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DEPARTMENT OF THE IHTEEIOE

BULLETIN

OF T1IK

GEOLOGICAL SURVEY

No. 17

WASHINGTON 

GOVERNMENT PRINTING OFFICE

1885
L
UNITED STATES GEOLOGICAL SURVEY

J. W. POWELL DIEECTOR

ON THE

DEYELOPMENT OF CRYSTALLIZATION

IN THE

IGNEOUS ROCKS OF WASHOE NEVADA

NOTES ON THE GEOLOGY OF THE DISTRICT

ARNOLD HAGUE AND JOSEPH P. IDDINGS

WASHINGTON

GOVERNMENT PRINTING OFFICE

1885

CONTENTS,

Page.

Letter of transmittal........................................................ 7

Introductory................................................................ 9

Diabase and augite-andesite............................................. 12

Sutro tunnel section........ .....*........................................ 18

Granular diorite......................................................... 2t

Porphyritic diorite and earlier hornblende-andesite....................... 22

Mica-diorite and later horublende-andesite ............................... 23

Quartz-porphyry, dacite and rhyolito .................................... 26

Younger diabase, black dike, and basalt................................. 27

Geological and chemical evidence........................................ 29

Conclusiona.......... ................................................... 39

ludex....................................................................... 43

(125) 6

LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR, 

UNITED STATES GEOLOGICAL SURVEY,

Washington, 1). C., February 16,1885.

SIR : I have the honor to present herewith a paper by Mr. Joseph P. 

Iddiugs and myself, on the development of crystallization in igneous 

rocks, the result of an investigation of the extensive and well-selected

lithological material collected by Mr. George F. Becker during his exanimation of the Coinstock lode.

Many field geologists have maintained that the acidic lavas and pumices occurring on or near the surface would be found changed to grnnite, if it were possible to trace them downward to great depth. This 

has been denied, especially of late years, by other equally eminent 

authorities, who have claimed that geological periods were characterized 

by crystalline rocks, differing in composition and structural features.

Although much has been written upon the relation between surface 

flows and deep-seated igneous rocks, the problems have never been 

solved. Indeed they could not be solved before the application of the 

searching methods of microscopical research now employed in the 

study of crystalline rocks. Few localities afford the necessary con ditions, and none surpass the Washoe district in furnishing the requisites 

for a proper investigation of such questions.

So far as I know, the results of no similar investigation showing the 

gradual transition in crystallization under pressure in continuous rockmasses have ever been published.

As the-results of the work lead us to definite conclusions upon some 

important questions and bear directly upon the geology of the Comstock lode, it seems desirable that they should be published by thernselves in a special bulletin as early as possible. 

Very respectfully, your obedient servant,

AKNOLD HAGUE,

Geologist. 

Hon. J. W. POWELL,

Director United States Geological Survey,

Washington, D. C. 

(127) 7

ON THE DEVELOPMENT OF CRYSTALLIZATION IN 

THE IGNEOUS ROCKS OF WASHOE.

BY ARNOLD HAGUE AND JOSEPH P. IDDINGS.

INTRODUCTORY.

> The Great Basin of Utah and Nevada, which has been the center of

such widespread volcanic activity, presents the greatest possible variety 

* of phenomena connected with the geology of igneous rocks. Within 

this well-defined area outbursts of lavas occur, forming high, rugged 

peaks, low, monotonous ridges interspersed with broad tables, extendV ing for such long distances and covering such wide tracts of country 

^ as fairly to deserve the title of plateaus. Again, extravasated lavas may 

be seen as small isolated craters, as massive eruptions breaking out 

through lines of fissure and faults, and stretching out for miles along 

the base of the longitudinal ranges, or they may occur in small irregular hills, and occasionally in narrow dikes scattered over the country 

without any apparent law as to their distribution. Probably most geologists who have studied the region would agree that the country has 

nowhere been subjected to any extensive erosion since the outbursts of 

the greater part of these lavas. In consequence nearly all the later 

extrusions exhibit on the surface much the same features as they presented at the time they were forced upward through the underlying 

sedimentary strata or older crystalline rocks. While the region is one 

characterized by lavas of both acidic and basic types, presenting various glassy forms, there occur innumerable rocks showing every variety 

of microstructure, from nearly pure glass to others not only wholly crystalline but with a decided granitoid structure in the groundmass. The 

dikes which penetrate the older rocks are found in most cases to show 

a more crystalline structure than the massive eruptions or surface flows. 

This is especially the case with basic rocks; and although their connection with larger masses can seldom be traced, the conclusion seems 

inevitable that they must form portions of the same original magma, 

crystallized under different conditions. Although there has been collected throughout the area of the Great Basin a large amount of material which pointed in one direction and suggested a common origin 

and close geological connection between these extreme forms of rock

(129) 9
10 DEVELOPMENT OF CRYSTALLIZATION. IBULL. 17.

structure, the subject has not been before closely studied, and direct 

proof of such an identity has been wanting. This is in part owing to 

the fact that erosion has failed to cut deeply into any great flow showing variations of structure, and in part to the want of sufficient data 

and material from any one locality upon which a searching mineralogical and microscopical examination could be undertaken, which would 

afford the necessary proof to warrant the assertion that difference in 

structure was mainly the result of consolidation under varying degrees 

of heat and pressure, or an expression of the rate of cooling.

In studying the collections of lavas from the Pacific coast volcanoes 1 

we were forcibly impressed with the insensible gradations in the microstructure in the groundmass of rocks of the same mineral composition 

from a purely glassy form to one wholly crystalline, and corresponding 

exactly in structure to a fine-grained granite-porphyry. The chain of 

microscopical evidence seemed so complete that we were convinced that 

the glassy and crystalline rocks were simply the extreme forms of the 

same magma, and that a highly crystalline rock brought in for diabase 

from one of the ravines of Mount Eainier could be nothing less than a 

crystalline variety of the hypersthene-andesjte, which, judging from the 

collections, makes up the greater part of the volcano.

In seeking a locality in the Great Basin which could afford the necessary conditions for carrying out such an investigation as we desired to 

make, showing the actual transition from the glassy to the granitic 

structure, it was readily seen that the Washoe district was the only 

place offering sufficient material for the work. Indeed, there are few 

localities in the world presenting conditions which would permit of such 

an inquiry, and so far as we know the investigation has nowhere been 

previously undertaken, at least upon material adequate for a satisfactory solution of the problem. For the means of making this investigation we are greatly indebted to Mr. George F. Becker, who placed at

our disposal the large lithological collections which he and his assistants had gathered in the course of their field-work preparatory to his 

report on the geology of the district. Owing to the unprecedented activity in mining which has characterized the district during the twentyfive years of its existence, we are enabled to examine the interior of the 

mountain by actual sections in a way unsurpassed in any other place. 

In addition to the magnificent horizontal section, over 4 miles in length, 

shown by the Sutro tunnel, and numerous vertical sections cut by shafts 

from 2,000 to 3,000 feet in depth, it is estimated from the official maps 

and records of the mining surveyors that over 180 miles of galleries and 

other openings have been run, and although a large part of these galleries pass through quartz and ore bearing material of the vein many 

miles of levels in exploring new ground penetrate in every direction the 

east and west country, laying bare the internal structure of the mountain.

'American Journal of Science, Vol. XXVI, September, 1883. 

(130)
HAGUE AND HIDINGS.] VIRGINIA RANGE. 11

If the Virginia range were split asunder across the center of Mount 

Davidson it would not be possible to gain a more accurate knowledge of 

the region than is presented by this net-work of underground explorations. In the former case we should get a section of the mountain down 

to the level of the surrounding plain, but now we have, by means of the 

many deep shafts, a knowledge of the lower rocks many hundred feet 

below the level of the Carson Desert. And instead of a section cut on 

a vertical plane we have a mountain honeycombed in every direction 

by the untiring miner in search after bonanzas. Thus no natural section 

could surpass it for purposes of studying internal rock structure at 

great depths.

A mining region which has proved of such vast economic importance 

as the Comstock lode has necessarily excited intense geological interest, 

and owing to the great liberality of the National Government, scientific 

investigation of the district has gone on, with brief intervals of a few

years, almost simultaneously with the exploitation of the precious metals. 

The amount of geological literature on the district which has been published is by no means inconsiderable, and a very large share of the 

space in the reports has been devoted to a consideration of the igneous 

rocks.

A brief summary of the results obtained by the more important investigators, including those of Baron von Eichthofen, 2 Mr. Clarence 

King, 3 Prof. Ferdinand Zirkel, 4 and Dr. John A. Church, 5 is given by 

Mr. Becker in his elaborate work on the Geology of the Comstock 

Lode, recently issued. Since the publication of von Eichthofen's report, 

nearly twenty years ago, followed quickly by his monograph on The 

Natural System of Volcanic Eocks, the knowledge of crystalline rocks 

has advanced with rapid strides, and the science of microscopical petrography has been almost wholly developed. In this advancement the 

study of the Tertiary volcanic rocks of the Washoe district has played 

a conspicuous part. In selecting, then, this district for the special purpose of this work, we have been guided by the knowledge that the principal geological features of the region are within the reach of all students of petrography. The results of our investigation, undertaken 

merely as incidental to a study of the volcanic rocks of the Great Basin, 

have proved so much more satisfactory and convincing than we were 

at first led to hope that we have no hesitancy in publishing them at this 

time. Moreover, the geological conclusions reached from the study of 

the lithological material are in many ways so diametrically opposed to 

those of our predecessors that we offer, no apology for adding another

2 The Comstock Lode, etc. Ferdinand von Richthofen. San Francisco, 1868. 

3 United States Geological Exploration of the Fortieth Parallel, Vol. III. " Mining 

Industry." Clarence King. Washington, 1870.

*Id., Vol. VI, "Microscopical Petrography." Ferdinand Zirkel. Washington, 1876. 

fi The Comstock Lode, etc. John A. Church. New York, 1879.

(131)
12 DEVELOPMENT OF CRYSTALLIZATION. [BULL. 17.

brief paper to the many already published on the geology of the Washoe 

district.

Mr. Becker, in agreement with his predecessors, divided the igneous 

rocks of Washoe into Tertiary and Pre-Tertiary species, differing with 

them widely, however, as to where the lines should be drawn, and adding one or two more varieties .of Pre-Tertiary rocks to those already 

recognized. His work possesses the very great advantage over the labors of those who preceded him, in that not only the Suttro tunnel, but 

all the deeper shafts and galleries were accessible to him, a large 

amount of ground having been opened since the earlier geological investigations were made. This recent material is of great value. The 

rocks obtained from these lower workings, when subjected to microscopical examination, so closely resemble in structure the so-called PreTertiary types, that Mr. Becker extended the domain of the "older" 

rocks over a considerable area at great depths below the surface. This 

same material from the deep-seated localities affords, it seems to us, 

abundant evidence that the so-called Pre-Tertiary and the Tertiary 

rocks pass by insensible gradations into one another, and that evidence 

of a difference in geological age is wholly wanting.

Mr. Becker classified the rocks under the following heads : Granular 

diorite, porphyritic diorite, micaceous diorite-porphyry, quartz-porphyry, 

earlier diabase, later diabase, earlier hornblende-andesite, augite-andesite, later hornblende-andesite, basalt.

