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i^DVEIlTISEM:E]SrT.
The publications of the United States Geological Survey are issued in accordance with the statute
approved March 3, 1879, which declares that —
The publications of the Geological Survey shall consist of the annual report of operations, geo-
logical and economic maps illustrating the resources and classifications of the lands, and reports upon
general and economic geology and paleontology. The annual report of operations of the Geological
Survey shall accompany the annual report of the Secretary of the Interior. All special memoirs and
reports of said Survey shall be issued in uniform quarto series if deemed necessary by the Director, but
otherwise in ordinary octavos. Three thousand copies of each shall be published for scientific exchanges
and for sale at the price of publication ; aud all literary and cartographic materials received in exchange
shall be the property of the United States and form a part of tbe Ubraiy of the organization : And the
money resulting from the sale of such publications shall be covered into the Treasury of the United
States.
ANNUAL REPORTS.
From the above it will be seen that only the Annual Reports, which form parts of the reports of
the Secretary of the Interior and are printed as executive documents, are available for gratuitous dis-
tribution. A number of these are furnished the Survey for its exchange list, but the bulk of them are
supplied directly, through the document rooms of Congress, to members of the Senate and House.
Except, therefore, in those cases in which an extra number is supplied to this office by special resolution,
application must be made to members of Congress for the Annual Reports, as for all other executive
documents.
Of these Annuals there have been already published :
I. First Annual Report to the Hon. Carl Schurz, by Clarence King, 8°, Washington, 1880, 79 pp.,
I map. — A preliminary report describing plan of organization and publications.
II. Report of the Director of the United States Geological Survey for 1880-'81, by J. W. Powell,
8*^, Washington, 1^82, Iv., 588 pp., 61 plates, 1 map.
CONTENTS.
Report of tbe Director, pp. i-lv., plates 1-7.
Administrative Reports by Heads of Divisions, pp. 1-46, plates 8 and 9.
The Physical Geology of the Grand Canon District, by Capt. C. E. Dutton, pp. 47-166, plates 10-36
Contribution to the History of Lake Bonneville, by G. K. Gilbert, pp. 167-200, plates 37-43.
Abstract of Report on the Geology and Mining Industry of Leadville, Colorado, by S. F. Emmons,
pp. 201-290, plates 44 and 45.
A Summary of the Geology of the Comstock Lode and the Washoe District, by George F. Beckfer,
pp. «91-330, plates 46 and 47.
Production of Precious Metals in the United States, by Clarence King, pp. 331-401, plates 48-53.
A New Method of Measuring Heights by means of the Barometer, by G. K. Gilbert, pp. 403-565,
plates 54-61.
Index, pp. 567-588.
The Third and Fourth Annual Reports are now in press.
MONOGRAPHS.
The Monographs of the Survey are printed for the Survey alone, and can be distributed by it only
through a fair exchange for books needed in its library, or through the sale of those copies over and
above the number needed for such exchange. They are not for gratuitous distribution.
So far as already determined upon, the list of these monographs is as follows:
I. The Precious Metals, by Clarence King. In preparation.
II. Tertiary History of the Grand Canon District, with atlas, by Capt. C. E. Dutton. Published.
m. Geology of the Comstock Lode and Washoe District, with atlas, by George F. Becker.
ii ADVERTISEMENT.
Published.
IV. Comstook Mining and Miners, by Eliot Lord. In press.
v. Copper-bearing Eocks of Lake Superior, by Professor R. D. Irving. In press.
"VI. Older Mesozoic Flora of Virginia, by Prof. William M. Fontaine. In press.
Geology and Mining Industry of Leadville, with atlas, by S. F. Emmons. In preparation.
Geology of the Eureka Mining District, Nevada, with atlas, by Arnold Hague. In preparation.
Coal of the United States, by Prof E. Pumpelly. In preparation.
Iron of the United States, by Prof. E. Pumpelly. In preparation.
Lesser Metals and General Mining Eesouroes, by Prof. E. Pumpelly. In preparation.
Lake Bonneville, by G. K. Gilbert. In preparation.
Dinocerata. A monograph on an extinct order of Ungulates, by Prof. O. C. Marsh. In press.
Sauropoda, by Prof O. C. Marsh. In preparation.
Stegosauria, by Prof. O. C. Marsh. In preparation.
Of these monographs, numbers II. and III. are now published, viz :
II. Tertiary History of the Grand Canon District, with atlas, by C. E. Duttou. 1882, 4°, 264
pp., 42 plates, and atlas of 26 double sheets folio. Price |10.12.
III. Geology of the Comstock Lode and Washoe District, with atlas, by George F. Becker. 1882,
4°, 422 pp., 7 plates, and atlas of 21 sheets folio. Price |11.
Numbers IV., V., and VI. are in press and will appear in quick succession. The others, to which
numbers are not assigned, are in preparation.
BULLETINS.
The Bulletins of the Survey will contain such papers relating to the general purpose of its work
as do not come properly under the heads of Annual Reports, or Monographs.
Each of these Bulletins will contain but one paper and be complete in itself. They will, how-
ever, be numbered in a continuous series, and will in time be united into volumes of convenient size.
To facilitate this each Bulletin will have two paginations, one proper to itself and another which
belongs to it as part of the volume.
Of this series of Bulletins No. 1 is already published, viz :
1. On Hypersthene-Andesite and on Triclinic Pyroxene in Augitic Rooks, by Whitman Cross,
with a Geological Sketch of Buffalo Peaks, Colorado, by S. F. Emmons. 1883. 40 pp., ti°. Price 10
cents.
Correspondence relating to the publications of the Survey, and all remittances, should be addressed
to the
Director of the United States Gkological Survey,
Washington, D. C.
Washington, D. C, March 1, 1883.
47th Congress, ) HOUSE OF EEPEESENTATIVBS.
Is* Session. |
( Mis. Doc.
( No. 52.
DEPARTMENT OF THE INTERIOR
MONOGRAPHS
United States Geological Survey
VOLUME III
■ -'^Ai' 2 1952
Wilbur Cros- r u
University ef c '^'^'^
y °i Connecticut
WASHIISrGTON
GOVERNMENT PRINTING OFFICE
1882
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UNITED STATES GEOLOGICAL SURVEY
CLAEENCE KING DIRECTOR
GEOLOGY
COMSTOCK LODE AND THE WASHOE DISTRICT
^WITH ATLAS
By GEOiiaE F. BECKER
WASHINGTON
GOVERNMENT PRINTING OFFICE
1882
American Museum of Natukal History,
New York City.
Hon. Clarknce King, Director:
Sir: In compliance with your instructions of March 6, 1880, directing-
nie to report upon the Geology, Mineralogy, Chemistry, and Physics of the
CoMSTOCK Lode, I have the honor to transmit the accompanying report'
Although several reports on the Comstock Lode have appeared during
the past twenty years, the great extension of the mine workings and the
advances in geological science made it probable that additional information
of value would result from a reexamination of this famous ore-deposit.
Administrative duties unfortunately prevented you from undertaking the
study of the lower portions of the Lode, the upper part of which you have
made so familiar to geologists. Under these circumstances, you did me the
honor to select me as your substitute; yet you did not abandon all share in
the investigation, since at every stage of it I have had the advantage of
}'our cordial support and wise counsel.
Very respectfully, your obedient servant,
GEO. F. BECKER,
* - Geologist-in-charge.
'See Second Auuiial Report of the Director U. S. Geological Survey, page xi.
(iii>
PREFACE
The field work for this report was begun in April, ] 880, and concluded
in March, 1881. In the spring of 1880 the Census of the Mineral Indus-
tiies West of the Rocky Mountains was placed in my charge in addition to
nay duties as geologist, and occupied much of my time both during the
period of field work in the Washok District and since.
My assistants were as follows: Dr. Carl Barus, physicist, who was
invited at my request to join the Survey for the express purpose of resum-
ing the question of the electrical activity of ore bodies, a subject in which
I had long felt an interest. He also made experiments on kaolinization,
and the two chapters in this volume devoted to these subjects sufficiently
attest how ably he has conducted the investigations to which he was
assigned. Mr. F. R. Reade, assistant geologist, made a large portion of the
collections, which embrace nearly three thousand numbers, and, with Dr.
Barus, earned out many of the computations involved in the discussion of
the increment of heat. I also contracted with Mr. R. H. Stretch to assist
me in mapping the underground geology. Mr. Stretch was for some 5'ears
one of the official surveyors of the Comstock, and his familiarity with the
old and inaccessible workings was of much assistance. In preparing the
sections it was necessary in many cases to infer the structure of localities to
which there was no approach from that shown in galleries on other planes,
a difficult task in which Mr. Stretch's aid was also very valuable. I visited
almost every foot of open ground, and the structural and lithological geology,
.as well as the conjectural portions of the sections, are my own. Mr. Stretch
was very zealous in the collection of the specimens necessary to prove the
lithology of the sections, and forwarded the work of the Survey in every
way in his power.
Vi PEEFACE.
The claim map was prepared by Messrs. Hoffmann ..& Craven, sur-
veyors, on contract, and the mine maps were obtained through the same
firm from official sources. Some additions have been made to the claim
map by Mr. L. F. J. Wrinkle.
All the mine officers were most courteous and offered every facility for
the examination, often at great inconvenience to themselves. Mr. I. E.
James, superintendent of the Sierra Nevada, had prepared a considerable
number of slides, which, as well as his microscope, he placed at my disposal.
Mr. Forman, superintendent of the Forman Shaft; Captain Taylor, super-
intendent of the Yellow Jacket; and Mr. I. Requa, superintendent of the
■ Chollar, gave access to their collections, and to their temperature observa-
tions, as did also Mr. C. C. Thomas, superintendent of the Sutro Tunnel.
Mr. George J. Specht, surveyor, compiled the temperature observations of
the Tunnel and many other data, most of which will appear in another
volume. Mr. Forman also presented the Survey with a duplicate collection
of the rocks encountered in sinking his shaft, a specimen having been taken
every five feet. Mr. W. H. Patton gave me extraordinary facilities in the
series of miues (from the Union to the Consolidated Virginia) under his super-
intendence; and Mr. Hugh Lamb, foreman of the Consolidated Virginia and
the California, spent much time in exploring with the party, and communi-
cated many acute and valuable observations gathered in his long experience
on the Lode. In short, from mine owners to common miners, an intelli-
gent interest in the objects of the Survey and a willingness to forward them
were manifested by all concerned. It is believed that the facts made out
with reference to the occurrence of ore will prove of sufficient practical
advantage to justify this interest.
The lithological illustrations were all drawn and colored under my con-
stant supervision. The endeavor was to reproduce the objects with absolute
fidelity, avoiding even the temptation to emphasize characteristic outlines or
tints, and the figures were not considei'ed complete so long as an addition
or a change could be suggested. The work was put on the stones, of which
no fewer than eighteen were requisite, by the same draughtsman who made
the drawings, Mr. G. K. Gardner, and the originals have thus not suffered
ill lithographic reproduction. It is safe to say that no lithographic illus-
PREFACE. vii
tnitions were ever more conscientiously prepared, and I have metwitli none
which seem to represent microscopical effects more exactly.
Special thanks are due from me to Dr. Barus and to Mr. J. P. Iddings
(assistant geologist on Mr. Arnold Hague's staff), with whom I have
repeatedly consulted on the subjects treated in Chapters IV. and III.,
respectively. But for the stimulus of their criticisms the proofs offered
would be less satisfactory; and in enabling me to meet the objections
which occurred to them, they have placed me more in their debt than if
they had made positive additions to the discussions of lithology and faulting.
The office work has been done at the American Museum of Natural
History, New York, that institution having courteously placed some of its
admirable working rooms at the disposal of the Survey.
G. F. B.
New York, May 6, 1882.
Oct. 16. — Mr. Albert Williams, jr., Statistician of the Survey, has
kindly given me the benefit of his extremely efficient assistance in the
proof correction of the volume.
G. F. B.
c o ]sr T E N T s .
Page.
Lettbr of Transmittal ^ III.
Preface V.
Contents IX.
List of Illustrations XI.
List of Atlas-sheets XIII.
Brief outline of results XV.
Chapter I.— The Comstock Mines 1
II. — Previous investigations of the Comstock Lode 12
III.— Lithology 32
Section 1 . The Rocks of the Washoe District 32
2. The Decomposition of the Rocks 72
3. Propylite 81
4. Detailed description of slides 91
Description of illustrations 145
Tables of Analyses and Assays 152
IV.— Structural Results of Faulting 156
V. — The Occurrence and Succession of Rocks 188
VI.— Chemistry 209
VII. — Heat Phenomena of the Lode 228
Section 1. General Discussion 228
2. Thermal Survey ' 244
Vni.— The Lode 266
IX. — On the Thermal Effect of the Action of Aqueous Vapor on Feldspathic Eock.s (Kao-
linization) by Carl Barus 290
X.— On the Electrical Activity of Ore Bodies by Carl Barus 309
XL— Summary 368
Note to Chapter III. (on the Determinatiou of Feldspars by Szab6's Method) 405
Index to the Mining Claims 409
General Index 413
(ix)
LIST OF ILLUSTRATIONS.
Page,
Plate I. — Weathered augite-andesite on divide between Mimiit Rose and Monnt Kate; Mount
Davidson and Mount Hutler in the distance Frontispiece.
II. — Single minerals, as seen under the microscope follows 151
III.— Ditto do . . . 151
IV. — Eock sections, as seen under the microscope do . . . 151
v.— Ditto do... 151
VI. — Bullion Ravine, looking east (diorite); Mount Kate in the middle distance laces 192
VII. — East flank of Monnt Rose (later homblende-andesite) do. 204
Fig. 1. — Area of extreme decomposition 73
2. — Orientation of slides J I45
S. — System of equal, vertical, movable sheets 160
4. — Calculated and observed curves 167
5. — Logarithmic curve referred to rectangular coordinates 168
6. — Logarithmic curve referred to inclined coordinates , 171
7. — Inclination of a faulted surface 172
8. — Double faul t-curve I73
9. — Fault accompanied by a strain I75
10. — Contour map of a faulted surface 177
11. — Rise of the hanging wall 179
12. — Ideal section across the Virginia Range 243
13. — Combination Shdf t and Yellow Jacket temperatures 249
14. — Yellow Jacket Shaft and Forman Shaft temperatures 251
15. — Forman Shaft temperatures . . .■- 253
16. — Rose Bridge Colliery and Forman Shaft temperatures 255
17. — Spereuberg Boring, Forman Shaft, and Rose Bridge Colliery temperatures 257
18. — Sutro Tunnel temperatures 259
19. — Boiler used in kaolinization experiments 292
20. — Disposition of apparatus 295
21. — Section of key 29ti
22. — Ideal line of electrical survey . .' 316
23. — Longitudinal section of terminal ...^ 325
24. — Terminal in jjosition 326
25. — Mode of suspending wire 327
26. — Vertical section through Richmond ore bodies 331
27. — Plan of the 400 and 500-foot levels, Richmond mine 333
28. — Earth-potential and distance, Richmond mine, 400 and 500-foot levels 343
29. — Plan of the 600-foot level, Richmond mine 344
30. — Earth-potential and distance, Richmond mine, 600-foot level 349
31. — Disposition of apparatus for measuring earth-potential 353
32. — Earth-potential and distance, Richmond mine, 600-foot level (later results) 354
33. — Potential of intermediate points 362
(xi)
LIST OF ATLAS-SHEETS.
Sheet.
Title I-
Contents II.
Map of the Washoe District, showing mining claims III.
Geological map of the Washoe District , IV.
Vertical cross-sections of the Comstock Lode through the Utah, Sierra Nevada, Union, and C.
&C. shafts V.
Vertical cross-sections of the Comstock Lode through the Sutro Tunnel and the Porman Shaft . . . VI.
Vertical cross-sections of the Comstock Lode through the Combination, Yellow Jacket, Belcher,
and Savage shafts .- VII.
Horizontal cross-section of the Comstock Lode on the Sutro Tunnel level (1,900 feet), north end. VIII.
Ditto, south end - - - IX.
Vertical longitudinal projection of the Comstock Lode, showing the position of ore bodies from
the Utah to the Potosi X.
Ditto, from the Bullion- Ward to the Baltimore Consolidated XI.
Ditto, from the Overman to the Silver Hill XII.
Comstock Mine Maps : No. 1, Utah, Sierra Nevada XIII.
Ditto : No. 2, Sierra Nevada, Union, Mexican XIV.
Ditto : No. 3, Ophir, California, Consolidated Virginia, Best & Belcher XV.
Ditto : No. 4, Gould & Curry, Savage, Hale & Norcross, ChoUar XVI.
Ditto : No. 5, Potosi, Bullion, Exchequer, Alpha, Imperial XVII.
Ditto : No. 6, Yellow Jacket, Kentuck, Crown Point, Belcher XVIII.
Ditto : No. 7, Segregated Belcher, Overman, Caledonia, New York XIX.
Ditto: No. 8, Lady Washington, Alta, Justice, Woodville, Silver Hill, Succor, Niagara .'... XX.
Ditto : No. 9, Knickerbocker, Baltimore Consolidated XXI.
(xiii)
BRIEF OUTLINE OF RESULTS.
The economical importance of the Comstock Lode appears from the fact that in twenty-one years a little over
$306,000,000 worth of bullion has been extracted from it. Of this about $132,000,000 worth was gold. The mines are the
deepest in America, reaching a distance of over 3,000 feet from the surface, and they contain about 185 miles of galleries.
Besides the scientific importance attaching to the occurrence of the immense accumulation of ore, the Lope and
District present other features of great interest. The nature of the rocks associated with the ores, some points of struct-
ure, and even the character of the deposit, have received different explanations at the hands of different observers. A
digest of the memoirs of Messrs. von Richthofeu, King, Zirkel, and Church forms one chapter of the volume.
The subject of rock decomposition has received especial attention in the examination described in this report.
This study has led to some lithological and mineralogical observations of interest, and to the identification of all of the
Washoe rocks with well-established rock species. The greater part of the hanging wall of the Lode is diabase ; the "black
dike" is also a variety of diabase, and the supposed trachyte of the District is a homblende-andesite. The so-called
propylite of "Washoe comprises a number of Tertiary and pre-Tertiary rocks, reduced to a nearly uniform appearance by
decomposition. The erroneous determination of these altered rocks as an independent species arose mainly from a confusion
between green and fibrous hornblende and chlorite. The supposed propylites from the other districts in the United States,
microscopical determinations of which have been published, were also examined and found to afford no sufficient evidence
of an independent rock species.
A discussion of faulting leads to an explanation of the similarity of the shape of the west wall of the Lode and the
form of the adjoining face of the Virginia range. The ravines of the latter are a direct result of faulting, and are only
slightly modified by erosion. A cross-section of the country on the Sutro Tunnel line shows that the surface forms a
logarithmic curve in accordance with the theory, which is further supported by experiments. The sheeted structure of the
country seems to be referable to faulting and not to eruptive bedding. The theory leads to rules applicable in prospecting
disturbed but not greatly eroded districts. The details of the topography of grassy hills are chiefly due to landslips, which
come under the law of faults in a modified form, and the characteristic curves of smooth hill-slopes are logarithmic.
The order of succession of rocks in the Washoe District is: Granite, metamorphics, granular diorite, porphyritic
diorite, metamorphic diorite, quartz-porphyry, earlier diabase, later diabase, earlier hornbleude-andesite, augite-andesite,
later homblende-andesite, and basalt. Homblende-andesite thus followed as well as preceded augitc-andesite.
Chemical evidence is offered to show that the pyrite of the region is a result of the action of soluble sulphides on the
ferro-magrtesian silicates of the rocks. Chlorite is held to be a product of the decomposition of hornblende, augite, or mica
while epidote forms at the expense of chlorite under certain conditions, but never from feldspar. There is extremely little
kaolinization at "Washoe, the feldspars having yielded to another kind of decomposition The diabase of the hanging wall
when fresh was argentiferous and auriferous, and the precious metals of the Lode are traced to this rock with much
probability, the lateral- secretion theory being thus affirmed. It is further supported by the dependence of the other ore
bodies of the District on the character of the inclosing rock.
The hypothesis that the heat of the Lode is due to the kaolinization of feldspar is not confirmed either by theory
or experiment. On the other hand, there is much geological evidence pointing to a deep-seated source of heat, probably of
volcanic origin. This conclusion is confirmed by extensive temperature observations, from which it appears that from the
surface downwards the increase of heat is uniform, about 1*^ F. for every 33 feet, while in a horizontal direction the heat
decreases in a geometrical ratio to the distance from the Lode.
Experiments on the kaolinization of feldspathic rock, conducted at the boiling point of water and extending over a
number of weeks, show that no heating effect due to this cause could be detected with an apparatus delicate enough to
j'egister a change of temperature of Qo.OOI C.
The numerous geological sections are discussed in Chapter Vm., and the application of the explanations suggested
in the preceding chapters is there shown in detail. All the important and profitable ore bodies of the Combtock, it appears,
have occurred at or close to the west face of the earlier diabase ; and it is near that surface, and there only, that exploration
is at all likely to be successful. The mode of occurrence of bonanzas is considered, and hopeful prognostications are made
for at least two portions of the Lode; bnt a series of bonanzas nearly on the same level, such as was found in the east
vein near the surface, is not likely to recur.
Electrical surveys were made both on the Comstock and at Eureka. At Virginia only negative results were
obtained. At Eureka a distinct though smMl difference of potential occurs near ore bodies, and with sufficiently delicate
' apparatus the method might there be used for prospecting. It is believed that sulphuret ores would have given results
of a more convenient magnitude than the carbonate ores of Eureka.
(XV)
GEOLOGY OF THE COMSTOCK LODE AND
THE WASHOE DISTRICT.
BY GEORGE F. BECKER.
CHAPTER I.
THE COMSTOCK MINES.
Importance of the Comstock mines. The geologj of the COMSTOCK LODE, thoUgh
of great interest from a purely scientific point of view, derives its chief
significance from the economical, industrial, and technical importance of
this extraordinary ore-deposit. The yield of the Comstock is supposed to
have exerted a seriously disturbing influence on the monetary system of the
civilized world, and its treasures have been exploited with an unexampled
rapidity. It is the chief focus of mining activity in the region west of the
Rocky Mountains, and represents the most highly organized phase of tech-
nical mining which has been reached west of the Mississippi River.
The present report deals exclusively with the geology of the Lode,
and of so much of the surrounding country as is supposed necessary to a
full comprehension of the occurrence of ore. The Geological Survey, how-
ever, will issue other volumes dealing with the Comstock from different
points of view. Mr. Eliot Lord has prepared a report upon the history of
mining on the Comstock; and Mr. "W. R. Eckart has in preparation a volume
on the mining machinery in use. The volumes now being prepared by
members of the Survey on the census of the mineral industries, will also
contain much technical information concerning the mines of the Lode.
Some of the readers of the present report, however, are unlikely to refer
to the other volumes relating to the subject, and to them a few introductory
1 c L 1
2 GEOLOGY OF THE COMSTOCK LODE.
remarks setting forth in the briefest possible manner some of the most impor-
tant facts concerning the mines may be of interest.
Geographical position. — The CoMSTOCK LoDE hes on the eastern slope of the
Virginia Range, a northeasterly offshoot from the Sierra Nevada. From
Mount Davidson the snow-capped peaks of the Sierra can be seen stretching
far away to the southeast, their flanks partially covered with trees; but to the
east and northeast lies the desert region of the Great Basin, visible through
the clear air for a hundred and fifty miles. Comparatively low ranges,
running north and south, break the surface of the Great Basin at short
intervals, and as seen from Virginia these appear in seemingly endless suc-
cession, like the waves on a stormy sea. They are clothed only by the low
growing, gray-green desert shrubs known as "sage-brush," and every detail
of the mountain sculpture is visible through the vaporless atmosphere at
great distances. White alkali deserts appear here and there in the valleys,
and now and then one catches a glimpse of the Carson River, which dwin-
dles almost from its source, and is at last wholly absorbed in the parched
earth. The Great Basin, which is five hundred miles wide, is bounded to
the east and west by high ranges. During the greater part of the year these
mountains precipitate almost all the moisture from the air-currents passing
over them, and at certain stations in the Basin ordinary meteorological
instruments sometimes fail to show any moisture in the air.
The parallelism of structure expressed by the disposition of the ranges
in California and the Great Basin finds a correspondence in the distribution
of metalliferous minerals, as was long since pointed out by Prof W. R
Blake. The coast ranges of California carry quicksilver, coal, and chromic
iron. On the western slope of the Sierra Nevada is a lower belt of copper
deposits, and a higher and more easterly one of gold. Along the eastern
base of the Sierra is a zone of silver deposits, the richest known point of
which is the Comstock, while still farther east in the Great Basin there are
less sharply defined belts carrying comj)lex silver ores and argentiferous
lead.
Difficulties of mining. — Miulug ou the CoMSTOCK begau in 1859, and has been
carried on ever since, but only in spite of obstacles of the most formidable
character. Only the scantiest supplies of potable water existed on the spot,
THE COMSTOCK MINES. 3
and that obtained from the mines was not fit even for the production of steam.
After many diificulties this want was overcome by laying lines of pipe to a
source in the Sierra Nevada, 25 miles from the Lode, at a cost of $2,200,-
000. Up to 1 870 not only all the machinery, but almost all the food of the
settlement was transported by wagon from beyond the Sierra, mainly from
Sacramento, a distance of 165 miles. The freight charges were of course
enormous; in the earhestdays as high as fifty cents a pound; but later from
five to ten cents. The Carson Valley, however, furnished a small portion of
the necessary food supply. In 1870 a branch railroad from the Central
Pacific was completed. The junction is at Reno, 22 miles from Virginia;
but the railway connecting the two points is 52 miles in length, a fact which
indicates the character of the country through which it passes. Fuel and
timber are obtained from the Sien'a at points from 10 to 30 or more miles
distant; but transportation down the slopes of the range is efi"ected in flumes
by water with a great saving of expense. The difficulties to be overcome
in mining on the Comstock were not less formidable than those met with in
establishing a settlement. The ground has been in great part very bad, th.e
size of the ore-bodies required the development of a new system of timbex'-
ing, and floods have burst into the mines which it took years to drain ; but
by far the greatest obstacle has been the heat, which increases about 3°
Fahrenheit for every additional hundred feet sunk, and which seems likely
eventually to put an end to further sinking. According to Mr. Church, the
amount of air passing through the mines is nearly 300,000 cubic feet a
minute, while, except at the change of shift, there are probably never 1,000
men below ground ; 3^et there are few spots where the miners can work more
than each alternate hour during the eight hours' shift, so that double gangs
to relieve each other are practically always necessary, and at many points
the conditions are still more disadvantageous. Besides every alleviation
which artificial ventilation can afford, the men must also be supplied with
unlimited quantities of ice-water both for drinking and washing. With all
these unheard-of easements, many men have died from overheating, and
some from contact with scalding water. Many more have fainted while
coming to the surface on the cages when they met the cool air, and have
4 GEOLOGY OF THE COMSTOCK LODE.
been dashed to pieces in the shafts.^ None of the miners in the hot
mines receive less than $4 a day (eight hours), and a few get more.
Good condition of the miners. — In spitc of the trying conditions the men are, with
very rare exceptions, in excellent physical condition. This appears to be
attributable to two causes. Even those who desire to practice close econ-
omy find themselves unable to live on the coarse fare on which miners in
other districts frequently subsist. The}' must have not only fresh meat but
fruit at any cost, and are large consumers of raw oysters brought from San
Francisco on ice, and similar delicacies. In short, they are compelled by
the physical effects of the conditions to which they are exposed, to employ
a much better diet than most workingmen. Moreover, while in the mines,
they are almost constantly in a perspiration as profuse as that induced by
a Turkish bath, a condition almost incompatible with bilious disorders.
They are thus much less liable than other workmen to derangements of the
digestive system, and are well nourished and extremely vigorous. The
average weight of the men is 166 pounds. It is said that short as the hours
of labor are, the work accomplished per man is as great as in cool mines.
In the California in 1877 the average amount of ore raised per man, includ-
ing employes of every kind, was 1.13 tons per day.
Population. — The average number of miners employed from 1860 to 1870
was, as nearly as can be ascertained, about 1,500. From 1870 to 1880 it
was probably as high as 3,200, but in January, 1880, the number had
fallen off to 2,770. The population of the towns of Virginia, Gold Hill,
and Silver City has fluctuated greatly with the condition of the mines
and the number of miners at work. Silver City has never had many inhab-
itants, while Gold Hill and Virginia long since extended over the space
which originally separated them, and are divided only by artificial lines. In
round numbers the population of the three settlements in 1860 was 4,000;
in 1870, 13,000; and in 1880, 15,500. The maximum number of inhab-
itants thus far was about 21,000 in the year 1876.
' It may not be improper to remark that geological examinations, which cannot of course be con-
fined to actual workings where everything possible is done to keep the air good, are exceedingly trying.
All the members of my jiarty were at times more or less overcome by heat and bad air. I once fainted
on the cage, and owe my life to the firm grasp of Mr. Hugh Lamb, foreman of the Consolidated Virginia
and California mines.
THE COMSTOCK MINES. 5
School statistics. — It would be easy to illustrate the wild life characteristic
of the mining camps of the far West by citing the liquor consumption of
Virginia and Gold Hill, or the statistics of gambling, which is a legal occu-
pation in the State of Nevada; but it is pleasanter and, in some respects, more
just, to turn to the school statistics of these towns. The methods employed in
the primary and grammar schools appeared to me fully equal to those in use
in the larger cities of the Union, and the results reached at least as good. The
proportion of children attending school is cei'tainly remarkable, when it is
considered that of those reported as not attending either public or private
schools a very large number must be considered by their parents too young
to be sent, while many more have left school after a number of years'
instruction. The official figures for Storey County are as follows:
School attendance.
1880.
Number of children between 6 and 18 years not attending school
Nnmber of children between 6 and 18 years represented as attending private schools .
Number of children between 6 and 18 years represented as attending public schools . .
211
493
763
543
2,565
The number of boys and girls in the schools is very nearly equal. The
proportion of children to adults is of course far smaller in these towns than
in ordinary settlements, a very large part of the miners being unmarried, and
some having families elsewhere.
Extent of the mines. — The total length of gallerics and shafts on the Comstock
up to January, 1881, is, as nearly as can be ascertained, between 180 and
190 miles. Of this about 154 miles is a matter of record on the official
maps, but though all more important galleries are run by survey and plotted
on the maps, many drifts of subordinate importance are cut without the help
of the surveyor. These are estimated at a total of 30 miles, after consulta-
tion with surveyors and superintendents. An immense consumption of tim-
ber is a necessity of mining on the Comstock. This is due to the shifting
character of much of the ground, to the great size of the ore bodies, and to the
necessity of keeping a large extent of workings open to secure rapid ventila-
tion, and as great a diminution of temperature as practicable. The timbers
are all sawn square, the commonest size being 12 by 12 inches. They are cut
in lengths and the ends fitted in shops on the surface, and they are placed
underground without the use of nails. The system is described in Mr. J,
6 GEOLOGY OF THE COMSTOCK LODE.
D. Hague's admirable memoir, "The Comstock Mines,"^ and has undergone
no essential modification since the date of that work. The consumption of
timber in the mines up to the close of 1880 is estimated at 450,000,000
board feet.
The only fuel used on the Comstock is wood, derived from the same
sources as the timber. The larger part reaches the town by rail, but a con-
siderable quantity is floated down the Carson River to convenient points,
and hauled to Gold Hill in wagons. The consumption of fuel at the mines
in hoisting and pumping is increasing rapidly, for the quantity of water is
greater year by year, as well as the distance through which it must be
forced. During the census year, ending May 31 , 1880, about 1 10,000 cords
were burned; and from 1860 to 1880 the consumption cannot have been
less than about 900,000 cords. The mills have burned about as much.
Milling. — In the early days of mining on the Comstock considerable quan-
tities of very rich and complex ores occurred, and these were treated by roast-
ing and barrel-amalgamation. Later the ores became more facile, and the
system of pan- amalgamation was developed and applied with success. For
many years it has been found practicable to beneficiate all the ores met
with by this process, with the aid of "bluestone" (cuprous sulphate) and
salt. The success of the process is unquestionably due in a large measure
to the chemical activity of the iron. Formerly the mills guaranteed a return
of 65 per cent, of the assay value of the ores, but of late years 72 per cent,
is guaranteed, and above 80 per cent, is often returned. The slimes and
tailings belong to the mills, which work them up for their own account or
sell them from time to time to other mills having especial facilities for their
treatment. Tailings not caught by the mills and deposited at considerable
distances in the streams have also been treated with success in a small way.
On the whole, however, it is improbable that more than 75 per cent, of the
bullion contained in the ore has been recovered from it, and it is therefore
fair to estimate that the ore received has contained at least four hundred
million dollars, of which about three-quarters has reached the market.
Relative quantities of gold and silver produced. The qUestioU of the prOpOrtioU of
gold to silver in the Comstock bulHon is one of considerable importance in
' Exjjloration of the Fortieth Parallel, Vol. III.
THE OOMSTOCK MINES. 7
discussions upon the price of silver and kindred subjects. It has often been
assumed that the product of these mines is almost wholly silver, but as will
appear from the tables it would be much neai-er the truth to assume that the
Lode yielded an equal value of each of the precious metals. I find that the
published and accessible mine reports give the assay values of nearly two-
thirds of the total product, and there is every reason to suppose that Baron
V. Richthofen's estimate, made when the total product was comparatively
small and very recent, was a very close approximation to the truth. Some
of the mining companies reported only the total value of bulhon produced;
and others gave the gold and silver assays in some years, but not in others,
or only for certain lots of bullion. The figures, however, cover portions of
all the important ore bodies excepting that in the Justice, and it appears
certain that not far from 57 per cent, of the product of the Lode has been
silver, or, say, $174,000,000, and that 43 per cent, or $132,000,000, has
been gold. The table from which this conclusion is drawn is given in con-
siderable detail, chiefly for the purpose of showing the differences in the
ratio of gold to silver in the various mines. In the Belcher, for the time
over which the record extends, about 57 per cent, of the value of the bullion
produced was in gold, while in the Yellow Jacket only about 31 per cent,
was in gold, Even in the great bonanza of the Consolidated Virginia, Cali-
fornia, and Ophir mines, the California or central portion of the body was
far richer in gold than the northern and southern ends.
The table of production is due to Mr. Eliot Lord, who has taken great
pains to sift the records and to ascertain the truth as closely as is now
practicable. This and the other appended tables need no further explana-
tion.
8
GEOLOGY OF THE COMSTOCK LODE.
SUPPLIES BROUGHT TO THE COMSTOCK TOWNS DURING THE CALENDAR YEAR 1879, AND ESTIMATED
CONSUMPTION.
Character.
Unit.
Total
amount.
Mine use.
Mill use.
Permanent
construc-
tion.
Domestic
use.
Cords
185, 6a2J
31, 443, 771
2, 373, 919
183, 366
725, 092
520, 319
51, 594
5, 910, 355
462,442
32, 514
1, 003, 808
29,251
119,207
16, 672
110, 000
23, 000, 000
873, 919
122, 244
725, 092
520, 319
51, 594
40,000
35, 622J
6, 443. 771
Board feet
1, 500, 000
61, 122
Steel
....do
...do
....do
!
....do
.. do
5,910,355
462,442
32, 514
1, 003, 808
Nails .
....do
Nut8
. do
do
do ...
27, 231
89, 405
11,115
1,000
29, 802
5,557
1,000
Gallons
do
ADDITIONAL, USED BT THE MILLS.
QolcksilTer .
Bluestone...
Salt
Pounds .
...do...
...do ...
300, 000
2, 500, 000
450, 000
MINE AND MILL SUPPLIES CONSUMED ON THE COMSIOCK DURING THE CALENDAR TEAR 1879.
COST.
Character.
Wood
Timber
Iron
Steel
Candles
Explosives
Quicksilver
Salt
Bluestone
Lard oil
Lubricating oil .
Sundries
Total.
Mine use.
$1, 100, 000 00
500, 000 00
52,435 14
22, 003 92
123,265 64
208, 127 60
89, 405 00
4, 446 00
♦106, 149 00
2, 205, 832 30
Mill use.
$400, 000 00
90, 000 00
11, 001 96
135, 000 00
25; 000 00
45, 000 00
29, 802 00
2, 222 80
t75, 000 00
813, 026 76
Total.
$1, 500, 000 00
500, 000 00
142, 435 14
33, 005 88
123, 265 64
208, 127 60
135, 000 00
25, 000 00
45, 000 00
119, 207 00
6, 668 80
181, 149 00
t3, 018, 859 06
* Including ice, "water, charcoal, coal oil, stone coal, tools, etc.
t Including water, tools, lights, chemicals, etc.
JDoes not include machinery, etc., entering into permanent construction.
THE COMSTOCK MINES.
9
PBOPOETIONS OF GOLD AND SILVER IN COKSTOCK BULLION.
[From official reports of the mining companies, so far as accessible. The product is not, in all cases, thus segregated into
gold and silver in the companies' reports. The figures quoted are of assay (not market) values.]
Gold.
Silver.
Total.
Percentage.
Source.
Gold.
Silver.
GOLD HILL GKOUP.
Crown Point, from May 1,1864, to May 1,1877
Belcher, from January 1, 1871, to December 31, 1873. . .
$10, 166, 656 88
8, 813, 196 06
170. 133 13
1, 973, 021 60
563, 121 83
$13, 762, 812 77
6, 716, 231 05
363, 123 80
2, 588, 138 85
786,713 69
$23,929,469 65
15, 529, 427 11
533, 256 92
4, 561, 160 45
1, 349, 836 52
Empire, from December 21, 1864, to December 16, 1868.
21, 686, 129 49
24, 217, 020 16
45, 903, 149 65
47.25
52.75
CENTRAL GROUP.
3, 661, 220 70
577, 729 22
2, 772, 468 28
3,868,<88 14
7, 090, 573 61
1, 219, 113 16
4, 774, 187 26
6, 314, 261 66
10, 751, 794 31
1, 796, 842 38
7, 546, 655 54
10, 182, 749 80
Gould & C urry, December 1, 1865, to November 30, 1867 .
Hale & Norcross, March 1. 1866, to January 31, 1874. . .
10, 879, 908 34
19, 398, 135 69
30, 278, 042 03
35.93
64.07
"BONANZA" GROUP.
29, 075, 338 97
23,308,012 69
2, 172, 600 57
35.895,438 98
23, 428, 818 75
2, 608, 744 28
64, 970, 777 95
46, 736, 831 44
4, 781, 344 85
64, 555, 952 23
61, 933, 002 01
116, 488, 954 24
46.83
53.17
RECAPITULATION.
21, 686, 129 49
10, 879, 906 34
54, 555, 952 23
24, 217, 020 16
19, 398, 135 69
61, 933, 002 01
45, 903, 149 05
30, 278, 042 03
116,488,954 24
87, 121, 988 06
15, 250, 000 00
105, 548, 157 86
32, 750, 000 00
192, 670, 145 92
48, 000, 000 00
45,22
31.77
54.78
Baron von Kiehthofen's estimate of the yield of the
68. 23
Xotal
102,371,988 06
138, 298, 157 86
240, 670, 145 92
42.54
57.46
10
GEOLOGY OF THE COMSTOCK LODE.
BULLION PEODTTCT OF THE COMSTOCK LODE TO JUNE 30, 1880.
[So far as ascertainable from the official reports of the mining companies and the assessors' returns.]
1 ounce 8ilver=$1.2929.
Mine.
Date.
Ore treated.
Tons.
(2,000 lbs.)
Founds.
Average
yield per
ton.
Product.
Alta
American
Andes
Bacon
Belcher
Bowers
Burke & Hamilton
Caledonia
California
Challenge
Chollar
ChoUar-Potosi
Confidence
Consolidated
Consolidated Imperial .
Consolidated Virginia. .
Crown Point
Eclipse
Empire
Gold Hill M. &M. Co..
Gould itCurry
Hale & Norcross
Hartford
Imperial
Justice
Kentuck
Luzerne
Mexican
Midas
Ophir
Overman
Plato
Potosi
Savage
Segregated Belcher
Sierra Nevada
SUverHill
Succor
Trojan
Union Consolidated —
Woodville
Yellow Jacket
Total
1879
1871.
1875
1867
1868
1867
1868.
1871
1876
1867
1879
1866
1867
1867
1876
1873
1864
1868
1864
1867
1860
1866
1871
1864
1873
1865
1871
1867
1871
1860
1866
1868.
1879.
1863
1867
1868
1873
1871
1877
1879
1872
1864
to June 30, 1880.
to 1878, inclusive
to 1869, inclusive .
to June 30, 1880...
to 1875, inclusive .
to 1873. inclusive .
to June 30, 1880...
to 1873, inclusive .
to 1878, inclusive .
and 1868
and 1868
to June 30, 1880...
to June 30, 1880...
to 1878, inclusive .
to 1877, inclusive .
to 1872, inclusive .
to 1873, inclusive .
to 1875, inclusive .
to 1876, inclusive .
to 1879, inclusive .
to 1872
and 1872
and 1872
to June 30, 1880...
to 1877, inclusive .
to June 30, 1880...
to 1871, inclusive .
to June 30, 1880...
to 1879, inclusive .
to 1873, inclusive .
to 1879, inclusive .
to June 30, 1880...
to 1875, inclusive .
to 1876, inclusive .
2,
2,
22,
702,
4,
1,
26,
559,
1,
1,
556,
10,
11,
37,
784,
815,
3,
162,
10,
306,
320,
2,
223,
183,
142,
11.
1,
*165,
77,
482,
4,
119,
13,
16,
12,
30,
7,
443,
250
.300
1,000
1,450
1,135
333
1,540
700
21
1,010
256
230
1,740
1,073
1,220
1,725
448
1,000
210
1,000
750
175
$10 38
14 38
16 86
24 99
46 52
17 36
28 22
12 50
82 72
22 56
14 21
24 21
20 74
42 65
15 42
82 26
36 84
25 65
21 05
23 70
50 70
24 91
9 02
23 42
19 40
34 47
5 08
35 32
9 97
37 79
18 18
19 71
15 53
34 32
20 44
8 64
10 53
10 03
11 27
38 84
17 21
29 29
6, 281, 885
221
44 26
$*,
32,
42,
570,
32, 672,
83,
28,
337,
46, 278,
43,
14,
13, 471,
217,
504,
583,
64, 508,
30, 049,
81,
3, 414,
240,
15, 525,
7, 986,
18,
5, 224,
3,554,
4,905,
57,
28,
12,
13, 659,
1, 411,
15,
16, 552,
101,
1, 035,
140,
162,
144,
1, 174,
121,
12, 998,
116 66
705 00
931 25
166 29
245 95
465 99
028 10
999 72
839 05
585 02
917 97
217 28
561 49
012 49
470 23
673 50
431 04
594 12
525 67
no 13
075 49
951 18
672 75
461 69
271 01
853 61
645 79
601 53
622 33
489 06
728 47
124 27
254 23
466 81
363 16
657 17
440 51
392 81
803 45
813 33
170 82
278. 012, 865 08
* Tonnage from 1860 to 1870 not ascertainable; average stated is for 16S,038^JJJ tons produced &om 1874 to 1880, inclusive.
THE COMSTGOK MINES.
11
BnXLION PRODUCT OF OTHEK MINES IN THE WASHOE DISTRICT TO JUNE 30, 1880.
Mine.
Date.
Ore treated.
Average
yield per
ton.
Product.
Tons.
(2,000 lbs.)
Poands.
1873
■ 240
3,425
1,004
7,849
2,161
$8 11
18 83
10 98
19 25
15 81
$1, 948 10
64, 507 96
1879 to Jnne 30, 1880
Occidental ...
1868 to 1873, inclusive
151 152 87
1875aDtll876
34 165 00
Total
14, 679
17 90
262 806 93
BULLION PRODUCT EROM TAILINGS OF COMSTOCEl ORES TO JUNE 30, 1880.
[So far as ascertainable from oflScial reports of the miuing companies and tlie assessors' returns. ]
1 ounce silver = $1.2929.
Mine.
Date.
Product.
1868
$229 97
4 811 96
Barke & Hamilton . ...
1868
1868 and 1869
45, 406 63
8, 342 45
334, 918 94
1 550 89
Gold Hill M. & M. Co
1871 and 1872
Gould &. Curry
1865 to 1869 inclusive
Hale &. Korcroas
1867 and 1868
Hartford
1871
195 50
1870
29, 362 01
81, 828 94
1866 to 1870, inclusive
1871
1868 and 1869
24 983 99
1871 to June 30, 1880
3 765 000 74
4, 306, 632 02
RECAPITULATION.
Product of the lode traceable by mines $278, 012, 865 08
Product of other mines in the district 262, 806 93
Tailings 4, .306, 632 02
Total product directly traceable 282, 582, 304 J)3
Estimated additional production, chiefly in early years 23, 598, 947 02
Total yield of the Washoe District to June 30, 1880 306,181,251 05
CHAPTER II.
PREVIOUS INVESTIGATIONS OF THE COMSTOCK LODE.
V. Richthofen's report. — III 1865, Baron Fei'dmand voH Richthofen made an
examinatiou of the Comstock for the Sutro Tunnel Company, a report of
which was issued by that corporation, but never published in the proper sense
of the word.^ It met the ordinary fate of mine reports, and is now scarcely
obtainable. It was, however, a very important contribution to American
geology, and no one who has studied the Lode has failed to acknowledge his
indebtedness to it. The mines are now about six times as deep as at the
date of von Richthofen's examination, but his opinions and predictions have
for the most part been verified in a very remarkable manner; and had his
lithological determinations been as accurate as his insight into structure was
keen, I should have had little to do beyond confirming and amplifying upon
his views, in spite of immensely increased facilities for observation. In litho-
logy, as is well known, there has been almost a revolution since von Richt-
hofen wrote, an*d nothing is less strange than that some of his determina-
tions should fail to stand microscopic tests. On account of the rarity of
Baron von Richthofen's report, I take the liberty of reproducing almost
entire and verbatim its geological portions. In these days of diffuse writing
its conciseness must be regarded as one of its important merits.
Rocks of the district. — The more important rocks of the Washoe district are
as follows :
Syenite, containing both orthoclase and oligoclase,^ mica and epidote,
' The Comstock Lode : its character and probable mode of continuance in depth. By Ferdinand
Baron Richthofen, Dr. Phil. (Nov. 22d, 18G5), San Francisco: published by the Sutro Tunnel Company.
Towne & Bacon, printers, 1866. 88 pp. 8vo.
''Professor Zirkel showed ten years later, by the help of the microscope, that this rock contains
exclusively plagioclase, and is therefore a diorite.
12
PEEVIODS INVESTIGATIOJfS. 13
but no quartz. It forms the prominent topographical feature of the dis-
trict, Mount Davidson. Adjoining the syenite to the north and south are
metamorphic rocks, the most recent of which Prof. J. D. Whitney has shown
to be Triassic. Overlying a portion of the metamorphic strata is quartzose
porphyry.!
The foregoing form the ancient series. Of the Tertiary rocks only
two^ have any close relation to the Comstock Lode. Propylite has this
remarkable peculiarity, namely: that it resembles many ancient rocks
exactly in appearance, and yet is among the most recent in origin. It is
prominent among the inclosing rocks of the Comstock vein and, besides,
incloses several, perhaps most, of the largest and most productive silver veins in
the world, as those in the Karpathian Mountains, of Zacatecas and other
places in Mexico, and probably several veins in Bolivia. Mineralogically
it consists of a fine-grained paste of ordinarily greenish, but sometimes
gray, red, and brown color, with embedded crystals of feldspar (oligoclase),
and columns of dark-green and fibrous, seldom of black, hornblende, which
is also the coloring matter of the base. A peculiarity of the rock is its
ferruginous character when decomposed. Probably it contains other metals
besides iron. Geologically it is an eruptive rock, but it is accompanied by
vast accumulations of breccia, which is- sometimes regularly stratified. The
flats of Virginia City, Gold Hill, American City, and Silver City consist of
propylite. It lies, in general, east of the mountains consisting of the ancient
formations, and contains several mineral veins besides the Comstock Lode.
Its distribution in other countries of the world is not very general. Sev-
eral different kinds of volcanic and eruptive rocks followed the outbreak of
propylite, but only one of them demands attention in reference to the Com-
stock vein, as it probably caused its formation, besides taking a prominent
part in the structure of the country. This is sanidin trachyte.
iln his memoir ou The Natural System of Volcanic Rocks, p. 41, Baron von Eichthofen says:
" Quartzose porphyry occurs to some extent in Washoe under circumstances which make the exact
determination of its age difficult, but render it certain that it is intermediate in this respect between
granitic and volcanic rocks." This rock was later regarded by Mr. King as quartz-propylite, and
determined by Professor Zirkel as dacite. (v. Quartz-porphyry.)
=In his Natural System of Volcanic Rocks, p. 31, Baron von Eichthofen, speaking of the
Washoe district, says: "Andesite is insignificant in bulk in that region. It composes a few hillocks on
the propylitic plateau, and in some cuts and tunnels andesitic dikes may be seen."
14 GEOLOGY OF THE COMSTOCK LODE.
The mode of occurrence of the trachyte shows that it has been ejected
through long fissures in a viscous or hquid state, and at a high tempera-
ture. In some places the eruptions were subaqueous, as in the vicinity of
Dayton. The entire table-land around that place is built up of stratified
trachytic tufa. The solid trachyte rises from it in rugged mountains, which
form an elevated and very conspicuous range, passing east of the inter-
section of Six-mile and Seven-mile Canons across the Seven-mile Canon
(where, for instance, the Sugar-Loaf Peak consists of it), and bending in a
semicircle round to "Washoe Lake. Farther north this rock covers the
country to a great extent. Sanidin trachyte has never been found to con-
tain silver-bearing veins, and in Washoe none occur in it; and yet it has
been mainly instrumental in the formation of the Comstock Lode and- other
veins in that region. No geological events after that ejDOch are worth men-
tioning for the present object.
Mode of occurrence of the Comstock. lu 1865 Only abOUt 11,000 fcCt of thc LODE
had been explored to any extent, mainly the ground lying between the
02)hir North mine and the Overman, and a few only of the mines had reached
a depth exceeding 700 feet. At an average depth of 500 feet both walls
were found dipping to the east at from 42° to 60°. Above this level the
west wall preserved the same slope, while the east wall curved rapidly
toward the vertical, and then to the east, giving the cross-section of the
vein the shape of a funnel, a great part of the space in the enlarged portion
next the surface being occupied by fragments of country rock or " horses,"
between which was the vein matter. The width of the belt in which these
branches came to the surface, and there form scattered croppings, is gen-
erally more than 500 feet. To the west of the Lode a number of small
veins show as croppings. They probably unite with the Comstock in depth,
and form with the latter what the Germans call a "Gangzug."
The course of the west wall, as far as explored, is somewhat dependent
on the shape of the slope of the range at the base of which it lies. It par-
takes of all its irregularities, passing the ravines in concave bends, and
inclosing the foot of the different ridges in convex curves; the greatest
convexity is around the broad uninterrujjted foot of Movmt Davidson itself.
These irregularities are of importance, as they influence the ore-bearing
PEEYIOUS INVESTIGATIONS. 15
character of the vein. The west wall of the main vein is well defined ; not
so the east wall, where, as is often the case with true fissure veins, the country
rock is impregnated with matter similar to that which fills the fissure. It
is frequently concentrated in channels running parallel to or ascending from
the vein, but in fact forming parts of it.
The rocks which accompany the Comstock vein change in its course.
They ai'e different varieties of propylite on the eastern side throughout its
whole extent. In some places the frequent and large crystals of feldspar
give it a porphyritic character, which in certain varieties is rendered more
striking by green columns of hornblende; at others, the rock has a very
fine grain, and the inclosed crystals are of minute size; again, the I'ock is
either compact and homogeneous, or it has a brecciated appearance from the
inclosure of numerous angular fragments; the color also changes, though
it is predominantly green, and the diff"erent degrees of decomposition create,
finally, an endless variety. The causes to which it is due will be consid-
ered presently.
The western country offers more differences. Along the slope of
Mount Davidson and Mount Butler, from the Best S Belcher mine to
Gold Hill, it is formed by syenite, which at some places is separated from
the vein by a fine-grained and crystalline rock of black color, having the
nature of aphanite, but altogether obscure as to the mode of its occurrence.
It is from 3 to 50 feet thick, and the elucidation of its real nature may
be expected from further developments. As syenite to the west, and propy-
lite to the east, occur just in that portion of the Comstock vein which has
been most explored, and where works, more than anywhere else, extend in
both directions into the country, it has been generally assumed in Virginia
that the Lode follows the plane of contact between two different kinds of
rock, and is therefore a contact deposit. But immediately north of Mount
Davidson, where propylite extends high up on the western hills, this rock
forms the western country as well as the eastern, as at the California and
Ophir mines, though at the latter metamorphic rocks and syenite are asso-
ciated with propylite on the western side. On Cedar Hill syenite again
predominates, but farther north propylite forms the country on both sides.
South of Gold Hill the syenite disappears from the western wall, and its
16 GEOLOGY OF THE COMSTOCK LODE.
place is taken to some extent by propylite, but in greater part by meta-
morphic rocks. Nowhere have syenite and metamorphic rocks been found
occurring on the eastern side.
The formation of the fissure was only in so far dependent on the con-
tact between propylite and syenite, as it follows the same accidentally in
part of its course, probably because the resistance along it was inferior to
that offered by the solid masses of rock on either side. It is characteristic
of fissure veins in general, not only that the country rock on one side has
moved downward on the other, but also that within the space formed by the
opening of the fissure powerful dynamic action has taken place. Few veins
present these phenomena so distinctl}" as the Comstock, the eastern side of
which has apparently moved downward on the western; and the action
within the vein is amply evinced by the brecciation of the vein matter, the
presence of masses and seams of clay, the crushed condition of the quartz, etc.
There is a marked difference between the western and the eastern crop-
pings. Those of the western branches of the vein carry principally crys-
tallized quartz of a very glassy appearance, light color, and comparatively
of a pure quality. Large angular fragments -of the country rock are em-
bedded in the quartz and form centers of its crystallization. Metalliferous
minerals are scarce, though nowhere entirely wanting Nothing indicates
underground wealth, nor indeed has such been found by subsequent mining.
The only exception is Cedar Hill, where native gold was found abundantly
in places, but its scarce distribution never justified great expectations. In
the eastern outcrop particles of countr}'^ rock, together with those of clayey
matter and metallic substances, occur finely disseminated through the quartz,
which is reddened by metallic oxides.
Contents of the Lode. — The vcin matter is composed of fragments of country
rock, clay, quartz, and ores. Near the surface about five-sixths of the mass
of the Comstock vein consists of horses, the shape and size of which vary
with the different nature of the rock of which they consist. Those of pro-
pylite are confined throughout Virginia City to the east side, and they are,
as a rule, longer and thinner than those of syenite. From the large horses
every variety occurs down to the smallest fragments. The quartz is often
so thickly filled with angular pieces as to have a brecciated appearance.
PEEVIOUS INVESTIGATIONS. 17
Propylite is much more common than syenite. Few large veins are so abun-
dant in clay and clayey matter as the Comstock. It forms the western and
eastern selvages from north to south in continuous sheets sometimes of from
10 to 20 feet in thickness. Other sheets divide horses from quartz, or different
bodies of the latter from one another. Most horses terminate at the lower
end in clayey substances. The differences mentioned before as prevailing
in the quartz of the outcrops continue downwai'd, but are not so conspicuous
in depth on account of the general white color of the quartz. Finely dis-
seminated particles of wall rock are always abundant where the quartz con-
tains ore. The quartz is generally fractured, and at numerous places the
effects of dynamical action on it are such as to give it the appearance of
crushed sugar. The principal silver ores of the Comstock are stephanite,
vitreous silver ore, native silver, and very rich galena. Quartz is the only
gangue, though carbonate of lime and gypsum occur in places. Zeolites are
limited to the northern portion of the vein, where chabasite and stilbite fill
small fissures and cavities in propylitic breccia within the body of the vein.
The ore is distributed in a different way in the northern and southern
parts of the vein. The passage between the two modes of occurrence is
gradual. In the northern part the ore is concentrated in elongated lenticular
masses, of which the greatest axis is not far from the vertical. The different
oi'e bodies often adjoin each other in such a way as to make a nearly con-
tinuous line, as was the case in the Gould & Gurry and the Savage mines.
The ore has been exceedingly rich in the center of the different bodies,
where, at the same time, it was soft and could be easily removed, while the
outer parts were hard, and consisted of second-class and low-grade ore.
Near the center of the Lode, at the Bullion mine, quartz fills the entire
width of the vein from the western to the eastern wall, though it is too poor
for extraction. The occurrence of ore in chimneys and in barren portions
between them ceases in this neighborhood. To the south the ore is concen-
trated in continuous sheets, the principal one of which is very near and
parallel to the eastern wall. The second sheet occurred farther to the west,
extending from the outcroppings to a couple of hundred feet in depth. This
sheet dipped to the west at an angle of about 60°, flattening in depth and
terminating in horizontal layers of clay. It was particularly rich in gold.
2 0 1.
18 GEOLOGY OF THE COMSTOCK LODE.
The yield of the ore has decreased in general from the surface down-
ward. The deposits of the Ophir and Mexican and of the Gould d Curry
were the richest. The former yielded on an average $107 per ton; the
latter $70 and $80, notwithstanding the imperfect processes of extraction
which were formerly applied. Ores of $600 to the ton were then no rarity,
and considerable shipments could be made of such as yielded from $2,000
to $3,000 to the ton. It would now scarcely be possible to collect one ton
of such ore. The general average of all the ores extracted in 186'^ will not
be more than $37 to the ton. The proportion of gold to silver decreased
during the early period of the working on the Comstock Lode, but is now
asrain on the increase.
Source of the ore. — The CoMSTOCK Vein has neither been filled from above
nor from the sides, as none of the surrounding rocks could have yielded the
immense quantity of vein matter and ore; and had it been formed in this
way, the mass would have a banded and comby structure, which is by no
means observable. The eastern rock may, on account of its extensive
decomposition, appear to favor the assumption of lateral infiltration; but
this decomposition was effected by ascending currents, which have left
distinct traces, and which could not have removed any matter in a lateral
way. Thermal springs, which are considered by many authorities as the
asrent which carried mineral matter from below into fissures and to have
formed every true vein, would not explain the formation of the Comstock
Lode. Silica, in such cases, is accumulated round the mouth of the fissures,
and though ordinarily removed by denudation, it could hardly be supposed
to be so at the Comstock vein, as since its formation the sui-face has under-
gone but slight changes. But, besides, the decomposition of the eastern
country for miles in extent cannot be explained by the action of thermal
springs.
The Comstock fissure is, of course, of more recent origin than the
rocks which it traverses; and as propylite is predominant in the latter, the
fissure must necessarily have succeeded it in age. The only event after
the outbursts of propylite capable of producing such powerful action was
the eruption of trachyte, which accomjDanies the vein at a distance of two
miles to the east. As there is other evidence of its intimate connection with
PREVIOUS LNVESTIGATIONS. 19
the CoMSTOCK vein, we may take it for granted that it caused the rending of
the fissure.
Applicability of the ascension-theory. — We have iu the elcments evolved during
the first two periods of solfataras, namely, fluoiine, chlorine, and sulphur,
all the conditions required for filling the CoMhTOCK fissure with such sub-
stances as those of which the vein is composed. Steam, ascending with
vapors of fluosilicic acid, created in its upper parts (by diminution of pressure
and temperature, according- to well-known chemical agencies) silica and sili-
cofluohydric acid, the former in solid form, the latter as a volatile gas, which
acts most powerfully in decomposing the rocks it meets on its course. The
chloride of silicon in combination with steam forms silica and chlorhydric
acid Fluorine and chlorine are the most powerful volatilizers known, and
form volatile combinations with almost every substance. Besides silicon,
the metals have a great affinity with them. All those which occur in the
CoMSTOCK vein could ascend in a gaseous state in combination with one or
the other of them. They must then be precipitated in the upper parts as
metallic oxides or chlorides, and in the native state. Thus the fissure was
gradually filled from its upper portion downwards, with all the elements
which we find chemically deposited in it. A fissure is ordinarily not sta-
tionary after its first opening; but by subsequent action may from time to
time widen and frequently contract again. New channels would thus be
opened where the old ones were obstructed. If such widening or opening of
an empty space within the matter filling the old fissui-e was followed by
emanations rich in metallic vapors, then the conditions would be given for
the formation of a body of ore of the shape of the newly-opened chasm,
which corresponds precisely to most of the bodies of ore in the Comstock
Lode. Contemporaneously with the filling of the fissure, the adjoining rock
would be acted upon by the ascending acid vapors, and its nature by them
entirely changed. Cracks would form in it, and be filled with substances
similar to those of the vein itself As the Comstock vein has an eastern
dip, and the action of forces manifests itself towards the surface, only the
rock on the hanging wall, or the eastern country, would be influenced in
this way. Crevices branching off from the main fissure would probably pen-
etrate into the hanging wall, and it may reasonably be expected that deeper
20 GEOLOGY OF THE COMSTOOK LODE.
workings will disclose such branches filled with vein matter, and probably
of oi'e-bearing character, east of the main body of the vein.
Alteration of minerals in situ. — A transforming actlon must necessarily take
place from the very commencement of the decomposition of matter in the
fissure. Svilphurous acid and sulphuretted hydrogen, which were among the
escaping gases in the first period, together with combinations of fluorine
and chlorine, gradually became predominant, marking the second period of
solfataric action. But little more matter could be introduced into the fissure,
as the combinations of sulphur with mineral substances are not volatile.
Chemical transformation was now the principal action within the vein. Silica
is deposited from its combinations with fluorine and chlorine in a gelatinous
state, very different in its physical character from those of the crystalline
quartz which fills the vein. It must undergo a solution in water, with which,
in the form of steam, it was impregnated, in order to assume this character.
Metallic oxides and chlorides were converted into sulphurets, and the pres-
ence of antimony caused the formation of sulph-antimoniurets, the principal
one of which is stephanite. By such processes the entire vein matter was
gradually converted from its former condition into that which it presents at
this day.
Fluorine and chlorine. — It is a fact worthy of notico that there is scarcely a
single chemical agent, excepting fluorine and chlorine, which would not
carry metallic substances into fissures in exactly or nearly the reverse quan-
titative proportion from that in which they occur in silver veins. Iron and
manganese are not only more abundant in rocks, but also much more easily
attacked* and carried away by acids than silver and gold. The proportion
of these to the former ought, therefore, to be still smaller in mineral veins
than it is in rocks, and lead and copper ought to be more subordinate, if
their removal from their primitive place had been effected by other agents
than fluorine and chlorine. Only these two will first combine with those
metals which are most scarce in rocks, and relatively most abundant in sil-
ver veins; and they are probably the only elements which have originally
collected them together into larger deposits, though these may subsequently
have undergone considerable changes, and water may have played altogether
the most prominent part in bringing them into their present shape.
PEEVIOUS INTESTIGATIONS. 21
Wide-spread soifataric action. — Thougli it seems that the CoMSTOCK fissui'e was
the principal theater for the emission of steam, and all those phenomena
which may be comprised by the name of soifataric action, yet the latter left
its traces over a wide extent of the adjacent country. The entire belt of
rounded hills, extending east of the vein for two miles, to the foot of the
trachytic range, shows its effects very conspicviously. It consists of propy-
lite, which, however, can scarcely be recognized on account of the complete
decomposition it has undergone, and which has transformed it into a clayey
rock of red and yellow color, but still showing distinctly the inclosed crystals
of feldspar and hornblende. It is traversed by numerous crevices from
which the decomposition originated, and shows everywhere evidences of
vertically ascending currents which caused it. Whoever has seen active
solfataras will be struck by the resemblance of chemical action on the sur-
rounding rocks to that displayed in the region east of the Comstock Lode.
Near some of the crevices the decomposed rock is strongly impregnated with
silica, producing the ranges of red-colored bluffs which accompany the
Comstock vein to the east, and which have been partly located as out-
croppings of veins, while at about two miles' distance real metalliferous
veins occur, promising in their outcrops, but not yet explored. Besides
this belt the former action of solfataras is plainly visible in many parts of
the country. The formation of the Comstock vein is but one of its mani-
festations.
Continuity of the Lode in depth. — As it has bccu shown that the vciu was filled
from a deep-seated source, it is certain that it is continuous in depth. The
inclination is not likely to vary considerably, for not only is the west wall
remarkably regular, tending to show its continuity, but the previously men-
tioned soifataric action to the east is an evidence that the vein underlies the
country rock in this direction for a long distance. As for the mean width of
the vein in depth no definite prediction can be made. In some places the vein
at a distance of 500 feet from the sm-face forms a channel of 120 feet or more
in width; at other points it is contracted. Such places must necessarily occur
in an inclined vein of some magnitude, since the hanging wall, during the
long periods of the filling of the fissure, required some support. The walls of
every true fissure vein are uneven planes. The downward movement of one
22 GEOLOGY OF THE COMSTOCK LODE.
side of the fissure on the other, at the time of the formation of the vein,
caused pi'otuberances of one wall to meet such of the other, and concave
places to come opposite to each other. This is the reason why every large
fissure vein is liable to repeated expansions and contractions, .though the
former prevail largely over the latter. It is to be expected that the Com-
STOCK Lode will exhibit the same feature in its downward course to indefi-
nite depth, as it has done heretofore, though its genei-al width will probably
remain nearly equal to that which it possesses in the lowest works. The
formation of large horses is, from the nature of their origin, more peculiar
to upper than to lower levels, since their. breaking down from the hanging
wall will in every fissure be most apt to take place where the latter is of
comparatively inferior thickness, than where it is hundreds or thousands of
feet wide. But small fragments may separate from it at any depth, and
their quantity will chiefly depend upon the nature of the rock and the power
of decomposing agents. If any change as to the inclosing rocks should
occur in depth, it is probable that propyhte will disappear on the western
side and syenite predominate there more and more.
Probable character of the Lode in depth. All thc CvideriCBS iu tllC UppCr IcVcls
justify the expectation that the foot wall will continue with its smooth and
regular clay-selvage, while the irregularity and indistinctness of the eastern
side will not diminish but rather increase as its true character as hanging
wall will become more conspicuous. The vertical sheets of clay which
have from time to time been cut in the adits east of the vein, rise undoubt-
edly from the hanging wall. Clay seams within the body of the vein will
probably diminish with the increase of unity in inclination. Those which
are at present observable at upper levels are particularly occasioned by
the vertical position of the vein-matter, which of course facilitates sliding
motions. Larger accumulations of clay will especially continue near the
old ravines.
The ores, through all the levels explored, retain the character of true
silver ores which they had near the surface. The amount of lead, copper,
iron, and zinc has never been large in the Comstock ores, and these metals
preserve now at the lowest levels nearly the same relative proportion as
formerly. Their increase, especially of lead, would be the most unfavora-
PEEVIOCJS nsrVESTIGATlONS. 23
ble indication for the future of the Comstock Lode ; as, besides the growing
difficulty of metallurgical treatment, the conclusion would be justified that
lead ores would more and more replace those of silver, and the limits of
profitable productiveness would soon be reached. But as it is, no deterio-
ration is to be expected, even if an impoverishment takes place. It thus
approaches in its ore-bearing character the great mother-veins of Mexico,
and is different from those of Hungary.
Conclusions. — Considenug these facts exhibited by the Comstock vein
itself, and comparing them with what is known about similar argentif-
erous veins, we believe we are justified in drawing the following conclu-
sions :
1st. That the continuity of the ore-bearing character of the Comstock
Lode in depth must, notwithstanding local interruptions, be assumed as a
fact of equal certainty with the continuity of the vein itself.
2d. That it may be positively assumed that the ores in the Comstock
Lode will retain their character of true silver ores to indefinite depth.
3d. That it is highly probable that extensive bodies of ore equal in
richness to the surface-bonanzas will never recur in depth.
4th. That an increase in size of the bodies of ore in depth is more
probable than a decrease, and that they are more likely to increase than
to remain of the same size as heretofore.
5th. That a considerable portion of the ore will, as to its yield, not mate-
rially differ at any depth from what it is at the present lower levels ; while
besides there will be an increasing bulk of low-grade ores. We are led to
this supposition by the similarity in character of all the deposits outside of
the rich surface-bonanzas, and the homogeneous nature which almost every
one of them exhibits throughout its entire extent.
6th. That the ore will shift at different levels from .certain portions of
the Lode to others, as it has done up to the present time. More equality
in its distribution may, however, be expected below the junction of the
branches radiating toward the surface, when the vein will probably fill a
more uniform and more regular channel. Some mines which have been
heretofore almost unproductive, as the Central, California, Bullion, and others,
have therefore good chances of becoming metalliferous in depth. But
24 GEOLOGY OF THE COMSTOCK LODE.
throughout the extent of the vein it is most likely that the portion which
lies next to the foot wall will continue unproductive, as it did from the sur-
face down to the lowest works, while the entire portion between it and the
hanging wall must be considered as the probable future source of ore. As
remarked in the foregoing pages, it is also probable that repeatedly, in fol-
lowing the Lode downward, branches will be found rising from its main
body vertically into the hanging wall, and consisting of clay and quartz.
Many of them will probably-be ore-bearing. Such bodies of ore should be
sought for at all mines, in what is generally supposed to be the eastern
country. Experience in the upper levels would lead to the supposition that
such eastern bodies might carry richer ores than the average of the main
portion of the vein.
7th. That the intervention of a barren zone, as is reported by good
authorities to occur at the Veta Madre of Guanajuato, at the depth of 1,200
feet, is not at all likely to be met with in the case of the Comstock Lode.
The argument which we have to adduce for this conclusion has some weight
from a geological point of view. It is a well-known fact that the inclosing
rocks have usually great influence on the quantity and quality of the ores
of cei'tain metals in mineral veins, and that a rich lode passing into a differ-
ent formation frequently becomes barren or poor. At the Veta Madre of
Guanajuato a sudden decrease in the yield of the ore, at the depth of 1,200
feet, attends the passage of the lode into a different formation, which from
thence continues to the lowest depth attained. No such change can ever
be anticipated for the Comstock Lode, since the structure of the country
seems to indicate the continuity of the inclosing rocks to an indefinite
depth.
King's memoir. — In 1867-'68 Mr. Clareuce King, Geologist-in-charge of
the Exploration of the Fortieth Parallel, made an examination of the Com-
stock LoDF.^ In regard to lithology Mr. King mainly followed Baron v.
Richthofen, but he recognized a much greater area of andesite than his
predecessor, and the affinity between certain propylites and andesites. Of
the latter he says: "The balance of probability points to a close alliance
1 Exploration of the Fortieth Parallel, Vol. III.
PEEVIOUS INVESTIGATIONS. 25
between this rock and propylite, and it will not be at all surprising if it
should finally prove to be chemically identical and, in reality, only a different
form." The rock determined as quartz-porphyry by Baron v. Richthofen,
Mr. King regarded as quartz-propylite.
In discussing the relation of Mount Davidson to the vein, Mr. King
calls especial attention to the agreement between the contours of the west
wall of the Lode and those of the exposed face of the range. He considers
this similarity as evidence that the wall is merely a continuation of the face
of Mount Davidson, and that the syenite has undergone very little erosion
since the opening of the fissure. It is not necessary to summarize Mr.
King's very graphic description of the structure of the Comstocic. Lode in
detail here, as I shall be obliged to use it in my own account of the vein,
the portion which he examined having long been inaccessible.
Mr. King closes his memoir with the following summary :
King's summary. — " Tlic ancicut Virginia Range, prior to the Tertiary period,
was composed of sedimentary beds of the great Cordillera system, which,
in the late Jurassic epoch, had been folded up, forming one of the corruga-
tions of that immense mountain structure which covers the western front of
our continent. Accompanying this upheaval were outpourings of granite
and syenite. The erosion which followed this mountain period escarped
the ancient rocks, and modeled the eastern front of Mount Davidson into a
comparatively smooth surface, whose average angle of slope sank to the
east at about 40°. In the late Tertiary, at the time of the volcanic era, the
Virginia Range shared in the dynamical convulsions which gave vent to suc-
cessive volcanic outflows of immense volume and very remarkable character.
The first and, so far as the Comstock Lode is concerned, the most import-
ant was of propylite, or trachytic greenstone, which deluged the range from
summit to base, covering large portions of its ancient surface, and leaving
here and there isolated masses, which rose like islands above the wide
fields of volcanic rock. Subsequently followed the period of the andesites
which, at their commencement, in the form of a thin intrusive dike, pene-
trated a new-formed fissure on the contact plane of the ancient syenite and
the propylite. This earlier andesite period gave birth to the solfataras,
which, bursting from a hundred vents, rapidly decomposed the surrounding
26 GEOLOGY OF THE COMSTOCK LODE.
rocks, and gradually filled the fissures of the Comstock with their remarkable
charges of metal-bearing quartz. The latest flows of andesite poured out
over the decomposed propylite; and since they are themselves unaltered,
their appearance marks the period when solfataric action over wide areas
had ceased. While it no longer maintained its energy through the broad
zone of propylite, it still continued intensely active within the chambers of
the Comstock Lode. Metallic contents were introduced into the quartz,
the clay seams were formed by a rapid decomposition of the neighboring
propylite materials, the horses reduced to a spongy, semi-plastic condition,
and at last the final solidification of the quartz took place. Outside the
vein two events of geological interest have occurred: first, the period of
trachyte eruptions, when from the ruptures of the crust, parallel to the
Comstock Lode, vast volumes of sanidin-trachyte overflowed the country;
and, secondly, the less powerful but still important outpouring of basaltic
rock, which marked the close of the volcanic era. Within the vein, and
probably caused by one or both of these latter volcanic disturbances, a
pressure has been exerted which has crushed and ground the masses of
quartz into minute fragments. It is interesting to observe that while this
force was great enough to crush quartz masses 1 50 feet in breadth into mere
angular pebbles, the disturbances were insufficient to cause any actual
faulting of importance. Both within and without the vein the solfataras
gradually came to a close. The heated currents of water which even yet
ascend into the lower levels of the mines, are evidence that at no very
great depth a considerable temperature is still maintained; but this is only
a faint relic of a once intense action."
zirkei's report. — lu ISTH Prof. Ferdiuaud Zirkel made a macroscopical and
mici'oscopical examination of the lithological collections of the Exploration
of the Fortieth Parallel.^ Among the slides which he described are thirty-
three from the Washoe district He confirmed the independence of horn-
blende-propylite and quartz-propylite as lithological species, regarded most
of the quartzose rock as dacite, coiTCCted the determination of the granu-
lar diorite (which had been considered as syenite), and added augite-ande-
' Exploration of the Fortieth Parallel, Vol. VI.
PREVIOUS INVESTIGATIOlSrS. 27
site, rhyolite, and a strange variety of basalt to the list of rocks previously
recognized. Professor Zirkel formulates the diagnostic differences between
propylite and andesite as follows:-^
propyiite. — "o The general color of the propylitic groundmass has more
of a greenish-gray, while the andesitic groundmass has more of a pure gray
or brown tinge.
"b. In structure and in the behavior of its constituents, the propylite
still resembles the older ante-Tertiary diorite-porphyries.
"c. The groundmass of the propylites is rich in minute particles of
hornblende, while in that of the andesites this mineral appears only in the
larger individuals, fine hornblende dust being wanting.
"d The propylitic feldspars are usually filled with a considerable
quantity of hornblende dust, while the andesitic feldspars are entirely with-
out it: the latter not infrequently containing glass-inclusions, which do not
seem to occur in the propylitic plagioclases.
"e. The color of the proper hornblende sections in propylite is always
green (never brown), while the color of those in andesites is almost with-
out exception brown ; and the propylitic hornblende never shows the curious
black border which is so common to that of andesites; and again, propylite
in some cases contains, besides the largely predominating green hornblende,
a few sections of the brown mineral, presenting, in many points, a strikingly
peculiar aspect, while in andesites two kinds of hornblende never occur
together.
"/ The propylitic hornblende is often distinctly built up of thin
needles or staff-like microlites, and therefore is not regularly cleavable ;
which has never been found to be the case in andesites.
"(jr. The production of microscopical epidote (mainly by the alteration
of hornblende), so very common in propylites, has, with one exception,
never been observed in these andesites, and it is also unknown in the Euro-
pean occurrences.
"h. Augite often occurs as an accessory constituent in andesites, but
it is comparatively very rare in propylites.
"i. The andesitic groundmass here and there seems to possess a half-
glassy development: a glass-bearing propylitic groundmass has never been
' Exploration of the Fortieth Parallel, Vol. VI., p. 132.
28 GEOLOGY OF THE COMSTOCK LODE.
found; and herein is another point of resemblance to the old diorite-por-
phyries.
"All these differences between propylitic and andesitic hornblende also
extend to both of the quartziferous members, quartz-propylite and dacite."
On page 1 1 7 Professor Zirkel states that the quartz of quartz-propylite
contains fluid inclusions, and "behaves exactly like that of the ante-Tertiary
dioritic porphyries, and differently from that of all other Tertiary quartz-
iferous rocks, dacites and rhyolites, which only contain glass inclusions."
Church's memoir. — lu 1877 Mr. J. A. Church made an examination of the
CoMSTOCK,^ as a member of the United States Surveys West of the One
Hundredth Meridian, under Captain Wheeler. Mr. Church accepted the
lithology of his predecessors with some modifications a little difficult to fol-
low, but thoucrh he mentions slides of the rocks, describes none. His
memoir contains a number of ingenious hypotheses which would possess
great importance if sufficient evidence in their favor could be adduced.
Lithology. — Mr. Church appears to^use the terms porphyrite and propylite
interchangeably for all strikingly porphyritic rocks of light color.^ Rocks
of dark color, whether from the presence of abundant hornblende or from the
transparency of the feldspars, he appears to have regarded as andesite,' and
asserts that it is quite safe to put the minimum number of north and south
dikes of this rock at between twenty-five and fifty. Besides the masses
which had hitherto been regarded as trachyte, he determined the rocks
about the new Yellow Jacket shaft, and at other points, as remnants of the
trachyte eruption. This leads to the supposition that he employed the term
merely to designate soft, rough, light-colored, porphyritic rocks. In Mr.
Church's opinion the diorite, propylite, and probably the andesite, were laid
down in thin regular layers, which he compares to those of sedimentary
rocks. This he considers proved by the sheeted character of the rocks in
the east and west country.
'The CoMSTOCK Lode, its formation and history. By John A. Church, E. M., Ph. D., member
of the American Institute of Mining Engineers, mining engineer. Illustrated by six plates and thir-
teen figures. New York : John Wiley & Sons. 1879.
= L. c., p. 40 to 42 and 52.
'Ii. c, pp. 37 and 47. The McKibben tunnel shows only diorites and quartz.
PEEVIOUS INVESTIGATIONS. 29
History of the lode. — Mr. Churcli dividcs the history of the Comstock Lode
into nine epochs :^
1. The diorite epoch. — The horizontal deposition of diorite, which is one
of the fine-grained, thin-running lavas,^ in stratified layers, by a series of
eruptions.
2. The subordinate pressure. — The system of diorite strata was acted
upon by a pressure which produced broad folds with east and west axes,
an uphft in Virginia, and a trough in Gold Hill. This important force
continued to affect the rocks through the greater part of their history,
and is the dynamic cause of the Lode.
3. The propylite epoch.— The horizontal deposition of the propylite, also
in stratified layers from successive fissures. The members of the new rock
are essentially parallel to the older layers.
4. The principal elevation. — After the propylite, came a movement by
which the two series of eruptive depositions were raised into a mountain
system. This elevation took place about a north and south axis, and its
folds are therefore at right angles to those of the former movement.
5. The andesite epoch. — A third period of eruption follows, the seat of
which is the upturned strata of the diorite and propylite. These are not
fractured except near the eroded surface, but the layers are separated, and
the andesite rises through the crevices, establishing an extensive system of
bedded dikes. The whole mass of erupted andesite is assumed to have
been some thousands of feet thick, and to have played an important part in
the history of the Lode by its weight and rigidity.
6. The opening of the strata — The crests of folds already produced were
lifted forcibly against the rigid andesite cap, while the intervening troughs
were bent downward, relieving them from its weight. Under this action
the uplifted portions of the strata were squeezed sidewise into the relieved
troughs, opening slightly the partings between the layers.
7. The silicious epoch. — Through the small partings of the strata thus
opened, rose currents of water holding silica in solution. The strata sub-
jected to their action were dissolved or carried off mechanically, and quartz
with "base" metals was deposited in their place. This action went on in each
'L. c, p. 128. «L. c, p. 153.
30 GEOLOGY OF THE COMSTOCK LODE.
of the open seams, the intervening rock being attacked from both sides
until the meeting of several depositions of silica composed quartz bodies,
which in many cases had a thickness of several hundred feet. This quartz
was not argentiferous, and no ore was formed. A second important result
of this appearance of silicious waters is the almost entire removal of the
immense andesite cap, which was decomposed in the same manner as the
deeper-lying rocks.
8. The trachyte epoch. — New crevices opened in the eastern part of the
district, and vast floods of trachyte poured out. Instead of resisting move-
ment hke the andesite, it loaded down the hanging wall of the Lode so
heavily that it slid upon the foot wall. This action resulted in an entirely
new system of openings. Near the surface the new crevices abandoned the
old line of quartz deposition, and broke through the hanging wall in a more
or less nearly vertical direction.
9. The argentiferous epoch. — Into these new crevices poured a second
stream of water containing minerals in solution, but differing from the first
in holding not only silica, but also silver and gold.
"The facts here brought forward," says Mr. Church, "show that no
vein and nothing like a real vein exists in ground that has for years been
supposed to contain the boldest example of true fissure vein formatioh in
the world; that the largest bodies of ore can be formed from deep sources
of mineral supply without the agency of a fracture even of the smallest
dimensions; and that it is quite unnecessary to seek for great dynamical
convulsions to account for the formation of thick masses of ore within the
solid rock, a sufficient cause being found in the quiet action of the same
forces whicli have everywhere molded the crust of the earth."
The Justice ore body Mr. Church regards as a deposit wholly distinct
from the Comstock, though attributable to the same general causes, and as
formed in a similar way.
Pi,y3i,3._0f the finely-divided quartz known as sugar quartz, he says ■}
"The grains are remarkable in never being crystalline, the microscope not
reveahng one crystal in millions of particles." And again i^ "The lesson to
be derived from the sugar quartz is not that it has been crushed, but that it has
iL. c, p. 85. »L. 0., p. 151.
PEEVIOUS INVESTIGATIONS. 31
been preserved from crushing. It was formed in the state of powder, and
since its deposition the Lode rocks have not received any addition which
could weigh it down. On the other hand, the barren quartz was probably
laid down in a similar state of powder, and has been consolidated by the
load of trachyte upon the surface."
The heat of the Lode Mr. Church ascribes to the kaolinization of feld-
spar, supporting this view by the statement, that as kaolinization involves
hydration, heat must be liberated, and by the assertion that flooded drifts
grow hotter. He believes the heat to be diffused by hot aqueous vapor
permeating the rocks. The latter, he asserts, are in large part perfectly dry.
Technical literature. — Thougli most of tlic scieutlfic aud tcchuical journals
contain papers on the Comstock, or items referring to it, and much space is
occupied by the same subject in the reports of the United States Mining
Commissioners and of the State Mineralogist of Nevada, I am not aware
of any further noteworthy contributions to its geology. The numerous
geological suggestions thrown out by engineers writing from a more or less
technical point of view, were never intended as matured geological opinions,
and it would be unfair to treat them as such.
CHAPTER III.
LITHOLOGY.
Section 1.
THE ROCKS OP THE WASHOE DISTRICT.
Importance of lithology to the theory of ore-deposits. ThoUgh the preSeilt memoir is
intended as a contribution to mining geology, the importance of the Hthology
of the' district is certainly not less than it would be, were no economical
problems involved. The slightness of the advances which have been made
in the theory of ore-deposits is regarded by business men as a reproach
to geological science. But the influence of the inclosing rocks on the char-
acter and tenor, and to some extent upon the occurrence of ore bodies, was
recognized before geology became a science; and the fact of this influence
has received confirmation from more extended observation. Whatever, then,
may be the true theory of the genesis of ores, the indications are clear that
exhaustive studies of the nature of the inclosing rocks, and of the influences
to which these have been subjected, are essential to its elucidation; for even
if it should prove that ores are derived from immense depths, and are brought
to the surface under conditions which are wholly removed from observation
and study, the influence of the wall rocks on their deposition is still within
the accessible field of, inquiry. The way to such investigations is already
paved. The microscopic analysis of rocks initiated by Mr. Sorby, and raised
to its present rank as a science by Messrs. Vogelsang, Zirkel, Rosenbusch,
Fouqud & L^vy, and their fellow workers, enables us to reach very definite
conclusions respecting the mineralogical composition and ph3'sical structure
of rocks; while Prof F. Sandberger and others have of late years made great
advances in proving the chemical relations which, in many cases, exist
between the wall rock and the contents of veins. On the other hand, the
mineralogical study of decomposed rocks under the microscope has made
32
THE EOCKS OF THE WASHOE DISTEICT. 33
but little advance. Geologists who do not deal with the phenomena of ore
deposits are commonly satisfied with determining the species of the rocks
with which they have to do, and recording the mere fact of decomposition.
They therefoi-e select only the freshest specimens for microscopical exami-
nation. If the I'esources of the microscope are to be fully brought to bear
upon the study of ore deposits, mining geologists nmst pursue a different
method. They must trace the mineralogical course of decomposition-pro-
cesses, and learn to recognize highly altered rocks, even when fresh speci-
mens are unattainable.
Disputed character of Washoe rocks. — There is a furtlicr reasou for the consider-
able and, as it may seem to some readei's, the undue space which this chap-
ter occupies. Baron von Richthofen based the independence of the new
rock propylite largely upon the occurrences in the Washoe District. Later
investigators in the same field without exception have adopted his views.
Professor Zirkel's characterizations of the microscopical peculiarities of pro-
pylite were also founded chiefly on the "Washoe occurrence. Though at
the beginning of the present investigation I was fully persuaded of the inde-
pendence of propylite, I subsequently found reason to doubt it; but to prove
a negative is notoriously difficult, and the great authority of my predeces-
sors made the task still more onerous. It was necessary to demonstrate that
the whole superficial area and all the accessible mine-workings were occu-
pied by other rock-species, and to give in this report a sufficient number of
instances, with detailed descriptions, to enable geologists to decide for them-
selves whether the elimination of propylite and the redetermination of some
other rocks is justified by the facts.^
1 Special localities — The rocks of the Washoe District may be advantageously studied in the fol-
lowing localities : Gramilar dioritc in nearly all varieties occurs along the line of the Virginia AVatcr
Comjiany's flume within a distance of a thousand feet north of Bullion Eavine. Poiylnjritio diorites can
he satisfactorily examined either in tbe ilcKibbin Tunnel or in Oi)hir Ravine, hetween the most west-
erly point of the flume and the more southerly of the bluff's marked " croppings" on the map. Earlier
diabase, in all varieties, is to be found from the Savage connection with the Siitro Tunnel to the junc-
tion of the maiu tunnel with the A'orlh Lateral, and from this point to the Mint connection. Younger
diabafie ("' black dike") is well seen on the west wall of the Belcher associated with black graphitic slates.
The foregoing are the rocks most important to miners on the Lodh.
• Granite is well developed close to the Eed Jacket, C. D. 6, and on the dump of that mine. Quartz-
porphyr>j is excellently exi)osed by a little quarry aboiit 2,000 feet southwest of the Justice. The felsitic
variety occurs near the drainage of Gold Canon (American Flat Canon is the name given on former
maps), just east of Roux' Ranch. The little basalt mesa in the same locality is very accessible. Meta-
3 C L
34 GEOLOGY OF THE COMSTOCK LODE.
GEAIJITE.
Character. — GrauitG doBs Hot plaj a lai-g-e part in the geology of Washoe.
Besides the small area laid down on the map, it has been struck by a tunnel
near McClellan Peak, and in the Rock Island and Baltimore mines; so far a^,
I know, nowhere else in the neighborhood. The rock is a fine typical granite,
consisting of orthoclase, quartz, biotite, a little oligoclase, magnetite, and
some accessory minerals. The apatites are colorless, the zircons are numerous
and beautiful, and the titanite occurs in typical rhombs, with well developed
cleavages. Finally, it contains a colorless regular mineral, seemingly in
ill developed rhombohedrons, which answers to sodalite. The microscopi-
cal characters of sodalite, however, are rather negative than positive, and
it may be some other physically similar mineral.
Near the Bed Jacket the granite shows yQvy distinct parallel partings,
suggesting, but by no means conclusive of, a metamorphic origin. Some
of the granite has been mistaken for diorite, and a part of the metamorphic
diorite has been called .granite; but these are errors which can readily be
avoided by careful inspection.
ERUPTIVE DIORITE.
General relations. — Tlic devclopmcut of dioHte in thc Washoe District is
very extensive, and the variations of lithological character which it presents
morpklc diorite occurs close to tlie granite. It is found as a very volcanic looking breccia, just east of
the Volcano at point 5,444, C. D. 6. The western portion of the small patch of this rock in C. 7 is
extremely similar to the erupiive diorite of Mount Davidson. Earlier korntlcnde-andesite in a fresh state
is found on the north Twin Peak C. T>. 4. An abandoned quariy 500 feet north of this point shows the
stages of its decomposition to admiration. The south Twin Peak is an occunence of loose texture and
gray color, somewhat resembling the younger hornblende-audesite of the Utah neighborhood. The variety
with lai-ge hornblendes is well developed at point 5,678, about 1,000 feet east of the Succor, J). 5. Other
varieties, including decomposition-products charged with epidote, may be found on the north flank of
Cedar Hill Canon, say 500 feet west of the Brewery. Fresh augiteandesite can be conveniently
examined at point 6,158, close to the Forman shaft. The cuts of the Occidental Grade, say from the For-
man shaft road to the Prospect, show many beautiful examples of the decomposition and disintegration
of blocks. The croppiugs of breccia marked 6,569 on the Ophir Grade, B. 4, show many transitions and
the development of epidote. Younger hornbhndc-andesUe is found as a purple porphyry at the quarries
2,000 feet northeast of Shaft III. of the Sutro Tunnel; as a red porphyry (very augitic) at a quarry 2,000
feet east of the Occidental mill ; as a gray, somewhat granular looking mass with fine columnar structure
in the quarry close to the Utah; as a dense, black, glassy rock at point 6,728 E 2. The tufa modifica-
tion is best seen on the Sutro road, where it crosses the divide between Mount Emma and Mount Rose.
THE EOOKS OF THE WASHOE i)J STRICT. 35
are numerous, and often perplexing. While the varieties often differ in
appearance from one another much more than is the case with separate
species of the younger rocks, there is strong evidence that they all formed
portions of a single extended series of eruptions. * They are so intermingled
that it is not even possible to lay down upon the map distinct areas of those
which differ most, but it seems best to describe the principal modifications
separately, and afterwards to discuss their transitions.
The mass of Mount Davidson is mainly composed of granitoid diorite
of a cold gray color, which resembles a syenite in habitus and, as has been
seen, was so considered until Professor Zirkel demonstrated the triclinic
nature of the feldspars. Two other modifications of the granitoid diorite
require attention. One of them is a very dark and fine-grained rock, rep-
resented to a alight extent upon the surface, and extensively underground.
It has sometimes been confounded with the andesites. The other is a coarse
black rock, much resembling highly graphitic pig-iron. It has been found
mostly at great depths, particularly at the bottom of the Union shaft.
Granitoid diorite. — The miueralogical constituents of the oi'dinary light-gray
and the dark, fine-grained, granular diorites are essentially plagioclase and
hornblende; magnetite, apatite, and zircon seem never absent, and quartz,
mica, titanite, and augite occur now and then. In one slide tourmaline has
been detected. The principal constituents seem all to be crystals of "sec-
ondary consolidation;" that is to say, they have all formed simultaneously
on the cooling of the rock, and have mutually interfered with one another's
growth, EO that there are scarcely any symmetrically developed crystals
present, but only irregular grains, each limited by surrounding imperfect
crystals of a similar character.
The hornblendes are generally green and fibrous. In many cases the
separate fibers appear to be independent microlites, loosely aggregated in
forms characteristic of hornblende crystals. In other cases they appear to
be distributed entirely without reference to one another. The impression
produced is as if the crystallization had taken place in a viscous or pasty
mass, which mechanically prevented the union of the hornblende molecules
to well defined crystals. The hornblendes give angles of extinction appro-
priate to that mineral, when the well known variations in this property are
36 GEOLOGY OF THP: COMSTOCK LODE.
taken into consideration. In certain localities underground, the granular
diorites contain much deep-brown and solid hornblende, and the speci-
mens which show this variety are manifestly fresher than those from
the localities wdiere green hornblende occurs exclusively. In some cases
an alteration of the brown to the green variety is strongly suggested, while
in one series of porphyritic diorites it can be actually proved. It is therefore
altogether probable that the surface diorite originally contained some brown
hornblende, which has been changed to the green, fibrous modification by a
process analagous to the formation of uralite. To what extent the fibrous
hornblende has been derived from the brown mineral, there is at present no
means of inferring.
Augite. — Augite is comparatively rare in the unquestionable granular
diorites, though I have observed it in a few instances. It is much more
common in the porphyritic diorites, and it may be that its absence from the
granitoid variety is due to conversion into uralite; for since determinable
crystal sections are seldom met with in this rock, it would be impossible to
distinguish secondary from primary green fibrous hornblende. Close to the
McKihhen Tunnel angular fragments of what appeared macroscopically to be
the dark fine-grained diorite frequently encountered in the district, and
especially well developed in this tunnel, were found embedded in light-
colored granular diorite. Under the microscope the inclosing mass exhibits
no peculiarity; but the inclosed rock, unlike the similar occurrences in the
same locality, sliows abundant augite and almost no hornblende, though
structurally resembling the dark diorite. As the distinct diabase eruptions
are manifestly later than those of diorite, I am wholly at a loss for an expla-
nation of this case, except on the supposition that it represents a local and
exceptional substitution of augite for hornblende. This hypothesis, how-
ever, is so contrary to ordinary experience as to be exceedingly objection-
able, though were it true it would also serve to explain the ill- defined patch
of diabasitic rock in Ophir ravine, which is, like that just mentioned, much
more granitoid than the mine diabases, and has no apparent structural con-
nection with them. There can be httle doubt that local modifications of
massive rocks in which the mineralogical composition is characteristic of a
distinct but allied rock-species, have been met with in various localities in
THE BOOKS OF THE WASHOE DISTEIGT. 37
the world. Such cases, however, demand very cautious treatment at the
hands of the geologist. Actual contacts are often exceedingly obscure, and
except where all the steps of a transition can be traced, such an explanation
of an anomalous occurrence is not justifiable. Fortunately nothing further
appears to depend either upon the specimen of diabasitic fragments inclosed
in a dioritic mass, or upon the diabasitic area in Ophir ravine, since the evi-
dence as to the succession of the rocks in the east country is decisive and
abundant.
Other constituents. — Tho fcldspars arc nearly or quite without exception tri-
clinic, and simple crystals are very rare, while pericline twinning is com-
mon. The stripes indicating polysynthetic structure are usually very
well defined, and of moderate width. The angles of extinction of a very
large number of favorably placed crystals have been noted, and seem to
indicate labradorite as the only feldspar present. Zonal structure is not
uncommon, but the feldspars are remarkably free from inclusions of any
kind, and are in general thoroughly transparent.
Quartz is present in a large proportion of these rocks, though its dis-
tribution is very irregular, some slides containing only one or two grains,
while others show hundreds of them. Secondary quartz also occurs, but it
can usually be distinguished from the primitive grains with ease. Primitive
quartz-grains are generally single, more or less imperfectly developed crys-
tals, around which grains of magnetite and other small crystals are so
ari'anged as to show that their disposition has been controlled by the pres-
ence of the quartz. Secondary quai'tz occurs in veins or in patches composed
of granules of different crj'stallographic orientation, and is not sharply
separated from the surrounding rock-mass. Secondary quartz, of course,
frequently carries fluid inclusions in rocks of all ages. The primitive
quartz of these diorites is rich in liquid inclusions, some of them vesicular
in shape, and others dihexahedral. The smaller ones show active bubbles,
and in some slides many contain salt-cubes. I have noticed none of the
appearances which accompany inclusions of carbonic acid, and in several
slides to which heat was applied no alteration in the size of the bubbles was
noticeable at a temperature considerably above 40° C.
The iron ore is certainly for the most pai't magnetite, and I was unable
38 GEOLOGY OF THE COMSTOCK LODE.
to make certain of any ilmenite, while sphene, in small and irregular masses,
is frequent. Apatite is not specially plentiful, and is of the ordinary color-
less variety. Many small but beautiful zircons are visible with the higher
objectives, mostly in eight-sided prisms terminated by the fundamental
pyramid.
The granitoid diorites resist decomposition better than any other rocks
in the district. On the surface erosion evidently proceeds with greater
rapidity than decomposition. Slides from beneath the surface, but near the
Lode, show that the hornblende is replaced by chlorite and epidote, and the
feldspars by calcite and quartz.
Dark varieties. — The dark fine-graiued diorite presents a much stronger con-
trast to the ordinar}^ gi'^y variety macoscropically than microscopically. The
difference in its appearance seems to depend simply' on the fineness of the
grain, and on the percentage of fibrous hornblende, which is greater in this
modification.
The dark coarse-grained dioi'ite from the lower levels is very peculiar
in appearance, and some of that from the Union shaft might be more readily
confounded with specimens of Scotch foundry-pig than with any other rock
occurring in the District. This variety also differs from the ordinary gray
diorite, principally in respect to the hornblende, which is more abundant.
It is not fibrous as a rule, and has consolidated in grains simultaneously with
the feldspar. As in the freshest gray diorites, the granules which show
no evidences of alteration are brown, and incipient alteration seems to be
accompanied by a change to a gi-een fibrous mass. The hornblendes also
contain numerous black inclusions, probably ilmenite. The feldspars are
very fresh and clear, and the black color of the rock is the natural conse-
quence of such a mineral composition. Although the difference in appear-
ance between the three varieties of diorite is very marked, it thus depends
on a variation of vinessential characteristics.
Structure of the granular diorites. — Tlic granular dioHtc Is exccedingl}^ hard and
tough, so much so that before the introduction of nitro-g-lycerine explosives
it was almost impossible to penetrate it where decomposition had not loosened
the texture. In the Chollar mine, many years since, when black powder
only was in use, an attempt to drive a gallery into this rock was abandoned
THE EOCKS OP THE WASHOE DISTRICT. 39
as wholly impracticable, charge after charge being shot from the drill-holes
as if they had been guns. Under tlie hammer it exhibits no tendency to
break in one direction rather than in another, but weathering develops con-
siderable differences in resisting power ; and in Bullion ravine, as may be
seen from Plate VI., ridges and pinnacles have been formed by the irregu-
lar disintegration of the rock. Immediately west of the Lode the diorite is
furthermore divided into a system of approximately parallel sheets. It will
be seen in the next chapter that I refer this system of fissuring to a faulting
movement.
Differences from other rocks. — The gray grauular dloHte is unlikely to be con-
founded with any other rock in the district, except granular diabase. This
variety of diabase seldom occurs underground, so far as the country is now
open to inspection ; and when it is met with, as at the Mint connection in
the Sutro Tunnel, it is commonly limited to a very small body which shades
off into finer-grained varieties. When decomposition has progressed too
far to permit a macroscopical determination of the mineral constituents, the
lath-like development of the feldspars, the tendency to cleavage in parallel
planes, and a certain waxy luster will usually be found characteristic of the
diabase. The dark fine-grained diorite has repeatedly been taken for ande-
site in the McKihhen Tunnel and elsewhere. The only resemblance, how-
ever, is in color, for the diorite shows to the naked eye a grannlai- structure
never observed in the andesites of the District, although the latter ai-e un-
commonly crystalline.
Porphyritic diorites. — From some peculiainty either in composition or texture,
the porphyritic hornblende-diorites have undergone very extensive decom-
position, and it was only after long and earnest search that two or tin-ee
small masses were found, which might furnisli a study of this diorite in
a fresh state. A close inspection of fresh specimens shows that the rock
is even macroscopically thoroughly crystalline, but that tolerably well-devel-
oped feldspars and good hornblendes are separated out in a finer ground-
mass of a dark color. In addition to these minerals the microscope sliows
magnetite, apatite, and zircon. Augite and mica also occur in limited areas.
The hornblendes when fresh are bright brown and well crystallized,
often showing terminal faces as well as the prism and clinopinacoid. In a
40 GEOLOGY OF THE COMSTOCK LODE.
slide from Cedar Hill the curious inclusions which seem probably ilmenite
needles, already referred to, are developed in great perfection. Most
of the hornblende substance is concentrated in the larger crystals, but
there are a few minute ones and some crystalline fragments interspersed
through the groundmass. The larger feldspars are fairly well developed,
but have not the sharpl}^ rectilinear outlines so common in the diabases and
the volcanic rocks, nor do they display any tendency to elongated lath-like
forms. They give the angles of extinction appropriate to labradorite; they
contain occasional fluid inclusions, of rounded forms, and of course no glass.
They are pierced by numerous apatite needles. The smaller feldspars are
in part crippled grains, similar to those of the granitoid diorites, and in
part elongated microlites. The angles of extinction of these latter render
it probable that they are oligoclase.
The iron ore seems to be exclusively magnetic.^ The apatite is in part
of the ordinary colorless variety, and in part brown and dusty. The inde-
terminable inclusions in the apatites are disposed very differently in different
individuals. An hexagonal brown core is sometimes surrounded by color-
less apatite, while in other cases this arrangement is reversed. Longitudinal
sections not infrequently show colorless ends, with a dusty middle portion.
Only a few zircons have been observed in this rock, in which respect it dif-
fers from the granular diorites. The groundmass consists of small feldspars
and magnetite grains, and its general effect is usually that of an excessively
fine-grained granitoid diorite. Occasionally the arrangement of the micro-
lites is such as to suggest fluidal structure.
Decomposition. — The decompositlou of these rocks forms an exceedingly
interesting study. It will be shown elsewhere in detail that the hornblendes
pass into chlorite, and this again into epidote, quartz, and calcite. The chlo-
rite evidently possesses a high degree of solubility, and soon diffuses itself
through the groundmass, and through the feldspars so far as these latter have
become porous from decomposition. The chlorite and epidote give the par-
tially decomposed rocks their characteristic greenish hue.
Another change of great interest appears in a small dike of porphyritic
diorite cutting granular diorite close to the Eldorado croppings. No effect
whatever has been produced upon the inclosing granular diorite, but for
'Excepting the acicular inclusions referred to above.
THE ROCKS OF THE WASHOE DISTRICT. 41
about an inch from the edge the intrusive porphyritic rock has an exces-
sively fine grain and close texture. In consequence of this physical char-
acter it has resisted decomposition, and close to the contact is very fresh.
Here it contains fine brown hornblende, but at a distance of half an inch
from the contact, the texture, as seen under the microscope, becomes coarser
and more open, and green fibrous hornblende makes its appearance. Cer-
tain hornblende individuals are brown towards the center, but gi'een and
fibrous near the edges and along cracks, and the dividing line is such as to
leave no doubt that in this case the green fibrous modification is to be
regarded as an alteration-product of the brown dense variety. This occur-
rence strongly confirms the indications of such a transformation mentioned
in describing the granular diorites.^
Structure of porphyritic diorites. — No special tcudeucy to parting in any direction
is perceptible in the porphyritic diorites, but they vary in coarseness of
grain and general appearance much more than the granitoid diorites. To
the south of Bullion ravine there are small localities where the rock is dis-
tinctly brecciated ; and in the ravine west of the Imperial there is a small
occurrence of excessively fine-grained diorite, with a closely laminated
structure, not unlike a calcareous slate. Similar spots are found in the
diabase and the andesites, but in no case was any explanation apparent from
the character oT the surroimding masses. Some such appearance might
ensue in a pasty mass if its compo.sition were locally altered and its fusibility
increased, say by the presence of a fragment of calcareous rock. There is
no relation between the direction of the lamellae of these spots and the
general fissure system. Where decomposition has proceeded far enough
for a diffusion of chlorite through the rock to take place, the granular texture
of the groundmass is much obscured, and the similarity between it and
certain partially decomposed andesites is great and misleading. A large
portion of what was supposed to be propylite in the Washoe District, is
porphyritic diorite in this stage of decomposition. Disintegration sometimes
accompanies decomposition of the rock, but an astonishing coherence is often
maintained when scarcely a particle of unaltered mineral is left.
Diagnostic points. — It is ofteu vcry difficult to distinguish between partially
' Confer Eosenbusch, Physiog. iler Min. u. Gest., Vol. II., p. :i33.
42 GEOLOGY OF THE COMSTOCK LODE.
decomposed diorite and hornblende-andesite ; and the only really safe course
is to continue the examination until comparatively fresh specimens are
obtained. The granular structure of these is not readily confounded with
that of andesite. The diorites are never fissile like hornblende-andesite, and
hornblende-andesite is usually pretty uniform over considerable areas, while
the dioritic porphyries var)^ in structure almost from yard to yard.
Mica-diorites. — Tlic micaccous diorite-porphyries do not differ greatly from
the hornblendic variety, except in the substitution of biotite for hornblende;
but the rock is of a looser texture, the porphyritic feldspars are generally
larger, and tend more to rounded forms. I met with no occurrence of this
variety in a fresh condition.
Relations of the diorites. — All varietics of dloHte pass over into one another.
Porphyritic and micaceous forms occur in the prevailing granular mass on the
front of Mount Davidson, directly opposite the Savage mine, and granitoid •
diorites occur, mixed with the porphyritic forms, on Cedar Hill and in the
McKihhen Tunnel. Especially in the latter locality gradations of the one form
into the other can be excellently followed out. The evidence of this character
is sufficient to prove conclusively that no absolute separation can be estab-
lished between the dioritic rocks. There is, however, also considerable evi-
dence to show that, as a whole, one variety succeeded another in the course
of the eruption of the mass. The first portion of the diorites appears to
have been of the dark, fine-grained variety, and cases have been met with
in which dikes of the lighter rock cut the darker. In the mines, too, the
excavations show that the dark rock frequently underlies the lighter, and
in the deepest workings the dark predominates over the light rock. There
ai-e not sufficient exposures of the coarse-grained diorites with brown
hornblendes to determine its relations to the varieties with fibrous horn-
blendes, but it evidently preceded the poq^hyritic rocks. The main mass
of the porphyritic diorite succeeded the granitic. Just to the south of
the Eldorado croppings there is a distinct dike of porphyritic diorite in
the granitic mass, with well-developed contact phenomena, extending about
an inch from each wall of the dike. To the north of Mount Davidson the
mine shafts, too, have gone down through porphyritic diorite into the granu-
lar variety. Indeed Mount Davidson, between Bullion ravine and Spanish
THE EOCKS OF THE WASHOE DISTRICT. 43
ravine, constitutes the principal area of the granitic diorite at tte surface;
while both to the north and the south porphyritic varieties prevail, and
nearly all the diorite to the east of the Lode is of the same character.
METAMORPHIC DIORITE.
Origin and association. — Tlic soutliem portiou of the dlstrict contains a large
area of this rather puzzling rock. It was mentioned by Mr. King as a "com-
pact, black, crystalline rock, which in hand specimens would unquestionably
be classed as a basalt," but which can be shown to be of metamorphic origin.
Professor Zirkel determined it as a peculiar basalt. The most ordinary
variety is of a black or iron-gray color, and shows an irregular crystalline
fracture; but certain varieties (the more feldspathic ones) are light in color,
and considerably resemble Mount Davidson diorite. There is no little diffi-
culty in determining whether this rock shall be regarded as of metamorpliic
origin, or as eruptive. To the west of the Florida mine the contact between
it and the underlying metamorphic rocks appears as sharp as possible. At
the Wales Consolidated, a mine opened on a deposit lying between this rock
and the granite, there is no evidence whatever of bedding. Near the Amazon
mine it is weathered in round boulders, precisely like those produced by the
action of frost on basalt; and close to the Volcano mine it forms a distinct
bi'eccia. On the other hand, in some of the railroad cuts, there appear to
be transitions into rocks of evidently sedimentary origin. But such appear-
ances need very cautious treatment, for between metamorphism and decom-
position, a contact might readily assume the appearance of a transition. The
microscopical character of the rock offers nothing decisive as to its origin,
and the point which has mainly determined me to regard it as metamorphic
is its relation to the quartz-porphyry. An inspection of the map will show
that it is invariably associated with the quartz-porphyry, and that if it had
resulted from the metamorphism of the sedimentary strata by porphyry
eruptions, subsequent erosion must have exposed it in relations almost iden-
tical with those observed. Its composition also indicates a metamorphic
rather than an eruptive origin.
