachytes. 

AnalysiM of rhyolitic pumice from Mono Lake, California, 



Per cent. 



L<ms 

SiO, 

AUO, 

>'e/), 

FeO 

c*o 

MjrO 

K,0 

Na,0 

Total 




No. GO. Rhyolitic Obsidian. 

(^oic Mono Lake, Mono County, California. Dkscribed bv Waldemar 

LlNDGREN.) 

Just south of Mono Lake, in the Great Basin, and a few miles east 
of the great fault scarp of the Sierra Nevada, there rises a long row of 
volcanic cones with a maximum height of 2,700 feet above the level 
of the lake. Probably in Pliocene, and certainly in Pleistocene times, 
these craters were in full activity, pouring out acid viscous lavas, which 
oousolidated as rhyolite and obsidian, and scattering volcanic bombs, 
^*PiDi, and ashes all over the surrounding country. The volcanoes are 
DO longer active, but their cones are still excellently preserved. 



150 THE KDUCATIONAL SERIES OF ROCK SPECIMENS. Ibuli^ISO. 

The lava flows are principally composed of a rhyolitic obsidian, which 
is a rapidly cooled and consequently glassy lava. The obsidian, which 
was collected at the foot of one of these ridges, is of deep-black color, 
has a glassy luster, and beautiful conchoidal fracture; in thin frag- 
ments it is translucent with a grayish or smoky color. 

Under the microscope the principal constituent is a colorless, isotropic 
glass, filled with a great variety of minute inclusions and crystals, very 
frequently arranged so a« to give the rock a banded or liuidal appear- 
ance. Once an inclusion of a glassy andesitic rock with feldspar and 
augite microlites was noted, showing that on its way up the magma 
broke through more basic eruptives. Although there are no large 
phenocrysts, the glass abounds with minute crystals and inclusions, to 
a degree not to be suspected by the appearance of the rock. 

While the mineralogical character of some of these may be distinctly 
recognized, others are so small and obscure that we can only infer from 
their form that they x>os8ess the physical characteristics of crystals. 
These are designated microlitic forms. 

Among the crystals which may be identified we note biotite, a few 
distinct augite prisms, small magnetite grains, pretty generally dis- 
tributed, and minute colorless prisms, with feeble refraction^ which 
almost certainly are feldspars. 

The most abundant mineral is brown biotite in long-drawn foils with 
a maximum length of 0.066*"*", although generally much smaller. 
Sometimes these biotite foils are bent or broken. Further we note a 
few yellowish-green, strongly refracting grains of augite, and some 
colorless rounded masses (maximum diameter 0.1™™) of a strongly 
refracting doubtful substance. Between crossed nicols these latter 
show undulous extinction and gray colors of interference. Besides 
these the glass swarms with microlites, which are usually of slender 
prismatic forms and act on the polarized light, but which can not be 
positively identified. 

Most prominent, however, are other microlitic forms which are desig- 
nated as trichites. They are defined as capillary crystals of exceed- 
ingly small thickness compared with their length. They are nearly 
always bent, broken, or curved in complicated forms. The trichites 
have a great tendency to combine in groups, radiating spider-like from 
a common center, usually a small grain of magnetite. On account of 
their minute thickness, rounded form, and the total reflection caused 
thereby, they usually appear opaque, but occasionally larger ones may 
be found which transmit the light with a dark-brown color. Sometimes 
the trichites appear isolated, as curved or straight hairs with a maxi- 
mum length of 0. 15™™, and it seems, indeed, as if there were a transit 
tion between the recognizable narrow biotite foils and the trichites, 
although such a connection has not been conclusively proved. At any 
rate, the trichites seem to be composed of some silicate rich in iron. 

Besides these microlitic forms the glass contains a great many gB& 
inclusions^ generally more or less drawn out in one direction. 



DUUB.] 



T>KSCRIPTIONS: NO. 61, RHYOLITIC PERLITE. 



151 



The stracture and microscopic appearance of the obsidian containing^ 
all these minute forms are by no ineans constant and similar through- 
out the mass. The principal cause of this is the microfluidal structure 
prevailing^ in the rock: and showing that the magma has been in motion 
after the separation of the crystals and the microlitic forms. Most 
slides have a more or less distinct banded appearance. In some bands 
biotite crystals abound, in parallel arrangnient; in others, trichites pre- 
dominate; in still other streaks, minute feldspar crystals may prevail. 
The bands are often curved in a complex way. 

Sometimes the glass may be comparatively free from microlitic forms, 
while in other places they accumulate in dense masses. 

A faint doable refraction of the glass is often noticed ; in most cases, 
however, this is doe to extremely small microlites, discernible with diffi- 
culty even with, strong objectives, but in a few places it really appears 
as if the glass itself were faintly double refracting. 

The appended analysis by W. H. Melville shows that the rock has 
the composition of a typical rhyolite, indicated by the high percentage 
of silica, small amount of lime, and relatively large quantities of soda 
and potassa. 

AnalffHa of rhyolitio obsidian from Mono Lake, California, 



Per c«nt. 



Ixwn 

SiO, 

Al.O, 

Fe,0, 

FeO 

CaO 

MgO 

K,0 

Na^ 

Total 



0.41 
75.78 
12.39 
0.22 
1.25 
0.81 
0.31 
4.M 
4.00 



99.81 



No. 61. Rhyolitic Pbrlite. 

(From Yt?lix>w8Tonr National Park. Described by J. P. Iddings.) 

A dark-gray rhyolite glavss forms a cliff on the east side of the Fire- 
hole River, opposite Excelsior geyser and the Midway geyser basio. 
^t is part of the great body of rhyolite which, as a surficial flow of lava, 
^^nstitutes the plateau of the Yellowstone Park. 

The rock varies in character in bands, which are alternately glassy 
*nd lithoidal. The glassy portions are distinctly perlitic, and also 
finely porous and vesicular, the cavities containing minute white pellets 
of tridymite. In places the centers of perlitic shells are black glass, 
^esurrounding thin shells being light gray. This is a common feature 
of perlites in many localities in the Yellowstone Park. The less glassy 
or lithoidal layers lack the vitreous luster, and are dull gray; they are 



152 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bull-IM. 

also porous, the vesicles or cavities in the rock bein^ unequally dis- 
tributed in diflPereut layers. The banding produced by the variously 
constituted layers of the rock, which express what is called the flow- 
structure, becomes more apparent in large masses of rock. The whole 
is filled with small phenocrysts of quartz and feldspar. In some parts 
of the rock there are small spherulites. 

In thin sections the microscopic character of the rock is quite varied. 
The groundmass consists, in places, of colorless glass, with numerous 
trichites and microlites with marked fluidal arrangement. Such places 
exhibit perlitic cracks. Through this glass are scattered spherulites 
that appear brown in transmitted light and show the optical character- 
istics of those in the lithoidite from Obsidian Cliff. They are close 
together in the lithoidal portion of the rock and have attained various 
degrees of crystallization. Occasionally they have formed around the 
phenocrysts as a spherulitic border. Their microstructure, as well as 
that of the clusters of tridymite, is the same as in the lithoidite of 
Obsidian Cliff, and needs no special desciiption. The phenocrysts are 
quartz, sanidine, plagioclase, and augite, with smaller crystals of mag- 
netite. Quartz occurs in idiomorphic crystals, whose sections are 
those of double hexagonal pyramids; some are rounded. They fre- 
quently carry a number of brown glass inclusions in negative crystal 
cavities, and less frequently bays of groundmass. 

Sanidine forms idiomorphic crystals, which sometimes have rounded 
corners. These crystals often carry numerous inclusions of ground- 
mass, and also of colorless glass. Sanidine in some instances surrounds 
plagioclase more or less completely, the two feldspars having parallel 
orientation. 

The plagioclase exhibits polysynthetic twinning, in thin lamellae with 
low extinction angles in symmetrical sections. It is probably oligo- 
clase. Frequently it is crowded with inclusions of groundmass and 
glass, so as to be fairly honeycombed, Augite occurs in comparatively 
large phenocrysts, but in small amount. It is light green and fre- 
quently incloses magnetite and glass. Magnetite forms crystals and 
grains of considerable size, 0.4*"'" in diameter. With it are generally 
associated colorless crystals of zircon and some of apatite. On the 
edges or walls of cavities and in places where tridymite and sanidine 
have crystallized in microscopic crystals there is a small amount of 
brown biotite in six-sided plates,* 

The chemical composition is shown by the following analysis, made 
by H. N. Stokes in the chemical laboratory of the United States Geologi- 
cal Survey. 



'For faller description of the rhyolites of the Yellowstone National Park see the forthcoming mono* 
grapli on that region : Mon. U. S. Geol. Survey, VoL XXXII. 



DIU.KK.] 



DESCRIPTIONS: NO. 62, LITHOIDITE. 



153 



Analjf9iM of rkyoUtic perlite from TttUowsUme National Park. 



Percent. 



SiO, 73.84 

A1,0, 12.47 

Fe,0| '. 32 

FcO ' .90 

MnO , trace 

C«0 ' 1.08 

MgO .25 

KgO 5.38 

X%0 2.88 

Ignition 2.76 

Total 99.88 



No. 62. LiTHOIDITB. 
(F«OM Obsidian Cliff, Yellowstonk National Park. Described by .T. P. 

iDDINGS. ) 

The litboidite in this series of rocks was collected from the iiortbeni 
end of Obsidian Cliff in the Yellowstone National Park. It represents 
the laminated and more or less crystalline portion of a great flow of 
rbyolitic glass which forms the picturesque cliff of obsidian, well known 
to the tourist by its glossy black columns that rise high above the road. 
The jet-black obsidian is filled with spherulites and litliophysa^, which 
are scattered through it in layers. In places the spherulites are minute 
and crowded together, so as to form light-gray layers having a stony or 
litboidal apx>earance. As these lithoidal bands become more numerous 
the rock is composed of alternating layers of black glass and gray lith- 
oidal ones. At the northern end of the cliff the lower portion of the 
whole mass is lithoidal, as in the specimen. 

The composition and structure of the sphemlites and lithophysae and 
the character and probable origin of the lamination of the lithoidite 
bave been described in detail by the writer in an article on Obsidian 
Chflf,^ from which the following extracts are taken : 

This lithoidite [loc. cit., p. 264] is a light purplisb-gray rock wbicb sbowR, on 
crots-fracturesy delicate bands of light and dark colored layers. The former are 
crystalline^ witb small cavities scattered along tbem, which form planes of weak- 
D«M snd permit the rock to split into thin plates, often one-sixteenth of an inch in 
thieknesB. Tbe dark layers are microspbemlitic and dense [and are sometimes 

S^MBy] The lithoidal rock is as |nll of sphernlitic forms as the obsidian, 

^ot it appears more porous and contains a multitude of hollow spherules of the 
Qtmoflt delicacy and beauty. An idea of their great abundance is given by PI. XIII, 
H'^ [PI. XXV of this bulletin, p. 154], which was drawn from a slab of lithoidite 
>nd it the natural size. Most of them are hemispherical and consist of a group of 
concentric shells wbich curve one over another like the petals of a rose. The shal- 

' J. P. Iddingfl, Obeidian CliiT, Yellowstone Kational Park : Seventh Ann. Rept. IT. S. Geol. Snrvey, 
^' A ]iart of this paper was previounly pablishfMl under the title, The nature and origin of litho* 
l^ya* nd tho lamination of acid lavas : Am. Jonr. Sol., Jan., 1887. 3d aeriea. VoL XXXHI, pp. 36^5. 



154 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bull. 150. 

lower ones present small, rose-like centers sarronnded by thin, circular shells. The 
disks are sometimes oval and sometimes composed of several sets of shells which 
have started from centers near together and developed in sectors, giving a scalloped 
form to the curves. Others are eccentric or send out long, curving arms, cross- walled 
like a chambered ammonite. 

The partition walls are generally very thin and often close together, in one instance 
50 occurring within a radius of 2 inches. They are very fragile and crumble under 
the touch, being made up of small and slightly adhering crystals with brilliant, 
glistening faces. 

The minerals occurring in the lithophysa^ and hollow spherulites and 
other cavities in the lithoidite are quartz, tridyiuite, sanidine, fayalite, 
and magnetite, the detailed descriptions of which will be found in the 
the paper referred to and in a more recent one in which the quartz is 
especially described.^ 

The microscopical character of this lithoidal rhyolite is extremely 
varied, and exemplifies the structure or phases of crystallization com- 
mon to many rhyolitic lavas in most parts of the world. It has been 
made the subject of a special paper on spherulitic crystaUi-zation ^ from 
which the following description has been taken : 

In thin sections the rock is irregalarly banded or mottled according as the sec- 
tions have been ground across or parallel to the layers of lamination. The most 
crystalline parts are colorless, the dark-gray portions are mottled with minute spots, 
and with a low magnifying power these dark portions are seen to be minutely spher- 
ulitic, exhibiting characteristic black crosses. The transparent parts are quite 
crystalline aggregations of tridymite or quartz and feldspar, often with nnmerous 
irregular cavities between the crystals. OccasionaUy these places contain irregular 
grains of fayalite, and still more rarely brown mica, besides scattered spherulites, 
which also border these more crystalline portions. 

The thiu sections show a few of the larger spherulites which are porous, and in 
some places branching, arborescent or feather-like growths of feldspar — in fact, all 
of the modifications of spherulitic crystallization described in the paper on Obsidian 
Cliff. 

Studied with higher magnifying power, it is seen that the finely spherulitic por- 
tions are crowded with trichites, which are more perfect as the spherulitic structure 
is more minute, but which lose their form and uniformly fluidal arrangement when 
the spherulites are more developed, sonietiuies being crowded out toward the margin 
of the spherulites and aggregated in opaque lines between the spherulite indi- 
viduals. The finely spherulitic parts of the rock section also exhibit an extremely 
minute granulation by trausmitted light and appear brown; but by incident light 
this granulated portion is white, evidently in consequence of the reflection of the 
light from inuumerable small surfaces or cracks. In the small spherulit-es that lie 
isolated in and also bordering on the more crystalline portions of the rock the cen- 
tei-s of the spherulites are granulated and brown while the margins are often color- 
less and transparent. In some cases the centers of the spherulites are colorless, and 
the brown color is confined to an outer zone. In such spherulites the fibers of the 
outer zone are more delicate than those of the central portion, showing a lower 
degree of crystallization. 

These small spherulites when investigated with the quartz plate prove to be opti- 
cally negative — that is, the axis of greatest elasticity, a, lies approximately parallel 
to the direction of the radial fibers. This is also true of the most minute colorless 



1 J. P. Iddings ami S. L. renfield. The rnineraU in hollow Apheniliteii of rhyolite from Gl»de Creek. 
Wyoming: Am. Jonr. S<'i.. July, 1891, 3d series, Vol. XLII, pp. 39-46. 

»J. P.Iddings, Spherulitic crystallization: Ball. Philos. Soc. Washington, VoL XI, 1891, pp. i4^ 
404,pU.7,8. 



DiuER 1 DESCRIPTIONS: NO. 62, LITHOIDITE. 165 

t<phenilite« which occor in the glass of the obsidian, and are represented by Hg. 1, 
PI. VII [fig. 1, PI. XXVI of this bulletin, p. 156]. In the lithoidite the spheruHtes 
have formed in juxtaposition^ so that they adjoin one another with more or less 
polygoDsl boundaries. Occasionally there is a small space between several spheru- 
]iteB\rfaere the magma has crystallized differently. These spaces may attain a con- 
siderable size, relatively^ or may constitute layers of the rock. The spherulites 
bortleriDg sach spaces frequently continue their crystallization a short distance into 
tiiem,and exhibit distinct prismatic rays that project beyond the apparent periphery 
of the Bphemle, and resemble the teeth of a cogwheel. Sometimes the projecting 
ravs assume a comparatively great size. These forms are illustrated by figs. 3 and 
5, PI. VII [PI. XXVI of this bulletin, p. 156]. In such cases the mineral character of 
the raya is clearly determinable. The projecting rays are prisms with parallel sides 
and ctyatallographic terminations. They extend with uniform optical orientation 
toward the center of the spherulite. They exhibit in a few instances distinct 
clearage parallel to the sides of the prism. The angle of extinction ranges from 
O'^ to 10^ or 12^, being usually low. The prisms are invariably optically negative, 
ud are therefore orthoclase crystals elongated in the direction of the clino axis. 
The high limit of the extinction angles, as well as the chemical composition of the 
rock and the spherulites, since there is no evidence of the presence of more than 
one species of fehlspar, indicates that the orthoclase is rich in soda, the molecular 
ratio of the potash to soda in the rock being 1 to 1. 

There is a diiference between the end of the projecting prisms of feldspar and the 
part of the ray within the spherulite. The former is transparent and clear, without 
ioelosioDs; the part within the spherulite proper is clouded and granulated, as 
already stated. In some instances the gr*inulation assumes a more definite charac- 
ter and has a radiating feather- like structure, which at once suggests the grano- 
pbyric arrangement of quartz in feldspar. This is unquestionably its true chciracter, 
ftlthoagh the quartz does not appear to afiect the optical behavior of the feldspar 

An examination of the microscopic granophyre groups of feldspar and quartz which 
^cor in the same thin sections, and which have been described in the article on 
Ohaidian Cliff (p. 274), shows the same optical characters and feather-like structure. 
8nch en intergrowth is represented in fig. 2, PI. VII [PI. XXVI of this bulletin.] 
It is made up of feldspar crystals, which cross one another at a common point, or 
vhich radiate from a common center. These feldspars invariably have the axis of 
latest elasticity, ^, approximately parallel to the direction of radiation. They 
have the same orystallographic orientation as the feldspar ra;ps of the spherulites. 
^ intergrown qnartz does not alter perceptibly the optical orientation ; therefore 
't mnat be either so oriented as to have its axis of greatest elasticity more or less 
^incident with that of the feldspar, or it is not present in sufficient amount to 
^Qence the interference phenomena appreciably. The latter is most probably the 
^*^i for in a large granophyric group with the same stnicture, which was studied 
w comparison, it was observed that the qnartz, though appearing to be present in 
^nsiderable amount, was not sufficient to change the character of the double refrac- 
tion of the feldspar, which was the predominant mineral. It modified it, however, 
^ ft variable extent ; and in places where quartz was more abundant its optical char- 
acter was predominant. 

The small spherulites of this rock are unquestionably composed of orthoclase 
Pnamt or needles elongated in the direction of the clinoaxis, which radiate from a 
center and are intergrown with quartz, after the manner of granophyre or raicro- 
Pcgmatite; and it is this microscopic intergrowth which gives them the granulated 
^ feather-like strncture. 

In the case of the spherulites with projecting rays of pure feldspar, it is evident 
that the free silica ceased to crystallize as quartz in intimate connection with the 
Ofthoclaseand allowed the latter to continue alone and project into a highly siliceous 
letidoal paste, which finally crystallized as tridymite in most instances. 



EXPLANATION OF PLATE XXVI. 

SECTIONS OF SPHERUUTE8. 

Fig. 1. Colorless microscopic spherulite, showing irregular dark cross between 
crossed niools, enlarged 153 diameters. 

Fig. 2. Simple form of granophyre gronp of quartz and feldspar between crossed 
nicols, enlarged 235 diameters. 

Fig. 3. Microscopic spheralite with projecting rays of orthoclase, enlarged 120 
diameters. 

Fig. 4. Like fig. 3, with crescent-shaped area of pure feldspar substance, enlarged 
130 diameters. 

Fig. 5. The same as fig. 3, enlarged 120 diameters. 

Fig. 6. Portion of large spherulite, showing different forms of feldspar needles. 

Fig. 7. Branching group of orthoclase needles occurring in the outer portion of 
the spherulite of fig. 6. 
156 







I 4 




SECTIONS OF SPHEftULITES FROW YELLOWSroNE U 



DUittl DESCRIPTIONS: NO. 62, LITHOIDITE. 157 

In certain caiHe& the zone of clear feldspar does not occur on the margin of the 
upheralite, bat forms a crescent-shaped transparent belt within it, as shown in fig. 4, 
P]. vn [PI. XXYI of tb.ia bulletin]. In ordinary light this belt appears to be a gap- 
leg, circular craclc, tliong:li its definition is lost at one end. Between crossed nicols it 
is found to consist of pare feldspar^ oriented in accord with the radiating prisms, and 
prwlncing no disturbance of the dark cross which passes regularly through it. At 
the lower end, fig. 4, it is seen to be part of the same crystallization as the purer feld- 
spar rays of that part of tbe sphemlite ; and in the upper part it differs ftom the rest 
of the sphemlite simply by being free from the granulated or micropegmatitic struc- 
ture. There can be no donbt that it is a part of the original crystallization of the 
iphemlite, and that from some cause the free silica ceased to crystallize for a short 
space and then continued as in other portions of the sphemlite. This is in harmony 
with the observation that the micropegmatitic structure and the other phases of 
rrystallization in tbis part of the rock are irregularly scattered in patches, so that 
ai^oining parts of adjacent spherulites are micropegmatitic, while other portions of 
them are free from tbis structure. Such crescent-shaped spaces in the spherulites of 
this rock have undoubtedly been produced at the time of the crystallization of the 
feldspar rays, by conditions which affected the quantity or distribution of the fne 
silica or of the mineralizing agencies engaged in its crystallization. 

In parts of the rock the small feathery spherulites bordering an area of tridymite 
do not terminate in well-defined feldspar prisms, but pass out into irregularly shaped 
feldspars and send out acicnlar rays of extreme delicacy. These transparent needles 
also lie in various directions in the tridymite area. They have apparently the same 
double refraction as orthoclase, and have the axis of greatest elasticity, a^ parallel 
to their length. A transverse parting is slightly developed. Their mineralogical 
character can not be made out with any degree of satisfaction from the smallest 
needles, but they can be traced to stouter ones which are undoubtedly orthoclase ; 
so that there would seem to be no question that the delicate acicnlar rays of these 
spherulites are needles of orthoolase elongated parallel to the cUnoaxis. They are 
then the same as the stouter prismatic rays, but have probably developed an acicnlar 
form because of some slight difference in the conditions under which they crystallized. 
The spaces between the spherulites already mentioned are in most instances occu- 
pied by tridymite in comparatively large crystals, often twinned and carrying 
DamerouM gas cavities. These patches may be completely filled with tridymite or 
may be but partially filled, there being open spaces betw(;en the crystals which 
cross one another in all directions, and in extreme caHCs the tridymite may simply 
coat the walls of a hollow cavity. In most instances the area is completely occu- 
pied, when the free silica is sometimes in the form of quartz. There are always other 
minerals present in variable amounts, including orthoclase and magnetite, with less 
regularly tourmaline, mica, and fayalite. The fayalite occurs in relatively large 
irregular individuals, with an opaque border, which at times entirely replaces the 
original mineral. 

The tourmaline and mica occur in minute crystals about 0.025 <"*" long and 0.01 °*™ 
thick. They are abundant in places and lie scattered through the tridymite or 
qnartz, and also in the margin of the bordering spherulites. They occur sometimes 
together, but usually one is present to the exclusion of the other. The tourmaline 
is recognized by its decided ploochroism, the strong absorption being acroHS the 
prisms, O > E. Its color is brownish -green ; colorless parallel to E. The double 
i«fra«'tion is strong and negative. Transverse sections exhibit a uniaxial cross, and 
are bounded by six sides, alternately three short and three long. It also occurs in 
the tridymite coating the walls of the hollow cavities in some cases. 

The mica is green and also yellowish brown to reddish. It forms stout tablets 
with six sides, and exhibits strong absorption, from colorless to almont opaque, 
which is of course in the opposite direction from that of the timrmaline, the axes 
of elasticity also in the long and narrow sections being reversed in the two minerals. 
The dark-green mica may be easily mistaken for the tourmaline. 



158 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bull. 150. 

The tourmaline and mica are idiomorpbic and must have crystallized just before 
the outer portion of the small spherulites and the tridymite and quartz in which 
they lie. They are confined to the region of these interspherulitic spaces, and are 
not found scattered indiscriminately through the compactly spherulitic portion of 
the rock. Their period of crystallization is therefore later than that in which the 
small spherulites began to crystallize and earlier than the final crystallization of 
the residual magma or paste. Their separation from the magma was preceded by 
that of quartz and orthoclase, and was also followed by the same. Their crystalli- 
zation in such a siliceous lava is abnormal, for they are locally abundant in very 
small spaces within the body of the rock, and not along a contact face of it. Tiie 
crystallization of the tourmaline at least involves the presence of a small amooni 
of boron and fluorine within the magma before its tinal •soliditication; but they 
were probably present in extremely small amounts and only locally. 

While the occurrence of the tourmaline, like that of the fayalite, may be referrt'd 
to the category of " fumarole action, '' still this is only correct when the term is ho 
defined as to include any mineralizing influence which heated vapors may have upon 
crystallization. It would thus include their primary action within fused magmas, as 
well as their secondary action on solidified ones. The effect of heated vapors which 
permeate the rocks in many places in the Yellowstone National Park is distinctly a 
destructive or metamorphosing process; and all such fumarole action is plainly 
secondary in the sense that it efieots changes in the crystalline character of rocks 
already solidified. It would seem advisable, therefore, in order to avoid confusion, 
to use some other terms for the primary mineralizing influence of absorbed vapors 
upon the crystallization of molten magmas. 

The second kind of spherulites occurring in this rock, which were described in 
the 2)aper on Obsidian Cliff under the head of porous eph^rulites (p. 278), arQ distin- 
guished by being composed of more or less distinct rays of feldspar, which are gen- 
erally branched, and a cementing material of tridymite, with numerous hollow 
spaces or gas cavities. The branching feldspars may also lie in an isotropic base,* 
which appears to he glass. The arborescent feldspar growth may form a complete 
sphere or only a plume-shaped growth, or it may even resemble the stem and 
branches of a shrub. The optical investigation of these feldspar rays shows them 
in some cases to consist of many small stout prisms of feldspar grown together with 
their longer axes parallel and forming long crooked rods in the direction of this 
axis. 

In thin section these rods or rays are partly positive and partly negative— that Is, 
the axis of elasticity, which is approximately parallel to their length, is sometimes 
less and sometimes greater than that at right angles to this direction. The prisms 
have a small extinction angle of variable size, and it is observed that the negative 
rays exhibit less double refraction than the positive ones and have a lower extinc- 
tion angle. From these characters it is evident that the rays are prisms of ortho- 
clase elongated parallel to the vertical axis c, and that the plane of the optio axes is 
normal to the plane of symmetry. In spherulites of this sort the positive and nega- 
tive rays are generally uniformly mixed throughout the whole. Such a sphernlit« 
sometimes has an outer zone or border of compact, finely fibrous structure, which is 
negative and is the same growth as the small granophyric spherulites. Tridymito 
is scattered through these spherulites, besides small grains of magnetite, and some- 
times a few grains of fayalite and a little mica. 

In other cases the branching rays are all positive. This indicates that the feld- 
spar prisms have the same development — that is, parallel to the axis c — ^but that the 
plane of the optic axes is in the plane of symmetry, which is frequently observed to 
be the case in prisms of orthoclase which have crystallized independently in the 
tridymite in other parts of the rock. 

The distinctly arborescent growths of feldspar in which the long slender rays 
branch off from a stouter stalk is shown in the figure on PI. VIII [PI. XXVII of this 
bulletin]. The prisms become thinner and more crowded together as they grow 



i»^ I>ESCRIPTION8: NO. 62, LITHOIDITE. 159 

itwftid, and temiiiiate in broad fronds like leaves. The stems are asnally twinned 
mougbont tlieir length, as are also the fronds, which are sometimes found as 
solated growtba. Tliese are twinned in the same manner, the composition plane 
iividingthe crystAl 111 two in the direction of its length. These twinned prisms of 
DTtboclMeare al\^aya negative, and the inclination to the twinning plane of the axis 
of {greatest elasticity is about 7^ or 8^. These characteristics could only be found in 
orthoclase prisma elongated parallel to the clinoaxis a, and twinned according to 
the Manebacli law, -which is the orientation already given for this form of feldspar 
in the article on Obsidian Cliff (p. 278). 
In the large porona sphernlites several forms of feldspar growths occur together. 
In one instance tbe center consists of an aggregation of partial sphernlites of small 
site and pi>8itive character. This is surrounded by a narrow zone of cloudy material, 
bot slightly doubly refracting, and with a positive character. Outside of this zone 
the porous portion of the spherulite begins. It is made up of nearly straight radi- 
ating fibers of feldspar, with weak double refraction, which are all positive. From 
points at varions distances from the center of the whole spherulite, within these 
positive fibers, there start stouter libers of feldspar with stronger double refraction 
and negative character. These branch out into radiating bunches, which unite to 
form the outer zone of the spherulite, where the fibers are partly negative and partly 
positive. In this outer zone it is observed, on closer inspection, that the negative 
feldspars form stems from which thinner positive feldspar fibers branch like the 
needles of a pine twig. These needles curve to a position parallel to the stem and 
to the radii of the sphere. All of the porous portion of the spherulite is thickly 
spotted with pellets of tridymite. The structure is very crudely represented by 
figs. 6 and 7 of PI. VII [PI. XXVI of this bulletin], the actual structure being 
extremely complex, formed as it is by innumerable delicate crystals. The first porous 
zone of weakly refracting rays of feld8p«ar, all of which are positive, must be com- 
posed of prisms elongated in the direction of the vertical axis, c, with the plane of 
the optic axes in the plane of symmetry. The branching groups of strongly refract- 
ing feldspars, which are all negative, must be prisms parallel to the clinoaxis a; 
they are twinned aecording to the Manebach law. In the outer zone these latter 
prisms send out thinner ones in the direction of the vertical axis, c, which are posi- 
tive. These thinner needles branch forward from both sides of the twinned stem; 
(onsequently the crystallization of the twinned prism must have advanced out from 
the angle 2 /? made by the c axes of the twinned halves of the crystal. 

The synchronous growth of crystals of the same mineral with two distinct habits 
i< a natural consequence of crystallographio branching as distinguished from that 
dne to the splitting or cracking of microlites.^ Its occurrence in this rock indicates 
how slight may be the difference in the conditions under which either form of crystal 
is indnoBd. This accords with the fact that we find no fixed order in which positive 
^d negative spherulitic growths have been developed. In the rock of Obsidian Cliff 
^^y alternate with each other, sometimes one having started first and sometimes 
the other. 



1 0. Lehman, Molekular Pbyaik, I^ipzig, 1888, p. 378. 



160 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [buli.150. 



The chemical composition of the Hthoidite at Obsidiau Cliff i8 showu 
by the following analysis, made by J, E. Whitfield : 

AHalysitt of lithoiditr from Obsidian Cliff, Yellowstone Xatioual Park, 



Per cent. 



SiO, ' 75.50 

; TiO,. 
AUG, 
Fe,0, 
FeO.. 
MnO. 
CaO.. 
MgO. 
LiaO . 
Na,0. 
K,0.. 
P»0,. 
SO, 
H,0 



none 

13.25 

1.02 

.91 

none 

.90 

.07 

.06 

4.76 

2.85 

none 

.32 

.41 




No. 63. LiPARITE. 

(From Pinto Peak, Eureka County, Nevada. Described by J. P. Iddixgs.) 

The rhyolite of Pinto Peak is x)art of a large body of rhyolitic lava 
that has been poured out over a limestone country, and that is accom- 
panied by breccia, tuff, and pumice of the same magma. It is mostly 
white, or light gray, and lithoidal, with abundant minute cavities, pro- 
ducing a rough fracture, and sometimes an earthy appearance. The 
rock is filled with phenocrysts of dark-colored quartz and colorless 
and white feldspars, besides minute crystals of biotite. In some cases 
minute red garnets may be seen with a pocket lens. The most con- 
spicuous constituent is quartz, whose dark-colored substance is strongly 
contrasted with the white groundmass. The quartzes are cracked, 
and seldom exhibit crystallographic forms. Feldspar, though equally 
abundant, is less noticeable because of its color. It is brilliant and 
glassy, and is partly sanidine- and partly plagioclase. Biotite occurs 
in distinct six-sided crystals. 

In thin section the rock consists of a holocrystalline groundmass and 
porphyritic crystals and angular fragments. The groundmass exhibits 
fiow-structure, which is expressed by streaks of semiopaque, faintly 
yellow material with no definite characteristics. The arrangement of 
the crystal particles presents no special connection with the flow- 
structure, except in some instances. The form of the grains in the 
groundmass is indefinite and allotriomorphic; their size varies from 
microscopic to submicroscopic proportions, so that their existence is 
inferred from their action on polarized light. The structure is then 
known as mlcrocryptocrystalline. 



ram] I>ESCRIPTI0N8: NO. 63, LIPARITE. 161 

A study of favorable spots sliovvs the groundmass to consist of micro- 
Bcopic interpenetrating grains of quartz and feldspar, through which 
are scattered larger grains, about 0.06 ""* in diameter, for the most 
part quartz; a x>ortion is orthoclase and plagioclase. There are also 
minate crystals of biotite in some cases, and a small amount of light 
red garnet in irregularly shaped grains. The latter is quite uniformly 
distributed tbrougli the mass of the rock. 

The pbenocrysts of quartz occur in well-developed dihexahedrons, 
and in angular fragments; rarely as rounded grains. They are irreg- 
ularly cracked and free from impurities, but sometimes contain bays 
of groundmass and a few colorless glass inclusions. Around the inclu- 
Bion may sometimes be seen double-refraction phenomena, which have 
been produced by internal strain. This in some cases has caused small 
cracks that pass through the center of the dihexahedral glass inclusion 
and extend short distances into the quartz crystal along three planes 
corresponding to three planes of symmetry in the crystal. Occasion- 
ally there are fluid inclusions with moving bubbles. 

The feldspar pheuocrysts are sauidine, and also plagioclase in varia- 
ble proportions. The substance of the feldspars is fresh and very free 
from impurities. Their forms are sometimes crystallographic, often 
angular and fragmental. The twinning of the feldspars is character- 
istic. Many of the sanidines are Oarlsbad twins; the plagioclase 
eihibits polysynthetic twinning according to the albite law, and some- 
times according to the pericliue law. Zonal structure is rarely observed. 
Cleavage is frequently perfect, though often wanting; conchoidal frac- 
tures occur, and the resemblance to quartz is so strong that optical tests 
are necessary to distinguish them. The sanidine often exhibits a very 
small angle between the optic axes, and the plane of the optic axes is 
sometimes normal to the plane of symmetry and sometimes parallel 
to it. 

The plagioclase feldspar is probably oligoclase, its substance is like 
that of sanidine, and the polysynthetic twins are also twinned accord- 
'^g to the Carlsbad law. Gas cavities sometimes occur, but none of 
glass. Biotite occurs in small crystals, dark brown, with small optical 
angle. Zircon occurs sparingly. Magnetite is almost entirely absent; 
occaaionally a minute crystal of it may be seen.^ 



'For farther de«cription aee Monograph XX, XJ. S. Geological Sur\'ey, Geology of the Eureka Bis- 
Wet, by Arnold Hague, pp. 237, 2«4, 374. 

Bull. 150 11 



162 



THE EDUCATIONAL SERIES OP ROCK SPECIMENS, [bull. 150. 



The chemical composition is given in the following analysis by Dr. 
Edward Hart: 

Analysis of lipariU from Pinio Peaky Nevada. 



SiO, 

TiO, 

AltOi 

FejOs 

FeO 

MnO 

MgO 

CaO 

Na,0 

K,0 

P,05 

Ignition ... 

Total 



iPer <-«ut. 



73.91 



15.29 



0.89 



0.77 
3.62 
4.79 
0.07 
1.19 



100.53 



No. 64. Nbvaditb. 

(From Chalk Mountain, Eagle County, Colorado. Described by Whitman 

Cross.) 

Geologic occurrence. — The nevadite of which No. 64 is a specimen 
forms a low mountain mass between the headwaters of the Arkansas, 
Eagle, and Tenmile rivers. The mountain lies on the continental 
divide, and has steep slopes with a cliff at the top on the south, east, 
and north, while cut into by stream branches on the west. The nearly 
level summit lies at an elevation of a little less than 12,000 feet on the 
eastern side of the mass. The mountain derives its name from the 
white color of the cliffs. 

All about the mass, which is 2 miles in longest diameter, Carbon- 
iferous sandstones and conglomerates of the Weber formation are 
exposed, but the contacts with the nevadite are so much obscured by 
talus slopes that the jrelation of the sedimentary to the igneous rock 
can not be fully made out. From the structure of the nevadite it is 
inferred that the mass seen must*have consolidated very near the sur- 
face. The body is interpreted a« an irregular intrusive sheet cutting 
obliquely across the Carboniferous strata, and it is assumed that parts 
of the magma may have reached the surface and formed lava streams. 

Definition. — The term nevadite is here applied, as defined by Hague 
and Iddings,^ to a rhyolite " characterized by an abundance of porphy- 
ritic crystals embedded in a relatively small amount of groundmass.'' 

General description. — This rock has many phenocrysts of sauidine, 
oligoclase, smoky quartz, and a few of biotite, embedded in a light- 
gray groundmass which a hand lens shows to be evenly granular and 
specked by numerous small biotite leaves and a dust of magnetite. 



> Am. Jour. Sci, 3d eeries, Vol. XXVII, 1884, p. 461. 



ma.] I>ESCRIPTIONS: NO. 64, NEVADITE. 163 

Twosomew\iat difterent modifications are represented in the collection. 
One has large Bauidine crystals, reaching a maximam length of 3^*^', 
which usually bIiow a delicate satiny luster parallel to a steep ortho- 

dome, apparently — P oo, (15.0.2). In the other form of the rock the 

sanidines are smaller and clearer with faint Inster, if any. Both yarie> 
ties have numerous smoky quartz crystals, generally much fractured, 
and showing prismatic and pyramidal faces. Oligoclase is present in 
many small phenocrysts, and these are often included in the larger 
sanidine crystals. The mica is a very subordinate element of the rock. 
The phenocrysts, — ^The especially noteworthy mineral of the rock is 
sanidine. Its crystals are fresh and glassy, usually developed as thick 
tabletd parallel to the cliuopinacoid, with prism and two or three 
orthodomes as the terminal planes. Carlsbad twinning is common. 
The delicate satiny luster referred to is very beautifully developed in 
many crystals. The luster is due to films of air between the thin 
plates of a very perfect parting which is not exactly like a cleavage. 
When actual separation of these plates has not occurred there is some- 
times an exquisite blue color to be seen parallel to the steep orthodome. 
This color is most brilliant in the small sanidines of certain rhyolites, 
where no cleavage parallel to the dome in question can be detected. 

The numerous small crystals of oligoclase in this nevadite are quan- 
titatively much less than the sanidines. They are frequently included 
in the latter. 

Quartz occurs very abundantly in more or less rounded or resorbed 
crystals. The material is very pure, free from inclusions of other min- 
erals or of glass, and from penetrating arms of the groundmass. Gas 
pores of very minute size are numerous, and also fluid intrusions with 
relatively little fluid in them. 

Biotite is sparingly developed in rudely hexagonal leaves, containing 
many included grains of magnetite. 

The groundmass. — Quartz and feldspar in very even-grained mixture, 
with flakes of biotite, constitute the groundmass. Quartz is very often 
developed in double pyramids, leaving the feldspar to occur in irregular 
grains. But little triclinic feldspar can be detected. There are some- 
times small gas pores between groundmass grains, and in some sections 
tbere appears to be a very small amount of clear amorphous substance 
in angular spaces between the quartz and feldspar particles of the 
gfoondmafls. This is never sufficiently developed to form a basis, and 
vhile it seems to be residual glass its nature is not clearly determinable. 
It is identical in appearance and development with what has been 
determined as glass in the rhyolite of the Hohenburg, near Berkum on 
the Bhine. 

OccasionaUy there are drusy cavities in this nevadite which contain 
many thin sanidine tablets and, rarely, a few stout crystals of quartz 
ftnd topaz. This occurrence of topaz was the first to be described as 



164 



THE EDUCATIONAL SERIES OF BOOK SPECIMENS, [mru^in. 



primary in igneoas rocks. Topaz is now known in lithophysal cavitiefl 
of many rhyolites, and the development of these drusy cavities in the 
Chalk Moantain uevadite, with the occurrence of topaz in them, is one 
of the principal indications that the rock consolidated near the earth's 
sarface. 

Chemical composition, — A specimen of the finer-grained rock of Chalk 
Mountain was subjected to quantitative analysis by W. F. Hillebrand 
with the following result, under I. The clear sanidine phenocrysts 
have the composition given under II. 

Analyses of fievadite and sanidine from Chalk Mountain, Colorttdo, 



SiO, . 

A1,0, 

IV), 

FoC. 

MnO, 

CaO . 

MgO. 

KjO . 

Na^O 

Li^O. 

H,0 . 

P,0.. 



Total 



I. 


TT. 


Percent 


Percent. 


74.45 


65.04 


14.72 


2a 40 


none 
0.56 
0.28 
0.88 








0.7» 


0.37 




4.53 


0.74 


1 3.97^ 
trace/ 


4.11 


0.66 

1 


.29 


O.Ol 




* 




100.38 


100.37 



SiHJcific gravity of rock, 2.61 at 32° C 

There probably should have been a very small amount of F^Oj 
found, but the rock analyzed was nearly free from magnetite, and 
contained little biotite. MnO^ is present as a stain of pyrolusite in 
patches through the rock. It is clear that the rock has a large excess 
of silica as quartz and that much of the soda is present in the sanidine. 

The following publications by the writer refer to the nevadite of 
Chalk Mountain : 

Bull. U. S. Geol. Survey, No. 20, Contribntions to the Mineralogy of the Rocky 
Mountains, pp. 75-80. On the luster exhibited by sanidine in certain rhyolites. 

Ibid., pp. 81-82. An unusual occurrence of topaz. 

Mon. U. S. Geol. Survey, Vol. XII, Geology and Mining Industry of Leadville, 
Colorado^ by S. F. Emmons. -In Appendix I, Petrography, pp. 345-349. 

No. 65. Metabhyolite. 

(From Utley, Green Lake County, Wisconsin. Described bt S. Wbidman.O 

The Utley metarbyolite (qaartz-keratophyre) is of preCambrian age, 
probably Keweenawan, and occurs as a single knob in unoonformable 



1 Adapted from an article on the Utley metarhyolite: Boll. No. S, Q«oL and Nat. Hiat. Smrvyef 
Wiaoonain. 



MUM-l DESCRIPTIONS: KO. 65, METARHYOLITE. 165 

contact with lower Ordoviciau Bandstooe and limestone. It is intruded 
by a few small dikes of diabase of pre-Cambrian age. 

The normal rock of this area has a dark — almost black — groundmass, 
in which are embedded nnmeroas phenocrysts of feldspar and quartz. 
The rock is very compact and brittle, and breaks easily, with a con- 
choidal fracture. 

The phenocrysts of feldspar, which are more numerous than those 
of quartz, are grayish white in the fresh specimens, but are somewhat 
reddish on the weathered surface, owing to alteration. They vary in 
size from 1"™ to 5"" in diameter, and sometimes in certain coarse spots 
^ey attain a thickness of 8"™ and even 10°'™. Ttiese feldspar crystals 
are tridinic, as is indicated by their well-develop d cleavage, the angle 
between the two most important cleavage faces Ueing about 93^ 32'^ 

The quartz crystals, as seen in the band specimen, have the char- 
acteristic limpid appearance, and usually possess good crystal forms, 
bat are sometimes rounded. They vary in size and are approximately 
equidimensional with the feldspar phenocrysts with which they are 
associated. 

The dark-colored groundmass is megascopically very dense and 
apbanitic and always breaks along curved planes. Thick masses of 
Qodoles or spheroids, produced by spheroidal parting, which pheno- 
menou is brought out clearly on the weathered surface, form an inter- 
esting feature of this rhyolite outcrop. The spheroids vary in size 
from i inch to 1^ inches in diameter, and do not differ in composition 
or texture from the normal rock in which they occur. They have sep- 
arated out as nodules by weathering or altering along the spheroidal 
partings. These partings are produced by contraction during the 
original cooling of the magma. 

Under the crossed nicols of the microscope and with low power the 
feldspar crystals of thin sections of the rock always have a mottled 
appearance. With a higher power, this mottled appearance is seen to 
be due to a close intergrowth of two varieties of feldspar, producing 
what is termed a microperthitic feldspar or microperthite.^ Besides this 
KBieroperthite there are a few phenocrysts of x)olysynthetically twinned 
pbgioclase, probably near the albite end of the series. There is a defi- 
Ait6 relation between this twinned plagioclase and the microperthite 
which will be referred to again. 

''Thetenn microperthite^ which is asnally meant to include an intergrowth of orthochtse with albite 
(^oUieraeid plagioclftae, ia here extended and meant to Include intergrowtba within the plagioclase 
"•••••weU. 



166 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bi 



The chemical compoBition of this feldspar, as detennined by 
Tolman, is as follows : 

Analyeia of feldspar of the Utley metarhyolite. 



SiO, . 
AlgO, 
FeO .. 
CaO .. 
K,0. 
Na,0. 



ToUd 



Per cent. 


62.57 


18.12 


1.32 


5.52 


5.11 


7.20 


99.84 



The microperthite is distinctly triclinic, as shown by it« clei 
system. Its chemical composition and optical properties are such 
place it entirely within the plagioclase series, the intergrown plagi( 
species ranging from albite to andesine. 

The microperthite phenocrysts are usually composed of intergr< 
of two varieties or species of feldspar. One species that is a 
present is albite^ having an extinction angle of + 19° on the br 
pinacoid (oo Pdb). Another species that is usually if not always pi 
is a variety of oligoclaseandesine, having an extinction angle of 
+ 50 to 70 on the same face (oo P ob). In one case the micropei 
proved to be albite and andesine, the latter having an extinction 
of - 30 on 00 P3b. 

The nature of the microperthite is that of a lamellar intergro^ 
albite with a more basic plagioclase, each species tending to have s 
nite crystal form with vertical axes in a common direction. It 
regular and invariable arrangement of its component feldspars, ui 
as in a few cases, it has been interfered with by excessive prej 
The appearance of the microperthite in a section cut parallel t 
brachypinacoid (00 P dc) is shown in Fig. B of PI, XXVIII. 

The microperthite is wholly secondary — ^that is, its developmei 
processes of metamorphism has been long subsequent to the ori 
crystallization and solidification of the magma. This process of cl 
in the upbuilding of the microperthite may be said to have been a< 
plished by three somewhat different, yet rehited, methods. 

First, by outgrowths on older feldspar. Growth of this kind is s 
in Fig. Ay PI. XXVIII, photographed in polarized light. The 
shell of microperthite is seen to inclose a twinned plagioclase of p< 
crystal form. The outer boundary of the enlargement is some 
irregular. The inner crystal contains fractures which do not e: 
into the microperthite. The outer zone of microperthite is fresh, 
the inner core shows much decomposition. The relation of the b 
perthite to the twinned plagioclase, as seen in this photograph, g 
that the latter was fractured and considerably altered before the £ 
tion of the rim of microperthite. 



IHLL1R.1 DESCRIPTIONS: NO. 65, METARHYOLITE. 167 

Second, by regeneration of older plagioclase. In this process the 
perthitic intergrowths of albite and oligoclase-andesine are products 
of tbe recrystsdlization of the older twinned plagioclase. This process 
begins along the enter boundaries and fractures of the older crystal 
and very probably takes place along the easy solution planes of the 
latter. The c axes of the perthitic growths are, except in a few cases, 
parallel to the c axes of the original crystal. This relation is shown in 
Fig. Bj PL XXVIII. This process, to a small extent, has gone on in 
the crystal shown in the photograph Fig. A, PL XXVIII. In the 
lower left-hand part of the inner crystal core, above the dark line sepa- 
rating the original from the secondary feldspar, is a small area of perth- 
ite forming out of the older crystal. Numerous other cases could be 
cited' where the alteration of the twinned x>lagioclase to microperthite 
has been brought about. The unperthitized feldspar is always in an 
advanced stage of alteration, while the microperthite is always fresh. 
Third, by new growth from the groundmass. This process has devel- 
oped small irregular crystals. These irregular crystals of microperthite 
have a medial suture from which extend normally radiating fibers of 
microperthite. 

The«e processes of change have very probably been aided by pres- 
sure. In some cases cross twinning after the pericline law has been 
indaced in the original crystal by pressure. In such cases the later 
growth of perthite seems to follow along the composition faces of the 
pericline twins. When the latter phenomenon occurs, the laths of 
perthite have their longer direction almost normal to the c axis of the 
older crystal. 

The quartz phenocrysts have a fairly perfect crystal form, but are 
sometimejj rounded. They are often corroded and have embayments, 
and, Kke some of the feldspar crystals, show the effect of a moving 
inagma by being cracked and broken, the dismembered parts being 
separated by films of groundmass. They often contain rounded and 
rhofflhic-shaped areas of included groundmass. Ehombic cleavage is 
developed in some of the crystals. 

Biotite, although not apparent in the hand specimen, is seen to be 
nther abundant in the thin section. It occurs as minute, short, tabu- 
^ crystals, to some extent scattered throughout the groundmass, but 
alik) seems to be congregated in little patches and in zones about the 
f^^ldspar crystals, and in the fractures as vein material. It is probably 
wholly secondary. 

Hornblende was once present in this rock, but has been completely 
altered. The fact that it was once present is Indicated by angular 
areas having the typical outlines of basal sectionsof this mineral. The 
hornblende is replaced by other minerals, such as magnetite, calcite, 
epidote, biotite, sphene, apatite, garnet, and quartz. These minerals 
also occur scattered throughout the groundmass. Minute fiakes of 
brown hornblende, of secondary origin, occur to some extent in the 
rock. 

'A more comprehcnMv© disciiMion of the origin and naturu of the microperlMlo \b g;W<m\ti Wie 
(ttper cited on pa^e 1^- 



EXPLANATION OF PLATE XXVIIL 

SECTIONS OP METARHYOLITE. 

Fig. A, Metarhyolite, Utley, Wisconsin ; slide 4379, University of Wisconsin Col- 
lection : Secondary growth of microperthite aboat plagioclase. In polarized light, 
X 30. Twinned plagioclase core with crystal form, fractured and containing many 
secondary inclusions of biotite in the fractures, and what is probably sericite distrib- 
uted through the plagioclase. The fractures and decomposition products are con- 
fined to the core. The outer rim is made up of short laths and wedges of feldspar 
lying in a parallel direction, giving a distinct difference in appearance to the 
ordinary twinned feldspar of the core. The secondary enlargement has been inter- 
fered with in its development by the small phenocryst above and to the right. In 
the lower left-hand corner of the older plagioclase crystal, above the boundary 
liueof core and rim, is a patch of microperthite, which is similar to that of the nm. 
The inner patch is a product of the regeneration of the twinned plagioclase; the 
surrounding rim is a development from the gronndmass. The position of the frac- 
tures and decomposition products within the plagioclase core establishes the earlier 
origin and advanced metamorphism of the plagioclase previous to the development 
of the microperthite. 

Fig. B, Metarhyolite, Utley, Wisconsin; slide 3846, University of Wisconsin Col- 
lection: Phenocryst of the microperthite in polarized light, X 30. Section cut 
parallel to ao P ob on the M face, showing basal cleavage. A bisectrix nearly nor- 
mal. The angle of extinction of dark part is from -f 3^ to -f 6^; of the light part 
about -f- 19°- "^^^ large amount of lime iu the feldspar coupled with the lower 
angle of extinction of the dark part indicates that the latter is oligoclase with a 
composition of Ab^Aui. The light part is nlbite. The microperthite is secondary, 
and the two varieties of feldspar are developed with their o axes parallel to the e 
axis of the original feldspar. 
168 




SECTIONS OF METAHHYOLITE FROM WTLEY. WISCONSIN, 
B. Fhtnocrytt of m'eioptnhrtt , in paimftied hgM, X 30. 



H •« 



DIUIft.1 



DE8CKIPTION8: NO. 65, METARHYOLITE. 



169 



The groundmass is composed of microscopic grains of interlocked 
quartz and feldspar, intermingled with which are minute opaque specks 
and grains of iron oxide. To this iron oxide and other dark-colored 
microscopic minerals is dae the blackish color of the groundmass as 
seen in the hand specimen. These small dark-colored minerals are 
arranged in sinuons lines and curve about the porphyritic crystals, 
producing the common fluxion texture characteristic of lava flows. 
There is also developed in some specimens a radial spherulitic texture 
of the gronndmass, and in other specimens the mottled pcecilitic tex- 
ture. The gronndmass was once partly and probably wholly glassy, 
and is now devitrified. This devitrification has been accomplished, in 
great part at least, by secondary processes since the original solidifica- 
tion of the magma. 

In the thin sections of the rock from the nodular or spheroidal beds 
occur abnndant examples of perlitic parting in the groundmass. The 
perlitic parting assumes the shape of curved fractures sometimes 
approaching circles, bnt more often only parts of circles or of other 
carvilinear forms. From these minute curved cracks there extend, with 
their longer axes perpendicular to the parting, small crystals of feldspar 
or of feldspar and quartz, giving the apx>earance of a radial or sphern- 
litic crystallization along the fracture. In the circular and curvilinear 
forms of perlitic parting, the radii of feldspar appear as zones sur- 
roanding an inner cryptocrystalline groundmass which is like that out- 
side of the zone. This phenomenon is in close similarity to the larger 
Btmctare of the spheroids of the hand specimens and indicates a common 
ori^Q of the two structures*. Throughout the area, as shown in both 
the hand specimen and thin sections, are numerous fractures that are 
filled with secondary quartz and feldspar. The vein material in the 
fracture of the feldspar is very often feldspar, while that in the ground- 
mass and in the quartz crystals is usually quartz. 

The chemical composition of the rock is as follows: 

Analysis of metarhyolite from Utley, Wisconsin, 



SiO, 

A1,0, 

Fe,Oa 

PeO 

MnO 

CaO 

MgO 

Na,0 

K/) 

H,0 

Total 



Per cent. 



} 



73.09 
13.43 

2.67 

trace 
2.29 
1.03 
3.85 
1.58 
.76 



98.60 



170 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [ami. 150. 



• 



Besides the constituents enumerated in tbe analysis, there are also 
present in the rock very small amounts of OO2, Zr02, Ti02, and P2O5, 
as indicated by the microscopic examination. The relatively large 
amount of lime and soda present explains the plagioclase composition 
of the microperthite. The proportionately large amount of soda would 
class this rock as a soda -quartz -porphyry or quartz -keratophyre. 
Applying the nomenclature of the more recent volcanic rocks to those 
of pre-Gambrian age, the Utley rock is in composition and texture a 
typical rhyolite, and in consideration of its altered condition may be 
called a metarhyolite. 

Ko. 66. Granite. 

(From Rocklin, Placer County, California. Described by Waldsmar 

LiNDGRSN.) 

The granitic area of Bocklin forms an irregular rounded mass about 
30 miles long and 20 miles wide, inclosed towards the north, east, and 
south by the auriferous slates of the Sierra Kevada. Westward it is 
bordered by the alluvial plains of the Sacramento Valley. 

As a rule, the rock in this area contains only black mica and horn- 
blende, sometimes, indeed, only hornblende together with quartz and 
feldspar, and is more closely allied to a diorite than to a granite. 
At Eocklin, which is a railroad station on the line of the Central 
Pacific, with an elevation of about 200 feet, the rock assumes a more 
granitic character and contains both black and white mica. It is some- 
what finer grained than in other parts of the area and makes an excel- 
lent building stone. The extensive quarries at Rocklin supply a large 
proportion of the granite used for building purposes in San Francisco. 

The rock is of light-gray color and of medium fine grain. Grayish 
quartz grains, white feldspars sometimes with striation on the cleavage 
planes, black or dark-brown biotite, and silvery muscovite in small 
scales may readily be distinguished by the naked eye. 

Under the microscope the following primary constituents may be 
discerned: Magnetite, apatite, and zircon are the oldest minerals 
occurring as minute inclusions in the later consolidated grains; then 
come, in order of succession, biotite, muscovite, plagioclase, orthoclase, 
and quartz. 

The quartz, being the last consolidated mineral, forms irregular 
grains, the outlines of which are determined by the older constitnentsi, 
although there are indications that the consolidation of orthoclase and 
quartz in part went on simultaneously. It is clear and fresh, being, of 
course, much less subject to decomposition than the other constituents. 
It often shows notable undulous extinction. Fluid inclusions with 
moving bubbles are common, as usual, in the quartz of the granites. 
Small needles and foils of biotite and muscovite occasionally abound; 
while there is a little apatite in more or less slender prisms and small 



DnuE-l descriptions: NO. 66, GRANITE. 171 

colorless crystals of zircon, recognizable by their strong doable 
refraction. 

The orthoclase occurs, as a rule, in clear, irregular grains, together 
with the quartz, between the more or less idiomorphic plagioclase crys- 
tals. More rarely it shows rough crystallographic outlines. Its period 
of consolidation is nearly identical with that of the quartz, and grains 
showing a micropegmatitic intergrowth of the two minerals are not 
aucommon. 

Many orthoclase grains are filled with minute quartz grains, all 
extinguishing together. Undulous and zonal extinctions occur fre- 
qaently. Fluid inclusions are not common. The orthoclase is less 
subject to decomposition than the plagoclase, to be described presently, 
but the alteration to white, fine, micaceous aggregates may often be 
observed, proceeding along the cleavage lines. As infiltrations from 
neighboring minerals chloritic aggregates and calcite may be noted. 

The plagioclase is found to be more abundant than in a perfectly 
normal granite. In many cases there is considerably more plagioclase 
than orthoclase. The plagioclase is a very acid soda-lime feldspar, 
belongiDg to the series of the oligoclases; in only one instance was a 
grain of microcline noticed. Ko albite appears to be present. Twin 
striation narrow, not infrequently absent, or shown by very few poly- 
synthetic lamelhe. Double refraction somewhat stronger than the 
orthoclase. Zonal structure exceedingly common. The plagioclase is, 
iu contrast to the orthoclase, more or less idiomorphic, showing short 
prismatic or lathlike forms, without well defined terminal faces. Decom- 
position has, on the whole, made greater headway than in the orthoclase, 
and proceeds along the cleavage line and also sometimes concentrically 
in crystals with zonal structure. As it begins the crystals grow cloudy 
and milky, by reason of minute, irregular air, more seldom fluid, inclu- 
sions. Small scales of hematite and other indeterminable microlites 
&Te noted in the clouded mass. The final products of decomposition 
are fine aggregates of white mica, calcite, and, where a migration of 
inagnesian solutions from adjoining minerals has taken place, single 
foils, and aggregates of chlorite and epidote. 

The muscovite occurs in large and small straight foils, showing the 
nsnal characteristics of the mitferal — strong double refraction, white 
color with notable absorption, etc. Idiomorphic foils of biotite may 
^metimes be noticed embedded in a larger foil of muscovite. The 
BJuscovite does not decompose readily. 

The biotite is rather more abundant than the muscovite; it Is from 
yellowish to greenish brown, has very strong absorption, and occurs as 
^regularly bordered straight foils. Inclusions are not abundant; apa- 
^te, magetite, and zircon are occasionally noted. 

The biotite is decomposed to a considerable extent, the resulting 
ininerals bein^ a green chlorite and a yellowish-green pleochroic epi- 
dote. In a brown biotite foil green chloritic lamella) are sometimes 



172 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bcllUO. 



intercalated, showing the way in which a complete psendomorph of 
chlorite after biotite may take place. 

The chlorite is deep green, pleochroic, and shows the nsoal deep bine 
or parple colors of interference. 

The stnictare of the rock is characterized as hypidiomorphic grann- 
lar, in contrast to the strnctnre designated as allotriomorphio granular. 
In the latter all of the constituents are irregular grains, while in the 
former one or more of them show crystallographic outlines. In this 
case the more or less lathlike oligoclase crystals, together with the 
micas, lie embedded in the clear, irregular grains of the orthoclase and 
quartz. 

The chemical composition is shown by the appended analysis, made 
by W. H. Melville in the chemical laboratory of the United States 
Geological Survey : 

Analysis of gi'anitefrom Bocklin, Placer County , California, 



Per cent. 



SiO, 

AliOg 

Fe,0, 

FeO 

CaO 

MgO 

K,0 

Na,0 

Tuition ... 

Total 



73 

16.38 

none 

.90 

2.42 

.48 

1.87 

4.53 

.62 



100.19 



From this and from a study of the thin section it is apparent that 
the rock has the composition of a normal granite, except that sodium 
is present in unusually large quantity and that a considerable part of 
the orthoclase is replaced by oligoclase. 



No. 67. BlOTITEGRANITE. 

(From Platte Cakyon, Jefferson County, Colorado. Lkscribed by 

£. B. Mathews.) 

The granites from the Platte Canyon are but a portion of the large 
granitic area which extends along the whole length of the Colorado or 
Front Kange. In the area described in the Pikes Peak folio of the 
United States Geologic Atlas a large portion of the district is occupied 
by a granitic complex which has been found to consist of several varie- 
ties of granites that can be grouped under four heads,^ of which the 
Pikes Peak type is the most prominent. To this type belong the rocks 
of this series. The fresh unaltered granites are coarse grained aggre- 



iThe innnites of Pikes Peak, Colorado, by Edward B. Mathews: BnU. G«ol. Soc. Am^^yi^.^ y^ 
VI 1804-95, pp. 471-473. 



BIME.1 DEgCRIFTIOKS: NO. 67, BIOTITE-GBAKITE. 173 

gates of quartz, perthitio Mdspar, and biotite with an oocasional flno- 
rite megascopically developed. The microscope also reveals apatite, 
siicon, ms^netite, rutile ( t), hematite, limonite, and epidote. 

The rocks are generally of a pinkish tone, which becomes more accen- 
tuated daring incipient alteration and fades away when the rocks 
kyenndergone considerable metamorphisin. The grain of the granite 
varies widely from a case in which the feldspar phenocrysts are 6 
inches in length to the more normal grain in which the length of the 
fe)d8|>ar individuals is not more than one-fourth of an inch. In the 
spedmens belonging to this series the usual dimensions are one-half 
an inch for the feldspar and one- fourth to one-eighth of an inch for the 
qoaitz. The biotite areas are generally smaller than the quartz indi- 
viduals, although sometimes they may be one-half an inch in width. 
The texture of the Pikes Peak type varies from an aggregate of min- 
mils in which the feldspar is only slightly larger than the quartz to one 
in which the feldspar stands out in large imperfectly formed porphy* 
ritic crystals. The mass of the rock is composed of three or four mineral 
species, of which microcline is by &r the most important^ 

The quartz is present in large, irregularly rounded grains, which are 
distinctly outlined against the feldspathic and micaceous constituents. 
Toward the feldspars the quartz appears normally younger, although its 
contact with the larger porphyritic microcline sometimes indicates that 
the qnartz had already consolidated before the formation of the pheno- 
erysta. In thin sections the quartz is seen to occur either as primary 
individual grains, or in pegmatitic intergrowths with the feldspar, or 
as secondary grains filling the interstices and cracks which were formed 
Bobseqaent to the consolidation of the rock. The largest fragments of 
the quartz may be one-fourth of an inch in diameter or the individual 
grains may scarcely be distinguished with the highest power of the 
mioosoope. Within the quartz are included numerous fluid and 
individaalized interpositions. They are arranged in long bands or are 
disseminated irregularly throughout the host. Sometimes there are 
long hair-like inclusions whose mineral characteristics can not be spe- 
cifically determined. These are the well-known << quartz needles," 
which have been considered by various authors as rutile, apatite, tour- 
naline, hornblende, or sillimanite. In a few instances their nature has 
Undetermined, but in the granites of this series the filaments are too 
fine to permit such specific determination. 

'A mechanical separation of 20 grams of the powered rook shows the constitaents to be present in 
^ fiAowiag proportion hy weight : 

« Per cent. 

Microcline 63.33 

<hisrtB 83.41 

Birtite 10.71 

OUjwslaao 2.55 

Under the term ** qnartz*' are included those fine filaments of olblte which were separated from 
^ nkfoeliae hi the eraahtaig, while in "biottte" are included all of those minerals which were not 
A^Med in a Thoolet eolntion when at its maximum density of 3.L 



174 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, tBmx.150. 

Microcline constitutes more than one-half of the rock mass, and in 
all instances shows the characteristic microcline twinning. The color 
of this mineral is the principal source of the pink tone noticed in the 
rock specimens, and is usually due to the presence of minute hematite 
flakes evenly disseminated through the feldspar, or to iron staining 
along the cracks. The size of the feldspar individuals varies widely, 
but in the rocks of this collection it may be large or small, since the 
microcline usually occurs both in phenocrysts and as a constituent of 
the groundmass. The phenocrysts may occur in rudely automorphic 
crystals, showing poorly defined pinacoidal and prismatic faces, which 
give to the mineral a prismatic habit in which the elongation is parallel 
to the clino-axis. In the groundmass the feldspar individuals are small, 
somewhat larger than the quartz and without crystal form, occurriDg 
only as irregalarly outlined grains or anhedrons. 

The microcline of these rocks may show twinning in addition to that 
according to the albite law which produces the so-called microcline 
structure. Such twinniug may be according to the Carlsbad law, in 
which the twinning plane is the orthopinaciod, or more commonly the 
twinning plane may be the basal pinacoid, according to the Manebach 
law. Throughout these potassium feldspars are numerous fine disks, 
lenses, or stungers of a plagioclastic feldspar, probably albite, in per- 
thitic intergrowths with the host. These fine albite stringers occur io 
the basal pinacoid normal to the clinopinacoidal cleavage and on the 
clinopinacoid they show an inclination of about 73^ in obtuse /?; accord- 
ingly these albite lamellsB lie approximately parallel to a steep positive 
orthodome of the value of 8 P 5c. In certain instances the albite occurs 
in small rounded disks, which oriented sections show are not cross 
sections of spindles, but of more or less spherical bodies. 

Micropegmatitic intergrowths between the quartz and the feldspars 
are not common in these rocks, although they are sometimes present in 
minute branching or arborescent areas. A more extensive study of 
the rocks of the surrounding country shows that micropegmatite is not 
common in the granites of this class (Pikes Peak type), but when pres- 
ent is probably the result of alteration of the original constituents. 

Oligoclase occurs in a few instances, but never constitutes any con- 
siderable portion of the rock mass. It is usually easily separated from 
the other constituents by the fine lamellae twinned according to the 
albite law. The fineness of the lamellae and the low extinction of one 
or two degrees clearly indicates the oligoclase nature. Within the 
micrbclines are also minute plagioclase disks which may or may not be 
twinned. These disks show the efiect of marked alteration and are 
clouded by the alteration products. The altered disks are separated 
from the microcline host by a rim of clear feldspar which is in physical 
continuity with the inner core. At times the twinning of the inner 
core is continued in the rim either directly or alternately, as in the Eoc 
Tourne twins described by Gustav Eose.^ 



Pogg. Ann., 1865. 



DiLLDul DESCRIPTIONS: NO. 67, BIOTITE-GRANITE. 175 

The cause of the altered plagioclase with sarrounding clear rims is 
not definitely known. The difference between the two parts may be 
the result of original differences of crystallization, the inclusion being 
more basic, more porous, or filled with interpositions; or the rim may 
be entirely secondary, a product of the alteration which the granites 
have undergone. 

Biotite is the third in importance of the constituents of the granites, 
and is present either in fine shreds closely aggregated or in single, 
somewhat larger, individual flakes. This mica shows a deep-brown 
color, metallic luster, and a strong pleochroism. The absorption is 
very marked for the ray vibrating parallel to the basal cleavage where 
the plate becomes practically opaque; the ray vibrating at right angles 
to the cleavage is yellow — sometimes even straw-yellow. The plane 
of the optic axes apparently lies normal to the leading ray of the per- 
cussion figure, and is therefore of the first sort — anomite. This deter- 
mination seems to be corroborated by the peculiar brown color, the 
small optic angle, and the brittleness of the perfectly fresh cleavage 
flakes. 

Flaorite is present in many sections of this series where it occurs in 
small irregular grains, and rarely in well-defined crystals, which sug- 
gest either cubes or octahedrons. The mineral is specially character- 
ized by a highly perfect octahedral cleavage, which is well developed 
in the larger areas, but is lacking in the minute crystals. The little 
anhedrons are clear, and may be colorless, purple, faintly pink, or 
green, and when the grains are colored the pigment is unevenly dis- 
seminated through them, and seems to be more intense about inclusions 
than in the clearer portions of the mineral. Between crossed nicols the 
areas show complete isotropism and no anomalous double refraction. 
The index of refraction of the fluorite In these rocks appears excep- 
tionally high, even as high as that for garnets. The cause of the 
anomaly is not apparent, but that the mineral is fluorite, and is not a 
garnet, is shown by the fluorine content in the bulk analysis, where the 
flnorine sometimes reaches 1 per cent. 

Zircon, apatite, and epidote are sometimes present in the sections, 
"^ey are usually in minute, more or less prismatic crystals, colorless 
or faintly yellow, that may be distinguished by the low double refrac- 
tion of the apatite, the high index, double refraction, and lack of 
cleavage of the zircon, and the irregularity of outline and cracked or 
shagreened ux)per surface of the epidote. 

The order of crystallization of the minerals in the rocks seems to have 
been as follows: The magnetite when present formed first, and was 

* 

immediately followed by the zircon and the apatite. Biotite formed 
somewhat later, including all of the minerals then existing in the 
magma. Probably the separation of the biotite was followed by the 
ciystallization of a small amount of plagioclase and a much larger 
amount of microcline. The microcline of the groundmass is clearly 
older than the quartz, but in the larger phenocrysts of microcline the 



176 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bull. 150. 

relative age of the quartz and feldspar is different, and it is very prob- 
able that the large phenocrysts of microcline succeeded the separation 
of the quartz and were the last of the original minerals to crystallize 
from the magma. The age relationships of the epidote and fluorite are 
unknown. The facts indicate that the epidote is a product of moderate 
alteration of the feldspar, while the fluorite is closely associated with 
the biotite or irregularly disseminated throughout the rock mass. 
When the former is associated with the biotite, tbe mica is clear and 
unaltered, and appears younger than the fluorite, and does not suggest 
that the fluorite is in anywise formed at the expense of a fluorine- 
bearing mica. On the other hand, the fluorite is not clearly a miarolitic 
fllling, but appears instead to be one of the older constituents of the 
rock, perhaps of the same age as the apatite and zircon. 

The granites from the Platte Oanyon very often show evidences of 
alteration as the result of dynamic metamorphism. The extinction of 
the quartz becomes undulatory or mottled, and very frequently the 
quartz has been crushed into a mosaic in which individual fragments 
are drawn out in a common direction, producing the well-known 
" striefenquarz." 

The feldspars have been deformed by a rounding of their angles and 
a drawing out or stretching of their material into long bands or flat 
ellipsoids. This deformation of the feldspars changes the well-formed 
phenocrysts into large lenses or eyes, so that the granite passes from a 
massive rock into a schistose augen-gneiss. Few, if any, new minerals 
are formed by the deformation of the original constituents which differ 
in specific character from the original minerals. The rocks when 
crushed show secondary quartz, microcline, and biotite, with rarely 
any other additional minerals, unless occasionally muscovite, which is 
formed through the bleaching of the biotite and the decomposition of 
the feldspars. 

The granite under discussion is one relatively rich in silica, sodium, 
and fluorine, and poor in calcium, iron, and magnesium. The following 
analysis, by W. F. Hillebrand, is that of a granite from the upper slope 
of Pikes Peak, closely related to the granite of the Platte Canyon in 
structure, and in mineralogical and chemical composition. 



van.] DESCRIPTIONS: N08. 67, 68, BIOTITB-OBANITE. 