Eepresenting these rocks from all parts of the Washoe district, we 

have had placed at our disposal more than two thousand hand specimens, six hundred from the surface, and over fourteen hundred from 

underground. From this collection there are now more than five hundred thin sections, including a number recently prepared to fill up certain gaps in the chain of our argument. Of these specimens those from 

underground, with the exception of the Sutro tunnel rocks, were collected mainly within the immediate neighborhood of the Comstock lode, 

while the surface specimens are confined to an area about 5 by 7 miles 

in extent.

Let us begin our analytical work by submitting the rocks to a searching microscopical and macroscopical scrutiny, to show what points of 

agreement exist between the so-called Pre-Tertiary and Tertiary eruptions, comparing rocks of the same mineral composition with each other.

DIABASE AND AUGITE-ANDES1TE.

In considering the relation between these closely related rocks we 

will first make a comparative study of the thin sections prepared from 

both series. In the collection there are about one hundred and forty 

thin sections determined as unquestionably diabase or augite-andesite,

(132)
HAGUE AND iDDiNos.] GRADES OF CRYSTALLIZATION. 13

and of these about twenty from each have undergone too much decomposition to allow of a satisfactory study of their feldspars, although they 

evidently agree in general characters with the fresher sections of the 

same class of rocks. Of the remaining one hundred thin sections forty 

were classed as diabase and sixty as augite-andesite.

Upon examining them somewhat hastily under the microscope two 

points are brought out in the most impressive manner. The first is the 

absolute identity in the nature and occurrence of the mineral constituents of the two rocks, and the impossibility of distinguishing many of 

those classed under one head from those classed under the other; the 

second is the gradual transition from a microlitic glassy groundmass 

to one holocrystalline, and from a microcrystalline groundmass of the 

minutest grain to one in which even the smallest grains are recognizable in thin section with the aid of an ordinary pocket lens. In studying them they may be arranged under eleven heads, according to the

size and development of the grains of their groundmass, placing those 

with glass base at one end and those of the coarsest grain at the other. 

The different grades of crystallization may be stated approximately as 

follows:

1. Groundmass consisting of colorless glass, through which are scattered relatively few microlites of feldspar and pyroxene, and magnetite 

grains with contorted trichites. PorpJiyritic crystals, abundant plagioclase varying from 3 mm. long to very small size, averaging 1 or 2mm long; 

pyroxene ctystals less numerous. Example, thin section 50.

2. Groundmass of colorless or light-brown glass crowded with microlites of feldspar and pyroxene, and magnetite grains, showing a typical "felt-like" structure. Porphyritic crystals the same as in grade 1. 

Examples, thin sections 239, 426.

3. Groundmass, holocrystalline, composed of feldspar microlites, and 

indistinct grains about .005mm in diameter, with grains of pyroxene 

and magnetite. Porphyritic crystals as in grade 1. Example, thin 

section 422.

4. Groundmass, holocrystalline, composed of feldspar microlites and 

grains of pyroxene and magnetite, the cementing material being crystallized in irregular, patches with uniform orientation for the space of 

about .03imu . Porphyritic crystals as in grade 1. Examples, thin, sections 121, 122.

5. Groundmass, holocrystalline, the same structure as in grade 4; 

but the indistinct patches about .05mm in diameter, though not of uniform 

size throughout the thin section. The feldspar microlites measure 

about .02mm long. It is sometimes composed of grains from .01 to .02inm 

in length with feldspar microlites. Porphyritic crystals as in grade 1. 

Examples, thin sections 35, 420.

C. Groundmass, holocrystalline, the same structure as in grade 5; the 

patches averaging as high as .lmm , full of feldspar grains .Olmm in diameter and feldspar microlites .05mra long by .Olmm wide. Porphyritic.

(133)
14 DEVELOPMENT OF CRYSTALLIZATION. [BULL. 17.

crystals, abundant plagioclase varying in different thin sections from an 

average of 1.5mm to 2.5mm j pyroxene as in- the preceding. Examples, 

thin sections 125, 389.

7. Groundmass, holocrystalline, composed of feldspar microlites about 

O.lmm long with granitoid grains of feldspar and quartz from .Oimm to 

.04ram in size, together with grains of pyroxene and magnetite. Porphyritic crystals, plagioclase average about 2.5mm , and begin to lose the 

sharpness of their outline. The same is true for the outline of the 

pyroxene sections. Examples, thin sections 169, 441.

8. Groundmass, holocrystalline, composed of granitoid grains of feldspar and quartz about .06mm in diameter, and lath-shaped plagioclase, 

in variable proportions, besides pyroxene and magnetite. Porphyritio 

crystals, plagioclase 2.5mm and smaller j the outlines of some rather indistinct; pyroxene as in grade 7. Examples, thin sections 53,173.

9. (Groundmass, holocrystalline, granitoid grains of about O.lmm and 

ill-defined lath-shaped plagioclase. Porphyritic crystals, plagicciase of 

3mm and smaller, some with very irregular border; pyroxene as in grades 

7 and 8. Example, thin section 215.

10. Groundmass, holocrystalline, granitoid grains, about O.lmm in size, 

with porphyritic crystals nearly equaling the groundinass in amount. 

Porphyritic crystals, plagioclase 3.5mro and smaller j pyroxene showing 

little change. Example, thin section 55.

11. Groundmass^ holocrystalline, composed of plagioclase feldspar of 

1.5ram and smaller, with fewer granitoid grains of about O.lmm . In 

places it shows fine pegmatoid structure, formed of quartz and plagioclase, surrounding plagioclase crystals. Porphyritic crystals, plagioclase 3mm long ; the sharply-defined zonal structure of the main mass 

of the crystal surrounded by a poorly-defined border of feldspar, which 

loses itself among the adjacent grains j pyroxene, sometimes in welldefined, frequently in irregular-formed, crystals. Example, thin section 213.6

 

6Agreeing with Professor Eosenbusch in general as to the origin of granular and 

porphyritic Structures in eruptive rocks; " Ueher das Wesen der kornigen und porphyrischen Structur bei Massengesteinen" in Neues Jahrbuch, 1882, II Band. We 

would briefly define the structural terms used throughout this paper as follows:

Granular. The structure presented by an aggregation of grains of nearly the same 

size, or an aggregation in which there is a range in the size of grain, but with such 

a gradual transition that no particular individual stands out from the others in strong 

contrast.

Granitoid. A structure in which the grains have an irregular form, produced by 

the mutual interference of adjacent crystals during their growth. The structure 

common to normal granites.

Porphyritic. The structure which is produced when more or less well-defined crystals are imbedded in a groundinass, which is either glassy or holocrystalline, composed of crystals or grains of much smaller size than the imbedded crystals.

Pegmatoid. The structure produced by the intergrowth of two or more minerals, 

usually quartz and feldspar, in such a manner that when seen in thin section they

(134)
HAGUE AND ionises.] TRANSITIONS IN GROUNDMASS. 15

Iu other words, we see that the porphyritic crystals show but slight 

modification in the different grades, the sharp outlines of the feldspars 

and pyroxenes being lost as the grains in the groundmass become larger, 

the nature of the inclusions in the feldspars also varying, as noticed 

further on. The essential variation takes place in the groundmass, 

which starting with a glass bearing few microlites, grows richer in them, 

then crowded until a very small amount of glass remains as a cementing base. This may be crystallized in microscopically cryptocrystalline 

particles, whose mineral composition is not determinate. As the size 

of the grain increases the cementing material assumes the appearance 

of uniformly oriented patches which, with increasing grain, are found 

to be in part quartz. In the higher grades the microlites have larger 

dimensions, and the cementing grains a granitoid form with or without 

pegmatoid structure.

In the highest grade (macroscopically a fine-grained porphyritic rock) 

it is difficult to draw any line be.tween porphyritic crystals and groundinass, the more or less well-defined feldspars ranging from 3mm in length 

to the size of the average granitoid grains of feldspar and quartz.

It is interesting to notice the appearance of a small amount of quartz 

as the last crystallizing mineral in the coarser-grained varieties of this 

pyroxene rock. It corresponds to the free silica., which remains after 

calculating the theoretical mineral composition from the chemical analyses of this rock, the excess being from 3 to 6. per cent.

Though the difference in the degree of crystallization between any 

two divisions of the scale is so slight, yet it represents more than actually exists, for the forms pass by imperceptible gradations into one another. If now the thin sections of both rocks are arranged according 

to this classification in columns as shown in Table I, we get at a glance 

some very striking results.

appear as groups of grains of more or less regular form, eacli group having one optical 

orientation throughout. It is similar to the structure in graphic granite.

Granopltyre. The variety of pegmatoid structure in which the intergrown minerals 

are arranged in radiating, plumose or feather-like forms.

Microcrystalline. An aggregation of crystalline grains which can only be recognized as such by the aid of the microscope. This includes many rocks which would 

be macroscopically cryptocrystalline, aphanitic, or lithoidal.

Microcryptocrij8talline. A.n aggregate of crystalline particles so small that the highest magnifying power fails to resolve them into individual grains.

(135)
DEVELOPMENT OF CRYSTALLIZATION. 

TABLE I. Grades of crystallization.

[BULL. 17.

1.

50

2.

d9R

49fi

239

45

43

37

3.

453

A99

316 100

1T7

41

4.

499

377

322

318

243

1Q7

186

184

147 

140

109

74

62

OQ

00

Q9

115

121

oog

374

410

5.

465

447

440

323

321

315

296

180 

178

177

143

44

42tin

35

308

344

346

365

405

420

276

6.

378 

181

175

125

124

69

38

31

16

114

341

357

389

7.

158

441

15

18

54

77

169

312

3:(6

339

340

350

386

8.

173

72

1

53

<?4Q

433

0.

14

215

348

10.

55

11.

27

280

The numbers in this and the following tables refer to the thin sections in the Washoe collection.

It appears that the coarsest-grained rocks with a well-defined granular structure are found only in the so-called diabase, while the extreme 

forms of andesite, as seen in column 8, show a tendency to a development of the same structure. On the other hand, the glassy and cryptocrystalline varieties are confined exclusively to the andesites. Moreover, the rocks of medium grain include much the larger number of 

thin sections from both series, the greater part of the diabase show, 

ing a coarser structure than most of the andesites, with, however, only 

a difference of about .02mm in the size of grain. Now, each of the five 

grades (4 to 8 inclusive) for one rock has its exact counterpart in the other 

in size and shape of grain and structure of groundmass. On comparing 

the thin sections it becomes apparent that not only is the grouudmass m 

each rock absolutely the same, but the porphyritic crystals are identical, giving the same results to every possible test that can be applied. 

They consist of the same species of triclinic feldspar, pyroxene, and 

occasional hornblende. The feldspars in rocks of corresponding grade 

present the same outlines, striations, similar extinction angles, zonal 

structure, and the same amount and character of inclusions. So nmch. 

stress has been laid upon the nature of the inclusions in the feldspars 7 

that an exhaustive study was made of them, comparing thin sections 

from rocks of the same degree of crystallization without discovering 

any difference. In each thin section the individual feldspars exhibit 

great diversity in the nature and amount of their inclusions, some bearing scattered rectangular glass bodies, others abundant smaller ones

Geology of the Comstock Lode, page 50. 