44 GEOLOGY OF THE CO]\ISTOCK LODE.
Character in detail. — The pniicipal constitiieiits are plagioclase, hornblende,
and mica, often with the addition of quartz; wliile the subsidiary minerals
are titanic iron, apatite, sphene, zircon, and in one case tourmaline. The
hornblende is for the most part fibrous and bluish green. But this does not
appear to have been its original color. The centers of considerable masses
of hornblende appear under the microscope wholly colorless, and the asso-
ciation of the two varieties described in detail under slide 295 is such as to
lead almost inevitably to the conclusion that the green color is secondary.
In many specimens the hornblende is pi-esent in great quantities, and micro-
lites so crowd the feldspars that their striations are almost imperceptible and
their species indeterminable. To a considerable extent the hornblende is
decomposed into chlorite and epidote, the latter mineral appearing in unu-
sually fine crystals. Mica is present in smaller quantities than hornblende,
and gives the interference figvire of biotite. Some specimens contain augite.
In the less hornblendic varieties the feldspars are well developed, many
of them with sharp, rectilinear outlines, like those in the more porphyritic
diabases. The lamellae are exceedingly attenuated; they show angles of ex-
tinction appropriate to oligoclase, and pericline twinning is occasionally
visible. A few orthoclase crj'stals also occur. Quartz grains are numerous
in the less hornblendic specimens, and do not appear to be of secondary
origin. They contain numerous nn'nute fluid inclusions, and with irreg-
ular grains of feldspar form a kind of coarse groundmass. The titanic iron
is accompanied by leucoxene, which in some cases appears to pass over into
titanite, though a transition cannot be demonstrated. The apatite presents
nothing peculiar, and the zircon is in no way remarkable except in the fre-
quency of its occurrence. Tourmaline was found onl}- in one slide. Of
course it suggests metamorphic origin, though the same mineral is known
to occur occasionally in rocks of eruptive origin, and, as has already been
mentioned, was noticed in one of the almost unquestionably eruptive
diorites of this very district.
Comparatively little decomposition has been noticed in this rock, a fact
which no doubt stands in intimate relation to its unusual hardness and
toughness, but in some Hmited areas it is highly chloritic, and certain speci-
mens would pass for propylite.
THE ROCKS OP THE WASHOK DISTRICT. 45
Diagnostic peculiarities. — SoiTie Varieties of" tliis vock, especially a portion of
the small patch shown on the map in square C. 7, greatly resemble Mount
Davidson diorite, and indeed the difference under the microscope is chiefly
in the species of feldspar. On the hills west of the Florida, and in some
other localities, it is much like augite-andesite or basalt in appearance, but
the macroscopical resemblance does not answer to any microscopical simi-
larity. It sometimes occurs in rounded shapes such as basalt often assumes.
In many cases weathered surfaces of this rock can be recognized by the
crystal outlines which they exhibit. These are often polygons of a variable
number of sides, and represent sections of hornblendes crystallized in re-
markably short prisms and provided with terminal faces.
QUARTZ-PORPHYRY.
General ciiaracter. — Quartz-jjorpliyry covers a large area in the southwestern
portion of the Washoe District, and extends for miles in the direction of
Washoe Lake. It presents a rough surface varying in color from white to
a yellowish or reddish gray, and is thickly set with quartz-grains the size
of a mustard seed, and smaller. Mica is nearly always visible, and horn-
blende occasionally. Only in one small area near the granite is the quartz
macroscopically suppressed, and here the rock is finer-grained than else-
where. Underground it extends to a considerable distance farther north and
east than its northern limit on the surface, and there underlies hornblende-
andesite and augite-andesite.
Composition. — Nouc of thc rock is really fresh and, though an earnest
search was made, not a single specimen could be found showing the con-
stituent minerals in an undecomposed state. Those which enter most largely
into the composition of the rock are feldspar, quartz, mica, hornblende, and
ores of iron. As accessory minerals, titanite, apatite, and zircon were
observed. The feldspars are not well defined, but occur in irregular or
rounded grains. They are in part striated, but the larger portion show no
trace of polysy nthetic structure, and while in some slides so large a quantity
of calcite is distributed through the feldspars that the striations might be
46 GEOLOGY OP THE COMSTOCK LODE.
supposed to be obliterated, in others the crystals are so clear that striations,
if present, could not but be apparent. Many of the unstriated feldspars show
cleavages, and extinguish light at angles which seem to prove their ortho-
clastic character. No unstriated feldspars were found to give angles of
extinction, reckoned from the cleavage planes, which would refer them to
either of the triclinic species. The triclinic crystals show for the most part
very narrow striations, and give angles of extinction which correspond to
oligoclase. The microlitic feldspars of the groundmass do not appear to be
triclinic. On introducing a portion of the rock in a condition of fine powder
into the well-known solution of mercuric iodide in potassic iodide, of a
specific gravity of less than 2.65, a large proportion rose to the surface.
This mounted in balsam ajjpeared to consist mainly of feldspar. The por-
tion which sank contained some feldspar and the other components of the
rock. The feldspars contain inclusions of glass and also of fluid, but a por-
tion of the latter I regard as of secondary' oi-igin.
Quartz. — The quartzes are bounded in part by straight lines and in part
by curves. In some cases the imperfectly developed crystals appear to
have been broken, and the fragments are now separated by narrow bands
of groundmass; in other cases they contain deep sinuous bays of the same
material. The quartz shows both fluid and glass inclusions, but their distri-
bution is somewhat uneven. In some slides they are present in nearly equal
numbers, while in others one or the other preponderates, or even occurs
exclusively; but this is exceptional. The inclusions are not thicklj^ set, but
a glass inclusion and a fluid inclusion, with a moving bubble, can often be
seen in the same field with a Hartnack No. 7 objective. Of the hornblendes,
which were all black-bordered, none now i-emain in a fresh condition.
They have been replaced by the usual products of decomposition — chlorite,
epidote, quartz, and calcite. The mica, too, is in great part decomposed,
but occasional scales remain, and these give the interference figure of bio-
tite. Tlie groundmass in every case shows fluidal and pseudospherolitic
structure. In some cases a base is also present; in others it is either wanting
or devitrified. When glass is present, it shows a preference for elongated
sinuous forms, and often the central line is marked by aggregations of iron
ore from which, as axes, black trichites sometimes spread into the sun'ounding
THE EOCK,S OP THE WASHOE UISTEIOT. 47
isotropic substance. The iron ore is in part magnetite, while in other cases
it appears to be ilmenite. Apatites of the usual colorless variety are fre-
quent. Zircons are not uncommon, and there are occasional small patches
of titanite.
Field habit. — The croppings of the quartz-porphyry are usually exceed-
ingly rough, and the nearest a2)j)roach to a structure is indicated in some
localities by the separation of the rock into uneven sherdy fragments. Its
appearance is almost identical all over the district, except in the small area
where the quartz is macroscopically suppressed. Here it shows various
brown and green colors, and sometimes a smooth fracture like a fine-grained
hornblende-andesite. In this area the color and texture vary every few feet.
This macroscojiical difference appears to correspond to no microscoiDical
peculiarity beyond a finer grain. As quartz-porphyry is the only quartzose
rock in the district, it is readily distinguishable.
Various determinations. — As has bccn Seen in the rdsumd of former memoirs,
the quartz-porphyry has been variously determined by the eminent geolo-
gists who have discussed the Washoe District. Baron v. Richthofen very
positively asserted that the circumstances of its occurrence rendered it cer-
tain that this porphyry was intermediate in age between the granitic and the
volcanic rocks, and I entirely agree with him. The absolute uniformity of
the rock fi'om the Overman mine to the southern extremity of the mass, with
the exception of a small felsitic area, utterly precludes the supposition that
it is separable into different species of Tertiary and pre-Tertiary origin.
The felsitic modification comes in contact only with granite and basalt, but
its microscopical character is identical with that of the coarser porphyry; it
strongly resembles well-known varieties of quartz-porphyry, and I can see
no evidence on the ground sufficient to separate it from that species. That
in the Overman and Caledonia mines and the Forman shaft the porphyry
vertically underlies hornblende-andesite is beyond question; both optical
tests and specific gravity determinations show that it is an orthoclase rock;
and the character and association of the inclusions in the quartzes are pre-
cisely those which are so very common in old quartz-porphyry. Professor
Zirkel determined the larger proportion of this rock as a dacite, but on
reexamining his slides I found that they corresponded in every respect to
48 GEOLOGY OP THE COMSTOCK LODE.
mine, and that the quartzes in each of them contained fluid inclusions with
moving bubbles. The one slide which Professor Zirkel determined as rhy-
olite differs, in that the quartz contains glass but no fluid inclusions; in a slide
of my own, however, from as nearly as possible the same locality, these con-
ditions are reversed, the quartzes showing fluid inclusions but none of glass.
I can see no difi"erence in the amount of orthoclase present in those slides
determined respectively as dacite and rhyolite. Professor Zirkel gives an
analysis of this rock made by Mr. Councler, showing two per cent, of soda
and three and six-tenths per cent, of potash. In discussing this composition
Professor Zirkel cites a number of analyses of Transylvania dacites, but in
none of these is the proportion of potash to soda so high as in the Washoe
rock.
DIABASE.
Earlier diabase. — There are two varieties of diabase in the district. The
older of these forms the hanging wall of the Lode; the other has beeri
known as "black dike." The east-country diabase varies considerably in
coarseness of grain and in color. When really fresh it is always dark, and
when also fine-grained it closely resembles an andesite. The coarser-grained
and somewhat decomposed occurrences are often confusingly like granitoid
diorite.
The rock consists of plagioclase, augite, and an iron ore, with a num-
ber of accessory and irregularly distributed minerals, quartz, hornblende,
mica, and apatite. The structure is not that most usually found, in diabases,
being somewhat porphyritic. The augite is of the usual pale-brown tint,
and occurs largely in well-developed crystals. These are often twinned
according to the ordinary la,w.^ The twinning attracts more attention than
usual, because polysynthetic structure is common, some of the lamellse often
penetrating only part way through the crystal. The ordinary cleavages are
well marked, and instances are common in which the pinacoidal cleavages
as well as the prismatic ones are developed. Some slides contain only sepa-
rate and well-formed crystals, while in others they occur in groups, and
these are apt to be gathered about branching masses of iron ore, almost like
' For a peculiar case, which might be interpi*ted as abnormal, see page 113.
THE ROOKS OF THE WASHOE DISTRICT. 49
close-growing bunches of grapes. In still other slides grains of the mineral
are distributed through the groundmass.
Mineral constituents in detail. — Tliis rock alwRjs contains porpliyritical feld-
spars. They are long, sharply rectilinear, and without exception triclinic.
They give angles of extinction pi-oper to labradorite. The lamellae are of
moderate width, and are often combined at the same time according to all
the common twinning laws. In nearly every slide they carry liquid inclu-
sions, generally of vesicular shapes. The smaller feldspars form granitoid
grains of "secondary consolidation, "and with the iron ores and more or less
augite, make up the groundmass. I have observed some of these smaller
feldspars which gave angles of extinction indicating a different species from
the larger crystals of first consolidation. The iron ore is in part magnet-
ite, and in part ilmenite, with the characteristic cleavage-lines and products
of decomposition.
Quartz grains of unquestionably primitive character are occasionally
met with. These show an arrangement of particles of magnetite, etc.,
about their peripheries such as secondary quartzes never exhibit. Almost
all of them show fluid inclusions, the smaller ones with moving bub-
bles. I have observed none in which the liquid appeared to be in the
spheroidal state, and the bubbles do not disappear at a temperature of
above 40° C; the fluid is therefore aqueous. I have met with no salt
cubes. Hornblende occurs sparingly, and is generally confined to closely-
limited areas. Where it is present great care is necessary in discriminating the
rock macroscopically from diorite. Mica is rare, and is seen only in almost
indeterminably small particles, which might even be secondary. The
apatite is of the usual colorless variety. Not a single zircon was detected.
Evidences of diabasitic character. TllO mlcrOStrUCtUrC of this I'Ock StrOUgly SUg-
gests that of some lavas, and I have sometimes been puzzled to say at the
first glance whether a particular slide was augite-andesite or diabase; but
the resemblance is superficial. As will be seen later, somewhat granular
augite-andesites occur in the district, but they are exceptional. Here as
elsewhere the younger rock generally shows a microlitic groundmass, and
frequently a glass base. This is the case equally on the surface, and in the
Sutro Tunnel more than a thousand feet beneath the surface. The diabase now
5 0 L
50 GEOLOGY OF THE COMSTOCK LODE.
under discussion shows in all cases a thoroughly crystalline structure, and
the groundmass is always composed of granitoid grains. The feldspars of,
I believe, every slide of the augite-andesite show glass inclusions; and I
have not met one fluid inclusion in that rock which appeai'ed to me of prij
mary origin.^ In the diabase the occurrence of fluid inclusions and the
absence of those of glass is equally universal. The auglte of the augite-
andesites shows no pinacoidal cleavages, and only one locality has been
detected at Washoe in which it has passed into uralite. The change even
there is so exceedingly local that although a dozen slides have been ground
from the same cropping, but one shows the alteration of augite into horn-
blende. In the diabase the passage of augite into uralite is the usual pre-
liminary to chloritic decomposition. Finally, if there is one point of struct-
ure incapable of two intei'pretations, it is that the black dike is of later
origin than the east and west country rocks. As will be shown, the black
dike is an ordinary diabase, and the hanging wall is consequently a pre-
Tertiary rock, and would necessarily be classed as a diabase were its resem-
blance to the volcanic series much more thorough than it really is.
Decomposition. — lu dccomposiug, the diabase shows few peculiarities. As
has already been mentioned, the augite is apt to be converted into uralite
and then into chlorite. Epidote almost always forms to some extent from
the chlorite, but the latter does not generally seem to pass so readily and
completely into epidote as does that which results from the degeneration of
hornblende. Instances occur, however, where the conversion is complete.
The decomposition of the feldspars presents no peculiarity. They change
slowly to quartz and calcite, and become porous and sufi"used with chlorite,
just as in the diorites. The final result is a mass showing aggregate polar-
ization with a few determinable grains of silica and carbonates, and par-
ticles of a whitish opaque substance, but nothing determinable as kaolin.
' It has been shown of late years that the evidence afforded by fluid inclusions needs to be treated
■with caution, for they are reported as present in all the younger rocks. No one, however, has claimed, so
far as I am aware, that such inclusions are frequent in or characteristic of the Tertiary oruptives. Pro-
fessor Rosenbusch, in liis "Pliysiog. der Gesteine," does not mention a single observation of his own on
fluid inclusions in augite-andesites, and cites only one instance of such an occurrence noted by others.
If my inferences as to the secondary nature of certain fluid inclusions (p. 79) are correct, a deduc-
tion may need to be made from the number of fluid inclusions, to which a genetic significance can prop-
erly be attributed.
THE ROOKS OF THE WASHOE DISTRICT. 51
Field habit. — The commonest variety of the east-country diabase is a fine-
grained blackish-green I'ock, the most noticeable macroscopical peculiarity of
which is its tendency to develop smooth fissui-e planes. Sometimes these
planes are parallel, and of course divide the rock into sheets. In other cases,
quite as common, they form all sorts of angles with one another, and divide
the rock into polyhedral fragments, almost like large crystals, or into prisms of
various angles; but I failed to find any law governing the angular relations.
There can be little question that the cleavages of the rock have been
developed by the dynamical action which has repeatedly racked the hang-
ing wall; but the tendency to jointing and the planes of cleavage may
have been involved in the original structure of the rock, for the hammer
develops only the imperfectly conchoidal and somewhat rough surfaces,
which other fine-grained rocks show when fractured, and not smooth planes.
Possibly, however, such might result from a slow but irresistable pressure.
The coai-se-grained diabases show much less of this jointing, but the fract-
ure of both presents the same appearance except in regard to scale — a
granular surface with fi-equent larger lath-like plagioclases. In a great
proportion of cases the feldspars are pellucid, even when the augite is
wholly decomposed; but when the coarser rocks are so far altered that
the feldspars become opaque, the rock looks very like diorite, a resem-
blance which is greatly increased by the comparative absence of joints.
The diabase on the south side of Ophir ravine looks very like a diorite,
though here the exposure is so large that the jointing is clearly visible.
In many cases under ground it is little developed, not more so than is fre-
quently the case with the diorite. In a few places, as for example the
2,700-foot level of the Yellow Jacket, there are limited occurrences of exces-
sively fine-grained, closely laminated diabase resembling slate. The diorites
and both the andesites show the same phenomenon.
It will be seen that the andesites behave very differently in subterra--
nean and subaerial decomposition. The behavior of the diabase in this
respect cannot be directly compared with the later rocks, because the ex-
posure in Ophir ravine is but little affected, and that near the Ward is obscure
and almost wholly covered with wash; but the protection of occasional
masses of diabase from decomposition by accidental arrangements of fissures
52 GEOLOGY OP THE COMSTOCK LODE.
and clay seams can be seen very perfectly in some of the mines, as well as
extensive disintegration of decomposed portions, and there can be little
doubt that the behavior under erosion would be analogous. The pistachio-
green so often seen in the diorites and hornblende-andesites is less common
in the decomposed diabases, simply because the prevalent secondary mineral
is not epidote but chlorite. The chlorite is sometimes peculiarly distrib-
uted in blackish, rounded spots on a lighter ground.
Diagnostic points. — Dlabasc Is Hkcly to be confounded with diorite chiefly
when the feldspars have lost their transparency. The best indication macro-
scopically is then the lath-like feldspars, which are rare in diorite. The
granular fracture, though it may be very fine-grained, is usually sufficient
to separate it from augite-andesite. Hoi-nblendic diabases in some cases
greatly resemble hornblende-andesites, which are often rather granular; but
hornblende is not very common or widely distributed in the diabase, and if
one specimen arouses a doubt, another can generally be found near by
which will set it at rest.
Younger diabase. — Tlic "black dlkc " Is a fcaturc which has long been ob-
served on the CoMSTOCK. It extends horizontally more than a mile through
some of the most important mines, and occurs from near the surface to the
lowest levels reached. It lies upon the foot wall, and is nowhere more than
a few feet in thickness. When fresh it is of dark-blue color and a granular
texture, without the least tendency to a porphyritic structure. Surfaces
which have been exposed only a few hours turn to a smoky brown tint,
a peculiarity shared, by no other rock in the district.
Under the microscope it is seen to be composed of triclinic feldspar,
augite, and magnetite. The feldspars are mostly developed in lath-like
shapes, and are of very uniform size. They give angles of extinction cor-
responding to labradorite. The augites are of the usual color, but seldom
well developed, and to a large extent occupy the interstices between the
feldspars. The rock is singularly free from inclusions of liquid or glass;
indeed, none such have been made out with certainty. The brownish tint
seems to arise from a suffusion of the minerals with brown oxide of iron,
and this substance is very likely produced by the oxidation of some chlo-
ritic mineral, of which, however, little is visible under the microscope.
THE EOOKS OP THE WASHOE DISTRICT. 53
Diabasitic character. — As tliis I'ock IS wliolly diflferent from the diabase of
the east country, and is evidently younger than either wall of the Lode, the
question naturally arose whether it might not be a peculiar form of augite-
andesite. This supposition, however, proves untenable on closer examina-
tion. The tendency of augite-andesite is to glassy forms, and this tendency
could scarcely fail to be developed to more than a usual degree, had it
been injected into so narrow a fissure as that which the black dike must
have filled ; and any hypothesis which might be invented to account for its
having crystallized much more uniformly and thoroughly than usual would
seem very forced.
The black dike, moreover, thoroughly resembles diabases from other
localities, and indeed represents a type of diabase which is much more widely
distributed than the variety which forms the east wall of the Comstock. The
rock from Orange Mountain, New Jersey, for example, possesses the same
color, turns brown in the same way, has the same microscopical characteristics,
and, in short, is indistinguishable from it except by the label. The analysis
of black dike is conclusive evidence of its diabasitic character.
Little can be said of the weathering of this rock beyond the fact that
it passes into a black clay; almost the only form in which it was observed
in the upper levels. To some extent it has been confounded in the Gold
Hill mines with underlying black slates, with which, however, it has exceed-
ingly little in common except the color.
Had black dike occurred in a fresh condition on the upper levels
former observers would assuredly have recognized its true character, and
the east wall would never have been supposed to be of Tertiary origin.
EARLIER HORNBLENDE-ANDESITE.
General character. — Tlic tlioroughly fresli homblendc-andesites are macro-
scopically dark-bluish rocks, showing porphyritical crystals of hornblende.
The feldspars are scarcely perceptible, except as they express themselves in
the crystalline fracture, on account of their transparency. Where the horn-
blendes are small, the appearance is consequently somewhat basaltic.
No base has been recognized in the earlier hornbleude-andesite of the
54 GEOLOGY OF THE COMSTOGK LODE.
District. The prevalent variety contains much augite; sometimes even
more augite than hornblende, but no mica. There are also micaceous oc-
currences, and these are nearly or quite free from augite.
Hornblende. — Tlic homblendc is always brown in the fresh rocks, occa-
sionally with a reddish, and often with a greenish, tinge. Of course it is
highly dichroitic, and the angles of extinction appear' in some cases to
exceed 20°. The crystal form is the ordinary combination of prism and
clinopinacoid ; terminal faces too, though rarer than in augite, sometimes
occur. The cleavages are usually developed, though in the freshest crystals
they are marked b}' such narrow lines that under a low power they seem
absent. In one case a clinopinacoidal cleavage was observed. Twins are
very common. Glass inclusions occur, generally as negative crystals, and
apatites are often inclosed. Very rarely indeed a slide shows a particle or
fragment of hornblende inclosed in another mineral, but as a rule all the
hornblende is concentrated in porphyritical crystals, and does not enter into
the groundmass. I discovered only a single very small area in which the
rock shows a large amount of hornblende distributed through the groundmass
in minute particles; and even in this case the difference seems to be one of
degree rather than of kind; for the minute hornblendes are in large part
well developed and appear to be "crystals of first consolidation." The black
border accompanies all the hornblendes in most of the andesites. Often it is
very heavy, and sometimes so encroaches on the crystal that little or none of
the mineral appears in the center. I have noticed no instances in which black-
bordered hornblendes accompany crystals of the same mineral w*ithout
black borders. In several cases a double black border is visible, the inner
one concentric with the outer, leaving a zone of hornblende between.
Such a case is described nnder slide 450, and shown in Fig. 17, Plate III.
I venture to offer some speculations on this phenomenon elsewhere. The
black border is readily soluble in chlorhydric acid, even where the slide con-
tains ilmenite A very few slides show hornblendes without black borders.
One such exception is from the Sutro Tunnel in a region of intense sol-
fataric activity. Here the hornblendes are in part very fresh, while the
' I say appear, because it is seldom possible to make absolutely sure that a crystal is cut exactly
in either of the three principal zones, and a very small obliquity often greatly alters the angle of extinc-
tion.
THE ROCKS OF THE WASHOE DISTRICT. 55
remainder of the rock is not. Cases occur on the surface in which it is
evident that the black border has been attacked before the hornblende, and
this slide may represent such an instance.
Augite. — The augites are essentially similar to those of the augite-andesites,
but it may be mentioned that in one case a pinacoidal cleavage was observed
which I have never noticed in the augite rock. In a slide from an area
which I have classed as hornblende-andesite, the augite also shows heavy
black borders like those of the hornblende. Augite is frequently present
in the groundmass in crippled crystals and irregular grains, which appear
to me referable to "secondary consolidation." The jDroportion of augite to
hornblende is always large except in the micaceous andesites, and, according
to Professor Rosenbusch, this is common elsewhere ; while in the augite-
andesites of the Washoe disteict thei'e must be more than one hundred
times as much augite as hornblende. I have not always seen my way,
however, to determining slides containing a decided excess of augite other-
wise than as hornblende-andesite, for such rocks occur in areas which appear
characteristically hornblendic. While in such cases, which are exceptional,
the endeavor has been made to take all the circumstances into considera-
tion, it must be confessed that where very augitic hornblende-andesites and
very hornblendic augite-andesites come together, the lines of contact laid
down may be somewhat inaccurate, though the error cannot be great; and
as these conditions appear to prevail only along Cedar Hill Canon it is of
small importance.
The mica of the andesites gives the interference figure of biotite. It
is frequently black-bordered, and the border is usuall}^ deeper than that
around the accompanying hornblende.
Feldspar. — The feldspars of the hornblende-andesites are nearly without
exception triclinic, and of course they can be divided into porphyritical
crystals of first consolidation and microlites of second consolidation. As for
the species, the porphyritical crystals are either labradorite or anorthite, and
the microlites either oligoclase or labradorite. Crystals giving anorthite
angles of extinction have been found in only a few cases, and in these I
suspect a mixture of anorthite and labradorite, because while many crystals
seemed so placed that had they been anorthite they must have given angles
56 GEOLOGY OF THE COMSTOCK LODE.
of extinction exceeding those of labradorite, only a few such sections gave
above 32°, while many of the remainder gave within a degree or two of
31°. But I know of no way of absolutely proving this point. The feld-
spars very often show a zonal structure. A beautiful case of this kind is
mentioned under slide 20. Simply twinned feldspars are rare, and most
are poly synthetic, according to the albite law ; pericline twinning is very
common, and both of these sometimes appear in combination with Carlsbad
twinning. The stripes are ordinarily fairly uniform, and of considerable
width; but sometimes one or both sets are exceedingly fine, and not uncom-
monly they do not penetrate the crystal, so that one end shows stripes while
the other does not. It need scarcely be said that in such cases the unstriped
portion if favorably placed may be proved to be triclinic by its optical
properties. The porphyritical feldspars are usually developed in long lath-
like forms. The feldspars contain inclusions of glass in almost every slide,
either as negative crystals or as rounded bodies, and these, when fresh, ordi-
narily carry bubbles. Inclusions of groundmass too are common, and
inclosed microlites occur both of apatite and of what appears to be augite.
The latter are not sharply crystallized, and are generally fresh, though occa-
sionally accompanied by chlorite. They are light yellow, and sometimes
give angles of extinction of above 30°. I have seen no fluid inclusions
in such feldspars as seemed to be unaffected by decomposition.
other minerals. — The apatitcs are usually colorless, but sometimes brown
and dusty. They seem to be universally distributed. Zircon occurs in only
one or two slides. The iron ore is for the most part magnetite, but occa-
sionally ilmenite is present. Fig. 19, Plate III., shows an excellent ilmenite
section from the highly augitic andesite in Cedar Hill Cauon, and the
application of chlorhydric acid established its presence with certainty in the
typical hornblende-andesite from near the ComhinaUon shaft.
The groundmass consists of feldspar microlites usually referable to
oligoclase, magnetite, and sometimes microlites of augite Fluidal structure
is common. Of course the groundmass must have crystallized in cooling,
and the question is suggested why the glass inclusions were not devitrified
at the same time; but it is evident that a large part of each porphyritical
crystal must have formed after the glass was inclosed, leaving a residual
THE ROOKS OP THE WASHOE DISTEICT. 57
magma of a different composition. In only one or two cases has anything
hke a thoroughly granular structure in the groundmass been observed.
The greater part of the feldspar microlites are generally well and sharply
developed. The same is true in the augite-andesites, and in cases of extreme
decomposition the shape of the feldspars, large and small, is an important
point of distinction between andesites and the older porphyritic rocks.
Field character. — In tlio uiost important part of the District lying in the im-
mediate neighborhood of the productive portion of the Lode, the hornblende-
andesite is dark and fine-grained, and contains only small hornblendes, which
are recognizable as such more often by their brilliant surfaces and evidences
of cleavage than by their crystal form. The rock breaks easily under the
hammer with a somewhat conchoidal fracture, and its luster is more or less
glassy. The hornblende-andesites which occur south of Gold Hill are much
more porphyritic, and the hornblendes are unusually well developed.
Crystals of an inch and a half in length are common, and one decomposed
crystal fully four inches long was observed. In none of the varieties are the
feldspars visible when fresh except on minute examination, simply because
they are transparent, and the dark color is therefore due to the bisilicates
and magnetite. Columnar structui'e is occasionally developed all over the
district, but in no great perfection.
Weathering. — Ordinarily the hornblende-andesite appears to possess little
or no structui'e in mass, while under the action of the atmosphere it develops
considerable fissility in certain directions, so that some croppings present
almost the appearance of upturned beds of sedimentary rocks with parallel
partings at a distance of one or two inches. That the fissile tendency does not
extend to an indefinite lamination is evident from the behavior of the sherds.
These do not continue to part parallel to their more extended surfaces, but
are gradually rounded by the action of frost. By this agency conchoidal
fragments are separated from the corners and edges of the loos6 blocks,
and when it is considered through how short a distance the action of the
frost can extend, the display of force is quite astonishing. Conchoidal chips
of three or four pounds in weight are often found at a distance of two or
three feet from the block on which they fit. Large masses of hornblende-
andesite breccia also occur, though this form is not so common as with the
58 GEOLOGY OF THE COMSTOCK LODE.
augite-andesites. Of coiirse, neither columnar structure nor fissilitj^, both
of which are probably to be regarded as results of tension from cooling,
are developed in the comparatively porous breccias, for the fragments of
unfused rock in breccia act like the chamotte in a fire-brick in preventing
density of structure.
Decomposition. — The Weathering of the hornblende-andesite seems to differ
in its nature,- as it takes place in direct contact with the air or under ground.
Croppings of the rock which on being broken prove internally fresh, are com-
monly coated with a very thin, deep-red or brown scale and, to judge by
fragments found in the immediate neighborhood of such croppings, the
change seems to consist mainly in disintegration by frost and in peroxida-
tion of the iron. Under ground, on the other hand, decomposition appears to
extend into the body of the rock. One of the first minerals to be afi"ected
is the feldspar, which loses its transparency and becomes a dead white.
This totally alters the appearance of the rock, which becomes a light-gray
porphyry, instead of a dark-bluish and basaltic-looking mass. Every varia-
tion in coarseness of grain also becomes apparent. The feldspars lose
their transparency when only a very minute portion of their substance
(certainly less than one per cent.) is altered. The next stage of decompo-
sition is the formation of chlorite from the bisilicates, which soon diffuses
itself tln-ough the groundmass and the feldspars. The chlorite is further
frequently decomposed into calcite and epidote without any special change
in the appearance of the rock. All these changes tend to diminish the
sharp definition of the porphyritical crystals and give the mass the look
rather of an olde)' dioritic porphyry than of a volcanic rock. It is easy to
suggest plausible explanations for the different behavior of the andesite
above ground and beneath the surface. The presence under ground of
water holding carbonic acid in solution is perhaps sufficient to account for
the formation of calcite in the feldspars, and the strong oxidizing action on
the surface may well explain the direct formation of ferric oxide in the
exposed rocks. When the andesites are not in the condition of breccia the
subterranean decomposition is commonly accompanied by a softening or
partial disintegration of the mass, though in some cases, as at the South
Twin Peak, rock not brecciated preserves great consistency, possibly from
THE ROCKS OF THE WASHOE DISTRICT. 59
an originally porous texture. The breccias remain hard and tough until
every mineral has been subjected to complete alteration. There is much
evidence and every analogy to show that this decomposition proceeds from
external surfaces, cracks, and fissures toward the centers of blocks or masses.
Very frequently where cuts have exposed altered rocks, blocks of small size
may be seen, which consist of concentric shells of loose decomposed rock-
substance, and still contain kernels of fresh andesite. The size of the blocks
is, of course, a matter of accident, and sometimes extensive masses decom-
pose only from their external surfaces.. When this is the case erosion
often acts more rapidly than decomposition and, as the decomposed rock
is comparatively soft, masses of the fresh andesite are frequently left standing-
above the general level. The fresh rock thus exposed has the appearance
of a cropping of a younger eruption penetrating and overlying an older and
different one; and this appearance is heightened by the weathering of the
pseudo cropping which, as already explained, results in a mass of reddish-
brown fragments quite unlike the product of alteration beneath the surface.
The andesite which had decomposed under ground used to be regarded as
propylite, but careful examination of exposed masses of andesite such as
those described, shows that a transition into the propylitic form may alwa)^s
be followed out at their base. As the course of the decomposition is depend-
ent on the presence of accidental fissures and, no doubt, on the texture of
the rock, the form of the residual masses of undecomposed andesite is fan-
tastically various, sometimes resembling dikes, again assuming the shape of
domes and cones.
Distinctive characteristics. — Hombleude-andesites are distinguishable from the
augite-andesites when fresh by the presence of abundant porphyritic horn-
blende crystals and by the luster, which in the augitic rocks is resinous.
From the porphyritic diorites they are distinguishable macroscopically by
a lack of the granular structure, which the older rock commonly shows.
In the propylitic stage of decomposition the three rocks are almost indis-
tinguishable.
Speculation on "black border." — Somc of tlic Washoe audesltes sccm capable of
throwing light on the conditions under which the black border forms about
hornblende crystals. In slides from different parts of the District two can-
60 GEOLOGY OP THE COMSTOCK LOBE.
centric belts of magnetite have been observed, separated by hornblende-sub-
stance. Much the finest instance is illustrated in Fig-. 17, Plate III. There
can be little doubt from direct observation on modern lavas that porphj'--
ritical crystals are formed prior to eruption, and a tolerably large and ver}^
sharply defined specimen, like that shown in the drawing, is not likely to be
an exception. At some time after it ceased to grow this crystal was broken;
but the external black border was formed at a still later period, for it is as
heavy on the fractured surface as on the crj^stal faces. It is difficult to imagine
a mass of melted lava in a state of agitation sufficiently violent to break crys-
tals suspended in the fluid magma, except during an actual eruption, and it
may be inferred with some probability tliat this was fractured in its passage to
the surface. If so, the external black border was probably formed as the rock
cooled after eruption. The inner belt of magnetite, on the other hand, indi-
cates a che(;k in the growth of the crystal, and must have bSen formed long
before ejection^. But it is impossible to suppose the temperature to vary greatly
in melted rock-masses, at the depth below the surface at which it is believed
that eruptions originate. The pressure upon subterranean fluid masses,
however, probably varies within very wide limits, and it is well known that
changes in pressure produce effects closely analogous to those caused by
variations in temperature. It seems on the whole, therefore, most likely
that this hornblende errew to the limits of the inner black border under
conditions which were uniform, or perhaps varied uniformly; that a sudden
change in pressure equivalent to a diminution of temperature induced the
secretion of magnetite; that the conditions for hornblende secretion were
then reestablished, and continued till the time of the eruption, when the
crystal was fractured, and became surrounded by a second border during
the cooling process. Other large hornblendes in the same slide also have
double black borders, though less symmetrically developed, but the smaller
hornblendes, though also of considerable size, and manifestly "crystals of
first consolidation," show only a single external belt of magnetite, as if their
formation had begun only after the temporary change in pressure. If the
hypothetical history suggested is correct, it is pi'obable that hornblende only
forms under conditions of pressure which have not yet been reproduced in
the attempt to crystallize the mineral artificially, and the comparative rarity
THE ROCKS OF THE WASHOE DISTRICT. 61
of the black border about augite may indicate that this mineral is less influ-
enced by differences of pressure. The basis of the whole speculation is,
however, exceedingly slender.
Discussion of a zonal piagiociase. — Zonal structuro is exceedingly common in the
feldspars of nearly all the rocks of Washoe, and not infrequently there is a
nearly uniform and progressive change in the optical properties from the cen-
ter of the crystal towards the periphery without demarkation into zones. Of
course such a feldspar may be regarded as consisting of an indefinite number
of zones, but while ordinary zonal crystals show recurrent layers these do
not.
A remarkable instance of zonal structure occurs in slide 20 from the
North Twin Peak. It is illustrated in Fig. 13, Plate III. This feldspar is
probably a labradorite cut on a plane at right angles to the brachypinacoid.
The outer edge and the interior kernel extinguish light almost simulta-
neously when the cleavage plane makes an angle of about 14° with the
principal Nicol section. The intermediate belt, on the contrary, extin-
guishes at an angle of only 5°, though in the same direction as the outer
and inner portions. The fine stripes are blackest at an angle of about 14°,
with an opposite inclination, but they show no zonal structure extinguish-
ing light at the same angle throughout their entire length. The persistence
of these stripes throughout the crystal seems to prove its crystallographic
unity, which is further confirmed by the ^parallelism of the zonal limits.
The section also shows very well the alteration in form of the feldspar
during growth, as well as the identity of the zonal inclusions with the
groundmass, there being a connection through an opening on one side.
The variation in the position of the optical axes of different portions of
a crystal, the effects of which are seen in zonal structure, must be due to
differences in crystallographic orientation, or in tension, or in chemical
composition.^ Checks in the growth of a crystal may produce demarka-
tions such as are shown in Fig. 1 7, Plate III., and described on p. 60, but
so long as composition, tension, and orientation are the same, the position
of the optical axes must be constant. In the feldspar under discussion the
orientation of the zones cannot be different, and variations in tension would
' Cf. Min^ralogie Micrograph, by Fouc[u6 & L6yy, pp. 36 and 130.
62 GEOLOGY OF THE COMSTOCK LODE.
be visible in the narrow lamellae as well as in the broad ones. The intru-
sive groundraass, too, is scarcely compatible with the supposition of variable
tension, and the zonal structure in this case must be due to modifications
in chemical composition. This may vary in two ways; there may be a sub-
stitution of isomorphous elements without disturbance of the characteristic
"oxygen ratio" (atomic ratio) of the mineral species, or this ratio may be
modified in the sense of Professor Tschermak's feldspar theory. Granting
the accuracy, or even the approximate accuracy, of Messrs. Fouqu(^ &
Levy's discussion of the optical properties of labradorite^ and other feld-
spars, the first supposition is impossible in the present case; for if, at the
position indicated by the angle of extinction of the thin lamellse and two of
the zones, this angle may vary 10°, the distinction of difii'erent species by
this property is illusory. Indeed the extinctions are consistent with the sup-
position that the intermediate belt is oligoclase, an hypothesis, however,
with which the crystallographic unity of the section is incompatible. I am
therefore forced to the supposition that the intermediate belt answers to a
variety of feldspar of a different oxygen ratio from labradorite, but crystal-
lizing in this mixture in the same form.^ The same explanation seems to
me indicated in most zonally -built plagioclases, and in those which display
progressive divergence of the optical axes.
AUGITE-ANDESITE.
General character. — Thc augite-andcsitcs prcseut the closest parallellism to
thehornblende-andesites; the resemblance being far closer than that which
exists, for example, between the diorite and the diabase. But for the fact
that they clearly belong to different eruptions it would seem more appropriate
to regard the two rocks as varieties rather than as independent species. In
the Washoe district the porphyritic augites are rarely macroscopically
noticeable, but their effect is perceptible in a certain resinous luster. While
the color of the earlier hornblende-andesite in a fresh condition is commonly
iL. c, p. 253.
°The influence of salts of analogous properties, when mixed, in modifying the resultant crystal
form is ■well-known.
THE ROCKS OF THE WASHOE DISTRICT. G3
a blue-gray, not unlike "teinte neutre," the augite-andesites are generally
a much deeper, somewhat brownish-blue. Certain glassy augite-andesites
strongly resemble the glassy hornblende-andesites, while another variety is
pinkish-gray, and bears no superficial resemblance to anything else in the
DiSTKiCT. Some gray vesicular modifications have a basaltic look. The
crystalline augite-andesites greatly predominate over the glassy ones.
Hornblendes occur in a majority of specimens, but in very small numbers
as compared with the augites, probably not one per cent., while mica is met
with only often enough to justify the assertion of its occurrence.
Augite. — The augite is of precisely the same character as that of the
hornblende-andesites. Its color is always a more or less brownisli-yellow,
which varies somewhat in shade but not in character, and is very like that
of bamboo. I have not observed a single case of pinacoidal cleavage, while
there is a decided tendency to the suppression of one of the prismatic cleav-
ages. In some specimens the proportion of augite is small, and the crystals
are then very well developed. In other cases they are very numerous and
occur in groups in which, owing apparently to interference, the crystallo-
graphic outlines are imperfectly developed. They frequently contain glass
inclusions, which sometimes assume the form of negative crystals, and
sometimes spheroidal shapes; but embedded microlites of other minerals
are rare. Besides the porphyritical crystals, the augite often appears to
form a portion of the groundmass, and microlites of it are common in the
feldspars. In one rock, which has been classified as a hornblende-ande-
site, an augite was noted piercing an ilmenite. These facts point to a very
wide range of time for the crystallization of the augite, which would seem
to have been among the first, and among the last, minerals to assume a
crystalline form. This is a strong contrast to the occurrence of hornblende,
but in conformity with the results of experiment, for, as is well known,
augitehasbeen artificially reproduced under a variety of conditions; whereas,
so far as I am aware, the eff"orts to reproduce hornblende have hitherto
proved unsuccessful. The augites very exceptionally show a trace of the
black border, so commonly accompanying hornblende.
Other minerals. — The homblendo is precisely similar to that of the horn-
blende-andesites. It usually occurs in minute crystals, with heavy black
64 GEOLOGY OF THE COMSTOCK LODE.
borders; but in one very glassy rock it lacks this accompaniment. The
feldspars are also entirely similar to those in the preceding rock. Anorthite
has been identified in a few slides among the larger crystals, but in most
cases the maximum angles of extinction correspond to labradorite. The
microlitic feldspars appear generally to be oligoclase. The iron ore is com-
monly magnetite, but in a few cases chai'acteristic ilmenite sections have
been observed. Apatite is invariably present, very frequently as brown or
dusty crystals. There is no inconsistency between the presence of the brown
apatite and the colorless variety, which often occur in profusion in the same
slide; but the brown crystals seem rarely to assume the acicular form which
so generally jjrevails among colorless apatites. I have not observed a single
zircon, nor anything which can be set down with certainty as titanite. The
groundmass of the augite-andesites is usually microlitic, though in one or
two cases granular structure has been noted. It is very frequently the
case that the microlites of feldspar are excessively minute, and with lower
objectives the groundmass then gives the impression of felt. This is an
appearance which the hornblende-andesites seldom present. The microlites
are often so arranged as to produce the effect called fluidal structure.
Field character. — Tlio Ordinary variety of augite-andesite in a fresh condition
is dark blue, or brownish-blue, in color, resinous in luster, and has a rough
fracture. The comparatively fine-grained varieties often show the lighter
colors and smoother fi-actures common in hornblende-andesites, and when
the rock is at the same time somewhat hornblendic it is readily confounded
with hornblende-andesite. Sometimes, when the feldspars are unusually
developed and the fracture is excessively rough, the rock might be mistaken
for trachyte ; but the absence of mica, the rarity of the hornblendes, and the
predominance of triclinic feldspars are generally sufficient to distinguish
it. In a few instances the augite-andesites are very granular and coarse-
grained, and when slightly decomposed do not greatly differ from some dio-
rites in appearance, but the likeness is superficial. An imperfect columnar
structure is occasionally met with, but is not characteristic of the rock.
Breccia is exceedingly common, and is sometimes tufaceous.
Decomposition and weathering. — As is the case with the homblonde-andesites,
when the rock is directly exposed to the action of the atmosphere the process
THE EOCKS OF THE WASHOE DISTRICT. 05
of decomposition is very different from that which it undergoes when buried
beneath the surface. Croppings of the fresh rock rarely exhibit the tendency
to divide into parallel plates so characteristic of the other andesite. The want
of homogeneity in structure displays itself in a different but very interesting
manner. Under the action of the weather it frequently becomes apparent
that large masses of augite-andesite are composed of thin beds of various
character. Some of these yield to weathering much more rapidly than
others, and the exposed face becomes indented with closely set parallel
grooves, such as are often observed in finely laminated sedimentary rocks.
There is, however, no perceptible tendency to the development of cracks in
the directions indicated by these grooves. The most natural explanation of
this structure would seem to be that they represent rapidly succeeding flows
of the melted rock, but it is hard to see in that case why differences of ten-
sion do not lead to the development of fissures. Other masses show an
analogous but different behavior in the development of grooves of sinuous
form, which cross each other at considerable angles, and give the surface
somewhat the appearance of an irregular pavement. If this structure were
found only upon opposite surfaces of blocks; it might be interpreted as an
expression of a tendency to separate into columns ; but when it occurs at
all, it is found equally on all the faces exposed. It appears to me that
solidification must have set in from numerous centers distributed through the
rock, giving it a coarse pseudo-spherolitic structure, and that the grooves
must represent a slight difference in chemical composition in that portion of
the lava which was the last to solidify. Whatever may be the cause of the
appearance, it is highly characteristic of the rock in this District. A good
example appears in the foreground of the frontispiece.
When fresh augite-andesite is exposed to the air, it soon becomes coated
with a yellowish-white product of decomposition. This is gradually con-
verted into a bright reddish-brown substance, no doubt largely ferric oxide,
the surface at the same time growing rough. In many cases this color is
succeeded later by a pitchy black. The rate of change is by no means
slow, and in some of the railroad cuts, made a dozen years since, decom-
position has penetrated the rock for about a quarter of an inch. There is
reason to suppose that after the rock has turned black the rate of change
5 0 L
66 GEOLOGY OF THE COMSTOCK LODE.
is greatly decreased. Whilfi the changes in direct contact with the air are
markedly different from tliose which take place in hornblende-andesite, the
process of decomposition under ground seems to be identical in the two
rocks; nor are the products of decomposition distinguishable after the pro-
pylitic stage has been reached. As is the case with the hornblende-andesites,
too, solid augite-andesite disintegrates, while brecciated masses retain their
consistency, and are consequently exposed as bold croppings by the erosion
of adjoining disintegrated rocks. I do not know of any cases of unbrec-
ciated augite-andesite retaining its consistency in spite of considerable
decomposition, as the hornblende-andesite of the South Twin Peak has
done.
LA.TER HORNBLENDE-ANDESITE.
General character. — This Tock, most of whicli lias hitherto been regarded
as ti'achyte, varies greatly in appearance in di£ferent parts of the field. The
more trachytic varieties, such as those of the quarries a couple of thousand
feet northeast of Sutro shaft No. Ill , are purplish or reddish soft rocks,
loose in structure, and thickly studded with large feldspar crystals, horn-
blendes, and flakes of mica. Near the Utah mine the color is gray, and
the texture firmer and finer-grained, while further north the rock is dense,
black, and glassy. It also occurs largely as tufa.
Fe-Mg silicates. — All the youuger hornblende-andesite contains mica, though
in some cases the amount of this mineral in comparison with the bisilicates
is small. Hornblende, too, is always present, and augite generally forms a
subordinate constituent. The feldspars are of course triclinic, and no deter-
minable sanidin has been detected. Much of the rock is thoroughly crystal-
line excepting inclusions, but the extent of the occurrences showing a glassy
base is considerable. The hornblende is entirely similar to that of the older
andesite, but there seems to be a relation between the physical structure of the
rock and the development of black border. In the coarser, more trachytic-
looking masses, the black border of both hornblende and mica almost wholly
replaces the original mineral, as maybe seen to some extent in Fig. 32, Plate V.
In the dense glassy rocks, on the other hand, the border is narrow, or alto-
THE ROCKS OF THE WASHOE DISTRICT. 67
gether wanting. The mica seems to be biotite in most cases, but in two or
three localities cleavage scales give an unmistakably biaxial interference
figure. It is as uniformly surrounded by a border of magnetite as the horn-
blende. The augite presents no peculiarities in structure. The amount of
this mineral is commonly inversely as the quantity of mica. Magnetite is
remarkable only for its abundance, and nothing which could be pronounced
titanic iron was noticed. Apatites are rarer than in the older volcanic rocks.
Feldspars. — Almost all the large porphyritic feldspars show abundant
striations, even under the lens, and few large crystals appear to lack them
under the microscope. Many feldspars which do not show polysynthetic
structure under an objective of low magnifying power, show striae under
higher powers. Many of the feldspars show zonal structure comparable with
that discussed on page fil and illustrated in Fig. 13, Plate III. The large
feldspars are manifestly crystals of first consolidation, while the groundmass
is in great part made up of microlitic feldspars. While the large crystals com-
monly give angles of extinction indicating labradorite, the microlites appear
to be chiefly oligoclase. There are also among the larger feldspars a
comparatively small number of Carlsbad twins, and simple crystals which
might be regarded as sanidin if no further test were applied ; but none such
which were cut in the determinable zones, gave angles of extinction ajDpro-
priate to orthoclase. As some of the possible sanidins were not so oriented
as to make optical determinations practicable, I submitted a specimen of the
most trachy tic-looking rock in the district to Dr. George W. Hawes,' curator
of the National Museum, for separation by Thoulet's method. The speci-
men sent was from a quarry 2,000 feet east of the Occidental mill, E 5, and
was in all respects identical with that described by Professor Zirkel under
slide 283. The following details are taken from Dr. Hawes' report on this
rock :
Feldspars determined by Thoulet's method. Tlie SpecimCU WaS pulvCnZed tO SUcll
an extent that it would pass through a sieve containing three meshes to the
millimeter; and from this mass of grains the dust that would not settle was
separated by elutriation. As the special object in view was to determine
• While this report was going through the press Dr. Hawes died (June 22^ leaving a vacancy in
the ranks of American geologists which it will he hard to fill, as well as a deep personal regret in the
hearts of all who knew him, however slightly.
68 GEOLOGY OF THE COMSTOCK LODE.
the species of feldspar, the mass of grains was first placed in a solution of the
double iodide of potassium and mercury, which possessed a specific gravity
of 2.95. A portion of the substance immediatel}^ fell to the bottom. When
examined with the microscope this was found to consist of hornblende,
augite, mica, and ii'on oxide. The specific gravity of the fluid was then
diminished to 2.85, when a small portion settled out. The precipitate was
found under the microscope to consist of composite grains including por-
tions of one of the previously mentioned minerals. At the specific gravity
2.75 only a few grains of the same character fell down, and these were
more largely feldspathic.
On reducing the fluid to 2.70, a large amount of clear white grains
fell from the fluid. At 2.68 another large portion was precipitated, and
these precipitates when examined under the' microscope proved to be com-
posed entirely of grains of feldspar. On reducing the specific gravity to
2.fi7 very little fell down, and this was of a red color, and consisted mostly
of grains containing clear feldspar, together with portions of the ground-
mass. Subsequent reductions of the specific gravity caused the remaining
substances to fall to the bottom in successive portions, and when the fluid
had reached the specific gravity of 2.61, only a very small amount of ma-
terial floated. This examined under the microscope was found to consist
entirely of groundmass. There appeared to be no portion of the glassy
feldspar crystals in any of the substances which had a specific gravity
below 2.65. As the amount of rock which will float at any specific gravity
which approaches that of orthoclase is very small, it would seem that
under no circumstances could this feldspar be considered as a preponder-
ating species, and that, if present at all, it must be in very small amount.
Mr. F. P. Dewey, at Dr Hawes' request, analyzed the feldspar which
fell when the specific gravity of the solution was 2.70 and found its oxygen
ratio 1:2.89:7.95. This I find would correspond to a mixture of 39 per
cent, labradorite and 61 per cent, oligoclase, supposing these the only feld-
spars present. He also analyzed the portion which fell at a specific gravity
of 2.68 and found the oxygen ratio 1:2.96:8.69, corresponding to 12 per
cent. labradorite,and 88 per cent, of oligoclase. The entire feldspar analyzed
was 3 1 per cent, of the rock, or 8 per cent, labradorite and 23 per cent, oligo-
THE EOCKS OF THE WASHOE DISTRICT. ' 69
clase, on the supposition of a mere mixture of species. It appears to me
more probable, however, from the character of the zonal plagioclases, that
many of the feldspars are not chemically referable to either species.
The results of the application of Thoulet's method agree excellently
with those of the microscopical examination, and together render it impossi-
ble to classify this rock otherwise than as a hornblende-andesite, in spite of
a macroscopical ajDpearance exceedingly like ordinary varieties of trachyte,
and very dissimilar to common andesite.
Remarkable glass inclusion in feldspar. The fcldsparS COUtalu glaSS incluslonS iu
all the slides of this rock, but these are most abundant to the north of the
Utah. In the quarry close to the hoisting works of that mine some of these
inclusions are of a peculiar character, forming negative feldsjiar crystals of
a more or less perfect shape. These were mentioned by Professor Zirkel
with admiration. No such fine example occurs in my slides as in that de-
scribed by him, and in his slide number 284 there is but one which can
have furnished his description. This is illustrated in Fig. 14, Plate III.
It is not a sanidin, however, but probably a labradorite crystal.
Groundmass. — The grouudmass of the more trachytic varieties is entirely
crystalline, though never granular like some of the older hornblende-ande-
sites; its texture is also very loose and open, a fact which often influences
the course of decomposition. To the north of the Utah patches of glass
similar to that which is included in the feldspars of the same locality are
distributed through the groundmass, and on the ridge running east by south
from the Geiger Grade toll-house, D. 1, as well as at the point whei'e the
Grade cuts the younger hornblende-andesite area, the glass prevails to such
an extent that the rock approaches an obsidian in character. Its pitchy
black color is due merely to the bisilicates and magnetite, the glass and
feldspar being transparent.
Field character. — Thc more trachytic varieties near Shaft III. of the Siitro
Tunnel, and on the southwesterly spur of Mount Rose, are red or purple,
and highly porphyritic, very soft and rough rocks, quite incapable of being
confounded with any other occurrence in the district. They do not exhibit
regular partings or columnar structure. Mount Rose and Mount Emma are
largely composed of tufa and tufaceous breccia. The tufa is not macro-
70 GEOLOGY OF THE COMSTOCK LODE.
scopically distinguishable from other tufas, such as that of augite-andesite,
but inclosed masses commonly indicate its character. The exposure repre-
sented in Plate VII. is made up of coarse porphyries and tufa, and the engrav-
ing shows a species of bedding in the rock, no doubt due to variations in
eruption. Gray, tolerably firm varieties, about as coarse as ordinary gran-
ular diorite, occur at the Sugar Loaf, F. 3, and near the Utah. At the latter
point columnar structure is very finely developed. Mount Abbie, C. 2, is
intermediate between the firm gray and the soft, highly porphyritic modifica-
tions, and the black glassy occurrences require no further description. None
of these bear much resemblance to the prevalent varieties of earlier horn-
blende-andesite, but there is a considerable area to the northeast of Mount
Emma, and just outside of the map, where the resemblance is almost perfect.
This area seems to be strictly continuous with the more typical one, how-
ever, and transitions occur. Lithologically the presence of more or less
mica seems characteristic.
The weathering of this rock is commonly confined to the separation of
ferric oxide, not merely on the surface, but often for considerable distance
into the mass, where the latter is of an open texture. In the neighborhood
of the Sierra Nevada mine, however^ chloritic degeneration of the bisili-
cates is perceptible.
Distinctive characteristics. — No csscutial pfopcrty dlstiuguishes the younger
from the older hornblende-andesite, but in the Washoe District it forms a
variet}^ of andesite readily distinguishable in most cases by its loose struct-
ure, and the presence of mica. The glassy modification is more likely to be
confounded with augite-andesite, but the luster is not resinous, as in the
augitic rocks. The distinction is hardest to draw in the wild canons east
of Mount Kate, a region wholly unlikely ever to have any importance.
BASALT.
Basalt plays a very small part in the geology of the district, but the
rock is a thoroughly characteristic representative of the species. It is dark
and compact, with many visible crystals of dark amber-colored olivine.
Microscopical character. — The basalt is a tlioroughly crystalline mixture of
THE EOCKS OF THE WASHOE DISTRICT. 71
olivine, augite, labradorite, and magnetite, showing no glass excepting as
inclusions in the augites. The olivine occurs in part as fairly well developed
crystals, with hexagonal and octagonal sections, and occasional perceptible
cleavages. It is almost colorless, but shows the faintest possible tinge of
yellow. The decomposition amounts only to a slight discoloration along
some of the edges and cracks. Augite is present, in part in crystals as large
as the olivine, and in part in minute grains forming a portion of the ground-
mass. The feldspar is crystallized for the most part in lath-like forms, and
is often twinned according to the Carlsbad law, but in one or two cases
both albitic and periclinic twinning are visible. The determinable crystals
seem all to belong to labradorite. The magnetite is in no way remarkable.
Field character. — Tho larger part of the basalt occurs in the form of ridges
with horizontal summits, giving the impression of tables, though they are
really very narrow. At the base of these ridges are numerous bowlders
which, under the action of frost, have assumed rounded forms. Besides the
areas visible on the map, there is a single bluff-like cropping near McClellan
Peak, where the bowlders have assumed an almost perfectly spherical shape.
It is hard, and rings like cast iron under the hammer, but is rather brittle
and chips readily. There is no considerable quantity of decomposed basalt
to be seen.
This rock cannot be confounded with any other in the district, for it
all carries visible olivine, a mineral not met with in any other Washoe rock.
The elevation laid down as Basalt Hill is augite-andesite, and the rock
described by Professor Zirkel as an unusual basalt^ is both macroscopically
and microscopically the same as that here considered as metamorphic diorite.
' Expl. of the 40th Par., Vol. VI., slide 528.
72 GEOLOGY OF THE COMSTOCK LODE.
Section2. (Chapter III.)
THE DECOMPOSITION OF THE EOCKS.
Sucli facts as have been established with reference to the decomposi-
tion of the Washoe rocks are necessarily mentioned in connection with the
lithological description of each species. The subject, however, is one of
such great importance in the geology of the District that it appears advisa-
ble to consider the observations bearing upon it as a whole, and in some
detail.
Area of extreme decomposition. — While fow absolutcly fresli Tocks occur iu tho
region surveyed, decomposition so great as to oppose a serious obstacle to
lithological determinations is confined to a smaller area. In the nature of
things this area is incapable of precise definition, but it is shown as nearly
as may be in its relation to the Comstock and the Occidental lode by the
accompanying sketch map, page 73. From this it appears that precisely
the area which is of the most importance in a discussion of the vein-geology
is that profoundly decomposed.
Effects of decomposition on various rocks the same. Willie tllC pliysical charaCtCr of
the different rocks has to some extent modified the physical results of decom-
position, the chemical and mineralogical changes and the degree of alter-
ation observed in the rocks of the decomposed area seems almost wholly
independent of their age or species. Granular diorites, porphyritic diorites,
the two diabases, earlier hornblende-andesite, and augite-andesite appear to
have been subjected to the same influences, with the same results. Quartz-
porphyry and younger hornblende-andesite come within the limits of the
chief area of decomjDosition only to a slight extent, either above or below
ground, but to that extent they show the same effects, as does also the met-
amorphic-diorite in limited spots more or less nearly related to the focus of
action. Only basalt and granite have escaped with mere traces of decom-
position, while the quartz-porphyry as a whole appears to have been sub-
THE DECOMPOSITION OF THE ROCKS.
73
jected to decomposing influences not shared by the other rocks in the same
degree. It is difficult to avoid the conclusion that the period of intense
chemical action cannot antedate the eruption of later hornblende-andesite,
and probably succeeded it.
Fig. 1. — Area of extreme decomposition.
Not only have the same minerals in the various rocks undergone iden-
tical processes of alteration, but similar groups of minerals have yielded
almost identical results. Hoi-nblende, augite, and mica have given place to
the same ultimate jiroducts, though in slightly different proportions, and the
74 GEOLOGY OP THE COMSTOOK LODE.
degeneration of each of the various feldspar species has taken the same
course.
Hornblende. — Homblcnde in the diorites is met with, both brown and green.
The brown variety is usually quite fresh, while the green exhibits a tendency
to a general degeneration throughout its whole mass. In one instance, at
least, it has been shown that the brown solid hornblende of a semi-porphy-
ritic diorite is altered into the green fibrous modification, and in other cases
there is strong reason to suspect a like change. Similarly, it has been shown
that the hornblende of the metamorphic diorite was in all probability once
colorless, and that it is now in part converted into a green modification of
a fibrous texture. The result in both cases is very similar to uralite. It is
by no means asserted that all the green fibrous hornblende of the diorites
in Washoe is an alteration-product of other varieties, though this seems
possible, but there is evidence enough to warrant calling the attention of
lithologists to the question how far green fibrous hornblende is to be con-
sidered the original form of the mineral. Professor Rosenbusch mentions
this change • in connection with the proterobases of Lusatia. In the
younger rocks I have not succeeded in detecting a similar change. The
hornblendes of the Washoe andesites are either full brown, reddish brown,
or greenish brown in color. The tint of those last mentioned it is somewhat
difficult to describe, and consultation has shown that the definition proposed
depends considerably on the susceptibility of different eyes. To some they
appear green with a tinge of brown, while to others the green admixture
is scarcely perceptible; but all agree that the color is very different from
the grass-green or bluish-green of the fibrous diorite hornblendes.
Alteration of hornblende to chlorite. — Thc fibrous dloHtic homblcnde, somc of the
brown variety in the por^jhyritic diorites, and all the hornblendes of the
younger rocks, appear to pass directly into chlorite. The attack seems to
take place from external surfaces and cracks. If the cleavages of the crystal
are well opened, each cleavage prism is attacked, and the result in longitu-
dinal section is a quasi-fibrated mass of rods of hornblende separated by
chlorite, and in cross-section a group of isolated rhombs or irregular patches
of the unaltered mineral embedded in chlorite, which often retains the outlines
of the oi'iginal crystal in great perfection. Figs. 1, 2, and 3, Plate II., are
THE DECOMPOSITION OP THE KOOKS. 75
exact representations of such cases. The chlorite, with only one or two ex-
ceptions, has the same characters described elsewhere.^ The Washoe chlorite
evinces a considerable solubility, which can be traced in many series of slides,
notably in those from the McKibben Tunnel. In the early stages the pseudo-
morphs after hornblende are verj^ fine; later the form of the hornblendes is
obscured, and irregular patches of chlorite appear in the groundmass; at
last this mineral appears diffused through the rock, settling in bands round
apatites, magnetites, or other solid minerals, and penetrating partially decom-
posed feldspars. Frequently too it occupies microscopic veins traversing the
slide. One such observation would perhaps be open to great question, but
scores of similar cases occur in the numerous slides examined.
Augite and mica. — Both augitc aud mica exhibit the same tendency to pass
into chlorite as hornblende, and neither the process nor the result commonly
differs in any way from that just described. A preliminary change of augite
touralite is, however, not uncommon in the diabases, and in a single slide of
augite-andesite the same alteration appears, though several other thin sections
from the same cropping show nothing of it. On the whole augite seems to
be somewhat more disposed to decomposition than hornblende, and cases are
numerous in which the hornblendes of a rock retain their freshness, when
the augites are completely altered; but in some instances the augites
have resisted longer than the hornblendes. Mica, on the other hand, cer-
tainly yields somewhat less readily than the bisilicates, to which it is so
closely allied, though it is often wholly changed to chlorite.
Formation of pyrite. — lu Very numcrous cascs pyrite has been observed in rela-
tions indicating that it is formed directly from hornblende and augite, appar-
ently at the same time as chlorite. The two products do not seem to me
dependent upon one another, for chlorite occurs where no pyrite is found,
and the process of conversion to chlorite is not visibly modified by the simul-
taneous growth of pyrite. The indications are, therefore, that the two pro-
cesses are wholly independent of and not inconsistent with each other.
Epidote formed from chlorite. — Epidotc is usually cousidcred as a direct result
of the decomposition of the bisilicates, but in Washoe such a transforma-
tion, if it occurs, must be exceptional, for it was not recognized in a single
1 See pp. 84, 211, etc.
76 GEOLOGY OF THE COMSTOCK LODE.
instance, though the paragenesis of the products of decomposition was a
subject of special inquiry. On the other hand, the formation of epidote at
the expense of chlorite is proved beyond a doubt. The development of
epidote usually begins near the centers of patches of chlorite, sending out
faggot-like masses of crystals in all directions, and ultimately, under cer-
tain conditions, occupying the whole space. In certain stages this process
can be admirably observed, long prismatic needles of epidote extending into
the chlorite and cutting the minute iibers of the latter at all sorts of angles.
This is peculiarly well seen in Figs. 6 and 7, Plate II. Sometimes the
chlorite-fibers, at the periphery of hornblende pseudomorphs, exhibit a spec-
ial arrangement, lying strictly parallel to one another and perpendicularly
to the crystal face. These fibers are often nearly of the same length, and
thus form a belt or zone. Such a belt must be denser than a spherolitic
mass, and not infrequently appears to offer a greater resistance to the forma-
tion of epidote. This is in accordance with the chemical conditions, for the
transformation cannot take place without the access of solutions. Complete
pseudomorphs of epidote after hornblende may result from this process under
favorable conditions, and such an one occurring in a slide from the McKihhen
Tunnel is shown in Fig. 9, Plate II. From a study of a series of slides from
tlie same locality it is evident that this pseudomorph is the last stage of the
process illustrated by Figs. 6 and 7, Plate II., and not a case of direct con-
version. Epidote is also constantly found developing in patches of chlorite,
which occur in the groundmass of the rocks, where they have apparently
been deposited but not formed; and microscopic veins of chlorite are
common in which various proportions of the mass are changed to epidote.
Feldspar docs not decompose to epidote. — When the feldspars bccomo porous, as
they do so soon as decomposition has commenced, they are subject to infil-
tration by chlorite, and the chlorite so deposited is converted into epidote
under the same conditions as in other portions of the rock. One distin-
guished lithologist has attributed the supposed, but confessedly mysterious,
alteration of feldspar into epidote, to the presence of plentiful hornblende -
needles embedded in the feldspar. In the section of this chapter dealing
with propylite it is shown that this determination is erroneous, the supposed
hornblende particles in the sHde upon which the suggestion is founded being
THE DECOMPOSITION OF THE ROCKS. 77
in fact chlorite; but that the epidote is to be attributed to the alteration of
these foreign jjarticles, and not to a transformation of the feldspar sub-
stance, seems to me certain. The grounds for this view, which has not
hitherto been entertained, are as follows: There is no question that chlorite
arises from the decomposition of the bisilicates, and that it may become
diffused through the groundmass is equally certain. Many slides from
Washoe show the feldspars in a very fresh state, while the bisilicates are
wholly chloritized. In such cases the chlorite cannot be due to feldspar
decomposition, but its diffusion through the groundmass is nevertheless
common. Carious feldspars appear to be impregnated with chlorite as a
rule when the neighboring bisilicates are undergoing chloritic decompo-
sition, but not otherwise; and epidote is found developing in chloritic
masses inclosed in feldspar when, and only when, the same process is
going on in chlorite patches not so inclosed, the origin of which is distinctly
referable to hornblende, augite, or mica. Many cases of the formation of
epidote have been observed in chlorite inclosed in feldspar, as convincing
as the instances of the transformation of chlorite arising from the bisilicates
which are illustrated on Plate II., though none so beautiful; but in no
instance in the Washoe District has epidote been seen sending its twig-like
crystals into feldsjDathic masses.
Other alteration-products of chlorite. — Chlorlte also dcgcuerates iuto quartz, cal-
cite, and limonite, and this change is sometimes to be observed in the same
slide which shows its alteration to epidote. In this case, also, the dense belt
of chlorite which occasionally forms at the surface of a crystal of hornblende,
seems to offer considerable resistance to attack. An instance is illustrated
in Fig. 10, Plate II. Sometimes this change seems to be referable to a dis-
tinct period in the decomposition of the rock, as in the case shown in Fig.
3, Plate II., and described under slide 464.
Character of the chloritic mineral. — The chloritc arisiug from the bisilicatcs and
mica shows the same optical properties, and the conversion of chlorite into
epidote is frequent from whichever primary mineral it may be derived.
There seems, however, a somewhat smaller tendency for the chlorite arising
from augite to change to epidote, than is displayed by that formed from
hornblende and mica; but the difference in this respect is not great or uniform.
78 GEOLOGY OF THE GOMSTOCK LODE.
Insolubility of epidote. — Epidote is usuallj classed as an insoluble mineral, and
the evidence to the contrary in the Washoe District is slight. Epidote, it
is true, frequently crystallizes in vugs, but since chlorite certainly possesses
some degree of solubility, their growth might be accounted for by supposing
them to form in a solution of the mineral from which they are derived.
There seems, however, to be a relation between the size of epidote masses
and of the crystalline grains of which they are composed, which is most
easily accounted for by supposing the mineral to be somewhat soluble. In
very small patches epidote is frequently so fine-grained as to reflect almost
all the light, and under low powers appears opaque. In larger masses,
formed apparently under similar conditions, the epidote often shows crystal-
line grains of considerable size, and transmits light readily. In a few cases
among the diabases epidote appears to be replaced by opaque mixtures of
iron oxide and other substances, but no certain instance of this sort was
made out, and there is usually no indication of any tendency to decompose.
Decomposition of feldspars. — The study of thc proccss of decomposition which
the feldspars of the Washoe rocks undergo is much less satisfactory. They
have offered a far greater resistance than the bisilicates, and no great con-
tinuous area exists in which they are not sufficiently fresh to be readily
determinable. Incipient decomposition is marked by the appearance of
specks of calcite, readily recognizable in polarized light. At a later stage
quartz grains make their appearance, accompanied by particles of a white
opaque substance, of a nature unknown to me. In the last stages of decom-
position nothing further than these three substances is recognizable. Kaolin,
according to Mr. H. Fischer, is an isotropic substance, accompanied in the
slides he studied by polarizing grains and scales.^ Nacrite is crystalline and
consists of an aggregate of six-sided scales of fibrous texture, each composed
of six triangular sectors. Nothing corresponding to the description of
either was observed in any of the slides, a fact which seems to prove that
kaolinization, if it has taken place at all, is a very subordinate phenomenon.
The analyses of the ''clays," too, show that they are not concentrations of
kaolin washed out of the surrounding rocks, but represent so much rock
crushed and degenerated in place. The water contents of some of them is
'Eosenbusch: Phys. derMin. u. Gesteine, Vol. I., p. .374.
THE DECOMPOSITION OF THE EOCKS. 79
such as to preclude the idea that even this material contains any notable
quantity of kaolin.
Secondary liquid inclusions. — The behavior of the parti cles of calcite which first
form in the feldspars is of some little importance, for these on leaching out
give rise to secondary liquid inclusions, as will be explained under slide 210.'
Such inclusions are met in all the decomposed andesites and appear to be
frequent also among the other rocks. If proper regard is paid to the con-
dition of the feldspars and to the shape of the inclusion, there is little
difficulty in discriminating between secondary and primitive liquid inclu-
sions, but a neglect of these precautions might readily lead to incoiTect
diagnoses.
Magnetite appears in certain cases to be converted into a yellowish-
white opaque substance, accompanied by polarizing grains, much resem-
bling calcite. The black border of hornblendes is sometimes wholly
removed in this manner, and the appearance of the rock considerably
modified. The phenomena suggest a conversion to a mixture of carbonate
of iron and limonite.
Decomposition of rock-masses. — The course taken by the decomposition of masses
of rock depends largely on their physical character, and is sufficiently dis-
cussed in connection with the general description of each rock. Only porous
masses suffer decomposition uniformly throughout, and these are apt to retain
their coherence. Blocks of dense rocks are attacked from their surfaces and,
as in all processes involving solution or substitution, the corners and edges
yield more rapidly than the flat faces, so that the fresh kernel tends to assume
a spheroidal shape. The altered portion of the dense rocks frequently dis-
integrates.
Condition of the quartz-porphyry. — The quartz-porphyry throughout the District
and far beyond its limits is so much decomposed that not a single fresh horn-
blende has anywhere been found in it. Except where increased by special
causes, such as propinquity to the Lode, the degree of alteration is also very
uniform. As it overlies very fresh granite and metamorphic diorite and is
overlain by fresh andesites, special causes must be sought to account for its
exceptional degeneration. None such have occurred to me except its phys-
'Page 119.
80 GEOLOGY OF THE COMSTOOK LODE. -
ical structure. The porphyry is, and seems always to have been, very
porous, and has permitt'ed a more rapid percolation of surface waters than
the other rocks. This is not improbably ascribable to the difference in the
coefficient of expansion of the quartz grains, and the other mineralogical
constituents.
The chemical aspects of the decomposition of the Washoe rocks will
be discussed in a separate chapter.
PROPTLITE. 81
Section 3. (Chapter III.)
PROPTLITE.
Historical statement. — The term propyHtc, as is well known, was introduced
into lithology by Baron F. v. Richthofen, mainly in consequence of observa-
tions made in the Carpathians and in the States of California and Nevada. In
his memoir on "The Natural System of Volcanic Rocks "^ greater prominence
is given to the Washoe occurrence than to any other. From his description
of the rock the following statement of its characteristics is taken almost
verbatim. Propylite is always porphyritic, and no prominent property dis-
tinguishes it from porphyritic diorite. The feldspars are oligoclase and the
hornblendes ordinarily dark-green and fibrous. The groundmass is usually
green and appears to owe its color to the profuse dissemination of small
particles of fibrous hornblende. It also presents a peculiar and recog-
nizable, though hardly describable, appearance- or habitus. It is extra-
ordinarily rich in mineral veins, both in Europe and in America. Geo-
logically it is the earliest of the Tertiary volcanic rocks. Mr. Clarence
King^ accepted Baron v. Richthofen's determination of propylite in the
Washoe District (a region which he visited in company with that geologist),
though with some limitations and additions. Outside of this District the
geologists of the Exploration of the Fortieth Parallel found only a few
obscure localities of the rock. In 1876 Prof F. ZirkeP confirmed the
independence of proi^ylite as the result of a microscopical examination of
the collections of the Exploration of the Fortieth Parallel. In 1880 Capt.
C. E. Dutton'' announced the presence of considerable areas of propylite
in Utah.
'Mem. Cal. Acad, of Sciences, Vol. I., Part II. ^Exploration of the Fortieth Parallel, Vol. III.
3 Ibid., Vol. VI. ^Tho High Plateaus of Utah.
6 C L
82 GEOLOGY OF THE COMSTOCK LODE.
Failure of the search for propyiite. — Washoe presenting tlio typical Ameiican oc-
currence of propyiite, a study of the rock necessarily formed a prominent
feature in the re-examination of the District; for while the structure and vein
formation of the Comstock are the objects of first importance and interest in
Washoe, the first step toward their elucidation was manifestly to clear up the
lithological obscurities as far as possible. Since Baron v. Richthofen and Mr.
King examined the District, the exposures of rock have been greatly increased.
Not only have the mines on the Lode been deepened by a couple of thousand
feet, but innumerable roads, quarries, prospect-holes, and the like, have ex-
posed more than the mere weathered surface in thousands of spots. It soon
became apparent that the area of andesite, which to Baron v. Richthofen
seemed inconsiderable and to Mr. King quite subordinate to that of the .
propyiite, had been unden-ated. Fresh andesites were found exposed by
cuts in many localities which had been laid down as propyiite; and since
the latter was supposed to underlie the former, the upper portions of these
exposures furnished a safe study of decomposed andesites, the results of
which could be applied elsewhere. It was found that even where a high
degree of decomposition and a thoroughly propylitic character prevailed,
reasonably fresh rocks could be discovered by diligent search, either as
masses protected by some accidental arrangement of fissures, or as nodules
at the centers of concentrically weathered blocks; and to the east of the
Lode, wherever fresh rocks were discovered among the propylites, they
always proved andesitic. Where andesite dikes or overflows had been
recognized, and had been supposed to succeed propyiite, careful examina-
tion and excavation showed that the change was through a transition, not
by a contact. In short, the propyiite area to the east of the Lode was
reduced almost foot by foot, until it disappeared altogether. The propyhte
from the head of Ophir ravine, one of the type-localities, had a slightly
diff"erent character from the eastern rock, yet the diff"erence was not greater
than seemed possible within the limits of a rock-species. Fortunately
there are many long tunnels penetrating the hills in the neighborhood; and
an examination, undertaken to establish contacts between propyiite and
diorite, resulted in a study of transitions between typical diorite-porphyries
and decomposed porphyritic forms of the same rock. At last even in the
PKOPYLITE. 83
huge "propylite" croppings of Ophir Ravine the industrious use of the sledge
revealed surfaces which were unmistakably dioritic, and so propylite dis-
appeared from the surface. Under ground it early became evident that the
east countiy rock was different from that upon the surface ; but a long time
elapsed before an accidentally protected mass was discovered, which was
fresh enough to serve as a basis for determination. It proved to be diabase.
Later, other localities of fresh diabase were found, but while in a new dis-
trict broad inferences might soon have been drawn as to the character of
the hanging wall, this was impossible in the face of previous determinations.
If a rock answering to the definition of propylite existed, it was necessary
to determine its precise area and occuiTcnce; and if there were no such
rock it was indispensable to prove that the whole area was occupied by
others. The state of decomposition of the underground rocks is so
advanced, that in not more than three out of a hundred of the specimens,
all selected with the utmost care, is there a fresh augite or hornblende, and
perhaps half of the 185 miles of underground workings are accessible.
The task was therefore a laborious one. The lithological examination
became a protracted study of decomposition-products, and resulted in
proving that propylite did not exist below the surface any more than
upon it.
propyiitic habitus. — The uiost Striking macroscopical points of distinction
between the rocks in the Washoe District which have been determined as
propylite,^ and the better established Tertiary and ante-Tertiary rocks, are
a greenish color which often tinges the feldspars as well as the groundmass,
impellucid feldspars, and a certain blending of the mineral ingredients
which helps to deprive the rock of those characteristics by which we are
accustomed to recognize fresh specimens as belonging to the older or to the
younger series. These appearances seem to me to constitute its "charac-
teristic habitus."
Fallacious nature of the distinguishing characteristics. BarOU VOn Richthofeil bclicVed
that the macroscopical character of the rock was due to green, fibrous
hornblende, and the diffusion of this mineral in fibrous particles through
the mass of the rock. This view was confirmed by Professor Zirkel, who
' See page 8cl.
84 GEOLOGY OF TBE COMSTOCK LODE.
founds the greater part of his diagnostic points of difference between pro-
pylite and andesite upon the color and structure of the hornblende, and its
distribution in the rock. What has been taken for green fibrous hornblende,
however, in a great majority of the propylite slides of the collection of the
Exploration of the Fortieth Parallel proves to.be not hornblende, but chlo-
rite. This mineral, which is probably the rhipidolite of G. Rose, is, like horn-
blende, green, fibrous, and strongly dichroitic, but it occurs largely in
spherolitic and felt-like masses, extinguishes light when either of the
principal sections of the polarizing apparatus is parallel to the fibers;
and, when the Nicols are crossed, usually shows only dark-bluish tints,
very different from those commonly transmitted by hornblende. In one of
the slides, indeed, there is abundant green fibrous hornblende, but the rock
is a granular diorite from Mount Davidson, while in the section from Storm
Canon, Fish Creek Mountains, there is both chlorite and hornblende, but
the latter is certainly uralite.
It has been shown in the preceding section of this chapter that chlorite,
which is a decomposition-product of hornblende, augite, or mica, is fre-
quently diffused through the groundmass and any feldspars which may
have become porous through decomposition. This fact, combined with the
mistake of chlorite for hornblende, explains the distinctions based upon the
greenish hue of the propylitic rocks, upon the color and structure of the
masses mistaken for hornblende, and upon the distribution of supposed par-
ticles of that mineral through groundmass and feldspars. Seemingly con-
clusive proof has also been offered elsewhere that epidote in the Washoe Dis-
trict is not an immediate product of the decomposition of hornblende, but
of chlorite ; which explains its absence in the comparatively fresh rocks recog-
nized as andesitic.
In one limited area of hornblende-andesite, not represented among the
slides of the Fortieth Parallel, minute spiculse of hornblende occur, distrib-
uted through the groundmass ; but they are brown, each microlite is solid,
and they are not grouped in crystal-like aggregates. In almost all cases
the andesitic hornblendes when fresh are black-bordered; but while the
magnetite usually resists decomposition longer than the hornblende sub-
stance, it sometimes yields first. The hornblendes of the diorite-porphyries,
PEOPYLITB. 85
though otherwise very similar to those of the andesites, seldom show even
a trace of the black border. Barring two or three exceedingly local excep-
tions, a division of the hornblende rocks of the District into those showing
black-bordered crystals, and those which do not contain them, would be
equivalent to a separation into andesites and diorites. The assertion that
propylite is characterized by the presence of hornblendes without black
borders is founded on the determination of chlointe as hornblende.
Much of the chlorite mistaken for hornblende is due to the decomposi-
tion of augite, but though fine pseudomorphs of this description occur in
the slides of the Fortieth Parallel collection, the significance of their out-
lines appears to have been ovei-looked. Some of these slides are from typ-
ical, though somewhat altered, augite-andesites. Augite also occurs in the
dioritic porphyries of Washoe.
Glass inclusions, in some cases partially devitrified, seem to occur in
nearly all the propylitic rocks of volcanic origin, while they are of course
absent from the dioritic rocks included among the propylites. The Washoe
andesites are somewhat unusually crystalline, and if those which have been
regarded as propylite ever contained any isotropic base, of which there is
no evidence from analogy, it is now devitrified.
Quartz occurs to a considerable extent among the granular diorites, as
an original constituent. One specimen of hornblende-andesite, from an
area not represented in the collection of the Exploration of the Fortieth
Parallel, however, contains a few minute quartz grains of indubitably prim-
itive character, and these carry fluid inclusions. Occasional fluid incliisions
have of late years been found in all volcanic rocks, and they do not conse-
quently form a conclusive point of difference unless they are widely dis-
tributed and are present in great abundance. In some of the rocks deter-
mined by Professor Zirkel as quartz-propylite, the quartz appears to me to
be secondary. It occurs in groups of grains of different orientation, and is
indistinctly separated from the surrounding mass. Secondary quartz, of
course, frequently contains liquid inclusions.
Value of habitus in rock-determinations. — The methods cmploycd to identify the
propylites of Washoe with other rocks were by no means confined to mere
mineralogical examinations under the microscope. It is but a few years since
86 GEOLOGY OF THE COMSTOCK LODE.
the only resources of the Hthologist were a study of variations and transi-
tions, and a keen perception of the habitus characteristic of rock-species,
aided only by the feeble help of a lens and an occasional chemical analysis;
and how much can be accomplished in this way is evident from the fact that
the chief features of lithological classification are still much what they were
before the introduction of the microscope. Nor are the methods of the
older lithologists antiquated; on the contrary, the proper use of the micro-
scope greatly increases their applicability and efficiency. The microscope
enables lithologists of the present day to give greater precision to their ideas
of macroscopical habitus, and to distinguish in most cases between essential
and non-essential characteristics, and, with this advantage, they should
become even keener field observers than their predecessors. Indeed the
relations of lithological varieties, and of the causes on which they are depend-
ent, can be successfully studied only in the field. In the present investiga-
tion slides were ground and examined from day to day as the exigencies of
the field-work seemed to demand. The microscopical and macroscopical
appearances were also diligently compared (for grinding slides without ma-
chinery was a serious addition to the labor of days spent in the saddle or
under ground) ; and it became possible at length to recognize at a glance a
unity of origin in specimens of very diverse appearance and to detect litho-
logical diff'erences in spite of advanced decomposition and great appai-ent
similarity. It proved possible to make the proper allowance for decom^w-
sition and to infer the original habitus when veiled by another of secondary
origin, as well as to identify the precise character of the change.
Typical propyiite localities. — The threc most important propylite localities men-
tioned by Professor Zirkel in the Washoe region are the head of Ophir
Ravine, Crown Point Ravine, and Gold Hill Peak. The last is represented
in the map accompanying the present report, as the southern Twin Peak
(C.4).
Head of Ophir Ravine. — The uppcr portiou of Ophir Ravine presents a very
great variety of diorite-porphyries, which are not related as separate
flows or sheets, but pass over into one another as if the whole heterogeneous
mass had cooled at once. The character of the rock changes every few
feet, and the same varieties recur in spots. Among them are some so gran-
PEOPYLITE. 87
ular as to be nearly indistinguishable from Mount Davidson rock, while
others are dark fine-grained porphyries closely resembling andesites. The
latter, however, can be shown from slides to be dioritic, while the granular
varieties are as like the Mount Davidson rock microscopically as they
appear to the naked eye. A portion of these rocks is altered, but the transi-
tions from the fresh to the decomposed state can be studied more satisfactorily
in the McKihhen Tunnel, because, in the ravine, decomposition is most preva-
lent in the bluffs near the andesite, which is also somewhat altered. There
is no evidence of any contact between these bluffs and the unquestionable
dioritic masses adjoining them, and in spots where the rock is comparatively
fresh, its character seems unmistakably the same ; but when the effect of
decomposition on the tunnel porphyries is considered in reference to the
ravine rocks, it becomes clear thdt the bluffs can be only altered forms of
the adjacent varieties of diorite.
Crown Point Ravine. — Ouc flauk of Crowu Poiut Raviue shows tolerably fresh
hornblende-andesites, the other excellent fresh augite-andesite. Near the
drainage the rock is largely a highly decomposed breccia, in part bleached
to whiteness, but the area occupied by propylitic rocks is very small and
could only represent an exposure by erosion. As is common in breccias,
the decomposition is not uniform. The matrix is so altered that its coher-
ence is a matter of surprise, and many of the included fragments are tinged
with epidote. Some, however, from superior density or accidental protec-
tion are less affected, and a few large unfissured blocks are tolerably fresh
at some distance within their surfaces. Wherever the inclosed masses are
fairly fresh they look like andesites, and under the microscope there proves
to be no distinction, when the course of chlaritic decomposition known
from other occurrences is allowed for.
South Twin Peak ("Gold Hill Peak"). — The South Twlu Peak looks more like the
younger gray hornblende-andesite of the Utah quarry than the more usual
varieties of the earlier eruption, while the northern peak is for the most part
normal; but it also shows occasional small patches resembling its southern
neighbor, and there seems a gradual transition from one to the other. Fresher
specimens from the South Peak show abundant lustrous, seemingly black,,
hornblendes with perfect cleavage, and these under the microscope prove to
88 GEOLOGY OF THE COMSTOCK LODE.
be deep brown black -bordered crystals. There is no green hornblende and
thei-e are glass inclusions. In short, there is no assignable reason for sepa-
rating this rock from the andesites. There are many other localities in the
District where the propylitic rocks are quite as puzzling as in the three
described, but it is sufficient to state that they Vrere studied with equal care
and with similar results.
Conclusions reached. — Field obscrvations, aided by microscopical examina-
tion, show that the mineralogical composition and the structure of the propy-
lites of the Washoe District in their original state were identical with those
of certain fresh rocks found in the same region, namely, granular diorite,
dioritic porphyry, diabase, hornblende-andesite, and augite-andesite. The
great and misleading similarity of the propylites to one another is due not
to original constitution, nor to their geological relations, but to the identity
of the decomposition processes to which they have all been subjected. The
failure to detect the lithological relations of these rocks arose principally
from a confusion between green hornblende and the green and dichroitic,
but uniaxial, minerals grouped under the term chlorite; but a neglect to
give due weight to evidences of pseudomorphism, partial devitrification
and other phenomena of decomposition, materially aided in obscuring the
true nature of the supposed rock-species.
Causes of error. — It appcars to mc by no means superfluous to consider how
so keen an observer as Baron v. Richthofen came to regard propylite in the
Washoe District as an independent rock-species, and as a volcanic of Ter-
tiary age; and while I have no authority for my suggestions, I offer the fol-
lowing explanation.^ Baron v. Richthofen regarded Mount Davidson as
syenite and the visible plagioclases as accessory. The rock does indeed
more nearly resemble ordinary syenite in its general appearance than ordi-
nary diorite, and the error was never detected until Professor Zirkel exam-
ined it microscopically. In the porphyritic diorites v. Richthofen saw a
plagioclase rock, but the triclinic character of the feldspars in the porphyry
aroused no known doubt in his mind as to those of the mass of Mount
Davidson. Porphyritic syenites are very rare, while the relation of the
' It would be superflnona to remind geologists that in 1865 the science of microscopical lithology
■was undeveloped.
PEOPYLITE. 89
diorite-porphyries of Washoe to the granitoid diorite is peculiar. Any
but a very thorough inspection would lead to the belief that the porphyries
are younger than the granular diorite. v. Richthofen had reason to suppose
that Mount Davidson was post-Jurassic, and the plagioclase porphyries were
therefore in his eyes younger than that period, and older than the andesites
which cap the range. As I have endeavored to show, the dioritic porphy-
ries, when in a certain stage of decomposition, are scarcely distinguishable
from the thoroughly crystalline andesites, when the latter are also somewhat
decomposed. These v. Richthofen found at considerable distances from his
"syenite," and so associated with Tertiary rocks as to prove them members
of that series. Tertiary leaves were also discovered in similar rock at no
great distance. These wackenitic andesites, too, stood in such a relation to
the fresher rocks that they appeared to precede them, and the chain of proof
seemed complete of a pre-andesitic Tertiary rock. The extension of the
propylite to the mines was natural and easy.
If propylite were older than andesite, where should we look for it but
in depth? And if there was no distinct lithological reason assignable for
pronouncing the underground rock, mostly in the last stages of decomposi-
tion, identical with that on the surface, there was next to no reason, macro-
scopically speaking, for supposing it different. This mistake having once
been committed, I do not believe it could ever have been corrected in ojopo-
sition to even a far less weighty authority than Baron v. Richthofen, had
not fresher rocks been opened up by the extensive lower workings, and had
the microscope not been sought as an auxiliary. That an association
between propylites and mineral veins should have been observed is natural,
for in mineralized districts we expect general decomposition.
Propylites from other di.tricts. — By the courtcsy of the gcologists of the Fortieth
Parallel Survey, I have been permitted to examine the specimens and slides
from all the localities laid down in their publications as propylite. Captain
Button, too, has kindly furnished me with specimens and slides from his
propylite localities in Utah. Rocks which are indeterminable in the field
are very apt to give uncertain evidence under the microscope, and as all
propylites are decomposed, I do not feel absolute confidence in my deter-
minations of the propylites occurring outside of the district which forms the
90 GEOLOGY OF THE COMSTOOK LODE.
subject of this paper. Nevertheless, I have given my notes upon them in
the section containing the "Detailed description of slides." There appear
to be fairly good grounds for the determinations there suggested, and the
specimens seem to offer no evidence even approximately suj0ficient for the
establishment of a new rock-species.
- No propyiite yet found in the United States. — Thc term propylite might bc retained
to express a certain macroscopical appearance and certain chemical changes,
just as we still speak of serpentine without denying its secondary character.
But a better name, and an older one, already exists for this very thing, for
the terms greenstone and greenstone-trachyte designate rocks in every way
similar. Considered as its originator intended it, as a pre-andesitic Tertiary
rock, I feel no hesitation in asserting that nothing answering to its defini-
tion has as yet been proved to exist in the United States.^
' European propylites. — The investigation of American propylites described in this report was Carried
out entirely without reference to the oj^inion of European lithologists regarding the Transylvanian
rocks. American geologists who have not followed the subject closely may be interested to learn, how-
ever, that the tendency of opinion in Europe is strongly against the independence of this rock-species.
Dr. C. Doelter upholds it in a paper " Ueber das Vorkommen des Propylits in Siebenbiirgen. Verhandl.
der k. k. Geolog. Keichsanstalt," 1875, p. 27. In reviewing this paper in the "Neues Jahrbuch fiir Min-
eralogie," etc., 187i), p. 648, Professor Rosenbusch incidentally considers Baron v. Richthofen's descrip-
tion and Professor Zirkel's views, and states his own conclusions as follows (translated) :
" The reviewer, in common with all other investigators, willingly recognizes the peculiar green-
stone habitus of the so-called propylites ; their Tertiary age, which in many cases must be further and
more sharply determined, being assumed. Since similar changes in habitus occur in many other series
of rocks, however, he does not feel himself compelled to accord propylite an independent position, but
rather to regard it as a mere pathological variety of qnartzose or quartzless hornblende-andesites, or of
the augite-andesites, as the case may be."
Professor vom Rath has published a paper in the " Sitzungsberichte der Neiderrheinischen Gesell-
schaft in Bonn," vol. 35, 1878, p. 26, in which he expresses a very positive opinion that the so-called
propylite of Scheranitz is diabase, and has no relation to the andesites of the neighborhood. He assorts
that this diabase has a very different look macroscopically and microscopically from andesites, but it is
to be regretted that he does not give the dififerences in sufficient detail to enable readers to judge for
themselves. Prof. J. Szab6 has read a paper before the Hungarian Geological Society, which is
reported in the Verhandluugen der k. k. Geolog. Reichsanstalt, 1879, Literatumotizen, p. 17. In this
paper Professor Szab6 maintains that various eruptive rocks and even sedimentaries have been altered
to what is called greenstone by solfataric action at Schemnitz, and he concludes with the following
statement (translated) : " There is no greenstone-trachyte formation proper in a geological sense ; there
has never been an independent propylite eruption." I infer that the conditions in Schemnitz are sub-
stantially similar to those in Washoe.
DETAILED DESCRIPTION OF SLIDES. 91
Section 4. (Chapter III.)
DETAILED DESCEIPTION OF SLIDES.
Reasons for this section. — In view of the considei'able alterations proposed in
the classification of the Washoe rocks, it appears proper to submit detailed
descriptions of a sufficient number of slides to enable lithologists to judge
whether the methods employed in the determinations are correct and the
grounds upon which distinctions have been drawn sufficient. Nearly but
not quite all the statements made in the foregoing sections of this chapter
concerning the microscopical character of the rocks may be substantiated
from these slides. It was considered that further descriptions were need-
less and would be burdensome.
Determination of feldspar. — The feldspars have becH determined optically ac-
cording to the rules laid down by Messrs. Fouqud and Levy.^ This method
is very tedious, and is, properly speaking, applicable only to the determina-
tion of the most basic feldspar present; but by applying it to a great num-
ber of cases the microscopist is able to satisfy himself of the prevailing
feldspars as well, and in this respect it appears to me more satisfactory than
the determination of isolated feldspar fragments by their specific gravity.
In two cases M. Thoulet's method has been employed. Professor Szab6's
method has not been attempted.^
An explanation of the method of reference to the slides by a system of
coordinates in millimeters, referred to the upper left-hand corner of the glass,
will be found in the description of the lithological illustrations, page 145.
GRANITE.
Slide 460. Close to Red Jacket mine.
Typical granite. — This is a moderately fine-grained gray micaceous granite.
The slide shows besides orthoclase, quartz, and mica, a few plagioclases,
' Miueralogie Micrographique, 1879. So far as I know this method was first suggested by Prof.
R. PmiipoUy, Proc. Amer. Acad., Vol. XIII., p. 258.
^ Tests by this method, subsequently made, are described on p. 40.'5, et seq.
92 GEOLOGY OF THE COMSTOGK LODE.
magnetite, and some accessory minerals. The structure is typically granit-
oid, none of the principal minerals showing either perfect crystalhne out-
lines or microlitic development. The orthoclase is for the most part trans-
parent, and in many cases shows good cleavages, which are usually parallel to
the extinctions. The plagioclases show very narrow stripes and no angles of
extinction exceeding those of oligoclase. The quartz contains abundant liquid
inclusions, many with moving bubbles. The mica shows the interference
figure of biotite, and is of course brown and highly dichroitic. A portion
of the biotite appears "bleached" to a lighter brown, and other fragments
are converted into chlorite. A few particles of epidote are visible, forming
from the chlorite. The iron ore is evidently magnetite, occurring mostly in
quadrangular forms, and being accompanied by hematite. There is also a
considerable amount of titanite, which in some cases takes the form of per-
fect rhombs, with an angle of somewhat less than 140°. It shows the
cleavages, the rough surface, high refraction, and dull colors between crossed
Nicols, appropriate to sphene. There are many minute zircons, and some
ordinary apatites. The slide contains two patches of a somewhat highly
refracting, nearly colorless, slightly yellowish, mineral, one of which seems
to be of an imperfect hexagonal outline, and the other nearly square. They
show a rippled surface, such as is often seen on augite. They remain
dark between crossed Nicols, and give no interference figure. The mineral
shows cracks, some of which are irregular; others seem referable to an im-
perfect rhombohedral cleavage. All these properties suggest sodalite. This
mineral, however, has been noticed, I believe, among the older massive
rocks only in syenite,^ and in combination with elseolite and zircon.* As
zircon is plentiful in this slide, I carefully looked for elseolite. If present
at all it must be in granitoid crystals, which might be mistaken for ortho-
clase. Many such are cut nearly at right angles to an optical axis, but I
failed to find one such which gave the interference figure of a uniaxial min-
eral.
1 As the name is now usually nnderstood. In Dana's Mineralogy the quartzless mica-orthoolase
rocks are stUl termed granite.
DETAILED DESCRIPTION OF SLIDES. 93
GRANULAR DIORITE.
Slide 213. Bullion Eavine, at Water Company's flume.
Typical diorite with green fibrous hornblende. Thls IS the tjpical dioHte Of MoUIlt
Davidson. Macroscopically, it is gi'ay in color and granitic in structure.
The sUde shows that it is composed of a mass of crystalline grains, filling
the whole space and without the most distant approach to a porphyritic
structure. It contains triclinic feldspar, fibrous hornblende, quartz, magnet-
ite, a few fragments of mica, and a number of accessory minerals. The
hornblende is present only in fibrous crystalline masses and patches, which
seem to have crystallized after the feldspar. Many of the masses of horn-
blende are cut at right angles to the main axis, and show excellent cleavages
at the characteristic angles. It is strongly dichroitic, giving tints varying
from buflf to sea-green. It polarizes with great brilliancy, showing the
whole range of prismatic colors. The angles of extinction observed reached
20°. lu parts of the slide the hornblende is decomposed, the products
being chlorite, epidote, quartz, and calcite. The disposition of the original
mineral is so irregular that the process of decomposition cannot be studied
to advantage.
The feldspars seem to be without exception polysynthetic plagioclases.
The twin striations are irregular in width, but very continuous and sharply,
defined. The angles of extinction of the twins, which extinguish light at
equal inclinations to the plane of the Nicols, are large. Very many such
were observed to exceed 20°, and one or two reach 29°. The feldspar is
therefore in the main labradorite, and I saw no indications of the presence
of any other feldspar species. There are no untwinned feldspars or feld-
spathic microlites. Besides the twins following the law of albite, there are
many instances of additional periclinic twinning. In several crystals there
is well-developed zonal structure. The feldspars are for the most part very
free from inclusions of any kind, and are clear and transparent.
Many grains of quartz are present, but I observed no crystal faces.
The quartzes are full of fluid inclusions, some of them dihexahedral. One
of these is so large that the movement of the bubble can be cleai-ly seen
with a magnifying power of 60 diameters. The bubbles of these inclusions
94 GEOLOGY OF THE COMSTOCK LODE.
do not disappear upon heating the shde to 40° C on Vogelsang's table, and
are therefore probably aqueous.
There is a considerable quantity of magnetite in this slide, characterized
by its square outlines and opacity. I observed no titanic iron. A few
crystals of apatite appear under the microscope, rather fewer than is usual
in the rocks of the Disteigt. They are colorless, and contain no determinable
inclusions. There are many minute zircons recognizable by their high refrac-
tion, brilliant polarization, and by their crystal form (the eight-sided prism,
terminated by the fundamental pyramid). One or two fragments of mica
appear in the slide — e. ^., at 21-21. -There are also a number of irregular
fragments of a mineral which can scarcely be anything but titanite. It
shows an uneven surface, brown color, perceptible dichroism, and high
refractive index. In polarized light it is only feebly chromatic. Plate IV.,
Fig. 25, shows a characteristic portion of this slide.
Slide 413. Union Shaft, 2,625 feet from surface.
Dark dioritc with some brown hornblende. — Macroscoplcally this is a vcry dark rock,
highly charged with scales of hornblende. It reminds one of freshly fract-
ured "No. 1" pig iron. Under the microscope it is seen to be composed
essentially of triclinic feldspar and hornblende, both minerals having con-
solidated nearly at the same time. A few grains of quartz, and an insig-
nificant amount of colorless apatite, complete the list of components.
The hornblende is in part of a brown tint, very slightly tinged with
green; in part it is of a light and vivid blue-green color. Many of the
hornblende crystals show both colors ; the green variety occurring along
the edges and cleavages, and sometimes leaving only small irregular patches
of the brown mineral surrounded by the green. The structure of the two
varieties is distinctly different. The brown mineral shows excellent cleav-
ages, but no tendency to fibration. In the green portions of the same indi-
viduals the hornblende seems to be composed of minute fibers, but the
tesselated appearance of the cross-sections is nearly obliterated. In fact all
the appearances are such as accompany a distinct alteration in mineral char-
acter. The brown hornblende is as usual very strongly dichroitic; the
DETAILED DESCKIPTION OF SLIDES. 95
green is less so. On the other hand, the green mineral polarizes in colors
of the utmost brilliancy, like those of the preceding slide.
The hornblendes contain a vast number of included microlites of a
black, wholly opaque mineral, crystallizing in needles and long pointed
scales, which can scarcely be anything but ilmenite or hematite. These
microlites are arranged in certain planes of the hornblende crystals, viz:
perpendicular to, and parallel to the base. In sections nearly parallel to the
vertical axis no further regularity is perceptible, but cross-sections show
that they are also parallel to the prismatic faces and to the clinopinacoid
The distances from these faces are wholly irregular, and the effect is there-
fore merely that the microlites form with one another angles of nearly 60°.
It is noteworthy that just the-faces most usually found in microscopic horn-
blendes are the ones emphasized by the position of these minute bodies.
The same microlites also occur in the feldspars, in which, too, their distri-
bution seems to be governed in part by some crystallographic law, but what
one is not evident from this slide. These microlites are, for the most part,
entirely unaltered in the brown hornblende, while in the green they are
replaced in part by very fine transparent yellowish crystalline grains. In
some places the black and the transparent inclusions are continuous with
one another, and everywhere the disposition of the latter is j^recisely that
of the former. In fact a narrow inspection does not leave a doubt that the
opaque microlites are decomposed into a transparent mineral. The minute
size of the grains found does not permit of absolute determination ; but the
product of decomposition is doubly refracting, possesses a high index of
refraction, is slightly dichroitic, and seems to polarize in rather feeble colors.
The only familiar minerals which it recalls are titanite and epidote, and the
probabilities are that it is sphene.
In one portion of the slide is a mass of a nearly colorless substance,
slightly tinged with green, which seems to be totally isotropic. Under
crossed Nicols it remains absolutely dark, and when the quartz plate is
introduced, and the Nicols are adjusted to the teinte sensible, no change
whatever in the shade is perceptible on revolving the slide This is one of
the substances grouped under the term "chloritic constituents," but it does
not appear to be certainly identical with the ordinary product of the decom-
96 GEOLOGY Ol^^ THE COMSTOGK LODE.
position, of hornblende. Embedded 'v. it are numerous small grains and
microlites, which extinguish light at a large angle to the plane of the Nicols.
They are arranged at angles of about 60°, and it appears to me that the
object must be supposed to be a decomposed hornblende, filled with micro-
lites of the mineral which results from the decomposition of the black
microlites. This opinion is strengthened by the occurrence of a number of
intermediate stages, as they seem to be, between fresh hornblende and the
last mentioned chloritic mass. As the black microlites alter, the hornblendes
become in some cases grayish and less and less pellucid, not apparently
from want of transparency on the part of the minerals, but through irregular
refraction of light.
The feldspar is undoubtedly for the most part labradorite, many of the.
finely twinned crystals showing angles of extinction of nearly 30° on each
side of the twinning jjlane. I see no evidence of the presence of any other
feldspar. There are a few grains of quartz, which contain some liquid in-
clusions. The apatites are few in number, colorless, and in no way remark-
able. I detected no other minerals in the slide.
Slide 81. Utah, 1,950.
Gray diorite with brown hornblende. — This is the freshest dioHte iu the coUectiou,
the feldspar being as transparent as it ordinarily is in andesite. Unfortu-
nately the slide is not thin. The principal difi"erence between this and
slide 413 is that the majority of the hornblendes are brown, many of them
without a tinge of green.
«
Slide 361. Savage 1,300. North drift, about 310 feet in.
Micaceous granular diorite. — lu this rock, whicli is uot a porphyrite, but gran-
ular, the hornblende has been almost wholly replaced by biotite, which is
of the usual structure, and gives an interference figure nearly like that of
a uniaxial mineral. The slide contains much quartz and many beautifully
sharp zircons. In other respects it is similar to slide 213.
Slide 291. Ghollar 1,700 ; 1,425 feet west of Combination shaft.
Diorite containing tourmaline, etc. — This is a diorito of the grauulsr crystalHue
type, but of a very quartzose variety. The quartzes contain innumerable
DETAILED DBSCEIPTIOK OF SLIDES. 97
fluid inclusions, many of them of unusually large size. Some are dihexahe-
dral in shape ; the bubbles of the smaller ones are active, and some contain
excellent salt cubes. The proportion of salt to water seems to be very high;
for on heating the slide to about 70° C, the only effect produced was to
round the edges of the cubes. The bubbles did not grow perceptibly
smaller at this temperature.
The slide is further remarkable for containing what appears to be
tourmaline. One small patch dichroizes between black and clear brown.
The mineral exhibits scarcely any structure, but there are traces of what
appear to be cleavage cracks parallel to the direction of extinction. No
distinct interference figtire could be obtained. The lack of structure and
the absolute extinction of the ordinary ray seem to separate this substance
from hornblende ; to mica it bears no resemblance.
POEPHYKITIO DIOEITE.
Slide 421. Center of Cedar Hill Ridge.
Fresh porphyry. — Thc mass of porphyritc forming Cedar Hill is very uneven
in composition, and, for the most part, greatly decomposed. Near the high--
est portion, however, is a small quantity of a comparatively fine-grained
variety, which, from one of the accidents so common in regions of decom-
position, has escaped nearly unaltered. Macroscopically it is a dark, leaden-
gray rock, rather fine in texture, and exhibiting porphyritical crystals of
feldspar and hornblende. Under the microscope it is seen that these min-
erals are separated out in a groundmass of tolerably fresh feldspar micro-
lites, and magnetite, to which the dark color of the rock is due. Numerous
colorless apatites form the only other prominent mineral ingredient. The
hornblendes are almost wholly undecomposed. They are of a slightly
greenish-brown color and fairly well-crystallized. Most of this mineral
occurs in crystals of large size, but there are a few minute crystals and
crystalline fragments interspersed through the groundmass. The horn-
blende is dense, though in many cases the cleavages are well developed,
and one crystal even contains fluid inclusions (10-24 J). There is no tend-
7 c L
98 GEOLOGY OF THE COMSTOCK LODE.
ency to zonal structure in this slide, but several of the hornblendes are
twinned according to the ordinary law. Decomposition has set in to a
slight extent ; and in one or two cases the degeneration into chlorite may
be observed starting from the cleavage fissures of the parent mineral. Where
the masses of chlorite have reached any considerable size, particles of epidote
have developed near their centers. In a large proportion of the hornblendes
occur inclusions of the same kind mentioned under slide 413. A group of
these microlites is shown in Fig. 21, Plate III. Their disposition is the same
as in slide 413, but this section contains nothing which throws light on their
natiire.
There are numerous good-sized but rounded plagioclases in this slide.
Those which show an approximately equal angle of extinction on each side
of the twinning plane, give angles of extinction which, in some cases, con-
siderably exceed 20°; no untwinned microlites were observed, and the
feldspar is probably labradorite. The feldspars contain a few fluid inclusions
of apparently primitive character, and are pierced by numerous apatite
needles. One or two fragments of hornblende are inclosed in feldspars,
but for the most part the feldspars are wholly free from that mineral.
The groundmass consists mainly of feldspar microlites and granules, and
traces of fluidal structure are perceptible. An abundance of magnetite is
recognizable as such from its crystal form ; and associated with and pene-
trating it are many colorless apatites. The slide also contains one poorly
developed zircon. There is further a small amount of chlorite and epidote.
Most of the former is concentrated in an excellent vein.
Except in the matter of inclusions, this rock bears a strong resemblance
to an andesite ; its groundmass, however, is less microlitic and the porphyritic
feldspars have not the sharp development almost invariably observable in
andesites. Its occurrence as a mass little more than a foot cube, embedded
in porphyritic diorites of an ordinary variety, forbids the supposition that it
is a volcanic rock. A portion of the slide is shown in Fig. 26, Plate IV.
Slide 278. Ophir Eavine, south side.
A second fresh porphyry. — This rock strongly rescmblos 421 in most respects,
but the hornblendes are noteworthy. They are unusually solid, often show-
DETAILED DESCEIPTION OF SLIDES. 99
ing scarcely a trace of cleavage. Indications of zonal structure are visible;
i. e., the exterior layer of the mineral exhibits a somewhat different texture
from the remaining mass. The polarization of these hornblendes is remark-
ably brilliant, quite equalling that of ordinary augite. Many of the crys-
tals are twinned, one of them (14-23) being polysynthetic. A crystal of
considerable size is divided into halves of identical orientation by a narrow
layer of the mineral in a reversed position.
In one part of the &lide (13-27) are some minute scales of epidote
which appear to represent the clinopinacoid, limited by the base, the ortho-
pinacoid, and the positive hemidome. The direction of extinction is sensibly
perpendicular to the orthopinacoid. The same form of epidote is found in
other slides, e. g., in 371 at 17^-19.