177 



Aualy9iB of kiotiU'granite from Pikea Peak, Colorado, 



SiO, 

TiO, 

AlA 

FM), 

FoO 

MnO 

CaO 

BaO 

MgO 

KjO 

No,0 

Li,0 , 

H«0 below 110° C 
H^ above IICP C 

P«CH 

F 

CO, 



Per cent. 



Total . 
LoM O for F 



Total 



77.03 
.13 

12.00 
.76 
.86 

trace 
.80 

trace 

.04 

4.92 

3.21 

str. trace 

.14 

.30 

trace 
.36 



100.55 
.15 



100.40 



The practical absence of phosphoric acid indicates that only a small 
proportion of the fluorine is needed for the apatite seen in the rocks, 
even though the apatite is a pure fluor-apatite. The remaining fluorine 
is sufficient to combine with most of the calcium and still be present in 
the biotite. The calcium occurring as a calcium-fluoride leaves little 
which may enter into the comx)osition of the feldspar, and consequently 
theoligoclase must be very near the albite limit. The strong pleoch- 
loism and the deep color of the mica suggest that almost all of the iron 
is used up in the relatively slight development of biotite. The percent- 
Age of alumina indicates that much of this is in the feldspar, and that 
only a small amount is in combination in the mica. After all of the 
eiemeuts are satisfied the silica is considerably in excess, as is evident 
from the abundance of quartz. 

From the texture and from the mineralogical and chemical com- 
positionit is evident that the granite from the Platte Canyon is a some- 
what porphyritic example of granitite (Bosenbusch), biotite-granite 
(Zirkel), and true granite (Michel L6vy). 

No. ^. HOBNBLENDE-BKABINa BlOTITEGBANITE. 

(From Fox Island, Maine. Described by J. P. Iddinos.) 

This rock occurs on Yinal Haven Island, one of the Fox Islands, in 
Penobscot Bay, Maine, where, according to George O. Smith,^ it occu- 
pies a roughly circular area nearly 5 miles in diameter. 

'Stt Geological Map of the Fox Islands, PL U of the Geology of the Fox Islands, Maine, by George 
Otiiaaiith,lB98. 

Bull. 150 12 



178 THE EDUCATIONAL SERIES OP ROCK SPECIMENS, (bull 150. 

It is medium grained and light colored, and consists of pinkish-gray 
feldspars and about an equal amount of brownish quartz, besides a 
smaller quantity of black ferromagnesian minerals, mostly biotite, but 
partly hornblende. These dark-colored constituents are in much 
smaller crystals than the feldspars, which are partly idiomorphic. 

In thin section the rock is hypidiomorphic-granular in texture, each 
of the constituent minerals x>08sessing some degree of idiomorphism. 

The large feldspars are for the most part microcline, with very 
delicate double polysynthetic twinning — ^that is, lamellar twinning— 
according to the albite and the pericline laws. There is also pronounced 
microperthitic intergrowth of lime-soda feldspar, whose index of refrac- 
tion relative to that of microcline, and whose low angles of extinction 
indicate that it is oligoclase. Some of the large feldspars are ortho- 
clase, and exhibit no polysynthetic twinning, while oligoclase also 
occurs in small partially idomorphic crystals. 

In some microclines the twinned lamellae are curved and crowded in 
certain spots, and since the quartz crystals exhibit a strain phenomenon 
of an analogous kind, it is probable that the microcline twinning is, in 
part at least, due to mechanical strains undergone by the rock. 

In places the feldspar is clouded with minute dots which have a lower 
index of refraction than the feldspar and are white by incident light 
Their exact nature can not be determined, but they are probably 
in part cavities, iu part, kaolin. In some cases very small flakes of 
colorless mica or muscovite are present. The same form of alteration 
occurs in the orthoclase. In the perthitic microcline the intergrown 
lime-soda-feldspar is more altered than the microcline. A zonal strnc- 
ture is present in some of the microclines, but is more noticeable in the 
oligoclase. 

Quartz exhibits the characters commonly found in the quartz in 
granite. It is allotriomorphic, and was probably the last mineral 
crystallized, though its crystallization undoubtedly began while the 
feldspars were being formed, and its growth was contem^xiraneons 
with tliat of the latter part of the feldspar. Some crystals exhibit 
irregular double refraction when examined between crossed nicols, 
showing that different parts of one crystal have dififerent optical orien- 
tation. In some cases the portions are very minute, and appear like 
indistinct lamination in two directions, as in microcline. 

Fluid inclusions are abundant and very small. They sometimes 
occur in planes which occasionally traverse several quartz crystals in 
one direction, proving that they are of secondary origin — that is, tliey 
were formed subsequent to the crystallization of the quartz and conse- 
quently of the rock as a whole. 

Biotite occurs in irregularly shaped plates. Its color is brown, with 
strong absorption for rays vibrating at right angles to a — that is, for 
rays vibrating nearly in the plane of cleavage. It is light brown for 
rays vibrating parallel to a or at right angles to the plane of cleavage. 



KLLtt] BMCItlPTIONS : NO. 69, HORNBLENDE-GRANITE. 179 

It incloses namerous crystals of colorless apatite and fewer of zircon 
and of magnetite. Hornblende is in smaller amount than biotite, and 
occnrs in irregular anbedrons. Its color is brownish green, with the 
asnal pleocbroism from greenish brown for rays vibrating parallel to 
a, the direction of vibration of the fastest traveling ray, to dark green 
parallel to fi, and c, JC > 6 > a. Biotite and hornblende are sometimes 
grown together. They are both quite fresh and unaltered. 

Sphene occurs in yellowish anhedrons, associated with the horn- 
blende. Zircon is comparatively abundant in small, short prisms, but 
apatite is much more abundant in small, stout crystals, which are color- 
less. These are readily distinguished from one another by their optical 
properties. Sphene, having very high index of refraction and com- 
paratively low double refraction, stands out prominently in the section, 
lias a roughened or shadowed surface, and is not brilliant between 
crossed nicols. Zircon, having both high refraction and high double 
refraction, but being in very small crystals, exhibits strongly marked 
outlines, has a strong relief, and is brilliant between crossed nicols; 
while apatite, with strong index of refraction and very low double 
refraction, exhibits strong relief, and very low color between crossed 
uicols. 
Iron oxide is sparingly present, presumably as magnetite. There 

are also small microscopic crystals, which are brown to opaque, whose 

character is indeterminable. 

No. 69. BlOTITE-BEARING HORNBLENDE-GRANITE. 

(From Cape Ann, Massachusetts. Described by J. P. Iddings.) 

This rock occurs along the coast of Massachusetts north of Boston, 
where, in the vicinity of Gloucester, it forms the greater portion of Gape 
Ann, and is extensively quarried for building purposes. Its texture is 
medium grained and its color light greenish gray. It is composed of 
greenish-gray feldspar, with somewhat pearly cleavage surfaces, about 
^ equal amount of brownish quartz, and a much smaller amount of 
black mineral, which is mostly hornblende. In thin section the texture 
is bypidiomorphic granular, the quartzes and feldspars having very 
i^gular outlines and an arrangement suggestive of that in gneisses, 
lie auhedrons of quartz have jagged outlines and interlock with one 
Mother, a number of quartzes being clustered together in places. 
Tbey exhibit irregular double refraction, and are often separated into 
patches having slightly diflferent optical orientation (feldererscliei nun- 
gen), which is the result of mechanical strain. There are numerous 
fl»inate fluid inclusions, and, rarely, inclosed crystals of zircon. 

The feldspars are microcline and lime-soda feldspar, which is mostly 
oligoclase. These are intimately intergrown, as in perthite. The micro- 
dine appears to form disconnected pieces, while oligoclase constitutes 
a continuous intersecting web between the pieces of microcline. The 
microcline looks as though a large crystal had been cracked and 



180 THE EDUCATIONAL SERIES OF BOCK SPECIMENS, [buu^iml 

had been pulled apart, the iuterstices being filled ^ritb oligoclase; bat 
all parts of the microcline have one orientation. The microcline is 
clouded with minute particles, indicating partial alteration, while the 
substance of the oligoclase is perfectly fresh and pure. These large 
complex feldspars are usually twinned after the Carlsbad law. 

In some cases there appears to be an intergrowth of two large feld- 
spars in much the same manner as in the graphic intergrowth of quartz 
and orthoclase — that is, the crystals cross one another at oblique angles. 

Between the large crystals of feldspar is a border of smaller feld- 
spars which are mostly lime-soda-feldspar, and probably oligoclase. 
With these small crystals are also small crystals of hornblende and 
biotite. These minerals occur in the same manner adjacent to large 
anhedrons of quartz. 

The hornblende is brownish green with the usual pleochroism. The 
shape of its anhedrons is very irregular agaiust both feldspar and quartz. 
Its outline is quite jagged, with minute projections of hornblende and 
of biotite, which have grown in the margin of the anhedron. It carries 
inclusions of biotite, magnetite, and zircon; besides larger ones of mono- 
clinic pyroxene, probably malacolite. These have irregular outlines and 
various orientations. Hornblende also occurs in clusters of minute 
anhedrons, together with similar anhedrons of biotite, inclosed in fdd- 
spar and quartz^ 

The biotite is brown with the usual absorption and other optical prop- 
erties. Its shape is quite irregular. Its anhedrons are not so large as 
those of the hornblende. It appears to have crystallized at the same 
time as the last part of the hornblende, being inclosed in the margins 
of crystals of the latter mineral. 

Monocliuic pyroxene is quite subordinate in amount, and occurs with 
irregular outlines, both as inclosures in feldspar and in hornblende. 
Its color in thin section is pale green, and its double refraction is fairly 
high. It is probably malacolite. 

Zircon occurs in comparatively numerous, minute, short crystals. 
Apatite is not abundant. It is in small stout prisms, and also in needle- 
like forms. It sometimes contains rounded isotropic inclusions resem- 
bling glass. Iron oxide, which is apparently magnetite, occurs sparingly, 
and is generally associated in clusters or in juxtaposition with erystala 
of the ferromaguesian minerals. 

Allanite is present in occasional anhedrons, irregularly shaped. Its 
color is chestnut-brown with strong pleochroism. 

The microstructure of the rock is that of a metamorphic granite or 
of a metamorphosed granite rather than that of a purely igneous or of 
an unaltered eruptive granite. 

The quarries at Gape Ann are said to be next to the largest in the 
United States, and have furnished stone for many of the buildings in 
the cities of the Atlantic coast. For further information on the geology 
of that region see paper by K. S. Shaler on The Geology of Gape Ann, 



wutt] DESCKIPTI0N8: NO. 70, TRACHYTE. 181 

Massachusetts, in the Ninth Annual Eeport of the United States Geo- 
logical Borvey, pp. 535 to 611. 

SYENITE TRACHTTE FAMIX.Y. 

No. 70. Trachyte. 

(From Gamk Ridgb, near Rosita, Custek County, Coix>rado. Described by 

Whitican Cross.) 

Oceurrem^e. — ^This trachyte forms a surface mass resting on audesitic 
toff and breccia. The rock was the last one in the sequence of seven 
distinct eraptions from the Eosita volcano, the others having been 
andesites of various types, dacite, and rhyolite. Game Eidge is a small 
mass of trachyte preserved from erosion by faulting, and none of the 
rock now remaining exhibits the outer structural features of lava 
streams. Dikes of the same rock seen near by show the channels 
through which the magma came to the surface. 

DeBcription. — The rock is light- gray, porphyritic, its most conspic- 
Qons constituent being sanidine in characteristic glassy crystals, much 
fissured and occurring both in tabular and stout prismatic forms, with 
a diameter rarely exceeding 1 centimeter. These crystals are not very 
abundant, the greater part of this feldspar being in the gray ground- 
mass. There are many small phenocrysts of oligoclase, but they are 
Bot prominent, as a rule; while the less numerous leaves of biotite, the 
only dark silicate of the rock, are naturally distinct. 

Microscopical study shows the predominant gray groandmass to con- 
sist chiefly of orthoclase feldspar in short prisms, which, by their more 
or less distinct parallel arrangement and the curving of the bands 
about the phenocrysts, cause a fluidal structure. This case illustrates 
very well that groundmass structure commonly called "trachytic,'^ 
which is due to the predominance of feldspar and its tendency to crys- 
talize in these microlitic prisms. 

The groundmass contains a small amount of quartz, usually found 
in little, elongated clusters of clear grains, or as matrix for the ortho- 
clase staves. This latter association is rarely so developed as to 
l^me truly a poikilitic structure. The amount of quartz is not 
enoagh to invalidate the reference of the rock to the trachytes. Oli- 
goclase appears in a very few minute crystals in the groundmass, 
And a scanty magnetite dust is found in the fresh rock. Ordinarily, 
there is much limonite in flakes all through the rock, often giving it a 
pinkish tinge. 
Apatite and zircon are very sparingly present, in characteristic form. 
The sanidine crystals are very free from included mineral grains 
except in the outer zone, where they contain groundmass particles, this 
zone being an added growth of the groundmass period. The plagio- 
ciase of the rock is oligoclase, as shown by the extinction very near to 
the albitic twinning line in all sections normal to the twinning pVaue. 



182 



THE EDUCATIONAL SERIES OP ROCK SPECIMENS, [bull. 160. 



Biotite is developed only in thin brown leaves wliicb seldom have reg- 
ular ontline and are often very ragged, with magnetite grains included 
in the border zone or lying adjacent to the leaf, as though resulting 
from recrystallization after partial resorption of the mica. 

Chemical composition. — The freshest available specimen of this tra- 
chyte has the composition shown in the following analysis, by L. G. 
Eakius : 

Analysis of trachyte from Boaita, Colorado, 



Per cent. 



SiOj 

Al^O, 

FeaO, 

FeO 

MnO. 

CaO 

MgO 

K,0 

Na,0 

HjO 

PA 

CO, 

Total... 
Sp. gr. at 290 C 



66.03 

18.49 

2.18 

.22 

trace 

.96 

.30 

5.86 

6.22 

.86 

.01 

trace 



100.24 
2.50 



This comx)osition confirms the deductions from mineral constitution 
that the rock is nearly a typical trachyte. Silica is higher than is 
normal, but not sufficient to place the rock with the rhyolites. The 
high percentage of alkalies and the low amount of lime makes it clear 
that alkali feldspar must make up the great bulk of the rock and that 
the oligoclase can not play a prominent r61e in the groundmass. The 
percentages of the iron oxides and of magnesia corresi)ond to the small 
amount of biotite and the change of magnetite into limonite. 

Relationship of the rock, — The Game Bidge trachyte is to be compared 
especially with the well-known occurrences of the Siebengebirge, on 
the Ehine, near Bonn. Many of the rocks of this classic locality are 
very similar to that of Game Ridge in structure, in relative importance 
of the oligoclase, and in general chemical composition. The similarity 
with the trachyte of the Drachenfels, as represented in specimens 
accessible to the writer, is particularly striking, especially in regard to 
amount and development of quartz and biotite and the structure of 
the groundmass. 

Literature. — On some Eruptive Bocks from Custer County, Colorado, 
by Whitman Cross, Proceedings Colorado Scientific Society, Vol. Ill, 
pp. 233-237 ; Geology of Silver Clitf and the Rosita Hills, Colorado, by 
Whitman Cross, Seventeenth Annual Report, United States Geological 
Survey, Part II, 1896, pp. 263-403. 



MU«^1 DESCRIPTIONS: NO. 71, SYENITE. 183 

No. 71. Syenite. 

(From Custer County, Colorado, two and one-third miles a little east op 
soRiH prom Silver Cliff, and one-half mile southeast of the Bull 
Domingo mine. Described by Whitman Cross.) 

• 

Geological occurrence and distribution. — ^This syeuite occurs in narrow 
dikes cutting gneiss or granite in a considerable area of mountainous 
country lying south of the lower portion of the Grand Canyon of the 
Arkansas River. The dikes have been specially observed in the vicinity 
of Silver Cliff, but they occur all through the area drained by Grape 
Greek, which enters the Arkansas near the mouth of the Boyal Gorge, 
and they probably have a still wider distribution in the Wet Mountain 
Kange, to which the district mentioned is adjacent. 

These dikes, standing in nearly vertical position, cut in all directions 
across the strike of the steeply upturned gneisses and schists. They 
are therefore younger than the folding of those gneisses, but are older 
than the great erosion which took place probably in very early Tertiary 
times and produced surface features not far different from those of the 
present day. The dikes are certainly pre-Tertiary, but their age can 
not be more closely fixed from known evidence. Associated with the 
syenite dikes through this area are numerous narrow dikes of normal 
diabase. 

The dikes of syenite vary in width from an observed maximum of 
about 20 feet down to 1 foot or less. They are continuous for distances 
of more than a mile, in several instances, and their courses are often 
nearly straight. The dike from which the specimens of this series 
came was traced for more than a mile, and varies in width from 2 to 12 
feet At the point where the specimens were obtained it was about 6 
feet wide and had been penetrated by a prospect shaft, furnishing the 
naaterial for coll ec t ion . 

Ascription. — The field habit of this syenite is very characteristic. 
The rock has commonly a strong brick-red color, sometimes with a 
yellowish shade. This causes outcrops of dikes to be distinguishable 
often at considerable distances as red lines, contrasting with the darker 
shades of the ordinary gneiss. 

The predominant mineral of the rock i^ red feldspar, developed in 
^liin tablets, and by the prevailing arrangement of these tablets 
approximately parallel to the dike walls a schistose structure is pro- 
duced. There is commonly a dense contact zone of three or four inches 
in width which grades off rapidly into the average coarser rock of the 
dike. These contact bands are usually of darker color, and have a 
dense porpbyritic structure. 

On macroscopical examination this rock is seen to be chiefly made 
^P of red feldspar tablets, with a subordinate amount of dark material 
^ fine particles, among which hornblende, mica, and epidote can here 
^i there be recognized. The feldspar tablets are like rounded disks, 
seldom more than 2">» across. The schistose structure arising from 



184 THE EDUCATIONAL SEKIES OF ROCK SPECIMENS. [euLulsa 

their rudely parallel arrangement is clearly illastrated by most of the 
specimens collected, for the rock fractures easily in the direction of this 
lamination and with difficulty across it. Occasionally larger crystals 
of orthoclase are developed, and, more rarely, long thin prisms of 
hornblende. 

A microscx>pical study shows the primary constituents of this syenite 
to be orthoclase, plagioclase, amphibole, biotite, and apatite, while epi- 
dote, chlorite, calcite, and a ferritic pigment are secondary products. 

The coloring pigment of the rock occurs in extremely small particles, 
which are doubtless limonite, and which impregnate nearly all of the 
feldspar crystals as a dense cloud. A large number of the feldspar 
tablets are plagioclase, as the common polysynthetic twin structure 
clearly demonstrates, but the pigment often so obscures the optical 
action that the angles of extinction can not be closely measured. In 
the zone normal to the twinning plane the extinction angle is always 
small, however, and it is probable that oligoclase is the only member of 
the soda-line series here developed. Orthoclase certainly predominates 
over plagioclase. 

The feldspar tablets are never perfectly idiomorphic. The crystalli- 
zation of the oligoclase individuals was finished before that of ortho- 
clase, and the predominant pinacoidal plane was then sharply defined, 
while the prismatic and terminal planes seem to have been irregular. 
The orthoclase tablets are throughout of irregular outline, owing to 
their mutual interference in the last stages of growth. 

The schistose structure referred to was doubtless produced by a 
movement of the magma during crystallization. By this means the 
feldspar tablets, which were already large, were forced into a position 
approximately parallel to the dike walls. Many angular spaces were 
naturally left, into which the free amphibole prisms and biotite leaves 
were crowded. The biotite, however, seems to have continued its 
growth after this movement, for it often fills the irregular spaces com- 
pletely, and has the usual hexagonal form only when included by some 
feldspar individual. Some of the angular spaces between clouded feld- 
spars are filled by a colorless mineral, and in polarized light this matter 
almost invariably resolves into segments having the optical orienta- 
tions of the adjoining feldspar individuals. The material seems in all 
cases to be orthoclase, and the ferritic pigment never penetrates into 
substance of this origin. Epidote and calcite also occur as fillings of 
these angular spaces, without their proper crystal form. Epidote occa- 
sionally appears as a direct decomposition product of biotite, but does 
not replace feldspar substance except in the more decomposed rocks. 

From the facts above given it seems probable that upon final solidifi- 
cation of the syenitic magma there remained some angular spaces 
between crystals, which were unoccupied by any mineral substance, and 
that epidote and calcite were deposited from solutions in these cavities 
at a later period. Whether the clear feldspar substance also belongs 



tHLLKL] 



DESCRIPTIONS: NO. 71, SYENITE. 



185 



to this later x>eriod, or to the final stage of crystallization of the magma, 
ia not fully evident. Porons strnctare originating in this way is known 
as the miarolitic strnctare. 

The amphibole of this syenite is nnlike any of the common varieties 
of the group. The prisms are seldom of perfect form in the coarser- 
grained parts of dikes, and are usually made up in part of a brown and 
in part of a dark-blue amphibole, the brown forming the inner portion 
and being surrounded in very irregular manner by the bine variety. 
The relations of the two are such as to indicate that the blue is merely 
the later part of the crystal, and grounds for considering it as a para- 
morphic product from the brown have not been observed. In other 
dikes of this syenite the brown and the blue varieties have been found 
in separate crystals. The blue amphibole has the following pleochro- 
iBm: a = strong yellowish-brown, b = dark blue; jc = dark greenish- 
blue; cAC = about IQo, absorption strong, jc>ft>a. The brown 
amphibole has dark chestnut-brown color parallel to h and c, and a 
lighter shade parallel to n. Extinction 12^ or more, not closely deter- 
minable. The brown variety also has a strong absorption parallel to h 
and JC. Both varieties seem to have the optical orientation of normal 
hornblende, and the blue is, therefore, unlike glancophane on the one 
hand, and arfvedsonite or riebeckite on the other, while the brown is 
apparently related to barkevicite. 

The biotite is almost uniaxial, and has h + c = dark green, and a = 
yellow-brown. Absorption is very strong. 

AUanite was observed in a few small characteristic prisms in the 
contact zone of one of the narrower syenite dikes. 

Chemical composition. — The freshest specimen of the average rock 
that could be obtained was analyzed by L. G. Eakins, with the follow- 
ing result: 

AnalifM of fytnitefrom near Silver Cliff , Colorado, 



SiO, 

AlA 

Fe,0, 

FeO 

MnO 

c»o 

MgO 

K,0 

Na,0 

H,0 

CO, 

Total 

Sp.gr. at 30OC., 2.680. 



Per cent. 



59.78 

16.86 

3.08 

3.72 

.14 
2.06 

.60 
5.01 
5.30 
1.58 

.76 



99.06 



The rock analyzed contains calcite and epidote as secondary min- 
erals, developed chiefly as fillings of the miarolitic cavities. The feld- 



186 THE EDUCATIONAL SERIES OF HOCK SPECIMENS, [buli.150. 

spar is not visibly decomposed, and it seems more likely that the lime 
of the epidote and calcite was brought in by solutions than that it was 
derived from the adjacent feldspars. The character of the feldspar 
contents of the rock analyzed must in any case be calculated after 
deducting lime for calcite, epidote, and amphibole. Nearly one-third 
of the lime is required for the carbonic acid to form calcite, and at 
least another third may be deducted for the other two minerals. This 
leaves one- third as belonging to an oligoclase. 

If one-third of the lime belongs to oligoclase of the composition 
AbsAU], and the remainder of the soda is referred with the x>ota8h to 
a pure alkali feldspar, the amount of the oligoclase in the rock will be 
nearly 30 per cent and of the alkali feldspar nearly 50 per cent. The 
percentage of oligoclase will rise rapidly with an increased amount of 
lime introduced into the calculation, but it is quite probable that the 
oligoclase molecule is richer in lime than has been assumed, and this 
would reduce the percentage of possible oligoclase in the rock. 

Literature. — This syenite was first described in an article entitled. 
On some Eruptive Rocks from Custer County, Colorado, by Whitman 
Cross, published in Proceedings Colorado Scientific Society, Vol. Ill, 
1887, pp. 237-240. It is further discussed, with details of geological 
relations, in a rei)ort on the Geology of Silver Cliff and the Rosita 
Hills, Colorado, by Whitman <3ross, in Seventeenth Annual Report 
United States Geological Survey, Part II, 1896, pp. 263-403. 

NEPHELITU-LiEXJCITE BOCKS. 

No. 72. Obendite. 

(From Leucite Hills, Sweetwater County, Wyoming. Described by Whit- 
man Cross.) 

Introductory. — When the project of gathering the rocks of this col- 
lection was first considered, the only leucite-beariug rock known in the 
United States was that occurring in the Leucite Hills, discovered by 
S. F. Emmons, and described by Prof. F. Zirkel.* It was decided to 
collect that rock for the Educational Series. When tbe material was 
brought in and subjected to microscopical study, it was found that the 
greater part was not like that described by Zirkel, but contained both 
sanidine and leucite. The rock collected has, however, been described 
under the name of orendite,^ being made the type of the kind of vol- 
canic rock rich in leucite and sanidine with diopside and magnesian 
mica as other essentials. The rock is still representative of the types 
especially characterized by leucite. 

Megascopical description. — The orendite of this collection has a dull 
reddish-brown color and is quite vesicular, the pores being small and 



» Reports of the Fortieth Parallel Survey, Vol. VII, MicroBcopical Petrography, I87«, p. 259. 
'Igneous rocks of the Leucite Hills and Pilot Butte, Wyoming, by Whitman CroM: Am. Joor. 
SoL, 4th series, VoL IV, 1897, pp. 123-126. 



wuJtM DESCRIPTIONS: NO. 72, ORENDITE. 187 

irregalar in shape, with divergent smooth-walled arms. The vesicles 
are iisaally much less than 1*="' in size, but vary considerably in dif- 
feient specimens. On examination with a hand lens the walls of these 
cavities may, in most specimens, be seen to be coated with a network 
of very pale yellowish needles of a peculiar amphibole, which will be 
described below. Hyaline opal, in clear globular droplike forms, is 
not uncommon in the pores. A dull white indistinctly crystalline sub- 
stance of undetermined character is also often present. 

Tbe only distinct megascopic constituent of the rock mass is a 
reddish-brown mica, found by chemical analysis to be phlogopite, 
occamng in hexagonal leaves only 3 or 4°^*°^ broad at most, and sink- 
ing to microscopic dimensions. The general color tone of the rock is 
due to this mica. 

In some specimens are occasional dull grains of orthoclase, which 
are corroded and belong to foreign rocks found in larger inclusions in 
many places throughout the mass. 

Mkroscapical description. — Microscopical study shows the rock under 
discussion to consist of leucite and sanidine in slightly predominant 
amount as compared with the ferromagnesian silicates, phlogopite, 
amphibole, and diopside. Apatite is unusually abundant and a few 
mach resorbed flakes of brown biotite may commonly be found in each 
section. Needles of a yellow mineral, rutile ( f ), are present in minute 
quantities, but magnetite, ilmenite, or pyrite have not been found. 

In quantitative development leucite and sanidine vary considerably; 
now the one, now the other, seeming to predominate. Of the darker 
silicates phlogopite is the most important, while diopside varies with 
IcQcite^and amphibole with sanidine. 

leucite occurs in this rock in a multitude of minute rounded grains, 
li€tween 0.01"" and 0.05"'" in diameter. Some of them have the com- 
mon icositetrahedral form, vnth a zone of minute inclusions, but more 
of the grains are anhedral. All are perfectly isotropic. In every way 
tbese lencite crystals and grains are identical in habit with those of the 
sanidine-free rock. 

Tiie sanidine of orendite occurs in stout, square prisms, of rough 
outhne and seldom exceeding 1°" in length. The prismatic axis is a, 
^ shown by the optical orientation. It is always the axis a of elas- 
^^ty which lies near the prismatic axis, and the maximum observed 
*»gle of extinction is lO^. Bude dome and prism faces terminate the 
crystals as a rule. Twinning after the Carlsbad law is not uncommon. 

The reddish-brown mica appears in thin sections to have a very 
^eak pleochroism, varying only from a pale salmon-pink to pale yellow. 
Sections normal to the cleavage often reveal a polysynthetic basal twin- 
^R which is made evident through the perceptible angle between the 
axis, a, of elasticity and the crystallographic axis, c. This reaches 3^ or 
^°. Thin basal sections, not twinned, when examined in convergent, 
polarized light show a negative bisectrix and an unusually large ov)tic 



188 



THE EDUCATIONAL SERIES OP BOCK SPECIMENS, [bdi 



angle. Tbe leaves are nsnally of hexagonal form, and they are 1 
and free from inclusions except rare ones of glass and still rarer ye 
ish needles, most probably of rutile. 

The mica of this rock was isolated and analyzed by W. F. Hilleb 
with the result given below, which shows it to have the compositi 
phlogopite, a species of mica not before identified as an original 
stituent of igneous rocks. The observed physical properties agree 
this determination. 

Analysia of phlogopiie of orendite from Wyoming. 



SiO,.. 

TiO,. 

A1,0, 

Cr,0, 

FejjO, 

FeO.. 

CaO . 

BaO .. 

MgO 

K2O . , 

Na/) 

H,0 . 

P.0,. 

F.... 



Per cent. 



Total. 
Lesa O for F 



42. 5« 

2.00 

12.18 

.73 

2.73 

.90 

.20 

1. 00 

22.40 

10.70 

.44 

2.35 

.06 

2.46 



100.80 
1.03 



Total 



9».77 



Diopside occurs in small prisms, pale green or colorless, which 
to microlitic needles not easily determinable. In some of the ( 
rocks of the region this mineral is developed in somewhat larger pri 
and W. F. Hillebrand has isolated and analyzed it, showing it to I 
entirely normal diopside. 

Analysis of diopside of orendiie from, Wyoming, 



Per cent. 



SiO, 

p,o,/ 

A1,0, 

Fo,0, , 

FeO 

MnO 

CaO , 

MgO 

KjjO 

Na,0 

H,0 

Total 

Sp. gr. at 200 C. 3.20. 



50.86 

3.03 

none 

1.19 

1.82 

.03 

23.32 

17.42 

.42 

.76 

.81 



99.16 



nun-l DE8CMPTION8: NO. 72, OBENDITE. 189 

The ampbibole of this rock is a very unusual one in its optical char- 
acters, and its determination as a member of this group depends largely 
on the strongly marked cleavage parallel to a prism whose angles 
measure about 124^ and 56^, together with the general habit of the 
mineral. It occurs in rudely prismatic individuals, between the sani- 
dine and leucite grains, producing structures soon to be described. 
Occasionally it has nearly regular crystal form with prism and pinacoid 
as in amphibole, and a termination apparently made up of pyramid and 
dome faces. Befraction and double refraction are of about the strength 
of actinolitic amphibole. Extinction is parallel to c, as far as has been 
ascertained, a = a is pale yellow, b = & is red, jc = o is bright yellow. 
Thereddish tone is similar to that of hjrpersthene, and all colors increase 
rapidly in intensity with increasing thickness of the sections. Absorp- 
tion, 6>c>a. 

The association of minerals in orendite leads to several interesting 
microstractures. Phlogopite appears to have formed first, and it is 
almost free from inclusioDS. Leucite and sanidine are as a rule devel- 
oped in separate patches or areas, the former in swarms of minute 
round grains, the latter in aggregates of a few irregular prisms seldom 
exceeding 1"" in length. Diopside is developed mainly in minute 
needles and microlites, a large share of which are included quite irregu- 
larly in the sanidines, producing a certain micropoecilitic structure. 
The remainder of the diopside occurs between other larger mineral 
grains. 

Tiie amphibole seems one of the latest crystallizations of the rock, 
and varies in development. In the angular spaces between the sani- 
dines the yellowish amphibole occurs exactly as does augite in ophitic 
diabase, acting as an oriented cement for several sanidine crystals. In 
tlie iencitic areas the same amphibole appears in stout prisms inclos- 

■ 

iiig nnmerous leucites, just as aegiriue holds the nex)helines in many 
pbonolites. In this manner another form of the micropoecilitic struc- 
^ is produced. It also occurs in very minute needles in many of the 
vesicles. 

In occasional spots and adjacent to the pores of the rock the min- 
erals are less intimately intergrown. Leucite is sometimes found 
inclosed in sanidine, but frequently the separation as described is very 
8^. There are thus in this orendite two kinds of micropcecilitic 
stractores, a curious separation of the analogous silicates, leucite and 
^idine, ophitic structure, and through the prominence of phlogopite 
leaves a porphyritic structure. 

OKemical compositiofi. — ^Two analyses of orendite have been made by 
^' P. Hillebrand, which are given below. Analysis I is of the rock 
from Fifteen Mile Spring, on the eastern edge of the Leucite Hills, the 
locality from which all the specimens of the educational collection were 
obtained. Analysis II is of a very similar rock from North Table 
^tttte, in the northern part of the Leucite Hills. 



J 



190 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [buu 



Analyses of arenditefrom Leucite HilU, Wyoming. 




SiO, 

TiOa 

ZrO, 

A1,0, 

CrjOs 

Fe,Oa 

FeO .^. 

MnO 

CaO 

SrO 

BaO 

MgO 

KaO 

Na^O 

LijO 

H2O below 1100 C 
HjO above llO© C 

PA 

SO, 

CI 

F 

CO2 

Total 

Less O for F 



Per cent. 

54.08 
2.08 



D.49 
.07 

3.19 

1.03 
.05 

3.55 
.20 
.67 

6.74 
11.76 

1.39 

trace 

.79 

2.71 

1.35 
.29 
.04 
.49 



Total 



99.07 
.21 



99.76 



Per eetit. 

54.17 

2.67 

.22 

10.16 

.05 

3.34 

.65 

.06 

4.19 

.18 

.59 

6.62 

11.91 

1.21 

trace 

.52 

1.01 

1.59 

.16 

.06 

.36 

.49 



100. 21 
.17 



100.04 



Tbe rocks are remarkable for the large number of rare elements < 
tallied in them in determinable quantities. Zr02 was not tested fo 
the first analysis. It does not belong in the mineral zircon, as that 
not be identified in thin sections, and it seems most probable that 
peculiar amphibole is allied to certain silicates which may be cla$ 
with the pyroxenes, in having Ti02 and ZrO» replace apart of the S 
Cr203 and most of the F belong in phlogopite, together with a 1: 
part of the BaO. SO3 represents some mineral easily soluble in \s 
acids, probably noselite, which may be indistinguishable from let 
in thin sections. Both the mica and the pyroxene contain TiOa, 
the rare yellow needles occasionally seen may be rutile. 

As regards the commoner rock constituents, the predominance of 
ash in orendite causes characteristic minerals contrasting with t 
of the corresponding soda-rich rock, phonolite. Leucite is deveh 
in place of nepheline. A potash-bearing mica appears instead of a?gi 
and arfvedsonite, which are common in phonolite. The pyroxeu 
orendite is almost a pure lime-magnesia mineral, and soda is preve; 
from entering into plagioclase. 

Occurrence and relationships. — The rock now under discussion oc 
in lava flows of Tertiary jige. In the same flows a part of the magma 
crystallized free from sanidine, but with leucite, diopside, and phi 



wiiXB.) DESCRIPTIONS: NO. 72, PHONOLITB. 191 

pite of the same development observed in orendite. This rock, the one 
originally described by Zirkel, has the same bulk chemical composition 
asoTendite. Since leacite contains less silica than sanidine, there is of 
necessity an excess of this acid radical in the pure leacite rock, appar- 
ently in the form of an obscure glassy base. This rock has been called 
"wyomiugite" in the cited paper on the rocks of the Leucite Hills. 

At Pilot Butte, west of the Leucite Hills, occurs a rock rich in diop- 
side, phlogopite, and perofskite, with a glassy base which the analysis 
of the rock shows must have nearly the composition of leucite. The 
name ^^madupite'" has been proposed for this type. 

Orendite belongs in the group called leucite-trachyte by Zirkel, and 
lencitephonolite by Kosenbusch. A special name has been proposed in 
tlie belief that both the compound terms above mentioned are objec- 
tionable, and that it is appropriate and desirable to have a distinctive 
name for the leucite-sanidine rocks corresponding to phonolite. 

No. 73. Phonolite. 

(Fbom Black Hills, South Dakota. Described by Whitman Cross.) 

Megascapical description, — This rock has a dense aphanitic texture 
except for a few small sanidine tablets. Its general color is a dull 
brownish green, mottled by numerous dark bluish-green spots of 
iodistiDct sheaf like form. These dark spots are due to the bundles 
of apgirine needles. A peculiar greasy or semivitreous luster is pro- 
duced by the reflections from the multitude of minute faces of nephe- 
lioe or sanidine grains forming the mass of the rock. 

Mi€ro8copi<:al description. — The principal constituents of this phonolite 
are nepheline, sanidine, and ajgiriue, with a characteristic accompani- 
ment of noselite and sodalite. Nepheline is very distinctly developed 
in short prisms causing the hexagonal or nearly square outlines seen 
in thin sections. The average diameter of these nepheline crystals is 
about O.!""™ and their length less than 0.2*°™. In this development the 
i^cphelines lie in a fine-grained mass consisting chiefly of sanidine 
scales and tablets, which overlap so much that very thin sections and 
Wgb powers of the microscope are necessary clearly to resolve the 
niass into its elements. 

To a subordinate degree in the sections examined the sanidine is 
'Jeveloped in more elongated staves and needles, and these are com- 
monly arranged in nearly parallel position, producing an ai)parent 
flnidal structure. It is noticeable that in these portions of the rock 
Depheline is developed in much smaller crystals than elsewhere. 

-^Igirine is the mineral causing the dark spots of the rock. It occurs 
ii» bundles or sheaves of minute needles which in the central zone seem 
Dioreor less of a common orientation or are united practically into one 
crystal which feathers out at each end. Owing to the great numbers 
of included nepheline or sanidine grains, even in the most massive part 



192 THE EDUCATIONAL 8EBIES OF BOCK SPECIBfENS. [bull.im. 

of these bandies, the thin sections rarely show the actual existiug con- 
tinaity of the segirine substance. Where the latter is most massive 
the abundant inclusioDS of other minerals produce a typical micro- 
poecilitic strnctare. The optical characters of this segirine are entirely 
normal. Its pleochroism is strong, as follows: a = pure green; b = 
olive green; jc = yellowish green, a lies very near the prismatic axis. 
!N^o considerable angle of extinction was observed, so that aegirine- 
augite is not developed in this rock. 

Scattered through the rock in amount greatly subordinate to nephe- 
line, but still an important constituent, is a mineral of the regular sys- 
tem characterized by a cloud of dark, dusty interpositions. While this 
substance is apt to be decomposed, it has the characters often found 
in noselite as occurring in phonolites. Its rudely irregular crystals are 
somewhat larger than the nephelines and may reach 1™°^ in diameter. 
From the chemical analysis, which shows both sulphuric acid and 
chlorine in the rock, it is to be inferred that sodalite is a companion of 
noselite here as in many other phonolites, but no means of distinguish- 
ing the two have been found in this case. 

In almost every section of this phonolite may be found small areas 
of clear, colorless, isotropic substance in angular spaces between well- 
defined crystals of other minerals. It is thought probable that this 
isotropic sabstance is analcite, through analogy with the occurrence of 
that mineral in the very similar phonolites of the Cripple Greek region 
in Colorado. 

Other minerals occur in this rock only in minute traces. There are 
occasional specks of magnetite and possibly of pyrite. The titanic and 
zirconic acids shown by analysis are most probably contained in a min- 
eral occurring sporadically in a manner much like the a^girine; that is, 
in irregular particles in residual spaces or inclosing minute grains of 
nepheline or feldspar. This mineral is almost colorless, of high single 
and double refraction, with no or very slight pleochroism and appar- 
ent parallel extinction. It resembles a mineral observed, but not 
positively identified, in the Cripple Creek phonolites, though there the 
association with lA^venite led to the supx)Osition that it was to be referred 
to that species or to some allied complex silico-titanate with zirconic 
acid also present. 

Chemical composition. — In the subjoined table of analyses. Column I 
represents the composition of this phonolite as analyzed by W. F. Hille- 
brand; II, an analysis by Pirsson of phonolite from the Devils Tower, 
near the Black Hills ;^ III, analysis by Hillebrand of a phonolite from 
Miter Peak near Cripple Creek, Colorado;* IV, analysis by vom Kath 
of a typical German phonolite from Zittau, Saxony.' 

1 Phonolitic rocks from the Black Hills : Am. Jour. Sci. 3cl series, Vol. XLVII, 1894, p. 341. 

* General geology of the Crliiple Creek district, Colorado: Sixteenth Ann. Rept. U. S. Geol. Surrey. 
Part IT, p. 39. 

* Quoted by Zirkel, Lebrbuoh der Petrographie, 2d ed., p. 446. 



DOLO.) 



DESCRIPTIONS: NO. 72, PHONOLITB. 



193 



AndtyBCB of phonoliiefrom various looaUUes. 



SiO, 

ZrO, 

TiO, 

A],0, 

FeA 

FeO 

MnO t 

C«0 

SrO 

BaO 

MgO 

K^ 

N%0 

LitO 

H,0— 110© C 
H,0 + 110° C 
P.0, 

CI 



CO, 

s .. 




III. 



Per cent. 
tfl.08 



IV. 
Per cent. 

se.i7 



.18 

18.71 

1.91 

.63 

trace 



19.74 
3.30 



1.58 


.92 


.05 . 




.08 


.15 


4.63 


6.45 


8.68 


8.88 



a 2. 21 



a 1.18 



trace 
.12 



DODO 



Total 



90.97 100.07 I 90.86 



99.88 



a, total water. 

The analysis shows the phonolite of this collection to be quite typ- 
ical, and Tery closely related to the rock so abandant in the Cripple 
Cieek district of Colorado. It was ascertained that the Cripple Creek 
pbonolites contained from 35 to 40 per cent of nepheline, and it is 
e?ideDtthat there must be abont the same amount in this rock. Sani- 
dbe probably constitutes at least 40 per cent of the rock, leaving 
^at 20 per cent for aegirine, noselite, sodalite, and the accessory 
nunerals. 

The presence of zirconic acid in determinable amount without dis- 
cernible zircon crystals, and the similar amount of titanic acid, seem 
to indicate the presence of some one or more of the rare minerals con- 
^ning these acid radicals, such as l^venite, which have been noted of 
f^nt years in eleolite-syenite or phonolite. 

LUeraiure. — ^The first phonolite to be discovered in the United States 
^ foand at Black Butte in the Black Hills, and was described by 
J- H. Oaswell.^ In 1894, L. V. Pirsson described the rock of Devils 
Tower, near the Black Hills, as a phonolite and gave an analysis 
of the same which has been reproduced in the above table. It is 
bown that there are many other occurrences of phonolite in and near 



' fi«port on the Geology and ^eaouites of the Black Hills of Dakota, by Henry Ke vton and Walter 
I* JeoMr. 1880, p. <192. 

Bull. 150 13 



194 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [BruuiM. 

the Black Hills, bat no systematic examination of their occorrenoe 
and relationships has been made. 

No. 74. PULASKITE (ELEOLITB-HOBNBLBNDESTENITB). 
(From Littlk Bock, Arkansas. Described by J. P. Iddings.) 

This rock is the so-called ^^blue granite" of Arkansas, which has been 
studied and fully described by J. F. Williams,^ from whose report the 
Ibllowing has been extracted. 

The rock is an intrusive body occurring in a wide, dike-like mass, 
whose eruption .took place about the close of the Cretaceous period, and 
which forms the main ridges of the Fourche Mountain region. It Is a 
bluish-gray, crystalline rock, in some parts dark in others light colored. 
It has a peculiar semi-porphyritic appearance when viewed megascopic- 
ally. The feldspar phenocrysts are conspicuous on account of their 
size and their highly perfect cleavage-planes and the light reflected 
from them. They appear to be crudely tabular parallel to the clinopin- 
acoid (010), and give a trachytoidal texture to the rock. The crystals 
are not sharply defined, since they interlock with the smaller crystals 
of the groundmass. The groundmass is subordinate to the feldspar 
phenocrysts, is phanerocrystalline, but dubiodiagnostic, consisting of 
whitish, gray, and dark-colored grains. Occasionally small plates of 
mica and crystals of dark amphibole or pyroxene are recognizable. The 
greater portion, however, is feldspathic. 

The texture of the. rock varies throughout the body from granitic 
porphyritic to hypidiomorphic granular. The following minerals are 
usually found in every specimen of the rock, but are present invariable 
quantities. Especially among the dark-colored minerals is this notice- 
able, since in many cases one of them predominates to the almost com- 
plete exclusion of the others. The minerals are orthoclase (cryptoper— 
thite), hornblende (arfvedsonite), augite (diopside), biotite, eleolite, 
sodalite (rare), titanite, apatite. 

Orthoclase is by far the most important mineral in the rock. It> 
appears usually in two forms as the result of two distinct periods oC 
crystallization. The crystals belonging to the first are the pheno- 
crysts, from 10 to 30"™ in length. They often show an idiomorphio 
form, although this is frequently impaired by the juxtaposition and 
mutual penetration of the smaller crystals of the second period of 
crystallization. Where crystal faces have been recognized they ai^ 
OP (001), ooP do (010), and ooP (110). Some crystals are twinned 
according to the Manebach law, others according to the Carlsbad. 
In one instance both kinds of twinning were observed in one crystal. 
The feldspar phenocrysts are impellucid as a result of more or less 
advanced kaolinization. 



iJ.F. Williams, The igneoas rooks of Arkansas: Ann. Bept. G«ol. Survey Arkanaaa, V(4.ll,im 
pp. 39-71 ; LitUe Rook, 1891. 



B£8CBIPTI0N8: NO. 74, PULASKITE. 



195 



The angle between the two cleavage planes is nearly 90^. Some 
(^stals exhibit delicate microcline twinning wbicb, taken in connection 
with tbe chemical comx)Osition, indicate soda- microcline. In some crys- 
tals there is a microscopic intergrowth with another feldspar showing 
ftlbite twinning and extinction angles of oligoclase. Tbe feldspar cor- . 
responds to Brogger's kryptopertbite. Some of tbe feldspars exhibit 
Dopolysyntbetic twinning. Some contain minate cavities elongated so 
as to appear like needles. These are generally in parallel groaps, 
either perpendicular to the cleavage planes or parallel to them. A few 
larger cavities appear to contain flnid with a gas bubble. There are 
also inclusions of the ferromagnesian minerals with apatite and mag- 
netite, besides more numerous grains of eleolite, irregularly shaped, 
and sometimes with hexagonal outline. Less frequently there are 
colorless isotropic inclusions with lower refraction than feldspar, which 
Are probably sodalite. 
The chemical analysis of the feldspar is as follows: 

AnalyH* offeUUpar of pulaskiie /ram Little Rooky Arkan9a9, 



Per cent. 



SiQ, 66.95 

AlA j 17.87 

FeA OW 

CaO 0.62 

MgO 0.24 

KjO 7.82 

KatO 5.20 

Igxiition 0. 80 

Total 99.80 



There is an excess of silica not accounted for. Ko quartz has been 
observed in the rock, and no individual crystals of lime-soda feldspar. 
The small feldspars constituting the groundmass have the same char- 
acters as the large ones. The greater part of the rock is orthoclastic 
feldspar. The ferromagnesian minerals are generally very small, and 
>n monoclinic pyroxene, amphibole, and biotite. The monoclinic 
pyroxene is pale green in thin section, without pleochroism. Its form 
is irregular, and it is generally surrounded more or less completely by 
Sf^nish-brown hornblende. In some parts of the rock the margin has 
> decided green color, probably segirite, the principal portion of the 
pyroxene being diopside or malacolite. The green margin shows an 
uicrease in the amount of sodium taken up from the magma toward the 
^Dd of the crystallization of the pyroxene. The cleavage is that common 
01 pyroxene; and the ordinary twinning parallel to the orthopinacoid 
(100) is present in some crystals. Among the inclusions noted are 
apatite, magnetite, titanite, biotite, and irregularly distributed gas- 
pores. 



196 THE EDUCATIONAL SERIES OF BOCK SPECIMENS, [buu^im. 

The amphlbole when seen in thin section is rich chestnat brown in 
some cases and greenish brown or dark green in others. When idio- 
morphic it is bounded by the faces oo P (110), oo Pob (100), oo P dc (010), 
and OP (001) and some orthodome (mOl) has also been observed. 
Amphibole cleavage is well developed. 

The pleochroism in some cases is as follows : 

6r = reddish brown; a = light yellowish brown; c-=dark reddish 
brown ; the absorption being jc > b > n. In other cases the pleochroism 
is: b = deep bluish green; a = brownish yellow; c = yellowish green; 
the absorption being b > c > a. A dark green edge, or border, sur- 
rounds many of. the brown crystals. Some of the brown crystals are 
completely free from such a border, but it almost always appears about 
the greenish ones. 

It is probable that these amphiboles belong to the arfvedsonite group, 
but no chemical tests have been attempted in proof of this supposition. 
Inclusions are similar to those contained in the pyroxene. 

The biotite is reddish-brown in thin section, with fairly strong pleo- 
chroism ^oni yellow to reddish brown. It forms plates parallel to the 
basal plane, which are idiomorphic in some cases and irregularly out- 
lined in others. It carries inclusions of magnetite, sphene, apatite, and 
zircon. It often surrounds comparatively large crystals of magnetite, 
sphene, and apatite. Sometimes it is on the outside of the pyroxene 
and amphibole, and sometimes it is inclosed by hornblende and pyrox- 
ene. Its crystallization appears to have begun before that of horn- 
blende. But in general the pyroxene appears to be older than the 
hornblende, and the biotite continued its growth to the end of the 
series of ferromagnesian minerals.^ 

Eleolite or nephelite occurs in variable quantities. In some parts of 
the rock it is quite insignificant, while in other parts it is an important 
constituent. It occurs in allotriomorphic anhedrons, often occupying 
spaces between large feldspars or forming irregularly shaped inclusions 
within them. It is colorless, with a higher refraction than the feldspar, 
and is sometimes more or less altered to analcite. Eleolite is nearly 
free from inclusions, but contains a few needles of apatite and occa- 
sionally slender crystals of segirite. 

Sodalite is found in some parts of the rock. It is generally fiUec 
with dust-like inclusions, but more often is altered to indeterminable 
decomposition products. The sodalite crystals are about half a milli 
meter in diameter. 

Sphene is the most important of the subordinate minerals. It fonnf 
light to dark yellow idiomorphic crystals, some of which are 1.5">" in 
length. In.thin section the crystals exhibit sharp outlines and a rongb- 
looking surface, and the mineral appears to have been the earliest to 
crystallize. 

' This la not the order ntated by Profee8«>r WilliamB, but ap)»earM to be that Bhowu by the Mctloni of 
»j)ecimen» collected for the Educatioual Series. J. P. I. 



wiiXRl DESCRIPTIONS: NO. 75, THERALITE. 197 

Apatite is qaite abundant in crystals, sometimes 2"*"" long. The 
larger crystals are comparatively short and stout, the smaller ones 
be'mg slender prisms, occasionally very thin and hair-like. In most 
eases colorless, some of the crystals are dnsted and contain rod like 
inclasions. Magnetite occurs sparingly in small anhedrons. Flnorite 
bas been observed in some varieties of the rock; also minute crystals 
of iegirite, which appear to be of secondary origin. 

According to Williams there is no zircon present. But certain 
luiDute crystals with high refraction and double refraction occur in the 
tbin flections from specimens in the educational series, which appear to 
be zircon. They may be easily confused with sphene. 

Williams points out the resemblance between pulaskite and the laurvi- 
kite of Norway, described by Brogger. The two rocks resemble one 
aDotljermineralogically and chemically, but differ somewhat in texture. 
Laorvikite is coarser grained. 

No. 75. Thebalite. 

(From Gk>RDON8 Buttr, Crazy Mountains, Meagher County, Montana. 

Dbscribrd by J. E. Wolff.) 

The Crazy Mountains, from the borders of which specimens 75 and 
76 were collected, constitute an outlying range situated a few miles east 
of the main mass of the Rocky Mountains in Montana and near the 
bonndary between the great eastern plains and the mountains. The 
Yellowstone Eiver flows around the southern end of the range and the 
Mngselshell, a branch of the Missouri, bounds it on the north. 

The mountains are composed of strata of Cretaceo-Tertiary age, 
Ijing for the most part nearly flat or at low angles, throngh which 
oaany kinds of eruptive rocks have been intruded as stocks, laccoliths, 
sheets, or dikes. Among these eruptive rocks certain peculiar dark 
basaltic rocks are found which occur generally near the periphery of 
the range, the center of which is formed of masses of dioritic rocks. 
• These basaltic rocks are coarsely crystalline when they occur in thick 
sheets or laccoliths, but fine grained and porphyritic when occurring 

■ 

in thin sheets or dikes, where the conditions of quicker cooling have 
b€en eflFective. They are characterized chemically by a low per cent 
^^ silica and high per cents of the alkalies and of lime, magnesia, and 
imn, together with exceptional amounts of barium and strontium, as 
^ill be seen in the accompanying analyses. Mineralogically they 
^I'e i)eculiar in the combination of augite, nephelite, a mineral of the 
halite group, and a feldspar which is chemically a potassium feldspar 
^Hh barium and some sodium, calcium, and strontium. In the original 
descriptions of the rocks this feldspar was determined as in part ordi- 
nary sanidine, in part a soda-lime feldspar, and the rocks were therefore 
niade the types of a plutonic rock characterized by the mineral combi- 
nation nephelite, soda-lime feldspar, and named theralite by Professor 
Hosenbasch. The feldspar appears to be, strictly speaking, Ti^\t\vct 



198 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [bulliSO. 

ordinary sanidine nor soda-lime feldspar, and hence for classificatory 
purposes the weight should be put on the chemical composition and 
the presence of nephelite and feldspar. 

Specimen 75 is from Gordons Butte, an outstanding hill on the north- 
western side of the range, which is oval in outline, with a diameter of 
2^ miles. The base of the butte is formed by sandstones, shales, 
and limestones, which lie nearly flat but dip gently in toward the 
center. Above these and toward the base, frequently interlaminated 
with the sedimentary rocks, is a great intrusive sheet of theralite which 
is nearly 600 feet in total thickness. The original cover of shale has 
been worn off and only little patches of loose, baked shale can be 
found. The great laccolith of eruptive rock ha« very perfect vertical 
prismatic structure or jointing, the columns of which lean gently 
inward, corresponding to the position of the contacts which conform 
to tbe dip of the containing strata. A great line of cliffs, nearly 
circular in outline, forms the outside edge of the summit, which in the 
interior is a basin with gently sloping sides. The specimens were gath- 
ered from the northwest base of the clififs and represent tbe coarsest 
variety of theralite found in the range. They come from the center of 
the mass, where the crystallization was coarsest. 

Description of the specimen {ha^id specimen), — The prisms of augite 
are tbe most striking characteristic. The}' have a roughly parallel 
arrangement, caused by the motion of the rock as it flowed parallel 
to the bounding country rock. The light-green border of segirite can 
be occasionally noticed. Large brown plates of biotite and occasional 
clear yellow grains of titanite are visible. The nephelite and feldspar 
make up the colorless part. The former is recognized by its yellowish- 
gray color and somewhat greasy luster, while the feldspar is white, 
with distinct glassy luster on the cleavage surfaces. In other places in 
this same laccolith the feldspar contains little crystals of haiiynite and 
nephelite distributed through it, and this causes a luster mottling or 
pcBcilitic structure on the cleavage surfaces. In the variety represented, 
here this character is obscure. 

In the thin section the augite is seen in stout prisms with both pina- 
coids developed and rarely terminal planes. It is pale green in color, 
passing into a deeper green toward the periphery, owing to mixture 
of the segirite molecule, which often results in a border of deep-green 
Fegirite in parallel orientation to the augite. The eegirite also occorfi 
independently in radiating prisms. The augite incloses crystals oi 
apatite and magnetite, and is often touched by plates of biotite, whicb 
may be oriented with the basal cleavage parallel to the augite prism. 

The biotite plates contain apatite, magnetite, and rarely augite. Their 
rounded outline and occasional embayment by the later-formed min- 
erals shows that they have been subjected to resorptive action. Small 
serpentinized grains of olivine inclosed in the biotite are uncommon io 
this occurrence, although found at other localities (see description of 
specimen No. 76). 



Miitt^l DESCRIPTIONS: NO. 76, THERALTTE. 199 

The feldspar occurs iu large, irregalar areas, which can be recognized 
as SQcb by their clear, transparent look and polarization tint, bluish 
white of the lower order. The two cleavages show distinctly only in 
very thin sections or near the edge, where the strain of grinding has 
developed it. It incloses little prisms of nephelite and rarely in this 
occnrrence square or hexagonal sections of bailjniite crystals, although 
this mineral is abundant in other parts of the same rock mass. 

The nephelite apx)ears also in large prismatic masses which have an 
imperfect crystal outline; the two cleavages, prismatic and basal, are 
well developed, giving generally two rectangular cleavages of which 
the prismatic is best develox)ed. It polarizes with almost the same 
tint as the feldspar, extinguishes parallel to the cleavages when both 
8bow, and basal sections are isotropic and give an indistinct cross in 
converging light. The nepbelite is in places ftesh, but generally more 
or less filled with decomposition products; some of these with bright 
polarizing tints and slightly yellow color (which is distinct in the hand 
specimen) are probably cancrinite, the rest zeolites, which give an 
^S^gate fibrous appearance. 

The feldspar in this rock has generally the optical properties of sani- 
dine, with the plane of the optic axes 5o from the base, small angle of 
the optic axes, and the axis of greatest elasticity an acnte bisectrix. 
Its chemical composition has, however, already been mentioned; the 
specific gravity seems to vary from 2.63 to 2.67. 

The order of crystallization is evidently first the apatite, magnetite, 
and olivine (since they are inclosed in the biotite and angite) ; then 
the aagite and biotite; then the nephelite, and lastly the feldspar. 
The»formation of the segirite at the close of the angite x)eriod and its 
occnrrence as the i)eripheryof augite crystals, suggests a caustic action 
of the magma which impregnated the forming or already formed angite 
^th the segirite material. The chemical analysis of the theralite from 
Gordon's Butte is given with that of the specimen from Alabaugh 
Creek, on page 197. 

No. 76. POBPHYBITIC THSBALITE. 

(Froh MouTn OF Alabaugh Crvkk, Crazt Mountains, Mkaqher County, 

Montana. Described by J. E. Wolfp.) 

Specimen 76 is from a sheet or bedded dike from Alabangh Creek at 
the north end of the mountains. The structure and mineralogical com- 
position are different from 75, although they represent the same rock. 

Description of the specimen (hand specimen), — A grayish-black rock 
contaming long slender prisms of augite with octagonal outline, hex- 
agonal plates of biotite and grains of olivine, which have a yellow core 
and msty red exterior. The groundmass is seen to glitter with little 
flakes of biotite, and small augite prisms can be detected in it. The 
augite crystals have a rongh parallel arrangement due to the flowing 
of the rock. 



200 THE EDUCATIONAL SERIES OF BOCK SPECIMENS, [bull. 150. 

In the thin section the angite phenocrysts have the same pale-green 
color and a^girite border as in 75, bnt the latter is mnch less marked. 
In addition to apatite and magnetite, they are honeycombed with incla- 
sions of the residual magma, which is generally individualized to nephe- 
lite or feldspar. Sometimes the groundmass has pushed into the augite 
in little bays which may contain biotite. The biotite is in distinct 
hesagoTial plates inclosing apatite, magnetite, and olivine; it has been 
corroded by the caustic action of the magma, which has produced the 
rounded forms sometimes seen. This biotite has a very marked obliq- 
uity of extinction to the cleavage. The olivine is in rounded grains 
touched or even surrounded by the biotite; between the two minerals 
there is a deep brown zone (iddingsite) which seems to grade into the 
biotite. The olivine incloses apatite, which occurs also independently 
in quite large crystals. 

The groundmass contains augite, biotite, magnetite, apatite, olivine, 
a mineral of the sodalite group in deep blue crystals with square or 
hexngonal outline (due to the fact that they are sections of the rhom- 
bic dodecahedron), and isotropic character. The colorless part of the 
groundmass is threefold; there are long colorless laths of feldspar, 
sometimes in Carlsbad twins, which optically correspond to sanidine 
(in other occurrences of this rock they may be anorthoclase) ; between 
these is a fibrous substance, often in prismatic aggregates, which is the 
zeolitized nephelite; and lastly clear glassy areas, which are nearly or 
quite isotropic, show traces of cubic cleavage and are analcite. 

It will be seen on careful study that there are two forms of feld- 
spar in the groundmass, one occurring in slender crystals, the other in 
larger irregular grains which seem to have formed later; there is proba- 
bly a chemical difference in these two forms corresponding to the 
different habit and period; whether one be sanidine and the other 
anorthoclase is not apparent. 

The distinction between the phenocrysts (crystals of the first gen- 
eration) and the same minerals in the groundmass is not very sharp, 
bnt the relations between the feldspar and nephelite are different from 
those in the other specimen, for here the nephelite is later than the 
prismatic feldspar, although perhaps contemporaneous with the other. 
We see that the olivine followed the apatite and magnetite and preceded 
the biotite. 

The following two analyses represent the chemical composition of the 
coarse and porphyritic theralite. No. 1, coarse theralite from Gordon's 
Butte, was made by W. F. Hillebrand in the laboratory of the United 
States Geological Survey, at Washington, and Nos. 2 and 3 by E. A. 
Schneider, in the same laboratory. Analysis No. 2 is of specimen No. 
75, and No. 3 of specimen No. 76. 



DiijLa.j 



DESCEIPTIONS: NO. 77, NEPHELITE-SYENITE. 



201 



Analyna of theralitefram Cragy Mountainif Montana, 



SiO, 

TiO, 

AV), 

FeA 

FeO 

MnO 

C»0 

SrOa 

BaO 

MgO 

K,0 

NajO 

Li,0 

H.OOielowllOoC). 

H,0 (above IKPC). 

P«0, 

COi 

CI 

S<),. 

Total 



I. 



Ptr cent. 

44.65 

.95 

13.87 

6.06 

2.94 

.17 

9.57 

.37 

.76 

5.15 

4.49 

5.67 

trace 

.95 

2.10 

1.50 

.11 

trace 

.61 

99.92 



Per cent. 
44.31 

(6) 

17.20 

4.64 

3.73 

.10 

10.40 

(6) 

(6) 

6.57 

3.64 

4.45 

(b) 

77 
3.30 

(&) 
(h) 
(b) 
(b) 

99.11 



Per cent. 

47.07 
ib) 

18.22 
3.65 
3.85 
.28 
8.03 
(b) 
(b) 
6.35 
3.82 
4.93 
ib) 

.38 
2.97 

(/') 
ib) 
(b) 
(b) 



100.16 



a Containe some CaO. 



b Undetermined. 



No. 77. Nephelitesyenitb (Eleolitb-syenite). 

(Pbom Litchfield, Kknkbbec County, Maine. Described by W. S. Bayley.) 

It is not certain that the rock represented by this specimen has been 

foQnd in place. It is known to occar in five or six localities in the 
Wcinity of Litchfield, Kennebec County, Maine, nsnally in the form of 
•wwlders lying on the surface, but sometimes in low ledge-like expos- 
^^^ uearly covered by glacial sands. The specimens in the collection 
^ere obtained from a pile of loose material lying on both sides of the 
^ranning from South Litchfield post-office, in the town of Litchfield, 
^ Kennebec County, Maine, to the city of Gardiner, on the Maine Cen- 
^I Railroad, about 6 miles south of Augusta. The distance of the 
locality from South Litchfield is about three-quarters of a mile, and 
^m Oardiner about 8 miles.^ 

The rock is a moderately coarsegrained crystalline aggregate of 
tlifee principal substances, none of which are porphyritically devel- 
^. Its texture is thus hypidiomorphically granular, or granitic. 
^Occasionally a general parallel arrangement of the constituents may 
^ detected, when the structure becomes schistose. 

The most abundant of the components is a white mineral, with bril- 
liant cleavage faces, often characterized by a pearly luster. This occurs 
both in large columnar grains from a quarter to a half inch in length, 

'Pordeecriptlon of other loctUtles see article by W. S. Bayley in Ball. GeoL Sec. America, Vol. Ill, 
P^2314S2. 



i 



202 



THE EDUCATIONAL SERIES OP ROCK SPECIMENS. D 



and in small irregular ones, so arranged in certain areas as to 
their aggregation to resemble a fine-grained marble. Tlie larger | 
which are albite, have a density varying between 2.600 and 2.6( 
a composition^ as follows: 

Analysis of alhiie of nephelite'Syenite from Litchfield, Maine, 




Another of the prominent constituents that may be seen in thi 
specimen is in irregularly shaped masses of a grayish color, and ^ 
oily luster. It possesses a well-marked cleavage, which is empli 
by the interposition of long black needles between the cleavage 
A fragment of this substance dissolves quite readily in hydnx 
acid, leaving a residue of gelatinous silica. Its composition, as 
mined by Dr. Clarke,^ is that of nephelite (eleolite) : 

Analysis of nephelite of nephelite'Sffenite from Lit^Jleldf Maine, 



SIC, 

A1,0, 

CaO 

MgO 

K,0 

Na,0 

H,0 

Total 



Per cent. 



43.74 
84.48 
trace 
trace 
4.S5 
16. 02 



100.25 



The only dark-colored mineral present in the rock is a lustrous 
lamellar one, which cleaves with such ease that large plates can I 
from it. These plates are elastic, and they show a nearly uniaxial 
ference figure in converged light between crossed nicols. The n 
is a biotite of the variety known as lepidomelane, as shown 1 
Clarke. 3 

> F. W. Clarke, Am. Joar. Sci., 3d series. Vol. XXXI, p. aoa. 

* Ibid., p. 262. 

* Ibid., Vol XXXIV, p. 138. 



descriptions: no. 77, nephelite-btenite. 



20s 



AntiljfwU of Upidamelane of nepkeliio-^enUe from LiichJUldf Maine. 



Per cent. 



SiO, 82.35 

AltQ, I 17.47 

F6,0, ; 24.22 

FeO 18.11 

MdO 1.02 

CaO ' .89 

K,0 . .70 

N%0 6.40 

HiO 4.67 

ToUl 100.83 



The three substances, albite, eleolite, and lepidomelane, occnr in all 
specimens of the rock wherever found, and are thus to be termed 
eneDtial constituents. The other components, though very common, 
tre not found in all specimens, and hence are accessory. They com- 
prise cancrinite, sodalite, and zircon, and are the minerals that have 
made the rock famous all over the world. The most common and at 
the same time the most striking of the accessory constituents is can- 
crinite, which is in very irregular lemon-yellow and orange grains, that 
ue scattered indiscriminately among the other components, but appear 
to prefer the neighborhood of the nephelite. The orange-yellow variety 
Tidded Dr. Clarke ^ the following analysis: 

AntUftU of cancrinite of nephelite'eyeniie from Kennebec County, Maine, 



SiO, 

AM), 

Mn/H 

Fe,0, 

C«0 

N«,0 

K,0 

HaO 

CO, 

Totol 



Percent. 



86.29 

30.12 

trace 

trace 

4.27 

19.66 

.18 

2.96 

6.96 



100.36 



l^'pon comparing this analysis with that of the nephelite. Dr. Clarke 
^hes the conclusion that the former is an alteration product of the 
^tter. The sodalite is much less common than the cancrinite. It is 
^sseminated in small blue grains throughout the rock mass, and occurs 

•Am. Jonr. Sci., 3d iieries, YoL XXXI, p. 263. 



I 



204 



THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [bcll.150. 



as coatings on its joint cracks. The composition of the sodalite has 
also been determined by Dr. Clarke. It is as follows : 

Analyaia of sodality of nephelite-syenit^ from lAtehfieldf Maine, 



Per cent. 



SiO, 37.33 

AljOj 31.87 

Na,0 24.56 

K2O ! .10 

CI 6.83 

H,0 1.07 

= CT 1.54 

Total 100.22 



_j' 



The zircon, the only remaining mineral that can be recognized in the 
hand specimen, is the single component of the rock that possesses a cryft. 
tal form. It is in hard, pinkish-brown grains, with an octahedral habits 
that may be found here and there among the older constitnents. It^ 
crystals are bounded by the tetragonal prism and pyramid of the sam^ 
order. An analysis by Gibbs ^ gave: 

AnalyHs of zircoti of neplielite-syenite from Litchfieldf ^faine. 




SiO, 35.20 

ZrO, 63.33 

Fe,0, ; .79 

irndetermlned .36 

Total 99.74 



The texture of the rock as seen under the inicroscojie is thorougli.1 
granitic, in that none of its essential components possess crystal ovm. 
lines, although many of the nephelite grains and the larger albitesha*^ 
quite well defined rectangular cross sections. In ordinary light 
appears to consist of plates of green mica embedded in a nearly hoiTB^ 
genous, colorless, transparent groundmass, clouded here and the^x 
with opaque white and yellowish substances, that seem to be decom|>< 
sition products of some constituent. Under crossed nicols, hower^< 
this groundmass is resolved into large and small grains of the mineral 
detected in the hand specimen, besides orthoclase and microcline. (3^ 
PL XXIX.) 

The mica, which is the oldest mineral present, with the exception 
of the zircon, occurs not only in the large plates already mentioned, but 
also as inclusions in the other components, more particularly the eleo- 
lite. In basal sections it is so dark as to be almost opaque, except in 
extremely thin pieces, when it is of a clear, deep-green color. The 
optical figure when tested with the mica-plate is found to be negative, 



1 Pogg. Ann., LXXIf p. 559. 



MLUi.] DESCRIPTIONS: NO. 77, NEPHELITE-8YENITE. 205 

ooDseqnently the acate bisectrix, which, in the biotites, is nearly coin- 
cident with the crystallographic axis c, is the axis of greatest elasticity, 
a. In cross sections the pleochroism is very pronounced. The ray 
TibratiDg at right angles to the cleavage is a bright yellowish green^ 
while that vibrating parallel to it is nearly all absorbed. The scheme 
for the absorption is therefore a < b = jc. The extinction in these sec- 
tions, as measured against the cleavage, varies from 0^ to 1^. In its 
optical properties, as well as in its chemical nature, the mica corresponds 
tobiotite. 

The largest of the colorless components is that clouded with opaque 
sabstaDce. This is present with rudely rectangular cross-sections, in 
which a single cleavage may be detected, parallel to which is the extinc- 
tioD. The relief is slight, consequently the index of refraction of the 
subiitaDce is low. Its opacity in places is due to the crowding together 
of litUe glass and fluid inclusions, in some of the latter of which are 
tiny, movable bubbles, and of small flakes of a brilliantly polarizing 
micaceous mineral that are visible only between crossed nicols, and are 
probably secondary in origin. The only other inclusions noticed are 
narrow flakes of lepidomelane. These are arranged with their long axes 
in parallel directions, that are likewise parallel to the extinction plane 
of their host. A few of the rectangular sections when revolved between 
crossed nicols remain dark during the entire revolution. These in con- 
verged light show a uniaxial interference figure. All other sections 
polarize in gray or bluish- gray tints, and these tested by the mica plate 
are discovered to be negative. If the slide be uncovered and treated 
with hydrochloric acid, and then washed and dipped in a dilute solution 
of aniline purple, the areas occupied by this mineral will be found to 
^ stained purple, indicating the presence of gelatinous silica. All 
these properties are those of nephelite. 