(136)
IIAGUB AND ICINGS.] PYROXENE-ANDESITE. 1 7

with rounded outlines, others being rich in grains of pyroxene and magnetite, others, again, combining all three, or, what is by no means uncommon, being nearly free from all inclusions. Fluid inclusions which might 

be considered primary are scarce in the medium-grained rocks, but were 

not found more abundantly in the "diabase" than in the andesite. As 

might be supposed, glass inclusions in the feldspars are more numerous 

in the glassy and fine-grained rocks, but upon careful search they may 

be observed in every thin section of the "diabase,'' from 1 he finest to 

the coarsest. No line of demarcation can be drawn between these rocks 

based upon the glass-inclusions in their feldspars. In general, however, 

it may be said that the number of glass-inclusions decreases with increase in the degree of crystallization. In other words, they are a function of crystallization. This includes all thin sections given in the 

table, those in which the feldspars arc clouded by decomposition being 

unfit for such a study. The fresh pyroxene consists of the same hypersthene and augite in both rocks, the hypersthene equaling or exceeding the augite in amount, and being the first to yield to decomposition.

Owing to the number of thin sections from these rocks arid their 

various degrees of alteration, it is easy to trace the decomposition of 

hyperstheue through all its stages, the augite in the same section reniiiining intact.8

This investigation leads then to the conclusion that in the thin sections of both rocks of the same degree of crystallization not only is 

the composition and structure of the groundmass identical, but that 

the relative abundance, size, and character of the porphyritic crystals 

scattered through this groundmass are the same.

Arranging now the hand specimens from which the thin sections were 

prepared in rows with the diabase and augite-andesite of the same degree of crystallization opposite each other, their macroscopic characters 

are seen to be identical. They agree in color, texture, fracture, and 

porphyritic crystals. Variations in one rock arising from stage ofdecomposition and amount and size of porphyritic secretions find their exact 

counterpart iu the other. The conclusion then is inevitable that the 

greater part of the diabase is identical microscopically and macroscopically with the greater part of the augite-andesite and presents the 

transition between the glassy forms at one extreme and the coarsely 

crystalline forms at the other, and further, that the presence of a large 

percentage of hypersthene removes these rocks from the type of augiteandesite, properly so called, and places them under the head of pyroxeneandesite.

8 Notes on the Volcanic Rocks of the Great Basin, American Journal of Science, 

June, 1884.

Bull. 17 2 (137)
18 DEVELOPMENT OF CRYSTALLIZATION. [BULL,IT.

SUTRO TUNNEL SECTION.

Having carefully examined all the specimens in the collection classed 

as diabase and augite audesite, and shown that they belong to identical 

rock masses, let us now take up and study step by step the transition

 which occurs in a continuous rock mass along a definite line. For this 

work the Sutro tunnel, constructed for the purpose of draining the Cornstock lode, furnishes an exceptionally fine geological section through 

20,400 feet, or nearly 4 miles, of igneous rocks lying east of the vein. 

The tunnel enters near the lower foot-hills in Carson Valley, and runs ;i, 

little north of west, approximately at right angles to the course of the 

vein and trend of the range, and directly toward Mount Davidson, the 

highest and largest mass of the mountain group. Probably few lines 

<could have been selected more advantageously situated for exhibiting 

the structure of the region. A collection of two hundred and seventy

 specimens, carefully selected at intervals of 100 feet and less, together

 ^vith their numerous thin sections studied in connection with the surface rocks immediately above ground, shows clearly the different kinds

 of rock which the tunnel penetrates and their variations iu structure. 

Referring to Sheet VI of the Atlas9 we observe that, starting from the 

mouth, the tunnel, according to Mr. Becker, passes through later hornblende nndesites with occasional masses of older augite-andesite, and 

^at 10,000 feet enters the main body of the latter rock. Thin sections 

from about this point show it to be holocrystallinc, of the grades 

4, 5, and 6. The rocks for the next 2,000 feet are much decomposed, 

onauy of them full of pyrite. Between 11,400 and 11,700 feet occurs a dike of dense white rock, resembling a broad dike met with 

further in the tunnel. From 12,000 to 13,700 feet the andesites show 

snore or less alteration, the porphyritic crystals being small, and the

iron magnesia silicates indeterminable in the hand specimens. Of the 

five rocks in this part of the tunnel, of which we have thin sections, 

four carry hornblende associated with pyroxene, two showing a largo 

amount of the former. At 13,700 feet a dike of quartz-bearing horn-

 blende-pyroxene rock is met with possessing quite a different groundmass structure from that of the andesite through which it cuts. Be-

 tween 13,800 and 13,900 feet another white dike occurs like that already 

'^mentioned, followed by a hornblende-pyroxene rock of coarse grain 

((grade 8). Here the tunnel cuts a dike 1,400 feet in width of a dense 

?wMte rock carrying relatively few porphyritic crystals of feldspar and 

^still less mica, but without hornblende or pyroxene, and having a 

.groundinass-structure wholly unlike any found in the andesites. In the 

.Sutro tunnel section this rock is indicated by lines of shading denoting solfataric action, but it is quite evident that it is younger than the 

andesite, as it is found penetrating it. The rock in every way corre9 Geology of the Comstock Lode. Atlas, Sheet VI. 

(138)
HAGUE A.ND IDDIKGB.] DIKES IN SUTRO TUNNEL. L9

spends in structure to the so-called felsitic quartz-porphyry from the 

surface near Koux's ranch and the Eed Jacket mine.

Following- this dike of white rock the next 1,000 feet of the tunnel 

cuts through augite-andesite, the nine thin sections of which range in 

.grades of crystallization from four to seven, three showing hornblende. 

At 17,100 feet a characteristeric rock is met, rich in fresh, brown hornblende, and having the same inicrostructure as the andesites previously 

 observed (grade 7), yet so much less altered than the surrounding rocks 

on both sides that it seems evident that L occurs here as a dike. According to the published map section, however, there begins here a 

continuous body 1,500 fV-et in width of horn blende-audesite, but the 

hand specimens studied nuu:roscopically show it to correspond in every 

particular to the augite-andesite; and of the eleven thin sections from 

this body, with the exception of the fresh rock above noticed, only two 

show small amounts of hornblende, the remaining eight not carrying any 

that could be definitely determined. They range in crystallization from 

grades 5 to 8 eight, and have the same microscopical character as the 

augite-andesite to the east. It appears to be cut in three places by 

mica-hornblende rocks differing essentially from the surrounding andesite, and corresponding in microstructure and mineral composition to 

the so-called mica-diorite from the head of Ophir Eavine (thin section 

459). .Referring again to Sheet VI, it will be seen that the tunnel after 

passing through the body represented as horubleude-andesite enters 

the diabase. Again, hand specimens from several hundred feet upon 

«ach side of the supposed line of contact are indistinguishable from one 

another, and the thin sections show identical structure and the same 

degree of crystallization.

Examining now the twenty-seven thin sections from the diabase cut 

by the main tunnel and the north and south branches, 10 the first of which 

is 4,400 feet, and the second 4,000 feet in length, they are seen to vary 

in degree of crystallization (with one marked exception) from grades 

5 to 10, averaging 8, or two grades higher than the average of augiteandesite. In the hanging wall west of the Savage mine connection 

the tunnel goes through diabase, and again also west of the vein, 

then passing through diorite it again cuts diabase 180 feet west of the 

Savage connection, and at the end encounters a mica-diorite like that 

observed further to the east. An examination of the thin sections recently made from the diorite showed in one case that the iron magnesia silicates, now completely altered to chlorite, were pyroxene and not 

hornblende, and in the other two cases, while their original nature is 

not determinate, the microstructure of the rocks is the same as that of 

the most crystalline diabase.

Let us reject for the present the thin sections which represent the 

dikes or later intrusions cutting through the main rock mass, and place

- lo Geology of the Comstock Lode. Atlas, Sheets VIII and IX.

: (139)
DEVELOPMENT OF CRYSTALLIZATION. [BULL. 17.

the others in columns (Table II) according to the scale of crystallization, 

already adopted, arranging them in the order in which they occur in 

proceeding from the mouth to the head of the tunnel.

TABLE II. Grades of crystallization.

(

I

Diorite ..................<

Diabase ................ .^

Distance 

from 

mouth of 

tunnel.

41 

49 

54 

403 

1,400 

2, 500 

5, 234 

9,761 

'9,800 

9,971 

10,000 

10, 055 

12, 000 

12,300 

12,500 

12,600 

13, 700 

14, 090 

15, 680 

16. OG6 

16, 384 

16, 700 

16, 758 

16, 800 

(?)16,884 

Hi, 92 1

16, 921 

17, 000

17. 100 

17,300 

17,500 

17, 600 

17, 700 

18, 100

18, 300 

18, 500 

18, 600 

18, 700 

19, 000

19,155 

19, 200 

19,464 

19, 540 

19, 700

19, 910 

*19,910 

19, 971 

20, 100

(t) 

20, 120 

20, 160

f

 § 

a

ca J J-t \

f

 3

3

o 

oa

11 ( 3 03 1 

*$

1.

....

2.

....

3.

138

316

151

338

4.

J40

318 

32

Q1

147

153 

154

74

5.

141 

144 

315

321

6.

31

71

34

323

324

63 

325

328

329

330 

331

344

34fi

149

95

69

290

335

341 

342 

343

16

347

7.

158

326

oqo

77

80

15

1 ft

169

 JOft

337

QQft

f\4.

340

OJK

3^fi

8.

79

007

332

334

00

506

17

9.

14

508

507

q<ifi

|

10.

55

11.

* Hanging wall of Savage.

(140)

t Lode limit Savage.
UACUH AND iDDisos.] UBALIT1C HORNBENDE. 21

The important points clearly brought out by the table are, first, that 

there is in general a gradual increase in the degree of crystallization of 

the rock from the outer foot hills as the tunnel approaches the central 

core of Mount Davidsoii; second, that while the rocks classed as augiteamlesite occur more at the lower end of the scale, and the diabase at 

the other, by far the greater part of both rocks overlap each other in the 

intermediate columns, and are indistinguishable from one another. The 

glassy iorms of pyroxene-andesite, abundant enough on the surface, are 

unknown in the tunnel. It is worthy of note that as the north branch 

approaches Mount Davidson there is exhibited a tendency toward a 

coarser crystallization, while on the other hand the south branch, as it 

turns away from the central mass, indicates an equally strong tendency 

to a finer-grained structure

GRANULAR DIORITE

Reference has been made in the text and in Table II to the diorite 

at the head of the Sutro tunnel upon the west side of the Comstock 

lode, which we have placed with the pyroxene rocks. This rock, classed 

by Mr. Becker as granular diorite, forms the cast front of Mount Davidson and presents the most conspicuous topographical feature in thedisl trict. Of the three specimens of granular diorite from Mount Davidson having thin sections only one (213) was originally determined as

^- unquestionably diorite, the others being left doubtful. Thin sections

L recently prepared from three typical localities, namely, the summit of 

Mount Davidson (488), Ophir Kavine (489), and the Sutro tunnel (507

^ and 508), show that they are identical with those previously examined.