Slide 252. Sierra Nevada, 1450. North drift 289 feet north.
Partially decomposed dioritic porphyry. — This is a grayish-grecu granitic-lookiug
rock, with brilliant hornblendes, and only a slight apparent tendency to
porphyritic structure. Under the microscope, however, it is seen to belong
among the porphyritic diorites. The feldspars are almost opaque, and it is
with some difficulty that they can be made out to be triclinic. The ground-
mass was evidently granular when fresh. There appears to have been a
little mica, now converted to chlorite and epidote. The hornblendes are
unusuall}^ interesting because present in all stages of decomposition. The
fresher ones are bright brown, without black borders, and solid except for
the well-marked cleavages. Other crystals seem to have undergone a spe-
cies of fibration in the direction of the cleavages. This fibration is accom-
panied by the presence of decomposition products, and each small elongated
cleavage prism seems coated with secondary minerals. Other hornblendes
are partially converted into chlorite, and a fine example is illustrated in Plate
II., Fig. 1 . Still others have passed completely into epidote. In some of
the partially decomposed hornblende crystals there are small crystals of
pyrite.
Slide 194. McKibben Tunnel, 480 feet from entrance.
Decomposed dioritic porphyry. — In hand spccimcns this rock is greenish-gray,
and somewhat porphyritic. Under the microscope it is seen to be greatly
100 GEOLOGY OP THE COMSTOCK LODE.
decomposed, but not in such a manner as to obscure its original constitu-
tion. When fresh it consisted essentially of well-developed crystals of tri-
clinic feldspar and hornblende, disposed porphyritically in a groundmass
mainly composed of feldspathic grains. A little mica, a small amount of
black ore (probably magnetite), and numerous colorless crystals of apatite,
were subordinate mineral ingredients.
No undecomposed hornblende now remains. It has been replaced by
chloritic material, epidote, quartz, and calcspar, but in such a way as to
leave the larger portion of the hornblende crystal outlines undisturbed.
All, or nearly all, the hornblendes seem to have been crystals of consid-
erable size and sharp definition, and there is nothing to indicate that they
possessed a fibrous structure. Some of the hornblende crystal outlines
are completely filled with the chlorite. This substance sometimes shows
an excessively fine, fibrous, imperfectly spherolitic structure. In other
cases the fibers near the peripheries of former hornblendes are arranged at
right angles to the crystal face. These fibers are of nearly equal length,
and they form a zone just within the crystal section. The chlorite is grass-
green, and very slightly dichroitic, varying between more and less yellow-
ish green shades. Between crossed Nicols it behaves almost like an isotropic
substance and shows, besides black, only dark purple tints. The chlorite
is not confined to the hornblende sections, but is difPused through the rock
in veins and patches. It also occurs in narrow borders about magnetite
and apatite, as if these minerals had mechanically obstructed its move-
ments.
The epidote occurs in a similar way both without and within the horn-
blende sections, which it sometimes wholly and sometimes only partly fills.
It is noteworthy that this mineral when it occurs in small patches is usually
finely granular, and that within certain limits, the larger the area, the
coarser the grain. When, as is often the case, the occurrences are wedge-
shaped, the granulation grows coarser from the point to the base. This
seems to indicate a more or less continuous recrystallization of the
mineral.
The relations of the chlorite and epidote in this slide are extremely
interesting, for it affords abundant proof that the epidote has formed at the
DETAILED DESCRIPTION OF SLIDES. ' 101
expense of the chlorite. This is well illustrated in Figs. 6 and 7, Plate II.,
especially in Fig. 7, where the growth of the epidote into the chlorite is
accurately and clearly shown. It is very noticeable that, as has already
been mentioned, the chlorite at the edges of the hornblende sections fre-
quently remains undecomposed longer than the interior mass. The behav-
ior of this peculiarly arranged chlorite seems to indicate a greater density,
and consequentlj^ a greater resistance to decomposition, than is possessed by
that with spherolitic structure. In a majority of cases the decomposition
of chlorite into epidote begins toward the center of the section, but there
are many exceptions. It is probable that the veins and patches of epidote
not connected with the hornblende sections have also been formed from
chlorite, for the latter appears to be the more soluble mineral. There is
evidence too, from other slides of the same rock, that, as decomposition
proceeds, the chlorite is replaced to an increasing extent by epidote, etc.
The chlorite in this rock also decomposes into quartz, calcite, and limonite.
Whether epidote, too, undergoes the same decomposition is uncertain.
Foinaaing, as it does, masses of irregular granules and imperfect prisms, it
would be difficult to .show that it had been encroached upon in any given
case by quartz and calcite, and had not formed simultaneously with
them.
There is no augite in this rock, but a little (5-23) mica, which, like
the hornblende, has been converted into chlorite and epidote. The feld-
spars still show twin striations, but are considerably decomposed, and under
high powers the mass is seen to be porous or even spongy. Particles of
chlorite, epidote, quartz, and calcite are disseminated through the feldspars.
In some of the freshest portions fluid inclusions may be detected. The apa-
tites are all colorless, and sharply crystallized. Fig. 18, Plate III., shows a
curious case, in which an intrusive bay of groundmass has reduced an apa-
tite section to the form of a horseshoe. There is a considerable amount of
pyrite in this rock (which occurs near ore), but only a trifling amount of
magnetite. The groundmass shows gray, semi-opaque markings, not dis-
similar to stippling. This appearance is caused in part by particles of cal-
cite, etc., but close examination shows that it is largely due to the spongy
structure mentioned above. >
102 GEOLOGY OP THE COMSTOCK LODE.
Slide 197. McKibben Tunnel, 488 feet from entrance.
Decomposed dioritic porphyry. — This sHdc is from the SRiue bodj of porphyrite
as 194, which it greatly resemhdes. Fig. 10, Plate III., from this slide, shows
a mass of chlorite bounded by the outlines of a former hornblende. A
portion of this chloiite has been converted into a mixture of quartz and
calcite, accompanied by limonite. This pseudomorph seems to prove that
the survival of a border of chlorite at the outer edge of the hornblende sec-
tion accompanies the decomposition of chlorite into quartz, etc., as well as
the change into epidote.
Slide 199. McKibben Tunnel, 488 feet from entrance.
This slide, from the same specimen as 197, contains a fine hornblende
section completely changed into epidote. In this case the formation of
epidote appears to have started from points near the edge. It is shown in
Fig. 9, Plate III.
Slide 281. Head of Ophir Eavine.
Decomposed diorite-porphyry. — This Tock strongly rcsemblcs that from the Mc-
Kibben Tunnel both macroscopically and microscopically. It forms very
extensive croppings, different portions of which vary greatly in degree of
decomposition and appearance. Where most decomposed it is reduced to an
almost uniform dull green color, but in the freshest portions it is granular,
greenish gray in tint, displays its feldspars and altered hornblendes in
marked contrast, and, in short, betrays its dioritic character. Under the
microscope this slide shows the original constituents to have been feldspar,
well crystallized hornblende, some augite, magnetic iron, and apatite.
The hornblende has been completely decomposed, and comparatively
little chlorite remains within the hornblende sections, which are mainly filled
with epidote. A definite geometrical relation is noticeable here, as in slide
194, between the outlines of the hornblendes and the progress of the
decomposition. Many of the outlines of hornblende sections are occupied
towards the center by a mass of epidote, between which and the periphery
is a band mainly filled by quartz. Either, then, the chlorite has been decom-
posed from the center into epidote, and simultaneously from the exterior
into quartz; or the epidote, after replacing the chlorite, has been decom-
DETAILED DESCRIPTION OP SLIDES. 103
posed from the periphery of the hornblende section. The former supposi-
tion is altogether the more probable. A portion of the epidote does not
show the usual crystalline structure, but forms a mass of small grains or
scales, of which so many are superimposed upon one another in the thick-
ness of the section, as to present perfect aggregate polarization; indeed it
is difficult to detect a difference between these masses in polarized light and
natural light. The change of the edges of the hornblendes to quartz has
been accompanied by the separation of minute particles of a whitish opaque
material of unknown character, and further by the formation of black
opaque particles which can hardly be anything else than hematite or mag-
netite. These particles are arranged in lines parallel to the crystal edges,
and now surround many of the interior masses of epidote with a black
border. This is interesting as evidence that the black border of decomposing
hornblendes is sometimes a secondary formation.
The slide contains a number of augites, some of them in very well
defined octagonal cross-sections. The presence of this mineral associated
in diorites with hornblende which was in all probability dense, is unusual
and interesting Like the hornblende, the augite has been completely con-
verted into chlorite, but the change from chlorite to epidote has begun in
only one or two cases. The augite is sometimes also surrounded with a
black border. Some of the apatites are dark brown and strongly dichroitic.
In all except a single case the outer edge is much more deeply colored
than the center, but in one instance this order is reversed. Many ordinary
colorless apatites are also present. The feldspars are triclinic; little more,
however, can be said of them, for they are much decomposed, and filled with
products of decomposition. The same is true of the groundmass, in which
secondary quartz and calcite, veins and patches of chlorite, and grains of
epidote greatly obscure the original structure, but it is still apparent that
it was granular and not microlitic.
Slide 233. Head of Ophir Eavine.
This slide is from the same locality and the same cropping as 281,
but from another specimen. In addition to the principal features of that
slide, it shows unmistakable mica sections, which have undergone precisely
104 - GEOLOGY OF THE GOMSTOCK LODE.
the same changes as the hornblende and augite described under 194 and
281. As would naturally be supposed, the change to epidote begins along
cleavage lines. The change is illustrated in Fig. 8, Plate II.
Slides 482, 485, 486. East-aod-west dike, just south of Eldorado croppings.
Dikeofdioritc-porphyry. — At tWs polnt & dlkc of porphjritic diorite about six
feet wide cuts the granular mass of Mount Davidson. Towards the center
the rock is fine-grained but evidently crystalline, with small porphyritic
crystals of feldspar and hornblende. For about an inch from the edge the
rock is very dark and crypto-crystalline. The contact with the granular
diorite is an absolutely sharp mathematical line, and the adhesion is vory
strong. Under the microscope the gray including rock is precisely such
as is described under sHde 213. The adjacent dark rock is manifestly
the same as 421. It is very andesitic in appearance, showing a microlitic
groundmass, with excellent flow structure, and solid brown hornblendes with-
out black borders. It also contains a few green fibrous hornblendes, and a
good deal of augite. Even within the limits of the slide, however, it is ap-
parent that the structure of the groundmass is more granular as the distance
from the contact increases. In slide 486, from the center of the dike, almost
the whole of the hornblende is fibrous, the structure is granular, and the
impression is simply that of an ordinary granular diorite with a few por-
phyritical crystals of feldspar. But a few of the hornblendes are partly
brown and solid, and these portions pass into and are surrounded by green
fibrous hornblende of the same crystallographic orientation.
MICACEOUS DIORITE-PORPHYRY.
Slide 101. 1,000 feet northeast of Silver Hill mine.
Typical example. — THs is a gray-grccu porphyry, in which crystals of feld-
spar of a very uniform size, about half as large as a grain of wheat, and
smaller crystals of mica and hornblende, are evenly distributed in a crypto-
crystalline groundmass. Under the microscope apatite, titanic iron, and
zii'con also make theii* appearance.
DETAILED DESCRIPTION OP SLIDES. 105
The mica is in part decomposed into quartz and epidote. One scale,
18-28, happens to be so exactly in the plane of the slide as to show no trace
of dichroism. This scale gives an almost absolutely constant interference
cross, and is optically negative. It is therefore biotite. The hornblendes,
which are much less numerous than the micas, are wholly decomposed to
chlorite and epidote.
The large feldspars are all striated. Several of them are cut in the
zone at right angles to ooPdb and show lamellae extinguishing at equal
angles on each side of the twinning plane. These angles correspond to
labradorite. One feldspar, which shows both albitic and periclinic twin-
ning, gives angles of extinction which differ by 75° in two successive
lamellae, but the angle on one side of the twinning plane is 8° larger
than that on the other. The crystal is cut in the zone oo Poo and oo Pdo ,
and of this zone so little is known that the crystal cannot be pronounced
anorthite. One of the feldspars contains a fluid inclusion with an active
bubble. The grains of feldspar in the groundmass are not well preserved,
but almost all those in which the angle of extinction is determinable trans-
mit least light when the twinning plane is parallel to the plane of the Nicols.
It seems probable, therefore, that they are oligoclase.
The groundmass is composed chiefly of partially decomposed feldspar
microlites and much secondary quartz, with some calcite. There is a con-
siderable amount of titanic iron in characteristic forms, accompanied by
much leucoxene. This decomposition product has the familiar want of struc-
ture close to the undecomposed ilmenite, but the edges of the patches show
a granular crystalline arrangement, as if the smaller particles gradually
united into comparatively large ones. The same appearance is often visible
in epidote. There are further many colorless apatites, and an unusual
quantity of zircons, which draw attention by their relief, and the brilliant
colors which they exhibit between crossed Nicols.
Slide 172. Sutro Tunnel, 20,424 to 20,434 feet from entrance.
This is a mica-diorite entirely similar to slide 101, except that it con-
tains large quartzes, in which are sinuous bays of groundmass. These
quartzes contain fluid inclusions with active bubbles.
106 GEOLOGY OF THE COMSTOOK LODE.
METAMORPHIC DIORITE.
Slide 295. Amazon dump.
Typical basaltic variety. — This Tock is of a Very dark iron-gray color, and is
full of bright scaly particles of bisilicates. It is intensely hard and tough.
Under the microscope it is seen to be composed chiefly of hornblende and
feldspar, but the former is present in great excess, and the feldspar is so full
of hornblendic microlites as scarcely to be recognizable. Mica, "chlorite,
and epidote are also present in considerable quantities.
The hornblende is of two varieties, green and colorless. The colorless
hornblende is wholly undecomposed, shows capitally marked prismatic and
clinopinacoidal cleavages. It absorbs light very faintly, but polarizes in
brilliant green and purple colors, like augite. Sections parallel to the ver-
tical axis show angles of extinction reaching 27°. The green hornblende
shows an equally high angle of extinction. It dichroizes strongly between
a bright, very slightly brownish, yellow and a dark grass-green. It is often
fibrous, and is frequently accompanied by decomposition products. The
two species of hornblende stand in the closest relations to one another. In
all cases the colorless variety is surrounded by the green ; in cross-sections
the white modification appears in polygonal spots in the green ; in the longi-
tudinal sections in irregular stripes. Where they occur together in this
way the optical orientation of the two is in all cases identical. In fact, the
relations are just such as would result from an alteration of the white into
green hornblende, and taking into consideration the fact that the green
variety alone appears to suff"er decomposition into any other mineral, I can-
not avoid the conclusion that the case is really one of alteration. The
association of colorless and green hornblende is illustrated in Figs. 11 and
12, Plate II. All the microlites of hornblende, which are present in great
quantities, are green. These microlites are so numerous in the feldspars
that the striations are only just perceptible, and the species cannot be satis-
factorily determined; indeed, hornblende microlites form the greater part
of the rock.
A considerable quantity of fibrous chlorite occurs between the micro-
DETAILED DESCRIPTION OF SLIDES. 107
Htes of green hornblende; it is strongly dichroitic, and extinguishes light
parallel to the fibers. There is also much epidote present in compara-
tively large crystalline masses. The dichroism, high colors of polariza-
tion, and the angles of extinction referred to the cleavages, leave no doubt
as to the mineral species. A few quartz grains are scattered through the
mass. The slide contains many minute scales of brown mica, but no well-
developed crystals. Its quantity is insignificant as compared with that of
the hornblende.
The iron oi'e is very characteristic ilmenite, occurring in groups of par-
ticles which look as if they had been produced by chopping a larger mass,
and is accompanied by a little leucoxene.
Slide 429. 3,000 feet southeast of Basalt Hill.
Granitoid variety. — This is a piukish-gray rock of granitoid structure, with
many lath-like feldspars, and a somewhat waxy look. In fact, its general
appearance resembles that of many diabases, but close inspection with the
unaided eye discloses small crystals of hornblende and mica. Under the
microscope quartz grains and some subsidiary minerals are added to the list_
The hornblende is for the most part green and fibrous, a few patches
showing a tendency to brown shades. It is all partially decomposed, and
is far inferior to the feldspar in quantity, and has evidently crystallized later.
Only a few flakes of mica are visible. The feldspar is for the most part
polysynthetic, and the lamellae are excessively thin. The angles of extinc-
tion of the sections cvit at right angles to the twinning plane indicate oligo-
clase as the species. There are no microlites of feldspar so developed as to
justify inferences concerning the species. A large part of the interstices
between the crystals are filled with quartz grains, which are evidently not
of secondary origin. They contain exceedingl}' minute fluid inclusions.
There is a large amount of titanic iron in this slide, recognizable by
its cleavages and accompanying leucoxene. This latter mineral is intimately
associated with sphene, and indeed possibly passes over into it. Sphene
also occurs in patches independently of decomposed ilmenite. Though
determinable crystals are not visible, the characteristically irregular shape-
of the masses both as to outline and surface, the high refraction, the feebly
108 GEOLOGY OF THE COMSTOCK LODE,
chromatic tints between crossed Nicols, and in one or two instances the
cleavage, make the diagnosis fairly certain. Besides the ilmenite thei-e
appears to be a certain quantity of magnetite, which "is not improbably
titaniferous, for while the crystal forms are referable to the cube there is no
accompanying limonite. Finally, there are numerous well-crystallized
zircons and a few ordinary apatites.
Slide 293. 700 feet southwest of Devil's Gate.
Intermediate variety. — This rock is intermediate in character between slides
295 and 429. It is crowded with green hornblende microlites, but not to
such an extent as to conceal the feldspar, which shows the angles of extinc-
tion proper to oligoclase. It also contains much quartz and ilmenite, as well
as many apatites and zircons.
This slide is chiefly remarkable for the presence of tourmaline. It
occurs in grains and in imperfect prisms. These extinguish light parallel
to their principal axis. They are very highly dichroitic, showing a clear
brown color when pai-allel to the main axis of the polarizer, and an almost
absolute black at right angles to this direction.
QUAKTZ-POEPHYRY.
Slide 354. 1,000 feet south of Lawson'8 Tunnel.
Typical variety. — Macroscopically this rock shows a purplish-gray ground-
mass in which are separated out porphyritically feldspar, mica, and quartz.
Under the microscope a few hornblendes, apatite, and iron ores also make
their appearance.
The feldspars, which in this slide are fairly fresh, occur for the most
part in irregular grains, rendering it difficult or impossible in many cases to
determine the crystallographic orientation. The larger part of the feldspars
are unstriated, and of these many are certainly orthoclase, as determined
by the angles of extinction referred to the cleavages. I was unable to find
. any unstriated feldspars which, tested in the same manner, gave angles
appropriate to either of the triclinic feldspars. There is also a considerable
DETAILED DESCRIPTION OF SLIDES. 109
amount of plagioclase present, which seems from the character of the band-
ing and the angles of extinction to be oligoclase. There is certainly less
plagioclase than unstriated feldspar. The feldspars contain fluid inclusions.
The quartz is present for the most part in macroscopical grains, which
are bounded in part by crystalline outlines and in part by curved lines.
One large mass appears to have been broken, and a narrow line of ground-
mass separates the parted edges. In many cases deep sinuous bays of
groundmass penetrate the quartz, and patches of groundmass are sometimes
surrounded by it. A considerable number of inclusions are sparsely scat-
tered through the quartz. Of these the fluid inclusions are somewhat in
excess of the glass. The glass inclusions, which all show bubbles, are rather
large, and often penetrate the slide, so as to extinguish light between crossed
Nicols. The glass is colorless ; its shape is often dihexahedral. The fluid
inclusions are smaller but very characteristic, many of them having exceed-
ingly active bubbles. None of them appear to be carbonic acid. Although
there are comparatively few inclusions in these quartzes, they are so dis-
tributed that both kinds are often in the field of a Hartnack No. 7 objective
at once.
The hornblendes are entirely decomposed to chlorite. They have a
black border, which does not appear to me to have resulted from weather-
ing of the hornblende substance. The mica too is entirely decomposed.
There is no large quantity of iron ore, and its character is somewhat
indefinite. It is present in irregular forms, but there are no sections with
the characteristic cleavages of ilmenite; neither is there any ferric oxide
accompanying the mineral. The groundmass shows traces of fluidal struct-
ure, and in places is pseudo-spherolitic. There is no base, but as the ground-
mass is impregnated with decomposition products, calcite, quartz, and minute
grains of epidote, it is probable that any glass that may have been present
would be devitrified. There are a few poor zircons and colorless apatites
in the slide. The rock is shown as it appears under the microscope in
Fig. 27, Plate IV.
Slide 304. 1,000 feet south by west of railroad tunnel above Bed Jacket.
A second example. — This Tock is uot distinguishable macroscopically from
that last described (slide 354). Microscopically it is also very similar. The
110 GEOLOGY OF THE COMSTOOK LODE.
relations of the feldspars are the same. A horizontal plate of mica shows
the interference figure of biotite. The quartzes contain good-sized inclusions
of glass and some exceedingly minute ones which appear to be liquid. The
groundmass shows a highly fluidal structure. The effect is produced by
elongated aggregations of iron ore, embedded in nearly colorless mateiial.
This colorless substance appears to be absolutely isotropic in some places,
in others it shows pseudo-spherolitic structure, but for the most part it exhibits
aggregate polarization as if it were a devitrified substance. In many places
it is full of black microlites, which seem to radiate from particles of iron
ore.
Slide 353. 1,700 feet south-southeast of the Amazon.
A third example. — This rock aud slide are entirely similar to the preceding;
the fluidal structure is less marked than in 304, but the pseudo-spherolitic
structure is well developed. The feldspar is mostly orthoclase, and the
quartzes with bays of groundmass, etc., contain some glass inclusions, and
a very few liquid ones.
Slide 351. Overman 1142, 200 feet north of Caledonia, shaft.
Specimens tested by Thouiefs method. — A gray rock entirely similar to those
already described. Under the microscope it is seen to be somewhat more
altered, the feldspars being clouded with calcite. Hornblende and mica
occur, and the groundmass shows the same fluidal and pseudo-spherolitic
structure. The quartzes contain more inclusions both of fluid and glass than
those of the surface rocks. One of them is of a very unusual character. It
is a glass inclusion in a glass inclusion, the inner one bearing a bubble. The
inner glass may differ slightly in composition, or may have solidified at a
different pressure. This cannot be a case of a cut bubble filled with balsam
and air, for if the instrument be focused on either surface of the quartz, the
inclosm-e and bubble are out of focus. The inclusion is shown in Fig. 24,
Plate III.
To test the nature of the feldspars in this rock a fragment was pulver-
ized and separated in a solution of mercuric iodide in potassic iodide of a
specific gravity of 2.65. A large portion of the rock rose to the surface.
DETAILED DESCRIPTION OF SLIDES. HI
and, on being mounted in balsam, proved to be groundmass and feldspar.
Hornblende and mica, most of the quartz, some feldspars and decomposition-
products sank.
Slide 461. West end of railroad tunnel above Red Jacket.
Feisitic variety. — Tliis is a greenisH gray, fine-grained, rhyolitic-looking rock.
Under the microscope, too, it differs in general appearance from the ordi-
nary quartz-porphyries of the District. In detail, however, it is found to
correspond with tliem. The quartzes, of which there are but few, and those
minute, carry numerous fluid inclusions, many of them with active bubbles.
One of the quartzes also carries a comparatively large glass inclusion with
a cut bubble, the hemispherical space being of course filled with balsam.
The groundmass shows traces of fluidal structure, and is pseudo-spherolitic.
in places. The feldspars are badly clouded, but a few are plagioelase, and
the remainder appear to be orthoclase. Hornblende, mica, titanite, and
ilmenite are present. In short, the rock appears to be merely a feisitic
variety of the ordinary quartz-porphyry.
Collection of the Exploration of the Fortieth Parallel. Slides 265 and 266.
40th Parallel slides. — Profossor Zirkel's description of these slides excellently
represents the phenomena, with one or two exceptions. While many of the
feldspars are clouded with decomposition products, others are nearly free
from extraneous matter. Most of these are unstriated and appear to give
the angles of extinction of orthoclase. The quartzes of both slides contain
fluid inclusions with moving bubbles, though they are neither very frequent
nor of large size. In slide 266 there are good glass inclusions in quartz,
penetrating the section and remaining dark between crossed Nicols. The
thin sections and specimens correspond entirely with those described in
this paper as quartz-porphyry.
Collection of the Exploration of the Fortieth Parallel. Slide 333.
4oth Parallel slide. — This sllde is vcry graphically described by Professor
Zirkel. It happens to be a very small one, and shows only two or three
minute quartzes, in which I have detected no inclusions. The specimen
from which it was taken, however, presents quartzes in abundance. The
112 GEOLOGY OF THE COMSTOCK LODE.
structure and mineralogical composition of this slide appear to me identical
with that of the rocks of the District described by Professor Zirkel as dacite
and by me as quartz-porphyries. The properties of the triclinic and ortho-
tomic feldspars are the same, the hornblende and mica are of the same char-
acter and of the same degree of decomposition, and the groundmass is
indistinguishable. Professor Zirkel draws special attention to the fluid
inclusions in the feldspars of this slide.
EARLIER DIABASE.
Slide 349. Sutro Tunnel, north branch, 50 feet south of Ophir.
Typical example. — This is a gray rock, which might readily be mistaken at
first glance for a diorite. On close inspection, however, a certain waxy
luster, characteristic of augitic rocks, is perceptible, as well as numerous
lath-like feldspars from !""• to 2"""- long. Under the microscope it is plain
that the rock consists of triclinic feldspar, augite, and an iron ore.
The larger feldspars are well developed; the smaller ones are granitoid
in structure, and appear to have occupied the interstices between the larger
crystals. The larger feldspars show polysynthetic twinning, according to
the albite law, the lamellae being of moderate thickness. In addition, many
of the individuals show pericline twinning, and in some cases polysynthetic
individuals are united as Carlsbad twins. The angles of extinction are all
within the limits appropriate to labradorite, and some of the macropinacoidal
sections recognizable by the shape, and by the angles of the two species of
twin lamellae, give almost exactly the theoretical maximum angle of extinc-
tion on each side of the twinning plane. Very few of the small feldspars
forming a sort of groundmass show crystalline outlines ; but almost all are
twinned, and many of them give angles of extinction indicating labradorite.
In fact, I was unable to find any evidence of the existence of any other
feldspar. There are a few fluid inclusions in the feldspars.
A considerable portion of the augite is fresh. It i& of the ordinary pale
brownish-yellow, only just perceptibly dichroitic, and in general exhibits
excellent cleavages. Some well-defined octagonal cross-sections show not
DET^ULED DESOEIPTION OP SLIDES. 113
only the prismatic cleavages, but both the pinacoidal ones. A large part
of the augites are twinned, and many of them show polysynthetic structure.
In one case in another slide, from the same region, I counted thirteen 1am-
ellse. In many cases, as is so frequent in feldspars, the lamellae do not
extend entirely through the crystal. An excellent instance is represented
in Plate III., Fig. 15. In this slide (and many similar cases have been found
in others from the Sutro Tunnel) there occurs a long, somewhat ill-defined
section of augite, showing a single cleavage parallel to the longer axis and
extinguishing at an angle of 38°, yet showing planes of twinning which
cut the direction of cleavage at an angle of 32°. At first sight this gives
the impression of a pinacoidal section, and a twin with a hitherto unob-
served face of composition. In reality it is a section at a considerable angle
to the principal axis, and cutting a prismatic face nearly parallel to the
edge 0 P, 00 P. The second system of cleavages does not appear in this
instance, because it cuts the section at a very low angle. Such sections
must occur in all augite rocks, but attract attention here on account of the
prominence of the twinning.^
A portion of the augite is converted into uralite. This product is
strongly dichroitic, light greenish-yellow in color, and of course fibrous in
texture. The crystallographic orientation is often the same over considerable
areas, and these show the angles of extinction characteristic of hornblende.
In some cross-sections, too, an excessively fine cleavage at an angle of
about 125° can be made out with high powers. The conversion into uralite
seems to have proceeded with little regularity, sometimes attacking the
augite from the outside, and sometimes along cleavages and fractures. The
direction of the fibers of uralite is not in general that of the augite cleavage,
but usually not very far from it.
The uralite is further often converted into chlorite of a darker green
color and equal dichroism. The fibers of this product extinguish parallel
'When there is reason to suppose that a section showing an oblique trace of a twinning plane
cuts one of the prism faces lying next to the clinopinacoid parallel to the edge between this face and
the base, the approximate position of the section can readily be inferred ; for if the prism angle were
90°, the tangent of the angle at which the trace of the twinning j)]ane cuts the longitudinal striations,
would be equal to the sine of the angle at which the section cuts the main axis of the crystal. As the
angle of ex P is only 87J degrees, the observed and the calculated angle will be too large, but the error
will reach a maximum of 2^ degrees only in sections at right angles to the main axis.
8 C L
114 GEOLOGY OF THE COMSTOOK LODE.
to their direction, and polarize for the most part in dark bluish tints. In
many cases the uralite seems to be attacked from innumerable points, and
the chlorite then shows a spherolitic sti'ucture. There are a few grains of
epidote in this slide, associated in a somewhat indefinite manner with the
uralite and the chlorite.
The iron ore seems to be ilmenite. It occurs in the characteristic forms
of that mineral, and is accompanied by a very little leucoxene. There are
also a very few apatites, a little quartz, which is probably secondary, and
one or two particles of sphene.
Slide 18. Sutro Tunnel. Hanging wall of Lode at Savage connection.
Fresh diabase used for experiments. — This iu hand spccimcns is a vcry black rock,
with less waxy luster than most diabases show, but with the usual lath-like
feldspars. Tlie feldspar does not differ from that in slide 349, and measure-
ments of the angles of extinction show it to be labradorite. It contains
some fluid inclusions. Most of the augite is fresh, and some crystals show
zonal structure ; a few are converted into uralite and chlorite. The ground-
mass of the rock contains many microlites of augite. There are a few
flakes of a brown, highly dichroitic mineral in this slide, which show none
of the structure of hornblende, and seem to be biotite. Its quantity is
insignificant. The iron ore is at least in part ilmenite.
This is the freshest diabase known to exist in the District, and as such
was selected for the experiments on kaolinization. Assays and a chemical
analysis of it will be given at the end of the chapter. Its appearance under
the microscope is illustrated in Plate IV., Fig. 28.
Slide 53. Sutro Tunnel, 19,200 feet from entrance.
Quartzose diabase. — Macroscoplcally thls rock entirely resembles that repre-
sented by slide 18. The slide is one of the few containing quartzes which
are unquestionably primitive. In this case the arrangement of the microlites
of iron ore round their edges, and the inclusions of groundmass, put their
character beyond question. These quartzes are remarkably full of fluid
inclusions; the smaller ones with spontaneous bubbles, which do not decrease
in size when the sHde is heated to above 40° C. The rock contains com-
paratively little fresh augite.
DETAILED DESCRIPTIO]Sr OF SLIDES. 115
Slide 346. Siitro Tunnel, south branch, 3,960 feet from fork.
Hornbiendic diabase. — Macroscopicallj this Tock looks much like those already
described, except that it contains a considerable number of clearly recog-
nizable hornblendes. Three or four of these occur in the thin section.
They are bright brown in color, and decomposition has scarcely set in.
Far more numerous are the augite sections, which, though wholly decom-
posed to uralite and chlorite, retain their characteristic outline. In some of
these the conversion of chlorite into epidote may be traced. The relations
of the porphyritical crystals to the groundmass in this slide are precisely
those met with in the ordinary diabases of the District.
Slide 396. Yellow Jacket shaft, 2,299 feet from surface.
Diabase containing epidote. — Thls Is a greonisli-gray granular rock, somewhat
unusual in color for Washoe diabase. In most cases in this District grains
of epidote may be observed under the microscope, developed in the chlorite
formed by the decomposition of the augite; but this change seems to cease
almost as soon as begun. In the more decomposed rocks the chlorite is
seen passing into calcite and quartz, while the epidote grains are replaced
by an opaque substance, which is probably iron oxide. In this slide, how-
ever, it is plain that augite has passed into uralite, this into chlorite, and
that a great part of the chlorite has been converted into epidote. All the
stages can be observed here, as in the McKibhen Tunnel diorite, and, as in
that rock, the crystals of epidote are seen eating their way into the chlorite.
Slide 134. Sierra Nevada, 1,450, north drift, 217 feet north of shaft.
Diabase containing diaiiage. — Macroscoplcally this Is a dark, fine-grained rock,
which looks more hke some of the dark diorites than it does like diabase.
Under the microscope it is seen to be composed of rather uniform grains of
plagioclase, diaiiage, and hornblende.
The plagioclase is broad-banded, contains fluid inclusions, and gives
the angles of extinction of labradorite. The hornblende is bright brown.
The diaiiage, which is much in excess of the hornblende, is dark gray and
feebly diaphanous. In general it is disposed in irregular patches between
the feldspars, but there are a few sections with the augite outline, and
116 GEOLOGY OF THE OOMSTOGK LODE.
showing close partings in a pinacoidal direction. It transmits light too
feebly to permit of exact determinations of angles of extinction, but angles
of about 30° were noted.
The occurrence of the rock is purely local, and I regard it as a mere
modification of the diabasitic rocks, and as not sufficiently independent to
be classified as gabbro.
YOUNGER DIABASE.
Slide 466. Ghollar, 1,900 foot level; 40 feet east of incline.
The only variety. — Thls is 9, bluish-black finc-graincd rock, without a trace
of porphyritic structure. Under the microscope it seems to be composed of
plagioclase, augite, and magnetite. The feldspars present lath-like forms
of nearly equal size ; they give angles of extinction corresponding to labra-
dorite, and show no distinguishable inclusions besides augite microlites. The
augite is mostly granular, and with the magnetite fills the interstices between
the feldspars. It is somewhat dichroitic.
The slide is considerably obscured by clouds of a smoky brownish sub-
stance, which possesses no visible structure and no dichroism, but shows
aggregate polarization. It is the formation of this substance which turns
fresh fractures of the black dike from the bluish color known by draughts-
men as "neutral tint" to a smoky brown after a few hours' exposure. There
is but one variety of the black dike, and it is almost impossible to distinguish
slides of this rock from different parts of the Lode. A characteristic field
of this slide is shown in Fig. 29, Plate V. A specimen of the diabase from
Orange, N. J., showed a tendency to the same alteration in color after a few
days' exposure, and a slide from it exhibits the same peculiarities.
EAKLIEE HORNBLENDE-ANDESITES.
Slide 309. Edge of plateau, northwest of Ophir Hill.
Typical rock. — This is a porphyritlc rock, in which crystals of feldspar
and hornblende are separated out in a bluish-gray groundmass. Under
DETAILED DESCRIPTION OF SLIDES. 117
the microscope a large amount of augite and some apatite and iron ore make
their appearance.
The feldspars aj^pear to be, without exception, triclinic. The large
crystals give labradorite angles, while the microlites appear to be for the
most part referable to oligoclase. The large feldspars in these andesites
very commonly show both albite and periclinic twinnings, and polysyn-
thetic individuals are frequently combined as Carlsbad twins. The feldspars,
which strangely enough, considering the fresh condition of the bisilicates,
are largely converted into calcite and quartz, contain some glass inclusions.
Hornblende is present only in large masses of somewhat irregular out-
line, surrounded by a deep black border. The substance of the hornblende
is for the most part quite fresh, and of a deep greenish-brown. It contains
minute opaque inclosures, which are probably of the same nature as those
described under slide 421. The augites are very numerous, but small.
The percentage of the two silicates cannot differ greatly, but the hornblendes
give the character to the rock. The augites are very perceptibly dichroitic,
and are often crystallographically well developed. Many of them are twinned
and some are decomposed to fibrous chlorite, which polarizes in dark bluish
colors. There is much of this mineral in the slide which has evidently
been transported, and has settled in patches in which there is a strong tend-
ency to spherolitic arrangement. The patches of chlorite are accompanied
by quartz, which usually occupies the periphery. In some, cases particles
of epidote may be seen in the chlorite.
The groundmass is made up of microlites of oligoclase, with a con-
siderable amount of augite, magnetite, and apatite. The last is almost all
of a deep brown color, and in consequence markedly dichroitic. There is
scarcely a trace of fluidal structure in this slide.
Slide 228. Knoll northwest of Combination shaft.
A second typical specimen. — This Is a purpHsh-gray rock, and in that respect
peculiar. Under the microscope it is very similar to that last described.
It shows a decided fluidal structure, but no glass base. The feldspars con-
tain good glass inclusions. Some of the hornblendes are twinned. In spite
of its purplish color this hornblende-andesite is microscopically typical of
the Washoe occurrences, and is illustrated in Fig. 30, Plate V.
118 GEOLOGY OF THE COMSTOOK LODE,
Slide 229. From the same locality as 228.
Specimen containing iimenite. — Tliis coiitains ail augltc wliich shows excelleiit
pinacoidal as well as prismatic cleavage. It also contains a few patches
of a finely granular mineral, whicli shows very feeble tints between crossed
Nicols, and might be taken for sphene. It is in reality epidote, which often
behaves in this way when finely divided.
The hornblendes in this slide are, as a rule, less decomposed than the
augites. Indeed, the hornblendes in the Washoe andesites frequently,
though by no means always, resist decomposition better than the augites,
perhaps on account of the heavy black border. Much of the chlorite formed
from the augite has further decomposed into calcite and quartz. One
pseudomorph of chlorite after augite has been attacked from within by epi-
dote, and from without by calcite.
To test the nature of the iron ore in this rock the cover of the slide
was removed, and the balsam well washed off with alcohol. Careful draw-
ings were made with the camera of certain portions of the slide, which were
then treated with strong chlorhydric acid. A drop of acid was placed upon
the area to be tested and the slide warmed over a lamp for several minutes.
The acid was then washed off with water, and the operation repeated five
times. After each treatment the slide was inspected, and the result showed
that while the black border and certain grains of iron ore were completely
soluble, others were only coated with a white film, and remained undis-
solved. The etching also brought out faint straight lines on the undissolved
grains, at an angle of approximately 60°, which seems to complete the
proof that the mineral is iimenite. I find myself unable to distinguish
under the microscope the difference in tint between magnetic and titanic
iron, which is so perceptible in the streak.
Slide 208." North Twin Peak.
Partially decomposed hornbiende-andesite. — A dark,bluish, fresh-lookingandesitc, but
in reality much more decomposed than those just described. The feldspars
contain glass inclusions, but no fluid ones were observed. They are but
slightly decomposed, showing a little calcite and a few porous streaks and
spots. They contain many yellow, rounded microlites, some of which extin-
DETAILED DESCRIPTIOlSr OF SLIDES. 119
guish light at an angle of above 30°, and are probably augite. The rest
of the augite is decomposed to chlorite, of which there are excellent pseu-
domorphs. The hoi'nblende, too, is decomposed. With a low power it
seems as if the space within the heavy black border were filled with calcite,
quartz, and magnetite ; but a No. 7 objective shows that the apparently
opaque paii;icles are in reality minute grains of a strongly refracting min-
eral, no doubt epidote. Epidote in determinable grains also occurs in the
chlorite masses. There is considerable magnetite in this slide, as well as
many colorless apatites and one or two zircons. The groundmass shows
well marked fluidal structure, but no glass base.
This is the rock described by Professor ZIrkel as from the first hill
north of Gold Hill Peak, and analyzed by Dr. Kormann.
Slide 209. Quarry 1,000 feet west of Yellow Jacket east shaft.
Considerably decomposed hornblende-andesite. A gray-greCU pOrphyritic TOCk, imme-
diately overlying and passing into ordinary bluish hornblende-andesite.
Under the microscope it appears that the bisilicates are wholly decomposed,
the hornblendes being traceable only by the black borders now filled with
quartz, calcite, and oxides. A few pseudomorphs of chlorite after augite
remain. The feldspars also are considerably attacked, and contain second-
ary fluid inclusions.
Slide 210. 500 feet north of North Twin Peak.
Much decomposed specimen. — Thls specimeu is ft'om tlic samc mass of rock
as that represented by slide 209, and resembles it, except in the fact that the
feldspars have lost their transparency. Under the microscope it is also
plain that it is the same rock in a more advanced stage of decomposition.
The feldspars are in part filled with specks of calcite ; in part the calcite
appears to have been removed by solution, and in some instances the cavi-
ties thus formed seem to have been filled with liquid, accompanied by a
bubble; or in other words the feldspars contain secondary liquid inclusions.
Secondary fluid inclusions. — I basc tlic opiuiou that thcsc inclusions are second-
ary on the following grounds: They do not occur in the fresh hornblende-
. andesite from the same locality, or in unattacked feldspars in decomposed
120 GEOLOGY OF THE COMSTOCK LODE.
andesites. They are accompanied by particles of calcite, and by cavities
which entirely resemble them in outline and general character. While
primitive fluid inclusions are either negative crystals, or more or less dis-
torted vesicles, and are bounded by smooth curves of greater or less com-
plexity, these inclusions, as a rule, show irregular edges composed of broken
lines. It is of course necessary that these inclusions should at some time
have had a connection with the minute water channels of the rock mass
through capillary fissures, but it by no means follows that these would
appear even under high powers if open, and nothing is more probable than
that they should often be closed by decomposition products. I have but
rarely observed an active bubble in inclusions of this class.
Similar inclusions have been observed in the decomposed andesites
of other localities in the District, and in the same relations to the decompo-
sition of the feldspars. While in typical instances it appears to me easy to
discriminate between primary and secondary liquid inclusions in feldspars,
cases may arise in which a confusion is possible. There is no reason why
such inclusions should not occur in the older rocks as well as in the andes-
ites, and indeed they appear to me to do so, especially in the quartz-por-
phyries. I have not alluded to them in describing slides of the older rocks,
because they are there accompanied by primitive inclusions, and it seemed
best to mention the subject in connection with a rock in which primitive
fluid inclusions are very exceptional. I have borne the matter in mind,
however, and have not used the presence of fluid inclusions as a diagnostic
point, except where their primitive character appeared certain. A secondary
inclusion from slide 210 is shown in Fig. 22, Plate III.^
Slide 311. 1,200 feet northwest of Geiger Grade Toll House.
Specimen showing disseminating hornblende. This is a light blulsh-gray, Ordluary-
looking andesite, with rather a large number of visible hornblendes. Under
the microscope it is remarkable from the fact that, besides the hornblendes
which are apparent to the unaided eye, it contains a vast number of spiculse
of the same mineral disseminated through the groundmass. In the thin sec-
' Mr. C. W. Cross, In his Studien tiber Bretonische Gesteine, Vienna, Holder, has called attention
to secondary fluid inclusions of a different origin and character.
DETAILED DESCRIPTIOIT OP SLIDES. 121
tion these hornblendes are of a light yellowish-brown color, and are not ac-
companied by black borders. Very many of the hornblendes are twinned,
and a few show zonal structure. The hornblende is strongly dichroitic and
gives angles of extinction reaching 20°. The augites are few in number
and minute; indeed, at first sight, there seems to be no augite at all.
The feldspars are all triclinic, but are considerably decomposed, and it
is not easy to determine their angles of extinction with accuracy. Most of
the large crystals, however, give angles which fall within the limits of lab-
radorite, while the microlites seem to be oligoclase; but one crystal show-
ing the two ordinary striations gives angles of almost exactly 37°. This
then must be anorthite. It is impossible to say that the other large crystals
are not so, but the probabilities are that others would have been found
exceeding the limits of labradorite had such been the case. In one of the
large feldspars fluid inclusions of the kind called secondary were observed,
and one of these contained a slowly moving bubble. Some of the feldspars
also contain partially devitrified glass inclusions.
The slide shows two or three small grains of quartz which, from the
arrangement of the particles of the surrounding groundmass, appear to be
primary. They contain liquid inclusions with moving bubbles. This is
the only case in which primitive fluid inclusions have been detected in the
Washoe andesites. The opacite is, for the most part at all events, magnetite.
A very perfect hexagonal crystal may be a section of a dodecahedron.
The apatite is colorless and without peculiarities. The groundmass polarizes
throughout, though in places only very feebly. If it ever contained any
base, the glass is now nearly or quite devitrified.
Slide 464. 1,200 feet northwest of Geiger Grade ToU House.
Coarse-grained trachytic-looking hornblende-andesite. This slidc Is frOm the SamO Crop-
ping as 311, and beyond the possibility of a doubt the same rock, but it
diff"ers greatly in appearance, being coarse-grained, gray, and more like an
ordinary trachyte than a common andesite in habitus. Under the mi-
croscope it is manifestly the same rock, though with a modification of
structure, for the groundmass is granular instead of microlitic. There are a
few grains of quartz which carry fluid inclusions. Slide 311 contains
122 GEOLOGY OF THE COMSTOCK LODE.
remarkably few augites, while in this I was unable to find a single one, in
which respect it and slide 375 form the only exceptions among the earlier
andesites of the District. The hornblende is of the same color and general
character as that in slide 311, but the crystals are fewer in number and
larger in size. The decomposition of the hornblende in this specimen is
peculiar and interesting. The first change appears to have been to chlorite
masses, of which a few are still surrounded by fresh hornblende. Some spots
of this chlorite contain bunches of epidote, evidently formed from it, but
much of the chlorite has been converted into, or has given place to, quartz.
Decomposition has set in along cracks or cleavages of the hornblende crys-
tals, producing little veins of chlorite, and the substitution of quartz for
chlorite has subsequently taken place from the hornblende walls of the veinlet
towards the central line, but has sometimes left a narrow seam of chlorite
along the middle of the vein. This is shown in Fig. 3, Plate II. A question
might be raised as to whether the quartz had not first partially filled the veins,
the chlorite representing a subsequent infiltration ; but the thoroughly fresh
condition of the hornblende walls seems to forbid such a supposition. In some
of the smaller crystals a fresh kernel of hornblende is seen surrounded by
a zone of quartz, and this again by a narrow border of epidote. Taking the
appearance just described into account, it appears to me probable that these
hornblendes were in process of conversion into chlorite from the edges, and
that an alteration to epidote had begun on the periphery, when the silicify-
ing action set in, leaving the hornblende and the epidote unaffected. The
hornblendes carry small bubble-bearing glass inclusions. The slide also
contains much decomposed mica. That mineral has been replaced by chlo-
rite and this, again, is full of patches of epidote, evidently parasitic on the
chlorite. A portion of this chlorite, as well as that derived from horn-
blende, appears to have been converted into quartz. The feldspars are
much decomposed, but are evidently triclinic.
Slide 454. Cedar Hill Caiioa, 1,500 feet due west of Water Tunnel.
Highly augitic variety. — This is a dark bluish rock, which shows a considerable
number of macroscopical hornblendes. Under the microscope the augite is
seen to predominate over the hornblende, but as it occurs in a typical horn-
DETAILED DESCEIPTIOlSr OF SLIDES. 123
blende-andesite area, has a microHtic groundmass, and shows considerable
hornblende, I have regarded the excess of augite as local. The slide is
remarkable for the fact that much of the strongly dichroitic augite is sur-
rounded by a black border quite as broad as that which ordinarily occurs
about andesitic hornblendes, though not so broad as that accompanying the
hornblendes in this specimen. This slide contains unquestionable ilmenite
with rhomboidal cleavage marked by translucent lines. One of these is
shown in Fig. 19, Plate III. O-ne of the masses of ilmenite incloses a twin
augite crystal, just as the same mineral so commonly includes apatite. The
apatites are mostly deep brown and dusty.
Slide 450. 1,000 feet east of station at jimction of Silver City Eailroad.
Specimen showing hornblende with double black border. Thls rOck Is Ouly OXpOSed by
the railroad cut for a few yards, and undoubtedly underlies the adjoining
augite-andesite. It is dark purplish gray, and contains a very large amount
of visible hornblende. The slide is chiefly remarkable for the light which
it throws on the character of the black border. The hornblendes are devel-
oped with unusual sj^mmetry, but many of the crystals have been broken, and
all the fragments are surrounded by black borders. In one case a beau-
tifully fresh, highly dichroitic, dark brown hornblende fragment shows not
only a black border but a parallel band of magnetite at some distance from
the edge — a zonal structure marked by an interior black belt. This crystal
is shown in Fig. 17, Plate III. Other of the large hornblendes in the slide
show the same phenomenon, though imperfectly; but the small crystals
have but a single border.^
The specimen contains considerable augite, arid the groundmass shows
fluidal structure, as well as the peculiar felt-like texture so common in augite-
andesites. It is possibly not a hornblende-andesite, in spite of the great
predominance of hornblende, but an augite-andesite with a local segre-
gation of hornblende. No other hornblende-andesite occurs for a long
distance, and a glance at the map will show the improbability of any con-
siderable amount of that rock being entirely covered by the limited areas
of augite-andesite.
'For some speculations on this occurrence see page 59.
124 GEOLOGY OF THE COMSTOCK LODE.
Slide 375. Outcrop at junction of Sutro and Quarry roads.
Micaceous hornbiende-andesite. — This is a somewhat trachytic-looking rock, with
very white feldspars embedded in a rough gray groundmass. Mica is also
visible, though not prominent. Under the microscope it is plainly only a
micaceous variety of the surrounding hornbiende-andesite. The feldspar is
wholly triclinic, and the large crystals give the angles of extinction of
labradorite. They are much decomposed, but contain recognizable glass
inclusions. There are also numerous secondary fluid inclusions. The mica
is decomposed, largely to chlorite and epidote. There are also hornblendes,
or rather their outlines. Much of the groundmass is devoid of microlites,
shows a feeble aggregate polarization, and is probably a partially devitri-
fied glass. I could find nothing which could be interpreted as augite.
Slide 326. Sutro Tunnel, 17,100 feet from entrance.
Specimen showing stages of decomposition. — This is a grceuish-gray rock, with por-
phyritical crystals of feldspar and hornblende. It also shows some pyrites,
and has evidently undergone considerable decomposition. Under the micro-
scope the slide shows much brown hornblende, some augite, triclinic feld-
spars, and an andesitic groundmass. The hornblendes are peculiarly inter-
esting because they exhibit the process of decomposition in all its stages.
The hornblende is brown, much of it is twinned, and none of it shows black
borders. The first step in the degeneration is the formation of chlorite, which,
of course, largely follows the cleavages. In some cases narrow, even bands
penetrate a crystal nearly from one end to the other like twin lamellae,
while in other instances irregular patches of chlorite occur in the hornblende.
In some such patches, and still better in others which are distributed through
the groundmass, and may or may not represent former crystals of horn-
blende, the formation of epidote may be followed; its prismatic microlites
are to be seen invading the chlorite, just as in the McKihhen Tunnel diorites.
Other patches of chlorite are in process of conversion into, or substitution
by, calcite. Where this change goes on in a partially decomposed crystal
of hornblende, the central portion of the area is generally occupied by
the fresh mineral and chlorite ; whereas the calcite, sometimes accompanied
by a little quartz, occupies the border of the pseudomorph. When the
DETAILED DESCRIPTION OF SLIDES. 125
substitution of calcite for chlorite begins, the conversion of hornblende
into chlorite seems to cease, and this slide shows many bright, fresh frag-
ments of hoi'nblende embedded in calcite. There is no indication that the
hornblende tends to pass directly into calcite. Were such a process going
on, we should find denticles of calcite penetrating the hornblende. Neither
do I see any reason to suppose that the epidote in this slide passes into
calcite; it appears rather to give place to clouds of dark-colored opaque
matter, which may be oxides or earthy silicates. In this and the other slides
of andesite which contain hornblende free of black borders, I see no indica-
tion that magnetite, or anything resembling magnetite, results from the
decomposition of hornblende. In the black-bordered hornblendes I have
often suspected such a change, but I see no way of proving that the particles
in question may not have formed a part of the original border. A horn-
blende in process of decomposition is shown in Fig. 2, Plate II., from this
slide. A portion of the augites are also partially converted into chlorite, and
in the pseudomorphs epidote is certainly developed parasitically. The large
feldspars are triclinic, and give angles of extinction answering to labradorite.
In one of them a bubble-bearing and only partially devitrified glass inclusion
was observed. The microlitic groundmass contains some magnetite, p5'^rite,
and ordinary apatite.
Slide 116. Crown Point Ravine.
propyiitic variety. — Tliis is a Very black, fine-grained rock, which, however,
proves, under the microscope, to derive its color from an unusual amount
of magnetite in the groundmass. The hornblendes are altered to chlorite
and epidote, and only a few sections have retained characteristic outlines.
It is evident that the chlorite preceded the epidote, and in some cases the
encroachment of the latter can be very well observed. A portion of the
chlorite has been replaced by quartz and calcite.
As I shall have occasion to refer to slides of the Fortieth Parallel Sur-
vey collection from Crown Point Ravine, and from the South Twin Peak,
it would be an unnecessary repetition to say more of my own sections from
these localities than that there is no notable difference between them and
those described by Professor Zirkel.
126 GEOLOGY OF THE COMSTOCK LODE.
AUGITE-ANDESITE.
Slide 122. Peak south of Crown Point Ravine, marked 7075.
Typical variety. — Tliis Is a blaclc, rather fine-grained, apparently crystalline
rock, with a somewhat pitchy luster. Under the microscope it is seen to be
composed of augite and triclinic feldspar, with apatite and magnetite as
accessory constituents. The feldspar is sharply angular, but there is no
special tendency in the larger crystals to elongation. The large crystals
give very high angles of extinction, many of them exceeding the labradorite
limits, and they must all therefore be regarded as anorthite. Among the
elongated microlites I noticed many which gave too high an angle for oligo-
clase, but none which exceeded the labradorite limits. The feldspars contain
partially devitrified glass inclusions and augite microlites. The greater part
of the augite is fresh. It is very light brown in color and slightly dichroitic.
It is not specially well crystallized, and shapeless masses are more abundant
than perfect sections. There is a decided tendenc}^ to the development of
only one of the prismatic cleavages, and I found no trace of pinacoidal
cleavage. There are numerous bubble-bearing glass inclusions. Manj^ of
the augites are converted in whole or in part into chlorite, of the same
properties mentioned so often in previous descriptions. There is a single
bright brown hornblende of small size heavily bordered. The apatite is in
part colorless and in part brown. The magnetite shows no peculiarities.
The groundmass is microlitic and in parts shows a felted structure.
Slide 137. Bench 400 feet southeast of intersection of Crown Point Eavine and.
Water Company's flume.
The same slightly decomposed. — This is macroscopically and microscopically the
same rock as the preceding, being merely somewhat more decomposed. The
feldspars contain secondary fluid inclusions; the augite is wholly converted
into chlorite, which for the most part retains the augitic forms ; and epidote,
quartz, and calcite are developing from the chlorite.
Slide 416. First peak above Ophir Grade, south of Crown Point Eavine.
Variety with augitic groundmass. — Tlus Is a gray porphyritic rock, with none of
the resinous look which augite rocks usually possess; and though it shows
DETAILED DESCRIPTION OF SLIDES. 127
no hornblende, it might readily be mistaken for a hornblende-andesite.
Under the microscope the slide shows little or no hornblende, but an unusual
amount of augite, which is present, not only as porphyritical crystals, but as
microlites in the groundmass in nearly the same quantity as the feldspar. A
majority of the augites are fresh, but many are deco-mposed to chlorite, which
in its turn is largely changed to epidote. The latter may be seen eating its
way into the chlorite, as it has been described in the diorites and hom-
blende-andesites. Only a single patch of chlorite suggests hornblende, and
there is none of that mineral in a fresh condition. The feldspars contain
devitrified glass and secondary fluid inclusions. They are much dimmed
by the presence of chlorite and calcite.
Slide 315. Sutro Tunnel, 1,400 feet from entrance.
Variety with felt-like groundmass. — Tliis is a dark, rcsiuous-looking rock, with
some large greenish feldspars. The slide shows many fresh augites well
crystallized, somewhat dichroitic, and with a tendency to develop only
one cleavage. Others have imdergone a somewhat peculiar decomposition,
the product of which seems to be chlorite, very heavily charged with
hydrated ferric oxide. There are few augite microlites in the groundmass,
but many in the feldspars. The feldspars are well developed, and a very
few only give angles of extinction answering to anorthite, in spite of the
fact that a considerable number show periclinic twinning, and seem to be
cut nearly in orthopinacoidal section. A good many such give almost
exactly the theoretical maximum angle of extinction of labradorite, and I
incline to the belief, for which there is no substantial pi'oof, that they really
belong to that species, and that consequently both of the more basic feld-
spars are present. There is much magnetite and many dark apatites. The
groundmass is a felt-like aggregation of tiny microlites, between which
there is certainly a small amount of glass.
Slide 481. Between summit of Mount Kate and Occidental Grade, near point 5639.
Glassy variety. — Tlils Is a gray glassy-lookiug rock, unlike the ordinary
augite-andesite. The slide, however, shows it to be decidedly of that
species, and to consist essentially of augite and triclinic feldspar, embedded
128 GEOLOGY OF THE COMSTOCK LODE.
in a true colorless glass. A few small hornblendes and some magnetite are
the subsidiary minerals. The feldspars show little tendency to elongation;
they appear to be all triclinic, and the maximum angles of extinction ob-
tained correspond to labradorite. The augites are not very sharply crys-
tallized, are largely massed in bunches, and are more than ordinarily
dichroitic. There are a few hornblendes which are bright brown in color,
and, like those in the glassy hornblende-andesite of the District, without
black borders. The glass which forms a large part of this rock is colorless,
and shows in places perlitic cracks. Many microlites and trichites are dis-
tributed through it, some of them transparent and very likely feldspathic;
others opaque. Some of the tiichites show a beaded structure. Embedded
in the glass are many curious spots of rounded shape, which are yellowish-
white by reflected light and feebly transmit yellow rays. In polarized light
they are seen to be wholly or partly crystalline; they are not sufficiently
diaphanous to say which. They are evidently irregularly radial in structure,
and in some favorable instances give a broad ill-defined cross between
crossed Nicols. The pseudo-spherolitic structure is further marked by more
or less curved opaque trichites which, starting from the center, preserve an
approximately radial direction, branching like twigs at short intervals One
of these masses is shown in Fig. 20, Plate III.
Slide 125. Above the Ophir grade, due west of Belcher hoisting- works.
Granitoid variety. — This is & bluish-gray grauular rock, looking almost like
an older crystalline species. Under the microscope, however, it reveals
itself as merely an unusually coarse-grained augite-andesite. The ground-
mass is granular. A portion of the augites are fresh, the remainder con-
verted into chlorite.
Slide 465. Crown Point Eavine ; on flume, near drainage.
Specimen showing stages of decomposition. — Macroscopically a bluish-gray, com-
pact, and rather granular rock, without macroscopically visible bisilicates.
The slide affords an unusually fine opportunity of studying the decom-
position of augite-andesite. It happens to contain a large proportion of
augites in octagonal sections, the outlines of which have been but little dis-
DETAILED DESCRIPTION" OF SLIDES. 129
turbed by the formation of decomposition-products. Some of the augites
are almost unattacked, and show thoroughly characteristic cleavages, ex-
tinctions, etc. Others are partially converted to chlorite, and yet others
are wholly replaced by the uniaxial, dichroitic, green mineral. Some of
the pseudoraorphs are partially converted into epidote, the characteristic
prismatic sprouts of which may be seen penetrating- the chlorite. A tine
example is illustrated in Fig. 5, Plate II. In some other cases the degen-
eration to epidote and to calcite is going on in the same chloritic pseudo-
morph. There were originally one or two small hornblendes in this slide,
now wholly converted into epidote. The feldspars seem to be labradorite.
Crown Point Ravine is the best of all the "propylite" localities, and the
specimen is an excellent representative of the rocks which have received
this name. A portion of the slide is very faithfully illustrated in Fig. 31,
Plate V.
Slide 428 500 feet southeast of Sutro Tunnel air-shaft.
Specimen with peculiar augites. — TWs is a black rock with au uncven fracture,
and a luster both vitreous and resinous Under the microscope it is seen to
be a fine augite-andesite with more augite than usual, and no hornblende.
The augite is of the common color and slightly dichroitic. One crystal
shows the uncommon phenomenon of multiple twinning, in which the surface
of composition of a portion of the lamellae is decidedly irregular. This
augite is illustrated in Fig. 1 6,' Plate III. The large feldspars give angles
of extinction corresponding to labradorite; the microlites correspond to
oligoclase, and many of them show a tendency to fibration at the ends.
The large feldspars contain inclusions of glass and microlites of augite.
There are a few brown apatites and some colorless ones in, the slide. The
groundmass has the well-known felted appearance in some portions and
shows tluidal arrangement in others. It contains a considerable amount of
isotropic glass.
Slide 31. Sutro Tunnel, 10,055 feet from entrance.
Specimen with unusual chlorite pseudomorph. — This is ail Ordinary augite-audcslte
in a somewhat decomposed condition, which most likely carried a little
9 0 L
130 GEOLOGY OF THE COMSTOCK LODE.
glass when fresh. Among the many pseudomorphs of chlorite after augite
which it contains, one is especially beautiful, and is illustrated in Fig. 4,
Plate II. Decomposition has evidently started from the cross-fractures,
and also from the centers of the fragments isolated by the o'acks, but these
two varieties of decomposition have proceeded somewhat differently. The
chlorite around the exterior of the crystal, and along the cracks, betrays
structure only by a very slight dichroism ; between crossed Nicols it is not
perceptibly luminous. The chlorite which has developed from the cen-
ters of the fragments is brownish-green, radially fibrous, strongly dichroitic,
and polarizes in dull-brownish colors.
LATER HORNBLENDE-ANDESITE.
Slides 472 and 473. Quarry 2,000 feet northeast of Sutro Tunnel Shaft III.
Trachytic-iooking variety. — TWs is a Very coarse-grained soft rock, with large
porphyritical feldspars and visible mica and hornblende. Its ground mass is
purplish-gray. This rock is that commonly employed on the Comstock for
engine foundations and the like.
Under the microscope it is seen to consist of jjlagioclase, hornblende,
mica, and magnetite, with a few Carlsbad twins and apparently simple
crystals which might be sanidin. Some of these last show minute stripes
under close examination, and others can be shown to be plagioclase by
their angles of extinction. The highest angles of extinction of the properly
oriented feldspars indicate labradorite ; but many of the larger crystals show
a strongly marked zonal structure. Inferences as to their composition
have been drawn on page 68. The feldspathic microlites appear to
be oligoclase. The mica is brown and intensely dichroitic. Cleavage-
scales give an hyperbolic interference figure, which could not for a moment
be confounded with a cross, and it is either a biotite in which the angles of
extinction are uncommonly large or another species The hornblendes
are brown and well crystallized, and are all black-bordered; but while some
have comparatively narrow borders, others are almost wholly converted to
magnetite, leaving only a particle of the fresh mineral near the center. The
DETAILED DESCRIPTION OF SLIDES. 131
same remark applies to the mica. Some of the hornblende crystals, too,
are decomposed, while the majority are perfectly fresh. This is probably
due to the structure of the rock, which must admit liquid currents more
easily on certain lines than on others. The grouudmass is thoroughly
crystalline and microlitic, and consists of feldspar microlites and magnet-
ite. This is the most important of the so-called trachytes of the District,
and was therefore selected for illustration. Plate V., Fig. '62, shows a
characteristic field.
Slide 474. Quarry 2,000 feet east of Occidental Mill.
A similar rock. — Thls is a reddish porphyry similar, except in color, to that
described under slide 472, and also used for building purposes. Under the
mici'oscope it is remarkable for its intensely dichroitic hornblende, which
shows an extremely light yellowish-brown tint, when parallel to the long
section of the analyzer, and a bright red-brown when at right angles to this
position. The slide also contains considerable poorly crystallized augite.
The mica gives the same decidedly biaxial interference figure as that in
slide 472. The feldspars also are similar to those in that slide. This is
the rock separated by Dr. Hawes by Thoulet's method, and found to con-
tain no sanidin.
Slide 230. Quarry above Utah mine.
More compact, gray rock. — A bluish-gray Tock of manifestly loose texture,
showing both mica and hornblende. Under the microscope plagioclase,
magnetite, and some brown glass are also visible. The feldspars are beau-
tifully fresh. Extremely few lack stripes, and these are not in determin-
able zones. Several of the larger feldspars show nearly square sections and
pericline as well as albite twinning. These give angles of extinction of
about 30° on each side of the albitic twinning plane. The small elongated
feldspars also give labradorite angles in many cases, and I see no reason to
. suspect any considerable quantity of any other feldspar. The feldspars con-
tain great numbers of colorless glass inclusions, most of them entirely fresh,
as well as patches of the brown glass and of groundmass with glass. A few
of the smaller feldspars seem to consist of negative crystals of brown glass
132 GEOLOGY OF THE GOMSTOOK LODE.
surrounded by thin shells of feldspar. The hornblende is in part perfectly
fresh, and so solid that the cleavages are almost imperceptible with low powers.
The color of the hornblendes is various, and seems to depend largely on their
position. Some crystals are nearly pure brown; others a slightly brownish-
green, or of intermediate tints. A few ai-e decomposed to calcite, quartz, and
epidote. A part of the crystals show no black borders, and others only a
very narrow line of magnetite. The mica is fresh biotite, giving the char-
acteristic interference figure, and like the hornblende shows a few glass
inclusions. It has a narrow black border. The groundmass is composed of
feldspar microlites and brown glass. In parts of the slide the arrangement
of the microlites seems wholly without order, while in others fluidal structure
is well developed.
Slide 462. 2,000 feet northwest of Geiger Grade Toll House.
Black, glassy variety. — This is a pitcliy-black rock, with a glassy luster, showing
some large hornblendes, but resembling certain augite-andesites in appear-
ance more than any hornblende-andesite of the District. Under the micro-
scope the reason of this unusual appearance is plain, for it contains a large
amount of glass base, which is not the case with any other Washoe rock
of this species examined. Nearly all the feldspars seem to be labradorite,
only the minutest untwinned microlites giving angles of extinction proper
to oligoclase. A few sections give angles of extinction which might be re-
ferred to anorthite, but I failed to find any such in which extinction took
place at equal angles to the trace of the twinning plane, and suppose the
crystals to be labradorites cut in one of the uninvestigated zones. Many
of the feldspars at first sight appear to be simple crystals, but show on
closer examination a few exceedingly minute strise. The feldspars contain a
very unusual abundance of glass inclusions, a large proportion of which
have polygonal outlines parallel to the sides of the feldspar section. They
also contain inclusions of the base, many of which assume fantastic forms,
some looking like ripple-marks, and others arranged as if the base had pene-
trated perpendicularly into the feldspar and spread between its zones. The
process must have been just the reverse, and the appearance is no doubt
due to an ineffectual effort of the feldspathic material to free itself from the
DETAILED DESCBIPTION OF SLIDES. 133
adhesive glass during crystallization. Some of these inclusions are shown
in* Fig. 23, Plate III. Zonal structure is beautifully developed in many of
these feldspars.
The hornblendes are of a somewhat dull yellowish-green color and
very solid. They are especially remarkable from the fact that scarcely
any of them show even a trace of a black border. There is a large quan-
tity of augite of exceptionally pale color. It is faintly dichroitic and
crystallized in unusually long needles. The sections and angles of extinc-
tion leave no doubt as to its nature. There is a single excellent biotite in
the slide. Many colorless apatites and a considerable quantity of magnetite
are present. The base shows a felt-like structure which appears to be due
to the presence of minute opaque microlites.
Slide 470. Mount ^.bbie.
Gray coarse-grained variety. — Thls Is a coarsc gray porous rock, strougly re-
sembling a hornblende-trachyte in appearance. Under the microscope,
however, it is plain that nearly or quite all the feldspars are triclinic, and
they give labradorite angles of extinction. Many of the smaller labrador-
ites are simple individuals. The hornblende and augite are such as are
common in the hornblende-andesites, but the hornblendes show only very
narrow black borders. There are about twice as many hornblendes as
augites. The groundmass is microlitic, and no base is visible.
Slide 467. 1,000 feet nortli-northwest of Flowery Peak.
Porphyry with dark groundmass. — This is a coarse-grained rock in which large
crystals of feldspar are separated out in a dark, rather compact groundmass.
Mica as well as feldspar is visible. Most of the feldspar crystals show twin
striations, but there are some Carlsbad twins and simple crystals, none of
which, however, are probably orthoclastic. Many of the larger crystals,
which are well developed, show zonal structure. The maximum angles of
extinction correspond to labradorite. The feldspathic microlites are shorter
and broader than is usual, and a few are possibly sanidin, but the great
majority are cei'tainly triclinic. I noticed no inclusions in the feldspars
beyond apatite. Hornblende and mica are almost wholly represented by
134 GEOLOGY OF THE COMSTOCK LODE.
patches of magnetite which are evidently exaggerated black borders.
Some of these patches are pseudomorphs after hornblende, but mica plainly
predominated. Of this enough is left to determine that its color was brown.
The slide contains a single grain of quartz, which is evidently primary.
The groundmass is thoroughly crystalline and microlitic. It contains much
magnetite, and shows fluidal structure in places.
Slide 476. Divide between Mount Eose and Mount Emma.
Light-gray porous variety. — A light-gray rock with a large amount of visible
mica and hornblende. The feldspar is largely in simple crystals, few, if
any, of which, however, are orthoclastic. The microlites are developed
with unusual sharpness. The feldspars contain bubble-bearing glass inclu-
sions and patches of groundmass. Hornblende is much more abundant in
this slide than mica, and is remarkable- for the fact that it shows scarcely a
trace of black border. Much of it occurs as minute brown spiculse dis-
seminated through the groundmass. The mica is present in well-developed
crystals, and cleavage scales show the ordinary sensibly uniaxial interfer-
ence figure of biotite. The iron ore is magnetite, and it and the feldspar
microlites of the groundmass seem to be imbedded in a colorless glass.
BASALT.
Slide 457. Basalt mesa, just west of Silver City.
Only variety. — Thls Is a dark Ordinary basalt, with numerous visible fresh
amber-colored olivines. Under the microscope it is seen to be a crystalline
mixture of olivine, augite, feldspar, and magnetite. The olivine is nearly
colorless in the section, but has a faint yellowish tinge. It occurs for the
most part in grains showing only one or two • crystalline faces or none at
all. A few of the larger crystals have good hexagonal or octagonal out-
lines. Besides irregular cracks, there are occasional indications of imper-
fect cleavage. The olivine is wholly undichroitic and polarizes brilHantly.
As inclusions it contains crystals of magnetite and a few particles of augite.
It shows only occasional traces of decomposition. The augite is of the
common brown color, but of a rather deeper tint than usual. Some of the
PROPYLITES OF THE FOETIETH PARALLEL. 135
crystals are as large as the olivines, but while there are many small augites
there seem to be no microscopic olivines. The augites are better crystal-
lized than the olivine, and often show characteristic sections and cleavages.
They are decidedly dichroitic. The aiigites carry a few bubble-bearing
inclusions, which seem to be glass. The feldspars are small, lath-like, and
often simply twinned. In a very few cases both periclinic and albite twin-
ning are visible. The angles of extinction are those of labradorite. I could
detect no orthoclase. The groundmass consists of feldspar, augite, and
magnetite in cubes, and contains no perceptible base. Slide 458 from 1,250
feet southeast of Roux's Ranch is identical with the above, and with the slide
described in the Exploration of the Fortieth Parallel, Vol. VI., as 528. The
slide and specimen described in that memoir as 529 is the same rock which
is here regarded as a metamorphic diorite.
PROPYLITES OF THE FORTIETH PARALLEL SURVEY COLLECTION.
Fortieth Parallel propyiites. — I havc bceu kiudly allowcd free use of the collec-
tions of the Geological Exploration of the Fortieth Parallel, and reproduce
in the following pages my notes on the specimens and slides described in
Vol. VI. of the publications of that survey as propyiites (212 to 225) and
as quartz-propylites (226 to 232). While I do not feel myself competent
to decide definitively the species of those rocks which I have not had an
opportunity of studying in the field, my opinion of each slide is indicated,
in order to convey a more complete impression of its appearance.
Exploration of the Fortieth Parallel. Slide No. 212, specimen No. 22,682, Crown Point
Eavine, Washoe.
This is a smooth, fine-grained rock, somewhat resembling a limestone
in texture. Its color is pistachio green. Seen under the microscope, it is
evidently much decomposed; indeed, the slide shows little besides epidote
and secondary quartz. Even the magnetite has almost wholly disappeared,
and the residual products are grouped within no outlines from which the
nature of the original bisilicates might be inferred Many very small feld-
spars are still fresh enough to make out with certainty that they are triclinic.
136 GEOLOGY OF THE COMSTOCK LODE.
Exploration of the Fortieth Parallel. Slide No. 213, specimen No. 22,684, Crown Point
Eavine, Washoe.
A somewhat more granular rock than the preceding, but of the same
color. The slide shows that it is slightly less decomposed. In a few cases
feldspars can be detected with striations not entii-ely obliterated, and with
rectilinear outlines, such as are ordinarily met with in andesites. Several
brown apatites are visible. The patches of decomposition products show
outlines here and there which are suggestive of hornblende and augite.
Besides quartz and epidote, this slide contains some calcite. I regard this
and the preceding rock as entirely indeterminable froiii the specimens and
slides, but from a study of their associations on the spot T believe them to
be hornblende-andesites.
Exploration of the Fortieth Parallel. Slide No. 214, specimen No. 22,686. Crown
Point Eavine, Washoe.
A gray coarse-grained rock, the feldspars of which are opaque, giving
it a superficial resemblance to pre-Tertiary rocks. Under the microscope a
glance shows it to be augitic. The slide contains several sections of the
undecomposed mineral with characteristic octagonal outlines and appropriate
angles and cleavages, as well as some longitudinal sections, giving angles
of extinction running up to above 30°. The color of this augite is the com-
mon brownish-yellow, not unlike the tint of bamboo. Much of the augite
has been decomposed to chlorite of fibrous structure, which shows dark
bluish tints between crossed Nicols, aggregate and sometimes spherolitic
polarization, and extinction when the microlites are parallel to the principal
sections of the Nicols. That the chlorite is a derivative of the augite is clear,
for in some cases augites are only in part converted into chlorite, and in
others the pseudomorphs are perfect, even retaining traces of the cross-
fractures of the augite j^risms. I found but one mass of decomposition
products that might with any probability be referred to hornblende. The
feldspars are tricliuic, and some of the large crystals show labradorite
angles of extinction. Apatite and magnetite are also present. The gi'ound-
mass contains no glass, but seemed to me to show traces of a felt-like struct-
ure, much obscured, however, by particles of chlorite and epidote. This
PEOPYLITBS OF THE FOETIETH PARALLEL. 137
rock is an augite-andesite, and one characteristic of the District, but is
partially decomposed.
Exploration of the Fortieth Parallel. Slide No. 215, specimen No. 22,689. Crovm
Point Eavine, Washoe.
A very dark, somewhat basaltic-looking rock. The slide resembles
that last described, containing, however, only pseudomorphs of chlorite after
augite, and none of the fresh mineral. The chlorite and the mineral from
which it was derived were carefully identified in the manner indicated in
the last paragraph. There is no fresh hornblende, but a few very minute
oval rings of magnetite grains probably represent the black borders of
former hornblendes. The feldspars, which are not distinguishable from
ordinary andesitic plagioclases, contain spots which look like devitrified
glass-inclusions. This, too, is augite-andesite.
Exploration of the Fortieth Parallel. Slide No. 216, specimen No. 22,690, from Crown
Point Eavine, Washoe.
This rock is much decomposed, and neither augite nor hornblende are
present in a fresh state, but the slide contains many black borders, which
retain the characteristic outlines of hornblende, though they now surround
only calcite, quartz, and a few residual grains of epidote. There is also
one good pseudomorph of chlorite after augite. From my acquaintance with
the rocks of the District I have no hesitation in pronouncing this a horn-
blende-andesite.
Exploration of the Fortieth Parallel. Slide No. 217. Gold Hill Peak, Washoe.
There is no specimen in the collection corresponding to this slide or
to the locality, which is represented on the map accompanying this paper
by the southern "Twin Peak". My own specimens are coarse greenish-
gray rocks of somewhat open texture. The feldspars are not thoroughly
transparent in consequence of incipient decomposition. The fresher por-
tions of the mass show brilliant hornblendes. The slide contains some fresh
brown hornblendes with black borders, and some black borders from which
the bisilicate has disappeared. A portion of the hornblende exhibits the
intermediate color between green and brown, which is seen in so many
138 GEOLOGY OF THE COMSTOCK LODE.
brown hornblende rocks; but I failed to find green hornblende, fibrous horn-
blende, or hornblende without a black border. There are a few excellent
augites and many capital pseudomorphs of chlorite after augite. This chlorite
shows the usual structure, dichroism, extinction parallel to the fibers, etc.
The feldspars are triclinic, the large ones seemingly labradorite, and they
appear to contain devitrified gla^s inclusions. There are many brown and
dusty apatites. The groundmass has the microlitic structure of hornblende-
andesites, nor can I see any reason for separating this rock from that species.
Exploration of the Fortieth Parallel. Slides Nos. 218 and 219, specimen No. 22,694.
Ophir Eavine, Washoe.
These slides I have sufficiently discussed in describing my own thin
sections from the same locality. I have there considered the rock as a dio-
rite-porphyry.
Exploration of the Fortieth Parallel. SKde No. 220, specimen No. 22,588. Hill east
of Steamboat Valley, Virginia Eange.
This is a brown rock which looks like an impure limonite. Under the
microscope nothing is visible excepting ferric hydrate and a little secondary
quartz.
Exploration of the Fortieth Parallel. Slide No. 221, specimen No. 22,574. Sheep
Corral Canon, Virginia Eange.
This is a light greenish, granular r^ck, evidently composed of feldspar
and hornblende. The slide shows that the hornblende is wholly decom-
posed. The crystals of this nnneral appear to have had black borders,
which are now in part replaced by higher oxides. When fresh it contained
great numbers of small augites, which are now converted into the ordinary
chlorite. The same product of decomposition is also disseminated through
the groundmass, and is accompanied by quartz and calcite. The feldspar
is fresh and striated, and the general character under the microscope is that
of an andesite. I can see no reason for calling it anything but hornblende-
andesite.
Professor Wiedemann analyzed this rock and found 64.62 per cent. siUca.
In discussing this analysis the fact should not be overlooked that a relative
increase in the quantity of silicic acid commonly accompanies decomposition.
PEOPYLITES OP THE FORTIETH PARALLEL. 139
Exploration of the Fortieth Parallel. Slide No. 221*, specimen No. 21,950. Between
the Truckee and Montezuma Ranges.
The specimen strongly resembles those from the head of Ophir Ravine,
Washoe. It is highly decomposed, but the bisilicates appear to have been
hornblende. The feldspars have not the sharp outlines usual in andesites,
and the toute ensemble is that of a porphyritic diorite.
Exploration of the Fortieth Parallel. Slide No. 222, specimen No. 21,542. Storm
Canon, Fish Creek Mountains.
This is a rather coarse-grained greenish rock, in which lath-like feld-
spars show prominently in a finer groundmass. The slide contains an
abundance of augites, some of which show pinacoidal cleavages as well as
the prismatic ones. The cleavages are very heavily marked. A portion of
the augite has been converted into grayish-green uralite, distinctly retaining
the crystal form of augite. Where it is favorably oriented it gives angles
of extinction of about 15°. The greater part of the augite has degen-
erated into chlorite, with the usual structure and optical properties. The
slide further contains much fresh mica, some of the scales of which are
horizontally placed, and give the biotite interference figure. There 'is also
a very little brown and intensely dichroitic hornblende, but I could find
none of this mineral which was green, except the uralite. The larger
plagioclases are well developed in lath-like crystals, but are nearly opaque
in consequence of the presence of "decomposition products.. The iron ore
occurs in irregular masses, but its nature is uncertain. The groundmass is
granular, not composed of well-developed microlites, but thoroughly crys-
talline. It contains much epidote and chlorite.
Exploration of the Fortieth Parallel. Slide No. 223, specimen No. 21,545. Storm
Canon, Fish Creek Mountains.
This is the same rock as the last, but in a different stage of decompo-
sition. The green hornblende shows in numerous cases the crystal outlines
of augite, and in my opinion i_s exclusively uralite. The plagioclases are
fresher than in the other slide and contain rounded fluid inclusions of small
size. While it may be somewhat rash to decide upon the age of this rock
140 GEOLOGY OP THE COMSTOCK LODE.
from these two specimens and slides, all the diagnostic points appear to
me to indicate diabase rather than augite-andesite as the proper determina-
tion.
Exploration of the Fortieth ParalleL Slide No. 224, specimen ISo. 21,259. Foothills
north of Tuscarora, Gortez Eange.
This is a green porphyry, with impellucid feldspars and brilliant horn-
blendes. I am almost inclined to doubt that this can be the slide described
in the "Microscopical petrography;" but the dark brown hornblendes tally
precisely with the figure and the description, the slide corresponds to the
specimen, as does the latter with the locality, and no other slide labeled
"propylite" bears any considerable resemblance to the text and the illustra-
tion. The slide contains a large number of unusually symmetrical brown
hornblende sections, with broad black borders. One or two of these exhibit
clinopinacoidal cleavage as well as the* usual prismatic one. Many of the
hornblendes are altered into chlorite, still retaining the -black border and
crystal outlines. This chlorite shows the usual aggregate polarization in
some places and spherolitic structure in others, and extinguishes light par-
allel to the direction of the principal Nicol sections. There are also many
fresh augites with characteristic sections, cleavages, and optical properties,
and pseudomorphs of chlorite after augite. As usual in decomposed rocks,
the groundmass contains irregular patches of chlorite, the properties of
which are identical with those of that in pseudomorphic forms. I could
find nothing whatever corresponding to the green hornblendes described by
Professor Zirkel, and figured as without black borders, and as showing
hornblendic cleavages and outlines. The feldspars and groundmass are like
those usually found in partially decomposed hornblende-andesite, and as
such I have no hesitation in regarding the rock.
Exploration of the Fortieth Parallel. SUde No. 225, specimen No. 21,314. Wagon
Canon, Cortez Eange.
This is a reddish rock, with well-developed porphyritic, impellucid
feldspars, visible mica, and greenish black patches, which are possibly
hornblendes. Under the microscope the rock is seen to be greatly decom-
posed. The slide contains fresh mica, numerous pseudomorphs of chlorite
PEOPYLITES OF THE FORTIETH PARALLEL. 141
after augite, and a number of patches of chlorite, which seem referable with
some probability to hornblendic forms. The feldspars are triclinic and very
closely striated. None of the angles of extinction which I observed exceeded
the oligoclase limits. The slide contains very little epidote, but the feldspars
and groundmass are clouded with calcite and limonite. While no satisfac-
tory determination can be made of this specimen, it seems to answer best
to a micaceous hornblende-andesiteJ
Exploration of the Fortieth Parallel. Slide No. 226, specimen No. 21,604. Hills east
of Golconda Station.
Macroscopically this rock is of a greenish-gray color tinged with yel-
low, and shows porphyritical crystals of mica, hornblende, and impellucid
feldspars. Under the microscope it is apparent that the feldspars are rend-
ered almost opaque by excessively fine grains of what is seemingly calcite.
Some of them are triclinic, others appear to me to be orthoclase, but which
are in the majority it is impossible to say. The rock contains quartz in
which there are numerous fluid inclusions, some of them containing carbonic
acid. The quartz also carries unquestionable glass inclusions of good size,
in which devitrification has proceeded only so far that between crossed Nicols
one or two bright points appear on the jet-black ground of the isotropic
substance. One of these is accompanied by the short cracks in the quartz,
which have often been observed, and which so beautifullv illustrate the
elasticity of silica. One of the numerous apatites, too, contains a glass
inclusion hung like a drop on an inclosed microlite which is probably
also apatite. The hornblende, and even the mica are wholly replaced by
decomposition products, largely oxides of iron. I could detect no trace of
augite. The groundmass contains some particles of epidote and chlorite.
It is nearly impossible to determine a rock so thoroughly decomposed with-
out a study of its occurrence. If the feldspar is triclinic it must be a dacite,
for the glass precludes the supposition that it is a diorite. The absence of
augite and of well-developed feldspar microlites, the appearance of the
orthoclase-like larger feldspai's, the abundance of fluid inclusions, and the
general air of the rock, seem to put dacite almost out of the question. Sim-
ilar arguments hold against its determination as rhyolite, and but for the
142 GEOLOGY OF THE COMSTOCK LODE.
presence of carbonic acid in the inclusions, which is, at all events, very
rare in quartz-porphyry, I should class it as a member of that group.
Exploration of the Fortieth Parallel. Slide No. 227, specimen No. 21,500. West Gate,
Augusta Mountains.
Macroscopically a gray, granular rock. Under the microscope it bears
a strong resemblance to the preceding. , The feldspars are almost opaque,
the hornblende and mica are wholly decomposed. The groundmass con-
tains some epidote and much chlorite, magnetite, and zircon. More than
one of the quartzes carry besides fluid inclusions, typical, fresh, colorless
glass inclusions which contain bubbles and are of sufficient size to remain
black between crossed Nicols. I noticed an apatite with good prismatic
cleavages. This rock seems to me an old quartz-porphyry.
Exploration of the Fortieth Parallel. Slides Nos. 228 and 229, specimen No. 21,308.
Cortez Peak, Cortez Range.
I entirely assent to Professor Zirkel's description of these slides. The
rock appears to me both macroscopically and microscopically to resemble
a porphyritic diorite in all respects. Slide 230 is a highly micaceous variety
of the same rock.
Exploration of the Fortieth Parallel. Slide No. 231, specimen No. 22,717. Gross-spur
below graveyard, Virginia City.
Macroscopically this is a greenish-gray, andesitic-looking rock, with
impellucid feldspars and brilliant hornblendes. Under the microscope the
slide shows a considerable number of hornblendes and some augites. The
hornblende is of the greenish-brown tint common among the andesites, but
brown enters very largely into the color. It is not fibrous, but decom-
position into chlorite has set in along the cleavages, and, in the longitudinal
sections, the cleavage prisms separated by chlorite might possibly be taken
for coarse fibers. It is a peculiarity of this rock that the iron ore has been
attacked more energetically than the bisilicates. Many of the smallest
grains of the ore, which is probably magnetite, may be seen throughout the
sHde, converted into a slightly diaphanous, whitish substance, which in so
far resembles leucoxene ; but between crossed Nicols it looks more Hke cal-
UTAH PEOPYLITES. 143
cite, and it strikes me as possibly iron carbonate. The quartz is indis-
tinctly separated from the groundmass, and seems to me secondary. The
feldspars and the groundmass have the usual characters of Washoe horn-
blende-andesi tes
Exploration of the Fortieth Parallel. Slide No. 232.
By some mistake this slide was labeled as from Berkshire Canon,
whereas the check-list of the survey, no less than the correspondence of
the slide and specimen, show that it should have been numbered 155, and
that the rock is the typical diorite of Mount Davidson, in Washoe. The
descriptions of this rock and of the Mount Davidson diorite, like the slides,
agree.
PEOPYLITES OF THE GEOGRAPHICAL AND GEOLOGICAL SURVEY OP THE ROCKY
MOUNTAIN REGION.
Utah propyiites. — Captain Dutton has also kindly furnished me with speci-
mens and slides of the propyiites mentioned by him in his memoir on "The
High Plateaus of Southern Utah."
Geology of the High Plateaus of Utah. Slide and specimen No. 226. Base of Mount
Dutton, Sevier Plateau.
Macroscopically a greenish, granular rock, in which lath-like feldspars
are separated out in a groundmass of a somewhat waxy luster. Under the
microscope it is seen that the rock is greatly decomposed, but also that in
a fresh state it consisted of plagioclase and augite, with an iron ore, quartz,
and apatite as subordinate constituents. A few of the augites are fresh, and
are in every way characteristic, but most of them have been converted into
chlorite, and the slide contains a large number of excellent pseudomorphs of
this character. The chlorite appears to be precisely the same as that to which
reference has been made so often in the foregoing pages. It is fibrous, ex-
tinguishes light parallel to the long axis of the microlites, dichroizes strongly,
and gives an aggregate polarization or a spherolitic cross, according to the
arrangement of the microlites. In places epidote may be seen forming at
144 GEOLOGY OP THE COMSTOOK LODE.
the expense of the chlorite. At least a part of the quartz is primitive. It
contains fluid inclusions, gas-pores, and possibly also glass-inclusions. The
larger feldspars are well developed, very closely striated, and appear to give
angles of extinction of about 18° 30' in orthopinacoidal section. The
smaller feldspars are granitoid rather than microlitic in their development,
and so much obscured by decomposition-products as to make it uncertain
whether they also are referable to oligoclase. The iron ore is probably
titanic, and is accompanied by both leucoxene and ferric oxide. The slide
contains much apatite, a large part of it in unusually long microlites, and a
little sphene. The rock appears to me to be a decomposed diabase.
Geology of the High Plateaus of Utah. Slide and specimen No. 274. Gate of Mun-
roe, Sevier Plateau.
The general character of this rock, both macroscopically and micro-
scopically, is almost identical with that last described, but the bisilicates
have been entirely decomposed, and the chlorite is much disseminated ;
there is a strong probability, however, that the oi'iginal mineral was augite.
The feldspar best answers in its optical characters to labradorite. It con-
tains fluid inclusions. The apatites are extraordinarily large and abundant,
and, strange to say, contain numerous fluid inclusions.
DESCRIPTION OF IliliUSTRATIONS.
In order that any object in a thin section, to whicli special reference
is made, may be readily found again by one studying the collection, thus
saving the time and patience of the student and leaving no room for doubt
Fig. 2. — Orientation of Slides.
as to the exact spot under discussion, the following method of determining
the locality of such an object has been employed and is recommended for
general use: Mark on the stage of the microscope two radii at right angles
and graduate them in millimeters, beginning from the center of the stage,
numbering them as in the figure. Place the glass bearing the rock section
10 C L 145
146 GEOLOGY OF THE COMSTOCK LODE.
on the stage, so that when the spot to be located is under the cross-wires
the upper left-hand corner of the object glass shall be within the quadrant
between the radii, and the sides of the object glass shall be parallel to the
same, as represented in the figure. The distances, then, of the spot in
question from the sides crossing the radii are read from the scales, and rep-
resent the rectangular coordinates of that spot referred to the upper left-
hand corner of the object glass as an origin, and are recoi'ded thus: 293""';
the number of the thin section being given, and the coordinates being placed
after it, with the vertical coordinate preceding the horizontal. The process
of finding any spot the coordinates of which are given in this manner needs
no further explanation.
FiGr. 1. Slide 252''^^ Brown hornblende passing into chlorite. The small stippled
white mass at the right of the cut is secondary quartz. The rock is a
porphyritic diorite. Sierra Nevada mine, 1,450-foot level; north drift,
289 feet. Magnified 170 diameters.
Fig. 2. Slide 326"'^^. Brown hornblende passing into chlorite, which is represented
as gray. The white patches are quartz and calcite. The rock is earlier
hornblende-andesite from the Sutro Tunnel, 17,100 feet from entrance.
Magnified 70 diameters.
Fig. 3. Slide 464^i'^. Greenish-brown hornblende, in longitudinal section. The cen-
tral portion of the veins is chlorite, between which and the solid horn-
blende the space is occupied by quartz. The rock is older hornblende-
andesite from croppings 1,200 feet northwest of the Geiger Grade toll-
house. Magnified 45 diameters.
Fig. 4. Slide Sl"-^'. Pseudomorph of chlorite after augite. The white intrusive mass
is feldspar; decomposition has gone on from the surfaces and cracks, pro-
ducing a green slightly dichroitic chlorite, which remains nearly black
between crossed Nicols. The fragments have also decomposed from their
centers into a greenish-brown, very fibrous, strongly dichoritic chlorite.
The rock is augite-andesite from the Sutro Tunnel, 10,055 feet from en-
trance. Magnified 95 diameters.
Fig. 5. Slide 465'^-^^. The outline is that of a cross-section of augite. The smooth
gray tint represents a felted mass of chlorite, composed of excessively
fine fibers. The coarsely granular mineral is epidote, which can be seen
sending denticular crystals into the chlorite. In the upper part of the cut
epidote has begun to develop from a second center. The rock is augire-
andesite from Crown Point Eavine. Another part of this slide is repre-
sented in Fig. 31. Magnified 65 diameters.
DESCRIPTION OP ILLUSTRATIONS. 147
Fig. 0. Slide 194"'". Pseudomorpli of chlorite after hornblende. Granular epidote
Is developing- from five distinct centers in the chlorite. The chlorite close
to the left-hand upper edge of the crystal is couiposed of fibers perpen-
dicular to the crystal face, and ajipears to resist the encroachment of epi-
dote. The rock is a porphyritic diorite from the McKibheri Tunnel. Mag-
nified 48 diameters.
Fig. 7. Slide 194^'^. A group of three hornblendes has been completely converted
into chlorite, and in these pseudomorphs epidote has developed from the
. centers in gr:inular masses and fagot-like bundles. The growth of epi-
dote needles into the chlorite (which is shaded a flat gray) can be excel-
lently observed at the right-hand edge of the cut, and between the left-
hand and the middle crystals. In the left-hand crystal there are two
small patches of secondary quartz. The rock is porphyritic diorite from
the McKihben Tunnel. Magnified 40 diameters.
Fig. 8. Slide 233^^-'^. Pseudomorph of chlorite and epidote, after mica. The conver-
sion to chlorite probably proceeded from the cleavages, and the conversion
of chlorite to epidote has begun upon the same lines. The chlorite as
usual is indicated by a flat gray tint. Minute denticles of epidote can
readily be seen under high powers, piercing the fibrous chlorite mass.
The rock is diorite-porphyry from the head of Ophir Eaviue. Magnified
30 diameters.
Fig. 9. Slide 199"-^'. Pseudomorph of epidote after hornblende. The epidote appears
to have crystallized from three different centers, and the radial needles
strike entirely across the crystal. The rock is a porphyritic diorite from the
McKibben Tunnel, part of the same mass the pseudomorphic phenomena of
which are illustrated in Figs. 6 and 7, and distant only eight feet from it.
It is the last stage of the conversion shown in Fig. 7. Slide 199 also
shows epidote developing in chlorite patches. Magnified 50 diameters.
Fig. 10. Slide l^V'^-'^^. Pseudomorph of chlorite and quartz after hornblende. The
quartz occupies the central portion of the crystal, and seems to have been
deposited by substitution for chlorite. The chlorite border is fibrous,
excessively fine, and, as usual where this structure occurs, transmits
scarcely a ray of light between crossed Nicols. The approximate uniform-
ity of the chlorite zone suggests that the resistance offered by it to decom-
position has exceeded that of the chlorite for which quartz has been substi-
tuted. The very dark spots in the quartz are limonite, and there are two
small granular bunches of epidote in the chlorite, at the lower left-hand
corner of the cut. The slide is from the same specimen as Pig. 9. Mag-
nified 100 diameters.
Fig. 11. Slide 295'°-'^. Colorless hornblende passing into a green variety of the same
mineral seen in cross-section. A large hornblende appears to have been
divided into cleavage prisms by chloritic decomposition, much as in Fig. 2,
but with the additional development of the clinopinacoidal cleavage.
148 GEOLOGY OF THE COMSTOCK LODE.
These prisms are colorless near the center, but green near the border. The
figure shows one of a vast number within the same crystal outline, the
shaded portion representing green. No change in the angle of extinction
is produced by the alteration. The rock is metamorphic diorite from the
Amazon mine. Magnified 270 diameters.
Fig. 12. Slide 295^^-^^. Colorless hornblende passing into a green variety of the same
mineral, longitudinal section. No longitudinal section so perfect as the
cross-section shown in Fig. 11 has been met with. Many, however, like
that portrayed in Fig. 12, show colorless fibers encroached upon by the
green mineral. This section also contains a little chlorite, shaded a
deeper tint than the remainder of the section. The rock is metamorphic
diorite from the Amazon mine. Magnified 40 diameters.
PLATE III.
Pig. 13. Slide 20'*-25. Zonal feldspar. The kernel and the outer zone extinguish light
when the principal plane of the Nicols is inclined at an angle of about
14° to the twinning plane, and the fine reversed lamellaj are blackest
when the angle measures about 14° in the opposite direction. The inter-
mediate zone extinguishes at an angle of 5° in the same sense as the
other zones. Just within the outer zone is a belt of nearly opaque inclu-
sions which connects with the grouudmass of the rock at the top of the
figure. The rock is hornblende-andesite from the quarry 1,000 feet west
of the Yellow Jacket east shaft. Magnified 50 diameters.
Fig. 14. Fortieth Parallel collection, slide 284'^^. Feldspar with rectangular glass
kernel. The two halves of this crystal extinguish light at angles of 24°
and 26° to the twinning plane, and minute twin lamellae are visible at
the lower end of the section. Magnified 140 diameters.
Pig. 15. Slide 349"'^^. Augite section showing discontinuous twin lamellae. These
are shaded dark gray. Two included crystals of iron ore are indicated
in black, and some chloritic patches in light gray. The rock is diabase
from the Sutro Tunnel north branch, 50 feet south of Ophir connection.
Magnified 40 diameters.
Pig. 16. Slide 428'2-'^ Augite with contorted twin-lamellae, which are shown in black.
The rock is an augite-andesite from near the Sutro Tunnel air-shaft (beyond
the limits of the map). Magnified 70 diameters.
Pig. 17. Slide 450'=*-^^ Fragment of brown hornblende with black border on the frac-
tured surface, as well as on the crystal faces, and a second parallel internal
belt of magnetite. The figure is from a hornblendic andesite from a cut
1,000 feet east of the railroad station at the Silver City switch. Magni-
fied CO diameters.
DESCRIPTION OF ILLUSTRATIONS. 149
Fig. 18. Slide 194"-^'. Horseshoe-shaped apatite cut so nearly at right angles to the
main axis as to remain almost black between crossed Nicols. It occurs
in a decomposed hornblende. The rock is dioritic porphyry from the
McKibhen Tunnel. Magnified 220 diameters.
Fig. 19. Slide dSi^^-^". Mass of ilmenite showing characteristic markings, from an
augite-andesite from Cedar Hill Canon. Magnified 70 diameters.
Fig. 20. Slide 182"'-2''. A iieculiar secretion in a glassy augite-andesite from the south-
west flank of Mount Kate. It is a brownish mass of pseudo-spherolitic
structure filled with black trichites. It much resembles a patch of brown
mold. Many others occur in Ihe same slide. Magnified 45 diameters.
Fig. 21. Slide 4:21"'-2''. Symmetrically arranged acicular black inclusions found in the
hornblendes of diorites and andesites. The illustration is taken from a
longitudinal section of brown hornblende, and the direction of the cleav-
age is indicated by the arrow. The rock is aporphyritic diorite from the
center of Cedar Hill ridge. Fig. 26 is from the same slide. Magnified
600 diameters.
Fig. 22. Slide 210'^-^". Secondary fluid inclusion in feldspar. These inclusions are
absent from the fresh jjortion of the same exposure. The rock is from
the quarry 1,000 feet west of the Yellow Jacket east shaft. Magnified
800 diameters.
Fig. 23. Slide 462"'-2^ The illustration shows the edge of a feldspar above a portion
of the groundmass of the slide. The feldspar contains inclusions of brown
glass, which are elongated in the direction of the edge of the crystal, and
seem thus to indicate a tendency to zimal structure in the formation of
the crystal. The inclusions also show a connection with the present face
of the crystal, and are continuous in a direction vertical to the face. Por-
tions of the viscid glass having become entangled in the feldspar during
its growth, the energy of crystallization seems to have been insulBcient
to expel or cut off the partially inclosed material. The rock is a glassy
younger hornblende-andesite from the Geiger Grade 2,000 feet northwest
of the toll-house. Magnified 200 diameters.
Fig. 24. Slide 35F'-". Double glass inclusion in quartz. No part of this inclusion
reaches either the upper or the lower surface of the slide, nor is there
any trace of a crack near it. The rock is from the Overman mine, 1,142-
foot level. Magnified 750 diameters.
PLA-TB IV.
In the description of the figures on this plate and tlie succeeding one,
the position of the minerals is given by their coordinates referred to the
lower left-hand corner of each figure, the ordinates being written before the
150 GEOLOGY OF THE COMSTOCK LODE.
abscissas. In seeking a mineral, it is convenient to lay a card, or rectan-
gular slip of paper, on the illustration with its edges parallel to those of
the figure, but intersecting the graduated edges of the latter at the given
distances. The corner of the card will then coincide with the point sought.
This method is capable of any desired degree of exactness and permits of
the indefinite multiplication of references.
Tig. 25. Slide 213'''''^. Grauular diorite from Bullion Eavine at Water Company's
flume. Nicols crossed. Magnified 30 diameters.
Geeen, FIBROUS HORNBLENDE : 20-22; 27-28; 22-13.
Labkadorite : 12-15 ; 14—28, and most of the unspecified grains.
Quartz: 8-14; 15-23; 17-18. The quartz carries fluid inclusions,
some of which show active bubbles.
Magnetite: 19-10; 25-27.
At 19-20 epidote is developing in a patch of chlorite, but cannot be well
observed with crossed Nicols or with so low a power.
Fig. 26. Slide 421"'-^'*. Porphyritic diointe from the center of Cedar Hill ridge. Nicols
crossed. Magnified 30 diameters.
Greenish-brown hornblende: 20-20; 20-27; 30-21; 10-22, etc.
A small feldspar is inclosed in the large hornblende, and chlorite
in small quantities is developing along the cleavages of the lat-
ter, producing with crossed Nicols the broad black markings
noticeable in the drawing.
Feldspars : The porphyritic feldspars in this slide, as at 10-18, ap-
pear to be labradorite. Some of the microlites give oligoclase
angles of extinction. The greater part of the small feldspars
are granular.
Magnetite: 8-24; 20-23, and many grains too small to appear indi-
vidually on this scale. The apatites are also too minute to be
shown.
Epidote developing out of chlorite occurs at 18-5, but requires a
higher power and different light for study.
Fig. 27. Slide 354'"-'''. Quartz-porphyry 1,000 feet southwest of Laicsoii\s Tunnel.
Nicols at 45°. Magnified 30 diameters.
Orthoolase: 22-5; 20-25; 26-23; 17-15; 15-10.
Quartz: 25-10; 15-25. The quartz contains bays of groundmass
and numerous fluid inclusions with moving bubbles.
Mica: 15-20; 5-24. The mica is wholly decomposed and replaced
by limonite and other secondary products.
i;H:iii,oi;irAi, ,sfi;vKY,
(;>.■()[. OGY OF THt: COMSTOfK LOUI-: fi-i PJ,,\TK II
• liiliuK Kim-tr^.lith
II. .s. (;K()i,()(;irAi. sir[-;vi':Y.
GEOLOWOF TFFE COAfSTOCK LODE &<■ Pr.ATK Wl
Fi^.14-.
Tig. 15.
Fig, 16
mg.i8.
Fig. 20.
Figig.
Kg. 21.
¥i§. 22.
Fig. 23.
'S^m^^srS^''^ ^^* ,i:i^^"^|3«5
Fig.?.
.TuliunBimXriiIiih.
U. S. GEOLOGICAL S[ITi'\'EY
GEOLOGTOF THE COlfSTOrK LODE &r FL.ini.
FIG^25. GRANULAR DIORITE
FIG, 26. PORPHYRITIC DIORITE
FIG. 27: QUARTZ PORPHYRY
FIG. 26: EARLIER DLA£ASE
■ loJii:.-. BiraiSCaliUi
i
U.S. GEOLor.irAl, SITJVKY.
RKOLnnYflF THE CDMSTOCK r.niJE H-r. pr. V.
FIG 29. LATER DIABASE (BLACK DIKE)
FIG. 30, EARLIER HORNBLENDE-ANDESITE
FIG. 3] : DECOMPOSED AUGITE-ANDESITE
FIG. 32 : LATER HORNBI.£NDE-ANDESITE
.luluuiIt>en.SI'(ilitli
DESCRIPTION OP ILLUSTRATIONS. 151
Pig. 28. Slide 34:9^"-^^. Earlier diabase, Sutro Tunnel, north branch, .50 feet south of
Ophir connection. Nicols crossed. Magnilied 30 diameters.
Labbadokite: 27-13; 27-23; 23-15; 19-27, and most of the grains
constituting the groundmass.
AtjGITE: 11-27; 5-20; 21-33; 21-12.
Ubalite: 22-8; 7-20. The augite at 21-12 is partly converted to
uralite.
Magnetite or ilmenite: 9-20; 10-12; 16-7, etc.
Pig. 29. Slide 466'*-2='. Later diabase ("black dike"). Chollar mine, 1,900-foot level.
Nicols at 45°. Magnified 120 diameters.
Labradorite: 12-18, etc.
Augite: 25-26; 14-7; 14-9; 16-20; 25-18, etc. The augite is all
more or less obscured by a smoky-brown decomposition product,
probably limonite.
Magnetite: 6-22; 16-18; 23-7, etc.
Pig. 30. Slide 228"-". Earlier horubleude-audesite. Knoll just northeast of Combi-
nation Shaft. Nicols at 45°. Magnified 23 diameters.
Hornblende: 27-23.
Labradorite: 22-10; 20-17; 17-25.
Magnetite and ilmenite : All the black spots.
Chlorite: 10-29.
Fig. 31. Slide 465i^*^^. Decomposed augite-andesite from Crown Point Ravine. No
polarizer was used, the sky light happening to be sufficiently polarized
to develope the lamellte of the feldspar. Magnified 20 diameters.
Labradorite: 25-25; 11-11.
Atjgite: 19-5; pseudomorphs of chlorite after augite, 7-19; 17-32.
In the first of these, epidote is developing as in Pig. 5, which is
also from this slide.
Epidote: 8-19; 18-10.
The mass at 25-7 is chlorite, calcite, epidote, and oxides. The black spots
in the groundmass are magnetite.
Pig. 32. Slide 473^°-^^. Later hornblende-andesite. Quarry 2,000 feet northeast of
Sutro Shaft III. Nicols at 45°. Magnified 35 diameters.
Hornblende: 19-18; 27-13; 23-3; 13-21; 14-25, etc.
Mica : 19-9 ; 15-30. The last is almost wholly represented by mag-
netite, leaving only here and there a particle of the original ma-
terial.
The one large feldspar and all the microlites appear to be labradorite.
The magnetite grains are readily recognizable.
152
GEOLOGY OF THE COMSTOCK LODE.
Table 2. — Silica determinations.
Dr. G. E. Moore, at my request, made the following determinations:
Porphyritic diorite, from the head of Ophir Eavine, much decom-
posed, contains - 58.56 per cent. SiOj
Earlier diabase, Sutro Tunnel, 19,100 feet from entrance, highly
decomposed, contains 59.26 per cent. SiO^
Later diabase, Belcher 1,145, very fresh, contains 49.79 per cent. SiOj
Table 3. — Analysis of Water from the GOO-foot level of the Savage mine, by Professor S.
W. Johnson, of Yale College.^
One liter contained —
Grammes.
Silica 0305
Alumina and ferric oxide - - 0009
Chloride of sodium 0021
Sulphate of lime 5044
Sulphate of magnesia 0308
Carbonate of potash 0148
Carbonate of soda 1297
Carbonate of magnesia 5012
.7644
Table 4. — Qualitative determination of Gomstock mine-waters.^
By Eugene S. Bristol.
1-5 O
a .
it
n >
o
Hi
3l
111
S <o
S '^
£="
o o
^a .
H
Dale & Norcross,
west drift 930-
foot level.
Savage, 5th sta-
tion, north drift.
Ophir, bottom of
new shaft.
Solid cont«nto, grammes^
Bases
0. 0553
Lime
Magnesia . . ,
0.3271
Lime
Magnesia . .
Soda
0. 0615
Lime
Magnesia . . .
Potash
Soda
0. 0924
Lime
Mapiesia . . .
0. 0784
Lime
Magnesia . . .
0.0660
Lime
Magnesia . .
Potash
Soda
0.080
Lime.
Magnesia-
Soda.
Soda
Soda
Soda
Carbonic
Sulphuric.
Phosphoric .
Carbonic
Carbonic —
Sulphuric.
Carbonic
Sulphuric. .
Phosphoric -
Carbonic
Carbonic.
Sulphuric- .
Sulphuric ..
Sulphuric. .
PhoBphoric
Sulphuric.
Chlorine. ]
1
Silicic
Silicic (trace)
1
lExploratioD of the Fortieth Parallel, Vol. in., p. 87. 2Esploration of the Fortieth Parallel, Vol. ni., p.
3 In 100 cubic centimeters of water.
Table l — Chemical analyses.
[From the publications of the Exploration of the Fortieth Parallel, excepting those by Dr. G. E. Moore.]
Detennination.
Diorite
Do
Mica-(liorite
Porphyritic diorite
Metamorphic diorite
£arlier diabase
Quartz-porphyry ("quartz-propylite") —
Quartz-porphyry ("dacite")
Horublende-andesite ("propylite")
Homblende-andesite
Augite-audesite ("homblende-andesite") .