The most abundant of the feldspars is the albite occurring in the 
columnar forms already mentioned. In the thin section it possesses 
long qaadraugnlar outlines. Its grains are characterized by a series 
of remarkably fine twinning lamella; that bend and curve, disappear 
suddenly at cleavage cracks, and reappear again in other parts of the 
S^ns— phenomena indicating that the mineral exhibiting them has at 
^metime been subjected to great pressure. A close inspection of the 
^n% will disclose the fact that some of them are made up of very fine 
Teliae of different feldspars, in which the extinction is different. 
Others consist partly of lamellae in which the gridiron structure of 
^icrocline is plainly apparent. The resemblance of such sections to 
the pictures of cryptoperthite and microclinemicroperthite ^ is so strong 
*8 to saggest the probability of their being, like the latter, inter- 
STowths of two feldspars. A feature well worthy of notice in connec- 
fion with the singly striated grains is the fact that they are penetrated 
in all du*ections by jagged embayments of a pellucid plagioclase with 
'broader twinning lamellft; than those of the rest of the grain. Small 

'^'•C. Brogger, Zeitttchr. fur KryuUillograpbie, Vol, XVI, Jt*l. XXII, lig. J, and PI. XXIll, tig. 4. 



206 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bull. ISO 



areas of this glassy feldspar occar all through the albites, so that th< 
latter appear to be completely saturated with it. The clear feldspai 
polarizes in gray and blue tints and always has ragged outlines agains 
the iuclosing albite. From the fact that the saturatiug material is 6< 
much fresher than the material of the large grains, and because of its 
peculiar saturating character, it must be regarded as probably youngei 
than the albite which it penetrates and as having been formed aftei 
the rock had consolidated. 

By revolution of the slide between crossed nicols the transparent 
colorless groundmass in which the mica and the cloudy albite and 
nephelite are embedded is found to break up into a mosaic of ver} 
brilliantly polarizing grains all of about the same size. This mosaic 
occupies all the space between the large grains and is so distributed ac 
to appear to fill what were at some time fissures in the rock. (Cf. fig 
Aj PI. XXVIII.^) The greater number of the grains in the mosaic an 
feldspar. Some are marked by a single set of twinning bars, others b; 
two sets crossing each other at nearly right angles, and still others ar^ 
untwinned. The large number of the latter noticed is an indication a 
the presence of orthoclase, though this can not be proved on accouo 
of the lack of cleavage lines, and of crystallographic contours in tlM. 
grains. Those showing two sets of twinning lamelL'e are probabl. 
microcline. The larger proportion of grains are of the first kind, ba 
their lamellffi are so bent and bowed that their extinction angles can no 
be read with any degree of accuracy. The only method by which t 
knowledge of the nature of the various feldspars may be obtained is bj 
their separation and analysis, and a comparison of the figures that 
obtained with those indicating the composition of the rock as a whola 
By use of the Thoulet solution, it will be found that two lots of feldspar 
fall when the density of the solution reaches 2.622 and 2.56, respectively. 
That which falls at 2.622 consists of grains usually striated in a single 
direction, and others in which no striations are visible. The latter 
extinguish at 19^ from the cleavage, and show in converged light the 
bar of an axial figure. An analysis of some of this powder made by 
Mr. W. H. Melville gave : 

Analysis of albite of nephelite-sifeHite. 



Per oent. 



Si0> 68.28 

AltOt I 18. 82 

FeO ': .23 

CaO ' .31 

MgO I .W 

K^ 39 

N»j0 i 10.81 

H,0 .09 

ToUl 99.82 



^The stmoture jirodiioed by the embedding of the larger oomponente of a rock in a finer grti^td 
aggregate, has been tenned the " mortar " structure by Tomebohm. It is thought to be the re«iil( o* 
pressure. 



Mual DE8CBIPTIONS : NO. 77, NBPHELITE-8YENITE. 207 

This is the comxMsition of a very pure albite. In a separation made 
by the vriter, the powder whose density was 2.56 comprised some 
ontwiimed grains, and many with the cross twinning of microcline. 
Its analysis gave the figures: 

AnalyBia ofmiorocline and orihotHase frtym nephelite-ByeniU: 

Percent. 



SiOi 65.14 

Al,Oi I 18.19 

FeO ' .25 

C«0 33 

MgO .10 

K«0 14.14 

K»,0 1.68 

HaO 17 



Total 99.82 



I 



This powder was thus a mixture of microcline with a little orthoclase. 

It is very evident that the feldspar grains of the mosaic are younger 
than the eleolite and the large grains of albite. Their smaller size, 
perfect transparency, lack of cleavage lines, and their method of occur- 
fBDce, in narrow stringers and small areas between the undoubted 
primary constituents, point to a secondary origin for them. The cause 
of the production of this new feldspar in the rock, which had already 
consolidated and been fissured before the formation of the mosaic, was 
probably the pressure to which it was subjected at some time in its 
lustory, and of the action of which we have ample proof in the bending 
of the twinning lamella of the feldspars and in the ^^ mortar" structure 
of the rock itself.* 

In addition to the feldspars in the mosaic there is also present in it 
uiother brilliantly polarizing substance forming prismatic and lentic- 
ular grains. In natural light it is indistinguishable from the newer 
feldspar, except in very thick sections, where it has a slightly yellowish 
^ge. It is transparent and free from inclusions of all kinds, save 
little liquid ones, inclosing movable bubbles. Two cleavages cross 
Q^ly sJl its grains at right angles to each other, and the extinction is 
P^lel to these. Sections that remain dark between crossed nicols 
show a uniaxial negative interference figure. The index of refraction 
^ BO low that grains have no relief. These properties sufficiently char- 
acterize the mineral as cancrinite. It is older than the other constit- 
uents of the mosaic, but is younger than the eleolite and the albite of 
the larger grains. 

A few other grains in the mosaic and in the neighborhood of the 
^cphelite remain completely dark in all positions between crossed 



fv » diacuMioift of the origin of secondary minemlft in dynamically metaroorphoBed rocks, M«e 
^■GOUway, Qnart. Joar. Geol. Soo. Londou, Aug., 1889, p. 475, and G. H.Williams, Bull. U. S. (iecl. 



i 



208 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. Ibullim 

nicols, and when examined in converged light show no axial figares 
These are sodalite. In ordinary light the mineral has a very pale-bla* 
tint that can be recognized only by the contrast afforded by the coloi 
lesH minerals in the field of view, wliich appear to be tinged witl 
yellow. 

The inclusions most abundant in it are grains of plagioclase, smal 
plates of lepidomelane, cancrinite, and a few flakes of a brightly polai 
izing micaceous substance. Kephelite is often intergrown with it ii 
such a way that a large number of apparently isolated areas of th 
former mineral polarize together. Since the sodalite includes all th* 
minerals of the rock except nephelite, even those that are younger thai 
this, and since it is intergrown with the latter, it must be an alteratioi 
product of it. It is the youngest of all the constituents. Not only i 
it present in irregular grains that include small particles of the othe 
components, but it is found also as a cement binding together thi 
grains of the mosaic. 

The only remaining mineral to be spoken of is zircon, but this is s< 
rarely seen in thin section that its discussion may be dismissed with t 
very few words. It occurs as small irregular grains, of a brownish 
yellow color, with a very high index of refraction, and strong donbl 
refraction. No cleavage cracks cross them, nor are crystal contour 
sufficiently sharply marked to aid in the determination of the extia^ 
tion. Occasionally a fragment remains dark during its revolutio 
between crossed nicols. This may be made to show a uniaxial figure 
which, when tested, is found to have a positive character. 

The composition of the rock as a whole as determined by Mr. L. G. 
Eakins, is as follows : 

Analysuf of nephelite-syenite from Kennebec Comii$ Maine. 

Per ot?nt. 



22. 5! 
2.20 



SiOa I 60.3U 

! A1,0, 

' FejO, 

FeO 

MnO I .08 

I CaO j .:J2 

MgO ' .13 

K,0 I 4.77 

NftjO I 8.44 

I HjO ." 57 

ro^ ' trHC© 



The small quantity of KaO as compared with the large amount of 
Na^O present would indicate a scarcity of orthoclase and an abuu- 
dance of albite among the feldspars. A calculation of the proportiom^ 
of the different constituents based upon the analyses given leads to tbe 
figures: 7 per cent of lepidomelane, 2 \)er cent of cancrinite, 17 per 



MUAl DESCRIPTIONS: NO. 78, NEPHELITE-SYENITE. 209 

cent of nephelite, 27 per cent of orthoclase and microcline, and 47 
perceut of albite. No other feldspars are present than albite, ortbo- 
ekse, and microcline. 

The rock from Litchfield is thus a granitic aggregate of the essential 
eonstituents, lepidomelane, nephelite, and albite, and the accessories 
arcon, cancrinite, sodalite, albite, orthoclase, and microcline. It has 
been shattered and fissured as the result of the action npon it of great 
piessore, and the crevices thas formed have been filled with new feld- 
spar and other minerals. As its original structure was granitic, and 
its original components are nephelite, biotite, and an acid feldspar, the 
rock must be classed with the nephelite-syenites. But as the acid 
feldspar is largely albite, whereas in normal nephelite-syenites it is 
principally orthoclase, the Maine rock represents a well-marked variety 
iD the nephelite-syenite group, a variety that has been given the 
descriptive name litchfieldite.^ 

Most of the eleolite-syenites that have been described from North 
America are normal phases, containing large quantities of orthoclase, 
snii, iu addition, pyroxene, amphibole, and sphene. For descriptions 
of these the reader is referred to the original articles.^ 

No. 78. Nephelite- SYENITE (Eleolite-syenite). 

(From Bebmervillk, Sussex County, New Jersey. Described by J. P. 

IDDINGS.) 

The Dephelite-syenite from Beemerville, New Jersey, is a medium- to 
^^arse-grained, dark-gray rock, with a somewhat greasy luster. Its 
texture varies considerably from a fine-graim d, evenly-granular mass 
to a coarse-grained one, with prominent feldspars, which present long, 
J^arrow sections on the surface of the rock. The rock forms a dike cut- 
ting Hudson River shales, and has been described in detail by Prof. 
^* E. Emerson.^ In places the rock contains 90 per cent of nephelite. 
lender the microscope it is seen to consist of nephelite, orthoclase, 
^girite, biotite, with melanite, sphene, apatite, and zircon in smaller 
aiDOQiits and in quite variable prox)ortions. In fact the mineral com- 
position differs considerably in different parts of the mass. 

^D general nephelite (eleolite) appears to have crystallized before the 
ft]dg|)ar, when the nephelite is idiomorphic or is inclosed within the 
feldspar in irregularly-shaped crystals. Its substance is quite fresh, 
^ith little or no indication of decomposition. In some places there 



'M. Ged. Soc. America, Vol. Ill, p. 243. 

'B< K. Emeraon, On a great dike of foyaiteor eleolite-syenite, cutting the Hudson River shales in 
"^livettem New Jersey : Am. Jour. Sci., 3d series, Vol. XXIII. 1882, p. 302. Lacroix, Sur la syenite 
^i^th^Qe de Montreal (Canada) : Comptes Rendns. ex. 1890, p. 1152. J. F. Williams, The igneous 
"*k»of Arkanaas: Ann. Rept. Geol. Survey Arkansas, 1890, Vol. II, p. 129 et neq. W. S. Bayley. 
'^dcotite-syenite of Litchfield. llaini\ and Hnwea* hornblende syenite from Red Hill, New Hamp- 
•'«W: BttIL GeoL So<- America, Vol. Ill, 1892, pp 231-252. 

^•K. Emerson, On a great dike of foyaite or eleolite-syenite. cutting Hudsou River shales in 
*''**»fiteni New Jersey : Am. Jonr. Sci., 3d series. Vol. XXIII, 1882, pp. 302-308. 

BuU. 150 U 



210 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bvu^I 

are minute irregular crystals of a transparent mineral, with stroi 
double refraction and lower single refraction than uephelite; this 
probably cancrinite, and may be secondary. The prismatic cleavage 
somewhat developed, but not marked. Its index of refractioD beii 
higher than that of orthoclase, the nephelite stands out in distin 
relief when in this mineral. The double refraction for both miners 
ranges from zero to grayish white of the first order, and is not a mea 
of distinction between thom. 

Tbe feldspar appears to be wholly orthoclase, in some cases exhib; 
ing microperthite intergrowth with plagioclase, presumably albit 
The outline of the feldspar crystals is generally allotriomorphj 
Twinning according to the Carlsbad law is common, that according 
Baveno law also occurs, in simple twins, and also in crossed twin 
furnishing cross sections with triangular quadrants, of which the opi: 
site pairs have like orientation. The ordinary cleavage is often d 
tinct. A slight decomposition has iiroduced a cloudy indeterminal: 
alteration product in some cases. In general the feldspars are Ic 
fresh than the nephelite. Both of these minerals are traversed 1 
veins filled with an isotropic medium, probably sodalite. ^giri 
occurs in crystals with irregular outlines, sometimes better defined 
the prismatic zone. The color is dark green with pleochroism 
brown. Cleavage is pronounced. The large crystals of eegirite inclo 
most all of the other rock constituents in small crystals or grains; tlii 
is, orthoclase and nephelite, and more often sphene, biotite, melanit 
and magnetite. In some places all of these minerals are so iutc 
mingled as to appear to be contemporaneous in growth. 

Biotite forms irregular crystals, with dark reddish-brown color an 
strong absorption. It may inclose sphene, iegirite, magnetite, apatite 
and zircon. Melanite forms irregular grains, with a dark-brown oolo 
high refraction, and isotropic character. It is not uniformly dissem 
nated through the rock. It incloses ^girite, magnetite, and spben 
and appears intergrown with feldspar and nephelite to some extent 

Sphene is generally idiomorphic, and is abundant. It occurs in mo 
all the other constituents in well-defined crystals, but is sometim 
allotriomorphic with respect to iegirite and biotite, and occasional 
incloses small crystals of cnegirite. Twinning is frequently observed. 

Apatite is abundant in more or less rounded, short, stout crysta 
associated with the ferromagnesian minerals. It is sometimes in sha 
crystals. Magnetite forms irregularly shaped grains intimately oc 
nected with the dark colored minerals. Zircon occurs in minute cr 
tals, sometimes short and stout and rounded, sometimes long ai 
narrow. 

On account of the variability of this rock in texture and in minei 
composition, it will be found that the specimens in the collection dif 
considerably in both these respects. The largest of the feldspa 
according to Professor Emerson, are 30°*"^ in length. In some pai 
of the rock he estimated the percentage of nephelite present at 90. 



descriptions: no. 79, ANDE8ITIC TUFF. 



211 



Tbe chemical composition of the average rock is given in the accom- 
pauyJDg aualysis, which was made by L. O. Eakins. 

Analysis of nephellie-ayenite. 



Percent. 



SiO... 

Al A 

Fe,0, 

FeO.. 

MnO.. 

NiC. 

CaO. 

MgO. 

K,0.. 

Na^. 

H,0.. 

CO,.. 



53.56 

34.43 

2.19 

1.22 

.10 



1.24 

.31 

9.50 

6.48 

.93 



Total 



99.96 



DIORTTE-ANDESITE FAMILY. 

No. 79. Andbsitic Tuff.^ 

(Fbox Stillwatisr Creek (near Binbhart's), Eight Miles Northeast of Red- 
, DING, California. Described by J. S. Diller.) 

This gray, earthy rock in the hand specimen is not conspicaously 
j fragmental, bat if examined closely it will be foand to contain lighter 
f and darker colored pebbles embedded in a gray groundmass. In the 
clifffrom which these specimens were collected it is readily seen that 
tbe white portions are fragments of pumice and the darker ones are 
andegitic lava, so that the material of which the rock is composed is 
evidently of volcanic origin. 

Od Stillwater Greek there is a small mass of this taff now exposed, 
and it is many miles from the nearest volcanic center. Formerly it 
was connected both eastward and westward with larger masses of taff, 
vhich have a wide distribntion nxK>n the borders of the Sacramento 
Galley. Upon the western side of the Sacramento Valley the material 
is very fine. Along the Stillwater it is intermediate in size, and on 
tbe eastern side of the valley, where the mass is very thick, it is coarse. 
Since sediments of volcanic origin are coarsest and their accumulation 
is tbickest very close to the point of eruption, it is evident that the 
aonrce of the tuff on Stillwater Greek is to be found to the eastward in 
the Lassen Peak district. Where best exposed the tuff is distinctly 
stratified, and was evidently dex>osited in a body of water which filled 
tbe Sacramento Valley. 

In the hand specimen, besides the small fragments of andesite and 
pomice already mentioned, there are small, dark specks, which, when 

'IiMlMd of tMg the word tufa is sometimes written. The former should be nsecl for f ragmen tal 
Tolcssie rocks only, and tbe latter for certain forms of carbonate of lime, as " caloareons tufa, " depos- 
tcdfrom station in water. 



212 



THE EDUCATIONAL SERIES OP BOCK SPECIMENS. Ibullim. 



removed from the specimen with the point of a knife blade, ernsbed 
and examined in polarized light, are found to have the cleavage, stroog 
pleochroism, and inclined extension belonging to hornblende. 

Grains of feldspar, too, are quite common in the hand specimeD, bat 
on account of the fragile nature of the matter in which they are 
embedded many of the crystals break out of the thin sections wben 
the material is being ground. In the thin section the remaining feld- 
spars are rarely idiomorphic. They are usually found to be very irreg- 
ular, corroded or broken crystals, containing many glass and liquid 
inclusions. Their tabular form, zonal structure, and angle of extinc- 
tion agree very closely with those of andesine from the andesitesof 
the Lassen Peak region. Fragments of magnetite and of hornblende 
crystals are not common, and those of hypersthene are rare. 

The gray gronndmass, which constitutes at least 7o x>er cent of tbe 
tuff, is made up of minute fragments of volcanic glass. This is easily 
discovered when examined with a higher magnifying power. It is then 
seen to be composed of such curiously formed particles as are repre- 
sented in iig. 15. a has the same tubulo-vesicular structure as the 





6 a ^ c 

Fig. 15.— Frainnents of Tolcanio glass In taff as seen under the microacopo, X 50. 

larger fragments, and in fact is a minute particle of pumice. Wbere 
thin, the glass is clear and transparent, but where thick it is sligbtly 
dark colored, b represents the curved wall of a broken bubble, (r isa 
small vesicle, still complete and surrounded by tubular glass, and d is 
interstitial glass between several bubbles and is not tubular. All are 
sharp, angular particles of glass, exactly analogous to volcanic dust. 
Although the macroscopic evidence indicating that the rock is tuffa- 
ceous is strong, the mi(TOscopic evidence is still stronger, and demon- 
strates comx^letely its volcanic origin. 

The analysis given below shows its chemical composition, as deter- 
nuned by W. H. Melville: 

Analysis of andeaitic tuff from Stillwater Creek, California, 



Loss . 
SiO,.- 
Al^O, 
Fe^O, 
FeO.- 
CaO-. 
MgO. 
KaO.. 
Na,0. 



Per cent. 

3.63 
69.51 
15. Gl 
0.56 
1.27 
2.80 
0.61 
2.81 
3.43 



Total ; 100.23 



^un.1 DE8CBIPTIONS: NO. 80, DACITE. 213 

Tuff, in its widest sense, is applied to all fragmental rocks composed 
f Tolcanic material, but it is more ireqnently used to designate ouly 
bo8e which are composed of fine volcanic detritus, such as lapilli, 
land, or dust. Those made of coarse fragments, somewhat assorted 
Micording to size, are called volcanic conglomerates; while others, 
which are composed of large, angular blocks as well as fine material — 
all intermingled, as they frequently are in the immediate vicinity of 
the volcanic vent — are designated volcanic agglomerate. 

Although much of the iragmental volcanic material about volcanic 
veote is more or less rounded, forming conglomerates, there are also 
many cases in which it is sharp and angular, forming breccias. These 
lesalt not only from accumulations of angular ejected material, but 
also, and perhaps frequently, from the breaking up or complete breccia- 
tioD of brittle viscous lava at the time of its eruption. 

Uicir-The so-called pozzolana, of Italy, and trass, of the Eifel, in 
Gennany, are tufis, and are extensively used in the manufacture of 
bydraalic cement. 

Near Paskenta, California, a tuff of the same stratum as that which 
occnrs on the Stillwater is used for making water coolers. As it is 
poioas it allows the water to evaporate rapidly through the sides of the 
vessel and thus cools the contents of the jar. Tuff, on account of its 
composition, is a x>oor conductor of heat, and does not readily crack 
vben exposed to fire. On this account it is used quite extensively in 
volcanic regions for constructing chimneys. It is soft and easily cut 
into any desired shape, but, as it is easily crushed, it can not be used for 
large structures where it will be subjected 'to great pressure. 

No. 80. Dacite. 

(Feom Bear Crkbk Falls, Shasta County, California. Described by J. S. 

DiLLER. ) 

In the field this dacite is of comparatively small extent, covering 
^^Y a few square miles. The forks of Bear Creek, in cutting their 
^^yons, have severed the original mass into five smaller masses. They 
^) rest upon andesitic tuff, which at the time the dacite was erupted 
^as DDconsoIidated, so that the overflowing dacite picked up and 
inchded a multitude of small fragments of andesite. The flow has a 
^ximam thickness of about 80 feet. Its upper portion is compara- 
^^elyfree firom included fragments, but below it is iiill of them, and 
^ closely resembles the tuff by which it is underlain that the line 
t^tween the two rocks is generally indistinct. Some of the cliffs of 
^^ dacite along Bear Greek exhibit a remarkably well-developed 
^lomnar structure. 

It is a rough, gray rock, containing short streaks of black glass, 
^PPToiimately parallel, giving to the rock a decided fluidal structure. 
fkis feature is usually much more conspicuous in the rhyolites than in 
^ke dacites. In the field this structure is parallel to the surface, and 



214 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [BrmlSft. 

is plainly seen to have been produced by the flowing of the lava at the 
time of its eruption. 

In the hand specimen it may be observed tbat the larger streaks of 
black glass are distinctly perlitic, and sprinkled with phenocrysts of 
feldspar. The gray portion is somewhat mottled, and envelops occa- 
sionally small fragments of andesite picked up at the time the lava wan 
efiused. 

Under the microscope, as seen in PI. XXX, the rock becomes plainly 
porphyritic, with angular fragments of feldspa.r, hypersthene, horn- 
blende, and quartz embedded in a light-colored groundmass of glass, in 
which the flnidal structure is well displayed. 

The feldspar occurs in irregular tabular crystals and fragments o: 
^rystalH, which generally show the twinning lamellae and angle of sym 
metric extinction belonging to andesine and labradorite. Polysyn 
thetic twinning occurs according to both albite and pericline laws, bai 
the former is much more frequent than the latter, and they are gener 
ally combined. The angle of extinction of cleavage plates on M and I 
ranges from zero to 20^, and lies chiefly about 13^^ indicating labra 
dorite. 

There is a small amount of sanidine present, indicated in part byth< 
absence of twinning lauiellae, but chiefly by the low specific gravity 
A56, and the presence of considerable potassium, as shown by thecbemi 
cal analysis of the rock. Only a portion of the x>otassium is to be rele 
gated to sanidine, for the greater portion, as has been shown b; 
chemical tests, is within the glassy groundmass. 

The feldspar contains numerous liquid and glass inclusions, beside 
a smaller amount of magnetite, quartz, pyroxene, and hornblende, whicl 
crystallized at an earlier stage than the feldspar in the solidification a 
the rock. A striking feature of the feldspar is the rarity of perfec 
crystals. In form it is irregular and fragmental, affording evident 
that the crystals were broken by the flowing viscous lava during it 
eruptions. Feldspar is more abundant than all the other mineral 
combined. 

In ferromagnesian silicates this rock is especially x>oor, and of thes 
hypersthene and hornblende are the only ones present. Hypersthen 
is most abundant, and, like the hornblende, it occurs in irregular pbei 
ocrysts and small grains. 

Quartz is rare in angular fragments and round grains, containin 
dihexahedral glass inclusions. 

The glassy groundmass constitutes nearly two-thirds of the rocl 
and has a marked fluidal structure due to irregular streaks of clea 
colorless glass, alternating with others that are clouded. The cles 
streaks are frequently full of elongated glass cavities, as in pumice, an 
have perlitic structure, as shown in PI. XXX. The clouded ones ai 
composed chiefly of glass containing a multitude of amorphous, dns 
like particles, and a few irregular grains of feldspar and other mineral 
Occasionally they contain spheruiitic portions. 



DESCRIPTIONS: NO. SI, DACITE. 



215 



The chemical composition of the dacite, determined by B. B. Biggs, 
as follows: 

Analysis of daciie from Bear Creek Falls, Shasta County, California. 



Per cent. 



SiO, M.10 

TiO, j 0.15 

AljO, 15.50 

Fe^s I 3.20 

FeO ! none 

P,0» : 0.03 

MaO I trace 

BaO ' 0.06 

SrO ! trace 

CaO 3.02 

MgO 0.10 

LiiO I none 

Xa^ , ! 4.20 

K,0 3.13 

HiO 2.72 



Total 

Drie<l nt 105° C. 



100.21 



The small amount of iron oxide and magnesia present is due to the 
small proportion of ferromagnesian silicates. The rather large amount 
of lime present is foand in the labradorite, while the soda and potassa 
are foaud, as in the Lassen Peak dacite, chiefly in the gronndmass. 

No. 81. Dacitb. 

(Fbom Spring Vaixky Road, Eureka County, Nevada. Described by J. P. 

IDDIN68.) 

Thig rock occnrs as part of a surficial body of andesitic perlite, 
which varies in mineral composition and character, the most siliceous 
•Bodification being dacite. 

The dacite is compact, with an earthy texture and rough fracture. 

It is bnflf, with lumps of pink, yellow, or white tuff, and is crowded 

^th phenocrysts of black biotite, small amounts of hornblende and 

Pyroxene, abundant feldspars, and numerous dark-colored quartzes. 

'^e quartzes resemble those in the rhyolite of Pinto Peak in this col- 

^^tion. The character of the rock differs somewhat in different speci- 

^^Q8. In thin section the gronndmass of the rock is seen to have been 

^^ which is more or less altered and devitrified. It exhibits char- 

^eristics of glasses that apx)ear as welded fragments and bits of tuff, 

*^ving a marked flow structure. It is seldom isotropic, but is faintly 

doubly refracting and in places consists of minute colorless globules. 

-Hiere is much semiopaque cloudy material of an indeterminable 

l^ture. Occasionally there is distinct spherulitic crystallization, and 

^ places the gronndmass is microcrystalline in irregular and iWd^&we^ 



216 THE EDUCATIONAL SEBIE8 OP BOCK SPECIMENS. Ibull15»- 

grains. The microstruoture of the rock varies considerably in differeim -t 
parts of it. 

The phenocrysts of feldspar are plagioclase: four-fifths of theind^x- 
viduals are striated, and the symmetrical extinction angles range froMin 
a few degrees to about 30^. They belong to the andesine-labradorifc^ 
varieties. The double refraction corresponds to that of felds]iars ricb^r 
in lime. For the most part they are in angular fragments, but sonde 
exhibit the customary crystal outlines. Inclusions of colorless glass in 
rectangular shapes, and with an inclosed gas bubble, are common, and 
in some individuals are abundant. Clouds and streaks of dust-like 
particles prove to be made up of minute glass inclusions, 0.002""*" in 
diameter, together with grains of iron oxide, magnetite, and opaque 
rods. The streaks are often in parallel lines, whose orientation in the 
feldspar crystal is not determinable. They suggest the dust-like mcln- 
sions characteristic of the plagioclase of many gabbros. Minute crys- 
tals of apatite and zircon, and of the accompanying ferromagnesian 
minerals are also included. The substance of the feldspar is fresh and 
unaltered. 

Quartz is much less abundant and occurs in rounded and irregular 
grains. Its substance is colorless in thin section. Olass inclusions in 
negative crystal cavities are common, and also those of groundroass. 
Other inclusions are seldom observed. 

Hornblende is quite abundant in crystals and fragments. The crys- 
tals are well developed in the prismatic zone, with unit prism and 
clinopinacoid, less often the orthopinacoid. Terminal planes are rare. 
Its prismatic cleavage is characteristic. The color is brown, with a 
tinge of green; the absorption is strong, and, as usual, c^b>a. It is 
free from dark border, and has no characteristic inclusions, beiug Id 
general quite free from them. Its substance is unaltered. 

Of the pyroxenes, the orthorhombic form is more abundant than tbe 
monoclinic. Hypersthene in irregularly shaped crystals is more or less 
altered to a fibrous decomposition product. The unaltered hyi)erstheD6 
exhibits a slight pleochroism in thin sections, becoming more marked 
in thicker sections. It is green parallel to the axis jc, light reddisb 
brown parallel to a and b. The substance is generally free from incla- 
sions, but sometimes bears numerous magnetite grains and glass 
inclusions, besides apatite needles and grains of hornblende. Wheu 
altered the hypersthene passes into a fibrous green mineral, who:^ 
fibers are about parallel to the crystallographic c axis. The alteratiou 
product has the optical properties of actinolire.' Augite occurs spar- 
ingly in pale- green grains and crystals, and is distinguished from tU^ 
hypersthene by its lack of pleochroism, monoclinic characters, and \>y 
its generally unaltered condition when near hypersthene which is mor*5 
or less decomposed. 

Biotite is the most prominent ferromagnesian mineral in the moi 



^See HgB. 5 and 9, PL III, Monograph XX, Q«ol. of the Buroka Dbtriot. WMhlngton, IM. 



Dun.] DESCRIFFIONS: NO. 82, DACIT£. 217 

qaartzose varieties of this rock. It forms six-sided ]>latc'S, is dark 
bruwBintbin sections, with strong ab.sorption. It possesses a small 
optic angle, behaving almost as a uniaxial mineral. It sometimes 
ex)iibit;8 the common twinning. Inclusions are rare. 

Mai^Detite, apatite, and zircon occur as subordinate minerals. The 
zircon crystals often have sharp outlines and well developed forms, 
inclading the two unit prisms and pyramids, and the ditetragonal 
pyramid 3 P 3 (311). The more common forms are shown in figs. 15-20^ 
PI. HI, Monograph XX, United States Oeological Survey. 

The chemical composition is probably similar to that of the dacite 
from northeast of South Hill in the same district, given on page L'64, 
Monograph XX. (Consult Monograph XX, U. S. Geol. Surv., pp. 236 
aQd3(j6et seq.) 

No. 82. Dacitb. 

(From Lasskx Pkak, Califorxia. Described by J. S. Diller.) 

Lava which is characterized by lime-soda feldspars and quartz is 
^cite. It is sometimes called quartz-andesite. 

Lassen Peak, like Mount Shasta, Mount Hood, and many other 
prominent peaks of the Cascade range, was once an active volcano, 
and of its newer lavas dacite forms by far the greater portion. It is 
widely distributed about the base of the peak, and at one place, known 
aa Chaos, it is of so recent eruption that it still remains in its original 
extremely rough, broken, rocky condition. 

It is a porphyritic gray rock, which has a rough fracture and porous 
structure. At first sight it looks somewhat like granite,^ from which, 
however, it dififers essentially in containing a large amount of glassy 



Among the phenocrysts embedded in the light-gray groundma»s, 
bomblende and biotite are the most conspicuous. Plagioclase is most 
abnndant. Quartz is common and pyroxene scarce. Idiomorphic 
phenocrysts are rare; the crystals are nearly all rounded or broken by 
the movements of the lava at the time of its eruption. 

The hornblende is black where fresh, becoming brownish by altera 
tion. It appears occasionally in well-defined crystals, but usually its 
form is broken or irregular. Pyroxene is occasionally found grown 
QI>on hornblende in parallel position. Grains of magnetite are fre- 
<)ueutly included in hornblende, but the black border so common about 
the hornblende and mica in many andesites is entirely absent here. 
Apatite, almost black, as seen in the hand specimen, by transmitted 
^'Rht appears deep brown and strongly pleochroic. It sometimes con- 
taiua inclusions of both magnetite and apatite. 

The feldspar phenocrysts, as shown by Hague and Idding8, are all 
plagioclase, and belong chiefly if not wholly to audesine. Well defined 

'Btnm r<m Richtbofen, The Natand System of Voloanio Rooks, p. 16. Hmgae and Iddings, Am. 
'^- ScL,M MriM, VoL XXVI, Sept., 1883, p. 281. 



I 



218 



THE EDUCATIONAL SERIES OP BOCK SPECIMENS. [bijll.1». 



crystals are rare. The asaal form is rounded or broken, and frequently 
contains numerous glass inclusions. 

The quartz, when seen in thin sections, is found to be well rounded, 
and generally contains the characteristic dihexahedral glass inclusions. 

The pyroxene is chiefly hypersthene, although augite is sometimes 
present. The crystals are usually so small that they belong rather 
to the groundmass than to the phenocrysts. 

The gray groundmass is porous, sometimes fibrous and pumiceous, a 
feature which can be best observed in the hand specimen with a small 
lens. In the thin section it is seen to be composed chiefly of clear 
glass swarming with acicular colorless crystallites. It contains com- 
paratively small crystals and grains of feldspar with a smaller propor- 
tion of hornblende, hypersthene, biotite, and magnetite, besides round 
or irregular dark gray felty spherulitic patches. 

The chemical comi)osition of the lava at Lassen Peak shows that it 
is a typical dacite. Analyses have been made of different flows and 
are given in the accompanying table. 

Analyses of Zara from Lassen Peak, California. 



SiO,. 
Ti0«. 
A1,0, 
Cr^Oa 
Fe,Og 
FeO - 
MnO. 
OaO . 
SrO.. 
BaO. 
MgO 

Na,0 

LiiO 

HgO 

p,o. 



1. 



Per eent. 
69.51 



Per cent. 
68.20 



15.75 



16.98 



3.34 



].71 




3.95 



4.33 



3. 

Per eent 

68.72 

0.31 

15.15 

none 

1.16 

1.76 

0.11 

3.30 



4. 



5. 



Per cent j Per eenL 



68.82 
0.81 
15.26 
noDe 
1.66 
1.26 
0.04 
3.26 



Total i 100.19 



100.27 



0.03 


trace 


0.07 


0.07 


1.28 


1.32 


2.78 


2.81 


4.26 


4.27 


trace 


trace 


0.74 


1.37 


0.09 


0.12 


99.76 


100.07 



55.14 
0.52 

19.10 

none 
6.16 
0.54 
0.11 
8.36 
0.07 

trace 
4.23 
1.04 
3.71 

trace 
0.91 
0.18 



1U0.07 



6. 


7. 


Percent 
85.77 


PereefU. 
76.76 


21.51 


12.32 






trace 


1.36 


5.72 


L18 


.......... 




none 
0.83 
5.92 


none 
3.98 
3.55 


0.84 


0.64 





100.09 


90.68 



Nu. 1. Gray dacite nt sontheant base of Lassen Peak. Analyst, T. M. Ghatard. 

No. 2. Reddish dacite at northeast base of Lassen Peak. Analyst, T. M. Chatard. 

No. 3. Dacite of latest eruption at Chaos, north base of Lassen Peak. Analyst, W. F. Hillebrand. 

No. 4. Gray dacite, west base of Lassen Peak, envelopes 5. 

No. 5. Nodule contained in 4. 

No. 0. Feldspar (Am. Jonr. Sol., 3d series. Vol. XXVI, p. 282, Sept., 1883). AnalyBt^ P. W. SMmer. 

No. 7. Glass base (Am. Jonr. Soi., 3d seriea, Vol. XXVI, p. 232, Sept., 1888} . Analyst, P. W. Shimer. 



In the first column is given an analysis of the material in this series. 
The composition of the feldspar, determined by Messrs. Hagne and 
Iddings, is given in colamn 6, and that of the glass base in oolamn 7, 
where it appeATS that the latter contains the chief portion of the potas- 




QC 
O 



< 
< 

UJ 

a. 



< 



UJ 

< 



UJ 

_l 
D 
O 

O 

z 



n 



] DESCRIPTIONS! NO. 83, HORNBLENDE-MICA-ANDESITE. 219 

and that none of the feldspar is sanidine. It is evident also that 
^lass base contains a larger percentage of silica than the crystal- 
portion of the dacite. It is almost always tme that in the process 
jBtailization the basic element8 become mineralized with greater 
lity, relatively, than the silica, so that the amorphons residne in 
tcryBtalline eruptive rocks is generally richer in silica than the 
age of that portion which is crystallized. 

thoagh the great mass of dacite at Lassen Peak contains a larger 
ortion of glass base and has attained a lower degree of crystalliza- 
than most of the other lavas in the same region, there are within the 
} isolated nodules, as seen in PI. XXXI, that are holocrystalline, 
mtaiu a relatively small amount of glass base. The complete crys- 
Eation of the mass at the time of its eruption was doubtless pre- 
ed by the condition of the magma taken in connection with its 
ronment. The irregular holocrystalline nodules, on account of their 
er degree of crystallization, may be regarded as the first portion of 
mass to solidify. 

these nodules hornblende is much more abundant than in the 
te which envelops the nodules. It is frequently allotriomorphic 
Dst the feldspar, as the augite in diabase;^ biotite and pyroxene 
isaally less abundant, and olivine is occasionally present, and some 
'mite. The feldspar are generally lath-shaped. A chemical analy- 
f one of these nodules, with that of the enveloping dacite, is given 
e table. 

nilar nodules are found in many eruptive rocks, especially such as 
ite, diorite, etc., which have crystallized at a considerable depth 
ath the earth's surface. They are generally more basic than the 
inclosing them, and are regarded by many petrographers, but not 
18 the part of a once homogeneous magma, separated out in the 
)ss of differentiation and first to crystallize, 
bcites are sometimes holocrystalline, but frequently contain more or 
amorphous matter. In a general way they are intermediate in the 
series between rhyolite and andesite. The structure and habit of 
f dacites appears most closely related to those of rhyolite, but in 
ield they are commonly found associated with andesites. In the 
en Peak region the dacites and rhyolites are of approximately the 
) age, and both are younger than the andesites which they have 
flowed. 

Kg. 83. HOBNBLENDE-MIOA-ANDESITE. 

M HooBAC Mountain, Eureka County, Nevada. Described by J. P. 

Iddinos.) 

t^i^ rock occurs in a small exposure on the road northeast of Hoosac 
intain, Eureka County, Nevada. It appears to be a lava flow or 

«daoHaga«Aiid Iddings, Am. Joar. Sci., Sept., 1883, 3d series, Vol. XX VI, p. 234, 286; and Roeen* 
'i ^ikrotkopiMhe Physiographie der nuiMJ^n Oetteine, 3d edition, 1887, p. Q1&> 



220 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [buu.150. 

poAsiblv ai) intraaive body, its contact with the limestone not having 
b*'eii discovered. It is a dense, compact rock having a recldishpaqile 
to purplish-gray groandmass, rich in megascopic crystals of feldspar, 
honiblfiide, and biotite, the feldspar predominating. Pyroxene is 
wholly absent. 

In thin sections the gronndmass is seen to be holocrystalline, com- 
posed of microlites of plagioclase, largely oligocla^e, in an aggregate of 
feldspar and (jnartz grains. These grains are nearly free from microlites 
at their centers. There are, besides, minnte crystals of magnetite and 
opaque microlites which correspond to shreds of brown mica in some 
instances. The lath-shaped feldspars show fluidal arrangement. The 
feldspar- phenocrysts are wholly triclinic. They are finely developed 
crystals, for the most part eqnidimensional, yielding sharply outlined 
sections having the usual forms. Zonal structure is pronounced and i& 
well shown in fig. 1, PL V, Monograph XX, United States Geological Sur- 
vey. They sometimes exhibit evidence of successive periods of growth 
and resorption. The customary cleavage is not well developed and ia 
often wanting, there being, instead, irregular cracks, as in sanidine. 
The polysynthetic twinning after albite and pericline is very unevenly 
developed. (This is shown by figs. 3 and 4, PL V, and fig. 2, PL VI, 
Monograph XX.) The largest feldspars have quite irregular outlines 
and abundant narrow stride; the medium-sized ones have sharp crystal 
outlines and fewer broader stride. Most individuals are also twinned 
in two parts alter the Carlsbad law (fig. 7, PI, III, Monograph XX), 
and frequently several have grown together in parallel orientation (fig. 
2, PL VI, Monograph XX). Most .of the feldspar phenocrysts are 
labradorite or andesine; the microlites are probably oligoclase. The 
precise character of the grains in the gronndmass is indeterminable; 
they may be in part orthoclase. The phenocrysts bear colorless glass 
inclusions and sometimes clouds and streaks of dust like particles, 
besides occasional minute apatites and zircons. 

The hornblende phenocrysts, which are recognized by their crystal 
form and characteristic six-sided cross section, are seen in thin section 
to be wholly decomposed. The opaque black border remains, but the 
interior of the crystal, when not lost during the process of giinding the 
section, is altered to a finely fibrous, yellowish-green mineral having 
the optical properties of amphibole (actinolite). Its fibers start from 
transverse fissures and run parallel to the c axis of the original crystal 
for a short distance. They form a network, the meshes of which are 
filled with a colorless snbsiauce, probably opal. There are sometimes 
opaque and also transparent globulites, besides ferrite and hematite. 
In some cases the decomposed hornblende is replaced by calcite, and 
in others by chlorite. 

Biotite occurs in fewer and larger crystals, tabular and six-sided in 
form, and often quite thick. The crystals are frequently twinned 
parallel to ooP (110), with O P (001) as the composition plane, the com- 



Dorni DESCRIPTIONS: NO. 83, H0RNBLENDE-MICA-ANDE8ITE. 221 



loon law. Absorption strong; pleochroism marked, sections parallel 
to the base being brownish red, those inclined to it, orange, yellow, or 
green. Optic angle small. There are sometimes opaqne, ueedle-like, 
possibly tabalar inclusions arranged parallel to the six sides of the 
mica plates; also apatite and zircon inclosures. In some cases crystals 
of plagioclase are inclosed in the pheuocrysts of biotite, and occasional 
intergrowths of these two minerals prove their crystallization to have 
been synchronous. It shoald be noted that the biotite is almost jyer- 
fectly Iresh, while the hornblende is completely altered. 

Quartz occurs in sporadic phenocrysts, rounded and often greatly 
corroded and cracked. Its substance is very pure and free from iuclu- 
«0D8. Microscopic quartz grains form a constituent of the ground- 
mass. Magnetite occurs in grains among the phenocrysts and as a 
microscopic constituent of the groundmass. Apatite is well developed 
in small crystals, dusted red and orange, with noticeable absorption 
parallel to the c axis. It contains in some instances glass inclnsions 
in negative crystal cavities (figs. 1, 4, 6, PI. Ill, Monograph XX). 
Zircon is a constant ingredient in small quantities. 

The chemical composition, determined by i{. W. Mahon, is as follows: 

AtuUy9%8 of harnblende-tnicthandeBiie from Hoo$ac Mountain, Nevada, 



SiO,. 
TiO,. 

Fe,0, 
FeO.. 
KnO. 
MgO. 
CaO . 

NM> 
K,0.. 
P«0.. 
Iga.. 



Percent. 



67.83 

1.04 

15.02 



5.16 



Toua 



.29 
3.07 
2.40 
3.20 

.26 
1.11 



90.38 



^or further details, see Monographs United States Geological Survey, 
VoL XX, Geology of Eureka District, Nevada, pp. 233, 264, 364, et heq. 

No. 84. Hornblende- ANDESiTE. 

(PfiOM Black Butte at Wkstern Bask op Mount Shasta, California. 

DBSCRIBED by J. S. DiLLER.) 

Andesites are lavas of intermediate chemical composition and are 
distiDgoished from the rhyolites and trachytes on the one hand and 
'>a«alt8on the other by the predominance of the soda-lime feldspar. 

Tliey are commonly divided into horublendeandesites, mica-andesites, 
Mgiteandesites, and hypersthene-audesites, according to the predouii- 



222 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [buu^ISO. 

nating ferromagnesian silicate. Homblende-andesite is one in which 
the characterizing ferromagnesian silicate is hornblende. 

Mount Shasta,^ California, during the latter part of the Tertiary 
period was an active volcano, and among the earlier lavas of its western 
slope homblende-andesite plays an imiK)rtant role. The later lavas are 
hypersthene-andesites (specimen No. 87) and basalts. Where unaltered 
the homblende-andesite is composed chiefly of a compact, light-gray 
groundmass, in which are sprinkled prominent crystals of black horn- 
blende. Upon the surface the rock is usually altered and becomes red- 
dish, owing to the liberation of oxide of iron in the process of alteration. 
It is a uniformly dense lava without any of the vesicular structure 
common to basaltic rocks of the same region. 

Under the microscope it is seen that the gray groundmass contains 
small phenocrysts of feldspar which can not be readily detected in i^ 
hand specimen. They are all banded, and their angles of extinction, 
indicate that they are anorthite and labradorite. In form they ar& 
commonly irregular but sometimes idiomorphic and nearly as broad as 
long. Their well-marked zonal structure affords an opportunity to 
observe the wide difference in angle of extinction between the outer 
and inner zones, a difference which is usually assumed to indicate that 
the central portion of the crystal is more basic than the outer zones. 
It may be sometimes observed, however, that the center and outer 
zones have the same angle of extinction, while the intermediate ones 
have a much larger angle. 

The hornblende phenocrysts are larger than those of feldspar. They 
are usually deep brown, strongly dichroic, and sorrounded by dark gray 
or black borders. Where the border can be resolved it is found to be 
composed chiefly of augite and magnetite, which originated in the 
caustic action of the magma upon the hornblende at the time of the 
eruption. 

The groundmass is a mat of microlites. It is composed chiefly of 
lath-shaped crystals or irregular grains of feldspar, with a smaller pro- 
portion of hypersthene, magnetite, and amorphous substance. The 
feldspars, many of which are very minute, have the small angle of 
extinction which belongs to andesine. They are occasionally arranged 
in streams about the older crystals of feldspar and hornblende. The 
minute slender crystals of hypersthene in the fresh rock are incon- 
spicuous. They are scarcely pleochroic, between yellowish and green- 
ish. When the rock begins to alter they become yellowish red and 
impart color to the whole mass. 

Homblende-andesite from Black Butte, at the western base of Mount 
Shasta, contains among its phenocrysts neither mica nor pyroxene, and 
is a good type. Such forms, however, are rather exceptional, for the 
most common phase of homblende-andesite, even In that region, oon- 

' Mount ShMiA a Typical Volcano, by J. S. Diller: Nat. G^«og. Monograph No. 8. Published 
nnder the auspices of the National Gkographio Society by tbe Amerioan Book Compuiy. 



NO. 83, HORNBLENDE-PTR0XENE-ANDE8ITE. 



223 



taiiiB phenocrystic hypersthene, bat it is aiways less oonspicuoas than 
tbe hornblende. Angite, which is common in hornbleudeandesites in 
many regions, is rarely if ever present in those of Monnt Shasta. The 
same is trne of mica. The groundmass, too, varies greatly; on the one 
hand it may be holocrystalline and on the other may contain an abnn- 
danceof brown, glassy base, forming the principal constituent of the 
Kroandmass. Generally, however, there is only a trace of clear, vitre- 
oos base present. In general habit, as well as in chemical composi- 
tioD, tbe hornblende-andesite of Mount Shasta is related to the tra- 
chytes, and on this account^ Eosenbusch classifies them as trachytoid 
hornblende-andesites. 

The following chemical analysis of si>ecimen No. 84 was made by 
W. H. Melville : 

AndffM of hornblende-andesite from weittem base of Mount Shaetay California, 



I SIO, 

AWOi 

I Fe,0. 

\ FeO 

CaO 

i MgO 

K,0 

N%0 

Total 



Per cent. 


0.06 


64.48 


19.28 


1.40 


1.78 


5.06 


1.64 


1.12 


4.41 


99.23 

1 



No. 85. HOBNBLBNDB-PYBOXENEANDESITE. 
(I^V KSAit THB COMSTOCK LODB, VIRGINIA ClTY, NEVADA. DESCBIBED BY 

J. P. Iddings.) 

Tbis rock, like the pyroxene-andesite from Virginia City, Nevada 
(specimen No. 88), forms massive volcanic lavas whose occurrence has 
^ described by the authors mentioned in connection with the 
deseription of that rock (q. v.). It is dense and dark gray, with a 
foagb fracture, an aphanitic groundmass carrying numerous small 
crystals of feldspar with brilliant cleavage planes. In thin sections it 
IS seen to be partly altered, the ferromagnesian minerals, hornblende 
and pyroxene, being almost wholly changed. 

It consists of a holocrystalline groundmass with abundant pheno- 
<^8t8 of plagioclase and few of altered ferromagnesian minerals. The 
Plagioclase phenocrysts are fresh and unaltered, and exhibit zonal 
stmctore and polysynthetic twining according to the albite law, besides 
Carlsbad twinning. The symmetrical extinction angles are those of 
^bradorite. The inclusions are minute rectangular glass ones, in some 

'HikrMkopiache Pbysiognphie, 3d edition. 1890, Vol. II, p. 888. See also paper by Hague and 
Wiliagi, Am, Jour. Sci.. Sept., 1883, VoL XXVI, pp. 222-235. 



224 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [Bvxi.iM. 

cases dervitrified, and opaqae grains and crystals, besides colorle^^s 
apatite prisms. There is also included in some cases a highly doubly 
refracting mineral in flakes and films, which is possibly calcite. There 
are occasional roanded grains which are probably aagite, and opaqae 
magnetite grains. 

Hornblende, which can be recognized by its forms in cross section, 
is altered to an aggregation of opaqae grains (magnetite), and color- 
less mica (mascovite), and yellow, strongly refracting grains (epidote). 
green chlorite, and calcite. In some cases the hornblende is largely 
replaced by calcite. These pseadomorphous aggregations sometimes 
inclose sm^l plagioclase feldspars and apatites, which were undoabtedlj 
originally inclosed in the hornblende. Pyroxene, which is recognized 
by the shapes of its sections, is altered to chlorite with seams of calcite 
and some quartz and numerous grains of magnetite, which latter niin 
eral may have been originally inclosed in the pyroxene. Epidote also 
occurs in these pseudomorphs. The chlorite is quite uniform ly oriented 
and sometimes exhibits marked fibration and cleavage, like that of 
bastite. It is somewhat pleochroic, being green parallel to the fibers, 
and yellow for light vibrating at right angles to them. A few small 
phenocrysts of unaltered pale-green augite were found in the tbiu 
sections of this rock. One showed the customary twinning parallel to 
the orthopinacoid (100). 

Microscopic phenocrysts of magnetite and apatite occur associated 
together in groups. The apatite is in the stout, colorless prisms usually 
found in audesites. 

The groundmass is holocrystalline, both microcrystalline granular 
and microlitic. It consists of micropoikilitic quartz, feldspiir microliters, 
and magnetite grains, besides chlorite aad some other secondary miu- 
erals. The micropoikilitic quartz is primary, having formed as the last 
act of crystallization of the molten magma. The microscopic anhedrons 
of quartz inclose microlites of feldspar and other minerals. The larger 
microscopic feldspars are lath-shaped plagioclases. The magnetite 
grains and crystals are abundant. There are scattered through the 
groundina^ small patches of calcite and aggregations of secondary 
quartz, in which the calcite occurs as minute rhombohedrons. Chlorite 
forms pseudomorphs after microscopic pyroxenes, and also occurs in 
spherical aggregates.^ 

No. 86. Hypebsthenb-andesite. 

(From Northeast Shoulder of Buffalo Peak, Park County, Colorado. 

Described by Whitman Cross.) 

Occurrence. — Buffalo Peak (elevation, 13,541 feet) lies between South 
Park and the Arkansas Eiver, a little southeast of the south end of 
the Mosquito Range. 

I Further information couoeming this rock may be found in Men. U. S. CriH>l. Survey, VaL III. 
pp. 53-02, and BulL U. S. Geol. Survey No. 17, pp. 22 and 23. 



MLLDu] descriptions: no. 86, HYPEBSTHENE-ANDESITE. 225 

It is a double pointed mountain whose base consists of easterly dip- 
ping Carboniferous strata, while the upper part is made up of liorizontal 
Ms of audesitic tuff, capped by andesitic lava flows. The summit 
sheet is a hornblende- andesite containing some hypersthene, and the 
northeastern shoulder is made up chiefly of the dark hypersthene- 
and^ite of this collection. The geology of the region has not been 
worked out in detail, but the mountain was visited in the summer of 
1880 by members of the Bocky Mountain Division of the Survey, then 
engaged in the study of the adjacent Mosquito Kange. 

General description. — ^This hypersthene- andesite is a dark, almost 
black, rock, exhibiting to the eye many very small white feldspar crys- 
tals embedded in a black groundmass having a dull vitreous luster. 
On careful examination a number of dull green grains and small i)rism8 
may be distinguished, which the microscope shows to be hypersthene 
or augite. Glistening particles of magnetite may also be detected. 

The rock varies somewhat in texture, but the specimens collected 
show a large nnmber of the white or clear glassy felds^iar phenocrysts 
which the microscope proves to be labradorite. In other parts of the 
mass the groundmass is more prominent and more clearly vitreous. 

The phenocrysts. — Microscopical examination reveals a lime- soda feld- 
spar, hypersthene, augite, and magnetite as very distinct phenocrysts, 
in a groundmass which has a glassy base holding numerous augite and 
plagioclase microlites and magnetite particles. The phenocrysts equal 
or exceed the groundmass in quantity. All are very small, few sur- 
passing 2"»» in diameter. 

Plagioclase is probably developed in several varieties, but the chief 
one is certainly labradorite. The crystals vary much in size and form, 
in number and character of inclusions, and in twinning. The larger 
(^stals are usually clouded by many original brown glass inclusions, 
now often devitrified, and obscured by an opaque ferritic dust. These 
inclasions are often connected and occupy more than half the space of 
the crystal. Zonal extinction is almost universal in the plagioclase, 
hot does not always represent great difference in optical character. 
^srioas twinning laws have been noted, the common albitic law being 
i^ceompanied by the Carlsbad, pericline, and other laws not determined. 

A gradation in size occurs between the largest phenocrysts and the 
SrouudmasB microlites, and these intermediate crystals usually carry 
few inclasions. 

Both hypersthene and augite are present, as phenocrysts of very 
similar development, but hypersthene greatly predominates. The 
pyroxenes occur in prisms usually two or three times as long as they 
are thick, but of variable size. The largest are, however, only 3 or 
^■■iu length. 

The hypersthene of this rock has been determined chemically and 
optically, and it is typical of the hypersthene now known to be a very 
frttjneut constituent of basic andesi tes the world over. The pleochcoi^m 
Bull. 150 16 



226 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [luiLlso. 

of the byperstbene, wbile not very strong, is safBcient to distingnishit 
from aagite, and the parallel extinction of the ortborhombic prisms is 
also a very commonly applicable test The augite is pale green in thin 
sections and not visibly pleochroic. The bypersthene gives a = a red- 
dish brown, b = b reddish yellow, jc = o green, almost identical with 
that of the aagite. In the thicker prisms the colors are quite strong. 
Cleavage is ordinarily not very markedly developed parallel to either 
piuacoid of the bypersthene. Sections parallel to the macropinacoid 
show that a is the acute bisectrix, but the optic angle is not verysmalL 
The bypersthene of the first specimen of this andesite collected at 
Buffalo Peak was isolated from the augite by Dr. Hillebrand tbroogh 
continued treatment of the rock powder with strong hydrofluoric acid, 
which attacks augite much easier than bypersthene. The purest mate- 
rial isolated was found on microscopical examination to be almost frcK 
from augite, and the analysis yielded Dr. Hillebrand the following 
result: 

Analysis of hypersthene of hypersikene-andosite from Buffalo Peak, CoUtrado, 



Per cent. 



SiO, 51.70 

A1,0, 

Fe,0, 

FeO 

MnO 

CaO 



1.72 
0.30 
17.09 
0.36 
2.87 
MgO i 25.00 



Total 100.08 

I 



This analysis, made in 1882, is very similar to many that have beei 
made since that time of bypersthene from other rocks. It appears tha^ 
andesitic bypersthene commonly contains magnesia and ferrous oxid^ 
in nearly equal amounts, but varieties richer in magnesia certainlj 
occur in some cases. 

The larger magnetite grains are to be considered as phenocrysts 
They are frecjuently included in the pyroxenes, which are free from th< 
more minute specks of iron ore characterizing the groundmass. 

The groundmass consists of an almost colorless glass base, appearini 
brownish by low powers through globulitic specks, holding numerott 
short microlites of plagioclase and augite, and very minute magnetit 
grains. No bypersthene could be found among the minute prisms c 
the groundmass. 



Munl DESCRIPTIONS: NO. 87, HYPER8THENE-ANDESITE. 



227 



Chemical compontion. — The following analysis of the fresh rock was 
made by W. F. Hillebrand : 

Anafysis of kjfpcrBthene-andenie from Buffalo Peak, Colorado, 



Per cent. 



I 



Si0«. 

AljO. 

Fe/),. 

FeO . 

MnO. 

CaO .. 

BftO . 

SrO.. 

MgO. 

Ka,0 

K,0.. 

H^O.. 

P,0,. 

CI ... 



M.19 

16.12 

4.92 

4.43 

trace 

7.00 

trace 

trace 

4.00 

2. 9v 

2.37 

1.03 

.27 

.02 



Total 

Sp. gr. at IQo C, 2.742. 



09.01 



The percentages of lime and magnesia explain the development of 
hyperstbene. The potash is so high as to indicate that if the rock had 
completely crystallized there must have been a considerable amount of 
orthocla^e in the groundmass, as this alkali does not normally enter 
into any of the phenocrystic minerals. 

Literature. — An outline of the geology of Buffalo Peaks and descrip- 
tions of the hyperstheneandesite were given in Bulletin No. 1, United 
States Geological Survey, On Hypersthene-andesite and on Triclinic 
Pyroxene in Angitio Bocks, by Whitman Gross, with a geological sketch 
of Buffalo Peaks, Colorado, by S. F. Emmons, 1883. The " triclinic'^ 
pyroxene referred to was augite, cut in sections nearly normal to the 
prism, and the erroneous determination was retracted in a note in the 
American Journal of Science, 3d ser.. Vol. XXVI, 1883, p. 70. 

No. 87. Htpbbstheneandesite. 
(Fbom Wkst Slops of Moui<rr Shasta, Caufornia. Dbscribkd by J. S. Diller.) 

The later lavas of Mount Shasta are chiefly hypersthene-andesite, 
>iid this specimen, No. 87, collected at Horse Camp, near the timber line, 
npon the western slope of Mount Shasta, represents one of the earlier 
of the late flows. The final flows upon the same slope are much darker 
<^lored and more basaltic in appearance. 

The lava illustrated by specimen 87 is a compact, even-grained non- 
P^n>byritic rock, of a light gray color, and contains only a few small 
crj'stalg of pyroxene, which are visible to the naked eye. 

h thin section, however, it becomes conspicuously porphyritic, as 



228 



THE EDUCATIONAL SERIES OF BOCK SPECIMENS. l»cu.l». 



illastrated in PI. XXXII, where the imtneroaa square and brick-shaped 
crystals of feldspar and grains of hyperstbene are seen distributed 
through a light-gray groundmass. 

The feldspar is clear and colorless, showing polysynthetic twinning 
and zones of growth. It is all plagioclase, apparently, and the angleof 
extinction, as well as its composition, judging from the chemical analyses 
below, indicates that it is labradoritc. 

The chemical analyses were made by W. H. Melville. For the par- 
pose of comparison, a chemical analysis of the later, dark -colored, more 
basaltic flow is given. In column I is an analysis of specimen 87, and 
in II is an analysis of a specimen taken from the latest flow.^ 

Analyses of hifperBthene-andeBite from Mount Shasta, California. 



Lo0S on ignition 

SiO, 

Al.Oj 

Fe/)s 

FeO 

CaO 

MgO 

K,0 

2?a^0 

Total 



I. 



U. 



0.20 
64.52 
18.31 
0.90 
2.M 
5.11 
2.35 
1.25 
4.64 



0.44 
63.03 
.17.72 
2.27 
1.92 
5.97 
3.63 
1.06 
8.92 



09.70 



99.96 



HyperRthene occurs in irregular grains and oblong crystals, lib 
those in the hypersthene-andesite on Buffalo Peak, Colorado. As iti 
sometimes included in the feldspar, some of the hypersthene must hav 
crystallized before the feldspar. Occasionally dark spots are foani 
to be composed chiefly of magnetite and pyroxene, and suggest th 
former presence of hornblende. 

The groundmass contains much glass, clouded by a multitude o 
feldspar microlites and minute grains of pyroxene and magnetite. 

1^0. 88. OLiyiNE-BEA.BINO PTBOXENBAMDESITE. 

(Fkom near the Comstock Lode; Virginia City, Nevada. Described b 

J. P. Iddimgs.) 

This rock forms a massive lava in the neighborhood of the Oomstoe 
Lode, Virginia City, Nevada. The rocks of this region have bee 
described by Mr. George F. Becker,' Prof. F. Zirkel,' and ilessr 
Hague and Iddings. ^ They form a great series of volcanic eruptioD 
the history of which has been variously given by the authors quotci 
The pyroxene-andesite in this educational series is one of these volcan: 
lavas. 



1 See also Hague and Iddings paper in Am. Jour. Sci., 3d ser., Vol. XXVI, Sept., 188S, pp.2tt-23S. 
« G. F. Becker, Geology of the Comstock Lode. ete. : Mon. U. S. Oeol. Snrvej, Vol. Ill, 1882. 
»F. Zirkel, Microscopical Petrography: IT. 8. Geol. Expl. Fortieth Parallel, VoL VI, 1876. 
*A. Hague and J. P. Iddings, The Development of Crystallization in the Igneont Roeka of Washt 
XevatlB, cU'. : Bull 17. S. Geol. Survey No. 17, l»ftb. 



DESCRIPTIONS: NO. 88, PYBOXENE-ANDESITE. 229 

[>ck is dense and grayish black, with an uneven fractare. It 
iwith minute crystals that crowd a dark apbahitic gronndmass. 
tgascopic crystals are feldspars with brilliant cleavage faces, 
k-colored minerals, more or less brilliant, which are pyroxenes, 
ueral comi>osition of the rock, however, can be discovered only 
sections with the aid of the microscope. The rock is then seen 
ist of a brown, glassy, and microlitic gronndmass, bearing innn- 
3 small phenocrysts of plagioclase feldspar, hypersthene,aagite, 
olivine, and a few anhedrons of quartz. 

phenocrysts of feldspar exhibit marked zonal structure and 
ithetic twinning according to albite, pericline, and Carlsbad 
Cross sections are rectangular, and also show evidences of pris- 
M domal faces. The symmetrical extinction angles read from 
id and albite twins, and, interpreted according to the method 
ed by Michel L^vy,' show that the feldspars are labradorite of 
he composition Ab^Aus. The feldspar is perfectly fresh, and 
fine glass inclusions, besides others of magnetite, rounded grains 
»xene (both angite and hypersthene), apatite, and portions of 
undmass. There are also minute rectangular inclusions, with 
>lorless margins and dusted centers. They have one orientation 
feldspar, and are apparently parallel to the crystallographic 
Borne are comparatively large, others very minute. In some 
hey appear isotropic, in others doubly refracting, and resemble 
r. 

e feldspar crystal there was observed a large inclusion of brown 
:ic glass, containing curved lines of opaque grains and rods 
ind at right angles to the lines of opaque grains. In some cases 
*oss rods api)ear to be transparent prisms, probably pyroxene, 
ppear to be connected with hair-like needles of pyroxene, and 
leedles of angite are present, studded with similar opaque rods 
lins, and also with crystals of magnetite. The glass base imme- 
surrounding fhese black grains is colorless for a short distance, 
wn coloring matter having been concentrated in the augite 
and opaque grains. Along the margin of the feldspar sur- 
g the brown glass inclusion are projecting crystals of feldspar, 
) teeth of a saw. They have lower refraction than the main 
of feldspar, and also lower angle of extinction, so that they are 
tedly more alkaline, and may be of the same substance as the 
rectangular inclusions which have just been mentioned, which 
letimes doubly refracting. 

lyroxene is hypersthene and augite in nearly equal proportions, 
Asibly more augite than hypersthene. The two minerals appear 
identical in thin sections, and are easily confused with one 
'. The hypersthene has slight pleochroism between reddish and 
b, while the augite is not pleochroic and is pale green. The 
ieristic prismatic and piuacoidal cleavage is less well developed 



bel LSrr, ^iadesarJa d^termiaation dej fcldapAtba daDs lea plaques minces, PatUA^iM' 



230 THE EDUCATIONAL 8ERIEB OP ROCK SPECIMENS. [bcluI! 

in the bypersthene than in the augite. General fonn^ twinning, ai 
iuclnsions are similar in both pyroxenes. The optical properties in oi 
case are those of orthorhombic crystals, in the other those of mon 
clinic crystals. 

The crystals are more or less idiomorphic, stont prisms, with the t\ 
pinacoids and unit prism in the prismatic zone, and terminal faces q 
readily recognizable. Gross sections show the characteristic 8 sid< 
outline of pyroxene crystals. Twinning, in which the orthopinacoid < 
the macropinacoid is the twinning plane, is common in both pyroxeuei 

Inclasions of magnetite are often observed, and also those of feldspai 
apatite, and of glass in negative crystal cavities or in irregnlarlj 
shaped cavities. They frequently carry a gas bubble. In some crys 
tals they are numerous, and often they are very minute. 

In certain spots in the rock are clusters of crystals of labradoriti 
and pyroxene with magnetite and some interstitial brown, globnlitic 
and trichitic glass. 

Serpentine derived from the alteration of olivine occurs in psendo 
morphs having the shai>e of sections of olivine, usually 4 or G sided 
with orthorhombic symmetry. The pseudomorph is surrounded by \ 
narrow border of augite grains. The serpentine is green in some case 
and brown in others. The amount of olivine originally present wa 
not great, so that the rock may be classed with the andesites rathe 
than with the basalts. 

Magnetite occurs in comparatively large grains or anhedrons, som< 
what irregularly shaped. In some cases the magnetite partly inclose 
pyroxene. It is frequently inclosed by the pyroxene. 

There are relatively few crystals of apatite in stout prisms, wliic 
are comparatively large. They have the usual hexagonal forms, aD 
are often dusted and slightly pleochroic, between brown and dark sepu 
E>0. In some crystals there are minute opaque needles lying parall 
to the prismatic axis c. 

In one thin section studied, there is a rounded anhedron of quart 
containing colorless glass inclusions with gas bubbles, and in one can 
what appears to be a colorless crystal with rhombic outline. Tl 
quartz is surrounded by brown glass and a shell of pyroxene crystal 

The groundmass is a fine example of glassy andesitic groan 
masses. It consists of brown glass full of stout microlites of pyro 
ene, probably monoclinic for the most part, and those of lathshaiH 
feldspar, together with grains of magnetite. These are not mingl< 
with complete uniformity throughout the rock sections examine 
there being sx>ots which are mostly glass and others mostly crystal 
The lath-shaped feldspars are plagioclase, whose exact position in tl 
albite-anorthite scale has not been determined. Besides these mici 
lites, there are small rectangular and lath-shaped feldspars with larj 
extinction angle, probably labradorite, and small anhedrons and prisii 
of pjroxene and crystals of magnetite. These are slightly larger thi 



DiuBi ) NO. 89, HORNBLENDE-DIORITE-PORPHYBY. 23 1 

tbe smallest microlites of the groandmaBS, but are still microscopic 
and may be classed as microlites. 

No. 89. HOBKBLEKDB-DIOBITE-POBPHYBT. 

(From South Sidb of Buckskin Gulch, 2^ Miles above Alma, Mosquito 
Eangb, Park Couimr, Colorado. Described by Whitman Cross.) 

(keurrence, — This rock occurs as an intrusive sheet in Silarian strata. 
Near the i)oint of collection the sheet is 20 feet or less in thickness and 
it has been traced for several miles with no great increase at any point, 
althongh often less than 20 feet thick on accoant of splitting into two 
or more parallel sheets. While following the same horizon for long 
distances, the rock locally cuts obliquely to slightly higher or lower 
piaoes of stratification, and in the cliff sections of the gulches it is not 
always continuous. 

Tbe rock maintains the structure and texture of the specimens in 
this collection except in narrow contact zones, where it is finer grained. 

General description. — The freshest rock obtainable has a general 
greenish-gray tone, and consists of numerous phenocrysts of plagioclase 
aod hornblende lying in a somewhat subordinate finegrained, greenish 
groandmass. The phenocrysts vary in size, but seldom if ever reach 
a diameter of 1 centimeter. Pinkish crystals of orthoclase are usua ly 
de?eloped in very small numbers, and biotite may occasionally be dis- 
tinguished, though commonly altered to chlorite or epidote. 

Microscopical examination shows the groundmass to consist of ortbo- 
dase, hornblende, and quartz, with a small quantity of plagioclase and 
magnetite. The accessory constituents, apatite and zircon, are present 
u nsoal, with a little titanite, and, sporadically, minute prisms of 
Bllanite, which can not be found in all the thin sections. AUanite was 
not observed megascopically. 

The phenocrysts, — The abundant plagioclase phenocrysts are devel- 
oped in various forms, but chiefiy in tabular crystals parallel to 
the brachypinacoid, often elongated somewhat parallel to the brachy- 
Axiaa. Twinning is common according to the albite, Carlsbad, and 
pericline laws, and all three are often combined. Zonal structure indi- 
cating change in comi>osition during crystallization is frequently very 
plain, but the variation in extinction between center and periphery is 
iiot great. Primary inclusions are few, apatite and magnetite being 
most common. 

Optical examination shows that labradorite is the predominant 
iBember of the series here developed. The maximum of extinction 
Dot^d is about 30^, and in crystals combining albite and Carlsbad laws 
of twinning the extinctions in the zone normal to tbe lamellae indicate 
^bradorite of the composition 1 albite + 1 anorthite to be most com- 
^n. Some andesine is thought to be present in crystals characterized 
by very fine albitic twinning. 

The hornblende is the common green variety, with normal pleochroism 
*iid extinction. Ite crystals are iiHually well terminated by t\ie c-om- 



232 



THE EDUCATIONAL SERIES OF EOCK SPECIMENS, [buu.15 



mon pyramid and dome faces. There is no resorption, and alteratic 
to chlorite has began cj^ternally and on many cleavage flssares. Tl 
phenocrysts vary in size and seem to grade rai)idly downward to tl 
needles of the gronudmass, which have the same characteristics < 
color and form. 

Biotite has not been seen in fresh condition, bat stont prisn 
believed to represent this mineral, now entirely altered to chlorite an 
epidote, may be foand in nearly all sections. 

The few orthoclase crystals are clinopinacoidal tablets, usually 
irregular boundary, including small plagioclase crystals. This 
regarded as an exceptional development of orthoclase during the ge 
eral groundmass period. 

Groundmass, — ^The groundmass is a distinctly granular mixture < 
predominant orthoclase with abundant hornblende needles, and som 
quartz and plagioclase. Orthoclase is developed in irregular grains 
plagioclase in small, stout crystals, and quartz either in clear grains oi 
most commonly in a cement for the other constituents, and then u^saallj 
oriented over small areas so that it practically includes several grains 
and causes a rudimentary micropoikilitic structure. 

The hornblende needles are not very acicular, but they are long 
enough and numerous enough to express a fluidal structure had there 
been movement after their formation. As such a structure is not 
found, these hornblendes are considered as crystallizing during the 
groundmass period. 

Plagioclase is very subordinate, and while its small crystals prob- 
ably belong to oligoclase or andesine, this has not been proved. Ma^ 
netite seems to have formed in two periods, a scanty dust through the 
groundmass belonging to the later period. 

Chlorite, epidote, calcite, and muscovite are secondary products ol 
variable development in the different specimens collected. 

Chemical composition. — ^The average rock has the composition of the 
following analysis by W. P. Hillebrand: 

Analy fits of h rnblende'dioriie-porphyry from Colorado, 



Per cent. 