. Moreover, they agree in every particular of microstructure and mineral 

character with the coarsest-grained pyroxene rock found to the east of

>- the vein from the north branch of the Sutro tunnel (55) and the 0. and 

O. shaft (27).

The light-green, fibrous hornblende shows its uralitic nature by 

having the characteristic eight-sided outline of pyroxene in cross-section; Both the diabase from Ophir Kaviiie (210) and the diorite from 

the Savage mine (463) show the transition of the pyroxene into fibrous

~ hornblende. While both rocks are at present hornblendic, it is evident 

that their hornblende is for the most part secondary, and that they are 

coarsely crystalline pyroxene rocks with the hyj.erstbene and augite

r altered to uralite. n

The identity of the granular diorite with the coarse-grained diabase

^ __________________._________.______________________

11 The diorite from Ophir Kavine (40fch Parallel Collection, No. 22620, thin section 

**>- 157) shows both nrnlitic hornblende aud fibrous hornblende, which appears to have 

resulted from primary hornblende. Some of the p'a^ioclase bears minute glass inelusions; the groundmass shows fine peguiatoid structure.
22 DEVELOPMENT OF CRYSTALLIZATION. [BULL 17.

is well brought out on comparing thin sections 507, 508. 488, 489, 213r 

214, 300, and 463 of tbe former with 14, 280, 215, 21G, 27, and 55 of the 

latter; also on comparing hand specimens 109, 215, 303, 567, 568, and 

1125 with 307, 264, and 364.

The difficulty of distinguishing between these rocks seems to have 

been realized by Mr. Becker, for he admits that the gray granular 

diroite may be mistaken for granular diabase; 12 and again, in speaking 

of tbe locality in Ophir Kavine, 13 says: "Ibis bears a very strong outward resemblance to a granular diorite, and it seems impossible to make 

out a sharp contact between tbe two rocks. I am by no means sure that 

it should not be regarded as a local modification of diorite rather than 

an independent eruption."

PORPHYRITIC DIORITE AND EARLIER HORNBLENDE-ANDESITE.

If we arrange the surface specimens on a table as nearly as possible 

in accordance with their relative positions on the map, we are at once 

impressed with the similarity between those labeled diorite and many 

classed as andesites and with the difficulty of establishing any line of 

demarcation between the two rocks. Another remarkable fact is tbe 

gradual transition from tbe most crystalline forms of which Mount 

Davidson is chiefly made up, outward as from a center to less crystalline and glassy varieties.

It soon becomes evident, upon a cursory examination carried on in 

the same raanuer as in the study of the proxeue rocks that the different 

varieties of porphyritic diorites have as to macroscopical habitus their 

exact counterparts among the earlier hornblende-andesites. The following surface specimens may be cited for the purpose of comparison1 1

Diorite. Hornblenfle-andesite.

481 like 157. .

478., 474 « 493.

125 " 439.

483

482

306

449,312

185,477,41

59,479,42,40

558

211,78,528,531.

213.

116.

486,487.

225,184,158.

217.

494,532,536.

Every variety is here represented; there may be found those with 

large porphyritic crystals, and those with very small ones, ranging 

from much-decomposed to quite fresh forms. They agree in texture and 

color of grouudrnass, in character of fracture and mode of weathering,

12 LOG. (At., p. 39. Loc. lit., \>. 197. 

(142)
HAGUE AND iDDiKGB.] HORNBLENDE-MICA-ANDESITE. 23

and in size and relative abundance of porphyritic crystals. In a wordr 

by every niacroscopical test that can be applied they are indistinguishable in the hand-specimens. Taking up now the sixty thin sections representing the surface collection of both these rocks and subjecting them 

to the same comparative microscopical scrutiny applied to the pyroxene 

rocks, they still fail to present any distinguishing features. The groundmass of the porphyritic diorites varies from cryptocrystalline to microcrystalline, formed sometimes wholly of lath-shaped microlites, others-, 

having a granular structure with microscopic feldspars scattered through^, 

it. Now, for every variety of groundrnass among the porphyritic; dio 

rites from the surface there can be found among the andesices identical] 

rocks of the same degree of crystallization, corresponding in structure,, 

size of grain, and mineral composition. The same identity holds good 

for the porphyritic crystals; the feldspars show the same inclusions, 

and the hornblendes and pyroxenes present precisely similar characters

in both rocks. Quite in accordance with the observations upon thepyroxene-audesites there are a few hornblende-andesite sections which 

show glass in the ground mass, and at the same time a corresponding 

increase, in the amount of glass-inclusions in the feklspars, but by far 

the greater number of rocks represented are holocrystalline.

MICA-DIOIMTE AND LATER HORNBLENDE-ANDESITE.

i*.

The rocks classed by Mr. Becker as later hornblende-andesite 14 we 

prefer to call hornblende mica-andesite, for the reasons that they are 

characterized in distinction from the earlier horublende-andesite by an. 

abundance of mica as an essential ingredient, and are at the same time-

^ more acidic in composition; and while the name later hornblende-

^ andesite answers well enough for the Washoe district, it is hardly ap.. 

plicable in other regions of the Great Basin where these rocks frequently

*- occur, forming large extrusions without any association on the surface 

with earlier hornblende-andesite. The specimens in the collection vary 

greatly in the abundance and size of porphyritic crystals, in texture and 

color of groundmass, as well as in the relative proportions borne by the 

constituent minerals to one another. This variation in habitus is greater 

than in either hornblende or pyroxene andesite. The mineralogical features and diversity of habitus have been well described by Mr. Becker 

in his monograph. 

The mica diorites are few in number and represent comparatively

r small masses scattered over the surface or exposed by the underground: 

workings of the mines. Nevertheless when considered in connection,

14 These rocks are the " tracbytes " of Baron A-on Ricbthofen, Mr. Clarence King, and 

Prof. Zirkel. Mr. Bocker has shown that according to the modern classification now 

generally adopted by lithologists they belong to the andesite group.

(143)
DEVELOPMENT OF CRYSTALLIZATION. [IJUT.U 17.

with the other mica bearing (liorites in the collection the.y form a much 

larger class, presenting an equally varied structure and composition^ 

aud for each variation, with the exception of the more crystalline forms, 

the exact counterpart may be found among the later boniblende-andesites. 13y placing the corresponding forms in parallel rows, as in the 

case of the diorites and earlier hornblende audesites, it is found quite 

impossible to distinguish them, the great variation in macroscopical appearance bringing out all the more forcibly the absolute identity of the 

two rocks.15

The thin sections clearly show the same correspondence in minute 

structure, although a thorough comparative study is not always possible, as the sections from the mica-bearing diorites are mainly from underground specimens exhibiting more or less decomposition, while those 

from the later horneblende-audesites are for the most part fresh. The 

only thin section (No. 101, 1,000 feet from the Silver Hill mine) of the 

fine grained mica-diorites in which the feldspars still remain uuattacked 

exhibits glass inclusions in the fresh feldspars, and in the relatively 

coarse-grained varieties (grades 8, 9, 10) they may be seen in profusion, 

while they are almost wholly wanting in those of the coarsest grain 

(grades 12, 13, 14) which abound in liquid inclusions, as might be supJ5 The following table represents the numbers of the hand specimens of the two 

rocks which correspond each to each:

Later hornblcDdeamlesite.

From Sutro tunnel collection.

7

10, 11, 12

29,30

31

32

33

34

36

43

45, 112

75

79

84

i 90, 91

100

; 103'

105

114

117

118

From surface collection.

26

79

92

97

101

103

291

292

293

302

Mica-bearing cliorite.

From surface collection. ,

505

561

563

156

556

554, 555

557

553

560

11

448, 475

564

159

From mining collection.

548,821,827,755

867, 1459

804

704

578

456

455

802, 808 (b)

803,1407

806, 807

458

809 (b) 810 (b)

388, 859, 1331

462

107,550 1 1417,1420

552

562

805

405, 1247, 1271

(144)
HAGUE AND HIDINGS.] URAD1 S OF CRYSTALL1ZATJON. 25

posed, lu this scries of rucks also we see that the range from glass to 

.liquid inclusions varies with the degree of crystallization, as already 

shown.

These last throe grades of crystallization have the following characteristics, it being borne in mind that the iron magnosia silicates in the 

series in question differ from those of the series described on pages 13 

and 14.

Grade 12. More or less well-denned crystals of plagioclase, 3.5"im 

and smaller, with very irregular patches of mica and hornblende, in a 

.groundmuss of most irregular1 }' shaped, interlocking grains of feldspar 

.and quartz, which range from 1.8mm to 0.6mm in diameter. Example, 

thin section 291.

Grade 13. The average size of the granitoid grains somewhat larger 

than in grade 12, and exceeding in many cases the size of the better 

outlined crystals. Example, thin section 460.

Grade 14. A thoroughly granitoid structure to all the mineral constituents, which range in size from 3.5inm to others quite small, the 

greater part of them averaging as high as 1.5mm. Example, thin section 268.

Arranging the thin sections according to their degree of crystallization (Table HI), the scale adopted being approximately the same as 

that given in the table for the pyroxene rocks, the relation of the later 

liornblende-andesites to the mica-bearing diorites will be seen at a 

glance:

TABLE III. Grades of crystallization.

Later hornblonde-andesite. 

I 

(

Mica-tearing diorite. ...... <

I

1.

;

2.

483

47S

477 

470 

474 

472 

48 

47 

46

3.

475 

470 

320 

319 

317 

5!) 

51

4.

471 

407 

390 

75 

C6 

05 

04

86 

91 

440 

172

5.

375" 

230 

49

101 

119 

194 

195 

197 

200 

203

205

399

41?

444

450

150

16?,

6.

39?" 

70 

67

252 

398 

402 

411 

443 

160

7. 8.

265

9.

247

10.

300 

4?0

11. 12.

291

13.

460

14.

85 

96 

268

The extreme glassy form represented in the table of pyroxene rocks 

does not happen to occur among the thin sections of later hornblendeandesite, but a large proportion of them represent a glassy groundmass 

crowded with microlites, while all of them belong at one end of the

(145)
26 DEVELOPMENT OF CRYSTALLIZATION. [BULL. 17.

series. On the other hand all of the so-called mica-diorites possess a* 

holocrystalline grouudmass, range from finely microcrystalline to a 

macrogrannlar or granitoid structure, in which the smallest grains arevisible to the naked eye. They reach a somewhat coarser grain than 

was recognized among tbe pyroxene rocks, the scale being extended, 

from eleven, in Table I, to fourteen grades. As indicated in the table, 

the three highest grades of the audesite correspond to the three lowest 

of the diorite and are identical with them in microscopical detail. Moreover more than one-half the thin sections examined fall within these 

tbree grades. A separation of the two rocks from a petrographic standpoint seems to us quite impossible. 16

QUARTZ-PORPHYRY, DACITE AND RHYOL1TE.

Baron vou Kichthofen,in his study of the Washoe district, evidently 

paid but little attention to these rocks and dismissed them with brief 

mention, regarding them as of slight importance in questions bearing 

upon the geology of the vein. He considered them, however, as earlier 

than the Tertiary volcanic lavas. Mr. Clarence King, after careful examination of the field, referred them upon geological evidence to the 

Tertiary series, and Professor Zirkel, after a microscopical examination 

of the thin sections, determined them as belonging to both dacite and 

rhyolite. The results of Mr. Becker's investigation led him to tbe conclusion that the absolute uniformity of the rock masses precluded their 

separation into Tertiary and Pre-Tertiary eruptions, while geological 

observation and an examination under the microscope of certain specimens, following the same methods of reasoning as adopted for other

rocks, induced him to agree with von Richthofen as to their age, From

our own knowledge of the district I7 and close study of the specimens 

and thin sections we have arrived at the conclusion reached by Mr. 

King as to the age of this series of rocks, and agree with Professor 

Zirkel as to their specific determinations.