Do
Later homblende-andesite ("trachyte") - .
Do
Do
f ("propylite")
"Propylite" horse
Clay
Do
Do.
Do.
Locality.
Eldorado outcrop, Mount Davidson
do
800' E. of Waller Defeat shaft, point 5,621 <D. 5) .
CenterofCedarHillKidge(D. 2)
Amazon mine (D. 7)
Main Sutro Tunnel, hanging wall of Lode
Hill west of American Flat, 'Waehoe
Hills above American City, 'Washoe
Cross-spur, below graveyard, "Washoe
First Hill north of Gold Hill Peak, Washoe
Kidge northeast of American Flat, "Washoe
Silver Terrace, "Washoe
Cross-Spur quarry, Washoe
Mount Rose, Washoe
do
Washoe ("Virginia City)
Yellow Jacket, 830-foot level
Tellow Jacket 6&at clay
Cftoliar west clay
Hale liNorcroBS east clay
5at!a/7e second station
Analyst.
R. W. Woodward .
...do
Gideon E. Moore. .
...do
...do
...do
W.G.Mixter
C. Coiincler
W. G. Mlxter
W.Xonnann
W.G.Mixter
...do
R. W. Woodward .
...do
...do
W. G. Mixter
...do
S. W. Johnson
W. G. Mixter .
-.-do;
S. W. Johnson .
SiO,
56.71
30.24
56.58
30.17
65.68
35.01
58.55
3121
46.65
21.86
56.40
30.06
68.44
3650
60.82
32.43
61.12
32. .W
68.33
31.10
59.22
31.58
63.13
33.67
63.13
33.67
58.66
31.28
80.27
60.02
69.71
65.69
39.52
TiOj
0.98
0.39
0.83
0.33
1.02
1.41
1.14
0l4«
AljOs
18.36
8.55
18.20
8.18
15.87
7.41
15.48
7.23
15.99
7.47
14.86
6.92
17.9
8.34
17.54
8.17
18.17
8.46
18.20
8.48
16.00
7.45
17.81
8.30
17.90
a34
0.39
12.15
17.59
15.39
15.97
FooOs
1.78
0.53
3.93
1.18
4.82
1.45
3.26
0.98
4.34
1.30
3.42
1.02
3.22
0.96
2.17
4.38
5.04
2.11
FeO
0.45
1.43
6.30
1.40
1.25
0.28
2.07
0.46
6.99
1.33
3.82
4.1
0.91
6.03
1.34
6.69
1.48
1.52
0.33
0.83
0.18
0.83
■ 0.18
4.11
0.91
Trace
0.11
0.02
0.10
0.12
0.03
Trace
CaO
6.11
1.74
3.60
1.00
6.44
1.84
LOO
0.54
5.65
1.61
6.19
1.77
5.51
1.57
4.45
1.27
5.12
1.46
6.15
1.47
6.87
1.67
0.54
6.00
0.73
3.92
1.57
1.79
0.71
3.60
1.42
3.64
1.40
1.3
0.52
1.76
0.70
0.61
0.24
2.40
0.96
2.90
1.16
2.07
0.63
2.06
0,82
0.81
Trace
4.41
2.85
3.40
Na,0
3.52
0.91
3.58
0.92
3.20
0.83
3.99
1.03
3.46
3.22
0.83
2.0
0.51
3.71
0.96
3.20
0.82
3.31
0.85
3.87
1.00
4.27
1.10
4.44
1.14
2.07
0.53
L94
0.45
LOl
2.36
K.iO
2.38
0.40
2.41
0.41
3.37
0.57
L69
0.29
2.02
0.U
L91
0.32
3.6
0.61
L41
0.24
3.52
0.60
1.30
0.24
2.65
0.45
2.26
0.38
2.22
0.37
3.10
0.54
2.19
L23
3.08
LijO
Trace
Trace
COj
Otlier components.
P205.
.0.23
0.13
.0.30
0.11
.0.44
0.25
Pyrite .
Pyritel.84; Pj Oj
0.34.
Pyrito3.58i PjOs
trace.
Pyiito2.84! PjOs
. trace.
Pyrite 9. 18; PjOt
trace.
Ignition.
L94
L96
3.10
3.62
2.44
2.47
2.26
2.1
2.31
4.35
0.70
. 2.80
2.00
0.88
0.95
6.53
L83
8.09 HjO
4.19
2.80
9.95 HjO
09 39
98.85
100.75
100. 61
100.24
99.78
100. 50
101.9
100. 39«
101. 03
99.954
100. 02
100. 03
99.96
99 54
100. 36
100. 02
99.07
100. 24
100. 34
101. 00
Specific
gravity.
2.65
2.71
2. 9582
2. 7972
2. 63, 2. 67
2. 72, 2. 76
2.6
2.4, 2. 5, 2. 6
2.5,2.4
Oxygen ratio of—
3.07
2.23
4.71
3.51
5.39
3.06
5.40
4.06
4.46
3.55
6.92
a 19
a34
9.70
&46
10.47
10.71
&75
8.34
9.71
SiO,
32.43
32.43
32.59
32.59
31.10
31.10
31.53
31.58
31.28
31.28
o
0.S73*
0.285'
0.307^
0.319^
0.397^
0.415*
0.331'
0.446'
0.46?'
aiog*
0.423'
' Including titanic and phosphoric acida. 'Supposing all the iron present as ferrous oxide.
tGeol. Comstock Lode, Vol. III.]
* Supposing all the iron present as feme oxide-
' Those totals do not agree with the items, no doubt in consequence of misprints ; the oxygen-contents of each constituent, however, cojrcspouds
to the percentage of the oxide given, and the errors therefore probably occur in the statements of the carbonic ..c.d or of the loss by igmfon.
ANALYSES.
153
Table 5. — Analyses of Gomstock ores.
\
Califomiamine.
California mine. Ophir mine.
i
Yellow Jaf'ket TeUow Jacket
mine. mine.
67.5
8.75
1.30
2.25
1.75
.0!9
12.85
6.75
65. 783
11.35
1.31
2.28
1.76
.57
11. 307
6.145
63. 38
7.919
1.596
5.403
2.786
.059
14. 455
4.151
.087
98. 310
.693
96. 560
.160
Copper
.575
.150
.005
2.800
.050
.001
Silver
Gold
Lead
.25
.267
.429
100.00
100.00
99. 896
100. 00
100. 00
London.
Swansea.
G. Attwood.
W. P. Eickard.
W.F.Kickard.
\
Table 6. — Analyses of Gomstock ores?
Savage.
Kentnck.
Silica
83.95
1.95
1.25
.64
2.82
.85
1.75
.30
.36
1.08
.02
1.80
1.28
2.33
91.49
.83
1.13
1.37
1.42
.13
.41
.02
.12
.0017
.98
1.05
.59
Alumina
Protoxide of manganese
Magnesia
Lime
Gold
Bisulphide of iron
Potash and soda
Water
100.38 i 90.48 1
•W.G.Mixter.
A. Hague.
Table 7. — Feldspars of the Yovnger Hornblende-andesite, from Mount Rose^ Slide 474.'
Grammes.
Weight of rock treated by Thoulet's uiethod 11
Weight of material of specific gravity above 2.75 1.7
Weight of material of specific gravity between 2.75 and 2.70 1. 8
Weight of material of specific gravity between 2.70 and 2.68 1. 6
Weight of material of specific gravity below 2.68 5. 8
• Exploration of the Fortieth Parallel, Vol. HI., page 80.
2 Exploration of the Fortieth Parallel, Vol. in., p. 80.
'See page 67.
154
GEOLOGY OF THE COMSTOOK LODE.
The following analyses were made for Dr. G. W. Hawes by Mr. F. P. Dewey:
Feldspar of specific gravity between 2.75 and 2.70.
Per ceDt.
Atomic ratio.
Oxygen ratio.
SiOa
69.51
23.83)
1.545
7.48)
0.96 5
1.12)
5.355
0.9918
0. 2312
0. 1576
0.0981
10.11
2.46
1.606
1.
31.738
11.567
I 3. 992
7.95
2.89
1.
AliOi
Fe,0.
CaO .. - .
MgO
K2O
NaaO
99.79
Feldspar of specific gravity between 2.70 and 2.68,
1 Per cent.
Atomic ratio.
Oxygen ratio.
SiOj
AUOs
Fe,0.
CaO
MgO
KaO
FasO
62.29
20. 74 )
2.19 5
7.01 )
0.65 5
1.22)
5.25 5
1. 0382
0. 2150
0. 14] 4
0.0975
10.65
2.20
1.45
33.22
10.32
I 3.82
8.69
2.96
1.
99.35
Table 8. — Assays of Gomstock rocks J
By J. S. Curtis.
Rocli.
Locality.
£.■
|i
■3
Granite
Granular diorite
Do :
Do
Do j Most westerly dioi'ite cropping ..
Darlf , fine-grained diorite ' McEibben tunnel
Dark, coarse-grained diorite j Bottom of Union shaft, 2,625 feet
Red Jacket mine
Bullion Ravine, at intersection of Water Company's flume .
Bullion Ravine, 200 feet above flume
Bullion Ravine, 2,000 feet above flume
Porphyri tic diorite
Do
Do
Granular diorite
Micaceous diorite-porpbyry.
Do
Earlier diabase
Head of Ophir Ravine, decomposed
Ophir, 2,500. Union connection, decomposed
Savage, 2, lUO, south drift 20 feet from east cross-cot .
Caledonia
Overman, 1,600, 250 feet east of shaft
800 feet east of Waller Defeat shaft
Sutro tunnel at Savage connection, fresh
$0.03
0.19
0.04
0.03
0.08
0.11
0.15
0.17
0.30
0.12
0.05
0.11
0.07
0.22
' For remarks on these assays see page 223.
ANALYSES.
155
Table 8. — Assays of Gomstock rocks — Continued.
Earlier diabase
Do
Do
Do
Do
Do
Do
Do
Do
Do
Do
Later diabase
Do
Black elate ,
Metamorphic diorite
Quartz-porphyry
Do
Earlier bomblende-andeaite
Do
Do
Do
Augite-andesite
Later honiblende-andesite .
Do
Basalt
Sutro tunnel, 50 feet north of junction with North Lateral, fresh
Overman, 1,600-foot level at main winze, somewhat decomposed
Sierra Nevada, 2,500-foot level, end north drift, somewhat decomposed
0. t£ O., 1,650, 116 feet from shaft, west drift, somewhat decomposed
Sutro tunnel, 19, 100 feet, somewhat decomposed
Sutro tunnel, 50 feet west of South Lateral, somewhat decomposed
Sutro tunnel, Noi-th Lateral, 1,000 feet north of C. t£ C. connection, much de-
composed -
Sutro tunnel, North Lateral, 600 feet north of O. d: G. connection, highly de-
composed, charged withpynte
Sutro tunnel. South Lateral, 250 feet north of Julia, highly decomposed,
charged with pyrite ,
Sutro tunnel, 50 feet west of South Lateral, highly decomposed
Sutro tunnel. North Lateral, 250 feet north of G. & G. connection, highly
decomposed, charged with pyrite
Gkollar, 1,900, 40 feet east of incline, fresh
Julia dump, fresh
Charged with pyrite
Amazon dump
Galedonia, 1,400-foot level, 350 feet east of Caledonia shaft
Quarry, 1,500 feet southwest of Justice
North Twin Peak
Spur northeast of Combination shaft
1,200 feet northwest of Geiger Grade Toll-House ,
Near Vivian mine
Forman shaft tank, point 6,158
Quarry northeast of Sutro shaft IH
Quarry near Utah mine
1,250 feet southeast of Eoux' Kanch
$0.20
O.IS
0.17
0.07
0.14
0.11
0.11
0.11
0.11
0.05
0.11
0.14
0.08
0.00
0.03
0.00
0.03
0.05
0.04
0.04
0.14
0.03
0.17
CHAPTER IV.
STRUCTURAL RESULTS OF FAULTING.
Views of previous observers. — Before proceeding to a description of the occur-
rence of the rocks forming the subject of the preceding chapter, it seems
necessary to discuss the faulting action traceable on and near the Lode, for it
has had an important share in determining the present position and relations of
the rocks. As has been seen in Chapter IL, Baron von Richthofen regarded
the Lode as a true fissure, only following the contact between the syenite
(diorite) of Mount Davidson and the east country rock for a portion of its
length because of the low resistance offered by this contact. He also insisted
that faulting both preceded and followed the deposition of ore. He does not
state, I believe, whether he regarded the west wall of the lode as a continuation
of the exposed surface of Mount Davidson, but implies that it is not, for he
speaks of the course of the vein as "somewhat" dependent upon the shape
of the slope. Mr. King, at the time of writing his memoir, considered the
vein as lying upon a continuation of the slope of the exposed west country,
an opinion to which he was led by the striking resemblance between the
contours of the west wall and those of Mount Davidson. Subsequently,
from an examination of the character of the west wall, he came to the con-
clusion' that the contact between east and west country was itself a faulted
surface. Mr. Church recognized abundant evidence of faulting action, but
regarded the contact of the east and west country as continuous with the
exposed surface.
' Privately communicated to me.
156
STRUCTURAL RESULTS OP FAULTING. 157
Evidence of faulting. — The evideiice of faulting is manifold. The irregular
openings of the vein, the presence of horses, the crushed condition of the
quartz in many parts, the presence of slickensides and of rolled pebbles in
the clays, are all conclusive on this point. Both to the east and west of the
vein, too, the country rock shows a rude division into sheets, and along the
partings between the plates evidences of movement are perceptible, decreas-
ing in amount as the distance from the vein increases, according to some
law not directly inferable. All the evidence points to a relative downward
movement of the hanffina: wall.
o ft
The question of the character of the west wall, whether it is a faulted
surface or a continuation of a former exposure of the east front of Mount
Davidson, is not to be settled by mere inspection. A cross-section, to scale,
taken from Mr. King's maps, shows immediately that while the dip of the
lode is 45° or more, the maximum slope of Mount Davidson is about 30°.
This fact, taken in connection with the character of the west wall where
exposed, indicates that the surface is a result of faulting. A natural surface,
too, sloping for a long distance, at an angle of about 45°, is very unusual.
On the other hand the coincidence between the contours of the west wall
and those of the exposed surface has been recognized from the earliest days
of mining on the Lode, and it seems a less violent supposition that the steep
face of the mountain passes over into the still steeper wall of the vein, than
that the range has experienced an erosion modifying its angle 15° and more,
and has still retained the details of its topography otherwise unaltered.
It is plain that the elucidation of the faulting action on the Comstock
is a very important structural problem, and that it is most desirable to
account quantitatively for the results as well as to prove the existence of a
notable dislocation, and no apology is therefore required for presenting to
geologists a somewhat detailed discussion of the principles involved.
Action offriction on the surfaces of a single plate. — The most Striking and widespread
evidence of the faulting is the apparent relative movement on the contact
surfaces between more or less regular sheets of the east and west country
rocks for a long distance in both directions from the Lode. Each sheet
appears to have risen relatively to its eastern neighbor, and to have sunk
158 GEOLOGY OF THE COMSTOCK LODE.
as compared with the sheet adjoining it on the west. The consideration of
a sheet or plate of rock imder the influence of friction of a relatively
opposite character on its two faces, therefore, forms the natural starting
point for an examination of the observed conditions.
Friction a force. — What is callcd frictlon^ is a complex phenomenon which
has never been satisfactorily reduced to a mathematical expression, and is
perhaps incapable of such a reduction. It is usually regarded as a mere
resistance, a force to which the negative sign is indissolubly attached. Pro-
fessor Reuleaux^ has insisted upon the incorrectness of this view and has
1 It is generally considered that the sensible movements, say of a rough block of stone dragged
over a pavement, are of the same character as those involved in the friction of smoother surfaces. On
the larger scale it is plain that projections of the moving body will meet those of the underlying sur-
face, and exert a pressure upon them precisely as in the case of the teeth of gearing. When the
draught has reached a certain intensity, and when the points of contact are small surfaces, approxi-
mately normal to the direction of translation, the projectious on one or the other surface will give way,
and heat will result. If the areas of actual contact are small surfaces, inclined at a considerable angle,
the moving body will rise to surmount them. In falling again a portion of the energy of position will
be converted into heat by the impact, but as all bodies are to some extent elastic, the energy of position
will not all be dissipated.
If a block of granite is at rest upon a pavement, it assumes the lowest possible position, the max-
imum number of points of contact are established and the projections on the two surfaces overlap to
the greatest possible extent. When the same block is set in motion, the energy imparted to it prevents
its settling into maximum contact.
It is plain that the resistance of the block will be greatest at the moment when motion begins,
or that the so-called friction of rest is somewhat in excess of the friction of motion. It would also
seem that the friction of rest is merely the maximum value of the friction of motion, and such is the
result of the recent investigations of Messrs. Jenkin & Ewing. The greater the velocity of the moving
body the less thoroughly will the projections of the two surfaces interlock ; on the other hand,
points which at a low velocity would meet one another nearly in vertical lines, will at high velocities
meet on a line considerably inclined, and the horizontal component of the elastic force developed by
impact will act as a resistance. Morin took the elasticity of carriage springs into consideration in deter-
mining the resistance of a pavement to the passage of vehicles. It appears to me that it must also
enter into the true expression for the coefficient of friction. The excess of the friction of rest over that
of motion is evidently due in part to the fact that when at rest the energy of position which must be
overcome is at a maximum, while after motion has set in a portion of this energy is elastically returned
to the moving body. Besides those elements of friction which have been mentioned, adhesion also
undoubtedly plays a part, at least in the case of very smooth surfaces.
The following deductions from the experiments of Coulomb and Morin are approximations only :
(1.) Friction is proportional to the pressure normal to the contact of the rubbing surfaces.
(2. ) It is independent of their extent.
(3.) It is independent of their velocity.
According to Rankin the excess "of friction of rest over the friction of motion is instantly de-
stroyed by a slight vibration." A vibration of course develops the elastic force.
The friction of hibricated surfaces appears to me wholly different from that of dry ones. A shaft
should not come in direct contact with its bearing, and the work done would seem to consist in a very
active stirring of a thin layer of oil. The amount of this work will be dependent on the adhesion of
the lubricator to shaft and bearing as well as upon the geometrical character of the solid surfaces.
^The Pneumatics of Machinery, by F. Reuleaus, translated by A. B. W. Kennedy, p. 595. The
translator states that similar views are maintained in Bell's Experimental Mechanics, a work I have not
met with.
STEUCTURAL RESULTS OP FAULTING. ] 59
given instances from which it appears certain that friction, like otiier forces,
may cause or accelerate motion as well as retard it. He does not, however,
explain how positive forces result from friction.
Transmission of energy by friction. — Material surfaces arc distluguislied from
mathematical planes by the presence of minute projections and depressions.
If a material sheet W is forced to move over a sheet Pj, the projections
interlock, and if the sheets are prevented from moving in the direction of
the normal to their contact plane, the projections must either be ground off
or be bent and compressed. If W begins its motion with a fixed quantity
of energy, and if P^ is fixed, the entii-e energy will ultimately be expended
in heat, sound, etc., on the contact. But if P^ is movable a portion of the
energy of W will be communicated to Pj, because the projections on the
under surface of W exert a pressure on those presented by the upper sur-
face of Pi, which is either in the direction of the motion of Wor which may
be resolved into two pressures, one of which is in the direction of the
movement and the other normal to the contact plane.
Distribution of energy through a system of sheets. If Pj is lH. COntaCt wlth a third plate
or sheet Pa the energy received by Pj will be expended wholly or in part
in overcoming the resistance on the contact Pj Pg. If these sheets are the
earlier members of a series of sheets W, Pj, P2, P3, , of indefinite
number, then each sheet which moves will communicate a certain amount
of energy to the next, and since the resistance of friction is proportional to
the distance through which it acts, each sheet which receives energy from
its predecessor must move.
The velocities of moving sheets may be treated as uniform. SuppOSe a SyStOfU of CQUal
sheets of indefinite extent vertically arranged and terminated at the top
by the horizontal plane A B. Let the system be under a compressive hori-
zontal pressure. If, through the action of some external force, W rises
through a distance b, it will communicate a certain energy to Pj, which will
in turn impart energy to Pg, and so on. Since the sheets are in all respects
alike and the pressure at each contact is the same, the frictional resistance
or negative force at each contact will also be the same, while, as more or
less vibration must always accompany faulting, the friction of quiescence
does not need to be taken into consideration ; but as energy is dissipated at
160
GEOLOGY OF THE COMSTOCK LODE.
each contact, the velocity of the sheets will not be equal. According to
Morin's law, however, the truth of which will be assumed, the frictional
resistance is independent of the velocity. The sheets will start and stop
Fig. 3. — System of equal, vertical, movable sheets.
at the same instant, and there is no error, except that inherent in Morin's
law in supposing each sheet to move throughout its path at a uniform
velocity.
Ratio of the movements of sheets. Lct tllC tOtal mOVemCUt of P„ bC 5„ and ItS
entire movement up to a given instant be y^. Then, since the velocities
may be regarded as constant,
— — r- — ^0
and
Vn+l
— 1 — ^n >
■'n+1
or the ratio of the movements of any two adjoining sheets is constant. Since
each sheet controls the movements of all its successors, of which there are
supposed to be an infinite number, each sheet bears the same relation to
those which follow it. If the first n sheets were rejected and P„ were forced
STRUCTURAL RESULTS OF FAULTING. 161
to move through a distance b, P„^.i would move through a distance h^. Hence
and
or the movements of any two successive sheets are in the same constant ratio;
Hence, too,
K+c
Locus of the edges of sheets. — If BC is takcu 38 the a:;-axis of the locus of the
edges of the sheets and TFP, as the ^-axis,
y» _ y _ ^dx
Vx-Ydx y+dy '
or
-^ z: 1 — m^^ z=z — In mdx;
y
whence
yz=.Am~'', (2)
which is the ordinary logarithmic curve and the equation of the locus of
the projecting edges of the sheets. The locus of the other edges found at
the reentrant angles is
Modification for case of a finite number of sheets. — lu auy natural Or experimental
case the number of movable sheets will necessarily be finite. The locus
whicli will be formed if P„ is fixed, can be obtained by supposing that after
the infinite curve
y' = Am~^
has been formed, P„ and all its successors are forced back to their original
position on the line AB. Each sheet from W to P„_i would then be drawn
down through a certain distance, which can readily be shown to be given
by the equation
y" =z Km'-'' — j:m^-2".
The locus actually assumed will therefore be
y — y' — y"—Am"' — Awi'-^'' — Am-^( 1 — Hi'<^-"').
Comparison of the two loci. — Thc variatiou of this equation from the logarith-
11 c L
162 GEOLOGY OP THE OOMSTOCK LODE.
mic curve is great when the number of movable sheets is small, but when
this number is great the effect of y" on the locus is imperceptible. If, for
example, m^\A and n =: 25,
fe„ — Am-'' — A lA-""'- - 0.00022^,
which on ordinary scales would be scarcely visible. The value taken for m
is one which has been noted in experiments to be described on a succeeding
page. If 0.0001^ is regarded as a negligible quantity, then the locus of
the edges of the sheets may be regarded as coincident with the logarithmic
curve y ■= Amr'^ when for the first fixed sheet P„
4
vC^
log w'
Logarithmic distribution of energy. — The force cxcrted at cach contact of a system
of sheets is that of friction, and when the friction is uniform throughout
the system, only the distance through which the force acts at each contact
varies with its distance from the first contact. If on the contact P„ P„+i the
surfaces are such as to present a greater or smaller number of opposing
projections per linear unit than exists upon other contacts, the force or friction
would also differ. But the energy received by P„ would be unaffected by
this difference, and the ratio of the energy expended upon the contact
Pti -P»+i to that transmitted to subsequent contacts will depend not upon the
number of projections but upon the physical (elastic) properties of the
material of which the sheets are composed. By Morin's law the friction,
and therefore also this ratio, are unaffected by the velocity, and the same
amount of work will consequently be done on the contact P„ F^+x as if the
friction were the same as on other contacts. If the whole energy applied
to the system is ^, and if the frictional resistance on the successive con-
tacts is/, /i, /a, etc.,
^=/(6-ii) +/(&,- 62)+ • ■ • +/„(^'«-^'n+i)+ • • •
The absolute movements of the sheets will be dependent upon the total
energy and upon the different resistances, and so also will be the curve or
broken line assumed by the edges of the sheets; but any term
/« (^„ — ^n+l)
STRUCTURAL RESULTS OF FAULTING. 163
is dependent only upon E. If w is the work done on any contact,
and if L denotes the work done on the first contact, WP^, the general
equation for the work on all contacts is
or the distribution of energy is logarithmic however the friction may vary,
so long as the material composing the sheets is the same throughout the
system, and supposing friction independent of velocity.
Morin's law is mei'ely an approximation, but should an exact relation
be discovered between friction and velocity it would be an easy matter to
give the variation of the friction its proper weight in the equation for a
faulted surface.
Locus of edges of sheets when the friction varies regularly. CaSeS may rCadily aHsO lu
which the friction varies regularly from contact to contact, as would hap-
pen for example in a system of sheets between which the pressure was pro-
duced by the weight of the sheets themselves. Suppose the case of friction
increasing from / at the contact TF P by a small increment ft. Then for
any distance x from the origin, the frictional resistance will be/(l+ ^0-
If dx is the thickness of a sheet, the relative motion at x will be dy and the
work done/ (1 + xt) dy. If the friction were constant and equal to/, the
work done on the same contact would be derivable from an equation, say
y-^z=.Am~^,
and would amount to
fdy-iz^ — -fA In m nr^ dx ;
and since it has been shown that the work on any contact is independent
of the frictional resistance,
f{l-\-xt)dyz:z—fA In mm~^dx;
or
^ , pmr^dx
y=-AlnmJj~^,
which is not integrable when m > 1.
Approximate equation. — If thc pressurc Is produced by the weight of the
sheets and if these are numerous, t is a very small quantity and its square
164 GEOLOGY OF THE COMSTOCK LODE.
may sometimes be to the senses a vanishing quantity. When this is the
case the equation
\-\-xt
sensibly represents the locus. For the value of iv may be written
Jyf{\+xt)=f(h-h,)m-^
or
^ l4-xt \l4-xt \+xty
'l+xt \l-i-xt I+rct
while the approximate equation gives
^ \l+xt . . xt J
and since
the two equations give the same results, if ^2 is inappreciable.
It has already been pointed out that, since the distribution of energy
is logarithmic, the sum of the relative movements is dependent on the vari-
ation of the friction. If therefore the friction is a minimum at the contact
W Pi, a greater amount of energy will be required to move W through a
distance A than if the friction were constant. The total energy required
will be the same as it would be if each relative movement took place by
itself Assuming the approximate equation deduced for this case, it can
readily be shown that, if W moves a distance A, the total energy required
by the system is
-^^O + ^fTT^)-
Since there is nothing essentially positive in the nature of t, all the
foregoing equations become applicable to the case of a decreasing frictional
resistance by merely reversing the sign of t. Landslides might furnish
cases of this character. Suppose a mass of material divided into sheets
resting on a hillside, and that through weakened coherence the mass de-
scended such a distance as miglit be necessary to do a work f A om. tlie
STEUCTURAL RESULTS OF FAULTING. 165
contact W Pj. This energy will be distributed through the system, and
were the friction uniform the resulting curve would be a simple logarithmic
one. But as the friction will decrease towards the surface, the locus will
be approximately
Amr"
To produce this configuration, however, an energy of only
is required, and the system will consequently reach it with a vis viva
fAt2^=fA,.
, 1 — xt
The system will continue its movement till this energy is expended and its
final configuration will be ^
y = {A + A,)^_
■xt'
Experimental verification. — If the various assumptious made are correct, a
fault under certain conditions will result in a surface, a vertical section of
which at right angles to the strike of the fault will present a logarithmic
curve. Before proceeding to any further deductions, it is evidently desir-
able to test the correctness of the postulates experimentally. I have sup-
posed the sheets of rock of infinite size as compared with their exposed
margins, because on this supposition the pressure per unit of area of each
parting will be the same. If the plates were thoroughly flexible, and if
the pressure were applied on a limited zone parallel to the croppings and
removed by a distance greater than h from either end of the plates, then
the pressure exerted on each plate would be the same, and would be dis-
tributed over an equal area, and the resulting curve would still answer to
the general formula deduced. These conditions we can approximately
reproduce. If a pile of, say, one hundred slips of very thin, flexible and
uniform paper, eight or ten inches long, with sharply cut edges, are laid
upon a flat surface, and a narrow weight of three or four pounds is placed
across them, the pressure under the weight may be considered as constant.
166 GEOLOGY OF THE COMSTOCK LODE.
In the experiments I have made the weight employed was about 5,000
times as great as that of a single slip. If a blunt edge, such as that of a ruler,
be now applied at right angles to the longer dimension of the slips, close to the
weight, with a light pressure, and be drawn away from the weight a fraction
of an inch, a slight relative movement will be perceptible. If this applica-
tion of energy to the system be repeated a score of times, the ends of the
pile of slips will be found to form a curved surface instead of a plane ^
If the frictional resistance is proportional to the pressure, this curve must
sensibly coincide with that given by the equation
y=
l+xt'
for i"— p—TT-a , and will altogether escape detection. The thinness of the
paper considerably obscures the character of the curve, but there is no
error in principle involved in plotting it on the assumption that the sheets
are of any thickness which may seem best adapted to bring out its geo-
metrical relations. For the given increment the curve will approximate
pretty nearly to the simple logarithmic curve. For the one hundredth-con-
tact the latter would give *
and the equation for increasing pressure
_ Am-""
^~1+U.02'
or
y,=1.02 y.
Unless the experiment is carried on until the lowest movable sheet has
traversed a sensible distance, the original position of the edges of the sheets
marked by the fixed slip gives the asymptote of the original curve. Fig. 4
on the next page shows a curve AB plotted from experiment with its asymp-
tote, and a logarithmic curve CD of the form y=iAmr-^ plotted from its equa-
'I noticed long since that pressmen in printing offices, by drawing the thnmb-nail across a pile
of sheets, force each of the upper sheets to project beyond the one next beneath it, so that one sheet at
a time can be removed conveniently and without delay. I observed that a regular curve resulted, but
jiresumed that it was a conic section. Having satisfied myself analytically that the curve produced
by faulting was log.arithmic, this observation recurred to me as a means of testing my results experi-
mentally.
STRUCTUEAL RESULTS OF FAULTING.
167
WMBk
tion. The deviation is exceedingly slight, and the experimental curve
stands almost as well as the other the very delicate constructive test of the
equality of subtangents.^
Variations of the experiment. — The sHps I liave cmploycd are of a nearly iincal-
endered paper. If for
one of them a highly
glazed slip is substituted
a comparatively large
relative motion takes
place on its surfaces, but
the only visible effect
which the introduction of
such a slip produces on
the locus of the others is
a dislocation at the point
where it is inserted.
There is in fact no evi- Fig. 4.— Calculated and observed eiirves.
dence that the work done on any contact is altered by the introduction of
a contact offering a smaller frictional resistance.
If the ends of the slips at the beginning of the experiment occupy an
inclined instead of a vertical plane, the result is a logarithmic curve referred
to axes inclined at the same angle In plotting it is well to reject the
upper three or four slips, because these are principally affected by inequal-
ities in the application of pressure and draught.
By employing a system of from three to ten slips of heavy writing
paper, using a thick pad of blotting paper for a support, and applying the
■Such an experiment forms a check upon the theory, but does not furnish absolute proof of it,
because arcs of other curves, known or unknown, might be constructed which would agree very closely
with the experimental result. Among familiar curves, that presenting the greatest general similarity
to the logarithmic curve is the hyperbola referred to in its asymptotes, and a hifperbolic arc very closely
agreeing with the experimental curve can be calculated. But the experiment gives the position of the
asymptote which for the nearest hyperbolic arc would occupy a distinctly different position, and the
supposition that the curve was hyperbolic would also lead to seemingly untenable hypotheses as to the
communication of energy. All that can be claimed, however, strictly speaking, is that the theory ac-
counts for the facts within the limits of the errors of observation, and that no other equally plausible
explanation of the facts has suggested itself to me,
168
GEOLOGY OF THE COMSTOGK LODE.
draught with great care, the locus
can be produced on such a scale that both its elements are sensible.
Reduction and interpretation of the equation. — A few data as to the com-
putation and representation of the logarithmic curve may be of use to
those who bave to do with special cases of faulting, either technically or
geologically.
Equation referred to the cropping as origin. In the form of the CqUatlon deduCed,
yzz:Am~'', (1)
the curve is referred to its asymptote and the fault line as axes. In ascer-
taining the value of the constants applicable to any given surface, however,
it will be more convenient to refer it to the fault line and a line perpendic-
ular to the latter at the point where it reaches the earth. If the fault dips
at 90°, and if the original surface was level, the equation will then be
y=A{m-^-\) (2)
If the original surface was not horizontal, but formed an angle 5 with
the a;-axis, then retaining the same axes each y will be diminished by x tan 3,
and the equation becomes
y—A (m-^— 1)— ictan 5; (3)
and in this case the asymptote of the curve would still cut the y-axis at —A,
but would make an angle 5
with the ic-axis or would be
parallel to the original sur-
face. Since the angle S^
merely expresses the rela-
tive directions of the .x-axis
and the original surface, this
equation is general, and ap-
plies, whatever may be the
dip of the fissure and what-
FiG.5.-j,=^(m-.-i)-xtau3. ^^,^^ ^^^ ^i^^^ been the
slope of the original surface. If f3 is the dip of the fissure and d is the
slope of the original surface, Ave also have
5 = 90°-/?zb<^,
STRUCTURAL RESULTS OF FAULTING. 169
in which d has the positive sign if the surface sloped in the same sense
as the fissure plane, and the negative sign if the dip and the slope were in
opposite directions.^ This formula therefore makes it possible to recon-
struct the original surface, in so far as it is unmodified by other causes.
Reduction of equation to simplest form. — Equatlou (3) is thc most couvenient form
for the calculation of the constants involved, because the direction of the
«/-axis, and commonly also the position of origin, can be directly observed,^
bat for plotting and for some purposes of discussion the equation can be
advantageously reduced to another form. The equation of the asymptote is
y-\-A^x tan S-.
If, therefore, we refer equation (3) to the intersection of the asymptote and
the «/-axis and adopt the asymptote as a new z-axis, (3) will reduce to the
form
' I have preferred to characterize these angles in this way rather than to adopt the ordinary hut
not universal oonventiou as to positive and negative augles, because this is a discussion of structural
geology. Tlie mathematical question involved is simply whether /3 and S lie in the same quadrant or in
adjoining ones.
For similar reasons common logarithms instead of natural logarithms have been used in all for-
mulas, the direct applicability of which to natural occurrences renders it possible that computations may
be based upon them.
^In computing the logarithmic curve which most nearly applies to a given surveyed section line
it is necessary to know the dip of the iissure and the position of three points on the surface relatively
to the rectangular coordinates the origin of which is the cropping and the y-axis the dip-line. The
computation is greatly simplified by so selecting the arbitrary values of x (xi, xi, X3) that Xi =^X2=^J3.
The three equations then become
j/i = ^{m~^' — 1) — x, tan 9;
1/1 = A (m~-^' — l) — '2xi tan 5;
y3 = A (»»-**'— l) — 4xi tan S.
Solving these equations for the three constants, it will be found that
log m-
A = — ^2/1— J/'!
tan ^^A
(m-="— 1)''
(m-»'-l)-y,
170 GEOLOGY OF THE COMSTOCK LODE.
Mere inspection also shows that
Xi-:^x cos 5,
and the equation refeiTed to the inclined coordinates indicated will be
y=A'm-'""'^\ (4)
By a proper selection of a unit and by removing the origin to a different
point on the a;-axis according to well known rules of analytical geometry,'
this equation may be reduced to the form shown in Fig. 6,
2/=lo-^ (5)
or
x=. — \ogy;
and the points on the curve may be directly plotted from a table of log-
arithms. The curve evidently cuts the ^/-axis at the point where y is equal
to the natural unit y, found as indicated in the foot-note. If the equation
were plotted on rectangular coordinates, ^ would also be the constant value
'As the COMSTOCK Lode excites a lively interest in many localities where books of reference are
rare, it may be a matter of convenience to some of my readers to give this reduction in fnll.
Let
A = cos 3 log m,
or
10'' = m'^<""»;
then introducing this value into (4), we have
Let the origin be transposed ou the x-axis by a quantity a, yet to be determined ; then
;i=A 10-''<*+"'=^ !()-'"' lU-*^.
Now let
logM-logj4
h
The introduction of this valne brings the equation to the form
%=io-'^,
because for the chosen valne of a
If, further, —is taken as the unit and x and y are each multiplied by it, we get
h
STRUCTUEAL RESULTS OF FAULTING.
171
of the subtangent, and the curve would cross the y-a\is at an angle of 45° ;
but this is not the case when the equation is interpreted on oblique coor-
dinates.
Fig. 6. ^ = lO--".
Point of minimum radius of curvature. The pOsitioU of tlie IJ-'dXlS of tile logaHth-
mic curve depends upon the unit chosen. There is, however, one fixed point
on the locus, that of minimum radius of curvature. This must be deduced
from the general equation referred to rectangular coordinates (3), and the
value of X corresponding to it is
_log (4 A Inm) —log (V8 + 9 tan^^ — tan 5)
log m
From this formula the value of Xo for all simpler cases can easily be
derived. For the simplest equation, viz:
ln2 J 1
Xo=-~- and«/o=— -.
^ v2
Spacing of contours. — As tlic topographj of a country is usually represented
for geological purposes by contours, it would be interesting to discuss the
spacing of the contour lines on a map of a faulted surface. For an origi-
nally level surface and a vertical fault we have immediately
^x=]ogy—log(y+Jy) ;
in which ^ a; is the variable horizontal interval between contour lines and
172 GEOLOGY OF THE COMSTOCK LODE.
J y the constant vertical difference between contour planes. But the equa-
tion for the case of an oblique fault is so complicated as to be of no value.
The ideal map would be one in which the contour planes were so close that
— would be sensibly equal to ~j- ; and, indeed, where the slope is consid-
erable this is often the case, but when the surface-line becomes nearly-
horizontal the difference between the two ratios is large.
Angle of tangent to the horijonai. — The angle which & tangent to the curve
y = \Q-'
referred to inclined coordinates makes with the horizontal may be found as
follows, without going through a
troublesome transformation of coor-
dinates. Let dx and dy be the differ-
entials at the point of tangency
obtained from the above equation,
and dx^ and dy^ the differentials for
the same point if the ?/-axis were
Fig. 7.— Explanation of a faulted surface. vertical and the ic-axis horizontal.
Consider ^ as a positive acute angle and S also as a positive acute angle
when it falls in the same quadrant with /?, but as negative when it falls in
an adjacent quadrant. Let a be the angle which the tangent makes with
the horizontal. Then, as appears from the figure,
tan a — — - ^,
dx-^
and the equation of the curve referred to inclined coordinates gives
ylnlOzz:-^^,
dx
and by a simple projection
_ — c?«/ sin y5 -fete sin <5_
~~ — dy cos /3 -\- dx cos 8^
or by reduction
. _ y\n 10 sin yS -f sin (5
«/ln 10 cos/? + cos(5
STKUCTDEAL RESULTS OF FAULTING.
173
If (5 is a minus angle (the case shown in the figure) the curve will be
horizontal when
— sin (5 — 2/ In 10 sin /?,
or when
y =
— sin S
In 10 sin A'
but if (J is a positive angle (falling in the same quadrant with ft) the curve
will have no horizontal tangent.
Fault involving double curvature. — As has already bccu pointed out, since gravity
is likely to be an insignificant force compared with other forces acting on
the sheets of a faulted country, it is a matter of indiflFerence whether we
regard the actual motion of the foot wall as upward or that of the hanging
wall as downward. If, therefore, contrary to the assumption thus far made,
the foot wall instead of the hanging wall were divided into sheets, and if the
latter were to sink relatively to the former, we should get a reversed loga-
rithmic curve asymptotic to the original surface of the foot wall; and other
things remaining equal, its equation would be
yzz. A{\ — trf) -\- x tan S^.
If the rock on both sides of the fissure is the same, or possesses the
Fig. 8. — Double fault curve.
saine physical properties, and is divided into plates of the same thickness,
the energy brought to bear at the fissure will be distributed in both direc-
174 GEOLOGY OF THE COMSTOCK LODE.
tions on the contacts between the plates, and the cross-section of the coun-
try will show two logarithmic curves with a common tangent at the origin
in Fig. 8. Each curve can of course be reduced to the form
Case involving different rocks. — If the fissurc wcre ou a contact between two
different rocks, the one might be divided into thinner plates than the other,
and they might have different coefficients of friction. If the coefficient
being the same the thickness of the plates varied, the origin would remain
unchanged, but the curves would be different. The curvature depends on
the throw of the fault and on the number of partings, and it can readily be
shown that the natural unit of the curves formed will be proportional to
the thickness of the sheets of rock. The two curves will therefore not
have a common tangent. Conversely it is evident that the relative thick-
ness of the sheets is calculable from the observed curvature, but the abso-
lute thickness of the one or the other is a matter of observation. If the
coefficients of friction are unequal, the inequality will manifest itself only
at the contact, for the fundamental equation of condition
is independent of / so long as / is constant. The curves, however, will not
be continuous with one another. There is reason to suppose that, at least
between similar rocks, the difference of the coefficients of friction is very
small.
Faulting accompanied by formation of parallel fractures. it a lault takeS place OU a
fissure in otherwise solid rock, and if lateral pressure accompanies the dislo-
cation, a great amount of energy will be brought to bear at the fissure.
If, as before, the foot wall is supposed to rise, the hanging wall as a whole
may be regarded as a fixed mass either from its cohesion with the surround-
ing country, or from the indefinite amount of inertia which it opposes to move-
ment. As has been shown earlier in this chapter, friction is a force which
produces motion as well as destroys it, and Professor Reuleaux is doubt-
less correct in asserting that motion always results from friction, although
STRUCTURAL RESULTS OF FAULTING.
175
it may be "only as small alterations of form in the body acted upon"
Rocks are by no means absolutely rigid or absolutely inelastic, and under
the conditions supposed a strain must be produced in the hanging wall.
Sedimentary strata, and especially the coal measures, furnish innumerable
known examples of this action, indicated by the permanent flexure of the
ends of the strata as indicated in Fig. 9. This is of course a familiar fact
which has from time imme-
morial furnished miners with
a practical rule for recovering
the seam beyond a fault.
When a fault takes place
in the comparatively rigid
massive rocks a similar strain
must also be produced. Its
effect will depend upon its in-
tensity and on the elastic pro-
perties of the rock. These
latter are so little known that
it is scarcely worth while to *'i«- 9.-Fault accompauiedby astiain.
investigate the conditions mathematically, but it is certain that if the strain
surpasses a limit defined by the cohesion of the rock, a sheet of the latter
will be sheared off from the main mass. If the compression attending a
fault in a massive rock is very great, and if the rock is very rigid, this action
may be repeated indefinitely, and either or both walls may be divided into
sheets of nearly equal thickness and divided by partings nearly parallel to
the original fissure. On the other hand, if the stress does not reach the ulti-
mate cohesive resistance of the rock, the energy must be expended in heat
and a strain which will be permanent or not as the rock is elastic or in-
elastic.
Evidence furnished by observation. — In coal mlncs there is abundaut evidence of
permanent strains produced by faulting. In massive rocks a division into
sheets sometimes accompanies faulting, but it might be asserted that the two
phenomena were unconnected. A very unobtrusive structural action serves,
however, to establish a relation. In hilly regions where the soil is deep, small
176 GEOLOGY OF THE COMSTOCK LODE.
landslips are common during wet weather, often involving the movement
of only a few square rods of ground for a few feet. The material in this
case is far from rigid, but on the other hand it possesses a minimum of elas-
ticity. I have examined hundreds of such slips in the Contra Costa Hills
of California, and noted with surprise the fact that they are almost invaria-
bly accompained by a separation of the moving mass into sheets far more
regular than might have been expected, and parallel to the initial surface of
motion.
It does not appear to me that the character of the curve assumed by
the edges of the sheets will be affected by the consumption of energy in-
volved in shearing them from the mass of country rock, for the work done
at each fracture will be the same and the effect will appear in the constants
of the equation, not in the form of the function.
Frequency of compressive strains in faulting. DisloCatioUS of the Carth's SUrfaCe may
no doubt occur under the most various dynamical conditions, and no gen-
eral law can be laid down as to the presence or absence of tangential press-
ure. It is evident, however, that the lateral extension of a faulted ai-ea is
increased by faulting whenever the hanging wall sinks or the foot wall rises.
If A is one-half of the total slip measured on the dip of the fissure, the in-
crease of horizontal distance between any two points on the logarithmic
surfaces of the rising and sinking countries respectively, so far removed
from the fault plane as to occupy positions which are sensibly on the asymp-
totes of the curves, will be
2 A cos /?.
It is evident that this increase in lateral extension will be accompanied
by lateral pressure and consequent friction, unless the fault is the result of
a tangential tensile strain. The general theories of dynamical geology, and
the study of sedimentary rocks, however, show that strains in the earth's
crust are commonly compressive.
Surface produced when the fissure is a plane.— It haS beCU sllOWU that Uuder Certain
conditions the sui-face line of the cross-section at any point of a faulted
country will be a logarithmic curve, or a combination of two logarithmic
curves. If therefore the fault fissure intersects the earth's plane surface on
STRUCTUEAL RESULTS OF FAULTING.
177
a straight line, the faulted surface will be that which would be generated
by the horizontal movement of the logarithmic curve or curves along the
^-axis of the equation
y=:A{mr'^ — 1) — a;tanS-
and in the case of a double curve in an area of a single rock, or of
rocks with the same coefficient of friction, this ^-axis will be found at an
elevation equal to half the vertical distance between the asymptotes of the
curves.
Surface produced when the fissure is not a plane. CommOUly, howeVCr,. the intersec-
tion of a fault fissure with the earth's surface is not a straight line, but an
undulating or broken one. If we still suppose the original sui'face of the
area a plane, the surface after faulting will be that which would be gener-
ated by the movement of the logarithmic curve or curves along the broken
or undulating line corresponding to the ^-axis, and this line will be the locus
Fio. 10. — Cimlonr iiiiip of a l'aiiltf<l siiif'aci'.
of the point of inflection of the double curve. The line corresponding to
the tff-axis will then be the intersection of a plane parallel to the original
surface of the earth with the surface as modified by the fault, and if the
original surface was level, the intersection will be a contour. Each inflec-
12 0 L
178 GEOLOGY OF TQE COMSTOCK LODE.
tion of the trace of tlie fissure on the original surface concave toward the
lower country will be represented on the faulted surface by a ravine, and
each inflection convex toward the lower country will result on the faulted
surface in a ridge.
Fig. 1 0 shows a contour map of the country shown in Fig. 8, the fissure
having reached the original flat surface of the earth on the undulating line
AB.
It is evident that if the form of the trace were capable of expression
by an algebraic equation, the equation of the faulted surface could be im- ,
mediately deduced, but such cases are not likely to occur, as deviations of
the trace from the right line are probably due to local variations in the
physical properties of the rock. Even when the original surface was irreg-
ular the same law holds, vuifatis mutandis; for the locus of the point of
inflection of the double logaiithmic curve will still be parallel to the trace.
The edges of the sheets on each side of the fault will be parallel to the
locus of the point of inflection, and where this is a contour they will also
be contours.
It frequently happens that the dip-line of a fissure is straight and nearly
constant for long distances from the surface, while the strike varies. When
this is the case the intersection with the foot wall of a sui'face parallel to
the original surface at any depth below it will give tU.e same line, and if the
locus of the point of inflection of the sui-face curve is a contour, the contour
of the foot wall of the fissure at any point will be identical with it and
with those of the altered surface, as far as the faulting action extends
unmodified.
Fissures into the hanging wall. — TliB diagrams sliowat a glauce that when a
fault takes place under the conditions specified, the rock of the lower coun-
try near the fault, as seen in cross-section, assumes the form of a sharp
wedge, which is exposed to the same heavy pressure as the rock at greater
depths. In an actual case in nature, it is scarcely possible to suppose that
this wedge would remain intact. A very slight obstruction to the smooth
rise of the foot wall would produce a crack across this edge at some consid-
erable angle to the dip of the fissure, and such a crack might very probably
be held permanently open b}' fragments of rock. Fissures diverging into
STEUCTUEAL EESULTS OF FAULTING.
179
the hanging wall might not unlikely form at greater depths as well, but
would partly close again, leaving behind only openings of limited size,
because the pressure and motion of the supei'incumbent mass would suffice
to grind to powder most of the intervening fragments.
Relation of chimneys to surface topography. If in faulting, the rislng COUntry shiftS
in the direction of the strike of the fissure, of course chimneys will form
where the strike undulates. Where the surface is modified by faulting in the
manner discussed, such chimneys will always lie on the same side of ravines
on the surface, and opposite them will be found crushed ground arising
from the pressure of the walls upon one another.
Infrequency of a rise of the hanging wall, TlirOUghoUt the foregolug dlsCUSSioU I
have supposed that the relative movement of the foot wall of the fissure was
upward, according to the well-known empirical rule. Were the reverse
case to occur, the resulting curve would still be a logarithmic one, but
would be constructed in the acute angle between the fault line and the
asymptote parallel to the original surface,
and unless faulting has gone on but to a very
slight extent, or unless the fault line dips at
very close to i)0°, the resulting surface will
not merely be precipitous, but form a reen-
trant curve, and the upper country will over-
hang the lower (Fig. 11). Countless faults
have been formed in past geological eras,
the surface indications of which have been
utterly obliterated, but there must be a very
great number which still exliibit their features ^^^^- n-Kise of the banging waii.
in a recognizable form; and if it were a usual thing for the hanging wall
to rise, overhanging surface would not form one of the rarest of topograph-
ical phenomena.
Applications of the theory to the Comstock, and other instances. — The evi-
dences, already alluded to, of the division of the east and west country
of the Comstock Lode into parallel sheets lend probability to the suppo-
sition that the faulted structure of the central portion of the vein may
180 GEOLOGY OF THE COMSTOCK LODE.
come under the conditions which have been explained in the preceding
portion of tin's chapter, and, as a matter of fact, if the Sutro Tunnel sec-
tion be taken as a representative one, it is easy to find a logarithmic curve
which shows a close coincidence with the surface. The eastern and western
branches of the curve referred to the fault line, and a perpendicular to the
fault line at the cropping of the vein are, respectively,
2/,=:147()"- (1.00161-"—!)— iCi tan 44° 27',
2/2 = 1470"- (1 -1.00298^0+^2 tan 44° 27'.
Knowing these values, the experiment on slips of paper can be modi-
fied to obtain a corresponding result. The only change needful is to pile
the slips in such a way that their ends instead of falling in a vertical plane
will lie in a plane forming an angle of 45° 33' with the table. The result
is a curve, which, when plotted on the assumption of a suitable thickness of
the sheets, is indistinguishable from that of one or other of the above equa-
tions. Precisely as in the former experiment, too, the position of the asymp-
tote precludes the supposition that the curve is hyperbolic. There is,
therefore, very strong reason to believe that the Sutro section surface line
is composed of two logarithmic curves, and no reason known to me to sup-
pose that it is not.
Atlas-plate VII. shows the surveyed surface line of the Sutro section
plotted from the contour map, and in the same figure the curve plotted
from the equations given above. The same plate also shows the curves
represented by the equations plotted by themselves with their axes and
asymptotes, and the curve obtained from experiment. By comparing the
surveyed line with the surface maps, it will appear that its deviations from
the curve given by the equations are the evident results of plainly limited
erosion, the section crossing two considerable ravines in the east country,
and passing along the flank of another in the west country.
Constants. — Tlic dislocatlou measured on the dip of the lode is 2 A, or, for
the present case, 2,940 feet. The dip of the lode at this section is 43°, and
the dislocation measured vertically is therefore 2,005 feet. The angle S- is
44° 27' and 6 is therefore 2° 33', or the original surface sloped contrary to
the dip at this angle. The natural unit of the east curve is 2,012 feet, and
STRCJCTURAL EESULTS OF FAULTING. 181
if the equation is referred to the asymptote and a Hne parallel to the fault
line and crossing the asymptote at 274 feet west of the fault line, it becomes
for tlie natural unit,
The natural unit of the west curve is 1,085 feet, and if it be referred
to its asymptote and a line parallel to the fault line and crossing the asymp-
tote at a point 143 feet west, its equation is
y= — iO^
The equation of the tangent for the Sutro section values shows that
the horizontal point of the east curve is at 2,840 feet from the fissure meas-
ured on a line parallel to the asymptote, and that of the west curve at 1,820
feet measured in the same way. The tangent to the east curve at the fault
makes an angle of 26° with the horizontal, while the tangent to the west curve
makes an angle of 32°. This sudden increase of inclination immediately
west of the croppings is a familiar feature of the landscape in Virginia.
Had the diorite been separated into plates of the same thickness as those
of the east country, the two curves would have had a common tangent at
the croppings.
The position of the points of greatest curvature presents no significant
peculiarity, so far as I am aware, and is expressed by a somewhat involved
logarithmic function. This point in the east curve is at a distance of 686
feet from the fault plane, measured on the asymptote. In the west curve
it lies at 951 feet from the same plane. The values of the minimum radii
are 6,640 feet and 3,580 feet in the east and west curves, respectively.
These radii are simply and directly proportional to the natural units of the
curves.
Topography chiefly due to faulting. — The wcst cropplngs of the CoMSTOCK, from the
Bullion to the OpJiir, are nearly horizontal, and the original surface, as has
been shown, sloped to the west at an angle of only two and a half degrees.
The theory of faulting propounded would therefore lead one to expect a
pretty close agreement between the contours of the faulted slope and those
of the west wall; for on the Sutro Tunnel section, at least, there is evidence
of but slight erosion. Such an agreement appears from a comparison of
182 GEOLOGY OF THE COMSTOCK LODE.
the horizontal sections with the surface map, and has long been well rec-
ognized among those who have had to do with the mines.
The ravines which furrow the range are not therefore the result of
erosion, but of faulting. Once formed through the dislocation of the
country, they have, of course, received the drainage, and have been modi-
fied thereby to some extent.
East vein. — It lias beeu shown that even if a fault takes place on a fissiare
perpendicular to an original surface, the hanging wall will assume the shape
of a sharp wedge, and that under the conditions of pressure necessary to
produce a logarithmic surface, it is unlikely that this wedge would remain
intact. Such a fractui-e occurred in the faulting of the Comstock, and
opened the famous "east vein", from which a large part of the ore produced
has been extracted. Baron von Richthofen regarded this structure as a
result of faulting, and as a surface phenomenon. I have simply shown in
addition how the east country came to assume the tapering form most
favorable to such a fracture.
Origin of the sheeted structure. Theory of eruptive stratification. i lie CharaCtCr 01 thC
sheets of rock into which the walls of the Comstock are divided is an open
question, for one observer has maintained that they form a series of thin,
bedded, regular layers of rock, presenting a fine example of eruptive strati-
fication. It is true that in confined spaces in several of the rocks a
stratified or laminated texture is visible; but in the half-dozen such cases
known to me the phenomenon extends for very short distances, often only a
few feet, and appears to be the result of some local variation in the compo-
sition of the rock; for not only can I perceive no general uniformity in the
direction of the layers in these different spots, but I have a single hand-
specimen which shows portions of two sets of them at an angle of nearly
90° to one another. These occurrences, however, cannot be meant in the
statement referred to, for they are rare. As applied to the great mass of
rock I am also unable to agree with it. To me it is nearly inconceivable
that a granular crystalline rock like the diorite of Mount Davidson, con-
taining only crystals of "secondary consolidation," should ever have been
sufficiently fluid to permit of eruptive bedding The face of Mount David-
son shows no lamination, though the division into parallel sheets is strikingly
STRUCTURAL RESULTS OF FAULTING. 183
apparent. The surfaces of the sheets in the same locah'ty are not similar
to those commonly formed by bedding, and are indisting-nishable from frac-
tures, nor is the persistence of the sheets comparable with that of sedi-
mentary strata. The McKibhen Tunnel in Spanish Ravine passes through
diorites in part somewhat porphyritic, in part of the dark, highly horn-
blendic variety. A quartz seam is cut by the tunnel, but no dikes of later
rocks. There is a greater superficial resemblance to a bedded structure
here than on Mount Davidson, but close examination shows that most of the
apparent differences in color and texture are referable to degrees of decom-
position. Decomposition has set in from the partings of the sheets of rock,
often leaving the central portion of a sheet less affected than its faces. In
the diabase, hornblende-andesite, and augite-andesite of the east country,
the phenomena are similar. There is ample evidence of fracture and of
decomposition following lines of fracture. Sometimes individual sheets or
portions of sheets have in a measure escaped decomposition on account of
the presence of protecting clay seams and the like, and these have been
mistaken for dikes, or flows of andesite or other rock; but careful examina-
tion shows that they differ only in the degree to which they have yielded to
decomposing agencies, and in no other respect. The partings are not such
as we should expect in bedded flows.' There is no trace of lamination
except the irrelevant local occurrences mentioned, and while it might well
be that the greater part of the seams had been reopened by upheaval, it
cannot be supposed that no adherent laminae would escape separation. In
short, my observations wholly fail to accord with the hypothesis that these
rocks were laid down in horizontal beds, and afterward tilted. Even if
observation furnished considerable grounds for such an interpretation of the
facts, I should hesitate to accept an explanation which appears to me wholly
at variance with what we know of the occurrence of similar rocks else-
where.' The deposition of a single igneous rock over several square miles,
in thin horizontal beds, implies a watery fluidity and a very high specific
heat. So far as I know only one or two of the later volcanic rocks are
'Mr. Church, indeed, states {I. c, p. 153) th.at "diorite is one of the fine-graiued, thin, ruuning
hivas." But he cites uo authorities for, or instances in proof of, this statement, which is .at variance
■with the coranionly accepted opinion, and with the indications of its composition and micro-structure.
184 GEOLOGY OF THE GOMSTOCK LODE.
known to flow in such a manner, and these only under exceptional condi-
tions, for even basalt commonly accumulates in large masses around the
orifices from which it issues; nor am I aware of any distinct evidence that
the granitoid rocks have ever flowed like a lava, or reached a higher degree
of fluidity than the plastic state.
Energy displayed in the fault on the Comstock. We haVe UO meanS of reducing tO
known units the pressure and resultant friction which accompanied the
faulting action on the Comstock, but the imagination at least may be
brought to bear upon the subject by considering the amount of disloca-
tion. If the west country is supposed to have revolved about a distant
fixed fulcrum, through a sufficient angle to account for its present relative
elevation, then the east country must have been pushed bodily eastward for
a distance of 2,150 feet. The maps and sections show that certainly not
less than a cubic mile of rock must have been thus driven out of place in
spite of all opposition, and the amount of horizontal dislocation involved is
not lessened by supposing the west country to have moved instead of the
east. Compared with the energy necessary to produce such a movement,
that requisite merely to raise each of the sheets composing the mass, in
opposition to friction through a mean distance of about 150 feet, certainly
seems small.
Dynamical theory of sheets. — I havc showu that the teudcucy of the faulting
movement is to separate sheets of rock, and that sheets thus separated will
arrange themselves along the logarithmic curve when divided from the
mass. The possibility thus presented does not conflict with my observa-
tions, and I am led to the belief that the sheeted structure of the east and
west country is due to the formation of fractures parallel to the faulting
surface, and that these fractures are the result of faulting under intense
lateral pressure.
Inferences from the fault as to the age of the Lode. SomC light is thrOWU UpOU tho age
of the Comstock as an oi'e vein by the relations of the fault to the ore, and
to the erosion. The " east vein," being a secondary fissure, cannot have
formed till faulting had made considerable progress, while the crushed con-
dition of the quartz^ and the phenomena attending it show that faulting
' The evidence that the "sugary quartz" is really crushed will he giveu later.
STRUCTURAL RESULTS OF FAULTING. 185
action has succeeded, as well as preceded, the deposition of the ore and
gangue. The regularity of the curve, on the other hand, shows that the origi-
nal surface line along the Sutro section was sensibly straight^ and lay on a
gentle western slope. The agreement of the contours of the range with
those of the west wall and of the cross-sections with the curve obtained
from theoretical considerations, proves that the erosion since the commence-
ment of the faulting action is sensible (on a scale of 800 feet to the inch)
only where most intensified — i. e., in the ravines. The faulting and the depo-
sition of ore have therefore occurred since the Distkict was subjected to any
considerable amount of general degradation. The level condition of the
country prior to the fault appears to me probably the result of erosion, and
if so the District must have been a plateau or a high mountain valley — in
short, an area of denudation.
Fault probably the result of a rise in the west country. It Is pd'hapS impOSslblc tO demon-
strate whether the absolute movement involved in the faulting was the rise
of Mount Davidson, or a sinking of the east country. If the east country
has sunk, the former level near the middle of the Lode must have been
nearly that of Mount Davidson, and the District must have occupied the
crest of a rather sharp undulation running nearly east and west. If the
main movement was an uplift of Mount Davidson, and its neighbors to the
north and south, the original general level was about that of the present
country east of the Lode. The District must then have been near the top
of a gentle undulation approximately parallel to the Sierra. The latter
supposition accords with the general character of the present topography of
the Great Basin area much better than the former, and seems to me much
more probable on general as well as local grounds.
Diminution of evidence of fault near the ends of the Lode. To tho UOrtll aud SOUth of
Mount Davidson the evidence of faulting diminishes. From the Overman
far into the Sierra Nevada claim, a distance of two and one-third miles, the
amount of fault has been great, and the indications are unmistakable. Be-
yond these points the disturbance of equilibrium has been to some extent
adjusted in a different manner. This is partly indicated on the surface map
by the union of the andesite fields, which are separated opposite the middle
portion of the Lode by diorites. Towards the ends of the Lode the dynamic
• I
186 GEOLOGY OF THE COMSTOCK LODE.
action seems to have been distributed in part by a forking of the fissure,
and in part by the formation of east-and-west cracks.
Coefficientof friction of rocks involved in the fault. Tho rOcks iuVOlved in tho faulting
action on the Sutro section are diorite, diabase, hornblende-andesite, and
augite-andesite. They must all have sensibly equal coefficients of friction,
for the curve in the western diorite is apparently continuous with that of
the other rocks which lie east of the vein, and there is no evidence of dis-
continuity in the eastern curve as it passes the contacts. All the east rocks,
too, appear to divide into plates of the same thickness, while the diorite has
split into sheets of less than half that of the others.
Rules applicable to prospecting in uneroded districts. It is, of COUrSe, mOSt Uulikcly that
the CoMSTOCK is the only vein in which the deposition of ore is recent, and
has been accompanied by faulting, and some conclusions as to the occur-
rence of veins in such cases may be welcome to some of the readers of this
paper.
In a locality modified by faulting action under lateral pressure, the fact
will appear in the parallelism of the exposed edges and faces of rock-sheets.
If erosion has not seriously modified the surface resulting from the
faulting action, the logarithmic curve will be recognizable to the observer
looking in the direction of the strike.
The main cropping of the vein is to be sought at the point of inflection
of the curve, which will be found nearly or exactly midway between the
top and bottom of the hillside. One or more secondary vein croppings
should be looked for below the main cropping, and these, so far as yield is
concerned (but not in regard to location of claim), may prove more impor-
tant than the main cropping.
The dip of the vein will be to the same quarter as the slope of the sur-
face, but, of course, greater in amount. The flatter the surface curve, the
smaller the angle of dip will be. The mean strike will be nearly or quite
at right angles to the direction of the spurs and ravines of the faulted area.
If besides the movement of one or other wall in the azimuth of the
dip, there has been a dislocation in the direction of the strike, chimneys will
open, all of them on the same side of the diff'erent ravines. Surface evi-
STRUCTURAL RESULTS OF FAULTING. 187
deuces will often enable the prospector to determine on which side the
chimneys are to be found. On the barren sides evidences of crushing and
of closure of the fissure are probable.
The fissure is more likely to have a constant dip (barring the second-
ary off"shoots) than a constant strike ; but, of course, irregularities in dip
like those in strike will open chambers which may be productire.
Ofishoots into the hanging wall may occur at any depth, but none except
those near enough to the main cropping to reach the surface, where it
has a very considerable slope, are likely to be continuous.
Application of theory to landslips. — Bcsidcs the dccp-scated fissures produced by
profound disturbances of the earth's crust, there are cortiparatively super-
ficial phenomena which seem to come under the laws deduced in this chap-
ter. In regions where the soil is deep and covered with low-growing
vegetation, such as grass, the details of the topography are not molded by
the direct action of the rain, but by landslips ; oftentimes, indeed, of very
small extent, but repeated or increased year after year. The hanging wall
of such landslips commonly separates into distinct layers, as has been stated
in a preceding paragraph. These sheets must arrange themselves on the
locus
if the arguments presented on p. I(i4, et seq., are correct. A yearly repeti-
tion of this action, sometimes modifying the hanging wall and sometimes
the foot wall of the slips, will eventually give the whole topography a log-
arithmic character; even the position of the gullies, and consequently the
lines of direct erosion, being determined as indicated on page 177. The simi-
larity between some of the logarithmic curves illustrated in this chapter and
the slopes of the gently-rounded hills common in grassy regions with
deep soil, needs only to be suggested.
OHAPTEE V. .
THE OCCURRENCE AND SUCCESSION OF ROCKS.
Methods of determining succession. — Determinations of the order of succession of
eruptive rocks involve considerable difficulties. Superimposition alone is
an insufficient indication of relative age, for intrusions and laccolitic accu-
mulations of younger rocks may underlie older ones. Neither are inclu-
sions of one rock in another always a safe guide. Cases are not unknown
where intrusive masses of a younger rock in an older might readily be mis-
taken for inclusions of an older rock in a younger one. I have even ob-
served instances, though not in the Washoe District, of slabs of older
rocks embedded in later eruptions in such a manner that but for other and
overwhelming evidence as to the order of succession, they might have been
interpreted as dikes of the older rock in the younger. Moreover, when the
rocks in question are closely allied, as is very frequently the case, local
modifications of one rock may readily be confounded with inclusions of a
different but similar species. Such an error is peculiarly likely to occur
where there is brecciation. As has been pointed out on page 82, masses of a
single rock subjected to partial decomposition may also simulate inclusions
or dikes of one rock in another. Thus while at first sight it might appear
that dikes and inclusions furnish the most unimpeachable evidence of suc-
cession, this class of evidence is peculiarly deceptive except where the rocks
are fresh and characteristic, the exposure perfect, and the cases abundant.
Where any of the rocks are very recent, evidences of erosion form an im-
portant argument as to succession, as will be seen from the remarks on the
later hornblende-andesite.
No single method of determining the succession of eruptive rocks is
ordinarily sufficient, and due weight must be given to all the facts bearing
188
OCOUEEENCE AND SUCCESSIOlSr OF EOGKS. 189
upon their relative age. Difficulties in the determination of succession,
however, are not peculiar to the geology of massive rocks; for there are
many instances of the reversal of sedimentary strata, and with sufficient
care the order of succession of eruptives can generally be established with
as much certainty as can that of sedimentary rocks in greatly disturbed
areas.
Order of succession. — Thc ordcr iu which the rocks of the Washok District
have appeared upon the surface is as nearly as can be ascertained the fol-
lowing:
Granite,
Metamorphics,
Granular diorites,
Porphyritic diorites,
Metamorphic diorites,
Quartz-porphyry,
Earlier diabase.
Later diabase ("black dike"),
Earlier hornblende-andesite,
Augite-andesite,
Later hornblende-andesite.
Basalt.
It is possible that strata since metamorphosed may have been laid down
upon the diorite as well as previous to it. The evidence of the succession
of diabase to quartz-porphyry would be more satisfactory if the contact
between them were more extensive, and of the age of the basalt there is no
direct evidence except that it is later than earlier hornblende-andesite.
The other points as to succession are clearly established. One of the most
interesting is the occurrence of hornblende-andesite after as well as before
augite-andesite, proving a recurrence in the character of eruptions. It thus
has a direct bearing upon the general theory of the succession of volcanic
rocks. In the following pages some notes are presented on the occurrence
and distribution of each of the series.
190 GEOLOGY OF THE COMSTOCK LODE.
Granite. — Granite is extensively develojied to the west of the Virginia
Range, but reaches the surface in the Washoe District only in a single
small area near the Red Jacket mine, C. D. 6. It occupies a considerable
space beneath tlie surface, however, for it has been met in the Baltimore and
the Bock Island, and by a tunnel, just beyond the limits of the map, to the
northwest of the Florida.
The granite must fall away very rapidly to the north and east, or it
would be encountered in the Gold Hill mines. Whether this is the conse-
quence of a fault or of a steep slope, there is no opportunity for deciding.
Near the Bed Jacket the granite here and there shows partings which might
be remains of a former stratification ; but a similar system of parallel cleav-
ages is not uncommon over small areas in rocks of an unquestionably
eruptive character, and I met with nothing which could be cited as definite
proof of a sedimentary origin.
In the Wales Consolidated, granite is directly overlain by metamorphic
diorite, at the Bock Island by schists and limestones, and at the Baltimore
apparently by eruptive diorite, metamorphics, quartz-porphyry, and augite-
andesite. It must, therefore, have been denuded to a considerable ex-
tent before each of several eruptions. It is nevertheless far fresher than
most of the rocks in the District, and no considerable quantity of ore
has been found associated with it, though some metalliferous quartz has
been met with at its contact with younger rocks ; but traces of ore are very
likely to occur at any contact in a district like Washoe, where every
point has been racked by dynamical action and the whole subterranean area
has been flooded with mineral solutions. It is possible that ore similar to
the Justice body may be found on the contact between metamorphic diorite
and granite south of that mine, but there is nothing to indicate that the
granite is likely to act otherwise than mechanically in the deposition of ore.
Metamorphics. — Tlicre Is a Small area of distinctly stratified rocks to the
south of American Flat, near the Florida. They are limestones and mica-
ceous schists, badly broken and contorted, and much metamorphosed. I
did not succeed in detecting anything like a fossil in them, in spite of an earn-
est search. They are colored as Mesozoicfrom the general. analogy of this
OCCURRENCE AND SUCCESSION OF liOCKS. 191
portion of the Great Basin, aw elucidated by the Exploration of the Fortieth
Parallel. In a cut on the American Flat road, just south of tlie Florida,
there occur two seams of coal-like matter half an inch in thickness. The
metamorphics extend into American Flat under the area laid down -as
Quaternary, where the detritus is too thick to permit of tracing the con-
tact between the metamorphic and eruptive rocks with certainty. The
Rock Island shaft is inaccessible, but a careful examination of the dump and
the descriptions of an employ^ leave no doubt that it passed through meta-
morphics into underlying granite. There is nothing to show that any
eruptive rock other than granite has been met with at the Bock Island. A
little coal is said to have been found well down towards the granite, and
was no doubt such an occurrence as that mentioned above. Metamorphics
of the same character appear to an insignificant extent north of American
Flat, and in the Caledonia, as is shown on the section through that mine.
In the Gold Hill mines black slates form the foot wall of the Lode to a
large extent. Thin sections made across the lamination show that the dark
color is due to absolutely opaque particles without metallic luster, and
these disappear on prolonged heating in an oxidizing flame, but are not
affected by acids. They are therefore graphite. The rock contains pyrite,
which is very irregularly distributed. The slate is often confounded with
"black dike" (younger diabase), with which, however, it shares only the
black color. In a fairly good light the slaty structure serves to distinguish
it without difficulty. The diorite at the Yellotv Jacket appears to overlie
these slates, though no single mine-opening shows a contact. The masses
of mica-diorite shown in the Yellow Jacket section can hardly be in their
original position, though very likely they have been transported but a very
short distance; but at the surface the dioritic mass is in sight to within a few
hundred feet of the Yellow Jacket, where it seems to disappear under the
andesites, and it is almost impossible to suppose that the great exposure of
slates in the Yellotv Jacket and the Belcher is not one surface of a body which
extends beneath the neighboring diorite. On the other hand, in the Cale-
donia diorite underlies the metamorphics, and it therefore seems probable
that the plastic diorite was forced horizontally between sedimentary masses
as well as vertically to the surface or, at all events, to higher points than
192 GEOLOGY OF THE COMSTOCK LODE.
any now occupied by stratified rocks. Further indications of sucli a history
are observable in the Sierra Nevada, where a thin and not very extensive
body of highly crystalline stratified limestone is completely inclosed in
diorites, which are granular on one side and porphyritic on the other. I
am able to offer no better suggestion than that this mass was carried into
its present position by the granular diorite, and covered over sooner or later
by a porphyritic outflow.
Eruptive diorite. — Besidcs tlic dioHtic mass forming Mount Davidson and
the adjoining hills, there is somewhat obscure surface evidence of a large
area of this rock beneath later eruptive masses. Near the Forman shaft are
several small patches of mica-diorite, which, however, might easily be passed
unnoticed; and in the Flowery district, about a mile and a half east of
Flowery Peak, dioritic porphyries again appear. Diorites occur in almost
all the CoMSTOCK mines from the Silver Hill north to the Utah, and are
also found in those of the Floweiy region. The dump of the Lady Bryan,
for example, consists largely of fresh, coarsely granular, quartzose diorite
To the west and northwest of Mount Davidson it also appears to be covered
by but a thin cap of andesite, so that at least two islands of the older rock
are wholly surrounded by the 5'ounger. Diorite forms the foot wall of the
Lode throughout the Vii-ginia mines and is replaced in this position by met-
amorphics in Gold Hill. On the hanging wall it is found in the Yellow
Jacket in masses apparently displaced, and in the Sierra Nevada and Utah it
forms both walls of the fissure which has been mainly explored. Frag-
mentary masses also appear embedded in diabase at intermediate points
but not to an important extent. Before the eruption of the earlier diabase,
the diorite no doubt formed a continuous mass, partly overlying and partly
underlying the metamorphic strata, and probably extended over the coun-
try now occupied by later rocks along the line of the Sutro Tunnel. If so,
this area has sunk under the subsequent outflows, but how far it is as yet
impossible to say, though it is a matter of importance to the future of the
Lode. At the time of the faulting the whole west wall in Virginia and
Gold Hill seems to have risen, the dislocating tendency having been adjusted
towards the ends of the fissure by diverging cracks. This action has moulded
the eastern face of the range opposite Virginia City and the northern por-
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OCOUEEENCE AND SUCCESSION OF EOCKS. 193
tion of Gold Hill. To the north of the Union shaft the porphyritic diorites
swing- to the northeast. On the surface they disappear under the andesites,
while underground the explorations north of the OpUr have been almost
wholly confined to the dioritic area, and afford no means of tracino- the
extension of the diorites beneath the cap. Near where the contact between
the diorites and the diabase probably occurs are the heavy croppings known
as the Scorpion. Whether these actually correspond to the contact or not
can only be told by exploration; but, if not, that contact has left no trace
upon the surface in this region, which would be very remarkable if the
deductions made in the last chapter as to the age of the Lode are correct.
It is not unlikely that the dioritic rocks are continuous, or nearly so, under
the Flowery Eidge, and are thus connected with the occurrences at and
near the Lady Bryan. Diorite seems to have preceded the quartz-porphyry,
for it occurs in the Justice, and in the Caledonia, beneath the porphyry.
Relations of porphyritic to granitoid forms. Thc TelationS of the dioHtic pOrphyHeS
to the granular mass are interesting. The former are constantly found over-
lying the granular rock, but a line of demarkation can seldom be drawn,
transitions and mixed masses being of constant occurrence. Roughly the
area between Bullion and Spanish ravines is granitoid, and the masses
beyond these limits porphyritic; but this is a very rude approximation, for
fine porphyries occur in the very midst of the mass of Mount Davidson,
and granular patches are to be found throughout the hornblendic porphyries.
The micaceous porphyries also appear to overlie the hornblendic variety,
into which, however, they merge. The conditions suggest a physical explana-
tion Some geologists now believe that the crystalline structure of rocks
depends solely on the pressure under which they have consolidated. Such an
explanation of the present case, however, seems to me unsatisfactory. The
variation in a horizontal direction is nearly as marked as that in a vertical
line, and though there is an exposure of at least 2,500 feet, vertically, allow-
ing for the displacement by faulting, the deepest granular diorites are not
more coarsely crystalline than those on the top of Mount Davidson. Nor
are the other rocks from the bottom of the mines in any perceptible manner
different from those collected at or near the surface. The cause of the differ-
ence between the granular and the porphyritic diorite, if these rocks are ad-
13 0 L,
194 GEOLOGY OF THE COMSTOCK LODE.
mitted to be of eruptive origin, must, I think, be sought in a period anterior to
the extrusion of the mass. The granular diorite is composed of crystals of
"secondary consolidation," interlocking grains, the relative position of which
cannot have changed subsequent to their formation. This rock must, there-
fore, have crystallized in its present position, barring, of course, any move-
ments to which it may have been subjected after solidification. The porphy-
ries, on the other hand, are composed of well-developed crystals in a granu-
lar groundmass. These crystals must have grown slowly in a magma suffi-
ciently fluid to permit of free movement, and this condition is not likely to
have been present after eruption. A state of considerable fluidity is also
indicated by traces of bi'ecciation in some of these rocks, and of fluidal
structure in the arrangement of microlites in a few slides. But the strong-
est evidence of a fluid condition is furnished by the little dike close to the
Eldorado croppings. The walls are granitoid, and the center o/ the dike is
semi-porphyritic, showing green fibrous hornblende and a granular structure,
though some porphyritical ciystals are imbedded in it. But for an inch
from the walls of the dike the rock is a dark, solid porphyry which contains
brown hornblendes, and is in all respects similar to the most porphyritic
varieties found in the District. The contact with the walls is perfect, and
the occurrence admits of no natural explanation but that of a hot intrusive
fluid.
Hypothesis suggested. — The porpliyritical crystals formed before eruption
must have sunk to the bottom of the fluid mass, for the specific gravity of
hornblende is far greater than the mean density of the diorite, and the
relation can hardly have been reversed at the temperature at which they
formed. Little as we know of the subterranean conditions of eruption, it
is probably safe to assume that the upper portion of a fluid or plastic mass
would be extruded before the lower, and that the portion holding the por-
phyritical crystals in suspension would be the last to appear. The dike of
porphyry between granitoid walls already referred to seems to show that
this was the case, while the frequency of transitions ig evidence that the
extrusion was a nearly continuous process. The granular groundmass of
the porphyries is finer-grained than the granitoid rock, but this does not
necessarily prove that it cooled under diff'erent conditions, for a certain dif-
OCCURRENCE AND SUCCESSION OF ROCKS. 195
ference in chemical composition would almost inevitably accompany the
supposed separation by specific gravity; and besides the porphyritical
crystals, other more minute solid particles would probably also sink, and
tend to the multiplication of centei's of crystallization.
Possibility of a metamorphic origin. — Whllc the evidcuccs of the oruptivo charac-
ter of this diorite are tolerably strong, they are not so conclusive as to
exclude a consideration of the possibility that the rock may be metamorphic.
As has been shown in Chapter III., one variety of the metamorphic diorite
is almost indistinguishable per se from the rock of Mount Davidson, and
another variety of the latter is distinctly brecciated. It is exceedingly diffi-
cult, if it is not in the present state of knowledge impossible, to comprehend
how the formation of pure and shai-ply developed crystals can go on in
media not svifficiently mobile to be regarded as fluid; yet we know that
tourmalines, garnets, and other minerals are sometimes beautifully developed
in metamorphic rocks, which have not only retained their lamination, but
have offered an efficient resistance to the pressure of thousands of feet of
overlying strata. Most of the indications of the eruptive character of the
Mount Davidson and Cedar Hill diorite, taken singly, are thus not absolutely
incompatible with a metamorphic origin. But until the origin of the granitoid
rocks has been more satisfactorily elucidated than heretofore, it is certainly
the duty of the geologist, while giving possible alternatives due weight, to
judge each occurrence on its own merits, and to seek explanations in compre-
hensible processes, rather than through unexplained analogies. At present an
eruptive origin can alone be regarded as probable for the Washoe diorites.
Metamorphic diorite. — Tlic grouuds for cousideriug the metamorphic diorite
as such, have already been given. It is a very puzzling rock in the field,
and may readily be mistaken in different occurrences for granite, diorite,
augite-andesite, or basalt. Wherever the underlying rock is exposed it is
sedimentary, except at the Wales Consolidated, where the metamorphism has
penetrated to the underlying granite. It is also associated in the most inti-
mate way with the quartz-porphyry, and does not appear between the
stratified rocks and eruptive diorite. If the area occupied by the quartz-
porphyry were made continuous, it would completely cover all the meta-
morphic diorite in the District; and the evidence is tolerably strong tliat
196 GEOLOGY OF THE COMSTOOK LODfi.
the metamorphism is due to the action of the porphyry on the strata over
which it flowed. Metamorphic diorite occurs on the Comstock only at the
extreme south end, in the Silver Hill and Justice mines. Mines have been
sunk in it south of Silver City — for example, the Amazon — and have struck
ore which was calcareous and carried mixed sulphurets. The Justice ore
associated with this rock was of a similar character.
Quartz-porphyry. — The quartz-porphyry which appears on the map is merely
the northeasterly corner of an extensive area of this rock. A noticeable
peculiarity is that it is everywhere decomposed, and everywhere to almost
precisely the same degree, while it is fissured only to a very slight extent.
It seems scarcely possible that this decomposition should have taken place
from below, for the underlying granite and metamorphic diorite are for the
most part very fresh. The decomposition would seem rather the result of
the action of surface waters, favored by a porous structure. This structure
is perhaps due to the unequal contraction of quartz and feldspar in cooling.
Before later eruptions covered it, the porphyry occupied the surface for. a
considerable distance farther to the northeast than at present, for it appears
in the Belcher ground and in the Forman shaft. In both these cases it under-
lies hornblende-andesite, while in the Belcher 1648, and in the Overman, it
also seems to underlie diabase. The accessible points at which these two
rocks come in contact, however, are so few that the order of their succession
is less satisfactorily made out than that of any other important members of
the series of rocks found in the Washoe District. The quartz-porphyry
does not appear to be intimately associated with the ore bodies of the Com-
stock, though occurring near to some of those in the Gold Hill mines ; nor
have any considerable quantities of ore been discovered in this rock in out-
lying mines. It also assays little or nothing. It is worthy of note that
quartz-porphyries in some mining districts have almost certainly supplied
the deposits with their charge of precious metals, though the Washoe
occurrence is so barren.
The felsitic modification of the quartz-porphyry is confined to a limited
area near the granite. To what cause the difference between its structure
and that of the ordinary variety may be due I cannot suggest.
OCCUERENCE AND SUCCESSION OF ROCKS. 197
Earlier diabase. — The diabases are almost wholly confined to the mines, only
two small patches having been discovered on the surface. Of these, that
between the Julia and Ward shafts appears normal in character though much
altered, and as it occurs at the bottom of a ravine vertically above the main
body of the rock nothing is easier than to account for its presence. Such
is not the case with the mass in Ophir Ravine. This bears a very strong
outward resemblance to a granular diorite, and it seems impossible to make
out a sharp contact between the two rocks. There is also no evidence of
any connection between this area and the main mass east of the Lode. As
has been explained in Chapter III., I am by no means sure that it should
not be regarded as a local modification of diorite, rather than an independent
eruption. Apart from the interest attaching to such an occurrence it is of
little importance, no further consequences, so far as I know, depending on
its determination. As may be seen from the sections, diabase approaches
the surface very closely immediately below the city of Virginia, so closely
that at least a few croppings would be expected in the ground covered by
the town. It is highly probable that a considerable area might have been
traced before the settlement was made, but the ground is now so graded and
built up that a careful search failed to reveal any rock in place.
Relations to the Lode. — The earlier diabase forms the east or hanging wall of
the Lode throughout its more productive portion; that is, from the Overman
to the Sierra Nevada, and from the surface, or very close to it, down to the
lowest depths yet reached. It also penetrates the west country, at the
north end of the Lode, in stringers, as may be seen on the horizontal sec-
tion on the Sutro Tunnel level. Atlas-sheets VIII. and IX. This fact scarcely
requires explanation, for that a single clean fracture of the diorite mass
should have been effected at the time of the diabase eruption is almost
inconceivable. If the diabase succeeded the diorite it would be natural to
expect diabase in fissures within the diorite masses, and fragments of diorite
inclosed in diabase. It has already been pointed out that these occur.
There is a considerable sheet of diorite east of the bonanza of the Califor-
nia and Consolidated Virginia mines, and similar masses were encountered
in sinking the new Yellow Jacket shaft. In the higher levels, too, it is
probable, from the accounts of former examinations, that diorite horses were
198 GEOLOGY OF THE GOMSTOCK LODE.
encountered. On this point, however, there is some uncertainty, for before
the identification of diabase in the east country much of the hanging wall now
exposed would undoubtedly have been recognized as an older rock and
confounded with diorite. The stringers of diabase in the Sierra Nevada and
the Utah mark fissures unquestionably belonging to the Comstock system,
and that in the former mine at least were accompanied by a very trifling
amount of ore. The history of the Lode and the chemical discussions
which form the subjects of other chapters, make it highly improbable
that bodies of any consequence will ever be found near these stringers.
The main contact of the diabase with the diorites swings sharply to the
northeast in the Sierra Nevada ground, and has not been explored beyond
that point. Diabase does not appear south of the Overman, and the Forman
shaft passed from hornblende-andesite into quartz-porphyry at 2,200 feet
from the surface. If, therefore, as there is reason to believe, the latter rock
preceded the diabase, this will not be encountered in the Forman shaft. The
extension of the diabase in an easterly direction is somewhat uncertain. On
the line of the Sutro Tunnel the diabase is only about 1,300 feet wide,
measured horizontally. It is certainly wider than this at the Oshiston shaft
and the new Yellow Jacket. The Oshiston is beheved to have met diabase at
a depth of about 1,000 feet, though the locality was not accessible, while
the new Yellow Jacket passed into it at less than 400 feet from the surface,
indicating an extensive body still farther east.
The lithological varieties of the diabase have been sufficiently described
in a former chapter. In structure it resembles the diorite, being split up
near the Lode into rough sheets parallel to the main fissure, as has been
explained in Chapter IV. I have been wholly unable to see any evidence
that this rock was not emitted at a single outbreak. Its position, lying as a
mass upon a diorite wall sloping at an angle of about 45°, together with
the details of the relations of the two rocks, shows that it is younger than
the dioiiites. That it is also probably younger than the quartz-porphyry
is shown by the occurrences in the Overman, which are not fully satisfactory
only because they are so limited.
"Black dike. — The younger diabase, as has been seen, is identical with
the trap of New Jersey. It has often been confounded with the black slates
OCCURRENCE AND SUCCESSION OF ROCKS. 199
of the Gold Hill mines, and black rocks and clays have sometimes been
classed with it in the north end mines. In the upper levels it was met with
only in an indistinguishably decomposed form. I was not able to authen-
ticate its occurrence north of the Savage, and found it, wherever struck, of
a very uniform width, always a few feet, never more than a couple of yards.
From the Savage to the Overman it generally marks the contact between the
older diabase and the west wall with precision, but on one level of the Chollar
it is 80 feet west of the contact, and in the Yelloiv Jacket a narrow belt of
slate sometimes lies east of it. In the Overman the dike diverges from this
contact, extending towards American Flat as far as the Caledonia. The uni-
form thickness of the dike shows that no considerable movement between
the diabase and the west wall took place at or previous to its eruption, for
otherwise the fissure which it filled must have presented the enlargements
and contractions characteristic of veins the walls of which have experienced
a relative motion. The divergence of the dike towards American Flat ex-
plains the so-called forking of the vein. A certain amount of solfataric
action is perceptible along the dike fissure, accompanied by the deposition
of quartz which is not wholly barren The American Flat vein is a stringer,
the position of which was predetermined by this fissure.
Earlier hornbiende-andesite. — The mine workiugs show that the coutact between
the earlier diabase and the earlier hornbiende-andesite is very steep, and
that it must be represented by a line something like that indicated in the
section through the Sutro Tunnel, Atlas-sheet VI. The inference from this
section is strong that the body of older hornbiende-andesite cut by the tun-
nel occupies a portion at least of the fissure through which it was erupted.
The eastern surface of the diabase is far too steep to admit of the supposi-
tion that it was ever exposed. Previous to the outbreak of the hornbiende-
andesite the diabase must either have extended much farther east than now,
or a mass of diorite must have occupied the place now filled by hornbiende-
andesite. In either case, the rock lying east of the present limit of the
diabase must have been submerged by the andesite eruption; and of the
two suppositions the former seems the more probable. The augite-andesite
stands in much the same structural relation to the earlier hornbiende-ande-
site as the latter holds to the diabase, and the east and west surfaces of the
200 GEOLOGY OF THE GOMSTOCK LODE.
hornblende rock, as seen in the tunnel, are parallel to one another and to
the Lode.
Even on the surface, indications of the parallelism of the contact be-
tween the two andesites and the Lode are observable. If a line is drawn
at a distance of 4,500 feet east of the vein, it will fall very close to the
easternmost edges of the earlier hornblende-andesite, and include only one
considerable tract of the aug-ite rock between it and the Lode. The For-
man shaft is nearly at the center of this tract, and the section through it
shows that hornblende-andesite exists below the surface. The contact be-
tween these two rocks in depth is therefore probably nearly parallel to the
Lode throughout the whole length of the latter.
Besides the area of earlier hornblende-andesite to the east of the Lode,
it covers a large extent of country to the west of the diorite. Before the
fault occurred the top of the range was probably about on a level with the
east wall, and it seems probable that the whole exposure of earlier horn-
blende-andesite is ascribable to a single eruption, or an unbroken series of
eruptions. I can find 'no indication of bedding, nor of the distinct lava
streams which give evidence of intermittent action in the neighborhood of
modern volcanoes. At first the andesite most likely buried the diorite com-
pletely, but the latter must have been reexposed by erosion before the
fault took place. The hornblende-andesite, as well as the diabase, is di-
vided into sheets by a system of parallel fissures. If the conclusions drawn
in Chapter IV. are correct, this fissure system was developed by faulting a1
a comparatively recent period, but the tendency to parallelism in the struct-
ure of the country was first exhibited as far back as the earher hornblende-
andesite eruption.
The area north of Silver City is remarkable for the unusual develop-
ment of hornblende crystals, which are frequently an inch and a half in
length, and occasionally more. In this area, too, there are several sharp
cones one or two hundred feet in height, which suggest volcanic vents, but
no craters are traceable ; and the evidence of degradation opposite Virginia,
especially the flatness of the surface previous to the fault, as is proved from
the present regular character of the fault-curve, makes it improbable that
distinguishable relics of craters or cones of eruption should remain. In the
OCCUERENCE AJSTD SUCCESSION OF ROCKS. 201
area north of Cedar Hill Canon this andesite is much less homogeneous
than usual, varying in texture from coarse to fine frequently, and almost
without transitions. These differences have been emphasized by decom-
position and erosion, which have carved out projecting- dike-like sheets,
fantastic columns, and the like, from the heterogeneous mass.
That this rock is younger than the quartz-porphyry and the diabase is
very evident from the sections, since it overlies these rocks vertically in
wide areas, while there is nothing in their relations suggesting laccolitic
masses.
There are some east-and-west veins in the hornblende-andesite near
Silver City, which are said to have yielded in the aggregate considerable
quantities of bullion. The only mines which could have thrown any light
on the origin of this ore, however, were closed at the time of the examina-
tion. They are near the Justice mine, which shows a great complication of
rocks in its ore-bearing region, and the ore of the east-and-west veins is
very probably due to the same general causes as the Justice ore-body. The
andesites themselves do not give considerable assays.
Augite-andesite. — lu its genei'al features, the occurrence of augite-andesite
closely resembles that of the preceding rock. It, too, appears to have issued
on a fissure nearly parallel to the Lode and to have spread very extensively
over the country; indeed, the present surface shows a greater area of it
than of the earlier hornblende-andesite. It is possible that its eruptions
were not confined to the fissure cut by the Sutro Tunnel. Basalt Hill, B 6,
for example, still some 300 feet high, may well 'be a relic of a still larger
eruptive cone, rather than a remnant of an overflow from a fissure at a con-
siderable distance. Like the older andesite the relations of the augite rock
to the faulted surface near Virginia seem to show that it was eroded down to
a level in that region before the fault occurred. Its character throughout
the District is, as a rule, very uniform. It is possible, however, that a
few localities described as hornblende-andesite are in reality local modi-
fications of this rock. Thus the rock containing the hornblendes with
two concentric belts of magnetite, a crystal from which is shown in Fig. 17,
Plate III., is exposed only by a cut 1,000 feet east of the railroad station, in
C 7. It is within but very near the edge of an area of augite-andesite,
202 GEOLOGY OF THE COMSTOCK LODE.
which appears everywhere to lie directly upon qiiartz-porphyry or still
older rocks. If hornblende-andesite proper occurs here, it should show at
the contacts; but the nearest area of the hornblende rock is 6,000 feet
away. If this is properly to be classed with the augite-andesites in spite
of its mineralogical composition, it is quite possible that the three small
patches of earlier hornblende-andesite shown on the map, each of them
entirely surrounded by augite-andesite, may also be of this character.
Independence of the augite-andesite eruption. The tWO rOCks arC SO mUch alike that
some hthologists doubt the propriety of classifying them as different species,
but in the Washoe District they are certainly diflPerent eruptions. The
contacts in the Sutro Tunnel, the Forman shaft, and at many points on the
surface are well defined, and the mineralogical character is persistent over
very large areas, in spite of a few doubtful localities. It has been seen that
there are also points where it is very difficult to say whether the rock is to
be regarded as diorite or diabase. The absence of such occurrences would
be a matter of surprise, for the character of a rock depends upon combina-
tions of chemical and physical conditions, which cannot be identical at any
two points. Each so-called rock species represents an endless number of
such combinations, and some of these are indistinguishable from those at-
tending the formation of allied species. The strange fact is not the occur-
rence of transitions, which are after all exceptional, but the persistence of
rock types not only within limited areas but throughout the world.
In determining the succession of the hornblende and augite-andesites
position alone can be relied upon, for the two rocks are so closely alhed
that it would be impossible to distinguish with certainty between an inclu-
sion and a local modification in composition. The indications of position,
however, all tend to the supposition that the hornblendic rock is the older,
as may be seen from an inspection of the sections.
Occidental lode. — The Occideutal lode occurs in augite-andesite. Unfortu-
nately the principal mines were closed at the period of the investigation,
and it could not be studied satisfactorily. The dump of the Occidental mine
seems to show that a contact with micaceous diorite is encountered in the
workings. This lode is plotted on the map from distinct croppings and
mine surveys, and its trace is a further remarkable illustration of the par-
OCCURRENCE AND SUCCESSION OF ROCKS. 203
allelism of structure so frequently referred to. Even the sinuous form of
the CoMSTOCK is almost exactly reproduced in the Occidental lode. Bedded
flows aggregating over a mile in thickness could never have resulted in so
nearly perfect a parallelism.
Later hornbiende-andesite. — The Sutfo Tunficl sectiou sliows a fourth vcry steep
contact between augite-audesite and younger hornbiende-andesite ; but the
eastern portion of the former, though covered for the most part, did not
sink in the younger rock below the level of the tunnel, and even reaches
the surface near the mouth of the adit. The manner in which the portions
of diabase and earlier hornbiende-andesite which lay to the east of the
masses now in place disappeared, is a matter of speculation; the Sutro
Tunnel section shows that the corresponding area of augite-andesite really
sank into the later hornbiende-andesite. Had it settled a few hundred feet
farther, it would have left as little trace behind it as did the earlier rocks.
So far as the Washoe District is concerned, however, the eruption of later
hornbiende-andesite was probably less violent and less voluminous than
that of either of the preceding andesites, and was therefore not so likely to
bury the east country to a great depth. Above ground, instead of lying
on a curved surface reducible to an original plain, it forms a range of
mountains extending to the north far beyond the limits of the map. These
do not appear to have suffered greatly from erosion, for even near the sum-
mits they are largely composed of tufa and tufaceous breccia, which could
off'er little resistance to water currents. It does not appear to me that the
existence of this range in its present form is compatible with the suppo-
sition that a large area of the same rock has been removed by erosion.
Making allowance for faulting, the older and firmer rocks have been worn
down to a tolerably smooth and uniform surface, upon which the present
younger hornbiende-andesite range lies in rugged masses. Had the older
andesites and the diorite been cut away after the formation of these hills, the
latter must have suffered at least as much as the older rocks.
Evidence of slight erosion. — Thd'c is uo evidcucc that they have done so; on
the contrary, if the contours of the map within the area laid down as younger
hornbiende-andesite are examined, it will be seen that these are not such as
commonly result from deep erosion. Compare, for example, the steep slopes
204 GEOLOGY OF THE COMSTOCK LODE.
of the Howery range with the older hornblende-andesite declivity west of
Ophir Hill. In the latter locality every water-way has eaten deeply into
the rock, and every slightest undulation in the line of cliffs has given rise
to an eroding streamlet during wet weather. On the Flowery range the
drainage channels are far apart, and very shallow, and many undulations
which in a deeply eroded district would be sure to be emphasized by water
carving show nothing of the sort. The contact line between this rock and
the augite-andesite seems to me unlike contacts developed by erosion. It
has a very different character from the other contacts in the District, and
reminds one strongly of the forms assumed by slag slowly oozing over the
floor of a smelting-works. The structure of the rock, as seen on large
exposures, appears to indicate subaerial rather than subterranean deposition.
Plate VII. shows the east flank of Mount Rose, and is accurately repro-
duced from a photograph. Rude, thick layers of eruptive material, mostly
tufa and breccia, are plainly visible in this locality, though they are trace-
able over no great distance. It is easy to see how such beds might form
in successive eruptions, or through the variations in activity of a single pro-
longed eruption; but it is difficult to account for such a structure in a mass
which has cooled beneath the surface, and has been exposgd by erosion.
Such a mass would be characterized by dike-structure rather than by beds.
The physical character of the varieties of this rock, considered with
reference to their occurrence, is also difficult to reconcile with the suppo-
sition that the range is a mere relic of erosion. As has been explained, in
Chapter III., some of the younger hornblende-andesite is dense and glassy,
and other modifications are firm enough to resist decomposition better
than ordinary augite-andesite. In an eroded district these harder rocks
would be looked for on the summit, and the soft tufas would be found, if
at all, in protected localities; but, as has been pointed out, the tufas are most
abundant at the summits. Deeply eroded areas of eruptive rocks almost
always show patches isolated, or patches nearly separated from the main
field, by the action of water. To a certain extent this is the case with the
younger hornblende-andesite, for the two little areas near the Sierra Nevada
mine were unquestionably cut off from the tongue of this rock extending
from the Flowery range towards the Utah, by the erosion of Seven Mile
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OCCUEEENCE AND SUCCESSION OF EOCKS. 205
Canon. Mount Abbie, on the other hand, shows amphitheatrical basins,
which are not impossibly relics of craters, and that mountain very likely
represents a separate, though unimportant, eruption. But had tlie rock
covered much more country than at present, it is almost certain that other
patches would have been cut off exactly as those near the Sierra Nevada
have been, and as various tracts of augite-andesite, quartz-porphyry, etc.,
have been separated from one another. It has been supposed that there
were such patches near the Combination shaft and the new Yellow Jacket,
but the statements rest upon erroneous determinations.
In the Sutro Tunnel the fissure system parallel to the Lode extends to
the younger hornblende-andesite, but though this rock, particularly near its
western limit, shows evidences of dynamical action, I was not able to make
certain of any regular partings within its mass. On mere geometrical
grounds it could hardly be expected that the fissures would be traceable in
this rock, for at so great a distance from the Lode the logarithmic curve
and its asymptote sensibly coincide.
There can be no doubt that the younger hornblende-andesite succeeded
the augite-andesite. In the Sutro Tunnel section it is seen directly over-
lying and inclosing the augitic rock, and on the divide between Mount Kate
and Mount Rose the augite-andesite can be traced passing horizontally be-
neath the trachytic-looking porphyry. The peninsular-like area near Sutro
Shaft III. would seem, too, to be a flow from the main body, not an inde-
pendent or subsidiary eruption; for the tunnel, though passing close by this
area, shows none of the younger rock west of Shaft II., nor is there any
sign of special disturbance of the augite-andesite in the tunnel near Shaft
III.
Basalt. — Besides the five little patches of basalt shown on the map,
there is another of about the same size directly west of these, and just be-
yond the limits of the map. It is said that a few miles farther south there
are considerable areas of this rock. Two of the five occurrences shown are
very characteristic mesas, and the rock is in every way typical. The only
remarkable fact connected with it is its small extension. No general eff'ect •
upon the history of the Disteict has been certainly traced to it. Though the
basalt comes in contact only with pre-Tertiary rocks and earlier hornblende-
206 GEOLOGY OF THE COMSTOCK LODE.
andesite, there can be little doubt that it is the youngest of all. Its relations to
the andesites have been observed in a great number of localities in the western
United States, and it has always been found to succeed them. This general
evidence is strengthened in the present case by the extreme freshness of the
olivine, which even under the microscope often shows no trace of decom-
position. As olivine is the most readily decomposed of all the lithologically
important minerals, this fact is evidence that the basalt is very recent.
Period of soifatarism. — The gcologists who havc studlcd the CoMSTOCK have
always sought to connect the solfataric action, which is so important a feat-
ure of the District, with one pr other of the volcanic eruptions. Since the
augite-andesite and the rocks which preceded it are deeply altered by soi-
fatarism, and even portions of the younger hornblende-andesite are also thus
affected, the general decomposition cannot be placed earlier than the erup-
tion of the last-mentioned rock. Portions of an eruptive rock may be im-
mediately decomposed by the emanations accompanying its ejection, but
before an extensive area can be decomposed throughout, it must probably
cool and be shattered by mechanical action sufficiently to admit a some-
what free penetration of active solutions. If the solfataric action is due to
one of the eruptions, it must then be either to that of the younger horn-
blende-andesite or to that of the basalt. But direct evidence of such a
connection is wanting. The focal line of soifatarism is at or close to the
Lode. The younger hornblende-andesite area shows no trace of it except
where it approaches the vein ; and, as has been mentioned, the basalt shows
no effects of solfataric decomposition. It is also somewhat difficult to under-
stand how an eruption can produce extraordinarily intense solfataric action
at a locality somewhat remote from the vent of its own fluid ejecta, and not
also at or close to that vent ; though I by no means deny the possibility
of such a coincidence.
While, ho.wever, the solfataric action appears to me, beyond question,
one of the series of volcanic events of which the history of the District
is so full, it does not seem to be necessarily connected immediately with an
eruption of lava. There are mud volcanoes; and solfataras are often active
at periods of time remote from those of eruptions in their neighborhood,
and though the emission of heated watei's frequently attends igneous erup-
OCOUERElSrCB AND SUCCESSION OF ROCKS. 207
tions, thei'e appears no reason to suppose that vast quantities of heated
fluids may not be driven to the surface without an accompaniment of lava.
The solfataric action and the fault are certainly contemporaneous, and may
together form the entire volcanic manifestation of the period in which they
occurred. If the3^ were independent of the eruption of younger horn-
blende-andesite, they must have been subsequent to it; and I beheve it
most probable that such was the case. Of their time relations to the basalt
eruption there is no means of judging.
Collections. — The disputcd character of a number of the rocks of the
District made verj^ full collections essential to the substantiation of the
views maintained in this report. A cabinet series of 200 specimens was
collected in tripHcate, one set being designed for the hthological collection
of the National Museum, a second for the geographical collection of the
same institution, and a third for the San Francisco office of the Geological
Survey. By order of the Director, the size of these specimens is 4 inches
by 5 inches, their thickness being from an inch to an inch and a half
Though these specimens, selected with a view to representing the District
as well as possible, amply suffice for the ordinary purposes of study, so small
a number was not found sufficient to justify the geological map and sections.
A working collection without duplicates was therefore also gathered. The
size adopted was only 1 1 by 2^ inches, in order to lessen the labor of
gathering them and to facilitate their use in the office. This collection con-
tains over 2,000 numbers. Slides were ground whenever they seemed likely
to afford desirable information, and the total number cut was about 600.
The locality of every specimen was recorded at the time of collection
on a map of the surface or of the mines as the case might be. The mine
maps employed were on a large scale, and the localities are usually accurate
to three or four feet. The surface map being on a comparatively small scale
the positions are less precise, but are recorded as accurately as practicable.
On the sections of the Lode, shown in the Atlas, the points from which speci-
mens were collected are marked by crosses, while each locality from which
there is a slide is indicated by a large black dot. On the surface map a
red cross shows the localities microscopically determined, a single cross
208 GEOLOGY OP THE COMSTOCK LODE.
often representing a number of slides. Accompanying the collections is a
copy of the surface map, showing the position of each specimen with its
number. The numbers of specimens of which there are slides are under-
lined, and cabinet specimens are distinguished from those of the working
collection. The collections are also fully labeled and completely cata-
logued.
CHAPTER VI.
CHEMISTRY.
General nature of the chemical activity. The general TCSVlltS of chemical aCtlvity
whicli have been observed in the Washoe District can be very briefly
stated. Decomposition is widespread, but while in the greater part of the
area it has not sei'iously modified the character of the rock, the alteration
within a certain portion of the region is profound, and often wholly obscures
lithological distinctions. This area of extreme decomposition is precisely
the most important, lying immediately about the Lode.-^ The characteristic
bisilicates of the eruptive rocks have been replaced by chloritic minerals,
epidote, quartz, and calcite; pyrite has been deposited in the mass of the
rock, and the feldspars have in great part undergone degeneration of a
complex kind; finally, oi-e-bearing quartz has been deposited in the Lode.
It is the purpose of the present chapter to give as rational an account of
these changes as I am able to suggest, and to trace their geological rela-
tions. Any such account must, in the present state of knowledge as to
the constitution of minerals, be lai-gely hypothetical; but, although future
investigations will probably greatly modify the present conceptions of the
nature of inorganic compounds, the best hypotheses at present are those
which put the least strain on well-proved theories. The Washoe District
affords, as has been seen, a remarkable opportunity for microscopic exam-
ination of the results of decomposition ; not, however, for their chemical
investigation, for no occurrences have been met with in which single alter-
ation products, excepting pyrite, are concentrated in sufficient masses to fur-
nish good material for analysis. Even were this the case, it seems to me that
' See Fig. 1.
14 0 L, 209
210 GEOLOGY OF THE COMSTOCK LODE.
little progress can be made until definite criteria are discovered by which
the state of combination of the elements in fresh minerals can be decided.
Formation of pyrite. — Pcrliaps tlio most stHking charactcrlstic of the decom-
posed rocks of Washoe is the presence of innumerable bright crystals of
pyrite disseminated through the mass. The unaltered rocks do not appear
to carry this mineral; if it occurs in them at all it is certainly a very rare
ingredient. In the altered rocks pyrite, when present, is abiindant nearly in
proportion to the degree of decomposition, and, except where it is exposed
to the direct action of the atmosphere, it is almost invariably perfectly fresh.
All the circumstances thus indicate that it is a product of decomposition.
The massive rocks contain iron, chiefly as magnetite and as a component of
the bisilicates; and the pyrite must have been formed by the action of soluble
sulphurets on one or both of these compounds. The slides of the pyritous
, rocks, however, frequently show large quantities of sharply defined mag-
netite, while the bisilicates are in a majority of cases wholly decomposed.
There is certainly nothing in the association of pyrite and magnetite to sug-
gest a relation ; but the pseudomorphs of decomposition products after the
bisilicates are very frequently studded with small pyrite crystals, and occa-
sionally real pseudomorphs of pyrite after augite or hornblende appear to
occur. Of these it is difiicult to be certain,' however; for the size of the
pyrite individuals is usually considerable, relatively to that of their hosts; the
original crystal form is consequently never unmodified, and is commonly
altered beyond recognition. The distribution of the pyrite in the rock also
reminds the familiar observer of the distribution of the bisilicates in the
same rock, and macroscopical comparison of suites of specimens from the
same localities shows that the pyrite to all appearances is associated with
the bisilicates, and in extreme cases replaces them. It is easy to lay too
much stress on an impression of this sort, yet when such an impression is
derived from the examination of many thousand instances it deserves some
weight. All the evidence thus tends to the supposition that the pyrite is
mainly a decomposition product of the bisilicates and of mica. Such an
alteration is quite possible in the presence of alkaline sulphides, or of
hydrosulphuric acid ; and, as has been seen, the waters even now entering
the mines three thousand feet from the surface are charged with the latter
CHEMISTRY. 211
reagent. Had oxidizing agencies been active to any great extent below the
surface tlie pyrite must have been decomposed, and the inference from the
facts is strong that such has not been the case.