SiO, ! 56.82 

A1,0, I 16.74 



Fe,0, 

FeO 

MnO 

CaO 

MgO 

K,0 

Na,0 

H,0 

CO, 

P.O. 

Total 

8p. gr., at 160 c, 2.77. 



4.M 
3.27 

.15 
7.39 
4.06 
l.»7 
3.50 

.02 

1.15 

trace 



lUO. 73 



DOittl DESCRIPTIONS: NO. 90, DACITE-PORPHYRY. 233 

The analysis agrees very well with the mineral constitution as do- 
scribed* It is clear that the plagioclase mast be one containing much 
lime, and the contents in x>otash is snfficient to represent a considerable 
amoant of orthoclase in the gronndmass. Galcite and chlorite mast 
have been well developed in the material analyzed. It woald seem nec- 
essary to have some free silica in the rock in view of the large propor- 
tion of hornblende and labradorite. 

Literature, — This rock and its occurrence, together with the associ- 
ated eraptives of the Mosquito Range, were described in Monograph 
XII, (Juited States Geological Survey, Geology and Mining Indastry 
of Leadville, Colorado, by S. F. Emmons, with Petrographical Appen- 
dix by Whitman Cross ; 1887, pp. 228-240, 334-340. The rock was there 
called hornblende-porphyrite. 

No. 90. Dacitb-pobphyby. 

(From Clear Ckb£k, Shasta Countit, California. Drscribbd bt J. P. 

IDDINOS.) 

This rock occnrs in Smiths Galch, Shasta County, California. It con- 
sists of a dense, greenish-gray, aphanitic gronndmass, filled withphe- 
oocysts of white plagioclase feldspar, 15"'"' long and smaller, and 
nnmerous rounded quartzes, the largest of which have a diameter of 7™", 
besides much biotite in small crystals, 2'"^"^ across, and numerous hom- 
Wende prisms, 4 or 5™™ long. There are also many very small crystals 
of these minerals in the gronndmass. The specimens collected are 
probably not all of the same degree of freshness, for in the specimen 
examiued tbe hornblende crystals are unaltered, while in the thin sec- 
tions prepared from chips of the same rock the hornblende is decom- 
posed. The rock presents a fine example of porphyritic texture, which 
^ emphasized by the contrasted colors of the feldspar phenocrysts and 
tbe gronndmass. By porphyritic texture is understood that appearance 
^bich is produced when the mass of a rock, having any texture what- 
cver^is sprinkled with crystals larger than those constituting the main 
^assof the rock. They usually stand out prominently in contrast to 
the gronndmass. 

lu thin sections of this dacite-porphyry the large feldspars are seen 
^ be twinned according to the albite law, with thin lamelliB, whose 
Symmetrical extinction angles range from 0^ to 4^, and are undoubtedly 
oligoclase. Other modes of twinning, after pericline and Carlsbad laws, 
^e rarely seen in these feldspars. A zonal structure, which shows 
itj^lf faintly between crossed nicols, is sometimes observed. The crys- 
^U are idiomorphic, and exhibit outlines in cross section due to the 
^Bces commonly developed in the feldspars of similar rocks, i. e. (001), 
(^10), (ilo), (110), (lOl), (201), and sometimes other subordinate faces. 

In a few cases the outlines of the sections of feldspar are irregular 
*«d are interrupted by "bays'' of the gronndmass. 
The feldspar is partly altered, and as a consequence is clouOleA v?\i\\i^ 



234 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. (Buii.l« 

in places by innumerable, minute, irregularly shaped inclusions, whic 
appear colorless iu thiu section and have a lower refraction than tb 
feldspar substance. Their exact mineral nature is indeterminabl 
These minute inclusions are sometimes confined to alternate lamell 
of the polysynthetic twins, the intermedite lamella; being free fro 
them. In other cases they occur in irregularly shaped clouds acroi 
all the twin lamellae of one crystal. 

Some of the feldspar phenocrysts inclose green hornblende and broiv 
biotite with sphene, zircon, apatite, and possibly epidote. Inoneinstam 
the biotite is the same size as that in the adjacent groundmass at 
lies near the margin of the feldspar. The inclosed mica is parallel - 
that in the groundmass, which exhibits a fluidal arrangement. Th. 
indicates that the feldspar phenocrysts continued to grow after tb 
motion in the groundmass ceased without showing signs of any breal 
or interruption in the act of crystallization. Hence, the growth of these 
phenocrysts probably took place in the molten magma at a period imme 
diately preceding the solidification (crystallization) of the groundmass 
of the rock. 

The quartz phenocrysts are almost as numerous as those of feldspar 
Their outlines are partly idiomorphic, partly rounded, and sometimei 
exhibit '^bays" of groundmass. In some cases the outline is qaiu 
sharp against the surrounding groundmass, in others it is jagged anc 
uneven in consequence of the merging of the quartz of the phenocrjst: 
into that of the small grains in the groundmass, proving that the quart: 
phenocrysts continued to grow in the period of crystallization of tli< 
groundmass. Sometimes several quartz phenocrysts have grown ii 
juxtaposition without there being any evidence of definite relatioi 
between their orientations. Most of the quartzps exhibit uniform extinc 
tion between crossed nicols and optical homogeneity. Others, howevei 
show an irregularly banded extinction, as though they were twinned i^ 
lamellae. These ill-defined, fairly broad bands appear to be parallel t 
a principal optic section, that is, they may lie in the prismatic zoni 
In one instance there are also extremely thin, poorly defined baud 
nearly at right angles to the broad bands. In another case these thi 
bands cross the broad ones at an inclination of 32^. They may b 
parallel to a rhombohedral face. A rhombohedral cleavage is marke 
in some cases and in others there are minute inclusions parallel to tb 
broader bands. The exact nature of these striations has not bee 
determined. They may be the result of molecular readjustment pn 
duced by dynamical strains. 

The quartz crystals carry inclusions of groundmass, zircon, spheiB 
monoclinic pyroxene, and very abundant, minute particles, irregular 
shaped, apparently fluid inclusions with gas bubbles; in some ca» 
simply gas inclusions. 

Biotite occurs in bent flakes, which are crudely hexagonal plates wi 
irregular outline. In thin sections they show an arrangement in lin 



Mini DESCRIPTIONS: NO. 90, DACITE-PORPHYRY. 235 

according to the flow of the magma before solidification. They are 
strongly pleochroic, being pale yellow for light vibrating i)arallel to the 
8 axis, i. e., nearly at right angles to the cleavage; and dark brown to 
almost black for light vibrating parallel to the cleavage and at right 
angles to the a axis. The angle between the optic axes is apparently 
zero, and the mineral behaves as an optically uniaxial crystal. 

The hornblendes in the rock sections stadied are completely replaced 
by calcite and chlorite, with numerous colorless prisms of apatite, for- 
merly exis* ing as inclusions in the hornblende. In some cases calcite 
prepoDderates over chlorite, or alone constitutes the pseudomorph after 
hornblende. In other ca^es chlorite occurs alone. 

There is some sphene present in small, sharply idiomorphic crystals, 
easily confused with those of zircon, which also occur. It possesses 
the nsual optical properties. Zircon is sparingly present in small 
4-8ided prisms with pyramids of the other order. There is considerable 
apatite in stout, colorless crystals of small size, having prismatic, basal, 
and sabordinate pyramidal faces. 

Epidote occurs in comparatively large, irregularly shaped individ- 
uals, and also in sharply defined prisms. Repeated twinning is present 
in 8ome crystals. There is faint pleochroism in thin sections, from 
colorless to yellow. From the arrangement of the epidote prisms it is 
evident that they existed in the magma before the crystallization of 
the groundmass, for they take part in the flow structure, and the mica 
plates bend around them in some instances. Either they are primary 
crystals of epidote that crystallized from the molten magma before its 
final solidification, or they are pseudomorphs after hornblende prisms, 
having the b axis of the epidote parallel to the c axis of the horn- 
blende. No cross sections of the prisms that might have shown the 
characteristic angles of either mineral were observed. While epidote 
is a frequent product of the alteration of hornblende, it does not 
appear probable that these epidote crystals have been formed in that 
nianner. They are simple or twinned crystals in each case, and not 
aggregations of minute anhedrons, as is usual when forming pseudo- 
inorphs. Moreover, no epidote was noticed in association with the 
chlorite and calcite pseudomorphs, which is its most frequent mode of 
wjcurrence when secondary after hornblende. The study of rock sec- 
tions from specimens in which hornblende is not altered might settle 
this question. 

Calcite occurs in comparatively large, but microscopic, anhedrons; 
sometimes alone, with numerous apatites, and probably replacing a 
f^rromagnesian mineral; sometimes with chlorite, replacing hornblende. 

Ilie groundmass of the rock is a microgranular aggregate of feld- 
spar and quartz in allotriomorphic anhedrons of various sizes from 
.0009 nm to .0025™™ in diameter. The larger anhedrons are mostly 
striated feldspar, a few only being unstriated and idiomorphic. With 
theae are mingled many microscopic crystals of magnetite^ mostly 



236 



THE EDUCATIONAL 8ERIE8 OF ROCK SPECIMENS, [buu 



4-Bided, and probably octahedrons, bits of chlorite, and carved pi 
of biotite, besides many minnte, rounded anhedrons having high rel 
tiop, and probably epidote. 

The very fine grain of the groundmass, which is microcrysta 
and megascopically aphanitic, together with its compositioDy pi 
the rock among the dacite porphyries. 

The chemical composition of this rock is shown in the accompan; 
analysis by J. Edward Whitfield. The absence of GO2 and the 
content of H^O indicate that the material analyzed was free i 
calcite and very slightly altered. The marked preponderance of c 
over potash is to be noted. 

Analysis of daeiie-porpliijry from Clear Creek, California. 



Percent. 



SiO, 

TiOj 

Al,0a 

Fe/), 

FeO 

MnO 

MgO 

CaO 

Na,0 

K,0 

Li,0 

PtOfi 

so, 

CI 

CO, 

HjO 

Total .. 
LesA O for 01 

Total . . 



No. 91. MiNETTE. 

(From Frankun Fuenack, Sussex County, New Jbrsst. Described by i 

Iddings.) 

This rock was first described as micaceous diabase by B. E. Emer 
in 18S2, and has been referred to subsequently as mica-diabase 
kersantite. It is, however, minette — that is, »more or less porphy 
holocrystalline rock, poor in feldspar which is some variety of al 
feldspar, orthoclase or albite, or both, and rich in biotite and monoc 
pyroxene. It is a lamprophyre in Eoseubusch's sense. 

The rock is dark greenish gray and dense, with a small hackly : 
ture. Abundant small micas are the only mineral constituent thf 
recognizable megascopically. There are besides occasional lumps 
white mineral which appears to be either feldspathic or zeolitic Aoc 



'B. K. EmenoD, On the dikes of mlcaceoas diabase penetrating the bed of sine ore at Fn 
FuTDMce, SoMaex Count/, New Jersey : Am. Jout.Scl..^^ Mt\%%,\oVXXttVlB82^ pp. 176-979. 



noia.] DE8CBIPTI0N8: NO. 91, MINETTE. 237 

ing to Emerson the rock effervesces freely with acid, owing to the pres- 

l»ii« e of calcite. In thin seeti n the rock is seen to be holocrystalliue, 

with a hypidiiiinorphic granular texture. The ferromagnesiau minerals 

atiil iron oxide are idiomorphic with respect to the feldspar. Together 

they also preponderate over the feldspar, which is more abundant, 

however, than any other one kind of mineral. Taken in the order of 

their relative abundance tbe constituents are feldspar, monoclinic 

pyroxene, mica^ magnetite, epidote, calcite, apatite, pyrite; but this 

order of abundance varies with the degree of alteration, and is different 

in different parts of the rock. 

The feldspar is in part twinned with polysynthetic lamellse. In some 
cases twinning is not recognizable. The index of refraction of the 
feldspar in both cases being lower than that of Canada balsam indi- 
cates that the feldspar is albite or orthoclase, or both. It is quite pos- 
sible that the unstriated crystals, more or less clouded by alteration 
products, are orthoclase. They sometimes yield long, rectangnlar 
sections arranged in fan-like groups, an arrangement often assumed by 
alkali feldspars. The less clouded, polysynthetically twinned crystals 
are undoubtedly albite, since their maximum symmetrical angles of 
extinction are about 15o to 17°, which corresponds to those of albite or 
ftiuiesine, but the lower index of refraction as compared with that of 
balsam proves it to be albite. The minute tlakes and anhedrons 
Scattered through the feldspar appear to be epidote, calcite, and some 
little chlorite. There are also microscopic crystals of apatite. 

Pyroxene is more abundant in some sections than in others, having 
become more or less altered to epidote and chlorite. The pyroxene 
ciystals are idiomorphic to a high degree, and are bounded by the 
Pfism and two pinacoids, the terminal planes not being determinable. 
'^^ crystals are short, stout prisms. The outlines are sometimes 
^(listinct, owing to the partial alteration to epidote. The prismatic 
cleavage is distinct, and a zonal structure is occasionally observed. 
^e color in thin section is pale brown, which renders it difficult to 
^discriminate between the pyroxene and epidote. It is monoclinic, with 
^high extinction angle^ and is probably augite. Pleochroism is not 
Noticeable. Inclusions of magnetite and pyrite are numerous. The 
Pyroxene has been replaced, in some cases almost completely, by epi- 
dote, and to a less extent by chlorite, quartz, and calcite. 

Biotite occurs in abundant crystals, which are six-sided plates, com- 
paratively thick. They are idiomorphic with respect to the feldspar, 
having crystallized before it; but they are allotriomorphic with respect 
^^ the pyroxene, sphene, and magnetite, having crystallized later than 
these minerals. It partly incloses these minerals, besides numerous 
BQiall crystals of apatite. Some of the mica crystals are bent and dis- 
torted. Twinning is present in some cases. The color in thin section 
^ reddish brown, with strong x^leochroism and absorption, between 
hgbt brown or yellow and dark reddish brown. In convergent polar- 
^ light the mineral appears nmaxial, the angle between t\v^ o^\>\a 



238 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS. 



axes is so small. Alteration has not affected the biotite to t1 
extent as the pyroxene. Many biotites exhibit no alteration. J 
crystals there are lenticalar inclusions, placed between the i 
which appear to be aggregations of minute grains of epidote, exl 
aggregate polarization. Chloritization has set in at the margin 
mica crystals. 

Spheue is very abundant in comparatively large idiomorpbic c 
yielding the characteristic sharply rhombic cross sections. C 
is pronounced. The color in thin section is pale brown, wit) 
abso]*ption. The index of refraction is somewhat higher than 
pyroxene, and the double refraction is somewhat lower. Other 
resemblance to this mineral is very close, and the two are eag 
fused. It is rather free from inclusions of other minerals, even 
tite, which occur rarely. 

Magnetite is present in numerous small crystals and in soni 
drons. Pyrite is quite abundant in crystals apparently bouii 
faces of the cube and rhombic dodecahedron in combination. . 
is also abundant in small, stout prisms, often sharply outlin 
bounded by the prism, a pyramid and the basal pinacoid. 11 
scattered through the other minerals — feldspars, micas, and pyi 

Epidote, a secondary mineral in this rock, occurs in aggref 
crystals usually having allotriomorphic outlines, sometimes oc( 
the place of former pyroxene, sometimes scattered irregularly i 
the rock in larger or smaller anhedrons. It has nearly the san 
as the pyroxene and sphene, but is noticeably pleochroic, with 
color. Its index of refraction is almost the same as the miner 
named, but its double refraction is higher. 

Oalcite is present in irregularly outlined anhedrons and to a > 
extent. It was not abundant in the thin sections studied. 'J 
also a little secondary quartz and chlorite. 

The chemical composition of the rock, as determined by L. O. ] 
is given in the accompanying analysis, except for the fact that ti 
has not been determined. 

AnaiysM of minetiefrom Franklin FHrnacej Sev Jersey. 



Per cent 



SiO^.... 

A1,0,.. 

FeiOa . . 
FeO... 
MdO . 
MjjO.. 
(;a() . . . . 
XajO. . 
KaO... 
CO, ... . 
Uf i) . • . , 



40.71 

10. iO 

7.46 

6.83 

.18 

0.21 

11.83 

1.80 

3.2« 

.74 

1.53 



Total ' 100. 01 

I 



J 



DUiiE.] DESCRIPTIONS: NO »2, CAMPTONITE. 239 

The rock is very low Id silica aud high in alumiiui, with conipara- 
tively low magnesia, high lime, and relatively high alkalies, chiefly pot- 
ash. The alumina must have entered largely into the augite together 
^rith lime and iron, which accounts for the abundant production of 
epidote. The formation of alkali feldspars in a magma so rich in lime 
and alumina and so low in silica is noteworthy. It followed the crys- 
tallization of these elements into pyroxene, the alumina having entered 
larpiely in the hypothetical subsilicate molecule- 

No. 92. CAMPTONITE. 

(Fbom Campton VALLSf Gbafton County, New HAMPsmKE. Described by 

J. P. Iddings.) 

''At Campton Falls there are several dikes which furnish handsome 
specimens for those who admire dark, porphyritic rocks. The black 
crystals of hornblende are not large enough to determine with the 
nnaided eye, but they are very brilliant and numerous.'' ^ These rocks, 
originally called diorite by Hawes, who recognized their extremely 
basic character, have become the type of Bosenbusch's camptonite 
group. The rock in the educational collection is compact and bluish 
black, with crystalline luster, reflecting light from minute needles. It 
carries numerous small phenocrysts of hornblende. 

It occurs in an 8-foot dike in mica-schist, according to a subsequent 
description by Hawes.^ In this description he also furnishes a more 
complete analysis of the rock, which will be cited later on. In thin 
section the rock is seen to be holocrystallinc and to consist of abundant 
bomblende crystals and a subordinate amount of feldspar, with some 
&Qgite and iron oxide, a little biotite and apatite and pyrite, and in 
some cases an isotropic mineral, which appears to be analcite or soda- 
lite. There is also a variable amount of calcite, serpentine, or chlorite. 
'Rie preponderant constituent is hornblende, which forms long prisms 
tbat are idiomorphic in the prismatic zone, having the prism faces (110) 
andclinopinacoid (010) well developed. In some cases the clinopina- 
coid is more developed than the prism faces. Twinning parallel to the 
ortbopinacoid (100) is sometimes present. The color in thin section is 
cbestnntbrown, with marked pleochroism jc and b strong brown, a 
'^gbt brown c>b>a. The inclination of x; to e appears to be about 
|2°. A study of the chemical analysis of the rock in connection with 
^ts mineral composition indicates that the hornblende is rich in alumina 
^Qd contains considerable soda. Its exact chemical composition has 
^ot been determined. 

lie next mineral in abundance is feldspar, which appears to be of 

'^•W. Hawes, Mineralogy and Litbology: Part IV of the Geology of New Hampshire. Vol. HI, 
*• '«!, Hitchcock, Conconl, 1878. 

^•W. Hawen, On a gronp of dUsiniilar eruptive rockH in Campton, New Hampshire: Am. Jour. 
***-.« twriea^ Vol, XVJI, 1879, p. 147 et. aeq. 



240 THE EDUCATIONAL SERIES OP ROCK SPECIMENS, [buli.: 

several varieties. The greater number of crystals exhibit little or 
polysyntlietic twinning, and have very low angles of extinction. Tb< 
crystals are probably oligoclase and orthoclase. They occur in loi 
slender prisms, sometimes with a tendency to divergent arrangeme 
They have a lower refraction than those feldspars in the rock wh 
exhibit more pronounced twinning in thin lamellae, and yield symm 
rical extinction angles as high as 20^. These feldspars may be an 
sine. They are subordinate to the more alkaline ones in amount, 
that it appears that the feldspars of this rock are decidedly alkal 
and are not basic, as they were supposed to be by Hawes.^ 

Some of the rock sections contain a colorless, isotropic mineral 
comparatively largo individuals. Its outline is irregular, and it is tr 
ersed by irregular cracks, and no distinct cleavage cracks. It is pi 
ably analcite, but no determination of its exact nature has yet b< 
made. Its quantity varies in different rock sections; from some it 
absent. 

There is a variable amount of monoclinic pyroxene present. It 
quite abundant in some sections and scarce in others. It is pale y 
low to violet in thin section, and is slightly pleochroic. The viol 
color may be due to titanium, which, according to Hawes's analyses/ 
present in the rock in considerable amount. The pyroxene is more 
less altered to serpentine. In some sections the change has been coi 
plete. When unaltered it has the usual form and cleavage, and son 
times the twinning parallel to the orthopinacoid. Some crystals 
pyroxene inclose brown hornblende. The pyroxene is probably augi 

A brown mica, biotite, is also present in comparatively few lai 
crystals; its outline is ill-defined, and it incloses numerous cryst 
of other minerals. The rock contains many small crystals of ii 
ores, mostly in well-defined crystal forms, which yield sections witl 
and 4 sides. They are white by incident light, and appear to be par 
altered to leucoxene. They may be titaniferous magnetite, or ilmeui 
Some of the opaque anhedrons of iron ore are pyrite, having a brai 
color in incident light. There are numerous microscopic needles 
colorless apatite. There is considerable calcite and serpentine wh 
do not occupy spaces that may be certainly referred to former pyr 
ene crystals. These are secondary minerals. 

1 Lov. cit. 



DESCRIPTIONS: NO. 93, DIOBITE. 



241 



The chemical composition of tbe rock is given by tbe following 
Kualyses, I by G. W. Hawes, and II by L. G. Eakins: 

Analyse* of camptonite from Campton Falls, New Hampshire. 



I. 



II. 



SiO, 41.94 

TiOt 4.15 

Al/), ' 15.38 

Fe,0, I 

FeO 

MnO 

MgO ' 

CaO .•... I 

N%0 

K/) I 

00, : 

H^ I 



Ptr cent. Per cent. 
38.45 



5.36 


19.68 


8.27 


4.01 


9.89 


11.15 


.25 


tnoe 


5.01 


6.65 


9.47 


9.37 


5.15 


2.77 


.19 


1.72 


2.47 


4>82 


3.29 


1.49 



Total I 100.44 



100.11 



These analyses differ from one another considerably, cliiefly in tbe 
alQiiiina and alkalies and in the determination of titanium oxide. Tbe 
^^^ varies in tbe content of analcite, but it is doubtful whether tbe 
'^'i'ii !ilainiiia percentage given in II is correct. Titanium is nndonbt- 
^lly present in (tonsiderable quantity. The differentte in carbon dioxide 
'Qay well be due to variability in the amount of calcite present. 

No. 93. DiOBiTB. 

(Fr^ Uiddle Brush Crbek, at base of the Teocalli Mountain, Gunnison 
County, Colorado. Described by Whitman Cross.) 

Occurrence, — This diorite occurs as a very irregular stock with rami- 

^5in|i: dikes catting through the Carboniferous in tbe southern Elk 

fountains. The part of tbe stock included within the Crested Butte 

Qoadraugle is more than 10 miles in length and it extends nortiiward 

into the Aspen quadrangle for several miles more. Large stocks of 

tlie same rock occur elsewhere in tbe Elk Mountains and in other parts 

**f Colorado. Tbe specimens collected show the average grain and 

^oin|x)sition of the diorite mass, but variations in grain are occasionally 

*^>et with, and a subordinate purpbyritic structure is locally developed. 

^criptian. — The rock is light gray, fine grained, composed chiefly 

^h^la^oclase, ortboclase, quartz, biotite, and hornblende. The light- 

**lore(l minerals predominate, with but slight variation in the ratio of 

"iotite to hornblende, over large areas; but, locally, quartz decreases in 

amount and then augite often appears as the associate of the other 

^^fk silicates. Magnetite is quite subordinate. The rarer accessory 

^iJstituents, seen only under the microscope, are apatite, titanite, 

^u, pyrite, allanite, and an unknown dark-brown mineral. 

Bull. 160 ^16 



242 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bull.1 



Plagioclase is the most importaut element of tbis diorite. It occn 
almost entirely in idiomorphic crystals, varying in size between (L5' 
and 1.5°"". These are developed in tabalar form parallel to the brach 
pinacoid. Twinning according to the albite laws is usual, united wi 
the Oarlsbad and pericline laws in many cases. Zonal extinctions a 
very marked, indicating a change in composition of the feldspar mo 
cnle as crystallization progressed. The main part of the crystals sec 
to be labradorite, as the symmetrical extinctions in the zone normal i 
the albitic twinning reach nearly to 40^. Tests on crystals showing bot 
Carlsbad and albitic twinning point to labradorite of the composit/oi 
3 albite + 4 anorthite as most common. Doubtless andesiue and oligo 
clase are present as outer zones in some cases. 

Orthoc]ase is much subordinate to plagioclase, but is still a ver} 
important (constituent. It occurs only in irregular grains, or added tc 
the plagioclase crystals in oriented position, especially in the zoueof the 
ortho (macro) axis. Quartz appears in irregular gi*ains exactly aualo 
gous to the orthoclase, these two minerals being the last elemeuts k 
form, and mutually interfering with each other. 

Biotite is reddish brown in color, in thin, irregular leaver. Horn 
blende occurs in irregular prismatic grains, green in color. Now one 
of these dark minerals predominates, now the other. They are ol 
similar size, seldom reaching 2*°*° diameter, and are both more or les^ 
altered, chlorite and epidote being common products. 

Of the accessory constituents magnetite, apatite, and zircon occur in 
common forms. Titanite appears in irregular grains, seldom in charac^ 
teristic crystals. Allanite was seen in two slides, out of a large uambei 
examined. It was there developed in irregular grains. 

An unknown mineral is present in some sections in minute, dftrk- 
brown, strongly pleochroic, and absorptive prisms, included in feldspar 
or quartz. 

Chemical composition. — An analysis of the fresh diorite by L G. 
Eakins yielded the following : 

Analytis of diorite from Elk Mountains, Colorado. 



SiO, . 
Ti0« . 
A1,0, 
Fe,0, 
FeO . 
MiiO. 
CaC. 
MgO. 
KjO.. 
NojO 
HO.. 



Per oent. 



Total 100.83 



Sp.gT.,at80oCM2.70. 



nam] DESCRIPTIONS: NO. 94, DIORITE. 243 

Tbese flgares show silica to be so high as to explain the quartz con- 
tent, and the relative strength of potash explains the abundance of 
orthoclaf^. The lime is strong enough to produce a plagioclase rather 
rich in this element. 

LiUrature. — The geological occurrence of this rock is illustrated in 
the Anthracite-Crested Butte Folio, No. 12, Geologic Atlas of the 
United States. 

No. M. DiOElTK. 

(From Electric Pkak, Yellowstonk National Park. Described bt J. P. 

Iddings-O. 

This diorite occurs as a part of a stock of igneous rock which was 
forced through Cretaceous strata in early Tertiary times. The sur- 
rounding sandstone and shale have been metamorphosed by the contact 
with the intruded lavas. The stock and connecting dikes were 
ODce part of a volcano which built its cone upon the surface of the 
Cretaceous rocks. The interior of the volcano has been laid bare by 
fanltiDg and erosion. 

The diorite mass has a variable grain, but is for the most part 
coarsely crystalline, the clusters of feldspars and of ferromagnesian 
aiiiierals ranging from S""* to 2"" in diameter. These clusters, which 
give the apparent texture to the rock, are composed of from two to a 
<iozen crystals of feldspar and an intermingled aggregate of ferromag- 
QeHJan minerals. The constituent minerals are hypersthene, augite, 
hombleode, biotite, lime-soda feldspar, orthoclase, quartz, magnetite, 
^ith apatite. Of these, augite, hornblende, biotite, and plagioclase are 
the most abundant. The structure is hypidiomorphic granular. The 
piagioclases are more nearly idiomorphic than the other constituents, 
bot are not strictly so. They are rectangular to lath-shaped. They 
^ere formed in large part early in the course of crystallization. Some 
^refonnd penetrating brown hornblende. Orthoclase is present in some 
easels in small amount, and either surrounds crystals of plagioclase as a 
1>artial border or forms irregular grains as cement between them. Quartz 
^so forms irregular grains between the other miuerals, as though the 
b&t mineral to crystallize. It carries fluid inclusions. Hornblende, 
pyroxene, and biotite rarely exhibit crystal boundaries. Their outlines 
^ Qsaally very irregular, and they penetrate one another in a most 
<:0Dip]ei manner. Magnetite is scattered through the rock in crystals 
^'grains, being most commonly found within the ferromagnesian min- 
^^^8. Apatite occurs in short, stout crystals, not very well formed, 
*^d is colorless. Zircon is rare. 

'^he plagioclase feldspars exhibit the characteristic poly synthetic 
twinning and sometimes zonal structure. Some carry abundant minute 
^^clusioQg. In some cases these are colorless rectangular bodies of an 



'^ ernptive rocks of Electric Peak and Sepulchre MoanuUn, etc. : Twelfth Ann. Re^t. \3 . %. 
^*- Scrrey, 18Q2, pp. 6«-«4. 



244 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS. 



indeterminable nature, bat sugi^esting glass. Others are c 
minute rods or needles in swarms. Small grains of the ferromt 
minerals may also be included. 

Hyperstbene is not always present, but occurs in parts of 1 
It exhibits a faint pleochroism in thin sections, green II c, yel 
and light red lib. Its outline is irregular, and it is genen 
rounded by hornblende, occasionally by augite or biotite. Th* 
is pale green in thin sections and is not pleochroic. It seldom 
crystallographic outline, but forms irregular grains, and is g 
surrounded by hornblende. Biotite is dark brown to yellow 
sections, with strong absorption. Its outline is irregular. ] 
outside of the pyroxenes in most instances, as though a youn 
eral, but it may be partly inclosed by them, as if to some 
contemporaneous. 

Hornblende is in part green, in part brown, the latter kin 
usually within the former. The green hornblende often so 
pyroxene and biotite wholly or partially. Generally they foni 
tricate intergrowth, as though they had crystallized at about t 
time. The problem of this intergrowth is discussed at lengt 
article on The Eruptive Eocks of Electric Peak and Sepulchr 
tain, Yellowstone National Park, by J. P. Iddings, in the 
Annual Report of the Director of the United States G^eological 
for 1890-91, page 606 et seq. 

The chemical composition of the diorite as determined by J. 1 
field is given in the analysis : 

Analy$i$ of diorite from Electric Peak, Yellowttone liaiioHal Park, 




SiO, ! 56.28 



TiO,. 

A1,0, 

Fe,0, 

FeO . 

MdO. 

CaO.. 

MgO. 

Li/). 

Na,0. 

K;,0.. 

P,0,.. 

SO,.., 

01.... 

H,0.. 



L«S8 O for CI 



Total 



.84 

14.23 

4.69 

4.06 

.16 
7.M 
6.37 

.01 
2.08 
1.23 

.40 
iraoe 

.17 

.93 



100.28 
.04 



100.24 



MUtt-l BSSCUPTIONB: NO. 96, VOLCANIC SAND. 245 

OABBBO-BASAXT FAMTLT. 

No. 95. Volcanic Sand. 

(Fbom Snao Laxk Cinder Conb, Lassbn County, California. Dkscribbd bt 

J. S. Diixer.) 

Yolcanic sand is sand that has been produced from molten rock 
material by a yolcanic explosion. Taken with volcanic dnst, it is often 
referred to as volcanic ash, bnt is not ash in the same sense as is that 
resoltiug from the baming of wood or coal. In order that its formation 
may he more clearly understood, it is necessary to consider some of the 
oonditions and surface features of volcanic eruptions. 

Deep borings, mines, and wells hare been sunk at many points on the 
earth's surface, and wherever observations have been made the temper- 
ature has been found to increase downward toward the earth's interior. 
The increase is by no means regular, and yet the rate generally does 
not vary greatly from an average of about 1° F. for every increase of 
57 feet in depth. 

If the temx)erature increased at this rate regularly to a depth of 20 
mileft, the temperature would be over 1,850^ F., and at 50 miles over 
4,6000 F.y or far higher than the fusing point of all rocks under ordinary 
conditions. 

We need not stop here to inquire further into the condition of the 
^h's interior, whether it be solid or liquid, or as to the source of heat, 
whether it is a residue of the original incandescent earth, or is due to 
chemical action, or is jiroduced by the mechanical crushing of rocks. 
Let it be sufficient for our purpose to know that the interior portion of 
the earth, below a depth of a few miles from the surface, is very hot. 

Bain falls on the mountain slopes. Some of it gathers into rills, runs 
into brooks, creeks, and rivers, and finally finds its way back into the 
^whence it came. Another portion enters the soil, and, under the 
^nfloeuce of gravity, passes through the pores, cracks, and fissures of 
the rocks to various depths within the earth. On the lower slopes of 
the mountains and in the valleys much of the water which entered 
^ve reappears flrom springs, most of which are cool and refreshing. 
In gome cases, however, the water penetrates so far into the earth 
before reappearing in springs that it is warmed by the internal heat. 
'Ring warm springs, hot springs, and boiling springs are produced. 

In those boiling springs in which the outlet is large enough to allow 
^^heat to escape, the movements of the water are comparatively uni- 
form; but in certain cases the outlet is narrow in proportion to the 
l«igth of the more or less vertical tube in the ground, and there is not 
sufficient opportunity for the heat to escape. The heat increases until 
the expansive force of the highly heated water and steam is sufficient 
^ produce an explosion. The overlying water and steam, as illustrated 
^u Pi. XIV (p. 92), are thrown into the air by the eruption. Such 
^^g% are geysers, and steam is the motive power in their eruption. 



246 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [boll^iso. 

Next more important than ernptions of water in geysers are emp- 
tioiis of mnd. A notable one occurred in 1888 at Bandai-san in Japan. 
Large quantities of mnd, saturated with steam or highly heated water 
under pressure, were developed a short distance beneath the surface. 
A great explosion occurred, removing the whole side of the mountain. 
A vast quantity of steam escaped, and streams of mnd flowed down the 
valley, damming water courses to form lakes, and destroying a number 
of villages. 

In true volcanic action the material transferred from the interior of 
the earth to the surface is neither simple water, as in the geyser, nor 
mud, as in the semivolcanic eruption at Bandai-san, but melted rock. 
It may come from greater depths than either of the others, where the 
temperature is higher, and the racks may be either in a molten condiiioii 
or so hot that when the pressure upon them is relieved they fuse aud 
become eruptible. 

A fine sample of a truly volcanic eruption is aflbrded by the stupen- 
dous explosion of Krakatoa, in the Straits of Sunda, in August, 1883. 
The explosion was heard for a distance of more than 150 miles, and a 
mass of matter 1^ cubic miles in bulk was blown to pieces and hurled 
high into the air in the form of pumice, ashes, and fine dust. Tlie 
dust, very like that of specimen No. 58, was thrown to a height of 17 
miles in the air and spread far and wide by the winds. Some of it 
fell hundreds of miles from its source. The air wave set in motion by 
this great explosion traveled around the earth three times, and the sea 
in the neighborhood was thrown into waves, one of which was com- 
puted to have risen more than 100 feet above tide level, destroying vil- 
lages and 38,380 people. 

The eruption at Krakatoa differed from that at Bandai-san chiefly in 
that the material was, at least in large part, not in the form of nrnd, 
but in a state of aqueo-igneous fusion, i. e., actually molten. In the 
geyser and the eruption of mud the material is impelled to the surface 
by steam. The same is true also in the final delivery of molten rock, 
or magma, in such eruptions as that of Krakatoa, where the explosion 
must have been due chiefly to the expansive force arising from water 
or its component elements. In that case, however, all the molten 
material was blown out by the expl sion. None of it flowed out 

The large quantities of steam given off by volcanoes in eruption, as 
shown in PI. XXXIII, is illustrated by the great clouds of vapor which 
rose from Vesuvius during its eruption in April, 1872. The steam 
clouds were given off not only near the summit of the mountain where 
the eruption took place, but also from the streams of molten rock 
coursing down its sides. These streams are marked by lines of vapor. 
On the left the lava reaches the very base of the mountain, while ou 
the right are two separate streams reaching nearly to its base. This 
eruption differed from those already noted in that some of the material 
was blown out by explosion at the mountain's summit, while the larger 




n 



DESCRIPTIONS: NO. 96, VOLCANIC SAND 247 

\ molten material flowed out and formed lava streams or 

not be supposed from the fact that attention has been called 
ion of steam in volcanic eruptions that it is the principal 
lived; snch a conclusion would be incorrect; steam has 
ittle to do with the raising up of great masses of molten 
rom the earth's interior; the cause of such upraising is not 
'Stood, but when the molten material has reached the earth's 
eam has much to do with the form of its delivery. 
I eruptions are of two forms — explosive and effusive. In the 
bhe material is blown to fragments and violently hurled into 

the second, the magma — that is, the molten rock material — 
rithin the volcano and flows out upon the surface, forming 
[n some cases the volcanic eruption is wholly explosive, as at 
; in others it may be wholly effusive, as at Mauna Loa, in 
it generally both forms occur together, as at Vesuvius. 
le was true also at the small volcano at Snag Lake, 10 miles 
of Lassen Peak, California, where the products of the explo- 
iffnsive eruptions are quite distinct, one forming the cinder 
the other a lava field, both of which are illustrated in part 
XIV. 

; sand, specimen No. 95, was obtained from the sand field 
the foreground in PL XXXIY. Lapilli, specimen No, 90, 

the cinder cone in the distance, and quartz basalt, specimen 
^s collected from the lava field to the right, where the lava 
^vered by sand as in the view. 

St material blown from t^is volcano, volcanic san^^ spread in 
ons from the vent, covering the ground as a sheet for a dis- 
bout 8 miles. The coarser fragments fell close to the vent, 
\ up, formed the cinder cone, surmounted by a cup-shaped 
m which its material was blown. 

dmum thickness of the sai)d at the base of the cinder cone 
be distinctly ascertained. It is loose and slides easily, so that 
al of excavating is required to get through it where the layer 
erable magnitude. One-fourth of a mile from the base of the 
re specimen No. 95 was collected, the sheet of fragmental 
\ 4 feet 4 inches thick. It consists of two portions. The upper 
jperly called volcanic sand; the lower part, however, is com- 
imall, light brown pumiceous fragments, ranging in size from 
»ea to an inch in diameter. The fragments are very vesicular 
ot Bufi&ciently light to float on water like ordinary pumice (No. 
Sr are very rough and jagged, with surfaces torn by the burst- 
mce filled with steam or other eruptive gases. Some of them 
iuced by concussion. The pieces hurled violently into the air 
jr struck others, and all were partially pulverized as in a great 
is sort of sediment is more abundant in the volcanic sand which 



248 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [umi-lM. 

overlies tlie pamiceous material, and was tlierefore ejected at a sabse- 
qaeut stage of the eruption. Many of the sand grains are vesicular, bat 
less so than the pamiceous fragments, and are generally rounded. 
Other grains are angular and composed of compact lava with few vesi- 
cular. Tliey are bounded by fracture surfaces, and were evidently pro- 
dnced by the violent concussion of larger fragments. A few grains of 
quartz, feldspar or olivine of early crystallization, are present. 

The sand exhibits irregular stratification, due to the sorting action 
of the atmosphere upon the subsiding particles during the eruption. 
The thin layers are lenticular in shape and continuous for short dis- 
tances only, not as sharply defined as beds laid down in water, but indi- 
cating a decided tendency to form beds under favorable conditions. 
This tendency is most clearl}'^ manifested a short distance away from 
the volcanic vent, where the air was not so much disturbed by the vio- 
lent current expelled from the crater. 

The selective influence of the atmosphere may be seen not only in the 
imperfect stratification of the sand, as exhibited in a vertical section, 
but also in its distribution upon the surface. Near the cinder cone the 
sand is coarse and the bed thick, but the bed becomes thinner and the 
sand becomes finer as the distance from the cone increases. 

On the borders of the large circular field covered by the sediment 
thrown out from the crater the fine material is sand, and there is no 
evidence tx) show that any considerable amount of volcanic dust or mate- 
rial still finer than the finest sand was formed. 

The almost complete absence of volcanic dust in this case-is surpris- 
ing when we consider the highly explosive character of the eruption, 
and it may be attnbnted in part, perhaps, to the viscous condition of 
the magma at the time of the outburst. Had the pumiceous fragments 
in the layer already referred to contained sufficient eruptible gases at 
the time of the ejection to blow them>to atoms, considerable dust would 
doubtless have resulted. 

For an illustrated description of the volcanic phenomena exhibited 
nt this recently active center, reference maybe made to Bulletin No. 79, 
United States Geological Survey, and to the Lassen Peak Folio. 

No. 96. Lapilli. 

(From Snag Lake Cinder Conk, Lassen County, California. Described bt 

J. S. DiLLRR.) 

The volcanic fragments next larger than grains of sand are lapilli. 
They are usually very porous and differ from the sand partic/es chiefly 
in size, ranging from that of a pea to several inches in diameter. Most 
of them are black, but shades of red and yellow prevail locally, and 
when abundant may give color to the whole cone. The lapilli of the 
cinder cone illustrated in PI. XXXIV, from which specimens d6 were 
obtained, are generally black, giving it a somber aspect. A cinder 
cone in Arizona, northeast of Flagstone, noted for the brilliant colors 



1 



DESCRIPTIONS: NO. 96, LAPILLI. 



249 



of itslainlli, ia called Sunset Peak. Lapilli are also called << volcanic 
dnden;^ hence the name cinder cone^ applied to all cones of loose vol- 
canic material aboat the vent from which it issued. , 

The lapilli of the Snag Lake cinder cone remained long enough in 
the air to become completely solidified before striking the ground, for 
they show no signs of flattening by impact, nor do they show \\\xm 
their surfaces any lines of flowage, such as are so plainly marked in 
the next specimen (No. 97), from the cinder cone of another locality. 

The following chemical analysis of lapilli from Snag Lake cinder 
cone, made by W. F. Hillebrand, shows that its composition is essen- 
tially the same as that of the lava in the lava field (So. 101). Both are 
qnartz basalt. 

AnalifMis of lapilli from Snag Lake Cinder Cone, Latten County, California. 



Percent. 



SIO, i 56.63 

TiO, .54 

ALQ, 17.50 

Cr»Oa ! trace 



FejO, 

FeO 

MnO 

CaO 

SrO 

BaO 

MgO>««>«* • 

K,0... 

Na^ 

Li^ 

H^ 

PiO. 

Total 



1.35 

5.00 

.12 

8.07 

trace (?) 

trace 

5.04 

1.55 

3.51 

trace 

.27 

.15 



1U0.5G 



No. 97. SCOBIA. 
(Fbox Ice Spring Cratbrs, Millard County, Utah. Described by J. S. Dillbr.) 

Tlie volcanic fragments of some cinder cones have surfaces like slag, 
allowing lines of flowage. Such fragments, especially when cellular, 
have been called scoria. The cinder cone from which specimen No. 97 
was taken shows many traces of the original molten condition of its 
material. Upon the surface of the lava which escaped from this cone 
there are distinct flow lines, and also ui)on the inner slopes of the 
crater where the subsiding magma left trickling remr.ants. 

The fragments of scoria in many cases show by flattening that they 
were yet soft when they fell, and other well-preserved flow forms indi- 
cate that they were not hurled violently from the crater. If that were 
the case they would have been broken by collision or rounded by impact, 
aH specimen No. 96. Instead they were ejected gently — that is, sputtered 



260 THE EDUCATIONAL BEEIES OP BOCK 8PECIMEOT. l»tn.i.MO. 

out — and were so soft and sticky as to mold themselves and adhere to 
that upon which they fell. Mr. Gilbert graphically describes them as 
'*' volcanic bombs whose aerial flight was too short to permit them to 
harden.'' For a description of the volcanic features of Ice Spring 
craters reference should be made to Gilbert's monograph on Lake 
Bonneville, Monographs United States Geological Sarvey, Vol. I, pp. 
320 to 325. 

No. 98. Volcanic Bomb. 

(Fhom near Mount Trumbull, Yavapai County, Arizona. Dxscribkp by 

J. S. Dillkr.; 

Besides the sand and various forms of lapilli ejected from an active 
volcano there are other fragments, generally larger, designated volcanic 
blocks or bombs. The angular ones are blocks and the round ones are 
bombs. The latter are illustrated in PI. XXXV, which is a photograph 
of those at the base of the Snag Lake cinder cone. They were hurled 
into the air, fell upon the steep slopes of the cinder cone, and rolled 
to its base. They are compact throughout, excepting upon the sur- 
face, where they are more or less vesicular. This vesicular covering 
shows that their form is not due to the impact of the neighboring 
particles during the eruption, but to some earlier cause. Some of these 
bombs are over 8 feet in diameter, and it is difficult to believe that they 
became round and cool while flying through the air, for that would 
postulate an unreasonable rapidity of cooling. That they were solid 
when they struck the ground is evident, because they did not flatten 
as did those ejected from the Ice Spring craters of Utah, the Mono 
craters of California, and many others. It appears probable that these 
large bombs were the first part of the lava to solidify and were sus- 
pended in the magma before the eruption, when they were hurled out 
of the crater in their present form. This view of their origin explains 
also their compact structure as compared with the vesicular character 
of the other ejected material. 

The volcanic bombs collected for this series were obtained from one 
of the cinder cones on the Uinkaret platform, a short distance south- 
east of Mount Trumbull, Arizona. On this platform are a number of 
symmetrical cinder cones, the result of geologically recent volcanic 
activity, the eruptions from which cascaded over the cliffs for thousands 
of feet to reach the bottom of Grand Canyon. Among the fragmental 
material of which these cones are made up are found great numbers of 
these << lava balls," ranging in size from that of a pea to 5 feet or more 
in diameter. The average size is nearly that of the specimen in the 
collection. Many of them are red from the oxidation of the iron, bat 
others are dark, with surfaces ramified by a series of cracks. 

The origin of these bombs is believed to be similar to that of those at 
Snag Lake cinder cone, where they were certainly ejected as independ- 



Diujil DESCRIPTIONS: NO. W, BASALT TUPP. 251 

ent fragments. Tbeir spherical form is generally attributed ' to rota- 
tion wliile flying throagh the air. Professor Dana^ says that at Hawaii 
sacli forms are produced " by the rolling movement of the front of the 
stream due to friction at the bottom.'^ Tiiey sometimes have a center 
of olivine or more scoriaceous lava. 

Some of the bombs near Mount Trumbull were found cont'^ining oli- 
Tine nodules, such as specimen No. 104, which were evidently solid at the 
time of the eruption, and may have been shot into the air like a bullet. 
The same mineral nodules occur also iu the lava stream, especially iu 
lis vesicular portions, and when they are released by weathering closely 
resemble volcanic bombs. 

The volcanic x>henomena of that region are described by C. E. 
Dotton in Vol. II of the Monographs of the United States Geological 
Survey, p. 101 to 112, to which the student is referred for further infor- 
mation. A fuller discussion. On the Fragmentary Ejectamenta of Vol- 
canoes, is given by H. J. Johnston-Lavis in the Proceedings of the 
Geologists' Association of London (1885-86) Vol. IX, pp. 421 to 432. 

No. 99. Basalt Tuff. 

(Peom Battle Crbek Meadows, Tehama County, California. Described 

BY J. 8. DiLLER ) 

Fine fragmental, volcanic material, snch as dnst, sand, and small 
lapilli, when cemented so as to be more ot less firmly coherent is tuff, 
Theterm, as explained under andesite tuii' (No. 79), is made to cover all 
pyroclastic rocks of which the component fragments are finer than 
coarse volcanic conglomerate. It varies greatly, owing to the character 
indsizeof the component parts, from a very fine granular, light-colored 
rock, such as would result from lithifying volcanic dust like specimen 
^'o.d8, to a conglomerate made up of lapilli, such as specimen. No. 96, 
and is designated according to the kind of lava fragments it contains, as 
basalt tufi", andesite tuff, rhyolite tuff, etc. 

B[)ecimen No. 99 was obtained near the summit of a cone from whose 
base issued a stream of basalt that flowed down the canyon of Battle 
Creek for miles. The cone was made up by the accumulation of vol- 
<^nic sand and lapilli about the orifice from which they issued, but 
instead of being loose material, as at the Snag Lake cinder cone, it is 
cemented and forms a tuff cone. The cementation probably occmrre4 
m connection with the volcanic outbreak. Eruptions are generally 
'"^mpanied by rains from condensed steam. When the rain is suf- 
ficiently copious the saturated mass of fragmental material may flow 
w mud and become consolidated, forming tuft'. Mud flows arc some- 
timeji disastrous. Pompeii was buried by such a stream from the slopes 
^^ Vesuvias. Cones made of tuff are tuft' cones. They are generally 
1*»* steep than cinder cones, and distinctly stratified. 



' ^kWi, Text Book of Geology, 8d edition, pp. 200 and 201. Alno Jadd'a Voloauoea, x>. 7u. 
' Dwt, Manual of Geology, 4th edition, p. 289. 



252 THE EDUCATIONAL SERIES OP ROCK SPECIMENS, [nwum 

The basalt tuff, specimen I^o. 99, is strongly contrasted with the andeo- 
ite tnff (No. 79), not only in the size and composition of its fragmentfl bat 
especially in its distribution with reference to the source of material 
The sand and dust of which the andesite tuff was made spread far aod 
wide over the country, and inay have been derived from numeroiu 
distant volcanoes about Lassen Peak. They were deposited in a body 
of water and formed part of an extensive bed. On the other hand, tb€ 
basalt tuff presented by specimen No. 99 is a land-made deposit, limited 
in its distribution to one small cone. It is somewhat stratified parallel 
to the slope of the cone. This arrangement is due to assorting dose 
by the air and the sliding of the material down the steep slopes. 

No. 100. Cellular Basalt. 

(From Ick Sprinq Craters, Millard County, Utah. Dxscribbd bt J. S. Dnxn.) 

By the expansion of gases contained in lava cavities are developed. 
If the cavities are small the lava is porous. If they are large and rather 
regular, either spherical or elongated, the lava is vesicular. When the 
vesicles are so abundant, as in specimen No. 100, that the space between 
them is reduced to a fine wall, the structure is cellular. 

Specimen Ko. 100 was obtained from the interior portion of a large 
ejected fragment or bomb near the eastern base of the Mitre, the same 
crater upon whose slope the specimen of scoria (No. 97) was ooUeeted. 
Much of the lava of this region is vesicular, but it is rarely cellular. 

The lava having the cellular structure (No. 100) is basalt. Yesicolar 
structure is common in basalts, but is rare or entirely wanting in the 
more siliceous lavas. The vesicles in many cases are elongated by 
movements of the mass, and show the direction of the flow. When 
the vesicles are filled with secondary minerals, such as oooar in sped* 
men No. 139, the structure becomes amygdaloidal. 

No. 101. QUABTZ-BASALT. 
(From Snag Lake Cindbr Conk, Lassen County, CALiroRNiA. Dbscribbd bt 

J. 8. DiLLER.) 

Quartz-basalt is basalt which is characterized by the presence of 
primary quartz. 

One of the latest volcanic eruptions in this country, so far as is yet 
definitely known, occurred a little more than 200 years ago near Snag 
Lake, 10 miles northeast of Lassen Peak, California. The cinder oone, 
ash field, and lava field, seen in PI. XXXIV, look as fresh as thoagh 
the eruption took place only yesterday. 

There were two lava flows from the base of the cinder cone. The 
older is partially covered by volcanic sand, as shown in PI. XXXIV. 
The surface of the younger flow is shown in PI. XXXVI to be without 
any covering of volcanic sand. Specimen No. 101 was collected trem 
the younger flow. 

The lava is quartz-basalt. It is a dark, compact, ipore or less por- 
pbyritic rock in which crystals and grains of quartz are distinct^ bat 




RFACE OF UtVk FrELD 10 MILES NORTHEUST OF LkSSEN PEAK, CALIFORNIA. 



[I 



r , 

)■ f 



n 
.'I 



f 



DESCRIPTIONS: NO. 101, QUARTZ-BASALT. 



253 



not conspidtoas. Uuder a microscope it is seen to be composed 
Lz, plajxioclase feldspar, pyroxeue, and olivine, together witli 
ry magnetite and a large proportion of unindi vidualized material 
s generally globalitlc. The feldspar is most abundant; then 
pyroxene in nearly eqnal amonnt, with less olivine and quartz, 
bnlitic base makes np nearly 25 per cent of the whole mass, 
olivine occurs in grains or well defined crystals, which often 
small coffee-brown isotropic crystals, supposed to be picotite. 
e of the first minerals to crystallize in the magma, and is rarely 
ided by a dark border. The pyroxene is chiefly hyperstheue, 
pte is sparingly present, especially in the shells surrounding the 

Teldspar occurs in two sets of crystals. The largest have their 
's clouded by a multitude of glass and globulitic inclusions, and 
regular outlines indicate that their initial forms were modified 
corrosive action of the magma. The other feldspar crystals are 
ftped, polysynthetic twins, such as are common in rocks of this 

18 and crystals of quartz in considerable numbers occur uni- 
distributed throughout the lava, and are especially remarkable 
mut of the shell of augite and glass by which they are envel- 
This feature is illustrated in PI. XXXVII, where it is seen th^t 
iriz is occasionally wholly resorbed by the magma, and its place 
1 by a group of granular augite. 

chemical composition of the quartz-basalt from Snag Lake is 
below, as determined by W. F. Hillebrand. It shows a large 
tage of silica, but this feature can not be considered as neces- 
bhe one which determined the presence of quartz ; its origin may 
irred to other causes.^ 

Uif9t$ of quartZ'hiuaU from Snag Lake laea field, La$»en County , California* 



Percent 



SiO,.. 
TIO,. 
AlA 

FeO.. 
HnO. 
CiK).. 
8rO.. 
BaO.. 
MgO. 

Na,0. 
Li/).. 
H,0.. 
PA 



Total 



57.25 


0.60 


16.46 


1.67 


4.72 


O.IO 


7.65 


tiaoe 


0.03 


6.74 


1.57 


8.00 


none 


0.40 


0.20 



100.88 



'See Ba21e£iD U, S. Geol Survey No. 66, by J. P. Iddiagt. 



i 



254 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [bcllUO. 

Quartz-basalts are relatively rare. They hold essentially the same 
relation to basalts that the dacites do to the andesites, or the rhyolites 
to the trachytes. A fuller illustrated description of the quartz basalt 
(No. 101) and its associated volcauic phenomena may be found in Bolle- 
tin No. 79, United States Geological Survey, by J. 8. Diller. 

No. 102. Basalt. 

(From Watchung Mountain, Orange, New Jersey. Described bt J. P. 

IDDINGS.) 

The basalt which forms a sheet intercalated in the red sandstone 
( Juratrias) of New Jersey and (M)nstitutes a capping to the ridge known 
as the First Mountain of the Watchung Bange was undoubtedly a 
sheet of lava that flowed over the surface of the country in Juratrias 
times. The rock in the collection is from the large columns exposed in 
John O'Rourke's quarry, near those illustrated in PL IV (p. 150), in 
West Orange Township, and is, therefore, from the lower portion of the 
lava sheet. 

The rock is dark bluish-gray when freshly fractured, usually taming 
greenish upon exposure. It is compact and breaks with an even- 
grained texture. Megascopically it is finely crystalline to aphauitic, 
sometimes slightly porphyritic, with small phenocrysts 1"™ or 2""" 
long. Owing to its durability and to the readiness with which it is 
broken into regular blocks, it is extensively quarried for paving stones, 
and is also crushed for road metal and used for macadamizing many 
miles of roads in this region. It is frequently columnar, or exhibits 
less regular prismatic jointing, and is the well-known '^ trap" rock 
forming the crests of the Watchung Bange, Bergen Hill, and tbe 
Palisades on the Hudson Biver. 

In thin sections, under a microscope, the rock is seen to consist of 
abundant monoclinic pyroxene and much plagioclase feldspar, with 
magnetite and scattered patches of microlitic and globulitic glass 
base, and a variable amount of serpentine or chloritei. The pyroxene, 
which is in excess of the feldspar, is mostly malaoolite, being pale 
green to colorless in thin sections, with high double refraction and 
poorly developed cleavage. It may easily be confounded with olivine. 
However, the occurrence of completely altered areas inclosed in per- 
fectly fresh pyroxene indicates that the serpentine represents a inach 
more easily altered mineral, such as olivine. The pyroxene of similftf 
basalts and diabases occurring in Connecticut was analyzed by G. ^^* 
Hawes ^ and shown to be an iron-limemagnesia pyroxene, low io 
alumina, corresponding to the composition of malacolite. In the basalt 
of Orange Mountain it does not exhibit tbe basal parting, or twinning' 
or the idiomorpbism that characterize salite. It is probable thatolivin^ 
was present in the rock before decomposition set in. A few part1> 



) O. W. Hawea, On the mineralogicid oompoaiUon of the nonaftl Mosoioic dUbsM apon the AtlcB^ 
border : Proo. U. S. Nat MoMoin, 1881t pp. 129-184. 



Ife 










nuiM,] DESCBIPTIONS: NO. 102, BASALT 255 

altered crystals of tbis mineral have been observed in some thin sec- 
tioDB. In others there are brown serpentine pseadomorphs which are 
anqnestionably decomposed olivines. It is jiossible that the scattered 
patches of serpentine which have been deposited in irregularly shaped 
spaces have resulted from the alteration of olivine. But serpentine 
may also be derived from the decomposition of the malacolite. 

The plagioclase feldspar forms lath-shaped crystals with polysyn- 
thetic twinning, often with only 3 or 4 stripes. The high extinction 
angles and relatively strong double refraction show it to belong to the 
more calcic 8i>ecie8, probably labradorite. Hawes has shown that two 
species of feldspar often occur together in these rocks, and has demon- 
strated the presence of labradorite and anorthite.^ 

The feldspar is in part altered to an almost colorless, brilliantly polar- 
izing mineral, without definite crystallographic boundaries, probably 
prehuite. 

Remnants of a glass base are occasionally observed. They form 
aogular patches, the glass being colorless with globulites and micro- 
liter, mostly of augite with attached grains of magnetite. The magne- 
tite is sometimes present in small aggregations. In places this residual 
base is holocrystalline, possibly through alteration. A study of the 
whole rock- mass showed that glass was more abundant in the upper 
portion of the lava sheet. 

The chemical composition of this rock is shown in the analysis made 
byL.G. Eakins: 

JnalifW of basalt from Watchung Mountainf Xetc Jersey, 

Per c«nt. '■■ 



"SlO.- 51.36 

Al/), 16.25 

Fe^Oj i 2.14 

FeO I 8.24 

MnO ^ 09 

XiO 03 

CaO 10.27 

MgO ; 7.97 

K2O 1.06 

Na,0 1.54 

H^ : 1.33 

CO, 



Total 100.28 

« ^^ _^ , 

For a description of the columnar structure of this rock, see The 
colunmar structure in the igneous rock on Orange Mountain, New Jev- 
^h by J. P. Iddings: Am. Jour. Sci., 3d series, Vol. XXXI, May, 
1886, pp. 321-331. 



1 Cr. W. Hawes, loo. oit, p. 131. 



256 THE EDUCATIONAL SERIES OP BOCK SPECIMENS. iBuii^lM. 

No. 103. Columnar Jointing in Lava.^ 

(From Karnak Ridgk, Montezuma R\nge, Churchill County, Nevada. 

Described by J. P. Iddings.) 

The occurrence of this rock, under the name of rhyolite, is described 
by Messrs. Hague ^ and King^ in the Reports of the U. S. Exploration 
of the Fortieth Parallel, where illnstrations of its columnar stracture 
are given. 

Along tbe crest of this [Kariiak] ridge the rhyolite fonns a aerieg of clnsten of 
prismatic columns of all sizes from 3 feet down to an inch in diameter. They show 
from 3 to 7 sides, most frequently 5, but iu many canes the fifth side will be mncli 
longer than the other four, with a slightly curving outline and a tendency to dovelop 
a sixth side. The four-sided figure would seem to be tbe least common. Csually 
they stand iu an approximately vertical position, that is, above an angle of6(F. 
The tendency to columnar structure shows itself in various degrees of perfection, 
from the symmetrical prism to a single set of parallel planes diagonal to the bedding 
of 1 he rock. The most perfect prismatic forms are found near the summit, becoming 
less and less sharply developed farther down the slopes. The exterior of tbe col- 
umns, generally of a dark, almost chocolate brown color, fades in many iiutancei 
into a reddish gray. The interior is an exceedingly brilliant, pure gray. 

The microscopical characters of the rock have been describee! by 
Zirkel ^ and referred to those of rhyolite, but they are those of bora- 
blende-mica audesite or possibly of some form of dacite. 

The rock has a light-gray, aphanitic groundmitss, with many sDiall 
phenocrysts of feldspar, biotite, and hornblende. The feldspar pheno- 
crysts are all lime-soda feldspar in idiomorphic forms, with m<arked 
zonal structure and numerous glass inclusions. The values of the sym- 
metrical extinction angles are such ns to indicate that the feldspars are 
labradorite in part.' The glass inclusions are colorless in some cases 
and dark brown in others. Other inclusions in the feldspars are tbiu 
prisms of apatite, magnetite, zircon, and occasionally hornblende and 
biotite. 

The hornblende phenocrysts are very small and have poorly devel- 
oped forms. Gross sections show the presence of the unit prism (HO) 
and clinopinacoid (010). The crystals are sometimes irregularly shaped 
anhedrons. Pleochroism is pronounced from greenish brown and 
brown to light brown : jc = greenish brown, b = brown, and it = lig^'^ 
brow!i. The absorption is c>b>;t. Some crystals in thin section 
exhibit a narrow border of magnetite grains and augite microlites; 
others are free from it. 

Biotite occurs in thin plates and also in comparatively thick crystals 
with more or less irregular outline. It has a brown color similar to 
that of hornblende, with strong absorption. It carries inclusions of 



1 Tbis rock, altliougli a qaartz-bearing micaandenite or dacite, is given this place in tbe 
because columnar jointing is most common in basalts.— J. S. D. 

» A. Il.njrue. U. S. Kxplnratiou of tbo Fortieth Parallel, Vol. II. WaabingtoD, 1877, p. 761. 

»C. King, ibid, V^ol. T. Washington, 1878, p. 644. 

* F. Zirkel, Microscopical Petrography : Vol. VI, U. S. Geol. Expl. Fortieth Parallisl. Washiifto** 
1876, p. 177. 



io.] NO. 103, COLUMNAR JOINTING IN LAVA. 257 

agnetite, sometimes in the central part, Bometimes in tlie margin; 
esides few opaque needles. 

Magnetite occurs in minute crystals. Apatite forms namerous long, 
lender prisms with dust-like inclusions. Zircons are very few and 
mall. There are a few phenocrysts of quartz with rounded and irreg- 
ilar outlines, carrying glass inclusions. 

The groundmass is holocrystalline, consisting of irregularly shaped 
iuhedrons of feldspar and quartz in niicropoikilitic growth, the whole 
being clouded by minute particles, probably gas caFities in most cases. 
There are, besides, microscopic idiomorphic feldspars with rectangular 
OQtlines, sometimes forked at the corners. These exhibit polysynthetic 
twinning in some cases, but not in others. The smaller ones have a low 
extinction angle and are probably oligoclase. It is questionable 
whether any may be ortho<;lase. In places there are idiomorphic crys- 
tals of quartz surrounded by a clouded zone of quartz and feldspar in 
micropcBcilitic intergrowth. The groundmass also contains minute 
crystals of magnetite and scales of mica, besides microscopic prisms 
of apatite. 

Golomnar structure, or the separation of a rock into^ prisms more or 
less straight and parallel to one another, is not confined to any one 
Und of rock, although it is most frequently developed in igneous rocks 
iuid especially in basalts. The most familiar examples of columnar 
rocks are basalts, notably the Giant's Causeway and FingaPs Cave in 
Ireland, the columnar lavas in the Auvergne in central France, and 
those of the Snake River Canyon in Idaho and of the valley of the 
Oolamhia in Oregon. Excellent columnar structure occurs in the basalt 
or trap of the Palisades on the Hudson and of Watchung Mountain, 
west of Orange, as shown in PI. IV (p. 20), and at Paterson and Little 
Falls, in New Jersey. But equally good columns are found in the rhyo- 
htes and obsidian in numerous localities in western America, notably 
in the Yellowstone National Park and in Nevada. They also occur in 
indesites and other kinds of igneous rocks, and even in granite in rare 
instances. Less x)erfect columns are sometimes observed in limestone, 
iMiked clays, and coal. In all cases the prismatic cracking is the result 
^ia contraction of the rock mass, either through cooling, as in the case 
>f volcanic lavas, or through loss of substance ui>on drying or baking, 
^hen water or vajwrizable portions of the mass have been driven off 
^y tbe heat of adjacent intruded rocks. 

lu igneous rocks the prismatic cracks start at right angles to the 
plane or surface of cooling, and if the rate of cooling is uniform over 
tfce snrface the cracks continue in a straight line, producing straight 
prisms. If the cooling is not uniform at the surface, the columns or 
prisms become curved, often in diverging groups. When the rock mass 
^* perfectly homogeneous and the cooling is uniform, the columns are 
^^lagonal and of uniform thickness; but these conditions are seldom 
^lized in nature, and the number of sides may vary, as may also the 
BuD. 150 17 



258 THE EDUCATIONAL SERIES OF BOCK SPECIMENS, [bull MO. 

thickDesB of different prisms. The slower and more gradual the shriok- 
age of the mass, the larger the columns. And since the rates of cooliug 
at the upper and lower surfaces of a lava sheet resting upon the sarfaoe 
of the earth are usually quite different, it often happelis that the lower 
portion of a lava sheet will be separated into larger columns than the 
upper portion. Moreover, since the cooling at the bottom is generally 
uniform, while that from the top may be irregular, the bottom colnmiis 
are usually straight and normal to the bottom plane, and tbe apiier 
columns are curved in diverging groups. Small prisms in volcauic 
lavas sometimes correspond exactly in shape to the well-known starch 
prisms which have been produced by the drying of a mass of starcL 
For a discussion of the x)roduction of columnar structure, consult the 
following articles: 

On the origin and mechanism of production of the prismatic (or col- 
umnar) structure of basalt, by B. Mallet: Philosophical Magazine (4), 
vol. 1, 122, 201. 

On columnar fissile and spheroidal structure, by T. 6. Bonney: 
Quart. Jour. Oeological Society, 1876, p. 140. 

The columnar structure in the igneous rock on Orange Moantaio, 
New Jersey, by J. P. Iddings: Am. Jour. Sci., 3d serien, vol. 31, 1886, 
p. 321. 

No. 104. Olivine Nodule from Basalt. 

(From near Mount Trumbull, Yavapai County, Aiuzona. Described by 

J. P. Iddings.) 

The rock wbich carries these nodules is a recent lava flow wbos© 
place of occurrence is illustrated and described iu Monograpt l^? 
United States Geological Survey, pp. Ill and 112. These nodules are 
found not only in the lava flow, but also as bombs among the ejected 
material of the cinder cone from whose base the coulee issued. 

The rock is a basalt, in part highly vesicular and scoriaceous, having 
a dark gray to black color, and in part solid and compact. In the vesic- 
ular portion the nodules are several inches long and thick; in th^ 
compact rock they are small, nearly the size of lima beans or almond^ 
or walnuts. The shape of the nodules is quite irregular, some being 
rounded, others angular with smooth surfaces. A megascopic exami- 
nation shows that in some cases the nodules are composed of several 
kinds of minerals, though chiefly olivine. Some appear to consist ol 
diallage or a mixture of diallage and olivine. The small ones in the 
compact basalt seem to be nearly pure olivine in nearly all cases. Tb^ 
basalt also carries comparatively large crystals of black hornblende 
and feldspar, which are less noticeable than the nodular inolusions* 
These nodules are granular crystalline aggregates of pale-green olivine 



>In the field, -when the nodules of olivine and diallage were c<»llect«d, crvAtalN of hornblende 
feldspar, o<;ca8ionall3r an inch in diameter, were seen. The outer snrfaeoH in nearly all cams 
irregular, showing embayments due to the corrosive action of the magma. J. S. D. 



mn.] NO. 104, OLIVINE NODULE FKOM BASALT. 259 

mi\ a variable amoant of green pyroxene. The oliviiie crystals liave 
ayitieoQslaster and snbconchoidal fracture. '^' - 
The vesicalar basalt containing the nodnles when seen in thin section 
ifl found to have an almost opaqne groundmass, filled with small cavi- 
ties, abundant small olivines and occasional augite crystals, besides 
relatively few lath-shaped microlites of lime-soda feldspar. In places 
tbe«6 microlites are more abundant and the grouudmass is partly 
transparent and is crowded with minute opaque dots. The opai^ue 
matter is undoubtedly magnetite in very minute grains, or crystals so 
namerous as to prevent the transmission of light through rock sections 
of tbe ordinary thinness. In the very thinnest edges it is possible to 
make out the presence of other constituents, whose character, however, 
Ib not determinable directly. 

The almost microscopic phenocrysts of olivine scattered through this 
groandmass are generally idiomorphic, with the outlines usual to such 
crystals — that is, the sections are rhombs or six-sided figures; but some 
of the crystals are imperfectly developed and have irregular outlines, 
or are skeleton forms of growth, with jagged outline and numerous 
pockets or bays of groundmass, and also inclosures of groundmass. 
Sach forms result from rapid crystallization in a quickly cooling magma. 
The olivine is nearly colorless, with strong double refraction, yield- 
ing brilliant colors between crossed nicols. Some of the crystals are 
twinned, so that two crystals cross one another, or are in juxtaposition, 
vith their vertical axes inclined to one another at an angle of about 
^. This arrangement may be brought about when the twinning and 
composition planes are the unit brachydome (Oil), an observed mode 
of twinning in olivine. Augite crystals about the size of the olivines 
are scarce. They form somewhat rounded and irregularly shaped 
crystals, which are pale green in thin section and are not pleochroic. 
They are also strongly doubly refracting, and are brilliant between 
crossed nicols, being distinguished fh)m olivine by their color and 
more marked cleavage, which is the usual pyroxene cleavage in the 
prismatic zone. The inclined extinction angles measured against these 
cleavage cracks reach as high as 42^. The augites are full of irregu- 
larly shaped inclusions of glass, and of multitudes of minute dot-like 
inelofiions, which also may possibly be glass. 

There are still less frequently irregularly shaped crystals of dark 
brown hornblende with strong pleochroism, which contain inclusions 
of groandmass. Tbey are the same as the large megascopic crystals 
of black hornblende already mentioned. 

The feldspar microlites are exceedingly small, are in many cases 
forked at the ends, and are twinned in two strips parallel to tbe long 
axis of the microlite. The symmetrical extinction angles, reaching 32° 
in some instances, indicate that the feldspar is labradorite. 

The nodnles in thin section are seen to consist of anhedrons of oli- 
vine, with a subordinate amount of pyroxene, which is in part ortho- 
rhombic, in part monoclinic. 



260 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, (bull 150. 

Tlie olivine is colorless and has wholly allotriomorphic oatlines. Id 
some cases it is without cleayage* in others there is a more or less dis- 
tinct pinaeoidal cleavage. In general the substance is quite pare, but 
inclusions are numerous in some individuals. They are in x)art nega- 
tive cavities having the form of hollow crystals of olivine; in part tbcy 
are rounded, or form an irregular network of cavities. These seem to 
be filled with gas in some cases and with liquid in others, there being 
a moving gas bubble within the liquid inclusion. Occasionally the 
cavities are cut in making the thin section, and balsam enters and 
partly fills them. There are also rod like iuclusiontf of a browD min- 
eral, which lie in parallel lines. They appear to be the same as stouter 
inclusions of similar shape, which have the same direction and the same 
color. They also occur in irregular shapes and as rounded grains 
like the larger brown grains which have a higher index of refraction 
than olivine, are isotropic, and are either picotite or chromite. Some 
individuals of olivine, when seen between crossed nicols, exhibit a 
parallel banding, as though composed of comparatively broad laiiielliB 
with slightly different angles of extinction, which might be dae to 
polysynthetic twinning. 