Omitting for the present the geological evidences bearing upon the 

nge of the rocks plotted on the Washoe map as quartz-porphyry, letu& 

first consider their microscopical and rnacroscopical habitus. They are 

for the most part rich in porphyritic crystals of quartz, feldspar, and 

mica, with some hornblende, but vary greatly-in the relative proportions

16 Similar petrographic relations apparently exist among the rocks of the Banat 

described by J. Niedzwiedzki in an article entitled "Zur Kenutniss der Banater 

Eruptivgesteine." Tschermak's Mineralogische Mittheilungeu. 1873. iv, 255-261.

17 One of the writers of the present article was geological assistant to Mr. Clarence 

King when he made his study of the Comstock mines in the winter of 1867-'68 and is 

quite familiar with the occurrence of the igneous rocks of the Washoe district.

(146)
HAGUR AND IDDINGS.] BASALT DIKE IN MOUNT DAVIDSON. 27

of these minerals. Those with few quartzes, but rich in feldspar, plagioclase being in excess of orthoclase, accompanied by much mica and less 

hornblende, correspond to dacite in mineral composition, arid resemble 

the dacites from Mount Prometheus in the Toyabe Eange. In others, 

characterized by abundant quartzes, plagioclase seems occasionally topredominate over orthoclase; but in most of them the latter feldspar 

is in excess, particularly in those poor in mica and hornblende. The 

region is one where not only both dacite and ihyolite occur, but one 

where the transition members are well represented.

In macroscopical habitus they exhibit great variations, arising from the 

abundance or scarcity and size of the porphyritic crystals, and from the 

color and density of the groundmass. Many band specimens arc perfectly fresh and show the brilliant sanidins so characteristic of the 

dacites and rhyolites of the Great Basin, and a microscopical examination proves conclusively tlicir identity. The groundmass of many of 

them often shows within the small area of a square millimeter the greatest diversity of structure, being partly spherulitic, axioltic, and microcrystalline, with glass either clear or clouded (microfelsitic), presenting 

elongated, curved patches and streaks, a structure described in great 

detail by Professor Zirkel in his monograph on microscopical petrogiaphy 

already mentioned. The porphyritic quartz contains in most cases only 

glass, less frequently liquid inclusions, the latter, however, occurring in 

greater numbers than is usually met with throughout the Great Basin.. 

As these rocks are somewhat removed from the center of mining activity, there are no good underground exposures, and in the few places 

where they have happened to be cut in exploration they show so much 

decomposition or occur so fine-grained as to offer very little variation in 

degree of crystallization. It may be said, however, that there exist no* 

petrographic distinctions between these rocks and many of those found 

in the ranges to the eastward. We consider them as partly dacite and 

partly rhyolite, and, judging from the specimens in the collection, without any sharp line of demarcation between them.

YOUNGER DIABASE, BLACK DIKE, AND BASALT.

The rock considered by Mr. Becker as younger diabase and of PreTertiary age is nowhere recognized on the surface. It only occurs as a: 

narrow dike in the immediate neighborhood of the Comstock lode, where 

it has been followed for more than a mile along the lower levels of the 

mines. In the upper levels it has undergone so much decomposition 

that little could be made out of it by the earlier investigators, and foryears mining engineers have designated it simply as the "black dike."'

(147)
23 DEVELOPMENT OF CRYSTALLIZATION. [HULL. 17.

AccordiDg to Mr. Becker, who has given careful attention to the occurrence of this rock, it has not been traced north of the Savage mine, but 

thence south to the Overman, "it generally marks the contact between 

the older diabase and the west wall with precision." I8 It extends as far 

south as the Caledonia, presenting everywhere great uniformity, and 

never measuring more than C feet in width. The rock was represented 

by only three thin sections, showing but slight variations in structure, 

and composed of triclinic feldspar, augite, magnetite, and what Mr. 

Becker described as "clouds of a smoky brownish substance" which he 

did riot determine. Now it is quite evident that much of this substance 

occupies spaces with the six sided and lozenge-shaped outlines characteristic of olivine; this is well shown in thin section 274. Two new sections from the Yellow Jacket (1278) and the Belcher mines (1322) show 

in the former a coarser grain and in the latter a much finer grain, having a large amount of fine-grained groundmass with small porphyritic 

feldspars, angites, and decomposed olivines, the typical structure of 

many Nevada basalts.

The five thin sections, arranged in the following order, 509, 466, 289, 

274, 511, present a gradual transition from coarser to finer grain, the 

porphyritic crystals diminishing in size and number as the rock becomes 

more and more porphyritic and carries more grouudmass. The augites 

aud feldspars are fresh, while the olivines are wholly decomposed to a 

g-reeuish-browu fibrous substance, which is also disseminated through 

the grouudmass. It is to be noted that the augite in this rock is quite 

fresh, while in many of the andesites it exhibits much alteration.

Turning now to the surface rpck which has been regarded by every 

field observer as basalt, we find that it occupies very limited areas, only 

occurring in a few isolated patches, closely associated with the dacite 

and rhyolite in the neighborhood of American Flat. Thin sections from 

these surface rocks sbow a ground mass without the chlorite, similar to

that in the underground rock (thin section 511), but with fewer and 

smaller porphyritic feldspars. The augite is of the same character, and 

the fresh olivine presents the same form and size as the spaces occupied 

by the "smoky brownish substance" in the black dike. Comparing 

hand specimens of the two rocks, those from the black dike are seen, as 

might be expected, to be slightly denser than the surface basalt. In 

specimen 1202 from the dike the decomposed olivines are especially 

prominent, and give the rock exactly the same habit as the basalt specimen 398, from southwest of Eoux's ranch, the latter differing from it 

only in being somewhat porous, of a steel-gray color instead of greenishblack, and in having fresh, glassy olivines. We fail to see any petro- 

..graphic distinction between the two rocks.

18 Loc. cit., p. 199. 

(148)
HAGUE AND iDDiHGs.] DECOMPOSITION OF HORNBLENDE-ANDESITE. 2l>

GEOLOGICAL AND CHEMICAL EVIDENCE.

In the discussion of the igneous rocks of the Washoe district we have 

confined ourselves mainly to a comparative lithological -study of therocks of the same mineral composition which have been classed by Mr. 

Becker as Tertiary and Pre Tertiary eruptions, and have endeavored toshow that they were identical masses presenting insensible gradations 

from the glassy to the granular structure. We have omitted for the 

most part the geological and chemical evidences, which not only 

strengthen the position we have taken, but clearly explain many difficult problems in the structural geology of the region and the relations 

of many of the rock masses to each other. It is a fact of some interest 

that nearly all geologists who have examined the district have been impressed with the apparent great variety in the igneous rocks represented. They have all easily recognized the more characteristic types,. 

such as the granular rock which forms the bold front of Mount Davidson, the porphyritic rocks upon the flanks of the mountain variously 

designated as porphyritic diorite, propylite, and horubleride-andesite, 

and the glassy and fine-grained rocks mainly referable to pyroxeneandesite. But the moment they tried to outline their areas they were 

immediately beset with what seemed insurmountable difficulties. This 

is in part owing to the great amount of decomposition occurring in the 

neighborhood of the Oomstock lode, but in greater part to the gradual 

transition in crystalline structure. Extreme forms were readily determined, but where to draw the line between them seemed impossible to 

decide.

The supposed differences between the porphyritic diorite and hornbleude-andesite, which we can but regard as one rock, were so obscure 

as to make their determination one of great difficulty and led to a most 

confusing entanglement.13 That this confusion arose from superficial 

appearances produced by decomposition seems evident, the more decomposed forms being classified for the most part as the older rock. In 

his discussion of the hornblende-andesite, Mr. Becker says,20 that 

through the decomposition of this rock " every variation in coarseness of 

grain also becomes apparent," and " all these changes tend to diminish 

the sharp definition of the porphyritic crystals and give the mass the 

look rather of an older dioritic porphyry than of a volcanic rock." Now,, 

in speaking of the porphyritic diorite he says,21 "From some peculiarity 

either in composition or texture the porphyritic hornblende-dioriteshave undergone extensive decomposition" and only " two or three small 

masses " were found in a fresh state. From a study of these rocks it 

seems evident that the apparent coarseness of grain of most of the socalled porphyritic diorites is derived solely from the decomposition of

19 Geology of the Coiustock loclo, pp. 42, 87,89. 

z°Loc. cit., p. 58. 31 Xoc. cit., p. 39. 

(149)
 30 a DEVELOPMENT OF CRYSTALLIZATION. luuu.17.

hornblendc-audesite.22 It is a most noteworthy fact that the decomposed forms of andesite occur in areas of fresh rock, while the somewhat fresh diorites are found in areas of general decomposition. The 

propylite of Baron YOU Kichthofen and Mr. Clarence King for the most 

part belongs to this decomposed rock, the latter geologist remarking 23 

"nearly all the piopylite observed by us is decomposed." The distinguishing features of propylite were considered to be its high degree of 

crystallization and a certain characteristic habitus of the hornblende, 

the former closely relating it in structure to the hornblende porphyrites, 

while the latter character tended to separate it superficially from the an- 

<lesites with which it is unquestionably connected geologically. On the 

one hand we quite agree with Mr. Becker, who has ably shown after a 

rigid examination that a greater part of the propylite cannot be separated from the hornblende-andesite, and that there exists no evidence of 

its pre-andesitic eruption. He has clearly shown that the propylite of 

Washoe has no claim to be considered as an independent rock species, 

its change in habitus depending wholly upon decomposition. On the

 other hand our own work, showing the transition from glassy lavas to 

somewhat highly crystalline forms, compels us to agree with the earlier 

investigators, and to relegate his Pre-Tertiary porphyritic-diorite to the 

Tertiary hornblende-andesite. A comparison of the geological maps 

accompanying the works of Mr. King and Mr. Becker shows how embarrassing the difficulties were in drawing correct lines of demarcation between the supposed Pre-Tertiary and Tertiary masses. Over a very 

considerable part of the area they fail to agree. In general the latter 

geologist regards the south flanks of Mount Davidson as covered by 

horublende-andesite, and the north flanks by diorite, the former geolo- 

.gist considering both north and south flanks as covered by propylite 

and of Tertiary age.24

22 The collection contains several series of specimens showing the various stages of 

decomposition of the rocks, classed as hornblende-andesite and porphyritic diorites. 

Among the former are numhers 486, 487, and 569 to 574. Among the latter, 448, 449, 

451, 497, 498, 450, and 310, 311, 312.