The formation of pyrite might conceivably either take place immedi-
ately at the expense of the bisilicates or be formed from secondary minerals;
biit the ferruginous silicates, chlorite and epidote, are frequently deposited
in veins and patches quite free from pyrite, and nothing has been observed
in their association with pyi*ite to indicate an epigenetic connection. It is
therefore more probable that p^aite resulted immediately from the action of
hydrosulphuric acid and similar compounds on the bisilicates. This action
could not possibly be unaccompanied by the formation of other alteration
pi'oducts, for the whole stochiometric relations of the bisilicates would be
changed by the abstraction of iron. Since hydrosulphuric acid Is a pow-
erful reducing agent, it Is a priori probable that the accompanying products
would contain little ferric oxide, and as the bases in the bisilicates are fully
saturated with silicon a separation of silicic acid is indicated.
Formation of chlorite. — The clilorite, whIch, as has been seen in Chapter III.,
nearly always results from the decomposition of the ferro-magnesian sili-
cates, is of uncertain species, but it Is neither cllnochlore nor pennine, and
answers well to Werner's chlorite (the ripidollte of Gr. Rose). This mineral
has approximately the composition of a semisilicate, and contains little or
no ferric oxide. It is also accompanied In a great proportion of cases by
secondary quartz, and often also by calclte. The occurrence of this last
mineral shows that carbonic acid, as well as hydrogen sulphide, must have
been present during the decomposition of the rocks, and probably fronf the
commencement, for chlorite contains no calcium; and had hydrosulphuric
acid alone acted on the bisilicates a calcium silicate m^jst have resulted in
the first instance. Of such a preliminary change, however, there is no
trace, although, as has been seen in Chapter III., it appears possible to fol-
low the course of decomposition mineralogically from its Incipient stages.
Calcite, however, Is not usually prominent among the decomposition pro-
ducts of the bisilicates in specimens collected under ground, unquestionably
owing to its great solubility.
Circumstances favoring the formation of epidote. That chloritC Or clllorltlc Uliuerals
212 GEOLOGY OF THE COMSTOCK LODE.
very usually result from the decomposition of hornblende, augite, and mica
is a well-known fact, and pseudomorphs of epidote, after these minerals, are
also common. The difference is great, for while chlorite contains little or
no ferric oxide and no calcium, epidote contains both, but is free from mag-
nesium. It would seem, therefore, as if epidote must be formed under
such conditions that fen-ous compounds might be oxidized, or such that
ferric compounds, at all events, would not be reduced, and, further, under
conditions favoring the solubility of magnesian salts rather than those of
calcium. It appears to me somewhat difficult to suppose the bisilicates
exposed to a sulphidizing action so strong as to result in the formation of
pyrite, and yet not sufficiently reducing to prevent the formation of ferric
compounds. On the other hand, pyrite, though of very variable stability,
often oxidizes with great difficulty, and the oxidation of ferrous compounds
may sometimes be effected in its presence. Epidote might, therefore, form
in the presence of pyrite, but hardly contemporaneously with it. The
behavior of the salts of magnesium and calcium salts towards one another
is known to var}^ greatl}^ with the physical conditions, especially with tem-
perature, and presumably also with pressure, and it is further affected by
the concentration of solutions. Thus, Dr. T. S. Hunt^ found that when solu-
tions of the chloi'ides and carbonates of these elements are evaporated at
ordinary temperatures, calcium carbonate alone is first precipitated; while,
when the solution is boiled, magnesium carbonate first separates. This and
similar facts tend to the supposition that high temperatures would favor the
formation of magnesian chlorite rather than of calciferous epidote.
'conditions under which epidote occurs. — The undcrgrouud rocks at Washoe all con-
tain chlorite in abundance, but epidote is uncommon. Thus, a special search
was necessary to discover epidote in the underground diabases, while per-
haps half the augite-andesites from the surface contain it in considerable
quantities. When it occurs at a considerable depth it seems to be either
close to the Lode or near strong seams extending towards the surface, as in
one or two localities in the Sutro Tunnel. On the surface epidote is extremely
common, tinging whole areas of the various rocks with its peculiar green,
and occurring in many and widely separated localities. Where epidote is
' Chem. and Geolog. Essays, p. 138.
CHEMISTRY. 213
best developed, as in Crown Point and Ophir ravines, the accompanying
pyrite is usually decomposed either wholly or in part; and in localities at a
small distance beneath the surface, like the McKibben tunnel, it is in those
belts of rock which are evidently most highly decomposed that epidote is
found replacing chlorite.
Probable course of the alteration of chlorite to epidote. StrOHg minOralogical evldeUCe
has already been offered to show that epidote at Washoe is an alteration
product of chlorite. The indications of relative solubility are worth con-
sidering in this connection. Chlorite is manifestly rather easily soluble, and
soon after its formation becomes diffused through the groundmass and any
porous crystals which may be present, settling, too, in veins when cracks offer
an opportunity for such a concentration. Epidote appears to be soluble only
in a greatly inferior degree ; indeed, its faggot-like masses of crystals seldom
show anything which can be interpreted as attack by a solvent. If, there-
fore, chlorite and solutions of calcium carbonate containing free oxygen are
brought together under physical conditions compatible with the formation
of epidote, it seems inevitable that epidote should be precipitated, unless
still more insoluble substances may also be thrown down under the same
conditions.
It is well known that chlorite is frequently altered to a mass of quartz,
ferric hydrate, and carbonates. When this change takes place it is prob-
able that at least a portion of the alumina is mingled in some form with the
iron oxide, and the carbonates most likely contain magnesium as well as
calcium. In the numerous cases of this change which have been observed
at Washoe, the carbonates form a large portion of the resulting mixture, a
fact which appears to prove that the active solutions were but slightly charged
with carbonic acid, since, had it been otherwise, calcite and magnesite, if
separated out at all, would have been redissolved. Cases of the conversion
of chlorite to epidote and to carbonates, etc., often occur in the same slide,
and presumably under nearly the same physical conditions. It may be
that the decisive point is the quantity of carbonic acid present. If the two
processes went on at different times such a difference would be readily expli-
cable, and if simultaneously it is not difficult to understand how the quantity
of carbonic acid might vary. Though rocks are permeable, the aqueous cur-
214 GEOLOGY OF THE COMSTOCK LODE.
rents are greatly obstructed, and move in labyrinthine paths of least resist-
ance. Of this the lithologist is constantly reminded by meeting wholly fresh
crystals and entirely decomposed ones of the same mineral close together.
One tiny current percolating through the rock may meet with comparatively
large quantities of carbonates and become saturated, while another in the
same neighborhood remains well charged with carbonic acid and oxygen.
If the suggestion made is correct, the former coming in contact with chlo-
rite would convert it into a mass of carbonates, quartz, and ferric oxide;
while the latter, which would be a solvent for carbonates, would convert
chlorite into epidote.
Nature of the decomposition of the bisiiicatcs. — Qualified by all tho doubts wliich have
been expressed, the observations considered in connection with the chemical
possibilities lead to the following as the most probable statement of the
decomposition of the bisilicates of the Washoe rocks. Waters charged
with hydrosulphuric and carbonic acids, but containing no free oxygen, at
temperatures probably very near the boiling-point, acted upon the fresh
augite and hornblende ('or mica), producing from them pyrite, chlorite,
quartz, and carbonates of the alkaline earths simultaneously Of these a
large portion of the carbonates passed into solution. At a later period sur-
face waters at lower temperatures, containing carbonic acid and free oxygen
in solution, produced a further alteration of a portion of the chlorite in the
rocks near the surface, or peculiarly accessible from it. Where carbonic
acid was present in excess epidote resulted ; where, through saturation with
carbonates, the carbonic acid was deficient, the chlorite was altered to car-
bonates, quartz, and metallic oxides, no doubt with admixtures of less impor-
tant compounds.
Magnetite. — No place has been given to magnetite among the decomposi-
tion prodvicts of the bisilicates. As all the rocks contain large quantities
of this mineral constantly associated with the bisilicates, and often so thickly
distributed in perfectly fresh crystals (particularly of hornblende) as to leave
but little of the host visible, it is difficult to distinguish sharply between
the primitive and the secondary occurrences of the iron ore. In fact, I have
not been able to make absolutely sure of more than one or two instances
of secondary magnetite, though such an origin seems probable enough in
CHEMISTRY. 215
many cases. On the other hand, it seems certain that the black border of
many hornblendes has been attacked, and has given place to a transparent
mineral, which is more or less diffused in and obscured by the groundmass.
The natural supposition is that it is ferrous carbonate.
iimenite. — Titauic iron ore may often be observed in slides from the Dis-
trict passing into leucoxene. The nature of this substance is doubtful,
and no occurrence in the District is conclusive as to its nature, yet many
cases have been observed the character of which would be very satisfactorily
accounted for if the supposition of Messrs. Fouquti & L^vy, that leucoxene
and titanite are identical, were accepted.
Decomposition of the feldspars. — The feldspars of tlic Washoe regiou have offered
a far more effectual resistance to decomposing agencies than the bisilicates,
much more, too, than would be supposed from a macroscopical examination
of the rocks. In the mines it is very rarely that a particle of auo-ite,
hornblende, or mica, can be found, these minerals being nearly always
wholly replaced by alteration products; but it is the exception when a
moderately hard rock does not show under the microscope well defined and
fairly fresh feldspars. When wholly unattacked the feldspars of diabase,
of some diorites, and of the older andesites are transparent, and the rocks
then show only the tints due to the presence of magnetite and the bisilicates.
They are then dark, somewhat basaltic-looking masses. But when only a
very minute amount of change has taken place in the feldspars, they become
opaque through irregular reflection, and form the most prominent feature
of the rock. Rough estimates, made with the help of the microscope, indi-
cate that the decomposition of much less than one per cent, of the feldspar
substance suffices to destroy the transparency of the crystals.
The nature of the decomposition of the feldspars is still very obscure.
It is usually considered that the triclinic feldspars as well as orthoclase are
sometimes converted into kaolin, though Professor Tschermak maintains,
as an analytical result, that the hydrated aluminium silicate resulting from
the alteration of plagioclase contains but a single molecule of water, and
not two, as is the case with kaolin. Saussurite and pinitoid are the names
given to complex silicates, or mixtures of silicates and other substances,
216 GEOLOGY OF THE COMSTOCK LODE.
which often result from the decomposition of feldspars; and mica and epidote
are counted among the pi'oducts of alteration.
Kaolin. — Kaolin is microscopically an obscure mineral. According to
Mr. H. Fischer it is amorphous, while Mr. A. Knop found it to consist of
delicate hexagonal plates of the rhombic system. Breithaupt named this
crystaUine modification nacrite, and M. Des Cloizeaux pholerite. If saussurite
and pinitoid are really independent minerals, it is certain that these names
have also been given to mere mixtures resulting from the extraction of por-
tions of the silicic acid and of the stronger bases.
Evidence of the microscope. — lu tlic Washoe rocks, as is usual elscwherc, the
first indication of decomposition is the appearance of calcite and quartz in
the more or less carious crystals. This is doubtless attended by the forma-
tion of soluble alkaline silicates, which, however, are not recognizable under
the microscope. As the process continues the striations are obliterated, and
the final result is a heterogeneous mass showing aggregate polarization,
sometimes only faintly translucent, and containing in a recognizable form
only grains of calcite and quartz. No amorphous substance has been ob-
served, nor any hexagonal lamellae answering to the description of nacrite.
Mica, too, appears to be absent, although occurring among the decomposi-
tion products of similar rocks at no great distance from Virginia. Chlorite
and epidote are common in decomposed feldspars, but in many cases it seems
certain that chlorite due to the decomposition of the bisilicates has merely
permeated the spongy mass; and epidote has repeatedly been observed
developing in patches of chlorite, which were surrounded by feldspar sub-
stances, just as it has been described and illustrated as occurring in altered
bisilicates. No case has been met with in which either mineral was dis-
tinctly parasitic on feldspar.
All lithologists agree that chlorite forms from the bisilicates, and that
feldspars become carious; it is also acknowledged that chlorite is diffused
through the portions of the rock mass in the immediate neighborhood of
the point at which it forms. It must therefore penetrate the feldspars where
these are partially decomposed, in all rocks in which the bisilicates are to
any extent converted into chlorite. It is, of course, by no means necessary
that the point at which chlorite gained access to the feldspar should be
CHEMISTRY. 217
visible, for entrance is as likely to have been effected above or below the
plane of a thin section as in it. If chlorite and epidote really occur as
results of the decomposition of feldspar, it should be easy to show the
parasitic growth of chlorite in feldspars, just as its development from horn-
blende has been shown in the present volume.
Chemical analysis. — The microscope glvcs mainly negative results concerning
the decomposition of the feldspars of the Washoe rocks. Chemical analysis
of the decomposition products could lead to no definite results, because no
reasonably pure material could be obtained, and the only remaining source
of information is the analysis of the rocks. The diabase from the hangino-
wall of the Lode, which was analyzed, is a very slightly altered rock, and
has been described under slide 18. Its feldspars are transparent and have
undergone only an inappreciable amount of alteration ; the rock nevertheless
contains a considerable quantity of water, as is shown by its loss in ignition,
2.47 per cent. Abundant fluid inckisions account for a part of this loss,
and the water of hydration of the small amount of chlorite it contains for
another portion. The ignition loss no doubt includes a small amount of
carbonic acid. The "propylite horse" analyzed by Prof W. G. Mixter was
in all probability decomposed diabase. An inspection of the analysis shows
either that silica had been deposited in the rock, or what seems more likely,
that the bases had been in large part extracted. It contained 1.83 per cent,
of water, or about two-thirds as much as the fresh rock. The bisilicates
must have been represented by chlorite, which contains about 12 per cent,
of water. The small quantity of aluminium not entering into the chlorite
may possibly have existed as kaolin, a supposition neither proved nor dis-
proved by the analysis, which, however, shows that the horse contained at
most a small percentage of that mineral. Four analyses of clays made for
the Exploration of the Fortieth Parallel" by Professors Johnson and Mixter
are available. It is here, if anywhere, that kaolin must be indicated. On
comparison of these analyses with that of the fresh diabase, it appears that
they do not represent concentrations of any special mineral, but merely
highly altered rock masses. Barring the pyrite and water, the first three
show very nearly the same composition as the fresh rock, while a portion
of the silicic acid has apparently been abstracted from the Savage clay.
218 GEOLOGY OF THE COMSTOCK LODE.
The quantity of pyrite corresponds fairly well with the deficiency of iron
in the clays. Taking into consideration that these clays must have con-
tained chlorite corresponding to about 18 per cent, of augite, it appears
from the water contents that those fi'om the Chollar and the Hale <£ Nor-
cross can have included little or no kaolin. Those from the Yellow Jacket
and the Savage, on the other hand, may have contained both chlorite and
kaolin, but the latter only to the extent of a few per cent.
Kaolinization not prevalent at Washoe. Tho Weight of CvideUCe iS thuS TCaSOnably
strong that in the regions thus far exploited on and near the Comstock,
kaolinization, if it has taken place at all, has occurred only to a very trifling
extent, and that the degeneration of the feldspars results almost wholly in a
mixture of silica, calcite, and unrecognizable minerals, earthy in texture, in
part nearly opaque, and of a light color.
Occurrence of ore and the accompanying rocks. As may be SeeU ft'Om the mapS aud
sections, the Comstock Lode is several miles long, and is found in contact
with various rocks. The fissure is not simple, but ramified, and might have
been represented as still more complex, for the quartz veins struck by the
McKibben Tunnel in Spanish Ravine, and by the Peytona and other work-
ings on Cedar Hill, are unquestionably either stringers joining the Lode at
unknown points, or subsidiary parallel veins due to the same chain of dy-
namical and chemical causes as the Comstock. It appears from the longi-
tudinal vertical projection that but a small fraction of the fissure has been
filled with ore. This statement, however, requires explanation and qualifi-
cation. Nearly all the vast mass of quartz on the Comstock contains con-
siderable quantities of silver and gold, but none, of course, is extracted
which will not pay for working. While auriferous gravels may yield a
handsome profit when they contain considerably less than ten cents per ton,
and gold quartz may sometimes pay which contains two or three dollars,
Comstock ores carrying less than about twenty dollars can usually be
extracted only at a loss. Geologically the Comstock must be considered
as filled with metalliferous gangue, enriched at numerous spots, which are
known by the Spanish mining term "bonanzas."
Vastly the most productive area has been that portion of the main
Lode between the Overman and the south end of the Sierra Nevada mine.
CHEMISTEY. 219
Bullion has also been produced at the Jws^ice to the south, and from the
veins on Cedar Hill to the north. In the Virginia and Gold Hill mines,
and on Cedar Hill, the gangue is quartz, only occasional masses of calcite
of insignificant size having been encountered. South of the Overman, on
the other hand, the gangue is largely calcite.
The quartz of Cedar Hill carries free gold, alloyed, of course, with a
little silver. Certain stringers from the main Lode and the "west vein" of
the CoMSTOCK, as that portion lying to the west of the great horse in Vir-
ginia City, above the line at which the two fissures join, is usually called,
are of the same character. The Justice ore was argentiferous, but very
"base," carrying large quantities of galena, zinc blende, etc. The ore
bodies on the main Lode in Virginia and Gold Hill, which have yielded
almost all of the bullion extracted, may profitably be considered as of two
classes. The greater portion of the bullion has been derived from minerals
disseminated in the quartz in microscopic particles. Ore of this kind is
often distinguishable from barren quartz by bluish stains, but not always.
The quality, and even the presence of ore, can in many cases only be told
by assay, and superintendents who have taken part in the mining opera-
tions almost from their commencement do not hesitate to confess that their
judgment of the quartz is often at fault. The behavior of this ore in amal-
gamation shows that its silver contents is mainly due to argentite. Its gold
contents constitutes from one-quarter to one-half its total value. Near the
outcroppings many bunches of other ores occurred, such as stephanite,
polybasite, ruby silver, etc. These were in some cases accompanied by
relatively large quantities of galena and zinc blende. In the great Consol-
idated Virginia and California bonanza, several streaks or veins of very
rich black silver ores, said to be mainly stephanite, occurred. These were
sepai-ated from the surrounding ore-bearing quartz very sharply, as if of
later origin.
Pyrite is found everywhere, both in the country rock and in the ore
disseminated in small crystals. It is less frequent in the quartz than in the
country rock, but it is especially abundant in the east country, opposite the
ore bodies. It also occurs with frequency in the diorite west of and near
the Lode. In all these cases it forms but a small portion of the mass — say
220 GEOLOGY OF THE COMSTOCK LODE.
from ten per cent, downwards; but in the graphitic slates forming the west
wall in the Gold Hill mines, bunches are met with in which it is the pre-
dominant constituent. These are, however, usually only a cubic foot or
two in size, and appear to occur only close to the vein. As a rule, the slates
are not much more pyritiferous than the diabase.
Relations between ores and rocks. — There is au cvideut relatiou between the in-
closing rocks and the character of the ore. The rocks occurring at and
near the Justice, with its refractory ores and calcite gangue, are metamor-
phic diorite, mica-diorite, quartz-porphyry, and hornblende-andesite. The
Cedar Hill gold-quartz veins are in diorite: The ores of the more impor-
tant mines lie on the contact between diabase and diorite.
There seem to be but two probable ways in which these differences
can have come about.^ The ore deposits might have taken place at differ-
ent times, and therefore under different conditions, or the contents of the
fissures may have been extracted from their walls at the same time, and the
differences be due to the composition of the siirrounding rock. If the
Cedar Hill veins were deposited at a different time from the main mass of
the CoMSTOCK ore, it must have been at an earlier date, for the vast quan^
titles of solutions which reached the Comstock could not have failed to
penetrate the fissured diorite. Not only stringers from the Comstock, how-
ever, but even the "west vein," are of the same character as the Cedar Hill
quartz. When this west quartz was deposited the fissure below was cer-
tainly open, and had it been deposited before the argentiferous ore, it is
scarcely possible to suppose that it would not also have filled the vein at
lower points. If they were to be assigned to different periods, one would
also expect to find either gold veins in the east country, or silver veins in the
west. In short, there is much to show that these two classes of deposits
were contemporaneous; and I know of no evidence tending to show that
they are not ascribable to a single period. The Justice ore body is not
closely enough connected with the more important portion of the Com-
' It is also conceivable that the ores should have been precipitated from solution by the rock
forming the ■walls and the horses, and that the observed diflferences are due to the character of the
precipitant. All the evidence of ore deposits in general, and of the Comstock in particular, however,
appear to me to point to changes of temperature and pressure, evaporation and the action of liquid
reagents, as the causes of precipitation. In describing the Lode I shall be obliged to recur to this
subject.
CHEMISTRY. 221
STOCK to permit of a detailed comparison, such as that given above, but
in the absence of proof to the contrary it is probable that it too was depos-
ited at the same time.
Time relations of the ore. — During the poHod in which the field work for the
present volume was done, there was but very little ore in sight. What I have
seen of ore near the croppings exposed in a few reopened workings, however,
and recollections of the streaks of high-grade ore in the "great bonanza," lead
to the belief that these rich concentrations were of later origin than the mass
of the ore. The quartz in the Consolidated Virginia and California was
almost everywhere a crushed, powdery mass, while the thin and persistent
veins of black ore running through it wei'e very solid. A somewhat simi-
lar relation seems to have existed near the croppings, and it is not impossi-
ble that these ores were formed at the expense of others of the more usual
kind at a later date, and that they occupy spaces opened in the ore masses
bv faulting action.
Origin of the vein minerals. — It is wcll kuowu that the able aud laborious inves-
tigations of Prof. F. Sandberger^ have added greatly to our knowledge of
the distribution of the metals in unaltered rocks, and of the reactions by
which in many cases they have been concentrated in veins. Though not the
first to show that the bisilicates, as well as mica, sometimes carry small
quantities of the heavy metals, he has multiplied the known instances so
greatly as to establish the frequency of such a composition. In many
cases it is an exceedingly complex matter to prove a possible connection
between a vein and the surrounding rock, because the minerals present in
noticeable quantities are numerous. This is not the case at Washoe, for
quartz, silver, gold, and sulphur predominate so greatly over all other ele-
ments that if the presence of these is accounted for, the problem may be
considered solved, unless the solution offered is inconsistent with the pres-
ence of small quantities of calcite, galena, zinc blende, etc., and with the
general distribution of pyrite.
Origin of the quartz and ore. — No chcmical aualysis is uccessary to detect a
possible origin for the quartz of the Lode. Macroscopical and microscop-
ical examinations sufficiently show the enormous destruction of primary sili-
' Untersuchungeu iiber Erzgange, erstes Heft, 1882. Also, Berg- n. h.-Zeit.ung, 1877 and 1880.
222 GEOLOGY OF THE COMSTOCK LODE.
cates which has taken place throughout a large area. On the other hand,
minute quantities of gold and silver can be more easily and more certainly
determined bj^ dry assay than by analysis, provided that pure lead reagents
can be procui'ed But the selection of suitable material for the investiga-
tion of the gold and silver contents of the Washoe rocks was by no means
a simple matter. As has been seen, there is but one spot known in which
nearly fresh diabase can be collected, and that close to the Comstock fissure.
Moreover, the quantities of the precious metals to be dealt with are so minute
that a mere trace of infiltrating solutions of their compounds would impart
a comparatively important metallic contents, and that such impregnations
occur in some of the rocks there is very good reason to believe. This
occurrence of fresh diabase is therefore open to suspicion. If, however, the
diabase which forms the east or hanging wall of the Lode is the source of
its gold and silver, fresh portions of the rock will show a larger quantity of
the precious metals than decomposed samples; while, if the source of the
ore were independent of the diabase, decomposed portions of the latter,
being more porous, would have been more readily and fully impregnated
by the metalliferous solutions. Moreover, it has been shown that pyrite
forms at the expense of the augite of the diabase, and as pyrite is known to
have a very strong affinity for gold, the decomposed pyritiferous rock should
show a greater proportion of gold to silver than the fresh diabase, if this rock
is the source of the metals. Were the original distribution of gold and silver
and their subsequent extraction nearly uniform, the composition of the ore in
the Lode would correspond to the contents of the fresh rock, less that of the
decomposed rock and the pyrite, as shown by a limited number of assays.
The quantity of the precious metals occurring in the vein should also be
calculable from the extent of the decomposed rock. Such ideal conditions,
however, are not to be expected. The excessive difficulty of obtaining a
representative sample of any gold or silver deposit is familiar to all mining
men, and in the Comstock itself great variations, both in the relations of
gold to silver and in the total tenor, are of constant occurrence. On the
supposition that the metals have been extracted from the diabase these
variations indicate great irregularity in the leaching action or in the original
distribution of the metals, or, more probably, in both.
CHEMISTRY. 223
Precautions observed in assaying. — The assavs tabulated at the end of Chapter III.
were made by my assistant, Mr. J. S. Curtis, who, in addition to a thorough
training, has had many years of experience in accurate and responsible
assaying. In attempting to detect minute quantities of precious metals in
the Washoe rocks, the first difficulty experienced was in obtaining suffi
ciently 2:»ure lead or litharge It was found that even that imported fi-om
Germany and sold at a very high price as chemically pure was far too rich
in silver and too irregular in its silvei- contents to answer the purpose. In
this dilemma Mr. Rickard,of the Richmond Mining and Smelting Company, in
Eureka, was kind enoiigh to place a refining furnace with a new test at Mr.
Curtis's disposal, as well as the purest of the lead refined by the Luce &
Kozan process in the works under his charge. By careful manipulation Mr.
Curtis was able to prepare litharge assaying less than eight cents a ton and
of so regular a composition that, with the help of blank assays, the silver
contents of the rocks could be very exactly determined.
A series of experiments was then made to determine the time of reduc-
tion which would give a maximum result with material so poor in metals
as the Washoe rocks. It was found that this time was much longer than
that requisite for the reduction of ore. Refined cream of tartar was the
reducing agent employed, with sodium bicarbonate and borax in carefully
determined proportions as fluxes. The cupels were made with great care
of two parts of bone-ash to one of cedar-ash, the surface being formed of
elutriated bone-ash. In cupelling feather-litharge was invariably allowed
to form, and throughout the experiments no known pi-ecaution was neg-
lected.
Gold detected in the rocks. — Li addition to the sllver contents of the Washoe
rocks, gold also was detected, but in such minute quantities that little reliance
can be placed upon the relative tenor of diffei'ent samples. It was estab-
lished, however, that the fresh diabase cari-ies as much as four or five cents in
gold to the ton, and furthermore that the pyrite, so abundant in the decom-
posed rocks, carries both gold and silver, but more of the former than of the
latter. Thus pyrite washed from the decomposed diabase 250 feet north of
the C. c(- C. connection with the North Lateral of the 8utro Tunnel, assayed
three cents in silver and eight cents in gold, and pyrite from the Belcher
224 GEOLOGY OF THE COMSTOCK LODE.
slates gave eighteen cents silver and twenty cents gold. The diorite from
Bullion Ravine also showed an indeterminably small trace of gold, while
the andesites carry about as much as the diabase.
Silver traced to the augite. — It sccmed probablc from Professor Sandberger's
investigations that the augite of the diabase was the seat of its metallic con-
tents. To test this point, the feldspar and augite were separated by Thoulet's
method and separately assayed. It appeared that, for equal weights, the
augite was eight times as rich as the feldspathic material, and, as a per-
fectly clean separation by Thoulet's method is impracticable, this seems
substantially equivalent to a proof that the silver is a constituent of the
augite.
Results of the assays. — By comparisou of thc diiferent assays it appears that
decomposed diabase carries somewhat less than half as much silver as the
fresh rock. Where the decomposed rocks are pyritous, the experiments
made do not indicate any essential diminution of the gold contents. This
fact, however, is quite possibly due to irregularity in distribution and the
minuteness of the quantities of gold to be determined. As the decomposi-
tion of the rock in question has proceeded at a great depth beneath the sur-
face, it is highly unlikel}^ that silver should have been extracted unaccom-
panied by gold. Much of the decomposed rock, too, is nearly free from
pyrite, and had the gold contents of such specimens been determined a
smaller percentage would probably have been found. The omission was
not detected until too late to resume the investigation. So far as quantita-
tive relations are concerned, only the silver can be relied on, though the
qualitative detection of gold as well is both interesting and important.
Comparison with the yield of the Lode. If, then, the COMSTOCK LODE is SUppOScd
to have derived its precious metals from the diabase, we should expect to
find that it yielded dord silver containing a small quantity of gold. The
gold contents has actually been very variable, in some few cases exceed-
ing the value of the silver and in other instances amounting to only a fourth
of its value. The Lode has bee'n pretty thoroughly explored to a depth of
2,500 feet, and the extent of diabase exposed may be put roughly at a
length of 8,000 feet and a thickness of 2,500 feet. If about 13 cents per
ton, or, say, 1 cent per cubic foot, has been extracted from this mass, the
CHEMISTRY. 225
total amount thus accounted for is $500,000,000. Over $300,000,000 have
been actually put upon the market, and nearly $100,000,000 more have
probably been lost in tailings. The low-grade quartz not extracted most
likely contains more than another hvmdred milHons, but the sum obtained
by calculation is nevertheless a fair approximation to the amount which the
Lode must actually have contained. On the other hand, if an attempt be
made to account for the ore on any other supposition than that it was derived
from the diabase, it seems very difficult to give a plausible explanation for
the disappearance of the gold and silver which appear to have been extracted
from this rock.
Other rocks. — The diorite also contains precious metals; but 'while dioritic
vein matter is highly charged, and even that at the mouth of Bullion ravine,
which is very solid but contains some pyrite and is very close to the Lode,
carries a notable quantity, that from the head of the same ravine shows only
a trace of silver. These relations are the reverse of those observed in the
diabase and appear to indicate an impregnation from the Lode. The diorite
also contains a trace of gold. More could hardly have been e^cpected; for,
except on Cedar Hill, it has never been found worth while to treat the gold
quartz of the District, and the Cedar Hill mines have yielded but little.
The andesites and the quartz-poi'phyry show only very small amounts
of silver, but the metamorphic diorite contains eight cents per ton. The
analysis also shows that this rock is highly calcareous, and it seems not
impossible that the Justice oi"e body, which is associated with the meta-
morphic diorite, was derived from it. The basalt, on the contrary, is
nearly as rich in silver as the older diabase, but no ore is likely to have
been extracted from it, for the rock is not only the freshest in the Disteict,
but is remarkably fresh for any region, many of the olivines showing no
trace of attack.
Lateral-secretion theory affirmed. — Ou thc wholc, therefore, the chsmlcal aud geo-
logical evidence point to the lateral-secretion theory as the true explanation
of the Washoe ore deposits, and to the augite of the older diabase as the
source of the important ore bodies. It is worth while to note that, accord-
ing to report, many of the famous silver mines of the world are associated
with this rock.
15 0 L
226 GEOLOGY OF THE COMSTOOK LODE.
Nature of the solvents. — As has been sceii, there is reason to suppose that the
active reagents in the decomposition of the minerals of the diabase were
sulphhydi'ic and carbonic acids. These acids so usually reach the surface in
volcanic regions that there seems no necessity for examining their origin
here, but it may be pointed out that solutions of sulphates rising through
graphitic slates, such as form in part the foot wall of the Gold Hill mines,
would necessarily be reduced to sulphides. Both augite and plagioclase
would yield to the attack of carbonic and hydrosulphuric acids; carbonates
and sulphides of the alkalies and alkaline earths would be formed, and these
are solvents for quartz and sulphides of the heavy metals. There is no
difficulty, therefore, in accounting for the solution of the materials filling
the CoMSTOCK Lode. It is somewhat less easy to trace the precipitation of
the ore with certainty. Solutions of silica in water containing alkaline car-
bonates deposit silicic acid only on evaporation, not on cooling; but when
sulphides of the alkalies are also present a reduction of temperature is fol-
lowed by the precipitation of a portion of the silica. Solutions percolating
from the east country into the main fissure, where communication with
the outer air was less impeded, may have deposited some of the quartz in
consequence of cooling. This possibility, however, seems scarcely adequate
to explain the phenomena. Vast quantities of the solvent must have been
necessary to carry all the silica occurring on the Lode ; and it is difficult to
understand how any great amount of cooling can have taken place. If hot
solutions are supposed to have issued as springs along the croppings, the
influence of exterior conditions on the temperature of the water below the
surface must have been insignificant, and Sandberger has found that copious
mineral springs deposit sinter about their orifices, but not in the channels
leading to them. Even if the solutions may be supposed not to have over-
flowed, being, as they must have been, in communication with an active
source of heat, they would have been maintained at a nearly, constant tem-
perature by convection.
Precipitation. — Silica Is vcry readily precipitated from solution, and it is
well known that when both silica and carbonate of calcium are dissolved
in the waters of hot springs, the acid is deposited near the source and calcite
at a greater distance. Sandberger states that when such solutions become
OHEMISTET. 227
saturated with carbonates the siUca is precipitated. If so/ it is not difficult
to understand how a continuous precipitation of silica may have taken
place while the carbonates were carried off in solution.
It has been explained that the District shows very small evidences of
erosion since the deposition of ore began — less than one would suppose com-
patible with the deposition of quartz from flowing springs on so large a
scale. The District presents many points of similarity to the neighbor-
hood of Steamboat Springs, where but little water flows ofi", while abundant
columns of steam constantly rise from many vents. If, as seems probable,
the condition of things at Washoe was similar, the precipitation of silica
must have been greatly accelerated by concentration of the solutions through
evaporation. Precipitated silica is, of course, in great part amorphous, but
its conversion into quartz is a well-known change.
' This statement is no doubt founded on experiments, of which I have failed to find an acconnt.
CHAPTER VII.
HEAT PHENOMENA OF THE LODE.
Sp:ction 1.
GENERAL DISCUSSION.
High temperatures of the mines. — OiiG of the pecuHarities for which the CoM-
STOCK Lode has been famous ever since deep mining began upon it, is the
high temperature of the rock and of the water encountered. In this respect
it stands alone among ore deposits, though water heated to 125° F. has
been encountered in the CHfford mine in Wales, and very hot water is found
in the superficial workings of the cinnabar deposits in the coast range of
California. On the 3,000-foot level of the Comstock floods of water have
entered the mines at 170° F. Water at this temperature will cook food, and
will destroy the human epidermis. Even a partial immersion in it is there-
fore fatal. In spite of very rapid ventilation, the air in the underground
galleries is often intensely heated and is nearly saturated with aqueous
vapor. Many deaths among the miners have occurred from prolonged
exposure to these unnatural conditions, which also add immensely to the
difficulties of geological exploration.
Normal increment of heat. — A great many luvestigations have been made during
the last years, in many parts of the world, on the increase of the temperature
from the surface of the earth downward. The observations have not resulted
in establishing a uniform rate of increase in any locality, nor is such a result
to be expected from any future observations. If the temperature is deter-
mined in a freshly drilled hole the record will necessarily be too high,
because the surrounding rock is heated by the mechanical action of the drill.
But the moment the rock is placed in communication with air from the
surface, or with water from higher levels, it begins to cool ofi". Rocks are
228
HEAT PHENOMENA, 229
always more or less fissured, and a shaft or well of any depth commonly
drains the surrounding country, so that water from a higher level is almost
invariably present at the bottom. If a shaft is kept pumped out, the equi-
librium of waters at a lower level may be disturbed, and cixrrents from
greater depths will then rise into the excavation. Even when the surface
is unbroken it is well known that there are usually subterranean cur-
rents, the course of which is determined by the structure of the rock,
and which locally interfere with the regularity of the isogeotherms. While
absolute uniformity in the increase of temperature is nowhere to be expected,
a vast number of observations show that the variations are usually confined
to comparatively thin belts, and that they vibrate about a rate of 1° F. to
from 50 to 60 feet of depth. Sir William Thomson makes an increase of
1° F. for every 51 feet of descent the basis of his calculations on the secular
cooling of the earth. The marked exceptions occur in regions where there
are other evidences of an abnormal temperature, furnished by traces of
recent volcanic action or by the presence of hot springs.
Disturbing effect of local causes in mines. If the obsCrVatioUS takcU iu VCrtical
openings of small diameter, such as artesian wells and mining shafts, are
subject to fluctuations from local causes like those above mentioned, this
must be to a much greater extent the case in an extensive and complex
system of mines, such as those which are being worked on the Comstock
Lode. The country is honeycombed to a depth of 3,000 feet. Above 150
miles of galleries have been driven, besides stopes of a very extensive
character, and in many of these artificial ventilation has been going on for
years. On account of the great heat, the ventilation is naturally rapid, and
is artificially stimulated to the greatest possible extent. The air leaves the
mines nearly saturated with aqueous vapor, at an average temperature,
according to Mr. Church, of 92° F. In this way an enormous quantity
of heat has been abstracted from the rock. Although before the opening
of the mines the country was almost absolutely dry, about 7,000,000 tons
of hot water are now yearly pumped from the Lode. Mr. Church esti-
mates that the heat annually abstracted from the Lode by drainage and
ventilation, without considering evaporation, is as great as 55,472 tons of
anthracite produce in the best manufacturing usage. The distui'bance of
230 GEOLOGY OF THE COMSTOCK LODE.
the natural distribution of the waters, and consequently also of the heat, is
further indicated by the immense pressure which the water often shows on
being tapped by the drills in the lower levels. This not infrequently
amounts to a head of several hundred feet.
Scattered observations cannot agree closely. — Taking thesc circumstauces iuto Consid-
eration, it appears to me impossible to reach any accurate result by discuss-
ing in. detail the fluctuation of the temperatures observed at different times
in different portions of the Lode. Before ground was broken considerable
variations probably existed in consequence of the presence of convection
currents. Under the present conditions it appears from the foregoing that
great fluctuations from a regular law of increase, and great anomalies which
cannot be immediately traced to their sources, must inevitably occur.
A first approximation from such data. BarOU V. Richtliofcn, althoUgh insistiug
strongly on the abundant evidences of solfatarism, mentions no abnormal
temperatures. Mr. King gives a table of observations, from which it ap-
pears that the average temperature of the mine waters, from the surface to
the 700-foot level, is between 70° and 75° F. At a depth of about 1,100
feet he found water at 108° F. Mr. King remarks : "That to the waters is
due the temperature of the whole interior of the Lode is evident from the
fact that they average a few degi'ees higher than the clays or rocky mate-
rial." He notes only one instance in which the rock and water showed the
same temperature. Mr. Church made many careful observations, which he
has very fully discussed. He estimates the mean temperature of freshly
exposed surfaces on the 2,000-foot level at 130°. The water with which
the Gold Hill mines were flooded in the winter of 1880-'81 entered on the
3,000-foot level. It was repeatedly tested by the officers of the mines, and
by myself, and was found to have a temperature of 170° F. This water
was first struck at a depth of 3,080 feet, by a drill hole from the bottom of
the Yellow Jacket shaft. Taking into consideration that 170° is not an
average, but probably a maximum for this depth, these data indicate
roughly a nearly uniform increase of temperature of about 1° for every 28
feet.^ If the attempt be made to discuss the observations in detail, great
irregularities will be found. As Mr. Church very pertinently remarks, " the
' More exactly an increase of 1° in 28.7 feet for the interval of 1,650 feet between the 350-foot and
the 2,000-foot levels, and of 1" in 27J feet for the 1,100 feet between the 2,000 and 3,100-foot levels.
HEAT PHENOMENA. 231
mining works do not follow the lines of heat manifestation, but intersect
them in every possible manner."
Better data lately obtained. — Thanks to Mr. Churcli, better data have been ob-
tained since his memoir was written. At his suggestion frequent observa-
tions have been made on the temperature of the rock and the water
encountered in sinking the Combination, the Yelloiv Jacket, and the Forman
shafts. A long series of observations has also been made in the Sutro Tun-
nel These observations and their discussion will be found in the second
section of this chapter. Though they might properly be introduced here
their voluminous character makes it more expedient to consider them sepa-
rately. The two chains of reasoning may be regarded as parallel argu-
ments on the same subject.
Explanations of the heat, — Various cxplanatlons have been offered to account
for the prevalence of high temperatures on the.CoMSTOCK. The source of
heat has been sought in friction, in the oxidation of pyrite, in the kaoliniza-
tion of feldspar, and in volcanic action.
That heat must have resulted from the faulting action there can be no
doubt, but the whole tendency of the evidence is so strongly against the
application of Mr. Mallet's hypothesis of terrestrial heat to this instance,
that a discussion seems unnecessary. The oxidation of pyrite, too, is a very
subordinate phenomenon on the Comstock. It is well known that vari-
ous occurrences of pyrite differ greatly in their behavior toward oxidizing
agents. That found on the Comstock is for the most part very stable, and
often remains exposed for years with no greater effect than tarnishing.
Most of the water from the Lode, too, shows but a small amount of sul-
phates. Indeed, there is much more reason to suppose that the formation
of pyrite is still in progress, on a small scale, than that the decomposition
of this mineral is the source of heat.
statement of the kaoiinization hypothesis. — The hypothesis that the high tcmpera-
ture is due to the kaoiinization of feldspar, appears to rest on two positive
grounds, viz., that flooded drifts have been observed to grow hotter, and that
the solidification of water liberates heat. In the argument supporting this
hypothesis, its author makes the following statement :
" The direct evidence that heat is produced when water is brought in
232 GEOLOGY OF THE COMSTOOK LODE.
contact with these rocks is of constant occurrence in the mines, and is
offered, in fact, whenever a pump breaks or is stopped for any reason, and
water rises upon a partially decomposed seam. A case of this kind in the
Caledonia is of more than ordinary interest, for the reason that this was a
cool mine, both rock and water being but little above ordinary tempera-
tures. The heat of the air in the drift was probably not above 90° F., but
after lying twenty-four hours under water a very marked change took place.
The water had reached a thick seam of the kind that is solid enough when
dry, but swells with great force when wet. The 1 2-inch timbers were all
splintered, and the temperature of the level had risen probably to 110°,
though no observation was taken. Still the fact of increased temperature
and of increase from this cause alone was undoubted. Since that time the
Julia, Savage, and Hale <& Norcross mines have all been flooded and subse-
-quently drained. The Norcross has a fine current of fresh air, and I have
not observed any complaint of its condition, but both the other mines were
reported to be extremely hot after their submersion. They were very much
above their usual temperature, and work was frequently stopped to allow
them to cool down. Such evidences cannot take the place of exact labora-
tory experiments, but they are just as incontestable proof of the fact of
heat, and high heat, from kaolinization, as if we had its precise measure."
Criticism on the evidence. — It Will be obscrvcd that it is Hot Stated when the
flooding of the drift in the Caledonia occurred, or who estimated the temper-
atures; nor yet whether the water of this particular flood was warm or cold.
With regard to the flooding of the Julia, Savage, and Hale & Norcross
mines, the only event of the kind known to me was that which occurred in
1876. This flood lasted for three years. Soon after its commencement an
official report of the superintendent gave the temperature of the water at
139°, and Mr. Church reports it later (apparently early in 1878) as 154°.'
The great heat of these mines appears to require no further explanation.
I am not able" to confirm the observation that flooded drifts grow hotter,
except when the water of the flood enters the workings at a high tempera-
' This change of temperature is not remarkable and has not been advanced in favor of the chem-"
ical theory of the heat, for many millions of gallons were pumped from the flooded mines. Streams per-
colating from a large body of heated -water through new channels in comparatively cool rock will at
first be cooled ; but they will grow warmer as the rock ia gradually raised to the temperature of the
water at the source.
HEAT PHENOMENA. 233
ture. There are many miles of drifts on the Comstock flooded to a greater
or less extent; but a great number of observations made by my party show
that the water is hottest when it issues from the rock, and cools off by
standing in the workings. When the water at its entrance is tepid or cool,
it appears to remain so indefinitely, even though it may be stagnant
Examination of the theory of kaoiinization. — While uo fact Can be better established
than that the solidification of water liberates heat, no direct conclusions can
be drawn from it as to the relations of the complex process of kaoiinization.
The constitution of the unisilicates is still very obscure, and there is no
unanimity of opinion among mineralogical chemists, even as to the formu-
las by which they should be represented, while almost nothing is known of
the reactions which go on during decomposition. It may not be amiss,
however, to examine the question from a theoretical point of view.
Feldspar assumed as representative. — As has been showu in Chapter III., the feld-
spars of the diorite and diabase which form the walls of the Comstock are
apparently labradorite and oligoclase Whether Professor Tschermak's
theory of the feldspar group is correct or not, a mixture of these feldspars
may for the present purpose be regarded as a compound of one molecule of
anorthite and one of albite. The mixed or intermediate (andesine) feldspar
may then be written
Na^Al ) ^^^ )
lAn + lAb = (CaAl)Si'0«-|-^., Si^Oi^^Na^AlSSi^O^*.
^^" ^ SiM
First step of decomposition. — The examination of thin sections leads me tb be-
lieve that the first change in the feldspars of the Washoe rocks is the
formation of calcdte, accompanied by a separation of silica. The formation
of sodium silicate probably takes place at the same time, but is not traceable
by optical means, for it will dissolve, and either pass out of the rock or
become diffused through it. If from the above formula
CaO + SiO^ and Na'^SiO'
are subtracted, it becomes
Al
Al > Si*0^^
Si'
234 GEOLOGY OF THE OOMSTOCK LODE.
The feldspars of the massive and metamorphic rocks are ordinarily
fresh,^ and they appear to decompose only under peculiar conditions, the
details of which are not fully understood, but the circumstances point to
the intervention of external energy. Such behavior is characteristic of
compounds the formation of which is accompanied by a liberation of heat.
The silicates containing a single base appear to liberate but a very small
amount of heat, for the thermal effect even of the formation of sodium
sihcate is very small indeed, and that of calcium and aluminium siKcates
is, by inference, smaller still. The separation of the feldspars into sihcates
of the earths will probably, therefore, be accompanied by the absorption
of heat, and so will the solution of sodium silicate. I know of no experi-
ments to show precisely what is the thermal effect of the conversion of
calcium silicate into calcium carbonate, but the behavior of the carbonate
and silicate of sodium, and of calcic carbonate, leaves little doubt that it
must be the evolution of a small amount of heat, less than that evolved
by the formation of calcium carbonate.
Formation of kaolin. — If kaoliu rcsults from the decomposition of these feld-
spars, there must be a still further separation of silica, and an introduction
of hydrogen. The structural formula adopted suggests interesting possi-
bilities. It is, namely, by no means impossible that the siHcon represented
in the last formula as basic should be replaced by hydrogen by the reaction
2Si + 4H20 = 2Si02-|-8H.
Were this the case, the result would be silicic anhydride and
=41>Si*0i«,
w)
or twice
A120^2SiO^-^2H^O
(the ordinary formula for kaohn), if the water is regarded as combined.
The heat hberated by the reaction
2Si-f4H^Oz=2SiO^-f 8H
1 It has been already remarked that the decompositiou of .au iusignificant percentage of afeldspar
crystal rohs it of its transparency. Many dull, chalky-looking feldspars, when seen nnder the micro-
scope, prove to he very slightly altered.
HEAT PHENOMENA, 235
can be calculated; for the combination
2H + 0 = H^O (solid) liberates 70,400 units of heat,
Si + 20 = SiO' liberates 211,100 units of heat,
and therefore
. 2Si + 4H^0 = 2SiO- + 8H liberates 140,600 units.
The molecular weight of the andesine feldspar under discussion is 757.6,
and the heat liberated per unit of weight would be
140,600 1 o^ V
,^^ = 186umts.
The specific heat of feldspars varies from 0.183 to 0.196. If the ande-
sine under discussion is supposed to have a specific heat of 0.1 86, the tem-
perature resulting from the substitution of hydrogen for silicon would be
1000° C.
The aH^o is water of hydration. — If, then, the watcr ill kaoliu were chemically
combined, a temperature would be produced much above that known to be
sufficient to expel the water from clay, and the only inference I can draw is
that the water is not, as has sometimes been maintained, chemically combined,
but is merely water of hydration. The latter view (which is also generally
held) is further supported by the varying amounts of water which various
analysts have found in kaolin. As is well known, Tschermak even denies
that kaolin is a product of the decomposition of plagioclase, affirming that
the resulting hydrated aluminium silicate contains but a single molecule of
water.
Nothing known of the heat of hydration of kaolin. Thc pUrpOSC of the forCgoing argu-
ment is to show that if any considerable quantity of heat is evolved during
kaolinization, it must in all probability be due to the simple hydration of
aluminium silicate. But of the heat liberated by the hydration of salts little
is known, except (1) that the quantity is usually small, (2) that it is some-
times negative, and (3) that the different molecules of water combine with
differing amounts of energy, indicating that the nature of their union difiers.
Of the heat of hydration of kaolin we know nothing specific, nor am I
236 GEOLOGY OF THE COMSTOCK LODE.
aware of any analogy wHich indicates a likelihood that it is sufficient to
account for the heat phenomena of the Comstock Lode.
Experiments on kaoiinization. — lu the hope of reaching more satisfactory results
regarding kaolinization than observation or theoretical considerations yielded,
I requested Dr. Barns to undertake experiments with a view to testing the
asserted rise of temperature when the Washoe rocks are brought in contact
with water.
Material selected. — The rock Selected was from a mass cut by the Sutro Tun-
nel in the Savage claim, just before the tunnel strikes the vein. It is a dia-
base, and the freshest encountered under ground opposite that portion of
the Lode which has been considerably productive. It is described under
slide 1 8, and its analysis is given at the end of Chapter III. Lest it should be
objected that this rock had escaped decomposition through an exceptional
striicture or a local variation in chemical composition, it may be remarked
that no trace of such a difference is perceptible either macroscopically or
microscopically, while its exemption from decomposition is fully accounted
for by the character of its occurrence. This mass, like most of the fresher
rocks in the District, is protected by clay seams which have prevented the
access of aqueous currents. The hanging wall of the Comstock is to so large
an extent obliterated by decomposition that many observant miners deny its
existence, but at this particular spot no vein-wall could be better defined.
It is marked by a compact smooth clay a foot or more in thickness, imme-
diately over which lies the mass of rock referred to. This is further pro-
tected, though not so clearly, by other clay-seams to the east, and is much
less shattered than the rock elsewhere.
Method adopted. — The rock was reduced to a gravel and placed within a
well-packed steam jacket. Steam was supplied from a boiler beneath, in
which the water was kept at a constant level and constantly boihng. The
difference of temperature between the rock and the inclosing steam was
measured by a thermopile. The electro-motive force was so compensated
that a variation of 0.001° C. was clearly indicated, and the experiments
extended over five weeks with only four interruptions. The whole plan of
the investigation was worked out by Dr. Barus, who wiU describe it in
detail in a separate chapter. The appliances at his command were few and
HEAT PHENOMENA. 2^7
simple, but they were employed with such ingenuity as to enable him to
obtain very accurate results. The execution of the experiments was most
conscientious and laborious.
No positive results obtained. — The tcmpcrature of the rock-mass never rose
above that of the surrounding steam. The rock seemed wholly unaffected
by the process, except that the fragments were more or less coated with
a fine dust, probably due to the salts contained in the water, which was
obtained from the Virginia Water Company's pipes.
Little kaoiinization at Washoe. — Some time after tho oxecution of these experi-
ments a special examination of the slides and a comparison of chemical
analyses led me to the conclusion that there has been only a trifling amount
of kaolinization in the Washoe rocks. This fact makes the experiments
none the less important, for the heat of the Lode might be due to other
chemical changes than kaolinization.
Conclusions regarding the hypothesis. lu short, the obserVatloUS aS tO thc risC of
temperature of flooded drifts lack confirmation; experiment fails to show
that hot water or steam have any action on the east country rock of the Lode;
there appear no theoretical grounds for the assertion that kaolinization
would produce a considerable amount of heat, and no evidence that any
considerable amount of kaolinization has gone on in the District. It is still
possible that when kaolinization occurs heat is liberated. It is also possible
that at temperatures above 212° and at pressures above one atmosphere,
feldspars are kaolinized near the Comstock fissure, but it no longer appears
reasonable to ascribe the heating of drifts, which are nearly at the normal
pressure, to the action of water below the boiling point upon the rock.
The scene of kaolinization, if it exists at all, must therefore be at great
depths, such as are indicated in the discussion of the increase of tempera-
ture from the surface downward. It cannot be demonstrated that the heat
of the Comstock is not due to the prevalence at unknown depths and press-
ures of a chemical change of unknown thermal relations, neither is there
any evidence that it does arise from such a cause; and the suggestion that
the heat of the Steamboat Springs and the ordinary variations of earth tem-
peratures are induced by kaolinization, is therefore foreign to the subject of
this memoir.
238 GEOLOGY OF THE OOMSTOCK LODE.
soifataric action. — The Only remaining supposition is that which connects
the heat of the Comstock with the chain of volcanic phenomena. What
is known as soifataric action is ill understood, and must remain so until
many of the mysteries of vulcanism have been made plain; but of certain
facts there is no doubt. In the neighborhood of active volcanoes, and often
also in regions where eruptions have ceased, gases and water charged with
more or less active reagents reach the surface through crevices. In its
earliest stages a soifataric spring frequently emits gas or water charged
with fluorine and chlorine compounds, which are replaced at a later stage
by hydrosulphuric and carbonic acids. The action of these reagents on the
rocks is manifold, but usually gives rise to characteristic appearances, such
as bleaching, accompanied by an extraction of a smaller or greater por-
tion of the bases. The appearances due to solfatarism are, of course,
accurately known, from immediate observation in the neighborhood of active
volcanoes. On the other hand, it is very seldom that effects likely to be
confounded with those of solfatarism are found at any great distance from
localities marked by the occurrence, present or past, of volcanic eruptions.
No two phenomena in geology are more intimately connected than vol-
canoes and solfataras. The connection between ore deposits and eruptive
rocks is also in a large proportion of cases a very close one, and where
ore deposits and evidences of soifataric action are found together in a vol-
canic region, it is certainly natural to conclude that an abnormal temperature
of the rock and water is also due to vulcanism. The burden of proof
rests on him who offers any other explanation.
Decomposed area at Washoe. — Extremo alteration is for the most part limited to
the area lying between the Comstock and the Occidental Lodes, though it
also extends up some of the ravines to the west of the great vein.^ Even
within this area there are great variations in the degree of decomposition.
While a portion of the rock on the surface is tolerably well preserved, there
are belts nearly parallel to the Lode, in which it is so altered that it might
be mistaken for more or less discolored chalk. These belts can be followed
under ground, and retain in dip as in strike an approximate parallelism to
the vein. Towards the edges of the surface area it is common to find
nodules of rock in place which are fairly fresh at the center, but show pro-
— • " ■
1 See Fig. Ij page 73.
HEAT PHENOMENA. 239
gressive decomposition towards the outside. Large masses of fresh rock
also occur in a similar way, as has been described in the discussion of pro-
pylite. It is clear from these occurrences that had the decomposing action
been prolonged sufficiently, no undecomposed rock would .have remained.
Under ground the decomposition is more universal, if one may judge from
the Sutro Tunnel. From Shaft II. to the Lode no fresh rock is exposed by
the tunnel, except the small mass of diabase close to the hanging wall which
has been referred to. This marked difference between the superficial and
subterranean rocks should be considered in connection with one of the
deductions made in discussing the structural results of faulting — viz., that
the country has undergone but little erosion since the deposition of the ore.
Indeed, it may be regarded as independent evidence tending to the same
conclusion.
Rocks involved. — The three rocks which occur in the belt of highly decom-
posed east country are diabase, hornblende-andesite, and augite-andesite.
The andesites are found extensively in other portions of the District,
where, however, they are decomposed to but a trifling extent. There is
no reason known to me to suppose that the decomposed andesites are of
different eruptions from the fresh occurrences ; on the contrary, the decom-
position dies out gradually in continuous areas. Neither is there any
evidence that the fresh and the altered masses are of a different composition.
Evidence of an external cause. — Had thc resolutloH of the complcx rock min-
erals into simple compounds been spontaneous, the nodules of rock described
could not have foi'med, for the action must have been nearly uniform
throughout. Neither could they have been formed if the presence of moist-
ure had been sufficient to induce decomposition, for all rocks, excej)t perhaps
obsidian, are permeable by water. Solutions of carbonic acid, hydrosul-
phuric acid or the like, on the other hand, if brought in contact with com-
pact masses of material susceptible to their action, would grow weaker as
they penetrated towards the centers of blocks, and would bring about just
such results as those referred to.
Evidence that the solutions ^scended. — If surface watcrs had producod tlic decompo-
sition, the andesites at the surface throughout the District would have suf-
fered nearly uniformly, and the amount of decomposition must have decreased
240 GEOLOGY OF THE COMSTOOK LODE.
as greater depths were readied. If, subsequent to the decomposition, erosion
had taken place, the rocks at lower elevations would be found fresher than
those on the hills. The reverse is the case. But if decomposition was pro-
duced by waters rising from great depths, the area of alteration would
dejjend on the structure of the rock, on the existence of fissures through
which they could reach the surface, and from which the}^ could act upon
the material bounded by these fissures; which accords with the observations.
Moreover, the resemblance of the products of decomposition in this Dis-
trict to those occurring in solfataric regions is very strong, and their
dissimilarity to those produced by ordinary surface action equally great.
These considerations appear to me conclusive that the decomposition
was efi'ected by aqueous currents rising from lower depths, and that these
currents carried in solution reagents capable of producing the effects familiar
in solfataras.
Nature of the reagents. — There is somc positive evidence as to what these
reagents were, for the water struck in the Yellow Jacket at 3,080 feet from
the surface was so strongly charged with hydrogen sulphide as seriously to
inconvenience the miners, and evidence is given in the chapter on chemistry
that hydrosulphuric acid must have played an important part in the rock
decomposition. The Steamboat Springs, which lie on a fissure parallel to the
CoMSTOCK, and on the opposite side of the Virginia range, are also charged
with solfataric gases.
Origin of the reagents volcanic. — There is uo conccivable reaction between water
and the components of the eruptive rocks, which would have produced
hydrogen sulphide, and the other solfataric gases. Their origin must, there-
fore, be sought outside of and below these eruptive rocks. It would cer-
tainly be permissible to argue immediately from the agency of solfataric
gases to volcanic action, but it may also be suggested that the vast quantity
of hydrosulphuric and carbonic acids which have been consumed could not
have been produced at low temperatures, and that, when formed at unknown
but certainly great depths, they could have been brought to the surface or
the mines only by convection currents, which were stimulated by heat.
These considerations force me to the belief that below the Comstock, per-
haps at a depth of three or more miles, there is a large body of highly
HEAT PHENOMENA. 241
heated rock in contact with sedimentary material. The well-known reac-
tions which take place under such circumstances in the presence of water
have produced soHataric gases as long as the supply of sulphates and of
reducing agents held out. Of these there is now a mere trace. Whether
this highly heated rock is part and parcel of the surface rocks of the
Washoe District is a question which can only be answered in terms of
probabilities ; yet as these rocks must have come from a focus of volcanic
action in about the same vertical line, the chances are certainly in favor of
the supposition that the high temperature of the Lode is a later member of
the series of phenomena, of which the ejection of the younger hornblende-
andesite, or possibly of the basalt, was an early manifestation.
The rocks all moist. — The disscmiuation of heat through the rocks of the
CoMSTocK has been regarded by one geologist as a point very difficult of
explanation. He regarded the rocks as dry, and assuming their conductivity
to be the same as that of the Calton Hill trap, which Sir William Thomson
has made famous, he found the transmission of heat insufficient to account
for the facts. The rocks are in great part dry, as miners use the word — i. e.,
many exposures do not drip water; but though paying especial attention to
the subject, I found none which were not moist. Chips and specimens, for
example, always changed color after half an hour's exposure to dry air,
except when taken from flakes which were already partially separated from
the mass and exposed to a drying current. The rocks of the District are
not glassy but crystalline, and that such rocks in the immediate neighbor-
hood of vast bodies of water at pressures equivalent to a head of, say, from
1,000 to 3,000 feet, ever since they cooled many thousand years ago,
should remain dry, would be strange indeed, and quite opposed to all that
is known of the permeability of rocks by water. But when it is taken into
consideration that far more than 99 per cent, of this rock is highly decom-
posed, it is almost inconceivable.
Source of the water unexplained. — Tho source of tho Water convcyiug the heat to
the CoMSTOCK is somewhat mysterious. The country is a sage-brush desert,
and the rainfall is not over ten inches. The slopes are steep and the evapora-
tion immense. The mines are now so deep that they might di-ain a large
extent of country, but great quantities of water were met with when the
workings were within a few hundred feet of the surface and could appar-
16 0 L
242 GEOLOGY OF THE COMSTOGK LODE.
ently drain but a very small area. Before mining began, however, little
or no water issued from the surface. When the first floods were encoun-
tered it was supposed that there must be great accumulations of water in
subterranean caves, and that water-ways leading to them had been cut
by the workings. But no such openings were ever reached in the mines,
and it came to be supposed that the water had accumulated in the inter-
stices of shattered rock masses. Broken as the rock is, however, it is very
closely packed, so that the interstitial space is but small, and considering
the vast quantities of water which have been pumped from the mines, I
cannot think the explanation adequate.^ The pressure under which the
water is frequently met is a significant feature of its occurrence. Though
there may be other workings on the same level, and though the country
above may be extensively opened up, a new source will sometimes show a
head of several hundred feet. The deeper the point at which the water is
struck the hotter it usually is, and there appears to be some tendency of the
temperature of the water from a single source to increase as it is drained.
But if it were accumulated in a mass of shattered rock of limited extent, the
water and the rock throughout the entire space would necessarily assume a
perfectly uniform temperature, and channels tapping such an accumulation
at different levels would emit streams of the same temperature. As has
been seen, the rock is commonly cooler than the water, and the general
reasoning in the foregoing paragraphs points to the rise of currents from
great depths. An attempt will be made to reconcile these facts.
Hypothesis of its origin in the Sierra. lu thc Gold Hill miuCS tllC foOt Wall of thc
Lode in the lower levels is composed of metamorphic rocks dipping to the
east, as do those also on the whole which occur at the southwestern corner
of the map. But from the Oomstock west the country, excepting one or two
small masses of granite, is completely covered by volcanic rocks, for a
distance of about 12 miles, or until the main range of the Sierra Nevada
is reached. This grand feature of the continent is far too complex to be
simply characterized as an anticlinal, but the declivities opposite the Com-
STOCK show more or less metamorphosed strata with an easterly dip, and
it is fair to infer that for some distance from its vast mass, i. e., in the coun-
try between it and Virginia, the strata underlying the fields of andesites dip
' Seveu million tons of water, the estimated annual discharge, is about 600 feet cube.
HEAT PHENOMENA.
243
in the same sense. If so, a portion of the drainage of the Sierra mu.st reach
great depths beneath the Washoe District, depths at which the tempera-
ture must be very high. It seems probable enough that meeting the fissure
of the CoMSTOCK and the partings subsidiary to it, the water thus conveyed
to the region of heat rises to the mines. The hypothetical structure sug-
gested is illustrated in Fig. 12.
Fig. ly. — Ideal section across the Virginia Range.
What it would account for. — lu a country SO aisturbed by volcanic action and
so highly metamorphic as that underlying the Washoe District probably
is, the circulation must be much obstructed. Comparatively open water
channels leading from the Sierra are likely to connect only with fissures
almost capillary near the Lode, and vice versa. This would account for the
fact that some springs in the mines yield a steady supply of water, while in
other cases a great body is eventually pumped out and leaves only an insig-
nificant flow. It would also account for the increase of the heat of the
water with the depth, and its decrease at considerable distances from the
Lode and its accompanying fissures; for if narrow water channels extend
from a distant source of heat towards a constantly radiating surface, equality
of temperature can never result. The rising currents must constantly lose
heat. Descending currents will also be established, which will, however,
cause only local irregularities in the increment of temperature. Where
great quantities of water are drained from a single source, the tendency
would plainly be to a rise of temperature, and the head which the floods so
often show would find an ample explanation in the supposed connection
with channels from the great range. No reasoning on such points, however,
can be conclusive, for the opportunities of establishing the truth of the
hypothesis are very meager.
244 GEOLOGY OF THE COMSTOCK LODE.
Section II.
THEEMAL SUEVEY.
Temperature observations. — Valuable temperature observations have been taken
on four lines near the Comstock, viz., in the Combination, new Yellow Jacket,
and the Forman shafts, and in the Sutro Tunnel. These observations were
all made at freshly exposed points as the excavations progressed, at a dis-
tance from all other workings, and while not unaffected by some of the dis-
turbing causes mentioned on page 229, form a far more trustworthy guide as
to the theoretical conditions of the Lode than a similar number of determi-
nations made in the mines. Each set, too, was observed as a matter of
routine duty, so that successive observations must have been affected by
nearly constant errors; and though many of them were made with less pre-
caution than a physicist would have employed, their great number goes far
toward compensating for any roughness in the method. Whoever is familiar
with the tone of speculative excitement which prevails in the mining i-egions
of the far West, a tone but little in harmony with scientific research, will
agree with me that great credit is due to the officers of the mines for
making and preserving these records. It would be well for the advance-
ment of pure and applied science if such a spirit were general among those
whose occupations bring them in contact with natural phenomena.
Computation of the observations. — The followiug tables and diagrams need but
little explanation. On plotting the temperatures taken in the shafts, no
indication of curvature could be perceived, and a straight line was therefore
assumed as expressing the relation of temperature to depth.
The equation of this line is
t^a -\-hd;
where f is the temperature in degrees Fahrenheit corresponding to the depth
d in feet, and a and b are constants to be calculated. The computations by
THERMAL SUEYET, 245
the method of least squares were performed by Dr. Barus and Mr. Reade.'
For the sake of comparison they also computed the observations made at
the Rose Bridge Colliery, and I add the Sperenberg observations with Mr
Heinrich's equation. The Sutro Tunnel data cannot be treated in the same
way, for they show an unmistakably curvilinear locus. A curve was drawn
empyrically through the plotted points, no weight being given to any pre-
conceived idea of the character of the law of increment. Subtangents were
constructed and found to be almost exactly equal; or, in other words, it
was found that the graphical approximation nearly coincided with the locus
of an exponential equation
in which I denotes the horizontal distance from the Lode.
The method of least squares is, of course, applicable to the computation
of an equation of this character, but the calculation is so serious an under-
taking as to be worth while only when a magnificent series of observations
is to be reduced. In the present case no exterpolation is desired, and a de-
termination of the character of the curve with an approximate knowledge
of the value of the constants is sufficient for the purposes of the discussion.
'The method of least squares famishes the formulas
n2d^—2d.2d '
^_n.2dt^2d.2t
n.2d^—2d.2d'
in which due preference is given to the temperatures corresponding to a greater depth. The observa-
tions become relatively more accurate as temperature and depth increase, and seem also to have been
made with greater care.
246
GEOLOGY OF THE GOMSTOCK LODE.
Table I.— COMBrNATION SHAFT. ROCK TEMPERATURES.
[Observations made by the superintendent.]
Columns 3 and 4.— Observatiooa of depth and temperature, respectively, as taken.
5 and G. — Means of consecutive sets, of five observations each, of depth nnd temperature, respectively.
7. — Temperature as calculated from the constants derived.
8. — "Error" or observed temperature minus the calculated result.
[Depths are given in feet; temperatures, in degrees Fahrenheit.]
a = 66.0. b = 0.0252 ± 0.0007.
No.
Date.
Depth
observed.
Temperature
observed.
Mean depth
observed.
Mean
temperature
observed. '
Mean
temperature
calculated.
Error.
1877.
Feet
°F.
1
July 17
1,476
106
a
18
1,479
104
3
19
1,482
103
4
20
1,485
105
5
21
1,489
100
1,482
103.6
103.4
+ 0.2
0
22
1,492
105
7
23
1,495
103
8
24
1,498
103
9
25
1,501
102
10
26
1,504
104
1,498
103.4
103.8
- 0.4
11
27
1,507
106
12
28
1,510
105
13
29
1,513
104
14
30
1,516
106
15
31
1,518
104
1,513
105.0
104.1
4- 0.9
16
Aug. 1
1,520
105
17
2
1,523
103
18
3
1,524
104
19
4
1,526
102
20
5
1,528
106
1,S24
104.0
104.4
- 0.4
21
6
1,530
103
22
7
1,532
106
23
8
1,535
104
24
9
1,539
105
25
10
1,541
106
1,535
104.8
104.7
+ 0.1
26
11
1,544
104
27
12
1,547
106
28
13
1,550
105
29
14
1,553
106
30
15
1,556
103
1,550
104.8
106.1
- 0.3
31
16
1,559
102
32
17
1,562
101
33
18
1,565
im
34
19
1,568
105
35
20
1,571
106
1,565
104.2
105.5
- 1.3
36
21
1,574
106
37
22
1,577
104
38
23
1,579
105
1
39
24
1,583
106
1
40
25
1,585
106
1,579
105.4
105.8
- 0.4
41
26
1,588
104
42
27
1,591
106
43
28
1,593
108
44
29
1,596
108
45
30
1,599
107
1,593
106.6
106.2
+ 0.4
' Probable error of one of these mean observations = ±0°.5.
HEAT PHENOMENA.
247
Table 1.— COMBINATION SHAFT. ROCK TEMPERATURES— Continued.
No.
Date.
Depth
observed.
Temperature
observed.
Mean depth
observed.
Me.in
temperature
observed.'
Mean
temperature
calculated.
1
Error.
1877.
Feet.
"F.
Fee^
° F.
°F.
°F.
46
Aug. 31
1,603
106
47
Sept. i
1,605
108
48
2
1,607
107
49
3
1,609
106
50
4
1,611
108
1,607
107.0
106.5
+ 0.5
51
5
1,613
106
52
6
1,615
110
53
7
1,617
109
54
10
1,620
107
55
11
1,622
108
1,617
108.0
106.8
+ 1.2
, 56
12
1,624
109
57
13
1,626
109
68
14
1,629
107
59
15
1,632
108
60
16
1,635
108
1,629
108.2
107.1
+ 1-1
61
17
1,637
106
62
18
1,640
109
63
19
1,642
105
64
20
1,645
107
65
21
1,647
106
1,642
106.8
107.4
- 0.8
66
22
1,649
108
67
23
1,651
105
68
24
1,654
110
69
25
1,656
109
70
26
1,658
107
1,654
107.8
107.7
+ 0.1
71
27
1,661
108
72
28
1,663
110
73
29
1,665
107
74
30
1,668
109
75
Oct. 1
1,671
110
1,665
108.8
108.0
+ 0.8
76
2
1,672
109
77
3
1,675
109
78
4
1,678
110
79
5
1,681
108
80
6
1,684
107
1,678
108.6
108.3
+ 0.3
81
7
1,687
110
82
8
1,690
106
83
9
1,693
109
84
10
1,696
108
85
11
1,698
107
1,693
108.0
108.7
- 0.7
86
12
1,700
109
87
13
1,703
110
88
14
1,706
110
89
15
1,709
108
90
16
1,711
110
1.706
109.4
109.0
+ 0.4
91
17
1,714
108
92
19
1,717
109
93
20
1,720
107
94
21
1,723
110
95
22
1,726
108
1,720
108.4
109.4
- 1.0
^Probable error of one of these "mean" observations = ± iP.b.
248
GEOLOGY OF THE COMSTOCK LODE.
Table I.— COMBINATION SHAFT. ROCK TEMPERATURES— Continued.
No.
Date.
Depth
observed.
Temperature
observed.
Mean depth
observed.
Mean
temperature
observed.'
Mean
temperature
calculated.
Error.
1877.
Feel.
oj..
Fett.
°F.
°F.
or.
96
Oct. 23
1, 728
109
97
24
1,730
110
98
25
1,733
110
99
26
1,736
110
100
27
1,738
111
1,733
110.0
109.7
+ 0.3
101
28
1,740
110
102
Nov. 22
1,744
110
103
23
1,746
108
104
24
1,748
109
105
25
1.750
109
1,746
109.2
110.0
- 0.8
106
26
1,752
110
107
27
1,754
107
108
28
1,756
108
109
30
1,758
110
110
Dec. 1
1,760
110
1,756
109.0
110.3
- 1.3
111
2
1,762
109
112
3
1,764
110
U3
4
1,766
108
114
6
1,768
110
115
6
1,770
Ul
1.768
109.6
110.6
- 0.9
116
7
1,773
112
117
8
1,T76
110
118
9
1,779
112
119
10
1,782
113
120
11
1,785
112
1,779
111.8
110.8
+ 1.0
121
12
1,788
111
122
13
1,790
U3
123
14
1,793
112
124
15
1,796
110
125
16
1,798
m
1,793
111.4
111.2
+ 0.2
126
26
1,800
U3
127
27
1,803
no
128
28
1,806
112
129
29
1,808
113
130
30
1,810
112
1,805
112.0
111.6
+ 0.5
131
31
187&
1,812
UO
132
Feb. 1
1,900
113
133
7
1,924
114
134
14
1,950
114
135
Mar. 1
1,986
U6
1,914
U3.4
114.2
-0.8
136
15
2,000
118
137
Apr. 5
2,070
118
138
27
2,135
127
139
May 27
2,207
128
140
Jane 10
2,230
112
2,128
120.6
119.6
+ 1.0
' Probable error of one of these "mean " observations = ± O^.S.
(249)
250
GEOLOGY OF THE COMSTOCK LODE.
Table II.— YELLOW JACKET SHAFT.
lObBervations taken by the official in charge, in drill-holes 3 feet deep; records Undly fomislied by Capt. Thomas Taylor. 1
a = 53.1. i)=0. 0334 ±0.0009.
No.
Date.
Depth of
drill-hole.
Character of
the rock, i
Depth.
1877.
Aug. 28
Aug. 30
Sept. 11
Sept. 14
Oct. 27
Oct. 30
Not. 4
Nov. 10
Nov. 14
Nov. 28
11 I Dec. 15
12 I Dec. 29
1878.
Jan. 20
Fob. 15
Mar. 22
Apr. 1
May 27
June 22
Aug. 10
Aug. 30
Dec. 7
Inches.
22
20
13
15
24
24
21
24
36
20
22
15
30
20
18
36
24
20
18
15
Wet
"Wet.
Wet
Dry..
Dry..
Dry..
Dry .
Wet.
Dry..
Wet.
Wet.
Wet.
Wet.
Dry-.
Dry..
Wet
Dry.
Wet.
Wet.
Wet.
Wet.
Fett.
845
849
874
932
945
960
966
1,000
1,054
1,095
1,167
1,212
1,316
1,333
1,451
1,600
1,660
1,720
2, 017
Temperature Temperature
i observed. I I calculated.
° F.
80.0
80.0
79.0
82.0
83.0
85.0
85.0
84.0
88.0
81.0
89.0
92.0
94.0
98.0
95.0
100.0
104.0
106.0
108.0
110.0
118.0
8L4
81.5
82.3
82.6
84.0
84.3
84.7
85.2
85.4
86.5
88.3
89.7
92.1
93.6
97.1
97.6
101.7
106.6
108.6
110.6
120.3
Error.
o J?_
-1.4
-1.5
-.3.3
-0.6
-1.0
+0.7
+0.3
-1.2
+2.6
-2.5
+0.7
+2.3
+1.9
+4.4
-2.1
+2.4
+2.3
-0.«
-0.6
-0.6
-2.5
• Probable error of an observation = ± 1°. 4.
:;■ I
■< ■
I I
? xl
•::• H
5 It'
S" ^
i ?
o
W
H
ai
W
!>
►^
a
o
>-
s^
CO
W
!>►
■^
K)
13
W
;>
1-3
d
03
(251)
252
GEOLOGY OF THE COMSTOGK LODE.
Table III.— FORMAN SHAFT. ROCK
TEMPERATURES.
(From 100 to 1,800 feet.]
a = 49.8. 5 = 0.0326 ± 0.0006.
No.
Depth.'
Temperature
observed. '
Temperature
calculated.
Error.
Feet.
"F.
"F.
OF.
1
100
50.5
53.0
- 2.5
2
200
55.0
56.3
- 1.3
3
300
62.0
59.6
+ 2.4
4
400
60.0
62.8
- 2.8
5
500
68.0
66.1
+ 1.9
6
600
71.5
69.3
+ 2.2
7
700
74.8
72.6
+ 2.2
8
800
76.5
75.8
+ 0.7
9
900
78.0
79.1
- 1.1
10
1,000
81.5
82.4
- 0.9
11
1,100
84.0
85.6
— L6
12
1,200
89.3
88.9
+ 0.4
13
1,300
91.5
92.1
- 0.6
U
1,400
90.5
95 4
+ 1.1
15
1,500
101.0
98.6
+ 2.4
16
1,600
103.0
101.9
+ 1.1
17
1,700
104.5
105.2
- 0.7
18
1,800
105.5
108.4
- 2.9
Table IV.— FORMAN SHAFT. ROCK
TEMPERATURES.
[From 50O to 2,300 feet.]
o = 53.2. 5=0.0296 ± 0.0002.
* Probable error of an observation = ± 1°.3.
No.
Depth.
Temperature
observed.'
Temperature
calculated.
Error.
Feet.
°F.
°F.
°F.
5
500
68.0
68.1
— 0.1
6
600
71.5
71.0
+ 0.5
7
700
74.8
74.0
+ 0.8
8
800
76.5
76.9
- 0.4
9
90O
78.0
79.9
- 1.9
10
1,000
81.5
82.9
- 1.4
11
1,100
84.0
85.8
- 1.8
12
1,200
89.3
88.8
+ 0.5
13
1,300
91.5
9L7
— 0.2
14
1,400
96.5
94.7
+ 1.8
15
1,500
101.0
97.6
+ 3.4
16
1,600
103.0
100.6
+ 2.4
17
1,700
104.5
103.6
+ 0.9
18
1,800
105.5
106.5
— 1.0
19
1, 900
106.0
109.5
— 3.5
20
2,000
111.0
112.5
- 1.5
21
2,100
119.5
115.4
+ 4.1
22
2,200
116.0
118.4
— 2.4
23
2,300
121.0
12L2
— 0.2
•Probable error of an observation = ± 1<>.4.
Table V.— FORMAN SHAFT. WATER TEMPERATURES.
o=45.8. 6=0.0373 ±0.0010.
No.
Depth.
Temperature
observed.'
Temperatnre
calculated.
Error.
Feet.
OF.
OF.
°F.
1
40O
62.0
60.8
+ 1.2
2
500
65.0
64.S
+ 0.6
3
60O
70.0
68.2
+ 1.8
4
700
73.0
72.0
+ 1.0
5
800
75.0
75.7
- 0.7
6
900
77.6
79.4
- 1.9
7
1,000
80.6
83.2
— 2.7
8
1,100
83.0
86.9
— 3.9
9
1,200
91.0
90.6
+ 0.4
10
1,300
94.0
94.4
— 0.4
11
1,400
100.0
98.1
+ 1.9
12
1,500
104.0
10L8
+ 2.2
13
1,600
106.0
105.6
+ 0.4
^Probable error ot an observation^ ±1°.3.
These temperatures were ascertained by drilling holes not less than three feet deep into the rock and inserting a Ne-
gretti Si. Zambra slow-acting thermometer (of the pattern adopted by the Underground Temperatnre Committee of the
British Association and standardized at Kew) into the hole, closing the hole with clay, and leaving the thermometer for from
12 to 24 hours. Not less than three holes were tried at each point.
254
GEOLOGY OF THE COMSTOCK LODE.
Table VI.— ROSE BRIDGE COLLIERIES, AT INCE, NEAR WIGAN.
[Observations given on tiie authority of John Arthur PbillipB. esq. It is not stated whether the temperatures are
those of the roclt or the water. The original data for depth, in fathoms, are contained in column 2. Observations on
Metalliferous Deposits and Subterranean Temperatures, by W. J. Henwood, p. 775.]
a = 56.0. 6 = 0.0149 ±0.0004.
No.
Depth.
Bepth.
Temperature
observed.'
Temperature
calculated.
° F.
Error.
Fathoms.
Feet.
° F.
OF.
1
80.5
483
64.5
63.2
+ 1.3
2
100.0
600
66.0
64.9
+ 1-1
3
279.0
1,674
78.0
80.9
- 2.9
4
302.5
1,815
80.0
83.0
- 3.0
5
315.0
1,890
83.0
84.1
- 1.1
6
331.5
1,989
85.0
85.6
- 0.6
7
335.5
2,013
86.0
86.0
±0.0
8
339.5
2,037
87.0
86.3
+ 0.7
9
367.0
2,202
88.5
88.8
- 0.3
10
372.5
2,235
89.0
89.2
- 0.2
11
380.5
2,283
90.5
90.0
+ 0.5
12
387.5
2,325
91.5
90.6
+ 0.9
13
391.5
2,349
92.0
91.0
+ 1.0
14
400.0
2,400
93.0
91.7
+ 1.3
IS
403.0
2,418
93.5
92.0
+ 1.5
1 1'robable eiTor of an observatiqu= ±lo.O.
o
H
H
W
S
O
o
tri
w
><
>■
O
§
n-
21
CO
W
H
H
q
w
(255)
256
GEOLOGY OF THE COMSTOCK LODE.
Table VH.— SPERENBERG.
[The observations were taken -with the geothermometer, and the column of water was cat off on both sides (Zeitschrift
for B.- H.- and S.-Wesen im preas. Staate, xx., 1872, p. 225). In the third and foarth colomns the data are converted into
terms of English units for convenienco in comparing them with those obtained at "Washoe.]
Depth
in Ehenish
feet.
Rock
temperature,
Keanmur.
Depth
in English
feet.
Bock
temperature,
I'ahrenheit.
100
10.16
103
55
300
14.60
309
65
400
14.80
412
65
500
15.16
515
66
700
17.06
721
70
000
18.50
927
74
1,100
20.80
1,133
79
1,300
21.10
1,339
80
1,500
22.80
1,545
83
1,700
24.20
1,751
87
1,900
25.90
1,957
90
2,100
28.00
2,163
95
2,300
28.50
2,369
9S
2,500
29.70
2.575
99
2,700
30.50
2,781
101
3,390
30.15
3,492
113
4,042
38.25
4,163
118
17 C I,
(257)
258
GEOLOGY OF THE COMSTOCK LODE.
Table VIIL— SUTEO TUNNEL.
IThe temperatures are usually the average of four observations on different days of the month. Observations taken by
the surveyor. Bock temperatures with Gall thermometer in regular drill-hole; water temperatures with common Kendall
thermometer.]
Date.
1875.
April
May
June
July
August
September .
October
November - .
December . .
1876.
January
February . .
March
April
May
June
July
August
September . .
October
November . .
December...
Mean distance
frora east wall
of Lode.
Feet.
10, 849
10, 575
10, 241
9,883
9,512
9,171
8,866
8,556
8,291
8,043
7,739
7,505
7,175
6,794
6,513
6,262
5,988
5,C51
5,326
5,008
4,687
Mean temper-
ature of water
at face.
o ^^
79
78
79
82
83
84
82
84
85
85
65
84
85
84
84
84
85
86
86
87
87
Date.
1877.
January
February
March
April
May
June
July
August
September —
October
November . . .
I December
I 1878.
i January
February
March
April
May
June
Mean distance | Mean temper-
fiom east wall ature of water
of Lode. at face.
Feet.
4,329
3,935
3,651
3,455
3,154
2,898
2,560
2,250
2,052
1,924
1,743
1,513
1,275
1,048
818
577
342
128
o p^
88
92
92
93
94
96
95
108
Mean temper-
ature of rock
at face.
102
108
110
111
110
110
Table IX.— COMPARISON OF RESULTS.
1. t = a + bd;
t in degrees Fahrenheit.
d in feet from top of shaft.
Table.
I
n
m
IV
V
VI
VII
Mine,
Combination shaft
Yellow Jacket
Forman shaft, 100 to 1,800
Forman shaft, 500 to 2,300
Forman shaft, 100 to 1,800
Rose Bridge collieries
Sperenberg, 700 to 2,700 (Heinrich)"
53
50
53
46
56
59
6X10'
25
33
33
30
37
15
17
Kemarks.
Temper.itnre of rock.
Do.
Do.
Do.
Temperature of water.
Not known.
2..
= , g*. J Tin degrees centigrade,
r *-rPoi ^fiin meters from top of
shaft.
Table.
Mine.
a
PXIO'
Bemarks.
1
I
19
12
10
12
8
13
15
46
61
59
54
68
27
31
Temperature of rock.
Do.
Do.
Do.
Temperature of water.
Not known.
n
m
IV
V
VI
vn
Forman shaft, 50O to 2,300
Forman shaft, 100 to 1,800
Koae Bridge collieries
Sperenberg, 700 to 2, 700 ( Hcinrich)
> Heinrioh's equation as given by himself in Khenish feet and degrees Keaomur is
( = 11.8277 + 0.0077828 d.
n
B
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ailBHH
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■■■■■■■■■■8
BSfflHH
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flinHnBEasss
■HiHjiBannBWiBBB
iBmui
■■■■■■■■^■■■■■■■■■■■■■■■ili
11
■I
■I
■■■
■■■
Si
8
BE
SE
■■■■■■LiSBBBnBHIHn
■■■■■■■{■■■■■■■■■■I
SHE
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■■■■■■■■■■ML
^iininiSi
isiai
BMBgaeaHKBs
L
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H
o
H
H
(25U)
260 GEOLOGY OF THE COMSTOCK LODE.
Reasons for some fluctuations. — A part of tliG fluctuations of the observatioDS in
the Washoe District can be reasonably accounted for. In the Forman
shaft it will be observed that the water temperatures are somewhat lower
than the rock temperatures above a depth of 1,160 feet. The upper portion
of this shaft passes through decomposed, and in part disintegrated, augite-
andesite. Near its under surface, however, this rock is somewhat fresher,
and is there unusually fine-grained and rhyolitic in structure. It therefore
offers some resistance to the rise of waters from below, and almost none to
the descent of the slight atmospheric precipitation. The point at which the
water grows hotter than the rock is exactly that at which the shaft passes
from augite-andesite into the underlying hornblende-andesite. At 1,700
feet the shaft became so hot that it was necessary to shower cold water from
the surface. The subsequent water temperatures were excluded from the
calculation, and it is most likely that the rock temperatures were somewhat
affected. This offers a probable explanation for the abnormally low tem-
peratures of the rock immediately below this point. Mr. Forman informs
me that, from the 2000-foot level on, the practice of showering water into
the shaft was abandoned.
As may be seen from the section through the Yellow Jacket (Atlas-sheet
VII.), this shaft passes through diabase and mica-diorite alternately, and such
changes are likely to exaggerate the ordinary disturbing influences. That
portion of the Combination shaft in which observations were taken is wholly
in diabase, but there is evidence of disturbed conditions. The water in the
face of the Sutro Tunnel opposite the Combination shaft was about 5°
cooler than the rock in the shaft, while a reverse relation would have been
expected. The shaft observations also fluctuate somewhat violently near
this level, while for the interval from 1,900 to 2,100 feet the increment is
sensibly the same as in the other shafts. It seems probable, therefore, that
the high value of a and the low value of b resulting from the reduction of
the observations is somewhat misleading, and that local variations of struc-
ture only cause them to differ essentially from those obtained for the Yelloto
Jacket and the Forman shafts.
Conditions in the Sutro Tunnel. — It will probably at oucc occur to the reader
that the depth of the Sutro Tunnel below the surface is far from iiniform.
HEAT PHENOMEIfA. 261
The smallest depth, however, of that section of the tunnel, 10,000 feet long,
for which the temperatures are plotted is above 1,000 feet, and for the last
5,500 feet of the tunnel the average depth below the surface is about 1,500
feet, with comparatively small surface variations. When it is considered
that the annual variation of temperature commonly ceases to be perceptible
at a depth of 100 feet, it appears that the irregularities of temperature in the
Sutro Tunnel due to the character of the surface topography, above this last
5,500 feet at least, must be insensible. The variations from the exponential
locus are, no doubt, due to the character of the rock, which, as is indicated
in the section (Atlas-sheet VI.), shows alternate belts of greater and less
decomposition. Rock temperatures would have been preferred to water
temperatures had they been recorded, but such was not the case.
Conditions in the laterals. — Rock tempcraturcs have been taken from time to
time in the north and south lateral branches of the Sutro Tunnel, but in
large part these branches pass close tQ mines which have been worked for
years to a much lower level than that of the tunnel. The mean of the
observations taken in the south branch as far as the Imperial ground is
almost exactly the same as that of the observations in the north branch as
far as the Ophir, 1 13° or 114°, thus confirming the fact, already well known,
that within the limits indicated the mines are all hot, and, on the whole,
pretty nearly equally so. The north branch near the Ophir is three or four
degrees hotter than might have been anticipated from the observations in
the main adit, and the Alpha and Exchequer claims are nearly as hot. Such
variations are certainly to be expected. They may indicate local peculiar-
ities of structure, such as the presence of diagonal fissures leading to the
Lode, or very possibly the prevalence of slightly higher temperatures
throughout the regions lying to the north and south of the main tunnel.
Regularity of the Forman curve. — A comparisou of thc diagrams shows that the
observations in the Forman shaft reveal an increment not greatly more
irregular than those observed at the Rose Bridge colliery, and at Sperenberg.
In view of the local character of the abnormal temperatures near the Com-
STOCK this fact is remarkable.
Results. — The five lines of temperatures near the Lode .form a tolerably
complete thermometric survey, and justify conclusions of a definite char-
262 GEOLOGY OF THE COMSTOCK LODE.
acter. The Forman and the Yellow Jacket shafts show that the characteristic
increment is very close to 1 "^ F. for every 33 feet of vertical descent, and,
since there is no evidence of curvature, this rate may be expected to con-
tinue for a long distance below the present workings. As the source of
heat is approached the vertical increment must increase, and the true
expression for the relation of depth and temperature is probably very sim-
ilar to that found for the horizontal increment in the Sutro Tunnel. It
appears hardly possible that were the source of heat within two miles of
the surface no trace of curvature would be perceptible in the diagrams for
a depth of 2,000 feet. The probabilities seem to be that the focus is several
miles from the surface.
Equations referred to the datum level. — The cquations for thc shafts are referred to
the surface at the points where they are sunk, and equal values oftidonot,
therefore, answer to the same level. The Forman shaft is 356 feet and the
Yellow Jacket 343 feet below the datum level employed in surveys of the
mines. Referred to that level, the equations become:
Forman Shaft: f=38-f 0.033 (i
Yellow Jacket : t=A2-\- 0.033 d
where d is the depth below the datum level.
Correlation with the tunnel equation. — The difference bctwecn the values of a in
these two equations is 4°. Now, the Yellow Jacket shaft is about 2,600
feet from the croppings of the Xode, and the Forman shaft is 950 feet
farther, and the curve obtained from the Sutro Tunnel shows that such a k
difference should exist. Indeed, if in the equation
^=80 + 34 e"""'^
— 2,600 and —950 are successively substituted for x the difference in the
values of t obtained will be 4°.^ This shows very clearly that the law of
decrease of the temperature to the east of the Lode holds good for other
sections than that taken on the hne of the Sutro Tunnel; and this inference
is strengthened by the similarity of the temperatures in the north and south
> To give a to fractions of a degree would manifestly be absurd. If the fractions resulting from
computation were to be retained the difference in the value of a calculated from the exponential equa-
tion would be the same as that derived from the observations at the shafts within half a degree.
HEAT PHENOMENA. 263
laterals of the tunnel. On the other hand, these equations give for the
Sutro Tunnel level (1,865 feet below the datum) temperatures five or six
degrees higher than are found at the corresponding points in the adit. This
agrees well with the supposition already suggested, that the isothermal
surfaces rise somewhat towards the south; but the data are too uncertain,
and the rock is too heterogeneous to warrant applications of the equations
implying their absolute accuracy. It appears to me, under the conditions,
extremely remarkable that the relations of temperature to depth and hori-
zontal distance from the Lode are capable of even approximate mathematical
expression.
Practical data. — Withiu thc belt of couutry between 2,500 and 3,500 feet
from the croppings, the relation of temperature of the rock to depth is
expressed approximately by the equation
t — M)-\-0.()B'id,
d being measured from the datum level in feet; and this equation may be
expected to hold good, with local fluctuations, for a long distance below the
present workings. The equation gives for a temperature of 212° a depth
of 5,200 feet. The water will be found commonly hotter than the rock,
and its temperature also more variable. It is not unlikely to be struck at
a boiling heat any time after the 4,000-foot level is passed, and will in all
probability be struck short of 5,000 feet.
inferencesfrom the Sutro curve. — The curve obtained froffi the observatious made
in the Sutro Tunnel is clearly a conduction curve, and proves that the east
country is heated from a surface at or near the Lode. If the Lode is sup-
posed to have assumed its present temperature suddenly, the radius of cur-
vature of this locus would be a function of the time, and if the coefficient
of conductivity of the rock, its initial temperature, etc., and all the condi-
tions of radiation from the surface were known, the time which has elapsed
since the Lode grew hot might be calculated. It is not likely, however,
that the temperature of the Lode has always been constant or nearly so,
and there is no means of inferring the constants, a definite knowledge of
which would be necessary to a mathematical discussion of this problem.
264 GEOLOGY OF THE COM8TOCK LODE.
But it is clear that as time goes on the radius of curvature of the conduction
curve will increase, and that no illimitable time has elapsed since the Lode
first assumed a temperature of above 110° F. on the 1,900-foot level.
Results independent of very accurate thermometers. It is WcU knOWn that the thermom-
eter is not an instrument which gives positively uniform results, and that
thermometric experiments aiming at a high degree of accuracy imply con-
stant rerating of even the best instruments, which probably change more
or less permanently at each fluctuation of temperature. The observations
discussed in the foregoing pages were only in part taken with first-class
thermometers, and some of them are very probably affected with errors of
1° or even 2° F. from this cause. This fact, however, does not at all impair
the general validity of the results obtained. Suppose the graduation of the
thermometers employed wholly arbitrary, and that the graduation of the
instruments used at each shaft bore no relation to those used at any other,
but that the calibration of each was good and permanent changes in volume
were absent; the results obtained would still show that the increment of
temperature from the surface downward was affected by no perceptible law
other than that of direct proportionality to depth, and that in the tunnel
the rise of temperature as the Lode was approached was best expressed by
a geometric ratio. Or suppose that imperfection in calibration and the per-
manent effects of expansion induced any error for which a precedent can
be found, say, even 3° F., between the highest and lowest readings in either
of the shafts or the adit ; the differences themselves are so large (from 30°
to 60° F.) that the same general conclusions as to the great distance of the
source of heat and the method of its communication to the walls of the Lode
would follow. In short, the indications are so positive that no probable
errors in the thermometers, however gross, could account for them or ob-
scure them.
Conclusions. — The couutry rock, then, is heated from the Lode or the sys-
tem of fissures closely associated with it, and the focus of this heat is at a
vertical distance which can hardly be less than two miles from the surface,
and is more probably four — in short, at a volcanic distance. Only fluid sub-
HEAT PHENOMENA. 265
stances, gas or water, could serve as a vehicle to transport this heat to the
upper portions of the Lode ; and while gas is absent, the immense volumes
of hot water form the most serious obstacles to mining. Water, then, has
been the vehicle of the heat. The same results, therefore, as were arrived at
in the first section of the chapter from geological and chemical arguments,
are reached by discussion of the thermometric observations.
CHAPTERVITI.
THE LODE.
Condition of the Lode. — Dui'ing the period in which the field-work for this
report was done the condition of the Comstodk was not flourishing. The
last remunerative ore from the neighborhood of the great Consolidated Vir-
ginia and California bonanza was extracted while the examination was going
on, and no other body of similar importance had been discovered. In the
course of the time covered by the stoping of the great bonanza several small
bodies were discovered in the Sierra and Union ground. These, however,
were speedily worked out. The old workings were, with few exceptions,
inaccessible, and the exposures of the vein were meager and unsatisfactory.
The study was therefore necessarily rather one of the conditions of the
occurrence of the great Lode than of the vein phenomena in detail. For-
tunately, the attention of previous investigators took the opposite direction,
and the vein has been amply and ably described as far down as the large
ore bodies extend. Aided by former descriptions and some few notes
and recollections of visits made when several of the most important bonanzas
were yielding largely, I am able to give a succinct account of the occur-
rence of ore on the Comstock, and to show to what extent the facts and
theories developed in the foregoing chapters throw light on the structure
observed in the upper portion of the Lode, as well as upon the probable
character of the regions below those as yet explored.
General outline. — The surface map, Atlas-sheet IV., shows a plan of the
Comstock as it would appear if the debris and talus were removed.^ The
main body of the Lode is a belt of quartz and vein matter 10,000 feet long
and several hundred feet broad, showing slight undulations in its course,
'The scale of the surface map is too small to show some of the minor irregularities iu the walls
which mjiy he seen in some of Mr. King's horizontal sections, or the outlines of the horses.
26G
THE LODE. 267
but with a general strike of about north 16° east. At each extremity of
this main fissure the Lode ramifies into diverging branches, of which there
are two at the south end, and a greater number, probably more than are
shown, at the northern extremity. These branches dwindle as the distance
from the main body increases, and finall}^ disappear, though it is not im-
possible that they might be traced somewhat farther than the map shows
them. The whole system produces upon the eye the impression of a crack
in slightly elastic material, due to a force acting near the middle and equal-
ized at the extremities by dissemination over a large ai'ea. This impression
is probably correct. The fissure has a comparatively constant dip of from
33° to 45°, though there are local irregularities of a trifling character.
Prismatic horse. — A vcry interesting and important feature of the Comstock,
observable in cross-section, is the forking of the vein at some distance be-
low the croppings. The foot wall continues in typical cases unbroken to
the surface, but a secondary fissure rises through the hanginj wall in a
more or less nearly vertical direction, leaving the foot wall at a depth of
several hvindred feet. A mass of country rock, which might be represented
diagrammatically as a triangular prism, is thus included within the external
walls of the vein. It is needless to say that very considerable modifications
in the direction, position, and geometrical form of the secondary fissure are
observable in different portions of the Lode.
Vein below the horse. — Exccptiug iu the region above the junction of the
east and west fissures, the vein in dip is of very uniform thickness; and
does not show as often or as prominently as many lodes the tendency to
open into chambers and pinch out again, which commonly accompanies a
faulting of one wall relatively to the other. This fact is by no means due
to the absence of a fault, but to its especial character. There is an un-
mistakable similarity between the configuration of the west wall and that of
the eastern face of the range.
The walls. — The hanging wall of the Comstock is diabase throughout the
entire 10,000 feet of the main Lode, for some distance on the southeast
branch, and along its northeast branch, as far as the explorations have been
carried. The east wall is almost all in an extreme state of decomposition so
far as the bisilicates are concerned, and the feldspars also are frequenti}'
268 GEOLOGY OF THE COMSTOCK LODE.
replaced by alteration products. The foot or west wall of the main fissure
is granular diorite for more than three-quarters of its length, but at the
southern end it is chiefly composed of metamorphic slates. The foot wall is
much less altered than the hanging. The northern branches, excepting the
most easterly one, are inclosed in porphyritic diorites, though stringers of
diabase also make their appearance in one or two spots on the fissure which
extends toward the Utah shaft. The southern branches pass along a variety
of contacts.
Black dike. — Accompanying the vein for about half its length is the nar-
row dike of younger diabase called "black dike." It is found only a little
north of the middle of the main Lode, extending thence southward and
following the southwest branch. It usually lies directly upon the foot wall,
but occasionally passes a short distance behind it. In the higher levels it
was so decomposed as to be unrecognizable as diabase.
Contents of the vein. — The coutcnts of tho veiu are simple, on the whole, con-
sisting of country rock in fragments varying in size from that of a grain of
sand to horses thousands of feet in length, clay, quartz, and argentiferous
minerals. The quantity of calcite, except in the Justice, is wholly insignif-
icant, and gypsum, zeolites, etc., are rare. Some of the quartz is said to
contain no silver or gold: but for the most part it carnes both, though in
varying quantities. That which lies upon or is inclosed in diorite carries
gold, but little silver; very little of this, however, will pay the expense of
extraction and treatment. The quartz associated with the hanging wall
carries more silver, accompanied by gold of a value nearly equal to that of
the silver.^ The variation in the tenor of the quartz is extreme, as it usually
is in silver veins; and it is only in certain spots that the quartz assays above
the fifteen or twenty dollars necessary to warrant extraction at the present
prices of labor and supplies; while occasionally the value per ton of com-
paratively small masses runs up to several thousand dollars. Masses of ore
which will pay for extraction are called throughout the region west of the
Rocky Mountains bonanzas, a Mexican mining term which avoids the ambi-
guity of the English term ore. The bonanzas, therefore, do not represent by
any means all of the quartz which carries a perceptible amount of precious
■See table of the proportions of gold and silver in Comstock bullion, p. 9.
THE LODE. 269
metals, and are often surrounded by low-grade ores in great quantities.
Though there are exceptions to the rule, large bodies of quartz commonly
contain bonanzas. The occurrence of these bodies depends on very com-
plex conditions, and no attempt can be made to account for their position
until the sections of the Lode have been passed in review. With two very
important exceptions they have all been found in the secondary fissure, not
on that with a constant dip. Excepting the Justice body they have all
occurred in contact with the east-country diabase.
Complex structure of the comstock. — The Ordinary couccptlon of a vein is a simple
crack in the earth's crust charged with ore and gangue. The Comstock does
not realize this conception even approximately. With the possible exception
of the east-and-west veins near Silver City, the whole fissure system of the
District is referable to a single mechanical cause and the charging of the
fissures is in all probability due to simultaneous lixiviation. The branches
of the Lode to the north and south are structurally integral portions of the
Comstock, but the Lode considered as a great ore deposit is limited to the
contact of the diabase with the underlying rocks.
Cross-section through the c. & c. — Thc ffiost iutcrestiug vcrtlcal cross-section of
the Lode is that through the C. & C, Consolidated Virginia, and. Andes shafts;
and fortunately this was pretty thoroughly accessible at the time of examina-
tion. The foot wall is diorite, and the hanging wall substantially diabase,
while the surface is capped with earlier hornblende-andesite. The secondary
fissure at this point was not simple but multiform, splitting the wedge of
country rock into sheets or sharper wedges. The intervening space is filled
with quartz, none of which has been stoped on the plane of this section,
though remunerative ore has been extracted in the Andes at a short distance
from it, and a very important ore body occurred near the surface some 500
feet to the north. The quartz contains numerous fragments of country rock,
too small to be shown in the drawing; and some of the horse is so silicified
as to be regarded in mining as quartz. At 400 feet from the surface the
different fissures unite, and the main fissure is supposed to continue without
interruption to the bottom of the Consolidated Virginia shaft, where it is a mere
crack. Why the vein has not been prospected for an interval of about
1,200 feet I cannot say. The great bonanza which has yielded over one-
270 GEOLOGY OF THE COMSTOOK LODE.
third of the product of the whole Lode stands in a vertical position and ex-
tends 500 feet from the fissure. Below it large masses of diorite are embed-
ded in indeterminable vein-matter and diabase.
In the funnel-shaped mass directly under the croppings a notable feature
is the variation in the character of the quartz. This is hard and firm where
it lies upon the west wall, and so far from it as the general structure indi-
cates that the quartz sheets are parallel to the line of the main fissure.
East of the horse, on the other hand, the quartz is in great part crushed to
a condition like that of commercial salt. The horse-matter in this portion
of the section is also accompanied by heavy clays. The ore near the crop-
pings in this region was heavily charged with galena, blende, and pyrite,
differing in this respect from the great bonanza in the same vertical plane,
and from the principal ore bodies of the Lode.
The "great bonanza.' — Tlic bouauza cousistcd of a group of three bodies, one
of them far larger than the others, and one of very small dimensions. The
cross-section under discussion and the longitudinal vertical projection, Atlas-
sheet X., give a better idea of the geometrical form and the position of this
important group than any description could do.^ It was composed of
crushed quartz, including fragments of country-rock, and carried a few
h M'd, narrow, vein-like seams of very rich black ores, consisting of ste-
phanite and similar minerals, while nearly the whole mass of "sugar-quartz"
was impregnated to a moderate extent with argentite and gold, the latter
probably in a free state. The immense volume of these soft ores more than
compensated for their moderate tenor,^ and much the greater part of the
entire yield of the bonanza was derived from them. They carried a mod-
erate amount of pyrite. A great part of the space stoped out consisted of
fragments of country-rock, impregnated, however, with ore, and assaying
well. These fragments were highly decomposed, but perfectly recognizable
by their green color and traces of porphyritic structure. They were not
rounded, and I never saw traces of the concentric structure which any pro-
cess of replacement must have imparted to them. On the contrary, they
were as sharply defined as if freshly broken. Comb structure was not visible
'Mr. Church gives excellent illustrations of the form of this body on difi'erent levels, but the
black rock west of the bonanza is not black dike.
"The ore of the great bonanza averaged abi>ut .$80 per ton, but this included the rich stringers.
the: lode. 271
in the bonanza on a large scale, but where masses of country-rock were
favorably placed, the space between them often showed this peculiarity,
indicating that the fragments had acted as centers of crystallization for the
quartz. The same appearance was noticed by Mr. King in the earlier
bonanzas. Clays were by no means a prominent feature of this body,
though not absent. The endless sheets of clay following and intersecting
the ore bodies which were so striking in the upper levels throughout the
Lode seem to have disappeared below the junction of the fissures. To the
east of the bonanza, especially in the region exposed by the north branch
of the Sutro Tunnel, the rock is very heavily charged with pyrite, as well
as greatly decomposed; and the sulphuret is clearly formed within the
augite crystals of the diabase. The dioritic masses east of and below the
bonanza are shattered and somewhat decomposed, but not to the same
extent as the augitic rock The material laid down as "vein-matter" on this
and the other sections is crushed rock, so highly altered that its original
character cannot be determined with certainty. The color underlying the
conventional markings which designate vein-matter indicates what, in my
opinion, is its probable lithological origin.
Inferences from the c. & c. section. — It is SO difficult to retain detailed descriptions
in the memory, that it seems advisable, in the interest of the reader, to draw
such inferences from each section as are justifiable, without waiting till they
have all been passed in review. The occurrence of the secondary fissures
on the CoMSTOCK appeared to Baron v. Eichthofen clear evidence that the
surface had not undergone great erosion since tlie formation of the vein. Mr.
King concurred in this opinion, and it also appears to me essentially a sur-
face phenomenon ; for had the east wall near the present top of the fissure
been backed up by thousands of feet of rock, it is difficult to see how it
could possibly have yielded in the manner shown by the section. The
secondary fissures must, too, have been caused by faulting action, for in no
other way can a tendency to rupture in a vertical direction be accounted
for. That the east country throughout the mines, and prominently in this
neighborhood, shows numerous signs of faulting, has already been explained
at length, as well as that the sheeted structure is not ascribable to eruptive
bedding.
272 GEOLOGY OF THE COMSTOCK LODE.
sugar-quartz. — The microscope further shows that the sugar-quartz is com-
posed of crushed crystals/ and this can also be demonstrated macroscopi-
cally. In interstices between fragments of country rock, bunches of quartz
crystals are not uncommon, and these though fractured are sometimes held
together by the support of the surrounding material. In such cases the same
crack can sometimes be observed running through a considerable number of
crystals, proving, if necessary, that they have not yielded to an internal
stress, but to an external force. Though the whole country is greatly
broken up, so that the average size of the blocks of country rock showing
no fissures is not much above the size of a man's fist, it is nowhere reduced
to the fineness of sugar-quartz. This need cause no surprise, however, for
miners and mill men are well aware that, in spite of its hardness, quartz is
very readily crushed, far more readily than volcanic rocks, or even than
limestone. The occurrence of sugar-quartz, then, is an evidence of move-
ment, and this can have taken place only in one direction, that of the dip
of the fissure ; for even if it were conceivable that the whole country in this
neighborhood might be compressed latterly, the behavior of the quartz in
the upper levels would prove the supposition inapplicable. The quartz-
sheets which are parallel to the fissure are solid, or, at most, according to
Mr. King, show a slaty structure ; while the masses which are not parallel
to the fissure are crushed. In some of the bonanzas in other portions of
the CoMSTocK, Mr. King noticed a parallelism to the Lode even in the
crushed masses, and such a phenomenon is also said to have been observed
in the great bonanza of this section.
Period of the fault. — Siucc tlic sccoudary or east fissure was filled with quartz,
the faulting action to which the existence of this fissure is due must have
preceded the deposition of ore on the Comstock; and since the ore was
crushed by a movement in the direction of the dip of the main fissure, fault-
ing must also have succeeded the deposition of ore. The faulting action
studied in Chapter IV. must therefore have embraced the whole or nearly
the whole of the period during which the deposition of ore was taking place,
' The finest portions of tho sugar-qu.irtz mounted in balsam and examined in polarized light nnder
the microscope are unmistakably anisotropic, and while portions of crystal-faces are occasionally visible,
most of the surfaces are conclioidal fractures. I have met with no evidence that any of the solid quartz
of the Comstock has resulted from the consolidation of sugar-quartz, either by pressure or any other
agency.
THE LODE. • 273
though movements may have occurred only at long intervals It is pos-
sible that the seams of rich ore in the great bonanza represent a deposition
posterior to the final cessation of movement.