The pyroxene resembles the olivine very closely, but has a grayisb- 
green tinge, with no pleochroism, but with marked cleavage as iit 
pyroxenes. Some of it has the parallel extinction and lower double 
refraction of orthorhombic pyroxene, and is probably eustatite. Otbcr 
individuals have higher double refraction and inclined extinction, and 
are probably diopside. Both pyroxenes in some cases carry numerous 
gas and fluid inclusions with moving bubbles, which have the shape of 
crystals of the matrix, or may be irregularly shaped. Occasionally the 
pyroxene is striated as though by polysynthetic twinning. 

In one section examined there is a little violet-brown glass between 
crystals of olivine within the nodule. When olivine lies in contact 
with this glass it has its proper crystal form, which indicates that the 
glass is a remnant of the matrix out of which the olivine crystallized. 
This kind of glass was not observed around the margin of the olivine 
nodule. 

The outline of the margin of these nodules in thin section is irregu- 
larly jagged, with minute bays and pockets of the groundmass of the 
surrounding basalt penetrating into the crystals of olivine and pyrox- 
ene. There is no sign of crushing of the olivine along the margin of 
the nodule. 

In mineral composition these nodules resemble certain varieties of 
peridotite, since they consist of olivine with picotite, or olivine witb 
diopside and enstatite and picotite, and might possibly be considered 
inclusions of fragments of such rocks. But the olivine is the samea^ 
that in the basalt, and in some cases there is brown glass within th£ 
nodule. It is probable that these nodules are segregations that hav* 
formed within the molten basalt magma, either in lumps the size 9^ 



nJJi.1 DESCRIPTIONS: NO. 105, DOLERITE. 261 

;he present nodules or in larger masses which have been cracked upon 
the eruptiou of the basalt magma. Analogoas segregations of horn- 
blende often occur in andesites. Similar nodales of olivine are found 
in basalt in various localities, some of them being very large masses. 
For a review of the different hypotheses that have been advanced to 
explain the formation of olivine nodules, the student is referred to Prof. 
Ziikefs liebrbuch der Petrographie, Second Edition, Vol. II, p. 931. 

No. 105. DOLBBITE. 

(From Valmont, Bouldkr Country, Colorado, Thrkr Miles East of the Town 
OF Boulder. Described by Whitman Cross.) 

Geological occurrence. — The rock to be described occurs as a vertical 
dike, rouuing east and west, 20 to 40 feet wide, and nearly 2 miles in 
length. The formation cut by the dike is the Fox Hills, or upper divi- 
siou of the Montana group of the Cretaceous, a series of sandy shales 
and friable sandstones. The locality is on the plains, 4 miles from the 
foothills, and the shales are nearly horizontal. The specimens were 
obtained from the west end of the dike, just above the village of Val- 
mont. Here the dike rises about 200 feet, with vertical walls near the 
top and debris-covered slopes below. 

It is supposed that this rock was erupted during the Denver period 
of the post-Lanimie, and that at the time of eruption there may have 
been a thickness of 2,000 feet, more or less, of sedimentary rocks above 
the horizon at which the dike is now exposed. Basaltic magma of 
practically identical composition was erupted at this time at several 
places, some 14 to 16 miles nearly south from Valmont, and the product 
is DOW seen in dikes and in the surface flows of Table Mountain at 
Golden. It is considered probable that the Valmont dike also repre- 
sents a channel through which the basaltic magma rose to the surface. 
Aside from analogy with the occurrence near Golden, a further support 
of this supposition is afforded by the uniform texture and structure of 
the Valmont rock from wall to wall and from end to end of the dike. 
Thi8 is interpreted to mean that the magma cooled at a very uniform 
fate, such as could be experienced only where the adjacent shales had 
become highly heated by the long-continued passage of lava through 
the fissure conduit. 

General description. — The Valmont dolerite is a very dark porpliyritic 
fock, whose most distinct phenocrysts are dark augite prisms, with a 
Biaximum length of l*^'", while most of them are less than 0.5*^™ long, 
and there is a regular gradation down to those less than 1'""' in length. 
% careful search a few glassy olivine crystals may be seen, especially 
where seri)entin]zation has begun. Eeflection from the curved fissure 
planes of the fresh olivine phenocrysts often gives them a metallic 
luster. Some olivine crystals reach a length of 3'""', but the average is 
about 1™"'. 

Feldspar phenocrysts are rarely discernible with the naked eye, but 



262 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [kllHH. 

examination with a lens shows the dark-gray mass in which olivine and 
angite crystals are embedded to consist very largely of plagioclase 
tablets, whose narrow cross sections exhibit a very fine striation on the 
basal cleavage plane. These tablets lie irregularly, and their boand- 
aries seem indistinct. The lens also shows the dark color of the mass 
to be largely due to many minute black and green particles regularly 
disseminated through the whole. 

Microscopical examination of thin sections shows the rock to consist 
of the minerals already mentioned, with the addition of orthoclaseand 
biotite and the accessories magnetite and apatite. The dark specks 
giving color to tbe mass are magnetite, augite, and biotite. The angu- 
lar spaces left between the plagioclase tablets are for the most part 
filled by orthoclase. No glassy or globulitic base exists in this rock. 

The constituents are fresh, excepting some of the olivines, which are 
partly changed into golden-yellow serpentine, and patches of feldspar, 
which are cloudy. 

Characteristics of constituents, — ^The augite is of a dull, greenish-gray 
color, occasionally exhibiting a yellowish tinge or becoming nearly color- 
less in certain zones. The phenocrysts are stout prisms of the usoal 
forms, but the outlines as seen in thin sections are almost always some- 
what irregular lines. Zonal structure is rarely seen, but a few crystals 
show a colorless zone near the outer border. This is free from inclo 
sions, while the green portion commonly contains numerous round or 
irregular sack-shai^ed brown glass inclusions, together with grains of 
magnetite, olivine, and prisms of apatite. Very rarely these are 
arranged in zones, though an irregular kernel of the crystal is fre- 
quently free from inclusions. Biotite leaflets are sometimes included 
in augite. 

There are many small phenocrysts of augite, but there is a decided 
gap between these and the irregular greenish grains of a second gen- 
eration, which average 0.05 to 0.20"™ in diameter. 

Olivine is developed in a manner closely corresponding to that of 
augite. Its most distinct phenocrysts show prisms and domes. Inclo- 
sions of magnetite, apatite, and glass are numerous. The latter are 
usually small and round and show black globulitic devitrification. 
They are sometiuies zonaliy arranged. Biotite leaves are rarely fonnd 
in the olivine. 

All stages of serpen tinization can be seen in almost every thin sec- 
tion, but the majority of the crystals are very fresh. The fibroo* 
golden-yellow product contrasts markedly with the colorless olivine. 
It develops in fibers normal to the various fissure planes in the crystal. 

Plagioclase is so developed in this rock that it is difficult to deter- 
mine what varieties are present. The larger crystals are tablets par- 
allel to the brachypinacoid, but their other boundaries are either 
imi>erfect or are obscured by the later growths. There are probably at 
least two varieties of the liuiesoda series present. The older of these 



wiixi.1 DESCRIPTIONS: NO. 105, DOLERITE. 263 

forms the larger tablets, 1 to 3"'"' long, and quite narrow. These usually 
pntsent very tUn lamina^, twinned according to the albite law, and 
such polysynthetic parts are often further twinned by the Carlsbad 
law. In addition, twinning by the pericline law is often found. The 
maximani observed extinction in the zone normal to the predominant 
pinacoid is nearly 40<=^, which indicates that such crystals must be at 
least as rich in lime as labradorite. 

Probably different from these larger crystals are the small staves of 
plagiodase, which are abundant. It is also difficult to make out the 
form of these crystals. 

Orthoclase is probably an element of considerable importance in the 
Valmont dolerite, but it is practically imx>ossible to prove its identity 
in most thin sections. The cause of this difficulty lies in the manner 
in which it is developed — in irregular grains, as the last crystallized 
mineral, between other constituents. But there are some cases where 
plagioclase crystals are more or less completely surrounded by an 
irre^lar border of apparent feldspathic substance which extinguishes 
nDiformly parallel to the pinacoidal line of sections normal to the lam- 
inae. That this substance and many of the simply polarizing grains are 
orthoclase is strongly indicated by the chemical analysis of the rock 
and by analogy with the basalts of similar comx)Osition near Golden, 
where the development is much more distinct. 

Biotite occurs abundantly and very regularly distributed throughout 
the rock in little greenish-brown flakes having a tendency to attach 
themselves to olivine and magnetite. They are apparently included 
both in aagite and olivine. No large leaves have been noticed. Only 
f&rely do the flakes have a hexagonal form. 

Magnetite is scattered through the rock, but is less abundant than in 
iDost basalts. It is the most abundant inclusion in the olivine and 
wgite crystals. 

Apatite is present in characteristic prisms, but is not so prominent 
98 in andesite. No zircon crystals have been observed, and no titanium 
oiinerals. 

StrMcture, — The large augite and olivine prisms distributed through 
^lie dense feldspathic mass of the rock give it a porphyritic structure, 
^Qt microscopical study shows that there is not quite that relation 
l^tween phenocrysts and groundmass which is commonly found in 
porphyritic rocks. The distinct crystals of augite and olivine lie in a 
inass which also contains crystals of labradorite, but these grade down- 
ward in size to correspond with a probably distinct feldspar occurring 
in staves, while orthoclase with some oligoclase Alls the interstices 
between the older tabular crystals. 

It is true that augite occurs in what is apparently a second genera- 
tion in small irregular grains, which are embedded in or lie between 
the feldspar crystals. This rock has therefore a peculiar structure 
iwt commonly met with, and it serves to illustrate one way in which 



264 



THE EDUCATIONAL SERIES OP ROCK SPECIMENS. lBuii.l«. 



porpbyritic structure may be produced witbout a Recond generation of 
the cbief minerals; for while the augite and possibly other iiiiiierals 
may have bad a second period of formation, the porpbyritic Btmctore 
seen is not due to that cause. 

Chemical composition, — Below is given the analysis of the typical 
rock (I), and that of augite isolated from the same (11). Both aualyoes 
are by L. G. Eakius. 

Analyses of dolerite from Valmoni^ Colorado ^ and of augite therefrom. 




SiO, 

TIO, 

A1,0, 

FejO, 

FeO . 

MnO. 

CftO. 

fiaO. 

MgO. 

K,0.. 

Nb,0. 

H,0 . 

P,0,. 

CI.... 

SO,.. 



Total 



I. 


11. 


Per cent. 


Per cent 


48.25 


49.10 


.89 
16.73 




7.95 


3.09 


trace 


6.28 


8.30 


trac6 




8.32 


22.54 


.013 




5.77 


12.37 


4.08 


trace 


3.24 


trace 


1.72 
.68 
.08 
.12 












100.163 


100.26 



The greater part of the titanic acid of the rock is undoubtedly ooa- 
tained in the augite, but it was not determined in the analysis of this 
latter. 

The analysis is that of a typical basalt, except as to the alkali6S> 
The high amount of potash found confirms the belief that the last feld- 
spar crystallizing in this basalt is orthoclase. In the closely allied 
basalts of Table Mountain at Golden there is likewise a larger amoaot 
of potash than soda. Perhaps the presence of biotite is also doe to 
this abundance of potash. This mineral is also developed in the Golden 
basalt. 

Literature. — The basaltic occurrences of this region are fully de- 
scribed in Monograph XXVII of the United States Geological Survey, 
Geology of the Denver Basin, Colorado, by S. F. Emmons, Whitman 
Gross, and G. H. Eldridge. 

No. 106. Diabase. 

(From West Rock, Nkw Haven, Connecticut. Described by L. V. PirssonO 

Northward from the city of New Haven there extends through Coa- 
necticut and into Massachusetts a long, narrow area of sandstones 
which, from their fossil contents, are known to be of Triassic age. The 



pniiil BB8CBIPTI0NS: NO. 106, DIABASE. 265 

length of this belt is about 110 miles and its width about 20. The 

rocks compoAing it are chiefly red sandstones which, pass, ou the one 

band, into coarse conglomerates and, on the other, into sandy shales. 

Tbe belt itself lies in a great trough of upturned crystalline meta- 

morpbosed rocks of nncertain, but probably Paleozoic, age. The thick- 

nesA of the Triassic strata in this trough is not known, but artesian 

borings down to 4,000 feet made in New Haven have not passed 

tliroQgh it. The belt is one of a system occurring in independent 

areas along the Atlantic border, which represent deposits made in 

Mesozoic time and to which the name of the Newark group has been 

given. The belt extending northward through Connecticut and Mas- 

ssKhasetts is known as the Connecticut Valley area. 

The general structure of this area has produced the belief that the 
trough was the former valley of the ancient Connecticut Eiver, which, 
tbrongh submergence, passed into an estuarine condition, and was 
then filled with the sediments. The character of these beds, their 
frequent cross bedding, and the sudden changes from sandstone into 
conglomerates show rapid shifting of currents and powerful stream 
ai^tioD. 

A small outlying area of these sandstones about 15 miles west of 
tbe main one indicates that the formation had once a wider area than 
at present, but denudation since its emergence has carried away an 
unknown amount, leaving, however, the thicker mass lying in the 
former trough. 

The beds in this area are not in their original horizontal position. 
-After their emergence, by the action of orogenic forces, they were 
halted into a series of great blocks, which dip eastward and have 
tbeir npthrow on the western side. They thus form a series of mono> 
clines which give rise to north and south ridges. 

The type of topography to which this structure has given rise is 
strongly accentuated by the fact that the upturned blocks of sand- 
stone contain numerous intrusions of diabase, or <^ trap rock," as it has 
l^n commonly called. These intrusions are of all sizes and of thick- 
nesses up to 250 feet. The sandstone which formerly covered the 
iQtmsions has been largely carried away, especially along the crests 
^ the faulted blocks, by denudation and glacial erosion, thus exposing 
^^e igneous rock. It therefore produces a series of curved north-and- 
%nth ridges which, on account of the upthrow on the westward side, 
faee toward the west and southwest with bold precipitous cliffs with 
columnar faces. These projecting masses of diabase dominate the 
topographic character of the region and form the most striking ele- 
loent in its scenic features. 

It is admitted by all geologists who have studied the region that the 
diabase occurs in intrusive masses, but in regard to the period at which 
tbe intrusions took place there has been much discussion. Concerning 
tWs, two views have been held. One of these supposes that the main 



266 THE EDUCATIONAL SERIES OP ROCK SPECIMENS [bullW. 

X)ortion, at least, of the intrusions took place after the upturning of the 
beds. The magma rose through fissures, often passing along betweeu 
the bedding until, nearing the top, it lifted the upper layers and, some- 
times abrading the upturned beds on the lower side, formed large 
intrusive masses. This view has been chiefly upheld by the late Prof. 
J. D. Dana. 

The other view supposes that the intrusions of diabase took place 
before the upturning of the strata, and that it was injected between 
them in immense horizontal intrusive sheets conformable with tbe 
bedding, or poured out in contemporary lava flows as the sediments 
were deposited. Later, when the sandstones were faulted into blocks 
with upthrow to the west, the trap sheets were brought to light. This 
idea has been chiefly urged and developed by Prof. W. M. Davis. It 
would be out of place here to enter into a discussion of these two con- 
flicting views; suflice it to say that some features of the area seem to 
be best explained by the former, others by the latter, and that it is by 
no means certain that either view is everywhere correct. The main 
point is that the igneous rock describe<l here is intrusive in origin, and 
that the peculiarities of structure and texture it exhibits are due to an 
igneous mass cooling and crystallizing under at least a moderately 
heavy cover of sediments. Those who desire more information in 
regard to the structure and occurrence of these intrusive masses of 
diabase will And it in the works given in the list of literature at the 
end of this article. 

West rock is the name given locally to the extreme southern end of 
the most western of the *'trap'' ranges mentioned above. The ridge, 
which has here a height of about 400 feet above tide, breaks off aloug 
the western front and southern end in a bold cliff from which the ma«8 
slopes back toward the east at a moderate angle until it merges into 
the lower country. 

Along the crest tbe rock exposures are those of the diabase itself, 
the sedimentary covering having been carried away; but down tbe 
slopes toward the north and east the covering sandstones are met 
with. Along the western side and southern front the cliff gives an 
excellent section, and it may be seen here that the diabase is in p&rt 
unconformable to the strata, and then dips and passes in between the 
planes of bedding of the upturned sandstones. The thickness of 
the mass above the sandstones on the west front is about 200 feet. It 
is cut by a series of joint planes which divide it into rude colamus, 
so that the clifl' front, viewed from below, has a pronounced columnar 
structure. 

Tbe rock has long been quarried and used as a building stone and 
for roail metal in the city of New Haven. For this latter purpose it is 
especially well adapted by its hardness and tough, resistant qualities* 
For building purposes material having smooth planes of the natural 
joint faces is especially sought, as these, from the oxidation of the iron- 
bearing minerals, are covered by a thin skin of varying shades of 



uaH DESCRIPTIONS: NO. 106, DIABASE. 267 

rown which produce a very pleasing efifect in surfaces of masonry, 
"be quarry ia at the extreme soatbem end of the cliff, and is in the 
illageof Weatville, one of the suburbs of New Haven, and about 2 miles 
rom the center of the city. It is from this quarry that the specimen 
a the collectiou has been taken. 

The diabase of the New Haven region possesses a special interest 
^m the x)etrographic standpoint in that it was, so far as known, the 
irst rock in America to be investigated by modern petrographical 
methods. This investigation was made in 1874 by Hawes and E. S. 
Dana, who showed by analyses and by examination of thin sections 
by the polarizing microscope, that the ^^trap^' rocks of the Connecticut 
valley area were composed of augite, iron ore, and a feldspar to which 
the composition of labradorite was assigned, with at times the addition 
of some chlorite. Later, by analyses and separations, Hawes showed 
that two varieties of feldspar were commonly present in these rocks. 

The specimen shows a rock of a very dark stone-gray color, heavy, 
and rather dense in texture. On a close examination with the eye, this 
dark color is seen to be due to the spotting of shapeless masses of a 
blaekish mineral (mostly augite) with tiny flecks of white (feldspar). 
It may be noticed that at times the white flecks take the form of minute 
nMl8. The clean firactured surface of the rock is nearly devoid of luster. 
The lens only serves to bring out these features more strongly, and to 
show that the light and dark minerals are mingled in shapeless masses, 
it does not bring out any pronounced feature which would serve to 
characterize the rock. The rock, indeed, does not megascopically show 
%ny pronounced features which would serve to classify it, beyond its 
▼eight and the dark color due to a large proportion of a ferromagnesian 
mineral, characters which would at once place it among the basic rocks 
of basaltic habit. 

On examining a thin section of the rock under the microscope, how- 
ever, its structure and mineral composition are at once clearly seen, and 
it also proves a very interesting one for petrographic study. 

The minerals composing it are found to be apatite, iron ore, biotite, 
pyroxene, plagioclase feldspar, orthoclase, and quartz, while chlorite 
^nd serpentine- like minerals occur as secondary products produced by 
feathering. The minerals are given in the order of their formation, 
^hich is told by the younger including the older, or by the latter i)ro- 
Ming into the former with crystal boundaries. The pyroxene and 
plagioclase feldspar are by far the most abundant and constitute the 
cbief mass of the rock. 

Apatite is not common. It occurs in small stout prisms, which are 
^pt to be associated with the colored components, and long slender 
needles, which occur almost wholly in the feldspar. It is colorless and 
^ly told by its high single refraction, low double refraction, and 
optically negative extension. It may occur in any of the other com- 
ponents, and is therefore the oldest. 
Iron ore, though well distributed throughout the section i\\ blavik 



268 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. Ibcllim. 

opaque masses, is by do means abundant. It is undoubtedly in part 
ilmenite, since the analysis of tbe rock shows a considerable amoautof 
titanic oxide present, and there is no other mineral to which it can be 
referred, except that pyroxene sometimes carries a small amoant. 
When it enters into their composition, however, they are almost iDvari- 
ably of violet color and have a marked dispersion of the optic axes, 
which is not the case in the pyroxene of this diabase. Both magnetite 
and ilmenite are present, as shown later. 

There are two varieties of pyroxene present. The one is the usoal 
brownish kind characteristic of this class of rocks; the other isawbite 
or colorless one, which has a tendency to be more idiomorphic than tbe 
brown, in columnar shapes. The difference between them, since tbe 
brown one is light in tone, is not extremely marked, but it can be easily 
seen by studying the section with a rather low power, so that a consid- 
erable number of the augites are brought into the field at once. Tbe 
exact nature of the white augite is not known. It occurs very com- 
monly throughout the diabase of the Connecticut valley area. It is 
also found in occurrences of diabase in Sweden, in northern England, 
at Kio Janeiro, Brazil, and in Nova Scotia. It was formerly 8ap[)a8e(I 
that the mineral was the same as that variety of pyroxene found at 
Sala, in Sweden, and hence called salite. From this the rock has often 
been calletl salite diabase.^ Recently, however, E. O. Hovey -^ has inves- 
tigated the salite from Sala and has found that the angle of the optic 
axes in air 2^^ = 112° 30', while the mineral under discussion is cbar- 
acterized by a remarkably small angle for a pyroxene 2Ba = 32° 39' 
(Brazil), 34^ 47' (Halleberg, in Sweden). Hence it can not be salite, and 
this name should no longer be given to this variety of diabase. The 
mineral has a good prismatic cleavage and also a pronounced cross 
parting parallel to the base. It is sometimes twinned with a (KO) ^ 
the twinning plane. It is more idiomorphic than the brown varietyf 
and therefore older. It supers from alteration much more easily tbau 
the brown, and while the latter is very fresh the colorless one is every- 
where beginning to be attacked by processes which are converting it 
into a fibrous serpentine-like substance. This fact also helps to distin- 
guish it from the brown. In this respect the section is a very instruct- 
ive one, as it shows very clearly how such processes of alteration by 
weathering go on, whereby minerals rich in magnesia are converted 
into serpentine-like substances. 

The process begins on the outside and works inward, or it starts from 
cleavage cracks, which have enabled the capillary moistnre to creep 
into the mineral, and works from both sides. The first stage consists 
in the production of a number of fine, parallel, colorless fibers which 
pass from the edge or crack into the mineral. They look like a series 
of fine parallel scratches on the surface of the otherwise unaltered 



1 See Kosenbuscb, Masn. Gest. 1887, p. 202. 

sTscherniak'a Mia. Mitt., vol. 13, p. 218. 1883. Also p. 213. 



»II1D.1 



DESCEIPTIONS; NO. 106, DIABASE. 



269 



BiBeral, and are to be seen best with very high powers. From this 
itagethey grow more and more numerous, until finally that portion of 
tbe mineral attacked is converted into a cloudy opaque substance of a 
brownish or yellowish color. 

Tbe brown pyroxene is of the usual aluminous variety of augite found 
in diabases. It is generally much fresher than the white. It has a 
much larger optic angle. Both varieties have a large angle of extinc- 
tion iu sections parallel to 6(010) or nearly so, which sections are easily 
told by the high polarization colors they exhibit. The brown variety 
has an excellent prismatic cleavage, a very poor parting parallel to 
b(OlO), which may be occasionally observed, and a much better one 
parallel to a(lOO), on which face it is also frequently twinned. 

The pyroxenes in this rock have been chemically investigated by 
Hawes. He did not know, however, that there were two species present, 
and the analysis probably represents a mixture of both of them. The 
material probably contained also a little feldspar. It is of value, how- 
ever, for it shows the nature of the average augitic component of the 
rock aod enables us to determine the average composition of the feldspar 
from the mass analysis. 

AnaljfHs of pyroxene of West Rocky New Haven j Connecticut; hy G, W. Hawes. 



SiO, (sHica) 

AljO, (alumina) 

FeO (ferrous oxide) 

MfiO (manganona oxide) 

CaO(lime) 

M{;0 (magoeaia) 

Ignition 

1, Halkaliaa and loas by difference) 
Total 



Per cent. 

50.71 
3.55 

15.30 
.81 

13.35 

13.63 
1.17 

1.48 



IGO.OO 



The analysis does not, of course, yield ratios which can be construed 
iDto satisfactory formulas. It must stand, however, until means for 
tbe separation of the two pyroxenes have been devised, which has not 
yet been successfully done. 

The plagioclase feldspar occurs iu the shaT)e of intergrown laths, 
sometimes long and slender, sometimes short and broad, which may 
attain a length of 1"". It is clear, colorless, and very fresh. Twin- 
ning according to the albite law invariable; according to the Carlsbad 
W very common; pericline twinning also occurs, but is much less 
common. 

The chemical and microscopic work of Hawes and Dana had early 
shown that the general composition of the feldspar was that of labra- 
^orite. Later, Hawes, by means of separations made by heavy liquids, 
^owed that the feldspars in a similar diabase from Jersey City con- 



270 



THE EDUCATIONAL 8ERIE8 OF ROCK SPECIMENS. 



Bisted of labradorite and andesite. Since in a dike catting Wi 
there occurred phenocrysts which analysis proved to be ai 
Hawes seems to have concluded that there was also more t 
feldspar present in the West Eock diabase, and that tberefoi 
made up of a mixture of anorthite and albite feldspars, not of i 
of these constituting intermediate species, as in the Jersey City 
This singular idea was justly and promptly combated by the L 
J. D. Dana. 

As a matter of fact, the feldspars do belong to intermediate 
and range from a rather basic labradorite to andesite, or s 
Ab2An3 to Ab3An2. This would give them as an average the • 
tion of an acid labradorite, which is exactly what the chemi< 
calls for. 

The section is very well adapted for the study of the plaj 
according to the excellent methods elaborated by Michel L^ 
Fouque,* based on optical properties. 

Thus, if sections oriented in the zone of a (100) on c (001), \v 
easily told by their equal illumination of the albite twins ^ 
twinning plane coincides with the cross hairs or is 45^ froi 
studied, the angles of extinction are found to be large, usual 
20<=^, and therefore indicative of a basic feldspar. Such secti 
be easily recognized by the disappearance of the albite twinnii 
450 position, while in parallel position to the cross hairs the 1 
lamellae, though equally illumined, are separated by fine, bl 
lines of the sutures. The 45^ iK>sition offers the best methoi 
crimination between the albite and Carlsbad twinning, thi 
disappearing and the latter becoming very evident, especial! 
basic feldspar. 

A number of such sections were selected and measured, of w 
following will serve as examples: 

Angles of extinction of lahradorites. 



Albite twin 1 . . . 
Albite twin 1'... 
Carlsbad twin 2' 



No.l. No. 2. 



35 
36 i 
22 



15 
15 
40 



No. 3. 


No. 4. 


No. 6. 


o 


o 


o 


20 


25 


30 


18 


27 


30 


6 


3 





Eeferring now to the tables given by ^lichel L^vy, it will 
that No. 1 is that of a labradorite of about the composition 
and that the section is inclined about 10° from a (TOO) on c (001 
is that of a labradorite a little more basic, the section cut inc 
from a (100) on c (001). No. 3 is a labradorite, AbsAua, the se( 
about 50O from c (001) toward a (100). No. 3 has the compo 

1 Determination des feldspaths, Paris, 18M. 

* L'^tnde des feldspaths des roches volcaniqnes, Paris, 1894. 



^ 



muni DESCRIPllONS: NO. 106, DIABASE. 271 

about Ab|A.ii3, tbe section being cut about ^^ from a (100), sloping 
toward c (001). No. 4 is AbiAus, tbe section inclined 50^ from a (lOO) 
one (001). No. 5, wbicb does not sbow tbe Oarlsbad twinning, indi- 
cates a probable labradorite. 

It will be noticed tbat the feldspars are frequently zoually bnilt 
OfUn in sacb sections as those mentioned above this is shown by a 
decreasing angle of extinction from center to periphery. If sections 
are chosen for study parallel to b (010), this is still more marked, and 
extinction angles referred to the trace of the cleavage parallel to 
c(OOl) will be found to vary from 20^ to 10° (negative direction), or 
from basic labradorites to andesites. It must be said, however, that 
everything indicates that labradorite is by far the predominant type of 
feldspar in this rock. 

The small amount of potash in the rock, shown by the analysis, indi- 
cates a little orthoclase to be present. The chance of recognizing a 
scattered grain or two in the section is naturally very small, but as 
the mineral mostly occurs associated with quartz in micropegmatite 
intergrowths it is easily seen. These intergrowths will be found in 
Httle angular interspaces between the laths of labradorite. They can 
be atndied only by the use of moderately high powers, and though well 
distribated, are not very common. Sometimes a little quartz alone 
will be found in the interspaces. This micropegmatitic intergrowth 
of qaartz and feldspar, or of quartz alone, is the very last product of 
erystallization in the rock. 

Biotite will only very rarely be found in the section, and may be 
wanting. It may sometimes occur as an irregular small leaflet of the 
Qsoal brown color and strong pleochroism. Sometimes it is bleached 
to a greenish color, but more often it is changed into chlorite. 

The distinct idiomorphic lath-shaped form of the plagioclase is char- 
iMsteristic for diabases, and it conditions angular interspaces which are 
sometimes filled with augite, sometimes with glass, and sometimes with 
g^nish fibrous material held to be chlorite. The occurrences of these 
masses has been called mesostasis. The exact nature of the greenish 
substance is not known. It has received tbe name of viridite and 
(^^loropitej which are, of course, only a cover for ignorance and under 
^bich various substances are thrown together. 

In the West Bock diabase it will be found, on studying the section 
^th a high power, that the angular interspaces between the feldspars 
^ frequently filled with material of this kind. It occurs in fibers, 
sometimes packed in bundles which extinguish simultaueously, some- 
times in divergent or radial masses, often in intertwined irregular little 
clumps. It will be noted to have a pale-greenish to yellowish color. It 
extinguishes, so nearly as can be told, parallel to the fibers, and they 
ftre extended in an optically i)ositive direction. The maximum double 
refraction of the quartz in one section reached a tone of very pale 
yellow, which shows that the section has a thickness of .03"™ (the 



272 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bdu^iMl 



average thickness of the sections). In this, the fibers under discassioo 
attain a doable refraction color of a brilliant yellow inclining to orange. 
This shows the maximum double refraction of the mineral to be aboat 
0.012, and therefore that it is not the ordinary variety of chlorite com- 
monly found in eruptive rocks and frequently an alteration prodactof 
mica, as the latter has an extremely low double refraction. With the 
means at present available it can not be said what the precise natare 
of this substance is; it resembles members of the chlorite gronp of 
minerals in many respects, and, as suggested by Rosenbusch,' is best 
designated as the '' chlorite-like substance." Its derivation from augite 
as an alteration product seems clear from study of the section. It is 
found around it, and in many interspaces the ratio of the amount of 
the mineral is inversely prox>ortional to that of the augite. 

The structure of the rock is conditioned by the amount and order of 
formation of its chief components and the conditions under which they 
crystallized. Pyroxene and labradorite are the chief constituents— all 
others in amount compared with them are insignificant — and they give 
the rock its character. Of them, the white pyroxene crystallized first, 
followed by labradorite, and finally the brown augite. The latter began 
crystallizing, however, before the labradorite had finished. It is this 
crystallization of the labradorite in idiomorphic lath-shaped forms, filled 
in and surrounded by the brown pyroxene, that gives to diabase its 
characteristic structure, termed "ophitic" by the French and well- 
named "intersertal structure" by Zirkel. The West Bock diabase 
shows this very well, but not in so marked a degree as may at times be 
found. 

The chemical composition of the rock has been carefully studied and 
discussed by Hawes, to whom we owe the following analysis of the 
West Eock diabase : 

AnalifHs of West Rook diabase, JVisir Haven , Connecticut. 




FejOs 
FeO. 
HnO. 
CaO. 
MgO. 

K,0. 



Ignition 
P,Oi 



2V/» 



51.78 

1.41 

12.79 

3.59 

8.25 

.44 

10.70 

7.«3 

2.14 

.39 

.63 

.14 



ToUl 



99.89 



I HaMige Gesteine, 1887, p. 183. 



I>BSCRIPTIONS: NO. 106, DIABASE. 



273 



is analysis expresses very clearly what the study of the section 
already iudicated — ^the large amount of ferromagnesian minerals, 
n by the bigli iron, lime, and magnesia, and that the feldspar 
; at least be a plagioclase of medium basicity, shown by the great 
onderance of lime over soda. The small amount of potash shows 
orthoclase can be present only in very limited amount, while the 
lie oxide sbows that part of the iron ore must be present as 
mite. 

iawes ^ on the basis of his analyses has calculated the proportion 
weight in which the various minerals enter into the composition of 
rock. With some rearrangement, this is shown in the following 
le: 

Minerals composing the West Bock diabase. 



nmenite 

Magnetite 

Augite 

Plagioclase (labradorite and andeaite) . 

Ort hoclase 

Apatite 

Total 



Per cent. 



1.8 
54.5 
37.7 

2.3 
.3 



99.3 



The above expresses the average composition very well; a minute 
mount of quartz is neglected, as are also the products of alteration. 
The rock from West Eock is a normal diabase, consisting chiefly of 
ibradorite and augite; the minerals in one generation arranged in 
Qtersertal structure. It is of rather fine grain and quite fresh. It is 
listingaished by containing an augite of a light color in addition to the 
^idinary aogite of such rocks. 

Literature of the diabase of the New Haven region, 
I>AHA, J. D. : 

Trans. Conn. Acad. Sci., vol. 2, p. 45, 1870. Amer. Jour. Sci., 3d eer., vol. 6, 
p. 104, 1873; vol. 22, p. 230, 1881 ; vol. 42, pp. 79 and 439, 1891 ; vol. 44, p. 165, 
1S92. Manaal of Geology, 4th edition. 
^>A»*, E. S. : 

I^. Am. Assoc. Adv. Sci., 23d meeting, Aug., 1874 ; Sec. B., p. 45. Aba. of same. 
Am. Jour. Sci., vol. 8, p. 390, 1874. 
^*vig, W. M. : 

Seventh Ann. Rept. U. 8. Geol. Survey, p. 455, 1885-^. Am. Jour. Sci., vol. 32, 
P 342, 1886 ; vol. 37, p. 423, 1889. Bull. Mus. Comp. Zool. Harvard Coll., vol. 
^ XVl,No. 6, Dec.,1889. 

Second Geol. Surv. Pa., Vol. C, p. 118, 1874. 
^^Ks, G. W. : 

Am. Jour. Sci., 3d ser., vol. 9, p. 185, 1875. Proc. IT. S. Nat. Mus., 1881, p. 129. 
Am. Jour. Sci., vol. 38, p. 361, 1889. 



Bull, 150 18 



' Proc. U. S. Nat. Mu«., 1881, p. 132. 



274 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. I 

No. 107. Olivinb-diabasb. 

(From Pigkon Point, Cook Cohnty, Minnesota. Dkscrided by W. S. B 

No. 107 is a representative of the rock that has always been 
an olivine-gabbro in the writings of students of Lake Superior g< 
The specimen was obtained from the north shore of Pigeon 
the northeastern extremity of Minnesota. Here the rock occn 
large dike-like mass, cutting Hnronian slates and quartzites. 1 
exposures are on the north shore of the iK)int at the water'i 
where the rock forms high cliffs whose face is always kept perpen 
through the action of frost, which penetrates joint cracks, and 
force of its expansion loosens large blocks that fall to the base of 
and there form a talus of fresh material. It is from one of thef 
of these blocks that the specimen in the collection was taken. 

The rocl&is a medium-grained, gray, crystalline aggregate, ii 
two components may easily be extinguished. One of these is a i 
nearly colorless mineral with glistening cleavage faces, often oo 
in long, narrow crystals, upon whose surfaces may usually be d 
longitudinal striations. Its color, cleavage, and structure inc 
plagioclase. The other component macroscopically distinguisbs 
jet-black substance also possessing glistening cleavages. Unde 
able circumstances this substance, which is a pyroxene, may be 
surround the feldspar crystals in such a way that the latter 
embedded in the former. Close inspection of the specimen in 
discover small areas of a finely granular texture and of a yc 
tinge. These areas consist chiefly of olivine. From a macr 
examination, then, we learn that the rock is a nonporphyritic, cry 
aggregate of augite and plagioclase; in other words, that it is i 
diabase or a gabbro. The long, narrow development of the pla| 
suggests a diabase.^ 

The density of the rock varies between 2.927 and 2.970, accor 
the specimen investigated contains a larger or smaller propoi 
feldspar. Its chemical composition, as found by Mr. W, F. HilL 
is as follows : 

lAng. Strong, l^euea Jahrb. f. Min., etc., 1877, pp. 113-138; R. D. Irving, Geol. of WIsch 
III. pp. 168-183; Alex. Jalien : Geology of Wisconsin, Vol. Ill (1880), pp. 233-238; R. D. I 
copper- bearing rocks of Lake Sajierior, Mon. U.S. G«ol. Sarvey, Vol. V, pp. 37-50. 

'Although the specimen in the collection is not porphyritic, in certain areas on Pigeon 
same rock is so developed that porphyritic crystals even 6 or 7 inches in length are not % 
The crystals are exactly similar to the crystals of plagioclase in the grouudmasa of the ro 
in point of size. 



^] 



DESCRIPTIONS: NO. 107, 0LIV1NE-DIABA8E. 



276 



AnalyaU of olivine-diabase from Pigeon Point, Minnesota. 




SiO, .. 
Tir),.. 
ZrO,.. 
A1,0,. 
Fe^ 
FeO.. 
MnO.. 

c«o.. 

SrO.. 
BaO. 
MgO. 
K,0.. 
N«,0 
Ll,0 . 
HsO . 

p,o,. 

CI... 



Total 



49.88 
1.19 

none 

18.55 

2.06 

8.37 

.09 

9.72 

trace 

.02 

5.77 

.68 

2.59 

none 
1.04 
0.1C 

trace 



100. 12 



s thin section, when examined under the microscope, is seen to be 
posed essentially of the three minerals above mentioned, viz, 
ioclase, olivine, and augite. The plagioclase comprises abont GO 
^Dtof the sections. In ordinary light it appears as a colorless 
admass in which everything else lies embedded. Under crossed 
8 tbis apparently homogeneous matrix breaks up into numerous 
, narrow crystals, ranging in length from 1 to 20'"™. Each of these 
mposed of a number of smaller individuals which, by their union, 
t give rise to broad lath-shaped forms, like those so characteristic 
»me of the European gabbros, notably those from Volpersdorf, in 
ia, and from the Harz.^ Each of the smaller crystals is polysyn- 
cally twinned according to the albite law, in which the brachypina- 
(x P do) is the twinning plane, and often two of them unite to form 
rlsbad twin with the macropinacoid (x P ob) as the twinning plane 
tbe brachypinacoid (x P oo) as the composition face. The material 
le crystals is very fresh, and is often almost as glassy as the feld- 
of modern volcanic rocks. It contains as inclusions only a few 
ides of dust and minute flakes of brightly polarizing fibrous 
in, except in those rare cases where slight alteration has developed 
rger quantity of the latter mineral, when tbe feldspar substance 
mes cloudy, and under crossed nicols is observed to be full of 
i needles, showing brilliant polarizing colors, 
le polysynthetic twinning lamellse noticed in nearly every piece of 
feldspar would lead to the suspicion that this is a plagioclase, but 
r occurrence is not positive proof of this, as a repeated growth of 
Isbad twins would give rise to tbe same phenomenon. The angles 
leby it« two systems of cleavage lines, bowever, are 85° and 95^ on 



' KoronbuBch, ^ikroBkopUcbe PhyBiographie, II, 1887, p. 155. 



276 



THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [bcllIM. 



tlie macropinacoid, and the cleavages are never at right angles to each 
other in any sections. Moreover, the maximum symmetrical extinction 
of lamellae on each side of a twinning line is 24^. These resolts iodi- 
cate a triclinic feldspar belonging in the lower i>ortion of the labra- 
dorite group.^ If a fragment of the rock is powdered and sifted, and its 
constituents are separated by means of one of the heavy solations used 
for this purpose, it will be found that most of the plagioclase will M 
when the density of the solution is between 2.700 and 2.716. If a 
portion of the material that falls at 2.700 be analyzed, it will give 
the figures in column I. In column II is given for comparison the 
composition of a labradorite^ of the formula Ab3An4, and in 111 that 
of a plagioclase^ with a specific gravity 2.700, from a European locality. 

Analyses of plagioclases of olivine-diahase. 



SiOt 

A1,0, 

Fe^Oj 

CaO 

N»,0 

Total 

Specific gravity. 



I. 

53.75 
30.39 

1.26 
10.84 

3.76 


II. 


III. 


53.60 
29.80 


54.55 

28.68 

1.03 


11.70 
4.90 


11.28 
4.62 


100.00 
2.700 


100.00 
2.703 


100.16 
2.700 



It will thus be seen that the density and the chemical composition of 
the mineral, as well as its optical and physical projierties, agree very 
well with those of a basic labradorite consisting of a mixture of three 
albite molecules and four of anorthite. 

. The olivine constitutes about a tenth of the rock. It is undoubtedly 
older than the augite, since it is surrounded by this mineral (see PI. 
XXXVIII, A), and is probably older than most of the labradorite, 
although in a few instances it may be seen to include portions of crys- 
tals of the feldspar. Its period of crystallization must have overlapped 
that of the plagioclase, i. e., while the chief portion of the olivine 
separated from the magma before the feldspar had begun to form, a 
small portion of it solidified after the labradorite began to crystaHiz** 
The mineral is in rounded grains of a light yellowish-green color. It 
has a high index of refraction and consequently a rough surface, and is 
traversed by irregular cracks filled with green decomposition products. 
Excepting the feldspar crystals included in the olivine above referretl 
to, no inclusions other than dust like particles, like those in the feld- 
spar, and certain green iicicular secondary substances are met with 
in it. In but very few sections does the olivine remain unaltered. 
In even the freshest varieties a little chloritization has taken place, 
and this shows itself as a green rim around the edge of the grains 
and in the cracks. The small, brown, strongly pleochroic fiakes, witl* 
a well-marked cleavage, sometimes intermingled with the chlorite* 



I lildinga's RoAenbnscli's Microscopical Physiography, p. 300. 
^SehufftPT, T8chermak'8 Min. u. Petrog. Mitth., Ill, 1880, p. 153. 
« Tacliermak, SitBnngsber. d. K. AkaA.^\%R. ^^l^ l.W^U Baml LX, Abt. 1, 1870, p. 145. 



BEBCRIPTION8: NO. 107, 0LIVINE-DUBA8E. 



277 



te plates, produced, like the chlorite, by decomposition of 
ue, and the buDches of dark-green fibers that penetrate the 
neral are probably some form of hornblende, 
oungest of the essential compcments, augite, is entirely allotri- 
c. It fills the interstices between the other constituents, and 
s its contours molded by these. (See Fig. Aj PI. XXXYIII.) 
3veral feldspar laths are included in a single augite plate, which, 
roken, reflects the light uniformly from its surface, leaving dull 
1 those places occupied by the feldspar. This kind of inter- 
gives rise to what is known macroscopically as luster mottling.' 
3rosco[»ic structure produced by it has been denominated poeci 
The greater part of the augite is perfectly fresh, and, like the 
r and the olivine, it contains no true inclusions other than 
stlike particles scattered throughout its mass. Its color is 
h'piiik, with a faint pleochroisni ; the a and c rays being a 
irplish-pink, in highly-colored pieces, and the h ray yellowish- 
The two series of prismatic cleavage lines are very distinct in 
cut parallel to the basal plane, where they make the usual 
ne angles of 87° and 93o. In other sections they form a series 
dlel lines that are sometimes gently curved as the result of 
•e. {See a, PL V, Bull. U. S. Geol. Survey, Ko. 109.) The 
ic parting parallel to the ortho])inacoid 1r only rarely seen, 
tinction on plates cut parallel to the clinopiuacoid is 44^ against 
g;le cleavage. 

lugite of nearly all Lake Superior rocks is uniformly tinged with 
wliereas that of foreign rocks is generally greenish, so that 
iplanation must be sought for the difference. Knop,^ who made 
iustive study of the augite of the Kaiserstuhl in Baden, found 
1 the varieties of this mineral rich in titanium and poor in iron 
purple color in thin section. Although investigations in other 
do not fully substantiate Knop's conclusion, it is interesting to 
at the purple augite in the Pigeon Point rock contains a large 
:age of titanium and a comparatively small ])roportion of iron, 
ial analysis of the augite jmwder separated from the si)ecimen 
ide in the laboratory of the United States Geological Survey by 
tiggs. His figures, indicating a very pure diallage, are : 

Analifsis of diallage of oli vine-diabase from Pigeon Pointy Minnesota. 



SiO, . 
.U,0, 
Fe/), 
FeO.. 
CaO.. 
MgO. 
TiO,. 



Per OL'Dt. 

48.34 

2.90 

4.68 

14.15 

15.10 

11.34 

1.98 



mpelly, Proc. Amer. Acad. Adv. Sci., XIII, p. 260; and R. D.Irving, Copper-bearing rocks, 



Wmiama: Am. Jour. Sci., 3d series. Vol. XXXI, 1886, p. 30. 
• ZeitaclL f. Rryat., X, p. 58. 



278 THE EDUCATIONAL SERIES OP ROCK SPECIMENS (Buu^isa 

In addition to the essential components, the rock contains only apa- 
tite and titaniferous magnetite as primary constituents. The apatite 
is only sparingly present in long, narrow, colorless crystals, with a 
parting parallel to the base. The magnetite appears both in idiomor* 
phic and in allotriomorphic forms, the latter predominating. A large 
number of the grains, especially in slightly altered specimens, are 
surrounded by rims of reddish brown mica, and not a few show the 
beginning of an alteration into cloudy- white or gray lencoxene. A 
little quartz and a few flakes of brown and red biotite, strongly pleo- 
chroic masses of green chlorite, and some undeterminate substanc&s 
filling cracks in the essential components and occupying corners 
between them, may be found in some sections. The quartz is in micro- 
pegmatitic intergrowths with feldspar; the biotite is an alteration 
product of olivine, augite, and magnetite, and the chlorite a product of 
the decomposition of the first two of these. 

As has already been stated, the rock has been pronounced a gab- 
bro by all geologists who have studied it. Its pyroxene has the 
composition of diallage, and it sometimes possesses the ortbopina 
coidal parting characteristic of this mineral.' Its structure, hoir 
ever, is more nearly that of the diabases than of the gabbros (see Fl 
XXXVIII, A). Its plagioclase is in lath-shaped crystals, embeddec 
in angite, so that its texture is not granitic, but ophitic. The textare 
then, if alone considered, would place the rock with the diabases, whih 
its mineralogiccomi)08ition would place it among the gabbros. Recent 
investigations have shown that many holocrystalline basic rocks, which 
no one would for an instant regard as gabbros, contain typical diallagei 
Therefore the possession of this constituent is not characteristic of 
gabbros; so there remains only the structure as a means of distingnislj- 
ing between these rocks and the diabases. Professor Judd' has 6Q|;- 
gested that we limit the term gabbro to those rocks composed of 
plagioclase and pyroxene, that possess the granitic structure, i e., 
that have their constituents developed in approximately equal sized 
grains, without crystal contours, and that we designate as diabases all 
rocks of this composition in which the structure is ophitic. According 
to this view, the I'igeon Point rock is a coarse-grained olivine-diabase.' 

No. 108. Gabbro. 

(From Mount Hope, Baltimore County, Maryland. Abstract ijy .1. P. Iddiscs 

FROM THE DESCRIPTION BY G. H. WiLLlAMS.) 

The occurrence and petrographical characters of this gabbro have 
been fully described by the late Prof. George H. Williams, and it will 

^For a r6sam6 of the opinions of different investigaton on the value of the orthopinaooidal F*(* 
ing as a characteristic of diallage, as distinguishing it from augite, see WadsworUi, Bull. No. 2. G«<1- 
and Nat. Hist. Survey, Minnesota, pp. 55-57. 

'Quart. Jour. GeoL Soc. London, 1885, pp. 354-418. 

*For a fuller description of the rock, both in its fresh and altered condition, see W. S. BstKt, T^^ 
eruptive snd sedimentary rocks of Pigeon Point, Minnesota, and their contact phenomena: Bull ^- ^ 
GgoI Survey No, 109, 1893. 




^ l^ht^olortd* long, fivTQw crymli ^ A an plag^ 






a^l DESCRIPTIONS: NO. 108, GABBBO. 279 

ly be necessary to quote what h§ has published on the subject in 
illetin 28 of tbe United States Geological Sarvey, which is entitled, 
He Gabbros and Associated Hornblende Bocks Occurring in the 
dghborhood of Baltimore, Maryland. To this paper the student is 
iferred for fuller details than can be given in this place. 
The gabbro is well exposed in a railroad cut at Mount Hope Station 
u the Western Maryland Bailroad, where it grades into hornblende- 
:ahbro-gnei8S (diorite). 

As ft rule tbe two rocks alternate with each other in every direction — horizontally 
od vertieally — in the most perplexing manner, and it is frequently impossible to 
ell on the spot whether a given specimen la gabhro or diorite (hornbende-gabbro- 
;Qein). . . . Just west of the bridge which crosses this cutting the gabbro 
bo¥B ft decided tendency to spheroidal weathering so often characteristic of basic 
Dusive rocks. [The hypersthene gabbro and the hornblende-gabbro-gneiss at this 
)lioeare clearly one geological mass.] 

Excellent exposures of the bypersthene-gabbro are very abundant within the 
Baltimore area, especially in the northern portion of it. ... Perhaps the best 
place to obtain fresh and typical specimens is at Mount Hope Station, although they 
ittj be secured from the huge bowlders which strew the surface at almost any spot 
rithin the area. 

The rock is always very massive in appearance, rarely exhibiting the banded and 
Dowhere tbe schistose structure, which is frequently seen in the associated horn- 
blendie rocks. Its irregular polygonal blocks are often covered with a thin coating 
of a deep red color due to decomposition, beneath which, howeyer, tbe rock is sur- 
prwingly fresh. . . . 

The eolor of the hypersthene-gabbro is on tbe whole uniform, although it may 
nry from a purplish black to gray. 

The most striking feature in the texture of tbe unaltered gabbro is tbe repeated 
UMlftbrapt change in the coarseness of the grain which is seen at some localities. 
Hiis phenomenon, as is well known, is one frequently observed in very ancient mass- 
^« rocks which cover considerable areas. . . . Irregular patches of the coarsest 
^iiuls lie embedded in those of the finest grain, without regard to any order. In 
>thfr cases a more or less pronounced banded structure is produced by an altema- 
ton of layers of different grains or by such as have one constituent developed more 
^undaotly than the other. Such bands are not, however, parallel, but vary consid- 
'^bly in dire<^tion, and show a tendency to merge into one another, as though they 
Ad been produced by motion in a liquid or plastic mass. . . . 
The mineral constituents of the gabbro which are discernible with the unaided 
^« are plagioclase, diallage, and hypersthene. A black hornblende, which is brown 
' trftDsmitted light, is also sometimes seen in good-sized crystals, and has every 
^pearanee of being a primary component. Magnetite and apatite are shown by 
^ microscope to be universally present, although in varying quantities. Olivine 
aa observed in only one specimen, collected near Orange Grove. With the excep- 
^n of certain indeterminable inclusions, no other minerals were discovered in the 
holly unaltered rock. 

The grain of the hypersthene gabbro is, as a rule, uniform and fine, the component 
inerals averaging from 1 to 2™°* in diameter. Exceptionally, however, the grain 
^omes coarser; in a few specimens tbe individuals of pyroxene and feldspar meas- 
^ from 25 to 35™™ in length. The coarsest varieties are rarely altogether fresh. 
The feldspathic constituent of the hypersthene-gabbro is bytownite, correspond- 
g to a mixture of six molecules of anorthite with one of albite. Stauroscopic 
eaanrements on the feldspar extracted from a Mount Hope specimen of tbe gabbro 
k^e extinction angles (measured against the cleavage lines) of — 16"^ to — 19'-' on OP 
«1) and of —28^ to — 30° on ^^ Pdb (010). 



280 



THE EDUCATIONAL SERIES OF EOCK SPECIMENS, [w 



A chemical analysis of this feldspar, made by W. S. Bayley, gave the folli 
percentages : 

Analysis of feldspar in gabbrofrom Mount Hope, Maryland, 



SiO, 

AlA 

CaO 

N«,Oa 

Totel 

Specific gravity, 2.74. 



Percent. 



46.17 

85.23 

16.29 

2.31 



100. CN) 



a Difiference. 

In the above-mentioned, specimen of gabbro from Mount Hope, the feldspa 
stitutes somewhat over one-half of the entire mass. This may perhaps be reg 
as a fair average, althongh p^reat variations in the proportions of the corny 
minerals caused the analyses of specimens of the gabbro from different looali 
differ widely. 

The feldspar individuals are generally quite irregular in shape, giving rise 
granular structure which is characteristic of gabbro in contrast to diabase, fi 
however^a lath-shaped crystal indicates aslight tendency toward the ophitic stru 

When seen between crossed nicols, the feldspar appears, as a rule, finely sti 
Sometimes two systems of strisB are seen to intersect at an angle of nearly 9C 
other cases the striations are very coarse or altogether wanting, in which ca 
extinction is more or less undnlatory nnd irregular. [In some instances the 
tions are slightly curved or bent, as though by dynamic stress. J. P. I.] 

The minute dust-like inclusions, which are so common in the feldspars > 
older basic rocks, are admirably developed in the Baltimore gabbros. They t 
very well to the descriptions given of them in rocks from ether localities by 2 
Hagge, Hawes, and other investigators. When viewed with a low magn 
power the plagioclase appears to be covered with a fine black or brown dust, ^ 
under the highest maij^nifying power, is resolved into a mass of very minute o 
dots and lines. The arrangement of these inclnsions is such that the needles o 
the center of the crystal; this is surrounded by a zone where only the d 
globulites are to be seen. The exterior of the crystal is generally free from 
sions of any kind. Frequently the acicnlar bodies are altogether absent, an 
the minute dots are arranged in lines which indicate the position of the twi 
lamellae. An occasional inclusion of another kind, like a fluid cavity, an s 
crystal, or a grain of magnetite, is surrounded by a narrow zone which is 
free from the dust-like particles. Neither a microscopical nor a chemical ess 
tion served to give any clue to the mineralogical nature of these minute \y 

The diallage is the constituent of the gabbro next in order of importance 
feldspar, although in some specimens it is not so abundant as the hypersthene. 
chemical composition of this mineral from a Gwynns Falls specimen is as folio 

Analysis of diallage of gabbro from Gwynns FallSy Maryland. 



SiO, 

AUO, 

FeO 

MnO 

CaO 

MgO 

Total 

Specific gravity, 3.26 



Percent. 


51.41 


4.32 


0.81 


.04 


20.60 


15.14 


100.82 



descriptions: no. 108, GABBBO. 



281 



pecimens the diallage appears black, but when seen by traoBmitted light 
ions it haa a lif^ht green color and exhibits no appreciable pleochroism. 
tueni, like all the others, shows no indication of a crystal form ; it is 
ly in rqanded or irregularly shaped grains, and appears to have crystallized 
later than the feldspar. The prismatic cleavage and the parting parallel 
kopinacoid are both well developed ; freqaeutly a second parting parallel 
opinacoid is also present. In sections approximately parallel to either 
r QO P 00 (100) an optical axis — not a bisectrix — ^appears, when the examina- 
ie with convergent polarized light. Clinopinacoidal sections exhibit an 
angle as large as 40^. 

iccording to the ordinary law for angite, where the orthopinacoid is both 
plane and composition face, are quite common. Instances where the 
trace is inclined from 25^ to B5° to the cleavage lines, as described by 
oeenbusch,^ and others, have been also not infrequently observed, 
rstems of fine striations, visible only between crossed nicols, are very often 
arsing the individuals of diallage in several directions. They are by no 
rays straight in their course nor continuous across the crystal, but appear 
to some molecular disturbance produced by pressure, to which, as is well 
lis mineral is peculiarly sensitive.^ 

reedom from inclusions the diallage, as a rule, presents a contrast to the 
oe, although when such inclusions are exceptionally present they do not 
iterially to differ from those of the rhombic pyroxene, 
bene is tf constant component of the Baltimore gabbros, although its 
iries widely in different specimens. It is readily detected in the rock by 
' luster. . . . This effect is, however, altogether due to inclusions in 
ithene, and, since these may be wanting, the absence of the metallic retlec- 
itself no proof that this mineral is not present. Its chemical composition 
fs: 

Analyns of hypertihene of Maryland gahhroB. 




he microscope this mineral exhibits no better crystalline form that the 
Tom which, however, it is easily distinguished by its marked trichroism. 
'hich vibrates parallel to the brachydiagonal axis (a == a) is brownish red, 
ibrating parallel to the macrodiagonal (& = !)) is light greenish yellow, 
X one which vibrates parallel to the vertical axis (c= C) is green. The 
a is a < C < b, C, being very nearly equal to a. . . . 
ftvage parallel to the unit prism oo P (110) is well developed, but even more 
Bd is a parting parallel to both pinacoids oo P o6 (100) oo P ob (010). . . . 
bhat the brachydiagonal axis is the acute bisectrix, and the optical angle 
ively small, is sufficient indication that it is hypersthene and not enstatite 
te. . . . 



itiiche B«8chreibang der Umgegend von Heidelberg, p. 69, 1881. Sammlung von Mikro> 

liflD. et«., Taf. XXVUI, fig. i, 1R81. 

^opische Physiographie. Vol. II : Die massigen Gesteine, p. 410. 

enreke: ^euee Jatarbucb fur Mineralogie, Geologie, und Palaontologie, II, p. 97. 1883. 



282 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [Brml». 

The peculiar ioelusiona bo geoerally cbara€teriBtic of hypersthcDe are, as a role, 
present in great perfection. . . . They are composed of line needles arrangHl 
parallel to all three crystallographic axes and of little plates of a reddub brown 
color lying parallel to the brachypinacoid. To these latter the peculiar metallic 
reflection seen in many sections is <lne. They vary considerably in thickness, as may 
be seen by the different depths of color which they ]>oesess. These plates all extin- 
guish simnltaneously with the hypersthene, which may be dne to their extreme 
thinnessortothefactthatthey have their axes parallel to those of their host. . . . 
Although generally abundant^ they are sometimes entirely absent from the hjper- 
sthene. 

A yellowish brown hornblende, undoubtedly of primary character, is sometimes 
met with in the hypersthene-gabbro. . . . This primary hornblende is, in the 
Baltimore gabbros, strongly pleochroic. The a ray, approximately parallel to the 
clinodiagonal axis, is light yellow ; the b ray, coinciding with the axis of symmetry, 
is brownish yellow, while the c ray, inclined about 12^ to the vertical axis, is yel- 
lowish brown. The absorption is, as usual in hornblende, C>l)>aorc = b)>8- 

This hornblende is sometimes so full of minute black inclnsions as to be nearly 
opaqne, even when in very thin sections. These interpositions are, however, not 
evenly distribute<l, but are massed in irregular patches. No trace of external crys- 
talline form is discernible. This mineral appears to have been the last to crystallizs 
from the magma, a single individual often covering a considerable space and inclos- 
ing smaller grains of both pyroxene and feldspar. It is not infrequently intergrown 
with pyroxene in such a manner that the orthopinacoids of both minerals are roin- 
oident. . . . 

The occurrence of this hornblende in the least altered specimens of the gabbro, 
its compact structure, and the other marked contrasts which it presents to the groen, 
fibrous amphibole, . . . are all a snflScient warrant for its original character. 

Green, more or less fibrous amphibole occurs in variable amoants in 
some sections of the gabbro from Mount Hope. It seldom exhibitH 
idiomorphic outlines, but is in confused aggregates of imperfectly 
developed individuals. The pleochroism is pronounced, and is that oom- 
mon in hornblende, i. e., the a ray, nearly parallel to the clinodiagonal 
axis, is light yellow ; the h ray, coincident with the orthodiagonal axis, 
is yellowish green, and the c ray is dark bluish green. The absorption 
is jc >ti>a. It appears to be of secondary origin. 

Of the other constituents occurring in the hypersthene-gabbro, the magnetite 
[pyrite] and the apatite present no peculiarities worthy of mention. The former is 
never very abundant; the latter rarely so, although in one specimen from Moont 
Hope it was found to compose 12 per cent of the entire mass of the rock. 

No. 109. Gabnetiferous Gabbro. 

(From Granitk Falls, Yellow Medicine County, Minnesota. Described bt 

W. S. Bayley.) 

No. 109, like Nos. 140 and 144, is from a rock foand interbedded witli 
gneisses and schists in the valley of the Minnesota Eiver, in MinneBota. 
The specimens in the collection were obtained from a pit some 20 feet in 
depth that had been blasted in the rock near a qnartz vein which h^i 
betn explored for gold. In the neighborhood of the vein the rock is 
quite massive, bat at a distance of a few feet from the contact it bas ^ 



Bl DESCRIPTIONS: NO. 109, GARNETIFEROUS GABBBO. 283 

itinctly fichistose character. The location of the pit is 1,500 paces 
irthof the SE. comer of sec. 4, T. 115 K, R. 39 W., MinnesotaJ 
In the hand 8x>^^^°^^^ ^^^ ^^'^^c^ ^^^ ^ massive aspect. Its iirevailing 
)lor is dark green, speckled with large patches of dark red and small 
reas of greenish yellow or white. Upon close inspection the yellow 
od white areas are discovered to be the glistening cleavage surfaces of 
striated plagioclase. The red areas are the surfaces of a dark-red, 
raasparent, hard mineral. It has no distinct cleavage, and is insoluble 
D acids. The properties are those of garnet. The nature of the dark- 
[reen matrix in which the garnet and plagioclase are embedded can not 
)e determined, though from its dark color it may be assumed to be very 
)a8ic. The specific gravity of the rock is 3.105. 

Fnder the microscope the thin section shows a granular aggregate of 
plagioclase, green augite, garnet, and magnetite, with small quantities 
}f green hornblende, a few large grains of quartz, and tiny crystals of 
ipatite. 

The plagioclase is in irregular allotriomorphic grains crossed by well- 
defined cleavage cracks and traversed by irregular fissures filled with 
decoini)osition products or stained by iron oxides. The material of the 
feldspar is clear and colorless except where rendered cloudy by inclu 
QODs. The most abundant of these are tiny flakes of chlorite and 
irregular masses of opac]ue, earthy substance, besides dust-like parti- 
cles of magnetite and little nests of a brightly |)olarizing calcite. All 
these are quite abundant in the neighborhood of the fissures, but are 
rare in other parts of the grains. Other frequent inclusions are long, 
narrow crystals of a strongly refractive, colorless mineral, with a par- 
>Del extinction and weak double refraction. In cross sections they 
possess the hexagonal outline and the isotropic character of apatite, 
n^eir average length is about 1""" and their thickness about 0.05'"™, 
though a few have cross sections measuring 0.2"'"' in diameter. Augite, 
hornblende, and large masses of magnetite are also included in the 
plagioclase. 

Under crossed nicols the plagioclase twinning becomes very apparent, 
M fairly wide bands, usually running entirely across the grains. Some 
!)f the bands curve slightly, others wedge out as they pass toward the 
interiors of the grains, and still others spring from the sides of cracks, 
itc. These phenomena, as explained in the description of No. 77, indi- 
cts that the rock has been subjected to pressure since it solidified. In 
*rtain restricted areas in the section, notably in the neighborhood of 
Arge garnets, there is often a second series of twinning lamellaa cutting 
the first series at some acute angle. This second series comprises 
■numerous lamellae, not as distinct as those of the first set, and not as 
sharply marked oif from one another. They cross the bars belonging 
to the first set and have a more or less undulous extinction. The 
specific gravity of the greater portion of the plagioclase is somewhere 



1 See map, pi. 28, Geology of Mixmesota, voL 1, of Final llejiort, p. 589. 



284 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [Bru-iSd 

aboat 2.68, or near audesine, thoagh probably a more basic member is 
also present, with a density of about 2.72. 

The next most abundant component of the rock is a pyroxene. This 
is in green, slightly pleochroic, allotriomorphic grains which are older 
than the plagioclase, since the latter conform to the former iu shape^ 
and occasionally include them. This augite when fresh is marked by 
two series of cleavage lines crossing each other at angles of about 90^ 
on basal sections, and on other sections by a single series of parallel 
lines. The pyroxene may easily be distinguished in these sections 
from hornblende by the large angles of extinction against the parallel 
cleavage lines, which often reach as high as 43^, whereas those of bom- 
blende rarely approach 24^, and by the lack of strong ])leochroisiD. 

In addition to the two cleavages mentioned as prominent on basal' 
sections, there is often another, presenting itself as a series of closely 
crowded parallel lines, bisecting the larger of the two nearly rectangu- 
lar intersections of the more prominent lines. Since the shorter of the 
two lateral axes in pyroxene is the orthoaxis, this cleavage mast be 
parallel either to the orthopinaeoid or to some orthodome. But since 
sections in the zone of the orthopinaeoid and the clinopinacoid show 
only a single series of parallel lines, it is evident that the cleavage 
under discussion must be ortbopinacoidal. This cleavage is character- 
istic of diallage. For some time it was regarded as original and the 
mineral exhibiting it was looked upon as a distinct species of pyroxene. 
Now, thanks to the investigations of Professor Judd,^ it is known that 
the cleavage is secondary and that its origin is often the direct conse- 
quence of pressure,'-^ to which the rock containing it was subjected. 

Though much of the diallage is fresh, a still larger pro|)ortion is 
more or less altered. The interior is often stained brown or yellowish 
brown, and at the same time is deprived of its power of polarizing 
brilliantly. A little magnetite in rounded grains separates around the 
edges ef the altered portion, and a more or less fibrous cleavage is 
developed iu it. In other places the yellow substance occurs iu little 
plates or needles arranged in parallel lines inclined to the cleavage^ 
thus giving rise to the appearance of a third set of fine cleavage \in^ 
In some cases, the lines are so straight and narrow that they are with 
great difficulty distinguishable from true cleavage lines, even under a 
high power. The origin of somewhat similar inclusions, so common ft 
feature iu diallage and hypersthene, has given rise to much discussion, 
Judd thinking them secondary infiltration products along planes of 
easy solution,^ and others regarding them as original inclusions taken 
up by the mineral during its growth.^ 

Around the outside edges of the diallage another alteration is 



I Quart. Jour. G«ol. Soc. London, 1885, pp. 378-379. 

'M. E. Wadsworth : Bull. Xo. 2, Geol. and Nut. Hist Sarvey Minnesota, ]>. 55 et seq. 
'Quart. Jour. Geol. Soc. London, 1885, p. 354. 

*G. H. WUliama: Am. Jour. Sci., 3d seriea. Vol. XXXI, Jan. 1886, p. 33;" and Vol. XXXII'. Feb. 1«7' 
p. 148. 



I descriptions: no. 109, gaknetiferous garbro. 285 

ved. Here the p roxene is surrounded by large plates or small 
iilesof a brigiit-green substance with strong plepcbroism, in yel- 
;li browu and dark bluish-green tints, and an extinction inclined to 
cleavage. This substance, which is a hornblende, surrounds the 
te as a narrow rim, which sometimes extends into the pyroxene 
n, and at other times is formed of granules that seem to have been 
A^ to the grain after its formation. In either case the substance 
> be regarded as secondary in origin. In a few cases, when the 
>xene grain was small, the entire substance has been changed, and 
» place is now an area of hornblende, that might be regarded as 
;inal, were it not for the fact that so much of the hornblende of the 
^ is undoubtedly secondary. 

Luother product sometimes formed by the alteration of the pyroxene 
biotite. This is in small reddish-brown Hakes, mingled with the 
mblende on the periphery of the diallage. 

rhe magnetite and pyrite appear as large irregular grains that are 
eaent more frequently near the pyroxene and garnet than elsewhere 
the rock. Both have resisted alteration, and both are equally 
)aqae. so that the only method of distinguishing between them is by 
leir lustre in reflected light. The magnetite is black and the pyrite 
rassy yellow. 

The garnet is the characteristic mineral of the rock. It is in large 
illalar masses, sometimes measuring half an inch in diameter. In 
resection it appears as a highly refractive, isotropic, deep pink sub- 
taiice, filled with inclusions and crossed by many irregular cracks, 
long the sides of which are stains of yellow iron oxides. So large 
lid 80 numerous are the inclusions that the garnet substance in its 
rraugement reminds one of the section of a coarse sponge saturated 
ith various colorless products. The largest and most striking of the 
■elusions are quartz grains. These are colorless and without cleavage 
'aces. They inclose little mica plates, apatite crystals, dust particles, 
Qd thousands of little liquid- filled pores, arranged in lines. Under 
^sed nicols most of the larger grains break up into an aggregate 
ith the lines of inclusions passing from one grain into another with- 
it interruption. Occasionally some of the clear inclusions in the 
arnet are discovered to be plagioclase, but these are comparatively 
^re. The other substances inclosed by it are small pieces of biotite, 
rystals and particles of magnetite, crystals of apatite, and thousands 
f tiny cavities filled with liquid. As in the quartz, these are arranged 
1 tines, and these lines are sometimes continuous in both substances. 
^is latter phenomenon would indicate that the inclusions are of sec- 
>ndary origin, and that they were formed after the quartz and garnet 
tad assumed their present positions. The age of the garnet is shown 
)yits associations to be younger than that of the other constituents. 

' For dMcriptions of added growths to pyroxene and hornblende, see C. R. Van Hiae : Am. Joar. Soi., 
34ierlM, Vol. XXXTIT, May 1887, p. 886, and G.P.Kerrill, lb, Vol. XXXV, June 1888, p. 488. 



286 



THE EDUCATIONAL 8EEIE8 OF ROCK SPECIMENS, [bclliml 



Another form in which the garnet exists is in small grannies sur- 
rounding magnetite. Here the mineral has the same properties as 
when in large pieces, except that it contains no large incloaioDS of 
quartz and feldspar. 

The quartz is in colorless masses, surrounded by garnet, as already 
mentioned, and also in larger pieces associated with the garnet, bnt 
not included in it. It usually occurs most abundantly near the garnet, 
in those sections containing a great deal of this mineral, and is almost 
if not totally absent from those sections in which there is none. 

Since the garnet is probably secondary, i. e., since it was probably 
formed after the main portion of the rock solidified, it is also prob- 
able that the quartz is of secondary origin, and that the original 
compouents of the rock were essentially diallage and plagioclase, with 
magnetite, pyrite, and apatite as accessory components. 

A rock of this composition is a gabbro, if its structure is not schis- 
tose. The principal mass of the rock under study is schistose, bnt it 
is very probable that this is an imposed structure, since, even in the 
apparently massive variety represented by the hand specimen, abund- 
ant indications of pressure are apparent. The massive phase, since it 
is characterized by garnet, must be denominated a garnetiferons 
gabbro, while the schistose variety is a squeezed gabbro or a zob 
tenite, midway in character between Kos. 135 and 137. 

The photograph (PI. XXXVIII, B) shows a large grain of plagio 
claae, surrounded by garnets, with the interstices between them filled 
with quartz. The black grains to the right are magnetite and pyrite^ 

The composition of the rock, as found by H. N. Stokes, is: 

Analysis of gametiferout gabbro from Granite Falls, Minnesota. 




SiO, 52,31 

A1,0, 18.35 

Fe,0, 5.90 

FeO 11.00 

CaO 7.33 

MgO I.OO 

KaO ' .40 

Na,0 ' 2.90 

Loss .35 

ToUI 99.09 



PERIDOTITE FAMILY. 

No. 110. Pyroxenite. 