23 U. S. Geological Expl. of 40th Parallel, Vol. I, p. 557.

24 In this connection it maybe profitable to review the literature upon the propylite 

of Schernnitz in Hungary, a region which present's in many respects a close resemblance 

in its geological history to Washoe. Dr. Doelter (Verhandlnngen der k. k. geologischen Reichsanstalt, 1879, p. 27), after a mineralogical and chemical study of the 

altered rocks recognizes the microscopical distinctions pointed ortt by Zirkel, especially in the quartzose rocks, but finds isolated masses which unite many of the characters of both andesite and propylite. He fails to find any difference in mode of occurrence or geological age between the two rocks. Vom Rath, probably after a study 

of the more crystalline portions, concludes that the propylite properly belongs to diabase and presents structural features quite unlike a Tertiary andesite. With his intimate knowledge of the Schemnitz region Professor Szabd, in the same number of 

the geologiscben Reichsanstalt, from a geological standpoint denies the independent 

eruption of propylite. Dr. A. Koch, in a pa.per entitled "Neue petrographische Untersuchuug der trachytischen Gesteine der Gegend von Rodna" (Verliandlungen der

(150)
AND IDDIKOB.] GEOLOGICAL SECTION OF MOUNT DAV1DSON. 31

The structure of the main body of the mountain is well brought out 

by the Sutro tunnel. It is shown to be for the most part a body of 

pyroxeue-audesite extending from the central core of Mount Davidson, 

"where it is well exposed, eastward to the outer foot-hills, where it is 

largely concealed byhornblende-rnica-andesite, which breaks through it 

and covers the surface, the relations between the two rocks being well 

exhibited by the tunnel section. As already shown, there is a gradual 

and steady increase in degree of crystallization as we pass toward the 

central core, the pyroxene andesite which forms the west country rock 

being one or two degrees coarser than the east country rock. In general the body west of the lode opposite the central portion of Mount Davidson is somewhat coarser than the rock to the eastward, but this does 

not hold true either to the north or the south. This is explained by the 

fact recognized by all geologists that the vein is situated along a line 

of great faulting, the displacement being quite sufficient to bring difiereut portions of the same rock mass into juxtaposition. Along the 

line of the vein solfataric action has produced so much decomposition,

 especially to the eastward, that comparative petrographic studies of 

the walls is a matter of considerable difficulty. Nevertheless, the distance is so great and the exposures in the many tunnels so numerous 

that the evidence of identity of both walls seems conclusive. Let us

-consider, for example, a cross-section in one mine. On the 1,450-foot 

level of the Sierra Nevada mine a small body of pyroxene rock has been 

identified to the west of the main shaft, but to the east of the shaft the 

country rock is so decomposed that sufficiently fresh material for accurate identification was not obtained for 1,600 feet. At this point, where 

unaltered material occurs, it is shown to be a pyroxene rock. Now, although the rock has undergone so much alteration au partially to preclude specific identification, there is no evidence at hand that thronghk. k. geolog. Reichsanstalt, 1880, Liceraturnotizen), is of the opinion that a separation of Griinstein-frachyto (propylite) from andesite is inadmissible, since he shows 

that both rocks are connected through transition forms. He points out that the principal difference between them is in the structure of the groundmass, the propylite being holocrystalline while the true andesite of Hungary always contains more or less 

glass. He maintains that the question whether there is any difference in geological 

age of these rocks is not yet satisfactorily determined. Dr. E. llussak, in a purely 

petrographic paper, " Giiinstein-trachyte oder Propylite," published in the same journal, states that the rocks are much decomposed and agrees with Zirkel as to their microscopical habitus. He says: "DieseGesteineiihneln den alteu sogenanten Dioritporphyren wie aucli den Diabas-porphyriten ungemein, sincl jecloch mit den tertiaren. 

Eruptivgesteineu eng verkniipft nnd wohl nicht davon zu trenneu." See also Prof. J. 

W. Judd's valuable paper "On the Ancient Volcano of the District of Schemuitz, 

Hungary," Quar. Jour. Geol. Soc., August, 1876.

Unless erosion at Schemnitz, unlike the conditions at Washoe, has worn away the 

greater part of the superincumbent rock, we are led to believe, without any personal 

acquaintance with the district, that a thorough investigation of a large number of 

thin sections prepared from judiciously selected rocks would show a complete transition from the glassy volcanic lavas to those corresponding in texture to crystalline 

diorite or diabase.

(151)
32 DEVELOPMENT OF CRYSTALLIZATION. [BULL. 17.

out its whole length it was not originally a pyroxeue-andesite. Wherever 

the rock occurs sufficiently fresh to allow a close study its identification 

with this type of andesite. seems established. All these facts convince 

us that the Coinstock lode cannot be considered as a contact vein between two different rock masses.

Owing to the intimate connection between hornblende and augite, 

and the fact that pyroxeue-andesites with varying amounts of hornblende occur, indistinguishable from many hornbleude-audesites with 

more or less pyroxene, it is impossible to draw a sharp line of demarcation between the two types. Great confusion arises when we underrake to map oat the Washoe district, where both rocks occur in every 

possible gradation between the two specific types. Such transitions 

were recognized by Mr. Becker, who, having outlined the different 

areas covered by the different volcanic masses according to geological 

age, often considered himself forced to determine a rock quite as much 

from its occurrence within certain areas as from its mineral composition, regarding the variations as local modifications.

In general the horublende-audesite here, as in many other districts,, 

seems to be more silicious than the pyroxene types, yet analyses show 

many marked exceptions. In the intermediate volcanic rocks, like 

those which make up the greater part of the mass at Washoe, where 

the range in chemical composition between the two is so slight, a variation in mineral composition may easily take place.

Doubtless much ot the confusion which exists in the Washoe district 

between the hornblende and pyroxene divisions of these rocks arises 

from the cutting of one rock by the dikes of the other in a far greater 

degree than has been heretofore recognized. Indeed, the region is penetrated by broad fissures and narrow dikes, varying in width from many 

hundred feet to those exposing only a few feet where cut by mining 

shafts and levels.

This is especially noticeable in the Sutro tunnel, which, after passing 

through the broad belt of hornblende-mica-andesite, enters, as has been 

already mentioned, a vast body of pyroxene-andesite forming the main 

rock mass as far as the tunnel runs. All other bodies, both large and 

small, cut by the tunnel present the appearance of occurring as dikes, 

many of them failing to reach the surface. These facts lend support to 

the theory that a large body of pyroxene-andesite forms the older and 

central mass of the mountain. At 17,100 feet from the mouth of the 

tunnel a remarkably fresh hornblende-andesite is cut with every indication, judging from the specimens, that it occurs as a dike. It seems 

quite possible that this may have served as a vent through which was 

extruded, a portion at least, of the hornblende rock found on the surface 

along the east base of Mount Davidsou.

In the subjoined table are presented a series of twelve chemical analyses representing the principal variations in composition and structure of 

the volcanic rocks found at Washoe.

(152)
TABLE IV.^-Chemical analyses. 1

Number.

II 

III

IV 

V

VI

VII 

VIII

X 

XI

xn

Determination.

Granular pyroxeneandesite. 

Pyroxene-andesite. . . .

Hornblende-ftndesite. .

Pyroxene-a n d o s i t o 

(hornblende bearing). 

Py > oxene-a n d e s i t o 

'(hornblende bearing). 

Hornblende -andesite .

Hornblende - mica-andesite. 

Hornblende mica-andcsite.

Locality.

North of American Flat 

Creek. 

Eldorado outcrop ........

Sutro tunnel, foot-wall, 

Savage connection. 

Center of Cedar Hill 

ridge. 

Ridge northeast of American Flat.

Silver terrace ...... .....

Cross Spur quarry below 

Graveyard. 

Mount Rose ...... ....

Crosg Spur quarry .......

800' ea st of Walter Defeat 

shaft. 

Spur northeast of McClellan Peak near American Flat road. 

S-Jiith-southc.ist of McClellan Peak.

SiOz

47.91 

50.71 

56:40 

58.55 

58.44

59.22

60. 82 

63.30 

63.13 

65.68 

69.96

73.07

TiOz

2.70

1.14

0.83

0.98

Alz03

14.26 

18.36 

15. 99 

15.48 

18.17

18.20

17.54 

17.81 

16.00 

15.87 

15.79

11.78

FezO3

1.65

3.26 

3.93

3.42 

4.34

1.78 

2.50

2,30

FeO

7.80 

6.45 

3.82 

2.07 

6.03

6:69

5.42 

0.83

1.25

MnO

trace

0.12 

0.11

CaO

9.60 

6.11 

6.98 

6.44 

6.19

5.C1

5.65 

5.12 

4.45 

3.50 

1.73

2.02

MgO

10.83 

3.92 

3. M 

3.60 

2.40

2.00

1.76 

2.07 

2. 07 

1.79 

0.61

0.39

NazO

3.01 

3.52 

3.83 

3.99 

3.20

3.31

3.71 

4.27 

3.87 

3.20 

3.80

1.19

KsO

1.89 

2.38 

1.91 

1.69 

1.97

1.39

1.41 

2.26 

2.65 

3.37 

4.12

6.84

Other components.

LizO .trace

P*O6 .-0. 32 

P2O5 ..0.30 

C01....2.87

C02....1.41 

LizO -trace

Ps05 -.0.23

Ignition.

0: 37 HzO 

1. 94 HsO 

2.47 

3:62 

0.76

2. 80 HzO

2.31 

0. 88 HzO 

2: 00 H20 

3.10 

1.53

2.24

Total.

100. 02 

99.39 

99.78 

100. 61 

100. 03

100. 02

100. 13 

99.96 

.99.54 

100. 75 

100. 07

09.83

Analyst.

Samuel L. Pen field. 

R. W. Woodward. 

Gideon E. Moore. 

Do. 

W. G. Mixter.

Do.

Do. 

R. W. Woodward. 

Do. 

Gideon E. Moore. 

F. A. Gooch.

Do.

CO

0

05

Number.

I

n

m

IV

V

VI

Designation in Mr. Becker's 

report.

Designation in Mr. King's 

report.

Number 

of thin 

section.

*528

(*)

1Q

421 (*)

-832

Number 

of specimen.

22647

OQR1 1\

270

409

OO71Q

22706

Number.

VTI

vmTY

XI

XII

Designation in Mr. Becker's 

report.

Designation in Mr. King's 

report.

Number 

of thin 

section.

*231

*283

*284

101

496

495

Numberbf speci- 

. men.

22717

O9C01

22637

506

251

249

1 These analyses have been revised by comparison with the original'records so far as possible; those which were found to be from much decomposed rocks have been omitted. 

* The numbers marked with an asterisk arc from the collection of tho Fortieth Parallel Erploration; the remainder are from Mr. Decker's collection.
34 DEVELOPMENT OP CRYSTALLIZATION. [BULL. 17.

They are mainly compiled from the table published in Mr. Becker's 

monograph and revised as far as possible by comparison with the original records. They are here arranged according to their acidity in order 

to bring out forcibly the great similarity existing in chemical composi 

tion between rocks which we have endeavored to show were identical 

in mineral composition and structural features. Numbers I, XI, XII, 

the extreme basic and acidic varieties of the series, are now published 

for the first time. The identity in composition of the hornblende and 

pyroxene-andesites, which make up the central mass of the mountain, 

is shown in analyses II to VII, inclusive, the extremes in silica (with a 

variation of a little more than 4 per cent.) lying quite within the range 

of most volcanic extrusions. Between the coarse-grained rock from 

Eldorado, outcrop (II), representing the mass of Mount Davidson, and 

the coarse-grained pyroxene-andesite (III) from the Sutro tunnel, near 

the;Savage connection, chemical analysis fails to detect any marked 

difference j while between the so-called porphyritic diorite (IV) and 

hornblende-bearing pyroxene-andesites the variations are such as might 

occur in different parts of the same volcanic flow. In a well characterized hornblende-andesite from the Cross-Spur quarry (VII) the silica 

reaches 60.82 per cent.

In order to strengthen and confirm these analyses silica determinations have been made of several rocks from other localities in the district, carefully selected to represent the same range in mineral composition. Those agreeing in the preponderance of hornblende or pyroxene are placed together as follows:

1. Pyroxene-andesite (granular diabase), 56.40 per cent. SiO2 .

2. Pyroxene-andesite (augite-andesite), 56.56 per cent. SiO2 .

3. Hornblende-andesite (porphyritic diorite), 55.66 per cent. SiO.2

4.; Hornblende-andesite (hornblende-andesite), 55.79 per cent. SiO2 . 