Tenor of the ore. — Tho Variation in the tenor of ore is probably ascribable to
two causes. The general poverty and the auriferous character of the quartz
associated with the diorite are probably due to the composition of that rock,
which in this locality nowhere secretes argentiferous ores. On the other
hand, the fluctuations in the composition of the ore associated with diabase
are most likely due to a combination of chemical and dynamical causes.
Whatever may have been the actual solubility of the silica and the argen-
tiferous compounds of the diabase, under the conditions which prevailed
when the solutions were formed, it is in the highest degree unlikely that it
was the same. When, by a renewed movement of the hanging wall, fresh
material was exposed to solution, either the silica or the silver would dissolve
with greater relative rapidity than after prolonged exposure to the solvent
action; and the ore deposited would vary correspondingly. It is also by no
means impossible that some of the richer ores have been redeposited, form-
ing at the expense of surrounding bodies of lower grade.
Indistinctness of the east wall. — The east wall is Very iudistiuct on this and on
most of the other sections. This is in accordance with the lateral-secretion
hypothesis. As has been seen, the fragments of country-rock certainly act
as centers of crystallization, and, had the solutions risen from great depths
along the fissure, quartz must also have crystallized from both walls equally;
but if the solutions percolated from the east into the fissure, this structure
would certainly not have resulted unless they passed the wall very gradu-
ally and gently.
Clays.— The clays of the Comstock appear to be for the most part mere
attrition mixtures of decomposed but not necessarily of kaolinized rock, as
has been explained in Chapter VI. In this section it is observable that the
horses near the croppings end downwards in clay sheets, and that the days
are most abundant where horse matter lies across the general direction of
movement.
Quartz deposited in openings. — The substitutiou hypothcsis of oro doposition
receives, as has been seen, no support either from observation or. theory.
18 o L
274 GEOLOGY OF THE COMSTOCK LODE.
It appears to me necessary, therefore, to suppose that quartz and ore have
been deposited in openings. The space occupied b)'' the bonanza can of
course never have been an vininterrupted cavern, but it would seem to have
been a space loosely filled hj fragments of country rock, which are now
rejDresented by the included horse matter. Though the country -rock is so
greatly fractured, a space of this kind is by no means impossible. If a
large opening were to be made anywhere in the diabase, fragments would
immediately fall from the sides and roof. The latter would assume the
shape of a dome, and though a complete arch of blocks would not form, a
portion of the weight of the overlying country would be distributed later-
ally, and the diminished pressure would most likely be insufficient to crush
the displaced fragments. The lenticular mass of diorite below the bonanza
does not appear to be in place. It was probably partly separated from the
west wall at the diabase eruption, and since that time it seems to me to
have moved downwards. Owing to the irregularity in the walls consequent
upon its presence, and to the difference between its resistance and that of
the diabase to lamination by faulting, it left a rent in the hanging wall,
which has afi'orded an opportunity for the deposition of quartz in the man-
ner just explained.
Cross-section through the Tunnel. — The next sectiou south of the C. (& C. is that
through the Sutro Tunnel and the Savage shaft. It fails to cross any ore
but, as may be seen from the longitudinal vertical projection, it nevertheless
passes through nearly the lowest point of a fan-like group of bonanzas,
the "Virginia group," as it is often called, extending from the Chollar to the
Gould d Curry. On this plane the secondary fissure leaves the west wall at
a lower point than in any other portion of the Lode, and all of the bonan-
zas were found in the secondary fissure. Throughout this portion of the
Lode the east and west fissui'es display the same general characteristics as
at and near the Andes. The west quartz was hard, according to Mr. King,
while the eastern quartz, as I have myself been able to observe, is crushed.
The great horse body is split by quartz-masses, which are not continuous,
however, thinning out in the strike and being replaced by others. Clay
seams are very heavy and intersect as well as follow the horses. The ore
was not "base," and much of it was extraordinarily rich. The bonanzas
THE LODE. 275
were very thin perpendicularly to the plane of the Lode as compared with
that previously described, and hence occupy a much greater space on the
vertical longitudinal projection. In detail the structure of these bodies was
excessively complicated, as may be seen from Mr. King's report. It is not
in my power to add anything to his description, to which the reader is
referred for more detailed information.
Virginia group of ore bodies. — The Virginia group of bonanzas lies in an undu-
lation of the west wall, the general shape of which may be clearly traced on
the surface map; but by inspection of the horizontal section on the Sutro
Tunnel level it will be perceived that this depression has flattened so as almost
to disappear at a vertical distance of about 1,900 feet from the datum point.
Before the walls were disturbed in their relative positions, a solid mass of
diabase lay in this local depression. The fact that the depression is limited
to the neighborhood of the surface must have brought an extraordinary strain
to bear upon the tongue of east country rock lying within it when the fault
took place. The lines of secondary fracture, instead of I'unning nearly paral-
lel to the Lode, appear also to have crossed the continuous prismatic horse so
often referred to, and to have reached the foot wall at the extremities of the
undulation. The mass thus separated would be canted eastwards by the
same force which effected the separation, and between it and the main body
of the east country there would form a crescentic opening, the points of
which would he at the croppings on the west wall, while its greatest width
would also be on the west wall at the bottom of the tongue of east country.
From the west wall vertically, or in the direction of the secondary fracture,
the opening would everywhere taper, ending in a mere line at the surface
or more probably somewhat below the surface, since the crushing stress in
an east-and-west direction would be powerful. This opening once formed
would be immediately blocked by fragments of rock, and could never close.
Such I conceive to have been the nature of the case in the region of the
Virginia group, modified in detail by more or less important irregularities
of structure ; and it will be observed from the Tunnel section that the west
quartz tapers from the surface downward, while the east quartz thickens ;
showing that the horse has revolved slightly on a horizontal north-and-south
axis, remaining firmly in contact with the east wall at the top and with the
276 GEOLOGY OF THE COMSTOOK LODE.
west wall at the bottom. By inspection of the longitudinal vertical projec-
tion and of the mine maps, it will also be perceived that the ore bodies lay-
within such a space as is suggested by consideration of the probable results
of faulting.
The occurrence of the rocks in the Sutro Tunnel has already been suf-
ficiently discussed in Chapter V. The various belts of decomposition indi-
cated have all been located as veins upon the surface ; but there is nothing
in this section to indicate any hope of ore away from the Comstock, except
upon the Occidental lode.
Cross-section through the H. & N. — Tho Httlc (& NoTcross scction passBS through
the edge of the largest bonanza of the Virginia group. Its thinness, com-
pared with the Consolidated Virginia and California bonanza, is striking, but
would be somewhat less so were the plane of the section nearer the axis of
the body. The structure of the horse is much less regular than on the Sutro
section, but it is again noticeable that the western quartz diminishes in
width as the depth increases, while the openings at the east increase. The
horse is intersected by a nearly vertical quartz body. In the Chollar these
two eastern fissures come together. The black dike makes its appearance
in this section, and is found running into the Savage, but no farther north,
nor is it known to reach the surface at any point. The andesite contact is
laid down from inferences drawn chiefly from observations made at the
Savage, 700 feet farther north, the Santa Fe adit being closed. Most of the
lower workings of the Hale dt Norcross were also inaccessible at the time
of the examination, and it is not impossible that the vein is drawn somewhat
wider than a careful examination would justify.
Cross-section through the Jacket. — In the Iviperlal grouud the diorite swings to
the west, leaving metamorphic slates with an easterly dip as the foot wall
in the Gold Hill mines.
But a small portion of the Yellow Jacket workings was accessible at the
time of the investigation; but a preliminary examination of the lower levels
had been effected before the Gold Hill mines were flooded, and an excellent
collection, kept by the company while sinking the new shaft, supplemented
by visits to the accessible tank stations, furnished all the necessary informa-
tion concerning the eastern portion of the section. The old workings had
THE LODE. 277
been carefully examined by Mr. King's party, and the information recorded
by him, with additional facts from the surface, and from a few levels below
the bottom of the old shaft, make the section fairly satisfactory.
Several masses of micaceous diorite crossing the new shaft are repre-
sented as embedded in diabase. The evidence already adduced of the rela-
tive age of these two rocks precludes the supposition that these bodies can
be intrusive, and the only tenable supposition seems to be that they are frag-
ments detached and moved into their present position by the diabase erup-
tion. That such an event is quite possible is evident, the wonder being
that it is not of more frequent occurrence on the Comstock.
Fissure dipping west. — A vcry pecuHar phenomenon is the occurrence of an
ore body in the Yellow Jacket dipping west and ending abruptly on the
west wall. The following is suggested as a possible solution. The earlier
hornblende-andesite cap is in this region of considerable thickness, and its
under and upper surfaces seem to be nearly parallel, while the diabase
contact slopes at an angle of some 33°. The direction of the faulting
movement was at least as nearly vertical as that of this contact. To this
movement the tenacity of the andesite offered a resistance, but as it con-
tained no parting in the direction of motion it yielded in the direction of
least resistance, or nearly at right angles to the surface. This action gave
rise to the fissure dipping westward. As the faulting movement continued,
a second eastern fracture formed exactly as in the Virginia mines.
Cross-section through the Belcher. — The Belclier sectioii IS madc out from fcwer
data than most of the others, in spite of the fact that the ore-bearing levels
were open to inspection. No galleries have been run into the east wall on
this plane, and there are no workings where the croppings should appear.
The quartz is continuous on the slope of the main fissure above its junc-
tion with the secondary fracture, but how far is not known. I believe,
however, that the fissure might be followed to the surface, though it is
improbable that ore in any quantity would be found. From the sketch
map. Fig. 1, it appears that the evidences of solfataric action run high up
Crown Point ravine, and back of the Belcher; and the decomposition seems
almost necessarily to indicate a structural connection between the surface
and the deep-seated fissures. The secondary fissure appears to represent
278 GEOLOGY OF THE COMSTOCK LODE.
the east fissure of the Yellow Jacket, the west fissure here coinciding with the
slope of the Lode.
The fault at the Belcher. — There is uiuch Icss evidence of faulting at this sec-
tion than on any of the preceding. The topography does not show a
logarithmic character; the lamination of the surface rocks is not percepti-
ble, nor is there much evidence of such a structure in the mine; and far
more of the ore was solid or composed of bunches of large interlocked quartz
crystals, with spaces between them, than in the Virginia mines. There is
some crushed quartz, however, and the character of the bonanza, which was
largely made up of angular fragments of country-rock, seems to indicate
faulting, though of a less violent and extensive character than that which
occurred on the flank of Mount Davidson. The bonanzas hitherto described
appear to have filled spaces due to secondary fracturing, while that in the
Belcher seems to have occupied an opening due to changes in dip, combined
with a relative movement of the walls, concave surfaces being brought into
opposition. An inspection of the section can hardly fail to produce this im-
pression and, if it be a fact, it furnishes another proof of the comparative
gentleness of the faulting action in this locality. Since the dislocating force is
manifestly dissipated at the ends of the Lode by distribution over a large
area, it is likely to grow less intense as the extremities are approached. The
diminution indicated at the south end of the main Lode is greater than
at the north end.
Small stringers of good ore have been met on the 3, 000 -foot level of
the Belcher, the deepest level yet reached.
Cross-section through the Forman shaft. — The scction through the Baltimore and
Forman shafts is more valuable as a study of the succession of the rocks
than for any positive information it furnishes regarding the Lode. The
contacts in this portion of the country are much more numerous than near
Viro-inia, and one of these, seemingly continuous with the main Comstock
fissure, has been sufficiently opened to admit the deposition of quartz. The
dynamical action must have been very slight, however, for thei'e are no
certain evidences, either in the shape of croppings or of Hues of profound
decomposition, that fissures from the surface connect with this contact in
depth. But croppings reappear just below the Justice, and the surface and
THE LODE. 279
subterranean phenomena together render it, to my mind, altogether proba-
ble that the fissure is continuous, as shown upon the surface map.
The evidence of the structure of the country on this section is, for the
most part, far less detailed than that obtained for some of the others; but
it is sufficient to justify considerable confidence in the general features
shown. The Forman shaft leaves nothing to be desired, thanks to the thor-
oughly scientific spirit in which the management has preserved accurately
labeled specimens from all levels, as well as temperature observations. A
very important point proved by the shaft is that the diabase does not extend
so far south as this line, for had it done so it must have been encountered
between the hornblende-andesite and the quartz-porphyry. The Caledonia
works were also open to inspection, and were carefully examined. The
three other shafts were closed, but the information afforded by the dumps,
in connection with the maps of the workings and the statements of employes
as to the drifts from which the different divisions of the dump-piles came,
and correlated with the data obtained on the surface and in the mines stiU
open, gave ample evidence as to the order of occurrence of the rocks.
Diabase nowhere appears on this section, but is found overlying quartz-
porphyry at the Overman, a short distance to the north, and a small partial
section is given to illustrate this occurrence.
Cross-section through the Union shaft. To the UOrth of tho main LoDE, aS tO the
south of it, the evidences of dynamical and of chemical action grow sHghter,
though much less rapidly. From the section through the Union shaft, for
example, it appears that on the main northerly branch no secondary fissure
has formed, and since the Lode is here divided at the surface into at least
three stringers, a sufficient intensity in the faulting action to produce a well-
marked secondary fissure could scarcely be anticipated. The south branch
of Seven-Mile Canon has cut deeply into the surface here presented. If the
eroded ground were restored some traces of a logarithmic surface would be
visible. The lower workings from the Union shaft are entirely accessible,
and prove that the diabase contact is not on the fissure which has been
chiefly explored to the north of this plane, but diverges from the strike of
the main Lode towards the northeast. A line of heavy croppings exists in
this general direction, and probably marks the contact. A comparison of
280 GEOLOGY OF THE COMSTOCK LODE.
this section with the surface map and with the horizontal section on the
Sutro Tunnel level shows that the contact between the diabase and the diorite
being steeper than the dip of the northern branch of the Lode, the fork of
the vein is met much farther north on the lower levels than at the surface.
The disturbing influence of the sharp bend in the diabase-diorite contact
upon the regularity of the faulting action is visible in the larger amount of
crushed rock, and the apparently displaced diorite masses on this section.
Most of the diorite east of the northerly fissure and nearly all of that on
the lower levels is porphyritic. A small ore body occurred near the crop-
pings on the northerly branch. Mr. King describes the ore there found as
"fragmentary masses of blocky quartz, impregnated with native gold,
closely resembling the California auriferous quartz." The little ore bodies
on the 2300 and 2400-foot levels are more like the ordinary Comstock
ores. The evidences of solfataric action are very strong on the lower levels
of this section; indeed, the decomposition is so profound as to make litho-
logical determinations a matter of the utmost difficulty.
Cross-section through the Sierra Nevada. The Sierra NcVUda SeCtioU shoWS Cvi-
dences of very powerful dynamical action, yet of but a small amount of
faulting; for the dip of the north fissure is here so irregular that no move-
ment whatever could occur in the ordinary direction without extensive frac-
turino". The occurrence of limestone on this section has already been
noticed. The diorite beneath it is mainly granular, and that resting upon it
is for the most part porphyritic, though no sharp line can be drawn for any
considerable distance between these varieties. It appears to me that this in-
cluded sheet of stratified rock was largely instrumental, by its weakening
effect, in determining the course of the north fissure. Beneath the lime-
stone is a small stringer of diabase, no doubt connected somewhere with
the main body to the east, but at what point is uncertain. It is accompa-
nied by a minute quantity of ore, not unlike that of the Comstock bonanzas,
but it would be difficult to gather five pounds of it, and there is no likeli-
hood of any ore body of importance being found here. The same stringer
of diabase, or a similar one, occurs further north in Utah ground, on the
north fissure. The main body of diabase seems to have been struck on the
1450 level of the Sierra Nevada by a drill hole, the cores of which were
THE LODE. 281
fortunately preserved. The drift itself was inaccessible, and could not have
been opened at any moderate cost.
The east-and-west fault. — There are clear evidences of a slight downward
movement to the north of the Sierra Nevada, or an equivalent rise of the
region to the south. It is impossible to state definitely that this was not
independent of the great fault, but after considerable study of the case it has
seemed to me unlikely, on the whole, that the two movements were uncon-
nected. Everything shows that the enaptive rocks of the District are exceed-
ingly rigid, and canaot be flexed perceptibly without breaking. At the
same time there is, as has been seen, strong proof that the faulting dimin-
ishes rapidly to the north and south, beyond the points at which the main Lode
ramifies. In part the strain was weakened by distribution over vai-ious
fissures, but this would have been insufficient to efffect adjustment in the
absence of flexibility. This argument would therefore point to the proba-
bility of east-and-west fractures as a means of relief, and it is to this action
that the little slips in the Sierra Nevada appear to me attributable.
Cross-section through the Utah. — In the Utttk the north fissure again straightens,
so as to exhibit approximately the usual dip of the Comstock, and though
the fault was slight it left a trace of a secondary fracture. Diabase appears
in several levels, but only as an irregular dike, backed by micaceous
diorite, which also shows extensively on the surface in this neighborhood.
As nearly as can be made out, this diabase comes in on a cross-fissure from
the southeast and not on the branch of the Lode. The evidences of solfa-
taric action are not great in this mine, much of the rock being even fresher
than that to be found on the surface at any point in the District. In the
lowest levels, however, there are belts of somewhat decomposed rock.
Horizontal section. — It was intended to make horizontal sections of the Com-
stock on three levels, but this proved wholly impracticable on account of
the inaccessibility of the older workings. Fortunately it was possible to
explore nearly all of the Sutro Tunnel level, 1,900 feet below the croppings.
The result is recorded in Atlas-sheets VIII. and IX., where the inaccessible
drifts are shown in hair lines; while the projection of the principal workings
on other levels, of which use was made in drawing inferences as to the
conditions existing on the 1900-foot level, is shown in dotted lines. The
282 GEOLOGY OF THE COMSTOCK LODE.
care with which the determinations were made is shown by the abundance
of the marks indicating the points from which specimens were collected,
and slides ground. This very laborious collection was necessitated chiefly
by the extreme state of decomposition of the rocks, which here almost
wholly effaces their distinguishing characteristics. It was also necessary to
prove the presence or absence of any rock which could properly be brought
under the definition of propylite.
Ore bodies occur at the diabase contact. — It appears from this scctiou that the east
wall of the Comstock, from the Overman to the Sierra Nevada, is diabase,
while the west wall is diorite for only a part of the distance. By comparison
with the vertical sections and the vertical projection of the Lode it will
be seen that all the ore bodies of any importance, except that in the Justice,
are at or close to the diabase; while the Gold Hill bonanzas rest upon met-
amorphic rocks. The forking of the vein at the Overman is well exhibited
on this level, with its cause, the divergence of the black dike from the main
diabase mass. To the north it is evident that the north fissure is on the
strike of the Lode, and that its formation was probably facilitated by the
presence of the hmestone body in the Sierra Nevada ground.
Faulting. — The evidence with regard to faulting offered by this level is
interesting. The course of the Lode is very closely the same as the line of
the croppings, with the exception of the undulation shown at the surface
opposite the Virginia group of bonanzas. The disappearance of this undu-
lation was discussed in connection with the vertical section through the
Sutro Tunnel. The effect of the compression produced by the sharp bend
of the diabase contact to the eastward at the north end, in conjunction with
the southeasterly dip, is seen in the great mass of crushed rock in the
northern mines. This crushed rock has been denominated vein matter, in
accordance with local mining usage, because it is decomposed past certain
lithological determination; it is not laid down as forming a part of the
vein, however, because it is not a loose aggregation of fragments considerably
removed from their original position, but consists of huge rock masses fis-
sured in every direction.
Close contact of the walls. — Considering the extent of the vein and the indu-
bitable evidences of an extensive fault, it is at first sight very remarkable
THE LODE. 283
that the walls are almost everywhere in such close contact, and that the
only large opening due to mere relative displacement of the walls is that
occupied by the Gold Hill bonanza. If the theory of the fault propounded
in Chapter IV. is correct, howe<^er, this state of things follows as a necessary
consequence, for the vein represents only a single parting, and the relative
motion between its walls is the relative motion of two successive sheets.
The actual amount of displacement must depend on the thickness of the
sheets, which on the Comstock is certainly not above twenty-five feet. This
would answer to a relative movement of the actual walls of something like a
hundred feet. The opening of the vein in Gold Hill is probably in part
attributable to the character of the foot wall, which, being stratified at an
angle to the Lode, would be, as all experience shows, less rigid and less
easily spHt into sheets. The dip of the west wall in Gold Hill is also con-
siderably smaller than in Virginia, about 10° less, and this fact must have
had a tendency to ease the pressure in the southern mines. •
Influence on the path of rising waters. — Oh account of thc Small relative movemeut
of the walls of the Lode these are sometimes found nearly or quite in con-
tact with one another over considerable areas; and at points where the walls
are perceptibly, but not distantly, separated the intervening space is often
closely packed with clay and rock fragments. The vein is therefore not an
open water channel throughout, and it is highly probable that on some
straight or sinuous line it may be impenetrable to liquids from one end to
the other. With the east country rock the case is different. As has been
noticed frequently in the foregoing pages, it shows an endless number of
partings parallel to the Lode and innumerable fractures across the sheets.
Few of these partings show any clay, and as capillary fissures can never be
stopped except by plastic material, there is httle obstruction to the circula-
tion of water in the country rock. This condition of things has most likely
had not a little to do with the deposition of ore. The waters, rising from a
depth which the heat relations show must be measured in miles, were pre-
vented from following the Lode fissure and were forced to permeate the coun-
try rock, reaching the open spaces of the vein laterally, and there depositing
the quartz and ore minerals dissolved.
Partial section on the asoo-foot level. — The uorthem miues wcrc accessible on the
284 GEOLOGY OF THE COMSTOCK LODE.
250U-foot level for a considerable distance, and a horizontal section of
these workings is presented. It shows, in connection with the parallel
section 600 feet above, the growing tendency of the diabase contact lo dip
towards the southeast and the great increase of crushed rock with increasing
depth. All preparations had been made to lay down the geology of the
Gold Hill mines at the corresponding level, when a flood rendered the
workings inaccessible. The map, however, at least indicates the continuity
of the vein in depth and parallelism of structure between this and the Sutro
Tunnel levels.
Vertical projection of bonanzas. — Thc lougitudinal vcrtical projection needs no
explanation, supplementing in an evident manner the other Atlas-sheets.
The disposition of the various bonanzas which it shows has been mentioned
in connection with the cross-sections of the Lode.^
Mine maps. — Tho cutirc official mine maps are also presented, and will
enable those specially interested in the Lode to follow out many details of
structure. The notes on these maps as to walls, clay seams, etc., represent
the deliberate judgment of the surveyors and superintendents, and I have
found them, where accessible, for the most part, correct. They are left as
they stood on the originals, because the greater number of the locahties
where they occur are inaccessible, and as a record of opinion of those
technically engaged in mining they have a distinct value, which would be
lost if partially replaced by my own determinations. Not all the galleries
appear on the maps, for, though the main workings have been carefully
plotted from the earliest times, unimportant drifts are often run without the
cooperation of the surveyor, and these sometimes escape record. The sur-
veyed galleries, shafts, and winzes aggregate about 165 miles, and the un-
recorded ones probably 30 miles additional.*
Claim-map. — Thc claim-map of the Washoe District forms a proper com-
plement to the mine maps. It shows the claims up to 1 881 and distinguishes
'In preparing all of the geological sections of the Lode, I was assisted by Mr. K. H. Stretch,
■who is responsible only for the mapping, the geological determinations being my own. My determina-
tions, however, were greatly faciUtated by Mr. Stretch's familiarity with the old workings, now for tha
most part inaccessible, and by his zealous assistance in gathering data as to structure and lithology.
The longitudinal vertical projection of the Lode is entirely Mr. Stretch's work.
i'The official surveyors of the Comstock have been Messrs. I. E. James, E. H. Stretch, Marietta
& Hunt, T. D. Parkinson, Browne, Hoffmann & Craven, and L. F. J. Wrinkle. The contract for the
maps was made with Messrs. Hoffmann & Craven.
THE LODE. 285
claims for which patents have been issued, those on which applications for a
patent have been made, those determined by U. S. survey, but on which no
applications for patents have been made, and finally claims the boundaries
of which have merely been determined by private survey. An index to
the claims, showing the position of each on the map, will be found at the
end of the volume.^
Conclusions, — Collcctively, the various observations made, if they are correct
and the inferences from them sound, throw considerable light on the history
of the Lode. After the eruption of the diorite the first event of importance,
so far as the vein is concerned, was the outburst of diabase, which involved
a rupture and dislocation of the earlier diorite, leaving a smooth contact
between the two rocks at an angle of about 45°. The contact was after-
wards slightly opened to admit the younger diabase or black dike. Erup-
tions of earlier hornblende-andesite and of augite-andesite afterwards
occurred, which probably caused fractures and dislocation in the eastern
portion of the diabase, but produced no traceable action on the Comstock
fissure. The country was subsequently so eroded as to reduce the surface
of these four rocks to a gently sloping plain, with an inclination of a little
more than two degrees to the west. After the commencement of the dry
period (dry, that is to • say, so far as this region was concerned) a great
movement began which may possibly have been a sinking of the hanging
wall, but was more probably a rise of the foot wall. The center of action
appears to have been near Mount Davidson. This dislocation involved an
enormous friction, one result of which was a separation of the foot wall and
the hanging wall into sheets parallel to the fissure for a long distance from
it. A secondary effect of the same force was the formation of inimmerable
cracks in these sheets nearly perpendicular to their partings. The edge of
the east country necessarily assumed the form of a wedge, and was broken
completely through at a point a few hundred feet below that at which the
primary fissure reached the sui-face. Openings were formed along the Com-
stock as a result of the movement of the walls, but under a variety of
circumstances. In Gold Hill a space was left by the non-conformity of the
wall surfaces brought into opposition. In the Virginia group a slight
' The claim-map was prepared by Messrs. HoflFmann & Craven. Some additions and corrections,
however, were made by Mr. Wrinkle.
286 GEOLOGY OF THE COMSTOCK LODE.
irregularity in the dip of the foot wall prevented the mass broken from the
edge of the east country from following the main body of diabase to its
final position ; while in the Consolidated Virginia and the neighboring mines,
at a depth of between 1,000 and 2,000 feet, a projecting mass upon the foot
wall gave rise to a local rent in the hanging wall. Besides these more
iinportant openings, numerous clefts formed in the prismatic horse which
had been broken off from the hanging wall, and between the horse and
the main body of the east country. Large quantities of rock were ground
to dust in the course of the faulting, especially at and near the great horse,
where the mechanical action was least regular.
Floods of heated waters now rose from a depth of two or more miles,
certainly carrying carbonic and sulphhydric acids, and possibly other active
reagents, in solution. The water followed the course of the main fissure as
closely as circumstances permitted, but was deflected to a great extent into
the fractured mass of the east country, where decomposition resulted, ^ihca
and metallic salts were set free from the mineral constituents of the rock,
and were carried into the comparatively open spaces near the main fissure,
where they were redeposited. The proportion of silica to ore minerals
varied greatly with time and local circumstances, which if Xhej are capable
of full explanation certainly have not received it in this report. Some of the
causes of the variations, however, can be indicated without difficulty. The
lithological character of the rock upon which the waters acted was evidently
of prime impoi'tance, determining both metallic contents and gangue; so that
the deposits of Cedar Hill, those of the Justice mine, and the bonanzas of the
main Lode, all show distinctive characters. The duration of the exposure
of particular rock masses to solvent action no doubt had much to do with
the tenor of the resulting ore. It is hkely, for example, that sihca under the
conditions then prevailing, is more readily soluble than silver compounds.
If so, the water first passing over a mass of rock would deposit low-grade
quartz in the vein, and subsequently, as the supply of soluble silica dimin-
ished, a better quahty. It seems clear that fresh movements occurred from
time to time, and that fresh rock surfaces were thus exposed. This would
have brought about alternations in richness, such as have sometimes been
noticed in the Lode. Pressure, too, if not temperature, may have varied
THE LODE. 287
from time to time, and so may the quantity of active reagents in the rising
waters. On the whole, the earlier deposits of quartz seem to have been of
lower grade than the later ones, but the phenomena are so complicated that
no considerable practical value attaches to this observation.
The ore was deposited on the walls and fragments of rock as in more
regular veins, but the currents percolating from the east and decomposing
the rock through which they passed, gave the east wall a somewhat indefi-
nite character. This indefiniteness was increased by the dynamical action
which followed the deposition of quartz, and probably also accompanied it.
After most of the quartz was precipitated, renewed movements occurred,
crushing the deposits in great part to so-called " sugar quartz." It was the
quartz bodies standing at a considerable angle to the west wall, and there-
fore crossing the fissure planes, which were most extensively comminuted.
More attrition products were of course also formed at the same time.
The solutions which so powerfully attacked the polyhedral fragments
of diabase were of course not without efi"ect on the pulverized rock masses
which were abundant, particularly in and near the secondary fracture,
or " east vein." The clays are the result. In a simple vein, attrition mix-
tures and clays are apt to occur only on the two walls. On the Comstock
such a regular formation is found on the west wall, but seldom on the east.
There is no necessary connection between walls and clays in spite of their
frequent association, some typical veins showing nothing of the kind. The
clays of the Comstock show little kaolin.
Probabilities. — Thc first couditiou for a deposit of ore is the formation of
an opening, and on the Comstock such spaces appear to have formed in
three distinct ways, already explained. The secondary fracture has been
worked out, and except in Gold Hill considerable nonconformity of the walls
is not to be looked for. There it is as likely to occur at greater depths as
above ; indeed, the fact of its occurrence in the Crown Point and Belcher, at
a mean depth of, say, 1,700 feet from the Gould d Curry croppings, leads
almost necessarily to the conclusion that there must be other nonconformi-
ties at greater depths, unless the rocks change to other species. Openings
of the type of that which contained the Consolidated Virginia and California
bonanza may occur at any point on the vein, and wholly without warning
288 GEOLOGY OF THE COMSTOCK LODE.
from above, as was the case with that body. The want of indications of
such an opening from above is due simply to the fact that from the nature
of the case the accompanying subsidiary phenomena are on lower levels than
the opening. At least one other type of opening may occur, which is as
likely to carry ore as those just mentioned. Where large bodies of rock
are broken and dislocated, interstitial spaces of considerable size may readily
form within the mass. An enormous volume of such material exists in the
lower levels of the north end mines, and nothing would be less surprising
than the discovery of one or more ore bodies in that locality. Attendant
upon the ore bodies and somewhat below them to the east, the hanging wall
will probably be more heavily charged with pyrite than the average rock
of the east country, as has been the case with former bonanzas.
Of the actual precipitation of ore and gangue from solution little is
known. It is very natural to connect it with surface influences, and hence
to suppose that ore must be limited to certain depths. Such an hypothesis
is frequently held by mining men, but experience does not confirm it ; for
though there are shallow deposits, there are many deep ones. The gold
veins of California and Australia show no tendency to give out in depth,
when affected by no other unfavorable conditions, such as a change of rock;
and the mines of Pribram, in Bohemia (the only ones, I believe, which are
deeper than those on the Comstock), were never so rich and profitable as
they have been since the 3,000-foot level was passed.
The western limit of the diabase is the only ground in which impor-
tant ore bodies ever have been or are ever likely to be found in the Com-
STonK mines, and exploration should, in my judgment, be confined to the
neighborhood of this contact. Money spent elsewhere will almost certainly
be wasted. As long as the east country continues to show an extensive body
of diabase, there is no i-eason for discouragement. Should this rock ever nar-
row to a mere dike between diorite walls, the outlook would be gloomy;
but it is highly probable that such a change occurs, if at all, only at a
point far below the limit which technical difficulties will set to exploration.
The whole contact between diabase and the underlying rocks is worthy
of careful exploration. Evidences of disturbance and decomposition are to
be regarded as indications of the possible neighborhood of ore, and regions
THE LODE. 289
exhibiting these characteristics should be thoroughly cross-cut ; while, where
the rock is comparatively firm and fresh, drifts or winzes should be pushed
on to more promising ground. The country northeast of the Ophir is par-
ticularly favorable. As may be seen from the horizontal sections, it pre-
sents a large extent of unprospected contact between diabase and diorite
directly adjoining a region of broken and highly altered rock where ore
in small quantities has already been found. Ore is not unlikely to be met
with in this unexplored area at depths of less than 2,000 feet, and therefore
under comparatively favorable conditions as to heat and water. The mines
near the Union shaft are also likely to find ore towards the bottom of the
mass of shattered rock in which the 1,900 and 2,.500-foot levels are exca-
vated. In the Best <& Belcher ground, too, there are signs of great disturb-
ance, though the decomposition is less intense than in the mines north of the
Ophir. A drift from the lowest levels of the Consolidated Virginia would
show whether the indications on this claim improve with depth.
CHAPTER IX.
ON THE THERMAL EFFECT OF THE ACTION OF AQUEOUS
VAPOR ON FELDSPATHIC ROCKS.
BY CARL BAKT7S.
Mr. Church/ in his report on the geology of the Comstock Lode, has en-
deavored to account for the abnormally rapid increase of the temperature
of this District with increasing depth^ by ascribing it to chemical action —
more immediately to the decomposition (kaolinization) of the feldspathic
rocks in consequence of the presence of moisture. This theory, however,
notwithstanding the ingenuity with which it has been discussed by its
author, is based on an assumption that has scarcely a single experimental
datum to support it; nor is the fundamental hypothesis upon which Mr.
Church bases his argument, namely, that the process of kaolinization is one
from which we may, a priori, expect the production of heat (as Mr. Becker
has already pointed out) by any means of a kind to be readily admitted.
General plan. — It appeared Very desirable, therefore, insomuch as from
theoretical grounds alone there is abundant room for difference of opinion,
to put the matter to a direct physical test. At the outstart, and with the
time and means available in camp, qualitative experimentation only could
judiciously be attempted, the necessarily complicated quantitative study
being reserved for more favorable opportunities ; if, indeed, the preliminary
investigation should furnish results of sufficient interest to warrant further
research.
The thermal effect of kaolinization (abbreviated T. E. K.) may be de-
fined as the quantity of heat produced by the action of aqueous vapor on
290
' The Comstock Lode, its formation and history, by John A. Church, 1879.
' This volume, Chapter VII.
EXPEEIMENTS ON KAOLINIZATION. 291
the unit mass of feldspathic rock in the unit of time. T. E. K. may, there-
fore, a priori, be either positive, zero, or negative. It must be regarded,
moreover, as a function of the time during which the action has been going
on, of the temperature and of the quantity of feldspar contained in a
given sample of rock.
The problem presented is none other than the measurement of very
small increments of temperature with all the accuracy attainable. For such
a purpose either thermometric or electrical means are applicable. The for-
mer requiring specially constructed apparatus, had at once to be discarded.
It IS a question, moreover, whether the thermometric method of research
will not, under all circumstances, offer obstacles of a very serious character.
In the measurement of small increments at the boihng point it becomes a
matter of great importance to keep the mercury column throughout at a
temperature as nearly as possible equal to that of the bulb— a condition
which can be reaHzed only with great difficulty, when a division of the stem
into very small fractions of a degree is also required.^ Electrically, there
are two methods applicable The first, however, based on the relation
between temperature and resistance, would have necessitated the measure-
ment of increments of the latter quantity amounting to scarcely 0.0005 per
cent, of the whole, in order to arrive at the accuracy desired. Though
even this is feasible in the laboratory, I despaired of being able to reach
such nicety with the means at my disposal. In view of these facts, it was
finally determined to try how far a thermo-electric method of research
might be successful in answering the question.
Processes of this kind, in which the effect observed is due to chemical
action, are usually accelerated by the application of heat. In other words
the assumption is warranted that the thermal effect of the action of aqueous
vapor on feldspar (T. E. K.) will increase, and will therefore be more easily
detected as the temperature of the vapor increases; provided, of course, that
this temperature is not chosen so high as to dissociate the products of de-
' A greater difficulty still \POuld probably be encountered from the fact that the reservoir of a
thermometer subjected to large differences of temperature is by no means constant in volume, but sub-
ject to variations dependent upon the glass chosen. (Phenomena of "after-action.")
292
GEOLOGY OF THE COMSTOCK LODE.
composition resulting in a normal case. Believing, therefore, that the phe-
nomena of kaolinization are reproduced at all temperatures below a certain
limit, and that the difference in effect is merely quantitative, the rock in
the experiments here described was subjected to steam at the boiling point
of water on the Comstock.' Besides this, it was intended to modify the
method of research sufficiently to trace the action of superheated steam
also. This must, however, be reserved for a future report.
Fig. 19. — Boiler (scale one-tifth).
Apparatus. — The apparatus (a boiler) in which the rock was subjected to the
action of steam is shown in longitudinal section, on a scale of one-fifth,
' About 93° C.
/
EXPERIMENTS ON KAOLINIZATION. 293
in Fig. 1 9. As will be seen, the well-known contrivance for determining the
boiling point of thermometers was made the pattern of construction. Steam
is generated in the interior conical compartment ahecdoi heavy tinned slieet
iron, 1 8 inches in diameter at the bottom and 12 inches at the top, and between
18 and 20 inches high. The top, dee, also conical,^ and provided with a
hole at c for the escape of steam, can be removed, and fits like a lid over
the walls of this compartment. The whole is surrounded by the cylindrical
mantle, fg i hf, of the same material. The top, g ih, of this can also be
removed, has the form of an ordinary lid, and is provided with tubulures
for the insertion of corks, etc., at h and i. The exterior compartment com-
municates with the air by the tubulures/ and y.
In the interior of the inner compartment, and held in position by a
suitable tripod (not shown in the cut), is the cylindrical chamber r s u t,\\
inches in diameter and 12 inches high, and provided — like a sieve — with a
bottom of wire gauze strengthened by radial supports of thick brass wire.
P q, finally, is a feed pipe for resupplying the boiler with water lost by
evaporation.
The rock to be tested was 'broken into small fragments, from the size
of a hazel-nut down to that of a pin-head, but excluding dust, and placed
in the chamber r s u t. Previously, however, the thermo-element x y z
(described on the next page) had been fixed in position, supported by suita-
ble cross-bars of wood covered with thick sheet rubber. In putting the
rock into the chamber care was taken to pack it sufficiently tight to prevent
currents of steam from possibly passing through the mass. Steam reached
the interior by a process of difi"usion, thoroughly saturating the whole. Of
this I had frequent occasion to convince myself Water having been poured
into the boiler to a level / /, approximately, and heated to ebullition, the
steam completely enveloped the rock chamber, permeating the material in
its interior. Passing through the hole c, and again around the greater part
of the apparatus, it finally escaped at / and f into the air.
As a source of heat, two small kerosene stoves were found excellent.
By means of the four broad flames thus obtained, the heat could be regu-
' Thus serving a second purpose, namely, to prevent steam condensed on the top of the boiler
from dripping into the rock below.
294 GEOLOGY OF THE COMSTOCK LODE.
lated as desired and kept constant during the whole time of experimentation.
Oil could be supplied without interfering with the flames. Trimming of
wicks was seldom necessary, and, there being four flames, gave rise to no
serious disturbance. To diminish the heat lost by radiation as much as
possible, the whole apparatus, with the exception of the bottom, was cov-
ered to a thickness of about three-quarters of an inch with cotton batting,
wrapped in layers and surrounded externally by heavy paper. Finally,
the water lost by evaporation was replaced drop by drop by means of a
pneumatic arrangement placed upon the boiler, but not shown in the
figure. The number of drops fed in a given time was so regulated by
the aid of a small faucet as to keep the level I I of the water in the boiler,
as indicated by the gauge m n, approximately at a constant height. The
ebullition was not allowed to become sufficiently intense to produce an
increase of pressure in the interior.
To recapitulate : By the aid of a fairly constant source of heat, the
ebullition from a water level of constant height could be maintained at a
nearly constant intensity. It was believed, therefore, that a stationary ther-
mal condition would soon set in and continue indefinitely. Errors due to
fluctuation of the barometric column, this being as likely to produce posi-
tive as negative efi'ects, could be excluded by proper methods of reduction.
Thermo-eisment. — To measurc thc Small increments of temperature, a thermo-
pile composed of three bismuth-silver elements was first used. Though
this acted well, there was danger, in consequence of the amount of sulphur
in the rocks (Fe S^), of complete destruction of the silver terminals during the
course of the experiment. This metal was therefore discarded, and platinum,
which is not thus afi"ected, chosen in its stead. The bismuth was cast in the
shape of three adjacent sides of a rectangle, the length and width chosen being
such as to allow the two ends to occupy the positions x and s shown in Fig.
19. Of course care was taken to insulate the whole thoroughly from the
walls of the boiler, this being accompHshed by surrounding the element on
all sides by strips of thick sheet rubber. The parts of the element were
kept from touching each other by pieces of glass tubing suitably placed.
The terminals — which, to prevent confusion, are not indicated in the figure —
were themselves insulated by a covering of rubber hose of small caliber.
EXPERIMENTS ON KAOLINIZATIOK.
295
They passed out of the boiler through tubulures (also omitted in the figure)
placed conveniently on its sides, the hose and wire being secured by small
perforated corks. Of course no attention was paid to the purity of the
metal employed. The silver and bismuth were fastened together by melting
a Httle globule on the end of the silver wire, and then applying it, while
still hot, to the end of a bismuth bar. The soldering thus produced was
very perfect. The platinum and bismuth had, however, to be soldered
together by ordinary means.
Method of measurement. — Thc relatiou betwecu thc electromotlvc force e, due
to the temperatures T and t (T>»^) of the ends of the thermo-element, can
be expressed with the aid of two constants, a and h, thus :
e:=z{T-t) \^a + h{T+ty\.
But as T— ^ in this case is a very small quantity (a few hundredths of
a degree),
where T is the temperature of ebullition of the water as given by the aid of
the barometer. Knowing, there-
fore, a, h, and the barometric
height we are able to find Jt,
or the diff"erence of temperature
between the interior and the
exterior of the rock chamber (x
and z in Fig. 19).
For the measurement of e
a "zero" method was employed.
In Fig. 20 a diagram of the con-
nections as actually made is
given, for the purpose of calling
attention to a few details of im-
portance in measurements of Fig. 20. -Disposition of apparatus.
this kind. The platinum terminals of the thermo-element e are soldered to
copper circuit wires at P, the points of junction being immersed in a reservoir
296 GEOLOGY OF THE COMSTOCK LODE.
filled with petroleum. Before each observation this liquid was stirred. The
copper wires pass through the commutator B, and thence the one through
a double key K to the point h, the other through the galvanometer G^ to
the point a, thus completing the first branch. The smaller resistance r
forms the second branch, also terminating at the points a and h. For the
convenient insertion of this resistance a number of small holes were bored
in a thick piece of wood and filled with mercury. The points a and h are
connected with the extreme holes of the series by means of strips of thick
copper foil. Finally, the terminals of a zinc sulphate Daniell E pass
through the commutator A, thence the one through the key K and directly
to fe, the other through a large rheostat R (from 1 to 10,000 ohms), and by
a thick wire, C, to a, completing the third branch.
When the current in (?i is zero
^—-^ Rj^r' ^^' ^^^^ simply, :=iE ^,
where e is the electromotive force ate, E at E in the figure; where, further-
more, B is the resistance at B, r s.tr in the figure, and where r is negligible
in comparison with B.
Having thus described the general method, it will be pertinent to men-
tion a few of the more important details. By means of K two circuits
conveying currents due to E and e, respectively,
are closed. It is, however, necessary that they
should be so closed as to act simultaneously (dif-
ferentially) on the galvanometer Gi; for if the cur-
rent due to the electromotive force e were to act alone
Fig. 21.— Section of key. gerious disturbances might be the result. This can
be accomplished by the following simple contrivance in the construction of
the key. Fig. 21 gives a section through the line of mercury cups c d, Fig.
20. Pieces of thick copper wire, bent as shown, are fastened to a thin
piece of board, capable of revolving partially about a horizontal axis parallel
to the line c d. In this way the pieces m and n can be dipped into the mer-
cury cups under their extremities or lifted out of them together. The board
is, moreover, provided with a spring so arranged as to keep m and n out of
the cups, and the circuit therefore remains open, unless closed by the ob-
n n
EXPBEIMENTS ON KAOLINIZATION. 297
server. The cups corresponding to w, and conveying the current due to the
Daniell E, are, however, filled vpith mercury to a level a little higher than
the rest. Hence, under all circumstances the circuit containing E, and not
passing through G^, is closed first. A moment after, however, that contain-
ing e and Gi is also completed, but it will be obvious that the effect from
e and E, if the directions of these electromotive forces are properly chosen,
will act differentially on G^, as was desired.
The electromotive force e, obtained as above, is never wholly due to
the thermo-element e alone, but contains also a disturbing electromotive
force, e, resulting from the accidental distribution of tempjerature in the
connections. For a short period of time (that of an observation) e may be
considered as nearly constant, or at least varying linearly. In order to elim-
inate the latter, very largely at least, Dr. Strouhal and myself, in analogous
experiments, inserted the two commutators A and B. In a series of corre-
sponding positions of the commutators, alternately opposite, direct meas-
urement would give
+E '
- --E -^
+e+eO.+2a)_
a.
+E --^^
— e-f£(l-|-3a)
E =""'
+e+e(l+ia)_
+E
=za,
where a is a constant. If now an odd number of observations be made,
and if Jfj be the mean of the odd right-hand members, M2 the mean of the
even right-hand members,
In the present investigation, the electromotive forces measured being
exceedingly small, at least five commutations of both A and B were made
for each value of M cited.
298 GEOLOGY OF THE COMSTOGK LODE.
The galvanometer (?i was one of low resistance, consisting of a few
hundred turns of wire around an astatic needle on silk fiber. The instru-
ment was quite delicate, and with the aid of proper methods of interpolation
would easily have enabled the measurement of increments as small as a few
ten-thousandths of a degree centigrade. Unfortunately, the silk was too thick,
and the zero point of the instrument, as a consequence, too variable ; while,
on the other hand, the strong winds of the region and the frail foundation of
the house itself rendered this accuracy unattainable, and we were obliged
to content ourselves with measurements accurate to a few thousandths of a
degree. Readings were made with a mirror and scale.
Thus far E has been considered constant. As this is not the case, its
variations were measured by the aid of a second galvanometer, G'g (Fig. 20),
made by Mr. Grunow, and described elsewhere.^ This instrument was
placed at a distance from the boiler, in Ae cellar, where the atmospheric
condition was tolerably uniform, and for convenience provided with a com-
mutator of its own, D. It will easily be seen that by breaking the circuit
at B and C and closing K, E will be in a simple circuit, including G^, and
that its value may be measured in terms of i2, which is also included.
The value of the constants a and h in the equation on page 295 was
determined by putting the ends of the thermo-element e in adjoining jars,
containing water at different temperatures.^ Ten or more observations were
usually made, from which a and h were calculated by the method of least
squares.
I cannot but consider this method of measuring differences of tempera-
ture as theoretically very perfect. First of all, discrepancies due to Peltier's
phenomena are avoided, while the constants a and h are used pi'ecisely in
the same way in which they were obtained. Moreover methods of interpo-
lation are particularly applicable ; even a method of multiplication might
be thus employed. There can be no doubt that under more favorable cir-
cumstances the minimum difference of temperature measurable with cer-
tainty would be much smaller than I have been compelled to consider it.
1 This volume, page 327.
2 Not having a reliable barometer, all temperatures are expressed in terms of the interval be-
tween zero and 100° C. of the best instrument at hand, this interval being arbitrarily assumed as correct.
On this assumption the stems of the thermometers used were calibrated.
EXPERIMENTS ON KAOLINIZATION.
299
Material experimented upon. — The Tock selected bj Mr. Becker for these exper-
iments was the freshest diabase encountered in the mines. The feldspars
show scarcely a trace of decomposition, and a large part of the augite is
unaltered. It was collected in the Sutro Tunnel, close to the hanging wall
of the Lode in the Savage claim. The same rock is described in Chapter
III., slide 18, and its analysis is given in the table following page 151
Results. — The results are reported chronologically, but with all correc-
tions, including those based on subsequent experiments. Temperatures are
given throughout in degrees centigrade, electromotive forces in volts.
From an inspection of the tables containing the results for the variation
of the electromotive forces of bismuth-silver and bismuth-platinum with
temperature, it will be seen that the relation is in both cases so nearly
linear that it may at once be assumed as such. One constant, a, only,
therefore, results from the calculation. Tables I. and II. contain the data
for the calculation of the thermo-electric constant a for the triple element
bismuth-silver, together with the results obtained. T is the temperature of
the warmer, t of the colder end of the element, e the electromotive force
corresponding to the temperatures of the respective observations observed
or calculated as specified, (5(e) finally the difference between observed and
calculated results. Two sets of observations were made in order to ascer-
tain to what extent a fixed value for a could be presumed — the bismuth bars
being cast and not pressed. In the calculations preference was given to
values of e corresponding to greater differences of temperature.
Table I,
No.
t.
T.
ex 10=
observed.
ex 103
calculated.
«(e)xlO».
1
6.1
75.1
15.61
15.63
-2
2
6.3
68.7
14.13
14.13
±0
3
6.4
63.8
12.89
13.00
-11
a~226. 5 : 106.
4
6.7
57.0
11.41
11.39
+2
5
6.9
60.5
9.92
9.88
+4
6
7.1
45.0
8.64
8.59
+ 5
.
7
7.2
39.5
7.32
7.32
+ 0
8
7.6
34.9
6.25
6.18
+7
9
7.8
30.4
5.18
5.12
+6
10
8.3
25.2
3.84
3.83
+1
300
GEOLOGY OF THE COMSTOCK LODE.
Table II.
No.
e.
T.
ex 103
observed.
6X103
calculated.
a(e)xlO».
1
11.0
73.7
14.22
14.22
±0
2
11.3
64.2
11.96
12.00
-4
3
U.5
58.7
10.70
10.70
±0
*
11.8
52.8
9.30
9.30
±0
a -226. 7: 10«.
5
12.2
47.8
8.10
8.07
+3
6
12.3
42.1
6.78
6.76
+2
7
12.5
36.9
5.52
5.53
-1
8
12.8
32.2
4.43
4.40
+3
9
13.0
27.2
3.19
3.22
: -3
10
12.8
16.4
0.87
0.82
i
Table III. contains the successive values of M, or the difference of
temperature between the interior and exterior of the rock-chamber. It
also shows the date of each observation and the number of hours which
had elapsed since ebullition first set in. Corrections for the variation of a
and the electromotive force of the normal element E have been applied.
During the time covered by the first six observations the water lost by
evaporation was supplied somewhat intermittently; subsequently, however,
as well as throughout all succeeding experiments, it was fed into the boiler
drop by drop, so that the feeding process may be considered as practically
continuous. M is positive, this sign having been chosen to indicate that
the space exterior to the rock-chamber— or the end of the thermo-element
in steam — is the hotter.
Table III.
No.
Date.
Eonra
t^t.
No.
Date.
HOQTS.
At.
ft.
h.
1
Dec. 10, 6
6
0.059
14
Dec. 14, 18
42
0.064
2
Dec. 10, 20
20
0.068
15
Dec. 14, 23
47
0.063 <
3
Dec. 11, 8
32
0.054
16
Dec. 15, 8
56
0. 062 ;
4
Dec. 11, 18
43
0.074
17
Dec. 15, 14
62
0.060
5
Dec. 12, 12
60
0.055
18
Dec. 15, 20
68
0.059
6
Dec, 12, 22
70
0.071
19
Dec. 15, 24
Dec. 16, 4
72
76
0.059
7
Dec. 13, 6
6
0.065
•n
Dec. 16, 8
80
0.060
8
Deo. 13, 10
10
0.005
22
Dec. 16, 12
84
0.058
9
Dec. 13, 14
14
0.062
23
Dec. 16, 18
90
0.060
10
Dec. 13, IS
18
0.068
24
Dec. 16, 24
96
0.057
11
Dec. 14, 3
27
0.063
V.b
Dec. 17, 4
100
0.059
n
Dec. 14, 8
32
0.063
'f»
Dec. 17, 8
104
0.057
13
Dec. 14, 13
37
0.062
EXPERIMENTS ON KAOLINIZATION.
301
Table IV. finally gives the data obtained for the calculation of a after
the experiments in Table III. had been completed, together with the results
of calculation. The nomenclature is the same as before.
Table IV.
No.
(.
I.
exlO^
observed.
6X103
calculated.
S(e)xl0».
1
10.0
74.8
14.12
14.18
-6
2
10.0
66.7
12.36
12.42
— 6
3
10.3
60.7
11.04
11.04
±0
4
10.4
55.8
9.95
9.95
±0
0 = 2191:100.
5
10.5
49.5
8.59
8.55
+ 4
6
10.7
44.3
7.39
7.36
+ 3
7
10.8
40.1
6.47
6.42
+ 5
8
11.0
33.3
4.97
4.89
+ 8
9
U.2
25.8
3.27
3.20
+ 7
10
11.8
20.0
1.88
1.80
+ 8
If the values of a in Tables I. and II are compared with that in Table
IV., a difi"erence of about 3 per cent, will be found. This may be due partly
to a change in the internal structure of the bismuth bars, partly to the fact
that both bismuth and silver were attacked by the sulphur fumes generated
in consequence of the presence of ii'on pyrites in the rock. In the case of
bismuth this action merely produced a thin, colored coating of sulphide on
the exterior. The silver, however, was so deeply corroded that its use liad
to be abandoned, and in subsequent experiments this metal was replaced by
platinum.
The data for ^t show a difference of temperature between the interior
and exterior of the rock-chamber, which is much greater than was antici-
pated. Moreover, the consecutive values of this quantity gradually de-
crease, indicating thereby an apparent increase of the temperature of the
rock itself
Tables V. and VII. contain the data obtained in the determination of a
respectively before and after the measurements of ^t made during the inter-
mediate week. In Table VI. these measurements are given, together with
the date, barometric height, and water-level, I (in inches from the bottom as
zero), corresponding to each /^t. The figures for barometric height were
obtained from a small aneroid. No reliance can therefore be placed on the
values as absolute, though the fluctuations are probably represented with
302
GEOLOGY OF THE COMSTOCK LODE.
tolerable faithfulness. Besides these data, the number of hours which had
elapsed since ebullition first set in are also given.
Table V.
No.
(.
T.
ex 10'
observed.
ex 103
calculated.
6(e) X 10».
1
12.6
79.7
14.51
14.51
±0
2
15.5
65.6
10.79
10.84
-5
3
18.4
61.1
9.28
9.24
+4
4
12.7
53.9
8.98
8.91
+7
0=216.3:106.
5
18.3
53.0
7.46
7.51
-5
6
13.0
46.4
7.22
7.22
±0
7
13.1
42.9 .
6.39
6.45
-6
8
15.6
39.8
5.23
5.24
-1
9
13.4
32.9
4.28
4.22
+6
10
15.3
31.9
3.52
3.46
+6
Tajble VI.
No.
Date.
Dec.
Dec.
Dec.
Dec.
Dec.
Dec.
Dec.
Dec.
Dec.
Dec.
Deo.
25,23'
26, S'
26, 9'
26, 14'
26, 22'
27, 3',
27, 9'
27, 14',
27, 23'
28, 10'
28,14'
Ht8.
23
27
33
38
46
51
57
62
71
82
86
0.098
0.093
0.088
0.088
0.083
0.082
0.078
0.077
0.072
0.068
0.066
Bar. H't.
23.30
23.30
23.24
2.3.15
23.10
23.06
23.15
23.23
23.19
4.9
4.2
4.9
4.8
5.0
4.7
4.5
No.
Date.
Dec.
Dec.
Dec.
Dec.
Dec.
Dec.
Dec
Dec.
Dec.
Jan.
28,23»
29, 5'
29, 14'
29, 24'
30, 9'
30, 18'
30,24',
31, 9'
31, 17'
1, 9*,
Hrs.
95
101
110
120
129
13H
144
153
161
177
0.067
0.067
0.061
0.062
0.062
0.062
0.058
0.060
0.055
0.048
Bar. H't.
2.
23.12
4.9
■J3. 17
4.9
23.24
4.8
23.30
4.4
23.35
4.2
23.36
4.4
23.40
5.2
23.30
4.4
23.35
4.0
23.25
4.4
Table vn.
No.
t.
T.
ex 103
observed.
exl03
calcnlated.
He) X 10«.
1
14.1
72.0
12.32
12.39
-7
2
14.1
65.8
11.13
10.06
+7
3
14.1
60.1
9.92
9.84
+8
4
14.1
54.1
8.56
8.56
±0
a=213.9:10«.
5
14.3
50.3
7.66
7.70
-4
6
14.3
45.0
6.54
6.67
-3
7
14.3
40.9
5.64
5.69
-5
8
14.3
36.1
4.66
4.66
±0
9
14.3
30.9
3.57
3.55
+2
10
13.9
38.1
5.18
5.18
±0
A large difference between the temperatures of the interior and exterior
of the cylinder, the former being the smaller, but increasing more rapidly
than before, is again apparent.
EXPERIMENTS ON KAOLINIZATION.
303
Table VIII. records an uninterrupted series of observations made by
Mr. Becker on the variation of M during an interval of three weeks
Table IX. contains the final check of the value of a.
Table VIII.
No.
Date.
Hra.
Bar. H't.
A«.
I.
No.
Date.
Hrs.
Bar. H't.
At.
I.
1
Jan. 4,21'...
3
23.25
0.024
26
Jan.l5, 7'....
253
22.83
0.062
4.5
2
Jan. 5, S'...
11
23.25
0.033
4.2
27
Jan. 15, 21'....
267
0.063
4.6
R
Jan. 5, 21'' . . .
27
0.033
4.7
••R
Jan 16 7'
277
22 94
0.067
0.068
0.063
5.0
4.6
4.7
4
Jan. 6, 7''...
J.on. 6, 16' . . .
37
46
23.15
0.044
0.042
4.4
4.8
29
Jan. 16, 21' ... .
291
301
23.29
fi
Jan. 6,21'...
51
0.042
4.8
1 31
Jan. 17, 21' ... .
Jan. 18, 10' ... .
7
Jan. 7, 7' . .
61
23.18
0.042
4.0
{32
328
23.36
0.068
5.2
8
9
Jan. 7,17'....
Jan. 7, 21'
71
75
0.041
0.044
4.8
4 3
33
11
Jan. 18. 21' ... .
Jan. 19, 20' ... .
Jan. 20, 7'....
339
362
0.066
0 063
4.8
4.5
4.6
23 30
10
Jan. 8, 7'....
85
23.21
0.044
3.8
35
373
23.36
0.021
11
Jan. 8,15'....
Jan. 8,20'....
93
23.50
0.016
4.8
1?,
98
0.042
5.3
37
Jan. 21, 22' ... .
Jan. 22, 7'....
Jan. 22, 22'....
13
14
Jan. 9, 7'..-.
Jan. 9, 17' . . .
106
119
23.18
0.045
0.041
4.0
4 ■>
38
39
421
436
0.048
15
Jan. 9, 21' ... .
Jan. 10, 7'....
123
0.039
4.6
0.034
0.081
5.2
16
133
23.15
0.042
4.5
41
Jan. 23, 21' ... .
459
17
Jan. 10, 19'
145
0.041
4.5
4''
Jan. 24, 7'....
Jan. 24, 21' . . .
Jan 25 8'
0.046
0.018
0 076
4.7
4 4
18
1<)
Jan. 11, 13' ... .
Jan. 11, 20'
163
171
0.040
0. 050
4.3
4.7
43
'I'l
483
494
2** 81
20
Jan. 12, 7'....
181
23.20
0.049
4.5
4.'i
Jan, 25, 21'....
507
0.051
?1
Jan. 12, 17' ... .
191
0.057
4.7
'16
Jan. 26, 7'....
Jan. 26, 21'....
Jan. 27, 7'....
517
22 92
0 149
4 6
n
Jan. 12, 21'....
195
0.055
4.5
47
531
23
Jan. 13, 8'....
206
23.12
0.060
4.6
48
541
23.15
0.094
4.5
24
23
Jan. 14, 7'....
Jan. 14, 17'....
229
239
22.90
0.067
0.063
4.5
4.9
49
Jan. 27, 15'....
549
0.126
Table IX.
No.
(.
T.
exlO^
observed.
ex 103
calculated.
6(e) X 106.
1
12.8
64.6
10.87
10.89
-2
2
12.9
59.5
9.79
9.80
-1
3
12.9
54.4
8.78
8.73
+5
a =^10.3: 10'.
4
13.0
50.1
7.74
7.80
-6
5
13.1
44.7
6.65
6.65
±0
6
13.1
40.7
5.81
5.81
±0
7
12.8
32.2
4.13
4.08
+5
8
13.1
30.1
3.62
3.57
+5
9
12.7
25.8
2.75
2.76
-1
10
12.7
22.1
2.02
1.98
+4
In the foregoing determinations of «, the temperature t was chosen to
coincide as nearly as possible with that of the room. Though this arrange-
ment furnished important practical advantages {t varying but slightly), only
304
GEOLOGY OF THE COMSTOOK LODE.
that part of the thermo-element lying near the hot end was really in action.
It was therefore thought desirable to reverse the element, so that the end
which was formerly in hot water would now be in cold, and vice versd.
Table X. contains the results thus obtained.
Table X.
1
No.
t.
T.
exl0»
observed.
«xlO»
calculated.
«(«) X 10*.
1
13.3
63.6
10.61
10.51
±0
2
13.4
57.4
9.23
9.19
+■4
3
13.5
45.0
6.54
6.58
-4
O = 208.9:10«.
4
13.5
40.6
5.65
5.66
-1
5
13.5
35.9
4.69
4.68
+ 1
6
13.5
30.7
3.60
3.59
+ 1
The difference between the values of a in Tables IX. and X. lies within
the range of unavoidable errors.
In Table VIII. there is a difference of temperatures between the inte-
rior and exterior of the rock-chamber analogous to that in preceding tables.
The former is, as usual, smaller, but in this case the temperature of the
rock apparently decreases as the action continues.
Between the observations No. 34 and No. 38 there appeared disturbances
of a kind which seemed to indicate that a break had occurred somewhere
in the insulation. Subsequent inspection showed that the parts of the rubber
hose around the platinum terminals, which were in contact both with air and
steam, had swollen to a spongy mass of many times their former bulk. It
is not improbable that the wire during the disturbances mentioned had been
more or less perfectly in contact with the walls of the boiler, the doughy
rubber protection having either given way or offering imperfect insulation.
Though this was partially remedied, yet the last week's observations are
nevertheless to be regarded as somewhat suspicious, and were consequently
omitted in the calculations below.
Discussion. — In the following discussion the observations in Tables III. and
VI., and the first two weeks in Table VIII., are to be considered. Together
these data correspond to an interval of four weeks. In the endeavor to
reach the most probable conclusion to be derived from the large number of
observations, the end in view will be attained most speedily, and perhaps most
satisfactorily, by assuming for the relation between the variables some ap-
EXPERIMENTS ON KAOLINIZATION. 305
proximate form, and calculating the constants by the method of least squares.
In the present case there is as much reason to adopt a linear form of func-
tion as any other, and this would have the advantage of greater simplicity.
Denoting the number of hours which have elapsed since the beginning of
the experiment by u, let
^t—a + ^u (1)
In this equation the constant a is without great interest. It simply
denotes the value of ^t when ^^ is zero, but is largely influenced by the
normal difference of temperature between the interior and exterior of the
rock-chamber, i. e., the difference which may be recognized by an inspection
of the foregoing tables, and of Table XIV. /?, however, is of importance,
representing the increment of temperature of the rock per hour in conse-
quence of the T. E. K. It will be noticed that j3 is either negative, positive,
or zero, according as the process of kaoHnization produces or absorbs heat
perceptibly, or is without appreciable thermal effect. In making the calcu-
lation for /? I had hoped to be able to derive this constant from the four
weeks' observations, as a whole. The problem is difficult, however, inso-
much as the results obtained do not form one continuous series. The problem
is not, in other words, that of a single straight line as in equation (1), but
one involving three straight lines, for all of which, however, the value of
/? is the same. Expressing the whole interval during which the observations
were made (four weeks) by ;r\ and regarding the values of ^t in Table III.
as being ordinates of the component line whose extreme abscissae are 0
and J , those in Table VI. as belonging to the line between J and I, and those
in Table VIII. to the hne between ^ and ;r; then the whole line between
0 and TT, expressed as a special case of Fourier's series, would be represented
by the equation
Jt = Aisinu-\-A2sin2u-{- + A^sinmu -}- . . ,
where
A^=-} I * {a^-^- ^ cp)smm<pdq)-\- /^ {a.^ + ^ (p)smmcpdq3
+ /„ ('^s -\- ^ g>) sin. m (p d <p > ,
^ J
'The assumption furnishes the constant for the reduction of the observations.
306
GEOLOGY OF THE COMSTOCK LODE.
and «!, Qfj. <^3 are the intercepts of the component Hnes on the axis of ordi-
nates. This finally leads to
n
* 1 c 'i
2_ \ ai(l — cosw;r) — /?— (cosm— +cosw^ + 2cosOT;r) >
0 m i 4 4 2 5
smwM,
an equation which, though linear with respect to a.^ and /? and capable of
further simplification, cannot be practically utilized.
In view of this fact, it was decided to calculate the constants ol^ and ft
for each set of observations separately. Tables XL, XII., and XIII. give
the results, these tables corresponding to III., VI., and VIII., respectively.
Table XI.
No.
u.
A( obs.
HA calc.
Diffi
No.
u.
At Obs.
At calc.
Diff:
7
6
0.065
0.065
±0
17
62
0.060
0.061
-1
8
10
0.065
0.065
±0
18
68
0.059
0.060
-2
9
W
0.062
0.065
-3
19
72
0.069
0.060
-1
10
18
0.068
0.064
-H
20
-76
0.060
0.060
+ 0
11
27
0.063
0.064
-1
21
80
0.060
0.059
+1
12
32
0.063
0.063
±0
22
84
0.058
0.059
-1
13
37
0.062
0.063
-1
23
90
0.060
0.058
+1
14
42
0.064
0.062
+1
24
96
0.057
0.058
-1
15
47
0.063
0.062
+1
25
100
0.059
0.058
+1
16
56
0.062
0.061
+1
26
104
0.057
0.057
±0
o =-(-0.064;
/3 = -0. 000082 + 0.000007.
Table XII.
No.
u.
At obs.
At calc.
Diff.
No.
u.
At obs.
At calc.
Diff.
1
23
0.098
0.090
+8
11
86
0.066
0.072
-7
2
27
0.093
0.088
+5
12
95
0.067
0.070
—3
3
33
0.088
0.087
+1
13
101
0.067
0.068
-1
4
38
0.088
0.085
+2
14
110
0.061
0.066
-5
5
46
0.083
0.083
±0
15
120
0.062
0.063
—1
6
51
0.082
0.082
±0
16
129
0.062
0.061
+1
7
57
0.078
0.080
-3
17
138
0.062
0.058
+4
8
62
0.077
0.079
-2
18
144
0. 058
0.057
+1
9
71
0.072
0.076
-4
19
153
0.060
0.054
+6
10
82
0.068
0.074
-6
20
161
0.055
0.052
+3
21
177
0.048
0.048
±0
o=+0.096;
P=-0. 000271 ± 0.000013.
EXPERIMENTS ON KAOLINIZATION.
307
Table XIII.
ITo.
u.
A« obs.
At calc.
Diff.
^0.
u.
At obs.
At calc.
Diff.
1
3
0.024
0.033
-9
18
163
0.040
0.030
-10
2
11
0.033
0.034
-1
19
171
0.050
0.051
- 1
3
27
0.033
0.036
-2
20
181
0.049
0,052
- 3
4
37
0.044
0.037
+7
21
191
0.057
0.053
+ 4
5
46
0.042
0.038
+ 5
22
195
0.055
0.053
+ 2
6
51
0.042
0.038
+4
23
200
0.060
0.055
+ 6
7
01
0.042
0. 039 ■
+ 3
24
229
0.067
0.057
+10
8
71
0.041
0.040
+ 1
25
239
0.063
0.058
+ 5
9
75
0.044
0.041
+3
26
253
0.082
0.060
+ 3
10
85
0.044
0.042
+2
27
267
0.063
0.061
+ 2
11
93
0.043
0.043
±0
28
277
0.067
0.062
+ 5
12
98
0.042
0.043
-1
29
291
0.068
0.064
+ 4
13
109
0.044
0.044
±0
30
301
0.063
0.065
- 2
14
119
0.041
0.045
-4
31
315
0.064
0.066
- 2
15
123
0.039
0.046
-7
32
328
0.063
0.068
+ 1
16
133
0.042
0.047
-5
33
339
0.066
0.069
- 3
17
145
0.042
0.048
-6
34
362
0.063
0.071
- 8
a= +0.033;
^ =+0.000106 ± 0.000006.
The constants /? in these tables are, however, of inconvenient magni-
tude, and it will be more expedient to represent these quantities on the
scale of a year. Let ^T, then, denote the apparent increase of the tem-
perature of the rock in the apparatus per year, the variation being sup-
posed to have continued during the whole of this time in the same manner
as during the time of observation. Then from Tables XI. and XII., which,
together, comprehend an interval of two weeks,
^r = -|-l°.5±0°.l;
and from Table XIII., corresponding to the same interval,
JT = — 0°.dzkO°.l.
These figures express the final result of the investigation. They indi-
cate that, as far as these experiments go, it would be about equally correct
to assume a positive or a negative thermal effect from the action of aqueous
vapor on the rock; and that for the present, at least, a thermal effect may
be assumed to be absent.
By comparing corresponding values of a in the tables above, it becomes
evident that the changes in the values of ^t cannot in any way be referred
to the thermo-element; nor is there, in the results taken as a whole, an effect
308
GEOLOGY OP THE COMSTOCK LODE.
due to the variation of the barometric pressure or to thewater level appa-
rent.
A series of experiments made with the rock-chamber empty, the rest
of the apparatus remaining, however, as before, gave the following results:
Table XIV.
No.
Sate.
Hrs.
At.
No.
Date.
Hrs.
At
1
Dec. 17, ll'.S
0.0
0.020
1
Dec. 17, 15'.0
0.0
0.019
2
Dec. 17, 12'. 0
0.5
0.022
2
Dec. 17, IS'.S
0.5
0.020
3
Dec. 17, 12'.5
1.0
0.024
3
Dec. 17, le'.O
1.0
0.029
4
Dec. 17, 13'. 5
2.0
0.028
i
Dec. 17, 17''.0
2.0
0.031
The interval of time covered by these experiments is, of course, too
small to justify any confidence in the constants which might be derived
from them. They are, however, sufficient to show that Jt undergoes
changes analogous to those noted in the preceding pages. It probably fol-
lows, therefore, that the final results may be regarded as giving an estimate
of the degree of accuracy attainable by the method in its present shape.
The chief source of error is the fact that the apparatus does not maintain
the constancy of temperature necessary. It is apparently impossible by
means of it to heat the large mass of rock to the same temperature through-
out. Furthermore, the thermometer employed is neither in sufficiently inti-
mate contact with the rock, nor are the junctures placed in circumstances
as nearly identical as is desirable. Finally, I am inclined to infer that a
stationary thermal condition was not reached in the experiments. Although
this supposition accounts for only a part of the anomalies met with, it will
nevertheless be necessary in future researches to extend the time of each set
of observations considerably beyond the duration of the above experi-
ments. I omit a detailed discussion of these matters, however, as a further
study of the subject is intended.
CHAPTER X.
ON THE ELECTRICAL ACTIVITY OF ORE BODIES.
BY CAEL BARUS.
GENEEAL STATEMENT.
In 1830 R. W. Fox communicated to the Royal Society a paper which
contained the results of a careful experimental study of the possible electric
activity of ore bodies. From this time until 1844 the matter was discussed
with some enthusiasm by Fox and Henwood, in England, and by von Strom-
beck and Reich, in Germany. After the publication of Reich's second paper
(1844), however, further research seems to have been altogether abandoned;
at least I have not, with some pains, been able to find anything that has a
bearing on the subject.^ This is all the more remarkable, as the general
line of investigation had already taken a promising direction. It would
also have been supposed that Thalen's^ work would have given the matter a
fresh impetus.
With the present investigation (undertaken at the suggestion of Mr.
Becker^) the question of a relation between local currents and ore bodies
is, as it were, resuscitated, so that a general review of the development which
it had attained previous to its abandonment seems pertinent. -^
'See, also, "Revue des Progrfes r^centes de I'Exploitation des Mines, etc., par M. Haton delaGou-
pillifere, Ingen. en chef des Mines, Professeur, etc.," in the Anuales des Mines, T. XVI., p. 6, 1879.
^R. Thalen; v. dela Goupilliere, I. c. : "Ou trace des lignes d'^gale intensitd, qui dans le voisi-
nage d'un glte prennent une forme caract^ristique consistant en deux systfimes de courbes ferm^es, con-
ccntriques, autour de deux foyers assez nettement indiqu6s."
3Cf. : First Annual Report of the U. S. Geolog. Survey, p. 46, 1880.
(309)
310 GEOLOGY OF THE COMSTOCK LODE.
BEIEF EEVIEW OF THE WOEK OF PEEVIOUS IISTVESTIGATOES.
Fox/ in his original experiments, secured electric contact with the vein
by wedging copper plates against it. These were put in connection with a
galvanometer by copper wire. Earth currents, if present, entered the wire
at one end, passing through the galvanometer and finally back into the earth
at the other.
As a general result of his investigation Fox found that the intensity
and direction of the currents bore no relation to the cardinal points, but
could be explained by a consideration of the distribution of ores.^ Between
two points of a continuous vein on the same level no cuiTent was observa-
ble; but when the points tapped were on different levels, or when there
intervened between them an area of barren rock (horse), or when two appar-
ently distinct veins were connected, the effect was invariably decisive. At
times the currents were so powerful as to throw the needle of his by no means
delicate galvanometer (3J-inch needle in twenty-five turns of wire) several
times around the circle. After enumerating a number of facts with reference
to the relative position of the veins. Fox remarks that "many of the phe-
nomena referred to bear a striking resemblance to common galvanic combi-
nations, and the discovery of electricity in veins seems to complete the
resemblance." In other parts of this paper, however, he expresses the opinion
that "mineral veins and internal heat are connected with electric action,"
and, moreover, anticipates greater effects with increasing heat and depth.
The experiments of v. Strombeck^ were made at Werlau and Holzappel
on a large vein, in which quartz, blende, galena, copper-pyrites, and tetra-
hediite occurred in irregular distribution, and are distinguished by the care
with which all known sources of error were avoided. Contact was secured
by drilling into the vein holes 2 to 3 inches in depth, into which the ends of
the wire, spirally wrapped and held in position by a cork, were inserted. In
'E. W. Fox, "On the electro-magnetic properties of metalliferous veins in the mines of Corn-
wall." Phil. Trans., II., p. 399, 1830.
= Galena, copper, and iron pyrites were the minerals met with.
»A. y. Stronibeck, "Ueber die von Herrn Fox angestellten Untersuchungen in Bezug auf die
elcctro-maguetischen Aenasernngeu der Metallgange." Karsten's Archiv., VI., 431, 1633.
ELECTEICAL ACTIVITY OF ORE BODIES. 311
other respects the method of research was identical with that of Fox.
V. Strombeck made a large number of experiments, but was unable to detect
any traces of electric excitation, and, consequently, concludes that Fox's
results are not applicable to veins generally, and that even in Cornwall
the matter requires further consideration.
In 1834 Fox again resumed his experiments, with special reference to
the objections which had been raised against the validity of his results.^ It
having been mooted that the currents observed might in some way owe their
origin to the copper contact-plates, he showed that by replacing these by
plates of zinc the results remained unaltered. This was the case even when
terminals of copper and zinc were used simultaneously. It was, moreover,
immaterial whether the contact was produced by plates or whether the ends
of the wire only were pressed against the vein. By inserting a copper-zinc
couple into his circuit Fox found that its effect was in some cases nearly, in
others decidedly, overbalanced by the lode currents. Finally, in the interval
of four years which had elapsed between these and his former experiments
the direction of the currents had remained unchanged.
In a subsequent paper Fox^ endeavors to classify minerals with refer-
ence to their electrical properties. A table of conductivities is contained in
his original paper.
In the Skeers lead mine, near Middleton, Fox^ obtained but feeble
currents; at the Coldberry mine, in the same locality, they were absent alto-
gether. Lead mines do not in general give evidence of electrical action
comparable to that of copper mines — a circumstance which Fox refers to
the positions of their ores in his scale.
Henwood'.s* experiments were made on a larger scale (at times as
much as 600 fathoms of copper wire were employed), but otherwise in a way
'E. W. Fox, "Account of some experiments on the electricity of the copper vein in Huel Jewel
mine." Rep. Br. Assoc, 1834, p. 572.
-R. W. Fox, "Note on the electric relations of certain metals and metalliferous minerals." Phil.
Tr.an8., I., p. 39, IS-IS.
"R. W. Fox, " Report on some experiments on the electricity of metallic veins, etc." Rep. Br.
Assoc, p. 133, 1837.
«W. J. Hen wood, "Surles conrants ^lectriques observes dans lesfilons de Comouailles," Annaleg
des Mines, [3], XI., p. 585, 1837.
312 GEOLOGY OF THE COMSTOCK LODE.
analogous to that of Fox. They contain a thorough corroboration of the
results of the latter. He, moreover, insists that currents are only obtained
in the case where the points tapped are in vein matter, being most decisive
for copper pyrites, vitreous and black copper ore, galena and blende; that
between points in barren rock electric action is altogether absent. After a
number of theoretical considerations — to which the paper is largely devoted —
he concludes that the currents are probably of thermo-electric origin, and
that they are certainly purely local.
Some time after, all of Fox's experiments were again repeated and the
results confirmed throughout by Reich.^ Although the heating of one of
the points of contact in the case where both were applied to the same vein
produced a decided thermo-electric effect, quantitatively this was so small as
to furnish grounds against Henwood's hypothesis. Reicli is convinced that
Fox's currents are hydi-o-electric phenomena. When a point in ore was con-
nected with one in rock, the currents were not only much smaller — proba-
bly on account of the greater resistance in this case — but if plates of copper
and zinc were used together as terminals, a commutation of these invariably
produced a corresponding change in the direction of the current.
In Fox's last paper^ on the subject, the effect of the contact plates is
again carefully considered. But even with one terminal of zinc, the other
of copper, "the current continued to deflect the needle from 50° to 60°,
notwithstanding that any action between the copper * * « * and the
zinc * * * * if it had existed would have been in the opposite direction
and have tended more or less to counteract the influence of the actual cur-
rent." The galvanometer referred to consisted of forty-eight turns of brass
wire wrapped around a 2-inch needle, on a pivot. The lode current in a case
observed was found to remain constant for a period of eight months. Toward
the end of the paper mention is made of experiments in which one or both
terminals were in rock. In this case the results were similar to those of
IF. Eeich, "NotizUber elektrische Strome auf Erzgaugen." Pogg. Ann., XLVIII., p. 287, 1839.
°E. W. Fox, "Some experiments on subterranean electricity, made at Pennance mine near Fal-
month." Pbil. Mag., [3], XXIII., pp. 457 and 491, ie43.
ELECTEICAL ACTIVITY OP ORE BODIES. 313
Reich, "there being still a tendency to deflection." The exchange of ter-
minals of different metals also produced a change in the direction of the
current.
In the next year Reich^ published his second paper, undertaken with
the especial object of studying more closely the currents probably existing
in the rocks surrounding the vein. His idea was that lode currents are
produced by the contact of the different ores in the deposit, the rock which
separates them more or less completely one from another performing the
function of the liquid of an ordinary galvanic couple. As Fox's method of
obtaining contacts with the earth was inapplicable, Reich had holes (12
inches deep) drilled in the rock, into which dilute sulphuric acid was poured.
Strips of copper foil plunged into the acid and connected with the ends of a
copper wire completed the circuit. Currents were obtained when at least
one point was near ore; they were completely absent when both points were
in barren rock. Though the deflections of the needle ranged from 2° to
30°, they seemed to obey no general law. The results are, moreover, diffi-
cult of interpretation, because the needle does not discriminate between
high and low grade, or between base and noble minerals,^ the deflection
being a function of both the quality and the quantity of the electrically
active material. Reich's mode of operation was derived from a considera-
tion of the currents of a galvanic cell in action. The paper' is interesting
and the reader's attention is especially called to it. I shall have occasion
to consider it again below.
The reader is finally referred to the Proceedings Roy. Soc. Lend., III.,
p. 123, 1832, and IV., p. 317, 1841, which were not at my disposal.
Remarks on the foregoing. — Froui 1830 Until 1844, therefore, the papers in
hand offer little more than a criticism of Fox's original investigation. In
1844, with the publication of Reich's second paper, in which the idea that
if local currents due to ore bodies are present at all they must be discover-
able in the rocks, was the basis of research, a second step may be considered
'F. Reich, "Versucte uber die Aufsuchung von Erzen mittelst des Schweiger'schen Multiplica-
tors." Berg- u.- huttenmiinn'sche Ztg., [3], pp. 342-346, 386-390, 1844.
"The term "mineral" wherever used throughout this chapter is intended to refer to those of the
heavy metals only — to those in short in which we may expect to find metallic properties.
'See, also, B. v. Cotta, " Erzlagerstiitten," Vol. I.
314 GEOLOGY OP THE COMSTOCK LODE.
as having been made. It is to be regretted that in none of the papers is
there even an attempt toward fully describing the phenomena quantita-
tively. Generally, conclusions are drawn from the deflection of a galvano-
meter needle without sufficient consideration of the very probable variation
of the resistance of different circuits. The experiments are, moreover, made
individually, not in series or with reference to any definite, preorganized
plan. Insomuch, however, as most of the work was done when methods of
electric measurement were still in their infancy, these matters are not to be
mentioned to the disparagement of the authors. In fact, the reader is sur-
prised at the broad view usually taken, at the cautiousness with which
hypotheses are stated, and at the number of details and chances of error
which are considered.
HYPOTHESIS UNDEELYING THE PRESENT INVESTIGATION.
There can be little doubt that the hypothesis which ascribes to ore-
currents a hydro-electric origin is perfectly correct. Fox and Reich them-
selves found in the case of terminals of copper and zinc used together, the
points tapped being in rock, that currents resulted, the direction of which
changed with an exchange of the terminals. I have actually measured the
electromotive force in action under these circumstances (see page 322), and
found it of the same order as that produced by combining these metals with a
liquid in the form of a galvanic element. If, then, there are also ores which
possess the electric properties of metals — and that this is the case Fox* went
to some trouble to show — the possibility of ore-currents due to hydro-
electric action follows as an immediate consequence. These currents will
in general have an origin analogous to those technically known as "local
currents " in batteries, while at times they may even be due to the occurrence
of a complete natural battery. Thermo-electric hypotheses are unnatural,
insomuch as with the temperatures met with, even in the Comstock, it
would be necessary to assume values for thermo-electric power which,
in comparison with those of known substances, are abnormally large. Such
'K. W. Fos, Phil. Trans., 1., p. 39, 1835.
ELECTRICAL ACTIVITY OF ORE BODIES. 315
a speculation is, therefore, remote, artificial, and forced, and, in cases where
there is a better hypothesis, deserves only very secondary consideration.
Suppose now that, in connection with an ore body, with reference to
which experiments are being conducted, electric action actually does occui-.
In the consideration of these currents we are at once confronted by the impor-
tant fact that insomuch as electric action has been going on for an indefinite
period of time the currents must have become constant both in intensity
and direction, and that therefore the equipotential surfaces corresponding to
this flow will have fixed and probably well-definable positions.
In view of the fact that with most geological readers the consideration
of electric phenomena will be merely an incidental matter, it may be well to
be more explicit than would otherwise be necessary. By far the greater
number of electrical phenomena can be explained by regarding electricity
as in the nature of an incompressible fluid. The analogy is, in fact, very
complete, and extends even into further detail than need be noticed here.
We speak of a liquid as having a tendency to flow from a higher to a lower
level; of electricity, as flowing from an equipotential of greater to one of
less value. In the former case the "levels" are approximately spheroidal
surfaces — "geoids" — parallel to the normal surface of the earth;, in the lat-
ter they may be closed, or may extend to infinity; they may be quite simple
or exceedingly complex. In order to exhibit the topography of a country in
detail, it may be repi'esented graphically by the aid of a series of equidistant
earth levels. In electricity an analogous problem is similarly solved, those
surfaces being chosen for which the potential value from surface to surface
increases by a definite amount.^ If a reservoir, the water in which is con-
stantly at a level, j), be joined by a pipe with one in which the water-level
is constantly g (both j) and g- being measured vertically upwards from some
fixed datum, and p'^q), the quantity of liquid traversing any right section
of the pipe in the unit of time would ccet. par. be dependent on the dimensions
of the latter and upon p — q. If a point on an equipotential of the value p
be connected by a thin wire with a point on one of the value q, analogous
remarks may be made with reference to the quantity of electricity (/) flowing
'Neither level nor potential imply the presence of matter or of electricity, respectively, at a
given point.
3 1 6 GEOLOGY OF THE COMSTOCK LODE. ^K^m
through any right section of the wire in the unit of time. Now HHB
it is upon I that the deflection of a magnetic needle surrounded H^^l
by a coil of wire, the plane of the windings being vertical and HHB
parallel to the needle, ccet. par., depends; whence it follows, even HBH
if the same arrangement of coil and needle were used throughout, H^H
that the deflection just mentioned would contain an incidental H^^H
element; in other words, that it depends upon the means which H^^l
have been adopted to efi"ect the connection between the equipo- HlH
tentials p and q. I^HI
Returning to the problem in hand, it will be found that the HBH
mere measurement of deflections would be of but little avail. An H^H
effort must be made to determine the values of p and q at the ^H^h
points tapped by the ends of a wire. These quantities, more- ^HBb
over, are particularly significant, insomuch as the potential at ^^^H
any given point in the vicinity of the ore body depends princi- ^^^H
pally upon the character and distribution of the electrically active B^9
ore-matter, and of the rock surrounding it, or wholly on con- H^H
ditions fixed by nature. Hence, instead of seeking for the ore HHH
body itself, an attempt will be made to add to the few clews H^H
available to the prospector by investigating some characteristic H^H
variation of the potential at consecutive, similarly disposed points, HH|
as indicating proximity to it. But what has been said of p and HHII
q applies equally well to p—q, which latter quantity is, moreover, H^H
easily measurable, either directly (electrometrically, or by cer- ^^^H
tain galvanometric methods) or indirectly, by the determination ^^^|
of the magnitude of deflection of the needle described above, ^^^H
under known conditions, p — q is technically called electromotive H^M
force. I^B
To an observer the equipotentials are accessible for measure- H^H
ment either on the surface or in those places where drifts pene- H^H
trate them. Let a, v, r. Fig. 22, be a line lying either upon or H^H
within the surface of the earth. Suppose the electromotive forces H^H
be measured between a point a, and consecutive points /?, y, 6 H^H
. . . fi, V, $, . . . . a, T, V, . . . taken at convenient, approximately H^H
equal, distances apart. The points ju, v, ^ . . . are supposed to fig. 22.
ELECTEICAL ACTIVITY OF OEE BODIES. 317
be near the ore body, whereas a, /3,y . . . and g, t, v . . are remote from it.
As I shall frequently have occasion to refer to the point a in contradistinc-
tion to the remaining points /?, 7, 5, . . . (>, t, t» . . , I will throughout this chap-
ter refer to the former under the name permanent contact (P. C), while to
any of the others the name temporary contact ( T. C.) will be applied. Then
will the electromotive force (e) between P. C. and any T. C. in general vary
with the distance (x) between these points. This relation will usually be so
complex as not to be easily expressible by mathematical means, but it can
nevertheless be indicated symbolically by
e=f{x).
If, however, x is supposed to increase from zero (in which case P. C.
and T. C. coincide) to the value it has for some remote point, v, then as a
field of electrical activity is encountered in the neighborhood of /',»', ^,
f(x) must pass through a single maximum or minimum, or a number of them.
It is therefore toward a characteristic variation of this kind that we must
look in endeavoring to define a position of greatest proximity to the ore
body. Analogously, though less generally, it may be stated that the incre-
ment of potential due to successive increments of distance a /?, /? y, y S, etc.,
will be small except in the neighborhood of the ore body. This is probably
the idea which Reich had in mind, and which he must have come upon had
he followed out the line of his argument to its consequences.
I will add here that local difficulties did not permit me actually to pass
linearly through an ore-region. I had to content m3'^self, therefore, with a
progress from the latter into barren rock.
EXPERIMENTS MADE IN SOME OF THE MINES ON THE COMSTOCK.
Method. — Experiments were commenced in the Consolidated Virginia, Cal-
ifornia and Ophir mines, the line at times extending into Union and Mexican
ground.
From the work of previous investigators I was naturally led to expect
currents due to electromotive forces of considerable magnitude, and as a
consequence, was satisfied with a method of obtaining contact with the vein
318
GEOLOGY OF THE COMSTOCK LODE.
in which the electromotive force due to the terminals alone was not greater
than a few hundredths of a volt. Bright steel gads, to the tops of which pieces
of thick copper had been firmly fastened, were especially convenient for
this purpose, as they could be driven into the vein or again withdrawn from
it expeditiously. These gads were from 8 to 10 inches long and about one inch
in diameter at the head, from which they tapered gradually to a point. As
it would be repeatedly necessary to use them in places where the earth was
naturally moist, the question arose whether it might not be desirable in all
the experiments to moisten the rock around the gads at once. Accordingly,
two sets of experiments, the results of which are contained in Tables I. and
XL, were made, the former above the surface, the latter below.
Two suitable positions in rock free from mineraP matter having been
selected, the gads were driven and the circuit completed. Measurements of
resistance and electromotive force were then made. The gads were now
exchanged and the measurements repeated, and so on. The relative posi-
tion of the gads to an observer facing them is indicated in the second column
of the tables. Resistance ( W) in ohms and electromotive force (e) in volts
are given in the third and fourth columns, respectively. The last column
shows the direction of the current, arbitrarily called "_j-" when flowing in
one way, " — " when flowing in the opposite.
Table I. — Experiments inade on south side of Bullion Ravine.
[Gads driven into quartz seams between walls of diorite, about 10 feet apart. Seams natnrally somewhat moist.]
Gads dry.
Gads wet.
No.
Position
of the
gads.
w.
€.
Direction
of the
current.
No.
Position
of the
gads.
w.
e.
Direction
of the
current.
1
I, n
7600
0.03
+
1
n, I
1560
0.01
■+
2
II, I
6300
0.09
+
2
I, n
1260
0.02
+
3
I, n
4300
0.01
_
3
n, I
1280
0.02
+
4
n,i
4500
0.06
+
4
I, n
1200
0.01
+
5
I, n
3700
0.00
—
5
n, I
1210
0.01
-
6
n,i
3400
0.01
+
6
1,11
1200
0.01
-
7
i,n
3200
0.00
+
7
II, I
1230
0.01
+
8
i,n
1240
0.01
-
9
n, I
1240
0.04
+
' See note, page 313.
ELECTKICAL ACTIVITY OF OEE BODIES.
319
Table II. — Experiments in the Con, Virginia and California^ 1750-foot level,
[Gada driven into rock, as free from mineral matter as possible, about 8 feet apart.]
GadB dry.
Grade vret.
No.
Position
of tbe
gads.
w.
€.
Direction
of the
current.
No.
Position
of the
gads.
w.
€.
Direction
of the
current.
Dale.
1
n,i
6000
0.04
+
1
II, I
550
0.03
_
Sept. 24, 1880.
2
I, n
3700
0.04
+
2
I, n
500
0.03
-
Sept. 24, 1880
3
n, I
2800
0.02
+
3
II, I
450
0.01
-
Sept. 24, 1880
4
I, II
2200
0.02
+
4
I, II
400
0.02
-
Sept. 24, 1880
5
n, I
1870
0.02
-
5
II, I
380
0.01
_
Sept. 24, 1880
6
I, II
1380
0.01
+
6
HI
390
0.01
+
Sept. 25, 1880
7
U, I
1030
0.03
+
7
I, n
270
0.03
+
Sept. 25, 1880
8
I, n
1060
0.01
-
8
n, I
280
0.01
+
Sept. 25, 1880
9
I, II
260
0.03
-
Sept. 25, 1880
10
n, I
270
0.01
-
Sept. 25, 1880
The results are highly in favor of wet gads. By their use a very
marked diminution of resistance is effected without increasing the values of
£. The direction in which e acts follows no observable law, probably being
conditioned by the electrical difference of the gads and by effects of polar-
ization due to the introduction of a Daniell.
Analogous experiments were also made with copper and zinc. These
metals were used in the form of strips cut from sheets. Each strip was
bent around the small end of a slightly conical stick of wood about one
foot in length. The plug was then firmly driven into a hole previously
drilled for the purpose, in such a way as to force the metal into thorough
contact with the rock Table III. gives the results, the notation being the
same as that used in Table I.
Table III. — Experiments in the Con. Virginia and California, nsOfoot level.
[Pings abont 10 feet apart in moist clay seams, repeatedly exchanged as indicated.]
Copper pings, wet.
Zinc plugs, wet. ]
No.
Position
of plugs.
«.
No.
Position
of plugs.
e.
1
2
3
4
5
6
7
8
9
10
I, n
n, I
I, II
II, I
I, n
n, I
i,ir
n, I
I, II
II, I
+0.02
+0.02
+0.01
+0.02
+0.02
+0.02
+0.01
+0.02
+0.01
+0.02
1
2
3
4
5
6
7
8
9
10
I, II
n, I
I, II
n, I
I. II
II, I
I, II
II, I
i,n
1,11
+0.02
+0.02
+0.03
+0.01
-0.01
—0.01
+0. 00 !
+0.01
+0.01 1
+0.00 1
320 GEOLOGY OF THE OOMSTOCK LODE.
Steel plugs are therefore not greatly inferior to those of copper or zinc
in cases whei'e a few hundredths of a volt are believed to be of minor im-
jDortance; whereas, on the other hand, their use for the purpose in view is
attended with much convenience. It was found, however, that great care
had to be taken in keeping them bright, as otherwise the electrical difference
between the gads themselves was apt to rise to many times the value given
above. It was also necessary to maintain a thorough contact between the
ends of the metallic circuit and the gads.
Great difficulty was encountered in avoiding leaks in the copper wire
connecting the plugs with the galvanometer. At first wire covered with a
double thickness of cotton and waxed was employed, but proved to be
wholly inadequate. Even gutta-percha wire scarcely offered as complete
an insulation as was desired, in the hot and damp atmosphere of the CoM-
STOCK, when laid in long lines without special precautions. After testing a
number of devices, it was finally found sufficient to suspend the wire from
silk or waxed cotton threads, care being taken to prevent it from anywhere
touching either rock or timbers. This plan of swinging the line was adhered
to throughout, in spite of the loss of time frequently occasioned thereby. In
short, the rule was finally adopted of arranging all the connections just as
though the experiments contemplated were to be made with frictional elec-
tricity.
The galvanometer used in these experiments was an ordinary instrument
with an astatic needle, capable of measuring intensities as small as 0.0001
in "Weber's electromagnetic scale {mg. mm. sec.) with certainty. Readings
were made directly, the needle swinging over a graduated arc.
For the measurement of electromotive forces a method of compensation
was first employed. But in the course of the investigation it was found
absolutely necessary to abandon all complications and to reduce the method
of research to the utmost simplicity. This will be evident to the reader
when he remembers that the heat of the mines is such as to cause profuse
perspiration, and thus seriously interfere with manipulation; that it was
desirable to make the first observations near or on the vein — hence in the
busiest part of the mine — so that expeditious operation was extremely
ELEOTEIOAL ACTIVITY OF ORE BODIES. 321
important; that, finally, the time during which exposure to high tempera-'
tures can be endured with safety is itself necessarily limited. A simple
method, analogous to one of consecutive substitution of two elements in
the same circuit of large resistance, was therefore adopted. If e and E denote
the lode electromotive force and the electromotive force of a normal element,
respectively, i and I the intensities due to the action of e and E^e in the
same circuit, we shall have, approximately,*
= ^, or eznE
E^e~r ~ I^i
Intensities were measured by the aid of the galvanometer above de-
scribed, the instrument having been carefully calibrated at the outstart — an
operation which was frequently repeated during the course of the experi-
ments, i and I could both be determined in the same circuit without
inserting auxiliary resistances.
Results. — By way of example, some of the results obtained in the mines of
the CoMSTOCK will now be cited. The plan has been indicated in a forego-
ing paragraph (page 3 1 6-7). It will be remembered that a permanent contact
placed conveniently in one end of the network of drifts, is successively con-
nected with points in positions of sufficient interest to justify measurement.
In the tables, unless otherwise stated, P. C. is to be understood as coinciding
with point I. The second column contains the distance, in feet, of the
points tapped below the level of the mouth of the shaft as a datum. " Dis-
tance" and "bearing" refer to the imaginary lines connecting P. C. (I)
with the remaining points of the series. An exception is, however, made in
Table VI., where the data contained in corresponding columns give the
horizontal distance and bearing of the lines joining consecutive points e,
the lode electromotive force, is expressed in volts, and is taken as positive
when it acts in the direction P. C. > Earth >- T. C.
'Approximately, because, in the case when the lode electromotive force acts alone, we have not a
true circuit, in the ordinary sense. Between the holes, both in the earth and in the wire, the direction
of the current is the same. But since the resistance of the rock, passing from the hole into the earth,
diminishes rapidly (.see page '^^9), the former may be considered, with a degree of accuracy sufficient for
the purpose, as acting through the same resistance as does the normal element, subsequently inserted.
21 0 L
322
GEOLOGY OF THE COMSTOCK LODE.
Table IV. — Experiments made in the Ophir mine.
[Steel gads.]
No.
Leyel.
Points.
Distance.
Bearing.
e.
Remarlcs.
1
2
3
4
5
«
1
8
9
10
Feet.
2,000
2,000
2,000
2,000
2,000
2,300
2,300
2.300
2,300
2,300
I
n
ni
rv
III, IV
I
n
m
IV
V
Feet.
0
116
170
415
260
0
230
370
415
470
±0.00
+0.02
+0.01
+0.02
+0.01
In quartz seam; barren.
In clay seam.
In quartz seam ; old stope; low-grade ore.
In quartz seam ; new stope ; ore.
In clay seam.
In small quartz seam ; barren.
In small quartz seam ; low-grade ore.
Do.
In quartzose clay.
S. 55° E.
S. 20° E.
S. 42° E.
S. 55° E.
N. 19° E.
N. 19° E.
N. 29° E.
N. 38° E.
+0.04
+ 0.01
+0.05
+0.02
Table V. Experiments in the Consolidated Virginia and California mines.
[Steel gads.]
1
1,750
I
0
+0.00
2
1,760
n
20
e * ° i;
HO o
+ 0.09
All points in the vein ; ledge very broad ;
3
1,750
in
60
Points
verti
above
anotb
+0.01
low-grade ore in quartz gangue.
4
1,750
rv
100
+0.08
Table YI. — Experiments in the Ophir and Mexican mines,
[Copper terminals.]
1
2
3
2,000
2,300
2,300
+0.00
+0 02
+0.03
In small quartz seam ; barren.
Do.
in
100
S.190W.
Do.
4
2,300
rv
100
S.19°W.
+0.04
Do.
5
2,300
V
100
S.19°W.
+0.04
In large quartz seam ; low-grade ore.
6
2,300
VI
80
N. 29° E.
+0.03
Do.
7
2,300
vn
85
X. 38° E.
+0.04
In quartzose clay.
Discussion. — From a comparison of Tables I. and II. vrith Tables IV., V.,
and VI. it appears at once that the electromotive forces due purely to chem-
ical difference and polarization of the terminals are of the same order as the
data expressing the electric activity of the Lode. The latter therefore can
serve no other purpose than that of aflPording information as to the magni-
tude of the forces to be determined. To assure myself as to the certainty
of this conclusion, I made a measurement of the electromotive force (e) ob-
tained by using terminals of copper and zinc conjointly, and found, as a mean
of three experiments,
£=0.82.
In consequence of polarization, the current speedily diminished in
ELECTRICAL ACTIVITY OF ORE BODIES. 323
strength, so that all the phenomena are identical with those which would be
obtained in the laboratory. The eflFect of polarization in distorting the true
value of the lode currents was frequently noticed, but it would be super-
fluous to repeat the data here.
It is necessary therefore, in order to obtain satisfactory results, to apply
all the refinements that have been developed for problems of this character.
In making an attempt of this kind in the mines on the Comstock, however,
unusually great difficulties would be encountered. At the outstart, the fact
that the observer is compelled to operate with wet hands must be considei'ed
as prejudicial to delicate physical experimentation. But there is a more
fundamental difficulty. It will be remembered that the ore of the Comstock
Lode is argentite accompanied by gold, probably in the metallic state, finely
disseminated in quartz. At the time of the experiments the mines without
exception were working in comparatively barren parts of the vein, so that
there was actually more mineral possibly possessing electrical properties
(iron pyrites, etc.) in the rocks than ore in the ore-stopes. In such a case
the term " ore body" is scarcely applicable at all.
The result of circumstances of this kind, regarded from an electrical
point of view, can be expressed as follows: Either there will be no electric
action at all, since each little granule of ore or pyrite may be considered as
surrounded by an insulating envelope of either quartz or country rock —
whether the latter be considered as an insulator or an electrolyte is imma-
terial— or the whole District, vein and rock, is to be regarded as the field
of electric action. In the latter case an equal difficulty occurs, insomuch as
within the limited space open to the observer the variation of potential will
be inappreciable. In short, from the peculiar distribution of mineral matter,
electric excitation is not local in comparison with the space accessible for
experimentation.
The unusual difficulty with which a correct interpretation of results
would be attended, not to mention the loss of time occasioned by the fact
that, in consequence of the heat, experimentation cannot be long continued,
finally induced me to abandon the matter at the Comstock altogether — at
least until definite results could be obtained in a more favorable locality.
324 GEOLOGY OF THE COMSTOCK LODE.
EXPERIMENTS MADE AT THE RICHMOND MINE, EUREKA DISTRICT,
NEVADA.
Opportunities for investigation. — III determining to make the study of local cur-
rents a part of the work to be done under his charge, Mr. Becker* had
selected both the Comstock Lode and the Eureka district as available local-
ities, in which to test the applicability of an electrical method as an aid to
prospecting. The former is a fissure vein, in which the ore, comparatively
free from base material, is scattered irregularly through a quartz gangue.
At Ruby Hill, Eureka, the ore is principally plumbic carbonate and sul-
phidp and oxide of iron — the whole containing more or less silver and gold —
occurring, moreover, in huge, apparently isolated masses in limestone. In
most of the cases fissures containing vein matter and connecting the cham-
bers have been traced. The facilities offered for the prosecution of the
investigation by the Eureka deposits were therefore, to all appearances,
unusually great. The immense ore bodies in sight were furthermore at a
mean distance of not more than 400 feet from the surface, and a series of
electric surveys could easily be carried out over, through, and under them.
Finally, it appeared not at all improbable, insomuch as the ore bodies in
places extend to within 100 feet from the surface, and are in fact to some
extent above the mean surface of the suri'ounding country,^ that local elec-
trical currents might actually be detected on the surface itself In consid-
eration of this encouraging prospect due pains were taken to work up all
the experimental details with corresponding care.
Arrangement of terminals. — Above all thiugs it was uecessary to devise some
method of obtaining electric contact between the ends of the metallic cir-
cuit and the rocks, which would be free from the difficulties met with in
the Comstock. Metallic plates, etc., used alone, ai"e objectionable (see page
358) ; but it is clear that through the intei'vention of a suitable liquid, effects
of polarization, etc., can be avoided. The following contrivance, based on
' C/. First Annual Report U. S. Geolog. Survey, p. 46, 1880.
'Being in Ruby Hill, an elevation of some hundreds of feet .ibove Ihe extensive plain partially
HurroHiKling it.
ELECTRICAL ACTIVITY OF ORB BODIES.
325
the well-known fact of the excellence of amalgamated zinc in a zinc sul-
phate solution, for the purpose in question, was finally adopted.
Into a large cork a,' Fig. 23 (longitudinal section), is inserted a strip
of amalgamated zinc, e/, about one-half inch
broad, to the top of which, e, a gutta-percha-
covered copper wire, hik, is soldered. Through-
out the greater part of its length it rests against
a stick of wood, cd, cylindrical above at c, which
end is to be thrust through a perforation in the
cork a, but wedge-shaped below, d. At i the
wire and stick are firmly tied together. A
smaller cork, b, secures the lower end of both
zinc and stick. The whole is surrounded by a
piece of beef-gut, gg (free from salt), tied to the
corks a and b, as shown in the cut.
Into the bag (6 to 10 inches long) thus
formed is poured a solution of zinc sulphate,
the wooden plug I being for this purpose re-
moved and a small funnel inserted. On replac-
ing the plug the terminal is ready for use. The
object of the stick is to obviate accidents due
to breakage of the zinc, this material becoming
very brittle by amalgamation.
Fig. 24 represents the terminal in place. A suitable hole, 6 to 9 inches
deep and 1 to 1 J inches in diameter, is drilled into the rock or vein, at an
angle of about 30° with the vertical, and filled with a solution of sodic sul-
phate or water; whereupon the bag is introduced as shown in the figure.
The dotted line mn indicates the level of the outer liquid.^ Solution of sodic
sulphate was at first used, because it increases the conductivity and is not
acted upon appreciably by the rock (limestone). It was found, however,
that ordinary water, which had previously been placed in contact with zinc
for some time, so as to precipitate all dissolved matter which might act upon
' 1 to li inches in diameter.
' The siilutioa poured into the hole will be referred to throughout this description as the " outer
liquid."
Fig. 23. — Terminal, longitudinal
section.
326
GEOLOGY OF THE COMSTOCK LODE.
it, was preferable (see page 357). When not in use the bags were kept
in a glass vessel containing a zinc sulphate solution; during the obser-
vations, however, they were transported from place to place in jars con-
taining water.^
The electromotive force between two similar bags placed in the same
external liquid was seldom
found to be greater than 0.005
volt, usually much less, and tol-
erably constant (see page 362);
whereas the electromotive force
of polarization, due to the ac-
tion of a Daniell under circum-
stances actually met with in the
mines, a number of data being
in hand, was in no case as large
as 0.001 volt and in the experi-
ments cited falls below this
limit. For comparison the bags
in a particular instance were
filled with water instead of zinc
sulphate, when an electromo-
tive force of polarization of
0.020 volt was obtained.
Fig. 24.-TerminaI in position. Wir=. — Gutta-percha-COV-
ered wire No. 19, of excellent quahty (Tillotson & Co., New York), was
used almost exclusively, the whole circuit nevertheless being suspended in
air from threads, as in the Comstock. In the long circuit on the 600-foot
level it was necessary, however, to employ cotton-covered wire for part
of the line, the supply of the other being insufficient. This could be
done without disadvantage, as follows: A hollow cyHnder of gutta-percha,
stripped from the end of a wire covered with this substance, was bent in
the form of a loop, Fig. 25, and kept bent by a thread passed through its
' It was desirable during the observation to have the outside of the bag as free irom zinc sulphate
solution as possible.
ELEOTRIOAL ACTIVITY OF ORE BODIES. 327
interior and tied. The cotton-covered wire used (a b in figure) was passed
through this loop, suspended by the other end of the thread.
A case in which gutta-percha-covered wire trailed on the ground a
distance of about 1,000 feet, was made the subject of measurement. A leak
was quite perceptible; the insulation offered, however,
was about 1,000,000 ohms.
In extending the line from point to point, accord-
ing to Reich's very convenient plan, the wire is wrapped
on a light wooden reel, but in such a way that the
inner end also remains accessible. The outer end being
in connection with the measuring apparatus, enough
wire is uncoiled to reach the desired hole, and a con- fig. 25.— Suspension.
nection (contact-bag) between this and the inner end of the wire is then
made. In the damp atmosphere the reel soon became saturated with
moisture, and, in spite of the insulation of the wire, care had to be taken to
insulate the former also.
Galvanometer. — For tho measuremcut of intensity I was fortunate in secur-
ing a magnificent instrument, made for me after the Wiedemann pattern, by
Mr. "Wm. Grunow, of New York. This instrument is exceedingly conve-
nient for the purpose, as by an adjustment of the coils the sensitiveness can
be varied over a very wide range. Readings were made with telescope,
20
mirror, and scale. In the adjustment adopted currents as small as —^
webers could be detected with certainty.