(From Pikksvillk, Baltimore County, Maryland. Abstract by J. P. IddiVj^ 

FROM THK DESCRIPTION BY GeORGK H. WILLIAMS.) 

Together with gabbros and peridotites, the pyroxenites break 

>Th« noufeldfiipathiciDtnisive rocks of Maryland and the conrseof theiralteratimi: Am.G«rf0f^^ 
J Illy, 1890. Alflo, Guide to Baltimore, with an account of tbe f^eology of ita environs, and three DtfP"- 
Aw. IdsL Min. Eng., Baltimore meeting, FebTuary , 1892. 



1 DESCRIPTIONS: NO. 109, GAENETIFEEOUS GABBRO. 287 

Qgh tlie gneisB and marbles of the eastern part of the Piedmont 
. of Maryland. The pyroxenites and peridotites are younger than 
gabbroB, bat are connected with them by intermediate varieties, 
. may be regarded as having originated from a gabbro-magma by a 
linntion in the alumina and silica. 

le pyroxenites of Maryland are of two types. One, having darker 
or and being heavier, consists of bronzite or hypersthene and dial- 
[e; the other, having lighter color, cousists of bronzite or hypersthene 
d diopside. The first is the more common. It is generally evenly 
aoalar in texture, with allotriomorphic crystals 1 or 2""" in diameter, 
d sometimes 8'°'" in length. Its fracture is hackly and very uneven, 
s color is a mixture of dark brown and greenish black. 
In thin section the rock consists almost wholly of anhedrons of red- 
Bh and greenish pyroxenes, both orthorhombic and monoclinic. The 
iiieral composition is monotonous and simple, there being in addition 
) pyroxene only occasional specks of iron oxide or magnetite. 
The pyroxenes are diallage and enstatite or bronzite. The diallage 
pale green in thin sections, with the customary optical characters; 
igh refraction and double refraction and inclined extinction in all 
M!tions except those parallel to the orthopinacoid oo Pqo (100). The 
leavage is prismatic and orthopinacoidal. Some sections of diallage 
xhibit between crossed nicols fine lines which are parallel to the pris- 
latic axis. These are due either to multiple twinning, probably parallel 

the orthopinacoid, or to lamellar inclusions of orthorhombic pjnroxene. 
)Oinetimes these fine striations are crinkled or crumpled as though by 
mechanical strain. In some sections of diallage there are inclusions 
f orthorhombic pyroxene in parallel lines crossing the diallage at an 
■icliDation to the prismatic axis. These are surrounded by minute 
Dclogions having relatively high refraction. In some places these 
itclasions take the form of parallel lamellae of pyroxene, which appears 
^ be orthorhombic, in others the two kinds of pyroxene are inter- 
J^wn, with quite irregular outlines. The orthorhombic pyroxene in 
Dme cases incloses irregularly shaped pieces of diopside or diallage, 
lid also thin lamellse of this mineral in parallel position. The diallage 
> diopside or malacolite. 

The orthorhombic pyroxene is pale reddish, with slight pleochroism, 
finish to colorless parallel to jc and the prismatic axis, and reddish 
srallel to a and h at right angles to this axis. Sometimes there 
% rodlike inclusions giving the mineral a bronzy luster. In the 
^fience of a chemical analysis it is not possible to determine whether 
'e mineral is more properly enstatite or hyi>er8thene. Professor 
Williams has called it bronzite or hypersthene. 
Both kinds of pyroxenes are more or less dusted by minute inclusions. 

1 some rock sections the orthorhombic pyroxene appears to be in 
icess of the monoclinic variety, but the following chemical analysis, 
«de by..!. E. Whitfield, of the pyroxenite from Johnny Cake road, 
H)W8 that in this rock diallage is slightly in excess. 



288 



THE EDUCATIONAL SERIES OF HOCK SPECIMENS. Ibi 



AnalyaU of pyroxenUe from Johnny Cake road, Maryland. 



Percent. 



810, 50.80 

AJjQg 3.40 

CrA 

re,0, 

FeO 

MnO 

CaO 

MgO 

N»,0 

K,0 

H^O (red heat) 

a 



Total .... 
Sp. gr., 3. 318. 



1.39 

8.11 

.17 

12.31 

22.77 

trace 

trace 

.52 

.24 



100.03 



Magnetite occurs sparingly in rounded anhedrons. There ar 
small brown grains of chromite or picotite. A characteristic fc 
alteration of this rock is brought about by the change of the pyr 
into secondary hornblende, and subsequently the change of tbii 
talc, giving rise to extensive beds of steatite, in which the talc is i 
with more or less pale fibrous tremolite and chlorite. 

In the vicinity of Pikesville, Maryland, the pyroxenite often 1 
extremely coarse grain, and not infrequently contains porpby 
crystals of orthorhombic pyroxene an inch or two in length. 

This type of nonfeldspathic rock, free Irom olivine and consist 
enstatite or hypersthene and diallage, has been called websteri 
Professor Williams.^ 



No. 111. Peridotite (feldspathio PeridoTitb). 

(From Sudbrook Park, Baltimorb County, Maryland. Abstract b\ 
Iddings from the description by George H. Williams.) 

The peridotites of the Baltimore district have been fully des< 
by the late Prof. George H. Williams in Bulletin 28, United \ 
Geological Survey, from which the following has been taken:' 

The rock [peridotite] sometimes occnrs in comparatively narrow, well cba 
ized dikes; sometimes in small oval or lenticular patches. In the latter ci 
nearly always associated with BerpentinCf which has originated from its alte 
. . . All the rocks of this class occnriug near Baltimore have a strong resen 
to each other. All are dark brown or greenish brown in color, and have a p 
rough appearance on tlie weathered sarface by which they may be recogniz* 
considerable distance. This is produced by the decay of the olivine, which lea 
crystals of the more stable bronzite stand ing out like knobs upon the surface. 

A macroBcopical examination discloses a compact greenish-black groundn 
which are embedded large crystals of a glistening yellow bronzit«. In som 
no other constitnent can be recognized with the unaided eye, but generall: 



Mjp. cit., page 47. 



"Pp. 50-64. 



^een; atMorptian c>a>D- ■ • ■ in tniD sections t bis bronzlte appears 
jellow tir colorless plutes. It boa often a fibroux btnictiirn parallel to tbe 
tiirt. Characteriatio inclusions like those foiled in the hyperBthena ilo not 
it brightly polarizing spots with a highly inrlined es:ti[ictiuu angle are 
minent between crossed dicoIh when the bronzite itself is dark, althongh in 
light tliHj- are quite invisible. They seem to bo due to an intergrowlh of 
rtions of diallage Bahatnnce, As alteration of the brouzite commences fine 
serpentiue are seen to he developed along criLcka which tniverae it. Minute 
Mmbling bastitc, are also of coutmon occmrence, 

illoge of the olivine rocks yields in iHolnted splintiTS cleavage plates par- 
jp* (100), and »P (110). , . . [Tbe latter] whoir an eitinction angle 
0° to 13° when mcasnred against the vertical axin. lu thin rock sections 
:tion angle as high as 35° was observed in this niiueial. 
tllage shows no pleochroisui. Thin sections of It are nearly colorless, while 
.re ijoit^,' thick, bat thin enungh to transmit light, remain green in all posi- 
th no perceptible change in the intensity of the color when the transmitted 
ide to vibrat« in difTereut directions tbrongh the crystal, 
illage hae a mncb more perfect cleavage than the bronzite. It likewise pre- 
[larked contrast to this mineral in the fre<|Ucncy with which twinning 
tccnr in it. These seem to be of secondary origin, ]>rodncL'd probably by 

as was also observed in the case of the diallage of the hyperstliene-gabbro 
) Baltiuioro district]. Such srcondary lamellio are often present in great 

and are especially distinct when the diallage crystal has been broken or 

dspar is shown by all its physical properties to be bytowoite. Its specific 
. . . is '2.722. The average of a large number of atauroscopic measnre- 
cleavage fragments gave extinction angles on 01' (001 ) of 19", and on ^ F db 
(2^, values trhich agree well with the spi-cific gravity. This feldspar is 
'ith dinicnlty in concentrated hydruchlorio acid and fuses readily iu tbe 
a Bunst'D borner. This mineral shows a marked freedom from inclusions, 
in this rrxpect fh>m the feldspar of tbe bypcrHtbeiie-gabbro. Wherever it 
contact with tbe olivine the pecnliar reactiouary rims of amphibole are 



290 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [b 



brilliant interference colors in polarized light, even when the section is exce( 
thin, an effect well known to be characteristic of the zeolites. . . . 

The olivine, the most characteristic constituent of this class of rocks, exhi 
very striking pecnliarities. It is present in small, rounded grains, forming tb 
compact groundmass in which crystals of the other minerals are porphyr 
embedded. The structure of these rocks is therefore quite an exceptional one 
family of peridotites : First, on account of the porphyritic character, which is 
members of this class, and, second, because of the unusual position of the oli 
the groundmass, indicating that it is here the youngest instead of the oldest < 
uent, as is generally the case. The grains of olivine are always more or le 
pletely changed to serpentine, although in many specimens cores of the < 
mineral remain intact. The serpentine is of a bright yellow color when seen ii 
section under the microscope, and shows in an admirable manner the well 
process of its formation out of the olivine substance. The fine serpentine fil 
developed along cracks and fissures in the olivine, working their way fro: 
inward until linally none of the olivine remains. A considerable quantity < 
netite in fine shreds and lines is always disseminated through the serpentine, 
been separated from the olivine during the process of its alteratign. 

No other constituents were observed in the olivine rocks of the Baltimore 
except in rare cases small quantities of a compact, brown, strongly pleochroi 
blende, which is filled with minute dustlike inclusions. This mineral is m< 
mately associated with the diallage, the two being completely intergrown 
same crystal. . . . 

Feldspathic peridotite is the commonest type of peridotites 
Baltimore region. The feldspar in it does not compose over 5 p( 
of the rock. The following chemical analysis, prepared by Dr. 
McCay, was made from a specimen from a dike on the Western 
land Kailroad, north of Howardsville. 

Analysis of peridotite from Maryland, 




No. 112. KiMBERLlTE. 
(From Elliott County, Kentucky. Described by J. S. Dflleb. 

This rock occurs in the midst of the coal fields of eastern Keni 
where the sandstones and shales of the coal measures are nearl] 
zontal. It is one of a series of three peridotites found under soin 
similar conditions west of the axis of the Allegheny Mountains \ 



DUill.] 



DESCRIPTIONS: NO. 112, KIMBERLITE. 



291 



the comparatively little disturbed sedimen tairy rocks. The other locali- 
ties referred to are the serpentine of Syracuse, New York, described 
by Dr. Williams,^ and the mica peridotite of western Kentucky.^ 

The kimberlite of Elliott County, Kentucky, outcrops at only three 
points in the sauie neighborhood, but the form suggested by the resid- 
ual material of the soil is that of dikes, and the large number of 
inclasions it contains in places shows that it has been erupted through 
Carboniferous strata. 

It is a compact, dark-greenish rock, with a specific gravity of 2.781. 
In it are embedded many grains of yellowish olivine, uniformly dis- 
tributed throughout the mass. Karely it is fine granular and dense, 
like many darker colored basalts, but generally the grains of which it 
is composed are medium sized. Occasionally the olivine grains dis- 
appear and the deep-green serpentine pervades the whole mass. 
Brides the olivine and serpentine, which together form nearly 75 per 
cent of the rock, there are other minerals which appear in the hand 
specimen. Most important among these are pyrope and ilmenite, the 
latter appearing in the form of irregular grains which sometimes attain 
a diameter of nearly 2*="". A few scales of biotite may be observed. 
>'ear the exposed surface the rock becomes yellowish, due to the oxi- 
dation of the iron, and softens so that it readily disintegrates. The 
ganiet and much of the ilmenite withstand the atmospheric influences 
aud are found quite fresh and abundant in the residual sand resulting 
from the disintegration of the peridotite. 
The following table is based directly upon estimates made under the 
microscope of the areal distribution of the various minerals in the 
freshest portions of the sections from the locality where the peridotite 
is least altered : 



Primarv minerals. 



Per cent J Si'condary ininorals. 



Percent. 



Olivine . . 
Enittatite 
Biotite... 
Pyrope .. 
Umenite . 
Apatite . . 



40 

1 

1 

8 

2.2 
trace 



Serpentine 
Dolomite.. 
Magnetite 
Perofskite 



30.7 


14 


2 


1.1 



It is not claimed, of course, that this table represents with a high 
*^gree of accuracy the mineralogical com[)osition of the rock, yet it 
<^fcsely approximates the real proi)ortions in tbe sections studied. The 
^l>Ie clearly indicates that originally at least 80 per cent of the rock 
^as olivine and that ultimately it will be nearly all serpentine — or, 
Waps, in some places dolomite — with a small proportion of magnetite, 
unienite, garnet, and perofskite. 



» Am. Joar Sci., 3d series, Vol. XXXIV, August, 1887, p. 137. 
* Am. Jour. Sci.. 3d series, Vol. XLIV, October, 1892, p. 286. 



292 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [k 

The general structure of the rock is illustrated in PL XXXIX 
A and B, which show the remaining more or less regular pheno( 
of olivine inclosed in a network of serpentine with other produ 
alteration. The high proportion of olivine in the rock places it a 
those peridotites which are generally designated dunites, but the 
ence of some enstatite shows its relationship to saxonite. The n)( 
less distinct porphyritic structure was regarded by Carville Lew 
I so important a feature that he designated a similar rock of the 

I' berly mines in the South African diamond field as "kimberlite." 

original structure of the rock was not wholly porphyritic. In 
places, at least, it has a holocrystalline even-granular structnn 
granite, the irregular grains of olivine interlocking like those of q 
and feldspar in granite. 

The olivine grains are generally irregular in form, varying fro 
to 1.5°*°* in diameter, and are penetrated by many fissures. Oc« 
ally, however, as shown in the upper figure of PL XXXIX, th( 
bounded by sharply defined crystallographic planes, a feature wh 
unusual for the olivine in peridotites. It occurs in the form, wh 
common in basaltic lavas, of a short prism terminated by brachyc 
without the base. 

The alteration of the olivine to serpentine takes place rapidly i 
cross fractures approximately parallel to the base, but very slowly 
the numerous minute fissures in the prism zone. Cleavage para! 
the brachypinacoid is scarcely discernible. 

In the process of alteration the olivine is transformed into serp< 
with the secretion of magnetite. Among the secondary pro 
there is much dolomite, which appears to result from the transf 
tion of the olivine. The abundance of the carbonate present su^ 
that the olivine might contain a considerable percentage of lime, 
suggestion was proved true by a chemical analysis of the olivine, 

Pyroxene plays so small a part among the minerals of this rocl 
it can not be considered an essential constituent. In the form of 
ularly corroded grains, it is distributed throughout the mass 
approximate uniformity, but it constitutes not more than 1 per e 
the whole. The cleavage is nearly rectangular, and the extincti 
prismatic sections is parallel, indicating with a high degree of ] 
bility that the pyroxene is orthorhombic. It is generally transpi 
with a sprinkling of fine, dark grains, and is surrounded by a cl( 
border conforming to the embayed contour. 

The embayments of the irregular enstatite sometimes contain ol 
demonstrating that the pyroxene is an earlier product of crystalh; 
than the olivine and owes its border, at least in part, to the subse 
corrosive action of the magma. 

The mica is dark colored, strongly dichroic, with a very small 
axial angle in the plane of the principal ray of the railial 
(schlagfigur) produced by puncturing a thin plate of the mica \ 



» Proc. Brit. Assoc. Ail v. Sci., 1887, p. 720. 





• KENTUCKY. »S SEEN UNOEH A 



DiLLBL] I>ESCHlPTION8: XO. 112, KIMBERLITE. 293 

sharp needle. This biotite is sparingly distribnted throughout the 
rock and is surrounded by a promiuent border composed of colorless 
mica and oxide of irou. 

Pyrope can not be considered one of the essential minerals in this 
rock, yet it is among the most prominent. It occurs in spherical and 
ellipsoidal grains varying from 1"™ to more than a dozen millimeters in 
diameter. They are found abundantly along the line of the dike in the 
soil resulting from its disintegration. The small, clear, deep-red grains 
have a specific gravity of 3.673 and are locally regarded as rubies of 
problematical value, but the paler red, much-fractured fragments of 
lar^^r size have attracted little attention. 

The most interesting feature of the pyrope is prominent under the 
microscoi)e, where it is seen to be surrounded by a border of radial 
fibers analogous to that described by Fr. Becke^ and A. Schrauf,^ and 
later critically examined by A. v. Lasaulx'.^ The general character of 
the border is represented in fig. 16. It is composed of two essentially 



\ 




Pio. 16. — Pyrope showing border of biotite and mt^ffjietMe. 

different substances, both of which are always present, although vary- 
ing much in proportions. First of these may be mentioned a dark 
powder, which is frequently so abundant as to render the border 
opaque. It occurs most abundantly in the outer portion of the border, 
and is chiefly, if not wholly, magnetite, for when carefully detached by 
a sharp needle from an uncovered section it is found to be strongly 
magnetic. The second, usually inner, substance of the ring is of a 
grayish or reddish brown color, and is generally fibrous in structure. 
Schrauf studied the fibrous substance enveloping the garnets in the 
serpentine of Kremze, Bohemia, and named it kelyphite. The investi- 
gations of Lasaulx have shown that in some cases the border, instead 
of being a single mineral, is a mixture of several minerals, chiefly of 
^'•e pyroxene and amphibole groups. In the example under considera- 
tion its composition appears to be exceptional. Although it is com- 
monly made up of closely compacted, very fine parallel fibers, perpen- 
dicular to the outer surface of the garnet, it frequently appears as an 
irregular nonfibrous fringe upon the inner side of the border, or even 



'Tucbennaks MittheOungcn, IV, 18«1, pp. 189, 285. 

*B«itngeKar Kenntniss dcs A88ociationt*krei»ea der Maf^ieaia-Silikate : Zeitschrift fur KryHtaUo- 
P»pl»ie. 1882. VI, pp. 321-388 ; alno Ueb«r Kelyphite : Neaea Jahrbuch, 1884, Bd. II, p. 21. 

^reberdi© UmriEdangen von Granat. Sitzungsberichto der niederrhein . Geaellschaft in Bonn, 1882, 
Jnlj3; T«rhand1iiDgen des natorhistorischen Vereines der preiissischen Rheiulande und Westfaleua, 
SiennunddrelaBigater Jahrgang, zweite Halfte, Bonn, J 882, p. lU. 



294 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [i 

completely incloses within tlie garnet, where it is usually of j 
brown color. Generally it is distinctly doubly refracting, and 
finely fibrous is sometimes strongly colored red and green \h 
crossed nicols. The nonfibrous form of the substance, although 
colored, is isotropic and consequently not dichroic, but when 
I the absorption parallel to the fibers is occasionally almost con 

I like that of biotite, which in one section, by its uniaxial, nej 

I and strongly dichroic character it was proved to be. IlmeDii 

' magnetite are common and uniformly distributed constituents 

Kentucky peridotite. 
i Abundantly scattered among the other secondary products 

1 serpentinous network enveloping the remnants of olivine are yel 

clouded grains ranging in size from .004 to .06'"™ in diameter 
intensity of the yellowish color varies considerably, with a 
' inclination toward brown. Its index of refraction is very higb 

ing it to rise above the neighboring minerals, but its low gr 
translucency scarcely more than allows the observer to discovc 
the mineral is distinctly doubly refracting without determinii 
tainly its degree. With a very sharp steel point a number oi 
grains were removed from an uncovered section and dissolved ii 
KHSO4. When the product was moistened with a solution of ] 
turned distinctly yellow, indicating the presence of titanium. 1 
these grains were supposed to be octahedrite, but Dr. G. H. Wil 
discovery of similar grains of perofskite In the serpentine of Sy 
led to the separation and chemical examination of those in sp^ 
112, and they also were found to be perofskite. 
; For a fuller description of this rock and its mode of ocenrreD 

student is referred to United States Geological Survey Bulletin 
Peridotite of Elliott County, Kentucky. 

No. 113. CORTLANDITE (HORNBLENDB-PEBIDOTITE). 
(From Stony Point, on tiik Hudson, New York. Abstract by J. P. 

FROM THK DESCRIPTION BY GeORGB H. WILLIAMS,) 

This rock was first described by Prof. George H. Williams, un( 
head of hornblende-peridotite,^ at which time it was sh«wn to 
to Bonney's class of hornblende-picrites, and to Cohen's class c 
sonites. The latter term, however, Williams showed had been g 
a variety of diallage, and so he proposed the name cortlandtite 
rocks ordinarily classed as hornblende peridotitea. The fol 
I description is taken from his paper just cited: 

f The best locality for Hpecimens of this type of peridotite is at Kings 1 

' Stony Point, a small prominence on the west si do of the Hudson River, so 

south-west of Verplanck. . . . [Many varieties of peridodite may be foun< 
the blocks thrown out of the railroad cutting near this place.] The most 



'Peridotitea of the "Cortlandt Soricn" on tho Iluilrton Uiver nrar Peektikill, New York: . 
Sci., 3a »erie«, Vol. XXXI, 1886, pp. 26-41. 



Diuia.1 DESCRIPTIONS: NO. 113, CORTLANDITE. 295 

tble amonj; these varieties is a dark green, at first sight apparently fine-grained, rock, 
which, bowever, "when held in a proper light, exhibits glistening, bronze-colored 
ckavage sarfacee, often measuring 3 by 4 inches. The reflection from these surfaces 
iaoot altogether continnons, being interrupted by small rounded grains of a dull 
green mineral whose nature can not be determined with the unaided eye, but which 
the microscope shows to be olivine or serpentine. 

Thia^luster-mottling," as a similar phenomena has been called by 
Pampelly,^ has been termed poecilitic structure by Professor Williams.^ 

The {{listening surfaces fin this rock], as the microscope shows, are those of a 
brown hornblende. The individuals of thin mineral are very large, being often 4 
mches in diameter; but, notwithstanding that they are so abundant as to be every- 
wiieie in contact with each other, so full are they of inclusions of the other con- 
itituenta that they do not of themselves make up one-half of the entire mass of the 
rock. [The only other minerals recognizable without a microscope] are frequent 
•particles of magnetic pyrites (pyrrhotite) and glistening flakes of a light green mica. 

The hornblende is recognized by its characteristic prismatic cleavage 
irith an angle of 124o 30'. 

In thin section it is rich brown, which is the color of basaltic horn- 
blende. Its pleochroism is pronounced. The color of rays vibrating 
parallel to c, nearly parallel to the prismatic axis, is dark chestnut; 
that of the rays vibrating parallel to b, parallel to the orthodiagonal 
axis, is a slight tinge lighter, while the color for rays vibrating parallel 
to a is a light yellow. The absorption is c = b > a. Hence some sec- 
tions exhibit marked pleochroism, and others almost none. The extinc- 
tion angle in clinopinacoidal sections is 9^ to 10° from the prismatic 
axis, indicated by the direction of the prismatic cleavage cracks in 
soch sections. 

The inclnsions in this hornblende are both numerous and characteristic. The 
Boit common are opaque black needles, ranging in size from the finest dust to about 
0.(]3"»a in length. The majority are arranged either parallel to the vertical axis, or 
^ 80 as to make an angle of about 45^ with this. Others appear quite irregular 
in their position. More rarely small transparent crystals, the largest of which are 
O-OS"*™ long and 0.02"»"' broad, c3cur with the opaque needles. The nature of these 
could not be determined. . . . Still more rarely than these transparent crystals, 
the hornblende contains inclusions of thin brown plates similar to those which are 
BO eharactenstic of hypersthene. All of these interpositions, of which the opaque 
D<^les M a rule occur alone, show a tendency to concentration toward the center 
of the hornblende, leaving a border near the edge comparatively free from foreign 
xi^«taiice8. Often they form irregular patches scattered like little clouds over the 
hrown background. 

The hornblende itself never shows any trace of crystalline form. It fills the 
>n«ga1ar spaces between the other constituents, a single individual often covering 
iBpAcesome inches square. From its relations to the other minerals in the rock it 
Me?ident that it was the last to solidify, while the great size of the crystals would 
B^^ni to indicate that the process of their formation went on very slowly. . . . 

The hornblende seems particularly subject to alteration, which in often far 
•dvanced before the olivine or the pyroxene are materially affected. The first 
citftDge which the hornblende undergoes is bleaching, accompanying which is the 

'MetaitomAtic derelopment of the copper-bearing rocks of Lake Superior. Proc. Amer. Acad., Vol. 
XIII, p. 290, 1878. 

*Loccit.,p, 30: also, On the use of the terniM poikilitic and micropoikilitic in petrology: Jour. 
G«»l.,VoLl,p.l7«,1893. 



296 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bixlIM. 

almost total disappearance of the characteristic inclusions. The mineral liecomet 
nearly colorless and consequently noupleochroic, while retaining the compact stnic- 
tare and optical behavior of the unaltered portion. Later there is developed, par- 
ticularly around the edge of the hornblende, a bright emerald-green substance, 
which, on account of its lack of dichroism and very feeble action on polarized light, 
may be regarded as chlorite. 

Next to the hornblende, the most important constituent of this rock is the olivioe, 
which is remarkable both for its freshness and for its beautiful inclusiona. It is 
present in rounded grains or in well-defined crystals, upon which the usual combi- 
nation of domes, prisms, and piiiacoids may be seen. These crystals vary from one- 
half to 2^^ in diameter. The mineral is quite colorless, with a high index of 
refraction, and is traversed by irregular cracks, along which serpentinization may 
be frequently seen to have commenced, although in many sections there is hardly a 
trace of this alteration. . . . They [the inclusions] are black and opaqae, 
having generally the form of minute, rounded grains, or long rods arranged parallel 
to one or more of the crystallographic axes of the olivine, although they are some- 
times more irregular in their distribution. Frequently these rods, inst-ead of being 
straight, are variously bent and tfv^isted, exhibiting the form of trichitee in obsid- 
ians. In such cases they show a tendency to form elliptical groups resembling a fine 
arabesque, as figured by Zirkel.^ The same author has observed that while these 
inclusions are very characteristic of the olivine of the older rocks [i.e., the coarser- 
grained rocks — J. P. I.] they are never found in that of the younger basalts. There 
seems little doubt that they are composed of magnetite, since they are readily 
decomposed by acid, and since such grains of olivine as contain them in abondanoe 
are attracted by the magnet. . . . 

[In the exceptional cases, in which feldspar is present, and is in contact with 
olivine, there is a zone between the two minerals consisting of two parts.] The 
inner portion, nearest the olivine, is composed of square grains of nearly colorless 
pyroxene; the outer one [adjacent to the feldspar, consists] of tufts of radiating 
actinolite needlcH of a beautiful bluish-green color and strongly pleochroic [The 
olivine is sometimes partly altered to serpentine.] 

The pyroxene constituent of the peridotite from Kings Ferry appears to be for 
the most part hypersthene. It sometimes forms small irregular grains not larger than 
those of the olivine, but in other cases it is present in individuals which are oTera 
centimeter in length, inclosing the smaller grains of both olivine and hypersthene 
like the hornblende. In all forms it possesses all the ordinary characteristics of hypff- 
sthene, except that it is singularly free from the usual inclusions. Its pleochroism 
is very strong : a = a ray is red ; ft = b ray is yellow ; c= c ray is green. Its cleav- 
age is well developed parallel to the prism (x P) and also still better parallel to the 
brachypinacoid (oo P db ). A common, nou pleochroic augite, in which a diallag^ 
habit is frequently developed by the presence of a parting parallel to the orthopin** 
coid [100] is also often to be observed in this rock, although in many specimens it is 
altogether lacking. As this constituent increases in amount the rocks grade Vi^ 
those of the next group [augite-peridotite or picrite]. 

Biotite is also present in small amounts. This mineral rarely retains its brown 
color. It is generally so completely bleached as strongly to resemble mascovite is 
thin sections. It is much bent and twisted, often having small lenses of calcite inter- 
posed between its lamellte. . . . Aside from mere bleaching, the formation of 
the bright green, chloritic mineral, noticed as an alteration product of the bom- 
blende, is also frequent. The true character of this mica is revealed byitstery 
small axial angle ... sis well as by the fact that rarely sections may be found 
which have escaped the bleaching. These have the characteristic color, and pleo- 
chroism of biotite, and sometimes contain acicular inclasions resembling needles of 
rutile. . . . 



• Zeit»chrift der deat«cb««n geologiscbeu Gesellschaft, XXIII, 1871, p. 69, PL IV, Fig, 11. Mflaw»k- 
Beacbaffbuheit, p. 214. 



DESCRIPTIONS: NO. 114, SAXONITE. 297 

lOQgh frequently accessory, is neyer an important constituent of these 
a basic lime-soda feldspar, probably bytownite. — J. P. I.] 
aside from composing the inclusions in the olivine above referred to, 
rains ivbich line the cracks in this mineral, and are especially abundant 
le, ^where it is in contact with the brown hornblende. The large opaque 
red through the rock are almost all pyrrhotite (magnetic pyrites (FctSs) ) ; 
picotite were not observed; another form of spinel, however, pleonast, 
y its dark-green color and isotropic character, is not uncommon. This 
lied with thin opac^ne plates almost exactly like the inclusions in the 
hercynite from Ronsperg, in Bohemia. Apatite was hardly ever observed. 

Fo. 114. SAXONITE.* 

R RiDDLxs, Douglas County, Oregon. Described by J. S. Diller.) 

:onite, of ^hich specimen 114 is a sample, occupies an irregular 
bout 2 miles in extent a short distance direetly west of Bid- 
glas County, Oregon. It is almost completely surrounded by. 
ch are considerably metamorphosed, although upon one side 
i for a short distance the Cretaceous shales, 
k has a dark yellowish-green color and high specific gravity, 
g at once that it is rich in ferromagnesian silicates. It 
rstalline granular, and composed essentially of olivine and 
with a small percentage of accessory chromite and traces of 
). 

nne predominates, so that the enstatite forms less than one- 
he mass. Nevertheless, it plays a much more important r61e 
le kimberlite of Kentucky. Here it is of sufQcient importance 
I an essential constituent of the rock, and occasionally shows 
lae twinning. Both the olivine and the enstatite are clear 
less, but may be readily distinguished by their cleavage and 
operties. They are allotriomorphic and do not contain inclu- 
;epting the coffee-brown grains of chromite with a small 
r magnetite and fine ferritic dust. 

istanding the comparatively fresh condition of the rock, it is 
completely permeated by a multitude of cracks filled with 
D resulting from its alteratiou. The combination of enstatite 
le would appear to be particularly favorable for the produc- 
erpentine, as it supplies to the other the material needed 
le water to make serpentine. 

ELxonite, including the serpentine derived from it, is of par- 
terest in being the seat of deposit of genthite, a nickel silicate 
lie importance. It was discovered by Will Q. Brown, and is 
i with quartz in more or less distinct veins, which, according 
. L. Austin, who has studied the deposits in the field, and 
hem, extend across the entire area in a northeast and south- 
stion. 



ma named by Dr. M. E. Wadaworth in his Lithological Studies in 1884, Cambridge, 
m. It is the same rock that Bopenbusch designates " harzbargite." 



298 



THE EDUCATIONAL SERIES OP ROCK SPECIMENS, [bull. 150, 



The genthite, like the vein quartz, is of secondary origin, and from 
the fact that the olivine in the saxonite contains nickel, it is regarded 
as the source of that in the genthite. 

The following analyses of the saxonite (I) and of the olivine (II) it 
contains were made by F. W. Clarke, of the United States Geo- 
logical Survey : 

Analyses of »<ixon\te and its contained olirine. 



Ignition ... 

SiO, 

AlaO, 

Cr,0, 

Fe,0, 

FeO 

NiO 

MnO 

CaO 

MgO 

Totel 



I. 


ir. 


Per cent. 


Per cent. ■ 


4.41 


0.57 


41.43 


42.81 


.04 




.76 


.79 


2.52 


2.61 


6.25 


7.20 


.10 


.26 

1 


none 


none 


.55 


, none 


43.74 


54.12 


99.80 


99.36 



For fuller description of the mode of occurrence of this rock and ix 
nickel ore the student is referred to United States Geological Sw 
vey, Bulletin No. 60, pages 20 to 26; also to a paper by W. L. Aust" 
in the Proceedings of the Colorado Scientific Society of Denver f 
January 6, 1896, and to a communication by Mr. H. B. von Foull 
Jahrbuch d. k. k. geol. Eeichsanstalt, Vienna, 1892, XLII, p. 223. 



METAMORPHIC SEDIMENTARY ROCKS. 

No. 115. Crystalline Limestone. 

(From east bask ok Modoc Prak, Eurkka County, Nevada. Described 

J. P. lDDIN(iS.) 

The crystalline limestone from the Devonian strata was collected 
the east base of Modoc Peak, Eureka, Xevada, and is called the Nevsv 
limestone (Monographs U. S. Geol. Survey, Vol. XX, p. G3 et seq.). 
is light-gray, quite crystalline and saccharoid, and contains 40.62 |^ 
cent of magnesium carbonate. It is not distinctly bedded, being m 
sive. In tliin section it is evenly granular and wholly crystalline, w 
almost no admixture of other minerals than dolomite. A small amo 
of quartz of subsequent crystallization occurs as scattered patclB- 
forming a filling in minute cavities between the dolomite crystals, 
other minerals were observed. The dolomite exhibits the charact:^ 
istic cleavage, but little or no twinning. There are numerous min 
inclusions of an indeterminate nature, probably gas or fluid. 





DILHB.1 DESCRIPTIONS: NO. 116, MARBLE. 299 

No. IIG. Marble. 

(From Lee, Brrksuirr County, Massachusktts. Dksckibkd by J. S. Dillbr.) 

Tnaltered limestones are well represented in the series by chalk 
(No. 39), patellina limestone (No. 40), cociuina (No. 42), shell limestone 
(No. 43), cherty limestone (No. 44), compact limestone (No. 40), litho- 
grapbic limestone (No. 47), and hydraulic limestone (No. 48). Speci- 
men No. 115 is an example of a limestone that has been completely 
crystallized since its deposition. This process is metamorphic in its 
nature. Owing to the fact that calcite is readily soluble and much 
more easily changed than the mass of other rock-making minerals, 
limestones may become completely crystalline (metamorphosed), while 
the adjoining rocks remain but little altered. 

Metamorphic limestone is sometimes called marble, but the name is 

usually restricted, as in the trade, to limestone which will take a polish. 

The marble of Lee, Massachusetts, is of Cambro-Silurian age, and is 

apart of the great limestone and marble belt extending from Georgia, 

through a number of the Southern, Middle, and New England States, 

into Canada. Specimens Nos. 46 and 150, and probably, also No. 117 

are from this belt. In the Middle States the rock is limestone; but in 

^ew England, as well as in Georgia and Tennessee, where it has been 

^objected to much greater disturbance and consequent metamorphism, 

't is converted into marble. 

The Lee marble is a white, uniformly fine-grained rock, which looks 
^o be a remarkably pure limestone; but if so, it should be completely 
Soluble. In Mr. E. A. Schneider's chemical analysis, given below, it is 
to contain only 0.19 per cent of insoluble matter, of whose compo- 
tiion the analysis gives us but little information. The analysis shows 
at the limestone is rich in carbonate of magnesia, containing over 40 
cent; so rich, indeed, that it may be classed among the dolomites, 
this respect it is closely related to the Cockeysville marble (No. 
7); also that of Eureka, Nevada (No. 115). 

Analysis of Lee marble. 



\ Per c<?nt. 



IdboIuUIo [ 0. 19 

Al.Oj. ^''OiOj 1 .24 ' 

CaO 30. H8 

MgO i 21.42 

f:()< I 46.72 

Organic matter 

n,(), 103^ 

ToUl »t).45 



^«'' 



'^ I y^der the microscope the Lee marble is seen to be completely crys- 

granular, with small grains, comparatively uniform in size. 



300 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [bill 

Many of these show twinning bands, but are generally quite freefi 
visible inclusions. The presence of 0.19 per cent of insoluble mate 
would be expected to reveal itself under the microscope. However 
minerals other than the carbonates were not readily detected in 
section, a fragment of the limestone was dissolved, and a resi 
obtained of clear colorless cleavage plates of a mineral which h; 
very small optic axial angle and is negative. The interference co 
of its cleavage plates are very low, in thin pieces, not even attaii 
yellow of the first order. 

According to Mr. Merrill,^ much of the marble of Lee and adjoii 
localities contains crystals of trejpolite, which weather out upon e 
sui'e, leaving the rock with a rough pitted surface. They are doubl 
due to metamorphism, Mr. Hobbs^ notes their abundant developr 
in the Canaan limestone of the same region along the line of a g 
fault. 

The Lee marble was used in the structure of the Capitol extern 
but Mr. Merrill reports that the weathering out of the tremolite cryi 
in the exterior walls is very noticeable. In England dolomite is 
sidered much more durable than pure limestone, but that view 
not appear to be shared in this country.^ 

No. 117. Marble (Dolomite). 

(From Cockeysville, Baltimore County, Maryland. Described B^ 

J. S. DiLLER.) 

This rock is known throughout the Baltimore region as the " Oocl 
ville marble" because it is extensively quarried at that place, 
completely crystalline, as shown in PI. XL, which rei)resents it as 
under a microscope of small amplification. The crystalline grains 
much larger than those of the Lee marble. The rhombohedral clea^ 
is especially distinct in the thin section, although not well shown ii 
illustration, which was taken between crossed nicols in order to h 
out the twinning lamelh^. Grains of quartz, several of which 
shown in the figure, are common, and account for the high percen 
of silica present. Farther westward in Maryland, according 
Williams, the limestones are much less crystalline and retain trac< 
their original sedimentary character. All of these features are o 
crated in the Cockeysville marble. The impurities may originally 1 
been distinct, but now, according to Williams, have crystallized 
appear as accessory minerals, the original sedimentary chara 
having been obliterated in the raetamorphism. 

There is little variation visible in the hand specimens, but in 
large mass, where quarried, there are considerable variations in 
size of the grains, as well as chemical composition and accessory i 



1 stones for Building and Decoration, p. 94. 

» Jour. GooL, Vol. I, p. 794. 

• Stonea for Buildiug and Decoration, by G. P. Merrill, p. 38L 





" *<WN Of MURBLE, COCKEVSVILLE, MARYLAND, AS SEEN 
A. In enJIiuijr ligliL Jt. 6e 



UNDER A MICROSCOPE, X JO. 



,1 



f 



/ 



IXSL.] 



DESCRIPTIONS: NO. 118, QUARTZITE. 



301 



tituents. The coarse-grained variety, which is much coarser than 
.pecimen 117, is locally called "alum stone," and is burnt for lime. 

Two analyses of this marble are given below. The first was made 
byJ.E. Whitfield,* and the second by E. A. Schneider. The second 
8\iows tlie composition of the specimen in this series. 

Analyses of Cofkejfsville marble. 



SiO, 

AUO, 

FeO 

CaO 

: MgO 

, CH\ 

Ignitiou . . . 

I Total 



Per cont. 

0.44 
1.22 
trace 
30.73 
20. 87 
4'}. 85 
1.22 

100.33 



Innolnble 

Al,03.Fe,0, ... 

CaO 

MkO 

COi 

Organic matter 
H,0 at 1050 . . . . 

Total . . - 



Per ceut. 

5.57 

.40 

29.08 

20. 30 

44.26 



99.61 



The amount of niagnesian carbonate present, as shown by these two 
analyses, is variable, and averages a little over 39 per cent. 

For information concerning the field relations of this rock, the student 
is referred to the "Guide to Baltimore, with an account of the Geology 
<»nt« Environs," pp. 97 to 102, by George H. Williams. Its distribution 
is shown upon an accompanying geological map. 

No. 118. QUAUTZITE. 
(^ROM Cakibou Hill, Eukkka County, Nevada. Described by J. P. Iddings.) 

Thequartzite from Caribou Hill, Eureka, Nevada, is of Silurian age, 
^"^ is designated on the atlas sheets accompanying Monograph XX, 
United States Geological Survey, as the Eureka quartzite.* It is dense, 
^^ite, and vitreous, in massive beds, with no distinct evidence of its 
^^gniental character visible to the unaided eye. 

h thin section it is seen to be a very pure quartz rock, consisting of 
^ins of (juartz about 0.25'"'" in diameter. In general they are very 
^ree from inclusions of foreign mineral matter, but usually exhibit 
minute fluid inclusions. Some grains contain hair like needles which 
^^ probably rutile. The fluid inclusions are often arranged in lines 
^r planes traversing the quartz grains in one or more directions. The 
*^)eaud outline of the quartz grains in some cases suggest those of 
UJ^anitic quartz, in which each grain has crystallized until interrupted 
".Vthe neighboring grains; but in many cases it can be seen that the 
outer portion of the grain is an addition of quartz substance upon a 
^^'ntral nucleus of quartz, which itself is a well rounded water-worn 
?''»n. The inner and outer portions have the same optical orientation 



' BuU. 60, U. S. Gool. Siu vty, p. 159. 
'See Monograph XX, pp. 54 and 212. 



302 THE EDUCATION-AX. SERIES OF ROCK SPECIMENS, [bull 150. 

and appear to be one continnoas individnal crystal. The line of 
demarcation between the two portions consists of minute iaclasioDS 
surrounding the original rounded grain. This secondary enlargement 
of quartz grains has been described by a number of petrographere,' 
and is due to the crystallization of quartz from aqueous solations 
l)ermeating the sandstone, the secondsiry quartz assuming the same 
crystallographic orientation as that of the nucleus.- This feature may 
be observed also in the Potsdam sandstone (PI. X, p. 80). By this 
l>rocess a sandstone may be converted into -a dense vitreous quartzite 
by the action of aqueous solutions without the accompaniment ot 
abnormal pressure or of any increase of temiierature. 

In the quartzite from Caribou Hill there are veins of chalcedony that 
traverse the rock in all directions and sink to microscopic proportious. 
The chalcedony is developed in spherulitic growths, radiating from the 
surface walls of the veins inward, usually as narrow bands in thin 
section. In some of the broader veins the chalcedony in the central 
portion exhibits definite spherulites and in some cases microcrypto 
crystalline aggregation. It is distinguished from the quartz by lower 
index of refraction, and by the fact that the axis of greatest elasticity 
is parallel to the long axis of the chalcedony fibers, which are in a 
sense optically negative. Minute prisms of quartz would be optically 
positive, the axis of least elasticity being parallel to the length of the 
prisms. 

It is evident that the sandstone, after being cemented by the second- 
ary enlargement of the quartz grains, and having been converted to 
quartzite, was subjected to dynamical strain which fractured it into 
small pieces and minute particles. It was then permeated by solations 
which deposited silica in the form of chalcedony in all the fissures and 
interstices and recemented it into a dense quartzite. 

The fractures, which are now indicated by the veins of chalcedony, 
traversed the grains of quartz and their quartz cement as across a 
compact mass, and incomplete parallel fractures, which accompany the 
more perfect ones, are shown by lines and planes of fluid inclusions. 
These are therefore of secondary origin, are subsequent to tlie first 
cementation and contemporaneous with the fracturing, and result from 
dynamic action. 

Ko. 119. Quartz- SCHIST. 

(From Stevenson Station, Grken Sprincj Valley, Baltimore Goi:nty, Mabv- 

LANi>. Described by W. S. Bayley.) 

The specimen numbered 119 was obtained from the Shoemftk^r 
Quarry, near Stevenson, a station on the Green Spring Valley Branch 



>A. E. Tornebohm, Eiu Uoitraj; zur Frage der Quartzltbildung: Geol. Foren. Stockh., 187«. ^•**- 
III, i». 35, reviewed in Xoues Juhrbuch fiir Min., etc. 1877, p. 210. B.C. Sorby: Quart. Joor.G*«*' 
Soc. London, 1880, Vol. XXXVI, p. 62. A. A. Young: Am. Jour. Sci., July, 1382. R. D. Ining: U- 
June, 1883. R. D. Irving, and C. R. Van Uise: Bull. 8. U.S. Geol. Survey, 1884. T.G. Bodd*"} *«^ 
J. A. IMiillips: Quart. Jour. Geol. Soc. London , Vol. XXXIX, p.l9. J.P. Iddinga: Monogrtpli^ 
Appendix B, U. S. Geol. Survey, 1892, p. 346. 



ua.] DESCRIPTIONS: NO. 119, QUARTZ -SCHIST. 303 

f the Northern Central Eailroad, in Baltimore County, Maryland, 
be rock is used for flagging, and for the foandatioDS and abutments 
f bridges. 

Geologically the rock is one of the members of the series underlying 
tie eastern portion of the Piedmont Plateau. This is an elevated base- 
iveled area, which is divided by Dr. Williams ' into an eastern and a 
resteru portion. In the western portion the rocks are unquestionably 
letamorphosed sediments (see description of No. 126), while in the 
astern portion they are nearly all holocrystalline, though they may 
Tiginally have been clastic. Some of them possess an obscure con- 
rlomeratic habit. They are beneath the metamorphosed sediments of 
be western plateau region, which are probably Cambrian and Lower 
Mlurian in age, and hence they are regarded as probably Algonkian, 
>r, at any rate, as pre-Cambrian.^ The rock represented by the speci- 
nen occurs along the contact between a dynamically metamorphosed 
lornblendic and micaceous gneiss and a crystalline dolomitic lime- 
itoue^known as the Cockeysville marble* (No. 117). It passes insensib y 
into tbe gneiss, of which it may be considered a phase. In the latest 
map* of the district no distinction is made between the two rocks, 
both being represented by the same color and described as metamor- 
phosed sediments. 

Different specimens of the rock present diflferent appearances. All 
are more or less foliated, and some are platy. The least foliated vari- 
eties are light-gray rocks with a sugary texture. They consist of an 
^^eTegate of small quartz grains and tiny flakes of a light-yellow 
gHstening mica. The more schistose phases contain much more mica, 
dod so have a little darker color than the less schistose forms. AU 
^eties of the rock are crossed by parallel joint planes whose surfaces 
dre covered with mica scales. In the massive varieties the joints are 
less numerous than they are in the more schistose ones, in which they 
^re often only a small fraction of an inch apart. The surface of a cross- 
^'actare through a si>ecimen of this kind resembles very closely that of 
a micaceous gneiss. 

^^0 traces of bedding planes can be detected in any of the specimens. 
Tfaeir platy character is determined by the joint planes passing 
through them. 

^' H. WiiliAms, The Petrography and Structure of the Piedmont Plateau in Maryland : Bull. 
^l-Soc. America, Vol. 2, 1891, pp. 301-322; and Guide to Baltimore, with an account of tbe Geologj' 
**iU£Brixons, and three mapa, Am. Inst. Min. Engineers, Baltimore Meeting, February, 1892. Pre- 
^'^ bj local committee for the use of the institute, pp. 77-124, with maps. 

^f-C.R.VanHifte, Correlation Papers^ Arc hean and Algonkian : Bull. U. S. GeoL Survey No. 86 ^ 
^■P^cully pages 411 and 415. 

/^^ specimens of this marble are seen in the columns and heavy platforms of the Capitol exten- 
''^fttTasbington. A large portion of this building, the Washington Monument, and the Post- 
**f*lwilding in the same city are constructed of it, as well as the spires of St. Patrick's Cathedral 
'"^'ewTork. 

JJ5eeinap in Gaide to Baltimore; and Geological Map of Baltimore and Vicinity, G.H.Williams, 
*J*^. published by Johns Hopkins University, 1892; also Baltimore sheet, Atlas U. S. GeoL Survey, 
**i»JiiiiADga8t, 1892. 

'-^ Preliminary Geological Map of Maryland, G. H. Willinms. ed]tor,.1893: Maryland, its Besources, 

■^"tttries, and Institutions, prepared for the Board of World's Fair Managers of Marvland, Baltl- 



304 THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [bcllWl 

A close inspectioii of the joint sorfiEM^es of most specimens will show 
the presence of small grains of coal-black tonrmaline in the midst of 
the mica scales. Dr. Williams,* in his account of the petrography 
of this schist, states that ''its most characteristic feature is imparted 
to this rock by long crystals of black tourmaline, which have beeu 
developed in these muscovite layers. These crystals are invariably 
broken and their fragments separated along one line, showing that 
the rock was compressed in one direction and elongated or stretched 
in another at right angles." 

The rock is described by Williams' in the following words: 

The least important of the rocks of probably sedimentary origin in the Baltimon 
region is a peculiar schist composed mostly of quartz and divided into beds of vvy* 
ing thickness by parallel layers of mnscovite. . . . Its quartz grains are of 
different sizes, but are so completely recry stall ized that they form an interlocking 
mosaic. Besides the flakes of muscovite, the only other constituents are iron stains aod 
occasional crystals of tonrmaline, microclines, and zircon. Sharply defined areas 
showing a minute spherulitio polarization are also common. They are identical with 
those occurring in the Saxon "greisen/' and probably represent altered feldspar. 
The rock shows the effect of pressure in the nndnlatory extinction of its quartz 
grains. The cleavage planes of the quartz-schist are due to thin layers of mnscoritc 
in good-sized scales, with their basal planes all parallel to the foliation. 

With respect to its origin he declares that the rock is probably a 
'' facies of the gneiss, produced by some dynamic agency, for it always 
shows the effect of internal mechanical action and motion. Moreover, 
the abundance of tourmaline points to the agency of fumaroles, which 
are always important factors in the recrystallization of deeply baried 
rocks.'' 

The platy character of the rock is plainly seen in the hand spedmeD; 
its schistosity, however, is not so apparent. Nevertheless, on breaking 
the rock the fracture is much more easily produced in a direction par- 
allel to that of the cleavage planes than in any other direction^ even 
when the separation is not along the cleavage plane. This is due to 
the arrangement of the mica scales throughout the body of the rock, 
with their flattened sides parallel to the cleavage planes. 

In the thin section the irregular grains with brilliant polarization 
colors are quartz; the long, narrow grains with a longitudinal cleavage, 
a very slight x)leochroism, an extinction parallel to the cleavage lin^ 
and brilliant mottled polarization colors are muscovite; the opaque 
crystals and rounded grains are magnetite and pyrite, altered in many 
cases to limonite or ocher, and the few clear, colorless grains with pale- 
green polarization colors are plagioclase. Some of these latter are 
crossed by indistinct twinning bars, and many of them, particularly 
those near the edges of the section, are traversed by cleavage cracks. 
In some sections the plagioclase is altered to a cloudy aggregate of 
scaly minerals and little dust particles. Grains of zircon are rare. 
They occur as small rounded or elliptical particles, with a very high 



1 Guide to Baltimore, p. 103. 
*Op.oit.,pp.l02.105. 



a.] BE8CBIPTION8: NO. 120, JA8PILITE. 305 

ractLve index and strong doable refraction. The latter property is 
iwn in tlie bright polarization colors and the former by the dark 
B8 bounding the grains. 

Che quartz is the most characteristic component. It is in irregularly 
erbcking grains. These are crossed by continuous lines of tiny dust 
doflions and larger liquid inclosures with movable babbles. The 
ijority of the grains exhibit undulatory extinction, though this i)rop- 
iy is best seen in the thinnest grains near the edges of the section. 
Along certain bands the larger quartzes are embedded in a rubble 
smaller grains, and in these bands the muscovite is most common, 
tie mica, the quartz grains, and the feldspars are all elongated in the 
[TOction of the banding, producing a well-marked schistosity. In 
her x>ortions of the section the muscovite also exhibits a tendency to 
parallel arrangement, but this tendency is more noticeable when 
le section is viewed under a low-power hand lens than when exam- 
led under the microscope. The mica^ and the feldspar occur between 
he quartz grains, the former lying along the boundaries between two 
irains and the latter occupying angular spaces between several 
[oartzes. Both the muscovite and the plagioclase show pressure 
iects— the mica in the bending of occasional flakes and the feldspar 
in the occurrence of secondary twinning lamellae. 

From the microscopic study of the sections we are led to the same 
GODclusion as that reached by the field study of the rock, namely, that 
itis a dynamically metamorphosed acid rock, which from its composition 
Appears more likely to have been a sandstone than an eruptive. The 
n)ck is now a mica-schist in its most micaceous phases, or a micaceous 
quartz-schist in its less micaceous forms. 

No. 120. Jaspilite. 

(From Ishpkming, Marquette County, Michigan. Described by C. R. 

Van Hise.) 

Macroseopical. — ^The jaspilites of the Ishpeming area of the Mar- 
Qnette district in the Lake Superior region occur in the Negaunee or 
ironbearing formation of the Lower Marquette series. They are banded 
focks, the alternate bands consisting mainly of small iron-stained par- 
ticles of quartz, or jasper, and of iron oxide. The exposures present a 
bnlliant apx>earance, due to the interlaminatiou of the bright red jasper 
And the dark red or black iron oxides. The iron oxide is mainly hema- 
^te,and includes both red and specular varieties; but magnetite is 
usually present. 

The jasper bands frequently have oval terminations or die out in 
to irregular manner. The rocks have been folded in a complicated 
^hion, as a result of which the layers present an extremely contorted 



'In » few sections there are present, in addition to the muscovite, a few flakee of a brownish-green, 
'^'^y pleochroic biotite. These, when present, are always cloNely associated with the muscovite. 
^'''Bihieral neTer occurs in quantity large enough to affect the general character of tho rock. 

Bull. 150 20 



i 



306 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bullUO. 

appearance. The folded layers frequently show minor faulting. Be- 
cause of their brittle character, at many places the bands have become 
broken through and through, and sometimes they pass into reibaogs- 
breccias. In some cases the movement of the fragments over one 
another has been so severe as to produce a conglomeratic aspect. 

In the folding of the rock the readjusrt;ment has occurred mainly in 
the iron oxide between the jasper bands. As a result of this, the iron 
oxide has been sheared, and when a specimen is cleaved along a layer 
it shows brilliant micaceous hematite. This sheared lustrous hema- 
tite, present before the last dynamic movement, is discriminated with 
the naked eye or with the lens from crystal-outlined hematite and mag- 
netite which have filled the cracks in the jasper bands and the spaces 
between the sheared laminae of hematite. The jaspilite differs maJnly 
from the ferruginous chert of the iron-bearing formation, with which it 
is closely associated, in that the siliceous bands of the former are stained 
a bright red by hematite and the bands of ore between them are mainly 
specular hematite, while in the cherts the iron oxide is earthy hematite. 
The jaspilite in its typical form, whenever found, always occupies one 
horizon, the present top of the iron-bearing formation just below the 
Goodrich quartzite. In different parts of the area the jaspilite has a 
varying thickness. With this jaspilite, or just above it, are the hard 
iron ores of the district^ hence it has been called by the miners, to 
discriminate it from the ferruginous chert, '^hard ore jasper.-' 

Microscopical. — In thin section the jaspilites are seen to have a 
minutely laminated chara^cter, each of the coarser bands, as seen in 
hand specimen, being composed of many laminse, due to the irregular 
concentration of the iron oxide. These laminae are of greatly varying 
width. They unite and part in a most irregular fashion, prodacin^r 
a mesh-like appearance, and frequently laminae disappear, as do the 
coarser bands. 

The complex, bright-red jasper bands are composed mainly of fiudj 
crystalline cherty quartz, but they are everywhere stained with minute 
particles of blood-red hematite. The grains of quartz average rather 
less than 0.01'"'" in diameter, and each of these minute grains contains 
one or more particles of hematite. These are concentrated in laminie, 
or are separate flecks included in the quartz grains. In some cases 
the hematite appears to be somewhat concentrated between the grains, 
but in general it is arranged in entire independence of them, as though 
it were present before the quartz had crystallized. The ferruginous 
bands contain a predominating amount of iron oxide, but in them is 
included much quartz, exactly similar to that of the jasper bands. The 
original, translucent, red, sheared hematite is easily discriminated from 
the secondary crystal outlined hematite and magnetite. 

The folding, faulting, fracturing, and brecciation spoken of in hand 
specimen are beautifully shown under the microscope. The resultao^ 
cracks and crevices are tilled with secondary quartz and crystal on tlin^d 



K'k, and because so mucb material lias eutered parallel to the 
amiaation this stractiire is emphasized by the secondary 
til > IIS. 

beeti noted that the jaspilite is chara<;teri8tic of the up[>er- 
zon of the irou-beariug formation; thati», it is itnrnediately 
I next overlying series. The contact /.one has heen one of the 
uea of aceomniodatioti, and thas the dyiiaoiic effects npou the 
ire explained. Between the Jaspilite horizon and that of the 
as cbert« is a transition zone. In this the layers of siliceoati 
sometimes have bonlern of red iron-stained quartz, 
>eeD explained that tlie chief differences between the jaapilites 
ferrngJDOUS cherts are the blood-red character of the miuute 
particles and the micaceous character of the ferruginous layers 
rmer. It appears liiglily probable, therefore, that dynamic 
ts have trausformed the ferruginous chert into jaspilite, the 
brown hematite being sheared into micaceous hematite and the 
« of brown hematite being changed into the blood-red variety.' 

Mo. 121. Magnbtitio Specular Hkmatitk. 



eciilar hematites of the Marqnette district occur at or near the 
if the TTpper Marquette and Lower Mjirqaette series, beiug 
the top of the Negaunee iron-bearing formation of the I.«wer 
te series, or at or near the base of the Goodrich quartzite of 
sr Marquette series. The greater quantities of the hard ori-s 
ibly in the latter position, and from this horizon the particular 
IS described below are derived. These hard ores are all asso- 



308 THE EDUCATIONAL SERIES OF SOCK SPECIMENS. [bullIMi 

tration, there was a mashing of the deposits^ and later a farther enrich- 
ment of the iron ore by infiltration. 

The specimens consist mainly of brilliant flakes of micaceoos hema- 
tite, which are arranged to a considerable degree with their greater 
dimensions parallel, thus giving the ore a distinct schistosity or rift. 
Between the flakes of hematite are miunte granules of magnetite. 
Under the microscope the slides are best studied in reflected light. 
They seem to be made up mainly of nnmerous closely fitting grains of 
hematite, the majority of which take an imperfect polish, and have 
therefore a gray, sheeny, spotted appearance. The grains, which are 
parted along the cleavage, reflect the light like a mirror. The hirge 
number of these reflecting surfaces is appreciated only by rotating the 
section, which brings successively diflerent ones into favorable \m* 
tions to reflect the light into the microscope tube. As both the mag- 
netite and the hematite are opaque, the two minerals in general can 
not be discriminated, although in some cases the crystal forms of mag 
netite are seen, and a small part of the hematite, much of it in little 
crystals, shows the characteristic blood-red color. 

The accessory minerals are quartz, feldspar, muscovite, and griiner- 
ite. Some of the small areas of quartz and feldspar appear to be 
fragmental. The muscovite occurs mainly in small independent flakes, 
but some of it is apparently secondary to the feldspar. The griiuerite 
is very sparse. The translucent, red hematite is closely associated with 
the feldspar, muscovite, and griinerite. 

The iron ores and associated rocks of the Marquette iron-bearing 
district are fully described in the Fifteenth Annual Report of the United 
States Geological Survey, pages 561 to 589, and in Monograph XXVII, 
pages 328 to 407. 

No. 122. Slate. 

(From Monsox, Piscataquis County, Mainb. Desckibed by W, S. Baylet.) 

Specimen No. 122 is an excellent sample of the rocks that are known 
as clay slates. It is a very fine-grained, bluish-gray variety, with a 
close crystiilliiie texture. It is so soft that it may easily be scratched 
with a knife blade, but at the same time it is so dense and elastic that 
it rings clearly when hit with a hammer. If struck on its side with a 
chisel whose cutting edge is i)arallel to the long edges of the specimen, 
it will split into plates whose minimum thickness is limited only by the 
skill of the workman. This peculiarity of splitting into thin slabs is 
the most characteristic property of slates, and is that which gives 
them their great economic value. The evenness with which they may 
be cleft and the size of the plates obtainable from them are important 
elements in determining the suitability of the material for roofing aod 
manufacturing purposes. In a good slate the grain is fine, and th© 
cleavage planes run for long distances without interruption. 

The principal localities in this country from which good slates cof*® 



ma] t)ESCHIPTIONS: NO. 122, SLATE. 309 

nethe Peacli Bottom and neighboring regions in sontheastern Penn- 
ijlvania and northern Maryland, the Arvon quarries aud mines in the 
western part of the upper peninsula of Michigan, various places in 
7ennont and Virginia, and the quarries of the Monsou Slate Companj-, 
it Monson, Piscataquis County, Maine.* The specimens* in the col- 
lection are from the last-named locality. 

Though not of as much importance as in the case of stone used in 
the foundation of large buildings, the strength^ of various slates serves 
as a rough measure of their comparative merits. In massive rocks the 
strength is nearly equal in every direction. In schists and bedded 
lOckR less i)ressure is required to produce crushing, when applied in a 
direction x)arallel to their schistosity or cleavage, than is necessary 
when applied perpendicularly to these structural planes. A cube of 
the Monson slate measuring an inch on an edge, breaks across the 
cleavage, pressure being applied at right angles to the cleavage, when 
the pressure reaches 30,426 pounds. A slab 12 by 6 by 1 inch, sup- 
ported on knife edges 10 inches apart, breaks under a stress of 4,000 
pounds when the pressure is applied along a line midway between the 
supports. 

The rock occurs in beds* of various thicknesses, ranging between 20 
feet aud 4 inches, interlaminated with hard, fine-grained, dark quartz- 
ites. The series strikes about 70^ east of north and dips at an angle 
of about 78^ to 83^ in a direction north of west. The cleavage is 
nearly vertical, perhaps inclining about a degree therefrom, and strikes 
Dearly in the same direction as the beds. The diflference in the inclina- 
taoD of the cleavage and bedding causes the course of the former to 
cross that of the latter at a small angle, but since the quartzite is so 
moch harder than the slate, the cleavage planes that are so marked in 
the softer rock stop abruptly when the contact of the quartzite is reached, 
or cross it as a few fractures or slight faults. Since the cleavage and 
the bedding are not coincident, and not even parallel, it is evident that 
the former is not in any way dependent upon sedimentation. A shale 
splits easiest along its bedding planes. The Monson slate can not be 
^lit along its bedding, but it is easily cleft along its cleavage planes. 
In other words, the cleavage of shales is an original characteristic, 
vhilethat of elates must be of secondary origin. 

The cause of cleavage in slates has been carefully studied by many 
geologists- Sharpe,'* as long ago as 1846, held that the cleavage of 



'For amonnts quarried in different districts see Mineral Eesonrces of tlie Unite<l States for 1888 
"sdUuT years, and for descriptions of the districts, consult G.T. Merrill, The Collection of Bnildiug 
•adOniMnental Stxjnes in the U. S. Nat. Mas. : Rept. Smithson. Inst. 1885-86, Pt. ir, p. 464, et Heq. 

*Th* Rpecimens in the collection and the thin slabs were kindly famished by the Monson Company, 
••^ere sUo the resoltM of the strength tests and the analyRia. 

'Foritatements regarding tests of building stones, see Merrill, loc. cit., p. 489. 

*Thoaght by Prof. C. H. Hitchcock to be Cambrian or Lower Silurian. See geological map of Maine 
"iColbT's Atlas of Maine. Houlton. 1884. 

•Daniel Sharpe, On Slaty Cleavage : Quart. Jour. GeoL Soc. London, 1846, Vol. Ill, pp. 74-105 ; Vol. V, 



310 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. fBrLum 

slates 18 due to the flattening of the constituent grains. Sorby ^ believed 
that slaty cleavage is caused mainly by the rotation of the mineral par- 
ticleSy and especially mica, until the flat grains assume a i>osition where 
they may best resist further rotation, namely, a position at or near 
right angles to the i)ressure. As favoring this view, he subjected a 
mixture of clay and iron oxide to pressure, and obtained a cleavage 
structure at right angles to the i)ressure. Tyndall* and Daubrce^ have 
also produced a cleavage in substances by simple pressure. The former 
used beeswax, clay, etc., and the latter clay mixed with scales of mica. 
In each case in the resultant cleavable product the flat particles were 
always fi)und with their broad sides in the plane of cleavage; but 
Tyndall held, with Sharpe, that in the case of beeswax, as in all cases 
of nature, the cleavage is produced mainly by the flattening of the con- 
stituent particles. 

In nature the pressure that produces the cleavage is the same as that 
which bows and bends the rocks of the earth's crust. Since this pres- 
sure is rarely perpendicular to tlie bedding, it follows that the super- 
induced cleavage is rarely jiarallel to the bedding. In many cases it is 
probable that the force tiiat upturned the rocks at the same time pro- 
duced in soft beds a cleavage; in other cases the cleavage was produced 
subsequent to the upturning of the rocks; and in rare cases two cleav- 
ages were produced by pressure acting at different times along two 
different directions.^ In addition to the cleavage produced in a rock by 
pressure, it usually happens that this agency causes such a change in 
the conditions under which the rock exists that crystallization is setup 
in its material, so that its original nature is largely obscured. Slates 
are usually interbedded with undoubted fragmental rocks like quartz- 
ites, grading into these and into conglomerates. Consequently, it is 
assumed that its original condition was fragmental, though perhaps no 
traces of this condition are now discernible. If the original bed was 
fragmental, the deposit must have been of a very fine grain, likenmd 
or silt. Mr. Hutchings^* has recently examined such beds in which 
schistosity has not been produced, and has obtiuned from them a greft^ 
deal of information with respect to the course of crystallization set up 
in them. He concludes that nearly all the mica of slates is a second- 
ary product, derived by the alteration of constituents of the original 
deposit. Other components of the slate are also new products, formed 
subsequent to the deposition, and hence the slate in its present condi- 
tion is composed largely of crystallized secondary material. lu this 
sense the slate is crystalline, and should be classed with the crystalliu* 
schists. Since, however, we know that it was originally a sediment, 
we usually class it among the fragmental rocks, thus placing more 



' Edinburgh ^'ew Phil. Jour., vol. LV., 18r.3, p. 137. 
2 PhiloH. Ma<:., IV, XII, p. 129. 
*G<iologio Exprrinunitah*, p. 391. 

* Cf. C. R. Van Hiee : Hull. Cnol Soo. America, vol. 2. p. 209. 

* Geological Magazine, Vol. VII, June and July, 1890, and VoL VIII, 1891, p. 164. 



pttial DESCRIPTIONS: NO. 122, SLATE. 311 

enpbasis upon the genetic relations of tbe rock thau upon its present 
coDdition. 

Yan Hise^ has recently rediscussed slaty cleavage. He concludes 
that the structure is due to the arrangement of the mineral particles 
with their longer diameters or readiest cleavage, or both, in a common 
direction, and that the cause of this arrangement is, first and most 
important, the parallel development of new minerals; second, the ilat- 
teniog and parallel rotation of old and new mineral particles, and third, 
aDd of least importance, the rotation into approximately x>arallel posi- 
tions of random original particles. He farther concludes that this 
structure is developed in rocks when they are so deeply buried as to be 
in the zone of plastic flow, and that the structure develops in the planes 
Donnal to the greatest pressure. 

Under the microscope the principal mineral constituents of the Mon- 
80D slate in the collection are discovered to be quartz, chlorite, musco- 
vite, biotite, magnetite, and a few little black organic particles. These 
are Dearly ail in the form of small lenses, with their long axes in the same 
direction. The largest grains are those of chlorite. This mineral is in 
light green masses, with a dark-green pleochroism. Its double refrac- 
tioB is weak, so that between crossed nicols it polarizes in gray or blue 
tints. Around its grains bend the flakes of muscovite, etc. — a proof 
that the former must have existed when the cleavage was superinduced 
in tbe rock. It is probable that the grains were then plagioclase and 
that the chlorite has since been produced by its alteration. Chlorite 
also exists in little shreds between the other components, where it is 
i^ways elongated in the direction of the foliation. 

Tbe next most abundant components are the micas. These occur as 
tiny shreds and flakes of muscovite that can be detected only between 
crossed nicols, when it appears with bright polarization colors and as 
Iftrger, ill-defined masses of bro^^n biotite. The latter sometimes occurs 
^80 as small flakes lying parallel to the muscovite plates. It is by no 
means so common as the muscovite, nor is it always arranged with its 
flat sides in the plane of cleavage. Since the flakes of muscovite are 
Dot crowded around the large masses of biotite as they are around the 
chlorite grains, it would seem that the biotite must have been formed 
after the potash mica. It includes shreds of the muscovite, and in its 
arrangement it by no means follows the rule that the long axes are 
parallel to the cleavage. Its genesis, consequently, was probably sub- 
sequent to the origin of the foliation. 

Tbe quartz is not in large quantity. Only occasionally can little 
pains be detected, when they have the usual elliptical cross section. 
The magnetite and the organic substance are both in comparatively 
^rge-sized grains. The former have badly defined crystallographic 
outlines, while those of the latter are irregular and ragged. If a small 

'Principles of North American Pre-Cambrian Geology, by C. R. Van Hiae : Sixteenth Ann. Kept. 
^' ^ Geol. Surrey, Part I, pp. 83a-«68. See also Deformation of Rocks, Part III, Cleavage and Fissility : 
Jow.G«L, VoL IV, 1896, pp. 449-i83. 



312 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [buii.IM. 

portion of the section is heated for some time on a piece of platinnm 
before the blowpipe the organic particles are burned out, while the 
magnetite remains unchanged. This is about the only method of dia- 
tingnishing between the two substances when present in such small 
particles. It is impossible to determine, except by careful chemical 
tests, whether the organic material is carbon or a compound of this 
element. Besides these opaque constituents there are in the mascovite 
shreds, between the lamellae, numerous little black particles, most of 
which are in tiny short needles, though a few are in larger round or 
irregularly shaped masses. The latter have a brownish color, bnt their 
dimensions are so small that the nature of the material composing them 
can not be determined. From analogy, however, we may conclnde that 
they are tiny grains of rutile. This mineral in small needles is so 
characteristic of slates that they were long known by the German 
petrographers under the indefinite name of thonschiefernadeln, nntii 
in 1881 Gathrein^ proved them to consist of rutile. 

ISo other components are important enough to merit mention as 
essential to the rock, though perhaps in an occasional section a single 
grain of pyrite or of some other widely occurring mineral may be 
detected. 

From the microscopic examination of the slate we easily discover 
why it splits so readily in one direction only and in such very thin 
plates. The different constituents, with the exception of the biotite^ 
are arranged in layers composed of flat pieces lying in parallel posi- 
tions and overlapping and dovetailing into one another. Across the 
lamina) fracture is difficult, and the fracture surface is rough, for the 
breaks, following the paths of least resistance, refuse to pass across 
the grains in their courses when they can so easily turn aside and pass 
around them. The crystalline structure noticed on the cross-fracture 
surface is caused by the projection of the grains. The cleavage surface, 
on the contrary, is smooth, because so few grains extend from one layer 
into the other that the easiest path for the fracture is between successive 
layers. 

In the following table the composition of the rock, as determined by 
L. M. Norton, is given in column I; in column II is given the composi- 
tion of a specimen of the Peach Bottom slates, Pennsylvania. 



1 Neues Jahrb. f. Min., etc., 1881, 1, p. 168. 



iiuit.1 DESCRIPTIONS: NO. 123, INDURATED JOINTED SHALE. 313 



Analytes of slates. 



SIO,. 
TiO, 
SO3. 



I. 



Per cent. 
56.42 



AJA 24.14 

FeO 4.46 

MnO 



c«o 

MgO 

K,0 

NhO 

HtO 

OrgRnic matter. 
Pyrite 



.52 
2.28 
5.53 
8.15 

3.88 



II. 



ToUl ! 100.38 



Per cent. 

55.880 

1.270 

.022 

21.849 

9.034 

.586 

.155 

1.495 

3.640 

.460 

^ .051 

t 1.974 

.051 



99.800 



The specific gravity of the Mouson slate is 2.851. 