5: Hornblende-andesites (horublende-andesite), 57.06 per cent. Si02.

They all occur near the base of Mount Davidson from the following 

localities: 

1. Ophir ravine, southeast of Cole water tunnel.

2. 1,000 feet clue west of quarry above Occidental grade.

3. Ophir ravine, southeast of Cole water tunnel.

4. Eidge north of Ophir Hill, 150 feet west of old flume.

5. Tank 600 feet northwest of Yellow Jacket shaft.

From these it will be seen that the pyroxeue-andesite from Ophir ravine is identical in silica percentage with the foot-wall, Savage connection in the Sutro tunnel, and differs from the pyroxene-andesite of 

the surface by only 0.16 per cent, of silica. The hornblende-andesite 

from near the Cole water tunnel differs from that of the ridge near 

Ophir Hill by 0.13 per cent, of silica. The refinements of chemistry like 

those of the microscope fail to detect any distinctions between the rocks 

of the same mineral composition, which have here been classed as Tertiary and Pre-Tertiary age.

(154)
HAGUE AND IDDINGS.] GRANITIC STRUCTURE. 35

We turn now to the later rock masses of both acidic and basic 

character found penetrating the main body of hornblende and pyroxene audesite of intermediate composition. We will first consider the 

hornbleude-mica-andesites (later horublende-andesite and mica-diorite), 

which, though presenting variations from a porphyritic lava with a 

glassy and microlitic ground mass to a granular structure recognized 

by the unaided eye, have already been shown to be indistinguishable 

petrographicly. An examination of the specimens shows that it is 

the coarsely crystalline and decomposed middle-grained varieties which 

have been classed as Pre-Tertiary rocks, while the fresh and unaltered 

middle-grained and the glassy ones were regarded as of Tertiary age. 

The broad high ridge, lying to the eastward of Mount Dayidson, of which 

Mount Eose and Mount Kate are conspicuous features, forms the principal mass of this rock breaking through the pyroxene-andesite, Upon

the slopes of the ridge a considerable variation in texture of the rock is 

easily recognized. It is only on the surface along the ridge that glassy 

forms are met with. Comparing the hornbleude-mica-andesite exposed 

through the 10,000 feet cut by the Sutro tunnel with the surface rocks, 

exact counterparts are observed with the exception that glassy forms 

are wanting. This is, however, what we should expect, and quite in 

accord with all the observations so far made upon the collections from 

Washoe, namely, that the glassy forms of the various rocks are only 

found at or near the surface, those from any considerable depth having 

cooled with sufficient slowness to become holocrystalline. Advancing; 

now a step further we find that the more crystalline rock from this large 

body of hornblende-mica-andesite bears the closest resemblance to parts 

of the so-called Pre-Tertiary mica-diorite, which passes, as has already 

been shown, by imperceptible gradations into rock possessing a gran-, 

ular structure.

Let us consider in this place what the intermediate forms.are which 

link together rocks of the chemical composition .of a glassy hornbleudemica-andesite possessing all the characteristics of a surface lava with; 

those having the structure of many granitic rocks. The results derived 

from a study of numerous rock sections 25 clearly indicate that the essential variation in the more porphyritic varieties takes place in the 

groundniass, which, starting with glass containing few microlities, gradually grows richer and richer in microlitic inclusions until only a small 

amount of glass remains as a cementing base. This in turn may be 

crystallized into microscopically cryptocrystalline particles. In a somewhat more advanced stage the cementing material assumes the appearance of uniformly oriented patches or of microcrystalline granitoid 

grains, which as the size of the grain increases is found to be partly 

quartz, partly feldspar. The dimensions of the cemented microlites also 

increase until they appear as small crystals embedded in a mass of ir2<'See Table III ; also Tublo I. 

(155)
36 DEVELOPMENT OF CRYSTALLIZATION. F«ULL. 17.

regularly shaped grains of feldspar and quartz. As the number of large 

crystals and those of medium size increases the porphyritic appearance 

of the rock is lost, especially as the sharp outline of the larger crystals 

is no longer preserved, as already described. The character also of the 

feldspars changes by reason of the variation in their inclusions, those 

of glass growing less numerous as the rock becomes more crystalline, 

until they disappear entirely, occasional fluid inclusions making their 

appearance. When finally the grains are nearly all of one size the 

structure is the even granular one of a granite.

Quartz, which in the glassy forms of these hornblende-mica-plagioclase rocks occasionally is seen in small porphyritic grains, first makes 

its appearance as an essential ingredient among the microscopic grains 

of the groundmass, becoming more and more apparent as their size increases. This quartz contains in the coarser-grained forms numerous 

fluid inclusions, indicating that the gases disseminated through the 

magma, having been excluded by the first crystallizing minerals, were 

finally inclosed in the quartz, the last mineral to crystallize, which would 

necessarily occur when a magma consolidates under pressure, the gases 

being unable to escape. The number of mineral constituents is still 

further increased by the development of acidic feldspars, the uncrystallized material of which, together with the silica of the quartz, makes up 

the residual glass base of the vitreous forms of the rock. It has already 

been shown 26 that the glass of a vitreous lava grows richer in silica and 

alkalies as the more basic minerals crystallize out of the magma.

Thus the rock which in a vitreous 'state is composed essentially of 

basic feldspar, hornblende, and mica, with a groundmass of glass more 

or less fuH of microlites, is, when coarse-grained, formed of both basic 

and acidic plagioclase with some orthoclaso, besides hornblende, mica,

and quartz the mineral composition of a quartz-diorite. The best examples of this decidedly granitic structure are from the 1,000-foot level 

of the Sierra Nevada and the shaft of the LadyBryan mine, rocks which 

some geologists would call hornblende-bearing granite, but to which 

Mr. Becker refers in his monograph 27 as "fresh, coarsely granular, 

quartzose diorite." Though we know but little of the actual occurrence 

and field relations of the rocks represented by these two specimens, one 

coming from the end of a drift and the other collected from the dump 

of a mine, still it is noteworthy that they both come from the immediate 

neighborhood of the most extensive area of hornblende-mica-andesite 

in the district, and though geological evidence of their direct connection 

is wanting, the chain of petrographic evidence is so complete as to leave 

no doubt in our minds that these coarsely granular rocks are parts of

26 F. Fouqud, "Sautorin et Ses Eruptions," Paris, 1879, p. 11. Hague arid hidings, 

"Notes on the volcanoes of Northern California, Oregon, and Washiugton Territory," 

Am. Jour, of Science, Vol. XXVI, Sept., Ib83, p. 230.

zr Geology of the Comstock Lode, page 192.

(160)
HAGUE AND IDD1NC8.] AGE OF HORNBLENDE-MICA-ANDESITE. 37

the same magma as the hornblende mica-audesites which have crystallized very slowly under considerable pressure.

With the exception of one body near the north end of the vein, the socalled raica-diorite only occurs on the surface, a,s shown on the map, 28 in 

a few isolated patches of no great elevation, whe?e it presents the appearance of having broken through the audesite. "That there is good 

reason for such a conclusion is borne out by the strongest geological 

evidence beneath the surface. It has been pointed out that the audesite of the Sutro tunnel is cut in three places by narrow dikes of hornblende-mica-andesite of a medium Crystallization, showing that here at 

least a similar rock is younger than the main body of Tertiary andesite.

Other instances of this hornblende-mica-rock cutting the pyroxeneandesite are shown in the vertical sections through the Yellow Jacket

and Belcher miues (atlas sheet VII), and can only, be reasonably explained as dikes and offshoots from the main body of a later rock penetrating an older one. By reference to the geological map it will be 

seen that the Eorman shaft is located in andesite between two small outcrops of hornblende-mica-rock only 3,500 feet apart. Now the fact that 

the shaft has been sunk for a depth of 2,340 feet without encountering 

either body seems to point to the conclusion that these hornblende-mica 

rocks are simply dikes cutting the andesite.

Let us consider one more fact. On the 1,950-foot level of the Utah 

mine a narrow body of medium grained hornblende-mica rock is exposed penetrating the pyroxene-audesite. Now this small body is vertically beneath the later hornblende-mica-andesite of the surface, with 

the evidence all tending to the conclusion that the two bodies are closely 

connected masses.

Such geological evidence as to the position of these rocks seems allsufficient to determine their a,ge, but taken in connection with the proof 

of identity afforded by rigid petrographic analysis the arguments appear 

indisputable.

As already mentioned, the hornblende-mica rocks vary considerably 

in their mineral composition within certain limits, and a corresponding 

variation is observed in their chemical composition. Analyses VIII 

and IX (Table IV) are from the hornblende-mica-audesites of Mount 

Eose and Cross Spur .quarry, which are rich in .iron magnesia silicates, hornblende being in excess of mica in one case and equaling it 

in the other. Analysis X from the " mica diorite," 800 feet east of the 

Waller Defeat shaft, shows less iron and magnesia, which corresponds 

to the small amount of iron and magnesia silicates found in the rock, 

mica greatly predominating over hornblende. It represents one of the 

most acidic of these hornblende-mica rocks.

The quartzose Tertiary rocks, as has been shown, belong both to 

dacite and to rhyolite, the rocks intermediate between the two being

28 Geology of the Comstock Lode, Atlas, Sheet IV.

... ,. (157)
38 DEVELOPMENT OF CRYSTALLIZATION. IUULL.IT.

well developed. The surface area covered by them lies wholly in the 

southern and southeastern portion of the district, mainly in the neighborhood of American Flat. Their geological position is best observed 

in the underground workings to the north, where the main body of 

pyroxene-andesite occupies the surface, and although the number of 

exposures is limited, their evidence is clear. No stronger proof can be 

desired of the Tertiary age of the rocks in question than the occurrence 

in the Sutro tunnel of a broad dike, 1,400 feet in width, with numerous 

offshoots cutting the main body of pyroxene andesite. Microscopical 

examination proves this rock to be the counterpart of alithoidal variety 

found on the surface near Gold Canon and the Eed Jacket mine.29 In 

the Overman mine a dacitic or rhyolitic rock apparently occurs as a 

narrow dike extending upward nearly to the surface from the 2,000-foot 

level. Again the occurrence in the Belcher mine can best be explained 

on the simplest and most plausible theory of an offshoot or stringer from 

the main mass. The Forman shaft, after passing through pyroxene 

and hornblende-andesite, exposes at about 2,340 feet from the surface a 

small body determined by Mr. Becker as an ortboclase rock, which he 

classes as quartz porphyry. Although our knowledge of the body is 

extremely limited, there seems to be every reason, judging from analogy, that it also occurs as an intrusive mass penetrating the andesite 

but failing by many hundred feet of reaching the surface. It is worthy 

of note that the rock is holocrystalline and presents certain characteristics common to igneous rocks cooled at considerable depths below the 

surface, and which many geologists regard as evidence of their PreTertiary age.

A recent analysis in the laboratory of the United States Geological 

Survey, by Dr. F. A. Gooch, of a dacite from near McOlellan Peak, is 

given in Analysis XI (Table IV). It will be noticed that the potassa is 

but slightly in excess of the soda, and the miscroscope shows that the 

plagioclase is far in excess of orthoclase among the porphyritic crystals. 