Measurement of electromotive force. The simple method of COUSeCUtivC Sub-
stitution for the measurement of electromoti v^e forces ( ezzE . \ — i
somuch as while there were no reasons for abandoning it there were a great
many in its favor — was adopted here as on the Comstock. The coils of
Grunow's galvanometer could easily be so placed as to enable the observer to
measure with sufficient accuracy both the lode current and that due to the
latter and the normal electromotive force conjointly, without making any
change at the instrument or inserting auxiliary resistances. By means of an
inclosed mercury commutator the current in the galvanometer could be
ui-
328 GEOLOGY OF THE COMSTOGK LODE.
reversed and the deflection thus doubled. All intensities (i and I) were
determined as a mean of five consecutive commutations — not that it was
desirable or necessary to increase the accuracy by such a process, but
because it appeared essential not to hurry the measurements and to test the
constancy of the current as appeax-ingin the five data obtained. Errors from
condensation of moisture on the commutator were avoided by excluding the
latter entirely from time to time, the measurements being made by simply
connecting the wires with clamp-screws.^
As a matter of especial importance it will be necessary to consider a
scheme of operations by which discrepancies due to extraneous causes can
be eliminated as comjiletely as possible. In the experiments the following
order of observations was adopted and rigidly adhered to throughout:
1. Measurement of the apparent intensity of the lode current (i').
2. The same, with the terminals exchanged (i").
3. Measurement of the current produced by the normal element and
lode conjointly (J).
4. With the battery left in place the circuit is broken at the temporary
contact; no deflection must ensue (E supposed to be acting with the lode
electromotive force).
If a mean of the intensities derived from the first and second opera-
tions [i=: J (*"+*')] ^6 taken, the intensity of the current (i) due to the
lode only will be obtained. That due to difi'erences in the amalgamated
zincs is thus eliminated. In by far the greater number of experiments three
exchanges were made, so that the first and third positions of the terminals
were identical. Analogously, then,
' = i
(-+"^'->
The fourth operation in this scheme insures the perfect insulation of
the circuit between the T. C. and the galvanometer. The part between
the latter and P. C. — the two being always placed in close proximity, this
'The commatator used wa8 made of wood boiled in linseed oil, and supported on three conical
feet of wood boiled in wax and resin. The holes, moreover, were coated with a thick layer of wax
(see page 354). Whole seta of observations had to be discarded on account of the insufficient insulation
of an earlier apparatus.
ELECTRICAL ACTIVITY OF OEE BODIES.
329
partial circuit, moreover, remaining fixed — is tested once for all before com-
mencing the experiments.
It is often desirable, before inserting the Danieli, to determine whether
the circuit is in order and without a break. This may be easily accom-
plished by touching with the finger a copper part of it, so that a secondary
circuit, T. C, wire, galvanometer, wire, body, earth, T. C, or P. C, wire
body, earth, P. C, is produced, respectively. The electromotive force acting
in this case is that of zinc-copper, but in consequence of the very large
resistance of the finger contact the current, though distinctly perceptible,
is too weak to produce any appreciable polarization.
In spite of all these safeguards, however, a close inspection of the
recorded values still revealed discrepancies which had not been avoided.
Accordingly the method of procedure was further improved by the follow-
ing additions: To eliminate as much as possible the effect due to the terminal
bags, a variation was introduced by which the results from different bags
could be compared. Four of these. A, B, C, and D, were generally employed,
which, when combined, two and two, in the manner shown in the diagram,
gave three separate and distinct values for the lode electromotive force e.
The electromotive force between any two bags, A and B, is represented h\
AB, between A and C by ^|C, etc.
Holes.
P.O.
A
A
A
T.C.
Electromo-
tive force.
P.O.
T.C.
Electromo- i
live force.
P.O.
T.C.
Electromo-
tive force.
First series . - .
Second series . .
Tliird series . - .
Ji
0
D
e±A\B
e±AiO
e±AD
B
0
D
A
A
A
e-fA\B
e^A\C
e^A D
A
A
A
B
C
D
etAB \
e*AC i
e^zAD
Original positioo.
First exchange.
Second exi'hiinKc
After the second exchange, the bags again have their original posi-
tion with reference to the holes. The corresponding measurements, there-
fore, check one another, while from their mean any linear variation of their
own electromotive force is eliminated. Each series gives a value for e.
With this method of triple measurement the series was completed by deter-
mining all the electromotive forces between P. C. and each of the T. C.'s,
starting with the one nearest P. C. and ending with the most remote. After
this the whole set was again repeated, starting, however, with the extreme
330 GEOLOGY OF THE COMSTOCK LODE.
T. C. and finishing with the one nearest P. C. The two sets, therefore, form
a symmetrical series, and from the means of all the values corresponding
to any particular T. C. any change which may have taken place in the hole
P. C. (see page 360), as well as in the electromotive force of the Daniell,
may be regarded as practically eliminated. A comparison of the two sets,
moreover, affords a good criterion of the constancy of the currents as well
as of the trustworthiness of the results obtained in general.
Resistance. — Besides the electromotive force, the resistance of the differ-
ent circuits was also measured, being an item of interest. The values usu-
ally ranged between 2,000 and 3,000 ohms, though at times they went as
high as 20,000, or as low as 700 ohms. Almost the whole resistance of the
circuit is encountered by the current in passing from the wire into the rock,
and from the latter back again into the former. In other words, the resist-
ance of the layers of rock immediately surrounding F. C. and T. C. is so
large that in comparison with it that of the rest of the circuit (never greater
than 20 ohms) can be completely neglected The total resistance is, there-
fore, essentially the sum of two terms, corresponding to the holes, respect-
ively. Suppose now that in a circuit P. C. (T. C.) these partial resistances
are w and r, respectively; in a circuit P. C. (T. C.)', w and r', respectively;
if it is found, experimentally, that
w-\-r=:a, ') \
w-\- r'— h, \ , and if s — a-{-b-{- c, then {■)■'=: ^ — a,
r+r'=c,) )
\w^- — c.
\ 2
These points have been described in considerable detail, being of such
importance that without them the results reached would be illusory. I was
twice obliged to discard whole sets of experiments because one or the other
of the disturbances set forth had found their way into the results in the most
insidious manner. It is true that Fox actually used uncovered wire; but
it must be remembered that the currents obtained by him were abnormally
large. Moreover, I am convinced that the currents found by Fox, when
connecting two different points in rock, were entirely due to, and that those
BLBCTRIOAL ACTIVITY OF ORB BODIES.
331
of Reich were very largely distorted by, discrepancies of the kind discussed
in this paragraph.
Relative position of the ore bodies. — Bcfore procecding furthcr, it will be neces-
sary to give the reader a general idea of the disposition of the ore bodies of
the Richmond mine. It will be convenient, and fully sufficient for the
present purposes, to consider them with reference to a horizontal and a ver-
tical projection. The former will be given with the different sets of obser-
vations which are to follow. For the latter I am indebted to Mr. R. Rickard,
superintendent of the Richmond Mining Company, without whose cordial
cooperation it would have been impossible, in the time allotted, to carry out
these experiments. To Mr. Rickard are also due the following details and
sketch
200'
400-
500'
600'
Fig. 26. — Vertical section through ore bodies.
In Fig. 26 the horizontals passing across the diagram represent the
levels in feet below the shaft-mouth as a datum. Different ore bodies are
differently shaded, the attached numbers depending upon the date of their
discovery. The sketch is intended to illustrate the relative positions of the
ore bodies one to another only, as seen from the extreme north.
Chamber No. 11 begins on the 200-foot level and continues to the 500-
foot level.
332 GEOLOGY OF THE COMSTOCK LODE.
No. 12 is a continuation of No. 11, beginning on the 500-foot level
and ending 70 feet below this level.
No. 16 commences 50 feet above and runs 70 feet below the 200-foot
level; the bottom of the present workings.
No. 15 commences on the 300-foot level and continues to the 500-foot
level.
No. 14 begins 50 feet above the 400-foot level and continues to within
50 feet of the 600-foot level.
No. 13 begins at the 500-foot level and continues 50 feet below the
600-foot level.
Chambers Nos 13, 14, and 15 are all connected and form one ore
body. No. 16 will undoubtedly connect also with these three, so that in
fact Nos. 13, 14, i5, and 16 are but lobes of one and the same huge deposit.
The greatest horizontal extent of these bodies is between the 400 and
500-foot levels, the plan showing the following dimensions:
N.toS. 520 feet.
E.to W. .600 feet.
No. 7 extends from the 400-foot level to 50 feet below this level.
No. 10 begins 20 feet above and ends 50 feet below the 400-foot level,
and is exhausted. No. 1 3 also is partially exhausted.
East of the group of ore bodies of the Richmond Company are those
of the Eureka Consolidated Company, which are also of unusually large
dimensions, the ore being the same in every respect.
Experiments on the 500 and 400-foot levels. — These scries of measurements were made
with the intention of observing the variation of potential met with in pass-
ing through the ore body, the line of electric survey beginning and termi-
nating in points as far distant from it as was practicable.
The plan of the position of the drifts on the 500 and 400-foot levels
relatively to the ore chambers, so far as is necessary for the present purposes,
is given in Fig. 27, on a scale of ~. Starting with the shaft at m, the drifts
are represented by broad black lines. The main drift on the 400-foot level,
passing from a point between VIII. and IX. on that level in an approxi-
mately semicircular path toward the shaft, has, as well as other workings,
ELECTRICAL ACTIVITY OF ORE BODIES.
333
been partially or wholly omitted. Instead of giving an outline of the hori-
zontal projection of the ore bodies themselves, it was thought preferable
to represent rather the position and extent of the actual workings. On the
map, chamber No. 11 is designated by ab, No. 12 by CD, Nos. 13 and 14
by rS, and No. 15 by tg. The position of chambers Nos. 7 and 10 is only
Fig. 27.— Plan of the 400' and .500' levels. Scale rihu-
indicated. Smaller patches of ore also occur at n, between the .500 and
600-foot levels, and at P, above and below the 500-foot level.
uv, on the 400-foot level, marks the position of a line of contact be-
tween shale and limestone. It may be remarked that the shale of the
334 GEOLOGY OF THE COMSTOCK LODE.
west country intersects the 400-foot level on a line approximately parallel
to the drift between P. C. and No. IV.
Unfortunately, local circumstances rendered it absolutely impossible
to make this survey in a single continuous series, however desirable such a
method of procedure would have been. But the object was accomplished indi-
rectly by selecting a permanent contact both on the 400 and on the 500-foot
levels, and carrying the two lines of measurement onward to the same inter-
mediate point. The differences of potential thus obtained from two fixed
points, respectively, can then be converted by a simple method of reduction
into those which would have been obtained had all the electromotive forces
been measured from one and the same P. C.
On the 500-foot level the permanent contact was placed in chamber
No. 12, in calcareous earth stained with iron, its position coinciding nearly
with the letter G in the plan of this chamber (Fig. 27, C. D.). The points
selected as T. C.^s are designated on the map by small circles, to which
Roman numerals are annexed, and extend from I., near the shaft m on the 500-
foot level, in a more or less broken line to XV., in chamber No. 15, about
30 feet below the 400-foot level. The following table will describe them
more completely. Column 2 in Table VII. contains the points, some of which,
to prevent confusion, were omitted on the map; column 3, the depth of each
below the mouth of the shaft, taken as zero. "Distance" refers to the length
of the lines joining consecutive points for which data are given.^ The
figures under "bearing" are to be similarly understood. (S. 81° W. refers
tothelinel.-IIL; S. 26°W.,toIII.-V.; N.67° W.,toV.-IX.,etc.) Itappeared
unnecessary to give more than the bearings of the main lines of direction
on which the points approximately he. The figures included under "re-
sistance" are the means of two determinations of this quantity made for
each of the points. They express the sum of the resistances of the rock
surrounding P. C. and the T. C. specified. The original results were always
greater than those made at a subsequent time; this from the fact that the
rock in the neighborhood of P. G. and T. G. became, during the progress of
the experiments, gradually more saturated with moisture.
' The points for which no data are given are distributed through various parts of chambers 12 and
1.5, in positions for wliich it was difficult to make measorements.
ELEOTEICAL ACTIVITY OJP ORE BODIES.
Table VII.
335
No.
Points.
P.C.
I
U
in
IV
V
VI
vn
VH'
vm
rx
X
XI
XI'
XII
xni
XIV
XVI
XVII
XV
Leyel.
500
500
600
500
500
500
600
600
500
500
600
500
500
490
480
460
450
460
440
430
Dis-
tance. I
Bearing.
Besist.
ance.
Feet.
0
84
13
47
101
101
94
Origin.
S. 81° W.
S. 26=> W.
N. 67° W.
S. 80° W.
S. 37° "W.
Ohms.
3620
1480
1560
1660
1670
970
2090
850
1520
1590
5660
3720
2240
3590
2620
1990
1140
6260
990
Kemarks.
\ Chamber No. 12.
Ferrnpnous, calcareous eartli, in chamber 12.
Hard, fissured limestone.
Limestone, compact, porous, moist.
Do.
Do.
Do.
Limestone, very porous, near contact of chamber
Ferruginous earth.
Red ocher, near bunch of ore
Ferruginous earth ,
Pocket of lead carbonate ore in limestone,
Hard, impervious limestone and calcspar.
Hard, solid limestone.
Ferruginous earth, with galena
Black iron ore, loose dry
Ferruginous earth, with galena
Ferruginous earth, without galena
Large breast of lead carbonate ore
Ferruginous earth, very dry
Large breast of lead carbonate ore
The results of the measurements of electromotive force between P. C.
in chamber 12 and the consecutive T. C.'s are given in Table VIII. The
general method of obtaining them has already been described (see page 329).
Intensities (i) are given in absolute electromagnetic units (C. G. 8.) ; electro-
motive forces (e) in volts, and these are arbitrarily considered positive
when the potential of T. C. is the greater, or when the lode current flows
(wire}
/earths
T. C.
->p. a
It will be remembered that, throughout, four terminal bags. A, B, C, JD,
were used. The results obtained with AB are given in Series I., where,
moreover, i' is the intensity observed with the bags A and B in any partic-
ular position (say A in hole P. C, and B in T. C.) ; i" the intensity observed
when the bags are exchanged (B in hole P. C, and^ in hole T. C); finally,
i'", the observed intensity when the bags again have their original position.
e is the corrected lode electromotive force between P. C. and the T. C.
specified. Series II. contains the corresponding results with the bags A and
C ; Series III., with A and D.
Finally, Series I., II., and III. were obtained in surveying from point
336
GEOLOGY OF THE COMSTOCK LODE.
I. to XV., series IV., V., and VI., on the other hand, on returning from XV.
back to I
In these experiments a solution of sodic sulphate was used as an outer
liquid.
Table VIII.
FIRST SERIES.
No.
P.C.
connected v' X 10»
with— 1
I
t" X 10»
v'" X 10»
«X10»
P.C.
No. connected
with—
j
t'xic
i" X 10«
1
1'" X 10" . « X lO"
1
2
3
4
1
I 1 + 16
n 1+41
in 61
- 10
- 26
_1_ flR
+ 13
+ 1
+ 1
± 0
± 0
+ 3
+ 2
11 ' X
X n
- 25
4- a
7
12 i XI -35
— 6
13 i XI' +41-46
14 XII -49+5
15 ! XIII - 25 - 25
16 ! XIV 1-5 - 12
17 '• XVI ' + 35 +10
± 0
TV J_ ifi 11
- 8 !
6 V -33+71
6 i VI +89 - 66
7 VII +59 - 39
8 VII' +7 - 57
9 i Vin 1 + 35 - 41
in 1 TV 1 117 41
- 30
+ 102
— 6
- 2
+ 3
- 1
± 0
- 13
19 ! XV
+ 112 j + 95
+ 121
+ 1.
1 i
1
SECOND SERIES.
1
2
3
4
6
6
7
8
S
10
I
n
m
IV
V
VI
VII
VTI'
vin
IX
8
30
53
39
31
71
39
0
36
120
+
+
33
46
61
33
26
57
49
38
+ 102
+ 43
- 5
- 1
11
± 0
12
- 2
13
- 1
14
+ 2
15
+ 3
16
+ 2
17
1 - 3
18
- 1
19
- 13
X
XI
XI'
xn
xm
XIV
XVI
xvn
XV
+ 2
- 38
+ 49
- 43
- 16
- 13
+ 53
- 3
+ 82
- 28
+ 25
- 56
+ 12
- 33
± 0
- 10
- 2
+ 118
+ uo
7
3
1
«
6
1
2
- 1
+ 11
+
THIRD SERIES.
I
n
m
IV
V
VI
vn
VII'
VIII
IX
12
38
33
33
- 11
+ 97
+ 54
+
13
34
31
46
- 34
+ 10 ■ -
+ 26
- 05
46
46
15
- 11
+ 105
+ 67
+ 5
+ 2
+ 2
- 2
- 1
- 11
11
12
13
14
15
16
17
18
19
X
XI
XI'
xn
- 5
- 26
+ 44
- 51
xm
- 20
XIV
- 8
XVI
+ 36
xvn
- 0
XV
+ 103
- 25
- 8
- 49
+ 11
- 28
- 11
+ 10
- 3
+ U8
8
6
0
7
6
2
3
- 1
+ 11
+
ELECTRICAL ACTIVITY OF ORE BODIES.
.337
Table VIII— Continued.
FOURTH SERIES.
No.
P. C
connected
with —
I
II
ni
IV
V
VI
vri'
VIII
i' X io»
+ t
- 26
+ 8
+ 16
+ 57
+ 116
+ 33
+ 8
i" X 10'
± 0
+ 25
- 8
- 16
+ 28
+ 66
+ 16
+ 15
%"' X 10«
+ 79
ex 103
No.
P. C
connected
■with-
IX
X
XI
XI'
xn
xnt
XIV
1' X io»
81
20
31
2
25
31
28
i" X 10«
57
31
16
11
15
3D
0
i'l' X 108
ex 10'
- 11
- 14
- 9
- 1
- 7
FIFTH SERIES.
1
2
3
4
5
6
7
I
II
m
V
VI
VII'
vrn
+ 7
- 23
+ 16
+ 56
+ 105
+ 36
+ 0
- 7
+ 20
- 13
+ 23
+ 51
+ 13
+ 10
± 0
+ 0
± 0
+ 6
+ 7
+ 2
+ 1
8
9
10
11
12
13
14
IX
X
XI
XI'
xn
xni
XIV
- 79
- 21
- 31
+ 2
- 20
- 38
- 16
- 61
- 31
- 18
- 18
- 18
- .20
- 15
- 11
- 14
- 9
- 2
- 7
- 7
- 3
+ 84
SIXTH SERIES.
I
II
in
V
VI
VTI'
vni
+ 0
- 25
+ 26
+ 56
+ 102
+ 34
+ 15
+
+
+ 51
+ 21
+ 90
IX
X
XI
XI'
xn
xm
XIV
72
25
26
3
26
20
5
72
31
20
23
16
25
25
11
15
8
2
7
7
400-foot level. — The permanent contact on the 400-foot level was placed m a
ferruginous clay seam, toward the southern end of the drift, and observa-
tions were made in a northerly direction from this point. The temporary
contacts have been designated on the map (Fig. 27), as in the previous case.
Point X. of the present survey coincides in position with XV. of the line on
the 600-foot level. The following table (IX.), in which full statements of
the position, etc., of the points are contained, will be intelligible without fur-
ther description. As before, the bearing of the main linear loci only have
been determined, the data referring to the lines joining the consecutive
points, for which figures are given. Resistances, as above, are mean values
22 c L
338
GEOLOGY OF THE OOMSTOCK LODE.
for the circuits P. C, earth, T. C, wire, P. C, and are essentially the
resistances of the layers of rock surrounding P. C. and T. C.
Table IX.
No.
Points.
Level.
Dis-
tance.
Bearing.
Eesist-
ance.
Eemarks.
1
2
3
4
5
6
7
8
9
10
n
P.O.
I
II
m
IV
V
VI
vu
Vlil
IX
1 X
Feet.
400
400
400
400
400
400
400
400
400
400
430
Feet.
0
100
140
139
85
37
88
94
89
68
37
Origin
Ohms.
2B90
1040
1820
710
2050
1760
2740
1280
1820
1030
Eed clay selvage.
Black, fissured limestone, dry.
"White calcareous pulp, very moist.
Gray limestone, compact, dry.
Shale, very moist.
Gray, fissured limestone, dry.
Limestone, compact.
Do.
Quartzite, very wet.
Bunch of lead carbonate ore in limestone.
Large breast of lead carbonate ore, chamber 15.
1^.7°^.
If.49oE.
N.71°B.
The results of the measurements of electromotive forces between P. C.
and I.-X. are contained in Table X. They are given in a way entirely
analogous to that adopted for the 500-foot level, and no further explanation
is necessary. Intensities are expressed in electromagnetic units (C G. S.),
electromotive forces in volts. Water was used as an outer liquid.
Table X.
FIRST SERIES.
No.
P.C.
connected
with —
i' X 10'
i"iy 10»
i'" X 10«
ex 103
No.
P.C
conn»Ci«d
with—
i'XlO*
i" X 10»
i'" X 10»
exlO>
1
2>
3
4
6
I
n
m
IV
V
+ 22
+ 54
— 49
+ 0
+ il
+ 35
+ 11
— 5
+ 10
— 7
+ 1
6
7
8
9
10
VI
vn
vm
IX
X
+ 125
+ 56
+ 34
+ i
— 30
+ 93
+ 50
+ 43
— 6
— 56
+ 116
+ 19
+ 15
+ 4
± 0
— 4
+ 52 +61
± 0
+ 2
+ 9
— 21
SECOND SERIES.
1
I
+ 22
+ 39 +34
+ 10
6
Vt
+ 118
+ 88
+ 116
+ 19
2>
3
n
III
vn
+ 60
+ 15
+ 45
+ 37
+ 15
+ 3
+ 65
+ 43
+ 63
+ 10
8
vm
+ 4
4
IV
— 71
— 130
— 67
— 7
9
IX
— 4
+ 9
— 6
± 0
5
V
+ 8
± 0
+ 13
+ 1
10
X
— 63
— 62
— 37
— 6
1
' In these cases three consecutive exchanges of the terminals were made, their positions in Nos. 1 and 3 and in l^os. 2
and 4 being the same.
ELECTEICAL ACTIVITY OF ORE BODIES.
Table X — Continued.
THIRD SERIES.
FOURTH SERIES.
339
No.
PC.
connected
with —
i'X10«
i" X 10»
i'" X 10»
ex 10'
No.
P. C.
connected
with—
i' X 10«
i" X 10«
i"' X 10«
eX10»
1
2>
3
4
5
I
n
ni
TV
V
+ 37
— 9 1 —
+ 77
— 73
+ 11
+ 30
47 1 — 28
+ 50
1 — 37
+ 11
— 4
+ 10
— 8
+ 1
6
7
8
9
10
VI
Vii
vni
IX
X
+ 138
+ 67
+ 4
+ 4
— 13
+ 82
+ 37
+ 28
— 6
— 73
+ 140
+ 21
+ 15
+ 2
± 0
— 4
+ 30
— 160
— 7
+ 80
— 52
+ 22
+ 6
+ 13
— 0
1
I
+ 41
+ 22
+ 43
+ 9
6
VI
+ 140
+ 88
+ 142
+ 20
2
II
— 76
— 142
- 88
— 13
7
vu
+ 76
+ 58
+ 84
+ 19
3
in
+ 86
+ 56
+ 82
+ 13
8
vm
+ 62
+ 13
•f 45
+ 4
4'
IV
—118 1 -
130 ] —120
1 —183
— 10
9
IX
± 0
— 21
— 2
— 2
5
V
+ 35
— 15
+ 24
+ 1
10
X
— 30
— 66
— 24
— 5
FIFTH SERIES.
1
I
+ 39
+ 28
+ 41
+ 9
6
VI
4- 108
+ 108
+ 104
+ 19
2
n
— 91
— 130
— 112
— 13
7
vn
+ 60
+ 71
+ 62
+ 18
3
ni
+ 87
+ 62
+ 82
+ 14
8
vni
+ 19
+ 41
+ 4
+ 3
4"
IV
—153 j —
45 1 — 4£
1 —220
— 8
9
IX
— 15
— 9
— 22
— 2
5
V
+ 7
+ 6
+ 0
+ 1
10
X
— 52
— 34
— 60
— 6
SIXTH SERIES.
1
I
+ 45
+ 22
+ 49
+ 9
6
VI
+ 134
+ 78
+ 121
+ 18
2
n
— 67
— 130
— 80
— 12
7
VU
+ 80
+ 45
+ 76
+ 17
3
m
+ 82
+ 50
+ 82
+ 13
8
VIll
+ 58
+ 4
+ 45
+ 3
4'
rv
- 91 1 -
108 1 —112
1 -168
— 9
9
IX
+ 7
— 28
+ 4
- 2
5
V
4- 24
— 15
+ 21
+ 1
10
X
- 13
— 67
— 11
— 4
^ In these cases three consecative exchanges of the terminals were made, their positions in Nos. 1 and 3 and in Nos. 2
And 4 being the same.
The values for electromotive force contained in Tables VIII. and X.
are now to be referred to one and the same origin. For this purpose it
will be convenient to select a point having an extreme position Point I.,
50()-foot level, is of this kind. As there is no means of assigning an abso-
lute value to the potential of this point, it may be arbitrarily called zero, in
which case the electromotive force between it and any succeeding point will
be identical with the potential of the latter. In the following table (XI.)
the potentials of all the points on the 400 and 500-foot levels have been
340
GEOLOGY OF THE COMSTOCK LODE.
calculated, that of No. I. (500-foot level) being zero. The values obtained
from the different series are designated by indices (e', e", e'", e", e^, e^'). e^
is the mean of the first three, 62 of the last three ; and e the mean of all the
series.
Table XL
No.
Points.
Level.
e' X 105
e" X 103
e>" X 103
ei X 103
e" X 103
e'X 103
e" X 10'
CjX 103
ex 103
Feet.
1
I
500
+ 1
- 1
+ 0
± 0
+ 1
± 0
— 1
± 0
+ 0
2
n
500
+ 1
± 0
± 0
± 0
± 0
± 0
± 0
+ 0
± 0
3
ni
500
± 0
— 2
± 0
+ 1
± 0
± 0
± 0*
• ± 0
+ 0
4
IV
500
± 0
— 1
± 0
± 0
± 0
± 0
+ 0
5
V
600
+ 3
+ 3
+ 3
+ 3
+ 5
+ 6
+ 6
+ 6
+ 5
■<
VI
500
+ 2
+ 3
+ 2
+ 2
+ 8
+ 7
+ 7
+ 7
+ 4
VII
500
+ 2
+ 2
+ 2
2
+ 2
+ 2
± 0
8
VII'
500
- 2
_ 3
— 2
+ 2
+ 2
+ 2
+ 2
9
VTTT
500
— 0
— 1
- 1
— 1
+ 2
+ 1
+ 2
+ 2
± 0
10
IX
500
—13
—13
—11
-12
-11
—11
-11
-11
—12
11
X
600
— 7
- 7
-8
- 7
-14
—14
-15
—15
—11
12
13
XI
XI'
500
490
— 6
± 0
- C
± 0
— 6
± 0
— 9
- 1
— 9
- 2
— 8
— 2
- 9
2
- 7
- 1
- 1
14
XTT
■ 480
- 8
- 6
— 7
— 7
— 7
- 7
- 7
— 7
- 7
15
xin
460
— 6
— 6
— 6
— 6
- 8
-7
- 7
— 7
- 6
10
XIV
450
- 2
— 1
_ 2
— 2
- 3
— 3
- 3
- 3
- 3
17
XVT
450
+ 3
1
+ 2
_ 1
+ 3
_ 1
+ 2
— 1
+ 2
— 1
18
XVII
440
19
X' or XV
430
+ 11
+ 15
+ 11
+u
+15
+11
+ 15
+11
20
IX or XVIII
400
+15
+13
+ 13
+13
+ 13
+ 14
21
VIII or nx
400
+ 19
+18
+ 17
+ 18
+19
+18
+19
+19
+18
22
VCCor XX
400
+30
+30
+30
+ 30
+34
+33
+32
+33
+32
23
VI or XXI
400
+35
+34
+36
+35
+35
+34
+33
+34
+35
24
V or XXII
400
+16
+16
+16
+16
+ 17
+ 16
+ 16
+ 16
+16
25
IV or XXUI
400
+ 8
+ 8
+ 7
+ 8
+ 5
+ ^
+ 6
+ 6
+ 7
26
ni or xxrv
400
+25
+25
+25
+25
+28
+29
+28
+28
+27
27
II or XXV
400
+ 10
+10
+ 11
+10
+ 2
+ 2
+ 3
+ 2
+ 7
28
I or XXVI
400
+ 26
+26
+26
+26
+24
+25
+25
+25
+25
29
P. c. or xxvn
400
+15
1
iTo facilitate tlie construction of Fig. 28, current numbers have been given to the points on ihc 400-foot level. Thenew
numbers are given with the original ones.
Table XII. has been prepared to show the character of e as a function
of distance (see page 342). In it e has the same signification as in the
preceding table. Under distance, however, is given the length in feet of
the imaginary line joining Point I. with the point to which the datum refers.
The data included under bearing also refer to this line. Current numbers
have been given to the points on the 400-foot level. (See "Points," Table
XL)
ELECTRICAL ACTIVITY OF OEE BODIES.
Table XII.
341
No.
Points.
LeveL
Dis-
tance
&om I.
Bearing.
eX10»
No.
Points.
Level.
Dis-
tance
from I.
Bearing.
eXlOS
Feet.
Feet.
1
I
500
Origin
+
0
16
XIV
450
635
S. 72° W.
— 3
2
n
500
84
S. 82° W.
±
0
17
XVI
460
600
S. 69° W.
+ 2
3
in
500
123
S. 81° W.
±
0
18
XVII
440
640
S. 75° W.
— 1
4
IV
500
168
S. 63° W.
+
0
19
XV
430
700
S. 72° W.
+ 11
5
V
500
216
S. 530 "W".
+
5
20
xvm
400
735
S. 72° W.
+ u
6
vr
500
228
S. 54° W.
+
4
21
XIX
400
805
S. 71° W.
+ 18
7
vn.
500
268
S. 60° W.
+
2
22
XX
400
890
S. 73° "W.
+ 32
8
VII'
500
275
S. 64° W.
4;
0
23
xxr
400
980
S. 71° W.
+ 35
9
vni
500
300
S. 70° W.
±
0
24
XXII
400
1066
S. 71° "W.
+ 16
10
IX
500
318
S. 77° W.
_
12
25
XXTII
400
1108
S. 70° W.
-1- 7
11
X
500
420
S. 78° W.
—
11
26
XXIV
400
1228
S. 65° W.
-1- 27
12
XI
500
515
S. 78° W.
—
7
27
XXV
400
1184
S. 69° W.
+ 7
13
XI'
490
595
S. 78° W.
—
1
28
XXVI
400
1276
S. 54° W.
+ 25
14
xn
480
600
S. 79° W.
_
7
29
XXVII
400
1332
S. 51° W.
+ 15
15
XTTT
460
610
S. 79° W.
—
6
Discussion of tlie results obtained on the 400 and 500-foot levels. FrOm & COmparisOIl of tllG
resistances of circuits between diflferent holes, as contained in Tables VII.
and IX., we find that in cases of fissured, of tough and impervious, or of
dry rock or earth, this quantity inclines toward a maximum ; whereas, on
the other hand, wherever the material is porous or moist minimal values
are obtained. It is to be remembered that under ground, from the exceed-
ingly damp atmosphere, as well as from infiltration of water, the rock form-
ing the walls of the drifts is throughout very moist, and at the surfaces of the
latter, at least, nearly saturated. Hence it follows that the conductivity of
the rock is largely, if not wholly, due to the presence of moisture in its
pores, and is therefore electrolytic. This important fact will be repeatedly
referred to hereafter.
Intensities. — lu Tablcs VIII. aud X. the intensities of the currents ob-
served in the different circuits have been very fully given, both because the
present measurements are the first of the kind made, and because the char-
acter of these data furnishes an important criterion of the validity of the
results subsequently derived from them. From an inspection of the tables, it
is moreover obvious that an exchange of terminals in measurements of this
kind, however tedious and laborious in case of long circuits, is indispen-
sable. The intensities i' and i'", which are measured with the bags in the
342 GEOLOGY OF THE COMSTOCK LODE.
same position relatively to the holes, are usually very nearly of the same
value, from which i" generally differs, frequently having even the opposite
sign.
Potential. — Betwceu the values of e for the first three, and for the last
three series, there is usually a good agreement. The means (ei and e^ of these
series, however, often show a lack of accordance which is greater than
was expected. The discrepancies occur principally in the results obtained
on the 500-foot level, and it was at first thought that they were largely to be
referred to the fact that a solution of sodic sulphate was used as an outer
liquid In the holes. In No. 11, Table XI., for instance, this liquid, instead of
soaking into the rock, as usual, remained in the hole, gradually becoming con-
centrated by evaporation. In the repetition of the experiment, therefore, the
exterior liquids in P. C. and X. were not of the same concentration, so that a
discrepancy would not seem remarkable. Subsequent experiments, how-
ever, hardly corroborated this supposition. Another large difi'erence occurs
in the case of No. 27 of the same table; but for this hole it was impossi-
ble to obtain constant results, though the experiments were many times
repeated. I am at a loss to account for this fact.
The actual relation between potential and distance will, of course, be
exceedingly complex, and it would be little short of a waste of time to
endeavor with the data at command to arrive at an empirical form for
this function. On the other hand, a graphic representation of the change
of potential due to a corresponding change of distance is certainly desira-
ble. Accordingly, I have discarded more elaborate mathematical means and
have represented the relation in question by the following simple plan : If all
points on the 400 and 500-foot levels be joined by straight lines with Point
I. on the 500, the horizontal projections of these will lie within a sector
whose center is at I. and whose bounding radii subtend an angle of 31°
approximately. It should be noted (Table XII.) that on passing through
the oi'e bodies the variation of bearing is much smaller; that it is large
both for points near I., where the actual length of arc subtended, however, is
small, and for points on the 400-foot level, where, though the actual length
of .subtended arc is large, as all points are remote from ore a smaller change
of potential may be expected. Bearing in mind, therefore, that the object
ELBCTEICAL ACTIVITY OP OEE BODIES.
343
is merely to represent in a systematic way the potential of consecutive
points, a curve may be constructed by representing the linear distance of
any point from I. as abscissa, the corresponding potential as ordinate. In
this way Fig. 28 was obtained. From an inspection of the curve it appears
that the ore body is in general at a lower potential than the points remote
from it.
Kegion of ore bodies.
Country rock.
+50:105
■AbJBIaBaBBkdPqaHB'BBaa '
■■■■■■■BaHaHtjaHBaiBIIBlBri
IBB
■■HUB
laarvjiBB— "s-'^Bawn
IBBBBrvj
■11 h
BHPI
■■■■■e J
I
HBBBBBBBBBBIbMBB
IBBBBBBBBBBBBadi
IBBBBBB«BBBBflflBH
^RBBBBBaBai
nr
0' 200' 400' 600' 800' 1000' 1200' 1400'
Fig. 28. — Earth potential and distance, Richmond mine, 400 and 500-foot levels.
Here it must be remarked that only the extreme points on the 400-foot
level (XXVII., XXVI., etc.,) can, so far as known, be considered actually
distant from ore. In the vicinity of Points I., II., etc., 500-foot level, there
are not only the streaks of ore, n and p (Fig. 27), but also chambers 7 and 10,
and still further east the large ore bodies of the Eureka Consolidated Mining
Company. This has been indicated by the dotted lin.e in Fig. 28.
The variation of potential is irregular, however — even more so than,
with the rough method of delineation, would have been anticipated — and its
amount is small. In fact, it will be seen that certain unavoidable errors
might conspire to produce an almost equivalent change. From results of
such a magnitude, in short, no prediction as to the occurrence of ore or
electroactive material would be justified. Not to mention minor matters,
the survey described suffers from a serious objection, due to the fact that
the temporary contact in progressing from I. to XXVII. passed through a
great number of varieties of rock, and therefore also, probably through a
great variety of absorbed liquids, holding more or less saline matter in solu-
tion. In such a case the electromotive force due to the contact of these
liquids would seem to come into play. As the matter will again be dis-
cussed (see page 356) I will add here only that electric effects thus produced
cannot, a priori, be regarded as negligible. Furthermore, the preference
344
GEOLOGY OF THE COMSTOCK LODE.
given to Point XV., in using it
alone as a basis for the coordi-
nation of the results of the sur-
veys on the 500 and 400-foot
levels, is to be criticised. It
was intended to use several
consecutive points for this pur-
pose; but in each case local
interferences prevented. As a
whole, however, the results are
-E sufficiently interesting to jus-
3 tify further and more careful
^ investigation.
■5 Experiments on the Soo-foot level. Results.
"i — This series of measurements
c
> 1 1 was made with the intention of
' '^_^ observing the variation of po-
I tential encountered in passing
1 across the ore body, without
§ actually entering it. Care was
^ also taken to place all the
S points, so far as practicable,
cj in rock of the same variety,
2 and to remove the ends of the
line of survey as far from the
ore body as possible.
The plan of the position
of the drifts on the 600-foot
level, relatively to the ore-
chambers, is given in Fig. 29.
As before, the points tapped
are distinguished by small cir-
cles, to which Roman numer-
als are annexed. P. C. in this
ELECTRICAL ACTIVITY OF ORE BODIES.
345
case coincides with Point VIII. and is in porous limestone. The great
ore bodies have been lettered as in Fig. 27. Ore is also found at G, above
and below the 600-foot level, and at n above it. Z7 F is a line of contact
between the shale of the west country and limestone. Table XIII. exhibits
more exactly the disposition, etc., of the points. It will be intelligible with-
out further explanation. (Cf Table IX., page 338.)
Table XIII.
No.
Points.
Dis-
tance.
Bearing.
Resist-
ance.
Remarks.
1
I
Feet.
0
Ohms.
1580
Shale, moist.
2
n
76
S. 49° E.
2350
Limestone.
3
in
77
S. 49° E.
1750
Do.
4
IV
65
S. 49° E.
3110
Do.
5
v
75
S. 49° E.
4015
Do.
6
VI
70
S. 49° E.
4420
Do.
7
VII
87
S. 62° E.
6300
Do.
8
vm
94
S. 74° E.
Do.
9
IX
85
S. 74° E.
2480
Do.
10
X
71
S. 74° E.
5150
Do.
11
XI
91
S. 74° E.
3420
Do.
12
xn
80
S. 74° E.
4170
Limestone, faintly stained witli iron.
13
xin
90
S. 74° E.
3480
Limestone.
14
XIV
75
S. 74° E.
3200
Do.
15
XV
78
S. 74° E.
9490
Limestone, with calcareous spar.
16
XVI
79
S. 74° E.
15950
Limestone, hard, impervious.
17
xvn
127
N. 72° E.
3440
Limestone, stained with iron.
18
XVIII
118
K. 85° E.
3380
Limestone.
19
XIX
72
K. 85° E.
2525
Pocket of fermginous earth in limestone.
20
XX
74
S. 49° E.
3275
Do.
1 21
XXI
121
S. 42° E.
4965
Limestone.
The results of the measurements of electromotive forces between VIII.
(P. C.) and the T. C.^s are contained in Table XIV. The nomenclature being
the same as that used above, the meaning of the data will be at once appar-
ent. As before, four terminal bags, A, B, C, and D, were used. Intensi-
ties are given in electromagnetic units (C. G. S.), electromotive forces in
volts ; and are arbitrarily considered positive when the potential of T. C. is
greater than that of P. C. (Point VIII.); or when the current travels
CpOTtJl)
T. C. >} . i >P. C. The experiments were made in continuous
( wire )
series, starting with Point I. in the extreme wes^ in shale, and ending with
XXL, near the shaft, in limestone. Water, which had previously been
kept in contact with zinc, was used as an outer liquid.
346
GEOLOGY OF THE COMSTOCK LODE.
Table XIV.
FIRST SERIES.
SECOND SERIES.
1
I
172
+
0
193
— 16
11
xn
— 76
— 93
— 79
— 33
2
n
+
40
—
55
+
38
2
12
xni
— 103
— 122
— 114
— 40
3
m
+
55
—
108
+
53
- 5
13
XIV
— 160
— 153
- 152
- 48
4
IV
+
64
+
71
4-
62
+ 18
14
XV
— 56
— 59
— 60
- 59
5
V
+
47
+
43
+
50
+ 19
15
XVI
— SO
— 55
— 43
— 90
6
TI
+
17
+
10
+
19
+ 6
10
XVII
— !i8
— 102
- 96
- 31
7
VII
+
12
+
5
+
10
+ 5
17
xvin
— 169
- 182
— 177
— 55
8
IX
+
9
—
9
+
5
± 0
18
XIX
- 81
— 72
- 81
- 18
9
X
. —
24
-
28
—
21
— 12
19
XX
- 60
— 69
- 64
— 17
10
XI
—
52
—
57
—
50
— 16
20
XXI
— 67
— 64
- 53
— 30
THIRD SERIES.
1
I
172
3
165
— 17
11
xn
- 72
— 103
- 79
- 34
2
n
+
33
—
38
+
21
— 2
12
xnr
- 107
— 134
- 110
— 41
3
m
+
40
—
83
+
36
- 4
13
XIV
— 160
— 167
-152
— 50
4
rv
+
65
+
71
+
02
+ 18
14
XV
- 55
— 57
— 62
— 59
5
V
+
47
+
34
+
48
+ 17
15
XVI
- 62
— 57
— 47
— 96
6
VI
+
19
+
9
+
17
+ 6
16
xvn
— 91
— 100
— 95
— 30
7
vn
+
9
+
5
+
9
+ 4
17
xvni
— 167
— 182
— 176
— 54
8
IX
+
10
■ —
19
+
10
— 1
18
XIX
— 79
- 71
— 76
-17
9
X
-
22
—
34
—
17
- 12 ,
19
XX
- 64
- 69
— 65
— 18
10
XI
—
48
—
71
—
48
- 18
1
20
XXI
— 64
— 62
— 64
— 30
1
FOURTH SERIES.
1
I
79
98
78
- 11
11
XII
- 83
- 86
- 81
- 37
2
II
-
0
-
19
-
9
- 3
12
xni .
- 96
- 105
- 95
- 35
3
m
-
12
-
31
-
21
- 4
13
XIV
- 138
- 146
- 141
- 46
4
rv
+
48
+
52
+
59
+ 19
14
XV
- 69
- 71
- 72
- 59
5
V
+
36
+
38
+
43
+ 15
15
XVI
- 65
- 69
- 64
- 95
6
VI
+
14
+
14
+
7
+ 5
16
xvn
- 72
- 95
- 69
- 30
7
vn
+
7
+
T
+
7
+ 4 1
17
xvm
- 148
- 143
- 143
- 52
8
IX
±
0
-
14
•
5
- 2 1
18
XTX
- 55
- 78
- 67
- 18
9
X
-
22
-
21
17
- 11
19
XX
- 60
- 84
- 60
- 21
10
XI
-
41
-
47
-
43
- 16
20
XXT
- 52
- 55
- 53
- 26
ELECTEICAL ACTIVITY OF ORE BODIES.
347
Table XIV— Continued.
FIFTH SERIES.
No.
P.C.
connected
with-
i' X 10»
i" X 10'
i'" X 10«
ex 103
No.
P.C.
connected
with—
i' X 10'
i" X 10»
i"i X 10'
«X103
1
I
- 84
- 83
- 96
- 11
11
XII
- 84
- 84
— 86
- 37
2
n
- 12
- ID
- 12
- 3
12
XIII
- 102
- 103
- 103
- 36
3
in
- 24
- 16
- 36
- 4
13
XIV
- 143
- 141
- 145
- 46
4
IV
+ 48
+ 59
+ 65
+ 19
14
XV
- 69
- 72
- 69
- 59
5
V
+ 33
+ 43
+ 38
+ 15
15
XVI
- 59
- 71
- 59
- 93
6
VI
+ 10
+ 14
+ 7
+ 5
16
xvn
- 74
- 95 .
- 65
- 30
7
VII
+ 5
+ 9
+ 5
+ 4
17
xvm
- 145
- 145
- 136
- 52
8
IX
- 9
- 2
- 12
- 2
18
XIX
- 60
- 76
- 64
- 18
9
X
- 22
- 21
- 21
- 11
19
XX
- 67
- 76
- 62
- 20
lu
XI
- 47
- 43
- 40
- 16
20
XXI
- 43
- 55
- 69
- 27
SIXTH
SERIES.
1
I
_
86
_
90
_
98
- 12
1 11
xir
- 64
- 86
- 84
- 37
2
n
-
16
-
12
-
24
- 3
12
xin
- 110
- 105
- 105
- 37
3
ni
-
36
-
21
-
38
- 5
13
XIV
- 138
- 152
- 138
- 47
4
IV
+
47
+
62
+
59
+ 20
14
XV
- 72
- 72
- 72
- 59
5
V
+
31
+
41
+
36
+ 15
15
XVI
- 00
- 71
- 64
- 93
6
VI
+
14
+
7
+
14
+ 5
16
xvn
- 76
- 93
- 74
- 31
7
YH
+
7
+
7
+
9
+ 4
17
XVIII
- 136
-141
- 141
- 51
8
IX
-
24
+
0
-
IC
- 3
18
XTX
- 65.
- 78
- 72
- 19
9
X
-
19
-
22
-
21
- 11
19
XX
- 67
- 76
- 60
- 19
10
XI
-
52
-
45
-
43
- 17
20
XXT
- 43
- 50
- 67
- 28
A comparison of the values of e obtained is given in Table XV. The
plan is analogous to the above.
Table XV.
No.
Points.
e'X10»
e" X 10'
e"'x 10'
ei X 10»
c" X 10>
e'XlOs
e"X10'
62X103
«X10»
1
I
- 16
- 16
- 17
- 16
- 11
- 11
- 12
- 11
- 14
2
II
- 3
- 2
- 2
- 2
- 3
- 3
- 4
- 3
- 3
3
m
- 3
- 5
- 4
- 4
- 4
- 4
- 6
- 4
- 4
4
IV
+ 17
+ 18
+ 18
+ 18
+ 19
+ 19
+ 20
+ 19
+ 18
5
V
+ 20
+ 19
+ 17
+ 18
+ 15
+ 15
+ 15
4 15
+ 17
6
VT
+ 6
+ 6
+ 6
+ 6
+ 5
+ 5
+ 5
+ 5
+ 6
7
VII
+ 5
+ 5
+ 4
+ 5
+ 4
+ 4
+ 4
+ 4
+ 5
8
Yin
± 0
± 0
± 0
± 0
± 0
± 0
± 0
± 0
± 0
9
IX
± 0
± 0
- 1
± 0
- 2
- 2
- 3
_ 2
- 1
10
X
- 11
- 12
- 13
- 12
- 11
- 11
- 11
- 11
- 11
11
XI
— 17
- 17
- IS
- 17
- 17
- 16
- 17
-17
- 17
12
xn
- 33
- 33
- 35
- 34
- 37
37
- 37
- 37
- 35
13
xni
- 40
-40
- 41
-40
- 35
- 36
- 37
- 36
- 38
14
xrv
- 48
-48
- 50
- 49
- 46
- 46
- 47
- 46
- 48
15
XV
- 57
-69
- 69
- 58
- 59
- 59
- 59
- 59
- 59
16
XVI
- 93
- 90
- 96
- 93
- 95
- 93
- 93
- 94
- 93
17
xvn
- 30
- 31
- 30
- 31
- 30
- 30
- 31
- 31
- 31
18
XVIII
- 57
- 65
- 55
- 55
- .13
- 52
- 51
- 52
- 64
19
XIX
- 17
- 18
- 17
- 17
- 18
- 19
- 19
- 19
- 18
20
XX
- 18
- 17
- 18
- 18
- 21
- 20
- 19
- 20
- 19
21
XXI
- 29
- 30
- 30
- 30
-27
- 27
- 28
- 27
- 29
348
GEOLOGY OF THE COMSTOCK LODE,
Table XVI., finally, contains the data necessary for the approximate
representation of earth-potential as a function of distance. By arbitrarily
assuming the potential of Point VIII. as zero the final means in Table XV.
are identical with the potential of the points to which the data refer. The
third and fourth columns of Table XVI. contain the length and bearing of
the imaginary lines joining I. with the succeeding points.
Table XVI.
Ifo.
Points.
Distance
from I.
Bearing.
ex 10"
No.
Points.
Distance
from I.
Bearing.
eXVfi
1
I
0
Origin.
- 14
12
XII
850
S. 6I0 E.
- 35
2
n
76
S. 490 E.
— 3
13
XIII
935
S. 630 E.
- 38
3
m
153
S. 49° E.
- 4
14
XiV
1010
S. 640 E.
- 48
4
VI
230
S. 49° E.
+ 18
15
XV
1080
S. 640 E.
- 59
5
V
295
S. 49° E.
+ 17
16
XVI
1160
S. 650 E.
- 93
6
TI
365
S: 49° E.
+ 6
17
xvn
1280
S. 680 E.
- 31
7
VII
450
S. 50° E.
+ 5
18
xvni
1380
S. 70O E.
- 54
8
vin
540
S. 540 E.
± 0
19
XIX
1450
S. 710 E.
- 18
9
IX
615
S. 570 E.
- 1
20
XX
1510
S. 70° E.
- 19
10
X
685
S. 580 E.
- 11
21
XXI
1630
S. 70O E.
— 29
11
XI
775
S. 6OO E.
— 17
Discussion. — From the results in Table XIII. for the resistance of dififer-
ent circuits similar conclusions to those on page 341 are deducible. Wherever
the structure of the rock and coexisting circumstances are favorable to the
absorption of moisture, there also minimal values for this quantity are found.
Unusually high values were obtained for the holes XV. and XVI. But the
rock at these points was so tough and tenacious that the miners complained
of the slow progress made in drilling.
Remarks analogous to the above are applicable to the values for inten-
sity on this level.
The results for earth-potential in Table XV. harmonize much better
than those for the preceding levels. The individual values in the two series,
as well as the means of the series themselves, are in fair accordance.
This might be ascribed to the fact that the holes were mostly in rock of
the same variety, and that strong salt solutions were discarded in completing
the contact between the terminal bags and the earth.
By a method of procedure similar to that already employed the relation
between potential and distance may be represented graphically. It will also
\
ELECTRICAL ACTIVITY OF ORE BODIES.
349
be seen, from an inspection of Table XVI , that the considerations involved
in constructing Fig. 28 are more pertinent in this case, as the main drift itself
is more nearly linear. Laying' off potential as or-
dinate, distance as abscissa (Table XVI.), Fig. 30 g g £
is obtained. 5 5 i
Both Fig. 28 and Fig. 30 demonstrate the
remarkable result that the region of ore bodies
coincides with a region of low potential. This is
all the more striking, as in the first case, taking
in general a northerly course, the ore bodies are
approached from barren rock (400-foot level). In
the second the course of survey, while passing
toward the ore region, was mainly in an easterly
direction. The two lines of survey may, roughly
speaking, be said to be at right angles to each
other. In one case, moreover, the sequence of
points tapped intersects the ore bodies, whereas in |
the other it remains exterior to them throughout g
its whole extent. |
a
Comparing the results of the two surveys, the
indications on the 600-foot level are found to be
much the more pronounced; in fact, they transcend
values which can be accounted for as an aggregate of
incidental errors. It may be remarked here, that it
is a very impi-obable chance which would place the
region of greatest electrical disturbance in coinci-
dence with the region of ore bodies, if the latter
were without influence in producing the former.
There is no reason apparent why the part of the
main drift on the 600-foot level, between Points I.
and X., should not be just as active as that between
Points X. and XXL, unless it be that these points lie nearest to ore, and
consequently that we are here rapidly approaching the seat of an electro-
motive force.
350
GEOLOGY OF THE COMSTOCK LODE.
Chambers No. 14 and No. 15 connected electrically. In the SUrVej Oil the 400 and 500"
foot levels, two ore bodies, Nos. 12 and 15, were indirectly connected,
but the indications obtained were much smaller than was anticipated. It
appeared desirable therefore to test this matter still more carefully by
connecting the huge ore-masses in chambers No. 14 and No. 15. Ac-
cordingly a P. C. in a large breast of ore (lead carbonate) in cham-
ber No. 15 was placed successively in contact with three diflferent points,
at a distance of about 100 feet one from another, in chamber No. 14. Each
of these was also in ore, the first in lead carbonate, the second in lead car-
bonate and earthy sulphide, the third finally in a mixture of carbonate,
sulphide, and ferruginous earth. Table XVII. contains the results of the
electrical measurements. A single set of observations, with one exchange
for each, was made.
Table XVII.
FIRST SERIES.
'So.
1
P.O.
joined
■with—
i' X 108
i" X 108
«X10'
1
- 16
- 3
- 6
2
n
- 8
- 10
- 7
3
m
- 10
- 13
- 7
SECOND SERIES.
1
I
- 18
- 2
- 6
2
n
- 7
- 10
- 6
3
m
- 11
- 13
- 7
THIRD SERIES.
1
I
- 11
- 8
- 6
2
n
- 11
- 10
- 7
3
in
- 11
- 13
- 7
In considering these results, it is strikingly apparent that the evidences
of electric action are almost altogether absent. It is true that in all proba-
bility chambers Nos. 14 and 15 are but parts of one and the same large
ore-mass, but in the place where the experiments were made they are to
some extent, at least, locally disconnected. The results lead to the inference
either that the ore of both chambers is remarkably similar in character, so
as to present no appreciable electric difference, or that it is here without
ELECTRICAL ACTIVITY OF ORE BODIES.
351
electrical properties altogether (earthy), the field of electric action being
confined to certain definite parts of the ore-deposit. (See also page 364.)
Experiments on the surface. — Encouraged by the resiilts on the 600-foot level,
it seemed not impossible that currents might also be observed on the surface
itself, insomuch as the ore extends in places to within 100 feet from the sur-
face, while vestiges of croppings, etc., still remain
A line of points lying in general in a north-and-south direction, and at
distances of about 1 00 feet apart, was chosen, the object being to extend
the electric survey from shale in the north, free from ore, over Ruby Hill
and the large ore bodies in its interior, to quartzite in the south, also more
or less free from ore. It was hoped that in this way a passage through a
field of electrical activity might actually be made. Unfortunately, the work
was interrupted by a heavy snow-storm and accompanying frosts.
P. C. was placed about half way up the hill in compact limestone.
Point I. is the most northerly of the series, and remote from ore; Point IX.
approximately over the Richmond ore bodies. The results are contained
in the following table, e is the mean of a single triple set. The potential
of P. C. (Point VI.) is arbitrarily put equal to zero.
Table XVIII.
No.
Points.
Eesistance.
«X103
Eemarks.
1
I
17, 000
- 20
Bibris; lowest point.
2
II
14, 000
- 30
Do.
3
HI
13, 000
- 30
Do.
4
IV
13, 000
- 10
Do.
5
T
13, 000
- 10
Shale.
fl
VI
dt 0
Limestone! (P. C).
7
vn
150, 000
+ 10
Do.
8
vm
40, 000
+ 20
Limestone; hif:Iiest point.
9
IX
20, 000
+ 40
Do.
10
X
25, 000
+ 50
Do.
In the table the unusually high values for the resistances of the cir-
cuits, P. C. earth T. C, are a striking feature. This may be due either to
the compact and impervious structure of the rock (the drill making very
slow progress), or, as the experiments were made in the early spring, to
the possibility that the moisture in the rock was still frozen. In either
case, however, the supposition that the conductivity of the rocks is princi-
pally due to the presence of moisture in their pores receives fresh support.
352 GEOLOGY OF THE COMSTOCK LODE.
The values for earth-potential again exhibit a marked variation in pass-
ing toward the ore-deposit. But, unlike former cases, the passage fi'om points
remote to those nearer the ore-region is one from lower to higher potential.
As nothing is known about the distribution of potential with reference to
ore bodies, this is not to be regarded as at variance with former results.
Not overmuch rehance, however, must be placed on the values of e in this
table. They were obtained under unfavorable circumstances, and not
checked as in the former cases.
According to Matteuci,^ a difference of potential exists between points
at different levels, in virtue of this fact alone. "Ce courant est ascendant
dans la partie mdtallique du circuit ; son intensity augmente h mesure que
les lignes sont plus longues, et que la difference de niveau entre ces ex-
tremit(^s est plus grande." But in the present case the direction of the cur-
rent is not only the opposite of this, but the electromotive force continues
to increase even in greater ratio after the highest point of the series has been
reached. The effects, therefore, are not such as Matteuci observed. The
reader is further referred to page 360.
EEPETITION OF SOME OF THE EXPERIMENTS AFTER AN INTERVAL
OF ABOUT ONE HUNDRED AND THIRTY DAYS.
The preceding experiments are to be regarded as incomplete in two
particulars. In the first place, the data are the results of but a single
method of measurement, the application of which is not immediately evident;
in the second, no criterion of their constancy in point of time has as yet
been obtained. The additional results now to be given were obtained on the
600-foot level of the Richmond mine, all of the former holes (points tapped),
with the single exception of No. I., being used over again. In place of
the latter, this having become inaccessible, a fresh hole, about 25 feet to
the east of the old one, but also in shale, was drilled.
The experiments were made after an interval of more than four months
from the time at which the original data were obtained.
Methods. — From an inspection of the magnitude of the electromotive
forces contained in the foregoing tables, it will be seen that they fall well
'Ann. de Chim. et de Pbys., (4), T. X., p. 148, 1867.
ELECTRICAL ACTIVITY OP ORE BODIES,
353
within the scope of a good electrometer. Such an instrument, properly
protected against the moisture of the underground air, would have been
most serviceable for the purpose. Unfortunately, one could not be obtained
in time for the work. The following methods were therefore resorted to :
In the first place the greater part of the data were checked by the
method already described. This, it will be remembered, was chosen because
of its simplicity and the comparative ease with which any fault in the con-
nections could be ascertained.
The potential of the same holes was noW measured by a method
in which the electromotive force is expressed in terms of the increment of
the reciprocal of intensity of current, and the corresponding increment of
the resistance of the circuit, to which the former is due. In order to vary
the resistance at pleasure a rheostat was introduced. If the resistances tVi
and W2 correspond to the intensities i^ and is, respectively,
to,—
W2
1
] '
h
«2
e =
where e is the electromotive force to be measured.
Finally, the whole of the experiments formerly made on the 600-foot
level were again repeated by a zero
method. Here great care had to be
taken to effect the complete insulation
of all parts. This was accomplished
in the manner previously indicated,
by suspending the terminal wires, as
well as all the connections, from
threads. The accompanying diagram.
Fig. 31, will show how this was done.
A and B are clamp screws, suspended
from the threads a and d, respectively,
E (rheostat) is the large, r the small
resistance, K a double key, C a com- i*''**. 31.— Disposition of apparatus,
mutator, O the galvanoscope. For a zero current in the latter (the effects
354
GEOLOGY OF THE COMSTOCK LODE.
due to tjie normal element E and the lode electromotive force compensat-
ing each other in G), approximately,
r
e = JE
R-\-r'
■■■■■I
■■■■■I
!■■«■■!■■■■
■■■■■ ■■«i|ilHB
■■■■■ ■■■■■■■■■B
■nliainiBH
nldiflBBHBI —
1Hb;s::i
'§■■■■ ■
IS
■■■■■
■■■■i
sSap
HI
- ■■■
■ HI
i
H
■i
■■■
■■■
■r
■
IP>"
li
liiS
I!
■■■■■■
■■■■■■■nBB
rssi
Sing ■■ ■
■«■■ ■■■■
ma :b:
b:::
!■■■
8s;s
The resistance r was wrapped on a small piece of wood and the whole sub-
sequently boiled in paraffine. The body of the key
i K, and that of the commutator C, were similarly pre-
pared, being boiled in linseed oil, and the mercury
cups covered internally with a thick coating of wax.
I -j Moreover, the wires of both in passing through the
I wood were additionally insulated from the latter by
I a covering of gutta-percha; the ends only being
i X uncovered and communicating with the mercury in
s the cups. In consequence of these precautions it
tS was found that this comparatively complicated
I § method could be em^jloyed in these wet drifts with
I complete success, and the adjustments having once
'§ been made, it proved to be nearly as expeditious as
i I either of the other methods.
^^ As the result obtained is derived from an ex-
I pression which is independent of the resistance of
§ I the circuit, the method could be used with advantage
I in studying the manner of variation of potential in
I passing, as it were, continuously from any T. C. to
* ^ the next. But the actual observations will be more
■3 appropriately cited in connection with another topic
^ (see page 361).
" n It will be remembered that in the former ex-
^ j^eriments four contact bags were used throughout,
■■■■Eoii I iBii which were so combined as to give three indepen-
|SMSS'S5S ■■■■■■■ „ dent values for the electromotive force to be meas-
s I I ured. The results thus obtained, however, being
o ^ "^
41 I 7 usually so nearly identical, it was thought that this
precaution might safely be dispensed witli. Two contact bags only, there-
■I
nsssn as
■■■InlaB ■■
■■■■■!■■
■■■■■■■!
■■■■■■nE
■■■■
■■■■
■■■■
■■■■
Hi
as:
ELECTRICAL ACTIVITY OP OEE BODIES.
355
fore, were employed. In all other respects, however, the former plan (see
page 328) was rigidly adhered to, with such slight variations, of course, as
the different methods rendered necessary.
Results. — The following table, containing the potential of the consecu-
tive points on the 6()0-foot level — that of No. VIII. being arbitrarily put
equal to zero, as before — will be intelligible without much further explana-
tion. The results of the different methods are arranged in parallel columns,
and in the order in which they were described. For the sake of comparison
those obtained in the foi'mer survey are also added, and a final column
shows the difference between the two.
Table XIX.
e X 10', determined—
No.
Points.
e X 103,
mean.
e X 103,
old value.
6(e) X 103.
Remarks.
By old
method.
With
rheostat.
By com-
pensat.
1
I'
+ ^
+ 1
+ 1
+ 12
+ 15
- 30
+ 7
+ 1
+ 1
— 14
New and old holes
2
3
n
m
- 3
_ 4
- 4
- 5
do not coincide.
4
rv
+ 12
+ 18
+ 17
+ 6
+ 6
5
V
+ 15
+ 2
6
VI
- 30
+ 36
7
Vll
— 3
_ 4
_ 4
+ 5
- 1
+ 9
8
IX
- 1
- 1
- 1
- 1
± 0
9
X
- 29
- 25
- 31
- 28
- 11
+ 17
10
XI
- 25
- 25
- 25
- 25
- 17
+ 8
11
Xll
- 13
- 12
- 14
- 13
- 35
- 22
12
Xlli
- 30
- 23
- 24
- 29
- 38
- 9
13
XIV
- 39
- 39
- 40
- 39
- 48
- 9
14
XV
- 41
- 40
- 47
- 43
- 59
- 16
15
XVI
- 72
- 75
- 76
- 74
- 93
- 19
16
XVII
- 15
- 15
- 23
- 18
- 31
- 13
17
xvm
- 48
- 47
- 50
- 48
- 54
— 6
18
XIX
- 8
- 13
- 7
- 9
- 18
- 9
19
XX
- 14
- 13
- 14
- 14
- 19
- 5
20
XXI
- 31
- 32
- 39
- 34
- 29
+ 5
The results obtained by different methods present throughout a fair
agreement, when it is remembered that errors amounting to a few thou-
sandths of a volt are introduced by circumstances beyond the observer's
control. Between the mean of the new and the mean of the former results
thei'e are a number of annoying discrepancies. In part, though by no means
wholly, these are due to a difference in the values of the standard electi-o-
motive force emiployed in the two cases. With the knowledge at present
356 GEOLOGY OP THE COMSTOCK LODE.
available it would be of little use, however, to attempt to assign reasons for
tlie remaining variations. A matter of greater importance is that the gen-
eral character of the curves, as derived from the two series of results, is
essentially the same.^
UNAVOIDABLE EEEORS AISTD MISCELLANEOUS CEITICISMS.
Moisture in the rocks. — Bv far the most seHous difficulty encountered in en-
deavoring to interpret the results obtained, is that due to the difference of
potential of two liquids in contact. The conductivity of rocks is, as has
been seen, largely, if not wholly, to be ascribed to the presence of moisture
in their pores. This moisture unquestionably holds saline matter in solu-
tion. Moreover, it is altogether probable that the solution in one rock of a
particular structure is in general different from that in another of different
structui'e and many hundred feet distant from the former, even if the com-
position of both is essentially the same. In tapping two points at some dis-
tance apart by the aid of two metals (plates or gads) supposed identical in
every respect, two members of the continuous sequence of solutions con-
tained in the rocks are, in fact, put in metallic contact. The difference of
potential thus obtained would be that due to the resultant action of the
series of liquids included between the points. This electromotive force is,
however, principally dependent on the extreme members of the sei-ies, i. e.,
those at the points tapped; and in the present investigation it was hoped
that the discrepancy thus arising might be very largely eliminated by put-
ting the same liquid in both holes, and by exchanging not only the metallic
terminals — amalgamated zinc — but also the terminal solutions (zinc sulphate).
Hence the "bag" form of the terminal.
It was thought not superfluous to test the matter with the aid of the
contact bags themselves; all the more as it would thus appear to what
extent the results obtained with the latter are trustworthy. The two liquids,
whose electromotive force was to be measured, were separated from one
another by a porous septum of animal membrane. As in the mines, the
terminal bags were exchanged. In passing them out of the first liquid into
the second, care was taken to wipe off the liquid adhering to the outside.
' Compare Figs. 30 and 32.
ELECTRICAL ACTIVITY OF ORE BODIES.
357
If now, e be the electromotive force of the two solutions in contact, e that
due to the difference between the zincs alone, in the first position of the bags
A and B (A in water and B in the liquid to be tested), the apparent force
would be
in the second position of the bags (B in water and A in the Hquid to be
tested),
the connections themselves remaining unaltered. A mean of both measure-
ments gives £; half the difference, e. The following are some of the results:
r Both bag8 in water
c Bags alternately in solntion of Na* SO* and in water.
fHoth bags in water
i Bags alternately in salt solution and in water
tBoth bags again in water ,
r Both bags in water
c Bags alternately in Zn SO'* solution and in water
eXlQS
1.0
1.0
2.8
3.4
3.4
ex 10=
0.4
2.2
0.6
+ 0.4
-0.4
It will be seen that in the different sets s is fairly constant. The value
of e is small, as anticipated, notwithstanding that nearly concentrated solu-
tions were used. In the case of zinc sulphate e is practically zero, as it
should be, the bags themselves containing this solution.'
The following table contains analogous experiments made in the mines.
Holes IX. and X., 600-foot level, were put in contact. Measurements were
made by a zero method:
Water in both IX. and X
Water in X. ; concentrjited brine in IX.
Concentrated brine in both IX. and X. . .
ex 103
- 28
- 27
- 27
Two other holes similarly treated gave:
exlC
- 14
- 17
Apparently, therefore, large discrepancies are not produced in this way.
358 GEOLOGY OF THE COMSTOCK LODE.
Of course all these experiments are only intended to furnish estimates
as to the probable magnitude of disturbances of an analogous kind, which
may possibly have influenced the data given above.
Mr. E. Kittler^ has recently commenced a new study of the question of
potential difference due to the contact of liquids. From a large number of
careful experiments he finds electromotive forces between them far exceed-
ing, as a rule, those met with in the measurements of earth currents here
described. These forces, however, obey the law of Volta's potential series.
From all these considerations, it seems to follow that in the present
investigation the discrepancies due to the presence of different liquids in
the rocks have been eliminated to a great extent. Certainly their effect
can hardly be estimated as much greater than a few thousandths of a volt.
It is obvious, moreover, that the use of simple metallic contacts (plates and
gads) is under all conditions unsafe. To this is to be added the fact that
metalhc plates are never identical in their electrical properties, and that their
difference (as large effects of polarization are also included therein) cannot
be eliminated by such a process of commutation as was employed.
The phenomenon of conduction of rocks being essentially hydro-electric,
the determination of the thermo-electric power earth|copper, for which it
was at first thought the high temperature on the lower levels of the Com-
STOCK Lode, in comparison with those at the sui-face, would offer excellent
facilities, has no further interest. No attempt of this kind was therefore
made.
If the hole drilled for the reception of the temiinals be regarded as
a cylinder with a hemispherical base, the directrix of the former as tangent
to the spliere corresponding to the latter, its axis as normal to the plane
face of the drift, approximate values may be derived for the specific resist-
ance of the rock met with. Let h be the height of the cylinder, a the common
radius of both the latter and the hemisphere. Let r be the radius of any
similar figure, the axis of whose cylinder and centerof hemisphere coincide
with those of the hole. Finally, let o be the specific resistance of the rock,
or the resistance in ohms between opposite faces of a cubic centimeter.
' E. Klttler : " Ueber Spanniingsdifferenzeii, etc." Wied. Ann., XTI.. p. .'i72 et seq., 1881.
ELECTRICAL ACTIVITY OF OEE BODIES. 359
The elementary resistance of a shell at the distance r from the axis and
of the thickness (Zr, is then
T a dr
'2 7r r{r-\-hy
and, therefore, the resistance of the layer of rock between coaxial and
concentric figures, the inner radius being a, the outer r, is
['"]: -^
_ iia+h)r
the symbol \^v^ being used to express the resistance of the layer of rock
between the similar surfaces just defined. If r is allowed to increase to
infinity, approximate values for a can be determined from the data given
above for the resistances of the circuits, and the known dimensions of the
holes. In this way it appears that the mean value of this quantity was about
0- = 40,000,
whereas values as high as 500,000 and as low as 20,000 were met with.
From the invariable presence of moisture, however, these figures possess
only minor interest.
If the resistance of layers of rock between consecutive similar sur-
faces be compared, the same notation being again employed, in round num-
bers,
w~\ \w'] [w]
Jl? _ A C . L JlOO _ A A7 . k_ Jj
= 0,6; ^^ = 0.07; "-^S =0.007, etc.,
]io — "■'-'> r -lliiO ".vi, r- -lie
all dimensions being expressed in centimeters, and a being 1.2 cm.; whence
it follows that the resistance of coaxial and concentric layers decreases,
though hardly as rapidly as might be desirable. In point of fact, however,
the convergence is more rapid than this approximate calcvilation indicates.
A drift may with greater accuracy be regarded as a cylindrical tunnel. Into
the sides of which the contact holes have been drilled, with their axes at
right angles to that of the drift. Now, it is obvious that as r (in the former
signification) increases, the values of dw will in this case decrease more
rapidly than in the previous one; this because the superficial area of the
infinitesimally thin shell increases much more rapidly. The actual analysis,
360 GEOLOGY OF THE COMSTOOK LODE.
however, is unnecesrary here. The points of greatest interest have already
been sufficiently illustrated by what precedes.
Earth-currents. — A second important consideration relative to the causes
which might have produced discrepancies in the present investigation is the
effect to be ascribed to earth-currents. Although numbers of experiments
have been made in different parts of the world as to the magnitude and
direction of such currents, I am unable to estimate their effect in this case,
especially as the constants for the currents probably vary with the position
of the field of observation on the surface of the earth. Most observers
have availed themselves of telegraphic connections between points very
many miles apart. Matteuci,^ I believe, was the only one who laid a care-
fully insulated line especially for this purpose, and it is to his investigation
that we can with greatest advantage refer. Yet, though his points were at a
distance of six kilometers apart, the currents obtained, so far as can be seen,
were certainly not much larger than those here recorded If, however, the
variation of potential in the above experiments were due to some normal,
non-local cause, it would be fair to assume a linear change of potential
with distance throughout the comparatively small area in which the experi-
ments were made. Such is by no means the case. In fact, some of the largest
variations observed occur within distances of a few hundred feet, while
elsewhere a range of 1,000 feet is without marked alteration of potential.
It is probable, therefore, that earth-currents have not perceptibly affected
the results.^
Drill-holes. — The angular and somewhat irregular curves (Figs. 28, 30,
32) might give rise to a suspicion that the difference of potential observed
is in some way to be ascribed to the accidental condition of the holes them-
selves. A priori, therefore, the presence of little pieces of steel, worn or
broken off from the drill, crystals of iron pyrites, particles of ore, etc., in
the walls of the hole should not be disregarded. That such material is,
however, entirely without disturbing effect will be seen from the following
experiments :
' Ch. Matteuci : " Sur les courants ^leotriques de la terre." Ann. d. Chim. et de PhyB., [4], T. IV.,
p. 177, 18ti5; ihid. [4], T. X., p. 148, 1867.
^Temporary disturbances, such, for instance, as are due to atmospheric induction, are obviously
without influence in the present case. Inductive action, moreover, is hardly to be expected from the
clear, dry air of Nevada.
ELECTRICAL ACTIVITY OF ORE BODIES.
361
In a particular case the intensity «i obtained by connecting two holes
in the ordinary manner was
ii = 101:10«.
A thin strip of platinum was subsequently introduced into one of the holes
and firmly pressed against its sides. The intensity ij then measured proved
to be
io = 99:10«;
or, ^practically, the same as before. An eifect due to the platinum was there-
fore absent.
Two holes, about 18 inches apart, were drilled in solid rock and con-
nected as" usual. The measurements made for difference of potential, by
the original method, gave, in four successive experiments, different bags
being used for each,
1) e = -f 1:10^
2) eir:— 1:10'
3) e = ±0:10^
4) e = ±0:10^
or zero, as from the proximity of the holes it ought to be.
Finally, the potential of a number of points lying between Nos. V. and
VII., on the 600-foot level of the Richmond mine, was determined. A zero
method being used, it was only necessary to put the terminal bags in con-
tact with the rock at the points chosen, by allowing them to recline against
the wall. Care was taken to prevent any part of the copper wire from
touching it. Two points, A and-B, were thus established between VII. and
VI. ; three, C, D, and E, between VI. and V. The following table gives the
results, the potential of No. VIII. being put equal to zero, as before:
Table XX.
No.
Pointa.
ex 103
Distance
from VII.
Kem.arka.
1
VII
- 4
0
DrUl-hole.
2
A
- 15
30
'
3
B
- 44
60
4
VI
- 30
87
Drill-hole.
5
C
- 23
105
6
D
- 9
120
7
E
+ 2
140
8
V
+ 15
157
DriU-hole.
362
GEOLOGY OF THE COMSTOCK LODE.
+ 50; 10'
0:10'
50: JO'
In Fig. 33 these results are graphically represented. It appears, not-
withstanding the different kinds of contact at V., VI., VII., and at ^, i?, C, D,
E, that the progress of the
curve in passing from
VI. to V. is continuous.
The experiments, there-
fore, failed to detect any
specific action due to the
holes. Nos. v., VI., and
Fig. 33. — Potential of intermediate points. VII. Were especially
chosen, because, as will be seen from a comparison of Figs. 30 and 32,
this part of the curve presents a curious and pronounced anomaly, the
newer results differing verj^ remarkably from the earlier. It was natural
to suppose that, in the time which had elapsed between the two series of
measurements, hole No. VI. had in some way been interfered with. The
results just cited, however, preclude such a supposition. Even larger masses
of metal seem to be without marked effect. Between the date of the earlier
and that of the newer observations, for instance, a track had been laid
from the vicinity of the hole No. I. to No. XV.
Terminal bags. — Tlicre occur a few cases in my notes in which, though in
every other respect the behavior was normal, different results were obtained
for the same hole at nearly the same time, by employing different bags, viz.:
IV. e = 0:10* VII. e = 2:10^
6 = 2:10' e=:6:10l
e = 6:10'
These cases are rare, however, and their effect is of minor importance.
More worthy of consideration are the successive diflPerences of potential due
to the bags alone when employed for a long period of time. The quantity
referred to has already been considered on page 357, under the symbol e.
It may readily be derived from the tables for intensity. The following table
(XXI.) probably contains good examples of its consecutive states, the data
given being deduced from those for the holes I.-XIV. on the 600-foot level.
If the bags are called A, B, 0, D, the electromotive force e between A and
B may be conveniently represented hj A \ B, between A and C hy A \ C
ELECTRICAL ACTIVITY OF ORE BODIES.
363
etc. The values of e, as derived both from the direct and return series, are
given in the table, the latter being primed. Heavy black lines across the
table indicate either that the bags were refilled or that the experiments had
to be temporarily discontinued.
Table XXL, containing ex 10".
So.
Points.
A|B
A|C
AID
A' IB'
A'lC
A' ID'
1
I
- 17
- 16
- 15
- 1
± 0
± 0
2
II
- 12
- 11
- 8
_ 2
± 0
+ 1
3
i
m
IV
- 16
- 15
- U
- 1
± 0
+ 1
+ 1
+ 1
+ 2
± 0
+ 1
+ 1
5
V
- 1
- 1
- 3
± 0
+ 1
+ 1
6
VI
- 2
- 2
_ 2
+ 1
+ 2
- 3
7
8
VII
IX
± 0
— 2
- 1
+ 0
+ 2
± 0
— 2
- 2
- 3
- 1
+ 1
+ 3
9
X
— 1
- 1
- 4
± 0
+ 0
- 1
10
XI
± 0
— 1
- 3
- 1
± 0
± 0
11
XII
— 4
- 3
- 5
- 1
± 0
J- 0
12
XIII
- 4
- 2
- 4
+ 2
+ 0
+ 1
13
XIV
- 1
- 1
- 2
- 1
+ 1
- 2
The successive values of £ are not constant, though in the majority of
cases they are so small as to be immaterial. At times, however, values suffi-
ciently large to be important are reached. An exchange of terminals is
therefore indispensable, especially as experiments will usually be sufficiently
extensive to involve interruptions. The gradual variation observed in the
value of e can most probably be referred to a corresponding change in the
concentration, etc., of the solution (zinc sulphate) contained in the bags. It
is hardly probable that it is due to polarization, or a change in the surface of
the amalgamated zinc strips. It is interesting that, in spite of the fact that
for the holes I., II., and III. the electromotive force between the bags in the
direct and return series differs largely, the lode-currents deduced from the
two sets of data are practically equal. (See Table XV.)
Wire. — In the above experiments especial care was taken to prevent
errors due to leaks in the wire. The galvanometer was sufficiently delicate
to register a fault of this kind of 5,000,000 ohms' resistance with certainty.
As has been mentioned, every leak introduces an electromotive force
zinc I copper;^ hence the great necessity, notwithstanding the fact that the
latter must act through a very great resistance, of avoiding leakage.
'For we have the closed couple: Copper (of wire); liquid (moist earth, etc.); zinc (of bag).
364 GEOLOGY OF THE COMSTOCK LODE.
General remarks. — TliG opinioii Has been expressed (page 35 1 ) that the field of
electric excitation is confined to particular parts of the ore body. That this
should be the case is not surprising, as the conclusion has already been
reached that contact between different kinds of material is necessary for the
production of currents. In the connection made between chambers No. 14
and No. 15, as well as in the survey on the 400 and 500-foot levels, the oi*e
actually met with was principally lead carbonate, at times stained with sul-
phide and ferric oxide. Now, disregarding the sulphide, which is here very
unfavorably associated, more pronounced electrical properties can hardly be
ascribed to the remaining constituents of the deposit than to the siirrounding
rock itself For, judging from physical properties, cerusite may be regarded
as an insulator with as much right as calcite, earthy lead carbonate as lime-
stone. In fact, it seems to follow that the feeble, though none the less
positive, reaction observed on the 600-foot level is already partially obscured
when the line of points on the 500-foot level is reached, and would perhaps,
coet. par., be equally obscured on the 700-foot level. I am also inclined to
infer that the currents observed on the surface are not due, or, rather, not
immediately due, to the deeper ore bodies (Nos. 11, 12, 13, 14, 15, etc.), but
to the deposits, also of considerable size, occurring in what are known as
the Lizette Tunnel workings. The entrance to the latter is on a level with
the mouth of the shaft, and the ore masses are distributed in a vertical range
from Point I. to a level even above Point X. on the surface. These ore
bodies, throughout their extent, are comparatively near the line of holes
used in the surface survey. It is, moreover, quite probable that an inti-
mate connection exists between these and the large group of ore bodies
below.
In consideration of the statements made in the foregoing paragraph,
and allowing as accurately as possible for discrepancies, the results thus far
reached may be regarded as agreeing well with the fundamental hypothesis.
ELECTRICAL ACTIVITY OF ORE BODIES. 365
CONCLUDING REMARKS.
On reviewing the results described it is strikingly evident that the
electromotive forces met with are invariably small, very frequently, indeed,
quite at the limit of the accurately measurable. It is true that the elec-
trically active material was probably galena, which, as Fox long ago ob-
served, is unfavorable for observations like the present. It is a question,
however, whether results much larger than these will generally be obtained.
I cannot believe that Reich's earnest appeal for genei'al research in the
direction of electric prospecting has been altogether disregarded. There
is much more to lead one to infer that many undertook the study of the
question, but, disappointed with feeble reactions and discordant results,
abandoned the matter altogether. Reich, at the end of his last paper,
gives a list of the apparatus desirable, which, however, except where the
action is so intense as it was found to be in Cornwall, and to a less extent
at Freiberg, would certainly be insufficient.
The very large currents obtained in the localities just mentioned ren-
dered it'not improbable, at the outstart of the present investigation, that
important conclusions might be drawn from the results of a minute mag-
netic survey^ of the interior of the mines, or across the vein on the surface ;
and preparations for such a purpose were, in fact, made. But the currents
obtained galvanometrically dictated the abandonment of this project.
The study of the electric activity of ore bodies should be carried out
on a broader basis than was possible in the present case, to reach the best
results. A single line of survey, or the investigation of the variation of
potential in a single drift, is far from sufficient. The endeavor should be
made to map the equipotentials as surfaces traversing the whole mine, care-
fully considering their position and contour relatively to any ore already
in sight, and their change of form on leaving it. The inferences to be drawn
herefrom would certainly compare in value with those of a purely geological
' Fox himself entertained an idea of this kind.
366 GEOLOGY OP THE COMSTOCK LODE.
character, even though dependence must be placed on the latter for a com-
plete interpretation of the results.
Furthermore, it will be desirable to carry out Fox's original idea, namely,
of investigating the electrical properties of ores and those minerals of the
heavy metals which are usually found associated with them; not that the
results of such an investigation could ever furnish a clew as to the par-
ticular ore to which an observed electric effect is due (it is here that our
knowledge of the locality must aid us), but that the class of ores, in pros-
pecting for which an electric method would be peculiarly applicable, could
thus be defined. The knowledge we possess of the conductivity and the '
position of ores in the electrical scale is largely the result of experiments
made a long time ago. Recent observers have made but few quantitative
additions, and even these — probably from improperly chosen methods — are
frequently discordant.
The method which has been described seems to me especially worthy
of consideration, from the fact that by means of it an electric survey, made
on the surface, may detect not only the presence but also the approximate
position of ore bodies under ground. With such an end in view the exper-
iments should be extended over a large area, and the potential at. all por-
tiqns of the surface determined. Suppose, now, that at each point of the
projection of the latter on a fixed horizontal plane, a vertical line is erected,
of a length proportional to the earth-potential at this point. The ends of
all such lines together make up a second imaginary surface, coextensive
with the first, which will represent the electric state graphically at each
point of the territory over which the survey has been carried.
The effect of normal earth-currents would then express itself in the
progress and contour of the imaginary surface as a whole, and would not
destroy its regularity. If its extent is not too large, this (normal) surface
will be a more or less inclined plane. As it has been observed that
earth-currents are not constant, even for short periods of time, the latter is,
moreover, to be regarded as slowly oscillating, more or less parallel to itself,
about a certain temporarily fixed position of equilibrium. But it is prob-
able that the limiting positions of the plane are so near to one another that
for this purpose they may be regarded as coincident.
BLECTEICAL ACTIVITY OF OEE BODIES. 367
Local action, however,, in contrast to the foregoing, would probably
manifest itself locally in the imaginary surface, as a hillock or depres-
sion. It is to such anomalies that attention should subsequently be
directed, the electric activity of ore bodies, differences of potential of liquids
in contact, and Matteuci's effects^ constituting the salient points to be consid-
ered. In the interest of expeditious work all measurements should be made
electrometrically, the lines of survey radiating from a central point (P. C.)
the potential of which is arbitrarily taken as zero.^
' Those mentioned, p. 352.
^The prosecution of the experiments described in this chapter was aided by the cordial coopera-
tion of Messrs. Patton, Lamb, anil Ballard, of Virginia City, and Messrs. Eickard, Westcott, Harris,
and Bryan, of Eureka, Nevada. The work is also indebted to Professor Micliie, of West Point, for the
loan of a Rowland magnetometer made by Mr. Wm. Grunow, of New York.
CHAPTER XI.
SUMMARY.
BY GEORGE F. BECKER.
Purpose of this chapter. — A verj lai'gc portion of the foregoing' pages is neces-
sarily occupied by detailed descriptions, written to enable readers to judge
whether the facts warrant the opinions expressed, and by discussions of
a somewhat technical character. There may be those, however, who
will be interested to know in brief what conclusions have been reached,
but who have no inclination to undertake the somewhat serious task of
weighing the evidence adduced, and of following the arguments in detail;
and for such tlie present chapter is written, but with the proviso that full
and fully qualified statements are to be found in the body of the report, and
there only.
History and statistics. — No more condcused statement of the technical and
economical relations of the Comstock mines can be presented than that
which is given in Chapter I., itself a meager abstract of reports which will
appear hereafter; nor is it necessary further to reduce the digests of the
previous memoirs on the Lode which constitute Chapter II. In some re-
spects the present volume is a tribute to the acumen of preceding observers,
upon whose investigations that here described is to a great extent built up.
That the recent development of the science of microscopical petrography
and the immensely increased facilities for observation, due to the extension
of the mine workings, should have led to some views different from those
heretofore entertained concerning the geology of the District is anything
but surprising.
368
SUMMARY. 369
LITHOLOGY.
Importance of the rock determinations. — In areas SO largely covercd bj massive
rocks as the Washoe District, lithological determinations form the neces-
sary preliminary to geological investigation, for few points in the history or
the structure of such a region are independent of the character of the rocks
involved. Moreover, the economical importance of the District, the ob-
scure character of some points in its geology, and the great weight of the
authorities whose investigations had already been published, made it essen-
tial that the work done under the new United States Geological Survey
should be supported by the strongest and most detailed evidence. The
collections embrace over 2,600 specimens and 500 microscope slides. The
locality of each specimen was fixed with great care on the maps at the
time of collection, and no time or pains was spared in preparing the geo-
logical maps and sections. In laying down the various formations the
microscope was in constant use, slides being ground as the occasion arose,
and the results obtained from them finding immediate application in the
extension of the work.
The area in which the Comstook lies is characterized by a wide-spread
and profound decomposition of the rock masses, and a study of the lithology
of the District resolves itself primarily into an investigation into decom-
position. In spite of the most painstaking choice of specimens, there is not
one in fifty of those collected underground which contains a particle of
either of the characteristic bisilicates or of the lithologically equivalent
unisilicate, mica, secondary minerals replacing them throughout. Even the
feldspars are rarely intact, and are sometimes wholly decomposed. When
the steps of these processes of degeneration are once understood, however,
it is comparatively easy to infer the original composition and structure of
the rock. Some of the results obtained are the following:
Decomposition. — Homblende, augite, and mica generally pass into a chlo-
ritic mineral, which, so far as can be judged by any optical tests now known,
is almost without exception the same, from whichever of the ferro-magne-
sian silicates it may have originated. This chlorite is generally green, but
in especially compact masses appears greenish-brown under the microscope.
24 c L
370 GEOLOGY OF THE COMSTOCK LODE.
It is strongly dichroitic, but except in dense masses appears nearly black
between crossed Nicols. It is fibrous, often spherolitic, and invariably ex-
tinguishes light parallel to the direction of the fibers. It thus bears a con-
siderable resemblance to fibrous green hornblende, but the cases are very
rare, if they actually occur, in which a careful examination will not serve
to discriminate between the minerals. This chlorite is decidedly soluble.
It occurs in veinlets and difi'used through the groundmass and through other
minerals when these have become pervious through decomposition. It is
especially striking as an infiltration in altered feldspars, where, of course, it
is readily visible. All the stages can be traced, from the first inconsiderable
attack upon the bisilicates or the mica through instances in which chlorite
occurs wholly or almost wholly as admirable pseudomorphs after the ferro-
magnesian silicates, and up to cases in which the secondary mineral is wholly
diffused through the mass of other products of decomposition.
Epidote is usually in Washoe a product of the decomposition of chlo-
rite. Comparatively very few occurrences of epidote are explicable on the
supposition that the mineral is the direct result of the decomposition of the'
primary siHcates; none are inexplicable on the supposition that chlorite
represents an intermediate stage in the alteration, and hundreds of cases
show beyond question that epidote develops in chloritic masses, sending
characteristic denticles and fagot-like ofi"shoots into the comparatively homo-
geneous chlorite. Several drawings illustrating these processes are shown
in Plate II. They are photographic in their fidelity. Epidote, too, is pos-
sibly soluble to a very slight extent, but certainly far less so than chlorite.
The veinlets of epidote are often, though perhaps not always, a result of
the alteration of chlorite. No evidence has been obtained that feldspars
are ever converted into epidote, and the dissemination of fresh hornblende
particles in feldspars in any considerable number has not been observed.
In many cases, on the other hand, it can be shown that feldspars have been
impregnated with chlorite, from which epidote has afterwards developed.
Chlorite does not always change to epidote, and appears often to be replaced
by quartz and calcite. This is frequently visible in shdes which also show
its alteration to epidote. No certain evidence of the alteration of epidote
has been met with.
STJMMAET. 371
In the decomposition of the feldspars, the first stage appears to be the
formation of calcite. This sometimes leaches out, leaving small irregular
cavities, and these cavities are not infrequently filled with liquid, some-
times carrying a bubble, which is commonly stationary, but occasionally
active. Thus secondary liquid inclusions are formed, which may mislead
in the diagnosis of a rock. Primary liquid inclusions are either more or
less perfect negative crystals or vesicular bodies. The vesicles often assume
strange forms through pressure, such as are often observed in air-bubbles
in the balsam of a slide, but their outlines are composed of smooth curves.
The secondary fluid inclusions are bounded by jagged lines. Inclusions of
this kind are never met with unaccompanied by other evidences of decom-
position, and thus are abundant in the altered outer crust of andesite masses,
the inner portions of which show none of them. There is every reason to
suppose that such secondary inclusions would form in older rocks, and it is
believed that many of them have been detected in the pre-Tertiary erup-
tives of the District; but in the older rocks their secondary character can
only be suspected, not proved.
Kaolin possesses so few characteristic optical properties that it is not
recognized with ease or certainty under the microscope. No kaolin has been
identified in the Washoe rocks, and while it is by no means asserted that
they contain none, it seems hardly possible that, had it formed a prominent
constituent, it would have escaped observation. The presence of enormous
masses of "clay" on the Comstock does not prove the existence of much
kaolin, for the so-called clays of veins are largely attrition mixtures.
An increase in volume appears to accompany the decomposition of the
"Washoe rocks. This is perceptible where dense masses, such as the more
compact andesites, are subjected to the process. Angular blocks are then
converted into a series of concentric shells of comparatively soft matter,
which approach the spheroidal shape more and more as the diameter dimin-
ishes.^ Often a nodule of undecomposed rock is found at the center, and
such masses aff"ord the very best opportunity for studying the macroscopical
appearances resulting from degeneration. When the attacked mass is large,
'Prof. R. Pumpelly has described the course of decomposition almost in the same words, in his
paper " On the Relation of Secular Rock-disintegration to Loess, etc." (Amer. Journ. XVII., 1879, 136.) I
did not happen to see Professor Pumpelly's paper until after this passage was written.
372 GEOLoaY OP the comstock lode.
erosion often exposes the fresh core, wliich then, oflfering greater resistance,
projects as a " cropping," or, if it has an elongated form, it protrudes hke a dike
above the surrounding country. As the tendency of the mere action of atmos-
pheric agencies is to the production of ferric hj^drate rather than of chlorite
from the bisilicates, the first impression which such a mass produces is that
of an older and a younger rock in conjunction. Nevertheless, sufficiently
thorough examination will reveal a transition. When the rock is not solid,
but bx'ecciated or loose-grained, sufficient space often seems available to
permit the requisite increase of volume without disintegration. Large and
often i^rominent masses of very strongly cohesive decomposition-products
derived from breccia are common in the District.
The mineralogical character and the microscopical phenomena of de-
composition seem to be identical in the different rocks. Those refined mani-
festations of physical character by which it is so often possible to discriminate
between older and younger rocks, and between the various rock species
when fresh, are nearly or quite obliterated by the decomposition process,
which impresses its own character on the pi'oduct.
Rocks of the District. — The rocks occurring in the Washoe District are gran-
ite; metamorphic schists, slates, and limestone; eruptive diorite of three
varieties; metamorphic diorite ; quartz-porphyry ; an older and a younger
diabase; an older and a younger hornblende-andesite ; augite-andesite, and
basalt.^ Chapter III. contains a discussion of each of these rocks and a
detailed description of about seventy-five slides, and is well illustrated.
Here they can be dismissed with a very few remarks.
'The signification attached to these names has varied somewhat as the science of lithology has
progressed. Some of the main points of their definitions as here understood are as follows :
Granite, pre-Tertiarynon- vitreous crystalline rock, of which the principal constituents are ortho-
clase, quartz, and mica or hornblende.
Diorite, pre-Tertiary non- vitreous crystalline rock, of which the main constituents are plagioclase
and hornblende. It may or may not contain quartz.
Quartz-porphyry, pre-Tertiary glass-bearing porphyritic rock, of which the main constituents
are orthoclase, quartz, and hornblende or mica.
Diabase, pre-Tertiary, more or less porphyritic rock, of which the main constituents are plagio-
clase and augite.
Andesite, Tertiary or post-Tertiary, glass-bearing, more or less porphyritic rock, of which the
main constituents are plagioclase and hornblende, mica, or augite. The andesites in which augite is
the characteristic bisilicate appear to be separate eruptions, while mica and hornblende replace one
another to a variable extent in the same eruption. In the andesites feldspar predominates.
Bas.alt, Tertiary or post-Tertiary plagioclase augite rock, with predominant augite, usually char-
acterized by the presence of olivine.
SUMMARY. 373
Concerning the granite and basalt there has scarcely been a question.
They are eminently characteristic occurrences. The metamorphic diorite
sometimes resembles eruptive diorite, and has been taken both for diorite
and granite ; usually it bears some resemblance to augite-andesite or basalt,
and has been determined microscopically as an unusual variety of the
latter rock. It is composed essentially of oligoclase and hornblende. The
hornblende was originally coloi'less, but through some chang-e (perhaps
absorption of water) it is in large part converted into an intensely green
variety. The hornblende polarizes in unusually intense colors.
The quartz-porphyry underlies both hornblende-andesite and diabase.
The microscope, Thoulet's method of separation, and analysis, show that
the predominant feldspar is orthoclase. It is characterized by the associa-
tion of liquid and glass inclusions usual in quartz-porphyry, to which also
the groundmass corresponds. In one locality, near the Red Jacket, the
quartz is nearly suppressed, and the rock is excessively fine-grained. It is
a felsitic modification of the ordinary variety. This rock, which Baron v.
Richthofen determined correctly, has since been called quartz-propylite,
dacite, and in its felsitic modification rhyolite. Most of the quartz-porphyry
is greatly decomposed.
The eruptive diorite is sometimes granular, sometimes porphyritic. In
the porphyritic diorite mica frequently predominates over hornblende.
Quartz is irregularly disseminated through the rock. In the granular dio-
rite the hornblende is sometimes green and fibrous, sometimes brown and
solid. In some cases it can be shown that the latter variety of hornblende
is altered to the former, and possibly this is ordinarily the case. Augite is
not uncommon, and a part of the fibrous green hornblende is very likely
uralite, but in the granular rock the outlines of the crj^stalline grains are
rarely sufficiently regular to determine this point. In the porphyritic dio-
rites the fresh hornblende is always brown. Even in this latter variety of
thediorites well-developed feldspars are rare. The porphyritic diorites have
for the most part been regarded as propylite, and some occurrences of the
granular rock have been classed in the same way. Some of the fresher
porphyritic diorites have been mistaken for andesites, the resemblance to
which is occasionally strong.
374 GEOLOGY OF THE COMSTOCK LODE.
The older diabase is porphyritic, and almost the whole of it is in a very
advanced stage of decomposition. When fresh, it considerably resembles
an augite-andesite; its groundmass, however, is thoroughly crystalline and it
contains no glass inclusions, but frequent fluid ones; the augites, too, show
both pinacoidal and prismatic cleavages, and a tendency to uralitic decom-
position. It is also manifestly older than the other diabase. An important
characteristic is the lath-like development of the porphyritic feldspars, for
in cases of extreme decomposition of the bisilicates this characteristic at
least serves to suggest whether the rock is dioritic or diabasitic. The older
diabase has been considered as propylite or andesite, according to the stage
of decomposition. The younger diabase ("black dike") is very highly
crystalline and not porphyritic. It is bluish when fresh, but in course of a
few hours turns to a smoky brown. It is identical with many of the dia-
bases of the New England and the Middle States.
The older hornblende-andesite and the augite-andesite where fresh are
typical rocks macroscopicall}^ and microscopically. When decomposed they
have been taken for propylite. The younger hornblende-andesite which
overlies the augite-andesite is a cross-grained, soft, often reddish or pur-
plish rock, with large glassy feldspars. It has always been supposed to be
trachyte; but when endeavoring to determine the different species of feld-
spar under the microscope, I was unable to include an}' satisfactorily deter-
minable sanidins in the list. Dr. G. W. Hawes was kind enough to under-
take the separation of the feldspars by Thoulet's method, and analyses of
the feldspars were made by Mr. F. P. Dewey. The specimen selected was
the most trachytic in appearance, that of Mount Rose, but no feldspar what-
ever was found corresponding either physically or chemically to orthoclase.
There is much reason to believe that trachyte occurs less often than had
been supposed in the Great Basin area.
Apart from the effects produced by decomposition the Washoe rocks
are typical of their kind, and correspond to representative specimens of the
same species from other parts of the world, even in the minutiae of miner-
alogical composition and physical structure. This persistence of rock types
in minor features, which would seem to be fortuitous, or at least unessential,
is one of the most remarkable facts established by microscopical lithology,
STJMMAEY. 375
and indicates a repetition of absolutely identical physical and chemical con-
ditions at distant points, which is far from having received an adequate ex-
planation.
propyiite. — Thc prcscnt investigation of the geology of the Washoe Dis-
trict has failed to establish the existence of propyiite. Full proof of this
responsible statement cannot of course be given in this summary of results.
It consists in a process of exhaustive elimination. A study of each of the
rocks of the District, in all stages of decomposition, has led to the identi-
fication of all of them with other and previously recognized species. The
reduction of rocks of originally different aspect to an apparently uniform char-
acter by chl critic decomposition is strikingly evinced by a mere list of the
species in the District, which have been grouped under the terms "propy-
iite" and " quartz-propylite." These are granular diorite, porphyritic diorite,
diabase, quartz-porphyry, hornblende-andesite, and augite-andesite. The
peculiar habitus which is always referred to in descriptions of propyiite
appears to consist in the impellucidity of the feldspars, the green and fibrous
character of the hornblende, the greenish color which often tinges feldspars
and groundmass, and a certain blending of the mineral ingredients. The
impellucidity of the feldspars (which surprisingly alters the appearance of
rocks originally containing transparent unisilicates) is due to incipient
decomposition, especially, as it seems, to the extraction of calcite. The
"green hornblendes" are simply pseudomorphs of chlorite after hornblende
or augite, as the case may be. Excepting the granular diorite, not one of
the rocks from which propyiite forms has ever been found in the Washok
District containing primitive green hornblende, though uralite is common.
The other characteristics are due to the diffusion of chlorite and the forma-
tion of epidote from it. The description of propyiite as a species arose from
the erroneous determination of chlorite as green hornblende — a very natural
mistake before the microscope was brought to bear on the subject, since even
with that instrument the same error may be committed if color and dichroism
are exclusively relied upon as diagnostic tests. The microscopical charac-
teristics of propyiite are illusory. Finely disseminated hornblende in the
groundmass of a Washoe rock is very rare, and far rarer is the presence of
particles of hornblende in feldspars. The propylites contain glass inclusions
376 GEOLOGY OF THE COMSTOCK LODE.
and primitive liquid inclusions, or not, according- to the rock from which
they were derived. Base is rare in propylites; where it originally formed
a constituent of the rock, it has for the most part undergone devitrification.
A reexamination has been made of all the slides of propylites from
other localities as well as from the Washoe District, descriptions of which
have been published in different government reports. These, too, can be
referred to other rock species with great probability, in spite of advanced
decomposition, and I do not hesitate to affirm that there is no proof yet known
of the existence of a pre-andesitic Tertiary eruption in the United States.
The term "propylite" should not be retained in the nomenclature of
American geology even to express certain results of decomposition, for the
equally loose term "greenstone" seems to cover the same ground and has
priority.
A few minor questions of interest were raised by the microscopic
examinations, in addition to those bearing directly upon the identification
of the rocks. Such are the occurrence of zonal plagioclases and their bear-
ing on Tschermak's feldspar theory; hornblendes with concentric belts of
magnetite, and the indications they furnish as to the conditions under which
"black borders '' form, and some other small points.
STEUCTUEAL RESULTS OF FAULTING.
Evidences of faulting. — The evidence of faulting on the Comstock is mani-
fold, and has been recognized by all observers. The irregular openings in
the vein, the presence of horses, the crushed condition of the quartz in many
parts, the presence of slickensides and of rolled pebbles in the clays, are
all conclusive on this point. Both to the east and west of the vein, too, the
country rock shows a rude division into sheets, and along the partings
between the plates evidences of movement are perceptible, decreasing in
amount as the distance from the vein increases, according to some law not
directly inferable. All the evidence points to a relative upward move-
ment of the foot wall.
SUMMAEY. 377
The question of the character of the contact surface, whether it is
a faulted sui-face or a continuation of a former exposure of the east front
of Mount Davidson, is not to be settled by mere inspection. A cross-sec-
tion to scale shows immediately that while the dip of the lode is 40° or
more, the maximum slope of Mount Davidson is about 30°. This fact,
taken in connection with the character of the west wall where exposed, indi-
cates that the surface is the result of faulting. A natural surface sloping
for a long distance at an angle of above 40°, too, is very unusual. On the
other hand, the coincidence between the contours of the west wall and those
of the exposed surface has been notorious from the earliest days of mining
on the Lode, and it seems a less violent supposition that the steep flank of
the mountain passes over into the still steeper wall of the vein than that
the range has experienced an erosion modifying its angle from 10° to 20°
and has still retained the details of its topography otherwise unaltered. It
is plain that the elucidation of the faulting action on the Comstock is a
very important structural problem, and that it is most desirable to account
quantitatively for the results, as well as to prove the existence of a notable
dislocation.
Discussion of faulting under certain conditions. The mOSt Striking and wide- Spread
evidence of the faulting is the apparent relative movement on the contact sur-
faces between more or less regular sheets of the east and west country rocks
for a long distance in both directions from the Lode. Each sheet appears
to have risen relatively to its eastern neighbor, and to have sunk as com-
pared with the sheet adjoining it on the west. The consideration of a
sheet or plate of rock under the influence of friction of a relatively oppo-
site character on its two faces, therefore, forms the natural starting point for
an examination of the observed conditions. It is shown in Chapter IV. that
if a country divided like the Comstock area into parallel sheets experiences
a dislocation on one of the partings under a compressive strain equal at
each parting, a vertical cross-section will show a surface line represented
by two logarithmic equations. The discussion is also extended to the case
in which the compressive strain is not uniform, but varies proportionally to
the distance from the fault-plane. This case also results in a logarithmic
equation of a more complex character.
378 GEOLOGY OF THE COMSTOOK LODE.
A discussion of the logarithmic equation as an expression of faulting
action leads to some very interesting results, some of which are as follows:
Where a fault of the class under discussion has occuiTed, and where
the resulting surface has not been obscured by deep erosion, the original
surface can be reconstructed or calculated, and the amount of dislocation
determined. This is also true where more than one rock is involved.
Where, as is nearly always the case, the movement on the fault-plane
is equivalent to a rise of the foot wall, the hanging wall seen in cross-sec-
tion will assume the form of a sharp wedge, and this wedge will be very
likely to yield to the compressive strain, and break across.
If the movement of the foot wall on the fault-fissure were downward,
a surface line would form which is scarcely ever met with in nature, and
the inference is that faults of this kind are of extreme rarity. This not
only confirms the observations made in mines, but places the fact on a wider
basis of observation.
If a fault, accompanied by compressive strain, takes place on a fissure
in otherwise solid rock, the walls are likely either to be distorted, if they
are composed of flexible material, or to be fissured into parallel plates if the
material is rigid. In the latter case the sheets of rock will also arrange
themselves on logarithmic curves.
If the intersection of a fault-fissure with the earth's surface is not a
straight line, but is sinuous or broken, the secondary fissures will be parallel
to the original one, and in the resulting surface each inflection of the trace
of the fissure on the original surface concave toward the lower country will
be represented on the faulted surface by a ravine, and each inflection con-
vex towards the lower country will result on the faulted surface in a ridge.
There is also a direct relation between the contours of the foot wall of such
a fissure and the surface contours. If the original surface was a horizontal
plane, the surface contours will be identical with the foot wall contours.
Application to the comstock. — The thcory, though worked out independently of
the Comstock, applies to it with much precision. Equations can be given
representing very closely the surface line of a cross-section, the amount of
the fault can be determined, etc. It can be shown that the erosion since the »
beginning of the fault is very slight, that the canons of the range were pro-
SUMMAEY. V 379
duced by faulting, and have been only slightly modified by erosion, whence
the correspondence of the contours of the foot wall with those of the sur-
face. The east fissure is a result of the faulting, and the ore has been de-
posited since Washoe became a region of insignificant rainfall. The sheeted
structure of the country is, in all probability, due to the fault.
It is, of course, most unlikely that the Comstock is the only vein in
which the deposition of ore is recent and has been accompanied by faulting,
and a repetition of a part of the conclusions as to the occurrence of veins
in such cases may be welcome to some readers.
Application to other veins. — lu a locality modlficd by faulting action, attended
by horizontal pressure, the fact will appear in the parallelism of the exposed
edges and faces of rock sheets. If erosion has not seriously modified the
surface resulting from the faulting action, the logarithmic curve will be
recognizable to the observer looking in the direction of the strike.
The main cropping of the vein is to be sought at the point of inflec-
tion of the curve, which will be found nearly or exactly midway between
the top and bottom of the hillside. One or more secondary vein-croppings
should be looked for below the main cropping, and these, so far as yield is
concerned (but not in regard to location of claim), may prove even more
important than the main fissure.
The dip of the vein will be to the same quarter as the slope of the sur-
face, but, of course, greater in amount. The flatter the surface curve the
smaller the angle of dip will be. The mean strike will be nearly or quite
at right angles to the direction of the spurs and ravines of the faulted area.
If, besides the movement of one or the other wall in the azimuth of the
dip, there has been a dislocation in the direction of the strike, chimneys
will open, all of them on the same sides of the different ravines. Surface
evidences will often enable the prospector to determine on which side the
chimneys are to be found. On the barren sides evidences of crushing and
of closure of the fissures are probable.
The fissure is more likely to have a constant dip (barring the second-
ary off"shoots) than a constant strike, but, of course, irregularities of dip,
like those in strike, will result in chambers which may be productive.
Offshoots into the hanging wall may occur at any depth, but none.
380 GEOLOGY OP THE COMSTOCK LODE.
except those near enough to the main cropping to reach the surface where
it has a very considerable slope, are likely to be continuous.
Finally, it is shown that the law of land slips is also capable of expres-
sion by logarithmic equations, and that a large part of the details of the
topography of grassy hills is formed in obedience to this law.
OCOUERENCE AND SUCCESSION OF BOOKS.
Succession. — Tho succcssiou of rocks made out in the Washoe District is
as follows: Granite, metamorphics, granular diorites, porphyritic diorites,
metamorphic diorites, quartz-porphyry, earlier diabase, later diabase ("black
dike"), earlier hornblende-andesite, augite-andesite, later horablende-ande-
site, and basalt.
Granite and metamorphics. — Granite occurs ou thc surface only in a very lim-
ited area near the Bed Jacket mine, but it is certain that it has a considerable
underground development, for it has been struck at the Baltimore, the Bock
Island, and by a tunnel to the southwest of the latter beyond the limits of
the map.
The granite is overlaid by metamorphic rocks, which, Jiowever, are
less metamorphosed close to it than at a distance from it, and the probabil-
ities are that the sedimentary strata were laid down upon the massive rock.
The sedimentary rocks are limestones, crystalline schists, and slate. They
are badly broken and highly altered, and the search for fossils was not
rewarded by success; but the general geology of this part of the Great
Basin leaves little doubt that they are Mesozoic. A considerable area
of metamorphics has been exposed in the southwest of the region by the
erosion of the overlying eruptive masses. North and east of Silver City,
however, the surface shows scarcely any metamorphics, while they play a
large part in the underground occurrences as far as the Yellow Jacket. In
the Gold Hill mines black slates form the foot wall of the Lode. They are
intensely colored with graphite, and often very highly charged with pyrite.