The analysis accords well witli the results of the microscopic study 
^ the thin section. The small amount of magnesia indicates a small 
proportion of biotite. The absence of plagioclase is revealed by the 
^all percentage of lime. The large proportion of potash points to an 
Abandance of mnscovite, while the water and ferrous iron show the 
presence of chlorite. The percentage of silica is so low that there can 
Dot be much quartz in the rock, and the alumina is just about sufficient 
to combine with the potash, magnesia, and iron protoxide to fown 
iDQBoovite, biotite, and chlorite. The soda may indicate the presence 
of a Tery alkaline feldspar, or it maybe present in the light-colored 
^ca, some of which may be paragonite instead of muscovite.^ 

No. 123. Indurated Jointed Shale. 

(FWll SOMKRVILLE, MIDDLESEX COUNTV, MaSSACHI'SETTS. DksCIUBED BY C. R. 

Van H18B.) 

VarioQs causes have been assigned for joints, of which the more 
|Oiportant are tension, torsion, and compression. It is believed that 
Joints may be classified into tension joints and compression joints, tor- 
^n joints being but a variety of tension joints. The first are ordinarily 
i& the normal planes, or at right angles to the stretching force; the 
^nd are in shearing planes, or are inclined to the crushing force.* 

Joints may be produced in rocks in the outer zone of the crust of the 
•*rth, where fracturing results from deformation. Rocks which are 
ooried to a great depth are under such a load that it is impossible 
^*t crevices and cracks can exist; therefore jointing does not occur. 

'» otlier deMsriptionii of Blates tee Irving and Van Hise, Tenth Ann. Rept. U. S. Geol. Survey, 
'*-''^<28; G«olog7 of Wii«conain, VoL III; and for general account of slate regions, etc., Report 
* Baflding 3t«qM of the United States, Census of 1880, Washington, 1884. 
'or » fbller diseaaaion of joints, see Principles of North American pro-Cambrian Q<M>logy, by 
•*• V»a Hise: Sfxteenth Ann. Kept. U. S. Geol. Survey, Part I, pp. e68-«72. 



{ 



314 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [bcllUB. 

Jointing is almost certainly confined to the onter 10,000 meters of the 
crust of the earth, and is perhaps confined to the outer 5,000 meters. 

Tertian joints. — Tension is often due to the contraction caased by 
cooling or by desiccation. It is well known that the peculiar colamDar 
jointing of igneous rocks is due to the contraction and consequent ten- 
sion caused by cooling, and the mud cracks of sedimentary rocks are 
due to the contraction and consequent tension caused by desiccatioD. 
However, it is probable that neither cooling nor desiccation is important 
in the production of systematic parallel sets of joints. 

When rocks are simply folded in the outer zone of fracture, the con- 
vex halves of the folds may be subjected to simple tension, and if this 
goes beyond the ultimate tensile strength of the rocks, radial cracks 
will form, which extend through the strata and strike parallel with the 
rocks. In rocks which are complexly folded or cross folded, there may 
be two sets of tensile joints which intersect each other nearly at right 
angles. Tensile joints in homogeneous rocks are exactly or nearly at 
right angles to the stretching force. 

Daubr^e^ has shown that if a brittle plate breaks when it is subjected 
to torsion, a double set of parallel fractures nearly at right angles to 
each other are produced. The forces which produce complex folding 
stretch the convex parts of the strata, where not too deeply buried, in 
two rectangular directions; or, in other words, they are subject to 
torsion. It therefore appears that Daubree's explanation of joints by 
torsion is but another statement of the production of joints by complex 
folding in the two principal planes of tensile stress. 

Compression joints, — Daubr^e^ and Becker^ have shown that joints 
may be produced by compression. Then there may be jointing in two 
sets of shearing planes when tji© rocks are subjected to a single stress, 
and, according to Becker, there may be joints in three or four planes 
when they are subjected to unequal stresses in diflferent directions, in 
the latter case one of these sets of joints being normal to tensile stress 
and the other in shearing planes. In all cases in which jointing occurs 
in shearing planes the joints are inclined to the pressure. In a simple 
ideal case the joints should be at an angle of 45^ to the pressure, bat 
this rarely occurs in nature. Fracturing along shearing planes is illas- 
trated when stones are crushed in a testing machine. The fractures 
do not form in the lines of pressure, but in general at any place in a 
homogeneous rock in two intersecting directions at angles somewhat 
less than 90° to each other. The direction of pressure bisects the acute 
angle. 

Becker has explained that minor faulting is a common phenomenon 
of compression joints. 



» Geologic Exp^rimentale, by A. Daubrce, jip. 306-:a4. Paris, 1879. 

«Ibld., pp. 315-322, Paris, 1876. 

■George F. Becker: Finite homoijeneous strain, flow, and ruptarn of rocks: Bull. G«^ S*^ 
America, Vol. IV, 1893, pp. 41-75. The torsional theory of JoinU: Trans. Am. Inst. Min. EngiB«*'* 
VoL XXIV, 18M, pp. 180-13S. 



ujOL] DESCRIPTIONS: NO. 124, CRUMPLED SHALE. 315 

In mauy cases when the rocks are jointed in more thau one direction 
le different sets of joints may have been produced by successive 
rogenic movements, rather than at a single period of deformation. 
The specimen (No. 121) fi-om Somerville, Massachusetts, belongs to the 
>called "Cambridge slates'' whose geologic age is not yet positively 
nowD. Crosby has referred them to the Carboniferous, but others 
ave considered them to be Cambrian. 

Tlie specimen is bounded by three sets of joints. One set is parallel 
ft the bedding; the other two sets intersect the b<^dding at acute angles 
lUd are at acute angles to each other. As a result of the three sets of 
oints, the rock is broken into rhomboidal blocks (or more accurately 

• 

ibliqne parallelepipeds) bounded by the three sets of joint planes, each 
>iece having as boundaries two parallel fractures along each of the three 
^t8 of joints. Probably each of the sets of joints is produced by com- 
pression rather than by tension, and therefore is formed along shearing 
planes. The joints parallel to the bedding are undoubtedly controlled 
in direction by planes of weakness along the bedding. As shown by the 
smoothed surfaces, there has been slipping along many of these joints 
daring the folding process. In some cases the evidence of readjust- 
ment iu slickensided faces is also seen along the sets of joints inter- 
secting the bedding. 

No. 124. Crumpled Shale. 

(From Hot Springs, Madisox (*oitnty, North Carolina. Dkscribf.d by Bailky 

Willis.) 

The specimen of crumpled calcareous shale illustrates the folding of 
laminated rocks. When this shale was deposited as a sediment its 
SQccessive layers of red and buflf mud were level and parallel. In that 
IK)8ition they hardened into firm rock. The shale was gradually buried 
beneath later sediments and sank into the earth's mass. At a depth 
of a few thousand feet the superincumbent pressure was such as to hold 
this shale in a condition iu which it o^uld not break apart. When 
compressed e<lgewise by a sufficient lateral force the layers were 
obliged to bend like paper. In thus bending, they slid one over 
aDother, they were squeezed thinner iu certain parts of the fold, on the 
limbs, and they thickened where the combined pressure of load and 
^gewise thrust were less intense. 

Folds of rock masses are not limited to small crumples such as cau 
be shown in a specimen. The beds of strata folded may be 5,000 feet, 
or five times 5,000 feet thick; then the folds are large. For example, 
iii central Pennsylvania there is a fold, known as the Nittany Arch, 
^bich is 25 miles across and about 150 miles long. Examples of still 
larger folds might be cited. 

The accompanying illustrations^ show two forms of single folds. 

^e of these (Fig. A, PI. XLI), which is open upward like a trough, is 



» Plate XLVni, Thirteenth Ann. Eept., U. S. Oeol. Survey. 






316 THE EDUCATIONAL SERIES OF ROC:K SPKCIMEXS. 

called a syncline. The other (Fig. B^ PL XLI), which is arche< 
is called an anticline, 

Synclines and anticlines usually occur in association, sid 
and overlapping end on end. They then form systems of foJ 
may be of great extent. The Appalachian system of foldi 
from Nova Scotia to Alabama, about 1,600 miles, and varies i 
125 miles in width. Along the eastern base of the Eocky 3 
there is a similar system of folds. 

Folds develop many different forms, both in cross section a 
gitudinal section. They are usually classified according to t 
of compression, which varies from gentle undnlatious to strc 
which the strata are pressed closely in vertical positions, or t 
which the beds are even overturned and squeezed out thin, 
also classified according to magnitude and complexity. Any 
includes smaller folds, each such smaller fold includes stil 
folds, and so on down to microscopic crinkles. 

Strictly speaking, in folds the relative order of strata is not 
beds a, ft, c, dj e, /, etc., after folding succeed one auothei 
original order. But there is another class of structures 
from compression, called faults^ ouerihrusts, or thrusts, which 
acterized by a change in the order of strata. When rocks I 
separate parts may be pushed one over another. When foldi 
are so intensely compressed that some strata, softer than o 
squeezed out, then that portion of the fold above the squeez 
pushed over the portion beneath. When rocks flow under tn 
pressure the movement of flow may be concentrated in a pla: 
number of planes in a narrow zone. Then the rock is divide 
plane, and the parts may be moved past one another. In 
cases the changes of form in the rock mass may result in 
lower formation a, over a higher stratum /. Such a struct 
overthrust, and the rock is said to be faulted.^ 

Overthrusts vary in magnitude from those which may be 
the specimens of crumpled shale to those of the southern App£ 
which exceed 300 miles in length and traverse strata 10,000 f 

The deformation of rock masses is more fully illustrated and < 

by the writer in the Thirteenth Annual Report of the Unit4 

Geological Survey, pp. 211 to 281, and by G. R. Van Hise in 

\ teenth Annual Report of the United States Geological Surve; 

to 872 ; also in the Journal of Geology, Vol. IV, 1806, pp. 1! 

I and 313 to 353. 



No. 125. Faulted Pebble of Cretaceous Conglomi 

(From Siskiyou Mountain, California. Describbd by Bailey A 

There is movement in the solid rock masses of the earth 
quakes are a violent but temporary effect of forces which aeto 
to modify at least the surface of the globe. The nature of th< 



Plate LII, Fig. 1, Thirteenth Ann. Kept., U. S. Geol. Survey. 



U. &. QCOLOGtCAL SURVEY 



BULLETIN NO. 1B0 PL. XU 




,1 
J 



— . •- .-<»*^ 







£// ^' '' -■/ ■ '''/ f ' ■ 



\^'-rM.^,.S^\., \ -V. -... ■ ■ .Vv.;a...v-...\ \ . \^Vs\VC^ 



SYNCLINE AND ANTICLINE. 
A. Syncline. B. Anticline 



( 



DESCRIPTIONS: NO. 126, PHYLLITE. 317 

ely understood. They originate in gravity, in the eartb's 
at, and in physical and chemical reactions at temperatures 
res which far within the earth greatly exceed human experi- 

forces are due the differences of elevation of sea bottoms 
lents; they raise mountain ranges, from which the atmos- 
uts, heat, frost, wind and rain, carve individual mountains; 
active now, and have been active throughout all the earth's 

3rces act upon rock masses whose size may be measured by 
iS, and whose weight may be expressed in millions of tons, 
r of these forces is beyond comprehension. 
t such forces the resistance of firm rocks is but as the plas- 
.Tax. In the movements within the earth's mass rocks break, 
even flow. Whether they break, or bend, or flow depends 
amount of superincumbent load under which they are forced 
3 form. The softening effect of heat is for rocks of the outer 
gniflcant as compared with the influence of pressure, 
jr any horizontal layer of the earth's crust. It rests with all 
t on the next layer below. The weight of the two layers is 
a third below them ; and so on into the depths. The pressure 
lis weight of the rocks increases rapidlj- downward. At 5 
>w the surface it is sufficient to crush most rocks; at 10 miles 
5 surface the pressure of load exceeds the strength of the 
)cks at the surface. 

asses which are lightly loaded ascompared with their strength, 
[xiks near the surface, break when forced to move. Rock 
bich are more heavily loaded as compared with their strength, 
>cks lying deeper in the earth, bend when obliged to change 
>ck masses which are loaded in excess of their strength, that 
it depths of 6 to 10 miles below the surface, flow from regions 
lelming force to regions of less resistance, 
icimen of a faulted pebble, No. 125, from a Cretaceous con- 
5 in northern California, illustrates the breaking of rock 
The pebble is composed of the very hard and brittle mineral, 
It has not been cracked by a blow, it was not suddenly 
, but it has been broken across by pressure. Afterwards the 
I parts have been cemented by the deposition of silica from 
Many pebbles over a large area in this particular conglom- 
t thus broken, showing that the effect of the force was dis- 
throaghout the mass of the rock. 

No. 126. Phyllite (Sericitk-schist). 

M LadiksburG; Frederick County, Maryland. Described by 

W. S. Bay LEY.) 

estern portion of the Piedmont Plateau in Maryland is under- 
ocks that are less thoroughly crystalline than are those of the 



318 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [b 



< i eastern portion.^ While they have been subjected to a certain ai 

I' of metamorphism and alteration, they still plainly show that tbe; 

once sediments of an ordinary type, such as limestones, shale 
The line of demarcation between these rocks and the more crys 
schists to the east is sharp. Where occasionally the less crysi 
rocks are infolded with the more crystalline ones, the former app 
be much younger than the latter. Moreover, they are interbed 
places with a lower Silurian limestone, and are always much less 
and distorted than are the eastern schists. The eastern schis 
regarded by Williams^ as Algonkian in age, and the western oi 
Cambrian and Lower Silurian. 

The principal rock of the western area is a phyllite^ of which 8 
types occur. That represented by the specimen is the most chai 
istic. All the phyllites were originally argillaceous sediments, 
their deposition they have been subjected to dynamic and ch 
metamorphism. 

Dr. Williams describes these phyllites as constituted princip 
a silky white mica (kaolin or sericite), whose individual scale 
greatly in size in diiierent specimens. This is sometimes wholl; 
part replaced by chlorite, forming a chlorite-schist. Quartz gr: 
varying size and outline are generally present, while felds 
extremely rare. It is probably the alteration of this mineral 
process of weathering that has yielded the micaceous component, 
oxide is also present in very small grains. Tourmaline in small ci 
is very common, as are also microscopic needles of ru tile (thonsc 
nadeln). The phyllites have always a perfect cleavage and a 
luster which increases with the crystallization of new mica. The! 
ranges from black through every shade of purple, blue, and gre< 
pale gray. The darker varieties are largely worked as roofing i 
Evidences of clastic structure are not infrequently preserved 
shape of rounded grains and small pebbles of varying api>earaD 
composition. Where least disturbed these slates are jointed and 
cross seams of chlorite or quartz. When they are more disturbe 
become greatly puckered and filled with eyes of quartz.^ 

The specimen in the collection represents one of the lightest c 
of the phyllites. It came from near the town of Ladiesburg, in 
erick County, on the western side of the area occupied by these 
near the contact of the phyllites with the overlying Newark 
stones.^ The rock is so closely folded and so perfectly cleaved t 



»Cf. cleHcrii»tion of No. 119. 

^G. H. Williams, Guide to Baltimore; prepared for Am. Inst. Min. Engiueers, Baltiiu* 
p. 80-87. 

'G. H. Williams, Tlie petro^rraphy and structure of the Piedmont Plateau in MaryUto 
Geol. Soc. America, vol. 2, 189], pp. 305-307. 

<The value of the slate quarried from these rocks in the State of Maryland amounted t«l 
the year 1894. 

*0p. cit., p. 306. 

«See Preliminary Geological Map of Maryland, G. H. Williams, editor, 1893: Mary 
Resources, IndustricM. and Institutions ; prepared fortlie Board of World's Fair ^iansfpen 
laud, Baltimore, 1893. Ladiesburg is at about latitude C9^ 35' aiid longitude Ti^ 15'. 



vnLO.] DESCRIPTIONS: NO. 126, PHYLLITE. 319 

bedding has been much obscured. From the differences in the geolog- 
ical stractare of the regions in which the phyllites and the mica-schists 
(No. 119) occur, it is argued that the former rocks have been squeezed 
in one direction only, by a single earth throe, while the latter have 
been sabjected to different squeezings at different times. 

The specimens are soft, argillaceous, slaty rocks of a light-green 
color when fresh and a pinkish-yellow color when weathered. Along 
tbe cleavage surfaces and along the sides of joint cracks the material 
is yeUow. All specimens are very fissile, their cleavage planes being 
very nnmerons and very close together, and nearly all are crossed 
either by joint planes perpendicular to the cleavage or by little puck- 
erings, which appear in cleavage surfaces as series of tiny waves. 
These are little contortions that serve as evidence that the rock has 
been at some time subjected to great pressure. When breathed upon 
tbe specimens emit the argillaceous odor so characteristic of clays and 
slates. 

When viewed under low i>owers of the microscope the thin sections 
occasioDally reveal the presence of rounded areas that appear to be 
cross sections of sand grains. Between crossed nicols they often break 
^) into matted fibrous aggregates of a strongly reft'acting mineral 
colored by yellow ocher. 

Under high powers in natural light tbe rounded and irregularly 
shaped colorless areas are seen to be embedded in a brownish-yellow or 
light-green cloudy mass, through which are scattered fibers of a light- 
green mineral, tiny grains of a very highly refractive one — probably 
zircon— small plates of red hematite, and little opaque particles of 
magnetite. Between crossed nicols the nature of the various compo- 
nents is not difficult to determine. The most prominent ones are quartz, 
sericite, chlorite, and masses of a finely fibrous substance, believed to 
be kaolin. 

In most sections quartz and chlorite are the most abundant. Some 
of the former is in little rounded or irregular grains. To these as a 
uacleos has been added new quartz material, which often extends in 
tbe form of long, narrow spicules into the surrounding mass of quartz 
sud chlorite. In this the quartz forms a groundmass that is so thickly 
''trewn with chlorite fibers and with opaque grains of diflferent kinds 
that its polarization can scarcely be distinguished. It is evidently a 
s^ndary product which has resulted from the decomposition of some 
ot therock's original components or is a substance that has been intro- 
'Jiced from without. The chlorite, which is a ferruginous variety, is 
Mainly in bunches of light-green fibers, with a very weak double refrac- 
tion. In many instances where the chlorite is thick it appears like an 
^^tropic substance. Occasionally the chlorite is intergrown with a 
"rightly polarizing, light-colored sericite. The two minerals together 
^'cupy areas that resemble the shapes of sharp-edged fragments, 
trough this quartz-chlorite groundmass are scattered large plates of 
*^cite, crystals of zircon, opaque reddish-brown grains of some iron 



320 



THE EDUCATIONAL SERIES OF ROOK SPECIMENS, [bou 



oxide that has been superficially chauged to limonite, small, irregi 
masses of ocher, innumerable particles of various opaque bodies 1 
may collectively be denominated dust, and thousands of very sn 
slender, apparently opaque needles. These are the rutile needles c 
acteristic of slates. They are most abundant in those x>ortion8 of 
slides in which chlorite predominates, being especially numerous aro 
the opaque granules of limonite. 

The yellow tint of most of the specimens is due to the present 
ocher. This occurs not only in the grains and irregular masses aire 
referred to, but it also occurs as a very finely divided pigment wl 
saturates the chlorite fibers and colors them. 

The structure of the rock is that of fine-grained slates which li 
suffered a large amount of alteration and which have been made lis 
by movement under pressure. All of its present constituents are 
ondary, except the larger grains of quartz, which probably reprei 
original sand grains. Its composition (I) as determined by Gee 
Steiger, of the United States Geological Survey, shows ihat how( 
much it may differ from its original condition, it still possesses 
composition of a clay slate. The analysis (II) of a typical clay & 
from near Clausthal ^ in the Har2 is given for comparison. 

Anahf$e8 of phylUte and clay $1ate. 



SiO, 

TiO, 

A1,0, 

FeaOg 

FeO 

MnO 

CaO.. 

BaO 

MgO 

K,0 

Na,0 

Water at 100° 

Water above 100°. 
S 



CO,. 



Total 



T 



Per cent 

57.24 

.06 

23.48 

3.19 

4.87 

none 

.09 

none 

.93 

3.55 

1.18 

.33^ 

4.65/ 

none 



.00 

none 



90.68 



II. 



Per cent 

57.99 

.32 

23.42 

.49 

5.06 



1.65 



1.20 
3.50 
1.32 

3.39 

.17 
.74 



1.12 



100.37 



'J.Hoili: AUgemeiue uiid Cheiiiische Geolo^e, II, p. 588. 



descriptions: no. 127, phyllite. 321 

No. 127. Phyllite (Chlorite-phyllitb). 

BON, GRAFTOX C'Ol'NTY, N«W HaMPSUIRE. DESCRIBED BY W. 8. BaYLEY.) 

len No. 127 is from a characteristic member of the Lisbon 
green schists, which C. H. Hitchcock ^ places in the lower 
jf the Haronian. The gronp embraces greenish schists, con- 
es, quartzites, jaspers, and dolomites. Van Hise^ donbts the 
ity of continuing to call these rocks Hurouian, in view of the 
so many of the New England crystalline schists are being 
be Cambrian or later in age. The term as used by Hit<;hcock 
have been chosen largely on lithological grounds. Whatever 
)f the rocks, they have been squeezed and altered until the 
of their origin has been more or less obscured, 
knd specimen shows a uniformly finegrained, crystalline rock 
yrish-green color. It has a typical though not very distinct 
structure. A study of the hand specimen reveals little 
g its composition. On surfaces at right angles to the 
:;y the rock has the appearance of a fine grained graywacke; 
that are parallel to the schistosity have a more or less silky 
lich is due to the presence of numerous small flakes of a green 
8 mineral with the appearance of chlorite. Here and there 
>f denser green occur. In these the green mineral is plainly 

thin sections of the rock are examined under a hand lens the 
the rock's schistosity is seen to be the elongation of its con- 
in a common direction. The elongation is, however, not very 
marked, and so the schistosity is not very prominent. 
QStituents of the sections, as seen under the microscope, are 
Eklcite, chlorite, epidote, feldspar, magnetite, pyrite, and hema- 
le order of their abundance, and in a few sections, in addition, 
iotite. 

artz is in irregular grains aggregated in long, lenticular areas, 
iry small grains that, together with a little feldspar, make up 
ix by which all the other components are surrounded. The 
ns like areas resemble large, flattened sand grains that have 
eezed until they were shattered internally, and the fine grains 
D be portions of the finer matrix of a sandstone in which the 
ains lay. Under high powers quartz is also seen forming a 
g mass uniting the small quartz grains and all the other com- 
3f the matrix. This form of the mineral may be a secondary 
resulting from the decomposition of some original constituent 



choock. Geology of the Connecticat Valley DiRtrict: (ieol. of Nt^w llamiwhire, Vol. II, 
p. 277-283. CoDcord, 1877. 
Viin HiHc, C«rrelatinn Pai>eni— Aroheaii aiid Algonkiiiii: Bull. U. S. iUnA. Siirvi'v No. 

ill. 150 21 



i 



Ill 



1 
I 

hi 
'f 



322 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. (Bt 

ef the rock. All of the quartz, except the larger grains, ooi 
numerous inclusions of chlorite and epidote. 

The calcite occurs in two forms. It is present as large, irre 
grains, including within its mass grains of all the other constiti 
and as the filling of little veins and nests that were at once tiny 
ices and cavities in the rock mass. A third form exists in soiix 
tions, where it appears to replace an original component whose u 
can not be determined. The calcite is easily recognized by it^j s 
white color in certain positions between crossed nicols, by the Ne\ 
rings of color around the edges of many of its grains, by the two 
of cleavage lines present in most grains, and by the parallel twi 
bars in the larger ones. The mineral is probably in all cases secoii 

Chlorite is present as flakes of a light green color and with a 
I)leoc*hroisin. Its double refraction is so weak that many flakes h 
almost like isotropic substances. In a few instances portions of 
of the plates have a strong pleochroism in green and brown 
These portions resemble biotite in some of their characteristici 
they suggest the thought that some of the chlorite is an altei 
product of this mineral. 

The epidote is in small, prismatic crystals, small, rounded grain 
larger, irregular masses that appear to be aggregates of g 
imbedded in the quartz and chlorite. The color of the mineral is 
greenish yellow, tlie tint being deeper in the large masses than i 
smaller grains and crystals. Pleochroism is noticeable in all ht 
smallest particles. It is always slight, the mineral appearing i 
ferent tints of the same color in difi*erent directions. The epid 
easily distinguished from the other components by its color, it^ 
refractive index, and its strong double refraction, producing I 
interference colors. Although the mineral is scattered every 
throughout the rock, it is accumulated more thickly in some areas 
in others. Together with chlorite and quartz, it forms aggrf 
whose outlines suggest that they once belonged to sand gi^ains 
have been replaced by the aggregate. 

Feldspar is rare. It is present as small, irregular grains seal 
among the quartz. In all cases it is altered, and in most cases so 
so that the traces of its twinning bars are very obscure. Like the 
components, it is filled with inclusions of epidote and chlorite, a 
addition it contains included grains of quartz. As a rule, the 
altered the feldspar, the more abundant the inclusions; hence it ^ 
appear probable that this mineral has in many cases given risel 
alteration to the aggregates of chlorite, quartz, and epidote menl 
above as possessing the outlines of sand grains. Whether all ( 
epidote, chlorite, and secondary quartz has resulted in this mam 
not is unknown, but it is probable that much of it was thus form 

Biotite is rare. It occurs in onl}' a few sections, where it appei 
large, reddish-brown flakes and plates, with a distinct cleavage,! 
strong pleochroism in reddish-brown and yellow tints. 






WLUB-] NO. 128, METAMORPHIC CONGLOMERATE. 323 

The magnetite and pyrite are both opaque. The former is in very 
irrefi^alar masses with a blue black luster, and the latter usually in well- 
defined crystals with an octahedral habit and a brassy yellow luster. 
The magnetite is more abundant than the pyrite. In some sections it 
is very plentiful. Like many of the other constituents, by its very 
irre^lar outlines it presents the appearance of a secondary substance. 

Hematite is only occasionally met with. It occurs as small blood- 
red plates included in quartz, chlorite, and feldspar, more particularly 
around the borders of magnetite grains. 

From the general appearance and the composition of the rock, it is 
apparent that it is a product of the alteration of some preexisting 
rock of an entirely different character. At present it is completely 
crystalline. The structure, however, suggests that it was originally 
fragmental. The shattered quartz grains and some of the feldspars 
may represent original sand grains, but all the other components are 
probably secondary. The rock as it now exists is a chloritic phyllite 
(Gf. No. 126) or a chlorite-schist. It possesses hardly sutiicient chlorite 
to be typical of the chlorite schists, consequently it seems preferable 
to call it a cblorite-phyllite. Were its grains fine and its cleavage sur- 
feces even, it would be a chloritic slate. 

No. 128. METAMORPHIC CONGLOMERATE. 

(Fkom H008AC Mountain (Tunnel), Massachusetts. Dksc^ribed hy J. E. 

Wolff.) 

In the hand specimen the rock varies in character according to the 
ilistinctness of the pebbles. These are of three kinds: First, quartz, 
either white or of a beautiful opaline blue; second, feldspar; third, 
a fine-grained quartz feldspar rock, either granite or gneiss. These 
pebbles are separated from each other by the cement, a crystalline 
aggregate of mica, quartz, and little glistening glassy feldspars (albite). 
The cement winds about the pebbles, sometimes cutting across them 
along little fault lines; at other times, little tongues of the cement, 
parallel to the general foliation or structure, cuts off thin strips from 
fte pebbles, giving them a sharpened, ragged look. In this case the 
cement appears to have followed planes of break and slipping. Thus 
a quite banded phase of the rock is x)roduced, iu which the dark mica 
bands, by their obliquity to each other, may simulate cross-bedding. 
However, in the large blocks at the dumps, bands filled with pebbles 
(original conglomerate bands) are plainly to be seen alternating with 
tiue-grained bands (original sandstone bands) in which little fragments 
ot the same blue quartz found in the larger pebbles can still be 
recognized. 

In the thin sections the quartz pebbles are distinguished by the 
*ame properties as those of the original quartz masses of the granitoid 
gneiss from which the former were evidently in part derived. The core 
of blue quartz, which can be recognized by its iridescence on holding 



324 THE EDUCATIONAL BEHIES OF ROCK SPECIMENS. LbollU 

the section ap to the light, is stained and cracked, passing at the edg< 
into a mosaic of fine quartz, evidently separated from the parent mai 
by pressure and motion. As soon as we pass from the core to thefie[i 
rate grains we find that the blue color disappears, so that we mi^j 
imagine this color was due to some peculiar effect on the light produce 
by the strained but not granulated quartz. This granulated quan 
becomes gradually mingled with mica and metamor)>hic feldspar, ao* 
so merges into the cement. 

The feldspar pebbles are also generally much strained and invaded 
by secondary quartz, mica, e})idote, etc., producing sometimes the 
appearance of an aggregate of individual grains of feldspar separated 
from each other by grains of quartz, but sucb aggregates are really a 
sort of feldspar breccia belonging originally to one piece, as is evident 
by the fact that the cleavage cracks run about parallel through all the 
grains which have been but slightly moved. The feldspar of the 
pebbles is niicrocline, orthoclase, or plagioclase. It is often difficalt to 
distinguish such aggregates, or those formed by the metamorphic feld- 
spar of the cement, from pebbles of original quartz-feldspar rocL 
The latter contain the feldspar and quartz in grains of more nnifora 
size, with sometimes a tendency to imperfect crystal form on the part 
of the feldspars (a characteristic of eruptive granites) ; moreover, the 
feldspar may be of several kinds in the same pebble, in different crys- 
tallographic positions, and free from the inclusions which abound in 
the metamorphic feldspar. 

The cement is composed of occasional small pieces of clastic qnartK 
and feldspar, but principally of metamorphic mica, feldspar, quartz, 
etc. The mica, both muscovite and biotite, and sometimes green chlo- 
rite, occurs in plates, sometimes in clumps or stringers. The musco- 
vite and biotite are often intergrown parallel to the base. The quarts 
is in isolated grains or interlocking aggregates. The metamorphic 
feldspar is in grains, often elongated, but without crystal outline; the 
grains are either simple, or in simple twins (albite law). • These feld- 
spars are similar to those developed on a larger scale in the albite 
schist, and are probably albite.^ They contain as inclusions grains of 
quartz, round fiakes of biotite and muscovite, dark grains of magnetite^ 
reddish-brown hexagonal plates of ilmenite ( f ), fluid inclusions, and 
dark aggregates of iron ore and graphite (f), among which occasional 
prisms of rutile may be recognized. Grains of calcite are fouod 
among the biotite or in the feldspar, and occasional large prisms of 
zircon, with high double refraction and positive character. Aggre- 
gates of dirty-white titanite grains are found near masses of black 
iron ore. Some of the little grains of microcline found in thecemCDt 
are probably formed in place, i. e., metamorphic. The contemporanc 
ous crystallization of these minerals of the cement is shown by their 

'Compare The nietamorphism of olastio feldspar in rongloinerate tichiat, by J. K. Wolff; ^''"" 
MiiR. Coiup. Zoology Hiirvanl Coll., Vol. XVI, No. 10. 



DESCRIPTIONS: NO. 129, ALBITE-SCHIST. 326 

sflociation, and the honeycombiiig of the feldspckifs by the 
ca, etc. 

Iier information concerning this rock, the student is referred 
Eiph XXIII, United States Geological Survey, pages 49-d9. 

No. 129. Albite-schist. 

HoosAC Mountain (Tunnel), Massachusetts. Described by 

J. E. Wolff.) 

hud specimen the rock has a perfect schistose structure, with 
n continuous layers separated by flat lenticular masses of 
lie the crystals of albite are scattered irregularly through 
There is a dark, almost metallic luster on the foliation plane, 
greenish color of the muscovite and its mixture with biotite 

tals of albite are of about the same size and generally rounded 
alar, their longer dimension not necessarily parallel to the 
of the rock. Sometimes the basal cleavage sections (P) are 
bounded by crystallographic planes corresponding to the 
ivage (M) and the prisms T and L 

iwing analysis of this feldspar was made by B. B. Biggs for 
I XXIII, before cited: 

U of feldspar of albUC'SchUt from Hooaac Mountain, MaasachusetU. 



Percent. 



SiO, 09.69 

AitOj ; 18. eo 

CaO ' trace 

MgO .20 

NaO 10.28 

K,0 .40 

Ignition .42 



[ 



Total 99.69 

CO, (cotnbnstion) 0.77 = 0.44 C. (graphitic ma- 
terial). 



imon to find the basal cleavage of a crystal reflecting the 

o parts, and this is due to the fact that the crystal is a simple 

lei to (M) — albite law. 

ravage pieces, showing a simple twin, give an extinction 4^ 

the twinning plane and second cleavage (M). Twins meas- 

e goniometer give angles of 172o 46' to 172^ 50' between the 

^ages of the two twins. The chemical and physical proper- 

erefore those of albite. 

lin section the albite grains are recognized by their large 

e development of cleavage when the slide is thin, and by 

teristic inclusions. The latter consist of quartz in droplets 



326 THE EDUCATIONAL SERIES OF UOCK SPECIMENS. [bullUO. 

or lenticalar grains, mascovite or biotite iu roand or long flakes, crys- 
tals of zircon, round grains of apatite, magnetite in cn^rstals or masses, 
rhombs of carbonate (calcite), grains of titanite, fluid inclusions, and 
irregular black masses or specks of iron oxides and graphitic sab- 
stances, among which little six-sided brown plates of ilmenite (t) can 
be observed with a high power. These inclusions may be few, or so 
abundant as to honeycomb the crystal, and are characteristic of many 
pseudoporphyritic crystals of the schists in general, such as ottrelite, 
staurolite, mica, etc., differing from true porphyritic crystals (pbeiK)- 
crysts) of eruptive rocks by the fact that they are of contemiwraneoas 
or even later origin than the constituents of the rock which lie ontside 
them, but which they inclose so abundantly. 

It is a peculiarity of the dynamic meta'iiorphism of both sediments 
and eruptive rocks that the new feldspar is sometimes a pure albite 
or a soda-lime feldspar near to albite in the series, and that the habitus 
is unlike that of similar feldspars in eruptive rocks, inasmuch as the 
crystal form is imperfect and the crystals uutwinned, even under the 
microscope, or in simple twins, unlike the multiple-twinned albite of 
eruptive rocks. An exception should, perhaps, be noted for the alkaline 
eruptive rocks (e. g., theralite). 

The mica, muscovite, biotite, and chlorite occurs outside the albite 
in thick plates or irregular aggregates of plates, or clumps. The mus- 
covite and biotite may be intergrown parallel to the base. The quartz 
grains are generally in the interlocking aggregates, which are crossed 
by fluid inclusions arranged in lines (really in planes). The quartz is 
otherwise generally free from inclusions near the center of the lenses, 
but at their edges, where individual grains lie in the meshes of the mica, 
they may contain flakes of mica. 

Magnetite, calcite, titanite, apatite, and zircon are found in the masses 
of mica. The clumps of mica and quartz have sometimes a rough con- 
centric arrangement. The muscovite has a distinctly greenish color, 
even in the thin section, and in part the flbrous appesirance due to the 
interweaving of irregular plates of the mineral, which is characteristic 
of the variety called sericite. 

Neither in the hand si)ecimen nor in the thin sections of the schist 
has any clastic material been observed, although such a derivation i* 
certain both from the field relations and from the rocks of this class 
observed elsewhere in this field which are transitional to clastic rocks. 
Neither can we state from what source were derived the elements which 
produced the large amount of albite (soda-alumina silicate) in this and 
in (probably) the two other rocks. The granitoid gneiss may have been 
originally an eruptive granite;^ the metamorphic conglomerate ^*'* 
originally a mass of granitic debris at the base, becoming a feldspathK* 
sandstone or arkose iu succeeding stages until succeeded by the schists 



iltH similarity should be uuted to tb& coarse eruptive graoite caUed "Bapaldwi," which M^**^ 
extensive areas in Finland. 



mutt] DESCRIPTIONS: NO. 130, MICA-SCHIST. 32 1 

which most have been a abale or slate, perhaps at the base an impure 
limestone. Metamorpbism has given all three rocks certain characters 
in comaion (regardless of diversity in origin and composition), such as 
achistosity, albitic feldspars, mica, etc., while the differences seem dne 
to the remnants of the original material or the influence this exerted 
on the process of the new crystallization. 

No. 130. MiGA-SCHIST (TOUBMALINE-BIOTITE-SCHIST). 
(From Black Hills, South Dakota. Described by W. S. Baylby.) 

This specimen is particularly interesting because it has every chai- 
acteristic of a typical mica-schist, while at the same time its mode 
of origin is well understood. The rock of which it is a sample occurs 
in the Black Hills, South Dakota. In the southern part of the district 
is ft large core of pre-Cambrian eruptive granite, forming Harney 
Peak.^ Immediately surrounding this on all sides are crystalline 
schists, striking everywhere parallel to the boundary between the gran- 
ite and themselves and dipping away from the eruptive mass. Close 
to the granite the schists are completely crystalline; at some little dis- 
tance from it they are less so. A few miles away from the contact 
they become very much less crystalline, and possess, in addition to 
tbeir foliation parallel to the granite boundary, a slaty cleavage which 
18 parallel to the cleavage of slates, existing at a greater distance from 
the granite mass, and into which the schists pass gradually. Since no 
nnoonformity exists between the schists and the slates, and since the 
microscopical examination of both shows that the mineralogical grada- 
tloQ between them is as complete as the gradation noticed in the Held, 
it is concluded that the schists are derived from the slates, and conse- 
<|oently belong to the same geologic horizon as do these, which are 
Algonkian, and probably Huronian.^ 

The position of the schists with respect to the granitic, their more 
complete crystallization as this is approached, and their passa<]:e into 
^e fragmental slates at a distance from the eruptive are all indica- 
tions that not only is their crystallization due to the proximity of the 
^uite, but their schistosity as well, since this structure disappears as 
tlie distance from the intrusive increases. 

The action by which an eruptive mass produces crystallization in 
clastic rocks through which it breaks is known as contact action, and 
tbe change effected in the fragmentals is known as contact-metaniorph- 
i»m. This kind of metamorpbism, while it may obliterate the proofs 
^Uhe original fragmental nature of the material acted upon, never of 
*Wf produces in the resulting rock a schistose structure. This is 
'^^aysa result of movement of the rock particles, a consequence, in 
^^Q, of mechanical action, hence known as dynamical metamorpbism. 



^ ipecimcn came from a point north of Harney Peak, about 1 or 1 J miles west of the Etta Mine, 
^or farther information concerning the rolations existing between the Blatoa and schists, see 0. B. 
^>i> Hiae, BaU. OeoL Soc. America, vol. I, p. 203. 



328 THE EDUCATIONAL SERIES OP ROCK SPECIMENS, (bcilmo 

The force producing the schistosity in the etise under discussion wm 
that generated by the intrusion of the granitic mass, as is plainly shown 
by the strike of the foliation of the schists and its dip sway from the 
core of granite. The mica-schist represented by the specuuen may 
therefore be regarded as a dynamically metamorphosed contact-rock, 
whose original character was fragmental. Its present condition is doe 
entirely to the recrystallizatiou of its originally clastic grains through 
processes carefully described by Professor Tan Hise in the article 
referred to in the footnote. It is now a typical crystalline schist 

It may be described macroscopically as a light gray, finegrained roct 
with a well-marked schistose structure which is best seen on thetrc 
long sides of the specimen. Here thin parallel bands of light and dari 
colors alternate. The grains comprising them are so small that tli< 
nature of the minerals forming the lighter bands can not be determined 
without the aid of the microscope. The dark color of the other band 
is plainly seen to be due to what look like flue black needles. On th< 
upper and lower flat surfaces of the specimen the needles are obBerve< 
to have more the appearance of narrow plates, with a brilliant lastei 
With a sharp knife thin flakes may be sprung from them. The miners 
therefore is very easily cleavable, and may be regarded presumably a 
mica. The only other features of the hand specimen worthy of notic 
are the little black flakes or bunches that dot the planes of cleavag 
and the small rusty spots scattered throughout the rock. The Iatt€ 
are garnets, and the former are accumulations of mica flakes, as ma 
be determined under a hand lens. They resemble very strongly tta 
"knoten" described by Kosenbusch ^ and other German jietrographei 
as characteristic of sedimentary rocks that have been made crystallic 
by the intrusion of eruptives through them. On some specimens thei 
are also little circular areas with diameters of about 6"*"', glisteniu 
brilliantly from many tiny facets. These consist of aggregates of 
light-colored mica, probably mnscovite. The rock is quite soft, and 
apparently much decomposed. The softness is due to the sligl 
amount of coherence existing between the component grains, in cons 
quence of which they fall apart very easily when rubbed. 

The microscopic investigation shows the rock to be remarkably Ires 
Its specific gravity is 2.806. tTnder the microscope the predomina 
light-colored mineral is discovered to be quartz, while some of t1 
black plates and needles are biotite and others are tourmaline. Game 
and mnscovite are also observed, but in very small quantity. Tl 
schistose structure is the result of the arrangement of the constitiiec 
with their longer axes approximately in the same plane, which is t> 
plane of schistosity. (See PI. XLII.) 

The lenticular forms of the light-colored quartz grains are so api^ 
ent in the photograph that they need merely be refeired to. Betw^ 
these grains is a matrix composed principally of large flakes of bio^ 

> RosenbuBch: Mikroskopische Physiographie der MasBigen Ge»teine, Stnttj^art, 18S7, pw4T > 



B, meaBuriDg from 0.15 to 0.35°'"' in length and about 0.1 to 
in widtb. It holds as inclusioos many little liiiuid-lilled cavities, 
ne filled with glass and devitriUcation products. Tiny specks 
uetite and other iron componuds and small crystals of apatite, 
I many toormaline crystals, are also inclosed by it. 
iark coustitaents, aa has been stated, are biotite and tourmalioe. 
ions cnt parallel to the plane of schistosity the biotite appears 
folarly outlined plates, most of which are basal sections of tbe 
1. These are dark brown in color and show no pteochroism. In 
sections the mica \s cat at some iuclinatiou to tbe vertical axis, 

its pieces are marked by parallel cleavage lines rauuing in the 
>u of tbe long axes of the flakes, which are, in the main, parallel 
long axes of the qnartz lenses. The biotite in these sectioos 
lishes piurallet to the cleavage. It is very dark bro«n, almost 
, when the cleavages and the loDgitudinaJ axes are parallel to 
(ration plane of the nicol, and is light yellow in directions at 
Qglea to this. It includes tourmaline and contains round dark 
lialos) that are more or less pleochroio. 

toarmaline is in small, well-developed crystals of a dark-brown 
>ud with nearly the same pleochroism as the biotite. These 
e 0.2 to O.S'""' in length and Crom 0.05 to 0.1""" in tbickuess. 
re of^n doubly termiDated in such a way as to show clearly 
amimorpbism. They may be distinguished from the biotite by 
i that their absorption is much less in tbe direction of their long 
lau at right angles thereto; consequently, tbey appear dark 
n that position in which the biotite appears light, i. e., when 
)Dg ases afe perpendicular to tbe vibration plane of the lower 

They may also be recognized by their basal parting running 



330 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [vrnjum. 

ress. Around tbe borders of the grains is often a yellowisb-green 
stain, resulting from the decomposition of magnetite or other iron com- 
pounds. It is this that causes the rusty color around the garnets in the 
hand specimen. 

Another yellowish-green substance sometimes stains the constitnents 
over small areas, and in some places forms little lenticular masses 
between the components. This substance possesses no definite strnc 
ture and shows no double refraction, hence its true nature is difficult 
to determine. It is, however, neither essential to the rock nor charac- 
teristic of it, and therefore is of little importance for purposes ofdasfti* 
fication. 

Muscovite flakes are very rare. Occasionally a tiny one may be seen 
lying between the quartz grains when the section is examined nudei 
crossed nicols. In ordinary light they are invisible. When the tliii 
section includes one of the little groups of plates noticed in the ban* 
specimen, the number of the flakes seen naturally becomes quite hrge 
though the area covered by them is limited entirely by the outliue o 
the group. 

Though all the mineral components of the schist have been described 
there remain a few structural features to be noted. The schistosity ha 
already been stated to be the result of the arrangement of the mineral 
with, their long axes ax}proximat«ly parallel, and the difierence in colo 
of alternate bands has been ascribed to the varying proportions o 
biotite and tourmaline in them. In some sections the difierence iu cola 
is found to depend largely upon the accumulation of black iron com 
pounds between the quartz grains in certain bands and its absence troo] 
others, in which case it is probable that there were variations iu the 
composition of successive laminae in the original sedimentary beds. 

Another structural feature of interest is the occurrence here and 
there of lenticular areas in which the quartz grains are larger tbau 
elsewhere in the rock, and the amounts of biotite and tourmaline are 
much less. These areas may represent pebbles that were originally 
scattered through the slates and were afterwards flattened hy the 
pressure that produced the schistosity,^ while at the same time cbemical 
changes set up in them gave rise to the quartz, mica, and tourmaliDe 
that now usurj) their places. 

The biotite, quartz, magnetite, and muscovite in the schist may all be 
looked upon as resulting from the decomposition of theorigiual materials 
of the slate under the influence of contact action. The garnets niaj 
likewise be ascribed to this action. The tourmaline, on the contrary 
contains a large amount of boron — a substance not occurring in an; 
large quantity iu slates. Its formation is probably due to a reactio 
beteen the constituents of the slate and the boron gases given off ^ 
the granite before it finally solidified.^^ 



> Cf. Geol. Soc. of America, Vol. I, pi. 4. 

'For accouut of gaaes exhaled from eruptive masses see Geikie, Text Book of Geology. ^^ 
pp. 180-184. 



Muo) DEbCRIPTIONS: NO. 131, HORNBLENDE-SCHIST. 



331 



The composition of the schist, which should be called a tourmaline- 
biotite-schist on account of the large amount of tourmaline iu it, has 
beenfoand by H. N. Stokes to be as follows: 

Analysis of schist from the Black Hills. 




The boron was not determined, but it is evident that it is present in 
considerable quantity in the rock, since very little of the loss of weight 
upon ignition is due to the escape of water.' 

No. 131. HOENBLENDE-SCHIST. 
(From Manhattan Island, New York. Described by J. P. Iddings.) 

Kather massive crystalline schist, greenish black, with tendency to 
cleave ill parallel plates. It occurs with mica-scliist and gneiss, exposed 
io the northwestern portion of Manliattan Island, in the neighborhood 
of Riverside drive and One hundred and twenty-fifth street, New York 

In thin section it consists of dark-green hornblende, with subordinate 
3&)oaDts of feldspar, quartz, magnetite, biotite, apatite, and a little 
zircon and pyrite, besides some secondary minerals in places — musco- 
^te, epidote, or zoisite. 

The lamination of the rock is produced by a somewhat parallel 
arrangement of the stout crystals of hornblende and by streaks of the 
other coDStituents in smaller grains. It is most pronounced in sections 
<^Qt across the lamination, but also appears iu those cut parallel to the 
«chisto8ity. 

The hornblende is strongly green in thin section, with tinge of brown 
and with pronounced pleochroism, the variation being from green 
through brownish green to light brown. Owing to the fact of easy 
<ileavage of the rock due to the parallel arrangement of the horn- 
blende, thin sections are ihostly i)repared parallel to the i)rismatic 
*xi8 of the crystals; consequently cross sections are scarce. Many 



'ror other deAcriptioDA of mica-schiata see R. D. Irviufr, Geology of Wisconsin, Vol. HI, 1880, pp. 
1«,\48. Plata XV. Fig. 1 and 3; A. A. Jnlien : lb. p. 232. A. wichmann, lb. p. 634. 
'Compgr* J. F. Kemp, The Geology of Manhat ton Island: Trans. New York AcAd.Sci., Vol. VII, 



V^4M5 



332 THE EDUCi.TIONAL SERIES OF ROCK SPECIMENS. [BmM. 

individuals of hornblende exiiibit no cleavage. The extinction angle 
is low. Twinning is seldom observed. The substance of the horn- 
blende is very pure. There are inclosions of occasional roanded 
crystals of zircon surrounded by brown halos, slightly pleochroic, 
considerable magnetite in places, besides feldspar, quartz, and biotite. 

Feldspar occurs in irregular grains without crystallograpbic bound- 
aries, and of much smaller size than the crystals of hornblende. It is 
mostly plagioclase with polysynthetic twinning according to.albite 
and pericline laws. Some of the extinction angles suggest andesioe 
labradorite. The lamellae are in places curved and may have been 
produced by dynamic processes. There is some undulatory extinction 
in places, but not much. Orthoclase may be present in small amounts, 
as there are a few unstriated feldspars. 

Quartz occurs in irregular grains which are sometimes rounded, 
especially when inclosed in hornblende. The substance is very pure, 
with almost no. undulatory extinction. Biotite is brown, with strong 
absorption, and occurs in irregularly shaped pieees, generally inclosed 
in the hornblende. Magnetite occurs in clusters and streaks of irreg- 
ular grains, and is relatively abundant. Apatite forms colorless grains 
or stout crystals with irregular or rounded outline. 

The rock in places is traversed by microscopic veins. When these 
cross crystals of hornblende they consist of aggregations of quartz 
and other minerals; or quartz may exist alone, having the same 
crystallograpbic orientation as that of adjacent grains of quartz; bat 
where the minute vein should appear crossing quartz crystals, it is 
represented only by rows of dots included in otherwise very pare 
quartz. The hornblende sometimes exhibits a slight fibration on botii 
sides of the vein, the direction of the fibers being parallel to that of 
the c axis of the hornblende. 

'So. 132. Schistose Btotite-Gneiss. 

(From Manhattan Island, New York. Described bt J. P. Idddtgs.) 

Schistose, finegrained, dark-colored rock associated with amphibo- 
lite-schist and gneiss at the same locality as that of the hornblende- 
schist on Manhattan Island, near Riverside drive and One huBdred 
and twenty-fifth street, New York City. It consists of biotite, feM 
spar, and quartz, with a small amount of muscovite, apatite, Bod 
zircon. The thin sections are most easily made parallel to tbe schis 
tosity of the rock and the foliae of the mica; hence they seldom sho^ 
cross sections of the mica plates. There is somewhat of a flaser- 
structure, caused by flakes of mica grouped in lines curving about 
larger crystals of feldspar and quartz. 

In thin section the biotite is dark brown, with strong absorption. 
Its form is irregular and seldom exhibits crystal boundaries. ^^ 
includes grains of quartz and feldspar and other minerals, and ^^ 
turn occurs as microscopic crystals included in quartz and feldsp^* 



wuB.] DESCRIPTIONS: NO. 133, STAUKOLTIIC MICA-SCHIST. 333 

Moscovite occurs sparingly as relatively large colorless plates, some- 
times surrounded by biotite. 

Quartz appears to be more abundant than feldspar. Its oatliue is 
quite irregolar, the smaller grains often rounded. It exhibits slight 
andalatory extinction and in places minute rutile needles, and some 
iuclasioDS of the other mineral constitaents of the rock. Fluid inclu- 
sioDS are quite scarce. 

Feldspar forms allotriomorphic grains like t.hose of quartz, but often 
attains larger dimensions. It is mostly plagioclase, with polysyuthetic 
twinning, in places curved, accompanied by undulatory extinction . The 
extiDctioD angles are low, indicating oligoclase. Orthoolase appears 
to be present in smaller amounts. No microcline was noted^ 

Apatite forms comxmratively large, irregular grains, colorless and 
quite pnre. Some grains are surrounded by a shell of brown, doubly 
Infracting mineral. A similar mineral, with paler color and low double 
lefiaction, occurs in spots through the rock. Its character has not 
been determined. Zircon occurs in rounded microscopic crystals. 
Magnetite is almost entirely absent. 

As the proportion of feldspar grows less the rock approaches a 
gaeissic mica-schist. 

No. 133. Staubolitic Miga-sghist. 

(^'«)M ChaRI^SSTOWN, SULU VAN COUNTY, NEW HAMPSHIRE. DESCRIBED BY W. S. 

Bayley.) 

The rock stratum from which specimen No. 133 was obtained consti- 

tates one of the members of 0. H. Hitchcock's Coos group,^ which 

<^iophses (1) a quartzit<e, (2) a staurolitic mica-schist, and (3) a stauro- 

Me and garnetiferous clay slate. In the Connecticut Valley the 

'niea-schists gradually lose their micaceous character and become 

argiUitic; hence there can be no question that the schists are rocks of 

^imentary origin. In the New Hampshire repoi'ts the " Coos group" 

i^ included in the Paleozoic system. In a later paper^ it is more defi- 

^tely assigned to some horizon above the Cambrian, possibly the 

I^evoniaD. 

As represented in the hand specimen, the rock is dark gray in color 
*nd crystalline in texture. In structure it is typically schistose, but 
the schistosity is not developed in parallel directions. In all specimens 
it is more or less contorted in short waves, which show themselves on 
<^lea?age surfaces as a series of approximately parallel crumplings. 
The luster of these surfaces is silky, suggesting the presence in the 
'XKik of some micaceous mineral with it« broiider faces parallel to 
tbe deavage surfaces. On cross fractures the specimens are charac- 
terized by the presence of rods of a very glistening, easily cleavablc 
iniueral in a dull, ligh^gray matrix, resembling in appearance a very 

• U. H. Hitchcock : <;eoloKy of New Haiiiptbire, Vol. 11. Chap. IV, pp. 316-32:1. 
» C. H. Hitchcock : Jour. Geol., Vol. IV, 1896, p, 59. 



1 



336 THE EDUCATIONAL 6EBIE8 OF ROCK SPECIMENS. [bi 



chlorite, a rod or a rounded i^ain of magnetite, a ragged grain of iln 
or of titaniferouB magnetite, a minute crystal of brownish-yellow 1 
and occasionally a small plate of greenish-brown tourmaline, 
tourmaline is recognized by its pleochroism and its absorption 
the titaiiiferous magnetite by its gray decomposition products, ai 
rutile by its color, its lack of distinct pleochroism and its high n 
ive index. Between crossed nicols the colorless or very light 
component of this matrix is the platy variety of muscovite kno 
n sericite. It is identified by its lack of color, its lack of pleocb 

its brilliant polarization colors, aud by the fact that it^ extinct 
parallel to its cleavage. The sericite occurs in long, acicular ai 
umnar crystals or scales, often grouped in little bundles. Be 
them are colorless grains of quartz, usually elongated in a dir 
parallel to the long axes of the sericite individuals. Ko pressure ( 
are noted in the quartz grains. Their elongation appears to 
original condition. Both the quartz and the sericite have the as] 
minerals that have crystallized in situ, and the entire rock see 
have undergone a complete recrystallization from its original st 
a sediment. 

The structure of the matrix is plainly schistose, the schistosity 
due to the elougation of the quartz grains, and the arrangement 
sericite flakes parallel to this elongation. The foliation, howe' 
not evenly parallel, as it is in specimens of some of the other rocks 
collection, but is contorted, little folds being crowded closely toi 
in some portions of the sections and opening out into very gently c 
folds in others. The quartz grains on the limbs of the folds are « 
much more elongated than those in their crests and along theii 
thus causing these microscopic folds in the thinning of their si 
resemble the larger folds noted in disturbed rock strata. 

Nothing in the microscopic structure of this rock suggests its oi 
fragmental character. As at present constituted, it is a thorc 
crystalline rock. No indications of a fragmental grain may be de 
in it. Since, however, it has been traced into slates, there can 
question as to its derivation from a sediment composed of the 
constituents as those composing an ordinary shale. The rock is i 
cal crystalline schist that has been derived from a sedimentari 
mental roc^k. In comi>osition it is a mica-schist containing a 
staurolite and garnet crystals. 
,1 Tbe rock's foliation is unquestionably the result of pressure 

ij elougation of the quartz grains and the bending of the mica 

'I point to this origin. There is, however, no fracturing of grains J 

[t distortion in their optical properties, as in the case of schists d 

r| by dynamic nietamorphism from igneous rocks (see No. 140). 

J' foliation of the rock under discussion was not produced bv the t 

'I ing of grains of minerals already existing in it, nor by their re 

• into positions of least resistance. It was caused by crystalliza 



I! 



HUH] DESCRIPTIONS: NO. 134, HORNPELS. 337 

its components while under pressure, attended by motion in the plastic 
crystallizing mass. 

Tho rock affords a good example of a schist formed from a fragmental 
rock through metasomatic agencies accompanying dynamic action. In 
dynamically formed schists like No. 140 the present constituents are 
practically the same as those existing in the original rocks; in meta- 
somatic schists the present components have been formed de uovo.^ 

No. 134. HORNFELS. 

(From Gexesek Vaixey, Plumas County, California. Described by H. W. 

Turner. ) 

By contact metamorphism is meant those alterations in a rock mass 
which take place along the border of an intrusive mass in cousequence 
of the beat and mineralizing solutions whose origin is due to the intru- 
sive igneous rock. 

lu the Educational Series collection this phenomenon is illustrated 
by two specimens from California — one a hornfels from Plumas County 
and the other a chiastoliteschist from Mariposa County. 

At both these localities areas of granitoid rocks are in contact with 
sedimentary masses, and along the border of the granite there is a 
zone of metamorphic rock which grades almost imperceptibly into less 
altered sediments at about the same distance from the granite contact 
at all points. 

In the metamorphic zone new minerals have been formed, chietly by 
therecrystallization of the material of the sediments. This phenome- 
non has been noted in many parts of the world, and careful investiga- 
tions have shown beyond all reasonable doubt that the formation of 
these minerals took place after the intrusion of the igneous magma 
and as a result of it. 

The investigations of Hawes^ have, indeed, made certain that in one 
instance there has been an addition of boric acid to the schists from 
the mineralizing solutions, resulting in the formation of abundant 
tourmaline in the zone nearest the granite. Rosenbusch,^ in a very 
thoroagh investigation of contact phenomena, however, found that, at 
the locahty which he studied, the new minerals formed were almost 
wholly due to the recry stall ization of the material of the sediments 
^hich had been metamorphosed. 

Hornfels is the term applied by Eosenbusch to contact metamorphic 
focks which are massive and not schistose. Such rocks usually break 
*ith a conchoidal fracture, especially when line grained. Often, as in 
tbe specimens in the Educational Series collection, the hornfels is a 



'Hradi»ctiftf»ion of the prorcsa«8 concerned in the metaiiiorphisDi of sefUmeutary rockn see C H. 

*» Hiae. The principles of North American pre-Canibrian geology : Sixteenth Ann. Kept. U. S. Geol. 
''''^«J',pp.68<>-694, 705. For u discnsflion of nu-tamorphiHtu iu general see Harkor'8 Petrology for 
^todent*. University Preiw, Cambridge, 1895, jip. 257-290. 

'Aw. Jour. Sci., 3d series, 1881. Vol. XXT, p. 21. 

'iHi'Steigersohiefer. Stra»8hurg, 1S77. 

Bull. 150 22 



338 THE EDUCATIONAL SERIES OF KOCK SPECIMENS. [blll.im. 

flue-^rained flinty rock, bat the name may also be applied to coarser 
varieties which were originally argillaceous sandstones. The horn- 
fels zone from which specimen No. 134 was obtained varies in width, 
according to Mr. Diller, who collected the specimens, from 160 to 1,300 
feet. This great variation in width is thought by Mr. Diller to be due 
to the fact that the contact of the granite and the sedimentary mass 
is probably not a vertical one; that is to say, he supposes the granite 
to pitch in under the hornfels, so that when the zone is wide the dis- 
tance of the metamorphic rock from the underlying granite surface 
may not in reality be any greater than when the zone is narrow. The 
igneous rock which has caused the original shale to be metamorphosed 
into a hornfels has been called above a "granite." In reality it should 
be called a quartz-gabbro or a quartz-pyroxene-diorite, for it contains 
little or no alkali feldspar. The intrusive rock is a gray, medium to 
rather coarse-grained granitoid rock, of granitic or hypidiomorphic 
structure, and is composed of plagioclase, hornblende, monoclinic 
pyroxene, quartz, iron oxide, biotite, and hyx)ersthene, the relative 
abundance of the components being roughly indicated by the order in 
which they are named, plagioclase being the most abundant. Apatite 
is present in some specimens, and in one thin section no pyroxene was 
noted. The plagioclase is chiefly labradorite, and if the kind of feld- 
spar be taken as the basis of classification, following Brogger and 
others, then the above rock must be called a quartz-gabbro. 

Microscopically the rock is dark, very fine- and even-grained, with 
couchoidal fracture. None of the individual constituents are recogniz- 
able with the unaided eye. 

Microscopically the rock presents, without the analyzer, a minutely 
mottled appearance, due to the presence of very abundant spots, lighter 
in color than the intervening spaces. These spots are often close 
together. With the analyzer they are seen to be made up of aggregates 
of translucent grains, with low, gray, interference colors. The grains 
of these aggregates often extinguish together; that is to say, they 
show aggregate X)olarization, indicating a like optical orientation of all 
of the grains of any one aggregate. No interference figure could be 
obtained in convergent light. There are very minute polarizing fibers 
scattered through these aggregates, which are presumably biotite. 
The aggregates appear to represent the incipient formation of some 
mineral. 

The spaces between the aggregates are dark colored and contain very 
abundant minute foils of a reddish-brown biotite and dull black graius, 
which are probably carbonaceous. 

Scattered through the section, both in the aggregates and in the 
intervening spaces, are clear grains of clastic (!) quartz and large foils 
of biotite, some of them 0,4'" "» long. 



BSSCBIPTIOKS: NO. 135, CHI ASTO LITE-SCHIST. 



339 



ITO. 135. OHlAJSTOLITE-SOmST. 
(FxoM KEAB Mabiposa, Mabiposa Coumtt, Califobxia. Described by H. W. 

TUBXKR.) 

The locality from which specimen No. 135 was obtained is in a zone of 
contact metamorphic rocks at the south end of a long belt of the Mari- 
posa slates which extends from Colfax, in Placer County, to the point 
at which the specimens were collected, a distance, following the curva- 
tui-e of the slate belt, of about 130 miles. To the west of Placerville 
this belt of Mariposa slates is in contact with an area of granite-por- 
phyry and does not seem to be greatly altered, but the contact was 
only cnrsorily examined. Only at its south end is this slate belt in 
contact with any large body of a coarsely granular rock of a granitoid 
nature, and at no other point in its entire extent does it show great 
alteration. The granitoid rock, which has caused the nietamorphism 
of the slates in Yaqui Gulch, extends at least as far south as the San 
Joaquin Eiver, a distance of 35 miles, with an average width of 12 or 
more miles. This granitic rock is not near tlie contact with the schists 
a true granite, but a subsiliceous quartzmica-diorite, as may be seen 
from the analysis in the table below, and indeed this is probably true 
of most of the area. 

A fine-grained granite, No. 372 in the table below, from Raymond, 
near the center of the area, is nearly a normal granite in composition, 
but analyses Nos. 369 and 851, from near the edge of the area, show 
the rock there to be much less siliceous. It has not, however, been 
determined whether or not No. 372 represents the average rock of the 
center of the area, or whether it is not a portion of an older or younger 
granite mass. 

Anal jf 969 of granite from the Raymond ffranitoid nrea. 



Ko. 851, 
Yaqui 
Gulcb. 




, Per cent. 

Silica ' 62.62 

Lime 5.40 

PotasM ; 1.76 

Sodft 3.49 



No. 372, 

Raymond 

Qiiarry. 



Per cent. Per cent. 

58.00 I 73.54 

6.24 ! 2.55 

2.02 I 1.89 

2.94 4.66 



In Yaqni Gulch, near the intrusive rock, the Mariposa schists have 
l^n irregularly displaced and their relation to the quartz-diorite is 
^ot a perfectly simple one, so that the study of the details of the meta- 
nwrpliism is not so satisfactory as it would otherwise be. The rocks 
>i^the granitoid rock (a quartz-diorite) are much obscured by soil. 

The specimen collected nearest the quartz-diorite is a rather medium- 
Sn^ined andalusite-hornfels, and was obtained at a point about 2,500 



340 THE EDUCATIONAL SERIES OF ROCK SPECIMENS, [mu^m. 

feet north of the main diorite contact This hornfels is presnmed to 
have been originally a sandstone. The expression << main diorite cod- 
tact " is used for the reason that a small mass of quartz-diorite, doubt- 
less an apophysis of the main area, occurs near the andalusite-hornfels 
noted above. After crossing the zone of andalusite-hornfels, the exact 
extent of which was not determined, we find in going away (or north) 
from the main diorite mass a cousiderable zone of knotted mica-schists 
and of chiastoliteschist. 

In Yaqui Gulch, about 3,800 feet from the main diorite contact, these 
schists are well exposed in the bed of the stream. The normal strike 
of the main belt of Mariposa slates is about N. 30° W., with a dip of 
from 50O to 90^ to the east. In the vicinity of the diorite these slates 
are much displaced and to some extent contorted. Along that portion 
of Yaqui Gulch, where the specimens were collected, the beds strike 
approximately north and soutb, and dip both east and west at an angle 
of GOO or more. The schists thus lie nearly at right angles to the course 
of the main diorite contact, which is nearly east and west. 

The specimens of chiastolite- schist were not obtained all at one point, 
but at various points along the gulch within a distance of about 1,200 
feet, or in a zone of from 3,800 to 5,000 feet from the main diorite area. 
The chiastoliteschist occurs in layers interbedded with mica-schists 
and knotted schists. Often these layers are but 2 to 6 inches in thick- 
ness. It is evident that their formation depends largely upon the 
original composition of the individual layers. Those containing a large 
percentiige of argillaceous matter appear to develop into chiastolite- 
schists, and the more sandy layers into micaceous schists without 
chiastolite, or with only imperfectly developed crystals. This is to be 
expected from the composition of chiastolite, which is a silicate of 
alumina, the latter being a prominent constituent of all argillaceoos 
rocks. 

Some of the specimens of chiastoliteschist collected in Yaqui Gulch 
contain distinct impressions ot AucelUi erringtonij or one of its closely 
allied varieties described by Professor Hyatt. This fossil is said by 
Professor Hyatt, Professor Smith, of Stanford University, and others, 
to be of Jurassic age. It is thus evident that the granitoid rocks 
which have affected the metamorphism of the clay-slates are of late 
Jurassic or post- Jurassic age. 

More than 5,000 feet away from the diorite the Mariposa beds are 
not greatly metamorphosed. They are no longer schists, but clay- 
slates. However, they still show some effects of the metamorphic 
action of the diorite in the presence of very abundant minute priH«»^ 
the distribution of which appears to be somewhat capricious, as soidc 
layers of the clay-slate show them and some do not, at exactly th® 
same locality. At a distance of about G,500 feet from the main diorit* 
contact the metamorphic action of the igneous mass api)ear8 to hav« 
ceased entirely, but the contact of the diorite and the schists is perbap* 



DOLEij DESCRIPTIONS: NO. 135, CHIASTOLITE-SCHIST. 341 

Dot a vertical one, and the distance of the schists from the underlying 
diorite mass in a vertical or inclined direction may be much less than 
the distances here given. Moreover, it should be stated that these dis- 
tances are all approximate. Some of them were measured by pacing^ 
others only estimated. A contour map has not as yet been made of 
this region, which lies directly south of the Sonora quadrangle. 

The specimen is from Yaqui Gulch, 2 miles southwest of the town of 
ManiK>sa. 

Megascopically, when fresh, it is a hard, blacky tine-grained schis- 
tose rock, with very abundant minute points with a silvery rejection 
and sleoder prisms which are square in cross section. These prisms 
are sometimes an inch in length, but usually shorter. In weathered 
specimens a dark center can be seen in some of the cross sections of 
the prisms. 

Microscopically, when seen without the analyzer, it shows a fine- 
grained, dark groundmass, composed of minute, clear grains, many of 
them rounded; abundant minute, black particles, and reddish brown 
biotite scales, arranged in more or less nearly parallel lines, giving the 
schistose structure to the rock. In this groundmass are long, clear 
prisms which are square in cross section, and minute, clear prisms 
which are nearly of a size, having a width of 0.02™™ and a length of 
aboat 0.2™"*. Many of these lie at an angle to the plane of schistosity, 
suggesting their formation after the rock had been rendered schistose. 
The large prisms with square cross sections are chiastolite, and 
are from 1 to 2™™ in diameter. The dark cross which distinguishes 
chiastolite from audalusite is feebly developed in many of the prisms, 
and consists in lines of minute granules extending from the center to 
the prism edges, bisecting the prism angles. The minute, black parti- 
cles show on some surfaces a metallic luster by reflected light. Some 
of the rock was powdered and washed. A fine, black dust, which col- 
lected on the surface of the water and which presumably represents 
the black particles seen under the microscope, was consumed when 
placed on a platinum spatula in a flame at high temperature. This 
powder is therefore assumed to be carbon, and the metallic luster indi- 
cates that it may be in the form of graphite. The minute, clear prisms 
seen in natural light probably represent the silvery points observed 
inegascopically. They are muscovite. With the analyzer these prisms 
(extinguish parallel to the direction of elongation and show bright inter- 
ference colors. 

The clear cross sections of the chiastolite crystals show the two 
prismatic cleavages intersecting nearly at right angles. In the cross 
tactions the extinction is diagonal, bisecting the prism angles and the 
intersections of the cleavage lines. In longitudinal sections the two 
cleavages are indicated by parallel lines, and the extinction is parallel 
to the cleavage. In favorable light a slight pleochroism may be detected 
in some of the chiastolite prisms, jc being faintly reddish. The outer 



342 



THE EDUCATIONAL J<ERIE8 OF ROCK SPECIMENS. (Bcu.iiio. 



edge of even the freshest chiastolite crystals is altered to a fibrous, 
colorless aggregate, which, according to Bosenbusch, may bea mixtare 
of sericite and kaolin, and occasionally irregular cracks extendiog iuto 
the crystals are filled with this same decomposition product. The til)er8 
of the decomposition rim usaally stand approximately normal to the 
prism planes. The particles of the groandmass, to a certain extent, 
exhibit a tendency to flow aroand the chiastolite crystals — that is to 
say, they are arranged in lines ronghly parallel to the sides of the 
prism. This is best seen in the oross sections, and may be taken to indi 
cate that the schists were in a plastic condition after the crystals were 
formed. A few minute veinlets cut the section, filled with a clear min- 
eral in little grains, which is apparently quartz. The powdered rock 
was tested for magnetite, but merely a few grains were found. 



Analyses of conUtct-meiamorphic rocks. 
[Analyst, Steiger.J 



i No. 851. 

I 

i SIC, ..' 58.09 

I TIO, I .95 

A1,0» ' 17.4« 

! Fe^O, ! 1.12 

I FeO 5.08 

MnO ' none 

i CaO 6.24 

I SiO ' .04 

BaO 07 

I MgO 4.06 

K,0 2.02 

' Na^O 2.M 

i Li^O ■ non« 

Water below 100" C 29 

' Water above 100° C 1.45 

I P,0, 17 

SO, 06 

CO, 21 

I 01 .02 

I F trace 

I BjOj none 

I Carbon .11 

Total 100.37 

^ LefisO .01 

Total I 100.36 



No. 433. No. 431 A. No. 856. 