In Analysis XII, also by Dr. Gooch, the potassa greatly predominates 

over soda, and the rock from which it was made is characterized by prevailing orthoclase, the habitus and chemical composition being those of 

a typical rhyolite.

Geological evidence having proved that the rock penetrated by the 

black dike is inseparable from the Tertiary pyroxene-andesite lavas, 

it of course follows that the later rock is also of Tertiary or still more 

recent age. A comparative study of the hand specimens and their sections has shown the similarity in composition and structure existing 

between the surface basalts, near American Flat, and the well-known 

black dike of the Comstock lode, they differing only slightly in degree of crystallization and state of preservation. By reference to the

map (atlas sheet IV) it will be noticed that there are only five surface

 . . 7

23 Compare tlio following hand specimens: Numbers 182 to 192 from the Sutro tunnel with 417, 370, 397, 399, 405, and 411 from the surface collection.

(158) '
HAGUE AND iDuiKos.] GEOLOGICAL HISTORY OF WASHOE. 39

occurrences of basalt, all of them insignificant in area and in height. In 

a direct line from the basalt masses, immediately to the westward of 

American Flat, it is only 4,500 feet to the Caledonia shaft, "where the 

black dike is last seen along the line of exploration on the Comstock.

The chemical composition of the surface basalt is given in Analysis I 

(Table IV). It is somewhat basic,'but in other respects is quite similar 

to many of the basalts of the Great Basin. The basalt of the black 

dike from the Belcher mine, 1,145 feet below the surface, yielded 

49.79 per cent, of silica, the difference between the two analyses being 

quite within the range of variation for the same flow. Now, although 

the black dike forms the only dike of basalt, known in the -district, 

and its direct connection with the small surface patches has never been 

traced, the evidence is quite sufficient to lead to the conclusion that it, 

like the intrusive bodies of hornblende-mica-andesite, dacite, and rhyolite, is an offshoot from a much larger mass at no great distance.

CONCLUSIONS.

As stated in the first part of this paper our investigation leads us, in 

many important particulars, to conclusions in respect to the geological 

structure of the Washoe district quite at variance with the opinions 

held by our predecessors. Throughout the body of the paper we have 

endeavored to place before the reader with considerable detail the facts 

upon which our opinions were based. Let us then in reviewing these 

results state in a few words what we consider to be the geological history of Washoe and the relations of the principal rock masses and intrusive bodies to each other, and then briefly summarize the conclusions 

maintained in this discussion as to the development of crystallization in 

the igneous extrusions which have built up this mountain pile.

We regard the main mountain mass of which Mount Davidson is 

the culminating peak to be formed of both pyroxene- and hornblencleaudesites, of much the same ultimate chemical composition, analyses 

showing a variation of only 4 per ceut. in silica. Both these types of 

andesite are so intimately connected geologically with each other as to 

render it extremely difficult to separate them on a basis either of physical structure or of geological age. So long as the conditions under 

which hornblende and pyroxene are formed in lavas of this class are so 

little understood it becomes a difficult matter at Washoe to determine 

with precision the relations of andesitic eruptions based solely upon 

these minerals. The fact, however, that the main mass of Mount Davidson is made up of coarsely crystalline material originally a pyroxene 

rock, and that the section through the Sutro tunnel after leaving the 

hornblende-mica-andesite body of Mount Eose and Mount Kate runs 

mainly through pyroxene-andesite, makes it highly probable that the

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40 DEVELOPMENT OF CRYSTALLIZATION. [BULL. 17.

earlier and larger part of the eruption was composed of this rock. 

Through this pyroxene-andesite hornblende-andesite is known to penetrate, and observations all point to the conclusion that vents have been 

opened through which both types of rock have been forced upwards, although not necessarily reaching the surface.

After these eruptions of pyroxene and hornblende andesites there was 

a period of rest, during which there was more or less removal of the superficial material by denudation, a study of the surface geology presenting many arguments in favor of a time of comparative inaction. 

These older andesitic rocks have been broken through by numerous intrusive bodies of both acidic and basic lavas represented by horublendeinica-andesites, dacites, rhyolites, and basalts. From masses of these 

later rocks offshoots were pushed out and forced upwards into the 

older rocks, in many cases failing to reach the surface, but now exposed 

by vertical shafts and horizontal workings.

We find the glassy forms of these rocks only on or near the surface. 

Eocks from the tunnels and mining shafts from any considerable distance beneath the surface present invariably a holocrystalline structure, 

while the more coarsely crystalline forms come from near the central 

mass, from intrusive dikes, or are brought up from the lowest depth 

reached by mining shafts. It is not of course to be expected that development in crystallization should show an unvarying increase in any 

given direction through a rock mass. On the contrary, changing conditions would necessarily bring about more or less local modifications. 

Variations in rapidity of cooling or amount of pressure would produce 

local centers of crystallization, with corresponding variations in rock 

structure. Moreover the degree of crystallization developed in different lavas under similar conditions appears to depend upon their chemical composition, the basic lavas showing a greater tendency to crystallize than the acidic.

When we consider the evidence offered by the 4 miles of the Sutro tunnel, the vertical sections 3,000 feet in depth, and the network of long 

galleries run at different levels from the main shafts, the innumerable 

facts, all pointing in one way, present such overwhelming proof that we 

are led to the conclusions maintained in this paper, and briefly summed 

up as follows:

That the degree of crystallization developed in igneous rocks is mainly 

dependent upon the conditions of heat and pressure under which the 

mass has cooled and is independent of geological time.

That there are abundant geological and petrographic evidences in the 

Washoe district -to show that the igneous rocks do not belong partly to 

Pre-Tertiary and partly to Tertiary eruptions, but are all of Tertiary 

age.

That the so-called granular diorite and diabase, augite-andesite are 

identical and belong to the same geological body.

(160)
HAGUE AND IDDIS08.] CONCLUSIONS. 41

That the so-called porphyritic diorite is identical with hornblendeandesite and should be designated by^the latter name.

That the so-called mica-diorite is identical with the later hornblendeandesite and both rocks are of the same geological age.

That the quartz porphyry is of Tertiary age and resolves itself into 

both dacite and rhyolite.

That the later diabase or black dike and the b.< salt are one and thv same 

rock, the difference being that the former is only known as a narrow 

dike while the latter occurs iu flat-topped bodies and low hills on the 

surface.

. That the Conistock lode occupies a fissure along a line of faulting 

in rock of Tertiary age, and cannot be considered as a contact vein between two different rock masses.

(161)

INDEX

	Pago. 

Augite-andesite ....... .............................. ..................12,1(>, 20,41

Sutro tnniiel section....................................... 20

Basalt....................................................................27,28,41

analysis of......................................................... 33

Becker,George F.,cited..................................10,12,22,23,28,29,30,33,3(5

Black dike..........................'.......... ........................... 27,41

olivine in ...................................................... 28

Chemical analyses...... ................................................... 33^34

evidences................................................ 23,33

Church, John A., cited ...................................... .............. 11

Conastock lode............................................................ 31,41

galleries of.......................... <........;.. it ........... 10

Conclusions....................... .... ..................................... 39

Dacito.................................................................... 26,41

analysis of......................................................... 33

Development of crystallization............................................ 9

Diabase.... .............................................................. 12,40

relations to augite-andesite......................................12,16,20,40

Sntro tunnel section............................................... 20

Dikes in Mount Davidson.............................. ..................... 32,40

Diorite, granular.......................................................... 21,40

mica.............................................................. 23,41

fresh feldspars in............................................. 24

porphyritic .................................................... ... 22,41

Sutro tunnel section............................................... 20

Doelter, Dr. C., cited...... ................................................ 30

Fluid inclusions in feldspars............................................... 17

quartz ................................. Vk ............... 36

Fouque*,F., cited .......................................................... 36

Geological and chemical evidence.......................................... 29

Glass inclusions in feldspars...... ......................................... 17

Gooch.F. A., cited.............................. ........................... 33,38

Grades of crystallization defined...........................................13,14,25

Granitic structure.................. ...................................... 35

Granitoid structure,defined ............................................... 14

Granophyre structure,defined ................................ ............. lf<

Granular structure, defined................................................ 14

Groundmass, acid character of............................................. 3P>

variations in ................................................. 13,14

Hornblende andesite, analysis of........................................... 33,34

(earlier) and porphyritic diorite..................... 22,41

(later) and mica-diorite..............................23.24,41

relation to pyroxene-andesite ........................ 34

Sutro tunnel section ........,,...................'...... 20

Horn blende-mica-andesite................................................. 23,41

analysis of ...................................... 33

Hussak, Dr. E., cited .......................... ............................ 31

Hypersthene.............................................................. 17

Introductory.... ................................^......,..........,.......' 9

Judd,.T.W., cited ...................... ................ ................... 31

(163) 43
44 INDEX.

	Page. 

King, Clarence, cited ................................................11,23,26,30,33

Koch, Dr. A., cited.....:...............................................___.. 30

Lady Bryan mine, coarse-grained diorite of................................ 36

Letter of transmittal...................................................... 7

Mica-andesite, analysis of.................................................. 33

Mica-diorite, and later hornblende-andesite ................................ 23,41

fresh, feldspars in.............................................. 24

MicrocryptocrystalHne structure, defined................................... 15

Microcrystalline structure, defined ......................................... 15

Mixter.W. G., cited ....................................................... 33

Moore, Gideon, cited........................................................ 33

Mount Davidson, structure of.............................................. 39

Prometheus, dacite of................................................ 27

Rainier, hypersthene-andesite of.................................... 10

Niedzwiedzki, J.,cited ........................ ............................ 26

Olivine in black dike........................................................ 28

Peginatoid structure, defined............................................... 14

Peufield, Samuel L., cited.............................1.. ....<.............. 33

Porphyritic diorite and earlier hornblende-andesite......................... 22

structure, defined ............................................... 14

Propylite, discussion of.................................................... 30

Pyroxene-andesite ..............................................."......... 17, 39,40

analysis of.... .......................................... 33,34

relation to hornbleude-andesite ......................... 34

Quartz-diorite, composition of.............................................> 36

porphyry, dacite, and rhyolite....................... ................ 26,41

Rath,G. vom, cited............ ............................................ 30

Eed Jacket mine, quartz-porphyry of....................................... 19

Richthofen, Baron von, cited ............................... ...^.. ......11,23,26,30

Rhyolite........................................................ .......... 26,41

analysis of....................................................... 33

Rosenbusch, H., cited ...... ................................................ 14

Roux's ranch, basalt near.................................................. 28

quartz-porphyry of.......................................... 19

Savage mine,rock of...................................................... 19

Schemnitz, Hungary, geology of............................................ 30

Sierra Nevada mine, coarse-grained di onto of .............................. 36

section through........................................ 31

Sutro tunnel, north branch ......................... ......................19,20,21

section through .............................................. 18

south branch................................................. 19,20,21

Szab6, Dr. J., cited......................................................... 30

Table I. Grades of crystallization....................................... 16

II. Grades of crystallization....................................... 20

III. Grades of crystallization....................................... 25

IV. Chemical analyses ................................. ......... 33

Thin sections, number of........ ...................... ................... 12

Transitions from glassy to granitic structure............................... 35

Virginia range ............................................................ 11

Woodward, R.W., cited.................................................... 33

Younger diabase, black dike, and basalt.................................... 27,41

Zirkel, Ferdinand, cited .................................................11.23,26,27

.(104)

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