They are frequently mistaken for "black dike," but a moment's inspection
in a good light shows their sedimentary origin. The presence of such car-
bonaceous rocks at greater depths would explain the formation of hydrogen
SUMMARY. 381
sulphide. There is also an obscure occurrence of metamorphic limestones
in the Sierra Nevada mine between granular and micaceous diorite. It
appears to be conformable to the face of the granular diorite. The meta-
morphics in and aboiit Gold Hill seem both to overlie and to underlie
diorite, and there is little doubt that sedimentary strata were present at the
period of the diorite eruption.
Between the metamorphics and the quartz-porphyry in the southwest
portion of the area is a considerable extent of metamorphic diorite. In
some occurrences this rock is a distinct breccia, and bears a strong resem-
blance to augite-andesites or basalts, while elsewhere it is extremely like
Mount Davidson diorite. Besides the surface occurrences, it is found par-
ticularly well developed in the Silver Hill mine.
Diorites. — The principal exposure of diorites is on the west of the Lode
through Virginia City, but there are several outlying occurrences about
the Forman shaft, and again far to the east at the Lady Bryan mine, which
show that the underground development of the rock is a very extensive one.
It forms the foot wall of the Lode from the Yellow Jacket north. The diorite
is excessively uneven in its composition, and in almost any area of a hun-
dred feet square sevei'al modifications are to be found. This fact, taken in
connection with the microstructure of the rock, is pretty conclusive evidence
that it has never reached a higher degree of fluidity than the plastic state.
The varieties can be roughly classified as granular diorite, porphyritic horn-
blendic diorite, and porphyritic micaceous diorite. But intermediate varie-
ties are of constant occurrence. There seems, nevertheless, to be a certain
amount of order in the disposition of the different varieties. Mount David-
son, from Bullion Ravine to Spanish Ravine, is almost altogether granular,
but to the north and south of these limits porphyritic forms prevail. In
the neighborhood of the Utah mine mica becomes the predominant ferro-
magnesian silicate, and this variety is also the one which occurs in the
neighborhood of the Forman shaft. How this orderly disposition of the
various diorites came about is a somewhat obscure question, as a possible
answer to which an hypothesis is advanced.
Diabases. — The diabaso appears but to a very trifling extent upon the
surface, though it is by no means unlikely that an exposure of this rock
382 GEOLOGY OP THE COMSTOOK LODE.
occupied the position now covered by Virginia City. Underground it is
extensively developed from the Overman to the Sierra Nevada, and from the
Lode to the Combination shaft, as is seen in the cross-section on the Sutro
Tunnel line, Atlas-sheet Vl. Its great importance is due to the fact that all
the important bodies of the Comstock have been intimately associated with
it, as are many of the other famous silver mines of the world. This diabase
is of a rather unusual character, being more than commonly porphyritic, and
containing comparatively little augite — a trifle less than twenty per cent.
In appearance it is often not dissimilar to the andesites, but the resemblance
does not extend to details. Almost the whole of this diabase is greatly
decomposed, and has hitherto escaped recognition on that account.^ >
Between the east-country diabase and the west wall of the Comstock
occurs a thin dike, which has long been known as " black dike." It is only
in the lower levels that fresh occurrences of this material have been met
with. The "black dike" appears to be identical with the Mesozoic diabases
of the Eastern States, from which it is scarcely distinguishable macroscopic-
ally, microscopically, or chemically. This younger diabase forms a remark-
ably thin and uniform dike, nowhere more than a few feet in thickness,
extending from the Savage southward to the Overman, and then branching
off to the southwest as far as the Caledonia shaft. This is the only dike
known in the District, excepting one of diorite in dionte, in spite of the
prevalence of eruptive rocks. Its presence shows that the fissure on which
the Comstock Lode afterwards formed was first opened in pre-Tertiary
times, and its uniform thickness indicates that its intrusion antedates any
considerable dislocation on the contact. This inference receives strong
confirmation from the evidence alread)' adduced that the faulting is a
comparatively recent phenomenon.
The occurrence of the two diabases also goes a long way toward
demonstrating the nature of the fork in the vein, which has always been a
mysterious point in the geology of the Lode. The prolongation of the
" black dike" beyond Gold Hill is toward American Flat, whereas the older
diabase extends in the direction of Silver City.
Andesites. — Much the larger part of the surface of the District is occupied
'Though diabase is the most iraportant east-country rock, it by no means coincides in position
either below ground or above with the rocks which have been regarded as propylite.
STJMMAEY. 383
by andesites, of which there are three varieties distinguishable both htho-
logically and geologically. These are a younger and a later hornblende-
andesite, the latter of which has hitherto been considered a trachyte, and
an augite-andesite intermediate in age. The older hornblende-andesite has
in part long been recognized as such, and is deceptive only when highly
decomposed. It occupies a belt immediately east of the older diabase (see
Atlas-sheet VII.), a large area on the heights immediately west of the
diorites, and a considerable area at and north of Silver City. The latter
occurrence is noteworthy for the unusual size of the hornblendes, which
are sometimes several inches in length. The augite-andesite occupies a
second belt of country east of the Lode and beyond the earlier hornblende-
andesite, and is also extensively developed to the north and south of the
diorite. The Forman shaft penetrates 1,200 feet of this rock before pass-
ing into the hornblende-andesite.
The reasons are given elsewhere for considering the rock heretofore
regarded as trachyte to be an andesite. Its roughness and softness, its red
and purple colors and large glassy feldspars made the mistake an easy one.
The Flowery Range, the Sugar Loaf, Mount Emma, and Mount Rose, are
all of this rock, which also occurs in two little patches close to the Sierra
Nevada mine. These latter have been cut off from the quarr}^ above the
Utah by the erosion of Seven-Mile Canon. The patches of rock near the
Combination shaft and the new Yellow Jacket which have sometimes been
regarded as trachyte are merely decomposed older hornblende-andesite.
Basalt. — The occurrence of basalt is exceedingly limited, and is confined
within the area of the map to two small localities, one at Silver City and
the other a mile west of it. It is a fine, fresh, and typical rock.
Area of decomposition. — Thc aroa of most profouud decomposition is shown
as nearly as may be on the sketch map. Fig. 1 . The amount of decomposi-
tion increases with depth. The period at which it was produced is almost
certainly the same as that of the faulting action and the deposition of ore.
It cannot have been earlier than the eruption of the later hornblende-ande-
site, and was more probably posterior to it. There is no indication of a
connection between the basalt eruption and the solfataric action, and it is
not improbable that the latter, though of volcanic origin, was independent
of any eruption of lava.
384 GEOLOGY OF THE COMSTOCK LODE.
CHEMISTRY.
The chemical history of the Comstock is no doubt a very complex
one, nor are there by any means sufficient data to trace it in detail. AH
that can be attempted here is to show that the results observed might
naturally follow from highly probable causes.
The decomposition of the rocks shows three important features — the
formation of pyrite from the bisilicates and mica, the decomposition of the
ferro-magnesian silicates into chlorite, which is in part further altered to
epidote, and a partial change of the feldspar.
Decomposition of the Fe.-Mg. sUicates. — The pyrite appcars to have formed at the
expense of the bisilicates or mica. The really fresh rocks contain no pyrite,
but minute crystals often occur in or are attached to partially decomposed
bisilicates. Sometimes distinct pseudomorphs of pyrite after augite or
hornblende are visible, but this is not common, because the average size of
the pyrite crystals is about one-half that of their hosts. A macroscopical
comparison, too, of series of rocks increasingly decomposed shows that the
pyrite is apparently associated with the ferro-magnesian silicates, and in
extreme cases replaces them with an entire correspondence of distribution,
so that the cumulative evidence is all in one direction. It is well known
that ferrous silicates in contact with waters charged with hydrogen sulphide
produce pyrite.
The transformation of the bisilicates and mica to chlorite is a familiar
fact, and the general character of the change is not obscure, though its
details are far from clear. It must be accompanied by a separation of
all the lime, and of much of the silica and magnesia. It probably took
place for the most pai-t in the absence of free oxygen.
Epidote is very common on the surface, while under ground it seems
rare and confined to the neighborhood of fissures. The conversion of
chlorite to epidote must be accompanied by a substitution of lime for mag-
nesia, and by the conversion of ferrous to ferric oxide It might very
readily occur in the presence of solutions containing carbonic acid and free
oxygen, or when surface waters mingled with waters lising from lower
SUMMAEY. 385
levels, for epidote is far less soluble than chloritg, and under these circum-
stances would form in obedience to the general law of precipitation. Its
occurrence is usually compatible with this supposition, but it is not so deci-
sive as to warrant a positive assertion that the conditions of its formation
are those indicated.
Decomposition of feldspars. — The tricliuic feMspars of the Washoe District
retain their optical properties in a recognizable form much longer than the
ferro-magnesian silicates. Among the mine rocks it is very rarely that bisili-
cates or mica occur undecomposed, but it is the exception when a slide of a
tolerably hard rock does not show recognizable feldspars. When the feld-
spars are altered they are replaced by an aggregate of polarizing grains,
which appear to be quartz and calcite with some opaque particles, but with
no transparent amorphous material. Kaolin could hardly be present in
large quantities without being recognized microscopically. The analyses
of the clays, too, show that when allowance is made for the presence of
hydrovis chlorite there is not enough water to correspond to any large
percentage of kaolin. In fact the analyses of the clays so exactly cor-
respond to the composition of the firm rocks that the great masses of clay
evidently represent only equal volumes of disintegrated rock. On the
whole, therefore, it appears improbable that there has been any great amount
of kaolinization in the Washoe Distkict.
Lateral-secretion theory. — As is wcll kuowu, Prof. F. Saudbergcr has very
ably maintained what is known as the lateral-secretion theory of ore depos-
its. With a view to testing the probabilities of this theory, with reference
to the CoMSTOCK, the rocks of the District have been assayed with all pos-
sible precaution. The principal rocks containing precious metals were also
separated by Thoulet's method, and the precious metals traced to their
mineralogical source. The results of this investigation show many inter-
esting facts, among which are the following: The diabase shows a note-
worthy contents in the precious metals, most of which is found in the augite;
the decomposed diabase contains about half as much of these metals as the
fresh rock; the relative quantities of gold and silver in the fresh and decom-
posed diabase correspond fairly well with the known composition of the
CoMSTOCK bullion; and the quantity of precious metals which has been
25 c L
386 GEOLOGY OF THE COMSTOCK LODE.
leached out of the diabase is comparable with that which the Lode must
have contained at its discovery. There are also relations between the
inclosing rocks and the ore deposits not found in contact with diabase.
The gangue of the Comstock is almost exclusively quartz, though cal-
cite also occurs in limited areas. The ore minerals elude investigation for
the most part because they are so finely disseminated as merely to stain
the quartz, but it is fairly certain that they are principally argentite, and
native silver and gold, accompanied in some cases by sulph-antimonides,
etc. The chloride has rarely been identified. Where ore is found in dio-
rite, or in contact with it, it is usually of low grade, and its value is chiefly
in gold. The notably productive ore bodies have been found in contact
with diabase, and they have yielded by weight about twenty times as much
silver as gold.
Reagents. — It would perhaps be legitimate to infer from the chemical
phenomena enumerated and the .association of minerals that waters charged
with carbonic acid and hydrogen sulphide had played a considerable part
on the Comstock. This is not, however, a mere inference, for an advance
boring on the 3,000-foot level of the Yellow Jacket struck a powerful stream
of water at 3,080 feet (in the west country), which was heavily charged with
h3-drogen sulphide and had a temperature of 170° F , and there is equal
evidence of the presence of carbonic acid in the water of the lower levels.
A spring on the 2,700-foot level of the Yellow Jacket, which showed a tem-
perature of above 1.50° F., was found to be depositing a sinter largely com-
posed of carbonates.
Baron v. Richthofen was of opinion that fluorine and chlorine had
played a large part in the ore deposition on the Comstock, and that this
is possible cannot be denied; but, on the other hand, it is plain that most
of the phenomena are sufficiently accounted for on the supposition that the
agents have been merely solutions of carbonic and hydrosulphuric acids.
These reagents will attack the bisllicates and feldspars. The result would
be carbonates and sulphides of metals, earths and alkalies, and free quartz ;
but quartz and the sulphides of the metals are soluble in solutions of car-
bonates and sulphides of the earths and alkalies, and the essential constitu-
ents of the ore might, therefore, readily be conveyed to openings in the
SUMMAEY. 387
vein, where they would have been deposited on relief of pressure and dimi-
nution of temperature. It is by no means unlikely that, as at Steamboat
Springs, evaporation aided in inducing precipitation
Substitution. — It has been claimed that the ore and qviartz have been
deposited by substitution for masses of country rock. This hypothesis is
exceedingly doubtful on chemical grounds, but there is also at least one in-
superable physical objection to it. In all processes involving the solution
of angular bodies it is a matter of common observation that points and
corners, which expose a greater surface than planes, are first attacked; conse-
quently masses exposed to solution, substitution, weathering, and the like,
always tend to spheroidal forms. Now, nothing is more common than to
find masses of country rock included in the ore-bearing quartz. These
masses, in all cases which have come under my observation, are angular
fragments, in form precisely such as result from a fresh fracture; not a
single instance has been observed in which a spheroidal rock was sur-
rounded by more and more polyhedral concentric shells of quartz and ore.
HEAT PHENOMENA OF THE LODE.
High temperatures met. OuC of thc faUlOUS pCCuliaritieS of the COMSTOCK
Lode is the abnormally high temperature which prevails in and near it.
This manifested itself in the upper levels, and has increased with the depth.
The present workings are intensely hot. The water which flooded the lower
levels of the Gold Hill mines dui-ing the winter of 1880-1881 had a tem-
perature of 170° F. This water will cook food, and will destroy the human
epidermis, so that a partial immersion in it is certain death. The air in the
lower levels more or. less nearly approaches the temperature of the water
according to the amount of ventilation. The rapidity of the ventilation
attained in the mines is something unknown elsewhere, yet deaths in venti-
lated workings from heat alone are common, and there are drifts which,
without ventilation, the most seasoned miner cannot enter for a moment.
Except where circulation of air is most rapid, and in localities not far
removed from downcast shafts, the air is very nearly saturated with moist-
388 GEOLOGY OF THE COMSTOOK LODE,
lire. It is a serious question how far down it will be possible to pusli the
mines in spite of the terrific heat.
The origin of the high temperature of the Comstock has been sought
in the kaolinization of the feldspar contained in the country rock and in
residual volcanic activity.^
Kaolinization hypothesis. — The theory that kaoliuizatiou is the cause of the
heat appears to rest upon two positive grounds — that the solidification
of water liberates heat, and that flooded drifts have been observed to grow
hotter. It is also claimed in favor of the kaolinization hypothesis by its
author that there is no evidence of an}' other chemical action proceeding
with sufiicient activity to afford an explanation, and that the retention of
igneous heat in the rocks is a sheer impossibility, while the hypothesis that
the heat is conveyed from some deep-seated source to the mines by means
of currents of heated water is characterized as somewhat violent and as
unnecessary.
So far as I am aware, there are no theoretical grounds upon which
the heat involved in kaolinization can be estimated. The decomposi-
tion of feldspar into kaolin and other products (supposing kaolin to result
from the decomposition of plagioclase) involves several processes, of
which some are more likely to absorb than to liberate heat. But sup-
posing an anhydrous aluminium silicate formed without loss of heat, the
thermal results of its combination with water are by no means certain.
Were the water contained in kaolin not water of hydration, but chemically
combined, it would be possible from known experiments to compute approxi-
mately the heat which would be produced. It is shown in Chapter VII.
that the corresponding temperature would be so high as to be utterly at
variance with known facts. The water is therefore the water of hydration.
Of the heat involved in the hydration of salts we know that it is usually
small, that it is sometimes negative, and that the different molecules of water
combine with differing amounts of energy, but of the heat of hydration of
kaolin we know nothing.
With a view to testing the theory of kaolinization as far as possible,
Dr. Barus, at my request, undertook some very delicate experiments
' Friction iind the oxidaliou of pyrite have also been suggested, but have not been seriously advo-
cated.
StlMMARY. 3g9
presently to be described. The result of these experiments, in a word,
was that finely divided, almost fresh east-country diabase, exposed to the
temperature of boiling water and the action of saturated aqueous vapor for
a week at a time, and for several weeks in succession, showed no rise of
temperature perceptible with an apparatus delicate to the i-„Vu of a degree C.
It is by no means certain that kaolinization was effected by these exper-
iments. The particles of rock were indeed coated with a white powdery
substance, but this was probably the residuum of the evaporated water.
It is still possible that, when kaolinization occurs, heat is liberated. It
is also possible that at temperatures above the boiling point and pres-
sures greatly exceeding 760°"°, feldspars are kaolinized, but it appears
no longer reasonable to ascribe the heating of drifts, which are at nearly
normal pressure, to the reaction on the rocks of" water below the boiling
point. The scene of active and heat-producing kaolinization, if it exists at
all, must, therefore, be at remote depths. As was explained in a previous
paragraph, the present examination has not i-esulted in tracing any consid-
erable amount of kaolinization on theCoMSTocK; while, had the heat of the
Lode been maintained ever since its formation at the expense of the feld-
spars, but little undecomposed feldspar could now remain. In short, while
it cannot be demonstrated that the heat of the Comstock is not due to the
prevalence, at unknown depths and pressures, of a chemical change of un-
known thermal relations, I have failed to find any proof that it is due to
kaolinization.
soifataric action. — Of tlic origiii of tlic heat of solfataras not very much is
known; yet, as they commonly occur either as an accompaniment of vol-
canic activity, or in regions characterized by the strongest evidences of past
volcanic activity, it is usual and seems rational to connect them as cause
and eff"ect, or as different effects of a common cause. There seems to be no
special opportunity on tlie Comstock for an elucidation of the wliole theory
of vulcanism, but considerable grounds for connecting the heat there mani-
fested with that chain of phenomena.
That soifataric action, as commonly understood, once existed on the
Comstock is certain. That the time at which the Lode was charged with
ore is not immeasurably removed from the present, seems to be demon-
390 GEOLOGY OF THE COMSTOCK LODE.
strated by the trifling character of the erosion which has since taken place.
The water entering at the bottom of the new Yellow Jacket shaft in the win-
ter of 1880-81, at a temperature of 170° F., was highly charged with
hydrogen sulphide. The Steamboat Springs, only a few miles west of the
CoMSTOCK, lie in a north and south line like the Comstock, close to the con-
tact of ancient massive rocks and andesites. Some of them are boiling hot,
are charged with solfataric gases, and are now depositing cinnabar and silica
as at the time of Mr. Phillips's visit many years ago. There is much evi-
dence in the structure of the country and in the relations of the fresh rocks to
the decomposed masses that alteration was effected by rising waters, and
the chemical changes traced are such as could have been effected only by
vast quantities of soluble sulphides and carbonic acid, which could hardly
have been produced on the necessary scale except by the aid of heat. A
deep-seated source of heat, therefore, probably gave rise to the decomposi-
tion, and the conditions point to vulcanism as its source.
Source of the waters. — Thc flood of waters stlU rcqulrcs explanation, and an
hypothesis is suggested to account for it. No meteorological station exists
at Virginia City, but the rainfall is so small that the country is a sage-brush
desert, and the precipitation is insufficient to account for the water met with
on the Lode. The main influx of water, and especially of hot water, is
from the west wall, and when encountered it is found under a head often
of several hundred feet. Between the Comstock and the main rana^e of the
Sierra Nevada, the whole country is covered by massive rocks, principally
andesites, with occasional croppings of granite. The general structure of
the country, and the exposures of sedimentary rocks in the mines, lead to
the supposition that the underlying strata dip eastward, and the inference
is that the Comstock fissure taps water-ways leading from the crests of the
great range. If the heat is conveyed to the Lode by waters from great
depths, the variations in temperature are readily explained. The distribu-
tion of the heated waters would be determined by the presence of cracks,
fissures, and clay-seams, and the uniformity of distribution of heat would
further be disturbed, even at considerable distances from the surface, by the
infiltration of surface water. One published observation, which is impor-
tant in this connection, is that a large proportion of the rocks in the Vir
SUMMARY. 391
ginia mines are dry. This is very true in the sense in which "dry" is used in
mining, i. e., there are many places where water does not drip from the
walls, but the present examination has failed to reveal rocks which are not
moist ; indeed, the occurrence of really desiccated rock thousands of feet
below the surface, near vast quantities of water, would disprove the gen-
eralization of the perviousness of rocks, which is one of the best established
in geology. Unless, therefore, very strong proof to the contrary can be
adduced, the conduction of heat on the Comstock must be considered as
taking place in moist rock.
Discussion of the thermometric observations. Thc rclatlon of the tCmperatUrC tO the
depth from the surface is evidently one of great interest, but not entirely
simple. If the rock were wholly uniform in character and unfissured, the
relation of temperature to depth would be wholly regular and would be
represented by a curvilinear locus. As the source of the heat was ap-
proached the rate at which the temperature rose would rapidly increase,
and under the ideal conditions supposed, it would be possible to deduce the
constants of the equation and to calculate the position of the source of heat.
But unless the source of heat were so close to the surface that the errors
introduced by the presence of fissures, the lack of homogeneity of the rock,
and the percolation of surface water were insignificant in comparison with
the rate of increase of the temperature, such a calculation would not be
possible. A careful record of temperatures has been kept at three of the
newer shafts to a depth of above 2,000 feet. On plotting these temperatures
as ordinates and the depths as abscissae no indication of regular curvature
appears, being wholly obscured by the fluctuations due to the disturbing
causes mentioned. In other words, there is as yet nothing in the observa-
tions to show any but local divergences from a strict proportionality between
depth and temperature. The source of heat must, consequently, lie at a
very great distance from the surface as compared with the depth yet reached,
and the curve is to be regarded as still sensibly coincident with its tangent.
In order to eliminate the fluctuations of temperatui-e as far as possible
Mr. Reade and Dr. Barus have computed the observations madci at the
Forman, Combination, and new Yellow Jacket shafts by the method of least
392 GEOLOGY OF THE COMSTOCK LODE.
squares, and also, for comparison with them, the observations of Mr. J. A.
Phillips at the Rose Bridge Colliery.
Chapter VII. contains the details for these localities and for the famous
deep boring at Sperenberg, near Berlin. Here it is sufficient to state that
on the CoMSTOCK the temperature of the rock rises about 3° F. for every
additional depth of 100 feet, or about twice as fast as in ordinary localities;
and that boiling water will probably reach the workings at some point not
long after the 4,000-foot level is passed.
Observations have also been made in the Sutro Tunnel, and these when
plotted give a very remarkable result, for the curve shows that the tempera-
ture rises in a geometric ratio as the Lode is approached. This is capable
of no other explanation than that the east country is heated from a plane
in the immediate neighborhood of the Lode. Combined with the results
obtained from the shafts this curve, without any reference to geological
reasoning, indicates that the source of heat is at a vast depth compared
with that of the mines, and that the heat is communicated upward along or
near the fissure, and thence to the country rock by conduction.
THE LODE.
General character of the vein. The COudition of the LODE duriug the period
in which the field work for this report was done was not what could have been
wished, for almost the only ore in sight was the remnants of the great bonanza
of the Consolidated Virginia and the California, and the accessible exposures
of the vein were meager and unsatisfactory. The study of the Comstock
was thus necessarily directed to the conditions of its occurrence rather than
to details of vein structure.
A glance at the' surface map shows that the Lode is a long and wide belt
of vein-matter ramifying at each end into divergent branches,' and the cross-
section exhibits a remarkably regular foot wall dipping to the east at an
' TBe scale of the surface map is not large enough to permit all of the minor fluctuations of the
walls to be shown, nor are the mine maps sufficiently complete to furnish data for a full exhibition of
these irregularities on a larger scale. For more detail the reader is referred to Mr. King's section on the
331-foot level of the Virginia mines.
SUMMAEY. 393
angle of from 33° to 45°. Near the top, in most of the sections, one or more
secondary fissures diverge from the main Lode and penetrate the east wall,
thus cutting off a body of country rock, or "horse," which is approximately
triang-ular in cross-section. This horse is of variable vertical and horizontal
dimensions, and often divided by sheets of clay or quartz. Below the horse
the vein is for the most part narrow.
The walls. — The hanging wall of the Lode between the points at which
it branches is older diabase, which also extends some distance on the south-
east branch towards the Justice, and towards the Scorpion on the northeast
branch. Its limits in the latter direction are unknown. Almost all of this
diabase is in an advanced stage of decomposition. The foot wall of the
main fissure is granular diorite, except in Gold Hill, where this rock is re-
placed by metamorphic slates, and is much less decomposed than the hang-
ing wall. The northern and southern branches of the vein pass through or
along the contacts of various older rocks. The black dike or younger dia-
base appears in the Savage and Hale & Norcross, but not to the north of these
mines, and has been followed on or near the foot wall to the fork at the
Overman, and thence in a southwesterly direction toward American Flat.
It' is the behavior of the two diabases which has given rise to this fork, the
older diabase forming the hanging wall of the easterly branch for some dis-
tance from its origin, while the narrow dike of the younger variety marks the
course of the westerly vein. To the north also there are indications that
the direction of the branches was predetermined ; the northeasterly one by
the contact between diabase and diorite, and that which has been explored
in the Sierra Nevada and Utah mines by the presence of metamorphic rocks
and some intrusive stringers of diabase.
Contents of the vein. — Thc contcuts of thc veiu is simple on the whole.
Besides fragments of country rock, practically the only gangue which it
contains is. quartz; though calcite occurs In insignificant quantities in the
main Lode, and is the prevalent mineral in the Justice. The principal ore
is argentite, accompanied by gold, probably in a free state, though sulphur
salts occasionally form rich stringers and pockets. The distribution of ore
is very variable. That associated with the diorite carries a little gold and
almost no silver, while that associated with diabase is regarded as a silver
394 GEOLOGY OF THE COMSTOCK LODE.
ore, though nearly half its value is usually in gold. The proportion of the
two metals varies greatly in different portions of the Lode and even in the
same ore body. It is probable that the Comstock contains but little quartz
which is wholly barren; while, as is usual in silver veins, it is only in cer-
tain spots that the tenor reaches a point at which extraction is profitable.
These concentrations, or "bonanzas," usually occur in masses of quartz of
lower grade, and large bodies of quartz usually contain "bonanzas" when
they are associated with the diabase, though to this rule there are exceptions.
The Justice bonanza is the only one of any moment which is not associated
with that rock. The quartz is in great part in a highly crushed condition,
resembling nothing so much as ordinary commercial salt. When the fine
dust from such masses is examined under the microscope in polarized light,
it is immediately seen to consist of fi-agments of quartz crystals; the larger
particles can be shown to have the same origin by direct examination.
Very solid quartz bodies are also met with in certain positions.
Crushing action. — The prcseucc of faults on the Comstock is abundantly
proved, as has already been shown. The secondary fissures form one evi-
dence of such a movement, and as a large portion of the ore occupies the
openings between the great horse and the east country, it is plain that the
deposition of ore was preceded by faulting. The only movement which can
have crushed the quartz must also have been in the nature of a fault, and
some of the bonanzas show a parallelism in the lines of dynamical action
to the dip of the Lodk. It is not probable that the solid masses of quartz
were formed at a later date than those now found in a crushed condition,
for it appears to be onl}^ when the quartz has been deposited in sheets par-
allel to the west wall that it has escaped comminution. Certain stringers of
rich ore in the bonanzas have seemed to possess great solidity, and may
possibly have been formed after the final cessation of movement; other-
wise the entire period of quartz deposition seems to have been embraced by
that of the faulting movement. It is much more probable that the total
fault was accomplished by a great number of small slips in the same sense
than that one large throw preceded and another followed the filling of the
vein.
Clays. — The clays of the Comstock are not largely composed of kaolin,
SUMMARY. 395
but represent sheets of rock triturated and decomposed without any great
translocation of material. This fact is determined chemically, but confirmed
by the relation of the claj's to the faulted structure; for while they are excess-
ively abundant in and near the secondary fissure where the influence of the
surface interfered with the development of the regular system of fissures
found in the lower levels, they are comparatively rare and thin below the
bottom of the great horse.
Infrequency of lenticular openings. Thc SCCtioUS shoW that the loWCr pOrtioUS
of the Lode, considering its enormous scale, are narrow and remarkable
for the absence of the lenticular openings which frequently characterize
faulted veins. If the hypothesis developed under the head of the structural
results of faulting is correct, this peculiarity is almost a necessary conse-
quence of the conditions under which the Comstock formed, for the slip of
the actual walls of the vein is on that theory only the relative movement of
two successive sheets, and if these are assumed to be twenty -five feet thick,
it would not amount to above a hundred feet. The intensity of the fault-
ing action was less toward the ends of the Lode than near the middle,
the force being distributed over a wide area by the branching, and probably
also to some extent by numerous east-and-west fractures, singly of small
extent. The south end of the main Lode seems to have been less forcibly
faulted than the north end This is partly ascribable to the character of the
foot wall, stratified rocks being less rigid than massive ones, and partly to
the fact that the dip is about 10° less
Character of the spaces occupied by bonanzas. TllC evideUCe appCarS COnclusive that
the ore bodies occupy spaces which once inclosed only fragments of country
rock, with numerous interstices. These openings seem to have been due to
faulting action variously modified by local circumstances. In the Consoli-
dated Virginia and neighboring mines a projecting mass upon the foot wall
gave rise to a local rent in the diabase. In the Virginia group an irregu-
larity in the dip of the foot wall prevented the broken edge of east country,
the great horse, from following the main body to its final position; and a
crescent-shaped opening resulted which furnished an oj)portunity for the
deposition of an extensive system of bonanzas. In Gold Hill, on the other
396 GEOLOGY OF THE COMSTOCK LODE.
hand, the opening appears to be a result of non-conformity of the wall
surfaces brought into ojDposition.
Lateral secretion. — The course of the asccnding waters appears to have
been much influenced by the narrowness of the vein in the lower levels. It
is highly probable that on some straight or sinuous line, at depths greatly
exceeding those yet reached, the vein is closed nearly water-tight from one
end to the other. If so, water ascending in vast quantities, as it must once
have done, would be forced into the network of capillary fissures which
pervades the east country. Having become saturated with soluble sub-
stances by contact with the immense surface here exposed to its action,
it would seek the main fissure once more as the path of least resistance,
and there deposit quartz and ore through changes in physical conditions, or
in virtue of chemical reactions. It is not unlikely that concentration by
evaporation was an important influence in accelerating precipitation. The
character of the deposited quartz evidently varied greatly from time to time,
but though the causes were probably very simple in their general nature,
the conditions under which they acted, considered in detail, must have been
exceedingly complicated. On the whole, the later deposits were probably
the richer, and it is not impossible that a part of the rich pockets and string-
ers was formed at the expense of older deposits of lower grade.
The east wall of the Lode is in most places very indistinct, though
occasionally, as at the Savage connection with the Sutro Tunnel, nothing
could be clearer. This is due in part to the percolation of strong currents
from the east country during the deposition of ore, and partly to dynamical
action on irregular deposits crossing the lines of motion.
Probabilities for lower depths. — Thc CoMSTocK Is essentlally a deposit at the con-
tact of diabase with underlying rocks, and so long as the hanging wall shows
a heavy body of diabase the prospects for ore are good, mere depth not being
likely to exert any prejudicial eff"ect upon the ore-bearing character of the
vein. In the search for ores explorations should be confined to a moderate
distance from the diabase contact, for no important bonanza except the
Justice body has been found which does not extend to within a very short dis-
tance from this contact; nor are any bodies likely to occur far from it which
will pay the expense of discovery. The first condition for the formation of a
SUMMAEY. 397
quartz body is an opening to receive it. The group of mines worked through
the Union shaft and the Jacket, Crown Point, and Belcher mines show peculiari-
ties of structure which point to the likelihood of such openings at lower levels.
Openings such as that which contained the Consolidated Virginia and Cali-
fornia bonanza, however, give almost no warning of their approach from
above, and may at any time be struck in the intermediate mines; but a
series of bodies nearly on one level, such as were found in the secondary
fissure (the "Virginia vein") is not likely to recui-.
THE THERMAL EFFECT OF KAOLIISQZATION.
Kaoiinization hypothesis. — -The vlew that tho hcat of the CoMSTOCK is due to
the kaoiinization of feldspar is new and ingenious, but purely speculative, for
there is no unquestionable, direct evidence in support of it; while the process
is so complicated and so little understood that there is abundant room for
difference of opinion in any discussion of the theory involved. Dr. Barus
contrived and executed expei^ments to test the assertion that a rise of tem-
perature followed the action of heated waters from the east-country rock
of the CoMSTOCK. These experiments he has described and discussed in
Chapter IX.
The thermal effect of kaoiinization may be defined as the quantity of
heat generated by the action of the aqueous vapor on the unit mass of the
given feldspathic rock in the unit of time. It is to be regarded as a function
of the percentage quantity of feldspar originally contained in the given
rock, and of the temperature of this material, as well as of the time during
which the action has been going on. A priori the thermal effect may be
either positive, negative, or zero. The experiments were undertaken to
ascertain in liowfar the fundamental principle of the kaoiinization hypothesis,
namely, that the thermal effect is jjositive, agreed with facts. Such a research
was also desirable because of the intrinsic interest which attaches to the
question.
Considered from a physical point of view, the question is rather a diffi-
cult one, and of a kind in which satisfactory results can be reached only
398 GEOLOGY OF THE COMSTOCK LODE.
by a laborious process of gradual approximation. As even in final experi-
ments the thermal effect may escape detection, the purpose of the first experi-
ments may be said to consist in reducing the positive and negative limits
within which this effect must lie to the smallest possible interval.
Character of the experiments. — In proccsses such as kaolinizatiou, action may
usually be accelerated by an increase of temperature, provided that the latter
is not sufficient to render the products unstable in a normal case. In the
experiments it would have been desirable to act upon the rock with steam at a
temperature from the boiling point of water upward, but with the primitive
facilities available in Nevada, the use of superheated steam was not practi-
cable.
The apparatus in which the rock was subjected to thp action of steam
closely resembles that usually employed for the determination of the boiling
point of thermometers. The rock to be acted upon was crushed fine and
packed into a cyHndrical receptacle open at the top, and provided with a
wire-gauze bottom. This was supported in the steam space of a boiler
provided with an external packing. Th*^ object of the arrangement was to
allow the heat, possibly generated in the mass of rock by the process of kao-
linization, to accumulate.
Measurements of temperature. — The difference of tempcraturc between the in-
terior of the rock and the steam surrounding it was determined by the
aid of a thermopile consisting of three bismuth-platinum couples, one junc-
tion being placed at the center of the pulverized rock, and the other in the
steam-jacket surrounding the rock receptacle. The electromotive force was
measured by a method of compensation. The constants of the apparatus
were frequently rechecked, and divers precautions were observed in the
experimentation, and in the mathematical treatment of the measurements,
as is explained in Chapter IX. The means employed enabled the observer to
detect a variation in the difference of temperature between the two ends of
the thermopile as small as 0.001° C.
Details of apparatus and method. — The boilcr was heated by two kerosene stoves,
each containing two broad wicks. The oil could be replenished without
interfering to an appreciable extent with the flames. The water lost by
evaporation was replaced drop by drop by means of a simple device, and a
SUMMARY. 399
glass water-gauge indicated the progress of evaporation. The whole aim
was to make the process a continuous one, and, had it not been for acci-
dents, a nearly constant source of heat and a nearly constant water-level
would have made it possible to keep up an ebullition of nearly constant
intensity for an indefinite period of time.
The rock used was earlier diabase from the hanging wall of the Lode
collected in the main Sutro Tunnel. It had undergone only a trifling amount
of decomposition.
The experiments were continued during a period of nearly five weeks,
unfortunately with an accident between the first and second, and another
between the second and third. On the average, three observations of tlie
difi'erence of temperature of the ends of the thermopile, or, say, T—t, were
made during each twenty -four hours.
Mathematical treatment and results. — lu ordcr to obtaiu a comprchensivc view of
the large number of data obtained it will be sufficient to assume the empirical
relation,
where a and /? are constants to be calculated by the method of least squares,
X the time in hours corresponding to any particular T — t, and dated from
the commencement of the series of experiments to which the results belong.
Under variation of oc, an apparent thermal eff'ect not due to kaolinization
may be conveniently understood.
For a a mean value of — 0.05° C. was found. The interior of the rock
was, therefore, invariably colder than the surrounding steam It follows,
also, that it is impossible, even after the lapse of a great interval of time, to
heat so large a mass of material to an equal temperature throughout. The
variation of oc will add itself algebraically to /?; and unless the thermal
eff'ect of kaolinization is comparatively large, will entirely vitiate the sig-
nificance of the latter constant. /? gives nominally the rate of increase of
the temperature of the interior of the rock per hour in consequence of a
thermal effect. Instead of reporting /?, however, it is more expedient to give
the corresponding rate B referred to a year as the unit, viz.:
B — S,lQb/3
For reasons which appear in Chapter IX. the experimental data may
400 GEOLOGY OF THE COMSTOCK LODE.
be conveniently divided into two portions. In the first of these it was
found that
5=+l°.5d=0°.l;
in the second
5=-0°.9±0°.l.
Hence it appears that the variation of a alone was observed. The
vakies of B are to be regarded as an index of the errors incident to the
method in its present form, and it is moreover probable that the effect of
kaolinization is negligible in comparison.
THE ELECTRICAL ACTIVITY OF ORE BODIES.
Preliminary statement. — It is Well known that Fox, Rcich, and others made
experiments of great interest upon the electrical phenomena of ore bodies.
Bernhard von Cotta eai'nestly recommended that these experiments should
be further pursued, as they seemed to him likely to lead to results of prac-
tical importance in the discovery of ore bodies. If this recommendation
has ever been followed out, no account of the investigation has been
published. It was my earnest desire to see the subject pursued, and Dr.
Barus was invited to join the Survey on account of his special fitness
for this inquiry. All the plans and details of the electrical surveys made
are due to Dr. Barus, the general scope of the work and the localities only
being prescribed; and a r^sumt? of his results is given below. Neither of
the localities chosen was the best possible for the purpose. It was evi-
dently necessary in such an inquir}- to begin by the examination of ore
bodies already exposed. At the date of the examination* there was very
little ore in sight on the Comstock. At Eureka large bodies of ore were
exposed, but being in an oxidized condition would be likely to give weaker
currents than sulphides of similar quantity and distribution. These two
localities, however, were the only ones practically available, and at the same
time accessible through extensive workings. The results are nevertheless
of great interest, and a considerable advance has been made towards a solu-
tion. It is one of the plans for the future to repeat these experiments under
STJMMAET. 401
more favorable conditions. The following summary is in Dr. Barus's own
words :
Nature of the problem. — Tlic problem offered is not apparently a difficult one,
and consists simply in determining the variation of earth-potential at as
many points as may be desirable within and in the vicinity of the ore body ;
or, in other words, in tracing the contour and position of the equipotential
surfaces.
It is practicable, however, to systematize the method of research, a
priori. In the first place, Reich's hypothesis that lode-currents, if present,
are due to hydro-electric action is quite a safe and natural one. It is known
that a number of ores — especially sulphides — possess metallic properties.
The presence of two or more of these in the same ore body is not an
uncommon occurrence, and electric action at their surfaces of contact
may fairly be anticipated. The currents thus generated have a very
close analogy to those technically known as "local currents" in batteries,
which are due to impurities in the zinc. In the second place, it is obvious
that if currents are met with in a region of ore deposits, such currents must
be constant, both in intensity and direction, because electrical action has
been going on for an indefinite period of time. The equipotentials cor-
responding to this flow will, therefore, have fixed and definable positions
relatively to the ore body.
Suppose, now, that from a point remote from the ore body a line has
been drawn towards it and prolonged beyond to about the same distance.
It is not necessary for the present purposes that this line should actually
pierce the deposit; but only that certain of its parts should be sufficiently
near ore, and more so than its extreme points, and that it should lie wholly
within or entirely upon the surface of the earth. Suppose, moreover, that
the ores are so associated as to generate electrical currents.
If, then, beginning at one end of the line the values of earth-potential
are determined at consecutive, approximately equidistant points, it is obvious,
inasmuch as the line passes by the seat of an electromotive force, or, in
other words, through the field of sensible electrical action, that progress from
one extremity of the line to the other must be accompanied by a passage
of the corresponding values of earth-potential, through a maximum or min-
26 c L
402 GEOLOGY OF THE COMSTOCK LODE.
imum, or both, or a number of such characteristic variations. In short, the
earth-potential at any point may be regarded as a function of the distance of
this point from the assumed origin of the hne. The assertion that this
function will pass through a characteristic change of the kind specified is
only another way of stating that the line may be chosen so long that,
in comparison with its extent, the field of sensible electrical action will be
local, or its linear dimensions in the direction in question small. Maxima
in a general sense are, therefore, to be regarded as criteria, and as indicat-
ing the part of the line nearest to the electrically active ore body.
Practically, since we possess no means of measuring potential abso-
lutely, it is sufficient to assume a value (zero) for one of the points of the
series. The electromotive force between this and any of the other points
is then the potential of the latter.
Methods employed. — lu making thc actual measurements, the simple problem
above enunciated became quite complicated, because the small lode electro-
motive forces were afi"ected by a number of errors, which, in the aggregate,
might possibly produce an effect in the same order. On the Comstock,
where the mine workings were, without exception, in very barren or nearly
exhausted parts of the vein, no definite evidence of currents due to the Lode
itself was obtained. Even at Eureka, in spite of the enormous ore bodies in
sight, the range of variation of potential corresponding to a distance of
2,000 feet in the underground experiments very rarely reached 0.1 volt;
while usually this variation lay within a few hundredths of a volt. These
limits, in a case where such disturbances exist as action between terminals,
polarization, earth currents (normal), bad insulation of circuit at any point,
difference of potential between liquids in contact, incidental effects due to
masses of metal distributed throughout the mine, etc., are to be considered
as comprehending a rather dangerously small interval. This small varia-
tion of potential is to be attributed to the earthy character of the Eureka
ores. For the manner in which the effects of the disturbances were to a
large extent eliminated, the reader must be referred to Chapter X.
Of the different surveys made, the one on the 600-foot level of the
Richmond mine, west drift, presents the greatest interest, because it was
here that all the precautions necessary could be satisfactorily applied. The
SUMMARY. 403
line of survey, moreover, lay completely outside of the ore body, and all
the points tapped were in rock essentially of the same kind. The measure-
ments were made in various galvanometric ways, and the results were subse-
quently checked by a "zero" method. It was found that the distribution
of potential along the length of the drift, even after an interval of four
months, had not materially changed, and that, on passing from barren rock
towards and across the ore body, small though decided variations of poten-
tial were encountered in its vicinity.
Results. — The electi-ical effects observed were too distinctly pronounced
to be referable to an aggregate of incidental eri-ors, and they were of the
character which must have been produced had the ore bodies been the seat
of an electromotive force. The experiments made cannot be said to have
settled the question as to whether lode currents will or will not be of prac-
tical assistance to the prospector. Indeed, as yet it cannot even be asserted
with full assurance that the currents obtained are due to the ore bodies.
What has been observed is simply a local electrical effect sufficiently coin-
cident with the ore body to afford in itself fair grounds for the assumption
that these contained the cause. Giving the investigations of Fox and Reich
proper weight, however, the supposition that the currents in the Richmond
mine were not due to the ore bodies is extremely improbable. But, unfor-
tunately, they are so weak as to require an almost impracticable delicacy in
the researches designed to detect and estimate them. It is highly probable
that under certain circumstances more powerful currents are generated than
those found at Eureka. It is not unlikely, for example, that galena, cinna-
bar, and the copper sulpho-salts produce electrical effects of far greater
magnitude, and that the method might be readily available for the discovery
of such ores. The results thus give much encouragement to further inves-
tigations in this direction.
Method proposed. — lu the experiments thus far made, the variation of poten-
tial along a single Hue of electric survey only has been determined. It is
obvious, however, that in order to derive the full benefits from such a method a
number of these surveys must be coordinated. An endeavor should be made,
by passing toward and from the ore body in all directions, actually to deter-
mine the contours and positions of the equipotential surfaces. It is not im-
404 GEOLOGY OF THE COMSTOCK LODE.
probable that the interpretation of the results would furnish clews for
the economical exploitation of mines, comparable in value to those of a
purely geological character. Both should go hand in hand. Under ground,
this general method of research is not always feasible, as it presupposes that
the mine has been already widely exploited. On the surface of the earth,
however, it may to some extent be applied; and in this case the endeavor
would be to obtain the traces of the equipotential surfaces on that of the
earth. Suppose, for instance, that the potential at every point in several
square miles of the earth's surface were known. Then let this surface
be projected on a fixed horizontal ("Xr") plane, and the value of earth-
potential corresponding to each of the points be constructed as "Z." In
this way a new (potential) surface would be obtained coextensive horizon-
tally with the former. Terrestrial electrical action would manifest itself
upon the new surface as a whole and would not affect its regularity. Local
action, on the other hand, would produce an effect circumscribed in com-
parison with the horizontal extent of the area under consideration. We
should expect to find a hillock or depression, or both, or a number of these
inequalities in the imaginary potential sui'face.
NOTE TO CHAPTER III.
FELDSPAE DETBEMINATIONS BY SZABO'S METHOD.
In the present state of lithological science it is most desirable both to determine
the feldspathic constituents of rocks with accuracy and to bring the evidence of inde-
pendent methods to bear for this purpose. Where rocks are extremely coarsegrained,
and at the same time carry feldspars the solidity of which is unimpaired by cracks,
the results of the examination of cleavage flakes under the microscope leaves little to
be desired ; but such rocks are exceptional. The determination of feldspars in rock-
slides is subject to two disadvantages. Crystals of above a millimeter in diameter are
very likely to be broken in grinding, and thus to escape examination ; and though the
microscopist may often infer the presence of two or more feldspars, he can be abso-
lutely certain only of the most basic species present.
Szab6's method,' on the other hand, discriminates with great delicacy, not only
the well-established feldspar species, but also the mixed feldspars, perthite and loxo-
clase, and the intermediate feldspars, andesine and bytownite, as to the independence
of which mineralogists are not agreed. It is also particularly applicable to the larger
feldspars, so seldom obtained in perfect form in slides. The weakness of the method
Ues in the fact that it is not applicable to very fine-grained rocks, or to the minute feld-
spars of coarser rocks, unless a separation has first been effected by Thoulet's method;
but this limitation does not prevent its being excellently adapted to confirm and sup-
plement the results of microscopic examination.
At the time of writing Chapter III. I did not feel competent to apply Szabo's
method, never having had an opportunity of seeing it carried into practice ; but while
the proofs of this volume were under correction. Professor Szabo visited the country
and was good enough to illustrate his method experimentally to some of the members
of the Survey, including myself. After convincing myself of the accuracy of the
results obtainable and acquiring familiarity with the manipulation by repeatedly test-
ing a series of classical feldspars, such as anorthite from Monte Somma, labradorite
from Labrador, orthoclase from Baveno, etc., I proceeded to an examination of the
feldspars of the Washoe rocks, the results of which are given below. From five to
ten crystals in each specimen mentioned were tested, and no results obtained are
omitted.
• Joseph Szal)6, Ueber eine neue Methode die Feldspathe auch in Gesteiuen zu bestimmen. Buda-
Pesth, Franklin-Verein, 1876. See, also, Fouqu^ et L€vy, Min^ralogie Micrographique, p. 108.
(405)
406 GEOLOGY OF THE OOMSTOCK LODE.
Granite, close to Red Jacket mine.
The orthoclastic feldspar gave reactions corresponding to perthite or loxoclase,
showing a mixture of amazonite with a triclinic feldspar. This mixture is also readily
recognizable under the microscope. The triclinic crystals are in part oligoclase and
in part answer to andesine.' Under the microscope I have noticed no crystals more
basic than oligoclase.
Granular diorite, Bullion Eavine at Water Company's flume.
All the feldspars tested gave reactions for andesine, with a tendency rather towards
labradorite than towards oligoclase. Under the microscope the maximum angles found
answered to labradorite, but the occurrence of zonal structure was noted.**
Granular diorite, Utah, 1950.
In this specimen labradorite, andesine, and crystals of intermediate composition
were found.
Porphyritic diorite, Ophir Ravine, south side.
Labradorite and andesine only were detected.
Metamorphic diorite, Amazon dump.
All the feldspars tested were oligoclase, according with the microscopic results.
Quartz-porphyry, 1,000 feet south of Lawson's Tunnel.
Amazonite and oligoclase were found, as well as a feldspar slightly more basic
than oligoclase, but not so much so as andesine. This mineral is therefore much more
closely allied to oligoclase than to labradorite. No angles of extinction exceeding
those of oligoclase were observed in the slide. Oligoclase was also found in the rock
described on page 109 (slide 304).
Earlier diabase, Sutro Tunnel, north branch, 50 feet south of Ophir.
All but one of the crystals tested proved to be labradorite. The exception was
an andesine.
Earlier horublende-andesite, North Twin Peak.
A single feldspar had a composition intermediate between labradorite and ande-
sine. The remainder were characteristic labradorites. The zonal feldspar described
on page 61, and shown in Plate III., Fig. 13, is from the same cropping, though not
from the same specimen.
Earlier hornblende-andesite, 1,200 feet northwest of Geiger Grade Toll House.
The microscopic examination led to the sui^position that anorthite, labradorite,
and oligoclase were all present, the last, however, only as microlites. The crystals
tested by Szabo's method proved to be andesine and a feldspar intermediate between
this and labradorite. No anorthite was met with. This fact, however, of course does
' It is usual to regard andesine as peculiar to volcanic rooks, and the plagioclase of granite is
often supposed to be exclusively oligoclase. Professor Rosenbusch, however (Physiog. der Massigeu
Gest., II., 121), mentions finding plagioclaaes in granites which showed angles of extinction correspond-
ing to all of the feldspar species excepting anorthite.
' For some remarks on the indications of zonal structure, see page 61.
FELDSPAR DETEEMINATIONS. 407
not ijrove that none is present in the rock, but only that it is comparatively infrequent.
That it was not the predominant feldspar was also inferred from the microscopic
examination.
Augite-andesite, peak south of Crown Point Ravine, marked 7075.
Anorthite, bytownite, and labradorite were detected in this specimen. The
anorthite was found under the microscope, and its presence prevented the detection of
labradorite, except among the microlites. But on page 64 it is stated of the augite-
andesite that, though " anorthite has been identified in a few slides, • * * in most
cases the maximum angles of extinction correspond to labradorite."
Augite-andesite, above Ophir grade, due west of Belcher hoisting- works.
This, too, showed anorthite, labradorite, and an intermediate variety. Anorthite
was found also in the augite-andesite from Basalt Hill.
Later hornblende-andesite, quarry 2,000 feet northeast of Sutro Tunnel Shaft III.
All of the feldspars tested (more than a dozen) gave very sharp reactions for
andesine.^ Nearly all of them show zonal structure.
Later hornblende-andesite, quarry 2,000 feet east of Occidental Mill.
An andesine, an oligoclase, and several crystals of intermediate composition
were found. This accords excellently with the analyses of these feldspars made by
Mr. Dewey.*
Later hornblende-andesite, quarry above Utah mine.
Labradorite, andesine, and an intermediate variety were detected.
It was not found practicable to examine the feldspars of the later diabase (black
dike) or the basalt, on account of the fine grain of these rocks.
On the whole, the examination strongly confirms the results of the microscopical
analysis, and the only rocks in which the flame-reactions revealed feldspars which
might have been detected by the microscopic method are the granite and the quartz-
porphyry, each of which shows, in addition to oligoclase, an unsuspected, more basic
feldspar, which is, however, more closely allied to oligoclase than to labradorite.
While the optical behavior of a few of the large feldspars in the Washoe
andesites indicates that they are mixtures of different species, this is exceptional.
They are ordinarily polysynthetic individuals, showing only two angles of extinction.
In a large proportion of cases the crystallographic relations of the twinned lamellae
are further emphasized by the presence of zonal structure. Granting the accuracy of
Szabo's method, it is therefore extremely difficult to suppose that the prevalence of
crystals giving the flame-reactions for andesine in the andesites, and particularly in
the rock from the quarry northeast of Sutro Shaft III., is due to aggregations of two
distinct species. The uniformity of the reactions obtained is also an argument against
1 Professor Szabd examined two crystals from this specimen, and prononnoed them very charac-
teristic andesine.
" See pages 67 and 154.
408 GEOLOGY OF THE COMSTOCK LODE.
such a supposition. In examining some fine instances of so-called perthite from the
original Canadian locality and from others on the Atlantic coast I have found the
reactions extremely variable, depending, as one cannot but suppose, on the relative
proportions of the two component feldspars which happened to be present in the little
fragment tested. In the andesite referred to, on the other hand, the reactions of the
feldspars were as regular as those obtained from different fragments of a single stand-
ard feldspar. The facts, therefore, do not appear to me to warrant the supi^osition
either that these crystals are mixtures of different feldspar species independently
crystallized or that they correspond in composition to some one of the unquestioned
varieties. They must rather be set down as isomorphous mixtures, in the sense in
which that term is to be understood in Tschermak's theory of feldspar-composition.
INDEX TO MTNINa CLAIMS.
[Atlas-sheet rrr. — Map of the Wabuob District showing Mining Claims.]
Mine.
Latitude,
N.39°.
Minutes. Seconds.
Alabama
Alexander
Alhambra
AUen, or Peruvian. .
Alpha
Alpine
Alta
Alta (Patent)
Amazon
America
American —
American
American Eagle
American Flat
Andes
Andrews
Arctic
Argonaut
Arizona
Atlantic
Bailey
Baltic
Baltimore American
BaltimoreConsolidated
Belchapin
Belcher
Belcher Extension ...
Benjamin
Benton
Best & Belcher
Bluejacket.
Boehler
Bonanza
Boyle
Brilliant
Browne
Buckeye
Bullion
Caldwell
Caledonia
18
17
16
15
19
17
17
16
16
14
19
14
15
14
16
18
18
19
15
16
15
18
16
16
16
16
17
16
15
16
18
19
14
18
16
16
19
16
17
17
18
Minutes. Seconds
53
50
27
05
27
40
SO
30
35
45
07
45
15
45
30
40
25
50
50
50
40
00
35
40
35
45
15
45
45
35
27
25
05
55
42
50
20
00
50
15
50
Longitude,
W. 119°.
37
38
38
38
39
38
37
41
38
38
40
39
38
37
37
40
38
37
40
40
40
39
39
39
39
39
38
38
38
38
38
38
15
25
46
16
30
15
20
00
55
40
20
00
25
60
20
00
13
20
43
30
55
25
38
10
05
15
30
18
16
00
50
22
30
58
50
57
17
35
08
39
60
Mine.
Minutes. Seconds.
California
California
California Bank
Capital
Capital No. 2
Carolina
Carolina
Carson
Cavonr
Cherokee
Chollar
Chonta
Chonta
City of Melbourne
Clemens
Cliff House
Clyde
Cole
Colorado
Colossus
Columbia
Columbia
Columbia
Comet
Comet Extension ....
Compromise
Concordia
Confidence
Consolidated Virginia
Cook & Gray
Cosmopolitan
Cosmopolitan
Coso
Coupon
Coupon No. 2
Coye
Crevice ".
Cromer .
Crown Point
Crown Point Extension
Crown Point Ravine
Latitude,
N. 39°.
18
16
19
16
16
19
16
15
19
15
18
16
16
17
IB
16
16
18
19
19
19
14
14
IS
15
16
18
17
18
16
18
16
16
IS
15
14
17
15
17
17
17
Minutes.
40
20
40
30
50
35
18
00
27
30
05
45
28
20
52
00
50
50
20
40
30
10
05
30
15
40
35
35
30
10
10
30
20
15
05
30
20
66
20
05
30
Longitude,
W. 119°.
38
39
38
38
38
37
38
37
37
39
39
38
39
39
38
37
37
38
38
38
38
38
39
39
39
Seconds.
60
30
00
43
45
62
08
60
10
17
00
12
07
20
12
30
00
18
00
00
25
22
26
61
55
20
26
20
50
21
15
00
40
55
66
25
06
10
20
18
33
(409)
410
GEOLOGY OF THE COMSTOCK LODE.
Index to mining claims — Continued.
TWinft.
Latitude,
N.39°.
Longitude,
W. 119°.
Minutes.
Seconds.
Minutes.
Seconds.
19
20
37
05
17
20
38
30
17
15
38
37
17
10
39
10
14
40
38
10
19
45
36
55
16
55
39
27
16
15
38
02
15
20
38
11
14
20
38
15
16
50
37
40
15
15
39
OS
IS
55
41
15
15
30
38
40
18
50
37
30
18
10
37
35
19
27
37
46
14
45
38
15
15
20
38
20
16
50
39
15
16
55
39
15
14
55
38
40
17
35
38
37
15
30
39
16
19
35
37
30
15
50
38
28
16
10
38
15
16
15
39
05
16
40
39
30
17
10
38
25
18
15
38
10
19
50
36
45
15
36
38
10
18
65
37
15
15
06
38
35
15
30
38
30
17
30
38
45
17
46
39
15
17
25
38
30
18
45
38
15
16
20
38
10
15
20
40
30
19
40
37
10
16
33
40
20
16
15
37
30
14
55
38
20
16
55
39
10
19
35
37
55
16
55
39
06
14
55
38
10
15
55
40
40
Mine.
Minutes. Seconds.
Latitude,
N.390.
Minutes. Seconds.
Longitude,
"W. 1190.
Crystal Eidge
Cuiver
Culver Addition
Curtis
Daney
Daniel Webster
Dardanelles
Dardanelles
Dayton
DaytonNo.2
Dean
De Forest
Delaware
DelEey
Dexter
Dexter
Diamond
Dies Senor
Drexel
E. Cliapin
E. Uomstock
Edinburgll
E. Europa
Elevator
ElUot
E migrant
Enderwood
English Company
Enterprise
E. Overman
E. Savage
Esperance
Esper.iuza
Essex
E.Star
Eslelle
Europa
Excliequer.
E. Yellow Jacket
Fairfax
Flora Temple
Florida
Francisco Mareano . .
Frankel
Frauklin-German
French
Fi-ont Lode Consoli-
dated
Fry
Fry
Genesee
Georgia
German
German
Gila Mina
Glasgow
Glen
Globe
Golden Arrow
Gold Hill Company . .
Gold Hm Tunnel . . . .
Gold Lead
Gold Leaf
Goodman
Gould & Curry
Grant
Great Eastern
Great Western
Green
Grosh
Grosh Consolidated . .
Hale & Norcross
Hale & Horcross, S. E.
Extension
Hardy
Harlem
Hartford
Hawkeye
Hawley Consolidated
Hayward
Hector
Henry Clay
Hercules
Hillside
Holman
Imperial
Independent
Industry
Insurance
Iowa
Irving
Jackson
Jacob Little
James
Joe Scates
Joe Scates
John Auer
Julia
Julia, E. Extension. . .
JuliaNo.2
Jura
Justice
Kate
Kennebec
16
16
16
14
19
16
19
16
17
19
15
15
18
IS
18
14
15
17
17
18
17
18
15
16
16
14
17
19
19
18
15
16
17
19
15
18
19
19
15
19
19
18
18
18
17
17
17
16
16
18
18
20
15
05
50
40
40
10
40
30
10
35
40
20
50
05
35
50
35
33
10
45
45
30
05
20
05
27
15
37
35
15
20
38
55
35
45
00
45
10
10
50
30
15
18
60
60
50
55
20
35
SO
38
38
38
38
37
40
37
37
39
38
38
37
39
39
38
37
38
37
37
38
38
39
38
37
39
38
38
38
38
38
38
38
37
38
38
38
40
39
37
37
18
20
32
30
20
38
10
25
10
46
35
10
SO
10
30
15
40
00
50
00
40
35
10
55
30
15
05
15
00
35
35
30
20
15
50
02
50
37
20
48
20
26
33
30
50
25
35
20
02
35
4S
CLAIM MAP.
411
Index to mining claims — Continued.
Mine.
Latitude,
N. 39°.
Minutes. Seconds.
Keystone
Keystone
Knickerbocki-i-
Kossuth
Kossuth Extension . .
Lady Wasliingtoii . .
La Fayette
Lanzac
La Plata
Lassen
Lawson
Leo
Leviathan
Lexington
Lincoln
Little G-iant
Little York
Lookout
Lord of Lome
Lowery
Low Kange
Lucorue
Mackey
Manhattan Consoli-
dated
Margarita
MargaritaNo.2
Marsano
Marvel
Mary
Mary Ar^n
Maryland
McErlain
McGinnis & Bazaii . -
McKibben
Memuon
Memphis
Metela
Metropolitan
Mexican
Miami
Midas
Mill Site
Mill Site
Mill Site
MiUSite
Mint
Mississippi
Miasouii
Mitchell
Modoc Chief
Monte Christo No. 2 .
20
16
16
15
15
16
18
15
17
19
16
16
17
16
19
16
19
11
15
16
18
16
19
19
19
19
19
19
18
18
16
16
16
18
14
16
20
15
18
18
16
16
16
16
16
18
16
17
16
17
18
Minutes. Seconds.
15
35
48
00
15
30
25
30
15
40
30
05
10
00
20
03
25
45
00
55
20
07
10
55
20
15
35
10
15
50
40
10
20
42
10
35
02
00
50
40
00
49
45
35
20
10
OS
35
17
20
20
Longitude,
W. 119°.
38
39
40
38
38
39
37
38
39
37
40
38
39
39
38
37
39
38
38
38
37
30
37
37
37
38
38
38
40
38
37
39
38
39
38
38
38
38
38
38
40
37
40
38
38
38
38
10
10
05
05
00
05
30
45
40
35
40
35
20
07
65
30
45
40
40
00
24
50
05
30
25
20
15
10
15
05
00
10
40
12-
15
03
10
10
45
10
20
25
25
37
20
30
25
25
47
45
10
Mine.
Latitude,
N. 39°.
Minutes. Seconds.
Monte Cristo
Montezuma
Monumental
Mooney & "Whiteman.
Moore '& Morga n
Morning Star No. 2 . .
Mountain View
Nagle
N. Chipman
N. Comstock
N. Consolidated Vir-
ginia
N. Dayton
Nevada
Nevada No. 3
New Empire State
New Oregon
New York
New York Mill Site . .
Niagara
Nigger Ravine
N. Knickerbocker
N. Lexington
N. Mexican
N. Milton
N. Occidental
N. Ophir
North
Northern Light
Northern Light
N. Prospect
N. Star
N Star
Occidental
Ohio
Ontario
Ophir
Original Gold HUl ....
Orleans
Oro
Oro
Overman
Overman No. 2
Overton
Palmetto
Patten
Pearson
Peruvian, or Allen
Peytona
Phcenix-Westem
Pictou
Pioneer
18
18
17
14
17
18
19
16
17
19
18
15
16
16
20
16
16
16
16
16
17
16
19
19
17
19
20
18
18
17
19
15
17
19
17
18
17
19
19
17
17
16
19
18
16
16
19
19
15
16
10
Minutes. Seconds.
35
30
10
25
35
57
15
00
17
25
55
20
30
25
30
50
45
43
05
00
00
20
08
20
20
05
15
20
53
20
15
40
05
50
20
45
35
50
65
12
05
40
00
42
35
25
27
25
16
10
20
Longitude,
W. 119°.
37
37
38
38
38
37
37
40
38
38
38
38
40
40
38
40
39
39
38
39
38
37
37
38
38
37
38
37
37
40
37
37
39
37
39
39
39
39
39
39
40
38
37
39
37
15
40
50
15
30
30
45
10
47
10
05
30
30
24
15
05
07
02
40
00
50
10
05
40
43
15
50
20
00
55
15
40
43
40
50
50
28
30
45
40
35
25
20
12
00
SO
30
35
50
20
50
412
GEOLOGY OF THE COMSTOC^K LODE.
Index to mining claims — Continued.
Mine.
Minutee. Seconds.
Piute «
Plato
PlutuB
Porphyry
Potosi
Pnde of Washoe
Prospect
Red & White Cross. .
Keno
Rock Island
Rocky Bar
Roman Capital
R. R. Consolidated . . .
Sacramento
Sadie
SallieHart
San Francisco
Santiago
Savage
S. Belcher
S. California
S. Chipman
S. Consolidated Vir-
ginia
Scorpion
Sec. Line
Seg. Belcher
Senator
Sewell &Sheel
S.Grosh
Shamo
Shanley
Sheridan
Sherwood
Sierra
Sierra Nevada
Silverado
Silver Central
Silver Hill
Silver Leaf
Silver Leaf
Silver Star
Solid Silver
South Buckeye
South Comstock
South End
South Jacket
South Lucerne
South Overman
South St. Lonis
Southern Star
Stevens
St. John's
Latitude,
N. 390.
Minntes. Seconds.
16
40
16
00
18
35
20
10
17
55
18
20
17
10
18
20
18
35
16
15
18
40
18
25
17
50
19
10
18
30
19
30
15
65
15
05
18
15
17
00
16
60
17
10
15
19
16
17
18
14
17
18
18
19
16
16
19
16
15
16
17
15
16
19
15
15
15
17
15
16
15
15
16
17
40
05
48
10
00
40
20
45
38
15
50
OS
10
05
00
10
30
45
55
05
10
55
30
00
55
30
40
15
26
30
Longitude,
w. 1190.
38
38
40
38
38
38
38
37
38
40
39
38
39
37
39
37
39
38
37
38
38
37
37
38
38
38
37
38
37
38
39
38
38
39
48
05
20
45
05
05
00~
20
10
37
00
28
20
43
10
10
25
20
55
18
15
46
05
50
25
30
35
47
00
13
45
25
53
53
20
45
56
50
15
30
20
55
35
30
00
35
45
25
45
40
55
35
Mine.
Minutes. Seconds.
St. Lawrence
St. Loiiis
St. LoniB
Storey
Storey
Succor
Sullivan
Sunrise
Sonrifle
Superior
Sutro
Swan
Table Mountain
TamO'Shanter
T.&C. Brooks
Tarto
Tehama
Thomberg
Thornton
Troy Consolidated . . .
Tucker
Twin
Twin Peaks
Tyro
Union Consolidated . .
Utah
Utah
Utah, 1st N. E. Exten-
sion
Utah, 2d N. E. Exten-
sion
Venis
Vermont
Victoria Garber
Virginia Standard . . .
Vivian
Volcano
Vulcan
■Ward
Ward
Wasatch
Washoe Consolidated
Waters
W. Belcher
W. Crown Point
Webber ..
Wells-Pargo
Western
W. Justice
Woodville
W.Star
Yankee
Yellow Jacket
Latitude,
N.390.
14
19
16
17
17
16
16
19
17
17
19
16
15
15
14
16
17
19
19
19
17
16
17
20
18
19
16
19
18
18
19
18
16
15
15
19
17
18
16
19
17
17
14
19
15
16
16
15
16
17
35
35
00
05
00
10
50
05
00
35
30
40
35
50
10
13
30
50
00
35
00
00
05
15
55
35
55
40
50
55
48
05
40
15
20
30
45
38
25
35
32
20
15
40
30
"30
20
20
06
60
ao
I^meitude,
W. 1190.
Minutes. Seconds.
37
38
37
39
38
38
37
37
39
38
40
38
38
37
37
38
38
39
38
40
38
38
38
38
37
37
38
39.
38
*38
38
38
39
37
39
39
38
37
38
39
38
30
50
46
42
15
00
31
45
35
30
56
40
20
23
34
06
52
11
30
50
40
20
10
05
40
46
30
30
00
05
20
35
05
10
00
35
20
45
05
55
35
40
35
00
45
10
10
56
45
20
M
GENERAL INDEX.
(To facilitate the use of the geologioal map, Atlaa-sheet IV., the positions of the more noteworthy localities mentioned
in the text are indicated in the index by letters and nnmbcrs attached in parentheses. The map is provided with a cor-
responding series of letters and numbers answering to each minute of surface.]
Page.
Bams, Carl, computations by 245
his experiments on kaolinization 236, 397
on the electrical activity of ore bodies . 290, 400
Basalt, description of slides of 134
lithological description of 70
occurrence and age of the 205,373,383
Basalt Hill (B. G), augite-andesite of 71, 201
metamorphic diorite, 3, 000 feet SE. of 107
Belcher, assay of slate from the 155
cross-section through the 277
fault at the 278
later diabase of the 33, 152
ore on the 3, 000- foot level of the 278
quartz-porphyry of the 196
slates in the 191
Belts, mineral, west of the Kocky Mountains 2
Berkshire CaSoD, propylite from 143
Bisilicates, course of the decomposition of the. , 214
Black border of hornblende. {<See illustrations.)
Black dike. {See Diabase, later.)
occurrence and age 199
Blake, "W. P., on mineral belts west of the Rocky Mount-
ains 2
Boiler (kaolinization), description of 292
Bonanza, definition of 268
of the Consolidated Virginia and OcUifomia
mines 270
of the Consolidated Virginia and California,
comb-structure in the 270
of the Consolidated Virginia and California,
origin of the opening occupied by the 274
of the Consolidated Virginia and California,
rock fragments in the 270
Bonanzas. {See Ore.)
character of openings occupied by 395
occurrence of, on the Comstock 218
proportions of gold and silver in yield from, by
groups 9
Virginia group of 274, 275
Bristol, E. S., analysis by 152
Bullion. {See Gold, Silver, product.)
Bullion, ijuartz of the 17
V. Kichthofen on the prospects of the 23
Bullion Kavine {C. 3), assaysof granular diorite from the 154
diorite of 39,41,93,150,406
(413)
Alteration of minerals in »i(u, von Bichthofen on 20
A malgamation, barrel 6
pan 6
Atnazon {T). 7), analysis of metamorphic diorite from the,
tiible following page 151
analysis of metamorphic diorite from the. 155
metamorphic diorite of the 43, 106, 196, 406
American Flat (B. 5), analysis of augite-andesite fi-om . . .
table following page 151
analysis of quartz -porphyry from,
table following page 151
Analyses, table of follows page 151
Andesite 374
iSee Augite-andesite and Homblende-andesite.)
and propylite, Zirkel on 27
assays of 155
occurrence of 382
Apatite. (>See illustrations.)
brown, in augite-andesite 64
brown, in earlier homblende-andesite 56
brown, in eruptive dioiite 40
Ascension theory, applicability to the Comstock, von
Richthqfen on 19
Assays of Comstock rocks 154, 223
of rocks, results of 224
Attwood, Gr., analysis by 153
Augite. {See illustrations.)
conditions of crystallization of 63
formation of chlorite from 211
formation of pyrite from 210
orientation of obliquely cut twins of 113
with black borders 123
with contorted twin lamellfe 129
with pinacoidal cleavage in homblende-andesite . 55
Augite-andesite. ((See illustrations.)
analysis of table following page 151
description of slides of 126
feldspars of 407
independence of eruption of 202
lithological description of. 62
occurrence and age of 201
occurrence of ore in 202
peculiar weathering of. 65
Bag, advantages of, as terminal , 356
Baltimore, granite of the 34, 190
414
GENERAL INDEX.
Page.
C. &. G. (D. 3), assay of diabase from the 155
cross-acction through the 269
diorite in the 197
Calcite, occnrrence as gangne 219
Calcium and magnesium salts, relative solubility of. 212
Caledon ia, assay of granular diorite from the 154
assay of quartz-porphyry from the 155
later diabase in the 199
metamorphics in the 191
quartz-porphyry of the 47
remarkable flood in the 232
Calibration of thermo- element 298
Caii/ornia. analysis of ore from the 153
V. Richthofen on the prospects of the 23
Calif omia amd CoTisolidated Yir^mia bonanza 270
ore of the 219
sugar qnartz of
the 221
Carbonic acid as a solvent 226
part played by, on the Lode 386
C arpath ian Mountains, propylite of 81
Cedar Hill (D. 2), analysis of diorite from table fol-
lowing page 151
dioritesof 40,42,97,150,195
lithological character of 15
ore deposits of 1-6
quartz veins of 218,219,220,225
Cedar Hill Canon (D. 2), andesites of 55,56
earlier homblende-andesite from. 34, 122, 201
Cedar Hill veins part of the Comstock 220
Central, von Kichthofen on the prospects of the 23
Chemistry of the Lode 209,384
Chimneys produced by faulting 179
Chlorine, part played by, on the Lode 20, 386
Chlorite. {See illustrations.)
alteration of, to epidote • 213
alteration of the quartz and carbonates 213
characteristics of 369
decomposition of 77, 384
formation of, from ferro-magnesian silicates .- 74,
210, 384
solubility and diffusion of 40
OhoUar, analysis of clay from the table following
page 151
a.ssay of diabase from the. 155
diabase of the 116
diorite of the 96
eastern fissures in the 276
later diabase from the 151
Church, J. A., denies that the Comstock is a vein 30
diagrams of the Consolidated Virginiaaiid
California bonanza 270
memoir on the Comstock Lode 28,29
on diorite 184
on faulting 156
on loss of heat in the mines - - 239
on sugar quartz 30
on the heat of t ho Comstock 31, 230, 290
on the Justice ore body 30
on the lithology of Washoe 28
on ventilation of the mines 3
Circuit, breaks in, how detected 329
resistances of -ISO, 334, 338
Claim-map, preparation i>f -84
Clays, analyses of tablo following page 151
character of the 273
Page.
Clays, not essentially kaolin 217
of the Comstock 394
of the Lode, von Eichthofen on 17
Clifford mine, "Wales, temperature of the 228
Coldberry lead mine, electrical activity of the 311
Collections, enumeration of the 207
extent of the 369
Ooim^nation shaft (D. 4), assay of earlier homblende-
andesite from near the 155
earlier homblende-andeaite near the 56,
117, 151
temperature observations in
the 231,244,246,260
Commutator 328,354
Comstock ffssnre due to trachyte eruption, von Eicht-
hofen on 18
Comstock Lode. (iSee Vein.)
age of 184
bullion product of the 10
character of the, below the great
horse 267
Church's memoir on the 28
complex structure of the 269
conclusions regarding the history of
the 285
conclusions regarding the, von Eicht-
hofen's 23
condition of the, during examination.. 266
contents of the 393
contents of the, von Eichthofen on 16
continuity in depth, von Eichthofen .. 21
cross-section of the, through the C. £ C. 269
cross-section of the, von Eichthofen on . 14
detailed description of the 266
dip of the, in Gold Hill 277,283
dynamical action in the, von Eicht-
hofen on 16
electrical activity of the 317, 321, 322
faulting on the, von Eichthofen on . . . 15
general character of the 392
general outline of the 266
geographical position of the 2
history and statistics of the 368
history of the, Church on the 29
King's memoir on the 24
King's summary of the geologyof the. . 25
mean width of, von Eichthofen on ... 21
mode of occurrence, von Eichthofen on 14
narrowness of the, on the Sutro Tunnel
level 283
not a vein, according to Church 30
probabilities for its future 287, 306
probable character in depth, von Eicht-
hofen on 22
report on, by von Eichthofen 12
summary of the geology of the 368
walls of the 267
Comstock mines. (See Mines.)
Conclusions, von Eichthofen's, regarding the Lode 23
Conduction curve, the Sutro TMnnc^ temperatures form a. 263
Consolidated Virginia, cross-section through the 209
Consolidated Tirginiu and California bonanza 270
electrical activity
in the 317,322
Contact, electrical, with vein.. 318,324
permanent - 317
GENERAL INDEX.
415
Contact, i)ermanont position of, on 400-foot leTel of the
Eiclimond mine 337
position of, on 500-footlevel of Rich-
mond mine 334
position of, on 600-foot leTel of Eich-
mond mine 344
position of, on surface of Richmond
mine 51
temporary 317
Contact bag, description of 324
Contacts, metallic (plates and gada), objectionable 358
Cooling, influence of, on precipitation 226
Cornwall, electrical activity of ore bodies in 310,311,312
Cortez Peak, propylito from 142
Range, foothills of, propylite from 140
Councler, C, analysis by table following 151
Country rock attacked by acids 226
Cross-spur below graveyard, analysis of earlier horn-
blende-andesite from - . .
table following page 151
■propylite from li'Z
Cross-spur quarry (D. 2), analysis of later homblende-an-
desite from table following page 151
Crown PointRavine (B.4), angite-andesite from. 87, 126,128, 151
earlier hornblende- andesite from. .87, 125
epidote from 213
propylite from 86, 87, 135, 136, 137
Crushing action, eflects of 394
Curtis, J. S. , assays of rocks by 155, 223
Dayton, subaqueous eruptions near 14
Decomposition. {See illustrations.)
area of 72, 238, 383
due to aolfataric action 240
evidence of an external cause for the. .. 239
of blocks of rock 79
of minerals. (See each mineral.)
of the rocks 72, 209, 369
((SeeUthological description of each rock. )
rocks affected by 239
Devil's Gate (D. 6), raetamorphic diorite near 108
Dewey, F. P., analysis by 154
on the feldspars of the later homblende-
andesite 68
Diabase, assays of -.154, 155
earlier. (See illustrations.)
analysis of table following 151
description of slides of 112
feldspars of
homblendic 52
laminated 51
lithological description of 48
occurrence and age 197, 374, 381
relations to the Lode 197
silica contents of 152
later. (See Black Dike.)
description of slides of 116
lithological description of 52
occurrence and age of 198, 381
silica contents of 152
of Ophir Ravine, possibly a diorite 36
of Orange Mountain, N. J 53
selected for experiments on kaolinization 236
silver traced to 224
the possible source of ore 222
Diorite, assays of 154, 155
Diorite, eruptive 378
(See illustrations.)
brecciated 41
containing tourmaline 97
dark varieties 38
feldspars of 406
granular, analysis of table following
page 151
granular, description of slides of 93
hypothesis concerning 194
lithological character of 34
micaceous, analysis of table follow-
ing page 151
micaceous, occurrence in the Yellow
Jacket 277
micaceous, porphyritic, description of
slides of 104
occurrence and age 192, 381
porphyritic 39
porphyritic, analysis of . . .table follow-
ing page 151
porphyritic, description of slides of .. 97
porphyritic, laminated 41
porphyritic, silica contents of 152
possibility of metamorphic origin 195
relations of varieties of 42, 193
metamorphic, analysis of table following page 151
brecciated 43
descriptions of slides of 106
feldspars of 406
lithological character of 43
occurrence and age 195
relations to quartz-porphyry 43
Zirkel on 12
Dcelter, C, on propylite 90
Drill-holes, disposition of 325
without specific electrical effect 360
Dutton, C. E., on propylite 81
Earth-currents, intensities of. 346
discrepancies produced by 360
Earth-potential, constancy of 352
graphic representation of 342, 348, 354,
362. 366
how measured 320, 327, 328, 352
maximal values of 317
observed at the Richmond mine 340, 347,
350, 351, 355
series of values of 329
values of on 400 and 500-foot levels of
the Richmond mine 342
values on 600-foot level of the Richmond
mine 349
variation of, on the surface 351
variation of, with distance 342, 348, 352
East vein, a result of faulting 182
inferences from the 271
Eckart, W. R., mining machinery of the Comstock 1
Eldorado croppings, analysis of diorite from the table
following page 151
diorite dike near the 40, 42, 104
Electric survey across au ore body 344
over the surface 351
through an ore body (Eureka) 332
Electrical action, confined to particular parts of the ore
body 351,364
416
GENERAL INDEX.
Page.
Electrical action, local effect of, graphically represented . 367
Electrical activity of ore bodies 309
of ore bodies, causes of.310, 311, 312, 313, 314
of ore bodies, method employed in de-
termining 402, 403
of ore bodies, results 403
of the Consolidated Virginia Rnd Cali-
fomia 319, 322
of the Mexican 322
of tho Ophir 322
of veins, earlier work on 313
Electrical difference between zinc strips of terminals 357
Electricity of metallifereous veins, Foxonthe-309, 310, 311, 312
Hen wood on the. . .309, 311
Keich on the ..309, 312, 313
von Strom beck on
the 309,310
Electrometer for measuring earth-potential 353, 367
Electromotive force, how measured 295
zinc, earth, copper 323
Empire- Jmperiai, analysis of water from the 152
Energy, logarithmic distribution of, in taulting 162
transmission of, by friction 159
Epidote. {jSee illustrations.)
circumstances favoring the formation of 21]
_ derived from chlorite 75, 213, 370, 384
insolubility of 78
mode of occuiTence of 212
not formed at the expense of feldspar 76, 216
Equipotentials, contour and position of the, relative to the
ore bodies 365
surrounding the ore body 315
Erosion in Washoe 285
Errors in the electrical results for the 400 and 500-foot
levels, Richmond mine 343
Eurr'ka District, electrical activity of 324
Evaporation as a cause of precipitation 227
E.-qieriments on electrical activity of veins, hypothesis
underlying 314
Fault, as shown on the Suiro Tunnel level 282
diminution of, near ends of Lode 185
on the Lode, probably caused by rise of the west
country 185
on the TItak section 281
period of the 272
to the north, in the Sierra Nevada 281
Fault-curve, angle which tangentmakes with theborizon 172
equations of 161, 163, 164, 165, 180
point ofminimum radius of, curvature of ... 171
reduction and interpretation of the equation
for..*. 168
where two or more rocks are involved 172
with double cnrvature 173
Faulted surface, character of 177
Faulting 285
accompanied by parallel fissuring 174
Church on 156
evidences of 157
experiments on 165
influence on the width of the Lode on the Belcher
section 278
King on 156
on the Union shaft section 279
atnictural results of 156,377
theory of, application to landslips 390
Page.
Faulting, theory of, application to the Gomatook, and
other instances 179
theory of, application to veins 379
von Eichthofen on 156
Feldspars, decomposition of 78, 215, 271, 385
determined by Szab(3'8 method 405
external energy involved ia the decomposi-
tion of , . 234
inferences concerning composition of 407
of later homblende-andesite, analyses of . . . 155
representative composition of 233
Ferro-magnesian silicates, decomposition of 384
Fissure, the Comatock, due to trachyte eruption, von
Richthofen on jg
Fissures in hanging wall produced by faulting 178
in the Yellow Jacket dipping west 277
Floods supposed to have induced kaolinization 232
Florida, metamorphics near the 190
FloweryPeak(F. 2), later homblende-andesite from near 133
Flowery Range (F. 2), erosion on the 204
Fluorine, instrumentality of, on the Gomstock, von Richt-
hofen ou 20, 380
Foot walls, rise of, in faulting 378
i^orman shaft (D. 5), assay of augite-andesite from near
the -7 155
augite-andesite at the 30, 200, 202
cross-section through the 278
earlier homblende-andesite at the . . . 200
equation between temperature and
depth in the 262
eruptive diorite near the 192
qnartz-poi-phyry of the 47,196
temperature observations in the 231,
244, 252, 260
Fox, experiments on the electricity of metalliferous
veins 309, 310, 311, 312
Freiberg, electrical activity at 312, 313
Friction, coefficient of, of Gomstock rocks 186
part it plays in faulting 158
Reuleaux on 158
Fuel, consumption of, by the mills 6
transportation of , 6
Gaobro-like diabase 115
Gads, steel and copper, electromotive force and polariza-
tion of 318, 319
G-alleries, mine, length of 5
Galvanometer » 298, 320, 327
Gate of Mimroe, propylite from 144
Geiger grade toll-house (D. 1), 1,200 feet north west of the,
earlier homblende-andesite from 120, 121, 406
Golconda station, propylite from 141
Gold and silver, proportions of, in Gomstock bullion . .0, 9, 18
Gold detected in "Washoe rocks 223 .
Gold Hill and Virginia, population of 4
Gold Hill mines, 2,500-foot level of the 284
Gold Hill Peak (c. 4) , propylite from 86, 87, 137
GovXd (£ Curry and Savage, bonanzas of 17
Goupilli6re, Haton de la, on exploitation of mines 309
Granite, assay of 154
description of slides of 91
feldspars of 406
lithological description of 34, 373
occurrence and age of 190,380
Great Basin, character of 2
Greenstone, meaning of the term 376
GENERAL INDEX.
417
Page.
Grunow, "W., galvanometer made by 208,327
HabitnB, value of, in rock determinationB 85
Hague, Arnold, analysis by 153
Hagae, J. D., on the system of timbering in the mines. . 6
Hale dk Norcross, analysis of clay from the
table following page 151
analysis of water from the 152
cross-section through the 276
flood in the 232
Hanging wall, rise of, a rare occurrence 179, 378
Hawes, G. "W., on the feldspars of the later hornblende-
andesite 67
Heat. (See Temperature.)
casualties from, in the mines 3
distribution of 230
loss of, in the mines 229
nonnal increment of 229
of the Comstock, Church on the 31
depth of the source of the. be-
low the surface 240
explanations offered 231
mines 3
relations to surface rocka 241
results of thermal survey con-
cerning 264
phenomena of the lode 228, 387
Heinrich, F., on the Siwrenberg boring 245
Hen wood, W. J. , on electricity of metalliferous veins . . 309, 31 1
History of mining on the Comstock, by E. Lord 1
History of the Lode, Church on the 29
Hoffmann & Craven, mapping by 284
Holzappel, v. Strombeck's experiments at 31 0
Hornblende. (See illustrations.)
Hornblende, black-bordered. (See illustrations.)
characteristic of andesite . . 84
brown, alteration to green 36
decomposition of 74
disseminated through groundmassof ande-
site 84
formation of chlorite from 211
formation of pyrite from 210
green, fibrous, chlorite mistaken for 84
green, resulting from alteration of brown . . 41
speculation on the black border of 59
with double black border 54, 123
with inclusions, probably of ilmenite 95, 98
Homblende-andesite, earlier. (See illustrations.)
analysis of table fol-
lowing page 151
containing finely dissem-
inated hornblende 120
containing ilmenite 118
description of slides of . . 116
feldspars of 406
lithological description of 53
occurrence and age 199
occurrence of ore in . . . 201
with disseminated horn-
blende 54
with excess of augite ... 55
with very large horn-
blendes 57,201
later. (See illustrations.)
analysis of table fol-
lowing page 151
27 0 L
Page.
Homblende*andesite, later, description of slides of 130
determination of 383
Dewey on the feldspars of. 68
feldspars of 153
Hawes on the feldspars of 67
lithological description of. . 66
occurrence and age of 203
slight erosion of 203
Zirkel on a feldspar In 69
Horse, the great 267
Horses, large, characteristic of upper levels, von Kicht-
hofen on 22
near surface, von Richthofen on 16
produced by faulting 179
Hydrosulphuric acid in YeUow Jacket water 240
Ilmenite. (See illustrations.)
decomposition of 215
in earlier homblende-andesite 56,118, 123
lithological description of 145
probable acicular inclusions of, in hornblende 38, 40
Imperial, diorite of ravine (C. 4.), west of the 41
Inclusions, black, acicular. (jS'ee illustrations.)
fluid, as diagnostic points 50
containing salt cubes in eruptive diorite 37
in homblende-andesite 121
secondary. (See illustrations.)
secondary 79, 119, 371
glass. (See illustrations.)
double. (See illustrations.)
occurrence in propylite 85
Intensities, constancy of 341, 348
observed on the 400-foot level of Kichmoud
mine 338
observed on the 500-foot level of Kichmond
mine 336
Investigation, previous, of the Comstock Lode 12
Johnson, S.W., analysis by table following page 151
Julia, assay of diabase from the 155
earlier diabase near the 197
flood in the 232
Justice, bonanza of the 394
Church on the 30
croppings near the 278
eruptive diorite in the 193
metamorphic diorite of the 196
ore bodies, probably of same age as the others . . 220
ore of the 219, 220, 225
quartz-porphyry near the 33
Kaolin, a hydrate 235
heat of hydration of, unknown ^ 235
not identified in the rocks 216
not prevalent on the Lode 78, 371
reactions leading to formation of 234
Kaolinization, criticism of the evidence that heat is pro-
duced by 232
discussion of experiments on 304
discussion of the theory of 233
disposition of apparatus for 295
errors in experiments on 308
evidences of heat caused by 231
experimental results discussed 304
experiments on 236
general plan of experiments 290
heat evolved by 235
heat of the Lode attributed to 231
418
GENERAL INDEX.
Page.
Kaolinization hypothesis 388
hypothesis, conclusions regarding the 237
not prevalent at Washoe 218, '237
results obtained from experiments on 304,
306, 307. 399
results of, final expression 307
thermal eflfect of 290,397
Karpathian Mountains, propylitic character of the 13
Kentuck, analysis of ore from the 153
King, Clarence, memoir on the Comstflck Lode 24
on fanlting 156
on propylite 81
on quartz-propylite 13
on the rela^tions of propylite and ande-
sit« 24
on the structure of bonanzas 271
on the west wall of the Comstock 25
summary of the geology of the Com-
stock = 25
Kittlcr, E., diflerence of potential of liquids in contact.. 358
Kormann, W., analysis by table following page 151
Lady Bryan, eruptive diorite in the 192
Lamination of country rock 182
Landslips, theory of 187,380
Lateral secretion theory 225, 385, 396
Lawson'g Tunnel (B. 5), quartz-porphyry 1,000 fe«t south
of 108,150
Leucoxene, occurrence of '. 215
Litbology. {See Rocks.)
importance to theory of ore-deposits 32
of the Washoe District 32
of "Washoe, Church on the 28
summary of 369
Tx)de, Comstock. (See Vein.)
Lode. (See Comstock Lode.)
the Comstock, detailed description of 266
Lode currents, hypothesis underlying experiments on. . . 314
opportunities forinvestigating at Eureka. 324
Lode electromotive force, bow measured 320, 327
Lord, Eliot, history of mining on the Comstock 1
on the product of the Lode 7
Machinery, mining, of the Comstock, by W. R. Eckart. . . 1
Magnetite, secondary 214
Mallet, R., theory of teirestrial heat applied to Washoe. 231
Matteuci, Ch., on earth currents 352, 360, 367
McClellan Peak (6 west), basalt near 71
MeKibben Tunnel, assay of granular diorite from 154
diorite of the 33, 39, 42, 75, 76, 87, 99, 102
epidote in the 213
rocks of the 28, 87
sheeted structure in the 183
Uetamorphic rocks, Whitney on the age of 13
Metamorphics in the Sierra I^evada 280
occurrence and age of 190, 380
Method of least squares, application to temperatures 245
Mexican^ electrical activity of 317,322
tenor of ores of 18
Mica. (See illustrations.)
Mica, biaxial 130,131
formation of chlorite from 211
Milling 6
of slimes and tailings 6
process used 6
returns guaranteed 6
Mills, table of supplies used by the, in 1879 8
Page.
Mine maps 284
Mines, bullion product of the 10,11
development of the, in 1865 14
extent of the 5
heat of the 3
importance of the 1
length of gaUerics 5
system of timbering in the 5
table of supplies used in 1879 8
the Comstock l
ventilation of the. Church on 3
Mineral belts west of the Rocky Mountains 2
Miners, average weight of 4
good condition of 4
hours of labor of 3
number employed 4
wages of - 4
work performed by 4
Mining, commencement of, on the Comstock 2
difficulties of, on the Comstock 2
Mister, W. G., analyses by table following page 151, 153
Moisture in the rocks, electrical effects of 356
Montezuma Range, propylite from 139
Moore, G. E., analysis by table following page 151
Mount Abbie (C. 2), later homblende-andesite of. .70, 133, 205
Mount Butler (B. 4), lithological character of 15
Mount Davidson (C. 3), age of 89
diorites of 33, 35, 42, 88. 193
infiuence on the Lode 156
King on 2£
probably an uplift 18£
propylite from 143
sheeted structure of 182
slope of 377
von Richthofen on 13, 14, 15
Mount Button, Utah, propylite from 143
Mount Emma (G. 4), younger hornblende-andesite of. . . 69
and Mount Rose, divide between, later
hornblende-andesite from 134
Mount Kate (F. 5), augite-andesite from 127
Mount Rose (F. 5), analysis of later homblende-andesite
from table following page 151
later hornblende -andesite of 69, 153, 204
Munroe.Gateof, Utah, propylite from 144
Occidental grade, augite-andesite of the 34
Lode, occurrence of 202
Openings, lenticular, infrequency of, on the Comstock . . . 395
Ophir, analysis of ore from the 153
analysis of water from the 152
assay of porphyritic diorite from the 1.^4
electrical activity in the 317,322
tenor of ore of the 18
Ophir grade, augite-andesite of 128, 407
Ophir Hill (B. 3), earlier homblende-andesite near 116
Ophir Ravine (C. 2), assay of porphyritic diorite from... 154
diabase of 51
diorite of 33, 36, 83, 98, 102, 103, 152, 406
epidote in 213
propyUteof S2,83,86,I3H
Orange Mountain, N. J., diabase of 53
Ore. (See Bonanzas.)
Ore and quartz, origin of 221
bodies, cause of electrical activity of, on the Com-
stock 323
electrical activity of 309,400
GENERAL INDEX.
419
Oro boilies, likely to bo found in the hanging wall, von
Richthofenon
Uepoaits, magnetic efifects of 309,
depoaitiou of
distribution of
found on the west face of the diabase
influence of the rocks on the
minevala of the Comstock 270,
near the surface on the Union shaft section
occurrence of, on the Comstock
precipitation of, from solution
probabilities of finding at great depths
probable cause of fluctuations in the tenor of the
relation of the, to tlie rocks 218,
small amoimt of, in sight during examination
source of, relations of decomposition to
source of, von Kichthofeu on
tenor of
Ores, analyses of
earthy character of, at Eureka
electrical properties of 311,
how dissolved
of diflerent grades, time relations of
of the Lode, von Richthofen on
uniform character of, von Richthofen on
jield of
yield of, von Richthofen on
Osbiston shaft (D. 3), diabase in the
Outer liquid
Overman, assay of diabase from the
assay of porphyritic diorite from the
earlier diabase in the
later diabase in the
partial cross-section through, the
quartz -porphyry of the 47, 110,
Plagioclase with zonal structure in eruptive diorite
zonal structure of. (See illustrations.)
Population of Virginia, Gold Hill, and Silver City
Potential, continuity in variation of, with distance. . .354,
ditference of, between terminals
Precipitation accelerated by evaporation
conditions affecting 226,
Product, bullion, from tailings
of the Comstock Lode, by mines, table.
of the Lode, Lord on ■
Propylite 81,
{See illustrations.)
analysis of table following page
and andesite, Zirkel on
association with silver ores, von Richthofen on
causes leading to its determination
European
' from other districts thau Washoe
from Utah, descriptions of slides of
King on its relations to andesite
typical localities of
von Richthofen on
von Richthofen's, based largely on Washoe oc-
currences
Zirkel's diagnostic points baaed largely on
Washoe occurrences
Propy lites of the Fortieth Parallel, description of slides of
Prospecting, rules applicable to
Pumpelly, R., on rock-weathering
Pyrite, fonnation of 75,
heat of the Lode attributed to tbe oxidation of. .
Page.
Pyrite, relations of, to ore 222
relations to the ferro-magnesian silicates 210
Quarry 1,000 feet west of the Tellmv Jacket ed&t shaft
(C. 4), earlier hornblende-andesite of the 119
500 feet N. of N. Twin Peak (C. 4), andesite of 34
1,500 feet SW. of Justice (C. 5), assay of quartz-
porpiiy ry from 155
2,000 feet E. of Occidental Mill (E. 5), later horn-
blende-andesite of: 34, 67, 69, 131 , 407
2,000 feet NE. of Sutro shaft III., later horn-
blende-andesite from 34, 66, 69, 130, 151, 407
2,000 feet NE. of Sutro shaft III. (E. 4), assay of
later hornblende-andesite from 155
near the Sierra Nevada {D. 2), younger horn-
blende-andesite of 70
near the Utah (D. 2), assay of later hornblende-
andesite from 155
near the Utah, later hornblende-andesite from ... 34,
131, 407
Quartz and ore, origin of 221
crushed, von Richthofen on 16
deposited in openings • 273
diflerence between east and west, von Richthofen
on 16
gold, on Cedar Hill, von Richthofen on 16
how dissolved 226
in earlier hornblende-andesite 121
occurrence of solid and of crushed 270
precipitation of, from solution 226
secondary, characteristics of 85
sugar. Church on 30
von Richthofen on 17
Quartz -poi-pbyry 373
(See illustrations.)
analysis of table following page 151
assays of 155
decomposition of 79
description of slides of 108
feldspars of 1 10, 406
felsitic variety of 47
lithological description of 45
occurrence and age of 196
separation of, by Tboulet's method 110
von Richthofen on the 13
Quartz-propy lite. King on 13
Rath, G. vom, on propylite 90
Ravines produced by faulting 177
Keade, F., computations by 245
Red Jacket, assay of granite from the 154
granite of the 33,34,91,190,405
Reich, F., experiments on the electricity of metalliferous
veins 309,312,313,317,365
Resistance of circuits 330, 334, 338, 345, 351
of layers of rock between consecutive similar
surfaces 359
of rocks decreases with porosity and moist-
ure 341,348,351
specific, of rock in place 359
Results for earth-potential from different methods 355
Reuleaux, F., on ftiction 158
Richmond mine chambers, Nos. 7, 10, 11, 12, 13, 14, 15, 16,
etc 332
Nos. 14 and 15 connected elec-
trically 350
disposition of points, 500-foot level 335
disposition of point.s, 400-foot level 338
420
GENEEAL INDEX.
Page.
Richmond mine, electrical activity of 324
plau of drifts and workings, 400 and 500-
foot levels 333
plan of drifts and workings, 600-foot
level 344
Kichinond mine ore bodies, extent, horizontal, of 332
relative position of the 331
Kichthofen, F.von, conclusioos as to the Comstock 23
estimated yield of the Lode to the
close of 1865 9
his predictions verified 12
on erosion 271
on faulting 156
on fluorine and chloi-ine 20, 386
on propylite 81
on the alteration of minerals in situ. . 20
on the applicability of the ascension-
theory 19
on the contents of the Lode 16
on the continuity of the Lode in depth. 21
on the east vein 182
on the mode of occurrence of the Com-
stock 14
on the probable character of the Lode
in depth 22
on the proportion of gold to silver in
Comstock bullion 7
on the quartz-porphyry 47
on the rocks of the Washoe District. 12
on the source of ore 18
on widespread solfataric action 21
report on the Comstock Lode ,. 12
ilickard, E., data concerning the Eureka ore bodies 331
W. F., analysis by 153
Kock subjected to action of aqueoas vapor, description
of 299
Kocks. (See Lithology.)
assays of 154
conductivity of 341, 348, 351
eruptive, means of determining snccession of 188
general character of the decomposition of the . . . 209
metallic contents of 223
moisture of the 241
occurrence and succession of the 188, 380
of the Distiict, typical character of the 374
of the Washoe District 32,372
of the Washoe District, Zirkel on the 26
special localities of, in the Washoe District 33
their relations to ore-deposits 32
Washoe, disputed character of the 33
which contain silver and gold 225
Hock-chamber 293
difference of temperature of outside and
_ interior of 300, 302, 303
Sock laland, granite at the 34, 190
metamorpbics in the 191
Rock-maases, concentric weathering of 371
Rose Bridge Colliery, temperature observations in the .245, 254
Rosenbusch, H., on propylite 90
Roux's ranch (C. 5), assay of basalt from near 155
basalt near 33
felsitic quartz-porphyry near 33
Sandberger, F., on the lateral-secretion theory 385
on the metallic contents of rooks 221
Page.
Savage, analysis of clay from the table following
page 151
analysis of oro from the 153
analysis of water from the 152
assay of porphyritic diorite from the 154
diorite of the 96
flood in the 232
later diabase in the 199
Savage and Gould t£ Curry, bonanzas of the 17
Savage shaft, cross-section through the 274
Schemnitz, propylite of 90
School statistics of Storey County 5
Scorpion croppings (E. 2), position of the 193,279
Shale in tbe Richmond mine 334, 345
Shafts, temperatures in various 391
Sheep Corral Cafion, propylite from 138
Sheeted structure of country rock discussed 182
on the C. d 0. section 271
Sierra Nevada, assay of diabase from the 155
cross-section through the .. - 280
earlier diabase in the 115, 198
eruptive diorite in the 99, 192
later hornblende- andesite near the 204
limestone in the 192
Sierra Nevada Range, water from the 243
Silica determinations of rocks 152
Silicates, ferro-magnesian, decomposition of the 214,369
Silver and gold, distribution of, in the Comstock 268
in rocks compared with yield 224
proportions of, in Comstock bullion . . .6, 9, 18
Silver City, basalt just west of 134
railroad (C. 7), earlier homblende-andesite
from 123
veins in homblende-andesite near 201
Silver Hill, eruptive diorite in the 192
near tbe 104
metamorphic diorite of tbe 196
Silver Terrace (E. 3), analysis of augite-andesite from
table following page 151
Silver traced to the augite of diabase 224
Slate, assays of 155
Slides, detailed description of 91
method of reference to . 145
SodaUte in granite 34,92
Solfataraa, von Richthofen on 19
Solfataric action, age of the 206
heat ascribed to 237.389
widespread, in the Washoe District, von
Richthofen on 21
Solfataric gases, part played by, on the Lode 386
Solutions in contact, electromotive force of 357
saline, in contact holes 357
Skeers lead mine, electrical activity of 311
Sperenberg boring, temperature observations in the. -245, 256
Sphene. {See Umenite and Titanite.)
Statistics of mining in preparation by tbe Census 1
school, of Storey County 5
Steamboat Springs, solfataric gases of 240
Valley, propylite from 138
Storey County, school statistics of 5
Storm Cauon, propylite from - 84,139
Stratification, eruptive 182
Stretch, R. H., mapping by 284
Stringers from the Lode 220
GENERAL INDEX.
421
Page.
Stiombeck, A. von, on eleotrioity of metalliferouB veins,
309, 310
Substitution, theory of 387
Snccession of eruptive rocks, means of determining 188
Sugar Loaf Mountain {F. 3), later homblende-andeaite of 70
lithological character of . . 14, 394
Sugar quartz, origin of the 272
Sulphydric acid in water from the Yellow Jacket 240
Sulphurets, formation in the vein, von Richthofen on. . . 20
Solphuric acid as solvent 226, 386
Summary 36ft
Supplies, table of, brought to the Lode in 1879 8
used by the mines and mills in 1879... g
Surface, electrical survey over 351
Survey, thermal. {See Thermal survey.)
Sutro road, earlier homblende-andesite from the 124
Sutro Tunnel, air shaft, augite-andesite near 129
analysis of earlier diabase from the
table following page 151
assay of diabase from the -.154, 155
assays of rock from the 223
augite-andesite from the 127, 129, 201, 202
cross-section through the 274
earlier diabase from the
33, 112, 114, 115, 151, 152. 107, 406
earlierhomblende-andesitefromthe.54, 124, 199
epidote in the 212
eruptive diorite in the 105, 192
experiments on kaolinization of diabase
from the 236
later hornblende-andesite of the 24, 203, 205
laterals, temperatures in 261
level, horizontal section on the 281
logarithmic character of section on 180
temperature curve influences from the 263
temperature observations in the 231, 244,
258, 260, 392
Syenite, von Kichthofen on 12
Zirkel on the supposed 12
Szab6, J., feldspars determined by method of 405
on propylite 90
Table of analyses follows page 151
analyses 153, 152
assays 154
school attendance in Storey County 5
supplies brought to the Lode in 1879 8
supplies used by the mines and mills in 1879 8
the bullion product from tailings 11
the bullion product of other mines in the District II
the bullion product of the Lode 10
Tailings, b»Uion product from 11
Temperature, difficulties in obtaining mean 229
disturbing influences affecting, in mines.. 220
equation between depth and 244, 258
in the Sutro Tunnel, equation between dis-
tance from the Lode
and 245
in dependent of surface
radiation 260
measurement of small increments of. . .291, 295
(Sea Heat.)
normal increment of 229
observations 231, 244
in the Sutro Tunnel laterals. . 261
resultsfrom 201
Page.
Temperature equations, agreement between, for the
shafts and the tunnel 263
high, of the mines 228
in mine workings not accordant 230
reasons for some fluctuations 260
source of high, on the Lode 264
Terminals, description of 318,324
electromotive force between 326
polarization of 323
Thalen, R.. magnetic indications of deposits of iron ore. 309
Theory of lateral secretion affirmed 225
Thermal survey 244,391
conclusions from the 264
results of, independent of accurate ther-
mometers 264
Thermo-electromotive force, correction for extraneous
effects 297
Themio-element 294
calibration of 298, 304
constants of 299, 300, 301, 302, 303, 304
Thermometers, results of thermal survey independent of
accurate 264
Thomson, SirW., increment of temperature adopted by. 229
Timber, consumption of, in the mines 6
source of supply on the Comstock 3
Timbering, J. D. Hague on the system of 5, 6
Titanite possibly identical with lencoxene 215
Topography r.ear the Lode a result of faulting 181
Tounnaline in eruptive diorite 35
in metamorphic diorite 44, 108
Trachyte, sanidin, von Richthofen on 13
Ti-ansformations in the vein, chemical, von Richthofen on . 20
Transportation, means of, to the Comstock 3
Transylvania, propylite of 90
Truckeo Range, propylite from 139
Tschermak's feldspar theory, evidence in support of 408
Twenty-five-hundred-foot level, partial section on the . . 283
Twin Peak, north (C. 4), analysis of earlier hornblende-
andesite from table following page 151
north, assay of earlier hornblende-andesite
from 155
north, earlier hornblende-andesite of 34, 61,
118, 406
south (C. 4), andesite of 87
propylite of the 86,87
Union shaft (D. 3), assay of granular diorite from the 154
cross-section through the 279
diorite of the 35,38,94
eiTiptive diorite northeast of the 193
JTtahf diabase in the 280
diorite of the 96,406
earlier diabase in the 198
younger hornblende-andesite nearthe 34, 66, 69, 70
Utah, propylite of 81
Vein, contents of the 268
{See Comstock and Lode.)
Vein-matter, angular fragments of country rock in 16
nature of the so-called 271,282
Vein-minerals, how dissolved 226
origin of 221
precipitation of, from solution 226
Veins, application of theory of faulting to 186,379
Ventilation of the mines, Church on d
Vertical longitudinal projection of bonanzas 284
Virginia and Gold Hill, population of 4
422
GENERAL ESTDEX.
Page.
Virt!;ima range, King on 25
Vivian, assay of earlier homblende-andesite from near
the 155
V'olcanic action as a scarce of heat at Washoe 231, 238
Volcano, metamorpbic diorite near the 34, 43
Wagon Canon, propylite from 140
Wales Consolidated, granite at the 190
^ metamorpbic diorite at the . . .43, 190, 195
Wall, east, impregnation of, von Kichthofen on 15
indistinctness of tbe 273
Wall rock, particles of, accompanying ore, von Richt-
hofen on 17
west, relations to slope of Mount Davidson, von
Ricbtbofen on 14
WaUs of tbe Lode 267,393
litbological character of, von Ricbtbofen on 15
Waller Defeat shaft (D. 5), analysis of diorite from near..
table following page 151
assay of porpbyritic diorite from
near 154
Ward, earlier diabase near the 51, 197
Washoe District, bullion product of tbe 10, 11
mines. {See Mines.)
Water, influence of the narrowness of the vein on the
path of rising 283
mine, analysis of 152
of tbe mines, possible explanation of the hea<l of. 243
possible origin in tbe Sierra 242
source of, unexplained 241
variations of temperature of the. - . 242
source of hot 390
supply of the Comstock 3
the vehicle of heat 265
Weathering of rock masses 371
Werlau, experiments made by von Strombeck at , 310
West Gate, propylite from 142
West wall, character of, King on 24
Whitney, J. D., on the age of the metamorpbic rocks of
tbe District 13
Wire, faults in 320,327,328,363
Page.
Woodward, R. W., analysis by table following page 151
Workings, extent of the 284
Wrmkle, L. F. J., mapping by 284
Yellow Jacket, analysis of clay from the table follow-
ing page 151
analysis of ore from the 153
analysis of propylite horse from tbe
table following page 151
analysis of water from the 152
cross-section through the 270
diabase in the 51,115, 198
eruptive diorite in the 192
later diabase in tbe 199
slates in the 191
temperatures of the 231, 244, 250
water of the 230
Yellow Jacket shaft, equation between temperature and
depth in the 262
temperature obsei-vations in the 230,
244, 250, *260
Yield. (iSeeProdnct, Gold, Silver, Bonanzas, Richthofen,
Lord.)
of ores, von Ricbtbofen on 18
Zacat-ecas, propylitic character of 13
Zeolites, occurrence of, von Richthofen on 17
Zero method 295,296,320,353
Zircon in earlier hombleude-andesit« 56
in eruptive diorite 38,39,40
in granite 34
in metamoi'phic diorite 44
in quai-tz-porph>Ty 45,47
in slides 92, 94, 96, 98, 104, 108, 109, 119, 142
Zirkel, F., on dacite 13
on diorite 12
on feldspar in later homblende-andesite 69
on propylite 81
on propylite and andeait^ 27
on quartz-porphyry 47,111
report on the Washoe rocks by 26
Zonal structure of plagioclase 37,01,67
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