100.80 
.06 

100.74 



100.55 
.10 

100.45 



99.76 
.01 

99.75 



65.10 


62.15 


60.35 


; -72 


.89 


.75 


17.77 


19.34 • 


17.62 


1. 95 


4.23 


5.64 


3.29 


2.25 


2.20 


none 


trace 


none 


1.88 


1.50 


.45 


none 


none 


none 


none 


.04 


.12 


1 1.43 


1.88 


1.14 


2,45 


3.07 


3.16 


2.25 


1.6U 


1. 


none 


none 


none : 


' .47 


.19 1 


1.02 


2.49 


1.79 


4.36 


.14 


.15 


.17 


! .03 


.13 


.06 ; 


none 


none 


none 


|/8trong\ 
i\ trace. / 


none i 


.01 ' 


.12 


.22 


trace 


* none 


none 


none 


1. 21 


1.12 


1.72 



No. 851 is a basic quartz- mica-diorite, collected near Taqui Creel^j 
about 300 feet from the border of the zone of contact-metamorphic rocl^^ 
It is the igneous rock which has caused the formation of the cont^* 
minerals. 

No. 432 is a knotted mica-schist collected in Yaqui Gulch, ab^^ 
3,800 feet from the main diorite contact. 



WLUB) DESCRIPTIONS: NO. 136, APORHYOLITE. . 343 

No. 431A (Educational Series ColIectioD, No. 135), is a cbiastolite- 
schist collected in Yaqui Gulch, a few feet to the north of No. 432, or 
iboQt 3,800 feet from the main diorite contact. 

No. 855 is a clay slate collected near the head of Yaqui Gulch, sonie- 
rhatmore than a mile from the main diorite contact. 
The above analyses were made with the object of showing, as nearly 
» practicable, whether the new minerals in the zone of contact meta- 
lorphism had formed as a result of certain elements being added to 
be schists from the quartz-diorite magma or from the mineralizing 
ohtions and gases accompanying its intrusion, or had resulted merely 
rom the recrystallization of the material of the clay slates. The speci- 
leos Nos. 432, 431A, and 855 may be regarded as having been all 
h^nally clay slates. Rosenbusch found that the amount of the water 
f crystallization in a contact metamorphic schist varied inversely with 
he distance from the igneous rock that caused the metamorphism. 
)lu8 same law appears to apply to the above series. In Nos. 432 and 
31A the amount of this water is 1.79 per cent and 2.79 per cent, resi>ec- 
ively, while in No. 855, which is farthest from the igneous mass, the 
faterof cr^'Stallization (lOQo C.+) is 4.36 per cent. It will be noted 
hat the amount of fluorine is greater in those schists nearest the 
iiorite. This element probably exists in the authigenic micas, and 
Day be regarded as having been added to the schists from gases that 
iseended along the contp.ct at the time of the intrusion of the diorite. 
tie carbon content (0.11 per cent) in the quartz-diorite may have been 
leriTed from carbonates of lime and magnesia formed by the action of 
orface waters containing carbon dioxide. This is more than probable, 
ioce carbonates were noted in small amount in the thin sections of 
he rock. It should be borne in mind, however, that carbon dioxide 
^org in a liquid form in minute cavities in the quartzes of diorite 
uidgnranite, and when great masses of granitic rocks have undergone 
ashing the amount of carbon dioxide liberated would be consider- 
able. The quartz-diorite No. 851, however, shows no evidence of crush- 
H* The large majority of the newly formed minerals in the contact 
ablets probably represents merely a molecular rearrangement of the 
i^nginal components. 

METAMORPHIC IGNEOUS ROCKS. 

No. 136. APORHYOLITE. 

(From South Moitntain, Adams County, Pennsylvania. Described by Miss 

F. Bascom.) 

The term aporhyolite, which has been recently introduced into petro- 
P^pbical nomenclature,^ is designed to cover those acid volcanic rocks 
^hich are similar in chemical and mineralogical composition and in 



'^' r. S. Geol. Survey, JN'o. 136. The structure, ori^, and nomenclature of the acid volcanic 
^^ of South Mountain : Jour. Geol., Vol. I, No. 8, pp. 813-«32. 



I 



i 



344 , THE EDUCATIONAL SERIES OF ROCK SPECIMENS. In 

structure to the rhyolite, but differ from that rock type in posses 
holocrystalline groundmass, presumably secondary. 

The presence of those microscopic structures peculiar to glassy 
associated with evidence of the secondary character of the crysl 
tion of the grouudmass, is considered indicative of the original 
character of the rock, and hence of a former identity with the rli 
I It is maintained that the present difference is due to changes 

f have taken place subsequent to the solidification of the rock 

among which has been devitrification. 

By devitrification (eutglasung) is meant the conversion of a gl 
partially glassy groundmass into a holocrystalline groundmass. 
molecular motion does not cease with the solidification of a roe! 
is an acknowledged fact. Daubr^e^ has shown experimental) 
I crystallization may take place in glass just as in a molten magm^ 

i action differs only in the amount of time required. In the formi 

it is exceedingly sluggish. Heat and moisture, which are not li! 
have been altogether absent in the history of metamorphic re 
any age, might be important factors in initiating, and accelerati 
process. 

Such then are the facts — the original character of the rock a 
subsequent alteration — indicated by the name aporhyolite.'^ 

The structures pointing to a glassy origin and the secondar; 
acter of the crystallization of the groundmass will be indicated 
description of the specimen included in this collection. 

The aporhyolite of the collection comes from a spur of South 
tain, about 2 miles west of the old Maria Furnace, in Adams C 
Pennsylvania, and about 3^ miles northeast of Monterey Stal 
Franklin County, Pennsylvania. The best specimens are founc 
of the junction of Toms Creek and Copper Kun, upon the mountaj 
They are of pre-Cambrian age, and have been subjected to pr 
rendering them more or less schistose. 

In color the aporhyolites of this locality show considerable ra 
blue-gray, bluish purple, and a reddish purple are the predomi 
colors. Both shades of purple are frequently present in a singl 
specimen, when there is a tendency to alternating bands of these 
Some specimens show a yellowish-green tone due to the prese 
\ some abundance of a secondary micaceous mineral which will h 

I acterized later. Phenocrysts are absent or inconspicuous. An 



mu.] DESCRIPTIONS: NO. 136, APORHYOLITE. . 345 

oat m relief, giving it a superficial resemblance to a conglomerate. 
They are irregularly distributed or are arranged in bands, and are 
often elongated by the movement of the magma during their formation. 
When seen in cross section the spherulites show a tlirecfold zonal 
banding of the blue and purple pigment. There is a dark center sur- 
rounded by light and dark zones, or this arrangement of zones is 
reversed. 

A flow structure, while not conspicuous as in many aporhyolites, is 
obscnrely indicated by the banding of the purple and blue sliades, by 
the elongation of the spherulites, and by their arrangement in chains. 

The bright cleavage surfaces of minute feldspar crystals occasionally 
reflect the light. These feldspars, magnetite, and the secondary mica- 
ceous mineral before alluded to are tbe only constituents readily deter- 
mined with the naked eye. 

Both the matrix and the spherulites are cryptocrystalline. 

The hardness of the rock is between 6 and 7. Its specific gravity is 
2.678. The specific gravity of specimens of spherulitic glass ranges 
from 2.385 to 2.394. These figures show the increase in density which 
accompanies devitrification. The presence of manganese oxide is 
denoted in a brown stain on the weathered surface of the rock. 

Under the microscope in ordinary light the spherulites appear as cir- 
cnlar, elliptical, or irregularly oval areas outlined by minute particles 
of red iron oxide (hematite) and dusted by the same pigment. Sue- 
ceBsive zones are faintly indicated and arise from the crowding in 
bands of innumerable particles of black and red iron oxide. Sometimes 
these circular areas merge one into the other, forming a chain of spher- 
ulites suggestive of those described by Professor Iddings^ as charac- 
lieristic of the Yellowstone National Park volcanics. 

The groundmass in which the spherulites lie, and which constitutes 
^lya small x>ortion of the field, is distinguished in ordinary light from 
the spherulitic areas by its comparative freedom from the i^on oxide 
particles. PI. XLIII shows the altered spherulites of an aiwrhyolite 
from the type locality in the South Mountain, Pennsylvania. 

With the analyser in place the field has a much more homogeneous 
tepect. The spherulites unexpectedly disappear. Instead of the radi- 
ating fibers of quartz and feldspar, which constitute tlie well-known 
aphemlitic structure, there is a finely granular quartz-feldspar mosaic, 
quite similar to the quartz-feldspar mosaic of the groundmass. 

A cloudiness due to the abundant iron oxide particles in the spheru- 
litic areas, the presence of brightly polarizing scales of a secondary 
^ii^eral, or the finer grain of the quartz-feldspar crystallization alone 
WfTe to distinguish the spherulites from the groundmass. 

Karely the aporhyolites of this locality show traces of a radial growth 
of quartz and feldspar, not yet entirely obliterated by alteration to a 
granular crystallization, while sometimes the groundmass still preserves 



I Obiiidian Cliff: Seventh Ann. Kept. U. S. G©ol. Survey. 1888, p. 277, PI. XVII. 



i 



346 . THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [Kvum. 

faint indications of perlitic parting. This is a stractare peculiar to f 
glass and consists in a concentric cracking due to the contraction 01 
cooling. Subsequent crystallization might readily obliterate such : 
structure. In this case iron oxide particles are so arranged as to pn 
serve the outline of the cracks. This structure is illustrated in Fig. J 
PI. XLI V, prepared i'rom a thin section * of aporhyolite from the Low< 
Keweenawan of the Lake Superior region. 

Aporhyolites from Eaccoon Creek in the South Mountain, aboi 
10 miles west of Copper Run, show spherulitic and perlitic structun 
still i)erfectly preserved, associated with lithophysal flow and rhyolit 
structures in great perfection. Fig. B, PI. XLIY, is aporhyolite fro 
Eaccoon Creek, showing spherulitic and perlitic structures. 

The association of any two of these structures is considered safficie 
proof that the rock which they characterize consolidated as a glas 
Such is the nature of the evidence that the finely granular crystalliz 
tion of the groundmass and the spherulites alike is secondary. 

The rock first consolidated as a glass (or in large part a glas< 
crowded with spherulites possessing the true radiating structure chs 
acteristic of spherulites. Subsequent to this consolidation, devitrifi* 
tion, the nature of which has already been explained, has broug 
about the uniform granular holocrystalline character which the ro 
now possesses. 

The mineral constituents of these rocks are few in number. Cli 
among the crystals of the first consolidation is a clear, well-preserv 
feldspar. Crystals of this mineral are inconsiderable in size. They fi 
frequently grouped, and are distributed without reference to the sphei 
lites. They may be twinned by the Carlsbad or Manebacher law a 
often show microperthic structure as result of pressure. They na 
contain inclusions of an original glassy magma. That they belongs 
the alkali end of the series of feldspars is indicated by the chemi^ 
analysis of the rock, while their specific gravity (2.6) and the emerger 
on the M face of a positive bisectrix very slightly inclined, as may 
observed in converging light on the Manebacher twins, suggest an 
thoclase. 

Occasional clear oval areas of granular quartz may sometimes r< 
resent granulated quartz crystals of the first consolidation, or the s 
ondary replacement of spherulitic centers, or they are, in many plac 
the filling of minute vesicles elongated by the movement of the roagt 
In the last case there are frequently forms along the walls of the v4 
cles, outlined by the iron oxide, which suggest minute tridymite cr 
tals or spherulites, such as are found lining the walls of vesicle^ 
modern lavas. They disappear in polarized light. Fig. J5, PI. XLl 
prepared from an aporhyolite from Eaccoon Creek, shows quartz-fil 
vesicles bearing tridymite spherulites on their walls. Earely trd 
parent colorless crystals of zircon, characterized by high index 

1 Thin B«ction farninhed by N. U. Winohell. 



DttixB.] DESCRIPTIONS: NO. 136, APOttHYOLITE. 347 

refraction and brilliant polarization colors, may be recop^nized, also 
minnte grains of pleocbroic hornblende. The only other original eon- 
Btituenta are the iron oxides, magnetite, and hematite. Tliey are both 
in part secondary. The latter is recognized by its nonmetallic luster 
and reddish color in incident light, the former by its rough metallic 
black surface. Neither show crystalline form. The former is magnetic 
in tlie powder. Hematite is a characteristic pigment for rhyolites. 

The conspicuous secondary constituent is the micaceous mineral of 
which mention has already been made and to whose presence the rock 
owes a light greenish tinge. Under the microscope, in ordinary light, 
it appears as transparent pale greenish yellow, irregular plates show- 
ing Hues of cleavage, with a low index of refraction and an oily 
luster. In parallel polarized light these plates show brilliant interfer- 
ence colors, and in converging polarized light a small axial angle. 
These plates are developed most abundantly around the feldspar crys- 
tals, filling the cracks in the feldspars that have suffered crushing, 
and around and in the spherulitic areas. 

It is undoubtedly an alteration product of feldspar under pressure 

and that species of mica known as sericite. This mineral can be formed 

from the acid feldspars by the replacement by hydrogen of a portion of 

the alkali constituent and the setting free of silica. This kind of meta- 

morphism is of common occurrence in the development of schistose 

n)eksfrom the massive acid eruptives, and has been carried so far in 

^meof the acid volcanicsof South Mountain as to form a sericite-schist. 

Another secondary constituent of these aporhyolites is epidote. This 

^neral is .of a deeper yellowish green than the sericite, and is easily 

distiognished from the latter by its high index of refraction. It occurs 

in irregular granular aggregates. Its high relief and bright interfer- 

^ce colors serve to distinguish it. It is a product of the weathering 

of the feldspars in the presence of solutions carrying alumina. 

That the magnetite of these rocks is titaniferous is plainly shown by 
its alteration products, of which there are two. The most abundant 
one is a cloudy, white (in incident light), or yellowish substance called 
lencoxene. Associated with this mineral there are, rarely, brown semi- 
transparent grains of titanite, showing a higher relief than epidote, and 
^ithoQt brilliant interference colors. 

The evidence that these rocks have undergone pressure and some 
ahearing lies in the development of a sericite and the resulting folia- 
tion, the cracking and pulling apart of the feldspars, the development 
of the perthitic structure, and the granulation of the quartzes. 



348 



THE EDUCATIONAL SERIES OP ROCK SPECIMENS. [buluUO. 



An analysis of aporhyolite fi:oin Monterey, Franklin Oounty, Penn< 
sylvania, as reported by H. 1^. Stokes, is as follows: 

Analysis of aporhyolite. 



SiO, 

TiO, 

P.O. 

AliO, 

Fe^Oa 

FeO 

MnO 

BaO 

SrO 

CaO 

MgO 

K,0 

NftaO 

Li,0 

Co, , 

H,0 below llOo c 
H,0 above llO© C 

Total 



Per rent. 

76. a4 

.26 

trac© 

11.60 

2.41 

.30 

trace 

.09 

none 

.55 

.06 

1'. 75 

5.50 

none 

trace 

.10 

.39 



100.35 



The analysis is essentially that of a typical rhyolite. The relation of 
soda percentage to the x>otash accords, in its indication of the character 
of the feldspar, with the optical determinations. The lime may be 
referred to the epidote, which doubtless also explains the trace of man- 
ganese oxide. A manganese epidote has been fonnd very abandantly 
in some of the aporhyolite near Monterey. The presence of titaniam 
in the magnetite is also substantiated by the analysis. 

Beside the locality from which these specimens were obtained typical 
aporhyolites also occnr in the South Mountain along Raccoon Greek, 
south of Caledonia Fnrnace, Franklin County, Pennsylvania. Else- 
where similar acid volcanics have recently been recognized, forining:^ 
with the South Mountain volcanics, a belt extending along the eastcTD 
border of the United States and Canada.^ 

In Newfoundland, Nova Scotia, New Brunswick, and on the Ga»p^ 
peninsula old volcanic rocks, both acid and basic, are extensively 
developed and have been described by the Canadian surveys. Hitch- 
cock and Shaler re{)ort their presence in Maine about Eastportand 
Mount Desert, along the coast,^ and on Moosehead Lake, in the interior.^ 

Dr. Wadsworth* and Mr. Diller'^ have.made the felsites (aporhyolites) 
of the Boston basin famous. 



iFora full account of theao localities see a paper by O. H. Williams, The dintribntlon ot meHttnt 
volcanic rocks along the eaateni border of North America: Jour. Oeol., Vol. II, No. 1, pp. 1-91, pi- 1. 
18W. 

«Am. Jour. Set., 3d series, Vol. XXXII, pp. 40-43, 1886. 

s Geol. Maine, 1801, p. 190 and 432 ; also id. 1863, p. 330. 

* The clavHsiflcation of rocks : Ball. Mus. Com. Zool. Harvard College, VoL V. p. 28S, 18T». 

'The felsites and their aasociated rocks north of Boston : Bull. Mas. Comp. Zool., Vol. VII, p. Itti KA* 



;!; 



DE8CKIPTION8 : NO. 187, GRANITOID GNEISS. 349 

itham and Orange coanties, North Carolina, ancient acid vol* 
lowing sphemlitic and flow structures have been collected by 
. Williams, and at Lancaster, South Carolina, devitrified glasses 
m found by Prof. S. L. Powell. With continued petrographic 
• the pre-Cambrian rocks of America aporhyolites may be 
ed at many other points. The similarity of the preCambrian 
» of the Lake Superior region to their modern equivalents was 
remarked by Dr. Irving.* 

Kewatin (Lower Huronian) of Minnesota holocrystalline vol- 
howing perlitic parting, spherulitic, and other structures com- 
iI)orhyolites, have lately been described by Dr. Grant.* 
aporhyolites have only recently been recognized in America, 
ind such writers as Allport, Cole, Bonney, Rutley, and Barker 
)g investigated similar rocks as they occur in Scotland, the 
[strict, and northern Wales. In Sweden and Belgium they 
en recognized and described by Nordenskjold ^ and De La 
oussin. ^ 

list of English, French, and German papers on this class of 
3 Bulletin 136, United States Geological Survey, pp. 87-91. 

No. 137. Granitoid Gneiss. 

oosAC Mountain (Tunnel), Massachusetts. Described by J. E. 

Wolff. ) 

iree rocks, granitoid gneiss (137), metamorphic conglomerate 
d albite-schist (129), form an ascending series, with the gran- 
liss at the base, and in their geologic relations give the key to 
iture of the main axis of the Green Mountains in north\Yestern 
Qsetts.^ 

anitoid gneiss occurs in two areas. The first (from which the 
IS are taken) is on Hoosac Mountain, where the rock occupies 
^al area on top of the mountain south of the tunnel line, and 
cut by the tunnel for a distance of several thousand feet 
liderable depth below the surface of the mountain. The sec- 
i lies a few miles northwest from the first, forming the crest 
>es of Stamford Mountain, in Vermont, and a x>^i't of its 
I continuation in Massachusetts, called Clarksburg Mountain. 
: has hence been called the "Stamford granite,'-'^ and occupies 

ing, Copper-bearing Rocks of the Lake Superior Il<*gioii : Mon. U. S. Geol. Survey, pp. 

! 5, p. 436. 

ant. Volcanic Rocks in the Kewatin of Minnesota: Science. Vol. XXIII, Jan. 12, ISM, 

rdenskjold, ITeber archaoische Ergnssgesteine aus Stnaland: Bull. Geol. Instit. Upsala, 

,1803. 

all6e Pouaain, Lea aaciennes rhyolites ditcs eurites de Grand Manil : Bull. Aca<l. R. de 

iea, Tome 10. 1885. 

Mon. XXIII, TJ. S. Geol. Sur\'ey. Part II, The Geology of HooMac Mountain nnd 

erritory, by J. E. Wolff. 

of Vermont, p. 601. 



\\ mica schist, so tbat the mass as a whole has a dome structure, wj 

;{ granitoid gneiss as a central core. On the east side of thisdom^ 

of the village of Stamford, Vermont, a remarkable contact b( 

the two rocks is found. The quartzite is conglomeratic, and : 

; granitoid gneiss there is a curious cleft due to the weathering oi 

i was originally a trap dike, some of the material from which is foi 

; the overlying sandstone, whose layers thicken over the cleft an 

down into it, the whole showing plainly the unconformable depc 

of the quartzite and the pre-Gambrian age of the granitoid g 

The latter rock is finely banded, almost schistose at the contac 

' away from this it becomes more and more massive and coarse, wi; 

! development of large pbenocrysts of feldspar in places. The sch 

structure is evidently an effect of crushing (stretching), combiuec 

mineralogical change, which decreases toward the solid core of mi 

' rock, but this change was probably assisted by the previous 

tegration of the granitic rock on the old pre Cambrian land surfs 

On Hoosac Mountain the rock occurs in smaller mass and Is 

what more metamorphosed than the rock of the Stamford are 

occurs, however, in the same relation, forming the original Can 

shore line and overlain by the representative of the Lower Cam 

here a conglomerate, itself extremely metamorphosed (the 

morphic conglomerate of the collection). It is noticeable hen 

that the gneissic structure is often better developed near the co 

and is then roughly parallel to the banding of the conglomerate. 

The second rock, the metamorphic conglomerate (Ko. 128), tyi 

developed, is found resting on the granitoid gneiss of Hoosac ] 

tain at the place where the latter rock leaves the surface of the i 

tain and plunges downward to the tunnel level. The rock foi 

bed 600 to 700 feet in actual thickness, verv coarse at the has 



I 
I 



MUB.] DSSCBIPTIONS: NO. 137, GRANITOID GNEISS. 351 

floathwest sides, so that the structure is that of ao anticlinal fold or 
dome overturned to the west, the axis of which inclines or pitches 
sorth, and therefore the formations successively leave the surface 
along the crest of the mountain and plunge northward and downward 
to the tannel level. The conglomerate, therefore, having been folded 
over the granitoid gneiss with overturning on the west side, we find 
that the rock has lost nearly all traces of its original character on the 
sides of the fold where the motion or stretching action was greatest, 
bat that this is better preserved on the axis or center of the arch, 
▼here the northerly pitch of the axis gives us the series in normal 
position. 

The conglomerate specimens are also taken from the dumps of the 
central shaft of the tunnel, which cuts the rock 1,000 feet below the 
sarface. They show conglomeratic character plainly, while the same 
rock found at the surface on the sides of the fold loses its character as 
a conglomerate and becomes a gneiss. 

The third rock, the albite-schist (No. 129), overlies the conglomerate 
conformably on the crest of the mountain, exposing a thickness of at 
least 1,000 feet, and mantles over the conglomerate on the east and west 
sides, as the upper layer of the fold. The basal layer, 50 feet or more 
thick, is very rich in garnet crystals, suggesting an original calcareous 
rock, and as the conglomerate represents the Cambrian quartzite, this 
overlying schist must represent the Stockbridge limestone and associ- 
ated schists (Cambro- Silurian) which lie in the Hoosac valley and 
moantain mass of Greylock to the west of Hoosac Mountain, so that 
there is some basis for this explanation of the presence of the lime- 
silicate (garnet) in such abundance. 

The albite-schist is cut by the tunnel on both sides of the anticlinal 
axis, and the specimens are therefore taken from the dumps of the 
central shait, which is a great storehouse for specimens of the various 
modifications of the three rocks, iu an ideally fresh condition. 

With this brief description of the geological occurrence of these 
specimens, we may proceed to their individual description. 

Iu the hand specimen the granitoid- gneiss contains large elongated 
masses of pale reddish feldpar, separated by thinner bands of white 
or blaish granular quartz, in the middle of which there are often 
larger grains of homogeneous blue quartz. Thin, dark-green, branch- 
ing bauds of mica and epidote traverse the specimen in rough paral- 
lelism to the quartz-feldspar bands, while within the quartz or feldspar 
masses little patches of the same green color are found. A broad, flat 
cleavage face of feldspar is often distinctly curved, or may be broken 
^P into several pieces which do not reflect the light simultaneously, 
an«l have therefore been moved from their original position in one 
plane. When we remember that the more massive rock, as found in 
^^^ center of the great Stamford core, has the same large feldspars 
in wellformed crystals, the blue quartz in large grains, and but little 



352 THE EDUCATIONAL SERIES OF RQCK SPECIMENS. [bcll.Uo. 

banding or development of thin mica bands, we can understand that 
the present structure is due to pressure, which strained and broke ap 
the minerals, the granulated minerals moving a little over each other, 
and in the interstices thus formed, or along the larger planes of 
motion, the mica, epidote, etc., developing to form the little secondary 
bands. In this way, by dynamic metamorphism, a coarse banded rock 
has been produced from a coarse massive rock. 

In the tbin section these mechanical and mineralogical changes can 
be studied more in detail. Many of the large feldspar areas are recog- 
nized as microcline by the fine double twinning (albite and perielioe 
laws), while others which show no twinning are partly orthoclase, partly 
microcline, which the section cuts parallel to the second cleavage 
( OD Pdb ). In some cases the large feldspars contain narrow, spindle- 
shaped masses of another feldspar, in parallel growth (microperthite). 
Other feldspar areas with but a single polysynthetic twinning are 
probably albite (primary). These large crystals are crossed by little 
vein-like aggregates composed of rounded grains of quartz, little irreg- 
ular grains of fresh, clean microcline, other grains of feldspar with a 
clear, even, polarizing tint, with often two straight parallel sides and 
a twinning line parallel to them which divides the crystal iuto two 
halves, all these properties being identical with those of the alhitesof 
the conglomerate and schist, so that it is probably the same feldspar 
in the case of the granitoid gneiss. In addition to these, the veiolets 
contain little grains of epidote and plates of biotite and muscovite. 
They may run irregularly or be roughly parallel in direction to the 
general banding of the rock. It often happens that the large feldspar 
is broken into several minor pieces, which are separated along the lines 
of break, or of micro-faulting by an aggregate of these minerals, whose 
later origin, after pressure and motion had acted, is thus evident. 

In the same way the larger masses of original quartz are identified 
by their large size and by the fact that they are surrounded by a zone 
of aggregate quartz broken off from the parent mass, which, in polar- 
ized light, is seen to be itself strained or cracked. This original quartz 
is moreover free from inclusions of any of the secondary minerals^ 
unless in cracks, while the metamorphic quartz of the little veinlets 
which penetrate the feldspar, and of the banded aggregates hetweeo 
the feldspar, frequently inclose biotite, showing the contemporaneons 
origin of the two minerals, and that the larger masses of original qaartK 
existed before the period of the biotite formation. 

The rest of the slide is composed of an aggregate of the following 
minerals: 

(1) Feldspar, in little imperfect crystals, which are often simple twins 
according to the albite law and are probably albite.^ These are either 
clear or are filled with little plates of mica, grains of epidote, and drop 
lets of quartz. There are also irregular grains of microcline, in pft^^ 



> Thoir refractive index is iiMually lower than that of tho UaUam of the section, or iMrely eqn«l **"" 
OhJitjnity of ^xfirictiou in the rone of the tvriuniiit; axix i« too high for oligooUuw, i. •., albito. 



WLLttl DESCRIPTIONS: NO. 138, EPIDOTE-MICA-GNEISS. 353 

broken off from the large primary crystals, in part probably metamorpbic. 
(2) Qaartz in interlocking aggregates or isolated grains. (3) Biptite 
and greenish muscovite in irregnlar stringers, inclosing in tbeir meshes 
litUe prisms or grains of yellow epidote with high polarizing colors, 
occasional grains of black opaqne magnetite, white grains or aggregate 
masses of titanite, and some areas of white calcite, recognized by its 
rhombohedral cleavage and white polarization. There are occasional 
small prisms of zircon with terminal planes and with distinct uniaxial 
positive character. 

This aggregate of minerals often invades the large original masses of 
feldspar, but it will be noticed that none of these minerals show signs 
of straining or breaking, and that the little albitic feldspars inclose 
epidote, quartz, and mica. Beasoning from this, we infer that they 
formed after or during the action of the forces which strained and 
broke the original feldspars and quartz, and under conditions of con- 
temporaneous crystallization, unlike those of eruptive rocks, where 
there is generally a certain succession in the formation of the different 
minerals — some earlier, some later. 

The original rock was a coarse, massive, granitic rock, perhaps of 
eniptive origin, and resembling the coarse granite of Finland, called 
rapakiwi, while dynamic metamorphism has produced the present 
peissic condition, the mineralogical change consisting essentially 
iu the production of new feldspar, mica, quartz, and epidote. 

No. 138. Bpidote-mica-gneiss. 

(From Lebanon, Grafton County, New Hampshire. Described by J. P. 

Iddings.) 

This gneiss from Lebanon, New Hampshire, was supposed to be 
80-caDed protogene-gneiss, the characteristic minerals of which are 
"cUorite, talc, rotten mica, or other decomposition products," accord- 
ing to Hawes.^ 

It is in fact an epidote-mica-gneiss, according to present classifica- 
tion. It consists of relatively large, irregular crystals of microcline, 
^th small grains of quartz in aggregates equaling the feldspar in bulk, 
l^esides brown biotite in aggregates of small plates, variable amounts 
of colorless muscovite, and much epidote in aggregations of microscopic 
crystals. Subordinate minerals, occurring in relatively small amounts, 
^ apatite, tourmaline, allanite, zircon, possibly sphene, and occasion- 
% green mica or chlorite and, rarely, calcite. The rock is quite fresh 
ftnd andecomposed, judging from the condition of the brown biotite. 
'Oie epidote is grouped in aggregations with quartz and sometimes 
^tb muscovite, or is scattered in various-sized crystals and grains 
^ugh the microcline, but there are no remains or other evidence 



'G.W.Hftwea: Mineralogy and Lithology of New Hampshire, Part IV, of Geology of J«ow Ilamp- 
»kfa«. Concord, 1878, p. 201. 

Bull. 160 23 



i 



854 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. [ButLlM. 

of any ferromagnesian mineral, more or less altered, from which it 
could have been in part derived. It appears as a primary constitaent 

The microcline exhibits microscopic multiple twinning with grating 
structure, and the fibration due to an intergrowth of plagioclaKe, or 
microperthite structure. In certain sections the microcline twiuuiog 
is scarcely recognizable. There is, besides, a simple Carlsbad twiu 
ning. Smaller feldspars occur mixed with the quartz. The feldspars 
are clouded to a greater or less extent by minute crystals of epidote, 
and sometimes of muscovite. The minerals are in some cases arranged 
in several directions in a feldspar, the substance of the feldspar being 
otherwise quite pure. Mioropegmatitic intergrowth of quartz aod 
feldspar is occasionally noticed. In some feldspars there are inDamer* 
able minute rectangular inclusions with a bubble of gas in each, 
presumably fluid inclusions of secondary origin. Microscopic grains of 
quartz with rounded outline are abundant in certain feldspars, the 
quartz being more numerous in the marginal part of the feldspar. 

Quartz forms irregular grains, somewhat rounded in outline, often 
very free from inclusions or impurities, sometimes bearing nameroiu 
minute crystals of epidote, and less often those of biotite and mus- 
covite. Fluid inclusions are not common. 

Biotite forms small stout plates, set at all angles, without approach 
to parallelism. Its color is dark bro^u with a tinge of green. Tbe 
absorption is strong, and the substance of the mineral quite pare. In 
a few cases it is altered to a green mica, and in some cases chlorite. 
Muscovite is almost absolutely colorless in thin section, with sligbt 
absorption of light for rays vibrating parallel to the cleavage of the 
mica. It is sometimes inclosed by biotite. Both micas are in places 
intergrown, or include one another, together with grains of qaartz 
and epidote. Inclusions of ax)atite and zircon are scarce. 

Epidote forms yellowish crystals or grains with faint pleochroism. 
Its high index of refraction and strong double refraction are character- 
istic. Occasionally the center of a crystal is chestnut-brown and 
pleochroic, and is probably allanite. It is closely associated with mus- 
covite in some places, in others is independent of it. Its size sinks to 
the minutest microscopic dimensions. It might be assumed to bean 
alteration product, as it often is, were it not for the fact tbat no par- 
tially decomposed calcium-iron mineral is to be found in the rock. 

Apatite is scarce, in colorless, irregular grains. Zircon is also scarce, 
and forms very small grains. 

Tourmaline occurs sparingly in short prisms, with a purplish-brown 
color in thin section, and exhibits complete absorption of the ordinary 
ray. Several grains of calcite were noted in one section, and in another 
an irregular grain supposed to be sphene. Chlorite is present in only 
a few instances, when it exhibits a strong green color and marked 
pleochroism. It is evidently an alteration product derived directly 
from biotite. Magnetite is entirely absent. 



Nuo.] DS8CRIPTION8: KO. 139, DIABASE AMYGDALOID. 355 

It is possible that this rock may be the same as, or similar to, that 
D6Dtioned by Hawes, in the report akeady referred to, as occurriDg at 
9^' alling's quarry, which contains epidote. 

No. 139. Diabase AMYaDALOiD. 

'From Grand MAkais, Cook County, Minnesota. Dbscribed by W. S. Bayley.) 

This rock forms one of the many flat-lying, sheets that, together with 
conglomerates aud other fragmental layers, make np the Keweenawan ^ 
or copper- bearing series of the Lake Superior region. On the north 
shore of the lake the eraptive beds of the series come down to the 
water's edge, forming cliffs, in whose sides several distinct layers may 
often be seen. Some of the beds are dense throughout; others, and 
among them the bed from which the specimen was taken, are com- 
pletely crystalline and moderately coarse grained toward their centers, 
and finer grained and vesicular at both the upper and the lower con- 
tacts. 

The vesicles in modern lavas are known to be due to the expansion of 

water contained in the molten rock magma. As long as this remains 

in the interior of the earth the pressure there existing retains the water 

in a liquid condition. When the magma escax>e8 to the surface, where 

the pressure is much less, the superheated water flashes into steam, 

which in attempting to escape produces bubbles in the liquid mass, just 

as babbles are produced in boiling water. If the rock solidifies quickly 

at top and bottom some of the bubbles are caught, the steam escapes 

trom them, and a vacuole results. If the vacuole is afterwards filled 

by materisJ deposited from waters circulating through the rock, amyg- 

dttles are formed, and the rock becomes an amygdaloid. The vesicles 

in old lavas are undoubtedly due to the same causes as give rise to the 

"Vacuoles of modern volcanic rocks. Hence, when a sheet of eruptive 

iDaterial is found with vesicles near its upper surface, aud especially 

^hen these are found near its lower sarface as well, we conclude that 

the sheet was a surface flow, like the lava sheets of modern volcanoes. 

The specimen was taken from near the upper surface of a flow or 

sheet that reaches the water's edge 1^ miles east of Grand Marais, on 

the north shore of Lake Superior. 

An inspection of the hand specimen shows it to be a very fine-grained 
purple rock, without any evidences of stratification or foliation. In it 
^ a large number of vacuoles. In a few of the specimens these are 
Arranged with their long axes approximately parallel, but in most no 
Bach regular arrangement is apparent. In the former case the regular- 
ity is due to the fact that movement continued in the pasty rock magma 
^ven after it had become viscous enough to prevent the escape of the 
babbles. These were consequently drawn out in the direction of 
^e motion. 

'for chM«ict«HMii«it of the Keweenawan, see Irring and Cbamberlin, BoU. U. S. GeoL Survey 
K«.a,»iid B. 1>. Irving, Copper-bearing Bocks of Lake Superior, Mon. U. S. GeoL Surrey, V«Jt'^ . 



356 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. iBcii^Ui 

Some of the vacuoles are entirely empty. Others have their walls 
covered with a thin coating of a fibrous pink mineral^ while others 
again are almost filled with larger needles of the same substance. A 
few are lined with a pistachio-green material resembling epidote, while 
still others have their sides lined with a layer of the green mineral 
under a coating of the pink substance. It is evident that the green 
mineral is older than the pink one, since the latter could not have been 
deposited upon the former until this had already existed. In a few of the 
vesicles a third substance may be seen. It is a soft, colorless, or white, 
easily cleavable material that effervesces strongly when moisteoed 
with cold hydrochloric acid. This mineral, which is evidently calcite, 
is younger than the pink mineral, since it occupies the central portions 
of the vesicles and is surrounded by the latter, which is present in 
little bundles of radiating needles, whose hardness is low. The needles 
easily dissolve in hydrochloric acid, yielding gelatinous silica, and give 
up water in the closed tube. Though no complete chemical analysis of 
them has been made, there is but little doubt that they are lanmontite. 

When placed under the microscope the mass of the rock is found to 
consist largely of small microlites of a twinned feldspar and a dark- 
brown interstitial substance or groundmass. The feldspar is often 
clouded with decomposition products and is reddened by stains of iron 
salts. It occurs usually in skeleton crystals with forked ends in place 
of crystal terminations, and in cross section it frequently appears as a 
hollow shell including portions of the brown groundmass. The micro- 
lites extinguish at low angles as measured against their long axes, and 
consequently are composed of a substance near oligoclase in composi- 
tion. The highest extinction to be noted is about 16°, while many of 
the tiny crystals extinguish nearly parallel. The microlites are 8ca^ 
tered irregularly through the section, except in those parts near the 
vacuoles, where they are absent. 

The interstitial substance or groundmass is opaque in most parts of 
the section and particularly so immediately around the vesicles. On 
the edges of the section, where it is thinnest, it is seen to be composed 
of many very slender feldspar needles aggregated into little radiating 
bunches or into sheaf like forms. Between these is a dark matrix 
consisting of a nonpolarizing substance — originally glass — darkened 
by little round grains and irregular masses of magnetite, hematite, 
or brown iron oxides. In other pla<;es this matrix is comparatively 
clear, when it exerts a feeble influence on polarized light, as if tbe 
glass of which it was composed had begun to differentiate into mineral 
species, whose nature, however, can not be determined because of 
the abundance of the iron compounds mingled with them. In the 
vicinity of the walls of the amygdaloidal cavities there is an absence of 
the larger feldspar microlites, as has already been stated. Under the 
higher powers small feldspathic microlit<5S may be detected in these 
portions of the groundmass, but otherwise no special features are to b^ 
noticed in it. 



nuAl DI18CKIPTI0N8: NO. 139, DIABASE AMYGDALOID. 357 

The general characteristics of the amygdaloid are those of a much 
decomposed glassy rock with feld spathic microlites. Its structure 
resembles that of a glassy diabase or basalt. Upon comparison with 
sections made from various portions of similar vesicular flows from 
other horizons in the Keweenawan, it is fimnd that the rock under 
investigation resembles very strikingly the fine-grained portions of 
those beds which in their interior have the composition and structure 
of typical diabase. Its fine grain and the microliiic character of its 
feldspars are due to the rapid cooling of the rock mass. The lack of 
the larger microlites in the neighborhood of the vacuoles is due to the 
fact that in those places comparatively large surfaces of the rock mass 
were exposed to cooling which there went on more rapidly than else- 
where, so that the time before the final solidification of the rock was 
not sufficiently long to enable the crystals to grow to the size reached 
by those in other portions of its mass. 

Not much can be learned from the section regarding the filling of the 
cavities. In most cases the softer materials contained in them are 
removed in the grinding. Along the walls may sometimes be seen 
adhering a small amount of a colorless mineral with a low double 
refraction. It occurs in little aggregates resembling very closely the 
qoartz mosaics of certain quartz porphyries. 

Other smaller cavities may be distinguished from the true vesicles by 
the fact that their sides are ragged and not smooth. The filling, a 
dirty green sabstance having very little effect on polarized light, 
extends far back between the microlites of the rock; and that portion 
of the rock mass immediately contiguous to them is not more compact 
than the portions away from them. These two facts taken together 
indicate that the cavities have an origin different from that of the 
larger vesicles. The irregular ])enetration of the filling mass into 
the rock material indicates that the boundary between the two has 
encroached on the material of the rock. Since the feldspar microlites 
project into the greenish substance filling the cavities, it is evident 
that the interstitial groundmass yielded more rapidly than the feld- 
spathic material. These cavities are consequently not true amygda- 
loidal ones, but they resemble these in that both are often filled with 
the same minerals. They are supposed to be secondary in origin, i. e.,. 
to have been formed by the decomposition of the groundmass of the 
^k containing them, and are distinguished from true amygdules by 
the designation psendamygdules. 

No analysis of the rock is given, because it is so much decomposed 
and so full of secondary products that very little regarding its original 
comijosition would be learned from it.' 



'rwde«criptioD8 of similar arayfrdaloidfl from the Lake Superior region see R. Pnrnpelly: Proc. 
A*. Acad. Adv. 8ci., 1878, XII. p. 282; and R. D. Irving, Copper-bearing rocks of Lake Superior, 
^Tj.S. G«ologicia Surrey, Vol, V, Waahington. 1884, pp. 87-W. 



358 THE EDUGAtlOKAL BERI^B Of ROCS SPECIMEKB. [Boii^iii. 

No. 140. QUABTZNOBITE-GNEISS. 
(From Odessa, Bigstonb Coukty, Mimnksota. Desoribkd bt W. S. Batut.) 

In the valley of the Minnesota River, in Minnesota, there exists an 
area of crystalline schistose rocks a few miles wide and extending ^ith 
very many interrui)tion8 from New Ulm in a northwesterly direction 
to the southeastern end of Big Stone Lake. Among the rocks occur- 
ring in this area are interbedded gneisses, crystalline schists, and 
other foliated rocks that may be regarded as squeezed ernptives. They 
are all thought to be pre-Algonkian in age, though so far as known no 
proofs of the correctness of this view have as yet been recorded. 

The specimen described in this article was taken from a small out- 
crop 1,200 paces north and 1,500 west of southeast comer, sec.9,T. 
120 N., R. 45 W., near Odessa, Minnesota.* 

In the field the rock appears to be quite schistose, with a strike 
about northeast and a dip of about 45^ to the northwest. According 
to Prof. C. W. Hall, of the University of Minnesota, who collected the 
specimen, the outcrop is isolated, being separated from all others by a 
distance of some two or three hundred paces. Consequently its rela- 
tions to the other rocks in the vicinity can not be made out. 

Very little can be learned from an inspection of the hand specimen. 
From the ease with which thin slabs may be broken from it we are 
justified in supposing it to be schistose, and this supposition is rendered 
certain by an examination of the specimen with a lens, especially on 
slightly weathered surfaces, where black acicular particles may be seen 
with their long axes lying in the plane along which the rock breaks 
most easily. 

We may, then, describe the rock as a very dark-gray crystalline aggre- 
gate with an obscure schistose structure and a sugary texture. Its 
grain is so line that but one of its constituents may be recognized with 
any degree of certainty. This is a black mineral with a tinge of brown. 
It is easily cleavable, and it glistens brilliantly from its cleavage snr 
faces. A flake of this mineral sprung from the specimen with the point 
of a penknife and placed under the microscope is found to have a nearly 
uniaxial interference figure, which, with the other properties above 
mentioned, indicates a biotite. In addition to this there can also be 
detected a granular white mineral, whose particles are so small that 
their nature can not be determined. The specific gravity of the rock is 
2.770. 

In thin section, the character of the rock becomes very apparent ^^ 
consists essentially of quartz, plagioclase, and a pyroxene, with biotite, 
garnet, pyrite, and magnetite as accessory constituents. 

In natural light there appears an apparently homogeneous ootorles^ 
groundmass, in which lie large, irregular grains of a very highly refra^' 
tive, almost colorless, mineral with well-marked cleavages, smaller dark- 

> See Map, PI. 29, in The Geology of Minnesota, Voi I of final report, Kina«apoUa, 1884, p. 9li. 



ai.1 DRSCKTPTIONS: NO. 140, QUARTZ-NORITB-GNEI88. 359 

own flakes, Bmskll irregalaror roanded grains of an opaqae substance, 
id occasionally isolated highly refractive grains of a very light pinkish 
Qge, or aggregates of these. When examined with a low power, a 
irtain parallelism may be detected in the arrangement of the various 
inerals (see PI. XLY, A), though this is not so well marked as in the 
iseof the mica-schist from the Black Hills (PI. XLII). The schistosity 
f this rock is beautifully shown in the photograph, where the pjroxene 
nd biotite grains, and to a less marked degree tlie plagioclase grains as 
rell, have their long axes running in the same direction. The light- 
olored substance without cleavage cracks is quartz, that containing 
lark inclusions is plagioclase, and the dark inclusions are its decom- 
)osition products; the dark-gray grains with heavy cleavage lines are 
3yT0xene, and the darker fibrous masses on the edges of these are bio- 
nte. In the upper right-hand corner is a large flake of the last-named 
nmeral, and on the right hand side of the center of the picture is a 
^D of magnetite. The supposition grounded upon the macroscopic 
examination of the hand specimen, viz, that the rock is schistose, is 
band to be correct, as is also the implied conclusion that tUe schistos- 
ity is not well marked. 

When examined under crossed nicols the apparently homogeneous 
gnmndmass bre^iks up into an aggregate composed of very irregularly 
^l2i\m\ interlocking grains of quartz and feldspar. Both are perfectly 
pellucid in natural light and both contain the same kinds of inclusions. 
They may be distinguished from each other by the brighter polariza- 
tion colors of the quartz and the twinning striations of the feldspar. 
From the intricate manner in which they interlock it is impossible to 
determine which is the older. Both include small particles of all the 
other components, and therefore they must be younger than these. 
"fhey contain, also, tiny liquid inclusions, a few with movable bubbles, 
large quantities of black, red, and green dust, and irregularly outlined 
green and reddish inclosures that appear to be either decomposition 
products of some substance whose nature can no longer be determined 
or secondary infiltration products. The latter supposition seems to be 
ifion* plausible, since the character of the inclusions is so varied, while 
>t the same time they are undoubtedly such as are produced by sec- 
ondary agencies. 

The twinning bars of the plagioclase are usually parallel, but some- 
times they appear in two series, crossing each other at nearly right 
angles. The maximum symmetrical extinction of two contiguous 
lamellft', measured against their line of contact, is about 26^, indi- 
(^athig a plagioclase somewhere in the labradorite series. The specific 
gravity of the mineral is 2,673. In any given position of the section 
^tween the nicols in most grains only one portion of the lamellae 
of any series is dark, the other ends of the same lamellce remaining 
tagbt. As the section is revolved the dark portions become bright, 
^edark zone gradually moving toward what were the bright ends in 



S60 THE EDbCAtlONAL SEftlES 6P ftOCK dPECIMENS. t"mi.lM 

the first position. This gradual movement of the dark portion of th 
field from one part of a mineral to another is known as nndolator 
extinction. It is supposed in most cases to be due to a strain nndc 
which the mineral exhibiting it exists. Its occurrence in the presei 
instance is probably evidence that the rock was produced by pressui 
from some previously existing rock of a somewhat similar character. 

The irregular, highly refractive grains have a slight pleochroism i 
pale greenish and pinkish tints, the latter, when the most prominei 
of the two cleavages, being perpendicular to the vibration plane of ti 
lower nicol. The extinction is parallel to the cleavage where on] 
one set of cleavage lines is observed, and on sections on which tw 
cleavages <are seen it bisects the angle between these. The axis o 
elasticity parallel to the single cleavage is smaller than the one g\ 
right angles to this, and the double refraction is not great. The polari- 
zation colors are consequently not brilliant. Since the long axes of 
nearly all the grains run in about the same direction, which is parallel 
to the plane of schistosity of the rock, and the sections are made 
parallel to' this plane, it is not surprising that so few basal sections of 
the mineral can be found. In the one or two that may be discovered 
in each section there are two series of cleavage lines, which make with 
each other angles of about 88^ and 92^. Ko inclined extinctions are 
to be seen in any sections where only one set of cleavage lines appears. 

The mineral, which is an orthorhombic pyroxene, is usually quite fresh 
and free from inclusions. In sections cut from weathered portions ol 
the rock, however, an interesting alteration is observed to be in prog 
ress. The cleavage cracks of the pyroxene are filled with a brown^ 
pleochroic substance without definite morphological characteristic^ 
This brown substance gradually spreads into the surrounding aagite. 
replacing it in part, so that often what at one end is a perfectly fiesli 
almost colorless, pyroxene is at the other end a mass of brown, pleo 
chroic substance. As the amount of the brown substance increases, it< 
characteristics become more pronounced, until finally it takes theforn 
of biotite. This biotite often lies embedded in augite, often it border* 
large grains of this mineral, and sometimes it occurs in the spaced 
between neighboring grains. There can be no doubt that this biotite 
is an alteration product of the pyroxene. 

The greater portion of the biotite in the section is in isolated graim 
between the colorless constitaents, quartz and plagioclase. Thelool 
axes of its grains are nearly always approximately parallel to tb< 
long axes of the pyroxene. The mineral is dark brown, almost opaqo^ 
in the direction of its single cleavage, and is a brownish yellow at rigl^^ 
angles to this. The pleochroic halos observed in the micas of otbc' 
rocks are found also in these, but their pleochroism is not very m»rk«<l' 
It is, of course, impossible to state positively that this biotite has th< 
same origin as that found in and near the pyroxene, but since it ^^ 



MM]. DESCRIPTIONS: NO. 140, QUARTZ-NORITE-GNEISS. 361 

icily the same properties as the secondary mica, we may infer that 
is probably of the same origin.^ 

Ihe only constituents remaining to be described are the garnets, 
rite, and magnetite. The first mentioned are in almost colorless, very 
ghly refiraiCtiYe grains, without cleavage and without inclusions, 
ley are irregular in cross section, but approximate more or less 
osely to circles. They may be easily distinguished from all other con- 
itaents^ since they are perfectly isotropic between crossed nicols. The 
iige grains are traversed by irregular fracture lines, distinguishable 
rom cleavage cracks by their lack of parallelism. The garnets are found 
requently as inclusions in quartz and feldspar, or in the interstices 
)etween these. They are evidently among the earliest formed of the 
rock^s comx)onents, as the contonrs of all the others, except pyroxene, 
where they touch the peripheries of the garnet are molded by them. 
The pyroxene is regarded as older than the garnet, for the reason that 
the contours of the garnet depend somewhat upon those of the 
pyroxene for their shape. 

The pyrite and magnetite are both irregular, opaque grains that may 
he distinguished from one another by the brassy luster of the former 
in reflected light and the bluish-black luster of the latter. 

In an attempt to name the rock we are met with the difficulty always 
experienced in discussing the classification of schistose rocks. No 
systematic scheme for their classification has yet been i)ropo8ed. It 
would seem best to confine the term schists to foliated aggregates of 
quartz and some bisilicate without feldspar, and the term gneisses to 
BcbiBtose aggregates of feldspar and some bisilicate with or without 
quartz. According to this distinction our rock is a gneiss, since it con- 
Biste essentially of plagioclase and pyroxene; i. e., it is a schistose rock 
with the comx)osition of qnartz-norite. Eoth^ calls such rocks ^^zob- 
tenites,'' while Dathe calls them "flaser-gabbros."^ Gordon* suggests 
that when the origin of a gneiss is known it receive a compound name, 
the first term of which shall designate the original rock from which the 
gneiss was derived, so that as much knowledge as possible concerning 
the rock may be comprehended in its name. If we accept this sugges- 
tion the name of our rock should be quartz-norite-gneiss. Professor 
Ball, who has made a careful study of the rock in the field, thinks 
there can be no doubt that it was once a typical member of the gabbro 
&mily. The microscopic examination confirms his opinion. It shows 
the rock to be a very slightly altered quartz- norite that has undergone 
pressure sufficient to render it schistose. 

Norite-gneisses are exceedingly rare, as are also gabbro-gneisses in 

' Cf. M. E. Wadnworth. Bull No. 2, Geol. Survey Minn ., p. W. For literature on alteration of augite 
V>biotite, see W. S. Bayley, BulL U. S. Geol. Survey, No. 109, p. 41. 
'AQgemeine and Chemiacbe G«oIO|(ie. Bd. II, p. 484. 
*Zdte d. dentsch geol. Oeaell. 1877. p. 327. 
*BaU. GeoL Soc America, vol 7, 1696, p. 122. 



362 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS. lBuii.150. 



which the p3rroxene and plagioclase are perfectly fresh,^ for the pressure 
that so squeezes a massive rock as to flatten its constitaent grains 
prodaces a coDdition of affairs very favorable to the alteration of 
pyroxene into hornblende. In the present case alteration has been 
effected in the rock, bat the change has been from pyroxene into 
biotite.' Schistose gabbros in which hornblende exists in large qaao* 
titles have been described in a number of foreign publications, and 
in several published in this country.^ 
An analysis of the rock, as reported by H. K. Stokes, is here given: 

Jnal^Hs of quariz-norite-gneiM from OdesHty Minnesota. 



SiOx. 
Al,0, 
Fe^Oi 
FeO.. 

c«o.. 

MgO. 
KaO.. 
Na^O. 
Ign .. 
FeS,. 



Totel 



Per c«nt. 



Gl.M 

16.97 

5.58 



5.W 
3.82 

.55 
1.96 

.43 
3.73 



99.87 



The l.d6 per cent of NaeO corresponds to about 34 i)er cent of labr» 
dorite among the rock's constituents. After making allowaDoe for the 
amount of Si02 required by the pyroxene, biotite, and garnet, there 
remains over about 20 per cent of the oxide, which must be regarded 
as existing in the rock as quartz/ 



No. 141. GABlTETIFEBOtTS HORNBLENDE SCHIST. 

(From Hanover, Grajton County, Nkw Hampshire. Described bt W. S. 

Bayley.) 

The specimens of this rock were collected at Hanover, Orafton 
County, New Hampshire, where it was originally thought by Professor 
Hitchcock^ to belong with the Lisbon group of schists. In a later 
paper, however, tbe same author is inclined to regard it as igneoas, 



iFor deacriptionn of hornblende freo gabbroitic gneisses, see accounts of augite-scliists, by Vi*"''' 
mann, Geology of Wisconsin, Vol. III. 18«0, p. «45: and R. D. Irving, ib., Vol. IV, pp. 6n and eM-*^: 
and Fifth Annual Report U. S. Geol. Survey, 1865, p. 211. 

»See G. H. Williams, Bull. U. S. Geol. Survey Xo. 62, for discussion of effect of presiiure upon *•* 
minerals of the massive rocks. 

»Cf. G. H. Williams, Bull. U. S. Geol. Survey Xo. 28, and description of specimens Kos. 109»ndl« 
in this bulletin. 

^This rock will be further described in a paper now in preparation by C. W. Hail, deslgo*!^ ^^ 
publication as a bulletin of the I*nited States Geological Survey. 

*C.H. Hitchcock, Geology of the Connectiout Valley District: Geol. of New Hampshire, Vo^ ^^ 
p. 363. Concord, 1877. 



KO. 141, GARNftTlFEftOUS H0RNBL£NDE-8CHI8T. 363 

occurs in *^ bunches, varying in size from a peck measure to a 
) miles long. . . . On the northwest side of the principal 
the schist comes successively in contact with mica-schist, hydro- 
shist, argillite, and chlorite-schist, all of which have been altered 
h h(^t into vitrified and indurated rocks, usually richer in silica 
rhen unaltered. On the southeast side the adjacent rock is 
]ibly mica-schist, somewhat indurated. . . . The present atti- 
ii the igneons hornblende is like that of the modern laccolite 
the cap has been worn away."* 

specimens of the rock are schistose, and some are banded with 
ating, narrow, dark and light bands. The rock as a whole has 
k greenish-gray color, which is produced by large numbers of 
^r black needles of hornblende embedded in a flue-grained white 
fty matrix. The bands of the banded specimens are portions of 
)ck in which these two substances alternately predominate. In 
licker of the lighter layers the white material is quartz, that 
to have been introduced along joint cracks in the rock.* 
ides hornblende and quartz, two other minerals are observable 
i hand specimens— garnet and biotite. The former appears as 
dark-red dodecahedral crystals, and the latter as dark bronzy 
1 with a very brilliant luster. The mica plates and the hornblende 
» are arranged with their longer axes approximately parallel to 
anding of the rock, thus accounting for the schistosity in this 
ion. 

ier the microscope the rock is discovered to be composed of 
rous green hornblende prisms and large plates of brown biotite, 
^Is of garnet, and grains of magnetite, in a colorless matrix which 
en crossed nicols is resolved into an aggregate of quartz and 

hornblende is usually elongated parallel to the c axis. Occa- 
ly a cross section with the characteristic prismatic cleavage is 
red, but not frequently. While the majority of the prisms are 
1^ with their longer directions approximately parallel, many of 
rger crystals lie in a direction inclined to this, thus interfering 
he perfection of the rock's schistosity. The hornblende is strongly 
iroic, as follows: c, bluish green; b, yellowish green; a, yellow, 
^vage is fairly well marked parallel to the longer axes of the 
9. In addition to this there is a parting, nearly at right angles 
t cleavage. 

I biotite is not found in all sections, though it occurs in greater or 
[uantity in most of them. It is in large flakes with a distinct 
ige and a reddish-brown and yellow pleochroism. The extinction 
irly zero. The biotite is evidently one of the latest formed of the 
» components. It not only incloses needles of hornblende and 



.GeoL Soc. America, Vol. VII, p. 511. 

Z. % Van Hise. Principles of North American PreCambrian Geology : Sixteenth Ann. Kept. 

BQl Barr^, pp. M2-«4, OM, 667. 



364 THE EDUCATIONAL SERIES OF ROCK SPECIMENS. Ibuu.180. 

grains of plagioclase and qaartz, but its plates occur mainly in the 
spaces between neighboring hornblendes. 

The garnets appear in thin section as light pink^ i>erfectly isotropic 
bodies, that include a few grains of quartz and some magnetite. Thoagh 
completely idiomorphio, they were plainly formed after the hombleDde 
and the biotite, as may be seen by the examination of their contacts 
with these minerals. 

The magnetite occurs as small irregular masses and grains scattered 
through the colorless groundmass and as comparatively large crystals 
with an octahedral habit. A noticeable feature in connection with the 
crystals is the presence around them of a zone free from hombleode 
and biotite. The iron that is necessary to the production of these 
latter minerals appears to have been appropriated by the magnetite 
in the manufacture of its crystals. 

The matrix in which these constituents lie is a hypidiomorphic gran- 
ular aggregate of triclinic feldspar and quartz. The latter mineral is 
the more abundant. It occurs in irregular grains of a larger size than 
the feldspar grains. The quartz grains are usually simple Occasioually 
they are complex, a single one sometimes breaking up between crossed 
nicols into a number of differently oriented parts. Very rarely a grain 
exhibits the undulatory extinction. Usually the quartz is water clear 
and is free from inclusions. In some sections, however, a few grains 
include small groups of colorless prismatic crystals that resemble a zeo- 
lite. The strong polarization of the quartz in which they are embedded 
has prevented the accurate determination of their nature. 

The feldspar of the matrix is mainly plagioclase; a few grains resem- 
ble microcline. The feldspars, like the quartz, are in irregular grains 
which show little or no effects of pressure. The majority of the grains 
are crossed by parallel twinning bars with a symmetrical extinction of 
from 18^ to 22^ against their line of junction. These are probably 
andesine or labradorite. In the darker bands, where hornblende is 
abundant, the plagioclase contains few if any inclusions; bnt in some 
portions of the lighter bands, where the plagioclase is most abundant, 
the grains are full of inclusions of hornblende, biotite, and magnetite. 
The microcline is represented by a few grains only. They are distin- 
guished from the plagioclase grains by the two series of twinning bars 
crossing each other at nearly right angles. 

The study of the thin sections throws but little light on the problem 
of the origin of the schist. The garnets, the biotite, and the horn- 
blende are probably secondary, i. e., they are not original secretion* 
from a molten magma. Whether the components of the matrix ar^ 
also secondary or not can not be told. It is impossible to discover^ 
therefore, from a study of the specimens in the collection, whether tb^ 
rock from which the schist was derived was originally a quartz diorit^ 
a diabase or a gabbro. From its mineralogical composition it may quit^ 
safely be inferred that it was originally one of these three types. ^ 



«ui.i descriptions: no. 142, steatite. 365 

;faemical analysis of a series of carefnlly selected specimens of the 
-ock would probably show that it was originally igneoas, and it might 
letermine the type of igneoas rock to which it belonged, for Adams^ 
las shown that in all probability sedimentary and igneous schists may 
»y this means be distinguished from each other, and Bosenbasch^ has 
)roved that a massive rock in its change to a schist suffers little chemi- 
cal change. Whatever may have been the original pature of the rock it 
s now a hornblende-schist, since it consists largely of hornblende and 
quartz. The structure of the rock from which it was derived has been 
completely lost in the changes which it has Huff(Bred in passing to its 
present condition. 

No. 142. Steatite, ob Soapstonb. 

(Fkom Frakcestown, H1LL8BORO County, New Hampshire. Described by 

W. S. Bayley.) 

Specimens Ko. 142 is from the quarries of the Francestown Soap- 
stone Company, at Francestown, New Hampshire. The rock has been 
qaarried at this place since 1802. It is nsed largely for the manu- 
facture of stoves, the refuse and waste being sold for packing mate- 
rial. The soapstone is said to occur as a bed in ferruginous and mica- 
eeoas schists belonging to the Merrimac group of sediments, which is 
placed by Hitchcock ^ in the Paleozoic. 

"The peculiarity of the stone," writes Hitchcock,* *'consi^^ts in the 
uniform distribution through it of spherical radiated aggregations of 
crystalline plates of talc. These make the stone uniformly strong in 
all directions." The rock in thin sections appears to be a fibrous mass, 
and where the fibers do not overlie one another so as to interfere they 
ve dark when parallel to the plane of vibration of the light.' 

The specimens in the collection are sawn slabs, consequently their 
sur&ces do not present the characteristic appearance of the rock as 
^Q in nature. On a fresh fracture, such as may be obtained by break- 

• 

H off an edge of the block, the rock exhibits an irregular aggregate 
of silvery gray plates, which here and there are grouped into the little 
^iierical masses referred to by Hitchcock. Occasionally a small grain 
of a dark metallic mineral, pyrrhotite, may be detected among the talc 
plates, but no other constituents are observable. So far as can be seen 
the rock is massive. There is no evidence of foliation in it. 

In thin sections the following components are noticed in addition to 
tbose mentioned above: chlorite, calcite, biotite, magnetite, quartz, 

' ^' D. Adams : Am. Jour. Sci., 3d series, VoL L, 1805, pp. 58-69. 

*H. BoMnboscb : Zar AnfEksimg der chemischen Natur dert Omndgebirges, Tsohermak's Min. and 
f«tiog.Mitth.,Xn,p.49. 

'CH. Hitchcock, Geology of the Merrimao District, East Part: Geology of New Hampshire, Vol. 
«i Chap. VI, pp. 688-591, Concord, 1877. 

'C.H. Hitchcock, Economic Geology, Pt V: Geology of New Hampshire, Vol. m, p. 87. 

*G.W.H»we8, Mineralogy and Lithology, Pt. IV: Geology of New Hampshire, Vol. HI, p. 118, 
««>«rt,U78. 



366 THE EDUCATIONAL SERIES OF BOCK SPECIMENS. [BUULliOL 

limoDite, and ratile. The rutile and limonite are present in very Rmall 
quantity, tbe former as tiny irregular grains and the latter as little 
crystals disseminated throaghont the chlorite and talc. Tbe magnetite 
is also iu irregular grains, but they are not evenly scattered over tbe 
section. They are much more abundant in the vicinity of calcite, some 
grains of this mineral being well filled with them. This fact suggests 
that the iron and the calcite are secondary substances derived from 
some original component by alteration. The quartz is not found ia aJI 
sections. When it is present it occurs as hypidiomorphic grains asso 
ciated usually with chlorite. From its general appearance and the 
shape of its grains, it appears to be a secondary infiltration. Biotito 
is more abundant than any of the other accessories. Cinder low powen 
its little plates look like brown stains on the almost colorless talc and 
chlorite fibers. When examined under high powers the ^< stains'' are 
discovered to be very thin scales of a reddish-brown biotite, with the 
usual properties of this mineral. 

The calcite, chlorite, and talc are the essential components. All are 
practically colorless. The calcite is granular and the other two min 
erals are fibrous. The former appears often as large irregular graDnles, 
with a higher refractive index than any other of the colorless minerals 
in the rock. It is distinguished by its strongly emphasized rhombo- 
hedral cleavage and by its brilliant polarization colors. The most 
usual occurrence of the mineral, however, is in the form of little veins 
interposed between the folia of the micaceous components and as nests 
filling the spaces between neighboring grains of talc and chlorite. 

The two micaceous constituents, talc and chlorite, constitute the 
greater portion of the rock, and the talc is the more abundant. Both 
minerals are found in plates, fibers, and folia. They are both colorless, 
or nearly so, both possess a parallel series of well-marked cleavage 
lines, and both extinguish parallel to these cleavages. The chlorite is 
slightly pleocbroic in very light-green tints, and has a very low double 
refraction. Its interference colors between crossed nicols are gray, 
blue, or white. The talc is nonpleochroic. Its double refraction is 
strong, and its interference colors are in brilliant red and green tints. 
There is no difficulty in distinguishing between the two minerals. 

In structure the rock is radially fibrous. The folia of talc and chlo- 
rite are rarely straight. They are usually curved and bent to such an 
extent that every plate shows the undulatory extinction. Large bent 
and crumpled folia lie in a felted mass of smaller fibers, which in tarn 
are curved and crumpled in the same manner. It is this interlacing o' 
fibers rather than the occurrence of spherical aggregates of talc tb^ 
gives strength to the rock. 

Nothing can be learned from thin sections of the rock concernioS 
its origin, except that, since its components are all secondary, the roc^ 
itself is probably secondary. There is nothing in the nature of the^ 
components, however, and nothing in the structure of the rook tk*^ 



ijo.] BE8CRIPTIOHB: NO. 143, OABBRO-DIORITE. 367 

iggests the natare of the original. Professor Hitchcock ^ in a recent 
kper regards steatite as an alteration product of material originally 
Tieons, and Koth^ declares that generally steatite is a secondary sub- 
ance produced from minerals that contain bnt little alnmina, especially 
om angite, hornblende, and olivine. The chlorite associated with the 
lie originated from aluminous hornblende and augites. If the original 
ock contained olivine this may have yielded magnetite in its decom- 
loaition. If Rothes statement expresses a fact the soapstone of Fran- 
«Btown may be an altered diabase or diorite, containing some olivine, 
[he only phenomenon observed in the thin section that may shed some 
ligbt on the question is the accumulation of magnetite and biotite in 
certain areas, which may possibly have once been occupied by olivine. 

No. 143. HOBNBLENDE-GABBBO-GNEISS (GABBBODIOBITB). 
(Fbom Franklin, Baltimore County, Maryland. Abstract by J. P. Iddinos 

FROM the description BY GeOROE H. WiLLIAMS.} 

This rock has been described by George H. Williams^ as a product of 
alteration of gabbros by processes of dynamic metamorphism. The 
Tock varies in character from place to place, grading from massive 
gabbro into massive hornblendic rock in some places. In other places 
hornblendic rock possesses a well-marked schistose structure. 



When thns foliated the rock cleaves with greater or less readiness aloDg one plane, 
which, BO far as observed, appears to be parallel to the foliation of the adjacent 
gneiMSB. Where this struotare is most eminently developed, all traces of the gabbro 
disappear, and the diorite has much more the appearance of one of the so-called 
<*n8talline schists than of a massive rock.-* 

The specimens in the Educational Series represent the more schistose 
phase of the gabbro-diorite, and might be described as a somewhat 
feldspathic amphibolite. The rock is greenish black and distinctly 
crystalhne, and consists of hornblende and a subordinate amount of 
feldspar. These are arranged in parallel layers not clearly distinguished 
except in thin section. 

The homblttDde is the usual grten irariety common in massive diorite and crystal- 
Uneichists. It almost never occurs in well-defined crystals, but in confused aggre- 
&>^i of imperfectly developed individuals. In the rock from Franklin, the horn- 
Mende is in compact crystals which are perfectly idiomorphic. The extinction 
'Qgle, measured on splinters parallel to the prismatic cleavage plane, gave an average 
^^t of about 13^. The pleoohroism is very pronounced and of the character nsnal 
in hornblende, i. e., the a ray, nearly parallel to the clinodiagonal axis, is light 
J«llow ; the h ray, coincident with the orthodiagonal, yellowish green ; and the c 
^y, dark bluish green. The absorption is here, as in all hornblende, C > h > a. 

Tke ordinary hornblende of the diorite is quite free from inclusions, but in certain 
^ImeDs crystals much larger and more compact than the average occur, in which 

'Joor.Oeol. Vol. Ill, p. 58. 

*J>Rotli: Allgemeine nnd ChemiBcho Geologic, Vol. II, p. 541. 

*0«Qrge H. Williams, The gabbros and aasociateil hornblende rocks occurring in the neighborhood 
^ Britiinore, Maryland : BulL U. & Geol. Survey No. 28, Washington, 1886, p. 27 et seq. 
*l^clt,p.32. 



368 



THE EDUCATIONAL SERIES OF ROCK SPECIMENS. (■cu-lM 



minnte black needles are so abundant as to render the mineral qnita opaqne, do 
matter how thin the section may be. These needles are short and stoat, being often 
BO rounded as to present an elliptical outline. They vary considerably in stze^ bat 
even the largest are not more than .04 ""^ in length and .002 ^^ in width. In prismatic 
sections of the hornblende, they are seen to be arranged parallel to the vertical axii, 
and also in two directions, making an angle of about 45^ with this. In basal sectioni 
they appear to follow, for the most part, the prismatic cleavage lines, though thej 
are also less frequently disposed in other directions, especially in that parallel to the 
cliuodiagonal. 

These inclusions furthermore are always central in their arrangement, the periphery 
of the hornblende crystal being free from them. When particularly abundant the 
smaller needles appear to run together, forming irregular, opaqne masses, much 
resembling magnetit<e. That these inclusions are in reality magnetite is indicated 
by the fact that such crystals of hornblende as contain them most abundantly m 
attracted by the magnet, while others are not. 

The feldspar of this rock is auorthite, yielding extinction angles oq 
the basal plane (001) as high as 22^ to 28^, and on the brachypinaooid 
(010) as high as 36o. The chemical composition of the feldspar, aft 
determined by W. S. Bayley, in the variety of this rock from Pikesville, 
is as follows: 

Analysis of feldspar from homhlende-gahhro-gneiss of PikesvilU, Maryland, 



RiOt. 
AWO. 
CaO.. 
Na,0 



Total 

Speoitio gravity, 2.74. 



Per oent. 



45.06 
35.60 
18.30 
.OSa 



100.00 



a By difference. 

This corresponds closely to the composition of a feldspar oomposed 
of eight molecules of anorthite to one of albite, Ans Abi. The crystals 
exhibit polysynthetic twinning, and occasionally dnst-like inclasionB, 
similar to those found in the feldspars in the gabbro from this region 
and elsewhere. The amount of feldspar in this rock is very small, more 
so than in the gabbro specimens. 

The subordinate or accessory minerals are very few in number and 
of very small amount. They are colorless apatite, pyrite, sphene, and 
minute crystals of epidote, and larger ones of irregularly shaped zoisite. 



] 



DESCRIPTIONS: NO. lU, GABBRO-DIOBITE. 



369 



The chemical composition of the gabbro-diorite from near Pikeeville, 
Maryland, as determined by Leroy McCay, is given in the following 
analysis: 

Analysis of gabhro'diariie from Pikeaville, Maryland. 



SiO, 

TiO, 

AlA 

Fe,0, 

FeO 

MnO 

MgO 

CaO 

Na,0 

K,0 

F,0« 

H^ , 

Total 

Sp. gr., 2. 006. 



Per cent. 



46.85 

.30 

20.02 

2.30 

4.60 

trace 

10.16 

13.84 

1.32 

trace 

trace 

.88 



100.27 



Its similarity to those of the gabbro of the region has been pointed 
out by Williams. 

No. 144. HORNBLENDIO GABBRO-GNEISS (GABBRO-DIORITE). 

{From Minnesota Falls, Yellow Medicine County, Minnesota. Described 

BY W. S. Bayley.) 

Specimen ^o. 144, like No. 140, was found among the crystalline 
>6bJ8t8 of the Minnesota Valley. It occars at Minnesota Falls of the 
Hiooesata Birer, 600 paces north and 100 west of southeast corner, 
•«c. 11, T. 115 K, K. 39 W., Minnesota.* In